Several accepted means exist to quantify pH using calibrated probes. Common options include glass membranes (e.g., in pH probes), colorimetric dyes (e.g., in pH paper), fluorescent molecules (e.g., using microscopy), proton-coupled electron-transfer electrochemical reactions (e.g., as a reversible hydrogen electrode), among others. Nevertheless, predicting and sensing local pH – and more broadly the local electrochemical potential of protons and/or hydroxides – in complex microenvironments is often nontrivial. Confounding factors include use of non-aqueous solvents, differences in local electrostatics, variations in species activity coefficients, and misinterpretation of electrochemical or spectroscopic data. During my talk, I will share several vignettes from our research where measurements were performed to inform about local pH or pOH near an electrified interface. Notably, using ion-exchange membranes, we measured Donnan electric potential differences, illustrating the role that local pH or pOH has on space–charge regions across membrane|solution interfaces. We then used this information to develop a new transient salt injection technique that demystifies the local pH or pOH at an ionomer-coated electrode. Lastly, we developed photoacid molecules that undergo a new photochemical fluorescence sensing mechanism, which enabled the first direct measurements of local pOH in a gas diffusion electrode driving electrochemical CO2 reduction at &gt;0.1 A/cm2. Collectively, our efforts form the foundational framework for increasing accuracy and spatial resolution in quantitative measurements of local species activities in complex microenvironments.

Advances in predicting and measuring local pH and pOH in complex electrochemical microenvironments

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ACS Spring 2023

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