Polydopamine-inspired nanomaterials for energy conversion and storage

Konggang Qu *a, Yinghua Wang a, Anthony Vasileff b, Yan Jiao b, Hongyan Chen a and Yao Zheng *b
aShandong Provincial Key Laboratory of Chemical Energy Storage and Novel Cell Technology, School of Chemistry and Chemical Engineering, Collaborative Innovation Center of Antibody Drugs, Liaocheng University, Liaocheng, 252059, China. E-mail: qukonggang@lcu.edu.cn
bSchool of Chemical Engineering, The University of Adelaide, Adelaide, SA 5005, Australia. E-mail: yao.zheng01@adelaide.edu.au

Received 4th June 2018 , Accepted 13th September 2018

First published on 14th September 2018


The development of renewable energy technologies is currently of global concern and calls for the exploration of novel functional materials. During the past decade, bioinspired polydopamine (PDA) has drawn tremendous attention in the advanced materials field. In this review, we provide an overall view of emerging PDA-inspired materials for energy conversion and storage applications. The peculiar advantages of PDA in the elaboration of desired functional materials are highlighted. We focus on the fabrication strategies used for PDA-based materials and their cutting-edge applications as electrocatalysts for oxygen-, hydrogen- and CO2- involving energy conversion reactions and electrode materials for lithium-ion batteries, lithium–sulfur batteries and sodium-ion batteries. The rational tailoring of structures and atomic level molecular design of PDA-based materials for specific functions are particularly demonstrated by a range of examples.


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Konggang Qu

Konggang Qu is an associate professor in the School of Chemistry and Chemical Engineering at Liaocheng University. He received his PhD degree in 2013 from Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. He had worked as a postdoctoral researcher in the University of Adelaide (2014–2015). His research interests include nanostructured materials for electrocatalysis, energy storage and sensors.

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Yao Zheng

Yao Zheng received his PhD in chemical engineering from the University of Queensland (Australia) in 2014. Currently he is a DECRA (Discovery Early Career Researcher Award) research fellow in the School of Chemical Engineering, the University of Adelaide (Australia). His research focuses on the fundamental studies of some key electrocatalytic processes such as oxygen reduction, hydrogen evolution, and CO2 reduction reactions by combining experiments and DFT computation.


1. Introduction

The consumption of conventional fossil fuels brings about a rapidly increasing energy crisis, environmental pollution and global warming, which have spurred unremitting effort by the research community over the past few decades to develop sustainable energy conversion and storage technologies.1–3 Normally, the performances of electrochemical energy conversion devices strongly depend on the reaction rates of the relevant electrochemical processes on their electrodes. Specifically, the oxygen reduction reaction (ORR) is an essential process at the cathode of the fuel cell.4–7 The electrolysis of water consists of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) on the cathode and anode, respectively.8,9 The electrocatalytic CO2 reduction reaction (CRR) can directly convert atmospheric CO2 to hydrocarbon fuels and chemicals, which is of paramount significance to alleviate global warming and our dependence on fossil fuels.10 All these electrochemical processes require high-performance electrocatalysts with appreciable efficiency and selectivity for feasible energy conversion technologies.11 At the same time, rechargeable batteries are the most popular form of energy storage devices and can harness intermittent renewable energy sources for stationary applications.12 For decades, lithium-ion batteries (LIBs) have dominated the portable electronics market.13 However, lithium–sulfur (Li–S) batteries and Na-ion batteries (NIBs) have become important supplements for LIBs in view of the limitations of LIBs regarding energy density and lithium resources.14,15 The successful development of high capacity, cycle-stable electrodes to maximize the energy density of energy storage systems still remains a stringent requirement.16 So far, substantial progress has been achieved in exploiting advanced functional materials as high-performance electrocatalysts and electrodes, which appears promising for the proliferation of related technologies.11,17,18

The development of advanced energy materials needs to consider both compositional and structural modulation simultaneously. Componential optimization aims to find the most effective active units among the materials, while rationally manipulating the structure or morphology to increase the surface area will maximize the number of exposed active sites.19–21 In this regard, multi-functional composite materials are highly desired as different components can work in concert to fulfill the multiple requirements usually present in practical applications. For example, cell configuration systems have to couple at least two kinds of catalysts together to achieve overall functionality, such as in HER/OER for water splitting and ORR/OER for metal–air batteries.22,23 Basically, it's relatively easy to find appropriate functional materials for one single purpose by optimizing the components and structures, but not for multifunctional composite materials which require a sophisticated design and complementary integration to achieve optimal synergistic performance.

Polydopamine (PDA) was first comprehensively researched by Messersmith et al. as multi-functional polymer coatings in 2007 and has attracted huge attention since then.24 PDA can deposit on the surface of any solid substrate due to its unique adhesive properties. In addition, PDA can combine non-metallic elements and metals through post-modification and strong chelation between the metal ions and PDA. As a result, nanomaterials with various components can be integrated and optimized for multi-functional purposes. Meanwhile, owing to its robust adhesion, PDA can be easily manufactured into any desired nanostructure depending on the engaged substrates. The unmatched adhesive properties, flexible componential tunability, and structural regulation are the most outstanding features of PDA and have not been found in any other material. In combination with its high carbon yield and excellent solubility, these multiple features are orchestrated to create the exceptional superiority of PDA in constructing various functional materials. The initial investigations into PDA mainly focused on surface modification and biochemical applications. There are several excellent reviews which summarize the relevant progress in these fields.25,26 In recent years, these unique properties of PDA and PDA-derived nanomaterials have also been widely utilized to develop a range of electrocatalysts and electrode materials in emerging energy-related applications. However, no comprehensive overview or engineering principles exist for these materials, which is a requisite for further advancing PDA- and energy-related studies.

In this review, we first highlight the intriguing physicochemical and structural versatility of PDA in facilitating the delicate design of multi-functional composite materials. Afterwards, PDA materials, PDA-derived carbon materials, and metal–PDA materials with tunable components and structures are discussed in detail together with their application in electrocatalytic reactions and rechargeable batteries. We particularly focus on the performance-oriented design strategies for addressing the specific issues therein. The engineering principles for the componential and structural regulation implemented via PDA are carefully examined and expected to be applied broadly as common methodologies. We then conclude with some perspective on pressing challenges and further research directions regarding PDA-based materials.

2. Unique features of PDA

2.1 Adhesive capability

PDA can be produced quite easily via oxidative self-polymerization of dopamine (DA), which contains both catechol and amine groups. Analogous to mussel adhesive proteins, PDA contains numerous catechol and nitrogen-containing groups, thus is characterized by extraordinarily robust adhesion.24 More interestingly, PDA can form continuous adhesive films with thicknesses that can be precisely controlled by adjusting the concentration of DA and the oxidizing conditions.27,28 By virtue of these unique properties, mussel-inspired PDA has been extensively exploited in energy-related fields for versatile surface functionalization and coating, and can be used for particular purposes including the follow-up integration of multifunctional materials as a nanoglue or nanobinder. Additionally, high-quality conductive carbon layers can be readily produced via pyrolysis of PDA coatings to improve the electrical conductivity and/or maintain the stability of active materials. This is indispensable when nanomaterials for energy-related applications are concerned.

