Previous research on iron oxides/hydroxides has focused on the crystalline rather than the amorphous phase, despite that the latter could have superior electrochemical activity due to the disordered structure. In this work, a simple and scalable synthesis route is developed to prepare amorphous FeOOH quantum dots (QDs) and FeOOH QDs/graphene hybrid nanosheets. The hybrid nanosheets possess a unique heterostructure, comprising a continuous mesoporous FeOOH nanofilm tightly anchored on the graphene surface. The amorphous FeOOH/graphene hybrid nanosheets exhibit superior pseudocapacitive performance, which largely outperforms the crystalline iron oxides/hydroxides-based materials. In the voltage range between −0.8 and 0 V versus Ag/AgCl, the amorphous FeOOH/graphene composite electrode exhibits a large specific capacitance of about 365 F g⁻¹, outstanding cycle performance (89.7% capacitance retention after 20 000 cycles), and excellent rate capability (189 F g⁻¹ at a current density of 128 A g⁻¹). When the lower cutoff voltage is extended to −1.0 and −1.25 V, the specific capacitance of the amorphous FeOOH/graphene composite electrode can be increased to 403 and 1243 F g⁻¹, respectively, which, however, compromises the rate capability and cycle performance. This work brings new opportunities to design high-performance electrode materials for supercapacitors, especially for amorphous oxides/hydroxides-based materials.

There is a pressing need to further increase the energy density of supercapacitors to meet the requirements of next-generation electronic devices. One promising solution is to develop advanced electrode materials with large capacitance at fast charge/discharge rates. Among the newly developed electrode materials for supercapacitors, iron oxides/hydroxides have recently emerged as a promising class of anode materials largely because of their attractive electrochemical performance, source abundance, low price, and environmental friendliness. However, the use of these emerging materials in practical set-ups is unfortunately curtailed by their relatively small surface area and poor electrical conductivity, which could pose detrimental effects on their pseudocapacitive performance. Recently, material scientists and chemists in the energy community have attempted to address these materials’ challenges by taking the advantage of the unique physical and chemical properties of iron oxides/hydroxides at the nanoscale size regime, which are the key topics discussed in this Focus Review. Here, we first summarize recent advances in the development of high-performance iron oxide/hydroxide-based electrode materials, and their use as anode materials in asymmetric supercapacitors. We then highlight and exemplify several effective design strategies, such as architectural design, chemical modification, and multifunctional composites of iron oxide/hydroxide-based electrodes, to further improve their electrochemical properties. In the last section, we discuss the challenges and perspectives in this exciting field, shedding some light on the design of iron oxide/hydroxide-based electrodes for practical applications.

For building high-energy density asymmetric supercapacitors, developing anode materials with large specific capacitance remains a great challenge. Although Fe₂O₃ has been considered as a promising anode material for asymmetric supercapacitors, the specific capacitance of the Fe₂O₃-based anodes is still low and cannot match that of cathodes in the full cells. In this work, a composite material with well dispersed Fe₂O₃ quantum dots (QDs, ≈2 nm) decorated on functionalized graphene-sheets (FGS) is prepared by a facile and scalable method. The Fe₂O₃ QDs/FGS composites exhibit a large specific capacitance up to 347 F g⁻¹ in 1 m Na₂SO₄ between –1 and 0 V versus Ag/AgCl. An asymmetric supercapacitor operating at 2 V is fabricated using Fe₂O₃/FGS as anode and MnO₂/FGS as cathode in 1 m Na₂SO₄ aqueous electrolyte. The Fe₂O₃/FGS//MnO₂/FGS asymmetric supercapacitor shows a high energy density of 50.7 Wh kg⁻¹ at a power density of 100 W kg⁻¹ as well as excellent cycling stability and power capability. The facile synthesis method and superior supercapacitive performance of the Fe₂O₃ QDs/FGS composites make them promising as anode materials for high-performance asymmetric supercapacitors.