Mechanistic Aspects of the Electrocatalytic Oxygen Evolution Reaction over Ni-Co Oxides

The electrocatalytic oxygen evolution reaction (OER) presents the key transformation in electrochemical water-splitting majorly determining energy efficiency and economics of hydrogen generation. In this study, the kinetics of the OER over Ni-Co oxide structured by KIT-6 templating and non-structured Ni-Co oxide catalysts in alkaline solution have been investigated aiming for insight with regard to the respective kinetically relevant surface reactions. Steady-state Tafel plot analysis and electrochemical impedance spectroscopy (EIS) were used to determine kinetic parameters, Tafel slopes and the order of reaction. A dual Tafel slope behavior was observed for both catalysts. Tafel slopes of ca. 40 and 120 mV dec(-1) and 90 and 180 mV dec(-1) at low and high overpotentials appear for structured and non-structured Ni-Co oxide, respectively. A reaction order of unity was observed for structured Ni-Co oxide, while non-structured Ni-Co oxide possessed a fractional reaction order in the high overpotential region. The kinetics of OER over structured Ni-Co oxide were governed by Langmuir adsorption with the rate-limiting step after primary adsorption of surface intermediates. In contrast, non-structured Ni-Co oxide obeyed the Temkin adsorption isotherm condition with the primary adsorption step being rate-limiting.