Switchable Charge Storage Mechanism via in Situ Activation of MXene Enables High Capacitance and Stability in Aqueous Electrolytes.

The need for reliable renewable energy storage devices has become increasingly important. However, the performance of current electrochemical energy storage devices is limited by either low energy or power densities and short lifespans. Herein, we report the synthesis and characterization of multilayer Ti4N3Tx MXene in various aqueous electrolytes. We demonstrate that Ti4N3Tx can be electrochemically activated through continuous cation intercalation over a 10 day period using cyclic voltammetry. A wide operating window of 2 V is maintained throughout activation. After activation, capacitance at 2 mV s-1 increases by 300%, 140%, and 500% in 1 M H2SO4, 1 M MgSO4, and 1 M KOH, respectively, while maintaining ∼600 F g-1 at 2 mV s-1 after 50000 cycles in 1 M H2SO4. This activation process is possibly attributed to the unique morphology of the multilayered material, allowing cation intercalation to increase access to redox-active sites between layers. This work adds to the growing repository of electrochemically stable MXenes reported for aqueous energy storage applications. These findings offer a reliable option for reliable energy storage devices with potential applications in large-scale grid storage and electric vehicles.

Superior cyclability of high surface area vanadium nitride in salt electrolytes.

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 µF 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 µF 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.