Hydrophilic microporous membranes for selective ion separation and flow-battery energy storage

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Membranes with fast and selective ion transport are widely used for water purification and devices for energy conversion and storage including fuel cells, redox flow batteries and electrochemical reactors. However, it remains challenging to design cost-effective, easily processed ion-conductive membranes with well-defined pore architectures. Here, we report a new approach to designing membranes with narrow molecular-sized channels and hydrophilic functionality that enable fast transport of salt ions and high size-exclusion selectivity towards small organic molecules. These membranes, based on polymers of intrinsic microporosity containing Tröger’s base or amidoxime groups, demonstrate that exquisite control over subnanometre pore structure, the introduction of hydrophilic functional groups and thickness control all play important roles in achieving fast ion transport combined with high molecular selectivity. These membranes enable aqueous organic flow batteries with high energy efficiency and high capacity retention, suggesting their utility for a variety of energy-related devices and water purification processes.

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Fig. 1: Ion-selective microporous membranes.
Fig. 2: Water adsorption, ionic conductivity and ionic dynamics of microporous membranes.
Fig. 3: Ionic and molecular sieving.
Fig. 4: Hydrophilic microporous membranes enable efficient and stable operation of aqueous redox flow batteries.

Data availability

The data shown in the plots and that support the findings of this study are available from the corresponding authors on reasonable request.


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This work was funded by the Engineering and Physical Sciences Research Council (EPSRC, UK, EP/M01486X/1), EPSRC Centre for Advanced Materials for Integrated Energy Systems (CAM-IES, EP/P007767/1), the Horizon 2020/FP7 Framework Program under grant agreement no. 608490, project M4CO2 and the European Research Council through grant agreement number 758370 (ERC-StG-PE5- CoMMaD). Q.S. acknowledges financial support by the Imperial College Department of Chemical Engineering Start-up Fund, a seed-funding grant from the Institute of Molecular Science and Engineering (IMSE, Imperial College) and seed-funding from EPSRC centres CAM-IES and Energy SuperStore (UK Energy Storage Research Hub). R.T. and C.Y. acknowledge full PhD scholarships funded by the China Scholarship Council. A.W. acknowledges a full PhD scholarship funded by the Department of Chemical Engineering at Imperial College. B.P.D. acknowledges the Statoil scholarship. K.E.J. acknowledges the Royal Society University Research Fellowship. A.I.C. and L.C. acknowledge the Leverhulme Trust for supporting the Leverhulme Research Centre for Functional Materials Design. T. Li is thankful for the support from the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub of the US Department of Energy. The work at the APS was supported by the US Department of Energy Scientific User Facilities under Contract DEAC02-06CH11357 with UChicago Argonne, LLC, and the operator of Argonne National Laboratory. The authors acknowledge E. Hunter-Sellars and D. Williams for facility support for the DVS measurements and A. G. Livingston for facility support for manufacturing polyacrylonitrile membranes. The authors acknowledge R. Woodward and R. Rinaldi for help with GPC measurements. The authors thank V. Yufit for helpful discussions. The authors acknowledge Q. Zhang, Y. Liu, S. Zhang and T. Juergensen for assistance with membrane preparation and battery tests.

Author information

Q.S., R.T. and A.W. developed membranes and redox flow batteries. A.W. synthesized PIM-1, AO-PIMs, amine-PIM-1 and DMBP-TB polymers, prepared membranes and performed characterizations. R.M. synthesized TB-PIMs and carried out characterizations. C.Y. synthesized PIM-SBF and performed modifications and characterizations. R.T. and A.W. carried out the ion transport and diffusion measurements. X.Z. and Z.F. helped with the crossover measurements. R.T. and A.W. performed electrochemical and flow battery experiments. B.P.D helped with membrane preparation, installation of the redox flow battery and electrochemical tests. E.Z., T. Liu and C.P.G. contributed to ssNMR measurements and interpretation and provided insights into the research. L.T., E.J., L.C., S.Y.C., K.E.J. and A.I.C. contributed to the molecular simulations and analyses. T. Li contributed to the small-angle X-ray scattering measurements. N.P.B. provided facility support and insights into flow battery systems. R.T., A.W., N.B.M. and Q.S. wrote the manuscript with contributions from all co-authors. All of the authors participated in the discussion and commented on the manuscript. Q.S. conceived the project, designed the research and directed the project with N.B.M.

