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Inorganic Porous Films for Renewable Energy Storage Kun Liang, Lei Li, and Yang Yang ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00666 • Publication Date (Web): 06 Jan 2017 Downloaded from http://pubs.acs.org on January 7, 2017

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ACS Energy Letters

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

NanoScience Technology Center (NSTC), Department of Materials Science and Engineering,

University of Central Florida, Orlando, FL, 32826, USA ‡

Department of Materials Science and Engineering, Northwestern University, Evanston, IL, USA

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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TOC


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Contents 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


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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 resource, environmental pollution and climate change.1-2 Current concerns about limited energy resources and the need to decrease soaring greenhouse gas emissions has brought about an urgent quest to develop renewable resources that offer sustainable energy without sacrificing environmental quality.3-4 Especially the emerging of 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 interface. The increasing demand for high-performance RESDs promotes to understand and manipulate physical and chemical properties of electrode and its interface with electrolyte. With the recent advance in nanotechnology, the 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 threedimensional (3D) network of nanostructured 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


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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 materials and conducting polymers, for example vertically aligned carbon nanotubes, 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 since they have been widely reviewed.12, 2023, 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 the non-expert but who are interested in nanotechnology.

2. Synthesis Methods and Particular Structural Features Inorganic porous films are generally produced via either top-down 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. 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 favourable for the applications of energy storage devices without using additional current collectors, binders and/or additives used in powdered materials. The substrates are not always necessary and can be removed after forming porous films in some cases. Freestanding and flexible porous films are also developed for the applications in flexible and wearable energy storage devices. In this section, we will introduce the main techniques


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developed to fabricate inorganic porous films. 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 aluminium oxide (AAO) which has been highly investigated as a template to fabricate low dimensional nanomaterials such as oxide nanotubes and nanowires.33-34 Anodization and AAO were first discovered by Buff at 1857. They were commercialized for protection of seaplane parts from corrosive seawater at 1923. This encouraged researchers on 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 certain anodic voltage on the surface of interested metal M under "high-field law" or a competition reaction between the oxide formation (eq 1) and dissolution (etching, eq 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 of the formation of anodized porous films is to maintain a reaction balance between oxide formation and dissolution processes.38 If oxidization formation reactions dominate the process, dense oxide layers would be formed. If dissolution reactions dominate the process, oxide layers would be dissolved and electropolishing process would take place. n M + H 2 O → MO n 2 + nH + + ne 2 ( m− n)-

+

nH MO n 2 + mF−  → [ MFm ]

(1)

+

n H 2O 2

(2)

( m− n ) -

Mn + + mF− → [ MFm ]

(3)

Some representative milestones of anodization technology developed in the past century are 6 ACS Paragon Plus Environment

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documented below. Porous anodized silicon was discovered accidently by Uhlir at the Bell Laboratories in 1956.39 It is currently one of the most popular multifunctional materials in photovoltaics, LIBs, and bio-imaging 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 Ti sheet in a fluoride-contained electrolyte.44 Owing to the great prospect of application in solar energy harvesting, biomedicine, and semiconductor industry, intense efforts have been devoted on 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, etc. 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 regarded as utterly impossible to prepare the self-ordered 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 electrolytes, such as [TiF6]2- and [BF4]-.50 Inspired by this work, Lee reported an 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 oxides porous films by using refined anodization technique.52-53 Beyond applying anodization to different metals, functionalization was new direction of the porous films. This involves both direct ways, for instance doping porous films by alloy anodization and post-annealing in different gas atmosphere, and indirect 7 ACS Paragon Plus Environment

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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 the corrosion protection. Some particular features make this technique attractive in nanotechnology and materials science: i) It can be precisely turned 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, e.g. aqueous, organic and ionic liquid solution.55-58 Several processes are involved in the electrodeposition. They are the mass transport of metal ions or complexes towards electrodes (substrates) under an applied electric field, nucleation and accumulation of deposits on the electrodes, and deposits 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, the electrodeposited porous films or nanostructured films have been extensively studied due to their particular electrochemical and optical properties. In 1997, Attard electrodeposited Pt film with a well-defined, long-ranged, porous nanostructure by using liquid crystalline plating solutions. It was suggested to be a versatile way to create porous electrodes for electrochemical applications such as renewable


