title: Osmotic Ballasts Enhance Faradaic Efficiency in Closed-Loop, Membrane-Based Energy Systems.
abstract: Aqueous processes for energy storage and conversion based on reverse electrodialysis (RED) require a significant concentration difference across ion exchange membranes, creating both an electrochemical potential and an osmotic pressure difference. In closed-loop RED, which we recently demonstrated as a new means of energy storage, the transport of water by osmosis has a very significant negative impact on the faradaic efficiency of the system. In this work, we use neutral, nonpermeating solutes as "osmotic ballasts" in a closed-loop concentration battery based on RED. We present experimental results comparing two proof-of-concept ballast molecules, and show that the ballasts reduce, eliminate, or reverse the net transport of water through the membranes when cycling the battery. By mitigating osmosis, faradaic and round-trip energy efficiency are more than doubled, from 18% to 50%, and 7% to 15%, respectively in this nonoptimized system. However, the presence of the ballasts has a slightly negative impact on the open circuit voltage. Our results suggest that balancing osmotic pressure using noncharged solutes is a promising approach for significantly reducing faradaic energy losses in closed-loop RED systems.
authors.affiliation: Department of Environmental Sciences and Engineering, Gillings School of Global Public Health, The University of North Carolina at Chapel Hill , Chapel Hill, North Carolina 27599, United States.
authors.forename: Ryan S
authors.identifier: 0000-0002-7168-3967
authors.initials: RS
authors.lastname: Kingsbury
authors.affiliation: Department of Environmental Sciences and Engineering, Gillings School of Global Public Health, The University of North Carolina at Chapel Hill , Chapel Hill, North Carolina 27599, United States.
authors.forename: Orlando
authors.identifier: 0000-0002-7018-391X
authors.initials: O
authors.lastname: Coronell
mesh_terms: D007474:Ion Exchange; D008566:Membranes; D009995:Osmosis; D009997:Osmotic Pressure