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 (SO 3 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/Fe 2 O 3 heterostructure further enhances intrinsic electron transport. As a result, the phosphorus-functionalized Fe 2 O 3 nanorod array electrode (Fe 2 O 3 –P 0.5 h) exhibits a significantly enhanced capacitance of 998.2 mF cm –2 at 1 mA cm –2 in 1 M Na 2 SO 3 electrolyte─2.3 times higher than that of pristine Fe 2 O 3 nanorods. When assembled into an asymmetric supercapacitor (Fe 2 O 3 –P 0.5 h/Na 2 SO 3 //Co–MnO 2 /Na 2 SO 4 ), 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 .