Metal (M = Co, Ni) phosphate based materials for high-performance supercapacitors

Abstract

With the ever increasing demand for clean, sustainable energy, electrochemical supercapacitors with the advantages of high power density, high efficiency and long life expectancy have become one of the major devices for energy storage and power supply, and have found wide application in hybrid power sources, backup power sources, starting power for fuel cells and burst-power generation in electronic devices. Metal phosphates with the advantages of abundance, environmental friendliness and low cost are emerging as a novel class of promising electrode materials for supercapacitors. This review summarizes recent progress with respect to cobalt and nickel phosphate based micro/nanomaterials applied as supercapacitors, covering ammonium/bimetallic phosphates, cobalt phosphates, nickel phosphates and cobalt/nickel phosphates. Much progress has been made using metal phosphate materials as supercapacitors; however, there is still much room for further improvement.

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Xinran Li

Xinran Li is now a Ph.D. candidate in Pang's group. She works in the field of nanomaterials for energy storage and conversion. She focuses on the synthesis of functional materials and their application in electrochemical conversion.

Xiao Xiao

Xiao Xiao is now a student under Professor Huan Pang's supervision, at school of Chemistry and Chemical Engineering, Yangzhou University, China. Her research mainly focuses on the field of micro/nanocoordination derived materials for electrochemical energy devices.

Huaiguo Xue

Huaiguo Xue received his Ph.D. degree in polymer chemistry from Zhejiang University in 2002. He is currently a professor of physical chemistry and the dean of College of Chemistry and Chemical Engineering at Yangzhou University. His research interests focus on electrochemistry, functional polymers and biosensors.

Huan Pang

Huan Pang received his Ph.D. degree from Nanjing University in 2011. He then founded his research group in Anyang Normal University, where he was appointed as a distinguished professor in 2013. Now, he is a university distinguished professor at Yangzhou University. He has published more than 130 papers in peer-reviewed journals including Chemical Society Reviews, Advanced Materials, and Energy Environ. Sci., with 4000 citations (H-index = 33). His research interests include the development of inorganic nanostructures and their application in flexible electronics with a focus on energy devices.

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1. Introduction

Environmental and energy problems have become increasingly severe in modern society.^1,2 In recent years, the development of new high-performance energy storage and conversion devices has become a hot research area.^3–5 Compared with solar, wind, hydraulic and geothermal energies, electric energy has attracted a great deal of attention due to its prolonged lifespan and intense power density.^6–9 Being novel energy storage devices, the energy densities of supercapacitors (SCs) are between those of conventional batteries and traditional capacitors, exhibiting a higher specific capacity as well as a higher specific energy than traditional capacitors.^10–12 Meanwhile, SCs show better power efficiency and much faster charge/discharge characteristics than batteries.^13–21 SCs also have a large power density, long cycle life and relatively high energy density.^22,23 With the development of hybrid cars and electric vehicles, SCs have been receiving more and more attention.^24,25 It is necessary to improve the energy density of supercapacitors.^26,27 The main approaches include the use of pseudocapacitive materials^28,29 and high-voltage electrolytes (molten and organic electrolytes, lithium salts, and ionic liquids)^30–35 as well as designing different materials for specific needs.^36,37 Recent research studies showed that combining the battery-type and capacitor-type electrodes can improve the capacitance and voltage of SCs.^38–42

SCs can be categorized into pseudocapacitors (PCs) and electrochemical double-layer capacitors (EDLCs). The differences mainly lie in the charge storage mechanisms; EDLCs use a carbon-based electrode material, which is not electrochemically active;⁴³ that is, no electrochemical reactions occur on the electrode materials while charging, but rather only the pure physical charges build up at the electrode/electrolyte interfaces.^44–46 Meanwhile, the electrode materials in pseudocapacitors are electrochemically active.^47–49 Typical pseudocapacitive materials include transition metal oxides^50–52 and conductive polymers which display a much higher specific capacitance than carbon materials for EDLCs.^53–56 As mentioned above, both rechargeable batteries and pseudocapacitors store charges based on redox reactions in the electrode-active materials.^57–59 It is common knowledge that the energy density of a SC is decided by the particular capacitance of the electrode materials as well as the overall cell voltage. Accordingly, one efficient method is to develop porous and nano-sized electrode materials for improving capacitance. Another approach is to build hybrid/asymmetric SCs, which can efficiently utilize the potential gap between the two types of electrodes to increase the overall cell voltage; for example, the negative electrode is made of a porous active carbon material, meanwhile, the positive electrode material is made of a pseudocapacitive material.^60,61

A nanoporous carbon electrode is an ideal EDLC electrode material, owing to its high conductivity, large surface area, and controlled pore structure.^62–64 Despite possessing large specific surface areas, the charges are mostly stored physically on the surface.⁶⁵ This results in a porous carbon electrode having a lower specific capacitance and energy density. Compared to carbon, pseudocapacitive electrode materials, like conducting polymers and transition metal oxides, have been much more widely investigated recently.⁶⁶ Among the transition metal oxides, cobalt oxide (Co₃O₄), ruthenium oxide (RuO₂), and nickel oxide (NiO) have traditionally been widely studied.^67,68 A RuO₂-type electrode involves fast reactions of Faradaic charge-transfer. Meanwhile, its cyclic voltammetry (CV) shape is very broad, showing the form of quasi-rectangular.^69,70 These reversible and fast Faradaic reactions with its large surface area, fast proton transport, and intense conductivity result in high capacitance.⁷¹ However, the high cost of RuO₂ hinders its large-scale application. Hence, important efforts have been made to find promising alternatives to RuO₂. For example, cobalt oxide and nickel oxide are considered ideal competitors to RuO₂. Moreover, such materials could withstand great current charge/discharge. Meanwhile, the redox actions of cobalt and nickel contribute to the energy storage.^72,73

Ni and Co are widely distributed as well as earth-abundant (ranked fifteenth and nineteenth on the earth as shown in Fig. 1a). In the total reserve of nickel resources in the world, laterite nickel ore accounts for about 55% while nickel sulfide ore and ferromanganese nodules account for 28% and 17%, respectively. Moreover, there are hundreds of kinds of minerals containing cobalt existing in nature; most of them exist in the form of linnaeitet (Co₃S₄), carrolite (CuCo₂S₄), cobalt glance (CoAsS), smaltite (CoAs₂), erythrite (3CoO·As₂O₅·8H₂O) etc.^74–80 Although Ni and Co are abundant compared with other metals, their exploitation and utilization by humans are very limited, especially for Co, as shown in Fig. 1b. The main reason is that with the evolution of the Earth, gaseous elements were lost gradually, and the fusible alkali metals and aluminum silicate were enriched on the Earth’s surface.⁸¹ Co, Ni, Fe, Cu and other metal elements are abundant in argillaceous sediments and deep sea sediments.^82–84 The average content level of Co in the crust is 0.001% (quality), while the total amount of cobalt in the ocean is about 2.3 billion tons. Above all, Co and Ni are promising materials for high performance energy storage and conversion devices; more importantly, they have huge reserves yet to be systematically exploited and rationally utilized.^85–88 At present, Co/Ni based hydroxides, metal oxides as well as their composites act as substitutes for those rarely available noble metals (Ru, Pt) for energy storage^18–23 and have received extensive research attention.^28–32 It becomes apparent that Co/Ni based materials have great prospects for deep study and exploration.

