Electrocatalytic Activity of Delafossite CuCoO2 for Alkaline Oxygen Evolution Reaction and Acidic Hydrogen Evolution Reaction

Electrochemical water splitting requires electrocatalysts that operate efficiently and durably under disparate electrolyte environments. Herein, pristine CuCoO2 particles were synthesized via a hydrothermal route as a single-phase rhombohedral (3R) delafossite structure composed of hexagonal, single-crystalline particles (~2.6 μm) with a uniform elemental distribution. The prepared CuCoO2 was then evaluated as a bifunctional electrocatalyst for the alkaline oxygen evolution reaction (OER) and the acidic hydrogen evolution reaction (HER), with a deliberate separation of electrode-level performance and intrinsic per-site activity. X-ray photoelectron spectroscopy revealed mixed Cu+/Cu2+ and Co2+/Co3+ states, together with signatures of copper and oxygen vacancies, indicating a defect-rich surface chemistry. In 1 M KOH, the CuCoO2 loaded on carbon fiber paper (CFP) delivered an OER overpotential of 404.38 mV at 10 mA/cm2 in 1 M KOH (Tafel slope = 102.39 mV/dec; charge-transfer resistance (Rct) decreased from 19.32 to 5.78 Ω with increasing potential) and an HER overpotential of 246.46 mV at −10 mA/cm2 in 0.5 M H2SO4, with sluggish kinetics (Tafel slope = 429.17 mV/dec; high Rct = ~1.0–1.1 kΩ). Despite this, CuCoO2 exhibited markedly higher intrinsic activity in acidic HER (ECSA = 82.97 cm2; TOF = 0.1432 s−1 at −0.2 V vs. RHE) than in alkaline OER (ECSA = 9.56 cm2; TOF = 0.079 s−1 at 1.5 V vs. RHE), indicating that acidic HER performance is primarily limited by electrode-level microstructural factors. This work provides, to the best of our knowledge, the first evaluation of acidic HER activity of delafossite CuCoO2 and underscores electrode-level microstructural engineering as a key route to better harness its intrinsic activity for practical water electrolysis.