Correspondence to Neil B. McKeown or Qilei Song.

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Extended data

Extended Data Fig. 1 Gas sorption and pore size distribution of PIM polymers.

a-b, N2 sorption isotherms at 77 K, and c-d, CO2 sorption at 273 K for PIM-EA-TB, PIM-BzMA-TB and DMBP-TB (a, c), and AO-PIM-1 with varied amidoxime contents, PIM-1 and Amine-PIM-1 (b, d). Solid symbols: adsorption; Open symbols: desorption. e-f, Pore size distributions derived from CO2 sorption based on density functional theory (DFT) calculations for TB-PIMs (e) and dibenzodioxin-based PIMs (f). With the increase of proportion of amidoxime group, partially functionalized AO-PIMs show a decrease in BET surface areas compared to PIM-1; however, their pore size distributions are similar to that of PIM-1 with a slight increase of frequency in ultramicropores. A similar phenomenon was observed in Amine-PIM-1, but it shows a significant shift of pore size distribution towards the ultramicropore range (<7 Å).

Extended Data Fig. 2 Structural analysis of amorphous polymer models.

a, PIM-1, cell size: 78.8 × 78.8 × 78.8 Å; b, AO-PIM-1, cell size: 78.8 × 78.8 × 78.8 Å; c, PIM-EA-TB, cell size: 66.8 × 66.8 × 66.8 Å. The surface shown is the Connolly surface with probe radius of 1.55 Å. d-f, Voids coloured with respect to the pore radius. g-l, interconnected (green) and disconnected (red) voids with respect to a probe 1.55 Å in radius (j-l), and a probe 0.85 Å in radius, respectively. The simulations show that PIM-EA-TB, AO-PIM-1 and PIM-1 present sub-nanometre-sized interconnected voids and channels. Apart from the higher surface area of PIM-EA-TB, its pores are also disconnected with respect to a relatively large probe of 1.55 Å, while PIM-1 shows a great portion of connected pores with a probe of the same size. When a smaller probe 0.85 Å in radius was used, the connected pores are observed in PIM-EA-TB and AO-PIM-1, suggesting the presence of ultra-micropores (<7 Å) in both PIM-EA-TB (due to chain rigidity) and AO-PIM-1 (due to hydrogen bonding). m, Normalized pore size distributions of three representative PIMs. The pore size distributions are qualitatively similar, but shift to smaller pores for PIM-EA-TB and AO-PIM-1. Particularly, all pores in the PIM-EA-TB have diameters smaller than 8 Å, whereas PIM-1 has a significant portion that is larger yet still within sub-nanometre range. The most common pore diameter in AO-PIM-1 is 4.5 Å in diameter, and its maximum pore size is intermediate with respect to those of PIM-EA-TB and PIM-1. The narrow distribution of ultramicropores enables the size-selective ion transport.

Extended Data Fig. 3 Fabrication and photograph of PIM membranes.

a, Thick symmetric membranes were fabricated by casting a polymer solution onto a clean glass plate using a doctor blade, followed by slow evaporation of solvent over two days. b, Thin films were fabricated by spin coating a dilute polymer solution onto a substrate. Thin film composite membranes were obtained by using a porous support as the substrate, while a glass plate was used to make freestanding thin films. c, Freestanding thick and thin films were peeled off the glass plates by floating onto water surface. d, Photo of a freestanding 300-nm-thick PIM-EA-TB film supported on a polydimethylsiloxane (PDMS) O-ring for ion dialysis test. e-f, Photos of AO-PIM-1 (e) and PIM-EA-TB (f) membranes in the dry (left) and fully hydrated (right) states. No significant swelling was observed after immersing the dry PIM membranes in DI water for 24 hours. The swelling ratios of AO-PIM-1 and PIM-EA-TB membranes in linear dimensions were 6.9 ± 1.0%, and 11.7 ± 0.3%, respectively. The swelling measurements were based on at least three membranes. g, Photo of an AO-PIM-1 membrane. The manufacturing of membranes was scaled up to about 20 cm × 20 cm.

