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.