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 (H${}_2$ER and H${}_2$OR) and atomic hydrogen evolution/oxidation (HER and HOR) reactions is made. It is suggested that the main reaction describing $reversible$ H${}_2$ER and H${}_2$OR in acidic and basic solutions is: ${\mathrm{H}}_3{\mathrm{O}}^{+}+2e^{-}\stackrel{{\left({\mathrm{H}}_2^{+}\right)}_{ad}}{\rightleftharpoons }{\mathrm{H}}_2+{\mathrm{OH}}^{-}$ and its standard potential is $E^0$ = −0.413 V (vs. standard hydrogen electrode, SHE). We analyse experimentally reported data with models which provide a quantitative match (R.J.Kriek et al., Electrochem. Sci. Adv. e2100041 (2021)). Presented analysis implies that $reversible$ H₂ evolution is a two-electron transfer process which proceeds via the stage of adsorbed hydrogen molecular ion H${}_2^{+}$ as intermediate, rather than H${}_{\mathrm{ad}}$ as postulated in the Volmer-Heyrovsky-Tafel mechanism. We demonstrate that in theory, two slopes of $potential$ vs. $\lg \left(current\right)$ plots are feasible in the discussed reversible region of H₂ evolution: $2.3RT/F\approx 60$ mV and $2.3RT/2F\approx 30$ mV, which is corroborated by the results of electrocatalytic hydrogen evolution studies reported in the literature. Upon transition to $irreversible$ H${}_2$ER, slowdown of H${}_2^{+}$ formation in the first electron transfer stage manifests, and the slope increases to $2.3RT/0.5F\approx 120$ mV; $R,F,T$ are the universal gas, Faraday constants and absolute temperature, respectively.

In this study, the processes taking place on the surfaces of nanostructured Cu/CuO and Cu/CuO/Pd electrodes at different potential, E, values in the solutions of 0.1 M KOH in H ${}_2$ O and D ${}_2$ O (heavy water) were probed by surface enhanced Raman spectroscopy (SERS), and the analysis of electrochemical reactions occurring under experimental conditions is presented. The bands of the SERS spectra of the Cu/CuO/Pd electrode observed in the range of E values from +0.3 V to 0 V (standard hydrogen electrode (SHE)) at 1328–1569 cm ${}^{-1}$ are consistent with the existence of species that are adsorbed or weakly bound to the surface with the energy of interaction close to 15–21 kJ mol ${}^{-1}$ . These bands can be attributed to the ad(ab)sorbed (H ${}_3$ O ${}^{+}$ ) ${}_{\mathrm{ad}}$ , (H ${}_2^{+}$ ) ${}_{\mathrm{ab}}$ , and (H ${}_2^{+}$ ) ${}_{\mathrm{ad}}$ ions as intermediates in reversible hydrogen evolution and oxidation reactions (HER/HOR) taking place on the Cu/CuO/Pd electrode. There was no isotopic effect observed; this is consistent with the dipole nature of the electron-ion pair formation of adsorbed (H ${}_3$ O ${}^{+}$ ) ${}_{\mathrm{ad}}$ and (H ${}_2^{+}$ ) ${}_{\mathrm{ad}}$ or (D ${}_3$ O ${}^{+}$ ) ${}_{\mathrm{ad}}$ and (D ${}_2^{+}$ ) ${}_{\mathrm{ad}}$ . In accordance with the literature data, SERS bands at 125–146 cm ${}^{-1}$ and ∼520–565 cm ${}^{-1}$ were assigned to Cu(I) and Cu(II) oxygen species. These findings corroborate the quantitative stepwise mechanism of water reduction.