Operando Electrochemical Fluorescence Microscopy Highlights Local Buffering and Mass Transport Effects in Polymer-Promoted Electrocatalysis

Abstract Many electrocatalytic reactions proceed via proton-coupled electron transfer (PCET) mechanisms, which can be promoted by protic polymer coatings. However, polymers modulate the local reaction environment in many ways. To deconvolute the effect of local pH in protic polymer coatings from other contributions, we developed a method for operando determination of in-polymer pH values using electrochemical fluorescence microscopy. Polyethylenimine (PEI) was equipped with a fluorescent pH probe, coated on polycrystalline gold, and electrochemical hydrogen evolution was applied as a pH-sensitive model reaction. Temporal changes in the average pH values across the entire thickness of the polymer film were quantified by widefield microscopy, while the spatial distribution of pH profiles within the films was determined by confocal laser scanning microscopy (CLSM). Complementing the experimental results with finite-element modeling identified two polymer-induced effects: (i) local differences in the mass transport of the proton donors and (ii) a pronounced local buffering by the highly concentrated ammonium groups inside the polymer coating. The latter was corroborated by linear sweep voltammetry (LSV): while the buffering PEI coating decreased the onset potential of hydrogen evolution, the control polymers poly(vinyl alcohol) and PiperION, which cannot be deprotonated in the aqueous pH window, did not promote electrocatalysis. These findings highlight the operando pH modulation by protic polymer coatings, and understanding this phenomenon will be crucial for rational design of polymer coatings for PCET reactions.