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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 in nano or micro scale. 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 lead 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 demonstrated that Cl- and CH3COOpreferentially adsorbed onto the (0001) plane of ZnO to produce platelet-like crystals,61 while EDA and NH4F tended 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


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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 three-dimensionally nanoporous sponge or bicontinuous interpenetrating nanostructure. Theoretically, various forming models, such as 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 Based on the 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 as catalysts for the oxygen reduction reaction (ORR). It was found that the electrochemical dealloying of non-noble base metals significantly altered surface catalytic activity of the remaining porous Pt.75 The results suggested that dealloying can be utilized as a versatile strategy to tune surface catalytic activity of noble metal catalysts in heterogeneous catalytic or electrocatalytic environments. Beyond noble metal porous films, Chen de-alloyed Cu30Mn70 alloy to prepare the porous Cu films with pore size distribution from 15 to 200 nm for photonic applications.76 He found that porous Cu films had a dramatic improvement in the surface-enhanced Raman scattering (SERS). At the range of 30-50 nm of pore size, 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 de-alloyed Pt-Co nanowires into highly porous


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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 for producing inorganic ceramic and solid materials from small molecules since 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 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 phase in order to convert sol to gel, for example naturally sedimentation, mechanically centrifugation and thermally drying. The porous structure and porosity of the gel are strongly determined by liquid removing 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 the 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


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cycles. Yang et al. prepared 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 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. Based on the templates used in this synthesis technique, two subcategories have been identified: hard-template and soft-template methods. The hard-template method employs the 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 hard-template method, one-dimensional nanostructures, for instance nanowires, nanotubes, and nanofibers, can be obtained after removing templates. Template-assisted electrodeposition, chemical vapor deposition (CVD) and atomic layer deposition (ALD) brought 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 nanotube arrays were prepared using different hard template. Although different kinds of porous nanomaterials can be prepared effectively by hardtemplate synthesis, removing 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 the advantage in the high yield, which is feasible for scalable nanomanufacturing of nanomaterials in large quantity. Currently, soft-template method has


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become a general way to fabricate ordered mesoporous materials. Typical soft-templates including anionic, cationic, non-ionic 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 sol-gel process, hydrothermal method, evaporation induced self-assembly (EISA) process, and post-synthesis treatment etc. Two general synthesis pathways were reported for soft-template method, including cooperative self-assembly and liquid-crystal templating process.82 In the cooperative selfassembly 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 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 porous structure. Liang et al. prepared LaNiO3/NiO mesoporous film with polyethylene glycol (PEG) as 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 the heat treatment. Soft-template method is 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 becomes an ideal method to prepare inorganic porous film. 2.5 Particular Structural Features


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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, controllable surface physical and chemical characteristics. With the development of portable and flexible electronics, it becomes urgent to develop lightweight, flexible, robust and free-standing 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 supercapacitors. Surface area is an important merit of the inorganic porous film. High specific area can provide more active sites and interfaces to enhance the electrochemical performance in the devices. Take hydrogenated NiCo2O4 hollow spheres as an example, Li et al. prepared single-shell and doubleshell NiCo2O4 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 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 supercapacitors. Highly porous films with open pathways/channels can facilitate the mass and/or electron transport at the electrolyte/electrode interfaces. These materials are suitable for the applications in renewable energy storage. Firstly, the porous structure can provide large surface area and enhance the mechanical properties which can relax the mechanical strain caused by the volume change in the electrochemical test. Secondly, the highly porous and interconnected channels can facilitate the ion/electron transport throughout the entire film. Zhu et al. prepared the


