Highly active macrocyclic nickel(II) complex for hydrogen evolution reaction in neutral aqueous conditions

Hydrogen (H₂) production via water splitting under completely aqueous conditions particularly using an earth-abundant component for both half reactions is very appealing and may reduce our dependence on fossil fuels. In this work, we present a water-soluble molecular nickel(II) complex bearing a macrocyclic N‒heterocyclic carbene (NHC)/pyridine hybrid ligand, ([NiL](PF₆)₂ (L = bis(2‒pyridyl-methylimidazolylidene)methane as a catalyst for hydrogen evolution reaction (HER) under completely aqueous conditions. The catalyst displayed an outstanding catalytic activity for HER in a phosphate electrolyte of neutral pH, as established by a series of techniques. Long-term (11 h) bulk electrolysis at 0.88 V overpotential provides a maximum of 173.5C charge and total ca. 165.08 μmol (∼15 μmol per hour) H₂ with Faradaic efficiency of 98%, while without a catalyst less than 3C was produced within the same time. To the best of our knowledge, the investigation of water reduction with Ni–NHC type catalyst under aqueous conditions has not been reported to date.