Supercapacitor electrode materials: nanostructures from 0 to 3 dimensions

Supercapacitors have drawn considerable attention in recent years due to their high speci ﬁ c power, long cycle life, and ability to bridge the power/energy gap between conventional capacitors and batteries/fuel cells. Nanostructured electrode materials have demonstrated superior electrochemical properties in producing high-performance supercapacitors. In this review article, we describe the recent progress and advances in designing nanostructured supercapacitor electrode materials based on various dimensions ranging from zero to three. We highlight the e ﬀ ect of nanostructures on the properties of supercapacitors including speci ﬁ c capacitance, rate capability and cycle stability, which may serve as a guideline for the next generation of supercapacitor electrode design.


Introduction
Before a ray of sun light peeked through the curtain and shined on John's face, everything was perfect for a quiet morning.Suddenly jumping off the bed, "Oh, my, I forgot to set the alarm last night."He got dressed and rushed to his electric car; "LOW BATTERY" exclaimed John, "wish I could charge it up in a few minutes."As a matter of fact, such energy storage technologies like supercapacitors which can charge very quickly have been around for decades, but their low energy densities have held them back in many such applications.The ever growing demand for energy urges researchers to look for new materials and processes to develop highly improved energy storage devices.Among energy storage systems, supercapacitors have drawn considerable attention during recent years due to their fast charge-discharge characteristics, high power density, and excellent cycle stability.Currently, supercapacitors are used along with batteries to provide the additional power required in many applications.However, they cannot be used as standalone units in these applications since their energy density is inferior to that of batteries.For example, commercially available supercapacitors can provide energy densities of less than 10 W h Kg À1 whereas Li-ion batteries can provide energy densities of more than 180 W h Kg À1 . 1,2Nevertheless, the power density and cycle life of supercapacitors are far superior to those of Li-ion batteries.At present, the scientic community is striving to considerably improve the energy and power density of supercapacitors by developing new electrode materials and electrolytes, and ingenious device design.
Achieving noteworthy improvements in supercapacitor performances by means of better electrode materials together with low-cost and environment-friendly production, requires a deep fundamental understanding of the charge storage mechanisms, transport pathways of electrons and ions, and electrochemically active sites.Advances in nanomaterials, ideal candidates for improved ion adsorption or faster surface redox reactions, have fostered extensive research efforts toward the enhancement of electrochemically active sites for charge transfer and controlled ionic and electronic transport at small diffusion length scales.
Supercapacitors are generally classied into two types based on their storage mechanism, the electrochemical double layer capacitors (EDLCs) and pseudocapacitors.EDLCs physically store charges via reversible ion adsorption at the electrodeelectrolyte interface, while pseudocapacitors chemically store their charges via redox reaction at the vicinity (a few nanometers) of the surface.Typically, electrode materials for EDLCs are made of carbon-based materials (including activated carbon, carbon nanotubes, graphene, etc.), while that for pseudocapacitors are transition metal oxides (e.g.RuO 2 , MnO 2 , CoO x , NiO, Fe 2 O 3 , etc.) and conducting polymers (e.g.polypyrrole, polyaniline, poly (3,4-ethylenedioxythiophene), etc.).
The criteria for designing a high-performance supercapacitor electrode include high specic capacitance (energy stored per unit mass, volume, or area of active materials), large rate capability (capacitance retention at high scan rate or current density), and high cycle stability.In addition, the toxicity and cost of the active materials used in an electrode design should be taken into account as well.Therefore, in order to realize a high-performance supercapacitor electrode, factors that determine the specic capacitance, rate capability, and cycle stability need to be mentioned here.(1) Surface area: Since charges are stored on the surface of the supercapacitor electrodes, an electrode with a higher surface area leads to an improved specic capacitance.Nanostructuring of electrode materials is a feasible method to considerably improve the surface area of the electrodes.(2) Electronic and ionic conductivity: As specic capacitance and rate capability are considerably dependent on both electronic and ionic conductivity, a high electronic and ionic conductivity will help to maintain the rectangular nature of cyclic voltammetry (CV) curve and symmetricity of galvanostatic chargingdischarging (GCD) curves.They also reduce the specic capacitance losses as scan rates/current densities are increased.Typical approaches to enhance the electronic conductivity include binder-free electrode design and nanostructured current collector design to provide efficient electron pathways for charge transport.To increase the ionic conductivity, precise control of pore size and prudent design strategies are used (e.g. more open structures for ion transport).( 3) Mechanical and chemical stability: The cycle stability is greatly inuenced by the mechanical and chemical stability of electrode materials during cycling.Phase change, dissolution, and side reaction of active materials are the major reasons for causing cycle instability.Sensible electrode surface protection may substantially boost the cycle stability.In addition to investigating new electrode materials, electrode design with nanostructures offers a promising avenue to fulll high-performance supercapacitors.This is because the aforementioned factors can be effectively manipulated by a nanostructured design, which will be discussed in detail in the following sections.
4][5] Particles which are more or less spherical in shape like fullerenes, quantum dots, nanoonions, nanoparticles, etc. are considered as 0D.Nanostructures with 1D are interesting due to their dimensionality dependence on its functional properties.They include nanotubes, nano-bers, nanowires, nanopillars, nanoribbons and nanobelts.Typically, materials with a thickness of a few atomic layers and with the other two dimensions beyond the nanometric size range are considered as 2D materials.Graphene and many other layered van der Waals solids like MoS 2 , CaGe 2 and CaSi 2 , fall under the 2D category.Materials with three dimensions beyond the nanometric size range but still preserve the advantages of nano size effect are regarded as 3D materials.Mesoporous carbon and graphene aerogel fall under 3D category.
Even though there are several reviews describing the material aspects of supercapacitor electrodes, 2,6-9 a comprehensive review of 0D, 1D, 2D and 3D materials used only in supercapacitors is still lacking.Although Wu and co-authors have reported the dimensional study of electrodes for 0 to 3D, 10 they only focused on carbon-metal oxide composite electrodes from 0 to 3D.This review covers a much broader range of electrode materials.In recent years, a range of materials congurations ranging from 0D particles to 3D nanostructured frameworks have been developed and tested in supercapacitors devices.This review is aiming to ll this gap and provide the readers a clear idea about the performance of these materials as electrodes in electrochemical supercapacitors.Fig. 1 summarized the key points of electrode materials from 0 to 3D and a detailed discussion is given in the following sections.

0D nanostructures
According to the strict denition, 0D nanostructures are spherical particles that have three dimensions constrained on the nanoscale (usually 1-100 nm).Nevertheless, particles with diameters up to 1 mm are usually regarded as nanoparticles as well, as the benet provided by the effect of their small size has been observed for this size range.In this section, three subcategories of 0D nanostructures used as electrodes in supercapacitors are considered, viz., solid, hollow, and coreshell 0D nanostructures.

Solid 0D nanostructures
As the foundation of 0D nanostructures, solid nanoparticles (nanospheres) are intensely investigated for designing supercapacitor electrodes.Carbon materials (e.g.activated carbon, 11 carbon nanospheres, 12 and mesoporous carbon 13,14 ) and transition metal oxides (e.g.MnO 2 , 15,16 NiO, [17][18][19] Fe 3 O 4 , 20 etc.) have been widely fabricated into solid nanoparticles for electrode materials.Among carbon materials, activated carbons (ACs) are distinct due to their high surface area (up to 3000 m 2 g À1 ) and economical fabrication process.In addition, ACs propose a wide range of pore size distribution including micropores (<2 nm), mesopores (2-50 nm), and macropores (>50 nm). 21ACs are generally produced by either physical or chemical activation of versatile carbonaceous precursors such as wood, coal, nutshell, and so forth.Larger surface area is usually expected to result in a higher specic capacitance.However, even with a surface area of up to 3000 m 2 g À1 , only small specic capacitance (<10 mF cm À2 ) could be obtained with ACs. 1 This is due to the fact that specic capacitance is not only determined by surface area but also by other parameters such as pore size distribution, pore shape and structure, accessibility of the electrolyte, and electrical conductivity. 6Therefore, an ideal design of ACs should meet at least two requirements: (1) large surface area for the enhancement of electrochemically active sites and (2) suitable pore size and structure for the transportation of ions in the electrolyte.In addition to ACs, Gogotsi, Simon, and their co-workers have prepared onion-like carbon by annealing nanodiamond powder at 1800 C. 22,23 The carbon onions were found to possess a moderate specic surface area ($500 m 2 g À1 ) compared to ACs; however, the surface can be fully accessed by ions (Fig. 2).As a result, micro-supercapacitors based on these onion-like carbons showed $70% rate capability from 1 to 100 V s À1 and an excellent relaxation time constant (26 ms) compared to that of activated carbon (700 ms).
The above mentioned criteria for designing ideal ACs are also applicable to transition metal oxide nanoparticles in pseudocapacitors as they rely on fast surface redox reactions.Additionally, as most of the transition metal oxides suffer from poor electrical conductivity, enhancing the bulk electrical conductivity should also be taken into account.For example, Chen and co-workers have fabricated nanoporous metal/oxide hybrid supercapacitor electrodes in order to enhance the electrical conductivity of MnO 2 . 24In this method, conductive nanoporous gold was rst produced by de-alloying Ag 65 Au 35 using HNO 3 , followed by growing nanocrystalline MnO 2 (with a grain size of $5 nm) into the nanopores.The resulting nanoporous Au/MnO 2 supercapacitor electrode showed a high specic capacitance of $1145 F g À1 at 50 mV s À1 , which can be attributed to nanoporous gold that allows easy and efficient access to both electrons and ions.Recent advances in using perovskite materials for solar energy harvesting have greatly stimulated the enthusiasm of researchers to exploit these materials for other potential elds.Lately, Stevenson, Johnston, and co-workers have prepared LaMnO 3 nanoparticles via reverse-phase hydrolysis method as supercapacitor electrode material. 25Fig. 3a shows an ideal LaMnO 3 cubic perovskite structure, where lanthanum atom locates at body centered position, manganese atoms at cubic corner positions, and oxygen atoms at mid-edge positions respectively.The dark-eld SEM image (Fig. 3b) presents the small ($20-50 nm) nanocrystals of LaMnO 3.09 that were sintered into agglomerated particles.They have found that oxygen intercalation has signicant effect on LaMnO 3 perovskite nanoparticles as supercapacitor electrodes.The specic capacitances of LaMnO 3.09 and LaMnO 2.91 reached 586.7 and 609.8 F g À1 , respectively.Interestingly, the capacitance difference ($4%) was found to be close to the oxygen content difference ($6%), implying that the oxygen intercalation is the major charge storage mechanism of LaMnO 3 perovskite supercapacitor electrodes.This assumption was further demonstrated by applying electrolyte with varying anion concentrations as well as different electrolyte types (i.e.neutral and alkaline electrolytes).As a result, perovskite-type materials with anion vacancies can be exploited to store charge through oxygen intercalation.Therefore, in addition to traditional solid 0D active materials (e.g.activated carbon), perovskite materials might open a new paradigm as supercapacitor electrode materials.