Extended Data Fig. 4 Stability of PIM-EA-TB and AO-PIM-1 in alkaline aqueous solution.

a, FTIR spectra. b-c, Molecular weight distributions. d-e, nitrogen sorption isotherm profiles. Symbols in (d, e): solid, adsorption; open, desorption. PIM-EA-TB and AO-PIM-1 powders were soaked in 3 M NaOH aqueous solutions for 4 weeks at room temperature. These powders were then collected by filtration and washed with DI water, and then freeze-dried before drying at 110 °C under vacuum. After NaOH treatment, no degradation or structural change of these polymers was observed in the FTIR spectra and molecular weight distributions. The BET surface areas of NaOH treated PIM-EA-TB and AO-PIM-1 were 905 and 498 m2 g−1, respectively, indicating the retention of microporosity.

Extended Data Fig. 5 Deprotonation mechanism of amidoxime groups in AO-PIMs for high ionic conductivity.

a, Temperature dependence of ionic conductivities of AO-PIMs with varied AO ratio and Amine-PIM-1 membranes measured in 1 M aqueous NaOH by electrochemical impedance spectroscopy. b, Temperature dependence of ionic conductivities of pretreated and pristine AO-PIM-1 membranes measured in 1 M aqueous NaOH or NaCl. Pretreatment is carried out by soaking AO-PIM-1 membranes in 1 M NaOH aqueous solution overnight at room temperature and then thoroughly washing them with DI water, before soaking the membranes in 50 ml DI water for more than four times with each time taking 2 h until the conductivity of the washing water fall below 1 µS to completely remove adsorbed NaOH. Symbols in (a, b) are experimental data; lines in (a, b) are fitting results. c, Electrolyte uptake and ionic conductivity of AO-PIM-1 membranes pretreated under varied pH conditions, and d, proposed chemical structure of these pretreated polymers. Lines are added to guide the eyes. AO-PIM-1 membranes were pretreated in NaCl or NaOH aqueous solutions with varied pH, respectively, followed by washing and immersion in DI water to remove any traces of adsorbed salts. To avoid further deprotonation, 1 M NaCl aqueous solution was used to explore electrolyte uptake and ionic conductivity of these AO-PIM-1 with varied deprotonation degree. The response of membrane electrolyte uptake and ionic conductivity towards pH changes of pre-treatment solutions is consistent with the pKa of amidoxime group. See Supplementary Information 3.1.1. for more detailed discussion.

Extended Data Fig. 6 1H and 23Na ssNMR spectra.

a, Dynamic adsorption of water molecules and b, sodium ions in the micropores of AO-PIM-1 (100%). c, Dynamic adsorption of water molecules and d, sodium ions in the micropores of AO-PIM-1 with 56% of amidoxime groups. e, Dynamic adsorption of water molecules and f, sodium ions in the micropores of AO-PIM-1 with 32% of amidoxime groups. g-h, 1H and 23Na ssNMR spectra of PIM-1 for comparison. See more detailed discussion in Supplementary Information 3.1.2.

Extended Data Fig. 7 Crossover tests of Nafion 212 and PIM membranes.

a-c, Photos and d-f, schematic diagrams of ideal dialysis experiments (a, d) using 0.1 M K4Fe(CN)6 in 1 M NaOH as feed solutions and 1 M NaOH as permeate solutions; contra-diffusion dialysis experiments (b, e) using 0.1 M K4Fe(CN)6 in 1 M NaOH as feed solutions, and 0.1 M 2,6-DHAQ in 1 M NaOH as permeate solutions; crossover measurements in an operating battery (c, f) with 10 ml 0.1 M K4Fe(CN)6 as the catholyte and 10 ml 0.1 M 2,6-DHAQ as the anolyte. g-i, Specific permeance of iron-containing ions (that is, Fe(CN)64− and Fe(CN)63−) through AO-PIM-1 (g), Nafion 212 (h) and PIM-EA-TB TFC (i). Specific permeance (mol m−2) was obtained by normalizing the quantity of molecules permeated through the membrane (in mol) by the effective area (H-cell: 1.54 cm2 and battery: 5 cm2). j-l, K4Fe(CN)6, 2,6-DHAQ and FMN-Na crossover tests (ideal dialysis experiments). Feed solutions are: 0.1 M K4Fe(CN)6, 0.1 M 2,6-DHAQ, and 0.06 M FMN-Na, respectively; permeate solution is 1 M NaOH solution. See more detailed discussion in Supplementary Information 3.3.2.

