Inorganic Porous Films for Renewable Energy Storage - ACS Energy

Inorganic Porous Films for Renewable Energy Storage Kun Liang,† Lei Li,*,‡ and Yang Yang*,† †

NanoScience Technology Center (NSTC), Department of Materials Science and Engineering, University of Central Florida, Orlando, Florida 32826, United States ‡ Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, United States ABSTRACT: Environmental issues and the depletion of unsustainable resources have triggered great research efforts on the development of renewable energy technologies. Electrochemical energy storage devices, including Li-ion batteries, supercapacitors, and some emerging rechargeable battery technologies beyond Li-ion systems, provide a way to use energy in a clean and sustainable manner. Inorganic porous films are the essential component for renewable energy storage technology owing to their unique merits compared with powder materials: (i) high surface area and three-dimensional open channels facilitate mass/ ion transport; (ii) additive-free features enable facile fabrication; (iii) porous structure can well relax the mechanical strain caused by the volume change; (iv) the porous structure can be integrated with portable electronic systems. Here, the synthesis methods and particular structural features of inorganic porous films are examined, and their applications in electrochemical energy storage devices are reviewed. The current limitations and future perspectives in novel inorganic porous films for advanced energy storage technologies are also discussed.

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CONTENTS

1. INTRODUCTION In the past century, advances in science and technology have improved the quality of our lives markedly but at the expense of the depletion of natural resources environmental pollution, and climate change.1,2 Current concerns about limited energy resources and the need to decrease soaring greenhouse gas emissions have brought about an urgent quest to develop renewable resources that offer sustainable energy without sacrificing environmental quality.3,4 Especially the emerging high-tech products, such as portable and smart digital electronic devices represented by Apple Watch and Google Glass, requires renewable energy storage devices (RESDs) to power them efficiently and reliably.5−7 Li-ion batteries (LIBs) and supercapacitors (SCs) are two types of energy storage devices, which work via electrochemical reactions in electrodes and electrode/ electrolyte interfaces. The increasing demand for high-performance RESDs promotes understanding and manipulation of physical and chemical properties of the electrode and its interface with an electrolyte. With the recent advance in nanotechnology, nanoscale engineering of materials surfaces has become possible by using nanomaterials such as quantum dots,8 nanowires/rods/ tubes/fibers,9−12 nanosheets/ribbons/layered, materials,13−15 and three-dimensional (3D) networks of nanostructured

(1) Introduction (2) Synthesis Methods and Particular Structural Features (2.1) Electrochemical Etching and Deposition (2.2) Chemical Etching and Dealloying (2.3) Sol−Gel (2.4) Template-Assisted Synthesis (2.5) Particular Structural Features (3) Inorganic Porous Films for Renewable Energy Storage Devices (3.1) Lithium-Ion Batteries (3.1.1) Anodes (3.1.2) Cathodes (3.2) Supercapacitors (3.2.1) Transition Metal Oxide (3.2.2) Transition Metal Hydroxide (3.2.3) Transition Metal Sulfide or Fluoride (3.3) Inorganic Porous Film for Other Rechargeable Batteries (4) Future Perspectives and Concluding Remarks Corresponding Authors Notes Biographies Acknowledgments References © 2017 American Chemical Society

Received: December 7, 2016 Accepted: January 6, 2017 Published: January 6, 2017 373

DOI: 10.1021/acsenergylett.6b00666 ACS Energy Lett. 2017, 2, 373−390

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materials.16,17 The subject of nanostructured materials was extensively reviewed in the past decade, and there were also more focused reviews on nanomaterials for renewable energy recently.18−23 The present Review will specifically focus on the recent progress in inorganic porous films for renewable energy storage of LIBs, SCs, and some other emerging rechargeable battery techniques beyond LIBs. Previous review articles mainly focused on some specific porous films for photoelectrochemical application.24−27 Few review papers covered different inorganic porous films for electrochemical energy storage. In the present review, therefore, we will compare the particular structural features of inorganic porous films with porous powdered materials. We will also discuss how different RESDs benefit from these particular structural features (Figure 1). Some carbon

In most cases, substrates are required to assist the formation of porous films on their surfaces. When the substrates are sufficiently conductive, the porous films on the substrates are favorable for applications of energy storage devices without current collectors, binders, and/ or additives used in powder materials. The substrates are not always necessary and can be removed after forming porous films in some cases. researchers for the experimental optimization and theoretical analysis of AAO, and a growth mechanism was built up.35,36 The anodized metal oxide porous films are formed under a certain anodic voltage on the surface of interested metal M under “high-field law” or a competition reaction between oxide formation (eq 1) and dissolution (etching, eqs 2 and 3).37 The electrochemical reactions in forming porous metal oxide films are very complex. They largely depend on the thermodynamics of metal M, including oxide stability and the solubility of products. The key to formation of anodized porous films is to maintain a reaction balance between oxide formation and dissolution processes.38 If oxide formation reactions dominate the process, dense oxide layers will be formed. If dissolution reactions dominate the process, oxide layers will be dissolved and an electropolishing process will take place. n M + H 2O → MOn /2 + nH+ + ne− (1) 2

Figure 1. Schematic illustration of structural features of inorganic porous films.

materials and conducting polymers, for example, vertically aligned carbon nanotubes (NTs), graphene papers, polyaniline (PANI), and polypyrrole (PPy), have also been employed to fabricate porous films for renewable energy storage owing to their enhanced electrical conductivity, but they are not covered in this Review because they have been widely reviewed.12,20−23,28−31 At the end of this Review, we will make a brief conclusion and the perspective for inorganic porous films in renewable energy storage. This Review article will have a broad appeal to those interested in transformative thin-film technology, inorganic materials, and renewable energy storage systems. This work will also be of interest to nonexperts interested in nanotechnology.

n H+

MOn /2 + mF− ⎯⎯⎯→ [MFm](m − n) − +

Mn + + mF− → [MFm](m − n) −

n H 2O 2

(2) (3)

Some representative milestones of anodization technology developed in the past century are documented below. Porous anodized silicon was discovered accidently by Uhlir at Bell Laboratories in 1956.39 It is currently one of the most popular multifunctional materials in photovoltaics, LIBs, and bioimaging due to its unique properties.40−42 In 1995, Masuda developed an approach to fabricate the highly ordered AAO and further optimized a two-step anodization process, which was widely used to produce the porous films.43 In 1999, Zwilling prepared the first self-ordered TiO2 porous films by anodizing a Ti sheet in a fluoride-containing electrolyte.44 Owing to the great prospect of application in solar energy harvesting, biomedicine, and the semiconductor industry, intense efforts have been devoted to tailoring the structure of TiO2 porous films and exploring their functional properties. In the following years, more and more metals, such as Zr, W, Nb, Ta, and so forth, and a wide range of alloys, were reported to form porous films on their surfaces by refining the electrolytes.45−49 There were still some “hard” metals that regarded as utterly impossible for prepatation of selfordered porous films on the surfaces, such as V, Co, Mo, and their alloys. Recently, scientists made the impossible possible by solving the issues of high etching susceptibility and formation of soluble complexes instead of oxide films. In 2011, Yang first successfully demonstrated the fabrication of self-ordered V2O5 porous films by anodizing V in complex fluoride salts containing

2. SYNTHESIS METHODS AND PARTICULAR STRUCTURAL FEATURES Inorganic porous films are generally produced via either topdown or bottom-up processing.32 Generally speaking, some conventional synthesis approaches belong to top-down processing, for example, electrochemical/chemical etching and dealloying. Bottom-up processing involves various (electrochemical, chemical, and physical) deposition techniques, template-assisted synthesis, and self-assembly. 2.1. Electrochemical Etching and Deposition. Electrochemical etching also known as anodization is a comparably green and century old process used for fabricating self-organized porous films on metal and silicon substrates. The most famous sample developed via this method is the porous anodic aluminum oxide (AAO), which has been highly investigated as a template to fabricate low-dimensional nanomaterials such as oxide NTs and nanowires.33,34 Anodization and AAO were first discovered by Buff in 1857. They were commercialized for protection of seaplane parts from corrosive seawater in 1923. This encouraged 374


