Multifunctional inorganic nanomaterials for energy applications

Abstract

Our society has been facing more and more serious challenges towards achieving highly efficient utilization of energy. In the field of energy applications, multifunctional nanomaterials have been attracting increasing attention. Various energy applications, such as energy generation, conversion, storage, saving and transmission, are strongly dependent upon the electrical, thermal, mechanical, optical and catalytic functions of materials. In the nanoscale range, thermoelectric, piezoelectric, triboelectric, photovoltaic, catalytic and electrochromic materials have made major contributions to various energy applications. Inorganic nanomaterials’ unique properties, such as excellent electrical and thermal conductivity, large surface area and chemical stability, make them highly competitive in energy applications. In this review, the latest research and development of multifunctional inorganic nanomaterials in energy applications were summarized from the perspective of different energy applications. Furthermore, we also illustrated the unique functions of inorganic nanomaterials to improve their performances and the combination of the functions of nanomaterials into a device. However, challenges may be traced back to the limitations set by scaling the relations between multifunctional inorganic nanomaterials and energy devices.

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Huilin Wang

Huilin Wang received his BS degree from Jilin University in 2018. He is currently pursuing MS under the supervision of Prof. Dongfeng Xue at Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. His research involves the preparation of rare earth crystals for energy applications.

Xitong Liang

Xitong Liang received his BE degree from Changchun University of Science and Technology in 2016. He is currently pursuing PhD under the supervision of Prof. Dongfeng Xue at Changchun Institute of Applied Chemistry, Chinese Academic of Sciences. His research involves the preparation of rare earth and transition metal oxide materials and their application for energy storage devices. He has published 6 papers since 2016.

Jiutian Wang

Jiutian Wang has a BE degree in Applied Chemistry from Harbin Engineering University, China (2013–2017). He is currently pursuing PhD under the supervision of Prof. Dongfeng Xue at the State Key Laboratory of Rare Earth Resource Utilization at Changchun Institute of Applied Chemistry, Chinese Academy of Science. His research involves the electrochemical performances and mechanisms of colloidal clusters for energy storage applications.

Shengjian Jiao

Shengjian Jiao received his BE degree in 2018 from the Harbin Engineering University. He is currently pursuing MS under the supervision of Prof. Dongfeng Xue at Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. He mainly works on the development of electrochemical energy storage materials and devices.

Dongfeng Xue

Dongfeng Xue is a full professor at Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, where he received his Ph.D. in 1998. He served as the associate editor of CrystEngComm and Fellow of the Royal Society of Chemistry in 2018. His research interests focus on the calculation and simulation of functional materials and the multiscale crystallization of inorganic matter for energy and optical applications. He has published over 400 papers in peer-reviewed journals with h = 64 and more than 20 invited book chapters.

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1. Introduction

Water, food and energy are always needed by society.^1,2 In fact, the history of human civilization is also a history of storing and utilizing energy.³ Today, the world's population is about 7 billion and is estimated to grow to 9 billion by 2050 and then to about 10 billion by 2100.⁴ Such rapid population and also economic growth will place additional demands on the global energy supply. The International Energy Agency has projected that the world's energy demand will increase from about 12 billion tonne oil equivalents (t.o.e.) in 2009 to either 18 billion t.o.e. or 17 billion t.o.e. by 2035 under their current policy or new policy scenarios, respectively.⁵ Carbon-dioxide emissions are expected to increase from 29 gigatonnes per year to 43 Gt per year or 36 Gt per year under the current and new policies, respectively. The data given above also reflect the fact that energy shortage and environmental deterioration resulting from insufficient fossil fuel supplies and increasing consumption have become two major global problems for human beings.⁶ As a result, developing new technologies to make full use of abundant “green” energy sources, such as solar energy, mechanical energy and thermal energy, is an effective way to meet our long-term energy needs and achieve sustainable environmental development.⁷ The use of energy in the twenty-first century must also be sustainable. Access to clean, affordable and reliable energy has been a cornerstone of the world's increasing prosperity and economic growth.

It is widely acknowledged that human life is closely related to various energy applications, which are needed to address the global challenge of producing plentiful and sustainable energy for the future. The various energy applications include energy generation, conversion, storage, saving and transmission applications.⁸ This review covers a wide range of applications in the renewable and conventional energy sectors. Looking for new ways of generating electricity in addition to traditional fossil fuel combustion remains a vibrant topic for addressing the demands on energy supply, in which the harvester generates electrical energy form heat, light and mechanical vibrations. Electrical energy is subsequently rectified, conditioned and stored within capacitors or batteries for various applications, such as wireless and self-powered sensors or low-power electronics.⁹ In addition to energy generation, human society also relies on energy conversion, which is the transformation of energy from forms provided by nature to forms that can be used by humans. In recent years, considerable attention has been devoted to certain direct energy conversion devices, notably solar cells and fuel cells.¹⁰ Although there are several types of renewable energies, such as wind, solar, tidal, biomass and geothermal energies, they are inherently intermittent and generate huge fluctuations when they are incorporated into the power grid. Therefore, we need advanced energy storage devices to make full use of the various energy sources.⁷ Before shifting from a fossil fuel economy to one based on renewable technologies, there is clearly a pressing need to significantly improve our ability to store energy.⁷ Recently, a lot of research has gone into batteries and supercapacitors. Furthermore, an additional major challenge is that the solar, wind and hydro energy sources are often located far from densely populated areas that are the customers for the renewable energy utilization, which means that electricity generation from these distant renewable energy sources must be combined with cost-effective energy transmission systems.¹¹

Multifunctional materials are a type of materials or material-based system that are designed to perform multiple tasks via a judicious combination of various functions. They are expected to deliver system-level efficiency beyond their individual parts. Generally, their function may participate concurrently or sequentially, and can be implemented on the same length-scale or hierarchically organized. Often, applications demand the multifunctional features of materials, as shown in Scheme 1. These remarkable features may include special optical, magnetic and electrical properties, among many others, which could potentially have great impacts on electronics, spintronics, medical applications and other devices.¹² Integrating different functions in one material system is a fundamental challenge. Such a system would need to be carefully designed to perform multiple responsibilities through combinations of different functions. Typically, each function contributes a distinct physical or chemical process that can deliver system-level improvements beyond the individual parts.¹³ Batteries require electrode materials with high electrochemical reactivity, good electrochemical reaction reversibility, excellent chemical stability and low toxicity. The additional functionality of the battery needs materials with special features. For example, in flexible batteries, the electrode must have a flexible function that maintains high performance even under severe bending. In solid-state batteries, the electrolyte material needs to have high ionic conductivity to ensure the ion transport rate inside the battery, which is not available in general solid material. In the nanoscale range, thermoelectric, piezoelectric, triboelectric, photovoltaic and catalytic materials have made major contributions to various energy applications. Due to the unique properties of inorganic nanomaterials, such as excellent electrical and thermal conductivity, large surface area and chemical stability,¹⁴ multifunctional inorganic nanomaterials play a key role in developing advanced energy applications with enhanced performance.^14,15 In energy generation applications, nanoscale materials have helped to nearly double the performance of thermoelectric materials.¹⁶ In energy conversion applications, nanostructuring is an advanced strategy for increasing the active sites of catalytic materials.¹⁷ These recent advances have the potential to propel further the prospects of tuning the hybrid properties at the nanoscale, which could lead to next generation materials for energy applications. Multifunctional nanomaterials research is the study of how the structures of materials determine their properties, including their design and fabrication.

Scheme 1 Various energy applications, such as energy generation, conversion, storage, saving and transmission, are strongly dependent on the different functions of materials. Thermoelectric, piezoelectric, triboelectric, photovoltaic, catalytic and electrochromic nanomaterials have made major contributions to various energy applications.

The use of multifunctional inorganic nanomaterials for energy applications faces significant challenges. These challenges are mainly from the exploration of novel materials, such as exploring new material structure systems, excavating new properties of existing structures and improving the performance of existing materials. Nanomaterials present chemical, physical and electrical properties that change as a function of the size and shape of the material (Fig. 1).^18,19 Nanomaterials have very high proportions of surface atoms and they are often subject to property variation as a function of size, owing to quantum confinement effects.¹⁸ Taking metals as an example, the evolution of energy levels is presented in Fig. 1b from that of individual and separated atoms to that in clusters of 2, 5, 20 and 10²⁰ atoms.¹⁹ The electronic state energy and electron delocalisation degree are shown in Fig. 1c. Valence electrons are able to move between the occupied and unoccupied orbits easily within the valence band, offering different levels of semiconductivity.¹⁹ In general, the study of multifunctional inorganic nanomaterials at the nanoscale provides richer possibilities, broader perspective, and more flexible approaches to overcome these challenges.

Fig. 1 (a) Conceptual free-energy landscape for the formation and interactions that define the typical state of nanomaterials in the environment. This figure was reproduced from ref. 18 with permission from The American Association for the Advancement of Science, copyright 2019. (b) Schematic illustrations of the band model for chemical bonding between metal atoms. (c) Corresponding energy levels of the valence electrons as a function of the degree of delocalization of valence electrons in the cluster of metal atoms. This figure was reproduced from ref. 19 with permission from Taylor & Francis Group, copyright 2017.

2. Energy applications

2.1 Energy generation applications

Energy generation, also known as energy harvesting, aims to convert ambient forms of energy, such as mechanical motion and heat, which are otherwise wasted, into useful energy (in many cases electrical energy).⁹ This can make a valuable contribution to solving the issues of the expected fossil fuel shortage in the near future as well as could help to reduce emissions and toxic waste into the environment.²⁰ In this section, the energy-generation approaches cover thermoelectric, piezoelectric and triboelectric effects. The realization of these approaches depend on the multifunction of materials. Over the past decades, scientists have focused on the research of multifunctional materials, which has promoted the development of the energy-generation field. However, the performances of devices, such as the output voltage, conversion efficiency and stability, still cannot meet practical demands. The development of nanotechnology has given rise to the dawn of resolving these challenges. When the size of materials is reduced to the nanometre, many of their properties, such as electrical conductivity, thermal conductivity, dielectric constant and mechanical quality factor, change due to unique superficial effects, bulk effects, quantum dimension effects and macroscopic quantum tunnelling effects. Therefore, multifunctional nanomaterials are of great significance for energy generation.

2.1.1 Thermoelectric materials. The thermoelectric (TE) effect enables the harvesting of thermal energy by means of a temperature gradient, typically across a semiconductor or metal, producing electrical current and voltage (Seebeck effect) or vice versa (Peltier effect). The scheme is shown in Fig. 2a. The most important parameter for evaluating the performance of thermoelectric materials is the thermoelectric figure of merit (ZT), which is defined bywhere σ is the electrical conductivity (S m⁻¹), S is the Seebeck coefficient (V K⁻¹), S²σ is defined as the thermoelectric power factor (W m⁻¹ K⁻²), κ_t is the thermal conductivity (W m⁻¹ K⁻¹) and T is the absolute temperature of the material (K). From the perspective of materials, in order to improve ZT, some effective approaches include increasing the electrical conductivity and the Seebeck coefficient, and reducing the thermal conductivity.

Fig. 2 (a) Schematic of a TE module made from a thermocouple of p- and n-type thermoelectric legs. This figure was reproduced from ref. 21 with permission from John Wiley and Sons, copyright 2019. (b) Schematic showing the interdependence of the various TE properties with the carrier density (over three orders of magnitude), illustrating the challenge of trying to optimize the material ZT. The shapes of the individual curves are extracted from actual data on single-walled carbon nanotube networks. This figure was reproduced from ref. 16 with permission from John Wiley and Sons, copyright 2018. (c) Evolution of the maximum ZT values for some typical TE materials. This figure was reproduced from ref. 22 with permission from John Wiley and Sons, copyright 2017.

It is noteworthy that these property parameters are dependent on each other. Since typical TE devices are based on the electromotive extraction and diffusion of the majority carriers, either electrons or holes, optimization of the parameters is related to the carrier concentration. As shown in Fig. 2b, with the increase in carrier density, the electrical conductivity increases, which is beneficial for ZT materials. However, the Seebeck coefficient decreases and the thermal conductivity increases, which are detrimental to ZT. At a certain temperature, a maximum ZT value of the material may be reached by tuning the interdependence of σ, S and κ_t.¹⁶

Classical theories and experiments have shown that nanomaterials can offer dramatic improvements to the ZT. The earliest theoretical study on thermoelectric nanomaterials was carried out by Hicks and Dresselhaus in 1993. They suggested that both nanoscale interfaces and quantum-confined nanomaterials would greatly improve the ZT.^23,24 Subsequent experimental results confirmed that nanomaterials have a nearly doubled ZT value to bulk materials. Among numerous thermoelectric nanomaterials, chalcogenides, such as tin selenide, lead telluride and bismuth telluride, and carbon-based compounds, such as carbon nanotubes and fullerene, have shown advantages and attracted extensive interests (Fig. 2c).

Tin selenide (SnSe) has sparked much attention since the discovery of its record ZT of 2.6 ± 0.3 at 923 K along the b-axis of a single crystal. However, due to the special demands of crystal growth techniques, the prospective high cost for production, and undesirable mechanical properties, SnSe single crystals are not well-suited for thermoelectric devices. Therefore, polycrystalline and doped SnSe has become a promising alternative candidate for single crystals. A thin film of SnSe nanosheets was fabricated by using a simple thermal evaporation method, and exhibited a ZT of 0.055 at 501 K.²⁵ Despite the value being lower than for the SnSe single crystal, it was the highest reported ZT for a SnSe film above 70 K. Such an excellent thermoelectric performance was due to the unprecedentedly low κ_t (0.08 W m⁻¹ K⁻¹ in the temperature range between 375 and 450 K), which was primarily due to the reduction of the lattice component.