2.2 Componential tunability

In nature, PDA has a variety of functional groups including o-quinone, carbonyl, amino, imine, and phenol. Thus PDA has abundant chemical characteristics and the following three points detail how they are exploited for advanced energy material applications. Firstly, alterations in the type and amount of these functional groups can be simply achieved by changing the reaction conditions, which can impart different properties to the resultant PDA material.29 Secondly, the composition of PDA-based materials can be adjusted by multiple post-modification processes. For example, PDA is extremely reactive to amine or thiol functional groups via the Schiff base or Michael addition reactions, and can also easily conjugate boric acid through the catechol groups.24,30,31 The positively charged N-containing groups of PDA can bind with negatively charged groups like phosphate groups through electrostatic attraction. Therefore, different heteroatoms of nitrogen, sulfur, boron, and phosphorus can be effectively incorporated into the PDA framework. This is a robust means of developing metal-free heteroatom-doped carbon materials for a range of electrocatalysis and battery applications.22,32,33 Thirdly, due to diverse functional groups, PDA can chelate strongly with a wide range of metal species such as Fe, Co, Ni, Mn, Mo, Ti, Cu, etc.34 Accordingly, a vast number of metal compounds with controlled components can be derived from this PDA–metal platform including (but not limited to) metal nitrides, carbides, sulfides, phosphides, borides, and composites, which have exhibited highly appealing performance in energy-related fields.35–37

2.3 Structural complexity

PDA itself can form different structures, including size-tunable spheres,38,39 nanofibers,40 nanofilms,41etc. PDA can also be manufactured into any desired structure from nano- to macro-size (depending on the template) due to its unique adhesive properties and film-forming ability. These properties have been utilized to develop a broad range of architectures for desired applications, such as nano/microcapsules,42,43 nanotubes,44 hollow structures,45 porous structures,46–48 and wafer-scale films,41 where the thickness of the layer-structured PDA in these structures can be tailored at the nanoscale. For example, flexible carbon structures are highly favored for combination with active electrode materials in rechargeable batteries. Such carbon structures play multiple roles for retaining excellent overall battery performance (e.g. improving the electrical conductivity and preventing the aggregation of active components) and especially for accommodating volume variations during the charge/recharge processes.

3. Electrocatalytic reactions

The development of advanced electrocatalysts has always been a priority for the practical application of energy storage techniques at any appreciable scale.8,49 PDA displays unique advantages for designing various electrocatalysts from both component and structural aspects. Due to the versatility of PDA chemistry, various non-metallic and metallic elements can be readily incorporated into the PDA framework to derive heteroatom-doped carbon and metal composite electrocatalysts, respectively. At the same time, diverse nanostructures of PDA can be manufactured according to the templates employed, increasing the extrinsic surface area and number of catalytically active sites.

3.1 ORR and OER electrocatalysts

3.1.1 PDA-derived metal-free electrocatalysts. The electrocatalytic ORR and OER processes contain complicated multiple electron transfer pathways, and thus have intrinsically sluggish kinetics.8,50 Meanwhile, the widespread use of benchmark noble metal electrocatalysts (Pt, Ir, Ru, etc.) for the ORR and OER is impractical considering their high cost and scarcity. As alternatives, heteroatom-doped carbons and earth-abundant metal-based materials have attracted extensive interest over the last few decades.11,51,52

The inherently high nitrogen content of PDA enables it to be an excellent N source to prepare N-doped carbons (N–carbons). Consequently, N–carbons with richly tailored structures have been synthesized and applied as ORR/OER electrocatalysts. In the early stages, hollow N-doped carbon microspheres (HNCMS) were prepared using PDA-coated SiO2 microspheres. The HNCMS exhibited enhanced ORR activity due to their large pore volume, hollow core, and porous-thin shell that worked together in facilitating diffusion and mass transfer of reactants and electrolyte.45,53 Later, Qiao et al. reported that PDA can be easily deposited on the surface of graphene oxide (GO) to prepare N-doped carbon nanosheets (NDCNs) via a simple self-assembly process (Fig. 1A).28,54 The resultant NDCNs possessed a well-defined mesoporous structure, which was highly dependent on the thickness of the PDA–GO intermediate hybrid and the annealing temperature.28 Specifically, the PDA–GO hybrid with a thickness of 5 nm and a 900 °C pyrolysis temperature derived the optimized NDCNs with a resultant thickness of 2.7 nm, mesopores of ∼3.9 nm and a large surface area of 272.3 m2 g−1 (Fig. 1B–D), which exhibited the best ORR activity (Fig. 1E). A disadvantage of this approach is that the mesoporous structure of the NDCNs is generated uncontrollably and randomly during the annealing process. In this regard, Feng et al. have synthesized NDCNs with size-controlled mesopores by first growing mesoporous silica nanoparticles and then assembling them on a GO surface by electrostatic interaction.46 After sequential PDA coating, pyrolysis, and silica etching, mesoporous NDCNs were obtained with different mesopore sizes (2, 7, and 22 nm), of which the NDCNs with a pore diameter of 22 nm exhibited the best ORR performance with a more positive onset potential than Pt/C and a high diffusion-limiting current approaching that of Pt/C in alkaline media. With the aid of its adhesive properties and film-forming ability, PDA can further be used to synthesize N-doped mesoporous carbons (NMCs) by using SBA-15 hard templates.47 The obtained NMCs possessed controllable graphitic N and pyridinic N species, large surface area (536 m2 g−1) and well-aligned 3D mesoporous architecture, thus displaying an ORR catalytic efficiency comparable to that of Pt/C.


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Fig. 1 (A) Schematic illustration of the synthesis of the PDA-based NDCNs and N,S–CNs. (B and C) TEM and (D) AFM images of NDCNs. (E) ORR LSVs of various electrocatalysts. Reproduced with permission.28 Copyright (2015), Royal Society of Chemistry. (F) TEM images, (G) XPS survey and (H) the overall LSV curve of the N,S–CN for bifunctional ORR and OER (inset: the value of ΔE for various catalysts reported previously). Adapted with permission.23 Copyright (2016), Elsevier.

