Copper-based molecular complexes: next-Gen electrocatalysts for the green hydrogen evolution reaction

Abstract

Electrochemical water splitting is a key technology for sustainable green hydrogen (H₂) production, yet its large-scale operation demands earth-abundant and cost-effective electrocatalysts capable of functioning efficiently with sufficiently low overpotentials with significant durability. In this scenario, copper-based molecular complexes are emerging catalysts which show promising results in the field of electrocatalytic green hydrogen evolution reaction (HER). It can be anticipated that these complexes upon further modification could replace noble-metals in coming years owing to their inexpensiveness, physicochemical tunability and industrial-scale compatibility. This review comprehensively consolidates and analyses the latest advances in copper-based molecular electrocatalysts for the HER, mainly featuring homogeneous complexes composed of polydentate N-donor, N/S-donor and N/O-donor ligands, as well as molecular precursors that are converted into catalytically active heterogeneous Cu species during electrolysis. It further meticulously summarizes various strategic ligand engineering approaches through proton-responsive functional groups and bioinspired coordination motifs, allowing technologically relevant catalytic performances featuring mainly turnover frequencies exceeding 10⁴ s⁻¹ and faradaic efficiencies approaching 98% across pH of ∼2–14. Beyond performance metrics, this review dives deep into mechanistic insights resulting from kinetic analyses, spectroelectrochemical studies, and density functional theory calculations to explicate proton-coupled electron transfer (PCET) pathways involving transient Cu-hydride intermediates or ligand-assisted protonation steps that optimize adsorption–desorption energetics for faster kinetics. Particular emphasis is placed on the technological translation of molecular catalysts into practical systems, including catalyst immobilization on conductive substrates, molecular-to-heterogeneous catalyst transformation and integration within electrode architectures relevant to overall water electrolyzers. Crucial challenges such as catalyst stability, structural evolution under operating conditions and the need for standardized benchmarking protocols are explicitly discussed. Collectively, this review provides a roadmap for rationally designing robust Cu-molecular electrocatalysts and highlights their potential to enable cost-effective and scalable green H₂ production technologies through electrolysis.