A p-type polycrystalline Cd-doped SnSe was synthesized by combining cation vacancies and through localized-lattice engineering, and its ZT was approximately as high as 1.7 at 823 K. A high cation vacancy concentration of about 2.9% contributed to a high hole carrier concentration of about 2.6 × 10¹⁹ cm⁻³, which in turn led to a high S²σ of about 6.9 μW cm⁻¹ K⁻². Meanwhile, the doped Cd resulted in massive nanoscale crystal imperfections, including dislocations, intensive local lattice distortions and point defects, which contributed to a low κ_t of approximately 0.33 W m⁻¹ K⁻¹.²⁶ Cation-doped SnSe nanomaterials could enhance the TE performance of p-type polycrystalline SnSe by Ag/Na dual-doping and Ag₈SnSe₆ (STSe) nanoprecipitates. The Ag/Na dual-doped p-type polycrystalline SnSe led to a two orders of magnitude increase in the carrier concentration, which in turn resulted in a sharp enhancement of σ and a high Seebeck coefficient. Moreover, an ultralow κ_t of below 0.3 W m⁻¹ K⁻¹ was realized for the Sn_0.99Na_0.01Se-STSe sample at 773 K, which represented an approximately 20% reduction compared to the pure SnSe. This was caused by the nanostructured SnSe matrix with dispersed nanoprecipitates of the compound Ag₈SnSe₆, which further strengthened the scattering of phonons. Consequently, Sn_0.99Na_0.01Se-STSe exhibited a high peak ZT of 1.33 at 773 K and an outstanding average ZT (ZT_ave) of 0.91 in the temperature range of 423–823 K.²⁷

Lead telluride (PbTe) is a state-of-the-art thermoelectric material at mediate temperature. The highly symmetric crystal structure assures it a high band degeneracy and thus a decent effective mass. The bond inharmonicity caused by the slight displacement of Pb atoms determines that the lattice thermal conductivity can be maintained at a relatively low value. Tremendous efforts have been devoted to improving the thermoelectric performance of PbTe-based materials. A PbTe-4%InSb composite was fabricated through introducing InSb multi-nanophases to the n-type PbTe matrix.²⁸ A highest ZT of 1.83 was obtained for the sample PbTe-4%InSb at 773 K. The highest average ZT reached about 1.0 for the sample PbTe-5%InSb, which is a new record for n-type PbTe-based materials. The outstanding performance resulted from the simultaneous enhancement in S²σ and reduction in κ_t across the entire temperature range. The enhanced S²σ was mainly due to the significantly increased absolute Seebeck coefficient via multiphase energy filtering effects, and the reduced κ_t can be understood via phonon scattering by the incorporated multi-nanophases.²⁹

A Sb-doped and GeTe-alloyed n-type thermoelectric nanomaterial was reported. GeTe alloying enlarged the band gap and electron effective mass, leading to a considerable enhancement of the Seebeck coefficient to −280 μV K⁻¹ at 673 K. Meanwhile the high density of point defects resulting from the supersaturated state led to greatly decreasing κ_t to 0.56 W m⁻¹ K⁻¹ at 573 K. Moreover, the formation of nanostructures could further improve the S²σ and reduce κ_t simultaneously. As a result, the Pb_0.988Sb_0.012Te-13%GeTe-nano achieved a ZT value of 1.38 at 623 K and a high average ZT_avg value of about 1.04 in the temperature range of 300–773 K.³⁰

In addition to cationic doping, the enhanced TE performance of anionic-doped PbTe nanomaterials has also been reported. As an example, a high ZT of approximately 1.4 at 900 K was realized in 1.25% Sb-contained and 12% S-introduced PbTe. The introduction of S opened the band gap of PbTe, which suppressed bipolar conduction and increased the electron concentration and electrical conductivity. Furthermore, point defects induced second phase nanostructuring, which led to a strong reduction of the κ_t to approximately 0.5 W m⁻¹ K⁻¹ at 900 K. An increase in the Seebeck coefficient of PbTe was also observed by S replacement for Te, due to the larger effective mass of electrons of PbS compared with that of PbTe. All these optimizations contributed to the improvement of the ZT.³¹

Following the success of nanotechnology in selenide and telluride thermoelectric materials, sulfide nanoparticles were also studied. An orthorhombic@cubic core–shell nonstoichiometric Cu₅FeS₄ icosahedral nanoparticle was reported, which contained high-density twin boundaries in the form of fivefold twins. Due to inclusion of the high-density twin boundaries and a tuned fraction of Fe-deficient cubic-structured Cu₅FeS₄, the thermal and electrical transport properties were synergistically optimized. The S²σ was enhanced to about 0.39 mW m⁻¹ K⁻² at 710 K. Meanwhile, the κ_t maintained a very low value of about 0.43 W m⁻¹ K⁻¹ within the whole temperature range from 300 K to 710 K. These outstanding properties of Cu₅FeS₄ icosahedral nanoparticles resulted in an enhanced ZT value of about 0.62 at 710 K.³²

Carbon-based nanomaterials, including carbon nanotubes, graphene and fullerene, are other potential thermoelectric materials in virtue of their unique electronic structures and excellent mechanical properties. A highly pure semiconducting single-walled carbon nanotube (s-SWCNT) network with equally large n- and p-type thermoelectric performance metrics was successfully fabricated. The results pointed to strong correlations between the ultimate power factor and the electrical conductivity, both of which also correlated well with the diameter of the bundles within the s-SWCNT networks. The κ_t appeared to decrease with decreasing the s-SWCNT diameter, leading to a peak material ZT of approximately 0.12 with diameters in the range of 1.0 nm.³³

The in situ growth of Cu₂Se on the surface of CNTs has also been studied. A series of Cu₂Se/CNT hybrid materials were fabricated. Due to the high degree of homogeneously dispersed molecular CNTs inside the Cu₂Se matrix, large reductions in lattice thermal conductivity and carrier concentration were observed, leading to a record-high ZT of 2.4 at 1000 K.

TiS₂/n-type fullerene hybrid films integrate mechanical flexibility and outstanding thermoelectric properties together. Assembling 0D C₆₀ nanoparticles onto 2D TiS₂ nanosheets not only significantly increased the Seebeck coefficient to about −130 mV K⁻¹ and thereby the S²σ to about 375 mW m⁻¹ K⁻², but also reduced the κ_t of TiS₂ to about 0.39 W m⁻¹ K⁻¹. The resultant TiS₂/C₆₀ hybrid films showed a ZT of approximately 0.3 at 400 K, far superior to the state-of-the-art flexible n-type thermoelectric materials.³⁴

Other multifunctional inorganic nanomaterials have also shown fascinating potential in the TE field. With the development of nanotechnology, some multifunctional nanomaterials have significantly improved the performance of TE materials. With the help of high-throughput calculations, the materials selection process can be accelerated and new nanomaterials with highly efficient TE performance may be identified.²²

2.1.2 Piezoelectric materials. Certain inorganic crystals, in which the charge centre of the cations and anions coincide with one another in its undisturbed state, are deformed and electrically polarized when mechanically stressed. Polarization charges appear on the pairs of opposite surfaces of the crystal, thus leading to a voltage difference between the same surfaces. If this deformed crystal is connected to an external load, the free electrons are driven to flow through the external circuit in order to partially screen the piezopotential and achieve a new equilibrium state, which is the energy generation process. The same crystal also exhibits the reverse effect, producing mechanical strain or distortion when it is subjected to an electrical field. In 1880, French physicists Jacques and Pierre Curie first discovered this phenomenon and defined it as the piezoelectric effect. Of the 32 crystal classes, 21 have no inversion centre of symmetry, and 20 of these exhibit piezoelectricity.^35,36

The electric energy available from piezoelectric conversion can be calculated based on a compressive force (F). Let us consider a piezoelectric material with area (A) and thickness (t). The induced voltage (V) is defined as the ratio of surface polarization charge (Q) to the capacitance (C) of the material.

where ε₀ and ε_r are the permittivity of the vacuum and material, respectively. The constant called the piezoelectric voltage coefficient is fixed by the material (Fig. 3).

Fig. 3 (a) Atomic model of the wurtzite-structured ZnO. (b) Piezoelectric properties and the different piezopotentials in the tension and compression modes of the material. (c) Numerical calculation of the piezoelectric potential distribution in a ZnO nanowire under axial strain. This figure was reproduced from ref. 36 with permission from John Wiley and Sons, copyright 2016.

Typical piezoelectric materials include zinc oxide, lead zirconate titanate (PZT) and barium titanate (BaTiO₃, BTO). The operation of a ZnO nanogenerator (NG) depends on a coupling of the piezoelectric and semiconducting properties. Since the first ZnO NG was demonstrated in 2006 by deforming a single ZnO nanowire with an atomic force microscopy tip, various great progresses, such as lateral ZnO nanowires, vertical ZnO nanowires and ZnO fibres, have been made to broaden the practical applications of ZnO NG.

A novel approach was developed by combining surface-coating and plasma-etching techniques to enhance the mechanical reliability of a Kevlar microfibre–ZnO nanowires hybrid structure for a piezoelectric nanogenerator (PENG).³⁷ The enhanced microfibre–nanowire hybrid structure showed high flexibility, robustness and durability. Moreover, the enhanced ZnO nanowires-covered fibre was conducive to improving the stability of the PENG. The open-circuit voltage and short-circuit current of the PENG were 1.8 mV and 4.8 pA, respectively. Also, no decay appeared in the output performance for the enhanced PENG after 3600 s of operation.

In previous studies, most 2D transition-metal dichalcogenides (TMD) materials exhibit piezoelectric properties, unlike their bulk parent crystal. Remarkably, according to density-functional theory, the calculation of the piezoelectric coefficient for monolayer MoS₂ revealed that the monolayer structure exhibited a stronger piezoelectric coupling than the bulk wurtzite-structured materials.

A sulfur-vacancy-passivated monolayer MoS₂ PENG was proposed. The S vacancies were effectively passivated by using the S-treatment process on the pristine MoS₂ surface. The S vacancy site had a tendency to covalently bond with S functional groups. Therefore, by capturing free electrons, a S atom forms a chemisorbed bond with the S vacancy site of MoS₂. S treatment reduced the charge-carrier density of the monolayer MoS₂ surface, thus the screening effect of piezoelectric polarization charges by free carriers was significantly revealed. As a result, the output peak current and voltage of the S-treated monolayer MoS₂ nanosheet PENG were increased by more than 3 times (100 pA) and 2 times (22 mV), respectively.³⁸

Piezoelectric ceramics have played an essential role in technology developments in many areas since the 1950s due to the extensive research and development of high-performance piezoelectric materials. PZT ceramics, excellent piezoelectric materials that have been widely used in piezoelectric electronic devices, have different electrical properties when their compositions are tailored with additives. However, increasing health and environmental concerns about the toxicity of lead in PZT ceramic materials have stimulated the search for high-performance lead-free piezoelectric materials. Potassium–sodium niobate (KNN) ceramics have been identified as one of the most promising lead-free candidates owing to their relatively good comprehensive performance. However, the deliquescence and alkali-oxide volatility deteriorated the piezoelectric properties of KNN-based ceramics. A novel (1 − x − y)K_1−wNa_wNb_1−zSb_zO_3−yBaZrO_3−xBi_0.5K_0.5HfO₃ ternary system exhibited a superior piezoelectric coefficient (d₃₃ = 570 ± 10 pCN⁻¹), the highest value reported to date in KNN-based ceramics. This high d₃₃ value could be ascribed to the co-existence of nanoscale strain domains (1–2 nm) and a high density of ferroelectric domain boundaries.³⁹

2.1.3 Triboelectric materials. Triboelectricity is an electrostatic phenomenon that has been well known since antiquity. When contact occurs, a charge separation takes place. This involves a physical contact between two materials with different electrostatic properties and leads to a positively charged material and a negatively charged one. If the materials are subsequently separated, a net potential difference occurs between them. From an energy point of view, those electrostatic charges constitute a capacitive energy device when the two triboelectric surfaces are separated. This led to the invention of early electrostatic generators, such as the “friction machine” and van de Graaff generator.⁴⁰ By coupling this triboelectric effect and electrostatic induction, the triboelectric nanogenerator (TENG) was first invented in 2012.⁴¹

Oppositely charged surfaces with a changing gap distance can be viewed as a capacitor with varying capacitance. As shown in Fig. 4, this perspective gave a more intuitive representation of the TENG using the capacitor model, whose current is given by

where σ_tr is the amount of transferred charges accumulated on the electrodes, which is a function of the gap distance (z), and A is the contact area between the electrodes. The result is equivalent to Maxwell's displacement current, which is the foundation for the capacitive model. The corresponding output voltage of TENG can be written as

Fig. 4 Theoretical models of a TENG. (a) Schematic illustration of the first TENG and its operation cycle. (b) The displacement current model of a contact-separation-mode TENG. (c) The equivalent electrical circuit model of a TENG. This figure was reproduced from ref. 42 with permission from John Wiley and Sons, copyright 2019.

Along with Ohm's law, this capacitive model of a TENG is the theoretical tool that enables the study, design and optimization of TENG.

