Redox mediator-enhanced charge storage in dimensionally tailored nanostructures towards flexible hybrid solid-state supercapacitors

Although extensive research has been performed on metal oxide-based supercapacitors during recent years, they remain lacking in their intrinsic conductivity and stability. To resolve this, 1D/2D heterostructure materials are being utilized, which significantly improves the performance and stability of both materials while employing their synergistic advantage consisting of morphologically tuned surfaces and superior electroactive sites. However, the performance remains unsatisfactory due to the sluggish faradaic reaction at the electrode/electrolyte interface. To address this challenge, we combined the synergistic advantage of morphological nanoengineering and the fast reaction kinetics of redox mediators, thus anticipating superior energy storage performance. A novel 1D/2D heterostructure of ZnCo2O4 (ZCO) and GaN was designed and implemented for the first time, and it demonstrated an excellent specific capacitance of 1693 F g-1 in the mixed electrolyte of KOH and K4[Fe(CN)6]. The all-solid-state flexible hybrid supercapacitor delivered an energy density of 92.63 W h kg-1 at a power density of 1287.52 W kg-1, with superb stability and mechanical endurance that outperformed previously reported ZCO-based materials. Additionally, we delineated the underlying mechanism governing the utilization of redox mediators along with morphological nanoengineering, which will facilitate the current development of state-of-the-art energy storage systems.

Redox-Mediator-Enhanced Electrochemical Capacitors: Recent Advances and Future Perspectives

Supercapacitors deliver exceptional power densities, high cycling stability, and inherent safety but suffer from low energy densities. Many methods to enhance the energy density are based on exploring electrode materials with well-developed structures and designing asymmetric systems with wide voltage windows. The energy density is substantially enhanced at the compromise of power density by utilizing the sluggish kinetics of pseudocapacitive materials. Redox-active electrolytes can contribute additional pseudocapacitance from the reactions of redox mediators at the interface, which have attracted increasing attention of researchers. Redox-mediator-enhanced supercapacitors deliver high energy densities while retaining high power densities. This Minireview highlights the recently prominent progresses of single-, dual-, and ambipolar-redox-mediator-enhanced supercapacitors, the challenges they face, and approaches to suppress self-discharge and develop high-concentration redox-active electrolytes for performance promotion.

Improved performance of a CoTe//AC asymmetric supercapacitor using a redox additive aqueous electrolyte

The performance of an ASC with CoTe as the positive electrode can be enhanced by introducing redox additive K₄Fe(CN)₆ into the alkaline electrolyte.