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

## TL;DR

This paper explores how CuCoO2 particles can be used as a bifunctional electrocatalyst for water splitting in both acidic and alkaline conditions.

## Contribution

The study presents the first evaluation of CuCoO2's acidic hydrogen evolution reaction activity and highlights the importance of microstructural engineering.

## Key findings

- CuCoO2 shows higher intrinsic activity in acidic HER than in alkaline OER.
- The acidic HER performance is limited by electrode-level microstructural factors.
- CuCoO2 exhibits mixed oxidation states and defect-rich surface chemistry.

## Abstract

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.