Mechanisms and Kinetics of H2 Dissociation and Open-Circuit Accumulation of H Atoms during the Formation of the Equilibrium H2/H+ Electrode on Platinum

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Abstract
The rate at which dissolved H2 dissociates and accumulates on a clean, H‐ and O‐free Pt electrode as the reversible H2/H+ electrode is formed has been studied under dissociation, diffusion, and mixed control. The kinetics of the open‐circuit hydrogen accumulation reaction (h.a.r.) in H2‐saturated (1 atm) 1M were determined by a galvanostatic double‐pulse method in which the first pulse strips the surface, and the second pulse measures the amount of H accumulated in a given time. Under dissociation control, the accumulation of the first 70% of the equilibrium concentration of H obeys a second‐order kinetic relation in which the maximum rate at zero time is 4.46 mA/cm2. As the h.a.r. proceeds, the initial reaching of zero volt cannot be equated with equilibrium of the H2/H+ reaction. A slower rearrangement of adsorbed and dermasorbed H atoms occurs before thermodynamic equilibrium is reached. The hydrogen oxidation limiting current density of 2 mA/cm2 (0.05–0.7V, vigorous H2 stirring) is shown to be the geometric average of . This mixed control indicates that diffusion causes the effective H2 partial pressure at the electrode surface to be reduced from 1 to 0.45 atm. Only a small fraction of the total number of Pt atoms which are associated with H atoms at equilibrium are active dissociation sites. Non‐faradaic charging corrections were found to be complex and involved adsorption/desorption processes as well as double‐layer charging.

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