Promotional effects on a PtRu/C catalyst-electrode interfaced with aqueous electrolytes: electrochemical metal support interaction (EMSI) and electrochemical promotion of catalysis (EPOC)

Abstract

The kinetics of the catalytic oxidation of H2 on PtRu/C gas diffusion electrode (GDE) was studied by interfacing the electrode with aqueous electrolytes at different pH values. The conducting electrolytes were aqueous solutions of varying concentrations of KOH and HClO4 so that the pH was ranging between 2 and 13. The open circuit catalytic reaction rates exhibit the lowest value at pH = 13, while the catalytic activity is progressively increasing with decreasing pH values. The enhancement of the open circuit catalytic reaction rate can be even an order of magnitude higher in the acidic solution with respect to the alkaline electrolyte. It is shown that the nature of the aqueous electrolyte plays the role of an active catalyst support for the PtRu/C electrode, which drastically affects its catalytic properties. This is substantiated through the electrochemical equilibrium charge transfer reactions at the catalyst-electrode/electrolyte interface: $${ \eqalign{ \hbox{H}_{3}\hbox{O}^{+}+\hbox{e}^{-}+\hbox{S}\leftrightarrow\hbox{H}_ {\rm ad}+\hbox{H}_{2}\hbox{O}\ (\hbox{acidic})\cr \hbox{OH}^{-}+\hbox{S}\leftrightarrow\hbox{OH}_{\rm ad}+ \hbox{e}^{-}\ (\hbox{alkaline}) }}$$ According to the aforementioned interaction, termed electrochemical metal support interaction (EMSI), the electrochemical potential of the electrons at the catalyst Fermi level is equalised with the electrochemical potential of the solvated electron in the aqueous electrolyte. The electrochemical promotion experiments carried out at various pH values showed that the non Faradaic modification of the catalytic activity is more intense when the catalyst-electrode is interfaced with electrolytes with high pH values where the OH− ionic conduction prevails. It was concluded that similar to the solid state electrochemical systems the non Faradaic electrochemical modification of the catalytic activity proceeds through the formation of a polar adsorbed promoting layer of $${\hbox{OH}_{\rm ad}^{\delta-}}$$ , electrochemically supplied by the OH− species, at the three phase boundaries of the gas exposed gas diffusion catalyst-electrode surface.