In principle, any material with distinct charge affinity can be used to construct a TENG, which results in a broad range of materials at opposite ends of the triboelectric series capable of high performance. Silver nanowires (AgNWs) were composited on the surface of PVDF as the triboelectric layers, achieving the development of high-performance TENGs. The addition of AgNWs to PVDF promoted the formation of the polar crystalline β-phase by introducing electrostatic interactions between the surface charges of the nanowires and the dipoles of the PVDF chains. Meanwhile the ability to trap the induced tribocharges increased upon the addition of nanowires to the PVDF matrix. The enhanced surface-charge potential and the charge-trapping capabilities of the PVDF-AgNW composite nanofibres significantly enhanced the TENG output performances. Moreover, the PVDF-AgNW composite nanofibres also exhibited excellent mechanical stability.⁴³

A nanocomposite material system having a superior surface charge density as a triboelectric active material was reported. The nanocomposite material consisted of a high dielectric ceramic material of barium titanate together with a ferroelectric copolymer matrix, poly (vinylidene fluoride-co-trifluoro ethylene), showing great charge-trapping capability, electrically manipulated polarization and strong triboelectric charge-transfer characteristics. A boosted power-generating performance was achieved to 1130 V output voltage and 1.5 mA output current by this ferroelectric composite-based TENG under 6 kgf pushing force at 5 Hz.⁴⁴

In order to overcome the interference of environmental factors on the performance of a TENG in practical applications, nanocomposite materials were developed to enhance the wear resistance and high-temperature resistance. A TENG was designed to deal with harsh environments and demonstrated a new wear-resistant triboelectric material by hybridizing a nanocomposite as the triboelectric layer. The layer was directly used for key wear-resistant parts. The nano-sized aluminum balls directly collided up and down between the two PTFE films enhancing the triboelectricity output. The results showed that the nanocomposite had a good wear resistance with a mean dynamic friction coefficient of about 0.69 μm at a low-friction of 8.1 N and room temperature. It had excellent high-temperature tolerance (temperature range of −30 °C to 550 °C), wear-resisting ability and high hardness (Rockwell Hardness about 63 HRM), making it capable of being used as a key supporting part, such as in automobile brake pads. In addition, it was found that the TENG output was 221 V, 27.9 μA cm⁻² and 33.4 μC cm⁻².⁴⁰

Direct-current output is another practical requirement. A conventional TENG converted frictional energy into electricity by producing alternating current triboelectricity, which was limited by a low current density and the need for rectification. A continuous direct-current with a maximum density of 106 A m⁻² could be directly generated by a sliding Schottky nanocontact without the application of an external voltage. Also, this was experimentally proved by sliding a conductive-atomic force microscope tip on a thin film of molybdenum disulfide (MoS₂). Finite element simulation revealed that the anomalously high current density could be attributed to the non-equilibrium carrier-transport phenomenon enhanced by the strong local electrical field (10⁵–10⁶ V m⁻²) at the conductive nanoscale tip. This study indicated that the rectifying Schottky barrier at the tip–sample interface played a critical role in efficient direct-current energy harvesting, which makes nanomaterials promising for efficient direct-current triboelectricity generation.⁴⁵

Energy-harvesting technology may be considered an ultimate solution to replace batteries and to provide a long-term power supply for wireless sensor networks. The development of multifunctional nanomaterials has greatly promoted developments in this field. Energy generation is moving towards more efficient, flexible, wearable and lightweight applications, where multifunctional inorganic nanomaterials bring new candidate materials for the development of energy-harvesting applications.

2.2 Energy-conversion applications

2.2.1 Solar cells. The key figures of merit of solar cells are the internal photocurrent efficiency, or the fraction of absorbed photons converted into electrical current, the external quantum efficiency, or the fraction of incident photons converted into electrical current, and the energy conversion efficiency.¹⁴ While many advances have been achieved in photovoltaic (PV) devices, efforts still need to be made to dramatically improve the conversion efficiency of solar cells.⁴⁶ Silicon is the most widely used absorber to date and currently dominates the PV device market. State-of-the-art silicon-PV devices based on p–n junctions, often referred to as first-generation solar cells, have an efficiency of up to about 25%. The development of second-generation PVs, based on thin-film technologies, has been driven by the need to increase the efficiency. However, the efficiency of second-generation PVs is below that of silicon. Third-generation PVs rely on the exploitation of emerging organic PV cells, dye-sensitized solar cells, and quantum dot solar cells.¹⁷ Among these materials uesd in solar cells, perovskites have become very attractive in solar cells around the world.

In recent years, perovskites have emerged as promising materials for low-cost, flexible and highly efficient solar cells.⁴⁷ Despite their processing advantages, before the technology can be commercialized, the poor stability of the perovskite materials with regard to humidity, heat, light and oxygen has to be overcome.⁴⁷ CsYbI₃ cubic nanocrystals have exhibited strong excitation-independent emission and a high photoluminescence quantum yields of 58%.⁴⁸ Perovskite solar cells (PSCs) have developed rapidly over the past few years, and the power conversion efficiency of PSCs now exceeds 20%. Such high performance can be attributed to the unique properties of perovskite materials, such as their high absorption over the visible range and long diffusion length.⁴⁹ Due to the different diffusion lengths of holes and electrons, the electron-transporting materials (ETMs) used in PSCs play a critical role in the performance of PSCs. ZnO materials have a similar energy band position and physical properties to TiO₂ but with much higher electron mobility, which can potentially improve the electron-transport efficiency and reduce the recombination loss as an ETM.^49,50 First of all, ZnO has a very high transmittance in the visible spectrum and more importantly, is low in cost. Second, ZnO is easy to crystallize and to be doped. At the same time, the layered structure of the ZnO crystal leads to different growth rates along different directions. As a result, various ZnO nanostructures can be easily fabricated.⁴⁹ High-efficiency methylammonium (MA) and Cs co-alloyed formamidinium (FA) triple cation perovskite-based ZnO solar cells have been reported. ZnO solar cells with a high crystallization multiple cation perovskite absorber showed a high efficiency of over 20%.⁵¹

A highly crystalline and compositionally photostable material, namely [HC(NH₂)₂]_0.83Cs_0.17Pb(I_0.6Br_0.4)₃, with an optical band gap of about 1.74 eV, was reported.⁵² The fabricated perovskite cells reached an open-circuit voltage of 1.2 V and a power conversion efficiency of 14.7% on 0.715 cm² cells. By combining these perovskite cells with a 19%-efficient silicon cell, the feasibility of 25%-efficient four-terminal tandem cells could be achieved. The incorporation of rubidium cations into the PSC improved the PV performance.⁵³ The cells achieved stabilized efficiencies of up to 21.6% (average value, 20.2%) on small areas (and a stabilized 19.0% on a cell 0.5 cm² in area) as well as an electroluminescence of 3.8%.⁵³ Perovskite films based on CH₃NH₃PbI₃ underwent rapid degradation when exposed to oxygen and light.⁵⁴ Fast oxygen diffusion into CH₃NH₃PbI₃ films was accompanied by the photo-induced formation of highly reactive superoxide species. Perovskite films composed of small crystallites show higher yields of superoxide and a lower stability. Ab initio simulations indicated that iodide vacancies are the preferred sites in mediating the photo-induced formation of superoxide species from oxygen.⁵⁴ A N₂ blow-drying method enhanced the hole-conducting effect of NiO in printable PSC.⁵¹ The best performing device demonstrated a remarkable PV performance with a short-circuit current density of 22.38 mA cm⁻², an open circuit voltage of 0.97 V and a fill factor of 0.50, corresponding to a photoconversion efficiency of 10.83%.⁵⁵ A solar cell based on this Sb₂S₃ film achieved a power conversion efficiency of 4.3%, which is the highest value reported in solution-processed planar heterojunction solar cells based on Sb₂S₃ films.⁵⁶ The partial substitution of FA⁺ by MA⁺ can give the α phase under low temperature, which is beneficial to perovskite stability. Moreover, the stability against moisture and light illumination can be remarkably enhanced by partly substituting FA⁺ by a caesium cation (Cs⁺); the stability could be further improved with a mixture of three cations: Cs⁺/MA⁺/FA⁺.⁴⁷ Highly efficient inverted solar cells based on perovskite were grown on nanostructures mediated by CuSCN.⁵⁷ Three different CuSCN nanostructures were first applied to inverted heterojunction PSC as p-type inorganic hole-transport layers using a moderate electrodeposition method at room temperature. It was revealed that the crystal structure and the thickness of the CuSCN layer could dramatically regulate the morphology and the crystal orientation behaviour of perovskite absorbing layers, which will further have a significant influence on the PV device performance.

A bication lead iodide 2D perovskite component could stabilize the inorganic α-CsPbI₃ perovskite phase for high-efficiency solar cells.⁵⁸ α-CsPbI₃, with the most suitable band gap for tandem solar cell applications, faces issues around the phase instability under ambient conditions. A small amount of 2D EDAPbI₄ perovskite containing the ethylenediamine (EDA) cation could stabilize α-CsPbI₃ to avoid an undesirable formation of the nonperovskite δ phase.⁵⁸ Stable high-efficiency 2D PSC could be achieved by caesium doping.⁵⁹ Cs⁺-Doped 2D (BA)₂(MA)₃Pb₄I₁₃ PSC could achieve a power conversion efficiency (PCE) as high as 13.7%, the highest among the reported 2D devices, together with excellent humidity resistance.⁶⁰ Cation-transmutation to design stable inorganic Pb-free halide perovskites for solar cells could address the key issue about the poor device stabilities associated with their intrinsic material instability. The idea here is to convert two divalent Pb²⁺ into one monovalent M⁺ and one trivalent M³⁺, forming a rich class of quaternary halides in a double-perovskite structure.⁶¹ The ionic defects at the surfaces and grain boundaries of organic–inorganic halide perovskite films are detrimental to both the efficiency and stability of perovskite solar cells. Defect passivation was consequently developed by using quaternary ammonium halide anions and cations in hybrid perovskite solar cells.⁶² An inorganic interlayer of spinel cobaltite oxides (Co₃O₄) could greatly enhance the carbon-based PSC performance by suppressing charge recombination and extracting holes efficiently. PCE is restricted by the charge-carrier transport and recombination processes at the carbon–perovskite interface. Devices realized with the screen-printed Co₃O₄ interlayer exhibited an 18% higher PCE of 13.27% as compared to standard carbon devices.⁶³ A monolithic perovskite/Cu(In,Ga)Se₂ tandem solar cell achieved an efficiency of 22.43%. Non-encapsulated devices under ambient conditions maintained 88% of their initial efficiency after 500 h of aging under continuous 1-sun illumination.⁶⁴ A major factor limiting the performance of nanostructured CuInS₂ photovoltaic devices is the current density, highlighting the poor charge-carrier transport in CuInS₂ nanoparticle films.⁶⁵ ZnS is typically chosen as a shell material for CuInS₂ core passivation, which leads to a significant enhancement of the photoluminescence quantum yield from the CuInS₂ nanoparticles. Replacing the typical divalent Zn cation surface termination with a monovalent Ag cation leads to small improvements in charge-carrier transport through nanostructured films.⁶⁵ This surface termination intentionally introduces lower energy electronic states directly at the surface of the CuInS₂ nanoparticles, reducing charge-carrier confinement and thus increasing the charge-carrier mobility between nanoparticles.⁶⁵

Halide perovskites, with a typical structure of ABX₃ (A = CH₃NH₃⁺, CH(NH₂)₂⁺, Cs⁺; B = Pb²⁺, Sn²⁺; X = Cl⁻, Br⁻, I⁻), have been demonstrated to be promising.⁶⁶ In particular, inorganic halide perovskites are attracting increasing attention because of their higher stability towards moisture, light and heat. Inorganic halide perovskites consist of inorganic A site cations, such as Cs⁺.⁶⁶ Inorganic halide perovskite nanomaterials provide a controllable morphology, tunable optoelectronic properties and improved quantum efficiency, as shown in Fig. 5.⁶⁶

Fig. 5 Schematic showing the design of some typical inorganic halide perovskite crystal structures (ABX₃) to various nanostructures, and then their integration in different applications. This figure was reproduced from ref. 66 with permission from John Wiley and Sons, copyright 2018.

Core–shell ZnO@SnO₂ nanoparticles were synthesized, and used as novel electron-transport materials in perovskite solar cells. Thanks to the high electron mobility of core–shell ZnO@SnO₂ nanoparticles, the PCE of solar cells reached 14.35%.⁶⁷ TiO₂–SiO₂ core–shell nanostructured anti-reflective coatings were introduced in Cu(In,Ga)Se₂ solar cells, and could prolong the optical path length by a scattering effect or by the formation of a refractive index gradient. The suppression of reflectance resulted in an improved PCE from 6.32% to 7.00% after applying the TiO₂–SiO₂ core–shell nanostructures.⁶⁸ The piezo-phototronic effect could be effectively applied to improve the relative conversion efficiency of a flexible solar cell based on n-ZnO/p-SnS core–shell nanowire array.⁶⁹ The efficiencies of quantum dot solar cells are limited by the high density of trap states caused by lattice imperfections on the quantum dot surface. PbS–CdS core–shell quantum dots were adapted to passivate the trap states. The results demonstrated that this may be caused by the improved passivation of the PbS surface by the CdS shell, leading to a lower electron trap density.⁷⁰ A nanostructured WO₃–TiO₂ core–shell electron-transporting material was applied to perovskite solar cells, leading to an observed improved performance, which may be due to the suppressed charge recombination at the WO₃/perovskite interface, faster charge separation and extended diffusion length of the charge carriers.⁷¹ PbSe/PbS core/shell quantum dots were used in solar cells, showing a short circuit current enhancement without the loss of open circuit voltage by shell thickness. PbS shell formation on the PbSe core mitigated the trade-off relationship between the open circuit voltage and the short circuit current density.⁷²