Hollow 0D nanostructures
Hollow 0D nanostructures have been recognized as promising candidates for electrode design due to their peculiar properties   View Article Online such as low density, high surface-to-volume ratio and shortened pathways for transporting both mass and charges. 26Hollow 0D nanostructures are generally synthesized via three distinct approaches: hard templating, so templating, and templatefree methods.Among them, hard templating methods have been widely adopted because of their remarkable advantages in controlling the size, shape, and structure of the products. 27ypically, a single-shelled hollow nanosphere is created by coating a thin layer of precursor materials onto a spherical hard template (e.g.silica spheres, polystyrene colloid spheres, carbonaceous spheres, etc.), followed by removing the template either by calcination or by chemical etching.For example, Yang and co-workers reported the synthesis of hollow carbon nanospheres by carbonizing its hollow nanosphere precursor obtained through a silica sphere-assisted hard templating method. 28The resulting hollow carbon nanospheres showed a high surface area (1704 m 2 g À1 ), large bimodal mesopores (6.4 and 3.1 nm), and large pore volumes (1.6 cm 3 g À1 ).With such a design, a specic capacitance of 251 F g À1 at 50 mV s À1 was achieved for the hollow carbon nanospheres.Hollow 0D structures based on other electrode materials such as MnO 2 , 29,30 NiO, 31,32 NiS, 33 etc. have been investigated as well.
In addition to single shelled hollow 0D nanostructures, multiple shelled samples have also shown fascinating characteristics (e.g.substantially improved surface-to-volume-ratio) for efficient electrode material design.Similarly, electrode materials like NiO, 34 Co 3 O 4 , 35 and Fe 2 O 3 , 36 have been successfully fabricated into multi-shelled structures.For example, Zhang and coworkers reported the synthesis of NiO hollow nanospheres via layer-by-layer (LBL) self-assembly for supercapacitor application. 34In their method, the number of shells was directly determined by the cycles of immersion-precipitation process before removing the carbonaceous template via calcination (Fig. 4a).Single, double, and triple shelled NiO nanospheres were prepared by repeating the immersion-precipitation process for 0, 1, and 2 times (Fig. 4b and c).The double-shelled NiO nanospheres exhibited the highest surface area (92.00 m 2 g À1 ), which ascribes to their more loosely-packed structure compared to their single and triple shelled counterparts.As a result, the double shelled NiO nanospheres showed the most enhancement in specic capacitance (612.5 F g À1 ).Since many other promising electrode materials have not been fabricated into 0D multi shelled structures, much effort still needs to be dedicated in the future.

Core-shell 0D nanostructures
Core-shell 0D nanostructures oen refer to a solid or hollow nanoparticle coated with a thin shell.Combining faradaic and non-faradaic materials into core-shell 0D nanostructures offer considerable advantages including enhanced electrical conductivity, less agglomeration, robust chemical and mechanical stability, etc. [37][38][39][40] For example, Zhao and co-workers have grown conducting polymer (polyaniline (PANI)) on hollow carbon spheres as supercapacitor electrode materials. 37Hollow carbon spheres (HCS) with high specic area (2239 m 2 g À1 ) were rst prepared using ferrocene as the carbon precursor and silica spheres as the template.PANI was then conformally grown on the surface of the HCS by polymerizing aniline at room temperature for 12 h (Fig. 5a and b).HCS-PANI core-shell structures with an optimized content of PANI showed a specic capacitance of 525 F g À1 , which is almost two times greater than that of pristine HCS (268 F g À1 ).However, the rate capability of HCS-PANI decayed with increasing PANI content, which can be attributed to the pore blockage due to the reduced accessibility of electrolyte ions.Zhao's group also synthesized core-shell nanoparticles with carbon core and MnO 2 shell by direct redox reaction between graphitic hollow carbon spheres (GHCS) and KMnO 4 solution at 70 C (Fig. 5c and d). 38The as-prepared GHCS-MnO 2 core-shell nanostructures showed a maximum specic capacitance of 190 F g À1 at 0.1 A g À1 with an optimized MnO 2 content (64 wt%), more than two times higher than that of pristine GHCS (95 F g À1 ).Moreover, the GHCS-MnO 2 coreshell nanostructures maintained 55% capacitance retention while current density increased from 0.1 to 10 A g À1 , indicating a porous structure of MnO 2 shell was formed to provide efficient diffusion pathway for electrolyte ions.

1D nanostructures
1D nanostructures are slivers of material with two dimensions constrained on nanoscale. 41][44][45][46][47] For instance, the longitudinal axis of 1D nanostructures can provide efficient transport pathway for both electrons and ions, 48 while maintaining the similar benets of 0D nanostructures in the two nanoscale dimensions.In this section, we place 1D nanostructures into two major categories: homostructures and heterostructures.

1D homostructures
1D homostructures are structures with only one singular structure such as nanorods, nanowires, nanotubes, etc.They can be divided into three groups, namely, nanorods/nanopilars with aspect ratios (length/diameter) less than 10, nanowires with aspect ratios more than 10, and nanotubes with hollow interiors. 49The methods for synthesizing 1D homostructures are versatile and can generally be divided into two major types, template-assisted and template-free methods.For templateassisted methods, sacricial templates (e.g.AAO templates) are commonly used to shape the initial structure, followed by the removal of the templates with an acidic or basic solution.For template-free methods, various approaches like hydrothermal synthesis, chemical vapor deposition (CVD), and electrodeposition are generally employed to directly grow 1D homostructures.Recently, vertical growth of active material on the conductive substrate either by a "bottom-up" or "top-down" method has become one of the most promising electrode design approaches.Such a design guarantees efficient transport of electrons between current collector and active material because of the binder-free design.It also enables fast transfer of ions between electrolyte and active materials because of the available void space between adjacent nanostructures.
3.1.1Nanorods/nanopillars.Although the moderate aspect ratio of nanorods/nanopillars limits the enhancement of surface area compared to nanowires/nanobers, the signicantly lower chances for individual nanorod/nanopillar to collapse on to its neighboring structures making them more accessible to the electrolyte, which effectively increases the ionic conductivity.
Tong and co-workers have reported oxygen-decient Fe 2 O 3 nanorods as supercapacitor electrode material. 50Nanorod-like FeOOH was rst hydrothermally synthesized in an aqueous solution containing ferric chloride, sodium nitrate, and hydrogen chloride.Subsequently, the FeOOH nanorods were heated at high temperature in air and N 2 , separately, which resulted in pristine Fe 2 O 3 nanorods and oxygen-decient Fe 2 O 3 nanorods.Interestingly, the oxygen-decient Fe 2 O 3 nanorods exhibited more than two times enhancement of the specic capacitance (64.5 F g À1 ) than that of pristine Fe 2 O 3 .This phenomenon may be attributed to the existence of oxygen vacancies which considerably enhance the donor density and in turn improve the conductivity and reactivity of Fe 2 O 3 nanorods.
Recently, Thomas and co-workers reported a facile method to produce highly ordered carbon nanopillars as supercapacitor electrodes. 512][53][54] In a typical SNAP process, a thin layer of polymer solution was spin-coated on an e-beam etched nanohole structured silicon mold, followed by curing and peeling off the polymer from the template (Fig. 6a).The SNAP technique was demonstrated to be simple, quick, and cost-effective since it requires just a spin coater and a pre-fabricated template.The entire printing process can be completed within a few minutes, but can be performed at a much faster pace if automated.Moreover, a large-scale nanostructure can be easily obtained by stitching several printed structures next to each other.Unlike commonly used sacricial template (e.g.AAO template), this template can be re-used many times without any noticeable degradation of nanostructures since all the polymer nanopillars can easily be removed from the template.The carbon nanopillar arrays were obtained upon post stabilization and carbonization of the polymer nanostructures.Fig. 6b and c show the SEM images of printed polyacrylonitrile (PAN) and carbon nanopillar array.The diameter and height of each carbon nanopillar is $95 nm and $200 nm, respectively.Signicantly, the specic capacitance of the carbon nanopillar arrays (3.4 mF cm À2 ) showed considerable enhancement compared to that of a planar carbon lm (0.05 mF cm À2 ).View Article Online 3.1.2Nanowires.Nanowires have been widely studied as electrode material due to their high aspect ratio that can provide large surface area to store charges and an efficient pathway to transport charges.Taking PANI nanowires as an example, Li and co-workers have synthesized vertically aligned PANI nanowires as supercapacitor electrodes by means of electrochemical deposition using AAO templates. 55The electrochemical deposition was carried out at a constant potential of 0.75 V for 5000 s in an electrolyte solution containing aniline and sulfuric acid.PANI nanowire arrays were obtained upon removing the AAO template in a 0.01 M NaOH solution.These samples showed a specic capacitance of 1142 F g À1 at a current density of 5 A g À1 , with 5% capacitance fading aer 500 cycles.Regardless of the superior electrochemical performance of PANI nanowires, the usage of AAO templates limits the practicality of this fabrication process for large scale production since a new AAO template is required for every array of nanostructures produced.Lately, Wei and co-workers reported a facile one-step template-free approach to synthesize PANI nanowire arrays. 56In their method, PANI nanowire could directly be electropolymerized by a simple galvanostatic deposition process at a constant current of 0.01 mA cm À2 for 1 h.Consequently, the asprepared PANI nanowire arrays showed 950 F g À1 at a current density of 1 A g À1 and 16% loss aer 500 cycles.
Lately, a ternary metallic oxide, spinel nickel cobaltite (NiCo 2 O 4 ), has attracted substantial attention due to its high conductivity (more than two orders of magnitude higher than single NiO or Co 3 O 4 ) and multiple oxide states that enable rich redox reactions originating from both nickel and cobalt ions. 57,58For example, Lou and co-workers have successfully grown NiCo 2 O 4 nanoneedle arrays directly on conductive substrates as supercapacitor electrodes. 59In their synthetic process, a conductive substrate was rst immersed into a solution containing nickel nitrite, cobalt nitrite, ethanol, and water in an appropriate ratio and heated to 85 C for 8 h, followed by annealing at 250 C for 90 min.Remarkably, the NiCo 2 O 4 nanoneedle arrays showed a capacitance of 1118.6 F g À1 and about 10.6% loss in capacitance aer 2000 cycles.
3.1.3Nanotubes.Compared to solid nanostructures (e.g.nanowires), tubular nanostructures typically offer higher surface area with less utilization of mass, resulting in more gravimetric specic capacitance.Nanotubes are usually synthesized via template-assisted method, CVD, hydrothermal method, etc.For template-assisted method, nanotubes can be easily prepared by partially inltrating the nanotube forming material into a nanoporous template.This is due to the fact that materials preferentially anchor on the walls of nanochannels at their early inltrating stage.Therefore, it is critical to manipulate the inltration process in order to produce tubular nanostructures.For example, MnO 2 nanotube arrays were synthesized using AAO template via electrochemical deposition for 10 min; whereas MnO 2 nanowire arrays were obtained aer 60 min of deposition. 60he MnO 2 nanotube arrays showed a specic capacitance of 320 F g À1 , while only 101 F g À1 was measured for the nanowire arrays.Using the same strategy, other electrode materials were synthesized with tubular nanostructure such as RuO 2 , 61 PEDOT, 62 etc.
The discovery of carbon nanotubes (CNTs) has led to several applications in energy storage eld.CNTs as supercapacitor electrode materials, and especially vertically aligned CNTs directly grown on conductive substrates, have drawn remarkable attention due to their moderate to high surface area (120-500 m 2 g À1 ), porous structure, superior electronic conductivity, and excellent mechanical and thermal stability.Ajayan and co-workers have grown aligned CNTs (400 mm long) on metal alloy (Inconel) using vapour-phase catalyst delivery. 63The CV curves showed rectangular and symmetric shape even at a scan rate of 1000 mV s À1 , suggesting ideal capacitive behavior and low contact resistance between the CNTs and the substrate.The specic capacitance of a supercapacitor based on these vertically aligned CNT arrays was found to be 18 F g À1 .Iijima and co-workers reported a method to prepare densely packed and aligned CNTs using zipping effect as supercapacitor electrode material. 64The specic capacitance of the aligned CNTs showed 80 F g À1 , which is roughly twice the value of 45 F g À1 for the entangled CNTs.Recently, Kim and coworker have grown vertically aligned CNTs directly on conductive carbon papers using an Al/Fe catalyst via water-assisted CVD. 65hese aligned CNTs exhibited a specic capacitance of about 200 F g À1 at 20 A g À1 .

