Cobalt phosphite microarchitectures assembled by ultralong nanoribbons and their application as effective electrochemical capacitor electrode materials

Inside–outside OH– incursion involved in the fabrication of hierarchical nanoflake assembled three-dimensional flower-like α-Co(OH)2 for use in high-performance aqueous symmetric supercapacitor applications

INTRODUCTION

The energy industry has been challenged by the current high population and high energy consumption, forcing the development of effective and efficient supercapacitor devices. The crucial issues until now have been high production cost, deprived cyclic stability, and squat energy density. To resolve these problems, various approaches have been taken, such as the development of long-life electrode materials with high capacity, rapid charging, and slow discharging to overcome poor life cycle stability.

OBJECTIVES

In the present work we focus on fabricating cost-effective unique-morphology, high-surface-area alpha-Co(OH)₂ for application in an aqueous-electrolyte symmetric supercapacitor.

METHODS

In this study, hierarchical nanoflakes assembled in three-dimensional (3D) flower-shaped cobalt hydroxide (HN-3DF-α-Co(OH)₂) electrode were synthesized using the solvothermal method with sodium dodecylbenzene sulfonate (SDBS) and methanol as solvents. Spectroscopic and microscopic techniques were used to characterize fabricated HN-3DF-Co(OH)₂, which revealed that the materials electrode exhibited the alpha phase with a hierarchical flower-like structure. A half-cell electrochemical assembly (three-electrode assemble cell) and symmetric full cell (two-electrode assemble cell) were examined in an aqueous electrolyte.

RESULTS

In three-electrode assembly cells, HN-3DF-α-Co(OH)₂ exhibited 719.5 Fg^-1 specific capacitance (C_sp) at 1 Ag^-1 with excellent cyclic retention stability of approximately 88% after 3000 cycles. In two-electrode symmetric supercapacitive systems, HN-3DF-α-Co(OH)₂ achieved a maximum C_sp of 70.3 Fg^-1 at 0.4 Ag^-1 with the highest energy density of approximately 6.25 Wh/kg at a power density of 328.94 W/kg. The fabricated two-electrode assembly cell with the HN-3DF-α-Co(OH)₂ electrode retained cyclic stability of approximately 85% after 5000 repeated charge and discharge cycles.

CONCLUSION

Solvothermally-synthesized, optimized HN-3DF-α-Co(OH)₂ showed outstanding electrochemical performance results in three- and two-electrode systems. This unique aqueous symmetric supercapacitor can be used to design cost-effective symmetric capacitors based on metal hydroxide.

High Energy Density in Combination with High Cycling Stability in Hybrid Supercapacitors

Hybrid supercapacitors are considered the next-generation energy storage equipment due to their superior performance. In hybrid supercapacitors, battery electrodes need to have large absolute capacities while displaying high cycling stability. However, enhancing areal capacity via decreasing the size of electrode materials results in reductions in cycling stability. To balance the capacity-stability trade-off, rationally designed proper electrode structures are in urgent need and still of great challenge. Here we report a high-capacity and high cycling stability electrode material by developing a nickel phosphate lamination structure with ultrathin nanosheets as building blocks. The nickel phosphate lamination electrode material exhibits a large specific capacity of 473.9 C g^-1 (131.6 mAh g^-1, 1053 F g^-1) at 2.0 A g^-1 and only about 21% capacity loss at 15 A g^-1 (375 C g^-1, 104.2 mAh g^-1, 833.3 F g^-1) in 6.0 M KOH. Furthermore, hybrid supercapacitors are constructed with nickel phosphate lamination and activated carbon (AC), possessing high energy density (42.1 Wh kg^-1 at 160 W kg^-1) as well as long cycle life (almost 100% capacity retention after 1000 cycles and 94% retention after 8000 cycles). The electrochemical performance of the nickel phosphate lamination structure not only is commensurate with the nanostructure or ultrathin materials carefully designed in supercapacitors but also has a longer cycling lifespan than them. The encouraging results show the great potential of this material for energy storage device applications.

