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

Abstract

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 Co₃O₄ 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_xO_y thin films inducing both oxygen and cobalt vacancies without relying on foreign dopants or ion–exchange processes. The Co₃O₄ 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 Co⁴⁺ 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 Co³⁺ → Co⁴⁺ 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.