Uncoupled non-linear equations method for determining kinetic parameters in case of hydrogen evolution reaction following Volmer–Heyrovsky–Tafel mechanism and Volmer–Heyrovsky mechanism

Abstract

Hydrogen evolution reaction following Volmer–Heyrovsky–Tafel mechanism and not under diffusion control can be characterized using Tafel polarization and alternating current admittance data at various frequencies and at various overpotentials. Such reaction has four independent kinetic parameters. One empirical constant related to charge required for complete surface coverage is also involved. In case of systems where only one time constant (related to charging/discharging of electrical double layer) is discerned, Volmer–Heyrovsky mechanism is assumed. Such systems have three independent kinetic parameters. Experimental data may provide maximum two independent conditions at a less cathodic overpotential and maximum one independent condition at a more cathodic overpotential. A new approach to determine kinetic parameters utilizing these data and neglecting Heyrovsky and Tafel backward reaction rates has been proposed. The conditions involved are coupled non-linear equations. Uncoupled non-linear equations have been derived and iterated using Newton–Raphson method. The approach has been validated using literature data. The graphical relationships between experimental parameters and kinetic parameters have also been shown.