Electrochemically Rechargeable Liquids in Highly Flexible Energy Storage Systems

Abstract

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. H2 can be used in a wide variety of both stationary and transportation applications, as envisioned by DOE’s H2@Scale program [1]. Although DOE has not yet established targets for stationary H2 storage, the key metrics for an “ideal hydrogen carrier” are [2]: liquid at all operating temperatures, high energy density, efficient conversions at relatively benign conditions, long cycle life, compatible with existing infrastructure for fuels, low cost, and safety (toxicological and eco-toxicological). Relative to thermal conversions, electrochemistry can enable relatively efficient conversions at more benign conditions. While electrochemically-rechargeable liquids (ERLs) are not a new idea, the focus of this talk will be on innovative conversion options, which includes 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 H2 or electricity on demand can enable highly flexible energy-storage systems that can potentially be used for both stationary and transportation applications. In short, an ERL can provide essential “energy-banker services” by simultaneously converting and storing these two major “energy currencies” of the future in a single aqueous liquid. These systems can be readily scaled up or scaled down. This talk will also present a very brief overview of potentially attractive ERL options, which can 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) [3]. The current status of this technology will be briefly reviewed, including a summary the key technical barriers, as well as some enabling concepts to potentially address each of these challenges.The goal will be to provide the audience with a sufficient overview of these ERL-conversion system concepts, such that a productive discussion can occur on additional improvements, potential end uses, and future collaborations. [1] B. Pivovar, N. Rustagi, and S. Satyapal, ECS Interface, 27 (2018) 47. [2] T. Autry and M. Bowden, “HyMARC; H2 Carriers for Bulk H2 Storage & Transport,” AMR slides, ST204 (2020). [3] M. L. Perry & Z. Yang, J. Electrochem. Soc., 166 (2019) A3045. Figure 1