2.2.2 Fuel cells. Fuel cells convert chemical energy from a fuel into electricity via a reaction with oxygen or other oxidizing agents. Their development goes hand in hand with hydrogen production and storage.¹⁴ The integration of fuel cells in electronics faces several challenges: (i) finding electrodes suitable for flexible electronics; (ii) the replacement of expensive noble metals, such as platinum, ruthenium, gold and their alloys as electrocatalysts; and (iii) the need to avoid metal electrode poisoning.¹⁴ To address these challenges, a new class of materials with low cost, high efficiency (both for the fuel oxidation reaction at the anode and oxygen reduction reaction at the cathode) and durability need to be developed before fuel cells can be considered as a serious technology for energy conversion in electronic devices.¹⁴ Fuel cells are presently receiving considerable attention as an alternative energy source because of their high efficiency without the emission of pollutants. Among the common types of fuel cells, direct methanol fuel cells (DMFCs) and proton exchange membrane fuel cells (PEMFCs) are widely considered to be the most promising for use in portable devices such as cellular phones, laptops and personal digital assistants due to their high power density and low operating temperature.⁴⁶ Traditionally, nanomaterials consisting of platinum and its alloys have been used as catalysts in fuel cells in view of their high catalytic activity. However, the high cost of the catalyst is one of main impediments to the commercialization of DMFCs. It is therefore one of the major endeavours of scientists to increase the catalytic efficiency of platinum while simultaneously decreasing the amount of catalyst needed. One method is to use a supporting material to improve the performance of the catalyst. When applied in fuel cells, catalytic supporting materials must be stable and dispersed uniformly.⁴⁶ In recent years, researchers have found that novel nanostructured carbon materials can be used as superior catalyst supports in fuel cells due to their good electric conductivity and low cost.⁴⁶ Various nano-architectures, with a ranges of size, shape, composition and structure, have shown good potential to catalyze the sluggish cathodic and anodic reactions.⁷³ An atomically dispersed Co-doped carbon catalyst with a core–shell structure was developed via a surfactant-assisted metal–organic framework approach.⁷⁴ The catalyst exhibited unprecedented oxygen reduction reaction (ORR) activity with a half-wave potential of 0.84 V vs. the reversible hydrogen electrode (RHE) as well as enhanced stability in corrosive acidic media.⁷⁴ A reversible protonic ceramic electrochemical cell based on a yttrium and ytterbium co-doped barium cerate–zirconate electrolyte and a triple-conducting oxide air/steam reversible electrode was also reported.⁷⁵ A reversible protonic ceramic electrochemical cell achieved a high faradaic efficiency of 90–98% and could operate endothermically with a >97% overall electric-to-hydrogen energy conversion efficiency at a current density of −1000 mA cm⁻².⁷⁵ N-Doped carbon nanomaterials were used as catalysts for the oxygen reduction reaction (ORR) in acidic fuel cells.⁷⁶ Rationally designed, metal-free, nitrogen-doped carbon nanotubes and their graphene composites exhibited significantly better long-term operational stabilities and comparable gravimetric power densities with respect to the best nonprecious metal catalysts in acidic polymer electrolyte membrane cells.⁷⁶

Multimetallic nanosheets with single or few atoms thickness, are attracting extensive research attention because they display remarkable advantages over their bulk counterparts, including high electron mobility, unsaturated surface coordination, a high aspect ratio and distinctive physical, chemical and electronic properties.⁷⁷ A non-Pt cathode catalyst was shown to be capable of simultaneously reducing both O₂ and H₂O₂.⁷⁸ The applicability of this unique catalyst was demonstrated by employing it in a fuel cell running on H₂/CO and O₂/H₂O₂.⁷⁸ Ultrathin PdCu alloy nanosheets were used as a highly efficient electrocatalyst for formic acid oxidation.⁷⁹ The excellent catalytic activity of PdCu nanosheets towards formic acid oxidation could be attributed to their ultrathin morphology, unique electronic structure and synergistic effect between Pd and Cu.⁷⁹ A carbon-nanotube-supported nickel catalyst was prepared for fuel cells. The functionalized redox nanomaterial exhibited reversible electrocatalytic activity for the H₂/2H⁺ interconversion from pH 0 to 9, with a catalytic preference for H₂ oxidation at all pH values.⁸⁰Fig. 6a shows a representation of the membrane-less fuel cell set-up using a 5 mm thick Teflon chamber instead of a classic proton-exchange membrane. The hybrid fuel cell was operated at 25 °C with humidified streams of H₂ (atmospheric pressure) and with air feeding the anode and the cathode, respectively. Fig. 6b displays the polarization and power curves, as determined from successive galvanostatic discharges measured for 30 s.⁸⁰

Fig. 6 Representation of a H₂/air fuel cell and polarization (black) and power (blue) curves for (a) single-walled carbon nanotubes|Myrothecium verrucaria (MvBOD)-carbon nanotubes (f-MWCNT) (phosphate buffer pH 5, 25 °C) and (b) single-walled carbon nanotubes|Pt/C (PEMFC, 60 °C, Nafion membrane). This figure was reproduced from ref. 80 with permission from John Wiley and Sons, copyright 2017.

Pt–C core–shell was grown on carbon nanofibres, and then used as a cathode catalyst for the ORR in proton exchange membrane fuel cells. The synthesized Pt–C core–shell catalyst almost fully maintained the unit cell performance over 30 000 stability test cycles with negligible cell voltage loss. The excellent performance may be due to the robustness of the carbon shells that secured the Pt nanoparticles.⁸¹ IrNi-PdIr/C was prepared via a galvanic replacement reaction. It exhibited enhanced hydrogen oxidation activity in alkaline anion exchange membrane fuel cells. XRD and XPS analyses suggested that the enhanced activity could be attributed to the weakening of the hydrogen binding to the PdIr overlayers induced by the IrNi core.⁸² Pd-PtCu core–shell nanoplates exhibited substantially improved electrocatalytic performance for oxygen reduction and methanol oxidation reactions in fuel cells.⁸³ A Pt-core and Rh_xS_y-shell catalyst was synthesized for HER and HOR in a H₂–Br₂ fuel cell. The results showed that the catalysts with a core–shell structure had a higher active surface area compared to a commercial catalyst. Compared to the platinum catalyst, the core–shell catalysts showed more stable performance in fuel cell cycling tests.⁸⁴

2.2.3 Catalysis. Electrocatalysts need appropriate efficiency and selectivity for the chemical transformations.^85,86 Active catalysts are required to minimize the overpotential to drive the hydrogen evolution reaction (HER). The four-electron ORR requires improved electrocatalysts to increase its rate and efficiency. Researchers typically focus on reaction intermediates, mechanisms and the number of electrons transferred.⁸⁷ The multifunction of materials requires many physical parameters to be considered, such as the hydrogen adsorption free energy. An active catalyst binds the reaction intermediates neither too strongly nor too weakly. Electrochemical barriers are required for proton-transfer reactions involving both hydroxide and hydronium ions. Support effects can also play a role in catalyst activity and stability.¹⁷ Electrocatalysts need to possess both high activity and high selectivity towards the particular product of interest.^88–90 Inorganic materials usually exhibit long-term stability. There are two strategies to improve the activity: (i) increasing the number of active sites on a given electrode or (ii) increasing the intrinsic activity of each active site. Nanostructure can provide an increased number of edge sites and enhanced charge transport. Disperse nanoparticles on supports increase the surface area.¹⁷

Carbon nanomaterials are promising metal-free catalysts for energy conversion and storage. Carbon nanomaterials are used as efficient photo-/electrocatalysts to facilitate the critical chemical reactions in clean and sustainable energy technologies.⁹¹ These reactions include the ORR in fuel cells, the OER in metal–air batteries, the iodine reduction reaction in dye-sensitized solar cells, the HER in water splitting and the carbon dioxide reduction in artificial photosynthesis.⁹¹ For carbon materials, the methods of tuning the electronic properties for creating active centres are illustrated in Fig. 7.⁹¹ The carbon atoms can be substituted by p-block elements, such as N, P, Si and B, or bonded with the heteroatoms, such as O, S, Se, F, Cl, Br and I.⁹¹ Intrinsic defects existing in carbon nanomaterials could induce charge transfer and a density of state change, thus generating active centres. The common methods of tuning the electronic properties of carbon materials for creating active centres are shown in Fig. 7.⁹¹

Fig. 7 Methods of tuning the electronic properties of carbon materials for creating active centres. (I) Heteroatom-doping on carbon materials, including: (a) 0D fullerene, (b) 1D CNTs, (c) 2D graphene, and (d) 3D hybrid hierarchical structures. (II) Intrinsic defect engineering on graphene, including: (e) Stone–Wales defects, (f) a vacancy, (g) an edge defect with a nonhexagonal carbon ring and (h) a line defect-558 carbon-ring grain boundary. This figure was reproduced from ref. 91 with permission from John Wiley and Sons, copyright 2019.

Atomically dispersed Zn–N–C nanomaterials are promising platinum-free catalysts for the ORR. However, the fabrication of Zn–N–C catalysts with a high Zn loading remains a formidable challenge owing to the high volatility of the Zn precursor during high-temperature annealing.⁹² Luckily, an atomically dispersed Zn–N–C catalyst with an ultrahigh Zn loading of 9.33 wt% could be successfully prepared by simply adopting a very low annealing rate.⁹² Electrochemical experiments showed that the atomically dispersed Zn–N–C catalyst exhibited not only comparable ORR activity to that of a Fe–N–C catalyst in both acidic and alkaline media, but also better durability than the Fe–N–C catalyst.⁹² Carbon-rich nanomaterials are fascinating hydrogen and oxygen electrocatalysts.⁹³ As illustrated in Fig. 8, the reversible water dissociation processes comprise four essential half reactions, that is, ORR, HER, OER and the hydrogen oxidation reaction (HOR).^93–97 For the ORR, nitrogen-doped carbon and atomic-metal-doped carbons have demonstrated a higher ORR activity than the Pt/C in alkaline solutions. For the HER, the HER activity of electrocatalysts is dominated by the adsorption free energy of the hydrogen intermediates in acidic solutions.⁹⁴ For the OER, among the previously reported electrocatalysts and noble-metal-based catalysts (IrO₂, Ir/C and RuO₂), MOFs manifest the highest performance in alkaline media. In order to further reduce the cost and improve the energy conversion efficiency, it is a highly promising strategy to develop multifunctional electrocatalysts.⁹³ Ultrathin PtPd-based nanorings with abundant step atoms enhance oxygen catalysis. Here, PtPdCo showed the highest ORR mass and specific activities of 3.58 A mg⁻¹ and 4.90 mA cm⁻² at 0.9 V versus RHE, i.e. 23.9 and 24.5-fold times larger than those of commercial Pt/C in alkaline electrolyte, respectively.⁹⁸ A porous silicon and nitrogen co-doped carbon (SiNC) nanomaterial prepared by a facile pyrolyzation was developed as a metal-free multifunctional electrocatalyst.⁸⁸ CO₂-to-CO conversion and the OER partial current density under neutral conditions were enhanced by two orders of magnitude in the Tafel regime on SiNC relative to single-doped comparisons beyond their specific area gap. The photovoltaic-driven CO₂ splitting device with SiNC electrodes imitating photosynthesis yielded an overall solar-to-chemical efficiency of 12.5% at only 650 mV overpotential.⁸⁸

Fig. 8 Schematic illustration of the water-splitting electrolyzer (left) and fuel-cell (right) devices as well as the corresponding electrochemical reactions. This figure was reproduced from ref. 93 with permission from John Wiley and Sons, copyright 2018.

Controlling the MoS₂ shell numbers can tune the tensile surface strain on Co₉S₈–MoS₂ core–shell nanocrystals for boosting the hydrogen evolution reaction activity. It was found that the tensile surface strain could be precisely tuned from 3.5% to 0%. Here, Co₉S₈–MoS₂ core–shell nanocrystals showed an overpotential of only 97 mV and a Tafel slope of 71 mV dec⁻¹.⁹⁹ CoP-PS exhibited a noticeable HER activity of approximately 80 mV with excellent durability.¹⁰⁰ PtPb-Pt core–shell nanoplates boosted oxygen reduction catalysis. The nanoplates reported also showed high electrocatalytic activity and stability towards anodic fuel cell reactions, such as the methanol oxidation reaction and ethanol oxidation reaction.¹⁰¹

2.3 Energy storage applications

The development of renewable energy storage devices is one of the most promising ways to address the current energy crisis, along with global environmental concerns. The exploration of suitable active materials is the key factor to the construction of highly efficient, highly stable, low cost and environmentally friendly energy storage devices.¹⁰² Nanomaterials with a large specific surface area were proposed to guide future research towards closing the gap between the achieved and theoretical capacitance, without limiting the loading mass.¹⁰³ Electrochemical energy storage devices, such as supercapacitors and batteries, are critical for enabling renewable yet intermittent sources of energy, such as solar and wind.¹⁰⁴ Charge is stored only at the surfaces in supercapacitors, which are not limited by diffusion processes, allowing high power to be achieved. Similarly, because charging and discharging do not involve a bulk-phase transformation, supercapacitors are much more reversible and have a longer cycle life.¹⁰⁵ Batteries store charge typically through redox reactions in the bulk phase of electrode materials, which leads to a higher energy density but lower power performance compared to supercapacitors. The demand for high-performance rechargeable batteries has become so tangible and ubiquitous in recent years that its numerous requirements and functions have nearly risen to the status of common knowledge.^106,107

2.3.1 Supercapacitors. Supercapacitors allow for high energy density, high rates of charge–discharge and a long cycle life, which help them bridge the gap between traditional capacitors and secondary batteries.⁴⁶ Supercapacitors store energy using either ion adsorption or redox reactions, in which most of the charge is transferred at or near the surface of the electrode material.¹⁰⁸ Supercapacitors are ideal for applications requiring a high power density of at least 10 kW kg⁻¹, one order of magnitude larger than lithium-ion batteries.¹⁴ The electrochemical performance of symmetrical supercapacitors based on carbon materials can be remarkably improved when hybridized with carbon dots (CDs), due to the large specific surface area of CDs.¹⁰⁹

