Thin-layer spectroelectrochemical kinetic study of viologen cation radicals reacting at hydrogen-evolving gold and nickel electrodes

The kinetics of viologen cation radicals reacting at hydrogen-evolving gold and nickel electrodes in pH 6–8 electrolytes have been investigated. Visible absorption spectroscopy was used to follow the course of the reaction in an optically transparent thin-layer electrochemical cell under quasi-steady-state conditions. The spectroelectrochemical data were analyzed using classical kinetics and yielded zero-order behavior with respect to the viologen cation radical. For methyl viologen cation radical at gold, a formal zero-order rate constant evaluated at zero hydrogen overpotential was found to be 1.0 × 10^?13 mol s^?1 cm^?2. At nickel the comparable rate constant was nearly two orders of magnitude larger than at gold. Increasing pH from 6 to 8 at gold electrodes shifted both the hydrogen evolution and the methyl viologen cation radical reaction 60–70 mV/pH unit in a negative direction. The diquat cation radical behaved in a similar manner. The proposed mechanism involves a fast, non-rate-limiting, chemical reaction between the viologen cation radical and adsorbed hydrogen atom(s). Results are interpreted in terms of previous proposed hydrogen evolution reaction mechanisms.