(528d) Is Potential of Zero Charge a Causal Descriptor for Solvent Dynamics in the Reversible Hydrogen Electrode?

While “beyond adsorption” phenomena are much less understood than the traditional volcano plot, their importance for activity and selectivity in heterogeneous electrochemical transformations is increasingly recognized. No example demonstrates the insufficiency of binding energies to explain kinetic trends more neatly than the anomalous effects of pH on electrocatalytic hydrogen evolution and oxidation (HER/HOR). It has long been recognized that both HER and HOR are multiple orders of magnitude slower in alkaline than acidic conditions for most (but not all) metals, most notably Pt. Despite decades of study, the community still lacks a unified explanation for this observation. Neither experiment nor simulations have been able to provide evidence that pH causes the intrinsic hydrogen binding energy (HBE) to change. Electric field strength has been hypothesized to be responsible for the sluggish kinetics of HER/HOR at high pH. If solvent molecules can move most freely when the field strength is minimized, the potential of zero charge should correlate to faster water rearrangement, a higher frequency of the Arrhenius prefactor, and lower activation barriers.

Here, we investigate through single-crystal voltammetry the importance of the interfacial electric field strength on hydrogen reaction kinetics using three complementary approaches: (1) correlating the potential of zero free charge (pzfc) with reaction kinetics using caffeinated Pt as a model surface; (2) varying the electrolyte ionic strength at constant pH; and (3) probing solvent dynamics via kinetic isotope effects. Our results show that pzfc seems to correlate with HER/HOR kinetics, but kinetic isotope effects on caffeinated Pt also suggest that the effect more likely originates from activation barriers to the reaction elementary steps than from electric field effects on solvent dynamics.Although the importance of interfacial phenomena such as the electrode’s pzfc and water reorganization energy are well established, the pzfc may not be a mechanistic descriptor of the alkaline hydrogen reaction kinetics. Surface additives such as caffeine may help elucidate the origin behind the slow alkaline HER/HOR kinetics and ways to manipulate it.