1D heterostructures
Unlike homostructures, heterostructures usually consist of more than one component.One major advantage of using heterostructured electrode design is the synergic improvement of intrinsic properties of each component for better electrical conductivity, faster ionic transport, greater electrochemical reversibility and cycle stability, and improved mechanical stability.Owing to these advantages, 1D heterostructures are currently considered as one of the potential candidates for the next generation electrode designs.
3.2.1 Core-shell 1D heterostructures.Core-shell 1D heterostructures oen refer to a 1D nanostructure (e.g.nanorod, nanowire, nanotube, etc) coated with a thin shell.Among various heterostructures, core-shell heterostructures have received considerable attentions due to their unique structural properties that can effectively decline the surface energy which reduces the aggregation possibility of active materials as well as relieves side reaction between electrode and electrolyte, resulting in better reversibility and cycle stability of electrodes.Therefore, rational synthesis of advanced core-shell heterostructures with fascinating synergetic properties offers a promising approach to improve electrochemical performance.Typically, most of the core-shell heterostructures are prepared by initially creating a 1D nanostructured core backbone, followed by sheathing the shell materials.Various methods have been explored in order to obtain such advanced conguration including sputtering, electrochemical deposition, hydrothermal synthesis, CVD, electrospinning, etc. Due to the versatile combinations of electrode materials in terms of core and shell in a core-shell design, herein they are simply grouped into two categories: (1) current collector coreactive material shell and (2) active material coreactive material shell.For the convenience of discussion, we denote material A (core) and material B (shell) as A@B for the remainder of the manuscript.
(1) Current collector coreactive material shell.In this conguration, 1D nanostructured current collector core can provide shortened pathways for electron transport, which effectively enhances the available electrochemically active sites.Owing to the high electrical conductivity as well as excellent mechanical stability, metals have been selected as pivotal current collector candidates.When metals are forged into 1D nanostructures like nanowires or nanotubes, active materials can be deposited onto these 1D metal nanostructures and hence form core-shell 1D nanostructured electrode materials.There are several approaches to synthesize 1D metal nanostructures including template-assisted preparation, 66-69 so solution processing, 49,70,71 vapor phase synthesis, [72][73][74] and self-assembling. 75,76Among them, template-assisted preparation is the most intensively studied method for 1D metal nanostructure synthesis.Particularly, 1D metal nanostructures can be fullled by either shaping with the help of sacricial templates (e.g.AAO) or directly coating active material on supercapacitiveinactive 1D nanostructures.For the former case, a pioneering work led by Teberna and co-workers reported Cu nanowire array prepared using AAO template as current collector, which was subsequently deposited with active materials (Fe 3 O 4 ). 69ollowing a similar concept, other works have been demonstrated such as Ni@MnO 2 , 77,78 Ni@NiO, 79 Au@MnO 2 , 80 Ni@Co 3 O 4 , 81 Mn@MnO 2 , 82 etc.For the latter, since the inert core does not give any contribution to the electrochemical properties, the metal coated structure is still regarded as the current collector core.
There are several on-going efforts by Thomas's group to deposit metal on supercapacitive-inactive 1D nanostructures as current collector.Recently, they have demonstrated the synthesis of AuPd@MnO 2 core-shell structures for supercapacitor application. 52AuPd was sputtered on PAN nanopillars prepared by SNAP technique, followed by electrochemical deposition of MnO 2 (Fig. 7a).Fig. 7b and c shows the SEM image of the as-printed structures, suggesting a highly ordered nanopillar structure.These nanopillars have diameter of $140 nm, height of $250 nm, and centre-to-centre distance of $200 nm.As shown in Fig. 6d, MnO 2 were conformally coated onto each pillar.A modelling of the 3D surface area enhancement indicated a $4.5 times surface area enhancement on the nanostructured region compared to a planar region (data based on the dimensions of nanopillars from SEM images).Accordingly, the specic capacitances of AuPd@MnO 2 core-shell nanopillars (603 F g À1 ) consistently exhibited more than twofold improvement compared to that of AuPd-MnO 2 planar electrode, at various scan rates.
Thomas and co-workers recently developed another strategy to prepare CuO@AuPd@MnO 2 core-shell nanowhiskers (NWs) as supercapacitor electrodes. 83This was accomplished by sputtering AuPd on high aspect ratio CuO NWs (with diameter $100 nm and length $15 mm) that were obtained by simply View Article Online heating a copper substrate in air at elevated temperature.MnO 2 was then electrodeposited onto the CuO@AuPd structures hence producing CuO@AuPd@MnO 2 core-shell NWs.Fig. 8a shows the schematic illustration of the fabrication process of CuO@AuPd@MnO 2 core-shell NWs.Fig. 8b clearly presents that CuO NWs were grown uniformly on the Cu wire and Fig. 8c conrms that each CuO NW was conformally covered by MnO 2 .
The as-prepared core-shell heterostructures showed excellent specic capacitance of 1376 F g À1 at a scan rate of 5 mV s À1 .This may be attributed to two reasons: (1) the CuO nanowhiskers as scaffold provides substantially improved surface area (more than 2 orders of magnitude higher than a planar CuO substrate) for the electrode materials and (2) the core-shell heterostructures considerably enhance the ionic and electronic conductivity.Additionally, they have revealed that the CuO layer not only serves as high surface area scaffold but also works as an insulating layer to block charge transfer.As a result, they have successfully demonstrated that the inner copper wire can be used for electrical transmission as normal while an independent supercapacitor can be built on the outside by using overlapping layers.This led to the development of a dual-function (electrical conduction and energy storage) device.
5][86][87][88][89][90][91] To date, most 1D current collectors are either nanowires or nanorods, while reports about nantubular current collectors are relatively few.However, electrodes based on nanotubular current collectors show promising results since both inner and outer surfaces can be utilized for depositing active materials.
(2) Active material coreactive material shell.Active material coreactive material shell presents another promising combination of 1D core-shell heterostructures since both core and shell materials contribute to electrochemical performance.Generally, active materials used in supercapacitors can be classied into two groups: EDLC materials and pseudocapacitive materials.There can be four types of core-shell combinations, namely, EDLC material core -EDLC material shell, EDLC material corepseudocapacitive material shell, pseudocapacitive material core -EDLC material shell, and pseudocapacitive material corepseudocapacitive material shell.Among these categories, EDLC material corepseudocapacitive material shell and pseudocapacitive material corepseudocapacitive material shells are the two major players.
EDLC material core and pseudocapacitive material shell constitutes an appealing combination of core-shell design due to the following merits: (1) EDLC materials as core have better cycle stability due to their electrostatic charge and discharge storage mechanism (i.e.no phase changes), which makes them robust backbones even for large number of cycles; (2) EDLC core material also have better electrical conductivity compared to pseudocapacitive materials, which facilitates the transportation of electrons; (3) pseudocapacitive materials as shell can effectively offset the low capacitance contribution from EDLC materials core.In this regard, Zhai and co-workers have demonstrated the feasibility of EDLC material core and pseudocapacitive material shell such as CNT@PPy-MnO 2 , 92 CNT@PEDOT-MnO 2 , 93 and CNT@PANI-MnO 2 . 94In CNT@PPy-MnO 2 , multiwall CNTs were rst well-dispersed and wrapped around using 18 wt% poly(4-styrenesulfonic acid) (PSS), which also provides attractive forces for pyrrole and metal ions upon negative charging.PPy-MnO 2 composite shell was subsequently prepared by adding pyrrole and KMnO 4 (Fig. 9).As a result, CNT@PPy-MnO 2 core-shell nanowires showed $268 F g À1 specic capacitance, which is much higher than CNT@MnO 2 ($170 F g À1 ) and CNT@PPy ($160 F g À1 ).Signicantly, 93% rate capability of CNT@PPy-MnO 2 was retained from 5 to 100 mV s À1 , while only 35% and 72% were retained by CNT@MnO 2 and CNT@PPy respectively in the same scan rate range when tested separately.Moreover, CNT@PPy-MnO 2 exhibited very stable cycle life (10% reduction aer 5000 cycles); in contrast, a specic capacitance reduction of 61% and 45% was observed under the same test conditions for CNT@MnO 2 and CNT@PPy, respectively.
A similar strategy reported by Ajayan and co-workers is the combination of CNT core and MnO 2 shell. 95,96First, MnO 2 nanotubes were obtained by vacuum inltrating manganese nitrate solution into an AAO template pre-electrodeposited with Au, followed by subsequent annealing in the air at 300 C for 10 h.CNTs were then grown on the inner void le by the MnO 2 shells by CVD process and hence obtained CNT@MnO 2 1D core-shell heterostructure upon removing the AAO template.Signicantly, the core-shell arrays showed almost two times enhancement of specic capacitance compared to pure MnO 2 .Moreover, no apparent structural change was detected even aer 1000 cycles, indicating remarkable mechanical and cycle stability.
Pseudocapacitive material corepseudocapacitive material shell presents another intriguing core-shell design because both core and shell materials have redox reactions during charge and discharge processes, which results in high specic capacitances.In this regard, Fan's group has synthesized versatile combinations of pseudocapacitive materials with 1D core-shell heterostructures including Co 3 O 4 @MnO 2 , 97 CoO@NiHON, 98 Co 3 O 4 @NiO, 99 CoO@TiO 2 , 100 and Co 3 O 4 @ PEDOT-MnO 2 . 101Taking Co 3 O 4 @MnO 2 core-shell nanowires as an example, Co 3 O 4 nanowire core was rst hydrothermally grown for 5 h, followed by coating a thin carbon layer and hydrothermal deposition of MnO 2 from a KMnO 4 solution at 160 C for 1-5 h.The as-synthesized core-shell heterostructures presented a specic capacitance of 480 F g À1 at 2.67 A g À1 and excellent rate capability (56% retention with current density up to 44.7 mA cm À2 ). 97Following a similar concept, Liu and co-workers further improved the performance by employing NiCo 2 O 4 instead of Co 3 O 4 due to its better electronic conductivity (as mentioned earlier). 102onsequently, the nal product of NiCo 2 O 4 @Ni x Co 1Àx (OH) 2 core-shell nanowires considerably increased the specic capacitance up to 1500 F g À1 and improved the rate capability up to 67% retention at 90 mA cm À2 .
4][105] Therefore, a rational design like a durable protective coating which can effectively inhibit the structural collapse or break-down of metal oxide core and alleviate the strain generated during cycling is essential.In this regard, conducting polymers are generally selected as promising protective shell materials due to their high stability, mechanical exibility, and excellent electronic conductivity. 106or example, Wu and co-workers reported core-shell structure of PPy grown on V 2 O 5 nanoribbon for supercapacitors. 103V 2 O 5 nanoribbons were rst prepared hydrothermally in a solution containing NH 4 VO 3 and poly(ethylene oxide)-block-poly-(propylene oxide)-block-poly(ethylene oxide) copolymer at 120 C. The as-prepared V 2 O 5 nanoribbons were dispersed in water by using anionic surfactant.The anionic surfactant also provided electrostatic force to absorb pyrrole at a later stage.Aer adding pyrrole and FeCl 3 , a core-shell structured V 2 O 5 @PPy was obtained (Fig. 10a).Signicantly, a supercapacitor assembled using V 2 O 5 @PPy as anode and activated carbon as cathode showed less than 5% capacitance fading aer 10 000 cycles, while one assembled under the same condition without PPy lm on the V 2 O 5 anode showed 17.5% capacitance loss.This result demonstrates the effectiveness of PPy shell in preventing the dissolution of V 2 O 5 upon cycling (Fig. 10b).Furthermore, the electrolyte for PPy@V 2 O 5 electrode aer cycling showed transparent color, while yellow electrolyte was observed for the V 2 O 5 electrode, indicating that vanadium ions diffuse into the electrolyte without a protective coating.Similarly, other works employing conducting polymers as protective shell, including, MnO 2 @PEDOT-PSS, 107,108 Co 3 O 4 @PANI, 109 MnO 2 @PPy, 110,111 V 2 O 5 @PEDOT-MnO 2 , 112 etc. have been  View Article Online demonstrated.Further studies are needed to address the relationship between the thickness of protective coating and electrochemical performances (e.g.cycle stability and rate capability).This is because, even though the protective coating could effectively prevent the dissolution issue, it could, in turn, block the accessibility of ions if it is too thick, which impedes the rate capability.
3.2.2Other 1D heterostructures.Composites containing 0D and 1D nanomaterials have been extensively studied for other heterostructures as well.For instance, by doping 0D nanomaterials into 1D nanomaterial matrix, Gu and coworkers have prepared vertically-aligned CNT arrays and doped them with MnO 2 nanoowers by electrochemical deposition. 113The MnO 2 /CNT composites exhibited a specic capacitance of 199 F g À1 which is much higher than pristine CNT (27 F g À1 ) and 97% capacitance retention even aer 20 000 cycles.This impressive result may be attributed to the employment of CNT arrays, which provides not only high electronic and ionic conductivity but also sturdy mechanical stability.Another example reported by Lee and co-workers was the loading of well-dispersed MnO 2 nanoparticles into PEDOT nanowires. 114PEDOT nanowires were rst prepared by electrochemical deposition of PEDOT in AAO template, followed by removal of the template using NaOH solution.MnO 2 nanoparticles were subsequently loaded into the PEDOT nanowires by soaking the PEDOT nanowires into KMnO 4 solution.The MnO 2 /PEDOT composites delivered a specic capacitance of 250 F g À1 and an 80% rate capability was maintained from 5 to 25 mA cm À2 .

