Recent development of metal hydroxides as electrode material of electrochemical capacitors

Abstract

Electrochemical capacitors, also known as supercapacitors, are energy storage devices, characterized by rapid rates of charging and discharging and high power density. In recent years, electrochemical capacitors have attracted significant attention as ‘bridges’ for the power/energy gap between traditional capacitors and batteries/fuel cells. The integrated performance of an electrochemical capacitor is essentially determined by its electrode materials. This is a review of electrode materials for electrochemical capacitors, chiefly concerning transition metal hydroxides. In this work, we focused particularly on recently published reports using cheap metal hydroxides as electrode materials for electrochemical capacitors, based on classification of metal hydroxide by composition and microstructure. Some important experimental data on this issue are indicated and summarized. Furthermore, a brief discussion of future development, challenges, and opportunities in this area is also provided.

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J. P. Cheng

J.P. Cheng received his PhD degree in Materials Science & Engineering from Zhejiang University in 2005. He presently is an associate professor in department of Materials Science and Engineering and State Key Laboratory of Silicon Materials, Zhejiang University. From 2011–2012, he was a visiting scholar at NUANCE Center in Northwestern University, IL, USA. His current field of interest is functional nanomaterials, such as electrode materials, sensor materials.

J. Zhang

J. Zhang graduated from Zhejiang University at Department of Materials Science & Engineering in 2013 to receive her BS degree. She is now a graduate student majoring in Materials Engineering, Zhejiang University. Currently, her work focuses on the fabrication of electrode materials of electrochemical capacitor.

F. Liu

F. Liu received her PhD degree of materials physics and chemistry at Zhejiang University in 2008. She is now a professor in the Department of Materials Science and Engineering and State Key Laboratory of Silicon Materials, Zhejiang University. She has been a visiting scholar at Kiel University, Germany during 2005–2007 and 2009. Her current researches focus on functional nanomaterials, such as electrode materials, and materials microstructure characterization methods including electron microscopy, X-ray diffraction.

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1. Introduction

Energy storage is critical for efficient, clean, and versatile use of energy, and is also key in energy harvesting for use as renewable energy. Batteries keep our electronic devices working for a long time because of their high energy densities. For rapid power delivery and recharging, electrochemical capacitors (ECs) are used. Electrochemical capacitors, also called supercapacitors, which can operate at a high charge/discharge rate over a large number of cycles, are one kind of important device for energy storage.¹ ECs can store electrochemical energy by either ion adsorption as an electrochemical double layer capacitor (EDLC) or fast surface redox reactions like batteries.² The performance of ECs is essentially determined by their electrode materials. The most widely known electrode materials for redox actions of ECs are RuO₂ and MnO₂. Recently, this list has been expanded to include other metal oxides, as well as hydroxides, nitrides, and carbides. Thus, transition metal oxides and hydroxides such as MnO₂, Fe₃O₄, NiCo₂O₄, V₂O₅, Ni(OH)₂, and Co(OH)₂, have been extensively studied as the electrode materials of supercapacitors, enabling fast reversible redox reactions on their surfaces.³ The charge storage mechanism of ECs is generally determined by EDLCs and pseudocapacitors. Although EDLCs based on carbon electrode materials have good stability, their charge mechanism limits specific capacitances at a range of low values. Thus, ECs based on pseudocapacitors usually exhibit much higher specific capacitances than EDLCs. Among the candidate electrode materials for pseudocapacitors, oxides and hydroxides of transition metals present high specific capacitance because of their rich redox properties involving multiple oxidation states.

In recent years, there has been great progress in the theoretical and practical research and development of ECs, as evinced by a large number of published research papers. Low-cost and environmentally friendly ECs with capability for storing large amounts of electrical energy have potential applications in electric vehicles, energy storage for renewable energy production, portable electronics, and large industry equipment. Thus, an elegant way of achieving high energy density is to design hybrid systems using earth abundant metal oxides or hydroxides as the electrode materials of ECs. Because there have been many review articles based on the advances of metal oxide electrode materials for ECs,^4–13 we will focus on the development of metal hydroxides such as cobalt hydroxide and nickel hydroxide.

In this review, we survey recent reports of earth abundant metal hydroxides as electrode materials of ECs, typically from 2009 to now. In an attempt to review transition metal hydroxide materials for ECs, we discuss their preparation and modification, followed by classification of metal hydroxides by composition and microstructure. They are grouped into several sections including single-, binary metal hydroxides, hydrotalcite-like compounds, hierarchical hybrids of metal hydroxide and metal oxide, and asymmetric supercapacitors based on metal hydroxide. Finally, we will discuss the potential future directions for EC research. We believe that the topic of this review will be interesting to scientists working in related fields.

2. Single Co or Ni hydroxide

2.1 Single crystalline hydroxide

Single crystalline metal hydroxide as an electrode material for ECs has been reported and investigated in many published papers, typically for transition metal hydroxides containing elements including nickel, cobalt, and iron. These metal hydroxides are often layered materials with a large interlayer spacing, and they can have very high theoretical specific capacitance. We focus firstly on interesting results reported using single metal (Co and Ni) hydroxide as electrode materials of ECs.

2.1.1 β-Cobalt hydroxide. Cobalt hydroxides are attractive as electrode materials of ECs because of their layered structure with large interlayer spacing (0.46 nm), their well-defined electrochemical redox activity, and the possibility of enhanced performance through different preparation methods. Their sheet or plate-like shapes are beneficial in improving electrochemical performance, providing a large inter-sheet spacing for ions transferring and high utilization of electrode materials. Uniform β-Co(OH)₂ crystals reported as electrode materials of ECs can be prepared easily by chemical methods,¹⁴⁻²⁰ hydrothermal methods,²¹ and electrodeposition.²² Usually, these Co(OH)₂ platelets have a hexagonal morphology lying on the (001) planes.¹⁸ The Co(OH)₂ nanoplates prepared by Mustafa et al. exhibited a high specific capacitance of 1012.7 F g⁻¹ and good capacity retention of ca. 92% after 1000 continuous charge–discharge cycles.¹⁴ Ji et al. prepared cobalt hydroxide thin film supercapacitor via screen printing onto a plastic substrate to make a flexible model, as shown in Fig. 1.²⁰

Fig. 1 Pictures depicting a flexible graphite screen printed electrode. Taken from ref. 20 with permission from RSC Publications.

It is known that the supercapacitive performance of electrode materials strongly depends on their morphological creation, and thereby on the specific surface area. Thus, porous three dimensional (3D), hierarchical structures of Co(OH)₂ such as coral-like shapes,^23,24 flowerlike shapes,^20,25−31 mesoporous nanowires,³²⁻³⁵ nanoflake films,³⁶⁻⁴² nanocone arrays,⁴³ and hollow core–shell structures were fabricated and developed for EC applications. The surfaces of these 3D Co(OH)₂ structures show randomly distributed, interconnecting configurations, resulting in a network-like structure with many cavities between the adjacent blocks. The high specific surface area and numerous pores in the 3D Co(OH)₂ can facilitate penetration by electrolytes, and thus contribute to the excellent capacitive properties. Thus, relevant methods have been developed for preparation of such electrode materials, including electrodeposition, hydrothermal method, liquid-precipitation, ionothermal synthesis,⁴⁴ ball-milling,¹⁹ and sonochemical synthesis.¹⁵

Ionic liquids (ILs, 1-butyl-3-methylimidazolium tetrafluoroborate) and Co(OH)₂ could form a nanohybrid with a large surface area of 400.4 m² g⁻¹ by ionothermal synthesis, as shown in Fig. 2. The IL–Co(OH)₂ based electrode exhibited a higher specific capacitance of 859 F g⁻¹ with a high-rate capability, better cycling performance, higher ion diffusion coefficient and lower charge transfer resistance than bare Co(OH)₂.⁴⁴ Theoretical calculations revealed that IL molecules consisting of anion and cation groups enabled an easier hydrogen desorption/adsorption process, leading to a favorable redox reaction on the Co(OH)₂ surface.

Fig. 2 (a–d) TEM image of IL–Co(OH)₂ (inset of image (d) shows a lattice image of individual IL–Co(OH)₂); (e) XRD patters of IL–Co(OH)₂, Co(OH)₂, and [Bmim][BF₄]; (f) nitrogen adsorption/desorption isotherms and pore size distribution of IL–Co(OH)₂. Reprinted with permission from ref. 44. Copyright (2013) American Chemical Society.

If Co(OH)₂ films could be directly deposited on a current collector, the Co(OH)₂ electrodes would have several advantages including not requiring the use of polymer binder and conducting agent. Therefore, preparation of Co(OH)₂ films is another excellent route to obtain high-performance electrode materials.⁴⁵ Among various synthetic methods, electrodeposition is popularly used. Li et al. compared various electrochemical tests of ordered mesoporous Co(OH)₂ films on a foamed nickel and titanium plate.³⁹ The Co(OH)₂ film on Ni foam had a much higher specific capacitance (maximum: 2646 F g⁻¹) than that on titanium plate (maximum 1018 F g⁻¹) because of the larger surface area of Ni substrate. More recently, Salunkhe et al. applied a chemical deposition method to synthesize Co(OH)₂ rod films on the current collector.⁴⁶ The direct growth of Co(OH)₂ rods gave an open-three dimensional structure for easy access of electrolyte throughout the material surface, being an efficient electrode for EC application. The electrode achieved the highest capacitance of 1116 F g⁻¹ at a current density of 2 A g⁻¹.

It is known that β-form Co(OH)₂ has a brucite-like origin, where octahedral with divalent cobalt cations six-fold coordinated by hydroxyl ions share edges to produce 2D charge-neutral layers stacked one over the other without any anion. Most as-fabricated β-Co(OH)₂ crystals have a lot of crystalline planes parallel stacking along the [001] direction. However, single-layer β-Co(OH)₂ nanosheets can be prepared by phase transformation from exfoliating layered α-Co(OH)₂ in formamide at 80 °C under nitrogen gas protection for 15 hours.⁴⁷ The obtained β-Co(OH)₂ nanosheets were measured with lateral size from several tens to several hundred nanometers, and thickness in the range of 0.9 to 1.3 nm. The selected area electron diffraction (SAED) image taken from an individual nanosheet corresponded to the crystalline structure of β-Co(OH)₂, as shown in Fig. 3. The as-prepared single-layer β-Co(OH)₂ nanosheets could be assembled with graphene oxide (GO) to form a two-dimensional composite. The reduced GO/β-Co(OH)₂ composite exhibited a high specific capacitance up to 2080 F g⁻¹ at a current density of 1 A g⁻¹.

