Abstract
The regulation of water content in polymeric membranes is important in a number of applications, such as reverse electrodialysis and proton-exchange fuel-cell membranes. External thermal and water management systems add both mass and size to systems, and so intrinsic mechanisms of retaining water and maintaining ionic transport1,2,3 in such membranes are particularly important for applications where small system size is important. For example, in proton-exchange membrane fuel cells, where water retention in the membrane is crucial for efficient transport of hydrated ions1,4,5,6,7, by operating the cells at higher temperatures without external humidification, the membrane is self-humidified with water generated by electrochemical reactions5,8. Here we report an alternative solution that does not rely on external regulation of water supply or high temperatures. Water content in hydrocarbon polymer membranes is regulated through nanometre-scale cracks (‘nanocracks’) in a hydrophobic surface coating. These cracks work as nanoscale valves to retard water desorption and to maintain ion conductivity in the membrane on dehumidification. Hydrocarbon fuel-cell membranes with surface nanocrack coatings operated at intermediate temperatures show improved electrochemical performance, and coated reverse-electrodialysis membranes show enhanced ionic selectivity with low bulk resistance.
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Acknowledgements
This research was supported by the Nano-Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2012M3A7B4049745). C.M.D. is supported by the Australian Research Council (DE40101359). C.M.D., A.W.T. and A.J.H. acknowledge the CSIRO Julius Career award, the CSIRO Office of the Chief Executive Science Leader Scheme and the Australia–Korea Foundation Early Career Researchers Program. M.D.G. is a BK21-Plus visiting professor at Hanyang University.
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Contributions
Y.M.L. conceived the study. C.H.P. and Y.M.L. designed the experiments and C.H.P., S.Y.L., D.S.H., Y.M.L., M.D.G., Tae-Woo K. and Tae-Wuk K. wrote the manuscript. C.H.P., D.S.H. and D.H.C. conducted plasma treatment experiments, X-ray photo electron spectroscopy analysis, and set the coating condition. C.H.P. and S.Y.L. conducted contact-angle measurements and scanning electron microscopy image collecting. D.W.S. conducted dynamic vapour sorption analysis and AFM image collecting. S.Y.L. and K.H.L. conducted electrochemical fuel-cell performances. C.M.D. and A.J.H. conducted PALS analysis. A.W.T. conducted mathematical modelling of water sorption through membranes. Tae-Woo K. and Tae-Wuk K. conducted microscopic observation of cactus stems. M.L. and D.-S.K. conducted the mathematical analysis of surface patterns using the Voronoi diagram program. D.S.H., D.H.C. and K.H.L. fabricated ion-exchange membranes and evaluated the electrochemical performance of the ion-exchange membrane for reverse electrodialysis. C.H.P., S.Y.L., D.S.H., D.W.S., D.H.C., M.D.G. and Y.M.L. discussed the results. All authors commented on the manuscript.
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Extended data figures and tables
Extended Data Figure 1 Cactus (Ferocactus schwarzii) stomata control mechanism for water retention, analogous to self-controlled nanovalve mechanism of plasma-treated membranes.
a, Transverse-section microscopic image of cactus stem (×400 magnification, Normarski differential interference contrast microscopy (Nikon, Eclipse 80i)) illustrates the structure of outer photosynthetic tissues which consist of the impermeable epidermal layer and stomata. b, Microscopic pictures illustrating the cactus water-conserving effect based on the stoma self-control mechanism by swelling of stoma cells. To obtain the transverse section images of the cactus stem, cross-sectional bulky pieces of cactus stem were fixed with fixation solution. After washing with 20% ethanol, thin transversely sliced tissues were cleared with chloral hydrate solution, and this was followed by staining with 0.05% (w/v) toluidine blue O solution for microscopic observation.
Extended Data Figure 2 Schematic outline of plasma polymerization.
a, Purple-lit atmospheric plasma of c-C4F8 and He (photograph) was generated from a radio-frequency (RF) glow discharger. b, Mechanism of the plasma polymerization and fluorohydrocarbon coating layer. c-C4F8 was broken down into various species of fluorocarbon monomers. Plasma polymerization formed a hydrophobic coating layer on the nanometre scale. c, Polymer structure of hydrocarbon aromatic ion-exchange polymer membranes. Two different degrees of functionalization (n = 0.4 or 0.6) were fabricated in sulfonated poly(arylene ether sulfone) in the protonated form (BPSH) or sodium form (CBPS) and an aminated poly(arylene ether sulfone) (ABPS), respectively. The chemical structures of various types of hydrocarbon aromatic proton-exchange polymer membranes are shown. XESPSN is an end-group cross-linked sulfonated random copolymer with a degree of sulfonation of 0.6. ‘Multiblock’ refers to a multiblock BPSH–BPS copolymer, which consists of 10 kg mol−1 BPSH and 5 kg mol−1 BPS with a spacer linkage.
