Valence state regulation of iron oxide composited with graphene towards negative electrodes in asymmetric supercapacitors

The asymmetric structure expands the voltage window of supercapacitors in aqueous electrolyte solutions, thereby significantly increasing energy density. However, exploring high-performance negative electrode materials is of utmost importance, on the premise that positive electrode materials have been extensively developed. This study explores the homogeneous composition of iron oxide and graphene via in situ laser induction on the precursor film of polyimide foam containing Fe3+ ions. 3D interconnected graphene was prepared under laser direct writing, while Fe3+ ions were thermally reduced to obtain iron oxide (Fe3+/Fe2+). By adjusting the concentration of Fe3+ ions in the precursor polyimide foam and laser parameter settings, the Fe3+/Fe2+ ratio was regulated to exhibit optimal electrochemical performance. When employed as a negative electrode material for assessing the electrochemical performance, it displayed a notable area capacitance of 654.3 mF cm−2 at a current density of 1 mA cm−2. In the assembled aqueous asymmetric supercapacitors, the specific energy density was 33.25 μW h cm−2 at a power density of 750 μW cm−2 and retained 86.8% of its capacitance even after 10 000 charge–discharge cycles. This method offers a new approach for synthesizing laser-induced multivalent compounds and serves as a reference for efficiently producing negative electrodes of an asymmetric supercapacitor.