Fig. 3 (a) AFM and height profile, (b) TEM and SAED images of single-layer β-Co(OH)₂ nanosheets. Reprinted with permission from ref. 47. Copyright (2014) American Chemical Society.

Traditionally, the theoretical specific capacitance of inorganic pseudocapacitors can be calculated according to the transferred electric charge and weight of active electrode materials. The theoretical specific capacitance of active material can be calculated as C_t = (n × F)/(M × ΔV), where n is the moles of charge transferred per mole of active material, F is Faraday's constant, M is the molar mass of the electroactive phase, and ΔV is the operating voltage range.^17a The theoretical value C_t of Co(OH)₂ is 3460 F g⁻¹ (within 0.6 V), assuming that all Co(OH)₂ phase in the bulk is electrochemically accessible and contributes to the redox reactions.^17a However, most reported experimental values are still lower than the theoretical values. The difficulty in approaching the theoretical specific capacitance is strongly associated with the limited ion diffusion within conventional dense electrode film, and poor electron transfer because of the semiconducting or insulating property of cobalt compounds, which means they do not take part fully in the electrochemical reactions. Thus, much effort still needs to be made to improve the electrochemical performance of Co(OH)₂.

2.1.2 Nickel hydroxide. Nickel hydroxide is widely used as an electrode material in commercial alkaline rechargeable batteries. Recent reports have proved that it is also an outstanding candidate for pseudocapacitors owing to its well-defined electrochemical redox activity, high specific capacitance, low cost, improved environmental compatibility, and various crystalline morphologies. Thus, Ni(OH)₂ is deemed as an active transition metal hydroxide material for ECs, too.

To obtain a high specific capacitance and good rate capability, nanoscaled Ni(OH)₂ crystals were prepared owing to its high surface area and abundant pores. Ni(OH)₂ nanosheets with high surface area and narrow pore size distribution could be synthesized facilely by a microwave assisted heating method, and were applied as electrochemical pseudo-capacitive materials for ECs,⁴⁸ exhibiting a high specific capacitance of 2570 F g⁻¹ at a current density of 5 A g⁻¹ with good cycling stability. Shao et al. reported that rare metal La-doped nano nickel hydroxide was prepared by electrodeposition and it could reach the highest discharge capability of 840 F g⁻¹.⁴⁹

It is well accepted that porous structures can accelerate the diffusion of active species and facilitate electron transportation. 3D Loose-packed porous Ni(OH)₂ for electrode materials is commonly prepared by various methods, including different morphologies such as coral-like⁵⁰ microsphere of nanoflakes,⁵¹ snow-ball like,^52-53 coin-like plates,⁵⁴ flowery architecture,^55,56 pompon-like,⁵⁷ nanotube arrays,⁵⁸ hollow microspheres,⁵⁹ and nanowire aggregations.⁶⁰ These porous structured Ni(OH)₂ with low densities and high surface areas have thus attracted much attention not only for their importance in achieving a better understanding of the formation process but also because of their numerous potential technical applications, such as ECs.

To date, many attempts have been made on the synthesis of 3D nano- and micro-structured hollow Ni(OH)₂. Some metal oxide nanoarrays (such as ZnO) can be used as a template to fabricate hollow Ni(OH)₂ nanotube electrode material, as shown in Fig. 4. The opening of the Ni(OH)₂ nanotube arrays provides many channels that facilitate penetration of the electrolyte and ions deep into the electrode, leading to an increase in surface area for electrochemical reaction.^58,61

Fig. 4 SEM micrographs of (a) ZnO nanorod arrays and (b) nickel hydroxide nanotube arrays after etching with 6 M NaOH. Inset in (b) is a top-view TEM micrograph of the nickel hydroxide nanotube. Taken from ref. 58 with permission from RSC Publications.

Because 3D mesoporous nanostructure films can enhance supercapacitive performance by reducing the diffusion resistance of electrolytes and enhancing ion and electron transportation,⁶² ultrathin Ni(OH)₂ nanowall films deposited on Ni foam with an ultrahigh capacitance of 2675 F g⁻¹ and up to 96% reversibility were reported by Sun et al.,⁶³ as shown in Fig. 5. This specific capacitance is beyond the theoretical value of β-Ni(OH)₂ (2358 F g⁻¹ within 0.44 V), and perhaps can be attributed to the combination of the Faradic capacitance from chemical reactions and the EDLC from the high surface area. A much higher specific capacitance observed on a Ni(OH)₂–nickel foam composite was 3152 F g⁻¹ at a current density of 4 A g⁻¹,⁶⁴ where loosely packed nanometer scale Ni(OH)₂ grains spread on nickel foam maintained a greater surface area for reaction and resulted in effective utilization of electrode material. Both α-Ni(OH)₂ and β-Ni(OH)₂ films can be electrodeposited directly on nickel foam at different temperatures. The sample synthesized at 65 °C possessed a porous honeycomb-like structure and the highest specific capacitance was up to 3357 F g⁻¹ at a current density of 4 A g⁻¹.⁶⁵ The above reports reveal the possibility of exceeding the theoretical Faradic capacitance of Ni(OH)₂. These Ni(OH)₂ crystal films usually have a 3D porous structure, ultrathin morphology, and direct connection to conductive substrates. In addition to electrodeposition,⁶⁶⁻⁶⁹ other methods such as chemical bath deposition,⁷⁰⁻⁷² and hydrothermal methods⁷³⁻⁷⁸ were reported to controllably fabricate thin Ni(OH)₂ films and they were applied as the electrode materials of ECs with a high specific capacitance.

Fig. 5 (a and b) Low- and high-magnification SEM of Ni(OH)₂ nanowall films; (c) schematic image of Ni(OH)₂ nanowall films. Adapted from ref. 63 after modification with permission from RSC Publications.

Lokhande et al. found that Ni(OH)₂ thin films with different nanostructures such as nanoplates, stacked nanoplates, nanobelts, and nanoribbons could be controllably fabricated by varying the deposition temperature.⁷⁸ These electroactive Ni(OH)₂ films grown on conductive substrates can be directly used as binder-free electrodes, and such electrode design has two advantages over conventional thin film electrodes: (1) the poorly intrinsic conductivity of the electroactive material is no longer a concern because of the direct attachment of electroactive materials to the conductive substrate; (2) this design makes auxiliary components like conductive agents and binders completely unnecessary. Thus, Ni(OH)₂ films in situ deposited on a conductive substrate have great potential for EC application.

Although Ni(OH)₂ has a lower theoretical specific capacitance than Co(OH)₂, the reported specific capacitance values of Ni(OH)₂ are usually much higher when compared with Co(OH)₂. We assume that the formed structure and morphology of Ni(OH)₂ are more suitable for electrochemical reaction than those of Co(OH)₂. Considering that Ni is much cheaper than Co, Ni(OH)₂ is thus a promising electrode material for ECs.

2.2 Amorphous monometallic hydroxide

Use of a metal hydroxide with poor crystallinity or amorphous phase may result in more transportation channels than use of a highly crystalline one, therefore some reports have been published on the fabrication and electrochemical properties of amorphous metal hydroxides. Tong and co-workers carried out some research on amorphous nickel hydroxides and cobalt hydroxides.^79a They used electrochemical methods to prepare amorphous nickel hydroxide nanospheres (as shown in Fig. 6) exhibiting a high specific capacitance (2188 F g⁻¹). Asymmetric pseudocapacitors made of the amorphous nickel hydroxide had a high capacitance (153 F g⁻¹), a high energy density (35.7  W h kg⁻¹ at a power density of 490 W kg⁻¹), and super-long cycle life (97% and 81% charge retentions after 5000 and 10 000 cycles, respectively).^79a Then the Cao group synthesized amorphous mesoporous Ni(OH)₂ nanoboxes with uniform size of 450–500 nm by a template-engaged route.^79b The nanoboxes showed high specific capacitance of 2495, 2378, 2197, 1993 F g⁻¹ at discharge currents of 1, 2, 5 and 10 A g⁻¹, respectively.

Fig. 6 (a and b) SEM images of amorphous Ni(OH)₂ samples synthesized on graphite electrodes. (c and d) TEM images of amorphous Ni(OH)₂ samples (the inset shows the corresponding selected-area electron-diffraction pattern). Scale bars, 1 μm (a), 100 nm (b), 0.2 μm (c) and 20 nm (d). Reprinted with permission from ref. 79a. Copyright (2013) Nature Publishing Group.

Amorphous Co(OH)₂ nanostructures with excellent electrochemical behaviors can be synthesized on graphite flakes by a simple and green electrochemical method in deionized water without using any chemical additives. These amorphous cobalt hydroxides having three-dimensional structure possessed a high specific capacitance to 1094 F g⁻¹ at a scan rate of 1 mV s⁻¹, with specific capacitance loss of only 5% after 8000 consecutive cycles at 100 mV s⁻¹.⁸⁰ Integrated electrochemical performances of the amorphous hydroxide were totally commensurate with those of Co(OH)₂ materials with crystalline phase.

2.3 Composites containing Ni hydroxides, Co hydroxides, and carbon materials

The major issues for metal hydroxides (Ni hydroxides and Co hydroxides) are their poor electrical conductivity and volume expansion during charge–discharge cycles, especially for high-rate applications. Therefore, considerable work has been performed to improve their electrochemical properties by tuning their morphologies at the nanoscale and modifying them to be nanocomposites with some conductive materials. An efficient method is to make a hybrid containing both a metal hydroxide and a conductive material such as graphene, carbon black, and carbon nanotubes. For these carbon materials, their capacitance primarily relies on an electrical double-layer mechanism without involvement of the Faradic process. Therefore, despite their high charge–discharge cycling stability and power density, most carbon materials still suffer from lower specific capacitance than transition metal hydroxides or oxides. To further increase the energy density of hydroxide materials, several kinds of carbon materials have been selected and incorporated into metal hydroxides to form pseudocapacitive composites.

Although Co(OH)₂ is considered to be an important transition metal hydroxide for ECs, it displays less satisfactory electrochemical capacity and reversibility compared with ruthenium oxides. Many composites containing Co(OH)₂, such as Co(OH)₂/carbon,⁸¹ and Co(OH)₂/graphene^82,83 have been prepared, with according improvement in electrochemical properties. The utilization percentage of Co(OH)₂ increases in the composite, the carboneous support of Co(OH)₂ yields a high conductivity and supports improvement of rate capability. So, a composite consisting of Co(OH)₂ and carbon support will have greatly improved capacitance value.

