Hexagonal (Cu,Co)Se ₂ Nanoflakes as Effective and Durable Bifunctional Electrocatalyst for Overall Alkaline Water Splitting: Understanding Local Structure Around Active Sites

Electrochemical water splitting is an eco-friendly method for large-scale production of high-purity hydrogen (H₂) and oxygen (O₂), and hence, pioneering the design of efficient and economic bifunctional electrocatalysts is necessary. Here, hexagonal (Cu,Co)Se₂ nanoflakes are fabricated to leverage the unique synergy between copper and cobalt for efficient hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). As HER and OER catalysts, (Cu,Co)Se₂ nanoflakes demonstrate low overpotentials of 104 and 250 mV, respectively, to reach current densities of 10 mA cm^-2 in 1 m KOH solution, with respective Tafel slopes of 117.7 and 61 mV dec^-1. The catalyst requires only 403 mV of overpotential to reach 1000 mA cm^-2 in OER. XANES analyses reveal average oxidation states of Co as 2.6+ and 3+ post-HER and post-OER, respectively, while Cu remains predominantly in 2+ states. Presence of Cu around Co induces orbital rehybridization through linking Se bonds; establishing a cooperative participation between Co and Cu to facilitate overall charge-transfer processes, eventually enhancing catalytic activity of (Cu,Co)Se₂. Further, (Cu,Co)Se₂/NF-based overall-water-splitting electrolyzer offers low cell voltage (1.65V at 10 mA cm^-2) and high durability even at a high current density (at η₅₀) in 1 m KOH, thus demonstrating its commercial application prospects.