Solar thermal water-splitting (STWS) cycles have long been recognized as a desirable means of generating hydrogen gas (H2) from water and sunlight. Two-step, metal oxide-based STWS cycles generate H2 by sequential high-temperature reduction and water reoxidation of a metal oxide. The temperature swings between reduction and oxidation steps long thought necessary for STWS have stifled STWS's overall efficiency because of thermal and time losses that occur during the frequent heating and cooling of the metal oxide. We show that these temperature swings are unnecessary and that isothermal water splitting (ITWS) at 1350°C using the "hercynite cycle" exhibits H2 production capacity >3 and >12 times that of hercynite and ceria, respectively, per mass of active material when reduced at 1350°C and reoxidized at 1000°C.

Electrochemical double-layer capacitors exhibit high power and long cycle life but have low specific energy compared with batteries, limiting applications. Redox-enhanced capacitors increase specific energy by using redox-active electrolytes that are oxidized at the positive electrode and reduced at the negative electrode during charging. Here we report characteristics of several redox electrolytes to illustrate operational/self-discharge mechanisms and the design rules for high performance. We discover a methyl viologen (MV)/bromide electrolyte that delivers a high specific energy of ∼14 Wh kg−1 based on the mass of electrodes and electrolyte, without the use of an ion-selective membrane separator. Substituting heptyl viologen for MV increases stability, with no degradation over 20,000 cycles. Self-discharge is low, due to adsorption of the redox couples in the charged state to the activated carbon, and comparable to cells with inert electrolyte. An electrochemical model reproduces experiments and predicts that 30–50 Wh kg−1 is possible with optimization.

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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.

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Research in electric double-layer capacitors (EDLCs) and rechargeable batteries is converging to target systems that have battery-level energy density and capacitor-level cycling stability and power density. This research direction has been facilitated by the use of redox-active electrolytes that add faradaic charge storage to increase energy density of the EDLCs. Aqueous redox-enhanced electrochemical capacitors (redox ECs) have, however, performed poorly due to cross-diffusion of soluble redox couples, reduced cycle life, and low operating voltages. In this manuscript, we propose that these challenges can be simultaneously met by mechanistically designing a liquid-to-solid phase transition of oxidized catholyte (or reduced anolyte) with confinement in the pores of electrodes. Here we demonstrate the realization of this approach with the use of bromide catholyte and tetrabutylammonium cation that induces reversible solid-state complexation of Br/Br. This mechanism solves the inherent cross-diffusion issue of redox ECs and has the added benefit of greatly stabilizing the reactive bromine generated during charging. Based on this new mechanistic insight on the utilization of solid-state bromine storage in redox ECs, we developed a dual-redox EC consisting of a bromide catholyte and an ethyl viologen anolyte with the addition of tetrabutylammonium bromide. In comparison to aqueous and organic electric double-layer capacitors, this system enhances energy by factors of ca. 11 and 3.5, respectively, with a specific energy of ∼64 W·h/kg at 1 A/g, a maximum power density >3 kW/kg, and cycling stability over 7000 cycles.

The performance of redox-enhanced electrochemical capacitors (redox ECs) is substantially improved when oxidized catholyte (bromide) and reduced anolyte (viologen) are retained within the porous electrodes through reversible counterion-induced solid complexation. Investigation of the mechanism illustrates design principles and identifies pentyl viologen/bromide (PV/Br) as a new high-performance electrolyte. The symmetric PV/Br redox EC produces a specific energy of 48.5 W·h/kgdry at 0.5 A/gdry (0.44 kW/kgdry) with 99.7% Coulombic efficiency, maintains stability over 10 000 cycles, and functions identically when operated with reversed polarity.

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