Graphene (or reduced GO) is a novel carbon material with 2D structure, and has attracted intense interest in ECs electrode because of its unique electrical and mechanical properties. Addition of reduced GO (rGO) sheets is expected to provide a large support surface area for metal hydroxide nanocrystals and to improve conductivity of the resultant composite materials. Zhu et al. prepared a graphene/Co(OH)₂ composite employing Na₂S solution as both the depositing and the reducing agent.⁸⁴ The composite had a remarkable specific capacitance of 972.5 F g⁻¹, leading to a significant improvement in relation to each individual counterpart (137.6 and 726.1 F g⁻¹ for graphene and Co(OH)₂, respectively). It is possible that aggregation occurs, resulting in lower utilization of Co(OH)₂ active material in the case of being dried. Decoration of graphene nanosheets (GNS) with Co(OH)₂ nanoparticles could effectively inhibit the aggregation, resulting in higher utilization of Co(OH)₂ and improved electrochemical performance. The specific capacitance of the rGO/Co(OH)₂ composite prepared by solution method (as shown in Fig. 7) can reach 474 F g⁻¹ at a current density of 1 A g⁻¹ and this value can even retain 300 F g⁻¹ at a high current density of 10 A g⁻¹, showing a relatively good rate capability. Meanwhile, the specific capacitance of the electrode remained at 90% after 1000 times of cycling, showing good cycle stability.⁸⁵ All above reports indicate that the enhancement of capacitive performance can be attributed to the synergistic effect between graphene and Co(OH)₂ components in the composite.

Fig. 7 STEM image (a), and carbon (b), oxygen (c), and cobalt (d and e) element mapping images of rGO/Co(OH)₂ composite. Reprinted with permission from ref. 85. Copyright (2013) Elsevier B. V.

Ni(OH)₂ is of particular interest as an EC materials because it is cheap and has high theoretical specific capacitance; however, one major drawback for Ni(OH)₂ is its poor conductivity (∼10⁻¹⁷ S cm⁻¹). A strategy to improve the electrochemical performance of Ni(OH)₂ is to form a composite with an electronically conductive material. Many graphene/Ni hydroxide based composites have been successfully prepared, in which graphene or reduced GO improves the conductivity of the electrode materials.⁸⁶⁻⁹⁴ Single-crystalline Ni(OH)₂ hexagonal nanoplates grown on graphene sheets showed a high specific capacitance (∼1335 F g⁻¹ at a charge and discharge current density of 2.8 A g⁻¹) and remarkable rate capability, significantly outperforming Ni(OH)₂ nanoparticles grown on GO and Ni(OH)₂ nanoplates simply mixed with graphene sheets,⁹⁵ as reported by Dai and shown in Fig. 8. Many functional groups of GO (hydroxyl, epoxy, carboxyl, and carbonyl groups) can act as anchor sites for Ni(OH)₂ crystal growth. Thus, a Ni(OH)₂/graphene composite prepared by an electrostatic method showed superior electrochemical properties, including high specific capacitance (1503 F g⁻¹ at 2 mV s⁻¹) and excellent cycling stability up to 6000 cycles even at a high scan rate of 50 mV s⁻¹.⁹⁶ This was attributed to its tailored properties, which were vital to the operation of ECs, including intimate bindings, high conductivity, structural stability, and good wettability.

Fig. 8 SEM and TEM characterizations of Ni(OH)₂/GS composite, Ni(OH)₂/GO composite, and Ni(OH)₂ + GS physical mixture. (a) SEM image of Ni(OH)₂ nanoplates grown on GS. (b) TEM image of Ni(OH)₂ nanoplates grown on GS. (c) SEM image of Ni(OH)₂ nanoparticles grown on GO. (d) TEM image of Ni(OH)₂ nanoparticles grown on GO. (e) SEM image of Ni(OH)₂ hexagonal nanoplates grown in free solution (without graphene). (f) SEM images of simple physical mixture of presynthesized free Ni(OH)₂ nanoplates and GS. Reprinted with permission from ref. 95. Copyright (2010) American Chemical Society.

In addition to graphene and reduced GO, carbon nanotubes (CNTs) are also suitable conductive supports for Ni(OH)₂ material.⁹⁷ Holze and co-workers developed a “bottom-up” chemical method to coat nanocrystalline Ni(OH)₂ onto the outer surface of multiwalled CNTs (MWCNTs) for flexible EC electrodes, where the high electronic conductivity of CNTs permitted their use as the supporting backbone onto which Ni(OH)₂ could be deposited,⁹⁸ as exhibited in Fig. 9. The high capacitance and excellent rate capability of Ni(OH)₂/CNT electrodes could be attributed to the unique structure of the materials, where sponge-like Ni(OH)₂ nanoparticles were supported on a high-packing density, porous multiwalled CNT network with adequate access to electrons and ions in the electrolyte. Ni(OH)₂/CNT composites also can be prepared using an electrochemical deposition method. The specific capacitance of the nanocomposite Ni(OH)₂/CNT electrode was as high as 2486 F g⁻¹ and stable over long cycling.⁹⁹ Meanwhile, active carbon also can be used to combine with Ni(OH)₂ to improve electrochemical performance.¹⁰⁰

Fig. 9 TEM images of (A) pristine MWCNT and (B and C) Ni(OH)₂/MWCNT (30 mg cm⁻²) and (D) XRD patterns of MWCNTs, Ni(OH)₂, and Ni(OH)₂/MWCNT powders. Reprinted with permission from ref. 98. Copyright (2013) American Chemical Society.

Zhao et al. first demonstrated preparation of 3D CNT-pillared rGO sheets with embedded Ni(OH)₂ nanoparticles for pseudocapacitor applications. The composite displayed specific capacitances as high as 1235 and 780 F g⁻¹ at current densities of 1 and 20 A g⁻¹, respectively.¹⁰¹ Graphene/Ni(OH)₂ composites also can be prepared by a scalable solid-state reaction method. The resulting composites with well-dispersed Ni(OH)₂ nanoparticles on rGO surface showed a high specific capacitance of 1568 F g⁻¹ and good cycling stability (>75% retention for 1000 cycles) at a current density of 4 A g⁻¹ in 1 M KOH.¹⁰²

3. Bimetallic hydroxide

Nickel hydroxide is deemed one of the most promising electrode materials owing to its high theoretical specific capacitance, up to 2358 F g⁻¹. However, such high capacitance is hard to realize in practice owing to its poor conductivity. It has been reported that incorporation of cobalt into nickel hydroxide can significantly improve the electrochemical and conducting properties of the overall electrode material, because of formation of highly conductive CoOOH during the charge–discharge process.¹⁰³ Thus, various nickel–cobalt double hydroxide electrodes have been prepared, and their enhanced electrochemical performances also have been reported.^104−121

3D porous Ni–Co binary hydroxides have been studied intensively because of their high specific surface area and porous structure. Urchin-like Ni(OH)₂–Co(OH)₂ hollow microspheres can be synthesized by a microwave-incorporated hydrothermal method.¹²² The hollow microspheres achieved a high specific capacitance of 2164 F g⁻¹ at 1 A g⁻¹ and long-term cycle life. Co_0.5–Ni_0.5 hydroxide nanocones can deliver a specific capacitance of 1580 F g⁻¹ at a galvanic current density of 10 A g⁻¹, measured from charge–discharge curves. The authors attributed this to enhancement of electroactive sites participating in the redox reaction because of possible valence interchange or charge hopping between Co and Ni cations.¹⁰⁶ Hollow rhombic dodecahedral NiCo hydroxide nanocages, composed of thin nanoplatelets, can be synthesized using ZIF-67 nanocrystals as templates. The porous nanocages exhibited superior pseudocapacitance properties because of their novel hierarchical and submicroscopic structures.^111,113

Ni–Co double hydroxide films also show great potential for EC electrodes. A high specific capacitance of 2682 F g⁻¹ at 3 A g⁻¹ based on active materials was obtained using Ni–Co layered double hydroxide (LDHs) hybrid film on nickel foam.¹⁰⁴ Co_0.72Ni_0.28 double hydroxides deposited on stainless steel by a potentiostatic deposition method had the highest specific capacitance of 2104 F g⁻¹ in 1 M KOH.¹¹⁴ The composition ratio of Ni–Co had great influence on the morphology of double hydroxide arrays deposited on stainless steel, where Ni : Co = 2 : 1 achieved a specific capacitance of 456 F g⁻¹ with an energy density of 12.8 W h kg⁻¹.¹⁰⁹ The nanocrystalline Co_1−xNi_x hydroxide thin films prepared by potentiodynamic deposition possessed different porous, nanoflake like morphology and superhydrophilic behavior by the composition influence. The maximal specific capacitance for Ni–Co hydroxide electrodes was found to be about 1213 F g⁻¹ for the composition Co_0.66Ni_0.34 LDHs in 2 M KOH electrolyte at 5 mV s⁻¹ scan rate.¹¹⁶

Zwitterionic p-aminobenzoate intercalated α-hydroxides of nickel and cobalt can be synthesized by ammonia precipitation, and have been shown to exfoliate in water. The monolayers from a mixture of aqueous colloidal dispersions of the α-hydroxides can be co-stacked instantaneously by addition of nitrate anions to give hybrids in which the two hydroxide layers were interstratified, as shown schematically in Fig. 10. The hybrid hydroxides having 80% nickel hydroxide showed the best electrochemical performance with a high specific capacitance of 990 F g⁻¹ and very good cycling stability.¹²³

Fig. 10 Schematic representation of the preparation of the nitrate intercalated interstratified α-hydroxide hybrids. Reprinted with permission from ref. 123. Copyright (2013) Elsevier Ltd.

