(29b) Nanopatterned Membrane and Prussian Blue Infiltrated Electrode Assemblies for Membrane Capacitive Deionization

Membrane capacitive deionization (MCDI) is a modular and electrified water desalination platform that does not generate significant acoustic, thermal, or electromagnetic signatures nor does it require high pressure piping. Flow-by MCDI, which is commercialized, uses electrical energy to remove ions from water. [1] This system architecture features two porous electrodes covered by ion-exchange membranes – the latter material is used to prevent co-ion adsorption and provide greater Coulombic efficiency than variants not utilizing ion-exchange membranes. This portable desalination platform is conducive for the U.S. Marine Corps during squad level field missions. Though reverse osmosis is the most reliable process for desalinating water, it is not conducive for portable missions due to piping requirements that can tolerate high pressure.

Our previous research improved the energy efficiency of MCDI by using micropatterned IEMs with lower area-specific resistance (ASR) values and porous ionic conductors in the spacer channel that augmented solution conductivity and curtailed ohmic losses. Reducing these resistances enabled the MCDI to operate at 700 mV lower cell voltage at 2 mA cm^-2 current density. This is important for shrinking the size of the MCDI unit and lowering the overall capital costs.

This talk presents our recent work examining ionic charge transport resistance at nanopatterned membrane electrode interfaces in MCDI. Membrane surface patterning increased the interfacial area between the membrane and aqueous stream promoting greater salt removal fluxes. Soft-lithography methods were used to nanopattern poly (phenylene alkylene) cation exchange membranes and anion exchange membranes with systematically varied topographies (e.g., hexagonal, octagonal and rectangular structures that vary from 100 nm to 300 nm). Initially, high performance borosilicate glasses were spin coated using Spin on Glass (SOG) solution and this coating was stamped with a PDMS mold generated by Laser Interface Lithography (LIL). [2] Next, this coated layer was annealed at high temperatures (1400 C) to create nanostructures on the glass substrate. After that, poly (phenylene alkylene) ionomers are drop cast on the nanostructured glass substrate. Finally, the nanopatterned membrane is hot pressed with the Prussian blue coated carbon cloth electrodes to prepare nanopatterned membrane electrode assemblies.

In our previous work, we reduced the MCDI cell voltage by 1 V when utilizing micropatterned ion-exchange membranes and ionomer infiltrated electrodes (operating at 2 mA cm^-2 with a 2000 ppm NaCl feed). This translated to a 42.72% greater energy normalized adsorbed salt (ENAS) value- a metric used to assess the energy consumption for MCDI. Furthermore, this strategy has been shown to be effective in reducing charge-transfer resistances in both proton exchange membrane (PEM) and AEM fuel cells and reducing the water dissociation kinetics resistance in bipolar membranes [3-6].

References

1. Palakkal, Varada Menon, et al., Low-resistant ion-exchange membranes for energy efficient membrane capacitive deionization. ACS Sustainable Chemistry & Engineering 6.11 (2018): 13778-13786.
2. Sansen, T., et al., Mapping Cell Membrane Organization and Dynamics Using Soft Nanoimprint Lithography. ACS Applied Materials & Interfaces, 2020. 12(26): p. 29000-29012.
3. Kole, S., et al., Bipolar membrane polarization behavior with systematically varied interfacial areas in the junction region. Journal of Materials Chemistry A, 2021. 9(4): p. 2223-2238.
4. Breitwieser, M., et al., Tailoring the Membrane-Electrode Interface in PEM Fuel Cells: A Review and Perspective on Novel Engineering Approaches. Advanced Energy Materials, 2018. 8(4).
5. Hee-Tak Kim, T.V.R., Ho-Jin Kweon, Microstructured Membrane Electrode Assembly for direct methanol fuel cell. Journal of The Electrochemical Society, 2007. 154(10).
6. Tomizawa, M., et al., Heterogeneous pore-scale model analysis of micro-patterned PEMFC cathodes. Journal of Power Sources, 2023. 556.