Kinetics of the hydrogen evolution reaction on NiMn graphite modified electrode

The mechanism and kinetics of the hydrogen evolution reaction (HER) on graphite modified with Ni and NiMn electrode (G/Ni and G/NiMn) in 0.1 M NaOH solution were studied using the methods of steady-state polarization, electrochemical impedance spectroscopy, cyclic voltammetry and open circuit potential transient. The addition of Mn to Ni significantly increases the catalytic activity in HER due to higher real surface area and higher intrinsic activity. The simulation of the data obtained from these methods, using nonlinear fitting procedure allowed us to determine the rate constants of Volmer, Heyrovsky and Tafel steps associated with the mentioned reaction. The kinetics results indicate that HER mechanism for G/NiMn electrode at low negative potentials is a serial combination of Volmer and parallel Tafel and Heyrovsky steps. At high negative potentials where the hydrogen coverage reaches its limiting value, a Tafel line with the slope of −125 mV dec−1 is obtained. In this potential region the mechanism of the HER follows Volmer-Heyrovsky while the Tafel step has negligible contribution. Open circuit potential measurements for G/Ni and G/NiMn at different charging currents show that hydrogen absorption into the electrode material occurs.