Investigation of Nitride Electrodes for Electrochemical Energy Storage

Abstract

High surface area vanadium nitrides (VN) have been extensively studied as materials for aqueous supercapacitors due to their 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 currently limited in analysis. Hence, I report 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. I observe that in salt electrolytes, the performance follows the trend: Mg2+ > Li+ > K+ > Na+ > Ca2+. Mg2+ systems provide the best performance at higher scan rates with areal capacitances of 294 μF cm-2 in 1M MgSO4 over a 1.35V operating window at 2000 mV s-1. Furthermore, VN in 1M MgSO4 maintained a 36% capacitance retention from 2 to 2000 mV s-1 compared to 7% in 1M KOH. The capacitance in 1M MgSO4 and 1M MgCl2 increased to 121% and 110% of their original values after 500 cycles and maintained capacitances of 77 and 66 F g-1 at 50 mV s-1 after 1000 cycles, respectively. In contrast, the capacitance decreases (37% capacitance retention) in KOH electrolytes, 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 2e- transfer pseudocapacitive mechanism between Mg2+ and the 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.

Additionally, I also investigated the synthesis and pseudocapacitive performance of multilayer Ti4N3Tx in various aqueous electrolytes. I demonstrate that Ti4N3Tx can be electrochemically activated through continuous cation intercalation over a 10-day period (over 10,000 cycles) to deliver a high pH-universal capacitance. After activation, the capacitance increases by 55% in 1M H2SO4 from 125 to over 190 F g-1, by 125% in 1M MgSO4 from 85 to over 190 F g-1, and by 470% in 1M KOH from 25 to over 150 F g-1 at 2 mV s-1. Each electrolyte also demonstrated a wide voltage window of up to 2.0 V throughout activation. Moreover, the activation process led to a switch in the charge storage mechanism of the material in H2SO4. This switch in the mechanism is likely attributed to hydronium ion intercalation accompanied by changes in the oxidation state of Ti resulting in an evolution from supercapacitor to hybrid capacitor-battery behavior. These findings offer a new and reliable option for energy storage, with potential applications in largescale grid storage and electric vehicles.