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electrolytes, such as [TiF6]2− and [BF4]−.50 Inspired by this work, Lee reported a highly ordered cobalt oxide porous film by anodizing Co in optimized anodization conditions. It avoided the oxygen evolution reactions during anodic treatment, which was supposed to be against forming ordered Co3O4 porous films.51 Shortly thereafter, Yang reported his success in producing Mo and Ni−Co oxide porous films by using a refined anodization technique.52,53 Beyond applying anodization to different metals, functionalization was a new direction of porous films. This involved both direct ways, for instance, doping porous films by alloy anodization and postannealing in a different gas atmosphere, and indirect ways, such as using porous films as templates to deposit other functional nanomaterials. Electrochemical deposition, or electrodeposition, electroplating, and plating, is an electrochemical process by which metals, oxides, or other compounds are tightly deposited onto the surface of conductive substrates from a solution containing the desired metal ion.53 It was first invented by Italian chemist Brugnatelli in 1805 and then spread around the world with the development of electrochemistry.54 Even now, electrodeposition technology is still widely used in various industries for coating metal products with a deposited layer that provides corrosion protection. Some particular features make this technique attractive in nanotechnology and materials science: (i) It can be precisely tuned in the composition, thickness, and morphology of the deposited nanostructure by adjusting the electrochemical conditions; (ii) the deposition is mostly performed at ambient conditions, which make this process suitable for industry application. The electrodeposits can be produced under both pulse and direct current conditions in different electrolytes, for example, aqueous, organic, and ionic liquid solution.55−58 Several processes are involved in the electrodeposition. They are mass transport of metal ions or complexes toward electrodes (substrates) under an applied electric field, nucleation and accumulation of deposits on the electrodes, and deposit growth via numerous modes. The electrodeposition mechanism turned out to be more complex, which may vary case by case. In addition to electrodeposited compact films, electrodeposited porous films or nanostructured films have been extensively studied due to their particular electrochemical and optical properties. In 1997, Attard electrodeposited a Pt film with a welldefined, long-ranged, porous nanostructure by using liquidcrystalline plating solutions. It was suggested to be a versatile way to create porous electrodes for electrochemical applications such as renewable energy devices and sensors. In 2003, Liu developed a general technique to electrodeposit metallic films with highly porous structure.59 In this technique, the electrolytically evolved hydrogen gas served as pore-forming agent, which had been applied to produce different metallic or alloy porous films. Most interestingly, the porous morphology of the electrodeposited metal compounds depends on the crystallographic characteristics of the deposits and their nuclei growth modes. This offers great promise for enabling the deposition of porous films with controllable nanostructure. Without the assistance of templates, the porous structures of the electrodeposited metal oxides (MOx) such as NiO and SnO2 are composed of agglomerated MOx nanoparticles forming an interconnecting porous network on the nano- or microscale, whereas MnO2 and some metal hydroxides such as Ni(OH)2 and Co(OH)2 tend to form porous nanosheet films under suitable conditions because of their particularly crystallographic layered structure. The porous structure and crystallographic orientation of the deposits can

be easily controlled by adjusting electrochemical conditions, which leads to different nuclei growth modes. Take ZnO as an example; the morphology of electrodeposited ZnO can be tuned by controlling the growth rates of the deposits with appropriate capping agents.60 It demonstrates that Cl− and CH3COO− preferentially adsorb onto the (0001) plane of ZnO to produce platelet-like crystals,61 while EDA and NH4F tend to form needle-like crystals.62,63 By using mixed capping agents, other morphologies such as hexagonal rods, woven needles, and rhombohedra rods have been prepared.64−68 Another representative deposit is Cu2O, whose morphology has been demonstrated to be readily controllable. At pH < 9, Cu2O prefers to grow with a preferential orientation in the [110] direction,69 whereas films deposited at pH > 9 have a preferred [111] orientation.70 2.2. Chemical Etching and Dealloying. Chemical etching/ dealloying is a method used to fabricate metallic porous films by dissolving undesired metals from alloy films using etching solution. In some cases, electrochemical treatments (electrochemical dealloying) are introduced to selectively remove undesired metals from corrodible components.55,71 Dealloying was widely used in metallurgy to identify the existence of stoichiometric intermetallic compounds. Dealloying can occur in any alloys, for example, Cu−Au, Au−Zn, Pt−Cu, and Pt−Si, to form a 3D nanoporous sponge or bicontinuous interpenetrating nanostructure. Theoretically, various forming models, such as the kinetic Monte Carlo model, dynamic roughening transition model, and corrosion disordering/diffusion reordering model, have been proposed to understand the physical mechanisms of dealloying in different alloy systems.72−74 On the basis of theoretical understanding of the formation mechanism of the nanoporous film during dealloying, different porous metal films with controllable morphology and composition were developed. Strasser prepared dealloyed Pt−Cu porous films and used them as catalysts for the oxygen reduction reaction (ORR). It was found that the electrochemical dealloying of non-noble base metals significantly altered the surface catalytic activity of the remaining porous Pt.75 The results suggested that dealloying can be utilized as a versatile strategy to tune the surface catalytic activity of noble metal catalysts in heterogeneous catalytic or electrocatalytic environments. Beyond noble metal porous films, Chen dealloyed Cu30Mn70 alloy to prepare the porous Cu films with a pore size distribution from 15 to 200 nm for photonic applications.76 He found that porous Cu films led to a dramatic improvement in the surface-enhanced Raman scattering (SERS). At a pore size range of 30−50 nm, the SERS enhancement factor was about 1.85 × 105, which was comparable to that of nanoporous gold. Gösele found more possibilities of dealloying technology in fabricating well-defined nanostructures when combined with other techniques.77 He electrodeposited Pt−Co alloy nanowires in AAO membranes and then dealloyed Pt−Co nanowires into a highly porous structure. Those nanoporous Pt−Co alloy nanowires showed distinctly enhanced electrocatalytic activities toward methanol oxidation. It was substantial promise for direct methanol fuel cells. 2.3. Sol−Gel. The sol−gel process was originally developed to produce inorganic ceramic and solid materials from small molecules in the mid-1800s. Sol or colloidal solution is a dispersion of colloidal particles in a liquid, which acts as the precursor for generation of gel. The gel is an interconnected and porous network composed of both liquid phase and solid phase with polymeric chains as basic frameworks. The morphology of gel varies with liquid content within the network. Generally, 375


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Two general synthesis pathways were reported for the softtemplate method, including cooperative self-assembly and liquidcrystal templating processes.82 In the cooperative self-assembly process, the surfactants and inorganic species were mixed together in a solution, and the surfactants interacted with inorganic materials driven by Coulomb force, covalent bond, and hydrogen bonding.83 The charge density match at the surfactants/inorganic materials interfaces determined the assembly process, resulting in phase separation to form an ordered 3D architecture precursor. The porous framework can be achieved after template removal. High-concentration surfactants were used in the liquid-crystal templating process.82 The condensation of inorganic precursors was improved due to the confined growth around surfactants. After that, the templates were eliminated to obtain a porous structure. Liang et al. prepared a LaNiO3/NiO mesoporous film with polyethylene glycol (PEG) as the template by a simple sol−gel process.84 These films were in a uniform porous structure generated by the removal of solvent and PEG during heat treatment. The softtemplate method is a low-cost and high-yield technique; however, the synthesis process is not very controllable owing to the complicated hydrolysis and polymerization. Therefore, a combination of hard-template and soft-template processes becomes an ideal method to prepare inorganic porous films. 2.5. Particular Structural Features. Inorganic porous films have attracted more and more attention and presented great potentials in many fields due to their outstanding properties (Figure 1), including high surface area, tunable pore sizes, 3D open channels, and controllable surface physical and chemical characteristics. With the development of portable and flexible electronics, it becomes urgent to develop lightweight, flexible, robust, and freestanding materials for these devices. Inorganic porous films synthesized by different methods possess the possibilities for flexible electronics owing to their unique structural features highlighted in Figure 1. Yang et al. fabricated edge-oriented MoS2 films through sulfurization of anodized Mo oxide porous films on flexible Mo substrates.52 These edge-oriented MoS2 porous films exhibited appealing energy storage performance when used as flexible all-solid-state SCs. Surface area is an important merit of the inorganic porous film. High specific area can provide more active sites and interfaces to enhance electrochemical performance in the devices. Take hydrogenated NiCo2O4 hollow spheres as an example; Li et al. prepared single-shell and double-shell NiCo2 O 4 hollow spheres.85 The specific surface area of double-shell spheres was 50% larger than that of single-shell spheres owing to the hierarchical pores and incremental shells that intercross with each other. This made the double-shell spheres store more than 62% energy at a current density of 1 A g−1 when used as electrodes in SCs. Highly porous films with open pathways/channels can facilitate mass and/or ion transport at the electrolyte/electrode interfaces. These materials are suitable for applications in renewable energy storage. First, the porous structure can provide large surface area and enhance the mechanical properties that can relax the mechanical strain caused by the volume change in the electrochemical test. Second, the highly porous and interconnected channels can facilitate ion/electron transport throughout the entire film. Zhu et al. prepared carbon/metal oxide nanocomposites.86 The porous structure with open channels of the nanocomposites are well retained after a calcination process.

