Unveiling the structure of interfacial water and its role in acidic and alkaline hydrogen evolution reaction at Au electrode by electrochemical in-situ infrared spectroscopy and theoretical simulation

Current for hydrogen evolution reaction (HER) in alkaline media has been generally found to be ca. one to two orders of magnitude smaller than that in acid. The origin is under hot debate. Different connectivity of the hydrogen bond network in the electric double layer (EDL) has been recently proposed to play a significant role in the significant kinetic pH effect. To verify the generality of such an effect, the structure of Au/0.1 M HClO₄ and Au/0.1 M NaOH interfaces and its correlation to HER kinetics have been investigated by cyclic voltammetry, electrochemical in-situ infrared spectroscopy and theoretical simulation. Our results reveal that, i) the current density and corresponding apparent rate constants for HER at Au/HClO₄ are ca. 1 to 80 and 1 to 800 times higher than that at Au/NaOH interface at the same $E_{\mathrm{R}\mathrm{H}\mathrm{E}}$$E_{\mathrm{R}\mathrm{H}\mathrm{E}}$, respectively, while the intrinsic rate constants for acidic and alkaline HER estimated after properly considering the EDL effect are comparable; ii) at potentials negative of the potential of zero charge, there is an enrichment of H₃O⁺ and Na⁺ near the surface, and the concentration of Na⁺ near Au surface is slightly higher than that of proton at the same $E_{\mathrm{R}\mathrm{H}\mathrm{E}}$$E_{\mathrm{R}\mathrm{H}\mathrm{E}}$ due to more free excess charge at Au/NaOH interface. Partial desolvation of hydrated Na⁺ occurs to balance the excess free charge; iii) water structure at Au/NaOH interface is more heterogeneous than that at Au/HClO₄ interface as evident by broader O–H stretching band with significant contribution at ca. 3590 cm⁻¹; iv) the superposition of the Onsager field induced by the enriched cations with the field generated by applied electrode potential across the electric double layer leads to a significantly higher Stark tuning rate for the O–H stretching vibration in the alkaline HER range compared to that under acidic conditions; v) instead of the difference in the connectivity of hydrogen bond network and the dynamics of water reorganization, smaller electrochemical driving force as a result of lower electric potential at the reaction plane is the origin for the smaller current as well as the apparent rate constant for HER at Au in alkaline media than that in acid under mild HER condition.