High energy density symmetric capacitor using zinc cobaltate flowers grown in situ on Ni foam

Metal-Organic Framework-Derived Zinc-Cobalt Oxide Materials as High-Performance Anodes for Direct Methanol Fuel Cell Application

Due to the exhaustion of fossil fuels and rising concerns about environmental pollution, direct methanol fuel cells (DMFCs) have emerged as one of the prominent green energy solutions in recent decades. However, the commercialization of DMFCs faces a significant challenge due to the dependence on expensive noble-metal-based electrode materials and the issue of methanol crossover. Therefore, there has been growing interest in developing cost-effective, high-performance anode catalysts to enhance the methanol oxidation reaction (MOR). In this work, unexplored non-noble transition metal oxide materials, such as metal-organic framework (MOF)-derived ZnO, ZnCo2O4, and Zn2CoO4, were directly synthesized on Ni foam using a simple solvothermal method, followed by calcination. The MOR activity of all the materials was tested in a 0.5 M methanol solution under alkaline conditions. Due to the synergetic effect of combined metallic composition, mixed metal oxides exhibited superior performance. The order of MOR activity was measured to be ZnO < Zn2CoO4 < ZnCo2O4. Particularly, ZnCo2O4 exhibited the highest mass activity (42.64 mA mg-1) and geometric current density (166.28 mA cm-2), outperforming Zn2CoO4 (27.44 mA mg-1) and ZnO (12.72 mA mg-1). It also demonstrated the lowest onset potential of 1.32 V (vs RHE) compared to Zn2CoO4 (1.35 V) and ZnO (1.39 V) and maintained excellent long-term stability for 12 h at 1.5 V (vs RHE). Additionally, to determine the optimal methanol concentration, all electrocatalysts were tested across a range of methanol concentrations from 0.1 to 1 M, showing 0.5 M methanol as the most suitable concentration. This study aims to develop cost-effective MOF-derived electrode materials and optimize methanol concentration to maximize catalytic activity. Furthermore, it establishes a foundation for the development of various MOF-derived electrocatalysts and the advancement of DMFC technology.

Carbon Nanotube Template-Assisted Synthesis of Conjugated Microporous Polytriphenylamine with High Porosity for Efficient Supercapacitive Energy Storage

Engineering of conjugated microporous polymers (CMPs) with high porosity, redox activity, and electronic conductivity is of significant importance for their practical applications in electrochemical energy storage. Aminated-multiwall carbon nanotubes (NH2 -MWNT) are utilized to modulate the porosity and electronic conductivity of polytriphenylamine (PTPA), which is synthesized via Buchwald-Hartwig coupling reaction of tri(4-bromophenyl)amine and phenylenediamine as constitutional units in a one-step in situ polymerization process. Compared to PTPA, the specific surface area of core-shell PTPA@MWNTs has been greatly improved from 32 to 484 m2  g-1 . The PTPA@MWNTs exhibites an improved specific capacitance, with the highest value 410 F g-1 in 0.5 M H2 SO4 at a current of 10 A g-1 achieve for PTPA@MWNT-4 due to the hierarchical meso-micro pores, high redox-activity and electronic conductivity. Symmetric supercapacitor assemble by PTPA@MWNT-4 has a capacitance of 216 F g-1 of total electrode materials and retains 71% of initial capacitance after 6000 cycles. This study gives new insights into the role of CNT templates in the adjustment of molecular structure, porosity, and electronic property of CMPs for the high-performance electrochemical energy storage.

Design of ZIF-67 nanoflake derived NiCo-LDH/rGO hybrid nanostructures for aqueous symmetric supercapattery application under alkaline condition

Well-defined polyhedral ZIF-67 metal-organic frameworks (MOFs) are usually synthesized using methanol as solvent. In this work, methanol is replaced with deionized water as a solvent to synthesize ZIF-67 MOFs with unique nanoflake morphology. The ZIF-67 nanoflakes are synthesized directly byin situmethod on reduced graphene oxide (rGO) to obtain ZIF-67/rGO-xprecursors which are further transformed into NiCo-layered double hydroxide nanocomposites (NiCo-LDH/rGO-x,x = 10, 30, 50 and 90 mg of rGO). The NiCo-LDH/rGO-xnanostructured composites are found to be excellent materials for battery type supercapacitor (supercapattery) applications. Among these samples, the NiCo-LDH/rGO-30 composite gives maximum specific capacity of 829 C g^-1(1658 F g^-1) at a current density of 1 A g^-1and high rate capability. The as fabricated 2-electrode symmetric Swagelok deviceNiCo-LDH/rGO-30NiCo-LDH/rGO-30delivered a high energy density of 49.2 Wh kg^-1and a power density of 4511 W kg^-1, and enabled us to glow red, blue and white LED bulbs using three coin cells. The device can show good capacity retention even after 3000 continuous charge-discharge cycles. The NiCo-LDH/rGO-30 composite,in situderived from ZIF-67 MOF in combination with optimal amount of rGO, is an excellent material to deliver both high energy density and high power density in supercapattery devices.

