Quantifying the trade-offs between energy consumption and salt removal rate in membrane-free cation intercalation desalination

Abstract

Electrochemical desalination devices that use redox-active cation intercalation electrodes show promise for desalination of salt-rich water resources with high water recovery and low energy consumption. While previous modeling and experiments used ion-exchange membranes to maximize charge efciency, here a membrane-free alternative is evaluated to reduce capital cost by using a porous diaphragm to separate Na 1+x NiFe(CN) 6 electrodes. Two-dimensional porous-electrode modeling shows that, while charge efficiency losses are inherent to a diaphragm-based architecture, charge efficiency values approaching the anion transference number (61% for NaCl) are achievable for diaphragms with sufficiently low salt conductance. Closed-form equations are thereby derived that relate charge efficiency to the non-dimensional Pèclet and Damköhler numbers that enable the selection of current and flow velocity to produce a desired degree-of-desalination. Simulations using these conditions are used to quantify the tradeoffs between energy consumption and salt removal rate for diaphragmbased cells operated at a range of currents. The simulated distributions of reactions are shown to result from the local salt concentration variations within electrodes using diffusion-potential theory. We also simulate the cycling dynamics of various flow configurations and show that flow-through electrodes exceed the degree-of-desalination compared with flow-by and flow-behind configurations due to solution stagnation within electrodes.

Dual‐Zinc Electrode Electrochemical Desalination

“…This technology can be used to achieve an ion capture capacity that is up to six times higher than conventional CDI . Various deionization designs based on the faradaic electrode reaction have been proposed, including rocking‐chair desalination, redox‐flow battery desalination, and some novel design desalination . These desalination devices operate by the use of individual capture/release of ions by two separate electrode materials.…”

Electrochemically mediated deionization: a review

“…These optimistic simulation results have motivated additional development of porous electrode theory coupled with fluid flow and ion transport across anion selective membranes for desalination with intercalation materials. [143][144][145] In experimental settings, the rocking chair design was implemented with two Prussian blue electrodes, NaNiHCF and NaFeHCF. 146 The Prussian blue electrodes were chosen for their higher capacity relative to that of NMO and their ability to reversibly intercalate Na + , K + Mg 2+ , and Ca 2+ .…”

Low porosity, high areal-capacity Prussian blue analogue electrodes enhance salt removal and thermodynamic efficiency in symmetric Faradaic deionization with automated fluid control

“…Techno-economic analysis of FDI and comparison with the more frequently studied capacitive deionization (CDI) shows that the former method outputs more desalinated water at lower cost while being a more compact technology, in part due to CIMs having greater capacity ( Metzger et al., 2020 ). Despite our early modeling that introduced a symmetric Na-ion battery architecture to desalinate seawater using CIMs ( Liu and Smith, 2018 ; Smith, 2017 ; Smith and Dmello, 2016 ), subsequent experimental demonstrations thereof failed to realize more than 30% salt removal from continuously flowing brackish water ( Kim et al., 2017 ; Porada et al., 2017 ; Reale et al., 2019 ). We posit here that efficiency losses within auxiliary systems must be understood in conjunction with electrode-scale transport processes to extend FDI using CIMs beyond brackish salinity.…”

“…The configuration of electrolyte flow through porous electrodes ( Liu and Smith, 2018 ; Reale et al., 2019 ; Smith, 2017 ; Son et al., 2020 ) and charge transport within porous electrodes ( Reale et al., 2019 ; Shrivastava and Smith, 2018 ) are known to affect rate capability and energy consumption in FDI when using symmetric CIM electrodes, but past results suggest that unknown mechanisms remain and prevent the salt-removal potential of CIMs from being accessed. In FDI, saltwater can flow through a gap between electrodes in a flow-by configuration or directly through the porous structure of electrodes sandwiched within a cell in a flow-through configuration.…”