Novel Carbene Anchored Molecular Catalysts for Hydrogen Evolution Reactions

Abstract

The development of low-cost molecular electrocatalysts for the HER from water remains scarce. The efficient electrocatalytic hydrogen evolution reactions (HER) with a series of sterically encumbered carbene ligated silver(I), gold(I) and nickel(II) complexes established here demonstrate the potential of molecular catalyst in hydrogen production from water. Tuning the benzannulation on the coumarin-substituted N-heterocyclic carbene (NHC) ligands afforded six new silver(I) (5–10), two new gold(I) (11 and 12), and two reported nickel(II) (13 and 14) NHC complexes, which differ by the steric bulk around the metal atom and the counterion. Benefiting from the desirable structure and appropriate porous morphology, complexes 6, 9, 10, and 11 exhibited significant electrocatalytic HER activity in acidic medium with an overpotential of −226.4, −445, −243 and −310 mV vs RHE, respectively, to drive a current density of 10 mA/cm2 when immobilized on glassy carbon electrode, which is analogous to that of several transition metal-based nanomaterials. The kinetic parameters such as Tafel slope value and exchange current density for the active complexes and the former observation authenticated the Volmer–Heyrovsky mechanism. Theoretical studies advocate the potential of developing novel series of carbene-ligated HER electrocatalysts based on the controlled ligand field following scalable and viable protocols.