2D nanostructures
Given the importance of properties such as diffusion ion length or contact area with electrolyte for utilization of active material in the performance of the electrochemical supercapacitors, the rise of 2D materials, and the fact that they exhibit only surface area (i.e.no bulk volume), has naturally attracted substantial interest and augmented a number of possible congurations of potential hybrid 2D congurations in the electrode material design.In addition, 2D materials exhibit unique properties such as mechanical stability, strength, exibility, transparency, and chemical stability.However, in supercapacitors involving 2D structures, little is known of the unique behavior and properties most 2D materials and derived hybrid in aqueous or organic solvents that will exhibit.Since capacitance is highly dependent on the surface areas of the electrode materials, 2D materials are envisioned to bring a paradigm shi in the eld of electrochemical capacitors. 115

2D homostructures
The most widely explored materials for 2D homostructuredelectrode fall under the following three categories.The rst one includes graphene-based electrodes, which have been the most extensively investigated since its emergence, and found to have extensive applications in EDLC.Secondly, electrodes made of metal oxides and hydroxides, which have also been widely studied as active materials for pseudocapacitors, will be discussed.Finally, recent developments in transition metal dichalcogenides (TMDs) including MoS 2 and VS 2 and transition metal carbides and/or nitrides (MXenes) will be overviewed.
4.1.1Graphene.Various forms of carbon have been considered for electrode materials in supercapacitors. 116,117oderate specic capacitance together with long cycle life represent the holy-grail for carbon-based electrodes.Currently low conductivity of porous carbon material, despite their high surface area, constitutes a major limitation for supercapacitors with high-power density. 118,119On the other hand, CNTs offer both high surface area and conductivity, but show high contact resistance at the electrode-current collector and expensive synthesis have hampered their scale up processing for large scale manufacturing.Thus, the emergence of graphene has provided an alternative to overcome the limitations of carbonbased electrodes and lead to new directions in the development and optimization of electrode materials.As a result, graphene is currently the most common material used for 2D materialbased electrodes in energy storage devices due to its chemical stability (i.e.resistance to oxidative processes), 116,[120][121][122] as well as its unique intrinsic electrical (i.e.fast transport of electrons), mechanical and thermal properties. 123Recently, the capacitance of graphene in its monolayer form was experimentally measured ($21 mF cm À1 ), 124 with a projected $500 F g À1 for an ideal single layer graphene-based EDLC, in which the surface area could be fully utilized.Such an electrode would constitute a record in carbon electrode materials for EDLCs but designing remains a great challenge.
In addition, as discussed by Ruoff and co-workers in one of the rst demonstrations of graphene as electrodes for supercapacitors, it eliminates the necessity of using conductive llers to improve the electrode conductivity. 125It also offers the possibility to increase the electrode thickness while greatly improving the performance including a higher ratio of electrode material to collector/separator and higher energy densities.6][127] The surface area of graphene is predicted to be as high as 2630 m 2 g À1 and all the more exciting is the fact that it intrinsically consists of structured, accessible pores and interlayer spaces (i.e.better accessibility for ions). 125,126Thin lms of graphene electrodes have been obtained by various methods aiming at large scale production, including CVD, [128][129][130] electrostatic spray deposition, 129 electrophoretic deposition, 23,129 and ink-jet printing. 94,129,131Nevertheless, to date, chemical approaches to make graphene oxide (GO) are the most cost-effective ton-scale method to produce graphene-based electrode materials.However, despite higher production yield, additional treatments are required to overcome its low conductivity.Among various treatments, chemically modied graphene (CMG) and reduced graphene oxide (rGO) are of particular interest as they offer a scalable and cost-effective chemical approach and produce a material with conductivity close to that of graphene. 116,125,132,133As a result, rGO has been extensively used as a highly conductive electrode material and the mechanisms of reduction of GO becomes highly important in the development of 2D electrode materials for supercapacitors.
(a) Performance of chemically prepared rGO electrodes.][136][137] Hydrazine is the most commonly reported reducing agent for GO reduction.Ruoff and co-workers described the synthesis of CMG by reduction of suspended graphene oxide in water using hydrazine hydrate. 125The resulting sheets showed a surface area