Ni–Co double hydroxides deposited on ZnO nanowire arrays¹⁰⁸ and zinc tin oxide nanowires¹⁰⁷ demonstrated outstanding performances with specific capacitances of 1624 and 1805 F g⁻¹, respectively. Li et al. also found that the atomic ratio between Co and Ni had a significant effect on their electrochemical activities. A specific capacitance of 2614 F g⁻¹ was achieved when the atomic ratio of Co to Ni was 0.57 : 0.43.¹¹⁰ Ni–Co binary hydroxide systems with controllable morphologies from nanosheets, to nanoplates-nanospheres, to nanorods, and to a nanoparticle geometry by simply tailoring the Ni and Co cation ratios in the initial reactants were reported by Lian.¹²⁴ The drastically different morphologies significantly affected the electrochemical performance of the binary hydroxides as electrode materials. A high capacitance of 1030 F g⁻¹ was achieved for the nanorod morphology at a current density of 3 A g⁻¹.¹²⁴ Even the lattice spacing and crystal size of Co–Ni hydroxide nanosheets could be slightly tuned by the metal ratio.¹²⁵ Although the optimum Co/Ni ratios are different in the above reports because of different preparation methods, we can still draw a conclusion that, to a large extent, bimetallic hydroxides of Ni–Co are superior to their individual components.

Liu et al. developed an accumulative approach to move beyond simple incorporation of conductive carbon nanostructures to improve performance of metal hydroxides. Ni–Co double hydroxide/graphene composites were first synthesized by co-precipitation, and then assembled into films by integrating with few walled CNTs that could be directly used as electrodes, as shown in Fig. 11. With 50% Co and 50% Ni, the composite exhibited a remarkable maximum specific capacitance of 2360 F g⁻¹ at 0.5 A g⁻¹. The control experiments showed that the double hydroxides outperformed either Co(OH)₂ or Ni(OH)₂ alone,¹²⁶ proving that graphene can greatly boost electron transfer during the redox reaction.¹²⁷

Fig. 11 Material characterization of the Co_0.5Ni_0.5(OH)2/graphene composite: (a) and (b) TEM images at different magnifications; (c) SEM image; (d) EDS spectrum and (e) XRD pattern. Taken from ref. 126 with permission from RSC Publications.

More recently, Ni–Cu double hydroxide spheres were synthesized by a chemical bath deposition method on different substrates including copper foam, nickel foam, and carbon paper. When the substrate was changed from a copper foam to a nickel foam or carbon paper, the deposited material became pure nickel hydroxide with a flower-like morphology. The Ni–Cu double hydroxide also demonstrated a high specific capacitance of 1970 F g⁻¹.¹²⁸

4. Hydrotalcite-like compounds

4.1 Layered double hydroxides

Layered double hydroxides (LDHs) are a kind of lamellar compound made of positively charged brucite-like layers and an interlayer region containing compensating charge anions and salvation molecules. The most widely studied LDHs usually contain both divalent and trivalent metal cations. A general formula for LDHs can be written as, [M²⁺_1−xM³⁺_x(OH)₂][Aⁿ⁻]_x/n·zH₂O, where M²⁺ may be common, such as Mg²⁺, Zn²⁺, Co³⁺, or Ni²⁺, and M³⁺ may be Al³⁺, Ga³⁺, Fe³⁺, or Mn³⁺. Aⁿ⁻ is a charge compensating inorganic or organic anion, e.g. CO₃²⁻, Cl⁻, SO₄²⁻, and x is normally between 0.2 and 0.4. As a result of the flexible ion-exchangeability and easily tunable composition, LDHs have a series of important applications in catalyst or catalyst precursors, anion exchangers, precursors to other materials and so on. Some earth-abundant transition elements including Co, Ni, Mn and Fe, can be used as the building blocks of LDH layers and there are wide and flexible galleries between host layers. These LDHs also have been applied as electrode materials in ECs as a result of the LDHs nanosheets facilitating increased surface area and remarkably decreased diffusion distances for ions.¹²⁹ The simplest and most common method for preparation of LDHs is co-precipitation of the chosen M²⁺ and M³⁺ hydroxides with diluted NaOH and/or Na₂CO₃ or NaHCO₃. However, they can also be synthesized using urea hydrolysis, hydrothermal or ion-exchange methods. Nickel and cobalt are the two most commonly studied elements in these systems because of their abundance and high capacitance.

Excellent capacitive materials should have large surface area and highly ordered dimensions. There are three important factors in realization of these goals. Firstly, larger specific surface area of active materials can accommodate more electrolyte ions for redox processes. Secondly, providing a suitable mesopore structure is important for mass transfer of electrolytes. Finally, electroactive materials with high electrical conductivity can deliver high capacitance under a high discharge current density, exhibiting an excellent high rate capability. Based on this, these factors have been intensively investigated by scientists. Mousty el al. found that the electrochemical behavior of Ni-rich LDH was mainly governed by an electron hopping mechanism, whereas in CoAl-LDH a multi-site pseudocapacitive behavior was observed.¹³⁰

4.1.1 CoAl layered double hydroxides. Among various LDH materials, CoAl-LDH is prominent because of its high specific capacitance and various production methods.^130−132 Yang et al. prepared a continuous CoAl-LDH nanosheet thin-film electrode by drying a nearly transparent colloidal solution of LDH nanosheets on an ITO substrate.¹³³ Partial isomorphous substitution of Co²⁺ by Al³⁺ is the key factor in improvement of the electrochemical behavior, because the Al³⁺ favors the retention of the original layered structure during the redox reaction. The Co_0.75Al_0.25-LDH thin-film electrode had a large specific capacitance of 2500 F cm⁻² (833 F g⁻¹) with a good high rate capability.¹³³ Multilayer films composed of two kinds of one-atom-thick sheets, GO and CoAl LDHs, were prepared by the Jin group via layer-by-layer assembly.^134,135 The hybrid films had well organized layered structure as well as finely controlled film thickness and uniformity, as presented in Fig. 12. The obtained CoAl LDHs/GO films displayed a long cyclic life and an extremely high specific capacitance, up to 1200 F g⁻¹ under a scan rate of 5 mV s⁻¹ after reducing the GO to reduced GO.^134,135 The films fabricated by this method also can be transferred to polyethylene terephtalate substrate to produce a flexible EC device.

Fig. 12 (a) Schematic of the formation and structure of Co–Al LDH-NS/GO composite. (b) Digital photographs of (left) an aqueous dispersion of Co–Al LDH-NS, (middle) an aqueous dispersion of GO, and (right) a mixture of Co–Al LDH-NS and GO. Taken from ref. 134 with permission from RSC Publications.

However, when CoAl LDHs were soaked in KOH solution, Al³⁺ ions were leached off from the lattice, transforming them into β-Co(OH)₂, and degrading the electrochemical performance of the electrode. The transformations were influenced by the CoAl ratio, soak temperature, alkali concentration, and also took place during the charge–discharge process.¹³⁶ The dissolution of Al(OH)₃ in the electrolyte and/or surface modification of Y, Er, or Lu can improve the cycling charge–discharge performance because they can retard transformation from LDH into β-Co(OH)₂.

CoAl LDHs usually suffer from low rate and poor cycling stability at high current densities. One strategy to deal with these drawbacks is doping LDHs with other carbon materials. Coating CoAl LDHs with conductive film or self-assembling them onto graphene or other conductive supports can greatly improve the high-current capacitive behavior.^137−139 Formation of CoAl LDHs on the graphene nanosheets can prevent restacking of the as-reduced graphene nanosheets,^140,141 as depicted in Fig. 13. Zhang et al. applied a refluxing method with urea as a basic precipitant for 48 hours to prepare graphene/CoAl-LDH, where the CoAl-LDH crystals adhered to the surface of graphene nanosheets to form a laminated structure with smaller size compared with the pure CoAl-LDH, which facilitated electrolyte soaking into electrode materials.¹³⁹ A time saving method is microwave-assisted irradiation; the reaction time can be reduced greatly to about 2 hours.¹⁴²

Fig. 13 (a) Low and (b) high magnification TEM images of nanosized LDHs/GO hybrid at high LDH nuclei concentration. Reprinted with permission from ref. 140. Copyright (2012) American Chemical Society.

A CoAl-LDH@poly(3,4-ethylenedioxythiophene)(PEDOT) core–shell nanoarray was grown on a flexible Ni foil substrate.¹³⁸ Its performances were superior to those of conventional LDH arrays without the PEDOT coating. The enhanced pseudocapacitor behavior of the LDH@PEDOT electrode was related to the synergistic effects of its individual components. The LDH nanoplatelet core provided abundant energy storage capacity, while the highly conductive PEDOT shell and porous architecture facilitated electron/mass transport in the redox reaction.¹³⁸ Pt film-coated CoAl LDH arrays also exhibited enhanced electrochemical properties.¹³⁷

4.1.2 NiAl layered double hydroxides. NiAl-LDH materials are another kind of LDH that have been extensively used as electrode materials for supercapacitors.^{130,143−146} They have shown great potential in EC applications as highly redox active, low-cost, and environmentally benign materials.¹⁴⁷

Core–shell NiAl LDH microspheres with tunable interior architecture have been fabricated and reported by a facile and cost-effective in situ growth method on Si microspheres, as presented in Fig. 14. The hollow NiAl-LDH microspheres with the highest surface area (124.7 m² g⁻¹) and a mesopore distribution (3–5 nm) can give a maximum specific capacitance of 735 F g⁻¹ and good cycle performance, as well as remarkable rate capability.¹⁴⁸

Fig. 14 TEM and SEM images of (A and E) SiO₂/AlOOH microspheres; (B and F) SiO₂/NiAl-LDH core–shell microspheres; (C and G) SiO₂/NiAl-LDH yolk–shell microspheres; (D and H) NiAl-LDH hollow microsphere. Reprinted with permission from ref. 148. Copyright (2012) American Chemical Society.

Although capacitance of up to 1000 F g⁻¹ has been reported for NiAl LDHs in various electrolytes, they often suffer from low electrical conductance and short cycle life during application. Thus, some efforts have been made to improve their performance by creating conductive nanostructure-supported NiAl LDHs,^149−155 as shown in Fig. 15. Incorporation of NiAl LDH platelets onto graphene can prevent restacking of graphene nanosheets (GNS) and improve capacitance of the composite electrode, the electrolyte/electrode accessibility, and the conductivity.^156,157 The prepared GNS/LDH composite exhibited a high specific capacitance (781.5 F g⁻¹ at 5 mV s⁻¹) and excellent long cycle life.¹⁵⁸ Using GNS as a support, a GNS/NiAl-LDH composite was prepared successfully by a liquid phase deposition method as an electrode material for ECs. Because of the improvement of conductivity and adhesion, facile electrolyte penetration, and better Faradic utilization of the electroactive porous surface, the as-obtained GNS/NiAl-LDH composite showed a considerably improved electrochemical performance with a specific capacitance of 1255.8 F g⁻¹.¹⁵⁹ An alternative method was use of layer-by-layer deposition of AlOOH onto GNS followed by an in situ growth process of NiAl-LDHs to form an interesting sandwich structure,^160,161 with the composite having a specific capacitance of 1329 F g⁻¹ at a current density of 3.57 A g⁻¹.

