title: Enhanced charge efficiency and reduced energy use in capacitive deionization by increasing the discharge voltage.
abstract: Capacitive deionization (CDI) is an electrochemical method for water desalination using porous carbon electrodes. A key parameter in CDI is the charge efficiency, Λ, which is the ratio of salt adsorption over charge in a CDI-cycle. Values for Λ in CDI are typically around 0.5-0.8, significantly less than the theoretical maximum of unity, due to the fact that not only counterions are adsorbed into the pores of the carbon electrodes, but at the same time coions are released. To enhance Λ, ion-exchange membranes (IEMs) can be implemented. With membranes, Λ can be close to unity because the membranes only allow passage for the counterions. Enhancing the value of Λ is advantageous as this implies a lower electrical current and (at a fixed charging voltage) a reduced energy use. We demonstrate how, without the need to include IEMs, the charge efficiency can be increased to values close to the theoretical maximum of unity, by increasing the cell voltage during discharge, with only a small loss of salt adsorption capacity per cycle. In separate constant-current CDI experiments, where after some time the effluent salt concentration reaches a stable value, this value is reached earlier with increased discharge voltage. We compare the experimental results with predictions of porous electrode theory which includes an equilibrium Donnan electrical double layer model for salt adsorption in carbon micropores. Our results highlight the potential of modified operational schemes in CDI to increase charge efficiency and reduce energy use of water desalination.
authors.affiliation: Wetsus, centre of excellence for sustainable water technology, Oostergoweg 7, 8911 MA Leeuwarden, The Netherlands; School of Chemical and Biological Engineering, Institute of Chemical Processes, Seoul National University, Daehak-dong, Gwanak-gu, Seoul 151-742, Republic of Korea.
authors.forename: T
authors.initials: T
authors.lastname: Kim
authors.affiliation: Wetsus, centre of excellence for sustainable water technology, Oostergoweg 7, 8911 MA Leeuwarden, The Netherlands; Department of Environmental Technology, Wageningen University, Bornse Weilanden 9, 6708 WG Wageningen, The Netherlands.
authors.forename: J E
authors.initials: JE
authors.lastname: Dykstra
authors.affiliation: Wetsus, centre of excellence for sustainable water technology, Oostergoweg 7, 8911 MA Leeuwarden, The Netherlands.
authors.forename: S
authors.initials: S
authors.lastname: Porada
authors.affiliation: Department of Environmental Technology, Wageningen University, Bornse Weilanden 9, 6708 WG Wageningen, The Netherlands.
authors.forename: A
authors.initials: A
authors.lastname: van der Wal
authors.affiliation: School of Chemical and Biological Engineering, Institute of Chemical Processes, Seoul National University, Daehak-dong, Gwanak-gu, Seoul 151-742, Republic of Korea.
authors.forename: J
authors.initials: J
authors.lastname: Yoon
authors.affiliation: Wetsus, centre of excellence for sustainable water technology, Oostergoweg 7, 8911 MA Leeuwarden, The Netherlands; Laboratory of Physical Chemistry and Colloid Science, Wageningen University, Dreijenplein 6, 6703 HB Wageningen, The Netherlands. Electronic address: maarten.biesheuvel@wetsus.nl.
authors.forename: P M
authors.initials: PM
authors.lastname: Biesheuvel
keywords: Capacitive deionization; Electrical double layer modeling; Nernst–Planck equation for ion transport; Porous electrodes; Water desalination