3D Macro-Micro-Mesoporous FeC\begin{document}$_{2}$\end{document}O\begin{document}$_{4}$\end{document}/Graphene Hydrogel Electrode for High-Performance 2.5 V Aqueous Asymmetric Supercapacitors

Distinguished from commonly used Fe\begin{document}$_2$\end{document}O\begin{document}$_3$\end{document} and Fe\begin{document}$_3$\end{document}O\begin{document}$_4$\end{document}, a three-dimensional multilevel macro-micro-mesoporous structure of FeC\begin{document}$_2$\end{document}O\begin{document}$_4$\end{document}/graphene composite has been prepared as binder-free electrode for supercapacitors. The as-prepared materials are composed of macroporous graphene and microporous/mesoporous ferrous oxalate. Generally, the decomposition voltage of water is 1.23 V and the practical voltage window limit is about 2 V for asymmetric supercapacitors in aqueous electrolytes. When FeC\begin{document}$_2$\end{document}O\begin{document}$_4$\end{document}/rGO hydrogel was used as the negative electrode and a pure rGO hydrogel was used as the positive electrode, the asymmetrical supercapacitor voltage window raised to 1.7 V in KOH (1.0 mol/L) electrolyte and reached up to 2.5 V in a neutral aqueous Na\begin{document}$_2$\end{document}SO\begin{document}$_4$\end{document} (1.0 mol/L) electrolyte. Correspondingly it also exhibits a high performance with an energy density of 59.7 Wh/kg. By means of combining a metal oxide that owns micro-mesoporous structure with graphene, this work provides a new opportunity for preparing high-voltage aqueous asymmetric supercapacitors without addition of conductive agent and binder.