The Electrode/Electrolyte Interface in MXene-Based Electrochemical Capacitors

Abstract

The operating voltage of electrochemical capacitor (EC) in aqueous medium is considerably limited due to the theoretical stability of water (1.23V). Hence, the delivered energy density of such systems is restricted compared to that of batteries. A traditional approach to tackle this issue is to focus on improving the capacitance rather than the working voltage of ECs. Especially, combining traditionally used carbons with pseudocapacitive materials has been investigated by many researchers. Adopting this strategy improves the delivered capacity of the system, but key metrics such as a power performance and cycle life of ECs may face a serious failure. The slow nature of faradaic reactions affects the frequency response, while the parasitic activities downgrades the favorable cyclability of the device. Hence, a realistic pathway to improve the energy performance of ECs must be taken. Transition metal carbides, carbonitrides, and nitrides (MXenes) are a new family of 2D layered materials. They have the general formula of Mn+1Xn, in which M indicates the transition metal, like Ti, Mo, Nb, etc., and X is carbon or nitrogen. Considering favorable properties such as a high conductivity and the ability of cation intercalation, many ECs designs based on MXenes have been reported. Nevertheless, there is a lack of clear elucidation of the charge storage mechanism in these materials. Here, the interface of electrode-electrolyte was studied to gain a better insight into the hydrogen storage mechanism and overpotential of MXenes (Ti3C2Tx) in acidic and neutral media. It has been found that MXene-based ECs are suffering from an unbalanced capacitive performance between negative and positive electrodes. Especially, a low capacitance in the positive potential range limits the cell voltage to 0.8V and 1.3V in 1M H2SO4 and 1M Li2SO4, respectively. To extend the cell voltage, self-standing composites based on 3D graphene and Ti3C2Tx were designed. Interestingly, it was observed that during the negative polarization, a significant redox couple activity was observed at the surface of 3DG/ Ti3C2Tx composite electrode. The working potential of the positive electrode was enlarged using micro/mesoporous Black Pearl (BP2000) as electrode material. The realized asymmetric cell was operated in a wider potential window. A higher expansion was realized after balancing the charge of both electrodes. The addition of redox-active salt (KI) to the electrolyte medium improved the capacitive performance of the positive electrode, due to the activity of iodide/iodine species. Excessive increase of the positive electrode capacity was further equalized by balancing the mass of electrodes. In the neutral medium, an asymmetric design of the cell based on Ti3C2Tx (negative electrode) and BP2000 (positive electrode) allowed the cell to operate in a broad range of voltage (up to 2V) with a stable cycling (more than 7000 cycles). The prepared electrodes were characterized using different physiochemical techniques, including XRD, XPS, BET, and SEM analyses. For the electrochemical characterization in two and three-electrode cells, electrodes in the form of pellets or self-standing discs were utilized. The performance of EC cells was studied by cyclic voltammetry, galvanostatic charge-discharge, and electrochemical impedance spectroscopy.