Electrolyte Structure Governs Formate Oxidation in Water-in-Salt Systems

We disentangle reactant concentration from local structural effects in water-in-salt electrolytes using the formate oxidation reaction on Pt. First, we observe that formate oxidation currents plateau at high concentrations. Using molecular dynamics and NMR spectroscopy, we attribute this observation to ion clustering of the kosmotropic formate reactant, which reduces conductivity and impedes reactant transport. Then, we demonstrate that this limitation can be overcome by introducing a chaotropic anion (perchlorate) that disrupts clustering and facilitates a further increase in formate oxidation currents. However, when perchlorate is introduced in excess, the hydrogen-bonding network is disrupted, which leads to hindered proton transport, local acidification, and enhanced CO poisoning, as evidenced by SEIRAS. Our findings demonstrate a direct link between bulk electrolyte structure and catalytic activity, which can be used to enhance catalytic performance at high reactant concentrations.