On overpotential and the photovoltaic process at polarized electrodes

Abstract

The acceleration of the electrodeposition of hydrogen and oxygen by light of wave-length from 4000 to 2000 A has been studied on different electrodes. The quantum efficiency of the photoreaction was determined as a function of the wave-length of the incident light and of the steady polarizing current. From these experiments it was concluded that the rate-determining step at these electrodes occurred after the discharge of the ions and that the overpotential corresponded to a stationary concentration of adsorbed atoms or radicals. It was further concluded that an electrode at which hydrogen is being evolved at a sensible rate is covered with a nearly complete layer of adsorbed hydrogen. The potential at which such evolution occurs is determined primarily by the negative potential necessary to displace adsorbed oxygen atoms or water molecules by hydrogen. Calculations of this potential, in the few cases where the necessary heats of adsorption are available, gave values in agreement with those observed experimentally. It was concluded that the rate-determining step on most cathodes, except platinum, was the electrochemical combination of an adsorbed atom with an ion in the double layer, and relations were obtained for the dependence of the overpotential on the current density and electrokinetic potential, which are in fairly good agreement with experiment. A similar mechanism is tentatively suggested for the electrode reaction at an anode at which oxygen is being evolved.