Spin-State-Modulated Interfacial Fe ²⁺ in Solid-Solution Heterostructures Promotes H ₂ O ₂ Selectivity and Electro-Fenton Antibiotic Degradation

In heterogeneous electro-Fenton (EF) catalysis, achieving selective H 2 O 2 generation with efficient • OH production remains challenging due to poor oxygen reduction reaction (ORR) selectivity and cycling stability. This study employs an interfacial spin-state engineering strategy to construct CoFe solid solutions/Fe 3 O 4 heterostructures (CoFe ss /Fe 3 O 4 ) through controlled Co doping. Co incorporation induces Fe 0 lattice expansion, forming CoFe ss phases that creates oxygen-bridged heterointerfaces with Fe 3 O 4 (CoFe ss –O–Fe 2+ ), altering the coordination environment and facilitating the Fe 2+ spin-state transition from high-spin ( t 2g 4 e g 2 S = 2) to intermediate-spin ( t 2g 5 e g 1 S = 1) configuration at the interface. This modulation optimizes electron transfer dynamics, steering ORR toward the 2-electron ORR pathway with H 2 O 2 selectivity of 75.9%. Co 0.1 Fe ss /Fe 3 O 4 (with a 0.1 Co/Fe molar ratio) exhibits exceptional tetracycline degradation performance, achieving 95.0% removal within 60 min, outperforming the undoped catalyst. The degradation involves • OH as the dominant reactive species, with • O 2 – and 1 O 2 contributing synergistically to drive tetracycline degradation through hydroxylation, demethylation, and ring-opening pathways. The catalyst demonstrates broad-spectrum pollutant removal and excellent cycling stability, with negligible metal ion leaching. Ecotoxicity assessments confirm that degradation intermediates exhibit low toxicity to aquatic organisms, ensuring high environmental safety. This work elucidates heterometallic doping as an effective spin-state engineering strategy through interfacial electronic reconstruction, providing insights for efficient EF catalysts in antibiotic wastewater treatment.