sol−gel monoliths are made by either gelation of a solution of colloidal powders or hydrolysis and polycondensation of precursors (inorganic or metal organic chemicals in the solvents). Hydrolysis and polycondensation normally occur at low rates, which are significantly influenced by steric and chemical factors. A number of approaches have been developed to separate the liquid and solid phases in order to convert sol to gel, for example, natural sedimentation, mechanical centrifugation, and thermal drying. The porous structure and porosity of the gel are strongly determined by the liquid removal rate during gelation, aging, and drying.78 Thereafter, a sintering process is applied to facilitate the polycondensation, crystallization, and grain growth processes. In order to form porous inorganic films, the sol precursors should be deposited on a substrate to form a film followed by dehydration and annealing.79 The sol−gel approach allows for fine control of the porous films’ chemical compositions, showing diverse applications in optics, electronics, renewable energy, and biomedicine. Pang et al. compared electrochemical performances of sol−gel-derived and electrodeposited manganese dioxide on nickel foil.80 The sol−gel-derived materials showed good capacitive behavior and high cyclability with more than 90% capacitance retention after 1500 cycles. Yang et al. prepared a nanostructured LiMn2O4 porous film by a polymer-assisted sol−gel method.81 These LiMn2O4 porous films exhibited high specific capacity of 131 mAh g−1 at 0.5 C and 105 mAh g−1 at 20 C. 2.4. Template-Assisted Synthesis. The template-assisted method is an effective way to synthesize porous nanomaterials. This method can be used to fabricate porous/hollow structures with any size, shape, and chemical composition from different templates. On the basis of the templates used in this synthesis technique, two subcategories have been identified: hard-template and soft-template methods. The hard-template method employs solid porous materials, for example, AAO and highly ordered mesoporous silica, as scaffolds to grow different target materials. Using these solid porous materials with uniform pore size, it is possible to fabricate oriented and highly ordered porous nanostructures. Other solid materials, such as zeolite and block copolymer film, can also be employed in hard-template synthesis. By applying the hardtemplate method, 1D nanostructures, for instance, nanowires, NTs, and nanofibers, can be obtained after removing templates. Template-assisted electrodeposition, chemical vapor deposition (CVD) and atomic layer deposition (ALD) brought the chance to fabricate novel inorganic porous films. The pore sizes of the resulting nanomaterials are typically between 15 and 150 nm. Mesoporous Pt networks and hierarchical TiO2 NT arrays were prepared using different hard templates. Although different kinds of porous nanomaterials can be prepared effectively by hard-template synthesis, removing the template always affects the porous nanostructures. Meanwhile, it is very difficult to achieve high product yields for the complicated hard-template synthesis. The soft-template synthesis shows an advantage in the high yield, which is feasible for scalable nanomanufacturing of nanomaterials in large quantity. Currently, the soft-template method has become a general way to fabricate ordered mesoporous materials. Typical soft templates including anionic, cationic, nonionic, and mixed surfactant, have been employed to synthesize porous materials with controllable structures and tunable architectures. Various synthetic processes have been combined with soft-template synthesis, including the sol−gel process, hydrothermal method, evaporation induced selfassembly (EISA) process, postsynthesis treatment, and so forth. 376


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anodization technology on Al foil and then used as a substrate for electroplating Ni. After removing AAO, TiO2 was deposited on a Ni substrate by the ALD method, forming 3D Ni/TiO2. In this structure, 3D Ni nanowires worked as the electrical conductivity network and TiO2 worked as the active materials for energy storage. The material showed good electrochemical performance with a high capacity of ∼0.16 mAh cm−2 and excellent cycling stability with no capacity decay after 600 extended cycles (Figure 2b,c). TiO2 porous films composed of different nanostructures, such as nanowires, nanofibers, and NTs, were fabricated through ALD, electrospinning, or anodization methods and demonstrated promising performance in energy storage used as anodes in LIBs. Beyond TiO2, there are many other inorganic electrode materials that store energy based on the same mechanism, such as Nb2O5, MgTi2O5, LiTiNbO5, TiNb2O7, and Li4Ti5O12 (LTO). Among these materials, LTO was another most invested material thanks to its high power performance and excellent thermal and mechanical stability.87 Liu et al. prepared a well-aligned LTO NTs forest on stainless steel foil via a ZnO template-based solution method. The thin carbon films were coated on the inner and outer surfaces of the NT forest, improving the electrical conductivity.89 The self-supported film exhibited super rate performance with a reversible capability of 135 mAh g−1 at 30 C and 80 mAh g−1 at 100 C and high capacity retention with only ∼7% capacity decay after 500 cycles at 10 C. Other methods, such as pulsed laser deposition or thermal annealing, were also used to prepared the porous LTO films, which also demonstrated promise in LIBs.

With this porous architecture, the carbon/metal oxide can deliver impressive cyclability even under high current density.

3. INORGANIC POROUS FILMS FOR RENEWABLE ENERGY STORAGE DEVICES 3.1. Lithium-Ion Batteries. LIBs are reigning over current rechargeable battery markets due to their high energy and long lifetime. They are widely used to power present-day portable consumer devices, such as smart phones, iPads, and laptops, and emerging electrical/hybrid vehicles. With the expected use of large-scale LIBs, the industry-projected demand and growth are expected to dramatically increase to a market value of US $53.7 billion in 2020.87 Therefore, developing advanced LIBs with high energy density, super power density, and excellent cycling performance becomes critical and necessary. The electrode materials are the most important components in LIBs. In this section, we will focus on some characteristic electrode materials with porous thin films used as the anodes and cathodes in LIBs. 3.1.1. Anodes. Anode materials in LIBs can be classified into three types based on energy storage mechanisms. They are the intercalation/deintercalation mechanism, alloying/dealloying mechanism, and conversion reaction or redox reaction mechanism. TiO2 stores energy based on the lithium interaction/deintercalation reaction.87 TiO2 is an attractive candidate of anode materials for LIBs thanks to its low cost, ready availability, and environmental friendliness. Wang et al. reported a 3D Ni/TiO2 nanowire network fabricated using AAO template-assisted electrodeposition of Ni, followed by TiO2 coating using ALD.88 As shown in Figure 2a, the AAO template was prepared by

Figure 2. (a) Schematic illustration of the preparation of the 3D Ni/TiO2 nanowire network. (b) Areal discharging capacity of 3D Ni/TiO2 at different current densities. (c) Cycling stability of 3D Ni/TiO2. Reprinted from ref 88. Copyright 2012 American Chemical Society. 377