Apart from the structural regulation of PDA, different heteroatoms like sulfur and boron can be easily introduced into the PDA framework to obtain dual-doped carbons in combination with intrinsic nitrogen in PDA. PDA can react with thiol groups under ambient conditions via the Schiff base or Michael addition reactions, with the rate constant ranging from 4 × 105 to 3 × 107 M−1 S−1 (in the case of cysteine at pH = 7).24,31,55 For example, Qiao et al. reported that a 2.5 nm-thick PDA–GO hybrid can graft about 16.7 at% of S by reacting with 2-mercaptoethanol, which finally yields mesoporous N,S-codoped carbon nanosheets (N,S–CNs) as shown in Fig. 1F.23 This S-doping strategy can realize uniform and in situ N,S-codoping and can simultaneously achieve much higher doping efficiencies (4.1 at% N and 6.1 at% S) than those by most post-treatment methods (Fig. 1G).23 The resultant N,S–CNs exhibited excellent performance as bifunctional ORR/OER electrocatalysts (Fig. 2H), which is attributable to the high level of dual doping, abundant porous architecture, and good electron transfer ability. In addition, boronic acid can bind with the catechol groups of PDA by the boronic acid–diol complexation.31 Zhong et al. have utilized this by reacting PDA-coated carbon and boronic acid to construct a N,B-codoped carbon, which showed enhanced ORR activity due to the synergistic coupling effect of B and N in the carbon framework.56


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Fig. 2 (A) TEM and (B) HAADF-STEM images of Co–ISAS/p-CN nanospheres. (C) R-space EXAFS and a schematic model proposed for Co–ISAS/p-CN nanospheres. (D) ORR polarization curves and (E) Jk at 0.83 V and E1/2 for different catalysts. (F) ORR polarization curves before and after 5000 cycles. Reprinted with permission.64 Copyright (2018), John Wiley and Sons. (G) Schematic synthesis diagram of carbon-supported and N-doped carbon-coated ordered fct-PtFe NPs. (H) TEM images of carbon-loaded and N-doped carbon shell-coated PtFe NPs with different PDA coating times. (I) ORR polarization curves and 100 h MEA tests for fct-PtFe/C (J) and Pt/C (K). Reprinted with permission.70 Copyright (2015), American Chemical Society.

Unexpectedly, in all these single- and dual-doped PDA-derived carbon materials, the intrinsic pyrrolic N of PDA was largely transformed into the graphitic and pyridinic N species (most active for oxygen electrode reactions) during annealing processes. For example, Ruoff et al. found that the ORR activity of N–carbon catalysts is mainly determined by the graphitic N while pyridinic N can lower the ORR onset potential.57 Dai et al. claimed that the electron-donating quaternary N sites are responsible for the ORR activity, whereas the electron-withdrawing pyridinic N moieties serve as active sites for the OER.58 Therefore, the N components in PDA-derived carbon materials are greatly conducive to enhancing the ORR and OER activities.

3.1.2 Metal–PDA based electrocatalysts. The inherent catechol and N–H groups in PDA can both interact with many types of transition metal ions such as Fe,35,36,59,60 Co,61,62 and Ni,63 enabling the fabrication of various metal–carbon/nitrogen composite materials. Lu et al. first reported that DA can self-polymerize into monodispersed and size-tuned PDA sub-micrometer spheres (SMSs) and consequently N–carbon SMSs by adjusting the amount of the ammonia solution used during synthesis.35 Similarly, monodisperse Fe@Fe3C-loaded N–carbon spheres (Fe@Fe3C/C–N) were fabricated through the simple mixing of an iron salt with PDA SMSs.35 The as-prepared nanocatalysts show comparable ORR catalytic activity to commercial Pt/C, but better stability. Wang et al. used a thermal treatment on the Fe@Fe3C/C–N spheres with ammonia, whereby the Fe@Fe3C phase transformed into Fe2N.36 The resultant mesoporous Fe2N/N-doped graphitic carbon spheres exhibited superior ORR performance to Pt/C, which primarily originated from the synergetic effect of highly active ultra-small Fe2N nanocrystals and the N–carbon support. Manthiram et al. applied a multi-step strategy to prepare a Fe–N/C composite with the assistance of PDA and metal–organic frameworks (MOFs).59 At first, porous tellurium nanotubes (Te NTs) were used as templates to grow Zn-based zeolitic imidazolium frameworks (ZIF-8). Then, DA self-polymerized into PDA in the presence of FeCl3 to obtain Fe,N embedded in PDA-coated Te NT@ZIF-8. After annealing in argon, both Zn and Te evaporated, and the final carbon framework embedded with Fe,N wired along 1D carbon nanotubes was obtained. The mass ratio of DA to Fe was carefully tuned to achieve the most effective Fe–N/C active sites and a hierarchical dual-porous structure with an extremely high surface area of 1112 m2 g−1. The presence of Fe3+ not only catalyzes the graphitization of PDA, but also facilitates the formation of the mesoporous structure during the acid leaching process. As expected, the optimized catalyst shows superior ORR activity compared to Pt/C in both alkaline and acidic media.

Single-atom metal materials have unique catalytic performance because they approach the limit of atom utilization efficiency. N–carbons have been widely studied as suitable supports to anchor single metal atoms and keep them from aggregating. Recently, Li's group used DA mixed with metal acetylacetonate to prepare single-atom metal/NC porous nanospheres (Fig. 2A and B), in which the metals selected were Co, Ni, Cu, Mn, and Pd.64,65 The X-ray absorption fine structure (XAFS) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) analyses confirmed that all of the metals exist in the form of positively charged isolated single atoms coordinated with four nitrogen atoms (Fig. 2C). As an example, the single-atom Co/N–carbon exhibited excellent stability under ORR conditions and comparable ORR activity to Pt/C (Fig. 2D–F). Song et al. have proposed a general space confinement strategy to achieve highly dispersed transition metal single atoms within typical graphene and carbon nanotube supports.66 Specifically, homogeneous ZnM (M = Co, Fe, or Cu) dual-metal hydroxide layers were first deposited in situ onto the surface of the supports. After PDA coating and thermal treatment, different single-atom M/NC materials were obtained with exclusive molecular sites in the form of MN4C8O2. Here, the PDA coating layer plays a vital role in protecting the isolated single-atom metals from aggregation and crystal growth.

Generally, one significant drawback associated with metal-based catalysts is their relatively poor stability caused by different mechanisms, such as agglomeration during pyrolysis treatment, fouling, and poisoning and leaching during long-term catalytic processes.67,68 It is well-recognized that protective carbon coatings can considerably improve the structural integrity and long-term performance stability of metal nanomaterials. Wan et al. used PDA to effectively confine iron carbide nanocrystals in N–carbon coated CNTs, which prevented the agglomeration of iron carbide during pyrolysis and thus provided sufficient active sites for the ORR.69 Although stability is improved, the presence of a protective layer may reduce the catalytic activity by impeding mass transport to the active sites. Therefore, controlling the thickness of the carbon shell is crucial, and can be readily achieved via PDA chemistry. Chung et al. found when the thickness of a PDA-derived carbon shell on PtFe NPs is less than 1 nm, the shell is highly permeable. Therefore, the catalytic activity is not affected while the NPs are well protected (Fig. 2G and H).70 For example, the “dual purpose” N–carbon shell not only prevents the coalescence of NPs during a thermal annealing process but also protects them under harsh fuel cell operating conditions. The ordered fct-PtFe/C nanocatalyst coated with a N–carbon shell shows a 11.4 times higher mass activity and a 10.5 times higher specific activity than commercial Pt/C (Fig. 2I). Additionally, the protected catalyst exhibits a superior long-term stability in a membrane electrode assembly (MEA) for 100 h without a significant activity loss compared to a loss of 27% for Pt/C (Fig. 2J and K).

