Copper Cobalt Selenide as a High-Efficiency Bifunctional Electrocatalyst for Overall Water Splitting: Combined Experimental and Theoretical Study

Nonprecious metal-based catalysts for full water splitting are still being sought after by several groups of researchers, owing to their promising practical application in energy conversion devices. In this article, nanostructured CuCo₂Se₄ comprising earth-abundant elements have been reported to exhibit superior bifunctional electrocatalytic activity for both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) under alkaline conditions. The CuCo₂Se₄ electrocatalyst with a spinel structure type requires low overpotential of 320 mV to reach current density of 50 mA cm^–2 for OER and 125 mV to achieve 10 mA cm^–2 for HER, respectively, which is lower than other reported transition metal chalcogenide electrocatalysts based on Co/Cu, and significantly lower than the well-known precious metal oxide catalysts (IrO_x and RuO_x). To understand the origin of high catalytic performance in CuCo₂Se₄, density functional theory (DFT) has been utilized to study the structural, electronic, and magnetic properties of bulk CuCo₂Se₄ as well as slabs with (100) and (111) surface orientations with and without OH^– adsorption. The theoretical results show that CuCo₂Se₄ is in a metallic state with a high electrical conductivity which plays a crucial role in the catalytic activity. Comparison between Co and Cu revealed that Co sites exhibit better OER catalytic activity. Importantly, a surface enhancement of the local magnetic moment on the Co atoms is found to be limited to the top layer in the (100) slab, whereas such variation of the local magnetic moment affects all layers of the (111) slab, strongly favoring OH^– adsorption on Co atom at the (111) surface and making the (111) surface more catalytically active. The different surface energies of (111) and (100) surfaces were also observed from DFT studies which will have a pronounced influence on the observed catalytic activity of these surfaces.