Electrocatalytic proton reduction by a nickel complex with a pentadentate pyridinophane ligand

Abstract

Development of efficient and stable molecular catalyst for H₂ evolution using earth-abundant materials is one subject of energy research. Inspired by the exceptional performance of [NiFe] hydrogenases in catalyzing H₂ evolution under mild pH conditions, the abundant nickel has emerged as a promising candidate for molecular catalysts design. While significant progress has been made in developing Ni-based molecular catalysts with sulfur or phosphine ligands, polypyridine ligand platforms remain much less explored for Ni-based H₂ evolution, despite demonstrated great promise of polypyridine ligands in cobalt-based H₂ evolution systems. Here, we report a Ni polypyridine complex based on a pentadentate N-methylpyridine-2,11-diaza[3,3]-(2,6)pyridinophane ligand, and evaluate its potential for proton reduction in weakly acidic media, aiming to functionally mimic [NiFe] hydrogenases. Electrochemical studies indicate that it exhibits a high Faradic efficiency of 98 ± 0.97 % for H₂ evolution at a modest overpotential of 540 mV when acetic acid is used as proton donor, highlighting its electrocatalytic activity even under weakly acidic conditions. The catalytic rate displays a first-order dependence on acid concentration, with a scan-rate-independent apparent rate constant of 903.58 ± 25.79 M^-1·s^-1. Unlike most reported Ni-based catalysts that require stronger acids or higher concentrations, this Ni polypyridine complex catalyzes proton reduction efficiently in weakly acidic conditions, representing a notable advancement. Consequently, this work describes a case of a Ni polypyridine molecular HER catalyst that emulates the proton reduction functionality of [NiFe] hydrogenases under mild acidic conditions. This study also expands the scope of polypyridine ligands in nickel catalysis, showcasing their untapped potential for developing stable, cost-effective H₂ evolution catalysts with tunable reactivity under mild conditions.