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improved electrical conductivity and facilitated electron transport in the porous structures. Cobalt oxide is another most investigated material that stores energy based on a conversion reaction due to its high theoretical capacitance (890 mAh g−1). Li et al. prepared the porous thin film with Co3O4 nanowires on different substrates, such as Si, Cu, and Ti, by the wet chemistry method.93 The thin film showed high capacity and good rate performance with 50% capacity retention when the current rate increased from 1 to 50 C. In order to improve the energy storage ability, Co3O4 can mixed with other metal oxides, such as NiO and Fe2O3, forming new hybrid materials taking advantage of both of these materials. Wang et al. developed one facile, rapid, and scalable method to prepare the FeCo2O4 on a Cu substrate.94 Graphene was introduced into the thin film for improvement of the electrical conductivity of the composite. The hybrid materials demonstrated high specific capacitance of 867 mAh g−1 at 1000 mA g−1, good rate capability, and improved cycling stability with no obvious capacity decay after 200 cycles. All of these improvements in the performance resulted from the synergetic effect between FeCo2O4 and graphene in the porous structure. Beyond these, other transition metal oxides, such as NiO and CuO, were also studied. Different strategies, such as electrodeposition, laser deposition, anodization, electrostatic spray deposition, and wet chemistry, were developed to prepare the porous films, and they also demonstrated promising performance. Silicon (Si) is an attractive anode material that stores energy based on am alloying/dealloying mechanism. The theoretical capacity can reach 3579 mAh g−1, almost an order of magnitude larger than that of conventional graphite. However, the huge volumetric changes in the Si anode during the alloying/ dealloying processes leads to limited cycling stability, reducing

To improve the capacity of the intercalation/deintercalation system, electrode materials that stored energy based on the conversion reaction were developed. This system, including most of the transition metal oxide, can store more than 3 times the capacity of graphite. Iron oxide is one of the most studied anode materials of this type because of its high theoretical capacitance, earth abundance, and environmental friendliness. 3D Fe2O3 electrodes on a Ni scaffold mesostructure were prepared by the combination of colloidal templating and pulsed electrodeposition technologies, as shown in Figure 3a−e.90 In this structure, 3D Fe2O3 electrodes played the major role of energy storage. The Ni scaffold worked as the substrate, loading the active materials of Fe2O3, and improved the electrical conductivity of the system. Fe2O3 was uniformly deposited on the surface of Ni inverse opal with a thickness of ∼10 μm (Figure 3f−i). The electrode showed high reversible capacitance of ∼1000 mAh g−1 at 0.2 A g−1 (Figure 3j). It gradually decreased to 450 mAh g−1 when the current density increased to 20 A g−1 and remained at 400 mAh g−1 after 100 cycles at such a high rate (Figure 3k). There was no obvious deformation of the structure after such hash test conditions, further supporting the stability of the electrode materials (Figure 3l). In order to enhance the energy storage ability of Fe2O3, thin films of hybrid materials of Fe2O3− Ta2O5 NTs and Fe2O3−Fe3C−Fe were prepared by the anodization method.91,92 These materials demonstrated improved performance because of the synergetic effects among the components in the hybrid systems. Other substrates, such as copper and SiO2, were also used as substrates to prepare porous metal oxide thin films through electrodeposition or wet chemistry methods. These materials showed improved performance in energy storage, especially in cyclability and rate capabilities due to the

Figure 3. (a−e) Schematic illustration of the preparation of the 3D Fe2O3 electrode. (f) SEM image of Ni inverse opal. (g−i) SEM images of the 3D Fe2O3 electrode at different magnification. (j) Rate performance of the electrode at different rates. (k) Cycling performance of the electrode at 20 A g−1. (l) SEM image of the electrode after 100 cycles at the high rate of 20 A g−1. Reprinted from ref 90. Copyright 2015 American Chemical Society. 378


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post-treatment.100 V2O5 uniformly loaded on AAO, forming the porous thin film (Figure 5a,b). V2O5 thin film demonstrated a high specific capacity of 142 mAh g−1 at 50 mA g−1, good rate performance with 83% capacity retention when the current increased to 600 mA g−1, and excellent cycling stability with no capacity decay after 105 cycles at a current density of 150 mA g−1. The improved performance resulted from the synergy between the crystallinity of V2O5 and the structure. Beyond V2O5, other cathode materials were also studied. For example, the ALD method was used to prepare the porous thin film of spinel lithium manganese oxide (LMO), which was one of the most attractive cathode materials due to its high voltage, high specific capacity, and minimal structural changes in operation.101 The dissolved manganese ion from LMO will result in quick capacity decay. In order to improve its cycling stability, one monolayer of graphene was transferred on the top surface LMO thin film, resulting in great improvement in the cycling stability. The LMO thin films can also be prepared by other methods, such as radio frequency (rf) magnetron sputtering, pulse laser deposition, sol−gel method, and chemical solution deposition. Lithium ion phosphate (LiFePO4) is another ideal cathode material because of the high theoretical capacity (170 mAh g−1), low cost, environmental friendliness, and thermal safety. For example, the nanoporous thin film of LiFePO4 was developed via template-supported sol−gel chemistry combined with the dip-coating approach, followed by thermal treatment.102 The material showed good cycling performance with high capacity of 158 mAh g−1 at the 1000th cycle. 3.2. Supercapacitors. SCs, also named electrochemical capacitors, have attracted great attention due to the high power density, long lifetime, and low maintenance cost. SCs have been widely used in consumer electronics, memory back-up systems, and industrial power and energy management. On the basis of the energy storage mechanisms of electrode materials, SCs can be categorized into two types. The first type of SC are electrical double-layer capacitors (EDLCs), which store energy at the

its wide applications. Developing the nanoporous structure for Si material is one effective strategy to address this issue, enhancing its cycling performance in LIBs. Cao et al. designed and prepared porous films with Cu−Si or Cu−Si−Al2O3 nanocable arrays through ALD technology.95 As shown in Figure 4a−e, CuO cable was first prepared on the Cu foil. Then, Si was deposited onto the surface of CuO. Cu−Si−Al2O3 was obtained after coating Al2O3 following the reduction of CuO. The morphologies of the composites were investigated by TEM and EDX elemental mapping, demonstrating the uniformity of the composites (Figure 4f−j). In this structure, Si was the major active material for lithium ion storage. Cu cores worked as both current collectors and a structural framework loading Si. The outer coating layer of Al2O3 can stabilize the solid−electrolyte interface (SEI) layer formed by decomposition of the electrolyte. Cu−Si−Al2O3 showed a high capacity of 1820 mAh g−1 at 0.3 A g−1. It remained at 790 mAh g−1 when the current density increased to 14 A g−1, demonstrating the good rate performance (Figure 4k−m). Figure 4n shows the cycling performance of these materials. After 100 cycles at a current density of 1.4 A g−1, the capacity of Cu−Si−Al2O3 was ∼1560 mAh g−1 with no obvious decay, showing promising cycling stability. The porous thin films can also be prepared by other methods, such as e-beam evaporation, radio frequency magnetron sputtering, plasma-enhanced CVD, and wet chemistry. All of them demonstrated promising improvements in lithium storage for LIBs. 3.1.2. Cathodes. Cathodes are the other important electrode component in LIBs. The energy density of LIBs was typically determined by the cathodes due to the limited capacity. Among all well-known cathodes, V2O5 showed promising possibility in improving the performance of LIBs due to its relatively high theoretical capacity (147 mAh g−1 at 2.6−4.0 V; 294 mAh g−1 at 2.0−4.0 V), good rate capability, and better safety.96,97 ALD is one of the most used methods to prepare V2O5 thin films.98,99 For example, Chen et al. prepared V2O5 thin film using ALD technology on an AAO substrate without any thermal

Figure 4. (a−e) Schematic illustration of the preparation of the Cu−Si Al2O3 electrode. (f) TEM image of the Cu−Si Al2O3 electrode. (g−j) EDX elemental mapping of O, Al, Si, and Cu in the Cu−Si Al2O3 electrode. (k) Voltage profile of Si−Cu for the first two cycles at a current density of 0.3 A g−1. (l) Voltage profile of the Cu−Si Al2O3 electrode for the first two cycles at a current density of 0.3 A g−1. (m) Rate performance of Cu−Si and Cu−Si Al2O3 at different current densities. (n) Cycling performance of Cu−Si and Cu−Si Al2O3 at 1.4 A g−1. Adapted with permission from ref 95. Copyright 2011 John Wiley & Sons. 379


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Figure 5. (a) SEM image of the AAO template. (b) SEM image of V2O5 deposited on the AAO template. (c) Voltage profile of the V2O5 electrode at a current density of 50 mA g−1. (d) Rate performance of the V2O5 electrode at different cycles. (e) Rate performance of the V2O5 electrode at different current densities. (f) Cycling performance of the V2O5 electrode at 150 mA g−1. Reprinted from ref 100. Copyright 2012 American Chemical Society.

