Dong-Yang Feng

Amorphous Mixed-Valence Vanadium Oxide/Exfoliated Carbon Cloth Structure Shows a Record High Cycling Stability

Previous studies show that vanadium oxides suffer from severe capacity loss during cycling in the liquid electrolyte, which has hindered their applications in electrochemical energy storage. The electrochemical instability is mainly due to chemical dissolution and structural pulverization of vanadium oxides during charge/discharge cyclings. In this study the authors demonstrate that amorphous mixed-valence vanadium oxide deposited on exfoliated carbon cloth (CC) can address these two limitations simultaneously. The results suggest that tuning the V4+ /V5+ ratio of vanadium oxide can efficiently suppress the dissolution of the active materials. The oxygen-functionalized carbon shell on exfoliated CC can bind strongly with VO x via the formation of COV bonding, which retains the electrode integrity and suppresses the structural degradation of the oxide during charging/discharging. The uptake of structural water during charging and discharging processes also plays an important role in activating the electrode material. The amorphous mixed-valence vanadium oxide without any protective coating exhibits record-high cycling stability in the aqueous electrolyte with no capacitive decay in 100 000 cycles. This work provides new insights on stabilizing vanadium oxide, which is critical for the development of vanadium oxide based energy storage devices.

Tri-layered graphite foil for electrochemical capacitors

Free-standing carbon structures are promising electrode materials for electrochemical capacitors. However, these electrodes usually have small mass that limits the amount of energy that can be stored. Increasing the electrode mass typically leads to reduction of gravimetric capacitance and rate capability due to the sluggish mass transfer kinetics and increased internal resistance. It has been a challenge to improve both specific capacitance and rate capability of an electrode with high mass. Here we demonstrate a new method to convert graphite foil (8.5 mg cm−2) with a compact layered structure into a unique tri-layered structure that consists of a top layer of partially exfoliated graphene sheets, a middle layer of intercalated graphite sheets and a bottom layer of graphite. This unique structure shows enhanced ion accessible surface area and pseudocapacitance. The seamless connection between the three layers ensures efficient electron transport across the electrode. The tri-layered graphite foil electrode delivers an excellent capacitance of 820 mF cm−2 at 5 mA cm−2 (corresponding to 96.5 F g−1), which is 400 times higher than the untreated foil. Moreover, it retains 75% capacitance when the current density is increased from 5 to 100 mA cm−2. These values are among the best values reported for carbon-based electrodes with comparable mass.

High Mass Loading MnO2 with Hierarchical Nanostructures for Supercapacitors

Metal oxides have attracted renewed interest as promising electrode materials for high energy density supercapacitors. However, the electrochemical performance of metal oxide materials deteriorates significantly with the increase of mass loading due to their moderate electronic and ionic conductivities. This limits their practical energy. Herein, we perform a morphology and phase-controlled electrodeposition of MnO2 with ultrahigh mass loading of 10 mg cm-2 on a carbon cloth substrate to achieve high overall capacitance without sacrificing the electrochemical performance. Under optimum conditions, a hierarchical nanostructured architecture was constructed by interconnection of primary two-dimensional ε-MnO2 nanosheets and secondary one-dimensional α-MnO2 nanorod arrays. The specific hetero-nanostructures ensure facile ionic and electric transport in the entire electrode and maintain the structure stability during cycling. The hierarchically structured MnO2 electrode with high mass loading yields an outstanding areal capacitance of 3.04 F cm-2 (or a specific capacitance of 304 F g-1) at 3 mA cm-2 and an excellent rate capability comparable to those of low mass loading MnO2 electrodes. Finally, the aqueous and all-solid asymmetric supercapacitors (ASCs) assembled with our MnO2 cathode exhibit extremely high volumetric energy densities (8.3 mWh cm-3 at the power density of 0.28 W cm-3 for aqueous ASC and 8.0 mWh cm-3 at 0.65 W cm-3 for all-solid ASC), superior to most state-of-the-art supercapacitors.