Fig. 1 (a) The reserves of Ni and Co and their production in 2013–2015; (b) the content with abundance ref. Si = 10⁶ atoms of atomic numbers 1–85.⁶² (Ref. 73https://www.usgs.gov/)

Meanwhile, metal phosphates, particularly the layered metal phosphonates and phosphates, have gained much attention from the research community from 1987 to 1990,⁸⁹ and show outstanding performance in catalysis, ion exchange, proton conductivity, interface chemistry, photochemistry, and materials science.^90–100 Carling and Yuan et al. obtained NH₄M^IIPO₄·H₂O (M^II = Mn, Fe, Co, and Ni) by precipitation from aqueous solution.^101,102 A successful preparation of metal ammonium phosphate microspheres was done by Zhang et al.¹⁰³ Li et al. prepared amorphous colloidal spheres based on transition-metal phosphates.⁹² Besides, transition metal phosphates like nickel/cobalt phosphates can act as potential anodes for sorbents, magnetism materials, rechargeable batteries, ion exchangers and heterogeneous catalysts. Meanwhile, PO₄ polyhedra with a zeolite structure have outstanding electronic and magnetic properties.^104–108 Hence, these materials have attracted great attention. Fig. 2 illustrates that phosphates have been employed as SCs since 2012.¹⁰⁹ It is clear that increasing attention has been paid to metal phosphate in SCs since 2012.^110–113

Fig. 2 Publications with the theme of metal phosphates applied as SCs in the years 2012–2016.

In this review, progress with respect to various morphologies of ammonium cobalt (nickel) phosphates, cobalt phosphates, cobalt/nickel phosphates and nickel phosphates is discussed mainly based on previously published papers (2011–2017). Furthermore, SC performances of metal phosphate materials have been tested in a three-electrode system, a two-electrode and asymmetry solid flexible electrode are introduced and compared. The disadvantages of SCs based on phosphate materials are also discussed. Due to their high electrochemical performance, high natural abundance, environment friendliness and low cost, metal phosphate materials are highly promising for SC applications.

2. Cobalt/nickel based phosphate materials

2.1. Ammonium cobalt/nickel phosphates

In recent decades, ammonium metal phosphates have been investigated extensively and employed as pigment protection for paint finishes of metals, fertilizers, and plastics’ or paints’ fire retardants.¹¹⁴ The NH₄⁺ cation inserts between the sheets and interconnects the layers by hydrogen bonds. Such material systems are quite appropriate for discovering the effect of framing, refining mixed transition metals as well as determining their synergetic effects. Nickel(II) phosphates possess numerous varied structures that can increase their ionic conductivity and ion exchange rate.^115,116 Nonetheless, there are barely any reports on the electrochemical properties of these materials, not to mention those for electrochemical capacitors. Generally, the pseudocapacitance of the battery type material results from the surface redox reaction, which depends heavily on the electrochemical active surface area of electrode materials. Therefore, ultrathin layered electrode materials with multiple functional atoms exposed to the electrolyte can create sufficient electroactive sites, which unfortunately suffer from poor crystallinity with sacrificed electric conductivity and chemical stability.

For the past few years, our group has been devoted to developing nanomaterials for energy storage and conversion devices such as SCs, oxygen evolution reaction catalysts, Li-ion batteries, oxygen reduction reactions, etc. We were the first to study metal phosphate based materials for high-performance supercapacitors. Various NH₄CoPO₄·H₂O nano/microstructures were synthesized by means of facile chemical precipitation without the use of templates and surfactants.¹⁰⁹ Varied sources of ammonium solvents were used to synthesize various NH₄CoPO₄·H₂O nano/micromaterials and the obtained samples show different morphologies such as microflowers, microplates, hierarchical architectures, and oblong plates. When pure water was used as a solvent, we obtained hierarchically architectured NH₄CoPO₄·H₂O with the size of 20–30 mm (denoted M5), as shown in the insert of Fig. 3f. Moreover, the supercapacitive performances of NH₄CoPO₄·H₂O nano/microstructures were also examined via CV curves, electrochemical impedance spectroscopy methods as well as galvanostatic charge–discharge measurements in 3.0 M KOH aqueous solution. It was found that NH₄CoPO₄·H₂O hierarchical architecture (M5) electrodes show outstanding supercapacitive characteristics: the specific capacitance of the NH₄CoPO₄·H₂O electrode (M1–M5) as measured from CV curves (Fig. 3f) at a scan rate of 5 mV s⁻¹ achieved 550 F g⁻¹, 418.2 F g⁻¹, 650 F g⁻¹, 365.7 F g⁻¹ and 1142.9 F g⁻¹, respectively. Moreover, the M5 electrode kept a capacitance of 711.6 F g⁻¹ when the scan rate reached 100 mV s⁻¹. The measurement of electrochemical properties of nano/microstructured NH₄CoPO₄·H₂O demonstrates that NH₄CoPO₄·H₂O materials can be utilized as excellent electroactive materials for SCs.

Fig. 3 (a) SEM and TEM images of layered NH₄CoPO₄·H₂O microbundles; (b) specific capacitances of layered NH₄CoPO₄·H₂O microbundle electrodes derived from the discharging curves at a current density of 1.5–15.0 A g⁻¹ in 3.0 M KOH electrolytes; (c) TEM images of large rectangular platelets; (d) galvanostatic charge/discharge (GCD) plots at different current densities of NH₄CoNiP//HPC asymmetric SC; (e) TEM images of the hexagonal structure; (f) SEM images of the M5 sample, and specific capacitances derived from CV tests. Adapted with permission from (a, b) ref. 100, © 2014 The Royal Society of Chemistry; (c–e) ref. 117, © 2016 Nature; (f) ref. 109, © 2012 Royal Society of Chemistry.

Other than 3D metal phosphate, 1D metal phosphates have also been studied as electrode materials for SCs. Wang et al. synthesized layered NH₄CoPO₄·H₂O microbundles consisting of 1D layered microrods through a facile hydrothermal method.¹⁰⁰ The novel layered structures (Fig. 3a) possess channels intended for the diffusion of electrolytes and ions. Besides, electrolyte access and ion intercalation/extraction into/out can also be promoted. The preliminary SC investigation shown in Fig. 3b indicates that the specific capacitance of the layered NH₄CoPO₄·H₂O microbundle electrodes is 662 F g⁻¹ at 1.5 A g⁻¹ and 520 F g⁻¹ even at 15.0 A g⁻¹. The layered NH₄CoPO₄·H₂O microbundle electrodes also exhibited an excellent cycling performance (92.7% of the initial specific capacitance after 3000 cycles). This is an excellent example showing that the definite control of the morphology of nanomaterials influences the electrochemical performance.

Zhao et al. prepared a novel hybridized phosphate via a mild hydrothermal method in order to fabricate asymmetric SCs.¹¹⁷ Single layered (Ni,Co)₃(PO₄)₂·8H₂O nanoslices (Fig. 3e) and single crystal (NH₄)(Ni,Co)PO₄·0.67H₂O microplatelets (Fig. 3c) were formed through a sacrificial template approach and a dissolution recrystallization method, respectively. When employed as asymmetric SCs with hierarchical porous carbon, excellent SC performances were obtained. Fig. 3d shows the GCD curves of the asymmetric SC cell at varying densities of current in a voltage window which ranged from 0 to 1.65 V with a closely symmetrical shape, indicating good electrochemical SC characteristics. The specific capacitance could be up to 1128 F g⁻¹ at a current density of 0.5 A g⁻¹. The asymmetric hybrid SC showed a high energy density of 35.3 W h kg⁻¹ at relatively low power density, but still it kept up to 30.9 W h kg⁻¹ at 4400 W kg⁻¹. Such an excellent electrochemical performance is explained by the synergistic effect of the surface redox reaction from ion intercalation and ultrathin nanoslices obtained from the single crystal bulk structure.

