Ultra-low platinum coverage at gold electrode surfaces: A different approach to the reversible hydrogen reaction

Abstract

The hydrogen reversible reaction (HRR) with ultra-low concentrations of platinum sites supported on a gold substrate is studied with decreasing amounts of active sites (estimate coverage range from 0.006 to 10− 5) at a rotating electrode in a 0.5 M H2SO4 solution. These particular experimental conditions allow the voltammetric study of the HRR without diffusion limitation in the whole range of potential. In the absence of a more precise determination of the amount of platinum surface sites, a method is given to compare results obtained from distinct experimental runs. Once eliminated the contribution of gold to the current recorded, it becomes possible to normalize the currents in the entire range of platinum coverage. It shows that below 0.2 V, the normalized HER and the HOR currents are linearly connected to the amount of active sites. At higher potentials the normalized currents indicate a systematic decrease of the efficiency of the active sites for HOR that deviates from linearity when their surface concentration decreases. Around the reversible potential, the voltammetric profiles obey to the Butler-Volmer formalism. The conditions of its applicability are discussed. The simulation of the experimental current allows the determination of the partial oxidation and reduction currents. The simulated reduction current is then compared to the HER current recorded in a solution without dissolved hydrogen and saturated with argon. The close fit of the two responses gives evidence for the possibility to have access to HER kinetic data free of diffusion. Finally a comparison is made between results obtained at ultra-low content of platinum active sites with pure platinum as well as for an intermediate coverage of gold with platinum.