Fig. 15 (A) SEM image of Ni foam; (B) SEM image and digital photo (inset in (B)) of NiOOH; (C) small and large (inset in (C)) SEM images of DOS-LDH; (D and E) SEM images and digital photo (inset in (D)) of NiAl-LDH; (F) lateral view image of NiAl-LDH. Reprinted with permission from ref. 151. Copyright (2013) Elsevier B.V.

As reported by Wimalasiri,¹⁶² delaminated NiAl-LDHs also can be incorporated between graphene nanosheets to form a layered hybrid structure as a new supercapacitor electrode material, GO and delaminated NiAl-LDH in aqueous medium, rather than in situ growth of NiAl-LDH on GO nanosheets. Both materials in their layered form provide more contact between electrochemically active areas and result in favorable conditions to realize their electrochemical energy storage capacities. The graphene/NiAl-LDH-based electrodes provided a specific capacitance of 915 F g⁻¹ at a current density of 2 A g⁻¹.¹⁶²

Similar to CoAl-LDHs, when a layered double hydroxide [Ni₄Al(OH)₁₀]NO₃ was aged at 60 °C in a concentrated KOH solution (around 7 mol L⁻¹), it transformed into β-form Ni(OH)₂; when it was cycled electrochemically, it could also transform into β-Ni(OH)₂ but finished much sooner, resulting in decrease of the maximum number of exchanged electrons per nickel atom as the charging–discharging cycling continued and degradation of the electrodes. The well-crystallized β-Ni(OH)₂ had a worse electrochemical activity than NiAl-LDHs; however, its high temperature performance could be improved by lowering the concentration of electrolyte and/or adding rare metal oxides such as Lu₂O₃, Er₂O₃, or Y₂O₃.¹⁶³

4.1.3 Other layered double hydroxides. In addition to CoAl- and NiAl-LDHs, various other LDHs have been also reported as electrode materials of ECs. Xu et al. found that the specific surface area of M^II₂M^III-LDHs (M^II: Co and Mn; M^III: Al and Fe) strongly affects their capacitance, but these LDH materials did not have a high specific capacitance (normally 100–200 F g⁻¹) at a scanning rate of 1 mV s⁻¹ in 6 M KOH solution.¹⁶⁴ Exfoliation of Mn^IIAl^III sulfonate and sulfate LDHs and their combination with GO by charge-directed self-assembly were reported.¹⁶⁵ Exfoliation of the bulk material in formamide yielded colloidal suspensions of positively charged LDH nanosheets with lateral dimensions of tens to hundreds of nanometers and thicknesses down to 1.3 nm. The hybrid materials were tested for ECs and showed significant increase compared with the pristine material.

CoMn-LDH nanowalls were deposited onto flexible carbon fibers (CF) via an in situ growth approach. The resulting CoMn-LDH/CF electrode could deliver a high specific capacitance (1079 F g⁻¹ at 2.1 A g⁻¹ normalized to the weight of the active LDH material) with excellent rate capability even at high current densities (82.5% capacitance retention at 42.0 A g⁻¹).¹⁶⁶ The dramatic performance is mainly attributed to the homogeneous and ordered dispersion of metal units within the LDH framework, which enriches the redox reactions associated with charge storage by both Co and Mn.

Magnetic films of CoFe-LDH nanoplates and porphyrin anions were fabricated by the layer-by-layer technique, with the assistance of an external magnetic field, and showed enhanced electrochemical behavior.¹⁶⁷ The Hwang group reported ZnCo-containing LDH phases and their exfoliated and calcined derivatives. The ZnCo-LDH materials showed not only interesting magnetic properties but also pseudocapacitance behavior with a discharge capacity of 160–170 F g⁻¹ and a good capacitance retention.¹⁶⁸

A NiTi-LDH thin film on nickel foam substrate was synthesized using two hydrothermal treatment steps with ammonia solution as a basic precipitant. NiTi-LDH crystallites perpendicular to the surface of the nickel foam substrates were formed and Ti atoms were dispersed homogeneously in the LDH lattice. A high specific capacitance of 10.37 F cm⁻² was achieved at a current density of 5 mA cm⁻² and 86% of the initial specific capacitance remained after 1000 cycles.¹⁶⁹ NiV-LDH composites were prepared by a co-precipitation process, during which small amounts of vanadium sources were decomposed by acid solution and mixed with β-Ni(OH)₂, and then a small quantity of nickel hydroxides transformed gradually to NiV-LDH composite. Compared with flake-like β-Ni(OH)₂, the NiV–LDH composite exhibits a much higher specific capacitance of 2612 F g⁻¹ at a scan rate of 2 mV s⁻¹.¹⁷⁰

Other LDHs such as CoCr- and CoIn-LDHs also have been investigated as electrode materials of ECs with good stability; however, their specific capacitances were not high.¹⁷¹

A hierarchical structure composed of NiMn-LDH microcrystals grafted onto a CNT backbone was prepared by Zhao and colleagues by chemical deposition,¹⁷² as shown in Fig. 16. The unique NiMn-LDH/CNT core–shell heterostructure can be seen in SEM and TEM images. The NiMn-LDH played the role of electrochemically active species, while CNTs served as both support and electron collector. Electrochemical measurement showed that the Ni₃Mn₁-LDH/CNT electrode was rather active, delivering a maximum specific capacitance of 2960 F g⁻¹ (at 1.5 A g⁻¹), together with good rate capability and excellent cyclability. Flexible ECs with asymmetric configuration can be fabricated using NiMn-LDH film and rGO/CNTs film as the positive and negative electrode, respectively, exhibiting a wide cell voltage of 1.7 V and large energy density of 88.3 W h kg⁻¹.¹⁷²

Fig. 16 (a) SEM image of the pristine CNT (inset: the enlarged image). (b and c) SEM images and (d and e) TEM images of the NiMn-LDH/CNT. (f) XRD patterns of: (i) the pristine CNT, (ii) the powdered sample of NiMn-LDH, (iii) the NiMn-LDH/CNT. Reprinted with permission from ref. 172. Copyright (2014) Wiley-VCH.

CoNiAl three-component LDHs with hydrotalcite-like structure can be synthesized successfully by homogenous precipitation using urea as a precipitant, whose capacitive behavior was influenced by the Co/Ni atomic ratio and electrolyte. The CoNiAl-LDH displayed the best capacitive performances when the molar ratio of Co/Ni reached 5.¹⁷³ The Co_0.55Ni_0.13Al_0.32-LDH delivered more specific capacitance in 1 M KOH than in 1 M LiOH or NaOH. Increasing the concentration of KOH from 1 to 6 M, the specific capacitance was elevated from 644 to 1124 F g⁻¹ at 1 A g⁻¹. After 1000 continuous charge–discharge cycles at 2 A g⁻¹, Co_0.55Ni_0.13Al_0.32-LDH exhibited a good capacitance retention (93.3%).¹⁷³ When ternary-component NiCoAl-LDH nanosheets and CNT composites were fabricated using urea precipitation, the small size CNTs were incorporated into the network of LDH nanosheets to form a homogeneous hybrid, as shown in Fig. 17. The specific capacitance can reach 1035 F g⁻¹ at a current density of 1 A g⁻¹, and increase by 33.3% in comparison with that of pure NiCoAl-LDH nanosheet.¹⁷⁴ In addition to precipitation, NiCoAl-LDH nanosheets with anisotropic morphology also can be synthesized by potentiostatic deposition, and their composition can be changed easily.¹⁷⁵

Fig. 17 FE-SEM and TEM images of (a and b) the NiCoAl-LDH nanosheets and (c and d) the NiCoAl-LDH–MWCNT nanohybrids, and high-resolution TEM images of (e and f) the NiCoAl-LDH–MWCNT nanohybrids. Taken from ref. 174 with permission from RSC Publications.

Well-dispersed LiAl-LDHs can be synthesized by a solvothermal approach by adjusting the concentration of ethanol. The hexagonal LiAl-LDHs calcined at 450 °C exhibited specific capacitance of 848 F g⁻¹ at a current density of 1.25 A g⁻¹.¹⁷⁶

4.2 α-Cobalt hydroxide

Cobalt hydroxide is well known to crystallize in two polymorphs, i.e. the metastable α-type and thermodynamically stable β-form. The first is isostructural with a hydrotalcite-like structure consisting of positively charged Co(OH)_2−x(OH)_x layers and charge balancing anions in the spaces between hydroxide layers. This structure is superior to that of the β-form for electrochemical reactions because it is a poorly and turbostratically crystallized structure, with large interlayer spacing. α-Type Co(OH)₂ materials used for EC can be prepared by co-precipitation,^177−181 hydrothermal method,^182−185 electrodeposition,^186−194 ionothermal synthesis,¹⁹⁵ and γ-ray irradiation.¹⁹⁶ A biomolecule-assisted method can be used to synthesize 2D nanoporous α-Co(OH)₂ mesocrystal nanosheets, where the amino acid L-arginine plays a dual role. It acts as a hydrolysis-controlling agent while stabilizing the synthesized Co(OH)₂ nanocrystallite building blocks.¹⁸³

The intercalated anions are exchangeable for α-Co(OH)₂, thus making it possible to adjust the interlayer spacing. Through an anion exchange reaction, the interlayer spacing and intercalated anions in α-Co(OH)₂ layers can be changed. Hu et al. reported that the intercalated anions in α-Co(OH)₂ had a critical effect on basal plane spacing, morphology, and specific capacitance.¹⁹⁷ Jin et al. also found that the interlayer space and electrochemical activity of α-Co(OH)₂ were closely related and that larger interlayer spacing allowed more electrolyte ions to be stored, leading to higher electrochemical activity.¹⁹⁸

However, the low conductivity of Co(OH)₂ usually limits its high rate capabilities. The hybrid synthesis of α-Co(OH)₂ on conductivity ITO nanowires forming nanoscale heterostructures can lead to improved high rate capabilities, originating from enhanced electrode and electrolyte conductivities, where ITO nanowires improve electron conductivity without using carbon black and polymer binder.^199−201 Meanwhile, using α-Co(OH)₂ and conductive carbon composites was also effective to improve electrochemical properties.^191,193 CNTs grown on carbon substrates offer an external surface area for supporting electroactive α-Co(OH)₂ materials, as presented in Fig. 18. Thus, nanometer-sized α-Co(OH)₂ sheets were attached to CNTs/carbon paper substrates using an electrodeposition technique, as reported by Zhang et al.¹⁹⁰ The composite electrode showed a large specific capacitance of 1083 F g⁻¹ at a current density of 0.83 A g⁻¹ in 6 M KOH, because the interconnected nanosheets of α-Co(OH)₂ facilitated contact of active material with electrolyte. The CNTs provide a short path for ion/electron transfer and sufficient contact between materials and electrolyte.¹⁹⁰

Fig. 18 SEM images of prepared Co(OH)₂ deposited at different times of 180 s (a and b) and 300 s (c and d). Reprinted with permission from ref. 190. Copyright (2013) Elsevier Ltd.

