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

Abstract

Highlights New colloid-inspired fabrication with calendering improved electrode utilization. FDI system used automatic water recirculation with calibrated valve timing. Water recirculation with dense electrodes improved salt removal. System removed 90% of salt from 100 mM NaCl brackish water. System can remove 80% of salt with thermodynamic energy efficiency of 80%.

Electrochemical Methods for Water Purification, Ion Separations, and Energy Conversion

“…Metzger et al conducted a comprehensive technoeconomic analysis on MCDI, HCDI, and intercalation CDI . This analysis showed that, compared to MCDI, intercalation CDI can achieve higher removal capacity and is more cost-effective and energy efficient. , To treat equal volumes of water with similar performance, for example, an intercalation CDI module would cost only 27% of an MCDI module and would be four times smaller in size.…”

Efficient, Selective Sodium and Lithium Removal by Faradaic Deionization Using Symmetric Sodium Titanium Vanadium Phosphate Intercalation Electrodes

“…These electrodes were assembled and operated in a custom-built FDI flow cell using galvanostatic cycling. The design, operation, and flow calibration of this cell were described in our previous work. , In brief, the cell had two NTVP electrodes separated by a Neosepta AMX anion exchange membrane (AEM, Astom Corp) sandwiched between two graphite current collectors (McMaster-Carr). Here, feed water was forced to flow through porous electrodes via a 3D-printed fluid-distribution unit.…”

“…Even so, the streamwise gradients in ion concentration that are inherent to deionization processes have been linked to streamwise gradients in the state-of-charge of intercalation materials, , where phase-separating NASICONs have exhibited pronounced gradients relative to solid-solution intercalation materials (e.g., NiHCF , ). While feedwater recirculation has been used to minimize such state-of-charge gradients, ,, the relative merits of phase-separating and solid-solution mechanisms are yet to be quantified among NASICONs, as we presently do.…”

“…Despite these limitations, the broader class of sodium superionic conductor (NASICON) materials to which NTP belongs show high charge capacity and fast solid-state sodium diffusion, as supported by their use in solid-state electrolytes, − lithium-ion batteries, and sodium-ion batteries as cathodes − and anodes. − These properties also make NASICON materials attractive candidates for the selective removal of Na + in FDI. Among NASICONs, sodium vanadium phosphate [Na 3 V 2 (PO 4 ) 3 ; NVP] shows high specific capacity (∼118 mA h/g) and density (3.25 g/cm 3 ), ,− theoretically producing 3-fold higher solid-state cation-storage concentration (14 mol-Na + /L) than the commonly used Prussian-blue analogues (5 mol-Na + /L) . Furthermore, the standard reduction potential of the V 3+ /V 4+ couple for the intercalation of Na + in NVP at +0.3 V versus Ag/AgCl is highly suitable for applications in aqueous electrolytes. , Despite earlier reports using NVP in a dual-ion type FDI process, NVP/C suffers from dissolution in water during oxidation of V 3+ to V 4+ , motivating its partial substitution with titanium to form Na 2 TiV­(PO 4 ) 3 (NTVP) to enable stable operation of the V 3+/4+ redox couple in water …”

“…Despite their promise based on battery-like experiments, the performance of cation intercalation materials in practical FDI cells additionally depends on the arrangement of flow channels and flow conditions. Flowing feed water through porous electrodes as motivated by modeling − has been shown experimentally to increase salt removal, salt removal rate, and thermodynamic energy efficiency , compared to feedwater flow either by or behind electrodes . Even so, the streamwise gradients in ion concentration that are inherent to deionization processes have been linked to streamwise gradients in the state-of-charge of intercalation materials, , where phase-separating NASICONs have exhibited pronounced gradients relative to solid-solution intercalation materials (e.g., NiHCF , ).…”