Structural engineering to maintain the superior capacitance of molybdenum oxides at ultrahigh mass loadings

Capacitance loss with the increase of mass loading, originating from the slow electron and ion migration kinetics through the thick electrode materials, has been the subject of intense investigation in the field of supercapacitors. In this work, we report the preparation of a mixed-valence molybdenum oxide (MoO3−x) electrode with an ultrahigh mass loading of 15.4 mg cm−2 on a functionalized partially exfoliated graphite substrate using a facile electrochemical method. In addition to the highly open graphene nanosheets atop, the unique layered structures of intercalated graphite sheets ensure efficient ionic transport in the entire MoO3−x electrode. The oxygen-containing functional groups on the exfoliated graphene can bind strongly with the MoO3−x via formation of C–O–Mo bonding, which provides a fast electron transport path from graphene to MoO3−x and thus allows high reversible capacity and excellent rate performance. The optimized MoO3−x electrode delivers an outstanding areal capacitance of 4.03 F cm−2 at 3 mA cm−2 with an excellent rate capability which is significantly higher than the values of other molybdenum oxide based electrodes reported to date. More importantly, the areal capacitance increases proportionally with the MoO3−x mass loading, indicating that the capacitive performance is not limited by ion diffusion even at such a high mass loading. An asymmetric supercapacitor (ASC) assembled with an MoO3−x anode delivers a maximum volumetric energy density of 2.20 mW h cm−3 at a volumetric power density of 3.60 mW cm−3, which is superior to those of the majority of the state-of-the-art supercapacitors.