Novel colloidal electrode materials were developed by Xue's group, and could deliver a higher specific capacitance than electrical double-layer capacitors and traditional pseudocapacitors.^110–117 The electrode materials in colloidal supercapacitors were formed in situ, transformed from commercial rare earth and transition metal salts in alkaline electrolyte by chemical and electrochemical assisted coprecipitation.¹¹⁰ In these colloidal supercapacitors, multiple-electron faradaic redox reactions can be utilized, which can deliver ultrahigh specific capacitance, often larger than one-electron capacitance. The multiple-valence metal cations used in these designed colloidal supercapacitors mainly include Ce³⁺, Yb³⁺, Er³⁺, Fe³⁺, Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Sn²⁺ and Sn⁴⁺.^{114,115,118–125} The colloidal supercapacitors can serve as promising next-generation high-performance candidates.¹¹⁰

A novel and creative in situ electrochemical activation method to transform vanadium ions into a highly electroactive colloidal cathode in KOH solution under an electric field was designed. The vanadium-based colloids//activated carbon asymmetric supercapacitor displayed a high energy density of 50.4 W h kg⁻¹ at a power density of 250 W kg⁻¹, which was higher than most reported vanadium-based supercapacitors.¹²⁶ Highly electroactive Mn₇O₁₃·5H₂O colloids were formed in situ by an electric-field-assisted chemical coprecipitation in KOH solution. The highly efficient faradaic redox reactions were confirmed in pseudocapacitors, which could deliver a high specific capacitance of 2518 F g⁻¹ based on active Mn cations at a current density of 5 A g⁻¹. The present results showed that Mn cations in the designed system could lead to two-electron faradaic reactions.¹²¹ The active central ions of Fe³⁺ in Fe(NO₃)₃·9H₂O could be fixed on the electrodes by the surrounding ligands (OH⁻ and NO₃⁻) and then could be in situ transformed into colloidal Fe₄NO₃(OH)₁₁ and α-FeOOH. The electrochemical results indicated that the current proposed colloidal pseudocapacitor system enabled the highly efficient utilization of electroactive central Fe³⁺ ions, showing a high energy density of 58.4 W h kg⁻¹ at a power density of 8.4 kW kg⁻¹.¹¹⁹ Water-soluble CoCl₂ electrodes showed the reversible redox reaction of Co²⁺ ↔ Co³⁺ ↔ Co⁴⁺ on the electrode and delivered a very high specific pseudocapacitance of about 1962 F g⁻¹. Commercial CoCl₂ salt was used directly as pseudocapacitor electrodes in an aqueous electrolyte, thus neglecting the complex synthesis procedures.¹¹⁶ Highly electroactive Ni-based colloidal electrode materials have also been synthesized by the in situ electrochemical activation of a NiCl₂ electrode. The highest specific capacitance of the activated Ni-based electrodes was 10 286 F g⁻¹ at a current density of 3 A g⁻¹, indicating that a three-electron faradaic redox reaction (Ni³⁺ ↔ Ni) occurred. Upon potential cycling and constant potential activation, a decrease in the charge-transfer resistance was found. The activation and utilization of multiple-electron reactions was shown to be an efficient route to increase the energy density of supercapacitors.¹²³ Water-soluble CuCl₂ electrodes showed a fast and reversible redox reaction of Cu²⁺ ↔ Cu⁺ and exhibited a very high specific pseudocapacitance of about 5442 F g⁻¹. It was identified that the Cu²⁺ was responsible for achieving this superior value. The chemical and crystallization transformation of the CuCl₂ electrode was presented.¹²⁴ An in situ crystallization method for tin chloride salt pseudocapacitors was developed. After undergoing coupled chemical/electrochemical crystallization and faradaic redox reactions, highly active SnO/Sn colloids were in situ crystallized within the carbon black matrix. Such an electrode configuration allowed a fast transfer of ions/electrons and efficient utilization of the active tin cations in the salt electrode. The SnCl₄ electrode could deliver an ultrahigh specific capacitance of 1592 F g⁻¹ at a current density of 1 A g⁻¹ during the potential range of 0.42 V, which is the highest value reported.¹²⁵

Rare earth elements have also been applied in supercapacitors. An ultrahigh specific capacitance of 2060 F g⁻¹ could be obtained by the direct use of commercial Ce(NO₃)₃ as an electrode material in KOH electrolyte without any additional processing. Ce³⁺/Ce⁴⁺ could deliver a high practical specific capacitance close to its theoretical value.¹¹⁸ A new ErCl₃ alkaline aqueous pseudocapacitor system was demonstrated by designing a commercial ErCl₃ salt electrode in alkaline aqueous electrolyte. The synthesis of the material occurred at the same spatial and temporal scale. Highly electroactive ErOOH colloids were in situ crystallized via an electric-field-assisted chemical coprecipitation of ErCl₃ in KOH aqueous electrolyte. These ErOOH colloids absorbed by carbon black and a PVDF matrix were highly redox-reactive with a high cation utilization ratio of 86% and a specific capacitance value of 1811 F g⁻¹, exceeding the one-electron redox theoretical capacitance (Er³⁺ ↔ Er²⁺).¹¹⁴ The electrochemical performance and fabrication scheme are shown in Fig. 9. YbCl₃ pseudocapacitor electrodes were synthesized in alkaline electrolytes, showing a high cation utilization ratio. The electrochemical reactive YbOOH colloids were crystallized through chemical coprecipitation and faradaic redox reactions. The effect of the crystallization kinetics on the electrochemical performance of the YbCl₃ pseudocapacitor was studied. The YbCl₃ pseudocapacitor showed an ultrahigh specific capacitance of 2210 F g⁻¹.¹¹⁵

Fig. 9 Electrochemical performance and fabrication scheme of the ErCl₃ pseudocapacitor. (a) Charge/discharge curves (time versus potential) measured at various current densities and a potential range of 0.55 V. (b) Specific capacitance of the inorganic ErCl₃ salt electrode versus the discharge current density at a potential window of 0.55 V based on the weight of ErCl₃·6H₂O salt and Er³⁺ ion. (c) Schematic drawing showing the in situ fabrication of the ErCl₃ pseudocapacitor. First, the electrode was fabricated with the use of commercial ErCl₃·6H₂O salts by slurry-coating manufacturing. This figure was reproduced from ref. 114 with permission from Elsevier, copyright 2014.

Crystalline-amorphous Fe₂O_3−δ core–shell architectures have also been successfully prepared. The core–shell architecture delivered a large capacitance of 701 F g⁻¹ at 1 A g⁻¹, which was almost double the capacitance of the conventional Fe₂O_3−δ nanorods without an amorphous surface layer on graphene.¹²⁷ Phosphorus-doped Ni(OH)₂ rods as the core and MnO₂ nanosheets as the shell were fabricated. The core–shell structure exhibited an ultrahigh areal capacitance of 5.75 F cm⁻² and great cyclic stability without capacitance loss after 10 000 cycles.¹²⁸ Zinc–nickel-cobalt oxide –Ni(OH)₂ nanowire arrays showed an ultrahigh specific capacitance of 2847.5 F cm⁻³.¹²⁹ Core–shell structured Co₃O₄–NiCo₂O₄ electrodes were grown on flexible carbon fibres. The core–shell structures presented an excellent specific capacitance of 1450 F g⁻¹ at 1 A g⁻¹. The unique structures supplied more pathways for accelerating fast electron and ion transfer.¹³⁰ A nickel–cobalt metallic core and nickel–cobalt layered double hydroxide shell was grown on carbon fibre cloth. The synthesized architecture showed a high capacitance of 2200 F g⁻¹ and retained 98.8% of its initial capacitance after 5000 cycles.¹³¹ MnCo₂O₄–Ni(OH)₂ core–shell flowers were reported, with the structure shown in Fig. S1.† The nanoflowers exhibited an ultrahigh specific capacitance of 2154 F g⁻¹ at a current density of 5 A g⁻¹.¹³²

2.3.2 Batteries. The battery represents an important component for energy storage, and can store electrical energy in the form of chemical energy through electrochemical reactions and then release this energy through the reverse reactions.^133,134 In the past few decades, batteries have achieved remarkable success in both research and commercialization. Nowadays, common problems and societal progress are placing higher demands on batteries. As such, advanced batteries require better performance, including high energy density, high power density, long cycle life and good safety. Meanwhile, convenience and comfort are another development direction for batteries, which require them to be flexible, wearable and lightweight.^135,136 Key components of a battery include positive and negative electrodes, an electrolyte and a separator. In order to achieve the above requirements, inorganic nanomaterials are playing an increasingly indispensable role as electrode materials.

The energy storage of the battery follows the ion insertion/extraction mechanism. Take the lithium-ion battery as an example: during the discharging process, the cathode material is oxidized, resulting in the extraction of lithium ions from the electrode bulk phase. Simultaneously, a reduction reaction occurs in the anode material, which causes the lithium ions in the electrolyte to form a metal or alloy. As for the charging process, the reversible reactions occur on both electrodes.¹³⁷ Therefore, the functions of the electrode material, especially the electronic and ionic conductivity, determine the electrode reactions. Electrode materials with effective ion migration channels, high electronic conductivity and abundant reactive sites are promising to achieve high capacity. In addition, the insertion/extraction of ions will deform the structure of electrode materials, resulting in volume changes, or can even collapse the channels, leading to a decay of capacity.¹³⁸

Previous studies have shown that reducing the size of materials to the nanoscale can expose more active sites, which is conducive to the electrode capacity approaching the theoretical value. Xue et al. investigated the effect of the size of the manganese dioxide electrode on the capacity of lithium-ion batteries. MnO₂ contains two types of Mn centres: one is the effective Mn centre where the faradaic reactions can take place, the other cannot make contact with the electrolyte. The specific capacitance of MnO₂ is closely dependent on the percentage of effective Mn centres in the faradaic reaction (X^effective_Mn). They calculated the relationship between X^effective_Mn and the particle size. Their results showed that as the particle size decreased to several nanometres, X^effective_Mn increased significantly. It is worth noting that when the MnO₂ size was smaller than 1 nm, X^effective_Mn equalled 1, indicating that the specific capacitance of MnO₂ should achieve its theoretical value. Moreover, the mechanism of charge storage in adsorption/desorption or insertion/extraction processes can be distinguished by calculations. As shown in Fig. 10b, 94.74% of the specific capacitance of α-MnO₂ with a size of 2 nm arose from adsorption/desorption processes, and only 5.26% of the specific capacitance arose from the insertion/extraction process.^139,140

Fig. 10 Size-dependent effective Mn centres in the faradaic charge storage of MnO₂. (a) Size-dependent percentage of effective Mn centres of MnO₂ polymorphs in the faradaic reaction. (b) Size-dependent percentage of effective Mn centres contributing to the adsorption/desorption and insertion/extraction processes of α-MnO₂. This figure was reproduced from ref. 139 with permission from Royal Society of Chemistry, copyright 2016.

2.3.3 Lithium-ion batteries. In addition to theoretical research, a large number of experimental results have shown that inorganic nanomaterials can effectively improve the performance of batteries, including lithium-ion batteries,^141–148 sodium-ion batteries,^149–152 lithium–sulfur batteries^153–156 and zinc–air batteries.^157–159

Nanoscale substitutional solid-solution Mn_1−xFe_xP compounds were synthesized as an anode for lithium-ion batteries. Enhanced electrochemical performance resulted from the in situ-generated nanocomposite of the Li–Mn–P alloying element and the Fe nano-network in combination with the surrounding amorphous lithium phosphide, which effectively buffered the accompanying volume variation, hindered the aggregation of the alloying element and ensured electron and ion transport. The Mn_1−xFe_xP solid-solution phosphide electrodes exhibited a capacity of 360 mA h g⁻¹ after 100 cycles at a high current density of 2 A g⁻¹.¹⁴⁴

A self-supported Cu₃Si-Si@carbon@graphene (Cu₃Si-SCG) nanocomposite anode was reported to mitigate the issues of the dramatic volume variation (>300%) during the lithiation/delithiation processes. The nanocomposite was composed of a Cu₃Si-Si core and carbon shell with core/shell particles uniformly encapsulated by graphene nanosheets anchored directly on a Cu foil. The carbon shell, the highly elastic graphene nanosheet and the formed Cu₃Si phase in Si served as buffer media to suppress the volume variation of Si during lithiation/delithiation processes and to facilitate the formation of a stable solid electrolyte interface (SEI) layer as well as to enable good transport kinetics. The optimized Cu₃Si-SCG nanocomposite anode exhibited good rate performance and delivered a reversible capacity of 483 mA h g⁻¹ (based on the total weight of Cu₃Si-SCG) after 500 cycles with a capacity retention of about 80% at a high current density of 4 A g⁻¹, rendering the nanocomposite a desirable anode candidate for high-performance LIBs.¹⁴⁵

2.3.4 Sodium-ion batteries. Sodium-ion batteries are emerging as promising candidates for grid energy storage because of the abundance of sodium resources and its low cost. However, the development of suitable electrode materials is far from satisfactory. MoP hollow nanospheres encapsulated in 3D reduced graphene oxide networks (H-MoP@rGO) were successfully prepared. Benefiting from the merits of the hollow nanostructure and flexibility of rGO, an enhanced electrode performance of H-MoP@rGO was demonstrated. The specific capacity of the H-MoP@rGO electrode was as high as 353.8 mA h g⁻¹ at 1 A g⁻¹ after 600 cycles. Besides, it also showed an extraordinary rate performance of 183.4 mA h g⁻¹ at an ultrahigh current density of 10 A g⁻¹ even after 3000 cycles.¹⁴⁹

A foam-like FeS₂ (F-FeS₂) nanostructure was reported by combining solution combustion synthesis and solid-state sulfurization. The obtained F-FeS₂ product was highly uniform and built from interconnected FeS₂ nanoparticles (approximately 50 nm). The interconnected feature, small particle sizes and porous structure endowed the product with high electrical conductivity, good ion diffusion kinetics and a high inhibition capacity against volume expansion. As a result, it achieved a high capacity of 823 mA h g⁻¹ at 0.1 A g⁻¹, a good rate capability of 581 mA h g⁻¹ at 5.0 A g⁻¹ and a cycle ability of 97% retention after 80 cycles.¹⁵⁰