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of 705 m 2 g À1 and provided specic capacitances of 135 and 99 F g À1 , respectively, in aqueous and organic electrolytes.Chen and co-workers discussed GO gas-based reduction by hydrazine at room temperature to lower agglomeration in the system. 116,117he electrode materials resulting from their protocol were shown to surpass CMG or CNT-based supercapacitors, 125,138,139 with a maximum capacitance of about 205 F g À1 , energy density of 28.5 W h kg À1 in an aqueous electrolyte solution and $90% specic capacitance retained aer 1200 cycle tests.C-N and C-O bonds, creating numerous hydrophilic polar sites, were present in the material, and seemed to improve the wettability of the electrodes. 116Despite the seemingly high packing of the layers, the agglomeration was shown to be lower than with other protocols, allowing ions from the electrolyte to penetrate inbetween layers and increase the capacitance of the system.Other reducing agents including hydroquinone 7,137 and NaBH4 137,140,141 have also been used to reduce GO.Interestingly, activated graphene using KOH in GO, another form derived from graphene obtained by chemical activation of exfoliated GO prepared by Ruoff and co-workers, revealed large surface area (3100 m 2 g À1 ), high specic capacitance (up to $170 F g À1 ) and great electrical conductivity. 132Despite the chemical processes involved in the formation of the activated layers, low oxygen and hydrogen content were found in the nal product.Furthermore, $97% of the capacitance was retained aer 10 000 cycles.
(b) Performance of rGO electrodes prepared by physical methods.Vacuum low-temperature exfoliation and laser treatments are among the physical methods used for the reduction of GO in energy storage applications.Chen and co-workers have recently demonstrated that graphene akes obtained as a result of low-temperature vacuum exfoliation demonstrated excellent energy storage performance. 142The devices were tested in aqueous and organic electrolytes and exhibited specic capacitance of 220 F g À1 and 120 F g À1 , respectively. 142hermal treatment of GO suspension from 150-200 C has also been shown to produce rGO by removing the oxygen from GO thus enabling high conductivity up to 5230 S m À1 . 137A specic capacitance of 122 F g À1 at 5 mA could be achieved aer thermal treatment at 200 C, corresponding to a chargedischarge rate of $1000 mA g À1 .This constitutes an improvement of the specic capacitance of $20% by direct comparison with the performance of a supercapacitor prepared using rGO obtained with hydrazine, 125 and compares well with other standard electrode-materials. 143The advantage of this approach is its potential for scalable "green" production of carbon-based supercapacitor electrode materials.
On the other hand, laser reduction of GO is also getting substantial attention, [144][145][146][147] in part due to the ability to pattern rGO-GO structures with customized geometries in the same piece of GO, in plane or in 3D.For example, Ajayan and coworkers have developed a laser writing technique to directly convert GO to rGO (Fig. 11a). 144In their study, the structure of the laser induced rGO was found to be porous, due to the decomposition of functional groups and water during laser treatment.Due to the anisotropic nature of ion mobility and transport distances, the capacitance was found to be highly dependent on the geometry of the patterned structures.The highest area capacitance of 0.51 mF cm À2 was obtained for inplane disc geometry with no electrolyte.Comparatively, an interdigitated rGO supercapacitor with an organic electrolyte and electrode of the same thickness exhibited a mean capacitance density of 0.4 mF cm À2 . 146A 30 to 35% drop in capacitance was found aer 10 000 cycles, but the same performance could be improved by long exposure to ambient environment or aqueous electrolytes. 144In contrast, Kaner and co-workers found that the laser-treated GO obtained using a LightScribe CD/DVD optical drive can provide an impressive conductivity ($1740 S m À1 ) (Fig. 11b). 145In addition, the nanosheets were found to be well aligned, with outstanding mechanical properties (only 1% change in conductivity aer 1000 bending cycles).The formation of rGO-GO interfaces by laser reduction paves the way to in-plane supercapacitors with new conguration of ion percolation for higher performance.
The laser reduction methods also addresses one of the several challenges that greatly impede the performance of the devices, in particular restacking of graphene nanosheets. 120his phenomenon is caused by van der Waals interaction between successive graphene layers. 120In fact this is one of many extensive research efforts that have been employed to develop protocols preventing graphene restacking.Curved graphene was also shown to improve the performance of supercapacitors by taking advantage of the maximum intrinsic surface capacitance and surface area. 127In addition, the formation of rGO-GO interfaces by laser reduction paves the way to in-plane supercapacitors with new conguration of ion percolation for higher performance.As demonstrated by Ajayan and co-workers (Fig. 12a), 130 the in-plane design fosters ions to interact with all the graphene layers resulting in the maximum utilization of electrochemical surface area.
Given the diversity of the results obtained with GO and reduced GO, a fundamental model describing the electrode polarizability, electrolyte dynamics, possible chemical processes, structural defects in the electrodes and pore characteristics would be of great interest to speed up the material discovery.However, this is a complex problem to model.Recently, Kim and co-workers considered the role of oxidation in molecular dynamics models (Fig. 12b) and demonstrated that the capacitance is highly dependent on the conguration of the oxidation, 133 as it modies the electrode surface accessibility of the ions.Although limited in the parameters considered for the calculations, the approach shows great promise to catalyse electrode optimizations.
Overall, the initial reports of graphene-based electrodes in electrochemical supercapacitors indicated specic capacitance of 117 F g À1 in aqueous H 2 SO 4 with operating voltage to 1 V for electrode preparation by graphitic oxide exfoliation and by nanodiamond transformation. 148Specic capacitance of 135 F g À1 and voltage up to 1 V in aqueous media were obtained with CMG 125 with energy density of about 32 W h g À1 . 148.1.2.Metal oxides and hydroxides.Metal oxides like RuO 2 , IrO 2 , MnO 2 , NiO, Co 2 O 3 , SnO 2 , V 2 O 5 and MoO 2 , are considered as important electrode materials for supercapacitors. 7Their implementation in the form of 2D electrodes is taking the centre stage of hybrid device development.Deposition methods such as spin coating, anodization, electrodeposition, atomic layer deposition, spray deposition, and sputtering have been employed to deposit smooth thin lms with optimal texture and low grain boundary density. 129,149D MnO 2 (up to 2 nm in thickness) has been considered for cathode electrodes.Using so template methods, Kang and coworkers showed high specic capacitance as high as 774 F g À1 up to 10 000 cycles. 150The MnO 2 electrodes were used in aqueous Ca(NO 3 ) 2 -SiO 2 gel electrolyte.The electrodes were prepared using a so template technique.MnO 2 nanosheets were obtained by mixing sodium bis(2-ethylhexyl)sulfosuccinate and water to form a binary lamellar structure, to which KMnO 4 is added.The dissolution of KMnO 4 occurs only in the aqueous phase of the mixture and the growth of MnO 2 is automatically initiated in the aqueous phase.Scan rates from 2 to 50 mV s À1 could be demonstrated.For RuO 2 thin lm electrodes, performances up to 730 F g À1 were demonstrated. 151,152.1.3.Transition metal dichalcogenides (TMDs).Layered TMDS, in their monolayer forms, have recently taken an important role in material sciences for applications in optoelectronics, nanoelectronics and sensors.However, they have not been actively considered as possible electrode materials for supercapacitors.Although graphene in-plane design has  View Article Online already demonstrated great results, its large scale implementation is hindered by high costs associated with the production of large-area low-defect sample.TMDs offer a variety of properties that could overcome the shortcomings of graphene.However, it remains to be seen whether TMDs may have an equivalent impact as graphene for in-plane supercapacitors.Among TMDs being considered for electrode materials, MoS 2 may be of special interest for EC, as its intrinsic conductivity was calculated to be higher than other similar metal oxides, while its theoretical capacity was predicted to surpass that of graphite. 153,154However, to date, the literature related to MoS 2 for EC electrodes is rather limited. 155,156Soon and Loh carried out electrochemical impedance spectroscopy to characterize the performance of an arrangement of dense edgeoriented MoS 2 lms for micro-supercapacitor and found that the performance is similar to that of CNT electrodes. 155,156Other studies with various conguration of MoS 2 lead to medium performance electrodes. 102,111,157More recently, ower-like MoS 2 electrode material developed by Geng and co-workers exhibited a specic capacitance of 168 F g À1 (at 1 A g À1 , 1 M KCl) and retained $93% capacitance aer 3000 cycles. 155mong other TMDs with intrinsically high conductivity, VS 2 represents a favorable alternative to graphene; further strengthened by the feasibility of exfoliation of layers aer intercalation with NH 3 (Fig. 13). 158Numerous tests were carried out on this new material.The conductivity remained constant throughout the 200 bending cycle test and is of great promise for exible applications.The fabricated in plane device exhibited a specic capacitance $317 F cm À3 , comparable to graphene based electrodes, with a good stability (90% aer 1000 cycles).
4.1.4.Transition metal carbides and/or nitrides (MXenes).][161][162] MXenes consist of highly conductive carbide and carbonitride layers of which surfaces are designed to be hydrophilic and primarily hydroxyl-terminated.High capacitance (over 300 F cm À3 ) can be achieved by cation (Mg 2+ , K + , Na + , Al 3+ , NH 4+ , Ba 2+ , Ca 2+ , Cs + , Li + ) intercalation, including those with various charges and sizes.Although ions intercalation is typically used in battery materials for energy storage, the slow diffusion rate hinder the charge-discharge performances.However, processes based on ion adsorption are oen used in supercapacitors.In MXene, cation intercalation rates are unusually high, and the process is referred to as the intercalation pseudocapacitance. 163Xenes derive from the family of layered hexagonal MAX phases, a group including more than 60 forms of ternary nitrides or carbides, where M represents an early transition metal (e.g.Ti, V, Cr, Nb, etc.), A stands for an A-group element (e.g.Al, Si, Sn, In, etc.), and X represents C and/or N. The synthesis of MXene involves removing the A of MAX compounds by selective etching (Fig. 14).While numerous MXenes were predicted theoretically, 164,165 only about eleven MXenes are tested experimentally to date.Theoretically, it has been demonstrated that the surface termination (hydroxyl, uorine) can be used to modify some of the MXene properties, such as their conductivity and bandgap.
Among the reported studies, a MXene, Ti 3 C 2 T x , was obtained by etching Ti 3 AlC 2 by hydrouoric acid (HF) at room temperature. 161A binder-free Ti 3 C 2 T x MXene material with surface area of 98 m 2 g À1 was obtained by delamination (single to few layers) and ltration of Ti 3 AlC 2 .It showed high exibility and high volumetric capacitance (as high as 350 F cm À3 ) in aqueous electrolyte NaOH. 159To date, research towards electrode materials based on MXenes are limited; nevertheless, as a new promising 2D supercapacitor electrode material, much effort needs to be dedicated in this direction.