Fabrication of composite material of RuCo2O4 and graphene on nickel foam for supercapacitor electrodes

Supercapacitors are energy storage devices with the advantage of rapid charging and discharging, which need a higher specific capacitance and superior cycling stability. Hence, a composite material consisting of RuCo2O4 and reduced graphene oxide with a nanowire network structure was synthesized on nickel foam using a one-step hydrothermal method and annealing process. The nanowire network structure consists of nanowires with gaps that provide more active sites for electrochemical reactions and shorten the diffusion path of electrolyte ions. The prepared electrodes exhibit outstanding electrochemical performance with 2283 F g-1 at 1 A g-1. When the current density is 10 A g-1, the specific capacitance of the electrodes is 1850 F g-1, which maintains 81% of the initial specific capacitance. In addition, the prepared electrodes have a long-term cycling life with capacitance retention of 92.60% after 3000 cycles under the current density of 10 A g-1. The composite material is a promising electrode material for high-performance supercapacitors.

MoO3 thin layers on NiCo2S4 substrate for efficient electrochemical charge storage

Ternary oxides/sulfides have long been investigated as promising electrode materials for charge storage applications. However, it is important to rationally design nanostructured hybrid composites for superior charge storage performance as electrodes in devices. In this work, MoO₃@NiCo₂S₄ hybrid composites materials are synthesized by the hydrothermal method followed by annealing at different temperatures. The charge storage properties of these materials are tested by cyclic voltammetry, galvanostatic charge-discharge curves and electrochemical impedance spectroscopy. It is found that the structure of the hybrid composite material not only assists electron and charge transportation but also precisely control the volume expansion during redox reactions, contributing to superior electrochemical behavior. Among all the electrodes, the electrode fabricated with MoO₃@NiCo₂S₄ composite material annealed at 400 °C (MoO₃@NiCo₂S₄-400) is the best for charge storage applications. At 400 °C, MoO₃ spreads as a thin layer of surface polymeric molybdates on NiCo₂S₄ as seen in the XRD pattern. Significantly, it delivers the highest capacitance of 1622 F g^-1 at 1 A g^-1 in 2 M aqueous KOH electrolyte compared to other hybrid composite electrodes, NiCo₂S₄ (962 F g^-1), MoO₃@NiCo₂S₄-500 (1412 F g^-1) and MoO₃@NiCo₂S₄-600 (970 F g^-1), under the same measurement conditions. Furthermore, the MoO₃@NiCo₂S₄-400 hybrid electrode shows better cyclic stability with 93% capacitance retention after 3000 charge-discharge cycles at 8 A g^-1. The synergistic effect of two components and annealing temperature plays important role in enhancing the charge storage performance. This work shows the importance of the synthesis temperature on the functional character of ternary sulfide/oxide composite materials for charge storage applications.

In situ construction of dual-morphology ZnCo2O4 for high-performance asymmetric supercapacitors

In this study, the controllable preparation of ZnCo₂O₄ with different morphologies in a reaction system and the orderly weaving of these morphologies into special structures was demonstrated, which might be impossible to achieve using other methods; herein, we successfully prepared a dual-morphology ZnCo₂O₄/N-doped reduced graphene oxide/Ni foam substrate (ZNGN) electrode by ultrasonic processing, a one-step hydrothermal method and a subsequent annealing process for high-performance supercapacitors. At first, ZnCo₂O₄ nanosheet orderly formed a honeycomb structure on the surface of Ni foam (NF); this improved the redox surface area of the electrode; then, feather-like ZnCo₂O₄ was evenly distributed over the honeycomb structure, playing the role of containment and fixation to provide space for material volume expansion during charging and discharging. The electrochemical test showed that the maximum capacitance of the ZNGN electrode was 1600 F g^-1 (960C g^-1) at the current density of 1 A g^-1 in a 6 M KOH solution. Moreover, the asymmetric supercapacitor ZNGN//activated carbon (ZNGN//AC) displayed the excellent energy density of 66.1 W h kg^-1 at the power density of 701 W kg^-1. Compared with the capacitance (233.3 F g^-1 and 326.6C g^-1) when ZNGN//AC was fully activated at 4 A g^-1, there was almost no loss in capacitance after 2000 charge-discharge cycles, and a 94% capacitance retention was achieved after 5000 cycles. Thus, this excellent electrochemical property highlights the potential application of the dual-morphology ZnCo₂O₄ electrode in supercapacitors.

