Cobalt pyrophosphate nano/microstructures as promising electrode materials of supercapacitor

Hydrothermal synthesis of dittmarite-group NH4(Co1-xMnx)PO4·H2O particles as inorganic violet pigments

Dittmarite-group NH4(Co1-xMnx)PO4·H2O particles were prepared via a hydrothermal route. Single-phase platelike NH4(Co1-xMnx)PO4·H2O particles were obtained from aqueous solutions containing MnCl2·4H2O, CoCl2·6H2O, and (NH4)2HPO4, where the [Mn2+]/([Co2+] + [Mn2+]) mole ratios in the products were controlled by changing the MnCl2 and CoCl2 concentrations of the precursor solutions. The vivid violet colour of the ammonium cobalt phosphate (NH4CoPO4·H2O) particles was maintained upon substitution of Co2+ with Mn2+ ions up to x = 0.8, thus achieving an 80% saving of cobalt in the preparation of violet pigments.

Dinitrogen Reduction Coupled with Methanol Oxidation for Low Overpotential Electrochemical NH3 Synthesis Over Cobalt Pyrophosphate as Bifunctional Catalyst

Electrochemical dinitrogen (N₂ ) reduction to ammonia (NH₃ ) coupled with methanol electro-oxidation is presented in the current work. Here, methanol oxidation reaction (MOR) is proposed as an alternative anode reaction to oxygen evolution reaction (OER) to accomplish electrons-induced reduction of N₂ to NH₃ at cathode and oxidation of methanol at anode in alkaline media thereby reducing the overall cell voltage for ammonia production. Cobalt pyrophosphate micro-flowers assembled by nanosheets are synthesized via a surfactant-assisted sonochemical approach. By virtue of structural and morphological advantages, the maximum Faradaic efficiency of 43.37% and NH₃ yield rate of 159.6 µg h^-1 mg_ca ^-1 is achieved at a potential of -0.2 V versus RHE. The proposed catalyst is shown to also exhibit a very high activity (100 mA mg^-1 at 1.48 V), durability (2 h) and production of value-added formic acid at anode (2.78 µmol h^-1 mg_cat ^-1 and F.E. of 59.2%). The overall NH₃ synthesis is achieved at a reduced cell voltage of 1.6 V (200 mV less than NRR-OER coupled NH₃ synthesis) when OER at anode is replaced with MOR and a high NH₃ yield rate of 95.2 µg h^-1 mg_cat ^-1 and HCOOH formation rate of 2.53 µmol h^-1 mg^-1 are witnessed under full-cell conditions.

Defect-rich cobalt pyrophosphate hybrids decorated Cd0.5Zn0.5S for efficient photocatalytic hydrogen evolution: Defect and interface engineering

Photocatalysts with highly efficient charge separation are of critical significance for improving photocatalytic hydrogen production performance. Herein, a cost-effective and high-performance composite photocatalyst, cobalt-phosphonate-derived defect-rich cobalt pyrophosphate hybrids (CoPPi-M) modified Cd_0.5Zn_0.5S is rationally devised via defect and interface engineering, in which the co-catalyst CoPPi-M delivers a strong interaction with host photocatalyst Cd_0.5Zn_0.5S, rendering Cd_0.5Zn_0.5S/CoPPi-M with a remarkably improved efficiency of charge separation and migration. Besides, Cd_0.5Zn_0.5S/CoPPi-M exhibits a hydrophilic surface with ample access to electrons and a strong reduction ability of electrons. Benefiting from these advantages, the integration of defect-rich cobalt pyrophosphate and Cd_0.5Zn_0.5S enables Cd_0.5Zn_0.5S/CoPPi-M-5% with high photocatalytic H₂ production rate of 6.87 mmol g^-1h^-1, which is 2.46 times higher than that of pristine Cd_0.5Zn_0.5S, and the notable apparent quantum efficiency (AQE) is 20.7% at 420 nm. This work provides a promising route for promoting the photocatalytic performance of non-precious hybrid photocatalyst via defect and interface engineering, and advances energy-generation and environment-restoration devices.

Co2P2O7 Microplate/Bacterial Cellulose-Derived Carbon Nanofiber Composites with Enhanced Electrochemical Performance

Nanocrystalline Co₂P₂O₇ and carbon nanofiber (Co₂P₂O₇/CNFs) composites with enhanced electrochemical performance were obtained by calcination after a hydrothermal process with NH₄CoPO₄∙H₂O/bacterial cellulose precursors under an argon atmosphere. SEM images showed that the CNFs were highly dispersed on the surfaces of Co₂P₂O₇ microplates. The diagonal size of the Co₂P₂O₇ plates ranged from 5 to 25 µm with thicknesses on a nanometer scale. Notably, with the optimal calcining temperature, the Co₂P₂O₇/CNFs@600 material has higher specific micropore and mesopore surface areas than other samples, and a maximal specific capacitance of 209.9 F g⁻¹, at a current density of 0.5 A g⁻¹. Interestingly, CNF composite electrodes can enhance electrochemical properties, and contribute to better electrical conductivity and electron transfer. EIS measurements showed that the charge–transfer resistance (R_ct) of the CNF composite electrodes decreased with increasing calcination temperature. Furthermore, the Co₂P₂O₇/CNF electrodes exhibited higher energy and power densities than Co₂P₂O₇ electrodes.