2D heterostructures
Regarding improvement of energy density in supercapacitors, making composites with transition-metal oxides and conducting polymers are being considered as they promise higher energy density than carbon materials.In addition, it has been shown that combining such high-energy metal oxides or conductive polymers with graphene based electrode materials can lead to high electrochemically accessible area.In addition, it leads to improvements in electrical conductivity, thermal stability and mechanical strength of the electrode, while minimizing the restacking of graphene layers.
4.2.1 Graphene-metal oxides and hydroxides.Graphene electrodes offer great potential for double layer capacitors.In addition, they were also found to be a versatile backbone to combine metal oxides nanostructures of known value for supercapacitors such as RuO 2 , MnO 2 , etc. Hybrids involving metal oxides offer great promise in the future of electrode materials as they can contribute to the total capacitance of the system by an added pseudocapacitance to the double layer capacitance from graphene 166,132 while the unique combination enable better chemical stability. 118[169][170][171] Fig. 16  As a very high conductivity metal oxide, RuO 2 has been known as a performing pseudocapacitive material. 104,121,172,173ecently, Dai and co-workers synthesized a capacitor-like RuO 2 -graphene ($30 wt% graphene sheets) hybrid in solutions, 121 which showed a specic capacitance of $370 F g À1 (at 2 mV s À1 ) and $280 F g À1 at a scan rate of 40 mV s À1 in 1 M KOH solution for a 0.2 V to À1.0 V voltage range.Another investigation of the performance of RuO 2 -rGO hybrid (Fig. 15) showed increasing capacitance with increasing Ru loading (at 16, 33 and 66 wt% Ru), 174 with a specic capacitance of 357 F g À1 at a current density of 0.3 A g À1 , which was attributed to the superior conductivity for rGO and an improved charge transport from RuO 2 to the rGO layers.This is supported by structural comparison to materials made of rGO (Fig. 15a and b), and rGO-PANI (Fig. 15c and d).Ni(OH) 2 nanoplates were also attached to graphene sheets to form a battery-like electrode material, and showed similar performance as RuO 2 -graphene structures. 121Finally pairing the two electrodes in an asymmetrical cell led to very attractive energy and power densities, suggesting that the performance of supercapacitors can be enhanced not only by hybrid electrode materials, but also by the combinations of electrodes with different properties.
6][177][178][179][180] Wei and co-workers developed a facile method for graphene-MnO 2 hybrid electrode material by microwave irradiation. 179With performance of 310 F g À1 at 2 mV s À1 (228 F g À1 at 500 mV s À1 ), MnO 2 -graphene surpasses pure graphene by a factor of three (104 and 103 F g À1 , respectively).About 95% of capacitance retention was observed even aer 15 000 cycles.Another facile synthesis by Gao and coworkers showed high specic capacitance of 324 F g À1 in 1 M Na 2 SO 4 with a retention of 97% aer 1000 cycles. 177Other metal oxides such as Fe 3 O 4 , 126 SnO 2 , 181 and ZnO 166 are also mixed with graphene to make supercapacitor electrode materials.
4.2.2Graphene-dichalcogenides.Although few attempts have been reported to add metal sulphides in the mix of electrode materials, their good electrochemical activity would suggest that they have great potential for energy storage devices. 182,183Therefore, much effort needs to be dedicated towards this direction in the future.
MoS 2 has been combined with rGO to investigate the performance of hybrid energy storage devices.Covalent bonds occur between MoS 2 and the oxygen atom in RGO, forming a Mo-O-C bond.The resulting hybrid 2D material is electroactive, and could be cycled in 1 M HClO 4 between 0.25 V and up to 0.8 V, with energy density of 63 W h kg À1 and a specic capacitance decrease of 8% aer 1000 cycles. 182S 2 -rGO hybrids, synthesised by a simple hydrothermal method, were evaluated for their performance as supercapacitor material (Fig. 16). 183Comprehensive testing showed interesting specic capacitance of 350 F g À1 at a scan rate of 2 mV s À1 in 1 M Na 2 SO 4 , signicantly higher than corresponding electrodes made of the bare materials measured at 130 and 70 F g À1 .In addition, a high energy density ($50 W h kg À1 ) and the capacitance remained almost the same aer 1000 cycles in the hybrid structure, due to the faster transport of electrolyte ions in the presence of WS 2 .
4.2.3Other hybrid congurations.Advances in device congurations and applications also require adjustment of the supercapacitor electrode materials.For instance, the implementation of 2D active materials in micro-supercapacitors faces new challenges due to the limited footprint of the device and subsequent loading mass limitations for the electrode.Therefore capacitance is highly limited in such devices, and calls for new electrode material congurations.To attain acceptable performance in such conguration, the emphasis is placed on high specic capacitance, conductivity and mechanical strength.Shen and co-workers used graphene and activated carbon as building blocks for thin electrodes made of an active material, a conductive agent and a polymer binder (carboxymethylcellulose sodium (CMC)). 184Graphene served as the active material and the conductive agent.The graphene-based composite exhibited larger specic surface area and pore volume than the AC-based composite (surface area: 2156 vs. 1547 m 2 g À1 , pore size: 2.6 vs. 2.2 nm).In the 2D conguration, specic capacitance of $60 mF cm À2 (100 mm thick electrode) was reported for the graphene-based electrode and up to $310 mF cm À2 for activated carbon (200 mm thick electrode).

3D nanostructures
3D porous structures of active materials provide large surface area, well-dened pathways to electrolyte access and mechanical stability for efficient supercapacitor electrodes.Generally, 3D electrodes are built either using metal foam as templates or organizing active materials into 3D nanostructures.View Article Online

Metal foams as 3D nanostructured current collectors
6][187][188][189][190] Compared with carbon based 3D substrates, metal foams have higher electrical conductivity and better mechanical properties but at the cost of larger density and lower specic surface area.3D Co/Ni oxides with different morphologies were controllably synthesized and deposited on nickel foam substrates. 186The synthesis involves a co-electrodeposition of bimetallic (Ni, Co) hydroxide precursors on a nickel foam scaffold and a subsequent thermal transformation of the precursors to Ni Ozkan and co-workers used nickel foams to grow graphene and CNTs followed by a deposition of ruthenium oxide (RuO 2 ) nanoparticles on CNTs (Fig. 17). 185The hierarchical and porous structures enable efficient access of electrolyte to the active materials (CNT-RuO 2 network layer) and provide a conductive framework to charge transport attributed to the embedded CNTs.The composite had a high energy density of 39.28 W h kg À1 and power density of 128.01 kW kg À1 and showed an excellent cycle stability of 106% capacitance retention over 8100 cycles.
Similarly, electrodes were prepared using well-aligned CoO nanowire arrays grown on 3D nickel foams with polypyrrole uniformly immobilized onto nanowire surface to utilize high electrochemical activities of CoO and polypyrrole as well as the short ion diffusion pathway in ordered mesoporous nanowires. 190Such synergy between materials and structures leads to a high specic capacitance of 2223 F g À1 , good rate capability, and good cycle stability (99.8% capacitance retention aer 2000 cycles).An asymmetrical supercapacitor device fabricated using the hybrid materials as the positive electrode and activated carbon lm as the negative electrode achieved a maximum voltage of 1.8 V, high energy density ($43.5 W h kg À1 ) and power density ($5500 W kg À1 at 11.8 W h kg À1 ), and outstanding cycle stability ($20 000 times with 91.5% capacitance retention).
Mn/MnO 2 core-shell 3D porous structures were fabricated from ordered polystyrene sphere templates.Mn was electrically deposited between polystyrene spheres from N-butyl-N-methyl pyrrolidinium TFSI ionic liquid solution of MnCl 2 .Aer removing polystyrene template using tetrahydrofuran, the 3D Mn template was anodized in a KCl aqueous solution to produce a Mn/MnO 2 core-shell structure.Such core-shell porous structures contribute several positive factors including larger surface area, high electrical conductivity attributed to Mn and high capacitance of MnO 2 for efficient supercapacitors.Especially, the electrolyte can easily access the active material, leading to high specic capacitance at high scan rate.The optimum specic capacitance of the electrode at 500 mV s À1 is 996 F g À1 , maintaining 83% of the capacitance measured at 5 mV s À1 . 188Composite supercapacitor electrodes based on metal foams have high electrical conductivity, good mechanical properties, and easy access of electrolyte to active materials, which leads to high specic capacitance and good cycle stability.

3D nanostructures based on carbon materials
Mesoporous carbon (MC) has been used as promising materials for supercapacitor electrode materials because of their large specic surface area and interconnected pores with tunable size.Since Ryoo and co-workers reported the fabrication of ordered MC from the ordered mesoporous silica molecular sieves as templates, 191,192 much effort has been devoted to exploring the application of such materials as supercapacitor electrodes.The studies of utilizing MC as electrode materials for EDLCs suggest that the pore size and symmetry have profound effect on the performance by inuencing the accessibility of electrolytes.The pore size distribution with best performance ranges from 0.6 to 3 nm depending on different types of electrolytes. 193,194While these EDLCs can reach the specic capacitance of 140 F g À1 which is much higher than those using commercial carbon, their performance is limited by the relatively low electrical conductivity. 195urther improvement can be achieved by doping 196 and depositing electrochemically pseudocapacitive materials such as metal oxides, [197][198][199] PANI, [200][201][202][203][204] and their mixtures 205 onto MC templates.For example, nitrogen-and sulfur-codoped 3D cubic ordered MC (KNOMC) materials with controlled dopant content (10.0-4.6 atom % for nitrogen and 0.94-0.75atom % for sulfur) have demonstrated a specic capacitance of 320 F g À1 because the incorporated S and N species function as pseudo-active materials. 196Metal oxides have also been added onto MC to increase the capacitance.For example, MnO 2 nanoparticles were imbedded in the MC through a redox reaction between permanganate ions and carbons. 199The ordered mesoporous structures were retained through the control of the concentration of permanganate ion.The resulted composite structure showed a specic capacitance of 200 F g À1 with decent electrochemical stability and reversibility.Nanostructured PANI has been deposited on various MC substrates through chemical oxidative polymerization and it greatly increased the specic capacitance due to its high capacitance and electrical conductivity. 206Li and co-workers have deposited PANI needle-like structures on MC and studied the effect of the structure on electrochemical properties. 202It was found that the composite with 60% PANI deposition has the best performance (517 F g À1 ), suggesting that a bimodal pore structure with some open pores facilitates the ion diffusion and shorten the charge transfer distance.PANI-CMK-3-MnO 2 ternary nanocomposites were built by co-deposition of conductive PANI and MnO 2 on MC and further improved the specic capacitance to 695 F g À1 . 205In such composite, MnO 2 nanoparticles stabilizes the interaction between the quinoid ring of PANI and the CMK-3-MnO 2 , while the PANI nanolayer retards the dissolution of MnO 2 nanoparticles during the charge-discharge process, leading to high cycle stability.
Theoretical studies 207,208 and experimental investigation 209 of the effect of pore size and distribution on ionic transport properties of porous structures suggest that the coexistence of macropore and mesopore would increase the efficiency of porous carbon as supercapacitor electrodes.Compared with MC, an ordered carbon structure with meso/macro/micropores can achieve a specic capacitance as high as 350 F g À1 . 209ierarchical carbon structures with a combination of meso/ macro/micropores have been produced through various approaches, [210][211][212][213][214][215][216][217][218] and used as electrode with and without metal oxides. 219,220These structures have demonstrated great potential in constructing high performance supercapacitors.Further improvement requires better control of pore size and distribution as well as increased electrical conductivity of porous carbon.

