Distinct electron-transfer processes at polymer electrolyte/electrode interfaces: Solvation-mediated versus proton-coupled pathways

Electron transfer processes at polymer electrolyte/electrode interfaces play a central role in modern electrochemical devices of energy conversion, however, current understanding of electron transfers through electrochemical interfaces was established exclusively based on the studies of liquid/solid electrochemical interfaces. Thus, similarities and differences of liquid and polymer electrolyte/electrode interfaces need to be mapped out to guide the design of device level electrochemical interfaces. In this work, we employ the sulfonate adsorption/desorption as a probe reaction to understand the electron-transfer steps in polymer and liquid electrolytes. Through cyclic voltametric investigations on the well-define single-crystal Pd_MLPt(111) electrode, we demonstrate that the oxidative adsorption and reductive desorption of sulfonates at the polymer electrolyte/electrode interface are chemically distinct from those in liquid electrolytes, with the former occurring mostly via the proton-coupled pathway while the latter proceeding mainly through the solvation-mediated pathway. Importantly, the sulfonate adsorption/desorption behaviors of alkylsulfonates become increasingly similar to those in Nafion with longer alkyl chains, suggesting that the interfacial hydrophobicity and solvation environment conferred by the perfluorinated polymer play a decisive role in the electron-transfer mechanism. Results reported in this study highlight the mechanistic distinctions between electron-transfer processes at electrochemical interfaces involving polymer and liquid electrolytes, and provide a framework for understanding electron-transfer processes at polymer electrolyte/electrode interfaces.