3.2 HER electrocatalysts

3.2.1 PDA-derived metal-free electrocatalysts. Owing to the unique molecular properties of PDA, the PDA-aided dual-doping strategy developed by Qiao et al. can act as a universal approach for the preparation of a range of dual-doped carbon nanomaterials.23 For example, N,S-doped carbon nanotubes (Fig. 3A) have dopant concentrations of 3.8 at% N and 5.6 at% S,22 and displayed superb bifunctional catalytic activities for both the HER and OER in an alkaline solution (Fig. 3B and C), even outperforming most reported carbon counterparts. Experimental characterization confirmed that the excellent performance is attributed to the dual doping together with the efficient mass and charge transfer, while theoretical computations revealed that the promotion effect of the secondary sulfur dopant enhances the spin density difference of the carbon to form highly electroactive sites for the HER and OER (Fig. 3D).20 Boron, sulfur, and phosphorus are generally used as secondary elements for dual-doped carbons with nitrogen as the primary dopant.20,71 However, evaluation of the promotional effect of B, P, and S in N–carbons has rarely been investigated, mainly due to the complicated N configurations and poor doping efficiency of the secondary elements. As aforementioned, PDA can derive N–carbons with homogeneous doping and identical N species. Moreover, the intrinsic functional groups of PDA can ensure highly efficient doping of secondary dopants via a series of chemical reactions. Therefore, a general platform using PDA has been proposed to synthesize N,B-, N,P-, and N,S-co-doped carbon nanosheets, characterized by identical N species and a high concentration of B, P, and S doping (Fig. 3E and F).32 Consequently, the promotional effect of B, P, and S to N–carbons in the HER electrocatalysts has been systematically investigated. Theoretical and experimental results confirmed that S promotes better HER activity in N–carbon compared to P, while B decreases the activity of N–carbon (Fig. 3G–I).
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Fig. 3 (A) TEM images of the N,S–CNT. (B) HER and (C) OER polarization curves of different catalysts. (D) Charge density (top) and spin density differences (down) on various single/double doped graphene models. Adapted with permission.22 Copyright (2016), John Wiley and Sons. (E) Fabrication of the co-doped carbon nanosheets. (F) TEM image of the N,S–CN. (G) HER and (H) OER polarization curves. (I) The relationship between the Cdl and ΔGH* values of different electrocatalysts and their overpotentials at a current density of 10 mA cm−2. Reprinted with permission.32 Copyright (2017), American Chemical Society.
3.2.2 Metal–PDA based electrocatalysts. Although a range of doped carbon materials have been developed for the HER, large disparity in performance compared to commercial Pt/C still remains. Consequently, the development of metal-doped carbon hybrid catalysts is a feasible strategy to bridge this gap. For instance, molybdenum carbide as a high-performance HER electrocatalyst has stimulated much interest because of its similar electronic structure to Pt-group metals. Li et al. prepared Mo–PDA microflowers using a simple mixture of Mo salt and DA, which can derive ultra-small Mo2C NPs uniformly dispersed on 3D N–carbon microflowers (Fig. 4A–C).72 The formation of such an ordered structure is presumably attributed to the strong chelating interaction of MoO42− with the catechol groups of DA. The resultant Mo2C/NCF possesses an open and accessible structure with hierarchical order at different levels. Similarly, hierarchical β-Mo2C nanotubes can also be synthesized using a MoO3 nanorod template and derived Mo–PDA nanotube intermediates.73 The resultant β-Mo2C nanotubes with wall thicknesses of ∼90 nm exhibited excellent HER performance, including overpotentials of 172 mV and 112 mV at 10 mA cm−2 in 0.5 M H2SO4 and 0.1 M KOH electrolyte, respectively (Fig. 4D–F).
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Fig. 4 (A–C) TEM images at different magnifications of Mo2C/NCF. (D and E) HER polarization curves of different catalysts in acidic and alkaline electrolytes. (F) Long-term operation stability of Mo2C/NCF in acidic and alkaline electrolytes. Reprinted with permission.72 Copyright (2016), American Chemical Society. (G) Schematic representation of the carbon-shell-coated FeP NP preparation. (H and I) TEM images of the as-synthesized iron oxide NPs and carbon-shell-coated FeP NPs. (J) HER polarization curves for FeP/C with and without a carbon shell and Pt/C, the corresponding Tafel plots are shown as the inset. (K) Long-term durability test of FeP/C. (L) DFT calculation for hydrogen bonding energy of FeP and FeP–O as a function of hydrogen coverage. Reprinted with permission.75 Copyright (2017), American Chemical Society.

Metal phosphides have been widely developed as efficient HER electrocatalysts, but are vulnerable to oxidation and thus are not appropriate for long-term operation.74 PDA-derived carbon shells have been applied as protective coatings for metal phosphides (Fig. 4G).70,75 The thickness of the carbon shell was optimized to less than 1 nm (Fig. 4H and I), which is thick enough to provide physicochemical protection but not interfere with the catalytic reaction at the surface of FeP NPs. As a result, the carbon-shell-coated NPs exhibited outstanding HER performance with an overpotential and Tafel slope measured as 71 mV at 10 mA cm−2 and 52 mV per decade, respectively (Fig. 4J). Remarkably, the shell-coated catalysts showed negligible activity loss even after 10[thin space (1/6-em)]000 cycles, while obvious deactivation was observed for the catalysts without the protective shells (Fig. 4K). DFT calculations further revealed that pristine FeP has a hydrogen adsorption free energy (ΔGH) much closer to zero than surface-oxidized FeP (Fig. 4L).

PDA can also assist in the synthesis of single-atom Pt combined with polystyrene-block-poly(ethylene oxide) (PS-b-PEO).76 By mixing with H2PtCl6, Pt-loaded PDA/PS-b-PEO was obtained. This was then subjected to a stepwise pyrolysis to gradually fuse Pt ions into the crystal lattice of the carbon matrix, finally producing the Pt@porous carbon matrix (Pt/PCM) hybrid. The Pt@PCM catalyst exhibited significantly enhanced HER mass activity (up to 25 times) compared to commercial Pt/C. Density functional theory (DFT) calculations disclosed that this high activity originates from the lattice-confined Pt centers and the activated carbon/nitrogen atoms around the isolated Pt centers.

3.3 Electrocatalytic CRR

The CRR is of great significance for mitigating the greenhouse effect and for producing renewable carbon-based fuels and feedstocks.77 Various nanostructured catalysts with favorable efficiency and selectivity have been designed for the CRR.78 Recently, the nucleophilic hydrogen bonds of organic molecules were found to be suitable as CRR active sites. However, their catalytic reactivity is poor due to their intrinsically low capability for electron transfer.79,80 Different to the conventional polymerization of PDA under ambient conditions, Stadler et al. reported a distinctive PDA thin film by vapor-phase polymerization,81 which had a conjugated polymer structure with hydrogen-bonded motifs (Fig. 5A). DFT calculations confirmed the energetically favored interactions between CO2 and the nucleophilic hydrogen-bonded sites (Fig. 5B). Therefore, the obtained PDA films deposited on carbon fiber networks exhibited impressive catalytic performance with a geometric current density of 18 mA cm−2 at an overpotential of 0.21 V for the production of C1 species (carbon monoxide and formate) and excellent stability over 16 hours of continuous operation at >80% faradaic efficiency (Fig. 5C–E). The CRR mechanism on this material likely occurred as a three step process (Fig. 5F): (i) electrochemical activation of the hydrogen-stabilized carbonyl group, (ii) formation of a nucleophilic center via the adjacent amine as the favored position for CO2 attachment, and (iii) creation of an amide with CO2 for the synthesis of C1 species.
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Fig. 5 (A) Synthesis of conductive-catalytic PDA. (B) BEs shown as functions of the distances between nitrogen and CO2 (1.99 Å) and oxygen and CO2 (2.11 Å). (C) CVs of conductive PDA on a CF electrode under different conditions. (D) Faradaic efficiencies of CO, H2, and formate as functions of time. (E) The long-term stability of PDA on a CF electrode under CRR conditions. (F) Initial steps driving the CRR in conductive PDA. Reprinted with permission.81 Copyright (2017), Science Advances.