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carbon/metal oxide nanocomposites.86 The porous structure with open channels of the nanocomposites are well retained after calcination process. With this porous architecture, 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 life-time. They are widely used to power present-day portable consumer devices, such as smart phone, iPad, and laptops, and emerging electrical/hybrid vehicles. With the expected use of large-scale LIBs, the industry-projected demand and growth is 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 film 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 intercalation/de-intercalation mechanism, alloying/de-alloying mechanism, and conversion reaction or redox reaction mechanism. TiO2 stores energy based on the lithium interaction/de-intercalation 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 a AAO template-assisted electrodeposition of Ni, followed by TiO2 coating using ALD.88 As shown in Figure 2a, AAO template was prepared by


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anodization technology on Al foil and then used as substrate for electroplating Ni. After removing AAO, TiO2 was deposited on Ni substrate by ALD method forming the 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 the 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 constituted with different nanostructures, such as nanowires, nanofibers, and nanotubes, 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 stored 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 the well-aligned Li4Ti5O12 nanotubes 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 nanotube 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 promising in LIBs. To improve the capacity of the intercalation/de-intercalation system, electrode materials stored energy based on the conversion reaction were developed. This system, including most of the transition metal oxide, can store more than 3 times of capacity than that of graphite. Iron 16 ACS Paragon Plus Environment

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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 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. Ni scaffold worked as 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 the thickness ~ 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 still maintained at 400 mAh g-1 after 100 cycles at such high rate (Figure 3k). There was no obvious deformation of the structure after such hash test condition, 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 nanotubes and Fe2O3-Fe3C-Fe were prepared by 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 the porous metal oxide thin films through the electrodeposition or wet chemistry methods. These materials showed improved performance in energy storage, especially in cyclability and rate capabilities due to the improved electrical conductivity and the facilitated electron transport in the porous structures. Cobalt oxide is another most investigated material stored energy based on conversion reaction due to its high theoretical capacitance (890 mAh g-1). Li et al. prepared the porous thin film constituted with Co3O4 nanowires on different substrates, such as Si, Cu, and Ti, by wet chemistry method.93 The thin film showed high capacity and good rate performance with 50% 17 ACS Paragon Plus Environment

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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 oxide, such as NiO and Fe2O3, forming the new hybrid materials taking both advantages of these materials. Wang et al. developed one facile, rapid and scalable method to prepare the porous FeCo2O4 on Cu substrate.94 Graphene was introduced into the thin film for the improvement of 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 these improvement 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 stored energy based on alloying-dealloying mechanism. The theoretical capacity can reach 3579 mAh g-1, almost an order of magnitude of conventional graphite. However, the huge volumetric changes in the Si anode during the alloying-dealloying processes leads to the limited cycling stability, reducing 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 the porous films constituted 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 invested by TEM and EDX elemental mapping, demonstrating the uniformity of the composites (Figure 4f-j). In this structure, Si was the major 18 ACS Paragon Plus Environment

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active materials for the lithium ion storage. Cu cores worked as both current collectors and structural framework loading Si. The outer coating-layer of Al2O3 can stabilize the solid electrolyte interface (SEI) layer formed by the decomposition of the electrolyte. Cu-Si-Al2O3 showed the high capacity of 1820 mAh g-1 at 0.3 A g-1. It can still maintain 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 showed 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 the 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 chemical vapor deposition, 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 the 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 method to prepare the V2O5 thin films.98-99 For example, Chen et al. prepared V2O5 thin film using ALD technology on AAO substrate without any thermal post-treatment.100 V2O5 uniformly loaded on AAO, forming the porous thin film (Figure 5a-b). V2O5 thin film demonstrated high specific capacity of 142 mAh g1

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 the current density of 150 mA g-1. The improved performance resulted from the synergetic between the 19 ACS Paragon Plus Environment

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crystallinity of V2O5 and the structure. Beyond V2O5, other cathode materials were also studied. For example, 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 the quick capacity decay. In order to improve its cycling stability, one monolayer graphene was transferred on the top surface LMO thin film, resulting in the 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 1000th cycle. 3.2 Supercapacitors SCs, also named electrochemical capacitors, have attracted great attention due to the high power density, long life-time, low maintenance cost. SCs have been widely used in consumer electronics, memory back-up systems, and industrial power and energy management. Based on the energy storage mechanisms of electrode materials, SCs can be categorized into two types. The first type of SCs is the electrical double layer capacitors (EDLCs), which stores energy at the electrode/electrolyte interfaces by the electrostatic charge accumulation. The second type is the pseudocapacitor stored energy through the fast and reversible faradic reaction near the electrode


