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

Abstract

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.

Understanding Synthesis–Structure–Performance Correlations of Nanoarchitectured Activated Carbons for Electrochemical Applications and Carbon Capture

“…g) Reproduced with permission. [ 74 ] Copyright 2021, American Chemical Society. h‐k) Reproduced with permission.…”

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Non-Debye impedance and relaxation models for dissipative electrochemical capacitors

“…Electrochemical capacitors are energy storage devices relying on the very large electric double-layer capacitance at the porous electrode/electrolyte interface for electrostatic charge storage, and/or fast and reversible faradic redox reactions for pseudocapacitive storage 1,2 . Their capacitance, potential window, energy and power capabilities are dependent on several geometrical and physical parameters, including the surface area, type and microstructural complexity of the electrodes being used, interfacial charge absorption/transfer, ions electrodiffusion and migration dynamics, types of supporting electrolyte and ionic strength, etc 3 .…”

Elaborating the Iodine/Polyiodide Equilibrium Effects in Nanoporous Carbon‐based Battery Electrode via Extreme Mass Asymmetry in Hybrid Cells

“…The shuttling of I 3 − and I 5 − is the main reason of performance loss in these hybrid supercapacitors. One way to properly store iodine and to tackle the issue of shuttling is to use carbons with different pores [9–11] . Due to very fast iodide/iodine redox reaction, there is also a high risk of polyiodides generation – as soon as iodine is electrodeposited under electrochemical polarization, it is converted to I 3 − and I 5 − [12] .…”

“…One way to properly store iodine and to tackle the issue of shuttling is to use carbons with different pores. [ 9 , 10 , 11 ] Due to very fast iodide/iodine redox reaction, there is also a high risk of polyiodides generation – as soon as iodine is electrodeposited under electrochemical polarization, it is converted to I 3 − and I 5 − . [12] According to the equations 3 and 4 and as shown in scheme 1 , an equilibrium tends to establish between the solid iodine and the polyiodides in presence of free iodide ions in the electrolyte.…”