Redox-Enhanced Electrochemical Capacitors: Status, Opportunity, and Best Practices for Performance Evaluation

Redox-enhanced electrochemical capacitors (redox ECs) are a class of augmented electric double-layer capacitors utilizing reversible redox reactions of soluble redox couples in the electrolyte. These systems offer increased energy density, efficient power delivery, and simple construction. In this Perspective, we provide an overview of the emerging field of redox ECs, including the current status, advantages, and outstanding problems confronting their development. Our discussion is primarily focused on operating mechanisms and how they affect performance. We also provide a perspective on the advantage of dual-redox ECs and how to improve them based on fundamental design principles including self-discharge suppression strategies. Finally, we comment on best practices for device characterization, suggest performance-reporting protocols for redox ECs, and examine future directions for the field.