4. Rechargeable batteries

As an energy storage device, LIBs have been successfully commercialized for decades. However, the energy and power densities of current commercial LIBs are far from satisfactory. Therefore, high-performance electrode materials are still highly desired.82–84 Recently, sulfur has received significant attention as a highly promising cathode material in Li–S batteries, due to its high specific capacity of 1673 mA h g−1 and natural abundance.14 Another issue concerning LIBs is the scarcity of lithium resources. Owing to the abundance of sodium resources and the similar electrochemical performance of sodium, SIBs have been reconsidered as a promising and lower-cost alternative to LIBs.15 In this section, we will first systematically review the major efforts of employing PDA in the development of high-performance anode materials and separators in LIBs. We will then discuss the latest progress of advanced PDA-based materials in overcoming the technological obstacles of Li–S batteries and SIBs.

4.1 LIBs

4.1.1 Anode materials. Beyond graphitic carbons, metals or metal oxides such as Sn, Si, and Fe3O4, with a higher theoretical capacity and low cost, are being recognized as promising candidates for next-generation anode materials in LIBs. Among these materials, typical problems associated with them are severe aggregation and significant volume expansion during Li+ insertion/extraction processes, which lead to the pulverization of the electrodes and rapid capacity decay. Coating of carbon shells is an effective strategy to tackle these issues with simultaneous conductivity improvements. As discussed in Section 2, due to the robust adhesion of PDA, uniform and continuous PDA layers can be easily formed on the surfaces of metals/metal oxides, which are then converted into highly conductive graphitic carbon materials after thermal treatment.

Fe3O4 is an attractive anode material for LIBs with a high theoretical capacity of 926 mA h g−1. However, its rapid capacity attenuation greatly hinders its practical application owing to its large volume expansion during lithiation. Paik et al. used a novel etching-in-a-box strategy to prepare unique Fe3O4@C yolk–shelled nanocubes with a hollow nanostructured core.85 In this work, PDA-coated Fe2O3 nanocubes were firstly prepared and transformed into Fe3O4@C core–shelled nanocubes. After a time-controlled hydrochloric acid etching, a series of Fe3O4@C yolk–shelled nanocubes with different void space sizes were obtained (Fig. 6A and B). The optimized samples with an etching period of 2 hours exhibited a high reversible capacity (1100 mA h g−1 at 100 mA g−1), an excellent rate capability (370 mA h g−1 at 20 A g−1), and an ultra-long cycling life (8000 cycles) as anode materials for LIBs (Fig. 6C). The enhanced lithium storage performance was attributed to the optimized void space, which can buffer the large volume expansion of Fe3O4 while maintaining a sufficient capacity. Jin et al. have carbonized similar PDA-coated Fe2O3 nanocubes in NH3 to obtain Fe2N confined in the carbon microcubes (Fe2N@C).86 The resulting composite consisted of a microporous Fe2N core and a ∼6 nm-thick carbon shell that can protect the air-sensitive Fe2N from oxidation to Fe3O4. The internal voids in the Fe2N microporous cubes coupled with the confined shell helped to buffer the volume expansion (only ∼9%) during lithiation/delithiation processes, which was much lower than that of bulk Fe2N (∼90%). Therefore, the Fe2N@C anode maintained a high volumetric capacity (1030 mA h cm−3), stable cycling performance (at 10 A g−1 with a capacity retention of over 91% for 2500 cycles) and an excellent rate performance arising from the typical pseudocapacitive behavior of Fe2N.


image file: c8ta05245j-f6.tif
Fig. 6 (A) SEM and (B) TEM images of Fe3O4@C-2. (C) The rate performance of different samples at various current rates from 100 mA g−1 to 20 A g−1. Reproduced with permission.85 Copyright (2016), John Wiley and Sons. (D) TEM and (E) HRTEM images of the Sn@aCNT composite. (F) Rate capability of the Sn@aCNT electrode. Reproduced with permission.88 Copyright (2016), John Wiley and Sons. (G) TEM and (H) HRTEM images of SnO2/NC submicroboxes. (I) Cycling performance of the SnO2 submicrobox and SnO2/NC submicrobox electrodes at a current density of 0.5 A g−1 and the coulombic efficiency. Reproduced with permission.91 Copyright (2016), John Wiley and Sons. (J and K) TEM images of C@MoS2 nanoboxes. (L) The rate performance at various current rates of annealed C@MoS2 nanoboxes. Reproduced with permission.92 Copyright (2015), John Wiley and Sons.

Si has an extremely large theoretical capacity of up to 4200 mA h g−1. The major challenge for Si-based anodes is to simultaneously realize adequate electronic conductivity and structural integrity during charge/discharge cycling.68,87 Kang et al. assembled PDA-coated Si NPs and GO through a vacuum filtration to prepare novel Si NP–carbon–graphene (Si@C–rGO) composites.68 The obtained anode material simultaneously achieved a high capacity and a long cycle life (600 mA h g−1 after 100 cycles at a current density of 1.5 A g−1), which were ascribed to the synergistic effect between the rGO sheets and PDA. Specifically, the graphene sheets can ensure good electrical conductivity, prevent the exposure of Si NPs to the electrolyte decomposition, and act as a buffer to accommodate the huge volume expansion of the Si NPs. Meanwhile, the covalent or hydrogen bonding between PDA and GO enables the anchoring of Si onto the graphene sheets, thus preserving the structural integrity and avoiding the detrimental aggregation during charge/discharge processes.

Sn has a high theoretical capacity of 993 mA h g−1 and a suitably low discharge potential. However, the practical implementation of Sn in LIBs suffers from pulverization caused by the large volume variation during lithiation/delithiation processes. This subsequently leads to an electrical disconnection of the active materials and instability of the solid-electrolyte interphase. To address this, Lou et al. developed a material composed of Sn NPs encapsulated in amorphous CNTs (Sn@aCNT) using MnOx nanowires as templates followed by sequential deposition of a SnO2 layer and a PDA coating. After the selective removal of MnOx and thermal treatment, Sn@aCNT was obtained with a peapod-like structure (Fig. 6D), where the Sn NPs with a mean size of ∼70 nm were well encapsulated by aCNT with wall thicknesses of ∼20 nm (Fig. 6E).88 The hybrid afforded a high specific capacity of 749 mA h g−1 at a current density of 0.2 A g−1 after 100 cycles and delivered a specific capacity of 573 mA h g−1 at a current density of 1.0 A g−1 up to 500 cycles (Fig. 6F). Unfortunately, the low melting point of Sn resulted in liquid coalescence during the carbonization and thus formed non-uniform and large-sized Sn NPs.89,90 Alternatively, SnO2 possesses a theoretical capacity of 780 mA h g−1, but a drastic volume expansion (>300%) during Li+ insertion/extraction. Lou et al. synthesized SnO2/N-doped carbon (SnO2/NC) submicroboxes by depositing SnO2 and PDA on Fe2O3 submicrocube templates. After the carbonization and template removal, the final SnO2/NC had an average size of ∼400 nm with 4–5 nm SnO2 NPs and highly porous shells of ∼40 nm thickness (Fig. 6G and H).91 These submicroboxes exhibited a high reversible capacity of 491 mA h g−1 at a current density of 0.5 A g−1 after 100 cycles and an excellent rate capability (Fig. 6I). Similar to the synthesis of SnO2/NC, carbon nanoboxes were prepared to continuously grow MoS2 shells (three to five layers) containing ultrathin nanosheets (Fig. 6J and K).92 These C@MoS2 nanoboxes exhibited a high specific capacity of ∼1000 mA h g−1, an excellent cycling stability up to 200 cycles and a superior rate performance (Fig. 6L).