Three-dimensional flower-like nickel doped cobalt phosphate hydrate microarchitectures for asymmetric supercapacitors

Development of asymmetric supercapacitors (ASCs) using hierarchical three-dimensional (3D) morphologies is becoming crucial in energy storage applications due to the greater power density rather than batteries. Herein, 3D flower-like Co₃(PO₄)₂·8H₂O (CPH) and nickel doped CPH (Ni-CPH) microarchitectures were synthesized by a silicone oil bath method at low temperatures without calcination. The synthesized microarchitectures-based electrodes (bare CPH and Ni-CPH) revealed battery-like properties during the electrochemical study. Importantly, the Ni-CPH electrode showed improved electrochemical performance compared to the bare CPH electrode material. The specific capacity values of the CPH and Ni-CPH electrode materials were calculated to be 74 and 108 mAh g^-1 at 0.5 A g^-1, respectively. Furthermore, for the Ni-CPH electrode, 78% of capacity retention was obtained after 9000 cycles at 5 A g^-1. Additionally, an ASC was developed while employing the optimized Ni-CPH electrode (positive-type) and activated carbon (negative-type) and it showed superior electrochemical results. The ASC device exhibited excellent capacity retention (94%) after 9000 cycles at 2 A g^-1. Also, this device delivered a high energy density of 23.4 Wh kg^-1 and a power density of 2103 W kg^-1. Finally, several portable electronic devices were successfully tested using the obtained good energy and power density results from the ASC device for energy storage applications.

Gold-Nanoparticle-Functionalized Cobalt-Nickel Phosphate 3D Nanoice Creams to Fabricate Stable and Sensitive Biosensors for the Cytochrome c Assay

Designing a stable and sensitive luminol-based electrochemiluminescence (ECL) analytical platform in the neutral condition has attracted a lot of attention. Here, gold-nanoparticle-functionalized cobalt-nickel phosphate three-dimensional nanoice creams (Au@Co₃Ni₃(PO₄)₄ NICs) are successfully prepared via electrostatic interaction. Generally, cobalt-nickel phosphate nanoice creams (Co₃Ni₃(PO₄)₄ NICs) are synthesized via a mild hydrothermal method and functionalized via polyethylenimine (PEI). Then, Au NPs are adsorbed on the surface of Co₃Ni₃(PO₄)₄ NICs via Au-N weak interaction to fabricate Au@Co₃Ni₃(PO₄)₄ NICs. Owing to the important roles of Au@Co₃Ni₃(PO₄)₄ in exhibiting excellent electrocatalytic activity, as well as preventing the deposition of negatively charged oxidation product induced electrode passivation, luminol in the nanohybrids (LH-Au@Co₃Ni₃(PO₄)₄) gives strong and stable ECL intensity in the neutral conditions. Moreover, the ECL emission of luminol is obviously quenched based on the resonance energy transfer (RET) between luminol as donor and cytochrome c (Cyt c) as acceptor. Hence, a sensitive ECL biosensor is successfully fabricated for the quantitative determination of Cyt c in cell lysates and exhibits wide linear ranges of 1.0 × 10^-4-0.5 × 10^-5 and 0.5 × 10^-5-1.0 × 10^-8 M as well as a low detection limit of 2.48 nM. This novel sensing strategy will broaden the application of transition metal (Co, Ni) phosphates in bioassays.

The morphology controlled growth of Co11(HPO3)8(OH)6 on nickel foams for quasi-solid-state supercapacitor applications

Co₁₁(HPO₃)₈(OH)₆ microstructures with different morphologies growing on NF were synthesized under different conditions, and the flower-like sample presents excellent electrochemical properties.

Electrochemical analysis of Na-Ni bimetallic phosphate electrodes for supercapacitor applications

Bimetallic sodium-nickel phosphate/graphene foam composite (NaNi4(PO4)3/GF) was successfully synthesized using a direct and simple precipitation method. The hierarchically structured composite material was observed to have demonstrated a synergistic effect between the conductive metallic cations and the graphene foam that made up the composite. The graphene served as a base-material for the growth of NaNi4(PO4)3 particles, resulting in highly conductive composite material as compared to the pristine material. The NaNi4(PO4)3/GF composite electrode measured in a 3-electrode system achieved a maximum specific capacity of 63.3 mA h g-1 at a specific current of 1 A g-1 in a wide potential range of 0.0-1.0 V using 2 M NaNO3 aqueous electrolyte. A designed and fabricated hybrid NaNi4(PO4)3/GF//AC device based on NaNi4(PO4)3/GF as positive electrode and activated carbon (AC) selected as a negative electrode could operate well in an extended cell potential of 2.0 V. As an assessment, the hybrid NaNi4(PO4)3/GF//AC device showed the highest energy and power densities of 19.5 W h kg-1 and 570 W kg-1, respectively at a specific current of 0.5 A g-1. The fabricated device could retain an 89% of its initial capacity with a coulombic efficiency of about 94% over 5000 cycling test, which suggests the material's potential for energy storage devices applications.

