Temperature-depended ion concentration polarization in electrokinetic energy conversion

Abstract

Previous studies on the electrokinetic energy conversion (EKEC) are limited to the isothermal condition at the environmental temperature. Here effects of temperature and membrane thermal conductivity are systematically investigated. Under isothermal conditions, elevated temperature can improve the electric power while the energy efficiency stays unchanged. Under non-isothermal conditions, at small membrane thermal conductivities, a negative temperature difference contributes to the electric power for dramatically enhanced streaming current as enhanced ion mobility along the streaming direction induces an internal ion concentration polarization (IICP) that generates a co-flow concentration gradient in the nanopore interior. At large membrane thermal conductivities, the positive temperature difference reverses the external ion concentration polarization (EICP) in the solution reservoirs due to the Soret effect, resulting in more obvious electric power improvement. Furthermore, a criterion to enhance the EKEC performance via employing asymmetric temperatures is proposed, and an alternative way to construct the tunable ionic current source is presented. Present study provides guidance for enhancing the EKEC performance by employing waste heat, and fabricating nanofluidic functional devices.