Self-discharge is known to have considerable adverse effects on the performance and application of electrochemical capacitors (ECs). Thus, obtaining an understanding of EC self-discharge mechanism(s) and subsequent derivation and solution of EC models, subject to a particular mechanism or combination of mechanisms during charging, discharging and storage of the device, is the only way to solve problems associated with EC self-discharge. In this review, we summarize recent progress with respect to EC self-discharge by considering the two basic types, electric double-layer capacitors (EDLC) and pseudocapacitors, and their hybrids with their respective charge storage mechanisms, distinguishable self-discharge mechanisms, charge redistribution and charge/energy loss during self-discharge. It was clearly observed that most of the voltage reduction is not purely due to the self-discharge effect but is basically due to redistribution of charge carriers deep inside pores and can therefore be retrieved from a capacitor during long-time discharging. Tuning the self-discharge rate is therefore feasible for single-walled carbon nanotube (SWNT) ECs and can be achieved by simply adjusting the surface chemistry of the nanotubes. The effects of surface chemistry modification on EC self-discharge are very important in studying and suppressing the self-discharge process and will benefit potential applications of ECs with respect to energy retention. Self-discharge can be averted by the use of redox couples that are transformed to insoluble species via electrolysis and adsorbed onto the activated carbon electrode in redox-couple EDLCs, thus transforming the EDLC electrolyte into a material that can store charge. Self-discharge in ECs can also be successfully suppressed by utilizing an ion-interchange layer (ion-exchange membrane), separator or CuSO4 mobile electrolyte that can be converted into an insoluble species by electrolysis during the charge/discharge process. This will help in producing a modern-day blueprint for ECs with high capacitance and improved energy sustainability.

The energy storage performance of one of the lightest-known MXenes, Ti 2 CT x (MX) combined with carbon nanospheres (CNS) has been investigated as a symmetric electrode system in an aqueous electrolyte (1 M Li 2 SO 4 ). The energy storage properties were interrogated using cyclic voltammetry (CV), galvanostatic cycling with potential limitation (GCPL), electrochemical impedance spectroscopy (EIS) and voltage-holding tests. The combined material (MX/CNS) demonstrated a higher specific capacity compared to each of the individual components. The material was fabricated with relatively high and low mass loadings, assembled into a symmetric device and performance compared. Specific capacitance, specific power and specific energy for the lower electrode mass loading of 180 F·g −1 , 37.6 kW·kg −1 and 14.1 W·h·kg −1 were all higher than 86 F·g −1 , 20.1 kW·kg −1 and 6.7 W·h·kg −1 for the higher mass loading. A wide voltage window of 1.5 V was obtained, but with limited long-term cycling behavior, suggesting the need for future improvement. Mathematical modelling and simulation of the supercapacitor showed good correlation with the experimental results, validating the model. The results reveal the potential of the Ti 2 CT x to be employed as a viable energy storage system for lightweight applications. As part of the increasing role of clean energy technologies, electrochemical capacitors (ECs) are continuously evolving components that contribute to meeting the demands of electronic apparatuses and systems and are projected to be even more significant in the future.1 In 2011, Gogotsi and co-workers, 2 first synthesized a two-dimensional (2-D) layered material called MXene, by selectively etching aluminum (Al), using hydrofluoric acid (HF) from the MAX phase material. MAX phase materials are layered ternary carbide and nitride materials with the general chemical formula of M n+1 AX n (where 'M' represents an early transition metal, 'A' represents a group IIIA or IVA element, 'X' is C and/or N, and n may equal 1, 2, or 3). There are more than 70 different MAX phase compositions currently known. 3,4 MXenes are formed when the 'A' element is etched out of the MAX phase material, and subsequently have a general formula of M n+1 X n T x (where T represent surface functional groups such as F, OH or O, and x is the number of functional groups that are attached to the surface following the etching process).5 This newly discovered family of materials has similar properties to graphene with good electronic conductivity and different surface terminations enabling the possibility to manipulate their properties to fit different applications. 2-D materials potentially have large electroactive surfaces and therefore attract research attention. MXenes are currently studied, both theoretically and experimentally, as potential electrode materials for energy storage devices such as batteries [6][7][8][9][10][11][12][13][14][15][16] and ECs 9,14,17-32 as well as other uses. 33,34Although the MXene materials do not possess such a high surface ...