Molecular precursor synthesis of the Rh2O3/Fe2O3 spherical architecture for enhanced acidic HER activity and durability

Abstract

Efficient hydrogen evolution catalysts that minimize noble metal content while maintaining high activity and durability are critically needed for scalable water electrolysis. Here, we introduce a molecular precursor strategy to synthesize intimately intermixed Rh₂O₃/Fe₂O₃ nanocomposites with precisely controlled 1 : 1 metal ratio. Thermal decomposition of heterobimetallic complex [Rh(acac)₃Fe(hfac)₂] (acac = acetylacetonate, hfac = hexafluoroacetylacetonate) at 300 °C yields 3D spherical Rh₂O₃/Fe₂O₃ architectures without high-temperature sintering. Electrochemical evaluation reveals that Rh₂O₃/Fe₂O₃ requires only 32 mV to reach −10 mA cm⁻², dramatically lower than Rh/Rh₂O₃ (140 mV), commercial Rh₂O₃ (260 mV), or α-Fe₂O₃ (210 mV). The Tafel slope investigation of Rh₂O₃/Fe₂O₃ indicates a Volmer–Heyrovsky mechanism with facile proton adsorption and electron transfer, while electrochemical impedance spectroscopy shows its charge-transfer resistance is an order of magnitude lower than that of Rh/Rh₂O₃. Importantly, chronopotentiometry at −10 mA cm⁻² reveals ultrastable performance with no observable decay over 120 hours, highlighting the exceptional long-term stability of Rh₂O₃/Fe₂O₃. Post-stability microscopy exhibits intact spherical architecture with no signs of sintering or Ostwald ripening. By integrating earth-abundant sesquioxide that promotes oxophilicity, oxygen-vacancy generation, and enhanced conductivity, the title Rh₂O₃/Fe₂O₃ catalyst uses less than half the Rh loading of Rh/Rh₂O₃ while delivering both superior activity and unmatched durability. This work establishes that although both individual Rh₂O₃ and Fe₂O₃ oxides exhibit poor HER activity and stability in acidic media, their intimately intermixed nanocomposite delivers dramatically enhanced performance and long-term stability. The reported mixed-oxide electrocatalyst overcomes the intrinsic limitations of single-phase oxides and provides general guiding principles for designing future high-performance mixed-oxide systems.