Wang et al. synthesized self-organized NH₄CoPO₄·H₂O microflowers through a mild chemical precipitation method.¹¹¹ The supercapacitive performances of NH₄CoPO₄·H₂O microflowers (Fig. 4a and b) and porous Co₂P₂O₇ (Fig. 4c–e) were studied schematically in 1.0 M KOH, NaOH and LiOH electrolytes. Results show that the crystallinity of these materials significantly affects the electrochemical activity of SCs in various electrolytes. Supercapacitive characteristics of NH₄CoPO₄·H₂O microflowers in 1.0 M KOH electrolyte were better than others (Fig. 4f and g). The specific capacitance of NH₄CoPO₄·H₂O microflower electrodes at a current density of 0.625 A g⁻¹ was about 525 F g⁻¹. It also showed a prolonged cycle life which remained 99.4% after 400 cycles in 1.0 M KOH.

Fig. 4 (a, b) SEM images of NH₄CoPO₄·H₂O microflowers (M); (c–e) SEM images of Co₂P₂O₇ samples: (c) P1; (d) P2; (e) P3. Specific capacitances of electrode materials at different scan rates derived from GCD in 1.0 M LiOH, NaOH and KOH aqueous solutions: (f) M; (g) P1; adapted with permission from ref. 111, © 2013 Springer.

Pang et al. obtained porous cobalt pyrophosphate (Co₂P₂O₇) nano/microstructures (microflower, oblong plates, hierarchical architectures, and microplates) via calcination of NH₄CoPO₄·H₂O nano/microstructures (Fig. 5a).¹¹⁸ The NH₄CoPO₄·H₂O nano/microstructures were well preserved after the thermal treatment where NH₄CoPO₄·H₂O was transformed into Co₂P₂O₇ (Fig. 5b). Besides, the supercapacitive performances of porous Co₂P₂O₇ nano/microstructures were studied in 3.0 M KOH solution as shown in Fig. 5c. Different nano/micro structured Co₂P₂O₇ samples have the possibility to have different surface-interface conditions, which assumes a pivotal function in ion intercalation/extraction into/out as well as electrolyte access. The charge–discharge curves of these electrodes with different morphologies at 0.625 Ag⁻¹ are shown in Fig. 5c. The symmetry of charging/discharging cycling curves shows that the Co₂P₂O₇ electrode material possesses perfect electrochemical capacitive characteristics indicative of reversible redox reactions. Compared to others, Co₂P₂O₇ hierarchical architectured electrodes (denoted P5) at 0.625 A g⁻¹ have longer charge–discharge times. P5 exhibited a high specific capacitance of 367 F g⁻¹ at a current density of 0.625 A g⁻¹ in aqueous KOH solution with good cycling stability.

Fig. 5 (a) SEM images of NH₄CoPO₄·H₂O nano/micromaterials (M1); (b) SEM images of the Co₂P₂O₇ samples (P5); (c) GCD curves of Co₂P₂O₇ electrode materials (P1–P5) at 0.625 Ag⁻¹; (d) TEM image of the sample Co/Ni 2 : 3; (e) a photograph of a red LED lighted up by two cells; (f) corresponding Ragone plot, inset showing the specific capacitance dependent on current densities. Adapted with permission from (a–c) ref. 118, © 2013 Springer; (d–f) ref. 112, © 2016 Springer.

The synergetic effects between mixed transition metal compounds have been proved to have great potential in improving the electrochemical performance of electrode materials. Xiao's group synthesized ammonium/bimetallic phosphates (NH₄Co_xNi_1−xPO₄·H₂O) (Fig. 5d) with layered nanostructures by a facile one-step solvothermal method.¹¹² They found that the synergistic effect between cobalt and nickel can significantly influence the crystal structure and morphology of NH₄Co_xNi_1−xPO₄·H₂O, compared with the single-component ammonium phosphates. As shown in Fig. 5f, the asymmetric device based on it presented a high energy density of 37.4 W h kg⁻¹ at a power density of 826 W kg⁻¹. Moreover, the insert of Fig. 5f indicated that the device also exhibited a good long-term cycling stability with 91.2% capacity retention even after 5000 cycles. Moreover, after charging each cell for 15 s, a light-emitting diode (LED, 2.0 V, 20 mA) can be illuminated for nearly 30 min, as shown in Fig. 5e.

As opposed to the conventional all-carbon supercapacitors (EDLC or electric double layer capacitors) which possess high power but lack energy density, pseudocapacitors embank on the high energy density innate to reversible redox reactions and simultaneously offer a facile way to improve the supercapacitors’ energy rate. Raju et al. reported novel NH₄NiPO₄·H₂O with unique morphologies (microplatelets in Fig. 6a denoted ANP_mp, microrods in Fig. 6b denoted ANP_mr, and microdendrites in Fig. 6c denoted ANP_md) with different supercapacitive properties.¹¹⁹ The ammonium nickel phosphate hydrate with 1D microrod structures sized about 36 nm gave extraordinarily high specific capacitance, power and energy densities in half-cell and full-cell configurations (i.e., symmetric and asymmetric cells, including all-solid-state flexible pseudocapacitors) in different electrolytes. The versatile all-solid-state ANP_mr-based pseudocapacitor exhibited high power densities (12.7 mW cm⁻²) and areal capacitance up to 66 mF cm⁻² with excellent energy (21.2 mW h cm⁻²) as shown in Fig. 6d. The study proves that rod-like morphology (with ∼36 nm width) provides a significant and promising direction for novel 1-D and 2-D materials to obtain high-performance pseudocapacitors, especially for flexible and wearable electronics.

Fig. 6 SEM micrograph of as-synthesized various morphologies of (a) ANP_mp (microplatelets), (b) ANP_mr (microrods) and (c) ANP_md (microdendrites), respectively; (d) specific capacitance vs. current density, i.e., rate capability test; (e) SEM images of as-prepared samples P1; (f) CV curves at a scan rate of 30 mV s⁻¹ with various bending angles (ranging from 0° to 180°) of the P1//graphene device; adapted with permission from (a–d) ref. 119, © 2015 Nature; (e, f) ref. 120 © 2015 Elsevier Ltd.

Other than traditional SCs that are bulky, sometimes environment-unfriendly (mainly owing to the organic electrolytes) and rigid, flexible solid-state electrochemical energy storage devices based on phosphates have also been built. Our group built an asymmetric SC based on amorphous nickel pyrophosphate (Ni₂P₂O₇) microstructures and graphene nanosheets as positive and negative electrode materials, respectively.¹²⁰ The amorphous Ni₂P₂O₇ microstructures (as shown in Fig. 6e) were obtained by calcination of the ammonium nickel phosphate hydrate microstructures in air at 350 °C. The flexible all-solid-state electrochemical energy storage device showed a maximum volumetric energy density of 0.78 mW h cm⁻³ as well as good mechanical flexibility under a bending angle of 0–180°. The electrochemical properties of the as-prepared device were measured when it was bent with different angles (0° to 180°) to test its flexibility (Fig. 6f). The shapes of the CV curves under different bending angles matched well with each other for extended bending cycles. Due to the advantages of the high energy density and flexibility originating from both the device architecture and the materials, various applications are anticipated by exploiting it in this work.

