Recent Progress on Copper-Based Electrode Materials for Overall Water-Splitting

Exponential increase in fossil fuel consumption demands an immediate alternative for a sustainable development. Furthermore, fossil fuel combustion releases a large quantity of CO₂ every day. In the quest for an alternative, although hydrogen is found to be a potent fuel with zero carbon waste, bulk-scale hydrogen production via steam reforming or partial oxidation of hydrocarbons produces tons of CO and CO₂ as waste. Perhaps, hydrogen production by means of electrocatalytic water splitting remains a viable and less energy-intensive approach. However, the potential bottlenecks of the water splitting are the large thermodynamic barrier and sluggish kinetics of the oxygen evolution reaction (OER) associated with the hydrogen evolution reaction (HER), a comparatively straight-forward reaction. Efforts over the last few decades have made it possible to design very reactive noble-metal-based catalysts, using Pt, IrO₂, and RuO₂, which dramatically diminish the working potential and enhance the rate of water oxidation. Nonetheless, the scarcity of these rare-earth metals precludes their physical implication and leads to the design of active transition-metal catalysts like CoO_x and FeNi(O)OH as key alternatives. However, copper, a highly conductive and one of the earth's most abundant metals, has not much been explored for electrode materials. Lately, copper-based materials have been employed as successful catalysts for not only the OER and HER (individual half-cell reactions), but also for overall water splitting (OWS) through the design of bifunctional copper catalysts. This review summarizes the recent developments of copper-based electrode materials for electrocatalytic water splitting, with emphasis on OER, HER, and OWS studies. Moreover, Cu materials are categorized by means of counter anions present and based on their catalytic activity (mono- and/or bi-functional behavior). Future scope and challenges to develop active Cu-based materials, as non-noble and earth abundant catalysts for sustainable energy studies, are highlighted.