Unveiling hydrogen evolution dependence on KOH concentration for polycrystalline and nanostructured nickel-based catalysts

Abstract

Nickel-based hydrogen evolution reaction (HER) electrodes have been widely used in alkaline and anion exchange membrane water electrolysis. Therefore, understanding the activity dependence on the KOH concentration (pH) of alkaline electrolytes is essential for designing durable and active HER catalysts. In this work, the HER activity and kinetics of polycrystalline and nanostructured nickel-based catalysts are evaluated in various pH and KOH concentrations. The results for nanostructured NiMo catalyst indicate that both electrochemical active surface area and reaction order have a promoting region under various pH’s and KOH concentrations (0.01–1.0 M, pH 12–14) accompanied by better HER activity (a lower overpotential for achieving − 10 mA cm−2) and Tafel slope decreases from around 180 mV dec−1 to 60 mV dec−1 in the same pH and KOH concentration range. The change in the Tafel slope indicates that the HER rate-determining step for HER at NiMo/C changes with pH and KOH concentration. The polycrystalline Ni displays different behaviours where a promoting (0.01–0.10 M, pH 12–13), stabilizing (0.1–1.0 M, pH 13–14), and an inhibiting region (2 M, pH > 14) are present. However, Tafel slopes of around 120 mV/dec are obtained for polycrystalline Ni at all KOH concentrations. The HER characteristics are inhibited at 2.0 M KOH for both catalysts due to slower OH* transport kinetics. The results confirmed the importance of tuning catalyst-pH/KOH concentration for better HER activity and kinetics. Graphical abstract: [Figure not available: see fulltext.]