Remarkable Bifunctional Oxygen and Hydrogen Evolution Electrocatalytic Activities with Trace-Level Fe Doping in Ni- and Co-Layered Double Hydroxides for Overall Water-Splitting

Large-scale H₂ production from water by electrochemical water-splitting is mainly limited by the sluggish kinetics of the nonprecious-based anode catalysts for oxygen evolution reaction (OER). Here, we report layer-by-layer in situ growth of low-level Fe-doped Ni-layered double hydroxide (Ni_{1- x}Fe _x-LDH) and Co-layered double hydroxide (Co_{1- x}Fe _x-LDH), respectively, with three-dimensional microflower and one-dimensional nanopaddy-like morphologies on Ni foam, by a one-step eco-friendly hydrothermal route. In this work, an interesting finding is that both Ni_{1- x}Fe _x-LDH and Co_{1- x}Fe _x-LDH materials are very active and efficient for OER as well as hydrogen evolution reaction (HER) catalytic activities in alkaline medium. The electrochemical studies demonstrate that Co_{1- x}Fe _x-LDH material exhibits very low OER and HER overpotentials of 249 and 273 mV, respectively, at a high current density of 50 mA cm^-2, whereas Ni_{1- x}Fe _x-LDH exhibits 297 and 319 mV. To study the overall water-splitting performance using these electrocatalysts as anode and cathode, three types of alkaline electrolyzers are fabricated, namely, Co_{1- x}Fe _x-LDH(+)∥Co_{1- x}Fe _x-LDH(-), Ni_{1- x}Fe _x-LDH(+)∥Ni_{1- x}Fe _x-LDH(-), and Co_{1- x}Fe _x-LDH(+)∥Ni_{1- x}Fe _x-LDH(-). These electrolyzers require only a cell potential ( E_cell) of 1.60, 1.60, and 1.59 V, respectively, to drive the benchmark current density of 10 mA cm^-2. Another interesting finding is that their catalytic activities are enhanced after stability tests. Systematic analyses are carried out on both electrodes after all electrocatalytic activity studies. The developed three types of electrolyzers to produce H₂, are very efficient, cost-effective, and offer no complications in synthesis of materials and fabrication of electrolyzers, which can greatly enable the realization of clean renewable energy infrastructure.

A New Class of Zn1 -x Fex -Oxyselenide and Zn1- x Fex -LDH Nanostructured Material with Remarkable Bifunctional Oxygen and Hydrogen Evolution Electrocatalytic Activities for Overall Water Splitting

The scalable and cost-effective H₂ fuel production via electrolysis demands an efficient earth-abundant oxygen and hydrogen evolution reaction (OER, and HER, respectively) catalysts. In this work, for the first time, the synthesis of a sheet-like Zn_{1- x} Fe_x -oxyselenide and Zn_{1- x} Fe_x -LDH on Ni-foam is reported. The hydrothermally synthesized Zn_{1- x} Fe_x -LDH/Ni-foam is successfully converted into Zn_{1- x} Fe_x -oxyselenide/Ni-foam through an ethylene glycol-assisted solvothermal method. The anionic regulation of electrocatalysts modulates the electronic properties, and thereby augments the electrocatalytic activities. The as-prepared Zn_{1- x} Fe_x -LDH/Ni-foam shows very low OER and HER overpotentials of 263 mV at a current density of 20 mA cm^-2 and 221 mV at 10 mA cm^-2 , respectively. Interestingly, this OER overpotential is decreased to 256 mV after selenization and the HER overpotential of Zn_{1- x} Fe_x -oxyselenide/Ni-foam is decreased from 238 to 202 mV at 10 mA cm^-2 after a stability test. Thus, the Zn_{1- x} Fe_x -oxyselenide/Ni-foam shows superior bifunctional catalytic activities and excellent durability at a very high current density of 50 mA cm^-2 . More importantly, when the Zn_{1- x} Fe_x -oxyselenide/Ni-foam is used as the anode and cathode in an electrolyzer for overall water splitting, Zn_{1- x} Fe_x -oxyselenide/Ni-foam(+)ǁZn_{1- x} Fe_x -oxyselenide/Ni-foam(-) shows an appealing potential of 1.62 V at 10 mA cm^-2 . The anionic doping/substitution methodology is new and serves as an effective strategy to develop highly efficient bifunctional electrocatalysts.