Charge storage on nanostructured early transition metal nitrides and carbides

[Display omitted] ► Capacitances in excess of 200Fg−1 for phase-pure VN and γ-Mo2N in KOH and H2SO4, respectively. ► Electrolyte ion isolation experiments used to identify key contributors to charge storage. ► Ion isolation results unambiguously link OH− and H+ to pseudocapacitance for VN/KOH and Mo2N/H2SO4, respectively. Phase pure, nanostructured V, Mo and W nitrides and carbides were synthesized and characterized in aqueous KOH and H2SO4 electrolytes. Capacitances for most of the materials exceeded that expected for double layer charging and suggested a pseudocapacitive storage mechanism. With the exception of β-Mo2C, the materials were stable in KOH and/or H2SO4 electrolytes. Capacitances for VN in KOH and γ-Mo2N in H2SO4 were the highest and exceeded 200Fg−1. The charge-storage species were interrogated using an ion isolation method. The results provide unambiguous evidence that OH− was the principal charge storage species responsible for the pseudocapacitance demonstrated for VN in KOH while H+ was the principal species for γ-Mo2N in H2SO4.