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.

Progress and prospects in reverse electrodialysis for salinity gradient energy conversion and storage

“…The results from the model calculations indicated that faradaic losses in the battery were mainly due to osmosis which also inhibits the system from returning to its initial state after charging-discharging cycles. With the aim to reduce undesirable osmotic water flux, a non-charged solute (referred to as 'Osmotic Ballast') was added to the dilute electrolyte solution to balance the osmotic pressure between the dilute and concentrated feed solutions[383]. The use of non-charged ballast resulted in a more than 50 % increase in the faradaic energy efficiency of the close-loop concentration battery compared to ballast-free battery operation.Van Egmond et al showed that the thermodynamic efficiency of CGFB increases with increasing current density and decreases with increasing HCC concentration: a maximum value of 75 % was recorded at 0.5 mol kg -1[104].…”

Performance of an environmentally benign acid base flow battery at high energy density

“…Bipolar membranes with high water permeability would be necessary to allow higher discharge current densities. Another way of increasing the discharge current density without causing delamination to occur could be to include osmotic ballast in the acid and base solution to increase the rate in which water is transported out of the bipolar membrane into solution …”

“…Another way of increasing the discharge current density without causing delamination to occur could be to include osmotic ballast in the acid and base solution to increase the rate in which water is transported out of the bipolar membrane into solution. 56 The voltage efficiencies of all experiments, as shown in Figure 4A, are measured to be between 41% and 63%. This is lower compared with other flow batteries and CGFBs.…”

Hybrid membrane distillation reverse electrodialysis configuration for water and energy recovery from human urine: An opportunity for off-grid decentralised sanitation

“…2). Consequently, the system was discharged at a constant current to mimic analogous discharge studies of galvanostatic cycling tests conducted on batteries and concentration gradient flow batteries [[34], [35], [36], [37]]. Constant current discharge experiments were conducted where 1 L of concentrate and diluate were recirculated through the stack until the potential across the stack reversed.…”