MnO2 film on a AuPd-coated substrate with a nanopillar structure, as shown in Figure 6. The material was prepared in three steps, which are fabrication of an inert polymer with nanopillar structure, deposition of AuPd on the substrate as a current collector electrode material, and electrodeposition of MnO2 on the nanostructured electrode, sequentially (Figure 6a). As shown in Figure 6b,c, it is obvious that porous MnO2 was uniformly deposited on the nanopillar structures. This material showed good electrochemical performance with a high specific capacitance of 603 F g−1 at 5 mV s−1, 93% capacitance retention after 5000 cycles, and superior energy density of 50.68 Wh kg−1 at a power density of 3.57 kW kg−1.103 Other substrates, such as nickel foam, hollow nickel dendrites, and Au-coated Co3O4 porous nanowalls, were also used as electrical conductive materials for MnO2 loading using the electrodeposition method, forming porous films with improved performance. Wet chemistry method is another strategy to prepare the porous MnO2 film. Chodankar et al. prepared the MnO2 thin film from KMnO4 precursor on stainless steel.104 The specific capacitance of MnO2 reached 614 F g−1 at 5 mV s−1 and slowly decreased to 366 F g−1 when the scan rates increased to 100 mV s−1. The MnO2 thin film composed of a 3D ZnO@MnO2 core−shell branched nanowire was prepared using ZnO nanowires as the substrate in KMnO4 solution. The porous film demonstrated good electrochemical performance with a high specific capacitance of 31.3 mF cm−2 thanks to the enhanced surface

electrode/electrolyte interfaces by electrostatic charge accumulation. The second type is pseudocapacitor stored energy through a fast and reversible faradic reaction near the electrode surface when a potential is applied. In this section, we mainly discuss several characteristic pseudocapacitive materials. They are the transition metal oxide, transition metal hydroxide, and transition metal sulfide and fluoride. 3.2.1. Transition Metal Oxide. Transition metal oxides, pseudocapacitive electrode materials, not only store energy like electrostatic carbon materials but also exhibit electrochemical Faradaic reactions between electrode materials and ions. MnO2 stands out as the most promising electrode material for SCs thanks to its remarkable features such as environmental friendliness, low cost, and high theoretical capacitance (1400 F g−1).103 However, the low electrical conductivity of MnO2 (∼10−5− 10−6 S cm−1) limited its application in SCs. To boost its electrochemical performance in SCs, nanostructuring and preparing composites with electrical conductive matrices are two major effective strategies to improve the electrical conductivity of MnO2-based electrode materials. The porous MnO2 film on an electrical conductive matrix, prepared by several different methods, is one promising electrode material taking advantage of the high surface area and good electrical conductivity, which facilitates electrolyte contact and diffusion in electrode materials. Electrodeposition is one of the most used methods to prepare porous films on different substrates. Yu et al. reported the porous 380


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Figure 6. (a) Schematic illustration showing the preparation of MnO2-coated electrodes. (b,c) SEM images of electrodeposited MnO2 nanoparticles on nanopillar arrays with (b) top view and (c) side view. (d) CV curves of nanostructured electrode (NE) and planar electrode (PE) at a scan rate of 100 mV s−1 in 1 M Na2SO4. (e) CV curves of NE at different scan rates of 5−100 mV s−1. (f) Specific capacitances of NE and PE at different scan rates. (g) Galvanostatic charge−discharge curves of NE in a current density range of 10−100 A g−1. (h) Cycling performance of NE at a scan rate of 1000 mV s−1. (f) Energy and power densities of NE and PE. Adapted with permission from ref 103. Copyright 2013 John Wiley & Sons.

(CV) curves and galvanostatic charge/dicharge experiments, the hybrid with 20 min depostion of MnO2 had the best electrochemical performance, with a high specific capacitance of 1145 F g−1, improved energy density of ∼57 Wh kg−1, and power density of ∼16 kW kg−1 (Figure 7c−e). In order to enlarge the working potential range and improve the energy density, asymmetric supercapacitors (ASCs) were developed. Qiu et al. reported a 3D MnO2 nanocone array on flexible substrates by the electrodeposition method.107 ASCs were fabricated using the Au/MnOx nanocone array as the positive electrode and a carbon-based material as the negative electrode. The device demonstrated good electrochemical performance with high device capacitance of 108.5 F g−1 at a current density of 1 A g−1, energy density of 46.8 Wh kg−1 at a power density of 0.72 kW kg−1, and excellent cycling stability with 96.5% capacitance retention even after 2000 cycles at a current density of 2 A g−1. Due to the limited capacitance of carbon-based electrode materials in SCs, pseudocapacitive materials were developed as negative electrodes in asymmetric SCs. Chodankar et al. prepared one Fe2O3 thin film by the wet chemical method. It was used as the negative electrode combined with the MnO2 thin film assembling ASCs.108 The capacity has no big variation at different bending angles, demonstrating superior mechanical stability. The energy density of the ASCs can reach 41 Wh kg−1 at a power density of 2.1 kW kg−1. Two ASC devices in parallel that were charged only 30 s can light 31 red LEDs for 2 min, exhibiting promising practical applications.

area and higher loading amount of MnO2 nanoparticle on the 3D ZnO nanoforest backbone. MnO2 can also be directly prepared by oxidation of Mn in air. Li et al. reported one porous thin film composed of a sandwich-structured MnO2−Mn−MnO2 tube by oxidation of Mn in air.105 In this structure, the middle Mn layer and direct contact between Mn and current collectors worked as the pathways for fast electron transport. The sandwich structure enhanced the contact between electrolyte and electrode materials, facilitating ion diffusion. The materials demonstrated superior performance with a high specific capacitance of 955 F g−1 at 1.5 A g−1 (937 F g−1 at 5 mV s−1), improved rate capability with an energy density of 45 Wh kg−1 and specific power of 23 kW kg−1, and excellent long-term cycling stability with high capacitance retention of 95% after 3000 cycles. Flexible SCs were also fabricated based on the porous MnO2 thin film for flexible electronics, such as wearable electronics, mobile phones, electronic papers, solar cells, and other collapsible gadgets. One MnO2 thin film on a nanoporous gold matrix was prepared by the electrodepostion method.106 The nanocrystalline MnO2 was uniformly deposited in a nanoporous Au matrix, forming hybrid mateirals, as shown in Figure 7a. In this hybrid, nanoporous gold allows electron transport through the MnO2, facilitating ion diffusion between the MnO2 and the electrolytes. MnO2, the pseudocapacitive material, played a major role in charge storage through the fast, reverisble redox reaction on its surface. The symmetric SC was fabricated as shown in Figure 7b. On the basis of the cyclic voltammogram 381


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Figure 7. (a) Schematic illustration of the fabrication process for nanoporous gold/MnO2 hybrid materials by direct deposition of MnO2 (orange) onto nanoporous gold. (b) Photograph of a SC based on nanoporous gold/MnO2 electrode materials. (c) CV curves for bare nanoporous gold electrodes and nanoporous gold/MnO2 electrodes with three different plating times of 5, 10, and 20 min at a scan rate of 50 mV s−1. (d) Galvanostatic charge−discharge curves of these samples at a current density of 0.5 A g−1. (e) Specific capacitance of these samples at different current densities. Adapted with permission from ref 106. Copyright 2011 Nature Publishing Group.

