The influence of solution pH on rates of an electrocatalytic reaction: Formic acid electrooxidation on platinum and palladium

We found for the first time that solution pH makes a significant difference in the rate of a prototypical electrocatalytic reaction, formic acid electrooxidation, under conditions where H+ and OH− do not adsorb strongly on the surface, and we demonstrate that the results can be explained using the Marcus model. We changed the pH of formic acid solutions and found enhanced oxidation on Pd and Pt black catalysts with increased pH. The interpolated current density for the oxidation of 1 M HCOOH in 0.1 M perchlorate electrolyte after 2 min at 0.22 V vs. SHE on Pt increased 30-fold from 0.005 to 0.17 mA cm−2 as the pH was increased from 1 to 5, while for Pd there was a 4-fold increase from 0.12 to 0.53 mA cm−2. The data was also interpolated at a current density of 0.1 mA cm−2, and the potential required to reach this current shifted negative 62 mV per pH on palladium and 56 mV per pH on platinum. A 24 h experiment compared two solution pH, in which the higher pH demonstrated remarkably stronger performance. In addition, the potential for oxidation of surface CO shifts negative on both catalysts, as much as ∼57 mV per pH on Pd.