Bio-inspired synthesis of nanomaterials and smart structures for electrochemical energy storage and conversion

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Bio-inspired synthesis of nanomaterials and smart structures for electrochemical energy storage and conversion Mei Ding a, Gen Chen b, c, *, Weichuan Xu c, Chuankun Jia a, **, Hongmei Luo c, *** a b c

College of Materials Science and Engineering, Changsha University of Science and Technology, Changsha, Hunan, 410114, PR China School of Materials Science and Engineering, Central South University, Changsha, Hunan, 410083, PR China Chemical and Materials Engineering Department, New Mexico State University, NM, 88003, United States

A B S T R A C T

Traditional synthetic methodologies are confronted with great challenges to fabricate sophisticated nanomaterials with delicate design, high efficiency and excellent sustainability. During past decade, bio-inspired synthesis has been extensively applied as an effective and efficient strategy for the fabrication of nanomaterials and nanostructures. Mimicking electrode materials at nanoscale either in the aspect of structure or functionality has been receiving surge interest because of their incomparable advantages and outperforming properties. In this review, we summarize the recent progresses on the bio-inspired synthesis of nanomaterials and smart structures in the field of energy storage and conversion. Firstly, an overall introduction of bio-inspired synthetic strategies will be presented, with focus on the biotemplates and bio-resources. Following that, a library of complex mimicking structures featured by high-order, hierarchical porosity, or bionic function are introduced, with discussion on their chemical and physical properties associated with the structure. The enhanced electrochemical properties such as energy density, cycling stability, etc. in different electrochemical systems will be also discussed. At last, we will expand the perspectives regarding the advantages and limitations of bioinspired strategy and possible solutions in the future.

1. Introduction Due to the fast consumption of fossil fuels and ever-growing environmental pressure, electrochemical energy storage and conversion systems have drawn increasing attention due to their sustainable characteristics such as renewability, high efficiency, long lifespan and lowcost [1–8]. However, their further development is strongly depended on the advancement of electrode materials featured by environmental sustainability, smart structure and intelligent function. Traditional synthetic methodologies are confronted with great challenges to fabricate delicate nanostructured electrode materials [9]. Therefore, the idea of bioinspired synthesis to overcome these barriers is undoubtedly promising and feasible due to the vast diversity of nature. Bioinspiration associates with the direct or indirect mimicry with a broader meaning, which is frequently substituted by “biomimetic” one another [10]. Dating back history, human have gradually evolved from the earliest direct use of natural biological materials to the upgraded application of bionic technology to improve or even develop new materials [11–14]. In the past decade, novel biomimicking materials have been investigated intensively and extensively, with the inspiration from nature [15–18]. An increasing number of scientists are seeking inspiration from the nature

that we are familiar with for synthesizing new materials. Ingenious combinations and arrangements from natural materials have been created with the development of high-resolution characterization techniques [19–27]. The discovery of novel bio-inspired structures has driven the research of biomimetic composites. In the specific field of electrochemical energy storage and conversion, bio-inspired synthesis of nanomaterials and smart structures have achieved many unprecedented results [10,28–35]. As summarized in Fig. 1, the efforts devoted into this area can be particularly generalized as the following categories. (1) Bio-inspired synthetic strategies. The conventional methodologies developed for the preparation of electrode nanomaterials usually suffer from high-cost, low efficiency and environment pollution. By taking advantages of the naturally abundant resources such as various biomass, many interesting carbon materials can be derived through clean processes and on a large scale [8,16,18,21,34,36–38]. In addition, researchers are now seeking opportunities from bio-templates such as bacteria, viruses, DNA, proteins, to name a few [39–47]. These methods can introduce favorable complexity into the products at nanoscale or atomic scale, which can remarkably improve the electrochemical performances. (2) Artificial bio-structures. Biological systems have been evolving through billions of years. Animals or plants are survived by

* Corresponding author. School of Materials Science and Engineering, Central South University, Changsha, Hunan, 410083, PR China. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (G. Chen), [email protected] (C. Jia), [email protected] (H. Luo). https://doi.org/10.1016/j.nanoms.2019.09.011 Available online xxxx 2589-9651/© 2019 Chongqing University. Production and hosting by Elsevier B.V. on behalf of KeAi. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Please cite this article as: M. Ding et al., Bio-inspired synthesis of nanomaterials and smart structures for electrochemical energy storage and conversion, Nano Materials Science , https://doi.org/10.1016/j.nanoms.2019.09.011

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Fig. 1. Scheme of the bio-inspired synthesis of nanomaterials and smart structures for electrochemical energy storage and conversion from biological nature with featured examples and advantages.

perfecting their structure and maximizing the performance in the environment, for examples, the sophisticated hierarchical structure of trees [48–51], the Bouligand structure of crab shell [52], and the highly ordered pores within butterfly wings [53,54], etc. These inspirations can be widely applied in the structural design of electrode materials with optimal properties and greatly extend our research scope. (3) Mimicking biofunction. Electrode materials frequently suffer from mechanical damage during cycling. Therefore, self-healing chemistry that mimics the function of skin has been applied to the fabrication of electrode with significantly enhanced stability [15,20,55–58]. Other functionalities inspired by leaves with complex venation or lung with abundant respiratory alveoli, etc. also display great capability of producing exciting electrode materials [17,59–61]. The synthesis of novel electrode materials for electrochemical energy storage and conversion can be deeply connected to the inspirations of nature, which provides more flexible and intelligent options. In this review, we put special emphasis on the recent progresses in this emerging field of bio-inspired synthesis of nanomaterials and smart structures for electrochemical energy storage and conversion. Firstly, we introduce the synthetic strategies associated with bio-resources, templates. Then various bio-mimicking electrode nanomaterials and nanostructures featured by different order, porosity, cross-linking network, and bionic function will be discussed. The corresponding chemical and physical properties rising from the special structure are also discussed.

