Abstract
The hydrogen oxidation and evolution reaction (HOR/HER) has been widely investigated due to its importance for a broad variety of applications especially in electrolysis and fuel cells. However, owing to its extremely fast kinetics, kinetic data can only be obtained with experimental setups that provide very fast mass transport, so that the effect of hydrogen partial pressure (pH2) and kinetic limitations at high overpotentials are not yet fully understood. Here we report detailed kinetic investigations on the temperature and pressure dependence of the HOR/HER kinetics on carbon supported platinum (Pt/C) using the PEMFC-based H2-pump approach. By using ultra-low platinum loadings, we could show that the apparent activation energy of the HOR/HER on platinum increases with increasing pH2, due to a diminishing effect of the hydrogen adsorption enthalpy with increasing coverage by adsorbed hydrogen atoms on the Pt surface. Consequently, the HOR/HER reaction order with respect to pH2 also depends on the temperature. We further observed that the HOR reaches a limiting current at high HOR overpotentials that showed a direct proportionality to pH2 and a pressure-independent activation energy. We ascribe this to a limitation of the hydrogen adsorption rate either by a rate limiting Tafel-reaction or mass transport limitations.
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