Another method that can be adopted is use of guest metal substitution to dope α-Co(OH)₂, such as Al and Zn.^179,189 Zn-substituted α-Co(OH)₂ nanosheet electrodes exhibited much better cycling performance than the pure α-Co(OH)₂ nanosheet electrode, as reported by He et al.¹⁸⁹ The sample with 21.1 at.% Zn substitution demonstrated high cycling stability with a capacitance loss of only 0.6% from 652 F g⁻¹ after 2000 cycles. Recently, exfoliation of α-Co(OH)₂ in formamide solution was shown to form graphene-like 2D ultrathin α-Co(OH)₂ nanosheets. The exfoliated α-Co(OH)₂ ultrathin nanosheets had much better electrochemical properties than the precursor, including a high specific capacitance of 1280 F g⁻¹, remarkable rate capability, and good cycling stability.¹⁸⁰ The ultrathin nanosheets obtained by exfoliation are suitable for industrialization owing to the simple synthetic procedures and reaction conditions.²⁰²

However, α-Co(OH)₂ is metastable in structure and easily undergoes a phase transformation into the more stable β-counterpart in strongly alkaline media. When immersed in KOH solution for a long time, this transformed to β-Co(OH)₂ and Co₃O₄, with a dramatic decrease in specific capacitance.²⁰³ To keep α-Co(OH)₂ stable in solution, inert atmosphere and weak alkaline media are necessary.

4.3 α-Nickel hydroxide

Similar to cobalt hydroxide, nickel hydroxide also has two polymorphs, i.e. α- and β-type Ni(OH)₂. α-Hydroxides of nickel have a structure similar to LDHs. β-Type is of the formula Ni(OH)₂ and is an ordered stacking of neutral layers of the compound with interlayer spacing of 0.46 nm. These have been studied extensively as they are widely employed as electrode materials of alkaline secondary batteries and supercapacitors.²⁰⁴

Methods used for preparation of α-Ni(OH)₂ as electrode materials of supercapacitors include electrochemical deposition,^205−209 precipitation,^210−212 and hydrothermal methods.^213−217 Microwave heating is another very useful technique that has been widely applied for synthesis of inorganic compounds including α-Ni(OH)₂. Mesoporous α-Ni(OH)₂ material was reported to be produced on a large scale with assistance of 2.5 minutes of microwave irradiation in ethylene glycol solution.²¹⁸

3D hierarchically structured α-Ni(OH)₂ materials are good candidates for electrode of ECs; the porous features facilitate easy transportation of electrolyte to reaction sites and lead to a further increase in specific capacitance.²¹⁹ Hollow spheres of α-Ni(OH)₂ were fabricated by Wu via electrochemical deposition using a polystyrene sphere as template, and showed greatly enhanced electrochemical performance in alkaline solution because of its opening structure.²⁰⁷ Without requiring sacrificed templates, flower-like α-Ni(OH)₂ microspheres composed of nanowires could be prepared by a solvothermal method using triethylene glycol and water as the mixed solvent. Compared with other conventional solvents, such as glycerol, alcohol, and water, triethylene glycol had a longer chain length, and moderate polarity and viscosity. Thus, the crystalline products obtained in this triethylene glycol/H₂O solvent tended to have kinetically slower growth rate. The as-formed nanocrystals had enough time to rotate adequately and found the low-energy configuration interface, then growth units with oriented growth direction were formed. These building blocks coalesced and aggregated together to form porous microspheres, as shown in Fig. 19. The sample had a high BET surface area of 318 m² g⁻¹ and showed a high specific capacitance of 1788.9 F g⁻¹ at 0.5 A g⁻¹ as well as excellent rate performance.²¹³

Fig. 19 (a and b) SEM and (c–e) TEM images of the microflower-like α-Ni(OH)₂. Reprinted with permission from ref. 213. Copyright (2013) American Chemical Society.

One simple method to improve the electrochemical properties of α-Ni(OH)₂ materials is annealing them under a rational temperature. Porous flower-like α-Ni(OH)₂ microspheres were synthesized by aqueous-phase reaction. Heating under a moderate temperature, i.e. 200 °C can deliver the highest specific capacity of 1551 F g⁻¹ in 6 M KOH, while a higher calcination temperature would decrease the capacitance because of formation of NiO.²²⁰ Yang et al. reported that annealing α-Ni(OH)₂ films on Ni foam at 100 °C delivered a specific capacitance of 2447 F g⁻¹.²²¹ It is supposed that the improved capacitance resulted from removal of physically absorbed water in α-Ni(OH)₂. Yu et al. found that flower-like NiO/α-Ni(OH)₂ composite had superior pseudocapacitive performance over individual α-Ni(OH)₂ and NiO, but poor electronic conductivity hindered its capacitance retention at high current density, where the composite was obtained by heating α-Ni(OH)₂ at 250 °C for different times. Another method that can be adopted is guest elemental doping in α-Ni(OH)₂ crystals. Al and Zn doped α-Ni(OH)₂ materials were reported to be prepared by electrochemical deposition and a hydrothermal method,^222,223 tending to formation of Ni-based LDHs.

However, poor conductivity is the major drawback for Ni(OH)₂ as a semiconductor material, leading to poor rate capability of the electrode, as well as low power output of the ECs. An efficient route to overcome this shortcoming is supporting Ni(OH)₂ on carbon materials, of which CNTs are optimum.^224−226 An α-Ni(OH)₂/graphite nanosheet composite was prepared via a homogeneous precipitation method to form a 3D hierarchical porous structure, of which fine α-Ni(OH)₂ nanocrystals as building blocks formed directly on the matrix of graphite nanosheets. The composite exhibited specific capacitance as high as 1956 F g⁻¹ at a current density of 1 A g⁻¹, and was able to endure the high discharge rate at current density of 40 A g⁻¹.²²⁷ α-Ni(OH)₂ nanosheets could be grown vertically on the surface of individual CNTs in CNT paper to form hierarchical nanowires by chemical bath deposition.²²⁸ This novel structure had a high electrochemical capacitance of 1144 F g⁻¹ at a current density of 0.5 A g⁻¹, and maintained 585 F g⁻¹ at 10 A g⁻¹. Flexible α-Ni(OH)₂ nanofibers also can be intertwined and wrapped homogenously on polypyrrole-based carbon networks, leading to formation of complex networks. A specific capacitance of 1745 F g⁻¹ could be obtained for Ni(OH)₂/CNT composite at 30 mA cm⁻².²²⁹ Uniform and conformal coating of α-Ni(OH) flakes on carbon microfibers was deposited in situ by a chemical bath method at room temperature. The microfiber-coated α-Ni(OH) flakes exhibited five times higher specific capacitance compared with non-conformal flakes, and the improvement was ascribed to the 3D network of the fibrous carbon fabric.²³⁰

Recently, graphene based composites were prepared by incorporating guest materials into 2D graphene sheets, for improving performance.^231,232 rGO/CNTs/α-Ni(OH)₂ composites were synthesized by a one-pot hydrothermal route. Electrochemical capacitance depended on the amount of CNTs, and the composite with optimized ratio exhibited high specific capacitance of 1320 F g⁻¹ at 6 A g⁻¹.²³³ Reduced GO/CNT formed a 3D conductive network in the composite, which promoted not only efficient charge transport and facilitated electrolyte diffusion, but also prevented effectively the volume expansion/contraction and aggregation of α-Ni(OH)₂ during the charge–discharge process.²³³ Low defect density graphene-supported Ni(OH)₂ sheets fabricated via a hydrothermal method were reported by Zhu et al.,²³⁴ where graphene simultaneously acts as both nucleation center and template for in situ growth of smooth and large-scale Ni(OH)₂ nanosheets. The specific capacitance of the as-obtained composite is 1162.7 F g⁻¹ at a scan rate of 5 mV s⁻¹ and 1087.9 F g⁻¹ at a current density of 1.5 A g⁻¹. Meanwhile, there was no marked decrease in capacitance at a current density of 10 A g⁻¹ after 2000 cycles.²³⁴ 3D Porous graphene hollow sphere frameworks were fabricated by integrating GO with amino-modified SiO₂ nanoparticles followed by etching, and were used as a support to combine with α-Ni(OH)₂ nanoparticles, as exhibited in Fig. 20. These porous graphene hollow spheres had formed a continuous framework and there was a homogeneous coating of Ni(OH)₂ throughout the 3D framework. The Ni(OH)₂ exhibited a specific capacitance of 2815 F g⁻¹ at 5 mV s⁻¹. Increasing the scan rate to 200 mV s⁻¹, α-Ni(OH)₂ still maintained a specific capacitance of 1950 F g⁻¹ with a capacitance retention of about 70%.²³⁵

Fig. 20 (a–c) TEM images at different magnifications and (d–g) STEM-mapping of porous graphene hollow spheres combined with α-Ni(OH)₂. Reproduced from ref. 235 with permission from the PCCP Owner Societies.

In addition to the carbon materials, ZnO is also a conductive candidate substrate for α-Ni(OH)₂ crystals to anchor to make a binder-free electrode for ECs.²³⁶ Fan et al. employed ZnO nanowire electrode as a 3D framework to support large-area α-Ni(OH)₂ growth and utilized ZnO nanowires with good electrical conductivity to provide a natural pathway for electron transport, as shown in Fig. 21.²³⁷ As each individual flake was connected to ZnO nanowire, the need for binders or conducting additives, which add extra contact resistance or weight, was eliminated. However, possible corrosion in a basic solution of ZnO might be one drawback after long cycling time.