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.

In situ fabrication of a rose-shaped Co2P2O7/C nanohybrid via a coordination polymer template for supercapacitor application

Using a coordination polymer as the template, the porous rose-shaped Co₂P₂O₇/C-X were fabricated by in situ hybrid Co₂P₂O₇ nanoparticles and nanocarbon for supercapacitor applications.

Preparation and characterization of manganese, cobalt and zinc DNA nanoflowers with tuneable morphology, DNA content and size

Recently reported DNA nanoflowers are an interesting class of organic-inorganic hybrid materials which are prepared using DNA polymerases. DNA nanoflowers combine the high surface area and scaffolding of inorganic Mg₂P₂O₇ nanocrystals with the targeting properties of DNA, whilst adding enzymatic stability and enhanced cellular uptake. We have investigated conditions for chemically modifying the inorganic core of these nanoflowers through substitution of Mg²⁺ with Mn²⁺, Co²⁺ or Zn²⁺ and have characterized the resulting particles. These have a range of novel nanoarchitectures, retain the enzymatic stability of their magnesium counterparts and the Co²⁺ and Mn²⁺ DNA nanoflowers have added magnetic properties. We investigate conditions to control different morphologies, DNA content, hybridization properties, and size. Additionally, we show that DNA nanoflower production is not limited to Ф29 DNA polymerase and that the choice of polymerase can influence the DNA length within the constructs. We anticipate that the added control of structure, size and chemistry will enhance future applications.

Ultrasmall Co2P2O7 nanocrystals anchored on nitrogen-doped graphene as efficient electrocatalysts for the oxygen reduction reaction

A novel composite electrocatalyst with ultrasmall Co₂O₂P₇ nanocrystals supported on nitrogen-doped graphene has been developed and exhibits remarkable ORR performance.

Fabrication of g-C3N4 Nanomesh-Anchored Amorphous NiCoP2O7: Tuned Cycling Life and the Dynamic Behavior of a Hybrid Capacitor

Developing a novel electrode material with better electrochemical behavior and extended cyclability is a major issue in the field of hybrid capacitors. In this work, we propose a novel strategy for the facile synthesis of nickel-cobalt pyrophosphate nanoparticles anchored on graphitic carbon nitride (NiCoP₂O₇/g-C₃N₄) via the simple solvothermal method. Field emission scanning electron microscopy and transmission electron microscopy analysis revealed the uniform anchoring of NiCoP₂O₇ nanocomposite on g-C₃N₄ nanosheets. Benefitting from the effect of amorphous nature and a conductive matrix of the NiCoP₂O₇/g-C₃N₄ (NP3) composite, the material achieves a specific capacitance of 342 F g^-1 at a scan rate of 5 mV s^-1. Impressively, the electrode shows long-term cycling stability with 100% capacitance retention over 5000 cycles. Employing activated carbon as an anode and as-prepared NP3 as a cathode, the assembled asymmetric hybrid cell exhibits high-energy density and exceptional cyclability (specific capacitance retention over 10 000 cycles). The outstanding electrochemical and cyclic stability is attributed to the shortest electron-ion pathway with effective interfacial interaction. The low electronic resistance of the NiCoP₂O₇/g-C₃N₄ nanocomposite is revealed by varying the bias voltage variation in the electrochemical impedance spectroscopy. Our results promise better utilization of the bimetallic pyrophosphate-anchored g-C₃N₄ matrix as a potential electrode for high-performance energy storage devices.

Redox Additive-Improved Electrochemically and Structurally Robust Binder-Free Nickel Pyrophosphate Nanorods as Superior Cathode for Hybrid Supercapacitors

For several decades, one of the great challenges for constructing a high-energy supercapacitor has been designing electrode materials with high performance. Herein, we report for the first time to our knowledge a novel hybrid supercapacitor composed of battery-type nickel pyrophosphate one-dimensional (1D) nanorods and capacitive-type N-doped reduced graphene oxide as the cathode and anode, respectively, in an aqueous redox-added electrolyte. More importantly, ex situ microscopic images of the nickel pyrophosphate 1D nanorods revealed that the presence of the battery-type redox additive enhanced the charge storage capacity and cycling life as a result of the microstructure stability. The nickel pyrophosphate 1D nanorods exhibited their maximum specific capacitance (8120 mF cm^-2 at 5 mV s^-1) and energy density (0.22 mWh cm^-2 at a power density of 1.375 mW cm^-2) in 1 M KOH + 75 mg K₃[Fe(CN)₆] electrolyte. On the other side, the N-doped reduced graphene oxide delivered an excellent electrochemical performance, demonstrating that it was an appropriate anode. A hybrid supercapacitor showed a high specific capacitance (224 F g^-1 at a current density of 1 A g^-1) and high energy density (70 Wh kg^-1 at a power density of 750 W kg^-1), as well as a long cycle life (a Coulombic efficiency of 96% over 5000 cycles), which was a higher performance than most of those in recent reports. Our results suggested that the materials and redox additive in this novel design hold great promise for potential applications in a next-generation hybrid supercapacitor.

