Electrochemical Kinetics of Nanostructured Nb2O5 Electrodes

Abstract

Pseudocapacitive charge storage is based on faradaic charge-transfer reactions occurring at the surface or near-surface of redox-active materials. This property is of great interest for electrochemical capacitors because of the substantially higher capacitance obtainable as compared to traditional double-layer electrode processes. While high levels of pseudocapacitance have been obtained with nanoscale materials, the development of practical electrode structures that exhibit pseudocapacitive properties has been challenging. The present paper shows that electrodes of Nb2O5 successfully retain the pseudocapacitive properties of the corresponding nanoscale materials. For charging times as fast as one minute, there is no indication of semi-infinite diffusion limitations and specific capacitances of 380 F g−1 and 0.46 F cm−2 are obtained in 40-μm thick electrodes at a mean discharge potential of 1.5 V vs Li+/Li. In-situ X-ray diffraction shows that the high specific capacitance and power capabilities of Nb2O5 electrodes can be attributed to fast Li+ intercalation within specific planes in the orthorhombic structure. This intercalation pseudocapacitance charge-storage mechanism is characterized as being an intrinsic property of Nb2O5 that facilitates the design of electrodes for capacitive storage devices. We demonstrate the efficacy of these electrodes in a hybrid electrochemical cell whose energy density and power density surpass that of commercial carbon-based devices.