Self-Supportive NiFe hydroxide with High Electrocatalytic Activity for Oxygen and Hydrogen Evolution Reaction

Nickel-Based Selenides with a Fractal Structure as an Excellent Bifunctional Electrocatalyst for Water Splitting

Nickel-based selenides are believed to be promising non-precious metal electrocatalysts, and have been widely used for both oxygen evolution reactions (OER) and hydrogen evolution reactions (HER). Here, we control the aging time to prepare Ni_xSe_y with different fractal structures as a bifunctional catalyst. An obtained sample with an aging time of 80 min shows outstanding electrocatalytic performance for hydrogen evolution reactions (HER) with an overpotential of 225 mV (η@10 mA/cm²) and for oxygen evolution reactions (OER) with an overpotential of 309 mV (η@50 mA/cm²). Moreover, to further improve catalytic activity, we doped Fe in Ni_xSe_y to obtain the ternary nickel-based selenide, Fe_0.2Ni_0.8Se (FNSs). The HER activity of FNS increased two-fold at 10 mA/cm², and the overpotential of OER decreased to 255 mV at 50 mA/cm². The synthetic strategy and research results of this work have a certain reference value for other low-cost and high-efficiency transition metal catalysts for electrocatalytic water splitting.

Strategies and Perspectives to Catch the Missing Pieces in Energy-Efficient Hydrogen Evolution Reaction in Alkaline Media

Transition metal hydroxides (M-OH) and their heterostructures (X|M-OH, where X can be a metal, metal oxide, metal chalcogenide, metal phosphide, etc.) have recently emerged as highly active electrocatalysts for hydrogen evolution reaction (HER) of alkaline water electrolysis. Lattice hydroxide anions in metal hydroxides are primarily responsible for observing such an enhanced HER activity in alkali that facilitate water dissociation and assist the first step, the hydrogen adsorption. Unfortunately, their poor electronic conductivity had been an issue of concern that significantly lowered its activity. Interesting advancements were made when heterostructured hydroxide materials with a metallic and or a semiconducting phase were found to overcome this pitfall. However, in the midst of recently evolving metal chalcogenide and phosphide based HER catalysts, significant developments made in the field of metal hydroxides and their heterostructures catalysed alkaline HER and their superiority have unfortunately been given negligible attention. This review, unlike others, begins with the question of why alkaline HER is difficult and will take the reader through evaluation perspectives, trends in metals hydroxides and their heterostructures catalysed HER, an understanding of how alkaline HER works on different interfaces, what must be the research directions of this field in near future, and eventually summarizes why metal hydroxides and their heterostructures are inevitable for energy-efficient alkaline HER.