R-017

Steffen Hardt

s.hardt@lic.leidenuniv.nl

Lars J.C. Jeuken, Marc T.M. Koper

Leiden University, The Netherlands

Monitoring Local-pH Effects in Polymer-Coated Gold Electrodes: Accelerating Hydrogen Evolution in Near-Neutral pH Conditions

Regarding current efforts to decrease society´s dependence on carbon-based fuels, the electrochemical hydrogen evolution reaction (HER) by water electrolysis is of outstanding importance. Commercial water electrolysers are, however, operating under very corroding conditions using either strong acids or bases. This requires the use of highly robust catalysts and support materials. Although being able to mitigate the aforementioned limitations, (near)-neutral water electrolysis is far from technological maturity due to low solvent conductivity and poor proton transport towards the electrocatalyst surface. Nature solves that problem by efficient proton hopping through dedicated channels in enzymes, having inspired novel approaches in molecular catalysis. Herein, we extend this strategy to heterogeneous electrocatalysis accelerating the HER on polycrystalline gold by a coating of the protic polymer, polyethylene imine (PEI). The PEI-coated electrodes exhibit a significantly decreased HER onset potential and increased current densities compared to bare gold electrodes and gold electrodes covered with control polymers. We rationalized that by the PEI serving as a highly concentrated, immobilized local buffer (pKa ≈ 7) ensuring a fast proton supply to the gold surface by proton hopping. 

To probe interfacial proton hopping, we equipped the PEI with fluorescent pH reporters and monitored in-situ changes of the local pH by electrochemistry-coupled confocal fluorescence microscopy (EC-CFM). The results were compared to those obtained from non-coated and aprotic-polymer-coated Au electrodes, emphasizing the crucial role of the proton supply in polymer-promoted HER. 

This work shows how polymer-coatings can accelerate proton transport, enabling mild reaction conditions for water electrolysis, but also serve as functional sensing platforms to extract local information. This combination bears great potential for the investigation of numerous pH dependent reaction including heterogeneous, molecular and bioelectrocatalysis.