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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, the 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 materials 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 (~ 10-510-6 S cm-1) limited its application in SCs. To boost its electrochemical performance in SCs, nanostructuring and preparing composites with electrical conductive metrics are two major effective strategies to improve the electrical conductivity of MnO2-based electrode materials. The porous MnO2 film on electrical conductive metrics, prepared by several kinds of methods, is one promising electrode materials taking the advantages of the high surface area and good electrical conductivity, which facilitates the electrolyte contact and diffusion in electrode materials. Electrodeposition is one of the most used methods to prepare the porous films on different substrates. Yu et al. reported the porous MnO2 film on AuPd coated substrate with nanopillars structure as shown in Figure 6. The material was prepared in three steps, which are the fabrication of an inert polymer with nanopillars structure, the deposition of AuPd on the substrate as current collector electrode material, and electrodeposition of MnO2 on to the nanostructured electrode, sequentially ( Figure 6a). As shown in Figure 6b-c, it is obvious to see that porous


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MnO2 was uniformly deposited on the nanopillars structures. This materials showed good electrochemical performance with the high specific capacitance of 603 F g-1 at 5 mV s-1, 93% capacitance retention after 5000 cycles, and superior energy density 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 metrics for MnO2 loading using electrodeposition method, forming the 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 can reach 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 constituted with 3D ZnO@MnO2 coreshell branched nanowire was prepared using ZnO nanowires as substrate in KMnO4 solution. The porous film demonstrated good electrochemical performance with high specific capacitance of 31.3 mF cm-2 thanks to the enhanced surface area and higher loading amount of MnO2 nanoparticle on the 3D ZnO nanoforest backbone. MnO2 can also be directly prepared by oxidization of Mn in air. Li et al. reported one porous thin film composited of sandwichstructured MnO2-Mn-MnO2 tube by oxidization 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 the fast electron transportation. The sandwich structure enhanced the contact between electrolyte and electrode materials, facilitating the ion diffusion. The materials demonstrated superior performance with 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 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. 22 ACS Paragon Plus Environment

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Flexiable SCs were also fabricated based on the porous MnO2 thin film for the flexible electronics, such as wearable electronics, mobile phones, electronic papers, solar cells, and other collapsible gadgets. One MnO2 thin film on the nanoporous gold metric was prepared by electrodepostion method.106 The nanocrystaline MnO2 was uniformly deposited in nanoporous Au metrics forming the hybrad 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 maerials, played the major role in charge storage through the fast, reverisble redox reaction on its surface. The symmetric supercapacitor was abricated as shown in Figure 7b. Based on the cyclic voltammogram (CV) curves and galvaostatic charge dichage experiments, the hybrid with 20 min depostion of MnO2 had the best electrochemical performance with the high specific capacitance of 1145 F g-1, imporved energy density of ~ 57 Wh kg-1 and power density of ~ 16 kW kg-1 (Figure 7c-e). In order to enlarge working potential range and improve the energy density, asymmetric supercapacitors (ASCs) were developed. Qiu et al. reported three dimensional MnO2 nanocone array on flexible substrates by 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, the 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 carbonbased electrode materials in SCs, pseudocapacitive materials were developed as negative electrodes in asymmetric SCs. Chodankar et al. prepared one Fe2O3 thin film by wet chemical method. It was used as negative electrode combined with MnO2 thin film assembling ASCs.108 23 ACS Paragon Plus Environment