One porous NiO network-like film on indium-doped SnO2 glass (ITO/glass) substrates was prepared by a combination of a green solvothermal method and the following heat-treatment in air. The film was assembled in the SCs and demonstrated high specific capacitance of 960 F g−1 with a Coulombic efficiency of ∼100% at 20 A g−1 and capacitance retention of 93% of its maximum value after 1000 cycles. A hydrothermal method was also used to prepare the porous NiO film. Huang et al. demonstrated one composite of single-crystalline NiO nanosheet arrays on nickel foam through hydrothermal reaction.111 The material showed improved electrochemical performance thanks to the synergy between the high surface area NiO nanosheets and good electrical conductivity Ni foam. Co3O4, the layer structured pseudocapacitive material, has several advantages for application in SCs, such as high theoretical capacitance (3650 F g−1), low cost, long-term stability, and good corrosion stability. The porous Co3O4 film can be directly deposited on the current collectors by a single-step solution precursor plasma spray route. In this approach, cobalt acetate solution was fed into a plasma plume, producing Co3O4 nanoparticles through an accelerated thermochemical conversion process. They were deposited on the current collector, forming a Co3O4 thin film with nanoporous structure. The device was fabricated and demonstrated high capacitance of 162 F g−1 and good capacity retention of 72% after 1000 cycles. Cathodic electrodeposition technology is another method to prepare the porous Co3O4 film. Yuan et al. reported a hierarchically porous Co3O4 film with a net-like structure of interconnected nanoflakes with a thickness of 15−20 nm.112 The porous film was prepared by cathodic electrodeposition via a liquid-crystalline template. When used as electrodes in a SC device, the material demonstrated good performance with high specific capacitances

NiO is another promising pseudocapacitive material for SCs due to its intriguing features, such as low cost, natural abundance, environmental friendliness, and high theoretical capacitance (2573 F g−1). Singh et al. reported a self-organized 3D architecture of NiO nanoblocks (NiO NBs) and hydrogenated NiO nanoblocks (H-NiO NBs) by the combination of electrodeposition and high-temperature annealing treatment of Ni on a Cu substrate.109 Figure 8a shows the schematic illustration of the preparation of samples. The electrodeposited Ni film on Cu foil was oxidized at high temperature in air to obtain NiO NBs, which were further annealed in a H2 environment, obtaining the H-NiO NBs. The morphologies are shown in Figure 8b,c. The electrochemical performance of the samples was studied by CV and galvanostatic charge−discharge experiments, as shown in Figure 8d,e. H-NiO NBs demonstrated the best performance with a high specific capacitance of 1336 F g−1 at 1.11 A g−1, good rate performance of 61% capacitance retention when the current densities varied 100 times from 1.11 to 111.11 A g−1, excellent cycling stability of 94.7% capacitance retention after 3000 cycles at 1.11 A g−1, and high energy density of 52.13 Wh kg−1 and power density of 19.44 kW kg−1 (Figure 8f,g). The great improved electrochemical performance of H-NiO NBs resulted from the high surface area of the porous film and the enhanced electrical conductivity by the hydrogenation of NiO NBs. The porous film of NiO can also be prepared by taking advantage of templates. Cao et al. used ZnO as a template and prepared the porous film with NiO hollow nanowires by electrodeposition and wet chemistry methods. The materials demonstrated good performance in SCs.110 Nickel was also used as a template, preparing the porous film arranged by hydrogenated Ni/NiO core/shell 1D nanoheterostructures. 382


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Figure 8. (a) Schematic illustration of the fabrication process for H-NiO NBs on a copper substrate. (b,c) SEM images of H-NiO NBs at different resolution. (d) CV curves of NiO NBs and H-NiO NBs at a scan rate of 100 mV s−1. (e) Galvanostatic charge−discharge curves of NiO NBs and H-NiO NBs at a current density of 1.11 A g−1. (f) Specific capacitance of NiO NBs and H-NiO NBs at different current densities. (g) Cycling stability of NiO NBs and H-NiO NBs at 1.11 A g−1. Reprinted from ref 109. Copyright 2014 American Chemical Society.

of 443 F g−1 at 2 A g−1 as well as excellent cycle life and superior rate performance with 75% capacity retention when the current density increased from 2 to 40 A g−1. The improved performance in SCs resulted from its unique hierarchically porous characteristics, which provided fast ion and electron transfer, large reaction surface area, and limited diffusion length. The porous Co3O4 film can also be prepared via the combination of hydrothermal reaction with thermal annealing. Duan et al. prepared one hierarchically porous Co3O4 film by a hydrothermal synthesis method based on a self-assembled monolayer polystyrene (PS) spheres template.113 The film showed noticeable performance with a high capacitance of 454 F g−1 at 2 A g−1 and good cyclability. In order to further improve the electrochemical performance of transition metal oxides, several of them mixed together forming the spinel cobaltites (MCo2O4), taking their individual advantages. MCo2O4 are promising pseudocapacitive electrode materials thanks to the presence of mixed valence metal cations that provide higher electronic conductivity and electrochemical activity in comparison with single-component oxides. NiCo2O4 is one of the most widely investigated MCo2O4 materials due to the low cost, environmentally benign nature, natural abundance, and high theoretical capacitance. For example, hierarchical NiCo2O4@NiCo2O4 core/shell nanoflakes on nickel foam were prepared via a combination of the hydrothermal method and chemical bath deposition.114 When used as electrodes in SCs,

they demonstrated high areal specific capacitances of 2.2 F cm−2 as well as excellent cycling stability and rate performance with 75% capacity retention when the current varied from 2 to 40 mA cm−2. The enhanced pseudocapacitive performances resulted from the unique core/shell structure, which facilitated ion and electron transfer and provided a large number of active sites and good strain accommodation. In order to further improve the electrochemical performance of NiCo2O4, it was combined with other metal oxides, such as MnO2, NiO, Co3O4, ZnO, and CoMoO4, forming new hybrids and demonstrating improved performance for energy storage. Other MCo2O4 species, such as ZnCo2O4, MnCo2O4, and CuCo2O4, were also studied and showed promising performance in SCs. 3.2.2. Transition Metal Hydroxide. Ni(OH)2, like NiO, is one promising pseudocapacitive electrode material due to its low cost, nature abundance, environmental friendliness, and high theoretical capacitance. It was also used as a positive electrode in asymmetric SCs, combing with the negative electrode, such as activated carbon or reduced graphene oxide. Yang et al. prepared one 3D nanoporous Ni(OH)2 thin film via the combination of anodization technology and the hydrothermal method.115 As shown in Figure 9a, a Ni(OH)2 thin film was prepared by anodization treatment of Ni following hydrothermal treatment at 100 °C for 4 h. The thin film was evaluated in a three-electrode system and demonstrated high specific capacitance of 1765 F g−1. Then, an ASC was assembled using this thin film as the positive 383


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Figure 9. (a) Schematic illustration of the fabrication process for a 3D nanoporous Ni(OH)2 thin film. (b) CV curves of Ni(OH)2 at different scan rates. (c) Galvanostatic charge−discharge curves of Ni(OH)2 at different current densities. (d) Cycling stability of Ni(OH)2 at 18 A g−1. (e) Ragone plot of the nanoporous layers (NPLs). Reprinted from ref 115. Copyright 2014 American Chemical Society.

electrode and a porous activated carbon film as the negative electrode. CV and galvanostatic charge−discharge experiments were carried out, and the device capacitance could reach 192 F g−1 (Figure 9b−d). More than 90% capacitance was retained after 10 000 cycles of charge−discharge processes at a high current density of 18 A g−1 (Figure 9e). The device possessed a high energy density of 68 Wh kg−1 and a power density of 44 kW kg−1 (Figure 9f). The synergy between the high surface area of the nanoporous Ni(OH)2 thin film and the good electrical conductivity contributed to the superior electrochemical performance of the Ni(OH)2 thin film in devices. The porous Ni(OH)2 can also be prepared by wet chemistry and/or hydrothermal reaction on the substrates, such as nickel foams and Au porous matrices, and assembled asymmetric SCs with other negative electrodes. In order to improve the electrochemical performance of Ni(OH)2, hybrid material was prepared by a one-step hydrothermal co-deposition method for growth of ultrathin Ni(OH)2−MnO2. The specific capacitance of the hybrid material could reach 2682 F g−1. When assembled in an ASC, the device demonstrated superior performance with a capacitance as high as 538 F g−1, an energy density of 186 Wh kg−1, and a power density of 778 W kg−1. Other metal hydroxides, such as Cu(OH)2 and Co(OH)2, were also invested in SCs. For example, the porous Co(OH)2 film can be prepared on different substrates, such as nickel foam, by electrodeposition, hydrothermal treatment, and wet chemistry. ASCs were also assembled using Co(OH)2 as positive electrodes and active carbon and carbon NTs as negative electrodes. The devices demonstrated improved performance in energy density. 3.2.3. Transition Metal Sulfide or Fluoride. Transition metal sulfides are suitable electrode candidates for SCs and have been extensively studied. Nickel sulfide can be prepared by different methods, such as the hydrothermal method and