complex hierarchical structure, which offers some meaningful strategic relationship such as the root and trunk, trunk and branch, branch and leaf. Nanostructures mimicking trees are developed and the bio-inspired synthesis can introduce favorable complexity into the products at nanoscale or atomic scale, which can remarkably facilitate the electrochemical performances. 2.1. Bio-resources Carbon-based electrode materials have been intensively and extensively investigated either for lithium/sodium ion batteries or electrochemical catalysis due to versatile structures [63–66]. Among them, the biomass derived carbon materials have been demonstrated with excellent electrochemical properties [13,16,18,37,67–69]. Moreover, these carbon materials are generally of high-sustainability, low-cost and environmental-friendliness, and can be produced from a variety of biomass such as peanut shells and roots [70–72], bamboo [73,74], sunflower seed [75,76], coconut shell [77,78], cellulose of different trees [18,79,80], etc. Particularly, considering the complexity of natural plants, additional merits can be expected, e.g. heteroatom doping of N, P, S and diversity of morphologies [81–84]. Ding and co-workers firstly reported a hybrid sodium ion capacitor with the active materials derived entirely from peanut shells [71], which are a green and highly economical waste globally generated at large scale. Treated by different processes as presented in Fig. 2, a hierarchically porous architecture and sheet-like morphology can be achieved through a hydrothermal treatment followed by KOH activation. The oxygen content that affects the pseudocapacitive behavior can be regulated at the same time. An ordered carbon with low surface area was derived by direct carbonization with mild activation in air. As-obtained carbons from single peanut shell precursor were applied as cathode and anode material respectively due to their controlled structure and surface area, which provide different ion storage mechanism. The capacitor delivered the most promising energy–power-cycling stability for either Na or Li ion hybrid systems. Liang et al. reported a highly active nitrogen-doped carbon nanofiber as electrocatalyst from direct pyrolysis of a low-cost, green, and scalable biomass of bacterial cellulose [41]. As shown in Fig. 3, the 3D carbon nanofibrous network is fabricated by direct pyrolysis of dried bacterial cellulose aerogels at 800
C under flowing N2 to generate black carbon nanofiber aerogels, followed by a second heat-treatment under an NH3 atmosphere. The structure inherits the three-dimensional nanofibrous network of bacterial cellulose and possesses high density of N-containing active sites (N content, 5.8 at%) as well as high BET surface area

2. Bio-inspired synthetic strategies To build sophisticated functional materials for energy related applications, multiple-steps or tedious experimental design may be required. Taking advantages of the biological raw materials directly or indirectly may facilitate the synthesis by jumping over the complex experimental control [11–14,17,18,36,51,62]. This can be regarded as bio-inspiration. For example, a typical plant leaf that offers natural hierarchical structure can be treated as a composite consisting of three fibers: the midrib corresponds to the central main fiber in the blade; the secondary vein is connected in a straight line parallel to the central main fiber; another set of tertiary fibers arranged in parallel is connected. These fibers are embedded in a matrix material to form a composite. Once the leaf falls and decomposes, the skeleton will be gradually revealed, which we are very familiar with. From a mechanical point of view, these venous fibers can reinforce the composite significantly, which therefore has become an important template for material structure design. The network or arrangement is generally superb, and maximum performances are correspondingly achieved. Another example is the tree, consisting of 2

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Fig. 2. Schematic illustration of the synthesis of different carbon materials from single peanut shell precursor employed for cathode/anode with different charge storage mechanisms in a sodium ion capacitor. Copyright 2015. Reproduced with permission from the Royal Society of Chemistry.

Fig. 3. Synthesis and morphologies of N–CNF aerogels. (a) Schematic diagram of the synthetic steps. (b,c) SEM and TEM images of N–CNF aerogels, respectively, showing the nanofibrous network structure. (d) HRTEM image of an individual N–CNF. Copyright 2015. Reproduced with permission from Elsevier Ltd.

(916 m2 g1). Other micro- and mesoporous carbon materials interconnected through macropores with high surface area and hierarchical structure can be obtained from chemical activation of biomass, such as foam-like activated carbon, sulfur and nitrogen self-doped carbon nanosheets, etc. Biomass of cellulose and alginate are widely and exclusively used as resources for the production of electrode materials, as well as separator and binder within the devices, indicating its high diversity [18,85].

biomass is not necessarily required for direct utilization. Instead, templated-synthesis based on their biostructure is highly motivated and promising, which greatly extends the scope of bio-inspiration [52, 86–89]. As mentioned above, the three fibers of a leaf and their structures work together to enhance the stability of the blade. To mimic the arrangement of the fibers or other bio-templates may produce materials that meet the needs of mechanical strength or other properties. Crab shell with Bouligand structure can be used as carbon polymer template for battery electrodes [52]. As shown in Fig. 4, crab exoskeleton consists of solid calcium carbonate with nano-channels with average diameter of 60 nm parallel to each other in a horizontal plane piled in a twist pattern, and chitin protein nanofibers, which can be removed by thermal treatment. Followed by dopamine self-polymerization, active

2.2. Bio-templates The electrode materials generated from biomass are mainly carbonbased, with limited tunability of the compositions. Therefore, the 3

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Fig. 4. Structural model and SEM images of crab shell templates. (a) Structural model of twisted plywood nanochannel arrays in crab shell templates showing the hollow channels created by removing organic nanofibers in crab shells. The hollow channels are arranged parallel to each other to form horizontal planes stacked in a helicoid fashion, creating a twisted plywood structure. (b–e) SEM images of the bio-templates from a Chinese hairy crab shell, a blue crab, a stone crab, and a Dungeness crab, respectively, demonstrating the universal nature of the nanochannel arrays in crab shell templates. Copyright 2013. Reproduced with permission from the American Chemical Society.

