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). Electrochemically-rechargeable liquids (ERLs) that can be charged using either electrical and/or hydrogen energy, which can also produce H2 or electricity on demand, can enable highly flexible energy-storage systems that may 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 also be readily scaled up or scaled down.This talk will present an 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]. A variety of ERL conversion system options shall also be presented, which includes combinations of the configurations shown in Fig. 1. The current status of this technology shall be briefly reviewed, including a summary of the key technical barriers, as well as some enabling concepts to potentially address each of these challenges. A perspective on potentially attractive end uses for ERLs shall be included, along with some recommendations on key performance indicators (KPIs) for ERLs in these applications. [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,” HFTO Annual Merit Review slides, ST204 (2020). [3] M. L. Perry & Z. Yang, J. Electrochem. Soc., 166 (2019) A3045. Figure 1