Fig. 21 (a–c) General SEM images, (d) TEM image and (c) cross-sectional SEM images of the Ni hydroxide–ZnO hybrid structure, (f) Schematic diagram showing the kinetic advantage of the hybrid array in electrochemical energy storage. Taken from ref. 237 with permission from RSC Publications.

5. Fe-based hydroxides

Compared with nickel and cobalt, iron is much cheaper and more abundant in the earth. Fe-based oxides and hydroxides (FeOOH) have also attracted great interest because of this natural abundance and their eco-friendliness, as electrodes of lithium ion battery and supercapacitors, in addition to acting as catalysts, adsorbents, and magnetic material.²³⁸

Cheng et al. first confirmed that FeOOH was suitable for use as a negative electrode in hybrid ECs with an activated carbon positive electrode. The hybrid EC exhibited an estimated specific energy of 45 W h kg⁻¹ based on the total weight of two electrodes with a good cycling performance, retaining 96% initial capacity after 800 cycles.²³⁹ Similarly, Jin also reported that a hybrid EC containing a nano-sized columned FeOOH negative electrode and MnO₂ positive electrode, demonstrated an energy density of 12 W h kg⁻¹ and a power density of 3700 W kg⁻¹.²⁴⁰

Various morphologies of FeOOH materials have been fabricated and used as electrode materials of ECs.^241−243 Marigold-like structured nickel doped (5 at.%) iron hydroxide thin film was deposited on stainless steel using electrodeposition, and used for EC application. The highest specific capacitance was 287 F g⁻¹ in 1 M Na₂SO₃ electrolyte at a scan rate of 10 mV s⁻¹.²⁴⁴ Large area self-standing γ-FeOOH nanosheets fabricated on iron foil were evaluated as electrodes of ECs in various electrolytes. These exhibited an areal capacitance of 0.3–0.4 F cm⁻² and good cycling stability in Na₂SO₃ electrolyte, but were not stable in Na₂SO₄ solution.²⁴⁵

Fabrication of a composite containing FeOOH and rGO was recently developed to obtain a high-performance electrode with good rate-capability and stability.²⁴⁶ FeOOH nanorod/rGO composites prepared by a solution method had a high electrochemical capacitance of 165.5 F g⁻¹ with an excellent recycling capability.²⁴⁷ Urea was employed to reduce and dope GO to N-doped graphene, and simultaneously hydrolyzed to fabricate metal hydroxide. Thus, FeOOH nanorods could be randomly dispersed on N-doped graphene sheets. Specific capacitance of N-doped graphene/FeOOH nanorods was improved to be 309 F g⁻¹.²⁴⁸

6. Other transition metal hydroxides

Beside Ni-, Co-, and Fe-based hydroxides, other transition metal hydroxides including Mn(OH)₂, Cu(OH)₂, and Cd(OH)₂ also have been reported as electrode materials of ECs.

Octahedral Mn(OH)₂ nanoparticles with a size range from 140 to 200 nm have been fabricated by a sonochemical irradiation method for energy storage applications. These exhibited a specific capacitance of 127 F g⁻¹ at a current density of 0.5 mA cm⁻² in the potential range from −0.1 to 0.8 V in 1 M Na₂SO₄ solution.²⁴⁹ Using spray coating, Mn(OH)₂/multi-walled CNT composite thin films were prepared on flexible indium tin oxide/polyethylene terephthalate substrate. The capacitance increased with the weight ratio of KMnO₄/CNTs up to 1.6. The highest specific capacitance obtained at a scan rate of 20 mV s⁻¹ was 297.5 F g⁻¹ for the composite thin film with weight ratio of KMnO₄/CNTs of 1.2.²⁵⁰

Copper hydroxide thin films on glass and stainless steel substrates were prepared by a soft chemical synthesis route at room temperature. The room temperature chemical synthesis route allowed formation of nanograined and hydrophilic Cu(OH)₂ thin films, which exhibited specific capacitance of 120 F g⁻¹ in 1 M NaOH solution.²⁵¹

A conductive additive-free and binder-less Cd(OH)₂ nanowires electrode was prepared via direct growth of Cd(OH)₂ nanowires onto a nickel foam current collector via a simple solution growth method. The as-prepared Cd(OH)₂ nanowires electrode showed excellent pseudocapacitive performance with specific capacitance of 1164.8 F g⁻¹ at 1 A g⁻¹ in 6 M KOH solution. However, the precursor solutions were harmful to the environment.²⁵²

The hydroxide decorated graphene composites displayed improved performance over pure MnSn(OH)₆ nanoparticles because the graphene sheets acted as conductive bridges improving ionic and electronic transport. The total specific capacitance depended strongly on the crystallinity of the MnSn(OH)₆ nanoparticles, where materials with poor crystallinity showed maximum specific capacitance of 31.2 F g⁻¹ (59.4 F g⁻¹ based on the mass of MnSn(OH)₆ nanoparticles) at a scan rate of 5 mV s⁻¹.²⁵³

7. Hierarchical composite materials containing metal hydroxides

Porous carbon materials, transition-metal oxides or hydroxides are promising candidates as electrode materials of ECs, but each of these materials has its own advantages and disadvantages. To achieve a breakthrough in energy-storage and energy-conversion devices for capacitors, facile fabrication of multiphase composites as electrodes is crucial, because these components enhance reaction kinetics and reduce cost. In this section, hierarchically porous heterostructure composites containing metal hydroxides as the electrode material of ECs are reviewed briefly.

Ion transfer and electron conduction are two main factors that determine electrochemical performance of electrode materials of ECs. The hybrid nanostructured electrodes with high specific surface area and porous configuration can lead to a large electrode/electrolyte contact area, short diffusion path to current carriers, and high electron conductivity in electrodes.^254,255 Huang et al. fabricated and tested the electrochemical performance of supercapacitor electrodes consisting of Ni(OH)₂ nanosheets coated onto NiCo₂O₄ nanosheets grown on carbon fiber paper current collectors. When the NiCo₂O₄ nanosheets were replaced by Co₃O₄ nanosheets, however, energy and power density, as well as the rate capability of the electrodes, were significantly reduced because of the lower conductivity of Co₃O₄ than NiCo₂O₄.²⁵⁶ Similar work also has been carried out by Xu et al.,²⁵⁷ where NiCo₂O₄@Co_xNi_1−x(OH)₂ core–shell nanosheet arrays on Ni foam had maximum areal capacitance of 887.5 mF cm⁻². The Co₃O₄/Ni(OH)₂ composite mesoporous nanosheet networks were synthesized on a conductive substrate for supercapacitor application by heat treatment of Co(OH)₂/Ni(OH)₂. The resulting products have been directly employed as EC electrodes, and exhibited predominant electrochemical performances, such as a high specific capacitance value of 1144 F g⁻¹ at 5 mV s⁻¹ and long-term cyclability.²⁵⁸

The core–shell nanostructures have shown promise in these systems recently, owing to the synergistic effects of the individual components, where the core materials generally have a high electron conductivity and the shells are porous oxides or hydroxides. A hybrid nanostructure of porous cobalt monoxide nanowire@ultrathin Ni(OH)₂ nanoflake core–shell was directly synthesized on nickel foam by a two-step hydrothermal route, which demonstrated a specific capacitance of 798.3 F g⁻¹ at a current density of 1.67 A g⁻¹ and good rate performance as electrode material for supercapacitors, as reported by Guan.²⁵⁹ Nanoarchitectured fibrous Co₃O₄@Ni(OH)₂ core–shell material grown on a nickel foam collector with excellent pseudocapacitive behaviors, was fabricated by combining hydrothermal synthesis and chemical-bath deposition methods. By combining the Co₃O₄@Ni(OH)₂-based electrode with reduced GO or active carbon, a series of Co₃O₄@Ni(OH)₂-based asymmetric EC prototypes were developed. These asymmetric ECs exhibited superior performance, such as high specific capacitance and high energy density. Because of the large mass loading and high energy density, the prototype could drive a minifan or light a bulb despite its very small size,²⁶⁰ as shown in Fig. 22. Co(OH)₂ and Mn(OH)₂ nanosheets also can be electrochemically deposited on the Co₃O₄ core nanowires to form core–shell arrays. The Co₃O₄/Co(OH)₂ core–shell nanowire arrays were evaluated as a supercapacitor cathode material, which exhibited high specific capacitances of 1095 F g⁻¹ at 1 A g⁻¹ and 812 F g⁻¹ at 40 A g⁻¹, respectively.²⁶¹ Co₃O₄@NiAl-LDH core–shell nanowire arrays with hierarchical structure have been synthesized by in situ growth of a LDH nanosheets shell on the surface of Co₃O₄ nanowire arrays, as reported by Duan group.²⁶² This structure exhibited promising supercapacitance performance with largely enhanced specific capacitance and rate capability, much superior to pristine Co₃O₄ nanowire arrays because of its hierarchically mesoporous morphology and the strong core–shell binding interaction.

Fig. 22 (a) Schematic illustration of the fabricated Co₃O₄@Ni(OH)₂//RGO asymmetric supercapacitor prototype in 6 M KOH electrolyte. Photographs of the Co₃O₄@Ni(OH)₂//RGO asymmetric supercapacitor prototype as a power supply for a minifan (b) and a bulb (c). Reprinted with permission from ref. 260. Copyright (2013) American Chemical Society.

In addition to these highly conductive metal oxides as the core of metal hydroxide, TiN nanowires were also applied as supports of Ni(OH)₂, because TiN has great promise as an electrode material owing to its desirable electrical conductivity and mechanical stability.²⁶³ TiO₂ nanorods were also selected as supports to combine with a Co(OH)₂ nanowall array, thus a novel hierarchical electrode with improved performance was obtained.²⁶⁴ Even K₂Ti₄O₉ nanowires grown on Ti substrate were coated by Ni(OH)₂ nanosheets to make a core–shell heterostructure, where the heterostructure delivered high specific capacitance in aqueous and solid-state electrolytes.²⁶⁵

Sun et al. designed and synthesized 3D hierarchical heterostructures of dense MnOOH nanosheets on porous hierarchical NiO nanosheet arrays, as shown in Fig. 23. In this configuration, porous hierarchical NiO nanosheet arrays serve as a fast ion and electron transport model, and MnOOH ultrathin nanosheets can enhance the contact surface area and assist ion penetration into the core region to realize release of potential electrochemical properties of NiO nanosheet arrays. These heterostructures provide intense necessary critical function for efficient use of metal oxide and hydroxide in ECs. As an electrode, the 3D NiO@MnOOH core–shell nanosheet hierarchies exhibited excellent electrochemical performances, i.e. high specific capacitance of 1625.3 F g⁻¹ at a current density of 4 A g⁻¹ with good rate capability and high energy density (80.0 W h kg⁻¹).²⁶⁶

Fig. 23 (a and b) SEM images of the NiO@MnOOH core–shell nanosheet hierarchies. (c and d) TEM and HRTEM images of the MnOOH nanosheets, two lower-right insets in (c and d) are the corresponding SAED and FFT patterns, respectively. Reprinted with permission from ref. 266. Copyright (2014) Elsevier Ltd.