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The capacity has no big variation at different bending angles, demonstrating the superior mechanical stability. The energy density of the ASCs can reach 41 Wh kg-1 at the power density of 2.1 kW kg-1. Two ASCs devices in parallel were only charged 30 s can light 31 red LED for 2 min, exhibiting the promising practical applications . NiO is another promising pseudocapacitive material for SCs due to its intriguing features, such as low cost, natural abundance, environmentally 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 Cu substrate.109 Figure 8a showed the schematic illustration of the preparation of samples. The electrodeposited Ni film on Cu foil was oxidized at high temperature in air to obtain the NiO NBs, which was further annealed in H2 environment obtaining the H-NiO NBs. The morphologies were 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 the 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 the advantage of templates. Cao et al. used ZnO as template prepared the porous film constituted with NiO hollow nanowires by electrodeposition and wet chemistry methods. The materials demonstrated good performance in 24 ACS Paragon Plus Environment

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SCs.110 Nickel was also used as template preparing the porous film arranged by hydrogenated Ni/NiO core/shell one dimensional nano-heterostructures. One porous NiO network-like films on indium-doped SnO2 glass (ITO/glass) substrates was prepared by the combination of a green solvothermal method and the following heat-treatment in air. The film was assembled into the SCs and demonstrated high specific capacitance of 960 F g-1 with a Coulombic efficiency of ~ 100% at of 20 A g-1 and promising capacitance retention of 93% of its maximum value after 1000 cycles. 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 the hydrothermal reaction.111 The material showed improved electrochemical performance thanks to the synergetic 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 plasma plume producing Co3O4 nanoparticles through an accelerated thermo-chemical conversion process. They were deposited on the current collector, forming 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 net-like structure of interconnected nanoflakes with a thickness of 15-20 nm.112 The porous film was prepared by a cathodic electrodeposition via liquid crystalline template. When used as electrodes in SC device, the material demonstrated good performance with high 25 ACS Paragon Plus Environment

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specific capacitances 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 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 a good cyclability. In order to further improve the electrochemical performance of transition metal oxide, 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 due to the low cost, environmentally benign nature, natural abundance and high theoretical capacitance. For example, hierarchical NiCo2O4@NiCo2O4 core/shell nanoflake on nickel foam were prepared via a combination of hydrothermal method with chemical bath deposition.114 When used as electrodes in SCs, it 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 its unique core/shell structure, which facilitated ion and electron transfer, 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 26 ACS Paragon Plus Environment

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oxides, such as MnO2, NiO, Co3O4, ZnO, and CoMoO4, forming new hybrids and demonstrating the improved performance for energy storage. Other MCo2O4, 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 materials due to the low cost, nature abundance, environmentally friendliness, and high theoretical capacitance. It was also used as 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 with hydrothermal method.115 As shown in Figure 9a, Ni(OH)2 thin-film was prepared by anodization treatment of Ni following the hydrothermal treatment at 100 oC for 4 h. The thin film was evaluated in three electrode system and demonstrated high specific capacitance of 1765 F g-1. Then ASC was assembled using this thin-film as positive electrode and porous activated carbon film as negative electrode. CV and galvanostatic charge discharge experiments were carried out and the device capacitance can reach 192 F g-1 (Figure 9b-d). More than 90% capacitance retained after 10,000 cycles of charge/discharge processes at high current density of 18 A g-1 (Figure 9e). The device possessed high energy density of 68 Wh kg-1 and power density of 44 kW kg-1 (Figure 9f). The synergetic 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 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 metrics, and assembled asymmetric SCs with other negative electrodes. In order to improve the electrochemical performance of Ni(OH)2, hybrid material was prepared by one-step hydrothermal co-deposition 27 ACS Paragon Plus Environment

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method for growth of ultrathin Ni(OH)2-MnO2. The specific capacitance of the hybrid material can reach 2682 F g-1. When assembled in ASC, the device demonstrated superior performance with the capacitance as high as 538 F g-1, the energy density of 186 Wh kg-1 and 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 nanotubes 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 hydrothermal method and electrodeposition technology. Yang et al. reported a porous Ni3S2 thin film with mushroom-like structures by 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 with 1190 F g-1 at 8 A g-1. When ASCs were assembled using the porous film as 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 sulfide, such as cobalt sulfide, and molybdenum sulfide, were also studied and demonstrated promising performance in energy storage.