carbon species within composites were cleared if calcined in air. The photonic hierarchical structure provides equal diffusion distance for Li ion transport due to the highly ordered arrangement and simultaneously provides buffering room for volume expansion during charge-discharge process. Carbon residue can improve the electrical conductivity and further prevent SnO2 aggregation. Besides, effective electrolyte flow was enabled because of the periodic structure. Comparison was made when Papilioparis butterfly wings, a honeycomb-like template without photonic hierarchical structure (Papilioparis-C-SnO2), delivering less stable electrochemical performance. Small templates of microalgae, bacteria, protein, DNA, etc. are also employed to fabricate electrode materials with interesting structure. For instances, bacteria were applied as template for producing hierarchical carbon materials for high-performance supercapacitor. Kim et al. reported a high capacity LIB anode consisting of the 1D NiO nanostructure by employing DNA template through a well-established metallization route [40]. As a more advanced technique, gene modification can intrinsically change the virus, making them capable of loading active electrode materials and electronic conductive percolating network. Lee

materials (sulfur or silicon) are loaded into the channels by thermal infusion or chemical vapor deposition. Finally, the CaCO3 is dissolved with HCl and polymer is annealed to form carbon. The plywood-like stacked nano-channels is suitable to store cathode or anode materials, leading to reduced solid electrolyte interphase (SEI) layer formation during cycles, less dissolution of active materials, rapid lithium ion transfer along the nano-channels, well accommodated volume change during cycles and other benefits. It is always meaningful and promising to transform bio-waste into high-value products. The synthetic strategy by using bio-waste as templates can be regarded as “trash to treasure” approach as Deng et al. reported [86]. They used eggshells as reactor and template for the synthesis of Co9S8 anode in LIBs, which can be also extended to different kinds of materials on various substrates. Similarly, Li et al. applied a butterfly wing template to fabricate a carbon/SnO2 composite with photonic hierarchical nanostructure as anode materials for LIBs [53], as displayed in Fig. 5. The typical synthesis process includes pretreatment of Euploeamulciber butterfly wing, impregnation of Sn precursor, calcination of as-prepared material. The product calcined in N2 was Euploeamulciber-carbon-SnO2; while the

Fig. 5. (a) Synthesis process of a bio-inspired carbon/SnO2 composite; the corresponding SEM and cross-section images (inset) of (b) Euploeamulciber-C-SnO2, (c) Papilioparis-C-SnO2. Copyright 2015. Reproduced with permission from the American Chemical Society. 4

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excellent ventilation capability, multiple diffusion paths of electrolyte, rapid reversible phase transition of the active materials, and swift pseudocapacitive lithium storage behaviors at high charging/discharging rates and ultrastable cycling performance can be simultaneously achieved for the electrodes of rechargeable batteries. Honeycomb structures offer high mechanical strength, while honeycomb-lantern inspired structures display interesting structural flexibility. For instance, a novel honeycomb lantern inspired 3D stretchable supercapacitor was fabricated based on an expandable honeycomb composite electrode composed of polypyrrole/black-phosphorous oxide electrodeposited on carbon nanotube film [96]. The 3D stretchable supercapacitors display device-thickness-independent ion-transport path. The stretchability was remarkably enhanced comparing with the 2D counterpart, which facilitated the integrability of wearable devices. Principally, the topological structure of honeycomb can be engineered to meet the specific requirements of morphology or relative density of the solid. The shape of unit cell is also feasible to mitigate the stress from different direction.

and co-workers equipped viruses with different peptide groups by manipulating two genes [90]. As schematically illustrated in Fig. 6, one-gene was modified for FePO4 materials loading. Amorphous FePO4 was fused on viral protein, where tetraglutamate were introduced to form carboxylic acid groups for materials growth along with improved ions interaction. The other one was adapted to improve graphene binding affinity. After modification, single wall carbon nanotubes show good connection to virus templates and make multiple contact to each other, forming a network and increasing overall electronic conductivity. This environmentally benign low-temperature biological scaffold provides interesting insights into the fabrication of hybrid electrode materials with combined advantages. 3. Artificial bio-structures Biological systems have been evolving through billions of years. Animals or plants are survived by perfecting their structure and maximizing the performance in their environment. They offer ingenious and artful solutions to many scientific and engineering problems. For example, common natural nacreous materials, such as nacre and enamel, usually contain a high proportion of minerals. But they often exhibit superior mechanical properties, durability and toughness far beyond their mineral content. Many engineers have been working to mimic these highly mineralized bio-composites and apply them to our buildings. Another example is the spider silk, which is exceptionally strong with both high mechanical strength and elasticity. It is capable to adapt massive kinetic energy before breaking. Scientists are therefore trying to figure out the polymeric structure of spider silks, which may boost the development of high-performance biomimetic fibers. In terms of electrochemical applications, the electrode materials and binders are expected with better performance. Mimicking bio-structure to artificially build more efficient microstructure is attracting, regardless of the specific compositions. In the last decade, numerous and distinctive nanostructures inspired by nature have been developed.

3.2. Pomegranate structure and associated Bio-inspired pomegranate structure is a typically 3D structure, generally involving multiple components. Rational design of pomegranate structure has been widely applied in rechargeable batteries with high energy density. Specifically, the anode materials based on alloy or conversion mechanism deliver high theoretical capacity, while suffer from large volume change during electrochemical reactions. The stability of electrode is significantly compromised because the electrode will crack and drop off the current collector due to large volume change. Fortunately, pomegranate structure facilitates the dispersion of active materials and offers controllable void space to accommodate the volume expansion [62,100–107]. For instance, Cui group developed pomegranate-like silicon-based anode materials to solve the stability issues of silicon-based anode materials in LIBs [107]. The pomegranate design displays two interdependent characteristics that ensure enhanced battery performance: (1) internally accommodated volume expansion, which retains the structural integrity of the secondary particles and stabilizes the SEI on the surface; (2) spatially confined SEI formation that reduces SEI quantity and retains the void space. The pomegranate-inspired nanoscale design can be regarded as an assembly of yolk-shell structure they previously reported [108]. Following that, many graphene or carbon caged silicon particles were created and the empty space can be engineered as well [109–111]. Recently, Yang et al. have firstly constructed an intriguing pomegranate-like microflower (PM-NixSy) confining core–shell binary nickel sulfide (b-NiS/Ni3S4) nanobeads [105]. As displayed in Fig. 8, the prepared graphene oxide (GO) solution was sonicated with nickel acetate (NiAc2) and the mixture was pumped into a spray drying apparatus and dried in the cylinder. Graphene flowers confining Ni atoms (Ni@G) were obtained through a

