Molecular Design of Electroactive Polyelectrolytes for Selective Electrochemical Separations

Designing selective interfaces is core to achieving efficient separation processes. Electrochemical separations can promote sustainability through the integration of renewable energy, plug-and-play modularity, and elimination of secondary waste. While electrochemical separations have been extensively explored for water desalination, their translation to value-added chemical manufacturing or resource recovery has been hampered by the lack of molecular selectivity. Here, we will discuss molecular engineering approaches to impart specificity to electrochemical interfaces, and the central role that polyelectrolytes and redox-active polymers play as tunable material platforms. 

We discuss the design of redox-active polymer electrosorbents for ion-selective separations. In-situ interfacial measurements and multiscale modeling are leveraged to elucidate the underlying mechanisms for selectivity.¹ In particular, we highlight how neutron-reflectometry can track solvation changes under electrochemical potential in stimuli-responsive copolymers, and the impact on ion electrosorption.^{2, 3} Next, we present the functionalization of ion-exchange membranes with polyelectrolytes. Through layer-by-layer coating or polyelectrolyte complexation, selectivity can be achieved between organic acids for their recovery from biomanufacturing.^{4, 5} The membranes are incorporated into redox-electrodialysis systems for the energy-efficient recovery of valuable organic molecules from fermentation streams.

Our work highlights the tremendous versatility of electrochemical separations, and its potential for decarbonization and sustainability across major areas of chemical and biochemical manufacturing, resource recovery and recycling, environmental management, and water purification. 

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(2) Chen, R.; Wang, H.; Doucet, M.; Browning, J. F.; Su, X. Thermo-Electro-Responsive Redox-Copolymers for Amplified Solvation, Morphological Control, and Tunable Ion Interactions. JACS Au 2023, 3 (12), 3333-3344. DOI: 10.1021/jacsau.3c00486.

(3) Candeago, R.; Wang, H.; Nguyen, M.-T.; Doucet, M.; Glezakou, V.-A.; Browning, J. F.; Su, X. Unraveling the Role of Solvation and Ion Valency on Redox-Mediated Electrosorption through In Situ Neutron Reflectometry and Ab Initio Molecular Dynamics. JACS Au 2024. DOI: 10.1021/jacsau.3c00705.

(4) Oh, W.; Kim, N.; Kim, H.; Mackie, R. I.; Su, X. Controlling Bicontinuous Polyelectrolyte Complexation for Membrane Selectivity: Redox-Mediated Electrochemical Separation of Volatile Fatty Acids. Advanced Functional Materials 2024, n/a (n/a), 2410511. DOI: https://doi.org/10.1002/adfm.202410511 (acccessed 2024/10/03).

(5) Kim, N.; Lee, J.; Su, X. Precision Tuning of Highly Selective Polyelectrolyte Membranes for Redox-Mediated Electrochemical Separation of Organic Acids. Advanced Functional Materials 2023, 33 (12), 2211645, https://doi.org/10.1002/adfm.202211645.