Extended Data Figure 3 Nanovalved plasma coating layers in response to humidity conditions.
a–c, AFM images showed that nanometre-sized cracks were formed upon hydration of plasma-coated membranes with different surface morphology, depending on the degree of sulfonation and coating cycles, when compared to uncoated membranes in hydration (a). The colour scale indicates the topology of surface (depth) according to colour gradation (with the deepest point shown as black and the highest point as bright yellow). R20, R30 or R40 indicates the number of repeated coating cycles. The water-conserving mechanism with self-control of nanovalves was investigated by the comparison of AFM images between hydrated (b) and dehydrated (c) plasma-coated membranes. The surface morphology of hydrated P-BPSH40 to P-BPSH60 was investigated at different scales from AFM images of dehydrated P-BPSH in order to clarify the entire surface morphology. P-BPSH60 R40 had the highest volumetric swelling ratio, making the morphology change particularly evident.
Extended Data Figure 4 Hydrophobic surface coating layer by plasma treatment.
a, Contact angles of the hydrophobic atmospheric plasma surface-coated membrane (BPSH40) in specific coating cycles, where the number of coating cycles increases from left to right images, 0, 10, 20, 30 cycles, respectively. b, d, Spectral change of P-BPSH40 (b) and P-BPSH60 (d) in fluorine peak after surface coating measured with X-ray photoelectron spectroscopy. c, e, Composition change of an F atom included in fluorocarbon layer and an S atom in sulfonic acid groups at surface of P-BPSH40 (c) and P-BPSH60 (e) with increased coating cycles.
Extended Data Figure 5 Reproducibility and regularity of nanocrack pattern evaluated via Voronoi tessellation entropy differentiation.
AFM images of P-BPSH60R40 (a, b, c) and P-BPSH60R30 (d, e, f) were analysed by area Voronoi diagram. a, b, P-BPSH60R40 hydrated in deionized water (size 10 μm, a), and dehydrated (size 10 μm, b) under 30% to 45% RH conditions. d, e, P-BPSH60R30 hydrated in deionized water (size 1.5 μm, d) and dehydrated (size 1.5 μm, e) under 30% to 45% RH conditions. The circularity of each pattern is indicated in different colours. Blue, yellow, and red colours present high, middle, and low circularity, respectively. The circularity of patterns is defined as the ratio of the circumference of circle in the same area of each pattern to the length of a pattern’s boundary. c, f, Tessellation entropy values were evaluated by probability of frequency distribution from the number of polyside patterns (approximately 20–40) in the AFM images for hydrated P-BPSH60R40 (c) and hydrated P-BPSH40R30 (f). The probability pn that a Voronoi cell will have n neighbour Voronoi cells, where n is a non-negative integer, is the number of n-polyside patterns divided by the number of total polyside patterns in an image, where n is 3–10. The value of the tessellation entropy was determined by the distribution of probability 
. The average of the number of neighbouring Voronoi cells is 
, where Nn is the frequency of the n-polyside pattern. Standard deviations of tessellation entropy were calculated from sixteen samples for each plasma treatment condition by repeating the same plasma coating twice.
Extended Data Figure 6 Mathematical model for water sorption–desorption of a plasma-coated membrane.
a, Variables and configuration for the ordinary differential equation that describes water sorption–desorption for a plasma-coated membrane. b, Simulated crack width expansion during DVS time. c, d, Simulated DVS as a function of time for the conventional membrane and the plasma-coated membrane for a single DVS cycle (c) and five pulsatile DVS cycles (d). Using equations (4) and (10) in Supplementary Discussion 2.5, with experimentally determined equilibrium water concentration Cw values, normalized A = 1, h = 1, Ds = 0.005, skin thickness l = 1 and step tolerance 0.01242. The hydrophobic skin of the plasma-coated membrane delays water sorption and more importantly inhibits the desorption of water such that the membrane remains hydrated for a longer time compared with the uncoated conventional membrane.
