(407e) Effects of Surface Additives and Electrolyte pH on Potential of Zero Charge and Electrochemical Hydrogen Kinetics

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, higher frequency of the Arrhenius prefactor, and lower transition state barriers. Our previous work has used single-crystal voltammetry to study the importance of the interfacial electric field strength on hydrogen reaction kinetics. By correlating the pzfc with reaction kinetics, varying the electrolyte ionic strength at constant pH, and probing solvent dynamics via kinetic isotope effects (KIEs), we found that pzfc seems to correlate with HER/HOR kinetics, but also suggest that the effect more likely originates from activation barriers to the reaction elementary steps than from electric field effects on solvent dynamics. Here, we further explore the role of surface additives such as caffeine on solvent dynamics and overall hydrogen electrocatalysis. Our work elucidates the role of “beyond adsorption” phenomena on importance for activity and selectivity in heterogeneous electrochemical transformations.