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Nickel fluoride (NiF2) is another interesting pseudocapacitive materials thanks to its high working potential and the possibility transformation between NiF2 and Ni(OH)2, which has theoretical capacitance (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 SCs device composited of the free-standing asprepared 3D nanoporous layer (NPL) on the substrate and the electrolyte of KOH in poly(vinyl alcohol) (Figure 11b-c). NPL not only showed the electrostatic adsorption effect during the low potential window of -0.8 to 0.8 V, but also the Pd-Cd battery type characteristic in the high potential window of -1.4 to 1.4 V (Figure 11d-g). The device demonstrated excellent performance in energy storage, with the 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. 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 firstly 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 the 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 the 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. 29 ACS Paragon Plus Environment

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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 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 nanotubes (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) is 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 ultra high theoretical energy density of 3500 mAh g-1, which is 5-10 times higher than the 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.

4. Future Perspectives and Concluding Remarks 30 ACS Paragon Plus Environment

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Li-ion batteries and supercapacitors 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 lithium-ion batteries and limited energy density of supercapacitors. To overcome these challenges, new electrode materials with novel nanostructures, tunable pore size and good conductivity, should be developed. Inorganic porous film have shown the promising advantages in high specific surface area, tunable pore sizes, controllable surface physical and chemical characteristics, providing prospects for the renewable energy storage. In this review, we summarized the different synthesis methods of inorganic porous films and their attractive applications in Li-ion batteries and supercapacitors. Although great progresses have been achieved in inorganic porous film 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 renewable energy storage devices. 3D current collectors offer advantages over conventional planar current collectors, because the active materials loaded in the 3D current collectors can form thinner electrode layer than 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 was widely used 3D current collectors recently. However, it was 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 the electrochemically inactive current 31 ACS Paragon Plus Environment

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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 improves the contact between electrolyte and electrodes and facilitates mass/ion transport inside electrodes. This characteristic will significant enhance the power density of device. Therefore, further effect should be devoted to develop the 3D hieratically porous structure (micropores on the surface and nanopores inside electrode materials) with open channels for inorganic porous films. Additive-free electrode is crucial in developing long lifetime energy storage devices concerning that the additives, including carbon black and organic binders, are one of the most frequently encountered battery failure sources in conventional LIBs, for instance 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 characteristic of an electrode for energy storage. Current non-carbonaceous electrodes are not robust because of the crispy nature of most metal oxide 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 highperformance 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 economic and effective than the other methods such as template-assisted fabrication processes and sol-gel method. More efforts in scalable nanomanufacturing of inorganic porous films are encouraged in the future.


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Quote-1, page 5 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 favourable for the 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.

Quote-2, page 33 Lightweight and free-standing electrodes have the capabilities to significantly reduce the weight of whole devices. They can avoid the use of the 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.

Corresponding Authors *

E-mail: [email protected] (Y. Yang); [email protected] (L. Li).

Notes The authors declare no competing financial interests.

Biographies Kun Liang received his Ph. D with Professor Wencheng Hu in Materials Science and Engineering from 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. 33 ACS Paragon Plus Environment

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His current research interests include nanoporous materials, energy storage and conversion devices, electrocatalysis and photoelectrocatalysis. Lei Li received his PhD 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/