3.1. Honeycomb structure Honeycomb structure, consisting of hexagonal channels and very thin vertical walls, is an important and representative biostructure with superior mechanical stability [15,54,85]. Moreover, owing to its unique porous structure featured by high-order, it has been widely applied in energy related applications such as solar cell, electrochemical storage and conversion, etc. [91–98] Mei et al. prepared heterogeneous bimetallic Co–Mo oxide (CoMoOx) nanoarchitectures assembled from 2D nanounits via a molybdenum-mediated self-assembly strategy as shown in Fig. 7 [99]. The composites display well-maintained honeycomb-like structure. By taking the advantages of structural stability, highly-aligned macrochannels, and interconnected open channels/porous structures for

Fig. 6. Biological toolkits: genetic engineering and biomolecular recognition. (a) A schematic presentation of the multifunctional M13 virus. The gene VIII protein (pVIII), a major capsid protein of the virus, is modified to serve as a template for a-FePO4 growth, and the gene III protein (pIII) is further engineered to have a binding affinity for SWNTs. (b) A schematic diagram for fabricating genetically engineered highpower lithium-ion battery cathodes using multifunctional viruses (two-gene system) and a photograph of the battery used to power a green LED. Copyright 2009. Reprinted with permission from AAAS.

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Fig. 7. Configuration of natural honeycomb and honeycomb-inspired CoMoOx nanostructures. (a) Photograph of a natural honeycomb and the inspired real-world construction with excellent structural stability and ventilation capacity, (b) schematic illustration of self-assembly of honeycomb inspired CoMoOx nanostructure, (c) scanning electron microscope (SEM) observation of as-synthesized honeycomb-inspired CoMoOx microspheres, (d) high magnification of the bioinspired cellular nanostructures, (e) SEM image of the calcined bio-inspired CoMoOx microspheres, and (f) enlarged bio-inspired cellular nanostructure after calcination. Copyright 2019. Reproduced with permission from WILEY-VCH Verlag GmbH & Co., KGaA.

The silicon@N, O-doped carbon hierarchical porous structure (Si@mNOC) sample was obtained after heat-treatment under N2 and HF etching. The structure induces additional capacitive charge storage, which works together with the diffusion-controlled Li-storage and shows synergistic effects. Si@mNOC therefore delivers boosting lithium storage properties, especially when cycled at high current density. This strategy represents an attractive method to address the problems of Si nanoparticles as anodes.

subsequent calcination in an Ar atmosphere. Finally, the Ni@G was subjected to a direct sulfidation and in situ formed PM-NixSy. The graphene with flower-like framework presents a typical crumpled and rippled morphology, which may prevent irreversible aggregation and restacking of graphene nanosheets. It is noteworthy that each pomegranate-like PM-NixSy microflower is cross-linked with graphene nanosheets, wherein graphene facilitates the electron transport and ensures that all NixSy nanobeads are interconnected and linked, thus preserving obviously enhanced structural integrity. Scientists have also developed some interesting bio-inspired structures that mimicking passion fruit, watermelon, custard-apple, frog egg, etc. for silicon-based anode materials [102,106,112–122]. Essentially, these bio-inspired structures share the similar advantages of pomegranate structure. For example, Xu et al. reported the ration design of hierarchically porous custard-apple-like structure to boost the lithium storage of Si@carbon [106]. As shown in Fig. 9, the SiO2 coating layer wrapping commercial silicon nanoparticles was synthesized by the addition of ethanol, ammonia and TEOS. Then the Si@SiO2@PANI sample was synthesized through the addition of HCl, aniline and APS.

3.3. Bio-inspired fiberous structure and associated As above-mentioned, a typical leaf consists of three fibers, forming an interconnected and strong framework. Hierarchical bio-inspired fiberous structure has been intensively investigated for various applications [24, 26,27,41,87,89,123–133]. Modification of the interaction between neighboring fibers can effectively tune the structure from rigid to flexible. Natural fibers such as cellulose and lignin in the wood or other plants can be directly used as bio-sources or inspire the synthesis of similar structural products [12,134,135]. Taking natural bamboo as an 6

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example, its geometry consists of mutually reinforced node and culm along the one-dimensional axis, leading to a light-weight structure [73, 74]. Due to the graded structure, bamboo offers excellent mechanical durability and good flexibility simultaneously, which makes them widely applied in our daily life. The structural advantages of bamboo can be readily transplanted to the scientific area. Sun and co-workers fabricated carbon nanofibers that mimicking the natural bamboo with balanced combination of macropores, mesopores and micropores via an electrospinning strategy [74]. As shown in Fig. 10, the special structure was fabricated by etching SiO2 with HF aqueous solution from nanofiber web, which was an electrospun product from polyacrylonitrile (PAN) and tetraethyl orthosilicate (TEOS) in dimethylformamide (DME) after thermal treatment in forming gas; resulting in hollow interiors with graded nanopores across the cross section. The hierarchical pores rendered a specific surface area of ~1912 m2 g1 and pore volume of ~2.27 cm3 g1 with optimized thermal treatment. Due to the bamboo-like shape, as-obtained carbon nanofibers are flexible, foldable and durable, which is fundamental to wearable devices and other applications. It is well-known that the bones of animal are mechanically durable and strong due to the sophisticated hierarchical structure consisting of fibril array, inorganic padding component, etc. [19,43,136–138] The inorganic padding species make the bone strong but brittle; while the well-patterned fibril arrays enable the tenacity by forming a light-weight scaffold. On this basis, many kinds of fiberous scaffold have been developed [139–147]. Bone inspired artificial fiberous scaffold can be either solely used or as container for holding active materials. Kn€ oller et al. reported a free-drying strategy to synthesize ultralight and hierarchically structured scaffolds with V2O5 nanofibers (Fig. 11) [146]. In a typical process, aqueous V2O5 suspensions was instantly frozen using liquid nitrogen. The building blocks are hydrated V2O5 nanofibers with a rectangular shape and lengths of several micrometers. Similar with the microstructure of bone, ice crystal platelets play an important role as the padding components in the scaffold, namely structural templates during the freezing. The following freeze-drying leads to the formation of self-supporting scaffolds with a water content of 13.47 wt%. The scaffolds were subsequently annealed at 350
C in air to remove the residue water and facilitate the evolution of crystal structure and morphology. Specifically, Fig. 10c and d indicates that the scaffolds of V2O5 nanofibers display a lamellar microstructure and the slab are interconnected by V2O5 nanofibers. As-prepared scaffolds exhibit macroscopically remarkable dynamic mechanical performance, which may be potentially applied as flexible electrode.