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.

Metal Phosphides and Phosphates-based Electrodes for Electrochemical Supercapacitors

Phosphorus compounds, such as metal phosphides and phosphates have shown excellent performances and great potential in electrochemical energy storage, which are demonstrated by research works published in recent years. Some of these metal phosphides and phosphates and their hybrids compare favorably with transition metal oxides/hydroxides, which have been studied extensively as a class of electrode materials for supercapacitor applications, where they have limitations in terms of electrical and ion conductivity and device stability. To be specific, metal phosphides have both metalloid characteristics and good electric conductivity. For metal phosphates, the open-framework structures with large channels and cavities endow them with good ion conductivity and charge storage capacity. In this review, we present the recent progress on metal phosphides and phosphates, by focusing on their advantages/disadvantages and potential applications as a new class of electrode materials in supercapacitors. The synthesis methods to prepare these metal phosphides/phosphates are looked into, together with the scientific insights involved, as they strongly affect the electrochemical energy storage performance. Particular attentions are paid to those hybrid-type materials, where strong synergistic effects exist. In the summary, the future perspectives and challenges for the metal phosphides, phosphates and hybrid-types are proposed and discussed.

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.

Coralloid Co2P2O7 Nanocrystals Encapsulated by Thin Carbon Shells for Enhanced Electrochemical Water Oxidation

Core-shell nanohybrids containing cheap inorganic nanocrystals and nanocarbon shells are promising electrocatalysts for water splitting or other renewable energy options. Despite that great progress has been achieved, biomimetic synthesis of metal phosphates@nanocarbon core-shell nanohybrids remains a challenge, and their use for electrocatalytic oxygen evolution reaction (OER) has not been explored. In this paper, novel nanohybrids composed of coralloid Co2P2O7 nanocrystal cores and thin porous nanocarbon shells are synthesized by combination of the structural merits of supramolecular polymer gels and a controllable thermal conversion technique, i.e., temperature programmable annealing of presynthesized supramolecular polymer gels that contain cobalt salt and phytic acid under a proper gas atmosphere. Electrocatalytic tests in alkaline solution show that such nanohybrids exhibit greatly enhanced electrocatalytic OER performance compared with that of Co2P2O7 nanostructure. At a current density of 10 mA cm(-2), their overpotential is 0.397 V, which is much lower than that of Co2P2O7 nanostructures, amorphous Co-Pi nanomaterials, Co(PO3)2 nanosheets, Pt/C, and some reported OER catalysts, and close to that of commercial IrO2. Most importantly, both of their current density at the overpotential over 0.40 V and durability are superior to those of IrO2 catalyst. As revealed by a series of spectroscopic and electrochemical analyses, their enhanced electrocatalytic performance results from the presence of thin porous nanocarbon shells, which not only improve interfacial electron penetration or transfer dynamics but also vary the coordination environment and increase the number of active 5-coordinated Co(2+) sites in Co2P2O7 cores.

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.

Ice-templated Self-assembly of VOPO4-Graphene Nanocomposites for Vertically Porous 3D Supercapacitor Electrodes

A simple ice-templated self-assembly process is used to prepare a three-dimensional (3D) and vertically porous nanocomposite of layered vanadium phosphates (VOPO4) and graphene nanosheets with high surface area and high electrical conductivity. The resulting 3D VOPO4-graphene nanocomposite has a much higher capacitance of 527.9 F g(-1) at a current density of 0.5 A g(-1), compared with ~247 F g(-1) of simple 3D VOPO4, with solid cycling stability. The enhanced pseudocapacitive behavior mainly originates from vertically porous structures from directionally grown ice crystals and simultaneously inducing radial segregation and forming inter-stacked structures of VOPO4-graphene nanosheets. This VOPO4-graphene nanocomposite electrode exhibits high surface area, vertically porous structure to the separator, structural stability from interstacked structure and high electrical conductivity, which would provide the short diffusion paths of electrolyte ions and fast transportation of charges within the conductive frameworks. In addition, an asymmetric supercapacitor (ASC) is fabricated by using vertically porous VOPO4-graphene as the positive electrode and vertically porous 3D graphene as the negative electrode; it exhibits a wide cell voltage of 1.6 V and a largely enhanced energy density of 108 Wh kg(-1).

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.