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


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Asymmetric Supercapacitors based on MnO2 and Fe2O3 Thin Films. Energy Technol. 2015, 3, 625-631. (109) Singh, A. K.; Sarkar, D.; Khan, G. G.; Mandal, K. Hydrogenated NiO Nanoblock Architecture for High Performance Pseudocapacitor. ACS Appl. Mater. Inter. 2014, 6, 4684-4692. (110) Cao, F.; Pan, G. X.; Xia, X. H.; Tang, P. S.; Chen, H. F. Synthesis of Hierarchical Porous NiO Nanotube Arrays for Supercapacitor Application. J. Power Sources 2014, 264, 161-167. (111) Gao, M.; Sheng, W.; Zhuang, Z.; Fang, Q.; Gu, S.; Jiang, J.; Yan, Y. Efficient Water Oxidation Using Nanostructured α-Nickel-Hydroxide as an Electrocatalyst. J. Am. Chem. Soc. 2014, 136, 7077-7084. (112) Yuan, Y. F.; Xia, X. H.; Wu, J. B.; Huang, X. H.; Pei, Y. B.; Yang, J. L.; Guo, S. Y. Hierarchically Porous Co3O4 Film with Mesoporous Walls Prepared via Liquid Crystalline Template for Supercapacitor Application. Electrochem. Commun. 2011, 13, 1123-1126. (113) Duan, B. R.; Cao, Q. Hierarchically Porous Co3O4 Film Prepared by Hydrothermal Synthesis Method Based on Colloidal Crystal Template for Supercapacitor Application. Electrochim. Acta 2012, 64, 154-161. (114) Liu, X.; Shi, S.; Xiong, Q.; Li, L.; Zhang, Y.; Tang, H.; Gu, C.; Wang, X.; Tu, J. Hierarchical

NiCo2O4@NiCo2O4

Core/Shell

Nanoflake

Arrays

as

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Supercapacitor Materials. ACS Appl. Mater. Inter. 2013, 5, 8790-8795. (115) Yang, Y.; Li, L.; Ruan, G.; Fei, H.; Xiang, C.; Fan, X.; Tour, J. M. Hydrothermally Formed Three-Dimensional Nanoporous Ni(OH)2 Thin-Film Supercapacitors. ACS Nano 2014, 8, 9622-9628. (116) Yang, B.; Yu, L.; Liu, Q.; Liu, J.; Yang, W.; Zhang, H.; Wang, F.; Hu, S.; Yuan, Y.; Wang, J. The Growth and Assembly of the Multidimensional Hierarchical Ni3S2 for Aqueous


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Asymmetric Supercapacitors. CrystEngComm 2015, 17, 4495-4501. (117) Yang, Y.; Ruan, G.; Xiang, C.; Wang, G.; Tour, J. M. Flexible Three-Dimensional Nanoporous Metal-Based Energy Devices. J. Am. Chem. Soc. 2014, 136, 6187-6190 (118) Xiong, H.; Slater, M. D.; Balasubramanian, M.; Johnson, C. S.; Rajh, T. Amorphous TiO2 nanotube anode for rechargeable sodium ion batteries. J. Phys. Chem. Lett. 2011, 2, 2560-2565. (119) Shen, W.; Wang, C.; Liu, H.; Yang, W. Towards Highly Stable Storage of Sodium Ions: A Porous Na3V2 (PO4) 3/C Cathode Material for Sodium-Ion Batteries. Chem.-Eur. J. 2013, 19, 14712-14718. (120) Wang, D. W.; Zeng, Q.; Zhou, G.; Yin, L.; Li, F.; Cheng, H. M.; Gentle, I. R.; Lu, G. Q. M. Carbon-Sulfur Composites for Li-S Batteries: Status and Prospects. J. Mater. Chem. A 2013, 1 (33), 9382-9394. (121) Seh, Z. W.; Li, W.; Cha, J. J.; Zheng, G.; Yang, Y.; McDowell, M. T.; Hsu, P. C.; Cui, Y. Sulphur-TiO2 Yolk-Shell Nanoarchitecture with Internal Void Space for Long-Cycle LithiumSulphur Batteries. Nat. Commun. 2013, 4, 1331. (122) Bruce, P. G.; Freunberger, S. A.; Hardwick, L. J.; Tarascon, J.-M. Li-O2 and Li-S Batteries with High Energy Storage. Nat. Mater. 2012, 11, 19-29. (123) Zhao, G.; Xu, Z.; Sun, K. Hierarchical Porous Co3O4 Films as Cathode Catalysts of Rechargeable Li-O 2 Batteries. J. Mater. Chem. A 2013, 1, 12862-12867.


ACS Energy Letters

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


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Figure 2. (a) Schematic illustration of the preparation of 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 reference 88. Copyright 2012 American Chemical Society.