Very recently, Kang et al. reported an approach for fabricating low-cost transition-metal based oxy-hydroxide@nanoporous metal electrodes by electrochemical polarization of a dealloyed nanoporous Ni–Mn alloy in an alkaline solution. The hybrid electrode had a high volumetric specific capacitance (505 F cm⁻³) and excellent rate-capacity performance.²⁶⁷ This study may pave a new way for fabricating oxide-hydroxide/nanoporous metal hybrid electrodes with unprecedented properties for high-performance ECs.

8. Asymmetric supercapacitors based on metal hydroxide

According to the cell configuration for ECs, there are two kinds of ECs, symmetric and asymmetric/or hybrid ECs. Symmetric ECs are formed with two similar electrode materials as cathode and anode, whereas two dissimilar electrode materials form asymmetric ECs. The asymmetric design is an attractive approach to increasing the energy density of ECs as it can lead to an almost doubling of device capacitance. Asymmetric ECs incorporate both a polarizable and non-polarizable electrode, consisting of a polarizable electrode (usually high surface area carbon) and a battery-type electrode (usually a Faradic or intercalating metal oxide).^268−270 It is difficult to have a higher capacitance with higher operating voltage in common symmetric ECs because of the limited potential window, whereas asymmetric capacitors can deliver charge with a higher operating voltage.²⁷¹ Asymmetric or hybrid ECs are regarded as the new trend in supercapacitors.²⁷²

Co(OH)₂ is an important electrode material for asymmetric supercapacitors, and there have been several reports on its application in asymmetric ECs.^273,274 Asymmetric capacitor CNT-α-Co(OH)₂ was prepared with a low electrode resistance of 0.42 Ω cm². Although α-Co(OH)₂ raises the electrode resistance, it enhances the cell energy capacity greatly to 7.8 W h kg⁻¹.²⁷⁵ Cheng et al. fabricated graphene–CNT and graphene–CNT–Co(OH)₂ electrodes and assembled them in asymmetric ECs. Single-walled CNTs could act as a conductive spacer as well as a conductive binder in the composite. A high energy density of 172 W h kg⁻¹ and a maximum power density of 198 kW kg⁻¹ were obtained in ionic liquid electrolyte EMI-TFSI.²⁷⁶

Use of nickel hydroxide or its composites is also popular as positive electrodes of asymmetric ECs with high specific capacitance and high energy density,²⁷⁷ typically for hydroxide films anchored directly on a current collector.^278−281 The asymmetric EC with the highest power density of 44 W h kg⁻¹ was made using porous β-Ni(OH)₂ films deposited on lightweight and highly conductive surface ultrathin-graphite foam as the positive electrode, and microwave exfoliated GO as negative electrode, as shown in Fig. 24. After 10 000 cycles, 63.2% capacitance remained. Its highest power density is comparable with or higher than that of high-end commercially available supercapacitors.²⁷⁹ An asymmetric EC consisting of Ni(OH)₂/CNTs directly grown on Ni foam positive electrode and active carbon negative electrode, can deliver cell voltage of 1.8 V and an energy density of 50.6 W h kg⁻¹, about 10 times higher than that of a traditional electrochemical double layer capacitor, as reported by Gao.²⁸⁰ The graphene-supported Ni(OH)₂-nanowires and CMK-5 were used as the positive and negative electrodes, respectively, to form a high performance hybrid supercapacitor which could deliver a maximum specific power density of 40 840 W kg⁻¹ with a high energy density of about 17.3 W h kg⁻¹.²⁸² Yan et al. assembled an asymmetric EC using α-Ni(OH)₂/graphene and porous graphene as the positive and negative electrodes, respectively. The asymmetrical EC showed a high specific capacitance of 218.4 F g⁻¹, high energy density (77.8 W h kg⁻¹), and good cycling stability at an operating voltage of about 1.6 V in KOH aqueous electrolytes.²⁸³ Wang et al. compared the asymmetric ECs made of Ni(OH)₂/graphene with RuO₂/graphene. The ECs made of Ni(OH)₂/graphene and RuO₂/graphene showed high specific capacitance and a high energy density with a 1.5 V operating voltage.²⁸⁴

Fig. 24 SEM images of the Ni(OH)₂/ultrathin graphite foam composite, (a) SEM image of the composite material, and the inset shows a higher magnification image of the Ni(OH)₂ nanoflakes; (b) a cross-sectional view of the composite; (c) SEM image and the corresponding EDS elemental mapping images of (d) carbon, (e) oxygen, and (f) nickel. Reprinted with permission from ref. 279. Copyright (2013) American Chemical Society.

Guest metal doping or substitution in Ni(OH)₂ was reported to improve the electrode materials of asymmetric ECs. The Kong group reported that Al-substitution could make a stabilized α-Ni(OH)₂ and 7.5% Al containing α-Ni(OH)₂ exhibited a specific capacitance of 127 F g⁻¹ and energy density of 42 W h kg⁻¹.²⁸⁵ The XRD results proved that addition of Al over 7.5% led to higher crystallinity and increased the structure stability in alkaline medium. Ni(OH)₂ prepared by co-precipitation with Zn and Co as positive electrode material of an asymmetric EC, both exhibited a higher specific capacitance than pure Ni(OH)₂.²⁸⁶ The improved properties were attributed to co-precipitated Zn and Co, because the formed Zn(OH)₂ with Ni(OH)₂ could inhibit formation of γ-NiOOH and the distributed cobalt in the lattice or on the surface of Ni(OH)₂ could maintain good conductivity.²⁸⁶

Meanwhile, some LDHs such as NiAl-,²⁸⁷ CoAl-,²⁸⁸ and CoNi-^{287,289−293} LDHs or their composites, and Ni(OH)₂^56,294−296 also have been fabricated as electrodes of asymmetric supercapacitors. They all showed great potential energy storage ability and high rate capability.

9. Conclusion and prospects

Earth-abundant metal hydroxides as electrode materials are suitable for development of ECs. These low-cost and high-performance active hydroxide materials realized by simple and scalable solution processes offer great promise in development of novel materials for future energy storage devices. An effective route is use of 3D porously hierarchical hydroxides. These 3D pseduocapacitor electrodes have a number of features, such as fast ion and electron transfer, easy access of pseudoactive species, and efficient utilization and excellent reversibility. Although 3D nanoporous architectures have been shown to have advantages in supercapacitors, design and fabrication of high performance electrode materials with 3D hierarchical structures with large specific area and excellent rate capability remains a challenge.

Although most transition metal hydroxides, such as Co(OH)₂, Ni(OH)₂, and CoAl-, NiAl-LDHs are promising electrode materials for electrochemical capacitors on account of their high theoretical specific capacitance and low cost, application is hindered by their low measured specific capacitance and poor cycle stability, which are often associated with low specific surface area and poor electrical conductivity. There are two methods that can be applied to overcome these problems. One method is combining the metal hydroxide into a composite with an electronically conductive material such as carbon nanotube, graphene (reduced GO), and active carbon. The conductive material acts as both support and electron collector, while the hydroxide plays the role of electrochemically active species to form a synergistic effect. Composites consisting of metal hydroxides and carbon materials are promising for EC applications. The other method is direct fabrication of metal hydroxide crystal films onto a current collector. These films of transition metal hydroxides deposited on conductive substrates can be used directly as EC electrodes (polymer binders and conducting agents are not required), with high specific capacitance and good rate capability. Therefore, various approaches have been employed to prepare nanostructure films of metal hydroxides on conductive substrates directly, typically for hydrothermal and electrochemical deposition. However, these two methods have some intrinsic shortcomings. The hydrothermal method is limited by volume size, in continuous procedure, and long reaction time. The electrochemical deposition technique has disadvantages including small area of deposition, extreme cleaning after each deposition, and high working cost. Moreover, the mass loading of active materials on the substrate is usually low and difficult to control precisely. Thus, these issues limit application of hydroxide films as electrodes on a large scale.

For practical application, cyclic stability is crucial for an electrode material in electrochemical capacitors. Excellent cycle performance can be attributed to a few factors. One of the most important factors is the stable structure of electrode material. The structure and phase of the electroactive materials should not change during repeated charge–discharge cycles. Typically for these porous materials, pore size and volume are of great significance for good ion diffusion in the electrolyte to the electrode surface. Some reports found that capacitance decay was caused by dissolution of active materials in the electrolyte. Facile electron transport through the electrode material to the current collector should be guaranteed, facilitating electrons to the current collector to improve electronic and ionic conductivities. Thus, low internal resistance of the electrode is necessary. Good wettability/or accessibility between electrolyte and electrode materials is also needed, therefore, consideration should be made of the functional groups on electrode materials.

It is clear that the structure of metal hydroxides has great influence on their electrochemical properties. Hydrotalcite-like structured hydroxides show greater potential than hydroxide materials with a brucite structure. Owing to their tunable composition, and flexible interlayer spacing, hydrotalcite-like hydroxides have been deemed one of the most outstanding electrode materials for ECs, including α-Co(OH)₂ and α-Ni(OH)₂. However, the susceptibility of their structure in an alkaline media as electrolyte is an inevitable disadvantage.

From the theoretical viewpoint, hierarchical heterostructure composites containing metal hydroxides as the shell and semiconducting metal oxide as the core, are promising as electrode materials of ECs. The hybrid nanostructured electrodes usually have a high specific surface area and porous configuration, leading to a large electrode/electrolyte contact area, short diffusion path to current carriers, and high electron conductivity. However, the fabrication processes for these heterostructure composites are multi-stepped and complicated, a large obstacle to their practical application in EC electrodes.

Acknowledgements

This work was financially supported by the Zhejiang Provincial Natural Science Foundation of China under grant no. LY13E020002, Experimental research project of Zhejiang University.

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