Zhao et al. fabricated uniform mesoporous ammonium nickel phosphate hydrate (Fig. 7a) nanostructures via a one-pot hydrothermal method.¹²¹ Owing to the distinct characteristics of a large surface area (418 m² g⁻¹) and controllable mesostructures, such a material showed an extraordinary performance-specific capacitance of 1072 F g⁻¹ (at 1.50 A g⁻¹) in 3.0 M KOH. As shown in the insert of Fig. 7b, the material exhibited a long lifespan, which hardly decayed after 3000 cycles with a retention of 95.0% of the initial specific capacitance.

Fig. 7 (a) SEM images of the as-prepared samples; in the inset: the optical photo of almonds; (b) specific capacitances of mesoporous NH₄NiPO₄·H₂O nanostructure electrodes derived from the discharging curves at a current density of 1.5–13.0 A g⁻¹ in 3.0 M KOH electrolytes. Adapted with permission from ref. 121 © 2013 The Royal Society of Chemistry.

2.2. Cobalt phosphate

Co based materials have gained interest and attention because of the lower cost; however, certain characteristics of Co-based materials such as the small surface area and poor conductivity limit the cycling performances and rate capability of Co-based SCs. Some porous/nanostructured Co-based materials have been designed and synthesized in such a way that they enhance the electrocatalytic performances, supply effective ion diffusion channels, and increase both electron migration and diffusion. Xie's group successfully synthesized hierarchical 3D cobalt phosphate hydrate (Co₃(PO₄)₂·8H₂O) architectures with flower-like morphologies built from 2-dimensional microsheets ranging from 5 to 12 μm at room temperature via a green precipitate process (free of surfactant).¹²² The SEM images of the as-prepared products (denoted P2) are shown in Fig. 8a. TEM images show the detailed and comprehensive structure of Co₃(PO₄)₂·8H₂O (Fig. 8b), indicating that the 3D architecture was assembled from plenty of interconnecting 2D microsheets. The architecture of as-prepared Co₃(PO₄)₂·8H₂O provided large nanoscale pore channels to improve ion-transportation. Moreover, excellent rate capability (227 F g⁻¹ at 10 A g⁻¹) and cycling stability (please see Fig. 8c for the capacitance increase to a retention of 102% after 1000 cycles) were observed. This material holds great promise for commercial applications due to the high yield, low cost, and outstanding capacitive behavior. Meanwhile, this synthetic method could be applied in the preparation of other metal phosphates.

Fig. 8 SEM (a) and TEM (b) images of Co₃(PO₄)₂·8H₂O (P2); (c) cycling stability of the Co₃(PO₄)₂·8H₂O materials at 1 A g⁻¹; SEM (d) and TEM (e) images of Co₁₁(HPO₃)₈(OH)₆; (f) CV curves of Co₁₁(HPO₃)₈(OH)₆ microarchitecture electrodes at different scan rates; (g) specific capacitance over 400 cycles under the curvature of 0°, 30°, 90°, and 180° at a current density of 0.5 mA cm⁻²; (h, i) SEM images of the as-prepared core–shell Co₁₁(HPO₃)₈(OH)₆–Co₃O₄ nanohybrids obtained after 8 h under hydrothermal conditions; (j) specific capacitances of CoHPO₄·3H₂O ultrathin nanosheet electrodes derived from the discharging curves; (k) at the current density of 1.5–15.0 A g⁻¹ in 3.0 M KOH electrolytes; (l) SEM image of the as-prepared CoHPO₄·3H₂O; Adapted with permission from (a–c) ref. 122, © 2015 Elsevier; (d–f) ref. 108 (g–i) ref. 114, © 2013 The Royal Society of Chemistry; (j–l) ref. 123 © 2015 Elsevier.

Pang et al. synthesized cobalt phosphate (Co₁₁(HPO₃)₈(OH)₆) microarchitectures built from super long nanoribbons under mild hydrothermal conditions without additives.¹⁰⁸ The uniform ultralong nanoribbon was sized around 100 nm in width and about 20–30 mm in length, as shown in Fig. 8d and e. Fig. 8f shows the CV curves of the Co₁₁(HPO₃)₈(OH)₆ microarchitecture at varying scan rates. The range is 0 to 0.60 V (vs. SCE) in 3.0 M KOH aqueous solution. It can be clearly seen that the Faradaic pseudocapacitive property of the Co₁₁(HPO₃)₈(OH)₆ microarchitecture is based on the surface redox mechanism of Co²⁺ to Co³⁺. In addition, the as-prepared materials were also successfully applied as a novel electrochemical SC possessing excellent specific capacitance (312 F g⁻¹ at 1.25 A g⁻¹), rate capability as well as cycling property (maintaining about 89.4% at 1.25 A g⁻¹ after 3000 cycles). Nonetheless, unlike other cobalt based SC electrode materials, the specific capacitance and rate performance of this Co₁₁(HPO₃)₈(OH)₆ microarchitecture need improving.

Zhang et al. fabricated a flexible solid-state asymmetric SC with graphene nanosheets as the cathode, KOH-polyvinyl alcohol (PVA) gel as the solid state electrolyte and core–shell structured cobalt phosphate–cobalt tetroxide [Co₁₁(HPO₃)₈(OH)₆–Co₃O₄] hybrids as the anode.¹²³ They developed a facile hydrothermal technique to fabricate core–shell structured Co₁₁(HPO₃)₈(OH)₆–Co₃O₄ hybrids, with the microporous Co₁₁(HPO₃)₈(OH)₆ microrods as the precursor. As shown in Fig. 8h and i, the 1D structure had a diameter of 380 nm. Moreover, the shell was made of the oxidation layers and the core was made of Co₁₁(HPO₃)₈(OH)₆ microrods. The shell was not close to the core so that efficient nanochannels were formed for electrolytes and ion diffusion. These as-prepared core–shell Co₁₁(HPO₃)₈(OH)₆–Co₃O₄ hybrids were further used for flexible solid-state asymmetric SCs. In order to test the influence of mechanical action on electrochemical performance, the device was bent with different angles (0°, 30°, 90°, and 180°, Fig. 8g) with the corresponding electrochemical tests conducted at the same time. The result showed that the specific capacitance changed slightly at different bending angles. Conspicuously, the specific capacitances of all devices remained during the bending processes. However, after 3000 cycles, the devices’ stable cycling performance was relatively different. It is worth noting that the specific capacitance of the 8H//graphene devices remained at 98.7% of the initial value after 3000 cycles.

Since the discovery of graphene, 2D layered materials (e.g., 2D compounds, metal chalcogenides, and transition metal oxides) have received renewed attention. Such materials exhibit interesting properties including thermoelectricity, topological insulator effects, and superconductivity; they even cover a relatively wide spectrum of electronic structures, from metals to insulators. Pang et al. synthesized ultrathin cobalt phosphate CoHPO₄·3H₂O nanosheets through a one-step hydrothermal process without any exfoliation.¹¹⁴ The CoHPO₄·3H₂O ultrathin nanosheets display a novel layered surface–interface structure (Fig. 8l), which is vital for electrolyte-access and ion-intercalation/extraction into/out. More importantly, novel CoHPO₄·3H₂O super-thin nanosheets were effectively gathered and integrated to construct the electrodes for SCs. The super thin CoHPO₄·3H₂O nanosheet electrodes possess a huge specific capacitance (Fig. 8j), which reached up to 413 F g⁻¹ at a current density of 1.5 A g⁻¹, and 338 F g⁻¹ even at 15.0 A g⁻¹. What's more, there was no apparent degradation even after 3000 charge–discharge cycles. The measurement of the electrochemical properties of CoHPO₄·3H₂O ultrathin nanosheets successfully illustrates that it was a promising SC electrode material; furthermore, the nano/microstructured materials’ chemical and physical properties as well as electrochemical performance can be tuned via accurate control of the morphology of materials.