Molybdenum dioxide has been suggested as a promising anode material for LIBs owing to its relatively high electrical conductivity, reversible capacity (838 mA h g−1), and chemical stability. Lou et al. used Mo–glycerate (MoG) spheres as the precursor to synthesize Mo–PDA hollow spheres via reaction with DA.93 By adjusting the amount of ammonia added and the size of the initial MoG spheres, the number of shells in the Mo–PDA hollow spheres was adjusted from one to four. A self-templating mechanism was proposed because the dissolution of the MoG template and the polymerization of DA take place simultaneously during the solvothermal process. The triple-shelled MoO2/carbon composite hollow spheres exhibited an appealing performance including a high capacity of ∼580 mA h g−1 at 0.5 A g−1 with a good rate capability and a long cycle life.

4.1.2 Separators. The separator is an integral module of LIBs, and can greatly affect the performance and safety of batteries.94 Currently available microporous polyolefin separators have hydrophobic surfaces and low surface energy, severely hampering the diffusion of a liquid electrolyte. Inspired by the versatile and wet-resistant adhesion ability of PDA, Choi et al. developed a simple dipping process to modify the polyethylene (PE) separator with PDA to overcome the poor compatibility of PE with electrolytes.95 PDA modification can retain the morphology and porous structure of the PE separator, while significantly reducing the contact angle from 108° ± 1.4° for bare PE to 39° ± 1.7° for modified PE. Benefiting from this enhanced hydrophilicity, the electrolyte uptake increased from 96 ± 3.2% to 126 ± 2.8%, accompanied by an increase in the ionic conductivity. As a result, at a current density of 15C, the discharging capacities of the modified PE cells maintained 84.1% of the discharging capacities at 1C. Additionally, to address the formation of uncontrolled Li dendrites that cause unnecessary consumption of lithium ions and the electrolyte, the continuous study by Choi's group further confirmed that PDA can also efficiently restrain the growth of Li dendrites on the surface of the Li metal.96 It was found that the PDA-modified PE cells exhibit a higher coulombic efficiency (97.1%) than both the bare PE cells (91.3%) and the gel polymer electrolyte (GPE) cells (94.1%) during the first cycle. After 100 cycles, the PDA-modified PE cells displayed a higher capacity retention with 94.9% of the initial discharge capacity maintained at a charging rate of 0.85 mA cm−2, whereas the GPE cells lost 46%, and the bare PE cells lost their entire discharge capacity after 30 cycles. The largely improved cycle life of the PDA-modified PE cells resulted from the uniform Li ionic flux and the strong catecholic adhesion of PDA onto Li surfaces, which both facilitated the suppression of the Li-dendrite growth.

Commercial PE separators have a melting point of ∼135 °C and lose their dimensional stability when exposed to temperatures above 100 °C. This could cause an internal short circuit, and consequently lead to combustion or even an explosion. Zhao et al. designed a PDA–ceramic composite modification of polyolefin membranes with substantially enhanced thermal properties.97 In their synthesis process, SiO2 particles were first applied to both sides of a PE separator. A self-supporting PDA film then formed on both the SiO2 layer and PE separator by a simple dip-coating process (Fig. 7A and B). The final PE–SiO2@PDA separator was found to retain its dimensional stability without any visual thermal shrinkage and kept its mechanical strength until 230 °C, while the bare PE and PE–SiO2 separators began to shrink above 100 °C and 140 °C, respectively (Fig. 7C and D). Such an enhanced stability was attributed to the overall-covered PDA film, which caused the ceramic layer and polyolefin separator to function as a single unit. Meanwhile, as shown in Fig. 7E, the modified PE–SiO2@PDA separator showed a similar cycle performance and rate capability compared to PE and PE–SiO2 separators.


image file: c8ta05245j-f7.tif
Fig. 7 (A) Preparation of the composite-modified separator with a simple dip-coating process. (B) SEM image of the PE–SiO2@PDA separator cross-section and corresponding elemental mapping of Si, C and N. (C) Thermal shrinkage of the pristine PE separator, PE–SiO2 separator and PE–SiO2@PDA separator as a function of temperature. (D) Tensile strength of the PE-SiO2 separator and PE-SiO2@PDA separator as a function of temperature, and the DSC curve of PDA. (E) C-rate capability of coin cells assembled with the PE separator, PE-SiO2 separator and PE-SiO2@PDA separator. Reproduced with permission,97 Copyright (2016), The Royal Society of Chemistry.

4.2 Li–S batteries

As alternatives to LIBs, Li–S batteries use abundant sulfur as cathodes and are attractive owing to their high theoretical specific capacity (2500 W h kg−1 or 2800 W h L−1). Different from the Li+ intercalation/deintercalation in LIBs, Li–S batteries contain a redox-driven phase-transformation process that involves a reversible interconversion between S and Li2S via an intermediate lithium polysulfide species (Li2Sn, n = 2–8).14 This process is the origin of the high specific capacity, but unfortunately suffers from the high electrical resistance of insulating sulfur, a large volumetric expansion (∼80%), and a shuttle effect triggered by the dissolution of Li2Sn intermediate species.98

