Atoms vs. Ions: Intermediates in Reversible Electrochemical Hydrogen Evolution Reaction

We present a critical analysis of the mechanism of reversible hydrogen evolution reaction based on thermodynamics of hydrogen processes considering atomic and ionic species as intermediates. Clear distinction between molecular hydrogen evolution/oxidation (H₂ER and H₂OR) and atomic hydrogen evolution/oxidation (HER and HOR) reactions is made. It is suggested that the main reaction describing reversible H₂ER and H₂OR in acidic and basic solutions is: H3O⁺ + 2e⁻ (H₂⁺)ad H2 + OH⁻, which proceeds via the formation of hydrogen molecular ion as intermediate and its potential is E⁰ = 0.414~V (at the standard Hydrogen electrode SHE scale). We analyse experimentally reported data with models which provide quantitative match (R.J.Kriek et al., Electrochem. Sci. Adv. e2100041 (2021)). The experimentally observed slope of (Current) vs. $Potential$ equal to ≈ 30~mV/dec corroborates the presented model of H₂ER/H₂ER based on {H2⁺} as intermediate.