Anodic molecular hydrogen formation on Ru and Cu electrodes

Electrochemical hydrogen adsorption and desorption, and the state of adsorbed hydrogen (*H) on metal surfaces are of fundamental interest as well as practical importance for the hydrogen evolution reaction (HER) and electrochemical hydrogenation reactions including CO₂ and CO electroreduction. Here, we report a previously undiscovered phenomenon whereby *H desorbs as H₂ during an anodic potential sweep at potentials anodic of (more positive than) the equilibrium potential of U⁰ (H₂/H⁺), hence at potentials where hydrogen desorption would be expected as H⁺. Using electrochemistry - online mass spectrometry, we observe, quantify, and characterize the phenomenon on two different materials in two different environments - Ru(0001) in acid and polycrystalline Cu in alkaline. For both Ru and Cu, the anodic H₂ formation seems to coincide with *OH adsorption, which would be consistent with a displacement mechanism. We propose that a high barrier for the Volmer step causes some of the displaced *H to desorb as H₂ (Tafel step) rather than the thermodynamically more favorable desorption as H⁺ (Volmer step).