Facile synthesis of Ni11(HPO3)8(OH)6/rGO nanorods with enhanced electrochemical performance for aluminum-ion batteries

The electrochemical behaviors of the ultrashort nickel phosphite nanorods supported on reduced graphene oxide (Ni₁₁(HPO₃)₈(OH)₆/rGO nanorods), as a candidate for cathodic applications in aluminum-ion batteries, are firstly investigated. Ni₁₁(HPO₃)₈(OH)₆/rGO nanorods are synthesized by a facile solvothermal process. Ni₁₁(HPO₃)₈(OH)₆ and Ni₁₁(HPO₃)₈(OH)₆/rGO cathodes both possess very high initial discharge capacities of 132.4 and 182.0 mA h g^-1 at a current density of 200 mA g^-1, respectively. In addition, the long-term cycling stability of the Ni₁₁(HPO₃)₈(OH)₆/rGO cathode is further evaluated, exhibiting a discharge capacity of 49.2 mA h g^-1 even over 1500 cycles. More importantly, the redox reaction mechanism of the Ni₁₁(HPO₃)₈(OH)₆ cathode for aluminum-ion batteries revealed that Ni₁₁(HPO₃)₈(OH)₆ is partially substituted with Al³⁺ to form Al_mNi_n(HPO₃)₈(OH)₆ and metallic Ni in the nanorod-like Ni₁₁(HPO₃)₈(OH)₆ cathodes during the discharge process. These findings are of great significance for the further development of novel materials for aluminum-ion batteries.

Ni-Doped Cobalt Phosphite, Co11(HPO3)8(OH)6, with Different Morphologies Grown on Ni Foam Hydro(solvo)thermally for High-Performance Supercapacitor

Ni-doped Co₁₁(HPO₃)₈(OH)₆ with different morphologies was directly grown on Ni foam hydro(solvo)thermally under different synthetic conditions. The optimum condition is solvothermal reaction for 6 h in an ethanol/water (EW) mixed solution, the molar ratio of NaH₂PO₂/Co(NO₃)₂ being 0.5:0.1, and the obtained S0.5-6 h-EW shows three-dimensional (3D) porous nanowire bundles. Whereas in the water-only solution, microrods are obtained, suggesting that the nanowires in bundles are aggregated together via the lateral (400) direction. Long reaction time and low molar ratio of reactants are all beneficial for the lateral growth of the nanowires, and the possible formation mechanism is proposed. All the obtained Ni-doped Co₁₁(HPO₃)₈(OH)₆/Ni foam samples are directly used as supercapacitor electrodes, and S0.5-6 h-EW shows the best electrochemical performance with a specific capacity of 159 mAh g^-1 at 0.5 A g^-1, which is close to the theoretical value of 212 mAh g^-1 for Co₁₁(HPO₃)₈(OH)₆, and it is the largest reported value so far. The excellent capacitive behavior of S0.5-6 h-EW is ascribed to the 3D porous nanowire bundles directly grown on a Ni foam collector without an additive and a binder, as well as to the doping of Ni into the cobalt phosphite. The S0.5-6 h-EW//activated carbon asymmetrical supercapacitor shows a maximum energy density of 58.7 Wh kg^-1 at a power density of 532 W kg^-1 and good cycling stability with the capacity retention of 90.5% after 10 000 charging-discharging cycles at 5.5 A g^-1.

Facile Synthesis of Ultrathin Nickel-Cobalt Phosphate 2D Nanosheets with Enhanced Electrocatalytic Activity for Glucose Oxidation

Two-dimensional (2D) ultrathin nickel-cobalt phosphate nanosheets were synthesized using a simple one-step hydrothermal method. The morphology and structure of nanomaterials synthesized under different Ni/Co ratios were investigated by transmission electron microscopy, scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. Moreover, the influence of nanomaterials' structure on the electrochemical performance for glucose oxidation was investigated. It is found that the thinnest nickel-cobalt phosphate nanosheets synthesized with a Ni/Co ratio of 2:5 showed the best electrocatalytic activity for glucose oxidation. Also, the ultrathin nickel-cobalt phosphate nanosheet was used as an electrode material to construct a nonenzymatic electrochemical glucose sensor. The sensor showed a wide linear range (2-4470 μM) and a low detection limit (0.4 μM) with a high sensitivity of 302.99 μA·mM^-1·cm^-2. Furthermore, the application of the as-prepared sensor in detection of glucose in human serum was successfully demonstrated. These superior performances prove that ultrathin 2D nickel-cobalt phosphate nanosheets are promising materials in the field of electrochemical sensing.