Traditionally, micro-supercapacitors are fabricated via an expensive and extensive lithographic approach. Alternatively, additive fabrication approaches such as inkjet printing offer a simple and versatile route to make complicated planar electrode structures that is crucial for their mass production and integration into larger systems of micro-supercapacitors. Compared with other printing methods such as screen printing, inkjet printing has advantages in versatility and cost and can be easily integrated into a roll-to-roll mass production system. Pang et al. fabricated a flexible all-solid-state asymmetric micro-SC based on lamellar potassium cobalt phosphate hydrate (K₂Co₃(P₂O₇)₂·2H₂O) nanocrystal whiskers (anode electrode) and graphene nanosheets (cathode electrode) using a solid state electrolyte [KOH-polyvinyl alcohol (PVA) gel].¹¹³ They provided a facile method to synthesize lamellar K₂Co₃(P₂O₇)₂·2H₂O nanocrystal whiskers under mild hydrothermal conditions. A low-magnification SEM image in Fig. 9c reflects the morphology of as-prepared samples: nanowhiskers which are 250–400 nm in length. The flexible solid-state asymmetric micro-SC (denoted a micro-device) was fabricated via an inkjet printing method, and the structure scheme of the device is illustrated in Fig. 9a. Silver ink was printed on the PET substrate, functioning as the current collector for the active materials (K₂Co₃(P₂O₇)₂·2H₂O and graphene). Due to the highly inter-connected structure of the lamellar K₂Co₃(P₂O₇)₂·2H₂O nanocrystal whiskers, these nanomaterials exhibited outstanding mechanical and electrical robustness, which enabled the device to function well under highly bent conditions. Fig. 9b shows the cycling performances of the lamellar K₂Co₃(P₂O₇)₂·2H₂O nanocrystal whisker electrode at disparate current densities (5.0, 3.0, 2.0, 1.5 and 1.0 mA cm⁻²). It is seen that the capacity changed from 1100 mF cm⁻² to 810 mF cm⁻² as the current density increased from 1.0 mA cm⁻² to 5.0 mA cm⁻². What's more, the cycle remained stable at each current density.

Fig. 9 (a) Schematic view of the printed flexible micro-device; (b) in a conventional three-electrode system, a charge/discharge cycling test at different current densities from 1.0 to 5.0 mA cm⁻²; (c) TEM image, and the model image of overlapping K₂Co₃(P₂O₇)₂·2H₂O nanocrystal whiskers (inset); (d, e) FESEM and TEM images of the as-prepared hierarchically porous NaCoPO₄–Co₃O₄ hollow microspheres; (f) cycling performances of the device at a current density of 1.0 mA cm⁻² for 5000 cycles (the insert picture shows the last ten cycles); EDX-elemental mapping images (g–j) of the hierarchically porous NaCoPO₄–Co₃O₄ hollow microspheres. Adapted with permission from (a–c) ref. 113, © 2015 Elsevier; (d–j) ref. 124, © 2015 WILEY-VCH.

Considerable effort has been devoted to developing methods for the preparation of hollow structured materials. Template-based methods are the most popular. Wei et al. developed a facile hydrothermal process to fabricate hierarchically hollow precursors with nanoplates acting as building blocks.¹²⁴ After a calcination process, hierarchically porous NaCoPO₄–Co₃O₄ hollow microsphere materials were obtained. Fig. 9d and e show the morphology of the as-obtained NaCoPO₄–Co₃O₄; the SEM images demonstrate that the product was made of microsphere particles sized from 5 to 8 μm, indicating that it inherited the morphology of its precursor. The hollow structure can be confirmed by the TEM image in Fig. 9e. The distinct contrast between the black and white ligaments also exhibits nonporous characteristics. The elemental mapping results (Fig. 9g–j) further affirm the even distribution of elements Co, Na, O and P within the hierarchical microspheres. More importantly, the cycling stability of the device was examined by means of a cycling charge/discharge process with a current density of 1.0 mA cm⁻² (Fig. 9f). Following 5000 cycles (the insert of Fig. 9f shows the last ten cycles), it retains 94.5% of its initial capacitance, revealing excellent cycling performance. The 5.5% capacitance decrease of the device can be attributed to the consumption of the gel electrolyte via an irreversible electrolyte–electrode material reaction. It is believed that these active NaCoPO₄–Co₃O₄ materials with unique structures can possibly act as potential alternatives for many other applications like lithium-ion batteries, catalysis, and sensors.

2.3. Nickel phosphate

Different calcination temperatures can affect the structural, morphological and capacitance characteristics of materials as electrode materials for SC applications.¹²⁵ Omar et al. synthesized Ni₃(PO₄)₂ particles using a chemical method to agitate the precursor solution avoiding particle aggregation during the chemical precipitation and followed by calcination at different temperatures.¹²⁶ The synthesized Ni₃(PO₄)₂ at different calcination temperatures (300, 600 and 900 °C) are represented as N300, N600 and N900, respectively. The SEM images (Fig. 10a) revealed that before the calcination, the particles displayed irregular-shape. However, the particles were not well separated with a high degree of agglomeration. The surface morphologies showed that the particle size increased with increasing the calcination temperatures. The electrochemical performances of N300, N600 and N900 were investigated using CV and GCD in a 1 M KOH electrolyte. It was found in Fig. 10b that N300 exhibited a maximum specific capacity of 620 C g⁻¹ at 0.4 A g⁻¹, which was significantly higher than N600 (46 C g⁻¹) and N900 (14 C g⁻¹). Additionally, the fabricated N300//activated carbon based asymmetric SC can be cycled reversibly at a cell voltage of 1.45 V. The device exhibited 76 W h kg⁻¹ energy density and 599 W kg⁻¹ power density with 88.5% capacitance retention after 3000 cycles.

Fig. 10 (a) FESEM image of N300; (b) cyclic voltammogram of N300, N600 and N900 at a scan rate of 1 mV s⁻¹; (c) TEM images of LVNP/C2.5; (d) cathode, anode and cell profiles at 0.33 A g⁻¹ (Tc ¼ charge time and Td ¼ discharge time; (e) specific capacitance as a function of current density; field emission scanning electron microscope images of NaNiPO₄ powders (f–h) synthesized at 300 400 and 550 °C, respectively; (i) charge–discharge curves of the Fe₃P₂O₈·8H₂O//Ni₃P₂O₈ asymmetric cell; (j) SEM images of Ni₃P₂O₈; (k–n) SEM images of M1, M2, M3, and M4, respectively; (o) specific capacitances derived from CV tests. Adapted with permission from (a, b) ref. 126, © 2016 The Royal Society of Chemistry; (c, d) ref. 127, © 2015 The Royal Society of Chemistry; (e–h) ref. 128, ©; (i, j) ref. 129, © 2016 The Royal Society of Chemistry; (k–o) ref. 130, © 2013 Wiley-VCH.

