Unveiling the pseudocapacitive charge storage mechanisms of nanostructured vanadium nitrides using in-situ analyses

The future of energy storage devices relies on energy storage systems with both high energy and power densities. Supercapacitors, a hybrid of capacitors and batteries, have the potential to deliver both high energy and power densities. Materials suitable for supercapacitors are those capable of storing charges via faradaic redox reactions or pseudocapacitive reactions in addition to electrostatic double-layer storage. High-surface-area early transition metal nitrides such as vanadium nitride (VN) have promising properties for use in energy storage devices, especially supercapacitors, due to their pseudocapacitive charge storage mechanisms. The nature of their pseudocapacitive function, however, remains poorly understood. Further development of these materials requires a detailed understanding of their pseudocapacitive charge storage mechanisms. This paper reports a detailed and comprehensive analysis of the pseudocapacitive charge-storage mechanisms of the VN material in aqueous alkaline electrolytes using in-situ small angle neutron scattering (SANS) and in-situ x-ray absorption spectroscopy (XAS) combined with physical and electrochemical characterization techniques. Contrary to conventional wisdom that large pores are more accessible to electrolyte ions and thus store more charge, our results showed that pseudocapacitive charge storage mechanism for VN in aqueous alkaline media is induced by the insertion/extraction of anions (i.e. OH- OD-) in and out of micropores. The anion insertion/extraction leads to the reduction/oxidation of the V metal, as the material was electrochemically charged and dis-charged within its operating potential window. These results suggest that high pseudocapacitances in excess of 1300 Fg(-1) could be achieved by the VN material in 1.2 V aqueous alkaline electrolytes. This work undoubtedly provides a clear and generic conceptual and fundamental characterization approach for energy storage devices and related systems.