Boosting Energy Storage by Sulfite Anion Confinement on Phosphorus-Functionalized Iron Oxide Surfaces

Iron oxide-based electrodes hold significant promise for supercapacitor applications; however, their poor electrical conductivity and sluggish ion transport hinder rapid current response. Additionally, the bulk-phase insertion/deinsertion mechanism induces substantial volume expansion, compromising structural stability. In this study, we introduce a phosphorus-functionalized iron oxide surface that acts as a "bridge" to immobilize sulfite (SO₃^2-) anions. This design enables dual-ion redox reactions at the electrode surface, thereby enhancing Faradaic pseudocapacitance while suppressing bulk-phase redox activity and mitigating volume changes. The formation of a FeP/Fe₂O₃ heterostructure further enhances intrinsic electron transport. As a result, the phosphorus-functionalized Fe₂O₃ nanorod array electrode (Fe₂O₃-P 0.5 h) exhibits a significantly enhanced capacitance of 998.2 mF cm^-2 at 1 mA cm^-2 in 1 M Na₂SO₃ electrolyte─2.3 times higher than that of pristine Fe₂O₃ nanorods. When assembled into an asymmetric supercapacitor (Fe₂O₃-P 0.5 h/Na₂SO₃//Co-MnO₂/Na₂SO₄), the device operates at a voltage window of 2.0 V and achieves an areal energy density of 177.6 μWh cm^-2 at a power density of 1 mW cm^-2.