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

Abstract Electrochemical water splitting is an eco‐friendly method for large‐scale production of high‐purity hydrogen (H ₂ ) and oxygen (O ₂ ), and hence, pioneering the design of efficient and economic bifunctional electrocatalysts is necessary. Here, hexagonal (Cu,Co)Se ₂ 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)Se ₂ 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)Se ₂ . Further, (Cu,Co)Se ₂ /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 η ₅₀ ) in 1 m KOH, thus demonstrating its commercial application prospects.