Nonetheless, LVP is quite a promising material for batteries for SC applications, and this is largely because of its high cycling stability and rate capability. The electrochemical performance of pure LVP/C has been effectively improved by Ni-doping. The advantages of introducing Ni in the LVP crystal structure include the following: a higher Li-ion diffusion capacity, a lower charge-transfer resistance on the electrolyte/electrode interface, and a decreased inorganic particle dimension. Secchiaroli et al. successfully synthesized Li₃V_2−xNi_x(PO₄)₃/C (x = 0, 0.05, and 0.1) nanomaterial, and the Li₃V_2−xNi_x(PO₄)₃ nanoparticle, both of which were well spread and integrated into the as-prepared carbon matrix with a mean particle dimension between 63 and 200 nm.¹²⁷ The carbon matrix was fabricated by decomposition of poly(acrylic acid) (PAA) and glucose, Fig. 10c shows the TEM image of the LVNP/C2.5, agglomerates of merged particles form the powder. The average particle dimension of LVP/C2.5 is about 300 nm. Ni metal doping has the ability to enhance the energy density of the pure LVP/C as shown for LVNP/C2.5, thus demonstrating the highest specific capacity among the three cathodes at the current density range from 0.033 A g⁻¹ to 0.33 A g⁻¹ (Fig. 10d). The system retained 99% of the initial specific capacity measured at 0.033 A g⁻¹ with 93 mA h g⁻¹ at 100 °C as well as a capacity retention of 97% after 1000 cycles at 100 °C.

Transition metal phosphates such as lithium and oxides have been studied for more than two decades. Minakshi et al. synthesized sodium nickel phosphate (NaNiPO₄) and measured the difference of products synthesized at different temperatures.¹²⁸ Both the morphology and structures of the polymorphs were characterized (Fig. 10f–h). According to XRD, NaNiPO₄ was in two varied forms (maricite and triphylite), which was determined by the synthetic temperature (300–500 °C). The triphylite form of NaNiPO₄ and the as-prepared form together with activated carbon in 2 M NaOH demonstrate a maximum specific capacitance of 85 F g⁻¹ at 1 A g⁻¹ and 125 F g⁻¹, respectively, as shown in Fig. 10e. Moreover, they both reveal remarkable cycling stability with a retention of about 99% of the initial capacity for more than 2000 cycles. Nonetheless, the blocky and coarse maricite form exhibited a lower capacitance of 70 F g⁻¹ with a 40% decrease in capacity after 50 cycles. Such a low performance can be attributed to a reduction in the accessible surface area, which hindered the penetration of the electrolyte. These findings indicate that the as-prepared triphylite revealed a 44 W h kg⁻¹ energy density which is considered a high performance and could be attributed to the hierarchical structure of the porous NaNiPO₄ nanosheets. The electrochemical performance of this new promising NaNiPO₄-based capacitor is better than that of an oxide or lithium-based phosphate electrode supercapaictor.

Liu et al. synthesized amorphous Ni₃P₂O₈via a facile chemical precipitation technique which acted as an anode electrode material for SCs; they also synthesized Fe₃P₂O₈·8H₂O using the same method to replace active carbon material.¹²⁹ The SEM images of Ni₃P₂O₈ and electrode materials are illustrated in Fig. 10j. It can be seen that these nanoflowers, which are sized about 10 μm in length, were composed of dozens of small individual petals with some degree of agglomeration. The flower-like structure was able to increase the surface area of Ni₃P₂O₈, which brought about a large C_m and excellent rate capability. The novel Ni₃P₂O₈ and Fe₃P₂O₈·8H₂O exhibited high specific capacitance as electrode materials for asymmetric SC with Ni₃P₂O₈ as the positive electrode and Fe₃P₂O₈·8H₂O as the negative one (see Fig. 10i). A specific capacitance of up to 94 F g⁻¹ was obtained in a high-voltage region of 0–1.6 V. Besides, a large energy density (32.6 W h kg⁻¹) was delivered at 420 W kg⁻¹ power density. The results indicate promising prospects of metal phosphate-based materials for SCs. Such an impressive Ni₃P₂O₈/Fe₃P₂O₈·8H₂O-based hybrid SC as well as electrode materials were expected to show good prospects for high performance energy storage systems.

Nickel phosphite (Ni₁₁(PO₃)₈(OH)₆) hexagonal polyhedrons were effectively synthesized by Pang's group under hydrothermal conditions.¹³⁰ It was found that different nano/microstructured Ni₁₁(PO₃)₈(OH)₆ samples possessed varied surface/interface conditions that assume pivotal functions in electrolyte access and ion intercalation/extraction. As shown in Fig. 10k, the hexagonal polyhedrons (M1) have a smooth surface, sized approximately 1200 nm individually. M2 have the same structure and size as M1, but take the form of a hexagonal prism and a hemisphere as reflected in Fig. 10l. In Fig. 10m, it is observed that some hemispheres have created several gaps and even become slightly decomposed. As is shown in Fig. 10n, the height of the as-prepared hexagonal prism structure has decreased greatly (less than 200 nm) and is not as smooth as M1 or M2. The Ni₁₁(PO₃)₈(OH)₆ hexagonal polyhedrons were prepared as SC electrode materials, as shown in Fig. 10o. M1–M4 achieved a specific capacitance of 356 F g⁻¹, 80 F g⁻¹, 25 F g⁻¹ and 141 F g⁻¹ at 5.0 mV s⁻¹, respectively. Moreover, when the scan rate was increased to 100.0 mV s⁻¹, the capacitance of the M1 electrode remained at 177 F g⁻¹ unlike the others, which underwent sharp decreases.

Another likely substitute for the traditional structured SCs are asymmetric supercapacitors (ASCs) that utilize a battery-type Faradaic electrode with higher power and energy density as the source of power. Microporous nickel phosphite [Ni₁₁(HPO₃)₈(OH)₆] nanocrystals were prepared through a hydrothermal process and then successfully used as an anode electrode in a versatile all-solid-state ASC by Gao et al.¹³¹ The magnified SEM images of a simple nanocrystal (Fig. 11b) demonstrated that the size of the as-prepared nanocrystal is 300 nm in length and its form is that of a hexagonal pyramid. Moreover, the big end of this pyramid has a diameter of approximately 195 nm, further confirming a hexagonal structure. Due to its specific micro/nanostructure, the flexible solid-state asymmetric SC, microporous Ni₁₁(HPO₃)₈(OH)₆ nanocrystals were successfully fabricated through a hydrothermal technique. The Ni₁₁(HPO₃)₈(OH)₆//graphene ASC device is for the first time reported as being applied for SCs and it exhibited excellent performance. The symmetrical characteristics of the charging/discharging curves are remarkable, indicating that the Ni₁₁(HPO₃)₈(OH)₆ nanocrystal electrode has a fantastic electrochemical ability, and the redox process is reversible, as shown in Fig. 11a. Because of the simple device fabrication and synthesis processes, this versatile solid-state ASC can have several applications in fields like solar energy harvesters, roll-up display panels, and power-on-chip systems.