Extended Data Figure 7 Current–voltage polarization curves for various types of hydrocarbon aromatic proton-exchange membranes in single membrane electrode assembly tests.
Fuel-cell performances of uncoated membranes (BPSH40, BPSH60, XESPSN, Multiblock) and plasma-coated membranes (P-BPSH40, P-BPSH60, P-XESPSN, P-Multiblock) were measured at various RH values and operating temperatures. a, At 80 °C under 100% RH and 1.5 atm of pressure. b, At 100 °C under 85% RH and 1.5 atm of pressure. c, At 120 °C under 35% RH and 1.5 atm of pressure by supplying H2 and O2.
Extended Data Figure 8 Current–voltage polarization curves of various types of hydrocarbon aromatic proton-exchange membranes in single membrane electrode assembly tests.
Fuel-cell performances of uncoated membranes (BPSH40, BPSH60, Nafion NRE212) and plasma-coated membranes (P-BPSH40, P-BPSH60) were measured at various RH values and operating temperatures. a, At 80 °C under 100% RH and 1.5 atm of pressure. b, At 100 °C under 85% RH and 1.5 atm of pressure. c, At 120 °C under 35% RH and 1.5 atm of pressure by supplying an H2/air feed.
Extended Data Figure 9 Effect of nanocrack pattern on electrochemical performances along with proton conductivites and water retention of plasma-coated membranes.
Proton conductivity (4-probe method, in-plane) of uncoated membranes (BPSH40, BPSH60) and plasma-coated membranes (P-BPSH40, P-BPSH60) were measured at various RH values and temperatures. a, Proton conductivities were measured at 80 °C as RH decreased from 100%, 70% and 50% to 33%. b, Proton conductivities were measured at 80 °C (100% RH), 100 °C (85% RH) and at 120 °C (35% RH) under 1.5 atm pressure. Values in a and b are averages of at least fifteen replicates; error bars represent 1 s.d. c, d, A conceptual crack pattern parameter (Cp = Ac/I) is correlated to electrochemical performance, and is defined by crack formation underlying parameters of crack surface area ratio (c) (Ac, average values of crack area to total membrane area, were calculated from the Image J program version 1.50b and image processing and analysis was done in the open-source program Java; https://imagej.nih.gov/ij/index.html) increased by membrane dimensional swelling ratio and thickness of coating layer (d) (I was measured by AFM image analysis). The average area of one Voronoi cell component (coloured domain with green boundary line in Extended Data Fig. 5a, b, d and e), Acell (in μm2) was calculated by using the open-source BetaConcept program for Voronoi diagram analysis (http://voronoi.hanyang.ac.kr/software.htm#BetaConcept)26. Values in c and d are averages of at least sixteen replicates; error bars represent 1 s.d. e, Electrochemical maximum power densities of P-BPSH40 and P-BPSH60 are proportionally enhanced with increasing Cp. The maximum power densities of uncoated BPSH40 (B40) and BPSH60 (B60) at various RH conditions of 100%, 85% and 35% RH are presented on the y axis, with their crack pattern parameters. f, The water retention effect by nanovalve control at low RH is reflected by the relative maximum power density of membrane electrode assembly (in Fig. 3a and b) with correlation of crack pattern parameters at various membrane electrode assembly operating conditions (100% RH at 80 °C, 85% RH at 100 °C, and 35% RH at 120 °C).
Extended Data Figure 10 Long-term stability test results.
Membrane electrode assembly single cell operation on conditions: current density measured at 0.7 V (constant voltage) at 120 °C under 35% RH and 1.5 atm of pressure by supplying H2 and air. Stability tests were performed three times for each type of membrane. Nafion (NRE212) reaches a maximum current density of 180 mA cm−2 after 20 h and shows a transient decline in current density. After 120 h, a complete loss of performance occurred and the Nafion membrane appeared to be decomposed. P-BPSH60 maintains its current density of about 150 mA cm−2 until 220 h, when the measurements were intentionally stopped for observation. The time until a 10% loss of current density occurs is determined to be 60 h and 220 h for Nafion and P-BPSH, respectively. After autopsy of each membrane electrode assembly, the Nafion membrane had a black colour indicating degradation, whereas the P-BPSH membrane still maintained its shape and original colour.
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Park, C., Lee, S., Hwang, D. et al. Nanocrack-regulated self-humidifying membranes. Nature 532, 480–483 (2016). https://doi.org/10.1038/nature17634
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DOI: https://doi.org/10.1038/nature17634
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