High-entropy oxides (HEOs), characterized by their multi-principal element compositions and configurational entropy stabilization, have emerged as a transformative class of materials for catalytic applications. Recent studies have demonstrated that HEOs, such as (Mg, Co, Ni, Cu, Zn)O, exhibit exceptional oxygen evolution reaction (OER) performance with overpotentials as low as 270 mV at 10 mA/cm² in alkaline media, surpassing traditional catalysts like IrO₂ (η = 320 mV). This enhancement is attributed to the synergistic interplay of multiple cations, which optimizes electronic structure and surface reactivity. Density functional theory (DFT) calculations reveal that the presence of five or more cations in HEOs creates a high density of active sites with tailored adsorption energies for intermediates, reducing the energy barrier for OER. Experimental results: OER overpotential = 270 mV, Tafel slope = 45 mV/dec.
The tunable composition of HEOs enables their application in CO₂ reduction reactions (CO₂RR), where they achieve remarkable selectivity and stability. For instance, a quinary HEO catalyst (Fe, Co, Ni, Cu, Zn)O exhibited a CO faradaic efficiency of 92% at -0.8 V vs. RHE, outperforming benchmark Cu-based catalysts (FE = 65%). The high entropy configuration facilitates the stabilization of key intermediates (*COOH and *CO) while suppressing competing hydrogen evolution reactions. In situ X-ray absorption spectroscopy (XAS) confirmed that the dynamic surface reconstruction under electrochemical conditions generates highly active sites with optimized CO₂ adsorption energies. Experimental results: CO faradaic efficiency = 92%, current density = 15 mA/cm².
HEOs also show promise in thermocatalytic applications, particularly in methane combustion and ammonia synthesis. A recent study on a hexanary HEO (La, Ce, Pr, Nd, Sm, Gd)O demonstrated a methane conversion rate of 90% at 400°C, significantly lower than conventional catalysts like Pd/Al₂O₃ (T₉₀ = 500°C). The high entropy-induced lattice distortion enhances oxygen mobility and surface reducibility, critical for activating C-H bonds. For ammonia synthesis under mild conditions (300°C, 1 bar), a quinary HEO catalyst (FeCoNiCuZn)O achieved an NH₃ production rate of 0.8 mmol/g/h with an energy efficiency of 20%, rivaling traditional Fe-based catalysts. Experimental results: CH₄ conversion rate = 90% at 400°C; NH₃ production rate = 0.8 mmol/g/h.
The structural flexibility of HEOs extends to photocatalytic applications, where they exhibit superior performance in water splitting and pollutant degradation. A recent breakthrough involved a quinary HEO photocatalyst ((TiZrHfNbTa)O), which achieved a hydrogen evolution rate of 12 mmol/g/h under visible light irradiation—nearly three times higher than TiO₂-based photocatalysts (4 mmol/g/h). The multi-cation environment narrows the bandgap to ~2.3 eV while maintaining robust charge carrier separation efficiency (>80%). Additionally, HEOs demonstrated exceptional stability over 100 hours of continuous operation without significant activity loss. Experimental results: H₂ evolution rate = 12 mmol/g/h; bandgap = ~2.3 eV.
The scalability and cost-effectiveness of HEO synthesis further enhance their industrial viability. Recent advances in mechanochemical synthesis have enabled the production of gram-scale HEO powders with controlled phase purity at room temperature within just two hours—a significant improvement over traditional solid-state methods requiring high temperatures (>1000°C) and prolonged durations (>24 hours). Life cycle assessments indicate that HEO-based catalysts can reduce manufacturing costs by up to 30% while maintaining superior performance metrics across diverse applications. Experimental results: Synthesis time = 2 hours; cost reduction = ~30%.
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