Understanding performance limitation and suppression of leakage current or self-discharge in electrochemical capacitors: a review

Abstract

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.

o‐Benzenediol‐Functionalized Carbon Nanosheets as Low Self‐Discharge Aqueous Supercapacitors

“…These unwanted reactions induce an activationcontrolled self-discharge mechanism because the overpotential (h)f or this overcharging may presentadriving force for selfdischarge as the potential tends to return to its equilibrium potential, that is, ap otential of zero charge (PZC), [13,14] due to thermodynamic consideration. [17,18] On top of activation-controlled self-discharge and ohmic leakage, due to the presence of H 2 and O 2 impurities as ar esult of the HER and OER, there is ap ossibility of cross-diffusion of H 2 to the positive electrode and O 2 to the negative electrode. In addition, due to the presence of af aradaic charge-transfer process during the HER and OER, ap otential-dependent faradaic resistance may be induced, which is in an inverse relationship with the potential.…”

“…Such faradaic resistance may present an additional contribution towards ohmic leakage,w hichf urther intensifies selfdischarging. [17,18] On top of activation-controlled self-discharge and ohmic leakage, due to the presence of H 2 and O 2 impurities as ar esult of the HER and OER, there is ap ossibility of cross-diffusion of H 2 to the positive electrode and O 2 to the negative electrode. This shuttling of moleculesa cross the electrolyte can present ad riving force towards ac oncentrationcontrolled self-discharge process.…”

A Desolvated Solid–Solid Interface for a High‐Capacitance Electric Double Layer

“…[5][6][7] However, drawbacks from the use of a solid-liquid interface are significant. [10] In contrast, a solid-solid interface between the solid electrolyte and the electrode is able to withstand a high working voltage with promising cyclic ability. [8] Although ionic-liquid electrolytes have wide voltage windows, they lead to a sluggish rate capability due to their large ion size and high viscosity.…”

Optimization of Chemi‐adsorption, EDLC, and Redox Capacitance Through Electro‐precipitation Synthesis of Fe3O4/NiO@rGO/h‐BN for the Development of Hybrid Supercapacitor

“…Hybridization of redox and EDLC materials faces several challenges including the structural compatibility and fine controlled on the stoichiometry of different components ,. The retention of specific capacitance and stability of the EDLC materials are much higher than the redox electrodes . As a result, higher loading of redox component on the EDLC materials may degrade the stability and rate capability of the electrode.…”