Fig. 8. (a) Schematic preparation process of PM-NixSy microflowers. (b) FESEM image and (c) TEM image. Copyright 2019. Reproduced with permission from the Royal Society of Chemistry.

3.4. Other structures It goes without saying that nature is an inexhaustible treasury, which provides interesting model or templates far beyond the aforementioned structures. Researchers have developed other novel bio-inspired structures such as flower-like, lung branching-like, venus flytrap-like, etc., in order to advance the performances of electrode materials [61,62,119, 134,148–150]. For instance, Xiong et al. designed a bio-inspired leaves-on-branchlet structure consisting of carbon nanotube arrays (CNT) serving as branchlets and graphene petals (GP) as leaves [50]. All-carbon CNT/GP micro-conduits were fabricated by a two-step process within an MPCVD chamber. Firstly, commercial plain carbon cloth was used directly as the substrate for CNT micro-conduit synthesis. Subsequently, GPs were further grown on CNT micro-conduits. CNT arrays form in the shape of micro-conduits (outer diameter of 30–40 μm) from the pre-deposited tri-layer catalysts on the surface of carbon microfibers. The diameters of the nanotubes in the array range from 20 to 30 nm. As presented in Fig. 12, the electrode materials work in the device similarly with the behavior in nature that leaves significantly increase exposed tree surface area to absorb carbon dioxide (like ions) from the environments (like electrolyte) for photosynthesis. In addition, the micro-conduit structures with hollow channels increase accessible electrode surface area to the electrolyte and facilitate fast diffusion of ions during

Fig. 9. (a) Schematic illustration of the formation process for the interconnected Si@mNOC, with the inset of (a) showing custard apple fruit. In the process: (I) Si nanoparticles are coated with SiO2; (II) the Si@SiO2 particles and aniline are self-assembled into Si@SiO2@PANI; (III) After carbonization and HF etching, the hierarchical porous interconnected Si@N, O-dual-doped carbon is formed. SEM (b) and TEM images (c) of Si@mNOC. Copyright 2013. Reproduced with permission from Elsevier Ltd.

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Fig. 10. Fabrication and characterization of the bamboo-like carbon nanofibers. (a) Schematic of the fabrication process of the bamboo-like carbon nanofibers. (b,c,d) TEM images of the initial TEOS/PAN composite nanofibers (b), SiO2/carbon composite nanofibers (c) and bamboo-like carbon nanofibers (d). The circles in (c) show the existence of SiO2 particles in the core of the SiO2/carbon composite nanofibers. (e,f,g) SEM images (e,f) and HRTEM image (g) of the asprepared carbon nanofibers. Copyright 2015. Reproduced with permission from the American Chemical Society.

inspired structures is to understand the unique properties arising from the structures, and correlate the electrochemical performances with the structural characteristics. There may incur some paradox that researchers and engineers need to strike a balance between e.g. high energy density and light-weight, high energy efficiency and large surface area, ingenious structure and sophisticated synthesis, etc.

charge/discharge, enabling high rate capability and power delivery. The hierarchical all-carbon micro-conduit electrodes with hollow channels exhibit high areal capacitance of 2.35 F cm2 (~500 F g1 based on active material mass), high rate capability and outstanding cyclic stability (capacitance retention of ~95% over 10,000 cycles). Inspired by natural coral with a porous structure, Wang et al. developed a conducting polymer deposited monolith carbon material [151], featured by coral-like mesopore dominated structure (Fig. 13). The hierarchical robust porous structure in monolith carbon barebones guarantees stable cycling performance even at high sweep rate, and nearly-ideal electrical double layer capacitor behavior. Due to the high specific area in the polymer deposited monolith carbon, conducting polymer with redox groups show weight percentage from 5% to 44%, which is fundamental for high energy storage (up to 1488 F g1 at 1 A g1 when pyrrole was chosen). Along with fast electrons transfer ascribed of thin conducting polymer and carbon matrix, the rich mesopores inside the material also favored electrolyte diffusion, thus the power density went up to 12 kW kg1 with energy density of 49.5 Wh kg1. Such combination exceeds most polymer deposited carbon electrodes. The inspirations from nature can be widely applied in the structural design of electrode materials with optimal properties and greatly extend our research scope. However, a major challenge in the research of bio-

4. Mimicking biofunction The concept of biomimetic not only refers to the bio-inspired structure, but also takes the biofunctionality into consideration. For instance, scientists are now able to synthesize bone-like materials. However, the real challenges may be the biocompatibility of the materials when being implanted. Over the past decades, researchers have devoted tremendous effort into the synthetic materials that can replace soft or hard tissues in the human body, resulting in ideal organ transplants and repairs [56,118, 152,153]. Hydrogels and biomedical polymers with high water content have long been considered as potential replacements for the soft tissues, whereas it is difficult to strike a balance between the mechanical strength and water content. The principle of materials design that mimicking biofunctionality can be readily transplanted, and the accomplishments are also intriguing in the preparation of novel electrode materials or

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Fig. 11. From single V2O5 nanofibers to highly ordered all-ceramic scaffolds. (a) Schematic depiction of scaffold formation. V2O5 nanofibers are randomly oriented in an aqueous suspension. During instant freezing with liquid nitrogen, lamellar ice crystals grow inside the suspension, compacting the nanofibers in between. This structural arrangement is preserved after freezedrying and thermal treatment, leading to highly organized, self-supporting V2O5 scaffolds. (b) Photograph of a cylindrical V2O5 scaffold with a diameter and height of ~8 mm. SEM images of (c) the scaffold's lamellar microstructure and (d) the filigree nanofiber assembly forming the nanometer thick lamellas and pillars. Copyright 2018. Reproduced with permission from the American Chemical Society.

