Comparing electrocatalytic oxygen evolution using laser-fabricated NiO and CuO – A look at the influence of fabrication, kinetics and the inevitable phase transitions

Abstract

The oxygen evolution reaction (OER) is the principal energy barrier in water electrolysis, often demanding overpotentials exceeding 400 mV on state-of-the-art catalysts and thus limiting sustainable hydrogen production. Here, we compare nanoporous NiO and CuO electrodes fabricated via ultrashort-pulse laser irradiation (ULPING) directly onto metal foils. By tuning pulse duration (∼150 ps), repetition rate (1.2 MHz), power (10 W) and scan speed (50 mm/s), we produced extensive hierarchical pore networks without binders or post-treatment. SEM and EDX mapping confirm distinct morphologies—elongated, fibrous ridges in CuO vs. globular, cauliflower-like clusters in NiO—and near-stoichiometric metal-to-oxygen ratios, with subsurface non-stoichiometries revealed by XPS (mixed Cu¹⁺/Cu²⁺ and Ni²⁺/Ni³⁺ species, abundant surface hydroxyls). In 1 M KOH, iR-corrected LSVs show NiO achieves 10 mA·cm⁻² at 430 mV and sustains 200 mA·cm⁻² at <800 mV, outperforming CuO (η₁₀ ≈ 511 mV). Tafel slopes of 127 mV/dec (NiO) vs. 168 mV/dec (CuO) reflect faster charge-transfer kinetics, corroborated by exchange current densities (j₀ ≈ 4.1 × 10⁻³ vs. 9.2 × 10⁻³ mA cm⁻²). EIS fits reveal lower R_ct and diffusion impedance for NiO, while representative free-energy diagrams illustrate how transition-state positioning governs kinetic asymmetry. Theory-guide information, such as phase-diagram modeling (Ni–O–H, Cu–O–H at 300 K) predicts stable NiOOH formation under OER conditions, contrasting with metastable Cu(OH)₂ that rapidly reverts to CuO/Cu₂O. Furthermore, this work highlights that transition metal oxides inevitably undergo oxidation-state transitions under applied potential, with the +3 states of Ni and Cu emerging as the most active for OER but prone to over-oxidation and instability at higher potentials. It emphasizes the importance of experimentally and theoretically tracking these dynamic oxidation-state transformations to distinguish between catalytic activity and degradation mechanisms. Future work will focus on operando XRD/XAS to track these dynamic phase changes and DFT-guided defect engineering to further optimize earth-abundant OER catalysts for large-scale green hydrogen.