Triple I: Electrochemically Rechargeable Liquids in Highly Flexible Energy Storage Systems
Authors: Perry Mike L. 
Published: December 2024

Hydrogen and electricity are expected to be the two major energy carriers of the future. Unfortunately, neither can be stored as an ambient-temperature liquid, which is the ideal energy-carrier medium. Electrochemically-rechargeable liquids (ERLs) are a low-maturity energy-storage paradigm that offer many compelling attributes over gases (e.g., H2), solid materials (e.g., batteries), or more conventional liquid fuels [1]. ERLs can be comprised of a wide variety of active materials, which may include conventional liquid-organic hydrogen carriers (i.e., LOHCs, such as organics that are liquids at room temperature, like methylcyclohexane), as well as aqueous solutions (e.g., diluted alcohols) or dissolved solids (e.g., redox-flow-battery electrolytes) [2]. An ERL-based system possesses the key attractive benefits of a redox flow battery (RFB), namely: fully decoupled power and energy (lower capital costs with longer durations, especially with a single, low-cost liquid), long cycle life (due to theoretically-invariant electrodes, which do not undergo morphology changes with charge-discharge cycles), and inherently superior safety (compared to conventional batteries). And, unlike a RFB system, an ERL system only requires the storage of a single liquid, which is especially attractive for long-duration energy storage (LDES) applications (i.e., > 10-h discharge times at rated power). The obvious disadvantage of any rechargeable-liquid paradigm relative to conventional fuels is the need to store and transport both the “charged” and “discharged” materials, but doing so is significantly easier and less expensive than recovering CO2 discharged into the atmosphere from conventional liquid hydrocarbons. The “charged” and “discharged” ERLs can be shipped in two parallel pipelines and the total transmission costs can still be far lower than electricity-transmission lines, since liquid-pipeline costs are an order-of-magnitude less than that of high-voltage electric lines [3], and pipelines are easier to get approved than overhead transmission lines. While ERLs are not a new concept, a primary focus of this talk will be on innovative conversion options and relevant developments since my presentation on this topic at the Fall 2021 ECS meeting[1]. The ERL conversion systems include combinations of the cell configurations shown in Fig. 1. In particular, it will be shown that ERLs that can be charged using either electrical and/or hydrogen energy, and which can also produce hydrogen or electricity on demand can enable highly flexible energy-storage systems that can potentially be used for both stationary and transportation applications, since ERL-based systems can have higher energy densities than state-of-the-art Li-ion battery packs. The goal of this short “Ideas, Interchange, and Initiative” (Triple-I) presentation will be to provide the audience with a sufficient overview of these high-level ERL concepts, such that a productive discussion can occur on additional improvements, potential end uses, and future collaborations.

References

M. L. Perry, “Electrochemically-Rechargeable Liquids in highly Flexible Energy Storage Systems,” ECS Trans., 104, 23 (2021).

M. L. Perry and Z. Yang, “Rechargeable-Liquid Fuel Cells with a Novel Recharging Method: Exploratory Results with IPA and VII/VIII Solutions,” J. Electrochem. Soc., 166, A3045 (2019).

D. DeSantis, et. al., “Cost of long-distance energy transmission by different carriers.” iScience,
24, 103495 (2021).

Figure 1. Possible configurations of ERL-conversion systems during Discharging (A and C) and Charging (B and D) operations: (A) Dehydrogenation of ERL on anode with oxygen-reduction reaction (ORR) on cathode; (B) Hydrogenation of ERL on cathode with hydrogen-oxidation reaction (HOR) on anode; (C) Dehydrogenation of ERL on anode with hydrogen-evolution reaction (HER) on cathode; (D) Hydrogenation of ERL on cathode with oxygen-evolution reaction (OER) on anode. Only acidic systems are depicted, alkaline-chemistry options are also possible.

Figure 1