Interfacial Water Structural Entropy as the Descriptor for pH-dependent HOR/HER Activity on Pt

The transition to renewable energy and variability in solar and wind production rates necessitates the use of an energy intermediary, well suited to acid based electrolyzer and fuel cell devices in an idealized hydrogen economy. However, high material costs have motivated a transition to alkaline based devices, exposing pH dependent performance losses on traditionally ideal catalysts. Understanding reduced hydrogen oxidation and evolution (HOR/HER) activity on platinum with increasing pH is essential from both a fundamental electrocatalysis perspective to the engineering of electrochemical interfaces. Here we show that reducing electrolyte water structural entropy (SStr) to promote hydrogen bonding with benzyl tri-alkylammonium cations and dibenzo crown ethers improves HOR/HER kinetics in 0.1 M KOH by 25%. In situ surface enhanced infrared spectroscopy verifies strengthened water hydrogen bonding through increased ν(O-H) stretching bands between 3000 and 3200 cm-1. Hydrogen bonding allows for facile transfer of H+/OH- between the electrolyte bulk and interface through the Grotthuss mechanism. Our results show that SStr is the descriptor for HOR/HER kinetics and engineering it through substitution and conversion of structure breakers or incorporation of structure makers, can increase HOR/HER kinetics.