Novel Iron Oxyhydroxide Lepidocrocite Nanosheet as Ultrahigh Power Density Anode Material for Asymmetric Supercapacitors

A simple one-step electroplating route is proposed for the synthesis of novel iron oxyhydroxide lepidocrocite (-FeOOH) nanosheet anodes with distinct layered channels, and the microstructural influence on the pseudocapacitive performance of the obtained -FeOOH nanosheets is investigated via in situ X-ray absorption spectroscopy (XAS) and electrochemical measurement. The in situ XAS results regarding charge storage mechanisms of electrodeposited -FeOOH nanosheets show that a Li+ can reversibly insert/desert into/from the 2D channels between the [FeO6] octahedral subunits depending on the applied potential. This process charge compensates the Fe2+/Fe3+ redox transition upon charging-discharging and thus contributes to an ideal pseudocapacitive behavior of the -FeOOH electrode. Electrochemical results indicate that the -FeOOH nanosheet shows the outstanding pseudocapacitive performance, which achieves the extraordinary power density of 9000 W kg(-1) with good rate performance. Most importantly, the asymmetric supercapacitors with excellent electrochemical performance are further realized by using 2D MnO2 and -FeOOH nanosheets as cathode and anode materials, respectively. The obtained device can be cycled reversibly at a maximum cell voltage of 1.85 V in a mild aqueous electrolyte, further delivering a maximum power density of 16 000 W kg(-1) at an energy density of 37.4 Wh kg(-1).