VS₄ microspheres were assembled from nano-units with different crystallinities via a facile template-free hydrothermal method to promote the electrochemical performance of 3D self-assembled nanoarchitectures. The results showed that the electrochemical performance of the VS₄ microspheres as anode materials for sodium-ion batteries (SIBs) largely depended on their crystallinity. A VS₄ electrode with the lowest crystallinity delivered a high reversible capacity of 412 mA h g⁻¹ at 0.2 A g⁻¹ after 230 cycles. The insertion mechanism was revealed within the selected voltage window of 0.50–3.00 V. Further analysis suggested that decreasing the crystallinity of the nano-units could dramatically enhance the pseudocapacitive behaviour of the VS₄ microspheres, which took the main responsibility for the improvement of the sodium storage properties.¹⁵¹

Ti₂Nb₂O₉ nanosheets with a tunnel structure can also be used as suitable anode materials for sodium-ion batteries. Ti₂Nb₂O₉ nanosheets were synthesized by liquid exfoliation combined with topotactic dehydration, delivering a high reversible capacity of 250 mA h g⁻¹ at 50 mA g⁻¹ at a suitable average voltage of approximately 0.7 V. It was found that a low energy diffusion barrier, enlarged interlayer spacing and exceptional nanoporosity together gave rise to a high rate performance, as characterized by a pseudocapacitive behaviour. The observed high reversible capacity, excellent rate capability and good cyclability of Ti₂Nb₂O₉ nanosheets make this material competitive when compared to other sodium-insertion anode materials.¹⁵²

2.3.5 Lithium–sulfur batteries. Lithium–sulfur (Li–S) batteries have attracted remarkable attention due to their high theoretical capacity of 1675 mA h g⁻¹, rich resources, inexpensiveness and environmental friendliness. However, a number of Li–S batteries are currently reported to suffer from severe drawbacks, such as the insulating nature of sulfur, sluggish redox kinetics and the shuttle effect of the intermediate polysulfides. To overcome these challenges, an efficient sulfur host in Li–S batteries comprising carbonyl group-functionalized porous carbon nanofibres assembled with nickel (Ni/PCNFO) was proposed. Nanostructured electrode materials offer many advantages for the electrode reaction. The large inner space of the PCNF can afford a high S content and accommodate the volume expansion. High electrical conductivity was provided by the carbon nanofibre skeleton and the electrocatalytically active Ni species embedded in the PCNF, which significantly facilitated the redox kinetics of the S species. Moreover, the carbonyl group anchored on the Ni/PCNF could effectively suppress the polysulfide effect via a strong chemical affinity/adsorption with polysulfides. With these advantageous features, Li–S batteries based on Ni/PCNFO-S cathodes exhibited a high specific capacity of 1320 mA h g⁻¹, excellent rate capability of 780 mA h g⁻¹ and long cycling stability.¹⁵⁴

A 3D nitrogen-doped graphene/titanium nitride nanowires (3DNG/TiN) composite was reported as a freestanding electrode for Li–S batteries. The highly porous conductive graphene network provided efficient pathways for both electrons and ions. TiN nanowires attached on the graphene sheets had a strong chemical anchor effect on the polysulfides, as proved by its superior performance. As shown in Fig. 11, the 3DNG/TiN cathode exhibited an initial capacity of 1510 mA h g⁻¹ and the capacity remained at 1267 mA h g⁻¹ after 100 cycles at 0.5 C. Even at 5 C, a capacity of 676 mA h g⁻¹ was reached. With a high sulfur loading of 9.6 mg cm⁻², the 3DNG cathode achieved an ultrahigh areal capacity of 12.0 mA h cm⁻² at a high current density of 8.03 mA cm⁻². This proposed unique structure represents a bright prospect for simultaneously achieving a high energy density and high power density in Li–S batteries.¹⁵⁵

Fig. 11 Electrochemical performances of 3DNG/TiN, 3DG/TiO₂ and 3DNG cathodes. (a) CV profiles of the 3DNG/TiN at a scan rate of 0.1 mV s⁻¹. (b) Cycling performance of the 3DNG/TiN, 3DG/TiO₂ and 3DNG cathodes at 0.5 C for 100 cycles. (c) Rate performance of the 3DNG/TiN, 3DG/TiO₂ and 3DNG cathodes. (d) Galvanostatic charge–discharge profiles of the 3DNG/TiN at different rates in a potential window from 1.6 to 2.8 V. (e) Cycling stability of the 3DNG/TiN, 3DG/TiO₂ and 3DNG cathodes at 1 C for 200 cycles. This figure was reproduced from ref. 155 with permission from John Wiley and Sons, copyright 2018.

Flexible batteries have become an indispensable component of many emerging devices, such as wearable, foldable electronics and sensors. Although various flexible batteries based on one-dimensional (1D) and two-dimensional (2D) platforms have been explored, developing a high energy density electrode with high structural integrity remains challenging. A flexible porous cathode for lithium–sulfur batteries was synthesized, consisting of reduced graphene oxide (rGO), graphene crumples (GCs) and sulfur powders. The electrode structures were tailored using GCs with different dimensions and functional features, which were critical to its robustness under mechanical deformation and electrolyte penetration into the battery components. The optimized rGO/GC/S composite ribbon cathodes delivered a high capacity of 524 mA h g⁻¹ after 100 cycles at a current rate of 0.2 C. A shape-conformable battery prototype comprising an rGO/GC/S cathode and a lithium anode demonstrated stable discharge characteristics under repeated bending/flattening cycles. The LSB prototype supported by an elastomer presented a stable discharge behaviour with high mechanical robustness against an extension of up to 50%.¹⁵⁶

A hollow Fe₃C-N/C with a frogspawn-like architecture was proposed as a highly efficient sulfur host in this paper. Derived from a Prussian blue self-template, Fe₃C-N/C possessed a metal-like Fe₃C spawn core and the high conductivity of an N-doped carbon shell. This unique structure enabled a large surface area, fast e⁻ and Li⁺ transport, as well as a large hollow space for volumetric expansion of the sulfur cathode. Moreover, with the N-doped carbon shell and the polar Fe₃C core, the trapping and catalytic conversion of intermediate polysulfides were also facilitated. The strongly coupled interaction of polar Fe₃C and polysulfides was confirmed by both theoretical calculations and the electrochemical performance. Specifically, the Fe₃C-N/C/S electrode presented a high capacity of 1351 mA h g⁻¹ at 0.1 C with Fe₃C-N/C as an integrated sulfur host. In particular, the rate capability and cycling stability of the Fe₃C-N/C/S electrode was outstanding. It displayed a high capacity of 792 mA h g⁻¹ at 5 C and a low capacity decay rate of 0.08% per cycle at 0.5 C after 400 cycles.¹⁶⁰

2.3.6 Zinc–air batteries. Zinc–air batteries offer a possible solution for large-scale energy storage due to their superhigh theoretical energy density, reliable safety, low cost and long durability. However, their widespread application is hindered by their low power density. A multiscale structural engineering of Ni-doped CoO nanosheets (NSs) was reported for zinc–air batteries with superior high power density/energy density and durability. In the nanoscale, a robust 2D architecture together with the existence of numerous nanopores inside the nanosheets provided an advantageous nanostructured surface for O₂ diffusion and a high electrocatalytic active surface area. In the atomic scale, Ni doping significantly enhanced the intrinsic oxygen reduction reaction activity per active site. As a result of the controlled multiscale structure, the primary zinc–air battery with an engineered Ni-doped CoO NSs electrode showed excellent performance, with a record-high discharge peak power density of 377 mW cm⁻², and further, it was stable for over 400 h at 5 mA cm⁻². A rechargeable zinc–air battery based on Ni-doped CoO NSs afforded an unprecedented small charge–discharge voltage of 0.63 V, outperforming a state-of-the-art Pt/C catalyst-based device.¹⁵⁸

A core–shell architecture of dual-phase FeCo-based nanoparticles-heteroatom-doped carbon microspheres (FeCo-C MS) was synthesized via a two-step carbonization process from a reactive multifunctional core–double-shell template. With the advantages of a heterogeneous composition and architectural structure, the obtained FeCo-C MS was shown to be an excellent cathode catalyst for the Zn–air battery, and FeCo-C MS exhibited a high discharge voltage of 1.27 V, high specific capacity of 503 mA h g⁻¹ and an energy density of 639 W h kg⁻¹ (Fig. S2†).¹⁵⁹

With the development of multifunctional inorganic nanomaterials, significant commercial and academic progress has been made on battery technologies. There is reason to believe that the invention of nanomaterials with novel structures will promote the better performance of batteries. Therefore, the use of efficient material screening methods using artificial intelligence is a growing trend in the development of energy storage applications.

2.4 Energy saving applications

Energy consumption by buildings accounts for a large proportion (about 40%) of the world-wide energy consumption, among which more than 50% is contributed by heating, ventilation and air conditioning.^161–164 Smart windows in modern buildings can significantly decrease energy consumption via an efficient modulation of the light/heat passing through these windows.¹⁶⁵ For smart windows, inorganic nanomaterials play an important role due to their unique properties. Smart windows can change their throughput of visual light and solar irradiation by using electrochromic materials characterized with a specific nanostructure, thereby avoiding excessive solar heating and controlling the heating, cooling, lighting and visual contact between indoors and outdoors.¹⁶⁶

As one of the most representative electrochromic materials, tungsten trioxide (WO₃) has been extensively studied and has become recognized as a very promising material for smart window applications due to its advanced features, such as reversible colour switching from optical transparency to deep blue and good electrochemical stability and electrochromic performances, as compared organic electrochromic materials.^{165,167–170} Tunnelled phosphorus-doped WO₃ films have been synthesized.¹⁷⁰ The elaborate architecture of the tunnelled structure without the formation of any cracks could improve not only the switching speeds and colouration efficiency values by increasing the electroactive contact with the electrolyte, but also the cycling stability to 91.5% after 1000 cycles by effectively accommodating the charge diffusion and the expansion–contraction of the WO₃ films. The accelerated effect of P doping on the WO₃ films enhanced the electrical conductivity to favour the electrochromic performance by a higher degree of electron sharing. It is worth mentioning that with the development of multifunctional materials, smart window materials not only have the function of saving energy, but also have certain functions for storing energy. One device made of this material is the electrochromic energy storage window and this has become a new focus of research.^171–173 Mesoporous WO₃ film, which exhibited enhanced electrochromic properties, showed a high transparency at the bleached state, with an optical transmittance of 99.5% at a wavelength of 633 nm.¹⁶⁵ The tiny WO₃ nanoparticles were filled into the granular voids of the substrate and simultaneously decreased the surface roughness, which may be a contributing factor for the high transparency and good stability of the mesoporous film. It exhibited a fast colouration switching response, in which the colouration time was found to be 2.4 s, and the bleaching time was 1.2 s. In addition, a high colouration efficiency of 79.7 cm² C⁻¹ and large optical modulation of 75.6% at 633 nm were achieved. The mesoporous WO₃ film also had the function of energy storage with a specific capacity of 75.3 mA h g⁻¹.

Tungsten bronze (M_xWO₃, M = Li, Na, K, Rb, Cs, NH₄) is a novel transparent conductive oxide with excellent visible transparency by selectively cutting off near infrared light and it showed near infrared shielding (NIR) induced by localized surface plasma resonance. It has potential applications in the field of energy saving smart window glasses.^174–177 Cs_xWO₃ nanorods were synthesized by controlling a variety of synthesis conditions to achieve a high light transmittance and high NIR shielding.¹⁷⁴ According to thermal insulation testing, the temperature of the box covered with ordinary glass was as high as 77 °C after 1 h of illumination, while the temperature of the box covered with the Cs_xWO₃ film glasses was only 50 °C.

Na_xWO₃ was obtained by annealing an amorphous non-stoichiometric WO_3−x (0 < x < 1) film prepared on common commercial glass.¹⁷⁷ NaWO₃ had a high transmittance for visible light as well as excellent shielding for near-infrared light. In the process of simulating a house illuminated by sunlight, the temperature difference was at least 4.5 °C for 60 min relative to blank glass. Further, the application of tungsten bronze observably reduced energy consumption, decreased the emission of carbon dioxide and thereby improved the living environment. However, from the viewpoint of practical applications, the VIS light transmittance and UV light blocking properties of hexagonal tungsten bronzes also require further improvement owing to their relative narrow band gap (2.5–3 eV).¹⁷⁸ Pt-Doped K_xWO₃ nanoparticles were synthesized.¹⁷⁶ Trace Pt doping could contribute to the uniform growth of hexagonal K_xWO₃ nanorods and could significantly improve the NIR performance. In heat insulation property tests, a sample coated with the Pt-doped K_xWO₃ film had a temperature of only 40 °C after 40 min of irradiation with a 250 W infrared light, and the sample temperature of the blank glass and the PVA film was as high as 60 °C, demonstrating the excellent heat insulation of Pt-doped K_xWO₃. To address the problem where the smart window glass is easily contaminated by dust, organic and inorganic particulate matter and microbial growth, the K_0.3WO₃/Ag₂O nanocomposites were synthesized with a self-cleaning function.¹⁷⁵ The self-cleaning function was studied by monitoring the decomposition of rhodamine B (RhB) under visible light. After 2 h visible light irradiation, the photodegradation rate of RhB by the K_0.3WO₃ film was only 29%, while the photodegradation rate of the K_0.3WO₃/Ag₂O film was as high as 74%. The photogenerated electrons of K_0.3WO₃ transfer to the conduction band of Ag₂O and the holes of Ag₂O migrate to the valence band of K_0.3WO₃. These well-separated photoelectrons and holes can further react with absorbed oxygen molecules and water molecules, generating ˙O²⁻ and ˙OH. Finally, the obtained ˙O²⁻ and ˙OH degrade the dye molecules.

