Understanding Structure-Performance Relationship of Nanoarray Structured Anodes in Water Electrolysis

Nowadays, to achieve carbon neutrality and utilize excess electricity generated from various energy systems, researchers in both industry and academia have made extensive efforts to explore a wide array of electrochemical reduction reactions for generating value-added chemicals and products from various green and renewable feedstocks such as water, biomass, and biogenic CO₂. Such electrocatalytic reduction reactions include hydrogen evolution reaction (HER) through water electrolysis, CO₂ reduction reaction (CO2RR) to hydrocarbons, nitrogen reduction reaction (NRR) to ammonia, and others. All these half-cell electrocatalytic reduction reactions attract significant attentions as they help produce desired products while reducing the carbon footprint, however, it should be noted that oxygen evolution reaction (OER) as the anodic reaction is commonly used as the pairing half-cell electrocatalytic oxidation reaction in order to achieve charge neutrality. With OER only requiring water as a source, it potentially could be applied for massive production. However, oxygen evolution is a complex 4-electron transfer reaction which suffers from sluggish reaction kinetics. As a result, a higher voltage than the theoretical voltage for water splitting (overpotential) is required to have the OER reaction occurred in a real scenario. Compared to expensive and scarce noble metal oxides such as RuO₂ and IrO₂, which are considered as the best catalysts for OER, nickel, as one of the earth-abundant metals, and its compounds also demonstrate excellent catalytic activities and stability in an alkaline environment. Here, a synthetic process based on anodic aluminum oxide (AAO) template is devised and presented to fabricate nickel-based nanowire array structured electrodes, which have been demonstrated to readily reduce the overpotential for the OER and increase the catalyst stability. The array electrode’s spacing/porosity, dimensionalities, and orientation alignment are tuned as well as electrolyte temperature and concentration to investigate the electrocatalytic OER mechanism and the relationship between nanoarray structure, electrolyte environment, and the demonstrated performance. Optimum nanoarray structure configurations are established for achieving better electrochemical activities while retaining the structure and chemical durability of the catalysts.