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

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.

Terephthalate-based cobalt hydroxide: a new electrode material for supercapacitors with ultrahigh capacitance

Terephthalate-based cobalt hydroxide, a layered hydroxyl derivative, is grown on Ni foam and first applied in supercapacitors with ultrahigh capacitances.

Hollow Co2P nanoflowers assembled from nanorods for ultralong cycle-life supercapacitors

Hollow Co₂P nanoflowers (Co₂P HNFs) were successfully prepared via a one-step, template-free method. Microstructure analysis reveals that Co₂P HNFs are assembled from nanorods and possess abundant mesopores and an amorphous carbon shell. Density functional theory calculations and electrochemical measurements demonstrate the high electrical conductivity of Co₂P. Benefiting from the unique nanostructures, when employed as an electrode material for supercapacitors, Co₂P HNFs exhibit a high specific capacitance, an outstanding rate capability, and an ultralong cycling stability. Furthermore, the constructed Co₂P HNF//AC ASC exhibits a high energy density of 30.5 W h kg^-1 at a power density of 850 W kg^-1, along with a superior cycling performance (108.0% specific capacitance retained after 10 000 cycles at 5 A g^-1). These impressive results make Co₂P HNFs a promising candidate for supercapacitor applications.

Vertically Oriented and Interpenetrating CuSe Nanosheet Films with Open Channels for Flexible All-Solid-State Supercapacitors

As a p-type multifunctional semiconductor, CuSe nanostructures show great promise in optoelectronic, sensing, and photocatalytic fields. Although great progress has been achieved, controllable synthesis of CuSe nanosheets (NSs) with a desirable spacial orientation and open frameworks remains a challenge, and their use in supercapacitors (SCs) has not been explored. Herein, a highly vertically oriented and interpenetrating CuSe NS film with open channels is deposited on an Au-coated polyethylene terephthalate substrate. Such CuSe NS films exhibit high specific capacitance (209 F g^-1) and can be used as a carbon black- and binder-free electrode to construct flexible, symmetric all-solid-state SCs, using polyvinyl alcohol-LiCl gel as the solid electrolyte. A device fabricated with such CuSe NS films exhibits high volumetric specific capacitance (30.17 mF cm^-3), good cycling stability, excellent flexibility, and desirable mechanical stability. The excellent performance of such devices results from the vertically oriented and interpenetrating configuration of CuSe NS building blocks, which can increase the available surface and facilitate the diffusion of electrolyte ions. Moreover, as a prototype for application, three such solid devices in series can be used to light up a red light-emitting diode.

Interconnected hierarchical NiCo2O4 microspheres as high-performance electrode materials for supercapacitors

Hierarchical NiCo₂O₄ microspheres with large tunnels and abundant mesopores have been prepared, and they exhibit excellent performance in supercapacitor applications.

Supercapattery Based on Binder-Free Co3(PO4)2·8H2O Multilayer Nano/Microflakes on Nickel Foam

A binder-free cobalt phosphate hydrate (Co₃(PO₄)₂·8H₂O) multilayer nano/microflake structure is synthesized on nickel foam (NF) via a facile hydrothermal process. Four different concentrations (2.5, 5, 10, and 20 mM) of Co²⁺ and PO₄^-3 were used to obtain different mass loading of cobalt phosphate on the nickel foam. The Co₃(PO₄)₂·8H₂O modified NF electrode (2.5 mM) shows a maximum specific capacity of 868.3 C g^-1 (capacitance of 1578.7 F g^-1) at a current density of 5 mA cm^-2 and remains as high as 566.3 C g^-1 (1029.5 F g^-1) at 50 mA cm^-2 in 1 M NaOH. A supercapattery assembled using Co₃(PO₄)₂·8H₂O/NF as the positive electrode and activated carbon/NF as the negative electrode delivers a gravimetric capacitance of 111.2 F g^-1 (volumetric capacitance of 4.44 F cm^-3). Furthermore, the device offers a high specific energy of 29.29 Wh kg^-1 (energy density of 1.17 mWh cm^-3) and a specific power of 4687 W kg^-1 (power density of 187.5 mW cm^-3).

Self-assembled graphene coupled hollow-structured γ-Fe2O3 spheres with crystal of transition for enhanced supercapacitors

Self-assembled hollow-structured γ-Fe₂O₃/graphene spheres were synthesized using a simple hydrothermal method combined with an easy annealing route.