As a result of the significant development of carbon materials, encapsulating sulfur composites with carbon has become the most popular strategy to address the above issues through suppressing the polysulfide diffusion and building a conductive framework for electron/ion transport. As an example, Manthiram et al. employed PDA and TiO2 hollow spheres to prepare nitrogen-doped double-shelled hollow carbon sphere (NDHCS) substrates.99 After sulfur loading and graphene wrapping, a flexible, hierarchical G–NDHCS–S hybrid paper was constructed (Fig. 8A). The dual confinement effect in the final system was provided by both NDHCSs and the graphene wrapping: the former affords plenty of internal voids to maximize the sulfur content, enhance energy density, and to accommodate the volume change of sulfur during cycling, while the latter can further enhance the confinement of sulfur, and at the same time offer a conductive network for the composite. As a result, the G–NDHCS–S hybrid cathode exhibited a high initial discharge capacity of 1360 mA h g−1 at a current rate of 5C, an excellent rate capability of 600 mA h g−1 at 2C, and a sustainable cycling stability for 200 cycles with nearly 100% coulombic efficiency. During the synthesis of carbon-encapsulated sulfur materials, sulfur needs to be impregnated into the hollow carbon structure via a heat treatment, which requires the presence of pores in the carbon shell. However, the oversized pores of carbon may lead to the outward diffusion of sulfur and a loss of confinement ability. Xiao et al. synthesized PDA-derived hollow carbon spheres with controlled pore sizes to study the influence of the pore size on the confinement of sulfur.33 The well-designed silica template consisted of a silica sphere core and a porous silica shell that permitted the deliberate tuning of the pore size. With the PDA coating, carbonization, and silica etching, hollow carbon spheres with controlled pore sizes were obtained. It was found that when the pore size is 2.8 nm, sulfur can diffuse into the internal void of hollow carbon and be effectively confined under ultrahigh vacuum conditions. On the other hand, when the pore sizes were 3.2 and 4.1 nm, the hollow carbons were observed to be partially filled or even empty. The underlying reason may be that overly large pores cannot facilitate a significant capillary force to draw the liquid sulfur into the inner space, and also will not effectively prevent sublimation of sulfur out of the pore.


image file: c8ta05245j-f8.tif
Fig. 8 (A) Schematic illustration of the fabrication of the NDHCS–S composites and the G–NDHCS–S hybrid paper. Reproduced with permission.99 Copyright (2015), John Wiley and Sons. (B) 3D view and simplified 2D cross-sectional view of the CB–HNSs–PDA double-shelled cathode before and after being discharged. (C) SEM and (D) TEM images of CB-81.5HNSs%-18.5%PDA showing the CB stuck to HNSs–PDA. Reproduced with permission.100 Copyright© 2016, John Wiley and Sons. (E) Synthesis procedure of the Ti4O7 NPs. Reproduced with permission.101 Copyright (2017), John Wiley and Sons. (F) Synthesis process of the TiO@C-HS/S composite. Reproduced with permission,102 Copyright (2016), Nature Communications.

Considering its robust adhesive capability, PDA has also been exploited as a nano-binder to glue conductive carbon black (CB) and hollow nano-S (HNS) at the nanoscale (Fig. 8B).100 HNS spheres were first coated with a PDA shell, which can then adhere CB particles via ball milling. The resultant material formed a stable cathode of double-shelled CB-decorated HNSs–PDA composites (Fig. 8C and D). The highly viscous PDA prevented CB from detaching from the cathode surface during cycling. Moreover, the hierarchical double-shelled structure (PDA and carbon black) acted as a physical and conductive barrier to effectively trap polysulfides and reutilize them. Additionally, the PDA coating layer effectively accommodated volume expansion due to its isotropy and good elastic constant (E = 6.748 kNm kg−1). These properties imparted ultra-long cycle performance of 2500 cycles at 0.5C, with only 0.014% capacity loss per cycle.

Other than carbon substrates, polar metal oxides can interact with polysulfides more strongly, therefore providing significantly improved cycling properties. For instance, Magnéli phase Ti4O7 has a high affinity for polysulfide through the polar O–Ti–O units. Using a soft-templating method, Lu et al. synthesized Ti4O7 particles with interconnected 2–5 and 15–25 nm mesopores (Fig. 8E).101 The abundant pores with large surface area achieved a strong physical and chemical confinement of sulfur. Meanwhile, a thin and porous PDA-derived carbon coating on Ti4O7 ensured a favorable contact between the Ti4O7 particles and CB, further improving the conductivity of the electrode. The carbon-coated Ti4O7 particles displayed a high capacity of 1411 mA h g−1 at 0.1C and a good cycling stability with a capacity decay of 0.099% per cycle over 300 charge/discharge cycles. Given the significantly higher electrical conductivity of rock-salt TiO than that of Magnéli phase Ti4O7, Lou et al. designed TiO@carbon hollow spheres (TiO@C-HS) as a sulfur host.102 As shown in Fig. 8F, polystyrene (PS) spheres first acted as the template with the sequential coating of amorphous TiO2 and PDA. After reductive annealing, TiO2 was transformed into conductive rock-salt TiO and PDA into an amorphous carbon layer. In this system, the PDA protective outer layer plays a crucial role in controlling the crystal phase and size of the inner TiO (the spherical morphology would collapse without PDA). Benefiting from the excellent conductivity and the strong polysulfide adsorption capability of the TiO@C shells, the TiO@C-HS cathode delivered a high discharge capacity of 1100 mA h g−1 at 0.1C and a stable cycle life up to 500 cycles at 0.5C with a small capacity decay rate of 0.08% per cycle.

4.3 SIBs

The natural abundance and low cost of sodium make SIBs feasible alternatives to LIBs. As an alkaline metal, sodium shows a similar electrochemical behavior to Li, but with a larger atomic radius (1.02 Å) than Li+ (0.76 Å). Therefore, conventional host materials of LIBs cannot hold true for SIBs and the development of suitable host materials to enable reversible Na+ insertion reactions is of great importance.15 Metal sulfides with high theoretical capacity and good electrical conductivity are promising anode candidates for SIBs. However, a structural collapse caused by a severe volume change during the de/sodiation process typically results in fast capacity decay, limited rate capability, and cycling instability. Considering the high theoretical capacity of pyrite (FeS2; 894 mA h g−1), Paik and Lou et al. developed a sulfidation-in-nanobox strategy to synthesize a unique FeS2@carbon (FeS2@C) yolk–shell nanostructure (Fig. 9A).103 Similar to the previous report,85 Fe3O4@C was first prepared and then transformed into yolk–shell FeS2@C. The resultant materials exhibited a high specific capacity owing to their FeS2 core, excellent conductivity owing to the PDA-derived carbon shells, and a large void space to accommodate the volume expansion (Fig. 9B and C). The yolk–shell FeS2@C exhibited a high specific capacity of 511 mA h g−1 at 100 mA g−1 after 100 cycles, a high capacity of 403 mA h g−1 at a current density of 5 A g−1, and an ultra-long cycle life with a capacity of 330 mA h g−1 after 800 cycles at 2 A g−1.
image file: c8ta05245j-f9.tif
Fig. 9 (A) Schematic illustration of the formation of FeS2@C yolk–shell nanoboxes. (B) SEM and (C) TEM images of FeS2@C-45. Reproduced with permission.103 Copyright (2017), Royal Society of Chemistry. (D) HAADF-STEM image of SnS2@C hollow nanospheres and (E) the corresponding elemental mapping image of Sn. (F) Schematic illustration of the synthesis of SnS2@CNSs. Reproduced with permission.105 Copyright (2018), Elsevier. (G) Schematic illustration of the formation of Sb@C coaxial nanotubes. (H) TEM and (I) HRTEM images of Sb@C-5. Reproduced with permission.107 Copyright (2016), Royal Society of Chemistry. (J) Preparation of the bifunctional PDA electrode/binder material. (K) Electrochemical performance of the O-PDA-2 electrode: discharge/charge profiles, rate performance and long-term cycling profiles. Reproduced with permission.111 Copyright (2016), John Wiley and Sons.

