Magnetic-field-induced rapid synthesis of defect-enriched Ni-Co nanowire membrane as highly efficient hydrogen evolution electrocatalyst

Metallic Ni-based materials are promising electrocatalysts for commercial alkaline water electrolysis towards hydrogen generation. It is therefore desirable to develop low-cost and controllable synthesis protocols for efficient Ni-based electrocatalysts. Here we report a rapid one-step method to fabricate self-supported membranes of highly-conductive Ni-Co nanowires, which are formed via self-assembly of reduced Ni/Co nanoparticles under a rotating magnetic field. The Ni-Co nanowires are composed of Co nanoparticles and NiCo alloy nanoparticles domains, with abundant inherent interface defects due to incomplete alloying and insufficient Ostwald ripening during the assembly. Nanowires with different Ni/Co ratios are tested as the HER electrodes in comparison to pure Ni mesh and Ni foam electrodes; And the Ni0.50Co0.50 nanowire electrode gives the most optimized performance. The HER activity shows little degradation for nearly 100 h. These nanowire electrodes are superior to the state-of-the-art metallic Ni-based ones. This facile technology may represent a critical step towards scalable production of highly active and durable metallic Ni-based electrocatalysts for industry applications.MOE (Min. of Education, S’pore)Accepted versio

Role of electrolyte pH on water oxidation for iridium oxides

Understanding the effect of non-covalent interactions of intermediates at the polarized catalyst-electrolyte interface on water oxidation kinetics is key for designing more active and stable electrocatalysts. Here, we combine operando optical spectroscopy, X-ray absorption spectroscopy and surface-enhanced infrared absorption spectroscopy to probe the effect of non-covalent interactions on OER activity of IrOx in acidic and alkaline electrolyte. Our results suggest the active species for OER (Ir4.x+-*O) binds much stronger in alkaline compared with acid at low coverage, while the repulsive interactions between these species is higher in alkaline electrolyte. These differences are attributed to the larger fraction of water within the cation hydration shell at the interface in alkaline electrolytes compared to acidic electrolytes, which can stabilise oxygenated intermediates and facilitate long-range interactions between them. Quantitative analysis of the state energetics shows that although the *O intermediates bind more strongly than optimal in alkaline electrolyte; the larger repulsive interaction between them results in significant weakening of *O binding with increasing coverage, leading to similar energetics of active states in acid and alkaline at OER-relevant potentials. By directly probing the electrochemical interface with complementary spectroscopic techniques, our work goes beyond conventional computational descriptors of OER activity to explain the experimentally observed OER kinetics of IrOx in acidic and alkaline electrolytes

Tip-enhanced electric field : a new mechanism promoting mass transfer in oxygen evolution reactions

The slow kinetics of oxygen evolution reaction (OER) causes high power consumption for electrochemical water splitting. Various strategies have been attempted to accelerate the OER rate, but there are few studies on regulating the transport of reactants especially under large current densities when the mass transfer factor dominates the evolution reactions. Herein, Nix Fe1- x alloy nanocones arrays (with ≈2 nm surface NiO/NiFe(OH)2 layer) are adopted to boost the transport of reactants. Finite element analysis suggests that the high-curvature tips can enhance the local electric field, which induces an order of magnitude higher concentration of hydroxide ions (OH- ) at the active sites and promotes intrinsic OER activity by 67% at 1.5 V. Experimental results show that a fabricated NiFe nanocone array electrode, with optimized alloy composition, has a small overpotential of 190 mV at 10 mA cm-2 and 255 mV at 500 mA cm-2 . When calibrated by electrochemical surface area, the nanocones electrode outperforms the state-of-the-art OER electrocatalysts. The positive effect of the tip-enhanced local electric field in promoting mass transfer is also confirmed by comparing samples with different tip curvature radii. It is suggested that this local field enhanced OER kinetics is a generic effect to other OER catalysts.Accepted versio