electrodeposition technology. Yang et al. reported a porous Ni3S2 thin film with mushroom-like structure by the hydrothermal method.116 As shown in Figure 10a, nanorod or mushroom structures were prepared by controlling the reaction time on Ni foam. The porous Ni3S2 thin film showed high specific capacitance of 1190 F g−1 at 8 A g−1. When ASCs were assembled using the porous film as the positive electrode and active carbon as the negative electrode, the device showed an enlarged working potential of 1.8 V, high device capacitance of 154 F g−1, improved energy density of 69.48 Wh kg−1, and power density of 9000 W kg−1 (Figure 10b−d). The device also exhibited improved cycling stability with only 20% capacity decay after 5000 cycles at 4 A g−1 (Figure 10e). Other metal sulfides, such as cobalt sulfide and molybdenum sulfide, were also studied and demonstrated promising performance in energy storage. Nickel fluoride (NiF2) is another interesting pseudocapacitive material thanks to its high working potential and the possible transformation between NiF2 and Ni(OH)2, which has a theoretical capacitance of 2082 F g−1. A freestanding, flexible 3D nanoporous NiF2-dominant layer on poly(ethylene terephthalate) had been developed using anodization technology by Yang et al.117 Figure 11a showed the sandwich-structured SC device composed of a free-standing as-prepared 3D nanoporous layer (NPL) on the substrate and an electrolyte of KOH in poly(vinyl alcohol) (Figure 11b,c). The NPL showed not only the electrostatic adsorption effect during the low potential window of −0.8 to 0.8 V but also Pd−Cd battery-type characteristics in the high potential window of −1.4 to 1.4 V (Figure 11d−g). The device demonstrated excellent performance in energy storage, with a maximum capacitance of 66 mF cm−2 (733 F cm−3 or 358 F g−1), energy density of 384 Wh kg−1, and power density of 112 kW kg−1. When being tested within the narrow potential window of −0.8 to 0.8 V, the capacitance of the device increased to 220% during the initial 900 cycles. 384


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Figure 10. (a) Schematic illustration of the fabrication process for mushroom-like Ni3S2. (b) CV curves of the Ni3S2/AC device at different scan rates. (c) Galvanostatic charge−discharge curves of the Ni3S2/AC device at different current densities. (d) Ragone plot of the Ni3S2/AC device. (e) Cycling performance of the Ni3S2/AC device at 4A g−1. Adapted with permission from ref 116. Copyright 2015 Royal Society of Chemistry.

After 900 cycles, the capacitance gradually decreased and finally stabilized at 150% after 10 000 cycles. When tested within the larger potential window of −1.4 to 1.4 V, the devices first increased to 105% for the initial 300 cycles and then decreased with the increase of cycle numbers and stabilized at 76% retention after 10 000 cycles (Figure 11h). The device also demonstrated good mechanical stability with little capacitance variation at different bending states and extended cycle numbers (Figure 11i−k). 3.3. Inorganic Porous Film for Other Rechargeable Batteries. LIBs can provide high energy densities; however, they are still challenged by high manufacturing cost and safety issues. Some emerging rechargeable battery systems beyond LIBs have therefore been extensively investigated as promising alternatives for energy storage.118 One promising candidate to replace lithium is sodium, which has low cost and earth abundance. Currently, Na-ion batteries (NIBs) developed for electrochemical energy storage share a similar energy storage mechanism with LIBs; Na ions shuttle between electrodes during charge−discharge processes. However, the radius of the Na ion is about 55% larger than that of Li ion, which becomes a barrier to ion insertion and diffusion in the electrodes. To address this issue, some inorganic porous films have been developed to facilitate Na ion reversible

insertion and diffusion. Xiong et al. synthesized densely packed, vertically oriented amorphous TiO2 NTs using anodically treated Ti foil.118 These TiO2 NTs were used as additive-free anodes for NIBs and delivered a high specific capacity of 150 mAh g−1 after 15 cycles. Shen et al. synthesized Na3V2(PO4)3 porous materials by the sol−gel method combined with a freeze-drying process.119 When employed as a cathode for NIBs, it exhibited excellent rate performance and cyclability. Li−S batteries (LSBs) are another alternative to conventional LIBs owing to a high theoretical capacity of 1672 mAh g−1, which is much higher than that of commercial LIBs.120 Using controlled hydrolysis of a sol−gel precursor, Seh et al. fabricated sulfur− TiO2 yolk−shell nanostructures, which delivered a high capacity of 1030 mAh g−1 at 0.5 C and Coulombic efficiency of 98.4% over 1000 cycles.121 Li−O2 batteries (LOBs) have been widely studied owing to their ultrahigh theoretical energy density of 3500 mAh g−1, which is 5−10 times higher than that of commercial LIBs.122 Zhao et al. prepared Co3O4 hierarchically porous films via electrodeposition followed by calcination.123 These Co3O4 hierarchically porous films showed a specific capacity of 2460 mAh g−1 when used as cathodes in LOBs. 385


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Figure 11. (a) Schematic illustration of the flexible device of a 3D NPL on a Au (∼40 nm)/Cr (∼10 nm)/PET substrate (∼35 μm). (b) Photograph of the NPL electrode. (c) Freestanding NPL after removing the substrate. (d) CV curves of the NPL device at different scan rates in the potential window of −0.8 to 0.8 V. (e) Galvanostatic charge−discharge curves of the NPL device at a current density of 0.1 mA cm−2 in the potential window of −0.8 to 0.8 V. (f) CV curves of the NPL device at different scan rates in the potential window of −1.4 to 1.4 V. (g) Galvanostatic charge−discharge curves of the NPL device at a current density of 1 mA cm−2 in the potential window of −1.4 to 1.4 V. (h) Cycling performance of the NPL device in potential windows of −0.8 to 0.8 V and −1.4 to 1.4 V. (i) Photograph of the bended NPL device. (j) Capacitance retention of the NPL device in different bending states. (h) Dependence of the capacitance retention on bending cycles to a 180° bending angle. The inset shows charge−discharge curves recorded before and after bending for 500 and 1000 cycles. Reprinted from ref 117. Copyright 2014 American Chemical Society.

4. FUTURE PERSPECTIVES AND CONCLUDING REMARKS LIBs and SCs are dominant energy storage systems to meet the increasing demands for clean and renewable energy. Key challenges are the low power density and limited lifetime of LIBs and limited energy density of SCs. To overcome these challenges, new electrode materials with novel nanostructures, tunable pore size, and good conductivity should be developed. Inorganic porous films have shown promising advantages in high specific surface area, tunable pore sizes, and controllable surface physical and chemical characteristics, providing prospects for renewable energy storage. In this Review, we summarized the different synthesis methods of inorganic porous films and their attractive applications in LIBs and SCs. Although great progress has been achieved in inorganic porous films regarding the discovery of novel nanostructures, there are still many problems and obstacles at the current stage. In the future, extensive efforts can be devoted to the following aspects in order to satisfying the demands of the RESDs. 3D current collectors offer advantages over conventional planar current collectors because the active materials loaded in the 3D current collectors can form a thinner electrode layer than those on planar current collectors at the same loading mass. That will result in much less interfacial stress and better adhesion between the electrode and current collectors. Ni foam has been widely used for 3D current collectors recently. However, it is too expensive and cannot effectively improve the loading efficiency of electrode materials owing to the limited surface area. Therefore, novel mesoporous or microporous films possessing the abilities of loading active materials in different

dimensions should be developed as advanced 3D current collectors.