Fig. 12. Structural characterization of CNT/GP micro-conduits. (a) Schematic illustration of CNT/GP micro-conduits in a leaves-on-branchlet nanostructure on CC substrates for highperformance supercapacitor electrodes (Note that the yellow shaded areas in the schematic indicate the selected areas to be magnified). (b) Bare CC substrate at low magnification (inset shows the surface of a single carbon fiber). (c) Uniform coverage of CNT micro-conduits on carbon fibers at low magnification. (d) A close-up of CNT micro-conduits on a carbon microfiber. (e) A CNT/GP micro-conduit in a heart shape. (f) A single CNT decorated with many GPs at high magnification (inset shows GPs on CNT microconduit array walls). Scale bars: b 500 μm (inset: 3 μm), c 300 μm, d 10 μm, e 20 μm, f 300 nm (inset: 2 μm). Copyright 2018. Reproduced with permission from the Nature Publishing Group.

electrochemical reactions. In recent years, the artificial self-healing materials have attracted intensive attention due to their capability of fully or partially recover a functionality after damage. Wang et al. applied self-healing chemistry to silicon microparticle (SiMP) anodes to extend remarkably their cycling lifespan [57]. In their research, SiMPs anodes coated with a self-healing polymer deliver good cycle life, which was

smart devices for electrochemical storage and conversion. One typical example of biofunction is “self-healing”, which refers the process of recovery. It is a significant characteristic in biology because it increases the survival probability and lifetime of most living creatures [56–58,154]. This feature is also very popular in the design of electrode materials that may suffer from mechanical degradation upon 9

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Fig. 13. (a) Optical image of the monoliths of silica template and the as-prepared carbon materials. (b and c) SEM images of silica template and carbon materials. (d) Optical image of coral. Copyright 2013. Reproduced with permission from the Royal Society of Chemistry. Fig. 14. Schematic of healing mechanism for SMiPs. Top, simplified schematic of coagulation cascade toward formation of thrombus (blood clot) and fibrinolysis of thrombus into soluble fibrin fragments, which are modulated by two trypsins: thrombin and plasmin; bottom, schematic of deposition/dissolution of micrometer-sized sulfur/Li2S particles with limited electrical contact, hindering full electrochemical utilization. To enable the stable and reversible utilization, a complementary route is introduced, through chemical reactions between an extrinsic healing agent in solution and insoluble sulfur compounds. Copyright 2017. Reproduced with permission from the American Chemical Society.

Fig. 15. Schematic illustrations of the biomimetic ionic channels in MOFs. (a) A Naþ-ions channel in biological systems with negatively charged glutamate ions. (b) Structure of HKUST-1 made from copper nodes (blue) and BTC ligands (black) with pore channels of 1.1 nm. (c) A schematic showing the formation of biomimetic ionic channels in HKUST-1 with ClO 4 anions bound to the OMSs and solvated Liþ ions in the channels with high conductivity (copper: blue; carbon: black; oxygen: red). (d) Schematic of biomimetic ionic channels in an MOF scaffold (dark gray) with bound ClO 4 ions (cyan dots), enabling fast transport of solvated Liþ ions (purple dots). Copyright 2018. Reproduced with permission from WILEY-VCH Verlag GmbH & Co., KGaA. 10

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Fig. 16. The schematic of the structure and the fabrication process of a spine-like battery. (a) Schematic illustration of bio-inspired design, the vertebrae corresponds to thick stacks of electrodes and soft marrow corresponds to unwound part interconnects all the stacks. (b) The process to fabricate the spine-like battery, multilayers of electrodes were first cut into designed shape, then strips extending out were wound around the backbone to form spinelike structure. Copyright 2018. Reproduced with permission from WILEY-VCH Verlag GmbH & Co., KGaA.

previously impossible with deep galvanostatic cycling. Cracks and damage of the coating layer can be healed spontaneously by the randomly branched hydrogen-bonding. When it turns to the next-generation secondary batteries within which high-capacity electrode materials are utilized, self-healing may play an increasingly important role in stabilizing these electrodes. For instance, lithium sulfur battery is promising because the sulfur cathode delivers a high theoretical capacity of 1672 mAh g1. However, the soluble intermediate of polysulfides and uncontrolled phase transfer will lead to the poor cycling stability. To solve this problem, Peng and co-workers introduced an extrinsic healing agent of polysulfide to stabilize sulfur microparticle (SMiP) cathodes by mimicking a biological self-healing process of fibrinolysis [155], which is quite different from the healing mechanism of skin. As schematically illustrated in Fig. 14, the undesirable deposition and accumulation of deactivated solid products can be analogous to coagulation of thrombus that hinders flow of blood within healthy vessels. Therefore, polysulfides were intentionally added to transfer the solid deposits into soluble species, mimicking the function of enzyme (plasmin) to solubilize the thrombus. The sulfur dissolved can be used again in the following electrochemical reactions. More importantly, the polysulfides are capable of self-regeneration, which enables the smooth running of Li–S batteries. As a result, an optimized capacity (~3.7 mAh cm2) with almost no decay after 2000 cycles at a high sulfur loading of 5.7 mg cm2 can be finally achieved. The comprehensive understanding of the bio-inspired self-healing process provides further insights into the design of novel healing agents. Another intriguing biofunctional analogy in biological systems are the ionic channels, which are gated aqueous pores whose conformational changes are driven by the electric field in the membrane [156,157]. Diffusion of ions can be very fast through the open channels, but involves many interactions of ions, pore, and solvent that lead to ionic selectivity, saturation, block, and flux coupling. The ionic selectivity of membrane is extremely important in the design of battery chemistry [158–162]. As illustrated in Fig. 15a, a representative structure of Naþ channels consists of α-helix domains folded from glutamic-acid-rich peptide chains, within which the carboxylic residues are deprotonated under the physiological environment. Hence, negatively charged glutamate ions (CH2CH2COO) are formed along the channels, enabling the selectivity of cations [163]. Recently, Shen and co-workers have developed a novel

Fig. 17. Schematic illustrations comparing Li–O2 battery with the human eye. The analogous situations of both systems suggest an approach of using pD, a synthetic melanin, as a superoxide radical scavenger. Copyright 2014. Reproduced with permission from the American Chemical Society.