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Figure 3. (a-e) Schematic illustration of the preparation of 3D Fe2O3 electrode. (f) SEM image of Ni inverse opal. (g-i) SEM images of 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 reference 90. Copyright 2015 American Chemical Society.


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ACS Energy Letters

Figure 4. (a-e) Schematic illustration of the preparation of Cu-Si Al2O3 electrode. (f) TEM image of Cu-Si Al2O3 electrode. (g-j) EDX elemental mapping of O, Al, Si, and Cu in Cu-Si Al2O3 electrode. (k) Voltage profile of Si-Cu for the first two cycles at the current density of 0.3 A g-1. (l) Voltage profile of Cu-Si Al2O3 electrode for the first two cycles at the 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 reference 93. Copyright 2011 John Wiley & Sons.


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


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ACS Energy Letters

Figure 6. (a) Schematic illustration showing the preparation of MnO2 coated electrodes. (b, c) SEM images of electrodeposited MnO2 nanoparticles on nano pillar arrays with (b) top view and (c) side view. (d) CV curves of NE and PE at a scan rate of 100 mV s-1 in 1M Na2SO4. (e) CV curves of NE at different scan rate range of 5-100 mV s-1. (f) Specific capacitances of NE and PE at different scan rates. (g) Galvanostatic charge discharge curves of NE at 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 reference 103. Copyright 2013 John Wiley & Sons.


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Figure 7. (a) Schematic illustration of the fabrication process for nanoporous gold/MnO2 hybrid materials by directly deposition MnO2 (orange) onto nanoporous gold metrics. (b) Photograph of a supercapacitor 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, respectively at the scan rate of 50 mV s-1. (d) Galvanostatic chargedischarge 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 reference 106. Copyright 2011 Nature Publishing Group.


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ACS Energy Letters

Figure 8. (a) Schematic illustration of the fabrication process for hydrogenated NiO nanoblocks (H-NiO NBs) on copper substrate. (b, c) SEM images of H-NiO NBs at different resolution. (d) CV curves of NiO NBs and H-NiO NBs at the scan rate of 100 mV s-1. (e) galvanostatic charge discharge curves of NiO NBs and H-NiO NBs at the 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 reference 109. Copyright 2014 American Chemical Society.


ACS Energy Letters

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Figure 9. (a) Schematic illustration of the fabrication process for three dimensional 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 density. (d) Cycling stability of Ni(OH)2 at 18 A g-1. (e) Ragone plot of the nanoporous layers. Reprinted from reference 115. Copyright 2014 American Chemical Society.


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Figure 10. (a) Schematic illustration of the fabrication process for mushroom-like Ni3S2. (b) CV curves of Ni3S2/AC device at different scan rates. (c) Galvanostatic charge discharge curves of Ni3S2/AC device at different current density. (d) Ragone plot of Ni3S2/AC device. (e) Cycling performance of Ni3S2/AC device at 4A g-1. Adapted with permission from reference 116. Copyright 2015 Royal Society of Chemistry. 57 ACS Paragon Plus Environment

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Figure 11. (a) Schematic illustration of the flexible device of 3D nanoporous layer (NPL) on Au (~ 40 nm)/Cr (~ 10 nm)/PET substrate (~ 35 µm). (b) Photograph of NPL electrode. (c) The freestanding NPL after removing the substrate. (d) CV curves of NPL device at different scan rates in the potential window of -0.8 to 0.8 V. (e) Galvanostatic charge discharge curves of 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 NPL device at different scan rates in the potential window of -1.4 to 1.4 V. (g) Galvanostatic charge discharge curves of 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 NPL device at the potential window of -0.8 to 0.8 V and -1.4 to 1.4 V, respectively. (i) Photograph of the bended NPL device. (j) Capacitance retention of NPL device at different bending states. (h) Dependence of capacitance retention on bending cycles to 180° bending angle. The inset shows charge discharge curves recorded before and after bending for 500 and 1000 cycles. Reprinted from reference 117. Copyright 2014 American Chemical Society.


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