Rational Synthesis of Rare-Earth-Doped Co3O4 Nanostructures: Insights into Eu3+-Induced Oxygen Vacancies for Enhanced Battery-Type Hybrid Supercapacitor Performance

Cobalt oxide (Co3O4) has emerged as a promising electrode material for energy storage due to its high theoretical capacity. However, its practical application is limited by poor electrical conductivity, sluggish charge transfer kinetics, and restricted electrochemical activity. To overcome these limitations, rare-earth (RE) doping has been explored as a strategy to tailor the physicochemical properties of Co3O4 and enhance its electrochemical performance. In this study, RE-doped Co3O4 nanostructures with abundant oxygen vacancies were synthesized via a simple and cost-effective solution combustion method. Cerium (Ce), europium (Eu), and dysprosium (Dy) were individually incorporated as dopants to evaluate their impact on the physicochemical and electrochemical properties. Among the dopants, Eu-doped Co3O4 demonstrated the most remarkable performance, attributed to a higher concentration of oxygen vacancies that facilitated enhanced charge transport and ion diffusion. These vacancies were identified and confirmed through Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and photoluminescence (PL), further supported by electron paramagnetic resonance (EPR) analysis. Further optimization was achieved by varying Eu doping concentrations (denoted as Eu1- and Eu2-Co3O4), with Eu2-Co3O4 exhibiting the highest specific capacity of 668.8 C g–1, significantly outperforming pristine Co3O4 (93 C g–1). Other RE-doped samples, including Eu1-Co3O4 (302.2 C g–1), Dy-Co3O4 (284.5 C g–1), and Ce-Co3O4 (252.8 C g–1), also showed considerable improvements in redox activity and charge storage capability. At a high current density of 10 A g–1, Eu2-Co3O4 retained 76% of its capacity, compared to only 65% for the undoped counterpart. Moreover, a hybrid supercapacitor (HSC) assembled with Eu2-Co3O4 and activated carbon (Eu2-Co3O4//AC) delivered a high energy density of 55.5 W h kg–1 and a power density of 999.5 W kg–1, along with excellent cyclic stability (93% retention). These results highlight Eu2-Co3O4 as a highly efficient electrode material, demonstrating strong potential for next-generation hybrid supercapacitors.