Inhibiting the Hydrogen Evolution Reaction (HER) with Proximal Cations: A Strategy for Promoting Selective Electrocatalytic Reduction

Most electrochemical reduction reactions require protons; however, direct reduction of protons to hydrogen is a common competitive side reaction that lowers the overall yield (Faradaic efficiency) for the desired product. Inhibition of the hydrogen evolution reaction (HER) by fixed, proximal mono- or dication is reported. An SNS tridentate ligand with a pendant aza-crown functionality and the corresponding Pd­(II) and Rh­(I) complexes were synthesized. For the Pd complexes, the presence of a Na+ or Ba2+ ion in the aza-crown increases the overpotential for hydrogen evolution by up to 260 mV compared to a congener that lacks an aza-crown. The increase in the overpotential for HER was observed with cationic and neutral acids in both acetonitrile and dimethylformamide. Additionally, for the Pd complexes, the inclusion of a Ba2+ ion into the aza-crown modestly improved the selectivity for electrocatalytic CO2 reduction to CO, whereas H2 was the only product for the congener lacking the aza-crown. The Ba2+-containing complex also had improved catalytic stability, although all complexes were ultimately unstable under prolonged electrolytic conditions. The increase in the overpotential for HER through the installation of a local charge exemplifies an effective catalyst design strategy for inhibiting the hydrogen evolution reaction.