Optimized Nickel-Cobalt and Nickel-Iron Oxide Catalysts for the Hydrogen Evolution Reaction in Alkaline Water Electrolysis

Ni and Ni-doped with transition metals (TM) such as Fe and Co represent the most suitable electrodes for hydrogen evolution reaction (HER) in alkaline media. Various compositions of co-precipitated Ni 1 + x Fe 2 − x O 4 and Ni 1 + y Co 2 − y O 4 nanoparticles were investigated. The intrinsic HER catalytic activity is the same for all the catalysts, which we relate to similar values of the iso-electric point (IEP). However, the mass catalytic activity of the catalysts changes through a modification of the electrochemical surface area. Fractional reaction orders for hydrogen evolution revealed in all catalyst compositions are due to double layer effects and surface acid-base equilibria. Reaction order and Tafel slope of the catalysts are compatible with electrochemical adsorption as the rate-determining step for the HER. Tafel slopes were also evaluated independently from impedance spectroscopy, in good agreement with the polarization curves. Electrodes prepared from catalyst inks containing an anion-exchange ionomer displayed inferior catalytic activity for the HER as compared to electrodes prepared with Nafion in the ink. Chronoamperometry confirmed the sustained superior hydrogen kinetics over time of NiFe 2 O 4 and NiCo 2 O 4 composition over that of NiO.