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

ChemRxiv

Abstract

Many environmentally relevant electrocatalytic reactions proceed via proton coupled electron transfer (PCET) mechanisms, which are not only dependent on highly active surface catalysts, but also on favorable local pH conditions. To that end, protic polymer coatings have been reported to promote PCET-reactions. However, they modulate the local reaction environment in numerous ways obscuring meaningful conclusions on the origin of this promotion. 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. Polyethylene imine (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 of the average pH values throughout the whole thickness of the polymer film were quantified by widefield microscopy, while the spatial distribution of pH-profiles within the films were 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, a control polymer (polyvinyl alcohol), 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.

Authors 6

1. Steffen Hardt Aachen Faculty of Science, Leiden Institute of Chemistry, Leiden University

   Leiden University

   Affiliation as printed

   Faculty of Science, Leiden Institute of Chemistry, Leiden University, Einsteinweg 55, Leiden, The Netherlands, 2333 CC

2. Katherine J. Levey Aachen Faculty of Science, Leiden Institute of Chemistry, Leiden University

   Leiden University

   Affiliation as printed

   Faculty of Science, Leiden Institute of Chemistry, Leiden University, Einsteinweg 55, Leiden, The Netherlands, 2333 CC

3. Mark Aarts Aachen Faculty of Science, Leiden Institute of Chemistry, Leiden University

   Leiden University

   Affiliation as printed

   Faculty of Science, Leiden Institute of Chemistry, Leiden University, Einsteinweg 55, Leiden, The Netherlands, 2333 CC

4. Bonho Koo Aachen Faculty of Science, Leiden Institute of Chemistry, Leiden University

   Leiden University

   Affiliation as printed

   Faculty of Science, Leiden Institute of Chemistry, Leiden University, Einsteinweg 55, Leiden, The Netherlands, 2333 CC

5. Lars J. C. Jeuken Aachen Faculty of Science, Leiden Institute of Chemistry, Leiden University

   Leiden University

   Affiliation as printed

   Faculty of Science, Leiden Institute of Chemistry, Leiden University, Einsteinweg 55, Leiden, The Netherlands, 2333 CC

6. Marc T. M. Koper Aachen Faculty of Science, Leiden Institute of Chemistry, Leiden University

   Leiden University

   Affiliation as printed

   Faculty of Science, Leiden Institute of Chemistry, Leiden University, Einsteinweg 55, Leiden, The Netherlands, 2333 CC