Optimized composition of non-equimolar high-entropy alloy catalysts for stable and high yield of hydrogen-production using methane pyrolysis

Here, we reports the design, synthesis, and catalytic evaluation of non-equimolar high-entropy alloy (HEA) and high-entropy oxide (HEO) catalysts for CO2-free hydrogen production via methane pyrolysis. Methane decomposition is an attractive hydrogen production route but typically requires very high temperatures and suffers from catalyst deactivation due to carbon deposition. To address these challenges, the current study explores the compositional flexibility and thermodynamic stability of high-entropy materials. A Co-Cr-Cu-Fe-Ni HEA system was compositionally optimized using FactSage thermodynamic calculations by imposing thermodynamic alloy design constraints. The optimized non-equimolar composition (Co24.8Cr15.2Cu8.3Fe34Ni17.7 at.%) was synthesized by vacuum arc melting and ball milling. Controlled oxidation at 550 °C produced the corresponding HEO. The equimolar HEA showed negligible activity, whereas the non-equimolar HEA exhibited moderate methane conversion. Notably, the HEO demonstrated superior reactivity, higher mass loss, and significantly enhanced hydrogen yields across the entire temperature range. Gas analysis revealed that methane decomposition dominates at elevated temperatures, with the HEO achieving substantially higher H2/CO ratios than the HEA. Post-reaction characterization showed uniform carbon coating on HEA surfaces, leading to active-site passivation, whereas HEO promoted directional growth of fibrous carbon. Density functional theory calculations elucidated the underlying mechanism, showing clustered d-band centers in HEO that favor optimal adsorption energies and preferential carbon binding at oxygen sites. The site-specific carbon deposition preserves active metal sites, explaining the enhanced stability and hydrogen yield of HEO catalysts. Overall, the work demonstrates that compositionally optimized nonequimolar HEOs are highly promising catalysts for efficient, stable, and CO2-free hydrogen production via methane pyrolysis