Buffers play important roles in aqueous electrocatalysis. Beyond maintaining the pH of the solution, they provide added acid/base species that may participate in proton transfer steps to or from the catalyst. The addition of buffer to CoMP11-Ac (a cobalt porphyrin-peptide catalyst) or CoGGH (a cobalt-tripeptide) in 100 % water and KCl leads to a large enhancement of the catalytic current relative to its value in the absence of a buffer. In the hydrogen evolution reaction (HER) catalyzed by CoMP11-Ac, two distinct catalytic regimes are identified as a function of the buffer pKa. In the presence of buffers with pKa ≤ 7.4, a fast catalysis regime limited by diffusion of buffer is reached. The catalytic half-wave potential (Eh) shifts anodically as the buffer pKa decreases from 7.4 to 5.6, proposed to result from fast protonation of CoMP11-Ac following the catalysis-initiating Co(II/I) reduction. With higher-pKa buffers (pKa ≥ 7.7) , a constant Eh is maintained independent of the buffer pKa, consistent with rate-limiting protonation of Co(I) under these conditions. We also report a Brønsted-type linear free energy relationship for the rate of H2-evolution from water with buffer-acid pKa in the HER catalyzed by CoGGH. Our data suggest that HER catalyzed by CoGGH uses water and buffer acid as proton donors for different steps of the catalytic cycle, with buffer acids involved in the rate-determining step, which is likely a concerted 1H+/1e- proton-coupled electron transfer. The values of the rate constants span over two orders of magnitude depending on the buffer-acid pKa, supporting the hypothesis that the acid strength of the proton donor impacts the rate of proton transfer to the catalyst in the rate-determining step. Our results highlight the important roles that buffers play in proton transfer reactions in water and offer a path to enhancing catalyst performance.

Buffer pKa impacts the mechanism of hydrogen evolution catalyzed by cobalt-peptide catalysts

ACS Spring Meeting 2021

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