Redox Additive Electrolytes for Supercapacitors: A Mini-Review on Recent Developments and Future Directions

Supercapacitors are promising energy storage devices that combine high power density, fast charge/discharge rates, and excellent cycling stability. However, their relatively low energy density compared to batteries remains a major challenge. To address this limitation, redox additive electrolytes have emerged as a key strategy to introduce reversible Faradaic reactions, significantly enhancing the energy storage capacity of supercapacitors. This mini-review systematically summarizes recent advancements in the use of redox-active species across aqueous, non-aqueous, and solid-state/gel electrolytes. We highlight the role of both inorganic and organic redox additives, detailing their mechanisms, advantages, and limitations in improving energy density and stability. Furthermore, we discuss the challenges associated with redox species, such as solubility, long-term stability, and parasitic side reactions, which hinder their practical applications. Future research directions are proposed to optimize redox-active materials and electrolyte systems, aiming to develop next-generation supercapacitors with superior energy density, extended cycling life, and enhanced applicability.