SnS2 has a high theoretical capacity of 847 mA h g−1 based on the stoichiometry of Na15Sn4, but suffers from a 420% volume expansion.104 To address this, Lou et al. synthesized SnS2 ultrathin nanosheets confined in the PDA-derived carbon nanotubes (SnS2@CNTs) from a MnOx nanorod template.105 To demonstrate the universality of this method, SnS2 confined in the carbon nanoboxes (SnS2@CNBs) and carbon nanospheres (SnS2@CNSs) were also synthesized by simply changing the template to Fe2O3 nanocubes and SiO2 nanospheres, respectively (Fig. 9D–F). Considering the rich redox reactions of bimetallic sulfides, PDA coated CoFe Prussian blue analogues were used in the synthesis of carbon-coated bimetallic sulfide hollow nanocubes (PBCS), in which the majority phase was Co8FeS8 distributed randomly with Co4S3, Co3S4 and Fe9S10.106 The optimum sample with a 4 nm thick carbon shell featured a higher surface area, smaller charge transfer resistance, and higher sodium diffusion coefficient than uncoated samples. A specific capacity of ∼500 mA h g−1 at 50 mA g−1 was observed with a good rate performance of 122.3 mA h g−1 at 5 A g−1.

Sb can afford a high theoretical capacity of 660 mA h g−1, but also suffers from a large volume expansion during sodiation. Paik et al. synthesized a unique material of Sb@C coaxial nanotubes via the thermal reduction of PDA-coated Sb2S3 nanorods.107 In this strategy, shown in Fig. 9G, the PDA coating was initially carbonized and then reduced Sb2S3 into metallic Sb. Accompanied with the partial evaporation of Sb, the final Sb@C coaxial nanotube product was obtained (Fig. 9H and I). The Sb content was tuned by adjusting the annealing time, of which, the optimized material with an annealing time of 5 min delivered a high specific capacity of 407 mA h g−1 at 100 mA g−1 after 240 cycles. Furthermore, a stable capacity of 240 mA h g−1 was retained at 1.0 A g−1 even after 2000 cycles.

Organic electrode materials have generated significant attention due to their high capacity, environmental friendliness, and abundance.108 As a polymer, PDA displays a catechol/o-benzoquinone redox couple, in which the oxygen atom in o-benzoquinone is suitably located for the coordination of lithium/sodium ions.109 Lee et al. loaded PDA on the surface of few-walled carbon nanotubes (FWNTs) to obtain flexible free-standing hybrid films via vacuum filtration, where the percentage of carbonyl species (quinone group) is as high as 26%. The hybrid cathode exhibited gravimetric capacities of ∼133 mA h g−1 in LIBs and ∼109 mA h g−1 in SIBs.110 In this system, PDA can stepwise bind multiple Li+ or Na+ ions, thus providing large charge storage capabilities while FWNTs contribute good conductivity and some specific capacity. Apparently, the higher o-benzoquinone content in PDA can provide more redox-active sites and thus a higher capacity, but an excessive oxidation or reduction compromises the electronic conductivity. In this regard, Zhang et al. synthesized pure PDA microspheres as advanced anode materials for LIBs and SIBs through the rational manipulation of the oxidant ((NH4)2S2O8) and heat treatment parameters (Fig. 9J).111 As shown in Fig. 9K, the PDA microspheres with 78.8% quinone content exhibited excellent electrochemical performance including a high specific capacity (1818 mA h g−1) and an excellent cycling stability (1414 mA h g−1 after 580 cycles with a capacity retention of 93% under 500 mA g−1 for LIBs and 500 mA h g−1 after 1024 cycles with a capacity retention of 100% under 50 mA g−1 for SIBs). In this system, the partially oxidized PDA created the optimal proportion of redox-active carbonyl groups and offered a large distribution of binding sites for sodium ions with the aromatic amine functional groups.112 Additionally, it is noteworthy to add that this kind of PDA electrode can simultaneously serve as a binder material due to the strong adhesion properties of PDA, avoiding the use of an insulating and inactive polymer binder.

5. Conclusions and perspectives

The componential tunability of PDA can provide abundant physicochemical properties. With the assistance of PDA, various heteroatoms can be effectively introduced into carbon materials as high-performance metal-free electrocatalysts. Due to the strong chelation between PDA and metals, different metal–carbon/nitrogen composites are also possible by converting metal–PDA intermediates. These composites are preferable for broader electrocatalysis due to the synergistic effects between the metal and heteroatom. Meanwhile, the abundant structural adjustability endows the PDA-derived active materials with optimized utilization efficiency. Further, the highlighted application of PDA can also be extended to the simple synthesis of single-atom metal materials covering transition metals (Fe, Co, Ni, Cu, and Mn), noble metals (Pd, Pt) and even bimetals (FeCo, FeNi). Additionally, the excellent hydrophilicity of PDA has been widely employed for the surface modification of nanomaterials, e.g. anodes and separators of LIBs. Of particular note, PDA-derived carbon coatings have unrivaled advantages given the thickness-controlled deposition of PDA onto the surface of any solid material. As a matter of fact, ultrathin carbon shells can preserve a catalyst's morphological stability without any hindrance to its intrinsic catalytic performance. Importantly, the PDA-derived carbon coatings can be considered a general strategy to buffer the volumetric expansion of electrode materials during charge/discharge cycling.

Despite the wide application of PDA in electrocatalysis and battery applications, its precise structure is still not fully understood. The debate is primarily centered on whether the monomer of PDA is held together by noncovalent interactions (hydrogen bonding or π stacking) or covalent C–C bonds. Moreover, the structure of PDA varies largely with reaction conditions, and significantly affects the properties and functions of PDA. Some evidence demonstrates that the PDA polymer can serve as an advanced functional material, but its structural regulation and underlying mechanism need addressing. Meanwhile, despite the ability to tune the thickness of PDA, PDA films can only reach a maximal thickness limited to tens of nanometers. PDA tends to detach from its substrates at a large thickness, and may restrict further improvement as an organic electrode. Additionally, the PDA-based porous materials still employ mesoporous silica or complex surfactants as templates. Therefore, continual efforts dedicated to exploring more simple and effective methods are required.

Due to the robust adhesive abilities and abundant physicochemical properties of PDA, the extended application of PDA in multifunctional composite materials creates many opportunities in advanced energy fields. For example, polyaniline and polypyrrole are well-known polymers used in secondary batteries. When incorporated with PDA, the composite polymers possess synergistic abilities including adhesion, enhanced conductivity and charge storage capacity. Additionally, PDA is a versatile platform that can integrate with various MOFs, Prussian blues, and polyoxometalates, all of which can easily derive high-performance metallic or multi-metallic compounds for electrocatalysis and energy storage. The PDA-based materials can even be applied to other energy technologies including solar cells and supercapacitors, but are not detailed here in this review. Given the significant accomplishments and opportunities afforded by the PDA-based materials, we believe that they will be instrumental for advancing different energy-related technologies in the future.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (21601078), Natural Science Foundation of Shandong Province (ZR2016BQ21) and Doctoral Program of Liaocheng University (318051608). The authors also thank the Australian Research Council (ARC) through its Discovery Projects of DP170104464 and DE160101163 for financial support.

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