Kinetics of the Hydrogen Oxidation/Evolution Reaction on Polycrystalline Platinum in Alkaline Electrolyte Reaction Order with Respect to Hydrogen Pressure

The electrochemical oxidation and evolution of molecular hydrogen are the key reactions at play in the anodes and cathodes of fuel cells and electrolyzers, respectively, which are likely energy conversion and storage devices for renewable energy concepts based on the use of H2 as energy carrier. Beyond this practical interest, the hydrogen oxidation and evolution reactions (HOR and HER, respectively) have also played a pivotal role in the historical development of fundamental electrocatalysis theories. Indeed, the exponential relation between current and overpotential that describes the kinetics of many electrochemical reactions, viz., the Butler-Volmer equation, was originally validated for the HER, 1 which also became the first electrochemical process for which the rate-determining role of the bond strength of adsorbed intermediates (following Sabatier’s principle) was verified. 2 Thus, the HOR/HER has become one of the most extensively studied electrochemical reactions, particularly at low pH values and on the catalytically most active platinum-based materials relevant to proton exchange membrane fuel cells (PEMFCs) and electrolyzers. In acid electrolytes, the HOR/HER kinetics on platinum electrodes are extremely fast, so that experimental methods which afford very high mass-transport rates are required in order to unambiguously differentiate kinetic- and diffusion-overpotentials (e.g., hydrogen pump experiments in PEMFC 3 or the recently developed floating porous gas diffusion electrode method 4 ). Using the hydrogen-pump approach, the very high exchange current densities (i0 )o f≈200 mA · cmPt2 obtained at 313 K 5 (or ≈600 mA · cmPt2 at 353 K) 3 are consistent with the fact that ultra-low Pt loadings of ≤0.05 mgPt · cm −2 can be used at the anode of PEMFCs, 3,6 while much higher Pt loadings are required for the cathode electrode due to the much more sluggish oxygen reduction