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Taking advantages of extraordinary properties of CNTs and graphene including large specic surface area, high electrical conductivity and superior strength, assembling CNTs and graphene into 3D porous structures offers excellent opportunities for building high performance supercapacitors.3D CNTs and graphene not only provide large surface area for double layer supercapacitors but also function as supports for metal oxides and conjugated polymers for pseudocapacitors.GO sheets dispersed in aqueous solutions offer promising materials for building 3D structures for graphene based supercapacitors.The GO can be reduced chemically or thermally to increase the electrical conductivity.To eliminate the aggregation of graphene sheets that limits the electron and ion transport, Huang and co-workers have transformed 2D graphene sheets into crumpled paper ball structures by isotropic capillary compression in rapidly evaporating droplets of GO dispersion using an aerosol spray pyrolysis setup. 221Fig. 18 compares the microstructures of mechanically compressed pellets of stacked graphene materials of at graphene sheets (a), heavily winkled sheets (b) and crumpled graphene balls (c) using surface and cross sectional SEM. 222The at graphene sheets pack like a stack of paper with a smooth featureless surface and an ordered lamellar cross section (Fig. 18a).The pellets of wrinkled sheets (Fig. 18b) have uneven surfaces with many attened islands in micrometers.The cross sectional SEM indicates a largely lamellar but signicantly more disordered microstructure.For the crumpled balls (Fig. 18c), the morphology of individual particles in the compressed pellet was largely unchanged.Compared to at or wrinkled sheets, the crumpled graphene balls have larger surface area and better dened voids for electron and ion transport.The electrodes based on crumpled graphene balls can deliver much higher specic capacitance (150 F g À1 ) and better rate capability.Furthermore, using at graphene sheets as binder for the crumpled particles eliminates the use of insulating binder materials like PTFE colloids and further improve device performances. 222raphene aerogel (GA) as electrode material for supercapacitor has drawn considerable attention lately due to its open 3D network structure that not only effectively reduces the aggregation of graphene nanosheets but also facilitates effective electrolyte ion transportation.As a result, more electrochemically active sites could be utilized compared to 2D graphene nanosheets.For example, Duan and co-workers have prepared a 3D holey graphene framework with a hierarchical porous structure by etching the GO sheets via H 2 O 2 molecules. 223The resulting holey graphene framework showed a large specic surface area of 830 m 2 g À1 , which is signicantly greater than that of non-holey one ($260 m 2 g À1 ).As a consequence, holey graphene framework electrodes delivered a specic capacitance of 310 F g À1 at 1 A g À1 , while 208 F g À1 was achieved for nonholey graphene framework electrodes.It is to be noted that the electrical conductivity of GA is generally lower than that of pristine graphene due to the formation of defect sites during the oxidation-reduction process. 224It is advantageous to fabricate a GA with enhanced electrical conductivity.Recently, Thomas, Zhai, and co-workers have reported functionalized GA composites as supercapacitor electrodes with reduced electrical resistivity by incorporating palladium nanoparticles (Fig. 19a  and b). 225The electrical resistivity of the functionalized GA (16 U cm) is substantially reduced compared to pristine GA (950 U cm).Consequently, the specic capacitance of the functionalized GA reached 175.8 F g À1 , while only 51.9 F g À1 was obtained for the pristine GA.
8][229][230][231][232][233][234][235][236][237][238][239] 3D macroporous electrodes of chemically modied graphene (CMG) were fabricated using polystyrene colloidal particles as a sacricial template. 227A thin layer of MnO 2 was deposited onto the porous electrodes to further improve the capacitance.The porous graphene structure facilitates fast ionic transport within the electrode while preserving decent electrical conductivity.Such combination grants the electrodes excellent electrochemical properties such as high specic capacitance (389 F g À1 at 1 A g À1 ) and 97.7% capacitance retention when the current increases to 35 A g À1 .In addition, an asymmetrical supercapacitor assembled using MnO 2 /CMG composite electrode and a CMG electrode achieves an energy density of 44 W h kg À1 , a power density of 25 kW kg À1 , and excellent cycle life.
As discussed previously, ordered 3D porous structures with controlled pore size and distribution would improve the electrochemical properties.1][242] Dai and co-workers prepared a VA-CNT-graphene hybrid by the intercalated growth of VA-CNTs into thermally expanded highly ordered pyrolytic graphite (Fig. 20b). 241The resulting VA-CNTgraphene hybridized with a nickel hydroxide coating also demonstrated a high specic capacitance with a notable charging-rate capability, as high as 1065 F g À1 (based on the active material) at a current density of 22.1 A g À1 .Sun and coworker have grown carbon nanotubes on graphene sheets through a facile one-pot pyrolysis strategy where cobalt ions and urea were mixed with GO followed by drying and heating the mixture at 900 C. The length of the carbon nanotubes was tuned by adjusting the amount of precursors (Fig. 20c and d).The resulting graphene-CNT composites showed a high specic surface area of 903 m 2 g À1 and maximum specic capacitance of 413 F g À1 . 242s an example of depositing pseudocapacitive metal oxides on graphene/CNT scaffolds, 3D ower-like b-Ni(OH) 2 /GO/ CNTs composites were prepared via a facile hydrothermal reaction in alkaline solutions.CNTs were introduced to the composite to improve the electrical conductivity.The electrochemical properties of the composites produced at different reaction time were evaluated to understand the effect of composite structures on energy storage efficiency.has the best performance with a high specic capacitance of $1815 F g À1 based on Ni(OH) 2 (nearly 96% of its theoretical pseudocapacitance) at 2 A g À1 and a good cycling performance of $97% capacitance retention aer 2000 cycles at 10 A g À1 , attributed to higher stability (vs.a-Ni(OH) 2 ) and more efficient ion diffusion (vs.stacked b-Ni(OH) 2 ). 228hai and co-workers have incorporated MnO 2 into MWCNT 3D networks as supercapacitor electrodes.More importantly, they have discovered the relationship between MnO 2 migration phenomenon and the scan rates.The structural evolution were examined by studying the structure and electrochemical properties of identical electrodes aer 500 CV cycles at various scan rates. 243By using Electrochemical Quartz Crystal Microbalance (EQCM) to monitor the mass change of the electrode in situ and SEM to examine the electrodes aer CV cycles, MnO 2 was found to "migrate" from the MWCNTs network and "deposit" on the electrode surface upon cycling.The structural change mechanism contains the dissolution of MnO 2 upon reduction, the diffusion of the reduced Mn species from the MWCNT network toward the electrolyte solution, and the deposition of MnO 2 on the electrode surface upon oxidation. 243The structural change of MnO 2 is highly dependent on the scan rate.For instance, the reduced Mn species have more diffusion time at a lower scan rate, while less reduced species are generated due to the lower associated current density and vice versa.A scan rate that might cause the greatest structural evolution of MnO 2 is expected based on this contradictory effect.Fig. 21a shows the cross section of as-synthesized MWCNT/MnO 2 composites sprayed on a graphite substrate (MCM0) as well as an X-ray line scan  View Article Online spectra of Mn element.The SEM and X-ray line scan spectral data clearly exhibit that MnO 2 nanoakes are uniformly embedded in the MWCNTs network as the active materials while the MWCNT network serves both as the support for MnO 2 nanoakes and current collector.In contrast, Fig. 21b-f shows the striking changes of electrode structures aer 500 CV cycles, and the structural change varies at different scan rates.As a result, a higher specic capacitance could be achieved with a lower scan rate; however, the specic capacitance considerably drops upon cycling due to the dissolution of active material.Therefore, either developing a protective layer as mentioned in the 1D section or manipulating a suitable scan rate that induces the least structural changes while maintaining high specic capacitance is required.

3D nanostructures based on pseudocapacitive materials
Different nanostructures of pseudocapacitive materials offer intriguing composition for 3D supercapacitors.Various types of metal oxide nanostructures produced through sol-gel, solvothermal and other approaches have been evaluated electrochemically to understand the impact of their structures on the energy storage properties.NiO nanostructures with three distinct morphologies (ower-like, slice-like and particle-like) were fabricated by a sol-gel method.The BET specic surface area of nanoower, nanoslice, and nanoparticle NiO was 159, 193, and 233 m 2 g À1 and the average pore size was 16.7, 6.3, and 8.8 nm, respectively (Fig. 22). 244With the lowest surface area and largest pore size, the nanoower NiO nanostructures formed a 3D network, offering the most efficient contact with and transport of the electrolyte.Therefore, this structure granted the best electrochemical properties among three different nanostructures.This study suggests that it is important to facilitate the transport of ions to active materials on the top of large surface area.
3D hierarchical Co 3 O 4 twin-spheres with an urchin-like structure were produced by a solvothermal synthesis of cobalt carbonate hydroxide hydrate followed by calcination.The morphology of the precursors evolved from nanorods to sheaf-like bundles, to ower-like structures, to dumbbell-like particles, and eventually to twin-spheres through a three-stage self-assembly of nanorods.Such structure provided larger surface area and better accessibility of electrolytes compared to nanorods and nanodiscs of Co 3 O 4 .The specic capacitances of the urchin-like structure evaluated from the discharge curves were 781, 754, 700, 670, and 611 F g À1 at current densities of 0.5, 1, 2, 4, and 8 A g À1 , respectively. 245Similar urchin-like MnO 2 nanostructures were also produced through a hydrothermal method.It was found that the phase of the products transformed from a-MnO 2 to 3-MnO 2 when Al 3+ was substituted by Fe 3+ in the solution.At the same time, the morphology of the product was changed from urchin-like clusters to 3D clew-like structures. 246The 3D clew-like 3-MnO 2 showed a specic capacitance of 120 F g À1 at the scan rate of 5 mV s À1 .
Current research in this eld focuses on the structural control of the products rather than achieving good electrochemical properties.These nanostructures, although present interesting shapes, have not demonstrated superior performance compared with pseudocapacitive materials deposited on porous carbons and metal foams.In order to advance the application of metal oxide nanostructures in supercapacitors, it is necessary to improve the electrical conductivity, electrolyte accessibility and cycle stability of the materials.

