High buffering capacity cobalt-doped nickel hydroxide electrode as redox mediator for flexible hydrogen evolution by two-step water electrolysis

Construction of Low-Crystallinity Three-Dimensional Flower-like Cobalt-Doped Nickel Hydroxide for High-Performance Nickel-Zinc Batteries

The main limitations of aqueous nickel-zinc batteries are their relatively low energy density and short cycle life, which are inextricably linked to the limitations of nickel-based cathodes. In this study, a low-crystallinity flower-like cobalt-doped nickel hydroxide (α-Ni(OH)2-0.2Co) is constructed by hydrothermal reaction and employed as high-energy-density cathode for aqueous rechargeable nickel-zinc batteries. Cobalt doping initiates the formation of a flower-like structure and lowers the material's crystallinity, conferring it with a larger specific surface area, more redox reaction sites, and shorter ion diffusion paths. The optimized α-Ni(OH)2-0.2Co electrode manifests a considerable specific capacity of 772 C·g-1 at 1 A·g-1 and remarkable rate performance, with a capacity retention of 75% at 10 A·g-1. The α-Ni(OH)2-0.2Co//Zn battery constructed with α-Ni(OH)2-0.2Co as the cathode exhibits a considerable specific capacity of 198 mAh·g-1 at 1 A·g-1 in an alkaline electrolyte. Additionally, the battery exhibits a substantial energy density of 326.7 Wh·kg-1 and a power density of 16.5 kW·kg-1, exceeding the performance metrics of most previously documented aqueous nickel-zinc batteries. This research presents a viable approach for developing advanced cathode materials for nickel-zinc batteries.

Decoupled Water Reduction and Hydrazine Oxidation by Fast Proton Transport MoO3 Redox Mediator for Hydrogen Production

Water electrolysis powered by renewable energy is a green and sustainable method for hydrogen production. Decoupled water electrolysis with the aid of solid-state redox mediator could separate the hydrogen and oxygen production in time and space without the use of the membrane, showing high flexibility. Herein, a MoO3 electrode with fast proton transport property is employed as a solid-state redox mediator to construct a membrane-free decoupled acidic electrolytic system. The MoO3 electrode exhibits high specific capacity (204.3 mAh g-1 at 5 A g-1) and excellent rate performance (92.8 mAh g-1 at 150 A g-1) in the acidic environment. Due to the dense oxide-ion arrays, MoO3 still exhibits excellent performance under high mass-loading. In addition, a hybrid decoupled electrolysis system is also constructed by combining water reduction and hydrazine oxidation, which can not only generate high-purity H2 but also remove hydrazine hazards in acidic wastewater with lower energy consumption.

Plasma-induced, N-doped, and reduced graphene oxide-incorporated NiCo-layered double hydroxide nanowires as a high-capacity redox mediator for sustainable decoupled water electrolysis

Although the recent emergence of decoupled water electrolysis prevents typical H2/O2 mixing, the further development of decoupled water electrolysis has been confined by the lack of reliable redox mediator (RM) electrodes to support sustainable H2 production. As energy storage electrodes, layered double hydroxides (LDHs) possess inherently poor conductivity/stability, which can be improved by growing LDHs on graphene substrates in situ. The proper modification of the graphene surface structure can improve the electron transport and energy storage capacity of composite electrodes, while current methods are usually cumbersome and require high temperatures/chemical reagents. Therefore, in this study, dip coating was adopted to grow graphene oxide (GO) on nickel foam (NF). Then, the GO was reduced using nonthermal plasma (NTP) to reduced GO (rGO) in situ while simultaneously implementing N doping to obtain plasma-assisted N-doped rGO on NF (PNrGO/NF). The uniform conductive substrate ensured the subsequent growth of less-aggregated NiCo-LDH nanowires, which improved the conductivity and energy storage capacity (5.93 C/cm2 at 5 mA/cm2) of the NiCo-LDH@PNrGO/NF. For the decoupled system, the composite RM electrode exhibited a high buffering capacity for 1300 s during the decoupled H2/O2 evolution, and in the conventional coupled system, the necessary input voltage of 1.67 V was separated into two lower ones, 1.42/0.33 V for H2/O2 evolutions, respectively. Simultaneously, the RM possessed outstanding redox reversibility and structural stability during long-term cycling. This work could offer a feasible strategy for using NTP to synthesize excellent RM electrodes for application to decoupled water electrolysis.

Matrix-type bismuth-modulated copper-sulfur electrode using local photothermal effect strategy for efficient seawater splitting

Constructing catalytic electrodes with green economy, stability, and high efficiency is crucial for achieving overall economic water splitting. Herein, a matrix-type bismuth-modulated nickel-boron electrodes loaded on sulfurized copper foils (Bi-NiBx@CFS) is synthesized via in situ mild electroless plating. This electrode features a 2-dimensional (2D) matrix-type nanosheet structure with uniform, large pores, providing more active sites and ensuring a high gas transmission rate. Notably, the crystalline-amorphous structure constituted by the photothermal materials Bi and NiBx is loaded onto sulfide-based heterostructures. This enhances the catalytic activity through the "local photothermal effect" strategy. A performance enhancement of approximately 10 % is achieved for the Bi-NiBx@CFS at a current density of 10 mA cm-2 using this strategy at 298 K. This enhancement is equivalent to increasing the temperature of conventional electrolyte solutions by 321 K. In addition, the overpotential required to catalytically drive seawater splitting at the same current density is only 1.486 V. The Bi-NiBx@CFS electrode operates stably for 200 h without any performance degradation at industrial-grade current densities. The Bi-NiBx@CFS electrode under the "localized photothermal effect" strategy is expected to be a new type of electrocatalyst for overall seawater splitting.