Buffer-Induced Electrocatalytic Hydrogen Evolution by a Cobalt Pentadentate Complex in Water

Elucidating proton transfer dynamics in water represents one of the most challenging problems in water splitting reactions due to the presence of multiple proton donors, which complicates the overall reaction kinetics. This study examines the impact of buffer pK_a and its concentration on catalytic performance for hydrogen evolution catalyzed by a Co^III complex (1). The results demonstrate that buffer increases the catalytic rate of the hydrogen evolution reaction. This enhanced activity is supported by the number of buffer acids possessing varying pK_a values, with 2-(N-morpholino)ethanesulfonic acid yielding the maximum catalytic current. A linear free energy relationship, a characteristic of a Brønsted-type mechanism, is observed between the buffer's pK_a and catalytic rate constants. This substantiates that the rate-limiting step is controlled by the proton delivery mediated by the buffer acids. Moreover, the observed inhibition in catalytic activity at a higher concentration of buffer reveals the possible binding interaction between buffer and the cobalt center, thereby impeding substrate access. These findings underscore the critical role of buffer identity and its concentration in optimizing the proton-dependent catalytic reactions in water.