Crystalline Copper Selenide as a Reliable Non-Noble Electro(pre)catalyst for Overall Water Splitting

Electrochemical water splitting remains a frontier research topic in the quest to develop artificial photosynthetic systems by using noble metal-free and sustainable catalysts. Herein, a highly crystalline CuSe has been employed as active electrodes for overall water splitting (OWS) in alkaline media. The pure-phase klockmannite CuSe deposited on highly conducting nickel foam (NF) electrodes by electrophoretic deposition (EPD) displayed an overpotential of merely 297 mV for the reaction of oxygen evolution (OER) at a current density of 10 mA cm^-2 whereas an overpotential of 162 mV was attained for the hydrogen evolution reaction (HER) at the same current density, superseding the Cu-based as well as the state-of-the-art RuO₂ and IrO₂ catalysts. The bifunctional behavior of the catalyst has successfully been utilized to fabricate an overall water-splitting device, which exhibits a low cell voltage (1.68 V) with long-term stability. Post-catalytic analyses of the catalyst by ex-situ microscopic, spectroscopic, and analytical methods confirm that under both OER and HER conditions, the crystalline and conductive CuSe behaves as an electro(pre)catalyst forming a highly reactive in situ crystalline Cu(OH)₂ overlayer (electro(post)catalyst), which facilitates oxygen (O₂ ) evolution, and an amorphous Cu(OH)₂ /CuO_x active surface for hydrogen (H₂ ) evolution. The present study demonstrates a distinct approach to produce highly active copper-based catalysts starting from copper chalcogenides and could be used as a basis to enhance the performance in durable bifunctional overall water splitting.