Conclusion and outlook
The quest for higher energy and power for supercapacitors to complement or compete with batteries has led to a major effort in developing advanced electrode materials.In this review article, we overviewed the fabrication and performance of 0-3 dimensional nanostructured electrode materials and discussed the impact of structures on the electrochemical properties.Table 1 exhibits a summary of the properties and characteristics of supercapacitor electrode materials based on various dimensional nanostructures.For quite some time, 0D materials like activated carbon have been the work horse for developing supercapacitor electrodes.However, the necessity to add an insulating polymer binder to make the actual electrode diminishes its performance.The emergence of perovskites as active materials for solar cell applications has inspired the research of using them for supercapacitor applications.It has been observed that oxygen intercalation in perovskite-type materials can be used to store the charges and has a bright future as the potential material to improve the performance of supercapacitor electrodes.Among other 0D materials, single and multi-shelled hollow structures stand out because of their highly enhanced surface area.1D nanostructure can also provide efficient transport pathways for both electrons and ions in the supercapacitor electrode.Particularly, binder-free and highly ordered designs can further improve the specic capacitance of the electrodes.Moreover, a rational 1D heterogeneous design helps to achieve enhanced rate capability and cycle stability.
Due to their high surface area, 2D materials offer a unique opportunity to surpass the current limitations of electrode materials.The great advantage of using 2D materials like graphene for supercapacitor application is its high conductivity and large surface area.In order to improve the energy density of pristine graphene-based devices, composites are made with 2D graphenes and conducting polymer or transition metal oxides.This resulted in improved devices with high thermal stability and mechanical strength with minimum restacking of 2D layers.However, stacking of these 2D layers to accomplish a considerable volume poses a major challenge.The availability of a large variety of van der Waals layered materials provides considerable opportunities to develop new 2D materials for future supercapacitor electrodes.Furthermore, MXene, as a new family of 2D materials, shows a great potential for supercapacitor electrode applications.
Building 3D structures with a controlled pore size distribution is very attractive as electrode materials.Utilizing 3D conductive materials as templates and depositing pseudocapacitive materials like conductive polymers and metal oxide on to these 3D structures is a very efficient method to construct high performing supercapacitor electrodes.Composite 3D metal foams are also very promising because of the high electrical conductivity and easy accessibility of electrolyte ions.Assembling prefabricated nanostructures into 3D structures is another very important direction to obtain highly efficient supercapacitor electrodes since nanostructures can be manipulated by controlling synthetic parameters.However, more effort needs to be focused on optimizing the interface between these nanostructures and charge collecting electrodes.
This review is focused on the supercapacitor electrode performance based on the dimensionality of the material used as the electrode.Due to the variability of conditions and parameters used to evaluate the materials, it is not easy to compare the performance of the electrodes with different dimensionality.Nevertheless, we can infer from this review that there are clear advantages and disadvantages for different dimensionalities.For example, it is possible to control the pore size of 0D carbon electrode to less than 10 nm to obtain better electrolyte accessibility than 1D materials, however, the conductivity of 0D materials remains low in the presence of an insulating polymer binder.In the case of 1D materials, it is possible to deposit active materials on a conductive nanowire, but precisely controlling the accessible space between high aspect ratio structures is not a simple task.A feasible future direction is the combination of different dimensionalities to generate a composite which can provide high conductivity, surface area, mechanical and performance stability and capacitance.2D graphene made into aerogels (3D) with incorporated active nanoparticles like NiO (0D) is an example of such hybrid system.Such composite could also combine the properties of both EDLCs (long cycle life) and pseudocapacitive electrodes (better specic capacitance).Recently, there is a concerted effort to develop 2D materials beyond graphene using van der Waals solids.Many of these solids constitute promising materials for supercapacitor electrodes.It is expected that the combination of these nanomaterials leads to a new generation of supercapacitors electrodes with high energy and power densities in the near future.

Fig. 1 A
Fig. 1 A schematic representation summarizing the key aspects of electrode materials from 0 to 3D.

Fig. 2
Fig. 2 (a) Cross-section of a charged 0D onion-like carbon capacitor.(b) TEM image of a carbon onion with lattice spacing close to 0.35 nm.Reproduced with permission from ref. 23.Copyright 2010 Macmillan Publishers Limited.

Fig. 6
Fig. 6 (a) Fabrication of carbon nanostructures by SNAP method.(b) SEM images of top view and side view (inset) of polymer nanopillars.(c) SEM images of top view and side view (inset) of carbon nanopillars.Reproduced with permission from ref. 51.Copyright 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Fig. 7
Fig. 7 (a) Schematic illustration of the development of MnO 2 coated electrodes.SEM images show top (a) and side (b) view of as-printed PAN nanostructures.(d) SEM image of electrochemically deposited MnO 2 nanoparticulates on pillar nanoarrays.Reproduced with permission from ref. 52.Copyright 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Fig. 8
Fig. 8 (a) Schematic illustration of the fabrication process of CuO@AuPd@MnO 2 NWs.(b) SEM image shows that the copper wire is completely covered by CuO NWs.(c) SEM image of MnO 2 that was uniformly electrodeposited onto NWs.Reproduced with permission from ref. 83.Copyright 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Fig. 10
Fig. 10 (a) Schematic shows the growth of PPy on V 2 O 5 nanoribbon surface.(b) PPy shell helps the electronic transport and prevents the dissolution of vanadium in electrolyte.Reproduced with permission from ref. 103.Copyright 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Fig. 11
Fig. 11 (a) rGO-GO-rGO in-plane and sandwich devices obtained by laser patterning.(b) Laser-scribed graphene-based supercapacitors.Reproduced with permission from ref. 144 and 145.Copyright: 2011 Macmillan Publishers Limited and 2012 American Association for the Advancement of Science.

Fig. 12
Fig. 12 (a) Conventional stacked geometry used for carbon based electrode material (top) and in-plane supercapacitor configuration made of graphene electrodes (bottom).The schematic depicts the mechanisms of increase in electrolyte percolation for higher performance.(b) Representation of a supercapacitor model consisting of GO electrodes (10% oxidation) with pure EMI + BF4 À .Reproduced with permission from ref. 130 and 133.Copyright: 2011 and 2014 American Chemical Society.

Fig. 13
Fig. 13 VS 2 nanosheets for supercapacitor electrodes.S-V-S multilayers with intercalated NH 3 molecules (a) is used to exfoliate thin layers of VS 2 (b).The transfer is accomplished by vacuum filtration and the layers are transferred on mixed cellulose membranes (c).SEM (d) and HR-TEM (e) images of the multilayer with NH 3 .Reproduced with permission from ref. 158.Copyright 2011 American Chemical Society.
Fig. 16 (a) Hydrothermal synthesis of the WS 2 -rGO materials.(b and c) TEM images of the resulting WS 2 -rGO electrode material.Reproduced with permission from ref. 183.Copyright 2013 American Chemical Society.

Fig. 17
Fig. 17(a) Schematic illustration of the preparation process of RGM nanostructure foam.SEM images of (b and c) as-grown GM foam, (d) lightly loaded RGM, and (e) heavily loaded RGM.Reproduced with permission from ref. 185.Copyright 2014 Macmillan Publishers Limited.
Fig. 17(a) Schematic illustration of the preparation process of RGM nanostructure foam.SEM images of (b and c) as-grown GM foam, (d) lightly loaded RGM, and (e) heavily loaded RGM.Reproduced with permission from ref. 185.Copyright 2014 Macmillan Publishers Limited.

x
Co 1Àx oxides.The crystal structure, morphology and electrochemical performance of the 3D Ni x Co 1Àx oxides can be readily manipulated by simply varying the Co/Ni molar ratio in the electrodeposition electrolyte.With the increase in the Co/Ni molar ratio, the structure changes from a NiO crystal dominant structure to NiCo 2 O 4 and nally to a Co 3 O 4 dominant structure.The unique 3D network architecture provides large solid/liquid interfacial area and facilitates efficient electron and ion transport.The electrode prepared with Ni 0.61 Co 0.39 oxide on Nickel foam showed a specic capacitance of 1523.0F g À1 at 2 A g À1 current densityand retained 95.30% of its specic capacitance at a current density of 30 A g À1 aer 1000 cycles.This electrode was used as a positive electrode with activated carbon as a negative electrode to build an asymmetrical supercapacitor and delivered a prominent energy density of 36.46W h kg À1 at a power density of 142 W kg À1 .186

Fig. 18
Fig. 18 Paper models and SEM images showing stacks of (a) flat graphene sheets, (b) heavily wrinkled sheets, and (c) crumpled graphene balls.Reproduced with permission from ref. 222.Copyright 2013 American Chemical Society.

Fig. 20 (
Fig. 20 (a) Schematic diagram of a 3D pillared VACNT-graphene nanostructure.(b) A SEM image of the thermally expanded graphene layers intercalated with VACNTs for 10 minutes pyrolysis times.(c and d) SEM images of CNTs grown on graphene using urea as carbon sources.Reproduced with permission from ref. 240-242.Copyright: 2008, 2011, and 2014 American Chemical Society.
Hydrothermal reaction converted a-Ni(OH) 2 to b-Ni(OH) 2 (1 h reaction time) and further 3D b-Ni(OH) 2 nanoowers to stacked nanosheets (10 h).The electrochemical characterization of three different structures indicates that b-Ni(OH) 2 composite prepared through one hour hydrothermal reaction

Fig. 22
Fig. 22 Schematic diagram showing the formation process of (a) flower-, (b) slice-, and (c) particle-like Ni(OH) 2 under different pH conditions; corresponding SEM Images of as-prepared samples are in the Right Panel.Reproduced with permission from ref. 244.Copyright 2013 American Chemical Society.