Superior cyclability of high surface area vanadium nitride in salt electrolytes.
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abstract
High surface area vanadium nitrides (VNs) have been extensively studied as materials for aqueous supercapacitors due to the high initial capacitance in alkaline media at low scan rates. However, low capacitance retention and safety limit their implementation. The use of neutral aqueous salt solutions has the potential to mitigate both of these concerns, but is limited in analysis. Hence, we report on the synthesis and characterization of high surface area VN as a supercapacitor material in a wide variety of aqueous chlorides and sulfates using Mg2+, Ca2+, Na+, K+, and Li+ ions. We observe the following trend in the salt electrolytes: Mg2+ > Li+ > K+ > Na+ > Ca2+. Mg2+ systems provide the best performance at higher scan rates with areal capacitances of 294 F cm-2 in 1 M MgSO4 over a 1.35 V operating window at 2000 mV s-1. Furthermore, VN in 1 M MgSO4 maintained a 36% capacitance retention from 2 to 2000 mV s-1 compared to 7% in 1 M KOH. Capacitance in 1 M MgSO4 and 1 M MgCl2 increased to 121% and 110% of their original values after 500 cycles and maintained capacitances of 589 and 508 F cm-2 at 50 mV s-1 after 1000 cycles, respectively. In contrast, in 1 M KOH the capacitance decreases to 37% of its original value, reaching only 29 F g-1 at 50 mV s-1 after 1000 cycles. The superior performance of the Mg system is attributed to a reversible surface 2 e- transfer pseudocapacitive mechanism between Mg2+ and VNxOy. These findings can be used to further the field of aqueous supercapacitors to build safer and more stable energy storage systems that can charge quicker compared to KOH systems.