solid-state electrolyte by mimicking the biofunction of ionic channels within membrane [164,165]. In their case, HKUST-1, one of the well-studied metal–organic frameworks constructed by Cu (II) paddle wheels and benzene-1,3,5-tricarboxylate (BTC) ligands, was investigated [166]. HKUST-1 has 3D pore channels with a pore diameter of ~1.1 nm (Fig. 15b), containing coordinated solvent molecules along the channels. The chemical environment of the ion channels can be regulated by complexing the anions of an electrolyte to the open metal sites of HKUST-1 (Fig. 15c and d). The MOF scaffolds can be herein transformed into ionic-channel analogs with capability of Liþ conduction and low activation energy. Mimicking biofunctions can be also realized through simple engineering process. For instance, flexible and wearable electrochemical storage devices are attracting more research interest due to the fast development of flexible electronics [8,15,118,126,138,167–170]. They 11

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Fig. 18. Electrochemical performances. CV curves of (a) a free-standing carbon nanofiber electrode using a conventional three-electrode configuration and (b) a typical all-solid-state flexible supercapacitor at different scan rate. (c) Electrochemical behaviors of the as-made allsolid-state supercapacitor under different mechanical deformation conditions. Digital images of the flexible device bended by (1) 0
and (2) 90
, and twisted by (3) 90
and (4) 180
; (d) the corresponding CV curves and capacitance retention tested at 100 mV s1. Copyright 2015. Reproduced with permission from the American Chemical Society.

Fig. 19. The electrochemical performance of the spine-like battery in different stress conditions. (a) Charge/discharge cycling test of the spine-like battery in different configurations at 0.2 C (28 mA g1). (b) Optical images of the spine battery in the state of flat, flexed, and twisted. Copyright 2018. Reproduced with permission from WILEY-VCH Verlag GmbH & Co., KGaA.

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Fig. 20. Electrochemical performance of LPC@MOF electrolyte and prototype lithium-based batteries. (a) Photograph of an LPC@UiO-67/PTFE membrane (LPC@UM) next to a coin cell (inset shows a bent LPC@UM). (b) SEM images of LPC@UM (top-left: cross-sectional view). (c) Current–time profile for LijLPC@UMjLi cell at 20 mV of polarization (inset: impedance spectra at initial and steady states). (d) DC miropolarization of LijLPC@UMjLi cell from 2.5 to 50 μA cm2. (e) Li symmetric cell test comparison between LPC@UM and LPC at a current density of 0.125 mA cm2 (0.25 mAh cm2). (f) Galvanostatic long-cycle stability tests at 1C (1C ¼ 170 mA g1, initially cycled at 0.2, 0.5, 1, and 2C for five cycles each) of prototype LiFePO4jLi batteries with LPC@UM electrolyte. Copyright 2018. Reproduced with permission from WILEY-VCH Verlag GmbH & Co., KGaA.

represent an important branch in the energy storage field. However, their wide applications are greatly hindered by the dilemma of energy density. How to simultaneously achieve the flexibility and high energy density is still challenging. On this regard, nature always provides the most successful examples that maximize the flexibility and strength. As mentioned above, flexible biotemplates can be directly used, while mimic flexibility of the biostructure through an engineering process is also feasible. For example, Qian et al. reported spine-like flexible lithium-ion batteries inspired by the structure of spine [138]. As presented in Fig. 16, thick and rigid segments to store energy through winding the electrodes are analogous to the vertebra of animals. Meanwhile the thin, unwound, and flexible components are mimicking marrow to string all vertebra-like stacks together, which guarantees excellent flexibility. In this case, conventional LiCoO2/graphite battery chemistry was used to demonstrate the shape and structural engineering and the electrochemical performances will be discussed in Section 5. Biofunctionality can be also extended to Li–O2 battery, which is a promising battery system for future applications. However, the electrochemistry of Li–O2 batteries is different from conventional lithium ion batteries and the detailed mechanism still needs further investigations. Researchers consider that the residual superoxide radicals may lead to irreversible side reactions and deteriorate the cycling stability [171–173]. To address this chronic issue, some functional characteristics of the human eye such as self-protecting were applied by Kim et al. [174] The human eyes can be easily attacked by reactive oxygen species (ROS) once exposed to sunlight. However, it naturally forms a self-protecting mechanism by using melanin as a radical scavenger, which was analogously used in the Li–O2 batteries. In this strategy, polydopamine (pD),

one of the most common synthetic melanins, has been introduced in the ether-based electrolyte (Fig. 17). As a result, the superoxide radicals were captured by the pD additive and the irreversible side reaction were remarkably suppressed. Cycling performance of Li–O2 batteries with pD significantly outperform the control cell without such treatment, providing a clue that simple bio-inspired routes may effectively solve the issues in electrochemical energy storage field. 5. Electrochemical energy storage and conversion With the rapid development of energy technologies, surging requirements have been proposed for current state-of-the-art electrochemical energy storage and conversion systems. As abovementioned, the key elements in these systems, e.g. active materials, electrolytes, membrane or even the structure of the devices, can be specifically engineered by the bioinspired strategies for enhanced performances. The major advantages of bioinspiration can be mainly described as natural resources, sophisticated bio-structure and smart bio-function as introduced previously. Therefore, the application of bioinspired materials or architectures may boost the efficiency of electrochemical energy storage and conversion and bring up a sustainable and clean energy supplyconsumption relationship. Apart from the briefly discussed electrochemical applications in previous sections, special emphasis is made on some typical cases that reflecting the superiority of bioinspired strategies. For instance, the bamboo like carbon nanofiber webs (Fig. 10) were demonstrated with excellent electrochemical performances in both traditional threeelectrode configuration and all-solid-state supercapacitor electrodes. 13

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Fig. 21. (a) Schematic illustration of the sorption behavior of oxygen for GNP catalysts analogous to the oxygen exchange in tunas' gills. The sub-micrometer pores on the gill lamellae facilitates the oxygen exchange between the vessel blood and seawater. Similarly, the micropores on the GNPs play an important role in the sorption behavior of oxygen species in the ORR process. (b) TEM images. (c) Polarization and power density plots for H2/O2 PEMFCs with GNP-based Fe/N/C as cathode catalysts at 80
C. Zero back pressure was applied. (d) Corresponding iR-corrected polarization plots of H2/O2 PEMFCs. Copyright 2016. Reproduced with permission from the American Chemical Society.

