Differences in oscillations and sequences of dynamical states caused by anion adsorption in the electrochemical oxidation of formic acid

Effects of anions on the electrocatalyzed oxidation of formic acid at polycrystalline Pt under constant current conditions are studied by varying the electrolyte composition. A subcritical bifurcation and an apparently unique sequence of oscillatory states are observed when the supporting electrolyte is H2SO4 Different behaviors, including a different sequence of oscillatory states, are seen when HClO4 is used as the supporting electrolyte. Effects of increasing the overall anion "adsorption strength" are also investigated by examining changes in the dynamical response when small amounts of HCl are added to the solution containing formic acid and HClO4. When the concentration of HCl is increased beyond a small critical value, instabilities occur that are not observed when using either the HCl free HClO4 solution or the H2SO4 solution. All the stated differences are in contrast to previously reported differences in potential control experiments that were caused by switching electrolytes. The differences in the latter experiments are quantitative. They were interpreted in terms of adsorption and blockage of surface reaction sites by anions. It is argued that the results of the current control experiments imply that anions are also directly involved in the oxidation process. They affect the formation and, consequently, the reactivity of surface water, surface bonded hydroxyl radicals, and Pt oxides.