What Structural Features Make Porous Carbons Work for Redox-Enhanced Electrochemical Capacitors? A Fundamental Investigation

The addition of redox-active molecules into electrochemical-capacitor electrolytes provides increased specific energy density. Here we illustrate the underlying operational mechanisms and design principles for carbons with hierarchical pore sizes in the micropore (0.6-2 nm) to mesopore (2-3 nm, 5-30 nm) range as electrode materials in redox-enhanced electrochemical capacitors. When using iodide as a model redox additive, we discover that the redox capacity is correlated to the pore volume of the carbon electrodes when void space is included. The fastest rates are typically observed with pore-sizes >1 nm, while slow self-discharge requires pores <1 nm. When used without an ion-selective-membrane separator, the delivered capacity correlated with the quantity of redox species held within the carbon. A commercial microporous carbon, MSC30, with substantial hierarchy in pore size, including small <0.8 nm pores and larger 1.1-3 nm pores, showed the best overall performance, illustrating key design principles.

Automated summary

The addition of redox-active molecules into electrochemical-capacitor electrolytes can increase specific energy density, with carbon materials designed for redox-enhanced electrochemical capacitors typically featuring hierarchical pore sizes and a correlation between redox capacity and pore volume. Performance is generally best with pore sizes >1 nm, but slow self-discharge requires pores <1 nm, illustrating key design principles. Commercial microporous carbons with a range of pore sizes, such as MSC30, demonstrate the best overall performance when it comes to holding redox species and delivering capacity.