Exploring the electrocatalytic oxygen evolution/urea oxidation activity of solution-combusted CuO/MnO2: machine learning insights from OER performance

Abstract

The growing demand for clean energy drives the search for efficient electrocatalysts for water splitting. This study reports the first synthesis of CuO/MnO₂ nanocomposites via solution combustion synthesis (SCS), offering rapid and energy-efficient production with precise control over material properties. Five compositions (100 : 0, 75 : 25, 50 : 50, 25 : 75 and 0 : 100 molar ratios of CuO : MnO₂) were synthesized at a fuel-to-oxidizer ratio of 0.3, having nanostructures with a tunable surface area and oxygen vacancies. X-ray diffraction, electron microscopy, FTIR spectroscopy, and XRD thermal analysis revealed the formation of crystalline phases with composition-dependent morphologies that exhibit stability up to 475 °C. Raman spectroscopy identified the characteristic vibrational modes of CuO and MnO₂, confirming phase purity and crystalline quality. Electrochemical evaluation identified the nanocomposite with an equimolar composition (50 : 50) as the optimal nanocomposite for the oxygen evolution reaction (OER), achieving a remarkably low overpotential (η₁₀₀ = 253 ± 6 mV), small Tafel slope (49 ± 3 mV dec⁻¹), and minimal charge-transfer resistance (0.75 Ω cm²). The bifunctional assessment of the urea oxidation reaction (UOR) showed a lower potential (1.32 V vs. RHE @ 100 mA cm⁻²) but unexpectedly slower kinetics (Tafel = 199 mV dec⁻¹), revealing that surface poisoning by urea intermediates limits the application of the CuO/MnO₂ nanocomposite for dual OER/UOR. Machine learning analysis quantified that surface area and adiabatic temperature contribute comparably to the composition ratio in governing the OER metrics, demonstrating that morphological and synthesis parameters warrant equal consideration in catalyst design strategies. Moreover, the equimolar composition exhibited true synergistic enhancement, while the other compositions demonstrated antagonistic behavior. Comprehensive post-stability characterization after 50 h of chronopotentiometry tests at 100 mA cm⁻² demonstrated the exceptional robustness of the nanocomposite through five independent techniques. XRD confirmed the preservation of the CuO and MnO₂ crystalline phases, Raman spectroscopy showed the maintained metal-oxide peaks, and FTIR spectroscopy revealed the enhanced metal-oxide band intensity, consistent with surface activation. EDS mapping confirmed the preserved morphology, while ICP-OES analysis of the electrolyte showed negligible metal dissolution.