0.2 C even with flexing to a diameter of 20 mm and twisting angle to 90
. Because of the unique architecture, the cell also delivered stable power and cycling performance during continuous dynamic mechanical load tests. The performance of cell was negligibly affected by the bending. As for the membrane in biology, the transport channels of specific ions are generally featured by high selectivity, efficiency and accuracy. Therefore, mimicking these biofunctional channels for Liþ ions conduction in LIBs seems operable as mentioned in Section 4. MOF materials offer versatile channels that can be engineered by complexing electrolyte anions to the open metal sites. Shen et al. demonstrated in their work that the complexing weakens the interactions between Liþ ions and the anions, enabling fast transport of Liþ ions through the channels [165]. As shown in Fig. 20, a flexible membrane can be produced by incorporating the MOF with 10 wt% of polytetrafluoroethylene (PTFE) as a binder, with thickness of 70–100 μm. The membrane was soaked in the solution of LiClO4 in propylene carbonate (LPC) to fill the channels with liquid electrolyte. Then the Liþ transference number (tþ Li) of soaked membrane was evaluated to be 0.65, which was significantly higher than that of the typical liquid carbonate electrolyte (0.2–0.4) [175]. The MOF embedded membrane was tested in both symmetric cells of Li/Li and asymmetric cells of LiFePO4/Li. The symmetric cell delivered a stable voltage plateau at ~20 mV up to 600 h operation and afterwards minor overpotential was accumulated till 1200 h. Within an asymmetric cell, a good specific capacity of 146 mAh g1 and 106 mAh g1 was respectively achieved at 0.2C and 2C. 75% of the initial capacity could be retained after 500 cycles. Coincidentally, the selectivity of ion transport channels is repeatedly

Without further support materials, the carbon nanofibers can be used as supercapacitor electrodes directly by sandwiching a cellulose separator in between. It was found that rectangular shapes of the CVs were wellmaintained at different scan rates up to 2000 mV s1, indicating a very fast and efficient charge transfer (Fig. 18a and b). As-obtained nanofiber web was applied and free-standing and highly flexible electrode without further requirement of current collector in symmetric supercapacitors. Due to the high conductivity, light weight and hierarchical pores, high energy and power densities can be achieved based on the entire devices. More importantly, the device was robust to work under different mechanical deformation conditions without degradation in performance. A capacitance retention of nearly 100% was observed for the as-prepared device under continuous dynamic operations of forceful bending and twisting (90
, 180
), and back to the initial status (Fig. 18c and d), demonstrating the superior structural durability and electrochemical stability, even after 10,000 charge/discharge cycles. The flexibility from bioinspiration is not only feasible in the active materials or electrodes, but also able to extend for the cell configuration. Qian group fabricated the spine-like flexible cell using the conventional LiCoO2/graphite active materials. The only modification was the spine-like connection between the cell units as discussed above. An energy density of 242 Wh L1 was obtained including the package, accounting for 86.1% of its prismatic cell counterpart with the same mass loading and packaging. Excellent cycling stability can be also achieved, even subject to different mechanical deformation, e.g. flexed and twisted configuration as presented in Fig. 19. A capacity retention over 94.3% after 100 cycles was achieved at 14

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proved to affect the performance of redox flow batteries [158–160,162]. These findings confirm the feasibility of biomimetic ionic channels for fast and effective transport of specific ions, which paves an interesting route to the novel membranes for energy storage. Recently, Yao et al. fabricated a lamellar porous graphene nanoplates (GNP) based Fe/N/C electrocatalyst for oxygen reduction reaction (ORR) by mimicking the Tunas’ gill structure [176], which consist of lamellae bestrewed with sub-micrometer pores that facilitate the interaction between hemoglobin and O2. As shown in Fig. 21, the unique lamellar porous structure offers high surface area for the efficient exchange of O2. The specific surface area and microporosity of the products can be well controlled by simply tuning the molten salt in the reaction, and accordingly structural sensitivity of oxygen adsorption/desorption. An optimized power density of 521 mW cm2 at 0.35 V could be achieved for proton exchange membrane fuel cell (PEMFCs), which was among the state-of-art high performance catalysts at zero back pressure. Many other bioinspired carbon based electrocatalysts, including but not limited to graphene, have been synthesized for ORR, mainly taking advantages of the controllable porosity and surface area [34,64,66,104].

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6. Summary and outlook Learning from nature to remove the obstacles to the development of modern society is undoubtedly intelligent, from the view point of either engineering or chemistry. With the fast blossom of nanotechnologies, researchers have now switched to more efficient and environmentfriendly synthesis methods for nanomaterials because the conventional physical and chemical methods encounter the bottlenecks. This review summarizes the recent representative advances in this field. The inheriting and favorable characteristics of materials obtained by a bio-inspired route may compensate the limitations of traditional materials. Bioinspirations can be firmly combined with the preparation of electrode materials for energy storage and conversion. However, we should pay special attention to combination mode. For instance, once the bioresources are selected and the coherence of products should be also taken into consideration because they are never the same in nature. There are some other aspects associated with the bio-inspired synthesis and structures that may lead to a different story. (1) The large-scale production of electrode materials. Carbon based materials can be transformed directly from abundant biomasses. Nevertheless, some smart structures from bio-templates offer only insufficient production rate. (2) The complexity of bio-resource derived nanomaterials or artificial biostructure offers a variety of possible solutions to complex problems, while it simultaneously sets great barriers for building general relationship between the synthesis and properties. (3) Following that, it may be very difficult to predict the performance through computational methods due to the complexity. The bio-inspired synthesis will be therefore empirical and laborious. Fortunately, by witnessing the continuous progress and in-depth understanding of the bio-inspired electrode materials, we believe in that the bio-inspired synthesis and smart structures will play an increasingly important role in electrochemical energy storage and conversion. Acknowledgement Q2

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