ZnO is an n-type semiconductor with interesting optical, thermal and electrical characteristics, and a very rich defect chemistry. Such features, coupled with its unique flexibility in adopting a large variety of nanoscale morphologies, have made ZnO an attractive candidate for optoelectronic and gas sensing applications.¹⁷⁹ A series of M_xWO₃/ZnO (M = K, Rb, NH₄) composites was prepared to achieve heat insulation, air decontamination, harmful UV light blocking and high VIS light transmittance simultaneously.¹⁷⁸ The combination of these two materials significantly improved the performance and increased the potential for tungsten bronze to become a commercial smart window material. A designed lithium-ion insertion-type material layer on a fluorine-doped tin oxide substrate with a TiO₂ mesoporous nanotube array film was prepared. It had rapid Li-ion insertion kinetics without sacrificing the window transparency.¹⁸⁰ Because of the rapid Li-ion insertion kinetics, including an enhanced pseudocapacitive effect and Li-ion diffusion coefficient, the device exhibited a high-rate capability over short galvanostatic charge/discharge times. At 1 A g⁻¹, it completed the charge or discharge process within only 232 s and delivered a high, reversible and stable specific capacity of 60 mA h g⁻¹.

Nanofluids are nanoparticle dispersions that can be dispersed in liquids to form a stable system. They do not affect the rheological properties of the system. Nanoparticles in nanofluids have a high surface area and therefore have more heat transfer surfaces than liquids.^181,182 This leads to a high thermal conductivity for nanofluids and so they are widely used in heat-transfer applications. The use of nanofluids to heat buildings can reduce the size of the heat-transfer system, the accompanying pressure losses and the need for subsequent pumping power. This can reduce the energy consumption of a power plant and indirectly reduce environmental pollution.¹⁸³

Refrigeration systems are one of the biggest reasons for the expanding pattern of energy consumption, with approximately 15% of the world's electricity consumed by refrigeration and air conditioning systems.^15,184 Energy savings in the refrigeration system can be achieved by reducing the amount of refrigerant required and by reducing the wear characteristics of the compressor. The refrigerants and lubricants used in the system need to have thermal, mechanical and chemical functions, so they require the use of multifunctional materials. At the same time, the materials must maintain high stability and non-toxicity, which highly conform to inorganic materials. The addition of nanomaterials can provide a higher thermal conductivity for the refrigerant, thereby reducing the amount of refrigerant required for the refrigeration system. It can also provide higher lubricity for the lubricant, thereby reducing the wear characteristics of the compressor and ultimately resulting in energy savings.^15,185 As a result, multifunctional inorganic nanomaterials are essential for energy saving applications. Boron nitride nanosheets possess high thermal conductivity and excellent insulation property. Boron nitride nanosheets nanofluids were fabricated and their effects on thermal conductivity enhancements were further investigated. The thermal conductivity of the boron nitride nanosheets nanofluids could reach 2.39 W m⁻¹ K⁻¹, which represent a 298% increase in comparison with water. By measuring the thermal conductivity from 25 °C to 50 °C, the temperature dependence of boron nitride nanosheets nanofluids was awaken, because the large size of boron nitride nanosheets led to insignificant Brownian motion.¹⁸⁵

Lubrication is defined as a process or technique utilized to reduce friction and wear between two surfaces in proximity and moving relative to one another by interposing a substance called a lubricant.¹⁸⁶ The use of lubricants in mechanical systems with a good tribology characteristic will minimize the wear and friction, and thus make it possible to reduce the power consumption. In the presence of a lubricant, a slippery film is formed, which substantially reduces friction, wear and tear between mating surfaces. Apart from the reduction of friction and wear, the other major functions of lubricants are to improve cooling and heat transfer. In order to serve these purposes, lubricants should possess certain characteristics, such as high thermal and chemical stability, adequate viscosity and resistance to oxidation and corrosion. Nanofluids serving as a lubricating medium have been widely referred to as “nanolubricants” in the literature.

In addition to its marvellous properties, graphene, being a 2D layered structure, offers unique wear and friction properties that are rarely seen in conventional materials.¹⁸⁶ Well-dispersed, clove-treated graphene nanoplatelet coolants were synthesized to achieve improved heat transfer and hydrodynamic properties. A clove-treated graphene nanoplatelet–water nanorefrigerant was tested in a horizontal tube heat exchanger to determine the heat transfer and hydrodynamic properties. A superior enhancement in the thermophysico properties of the clove-treated graphene nanoplatelet–water nanorefrigerant was found. A maximum thermal conductivity enhancement of 22.92% was attained for the sample containing 0.1 wt% of clove-treated graphene nanoplatelets at a fluid temperature of 45 °C. In addition, the dynamic viscosity and density of the clove-treated graphene nanoplatelet–water nanofluids were close to those for deionized water. There was a pronounced increase in the Nusselt number and convective heat transfer coefficient for the clove-treated graphene nanoplatelet–water nanofluid at a weight concentration of 0.1% with a maximum enhancement of approximately 18.69% and 37.54% when the Reynolds number was 15 927. More importantly, there was a negligible increase in the corresponding friction factor, with a value of 3.79%.¹⁸⁷

Graphene oxide–TiO₂ nanocomposites were synthesized by a facile solvothermal reaction between graphene oxide and tetrabutyl titanate.¹⁸⁸ Test results showed that spherical TiO₂ was successfully anchored on the graphene oxide nanosheets. The graphene oxide–TiO₂ nanofluids exhibited better friction and wear properties than a lubricant with graphene oxide nanosheets, TiO₂ nanoparcles and a mixture of graphene oxide and TiO₂. The film thickness of the lubricant with graphene oxide–TiO₂ was 28.07 nm, which was the largest in all the lubrication states.

Based on the experimental and test results, Fig. 12 presents the lubrication mechanism of graphene oxide–TiO₂. The good dispersion stability and hydrophilicity of graphene oxide–TiO₂ made it possible to lubricate in flakes, which means it can more efficiently enter the gaps between the rollers and strips. In addition, the presence of graphene oxide–TiO₂ at the rough peak can avoid direct contact between the roll and the strip, and the TiO₂ nanoparticles anchored on the graphene oxide nanosheet can only be deformed without rolling at will, ensuring the stability of the film formation. Finally, the increase in defects in the graphene oxide–TiO₂ nanosheets indicated that the graphene oxide–TiO₂ nanocomposites may be worn or torn into smaller particles during the lubrication process.

Fig. 12 Different views of graphene oxide–TiO₂ nanofluids rolling lubrication mechanism described in the reference. (a) Overall schematic, (b) rough peak view, (c) rough valley view. This figure was reproduced from ref. 188 with permission from Elsevier, copyright 2018.

Enhanced tribological properties of a mineral base oil containing an antifriction nano-additive based on reduced graphene oxide nanosheets were reported. The graphene oxide nanosheets exhibited no clear acute toxicity of industrial grade. Furthermore, the author proposed a workflow for the evaluation of reduced graphene oxide-based nanolubricant for human and environmental toxicity considering different steps of the material value chain and different reduced graphene oxide dispersions.¹⁸⁹ The stability of a nanolubricant is important to affirm the best performance in an automotive air conditioning system and to avoid system problems after long runs, such as clogging or sedimentation. SiO₂/PAG nanolubricants were successfully synthesized by a sonication method. The performance parameters and energy savings of an automotive air conditioning system using SiO₂/PAG nanolubricants were compared with PAG lubricants. It was found that the condenser pressure and the pressure ratio of the automotive air conditioning system decreased to an average of 10.8% and 5.6%, respectively. The volumetric heat absorb increased by up to 3% and the coefficient of performance increased by an average of 21%. The compressor work and power consumption of the automotive air conditioning system were reduced by 16.5% and 4%, respectively.¹⁹⁰ The purpose of the sonication stage was to break the agglomeration and to provide a uniform nanoparticle dispersion. The coolant side pressure drop, as well as pumping power, could be reduced by using hybrid nanofluids.¹⁹¹ The maximum pumping power reduction was with Al₂O₃ + MWCNT (11.2%) and minimum with Al₂O₃ + Ag (9.6%). Furthermore, the payback period was considerably higher with the use of the hybrid nanofluid. Al₂O₃ + Ag showed a maximum of 247 years, while Al₂O₃ + TiO₂ showed a minimum of 9.8 years. The payback period could be reduced by reducing the nanoparticle cost and increasing the nanofluid stability. This demonstrated the great potential of nanofluids in industrial applications.

To sum up, electrochromic materials and nanofluids are promising materials for energy saving applications. Electrochromic materials are the main materials for smart window applications and require high light transmission, reversible colour conversion, high chemical and electrochemical stability, and excellent near-infrared shielding properties. Furthermore, according to the additional functions of the device, the material also needs to have additional functions. For example, energy storage smart windows require the material to have an energy storage function, while the self-cleaning smart window requires the material to have a catalytic function. Nanofluids are mainly nanorefrigerants and nanolubricants. The addition of inorganic nanoparticles to traditional refrigerants and lubricants can significantly improve their thermal and lubricating properties. This is related to the size effect of the nanomaterials, where their large surface areas provide more heat transfer surface, and the addition of the nanoparticles thickening the film of the lubricant on the surface of the contact layer greatly improves the lubricating effect of the lubricant. Multifunctional inorganic materials provide a rich selection for smart window applications and nanorefrigerants and nanolubricants. Although the high performance of multifunctional inorganic materials is important, in energy saving applications, the price and toxicity of materials and the ease of material recovery should also be given priority.

2.5 Energy transmission applications

The development of reliable and environmentally friendly approaches for energy applications is one of the key challenges that modern society is facing.¹⁴ The large amount of greenhouse gases produced by burning fossil fuels are detrimental to global warming and climate change. The substitution of fossil fuels with environmentally friendly and renewable energy sources is therefore a promising way to reduce greenhouse gas emissions.¹⁹² A major challenge, however, is that such renewable energy sources, such as solar, wind and hydro energy sources, are often located far from densely populated areas, which means that electricity generation from these distant renewable energy sources must be combined with cost-effective energy transmission systems.¹¹ Multifunctional inorganic nanomaterials are an important class of materials with significant potential for energy transmission applications, mainly focused on superconductors and smart grids.⁸ Since the discovery of superconductivity, it has been the dream of researchers to realize a room temperature superconductor.¹⁹³ Recent experimental advances on inorganic nanomaterials have unveiled a range of exotic physical and electronic phenomena for superconductors, such as a reduced superconducting transition temperature upon reducing a sample thickness to the nanoscale and 2D superconductivity.^194–199 In addition, the voltage levels have to increase to further reduce the energy transmission losses in the research and development application of smart grids, which presents a great challenge for high-voltage direct-current (HVDC) insulating materials. Recently, nanodielectrics (nanocomposites) based on an ultralow loading volume of metal oxide nanoparticles (<1 vol%) have been introduced as a potential material group for electrical insulation materials due to their lower charge mobility and direct-current conductivity.^200–203

2.5.1 Superconductivity. From the discovery of superconductivity, it is now established that various metallic elements and chemical compounds, when cooled below a certain critical temperature (T_c), show the remarkable property of conducting electricity without offering any resistance to the flow of an electric current.¹⁹³ This discovery truly established a combined condition for this remarkable state of matter, that all known superconductors below T_c exist as electron- or hole-paired states and that these paired states are known to carry a supercurrent.²⁰⁴ Given the excellent conductivity of superconductors, this would be of great significance for energy transmission if we could realize a room temperature superconductor. The Nobel laureate NevillF Mott once noted that he believed that the first-ever room-temperature superconductor would come from the hands of someone skilled in the art of chemical synthesis, but having the intuition and knowledge of physics and the materials science of metallic materials, ranging from the chemical elements to complex inorganic materials.²⁰⁴ Superconducting nanostructures exhibit a number of intriguing phenomena, such as vortex ratchet and quantum size effects, which are absent in bulk superconductors.²⁰⁴ Shrinking the size of a sample close to its characteristic lengths of penetration depth (λ_L) and coherence length (ξ₀)¹⁹⁷ induces a range of phenomena, such as a decrease of T_c in thin films,²⁰⁵ superconductor-to-insulator transitions in thin films and nanowires²⁰⁶ and the coexistence of charge-density-wave (CDW) order and superconductivity.¹⁹⁴ Superconductivity at the nanoscale enables applications in the fields of quantum information, with superconducting circuits based on Josephson junctions or with nanowires working as single-photon detectors.^207,208 Moreover, high-sensitivity magnetometry based on superconducting quantum interference devices and studies on vortex dynamics in nanostructures, nanoarchitectures and nanopatterned superconductors have become a focus of intense research activity.^{204,209–211}

High-quality superconductor/semiconductor hybrid structures, such as Al/InAs and V/InAs, are of interest for ongoing research in the fields of gateable Josephson junctions (JJs) and quantum-information-related research.^212–214 Phase-dependent zero-bias conductance peaks measured by tunnelling spectroscopy at the end of Josephson junctions has been observed, as realized on a heterostructure consisting of aluminium on indium arsenide nanowires and its planar JJs were investigated as a function of the magnetic field, chemical potential and phase difference (Fig. 13). This work suggested that phase control could offer an additional tuning parameter to enter the topological regime that has not been explored so far.²¹⁴

Fig. 13 Planar JJs phase dependence of the critical field. Third harmonic of the current I_3ω(V_sd = 0) measured by the lock-in amplifier at zero bias as a function of B_∥ and Φ, for different values of gate voltage V₁ at (a) −116 mv, (b) −118.5 mv and (c) −120 mv. This figure was reproduced from ref. 183 with permission from Springer Nature, copyright 2019.