Extended Data Fig. 8 FMN-Na|K4Fe(CN)6 Battery performance.

a, Typical charging-discharging curves, and b, capacity and efficiency of RFBs using Nafion 212 membrane at current densities varied from 20 to 100 mA cm−2. c, Cycling performance for Nafion 212 membranes over 300 cycles at 80 mA cm2. d, Typical charging-discharging curves, and e, capacity and efficiency of RFBs using TB-based membranes at current densities varied from 20 to 100 mA cm−2. f, Cycling performance of RFBs using PIM-EA-TB TFC and PIM-BzMA-TB TFC membranes over 300 cycles at 80 mA cm−2. With a low area-specific resistance, PIM-EA-TB TFC membrane enables the operation of FMN-Na|K4Fe(CN)6 flow battery at high current of 80 mA cm−2 with a current efficiency of 96.1% and a discharge capacity of 2.52 A h l−1 (utilization ratio of 96.7 % in terms of the theoretical capacity). After 300 cycles, batteries using PIM-EA-TB TFC and PIM-BzMA-TB TFC membranes exhibited similar and remarkably high capacity retentions of 86.5% and 86.1% respectively. g, Capacity and efficiency at current densities varied from 20 to 100 mA cm−2 and h, cycling performance at 80 mA cm−2 for battery system using AO-PIMs membranes. The capacity retention of AO-PIM-1 over 300 cycles is 84.5 %, comparable to the performance of Nafion 212. i, Cycling performance for AO-PIM-SBF at 80 mA cm−2. AO-PIM-SBF membrane also proved the feasibility of membrane design strategy. Electrolytes: 10 ml 0.06 M FMN-Na and 10 ml 0.1 M K4Fe(CN)6 in 1.0 M NaOH solution. The batteries were operated in an open-air atmosphere.

Extended Data Fig. 9 2,6-DHAQ|K4Fe(CN)6 Battery performance.

a, Typical charging-discharging profiles, and b, capacity and efficiency of RFBs assembled with AO-PIM-1 membrane at current densities varied from 20 to 100 mA cm−2. c, Cycling performances over 100 cycles at 80 mA cm−2 of PIM-EA-TB TFC, AO-PIM-1 and AO-PIM-SBF membranes. d, Typical charging-discharging profiles and e, capacity and efficiency at current densities varied from 20 to 100 mA cm−2, and f-g, cycling performance of RFBs assembled with PIM-EA-TB TFC membrane. Electrolytes in (a-e, g): 10 ml 0.1 M 2,6-DHAQ and 10 ml 0.1 M K4Fe(CN)6 in 1.0 M NaOH solution. Electrolytes in f: 10 ml 0.5 M 2,6-DHAQ, and 10 ml 0.4 M K4Fe(CN)6 in 1.0 M NaOH solution. The batteries were operated in an open-air atmosphere.

Extended Data Fig. 10 2,6-DHAQ|K4Fe(CN)6 Battery performance in a glove box filled with argon.

a, Cycling performance of RFB using AO-PIM-1 membrane over 100 hours at 40 mA cm−2. b, The specific permeance of Fe-based ions in 2,6-DHAQ electrolyte. c, Cycling performance of RFB using Nafion 212 over 100 hours at 40 mA cm−2. d, The specific permeance of Fe-based ions in 2,6-DHAQ electrolyte. e, Cycling performance of RFB using PIM-EA-TB TFC over 40 hours at 40 mA cm−2. f, The specific permeance of Fe-based ions in 2,6-DHAQ electrolyte. Electrolyte: 10 ml 0.1 M K4Fe(CN)6 and 10 ml 0.1 M 2,6-DHAQ in 1.0 M NaOH solution. Sample for ICP-OES tests were picked from anolytes and diluted for 100 times so as not to exceed the detecting limit of ICP-OES. See more detailed discussion in Supplementary Information 4.4.4.

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Supplementary Figures 1–32, discussion and Tables 1–12

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Tan, R., Wang, A., Malpass-Evans, R. et al. Hydrophilic microporous membranes for selective ion separation and flow-battery energy storage. Nat. Mater. (2019) doi:10.1038/s41563-019-0536-8

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