Boosting large scale capacitive harvesting of osmotic power by dynamic matching of ion exchange kinetics†

Abstract

Osmotic energy is an untapped source of renewable and non-intermittent energy. However the performances of existing recovery technologies – e.g. pressure retarded osmosis (PRO) and reverse electro-dialysis (RED) – remain too low for sustainable industrial development, while the promising nanopore-based systems remain challenging to scale up. In this study, we explore an alternative osmotic energy harvesting methodology, based on a capacitive recovery process, furthermore at the membrane (large) scale, in contrast to nanopore-based investigations. The approach, here coined capacitive reversed electrodialysis, consists of using porous capacitive electrodes made of (low-cost) activated carbon and periodically alternating the flow through the electrodes with high-salt and low-salt water solutions on each side of a (commercial) cation exchange membrane. Periodic switches prevent full capacitive charging of the electrodes and the harvested power is found to be a non-monotonous function of the switching frequency. Theoretical modelling of the process demonstrates that this optimum results from the internal dynamic properties of the membrane, which has to match the switching frequency for maximal power harvesting. We then demonstrate that the power density accordingly reaches 5.3 W m⁻² under a 100-fold salinity gradient over a cm² membrane, surpassing both all large-scale studies in the literature and the commonly accepted (economic) break-even point of 5 W m⁻². This study demonstrates the feasibility of large-scale, high-power density osmotic energy recovery and suggests a systematic avenue for an informed screening of materials, thus providing a viable pathway for sustainable energy solutions.