Plasma–enhanced chemical vapor deposition of Co3O4 thin films as a new approach for improving oxygen evolution activity

The development of efficient and durable electrocatalysts for the oxygen evolution reaction (OER) remains a key challenge in advancing sustainable energy technologies. Among cobalt – based materials, spinel Co 3 O 4 is a widely studied catalyst; however, its bulk form often suffers from poor electrical conductivity and limited accessibility of active sites. In this study, we demonstrate that plasma–enhanced chemical vapor deposition (PECVD), followed by thermal annealing in air, provides an effective route for vacancy engineering in Co x O y thin films inducing both oxygen and cobalt vacancies without relying on foreign dopants or ion–exchange processes. The Co 3 O 4 films demonstrated exceptional OER performance, with the best sample achieving a low overpotential of 343 mV at 10 mA cm –2 and a high capability for Co 4+ formation, a key intermediate in the OER process. Additionally, the films exhibited catalytic activity not only at the surface but also throughout their entire volume, highlighting the significance of deposition time and film thickness in optimizing OER performance. Ab–initio calculations, including Bader charge analysis and density of states evaluations, further elucidate the role of defect chemistry: oxygen vacancies enhance *OH adsorption and drive structural reconstruction, while cobalt vacancies reduce the energy barrier for deprotonation and facilitate the Co 3+ → Co 4+ oxidation process. Overall, this work establishes PECVD combined with thermal treatment as a powerful and scalable strategy for tailoring defect concentrations in cobalt oxide catalysts. The approach offers valuable insights into the rational design of high–performance OER catalysts, with broad implications for electrochemical energy conversion technologies. • PECVD enables vacancy engineering for efficient cobalt-based OER catalysts. • Catalysts formed by PECVD and annealing refine defect structure across the thin film • High OER performance is linked to oxygen and cobalt vacancies in the thin films • OER overpotential decreases with longer deposition times • Obtained high Co³⁺ → Co⁴⁺ current density is linked to structural defects • Coexistence of cobalt and oxygen vacancies leads to OER improvement.