Lightweight and free-standing electrodes have the capabilities to significantly reduce the weight of whole devices. They can avoid the use of electrochemically inactive current collectors, which occupy a large portion of total weight in commercial LIBs. Therefore, lightweight and free-standing metal oxide porous films will considerably enhance the specific energy of the devices by reducing the device weight. Hierarchical pores provide gradients in pore size distribution that improve the contact between electrolyte and electrodes and facilitate mass/ion transport inside of electrodes. This characteristic will significantly enhance the power density of devices. Therefore, further effort should be devoted to develop the 3D hieratically porous structure (micropores on the surface and nanopores inside of electrode materials) with open channels for inorganic porous films. Additive-free electrodes are crucial in developing long-lifetime energy storage devices considering that the additives, including carbon black and organic binders, are one of the most frequently encountered battery failure sources in conventional LIBs, for 386


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(3) Armand, M.; Tarascon, J.-M. Building Better Batteries. Nature 2008, 451, 652−657. (4) Winter, M.; Brodd, R. J. What are Batteries, Fuel Cells, and Supercapacitors? Chem. Rev. 2004, 104, 4245−4270. (5) Liu, C.; Li, F.; Ma, L.; Cheng, H. Advanced Materials for Energy Storage. Adv. Mater. 2010, 22, E28−E62. (6) Dunn, B.; Kamath, H.; Tarascon, J.-M. Electrical Energy Storage for the Grid: a Battery of Choices. Science 2011, 334, 928−935. (7) Jiang, J.; Li, Y.; Liu, J.; Huang, X.; Yuan, C.; Lou, X. W. D. Recent Advances in Metal Oxide-Based Electrode Architecture Design for Electrochemical Energy Storage. Adv. Mater. 2012, 24, 5166−5180. (8) Kroutvar, M.; Ducommun, Y.; Heiss, D.; Bichler, M.; Schuh, D.; Abstreiter, G.; Finley, J. J. Optically Programmable Electron Spin Memory Using Semiconductor Quantum Dots. Nature 2004, 432, 81− 84. (9) Hu, L.; Deng, Y.; Liang, K.; Liu, X.; Hu, W. LaNiO3/NiO Hollow Nanofibers with Mesoporous Wall: a Significant Improvement in NiO Electrodes for Supercapacitors. J. Solid State Electrochem. 2015, 19, 629− 637. (10) Liang, K.; Tang, X.; Hu, W.; Yang, Y. Ultrafine V2O5 Nanowires in 3D Current Collector for High-Performance Supercapacitor. ChemElectroChem 2016, 3, 704−708. (11) Wang, D.; Yang, J.; Li, X.; Geng, D.; Li, R.; Cai, M.; Sham, T.; Sun, X. Layer by Layer Assembly of Sandwiched Graphene/SnO2 Nanorod/ Carbon Nanostructures with Ultrahigh Lithium Ion Storage Properties. Energy Environ. Sci. 2013, 6, 2900−2906. (12) Cao, Z.; Wei, B. A Perspective: Carbon Nanotube Macro-Films for Energy Storage. Energy Environ. Sci. 2013, 6, 3183−3201. (13) Zhu, Y.; Peng, L.; Chen, D.; Yu, G. Intercalation Pseudocapacitance in Ultrathin VOPO4 Nanosheets: Toward High-Rate Alkali-IonBased Electrochemical Energy Storage. Nano Lett. 2016, 16, 742−747. (14) Zhang, X.; Yang, W.; Liu, J.; Zhou, Y.; Feng, S.; Yan, S.; Yao, Y.; Wang, G.; Wan, L.; Fang, C. Ultralong Metahewettite CaV6O16·3H2O Nanoribbons as Novel Host Materials for Lithium Storage: Towards High-Rate and Excellent Long-Term Cyclability. Nano Energy 2016, 22, 38−47. (15) Li, H.; Peng, L.; Zhu, Y.; Chen, D.; Zhang, X.; Yu, G. An Advanced High-Energy Sodium Ion Full Battery Based on Nanostructured Na2Ti3O7/VOPO4 Layered Materials. Energy Environ. Sci. 2016, 9, 3399−3405. (16) Liang, K.; Tang, X.; Hu, W. High-Performance ThreeDimensional Nanoporous NiO Film as a Supercapacitor Electrode. J. Mater. Chem. 2012, 22, 11062−11067. (17) Liang, K.; Gu, T.; Cao, Z.; Tang, X.; Hu, W.; Wei, B. In Situ Synthesis of SWNTs@MnO2/Polypyrrole Hybrid Film as Binder-Free Supercapacitor Electrode. Nano Energy 2014, 9, 245−251. (18) Wang, G.; Zhang, L.; Zhang, J. A Review of Electrode Materials for Electrochemical Supercapacitors. Chem. Soc. Rev. 2012, 41, 797−828. (19) Bruce, P. G.; Scrosati, B.; Tarascon, J. M. Nanomaterials for Rechargeable Lithium Batteries. Angew. Chem., Int. Ed. 2008, 47, 2930− 2946. (20) Shi, Y.; Yu, G. Designing Hierarchically Nanostructured Conductive Polymer Gels for Electrochemical Energy Storage and Conversion. Chem. Mater. 2016, 28 (8), 2466−2477. (21) Shi, Y.; Peng, L.; Ding, Y.; Zhao, Y.; Yu, G. Nanostructured Conductive Polymers for Advanced Energy Storage. Chem. Soc. Rev. 2015, 44 (19), 6684−6696. (22) Liu, B.; Soares, P.; Checkles, C.; Zhao, Y.; Yu, G. ThreeDimensional Hierarchical Ternary Nanostructures for High-Performance Li-Ion Battery Anodes. Nano Lett. 2013, 13 (7), 3414−3419. (23) Pan, L.; Yu, G.; Zhai, D.; Lee, H. R.; Zhao, W.; Liu, N.; Wang, H.; Tee, B. C.-K.; Shi, Y.; Cui, Y.; et al. Hierarchical Nanostructured Conducting Polymer Hydrogel with High Electrochemical Activity. Proc. Natl. Acad. Sci. U. S. A. 2012, 109 (24), 9287−9292. (24) Choudhary, S.; Upadhyay, S.; Kumar, P.; Singh, N.; Satsangi, V. R.; Shrivastav, R.; Dass, S. Nanostructured Bilayered Thin Films in Photoelectrochemical Water Splitting-A Review. Int. J. Hydrogen Energy 2012, 37, 18713−18730.

instance, the thermal runaway and Li plating caused by temperature fluctuations. To develop advanced electrodes with reliable lifetime and good electrical conductivity, therefore, metallic porous films are desired. Good mechanical properties are essential characteristics of an electrode for energy storage. Current noncarbonaceous electrodes are not robust because of the brittle nature of most metal oxides and other metal compound active materials. Improving the mechanical properties of porous films becomes important to meet the requirements of electrodes in device. Scalable nanomanufacturing of inorganic porous films is desired to produce high-performance electrodes for energy storage in a low-cost manner. The synthesis methods of inorganic porous films, electrodeposition and anodization, are preferred because they are more economicial and effective than the other methods such as template-assisted fabrication processes and the sol−gel method. More efforts in scalable nanomanufacturing of inorganic porous films are encouraged in the future.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.Y.). *E-mail: [email protected] (L.L.). ORCID

Yang Yang: 0000-0002-4410-6021 Notes

The authors declare no competing financial interest. Biographies Kun Liang received his Ph. D with Professor Wencheng Hu in Materials Science and Engineering from the University of Electronic Science and Technology of China in 2015. He is currently a Postdoctoral Associate in Professor Yang Yang’s group, University of Central Florida. His current research interests include nanoporous materials, energy storage and conversion devices, electrocatalysis, and photoelectrocatalysis. Lei Li received his Ph.D. from Professor James Tour’s group at Rice University in 2015. After graduation, he moved to Northwestern University and joined Professor Mark Hersam’s group as a postdoctoral research associate, where his research included synthesis and application of printable materials in electrochemical capacitors and transitional metal oxide cathodes in lithium ion batteries. Yang Yang obtained his Ph.D. from Tsinghua University in 2010. From 2010 to 2012, he was supported by the Alexander von Humboldt Postdoctoral Fellowship and worked with Prof. Dr. Patrik Schmuki at the University of Erlangen−Nuremberg. From 2012 to 2015, he was supported by the Peter M. & Ruth L. Nicholas Postdoctoral Fellowship and worked with Prof. Dr. James M. Tour at Rice University. Since 2015, he has been assistant professor at the University of Central Florida. His current research interests cover nanostructured films, renewable energy generation and storage, and flexible electronics. http://www. yangyanglab.com/

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ACKNOWLEDGMENTS This work was financially supported by the University of Central Florida through a startup grant (No. 20080741).

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