Electrocatalysis under conditions of high mass transport: Investigation of hydrogen oxidation on single submicron Pt particles supported on carbon

The mechanism and kinetics of the hydrogen oxidation reaction (hor) has been investigated using carbon-supported single particles of Pt electrocatalyst with radii as small as 40 nm. The high mass transport rates on such small particles enable us to investigate the rapid kinetics of the hor in the absence of diffusion limitations. Surface kinetic controlled polarization curves during the electrochemical oxidation of hydrogen molecules in acid solution have been obtained in the entire H UPD region, showing features obviously different from those obtained on normal micrometer electrodes or in RDE experiments. For instance, two current plateaus rather than one are seen during the steady-state polarization of the hor on electrodes made of small particles. Upon decreasing the size of the Pt particles, the two current plateaus show greater separation and become better defined. A theoretical model for the steady-state polarization of the hor has been developed in which UPD H atoms of various states are considered as the reactive intermediates and the Frumkin adsorption mode is assumed for the atomic H on Pt electrodes. It is shown that the first current plateau represents the limiting reaction rate under adsorption or combined adsorption-diffusion control while the second plateau current corresponds to the limiting diffusion-controlled reaction rate. It is pointed out that Tafel plots that have been frequently used for kinetics analysis in the hor are meaningless, especially in the potential region below 0.05V vs RHE. The polarization curves are fitted with a general polarization equation derived according to our model. The fitting shows that the hor on Pt proceeds most likely via the Tafel-Volmer reaction mechanism rather than the Heyrovsky-Volmer mechanism. These results have significant implications on the understanding and modeling of the reactions in solid polymer electrolyte fuel cells.