Fig. 11 (a) The galvanostatic charge–discharge curves for current densities between 0.5 and 7.0 A g⁻¹ of Ni₁₁(HPO₃)₈(OH)₆; (b) SEM images of Ni₁₁(HPO₃)₈(OH)₆; (c) SEM images of the porous Na-doped Ni₂P₂O₇ hexagonal tablets; (d) specific capacitance at various discharge current densities of Na-doped Ni₂P₂O₇ hexagonal tablets; (e) specific capacitances of the cobalt/nickel phosphate electrodes calculated using discharge curves; (f) TEM images of Co₃(PO₄)₂, Co_2.30Ni_0.70(PO₄)₂, Co_0.86Ni_2.14(PO₄)₂ and Ni₃(PO₄)₂ samples before pyrolysis; SEM images of the Co_0.2Ni_0.8 pyrophosphates prepared at calcination temperatures of (g) 300 °C, (h) 400 °C, (i) 500 °C and (j) 600 °C; (k) calculated specific capacitance from the GCD curves for the products Co_xNi_1−x pyrophosphates at a calcination temperature of 400 °C; (l) TEM images of nickel–cobalt phosphate nanosheets; SEM images of (m) Ni₃P₂O₈, (n) Co₃P₂O₈·8H₂O and (o) Ni₃P₂O₈–Co₃P₂O₈·8H₂O; (p) specific capacitances of Ni₃P₂O₈–Co₃P₂O₈·8H₂O with various Co/Ni molar ratios at a current density of 0.5 A g⁻¹; (q) the dependence of the specific capacitances on Ni/Co molar ratios of Ni₃P₂O₈–Co₃P₂O₈·8H₂O; (r) SEM images of MOF-derived Ni_xP_yO_z; (s) charge–discharge curves of pristine MOF and MOF-derived Ni_xP_yO_z at different current densities. Adapted with permission from (a, b) ref. 131, © 2014 The Royal Society of Chemistry; (c, d) ref. 132, © 2015 Wiley-VCH; (e, f) ref. 12 © 2016 Elsevier; (g–l) ref. 134, © 2016 American Chemical Society; (m–q) ref. 135 © 2016 Elsevier; (r, s) ref. 133, © 2016 Wiley-VCH.

In recent years, inorganic materials treated with metal ions (e.g., Ni²⁺, Cu²⁺, Zn²⁺, Mn⁴⁺) have received much interest, which can positively modify the properties of the materials through a change in electrical conductivity and electron density. Our group prepared hexagonal tablet precursors though a hydrothermal method; then after calcination treatment, it was transformed into porous sodium-doped Ni₂P₂O₇ hexagonal tablets.¹³² It can be seen in Fig. 11c that the overall morphology and size of the Na-doped Ni₂P₂O₇ hexagonal tablet decreases slightly after annealing. The higher magnification images of individual particles indicate that the Na-doped Ni₂P₂O₇ hexagonal tablet has a porous structure after CO₂ and H₂O release during the thermal decomposition process. The obtained samples were applied as SC electrode materials, with the sample as the anode and graphene as the cathode. Galvanostatic charge/discharge measurements were tested at a current density of 0.5, 1.0, 1.5, 2.0, and 2.5 mA cm⁻². And the equivalent specific capacitances of the as-prepared solid-state hybrid SC were consequently measured (Fig. 11d). At 2.5 mA cm⁻², the device's specific capacitances may possibly extend up to 32.6 mF cm⁻². The Ni₂P₂O₇//graphene solid-state hybrid SC achieved an excellent energy density of 23.4 W h kg⁻¹ as well as good cycling stability, which proves that the porous sodium-doped Ni₂P₂O₇ hexagonal tablets are novel promising materials for flexible electrodes of SCs.

Metal–organic frameworks (MOFs) with controlled pore size distribution, high porosity, and abundant electroactive constituents have received great attention for SC applications. Bendi et al. reported a new technique for synthesizing Ni-MOF-derived hollow porous nickel phosphate (Ni_xP_yO_z) for high performance SCs.¹³³ They developed a new strategy to prepare an efficient and original MOF-templated hollow porous Ni_xP_yO_z composite (as shown in Fig. 11r) electrode for SC. The synthesized Ni_xP_yO_z was a possible electrode material for electrochemical SCs under three-electrode configuration in the solution of 2 M KOH aqueous electrolyte. Fig. 11s demonstrates the galvanostatic charge/discharge curves of MOF-derived Ni_xP_yO_z, within a possible voltage window of 0–0.45 V at different current densities. The discharge curve exhibits the properties of pseudocapacitance. The specific capacitance of the MOF-derived Ni_xP_yO_z at 1 A g⁻¹ was 1627 F g⁻¹. It is also found that phosphate ions can enhance the conductivity of MOFs by providing more Faradaic reactions throughout the interface between the electrolyte and the electrode. The increased redox reactions at metal centers may be due to the resultant microstructures, which have loosely adsorbed structures on microrods with intense conductivity and huge surface area of the MOF-derived Ni_xP_yO_z. Thus, such a hybrid-structured electrode inherits the strengths of the MOF template (high capacitances drawn from the internal surface areas) and metal phosphates (effective pseudocapacitance).

2.4. Cobalt/nickel phosphate

A uniform three dimensional (3D) mesoporous structure (2–10 nm) can shorten diffusion paths and increase the surface areas for electrolyte ions, but is not frequently reported in phosphate based electrode materials. Tang et al. synthesized cobalt/nickel phosphate nanospheres with honeycomb-like mesopores (Co_xNi_(3−x)(PO₄)₂) through a facile hydrothermal process integrated with low temperature pyrolysis.¹² The formation of honey-comb like mesopores may result from the dehydration process of the hydrated cobalt/nickel phosphates. The TEM images of Ni₃(PO₄)₂, Co_2.30Ni_0.70(PO₄)₂, Co_0.86Ni_2.14(PO₄)₂ and Co₃(PO₄)₂ samples prior to the pyrolysis are shown in Fig. 11f. Commonly observed in Co-rich Co_2.30Ni_0.70(PO₄)₂ and Co₃(PO₄)₂ samples are typically nanospheres with clear honeycomb-like pores (∼5 nm). Furthermore, the honeycomb-like mesoporous structure offers more active sites for Faradaic reactions. The synergistic effect between cobalt and nickel also occurred in the as-prepared porous cobalt/nickel phosphate electrodes. Fig. 11e shows the capacitance retentions of the electrodes: Co₃(PO₄)₂ at 90.0%, Co_2.30Ni_0.70(PO₄)₂ at 54.8%, Co_1.62Ni_1.38(PO₄)₂ at 85.9%, Co_0.86Ni_2.14(PO₄)₂ at 73.2% and Ni₃(PO₄)₂ at 10.0%, respectively. A home-made activated carbon was used as the cathode material for an asymmetric SC along with the as-prepared cobalt/nickel phosphates. High energy densities of 45.8 W h kg⁻¹ at 42.4 W kg⁻¹ power density and 2.8 kW kg⁻¹ at 30.7 W h kg⁻¹ have been obtained.

