Membrane-enabled ion selectivity in flow-electrode capacitive deionization: materials, architectures, metrics, and design rules

초록

Flow-electrode capacitive deionization (FCDI) uses recirculating carbon slurry electrodes to separate adsorption from regeneration, enabling continuous and modular electrochemical separations. While early CDI/FCDI mainly focused on bulk NaCl removal, more recent studies increasingly aim for selective-ion separation and resource recovery. In this context, membrane design is crucial because membranes program the ionic transport window that couples ion migration to the flow-electrode electrosorption sink. This review focuses only on membrane-enabled selectivity in FCDI, distinguishing it from classical membrane CDI (MCDI), where ion-exchange membranes are used primarily to improve salt-removal performance. We distinctively aim to treat the membrane as the primary selectivity element that programs the ionic transport window in FCDI specifically, and to organize the field's materials, architectures, and metrics around this principle, in contrast to MCDI and to electrode-centered selectivity reviews. We define scope and exclusion rules, recommend benchmarking metrics for selectivity, enrichment, recovery, charge efficiency, and energy, and classify membrane strategies into: (i) commercial monovalent-selective ion-exchange membranes (MS-IEMs), (ii) functionalized IEMs (layer-by-layer polyelectrolyte multilayers, surface coatings, and charge-density tuning), (iii) ion-sieving porous membranes (nanofiltration-type NF-FCDI), and (iv) non-conventional membranes, including lithium-ion conductors and carrier-facilitated/affinity membranes. We also review architecture-level integrations (liquid-membrane chambers, pH-gated designs, membrane stripping/contactors, multi-chamber stacks, and slurry barrier concepts) and synthesize application-driven design rules for lithium/critical metals, nutrient recovery, heavy metals/oxyanions, and carboxylate/VFA separations. We also conclude that the majority of reported selectivities are derived from short-term tests using simplified feeds, whereas practical performance is still limited by the selectivity-resistance trade-off, slurry-adjacent membrane degradation, fouling and scaling, and module-level losses such as shunt currents and concentration polarization. Finally, we highlight major research gaps in slurry-specific membrane durability, standardized selectivity reporting, and scale-up-aware module design to accelerate the transition from laboratory demonstrations to robust selective separation processes.

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