Electrochemical energy storage is increasingly important as the world decarbonizes and electrifies. Research in this area requires compromise between properties that are often mutually exclusive, including power, energy, lifetime, efficiency, operating temperature, safety, and cost. Electric double layer capacitors (EDLCs) exhibit high power and outstanding cycle life compared to secondary batteries, but have low specific energy, limiting applications. 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 to add faradaic charge storage. These systems offer increased energy density, efficient power delivery, and simple construction.
We study a range of redox-active electrolytes to clarify operating mechanisms and formalize design rules for high-performance. Our investigations focus on dual-redox ECs, which attain higher energy density by employing a pair of distinct redox couples, with each operating at a different electrode. Simplicity is important both in terms of the mechanistic aspects of the electrochemical redox chemistry and for system cost considerations. A single ionic molecular entity that intrinsically delivers both distinct redox couples is preferable over two separate redox-active electrolytes. In this context, we have identified specific viologen bromide salts as particularly promising aqueous redox-active electrolytes for dual-redox ECs. While charging, Br– is oxidized to Br3– at the positive electrode and the viologen dication (V2+) is reduced to the stable monocation radical (V+•) at the negative electrode. The reverse processes occur during discharge, providing a high-capacity faradaic discharge plateau and attaining an energy density of ~20 Wh/L.
The viologen bromide system shows unusually high Coulombic efficiency and low self-discharge rates for an aqueous redox-EC. This was initially attributed to strong adsorption of the Br3– and V+• to the activated carbon electrodes, but a more detailed analysis confirms that the behavior is due to two electroprecipitation mechanisms. In these mechanisms, each ion acts as a charge-storing redox couple at one electrode and as a complexing agent at the other electrode. The processes are highly reversible and cells show negligible capacity fade even after 20,000 cycles. The devices use conventional activated carbon electrodes, and because crossover is not a concern and self-discharge is suppressed, a simple inexpensive cellulose separator is sufficient and more costly ion-selective membranes are not required.
Based on our understanding of solid complexation in redox ECs, we studied in detail the confinement of charged redox species in porous electrodes with liquid-to-solid phase transitions to mitigate self-discharge. We demonstrate that in addition to viologens, tetrabutylammonium cations induce reversible solid complexation of Br2/Br3–. This mechanism slows cross-diffusion of Br3–, stabilizes the reactive bromine generated during charging, and can be broadly used as a positive electrode to balance the capacity of a wide variety of pseudocapacitive or battery-type negative electrodes.
In the final component of this work we consider the challenges of scaling up these systems for practical application. We address corrosion of metallic current collectors, which is a common device design challenge for high-power aqueous electrochemical energy storage devices, by designing a bipolar pouch cell using electrochemically stable carbon-polymer composite current collectors. In order to show the versatility of this approach, we construct a high-power redox EC/battery hybrid using a zinc metal anode and an activated carbon cathode with tetrabutylammonium-complexed bromide catholyte. This system achieves excellent power and energy performance, with negligible capacity degradation over more than 3000 cycles.
Throughout the dissertation we compare the performance and properties of our dual redox ECs to those of the current state-of-the-art conventional energy storage systems and other redox-enhanced energy storage systems and we close with comments on economic analysis and the future of redox enhanced electrochemical capacitors.
