(530g) Charge Storage Mechanisms of High-Surface-Area Carbides and Nitrides

High-surface-area early transition-metal carbides and nitrides possess very high capacitances and hold promise for use in high energy density supercapacitors as a consequence of their high surface areas, pseudocapacitive nature and high electronic conductivities. These materials are also stable in a variety of aqueous and non-aqueous electrolytes. Most of the work on carbides and nitrides for supercapacitors has been done in aqueous media. Vanadium nitride (VN) and molybdenum nitride (Mo₂N) have been reported to possess specific capacitances as high as 1340 Fg^-1 in 1.1 V aqueous KOH and 346 Fg^-1 in 0.8 V aqueous H₂SO₄ electrolytes, respectively.

Further development of early transition-metal carbides and nitrides for use in supercapacitors would benefit from a detailed understanding of their charge storage mechanisms. This work aims to characterize the charge storage mechanisms for high-surface-area early transition-metal (Ti, V, Nb, Mo, and W) carbides and nitrides in aqueous electrolytes. We have identified the active ions involved in the charge storage mechanisms (OH^- and H⁺), located the active sites for charge storage, quantified the extent of pseudocapacitance and the amount of inserted active ions, and proposed the pseudocapacitive charge storage mechanisms for these materials.

Given findings regarding the storage mechanism, we hypothesized that protic, non-aqueous electrolytes would enable increased energy densities, given the higher operating voltages afforded by non-aqueous electrolytes. We were able to expand the operating voltage windows for Ti, V, Nb, Mo and W carbides and nitrides (i.e. from 1.1 V to 2.0 V for TiN) using protic ionic liquid electrolytes instead of aqueous electrolytes. For most materials, a three- to four-fold increase in energy density was observed while maintaining similar power density.