The synergistic effect of nickel–cobalt species plays crucial roles in the high electrochemical performances of cobalt/nickel binary oxides/sulfides. The cobalt/nickel binary phosphates are thus predicted to give higher supercapacitive performance compared to either cobalt- or nickel-phosphate. Chen et al. incorporated two transition metals by an effective method to enhance the electrochemical performance in SCs for transition metal compound based electrodes.¹³⁴ Here, amorphous phase Co–Ni pyrophosphates are fabricated as electrodes in SCs. They adjusted the ratios of Co and Ni via changing the calcination temperature, and thereupon the electrochemical performance was also tuned. The morphology of the product was characterized by SEM. The Co_0.2Ni_0.8NH₄PO₄·H₂O precursor was composed of a plate-like structure. These microplates have nonuniform sizes from 15 to 1 μm. After calcination at 300 and 400 °C, a microplate-like structure was obtained as illustrated in SEM images in Fig. 11g and h. The SEM image in Fig. 11h shows that the product prepared at 400 °C has a smooth surface with a thickness of about 500 nm. Such a morphology was maintained when further increasing the temperature to 500 and 600 °C as shown in Fig. 10i and j. An optimized condition produced the amorphous phase of Co_0.2Ni_0.8 pyrophosphate at a calcination temperature of 400 °C. The specific capacitances were calculated by using GCD curves as shown in Fig. 11k which are 358, 849, 1056, 1259 and 945 F g⁻¹ at 1.5 A g⁻¹ for Co, Co_0.7Ni_0.3, Co_0.5Ni_0.5, Co_0.2Ni_0.8, and Ni polyphosphates, respectively. Obviously, Co_0.2Ni_0.8 polyphosphates exhibited the highest specific capacitance, which could be attributed to the electro-active sites involved in the redox reaction from either the valence interchange or the alternate charge between nickel and cobalt cations. This study provides a facile method to construct inexpensive bimetallic transition metal pyrophosphate as efficient electrodes in asymmetric electrochemical capacitors. Furthermore, Pang's group reported recent research studies about ultrathin nickel–cobalt phosphate nanosheets (Fig. 11l), which presented an outstanding pseudocapacitive performance for electrochemical supercapacitors under a solid-state electrolyte.²¹ The nickel–cobalt phosphate nanosheets were synthesized by a facile hydrothermal process, and exhibited a maximum specific capacitance of 1132.5 F g⁻¹. What's more, this solid-state device based on nickel–cobalt phosphate 2D nanosheets showed an energy density of 35.8 W h kg⁻¹ at 0.7 kW kg⁻¹, and an energy density of 32.5 W h kg⁻¹ at 0.6 kW kg⁻¹.

It is worth noting that the electrochemical performance of binary metallic–nonmetallic compounds is normally better than that of single metallic compounds; however, the ratio of metals also influences the performance. Liu et al. came up with a very basic and general synthetic method for the Ni–Co phosphates with varied cobalt and nickel components with no additional structure-directing agent or surfactant, not even an extra alkali source.¹³⁶ The SEM images of the samples – Ni₃P₂O₈, Co₃P₂O₈·8H₂O, Co₃P₂O₈·8H₂O, and Ni₃P₂O₈ – are shown in Fig. 11m, n, and o. Fig. 11m shows flower-like Ni₃P₂O₈ where the flower is composed of bundles of separate single flakes with pores. Meanwhile, Co₃P₂O₈·8H₂O shows a flake-like structure consisting of numerous nanoparticles with a certain degree of agglomeration. Similarly, several pores were observed between Co₃P₂O₈·8H₂O (Fig. 11n). Neither a flake-like nor flower-like morphology was formed as Ni₃P₂O₈–Co₃P₂O₈·8H₂O, which exhibited a particle-like form assembled out of several randomly-distributed particles of Ni₃P₂O₈–Co₃P₂O₈·8H₂O (Fig. 11o). Fig. 11p and q show the specific capacitances of Ni₃P₂O₈–Co₃P₂O₈·8H₂O with different cobalt–nickel mole ratios. Apparently, the cobalt–nickel mole ratios have a large influence on the electrochemical performance of Ni₃P₂O₈–Co₃P₂O₈·8H₂O. The Ni₃P₂O₈ electrode exhibits much higher specific capacitance than Co₃P₂O₈·8H₂O. Meanwhile, compared to Ni₃P₂O₈, the Co₃P₂O₈·8H₂O electrode shows a more stable rate capacity. Moreover, the specific capacitance of Ni₃P₂O₈–Co₃P₂O₈·8H₂O increased at higher cobalt–nickel mole ratios, indicating that Ni₃P₂O₈ with its higher specific capacitance is able to remarkably increase the specific capacitances of the Ni₃P₂O₈–Co₃P₂O₈·8H₂O electrode. Ni₃P₂O₈–Co₃P₂O₈·8H₂O with a cobalt–nickel mole ratio of 8 : 2 exhibited the best electrochemical performance, with outstanding cycling stability and rate capacity. In addition, its specific capacitance is much greater than that of pure Ni₃P₂O₈ and Ni₃P₂O₈–Co₃P₂O₈·8H₂O, which indicates a synergism in the given mole ratio. Interestingly, the further increase of the Ni/Co ratio to 9 : 1 resulted in a decrease in the specific capacitance of the electrodes because of the relatively small quantity of Ni₃P₂O₈–Co₃P₂O₈·8H₂O, which prompted the synergistic effect to be less obvious. Therefore, the 8 : 2 Ni/Co ratio is believed to be the most desirable ratio in such materials. Such a compound is expected to be one of the emerging novel materials to be applied as electrodes in high-performance energy storage systems.

Fig. 12 enumerates the list of the morphologies as well as the highest specific capacitance rates taken from the reviewed literature to present more vividly the SC performance of each selected material. Moreover, their electrolytes and cycling stabilities are also included. Phosphates of different nanostructures have been used for SCs, such as nanoflowers, nanotubes/rods, nanoplates, nanospheres, etc.

Fig. 12 Different morphologies of metal phosphate SCs materials and their SCs performances in different test environments.

3. Conclusions and outlook

This review summarizes some progress with respect to cobalt and nickel phosphate based micro/nanomaterials applied for SCs, covering ammonium/bimetallic phosphates, cobalt phosphates, nickel phosphates and cobalt/nickel phosphates. It is clear that much progress has been made in this direction. Moreover, the morphology of the material is relevant to the diffusion of ions in electrodes, and the surface area of the electrode materials. Furthermore, nanostructured phosphate materials are promising for SC applications due to their larger specific surface area and a reduction in diffusion pathways. Although phosphates as one of the newly-developed materials show great promise for SCs, with the advantages of abundance, environment friendliness and low cost, phosphate SC materials still show weaknesses including poor electrical conductivity and relatively low capacitance. Researchers could improve the performance of phosphate-based SC materials by making nano-sized porous structures and compositing them with highly conductive materials such as activated carbon nanotubes, carbon, graphene and conductive polymer (polyaniline, polypyrrole, and so on).

SCs with their high efficiency, prolonged lifespan, and great power density have become key emerging energy storage devices. The biggest challenge for SCs, however, is to increase the energy density. Thus a major focus of SCs studies is to develop novel electrode materials with high capacitance and to develop hybrid SCs with both a battery-type electrode and a SC-type electrode as the energy density is significantly increased due to both a high cell working voltage and a large capacitance in the hybrid system. As far as the design of phosphate based SC electrode materials is concerned, some desirable characteristics are: (1) low internal resistance; (2) better electrochemical and mechanical stability for better cycle performance; (3) a large specific surface area, implying more active sites; and (4) an appropriate pore network, length and size distribution to promote ion diffusion at high rates. Phosphate based materials hold great promise for achieving both high conductivity and high electrochemical activity, promising for high-performance SC electrodes.^137–142

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Program for New Century Excellent Talents of the University in China (NCET-13-0645) and the National Natural Science Foundation of China (NSFC-21201010, 21671170 and 21673203), Graduate Student Scientific Research Innovation Projects in Jiangsu Province (SJLX16-0588), Innovation Scientists and Technicians Troop Construction Projects of Henan Province (164200510018), Top-notch Academic Programs Project of Jiangsu Higher Education Institutions (PPZY2015B112), Program for Innovative Research Team (in Science and Technology) in University of Henan Province (14IRTSTHN004), the Six Talent Plan (2015-XCL-030), and a Qinglan Project, Training Program of Innovation and Entrepreneurship for Undergraduates (201711117011Z). We also acknowledge the Priority Academic Program Development of Jiangsu Higher Education Institutions and the technical support we received at the Testing Center of Yangzhou University.

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