Oxygen-tolerant hydrogen evolution in water using Schiff base copper complexes with tuneable secondary coordination environments

Abstract

Green hydrogen is increasingly acknowledged as a pivotal facilitator for the transition to a sustainable and renewable energy paradigm, presenting a clean alternative to conventional fossil fuels. Fundamental to this aspiration is the advancement of efficient and resilient electrocatalysts capable of functioning in aqueous media under ambient conditions, encompassing the presence of atmospheric oxygen. In the ongoing investigation, we elucidate a unique series of water-soluble copper complexes formulated for the electrocatalytic generation of hydrogen in nearly neutral aqueous settings. These complexes, constructed upon a shared N₂O₂ Schiff base architecture, exhibit nuanced modifications within their secondary coordination spheres through ortho-substituents (–H, –OH, and –OMe), thereby facilitating precise optimization of their catalytic efficacy. Among the investigated series, the hydroxyl-substituted complex (–OH) was identified as the most catalytically active, achieving a turnover number (TON) of approximately 1450 at a pH of 5.0, alongside a faradaic efficiency surpassing 80%. Comprehensive mechanistic studies utilizing electrochemical, spectroscopic, and spectroelectrochemical methodologies elucidated the essential role of the Cu(II/I) redox couple and the impact of proton-shuttling functional groups in augmenting hydrogen evolution activity. Significantly, these catalysts preserved their structural integrity and catalytic efficacy under aerobic and mildly acidic conditions, underscoring their applicability in practical scenarios. To conclude, the results of the study yield important revelations regarding the formulation of efficient, cost-effective, and oxygen-resilient copper-based catalysts, thereby setting the stage for their implementation in scalable green hydrogen production frameworks.