The in situ growth of crystalline aluminium and niobium shells on indium arsenide nanowires by molecular beam epitaxy (MBE) was reported. The realization of new material combinations of Nb/InAs in addition to Al/InAs extended the temperature range and the critical fields possible for further research.²¹⁵ Hybrid devices based on III–V nanowires with ex situ e-beam deposited vanadium contacts have previously been used for studies of Andreev bound states and superconducting quantum interference devices.^216,217 A combined analysis of the crystal structural and electronic properties of vanadium deposited on InAs nanowires has already been presented. Nanoscale superconducting vanadium had a high out-of-plane critical field, far exceeding the bulk value and that of Al on InAs nanowires. The wide variety of grain sizes dominated the electrical behaviour of devices fabricated from those crystal growths. This work suggested better hybrid devices need further optimization of the entire crystalline and that there is a need to find a more suitable semiconductor to achieve epitaxially matched vanadium/semiconductor hybrids.²¹¹ In addition, low-dimension and small-size superconductors are also candidates for studying electrical transport in nanoscale systems due to their unique size-and shape-dependent properties. YBa₂Cu_3−xNi_xO_7−δ (0.00 ≤ x ≤ 0.04) nanowires were prepared by an electrospinning technique. The temperature dependence of magnetization of these nanowires revealed a diamagnetic transition at temperatures close to the T_c, which was related to the superconductor transition in these compounds. The T_c values ranged between 70 K and 93.2 K for samples with x = 0.04 to x = 0.00, respectively, where the substitution of Cu with Ni resulted in a lower T_c. This behaviour could be interpreted as a distribution of the transition temperatures due to an inhomogeneous distribution of Ni atoms, granular effects and structural variations resulting from the synthesis process.²¹⁸

2D superconductors have many interesting properties, such as the localization of electrons and Cooper pairs, transition-temperature oscillations caused by quantum size effects, excess conductivity originating from superconducting fluctuations, Berezinskii Kosterlitz Thouless (BKT) transitions and quantum phase transitions (QPTs) at zero temperature.²¹⁹ With the continuous development of new preparation and stacking methods for ultrathin 2D materials with atomic layers, such as MBE, accompanied by new surface or interface reconstruction processes, mechanical exfoliation and methods for the production of field-effect devices, the emergence of van der Waals heterostructures of 2D materials has opened a new door for quantum materials.^{206,220–223} Novel physical properties, such as high temperature superconductivity, can be achieved by controlling the composition and stacking direction of different layered materials, or by changing the external electric field. Recently, signatures of tunable superconductivity in an ABC-trilayer graphene (TLG) and hexagonal boron nitride (hBN) moiré superlattice were reported. The electronic behaviour in the ABC-TLG/hBN superlattice depended sensitively on the interplay between the electron–electron interaction and the miniband bandwidth. Transitions from the candidate superconductor to a Mott insulator and metallic phases could be observed by varying the vertical displacement field (Fig. 14a). It was shown that ABC-TLG/hBN heterostructures offered attractive model systems in which to explore high T_c superconductivity's relationship to the Hubbard model.²²⁴ In addition, the heterostructures or hybrids of graphene and superconductors offered a very interesting platform to study mesoscopic superconductivity and the interplay of the quantum Hall effect with superconductivity. The direct growth of high-quality graphene/2D superconductor (nonlayered ultrathin α-Mo₂C crystal) vertical heterostructures with uniformly well-aligned lattice orientation and strong interface coupling by chemical vapour deposition was reported. As shown in Fig. 14b and c, based on these strongly coupled high-quality heterostructures, highly transparent Josephson junction devices were realized, in which a clear magnetic-field-induced Fraunhofer pattern of the critical supercurrent was observed. This work provided a promising platform for future studies of exotic quantum transport behaviours, such as Majorna modes.²²⁵

Fig. 14 (a) Tunable electronic phases with the displacement field. As a function of D, the system can be tuned across four different electronic states, from left to right: superconducting, correlated resistive state, metal and correlated insulating state. This figure was reproduced from ref. 224 with permission from Springer Nature, copyright 2019. (b and c) Highly transparent Josephson junction device based on a CVD-grown graphene/2D α-Mo₂C crystal heterostructure. This figure was reproduced from ref. 225 with permission from American Chimerical Society, copyright 2017. (d and e) NbSe₂ temperature (d) and magnetic-field (e) dependent superconducting behaviours. This figure was reproduced from ref. 205 with permission from Royal Society of Chemistry, copyright 2017.

The transition metal dichalcogenide niobium diselenide (NbSe₂) is one of the most studied van der Waals materials that exhibits both CDW and superconductivity at low temperatures.^195,205 There was a combined optical and electrical transport study performed on the many-body collective-order phase diagram of NbSe₂ down to a thickness of one monolayer. Both the CDW and the superconducting phase were observed down to the monolayer limit. It was reported the superconducting transition temperature decreased on lowering the layer thickness, but the newly observed CDW transition temperature increased from 33 K in the bulk to 145 K in the monolayer, which could be understood to be a result of significantly enhanced electron–phonon interactions in 2D NbSe₂. This work suggested van der Waals materials can provide an ideal platform for the investigations of CDWs and their relation with superconductivity in the 2D limit.⁹ High-quality NbSe₂ nanoplates were synthesized by a single-step chemical vapour deposition and their temperature and magnetic-field dependent superconducting behaviours were investigated (Fig. 14d and e). The NbSe₂ nanoplates showed the 2D characteristics of superconducting transitions and strong anisotropy with magnetic field orientation, thereby providing potential platforms for the exploration of new physics in nanoelectronic devices.²⁰⁵ In the future, the development of new technologies for the fabrication and characterization of 2D nanostructured superconductors will probably be in high demand.

As mentioned above, superconducting planar nanostructures are widely used in many applications. In contrast, 3D superconducting nanostructures, despite their potential in quantum information processing and nanoelectronics, have been addressed only in a few pioneering experiments.^226–228 A comparative study of planar nanowires and free-standing 3D nanowires have been carried out by focused electron- and ion (Ga⁺)-beam induced deposition (FEBID and FIBID) using the precursor Nb(NMe₂)₃(N-t-Bu). Electrical transport measurements showed that FEBID nanowires were highly resistive following a variable-range-hopping behaviour. In contrast, FIBID planar nanowires became superconducting at 5 K. In addition, the critical temperature of free-standing 3D nanowires was as high as 11 K, which was close to the value of bulk NbC, which suggested that FIBID-NbC was a promising material for the development of 3D superconductivity, with applications in quantum information processing and nanoelectronics.²⁰⁴ The growth of 3D superconducting WC_1−x hollow nanowires with the use of a He⁺ focused-ion-beam-microscope in combination with the W(CO)₆ precursor was reported, and this became superconducting at 6.4 K and showed a large critical magnetic field and critical current density, resulting from their quasi-1D superconducting character. The results paved the way for future nanoelectronic devices based on 3D nanosuperconductors.²²⁸ The unique capability of 3D nanosuperconductors to achieve extreme in-plane and out-of-plane magnetic-field sensitivity allowed them to be used as a local probe for nanomagnets and nanosuperconductors. Hence, the exploitation of the three-dimensions in nanosuperconductivity could represent a paradigm shift for the development of next-generation electronic nanodevices with applications in advanced sensing and quantum computing.

2.5.2 Insulating materials. In the process of electric energy transmission, the voltage levels must be increased to reduce the energy loss. However, these higher voltage levels inevitably cause considerable electrical stress and an increased temperature, which may result in a breakdown in current insulating materials. The heat generation (ΔT) in a cable insulation is proportional to the DC conductivity (σ) and the square of the applied voltage (U), hence an insulating material with ultralow DC-conductivity that prevents the risk of thermal runaway and electrical failure at higher voltage levels is required.¹⁹² Recently, nanodielectrics based on an ultralow loading volume of metal oxide nanoparticles (<1 vol%) have been introduced as a potential material group for electrical insulation materials due to their lower charge mobility and DC conductivity than pristine insulation materials.^{200,203,229,230} These metal oxide nanoparticles, being either insulating (Al₂O₃, MgO and SiO₂) or semiconducting (TiO₂ and ZnO), had a higher conductivity than that of the pristine low-density polyethylene (LDPE) matrix, and they reduced the charge mobility in LDPE nanocomposites through the trapping/adsorption of charge carriers, including electrons, holes and polar/ionic species.¹⁹² The final electrical properties of the nanocomposites depended on the degree of particle dispersion, and the size, morphology and crystal surfaces of the incorporated nanoparticles.

Among these metal oxide particles, ZnO is relatively unique due to the possibility of tailoring this particular metal oxide into a broad range of particle sizes and shapes by a simple nonsurfactant aqueous precipitation under controlled synthesis conditions. High-purity, narrow-size-distribution ZnO particles with different sizes and morphologies were synthesized and a series of silanes with terminal alkyl groups of different lengths (methyl, octyl, and octadecyl groups) were used to modify the surface of the ZnO nanoparticles. These modified hydrophobic particles showed excellent dispersion/distribution in the LDPE matrix due to the high particle/polymer compatibility. The hierarchical flower-shaped ZnO particles with a nanosized porosity accessible to the LDPE chains showed a high interfacial surface area and additional electron traps for charge carriers at the polymer/particle interface, while the addition of 3 wt% ZnO nanoparticles with controlled functionality was sufficient to decrease the conductivity of pristine polyethylene by more than two orders of magnitude.²⁰⁰

In addition, MgO has become a research hotspot because of its relevance as an emerging inexpensive industrial nanoparticle material and due to its insulating properties. MgO/PP/POE nanocomposites were fabricated by melt blending. An examination of the electrical properties revealed that the DC electric breakdown strength and space charge suppression effect were remarkably improved by the introduction of surface-modified MgO nanoparticles. In addition, obvious enhancements in the tensile modulus and strength were obtained as a result of the synergistic toughening of the POE and MgO nanoparticles. All of these enhancements in the mechanical and electrical properties were closely related to the large interfacial zones and good adhesion properties between the nanoparticles and polymer matrix. These results suggested that MgO/PP/POE nanocomposites with enhanced mechanical and electrical properties have great potential to be used as recyclable insulation materials for high-voltage DC cables with large transmission capacities and high operating temperatures.²²⁹ Also, 70 nm hexagonal MgO nanoparticles with 18 units long hydrocarbon functional silsesquioxane coatings were synthesized. The lowest volume conductivity was about 7 × 10⁻¹⁶ S m⁻¹ for 3 wt% surface-coated nanoparticles, which was a reduced conductivity of two orders of magnitude compared to pristine LDPE.²⁰³

In addition to metal oxide particles, carbon-based materials, such as fullerene (C₆₀), have also been studied for their high electron affinity and low ionization energy for efficient electrical tree inhibition.²³¹ In fact, the primary challenge in the processing of nanocomposites is achieving a homogeneous dispersion of the nanoparticles. The dispersion quality affects the interfaces between the organic and the inorganic components, which can determine the final properties of the nanocomposite.^229,232 Besides, the search for high purity nanoparticles without conducting counterions on the particle surfaces is also very important for the development of ultralow electrical conductivity nanocomposites.²³³

3. Perspectives and conclusion

Recently, inorganic nanomaterials have been well developed for energy applications. Energy applications require materials with high electrical, optical, mechanical and thermal properties. In order to meet the requirements of these energy applications, multifunctional inorganic nanomaterials have been extensively studied. For each energy application, we illustrated the unique functions of inorganic nanomaterials to improve their performances. We highlighted the combination of functions of nanomaterials in a device. In most energy applications, the limited success so far may be traced back to the limitations set by the scaling relations between multifunctional inorganic nanomaterials and energy devices.

Multifunctional inorganic nanomaterials play a key role in energy generation, energy conversion, energy storage, energy saving and energy transmission applications due to their unique properties, such as excellent electrical and thermal conductivity, large surface area and chemical stability. Higher performance, more functions, lower cost and lower toxicity are the development direction of multifunctional inorganic nanomaterials for future energy applications. Although they have great potential, the scientific community still needs to make great efforts to achieve their implementation and success in large-scale applications. The working mechanism of multifunctional inorganic nanomaterials is still fuzzy. Therefore, in-depth characterization using advanced instruments and methods combined with scientific interpretation models should be adapted to gain more insights into the structure–properties relationship of multifunctional inorganic nanomaterials. The functions of devices are closely related to the properties of materials.

Discovering novel materials is hampered by the lack of efficient approaches to the exploration of both a large number of possible elemental compositions and candidate structures.^234,235 Artificial intelligence (AI) is currently a hot research method, and is inextricably linked to the future development of multifunctional inorganic materials. In the field of materials, the use of AI to screen different combinations of elements in a fixed structure is a new way to discover novel materials and to simulate material properties.^236,237 At present, the exploration of novel materials is still empirical, and the screening of elemental combinations is influenced by human bias. Researchers often focus on the combination of several elements that they find interesting. AI can efficiently screen all possible combinations of elements, which could give a small number of candidate materials and greatly improve the efficiency of finding novel materials.

Finally, multifunctional inorganic nanomaterials intended for energy applications will move towards higher performance and more functions in the future. Core–shell nanomaterials show significant material advantages over individual materials in many energy applications. However, it is still difficult to fabricate high-quality core–shell hollow materials that satisfy all the requirements of energy applications. Compounding materials can significantly improve the performance of materials. The recombination of materials allows the functions of the individual materials to complement each other, resulting in a composite material with multiple functions at the same time. Screening the elements and combination of materials and predicting the properties of materials through AI is an important development direction. Although considerable achievements in multifunctional inorganic nanomaterials have been made in the fields of energy applications which clearly highlight their scope for future development, we still need to constantly strive to create higher performance multifunctional inorganic nanomaterials to meet future demands.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors greatly acknowledge the National Natural Science Foundation of China (Grant No. 51832007).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9nr07008g

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