High performance electrochemical capacitor materials focusing on nickel based materials

Of the two major capacitances contributing to electrochemical storage devices, pseudo-capacitance, which results from the reversible faradaic reactions, can be much higher than the electric double layer capacitance.

High capacitive amorphous barium nickel phosphate nanofibers for electrochemical energy storage

Ba_xNi_3−x(PO₄)₂(0 <x< 3) amorphous nanofibers with excellent supercapacitive performance were synthesized through a facile cation-exchange method.

Electrochemical performance in alkaline and neutral electrolytes of a manganese phosphate material possessing a broad potential window

An underlying Mn₃(PO₄)₂ material with superior electrochemical characteristics is developed as an electrode material for use in both neutral and alkaline electrolytes.

Ferric Phosphate Hydroxide Microstructures Affect Their Magnetic Properties

Uniformly sized and shape-controlled nanoparticles are important due to their applications in catalysis, electrochemistry, ion exchange, molecular adsorption, and electronics. Several ferric phosphate hydroxide (Fe4(OH)3(PO4)3) microstructures were successfully prepared under hydrothermal conditions. Using controlled variations in the reaction conditions, such as reaction time, temperature, and amount of hexadecyltrimethylammonium bromide (CTAB), the crystals can be grown as almost perfect hyperbranched microcrystals at 180 °C (without CTAB) or relatively monodisperse particles at 220 °C (with CTAB). The large hyperbranched structure of Fe4(OH)3(PO4)3 with a size of ∼19 μm forms with the "fractal growth rule" and shows many branches. More importantly, the magnetic properties of these materials are directly correlated to their size and micro/nanostructure morphology. Interestingly, the blocking temperature (T B) shows a dependence on size and shape, and a smaller size resulted in a lower T B. These crystals are good examples that prove that physical and chemical properties of nano/microstructured materials are related to their structures, and the precise control of the morphology of such functional materials could allow for the control of their performance.

Fabrication of V2O5 super long nanobelts: optical, in situ electrical and field emission properties

A low turn-on field of 1.4 V μm⁻¹, carrier concentrations of N_d = 1.48 × 10¹⁸ cm⁻³ and electron mobility of 1.26 cm² V⁻¹ s⁻¹ were obtained for V₂O₅ super long nanobelts.

Novel red blood cell shaped α-Fe2O3 microstructures and FeO(OH) nanorods as high capacity supercapacitors

Novel red blood cell shaped α-Fe₂O₃ microstructures were first synthesized through a hydrothermal method by using NH₄Cl as a structure-directing agent.

Cobalt vanadium oxide thin nanoplates: primary electrochemical capacitor application

Co₃V₂O₈ thin nanoplates are firstly described as a kind of electrode material for supercapacitors. More importantly, from electrochemical measurements, the obtained Co₃V₂O₈ nanoplate electrode shows a good specific capacitance (0.5 A g⁻¹, 739 F g⁻¹) and cycling stability (704 F g⁻¹ retained after 2000 cycles). This study essentially offers a new kind of metal vanadium oxides as electrochemical active material for the development of supercapacitors.

Assembling CdS mesoporous nanosheets into 3D hierarchitectures for effective photocatalytic performance

3D hierarchical CdS mesoporous nanosheets are successfully synthesized via a simple hydrothermal approach. Due to their specific 3D hierarchical structures, they exhibit good photocatalytic activity for the degradation of MO.

Microporous Ni11(HPO3)8(OH)6 nanocrystals for high-performance flexible asymmetric all solid-state supercapacitors

Microporous Ni₁₁(HPO₃)₈(OH)₆ nanocrystals were successfully applied to create a flexible solid-state asymmetric supercapacitor, which achieved a maximum energy density of 0.45 mW h cm⁻³ with high stability for 10 000 cycles.

Facile synthesis of cerium oxide nanostructures for rechargeable lithium battery electrode materials

A facile method is developed to synthesize cerium oxides with plate and brick morphologies by the thermal decomposition of nanostructured oxalate precursors.

Few-layered CoHPO4 · 3H2O ultrathin nanosheets for high performance of electrode materials for supercapacitors

Ultrathin cobalt phosphate (CoHPO4 · 3H2O) nanosheets are successfully synthesized by a one pot hydrothermal method. Novel CoHPO4 · 3H2O ultrathin nanosheets are assembled for constructing the electrodes of supercapacitors. Benefiting from the nanostructures, the as-prepared electrode shows a specific capacitance of 413 F g(-1), and no obvious decay even after 3000 charge-discharge cycles. Such a quasi-two-dimensional material is a new kind of supercapacitor electrode material with high performance.