Supporting Information

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1 Supporting Information Water uptake study of anion exchange membranes Yiwei Zheng, Uri Ash, Ravi P. Pandey, Amobi G. Ozioko, Julia Ponce-González, Michael Handl, Thomas Weissbach, John R. Varcoe, Steven Holdcroft, Matthew W. Liberatore, Renate Hiesgen, and Dario R. Dekel Experimental Synthesis of graphite oxide (GO) Graphite powder (1 g), NaNO 3 (1.0 g) and H 2 SO 4 (48 ml, 97%) was cooled to 0 o C by stirring in an ice bath for 15 min. Finely meshed KMnO 4 powder, 3.0 g, was added slowly with vigorous stirring while keeping the temperature below 20 o C. After half an hour, the mixture was warmed to 35±3 o C for 30 min. Then 180 ml water was added slowly, the temperature was increased gradually and kept at 95±3 o C for another 30 min. 400 ml water was then added. Finally, H 2 O 2 (30%, 12 ml) was added to convert the unreacted permanganate and manganese dioxide into soluble sulfates. The mixture was centrifuged (7000 rpm, 7 min.) and the precipitate was washed with 5% HCl solution (once) and with water (3 times). The final precipitate was dried at 60 o C for 24 h. It was then dispersed in water (1 mg/5 ml) by 2 h sonication, followed by aggregated carbon removal through centrifugation at 4000 rpm for 1 hour. Structure was confirmed by IR and 13 C NMR spectroscopy. Synthesis of DMAPSiGO [(N,N-Dimethylaminopropyl)silane graphene oxide] 1

2 DMAPSiGO was prepared by condensation reaction between GO and (N,N- Dimethylaminopropyl)trimethoxysilane (DMPTMOS). In a typical synthetic procedure, highly oxygenated GO (100 mg) was dispersed in anhydrous THF (100 ml) and sonicated to obtain a homogeneous dispersion. Then, DMPTMOS (200 mg) was added, and reaction mixture was refluxed for 15 h. Finally, reaction mixture was cooled to room temperature, filtered and rinsed several times with THF, and dried under vacuum overnight to obtain the DMAPSiGO. Structure was confirmed by IR, solid state 13 C and 29 Si NMR spectroscopy. Synthesis of imidazolium-functionalized (N,N Dimethylaminopropyl)silane graphene oxide (aminated graphene oxide, AGO) cross-linker One gram of DMAPSiGO, dispersed in 250 ml THF in a 500 ml round-bottomed flask by 30 min sonication and 30 ml of thionyl chloride were placed. The resulting mixture was kept for stirring at 30 C for 3 h. After that, the extra thionyl chloride and solvent was removed by centrifugation. Afterwards, 1 g of the resulting acylated DMAPSiGO, dispersed in 200 ml THF in a 500 ml round-bottomed flask by 30 min sonication and 30 ml of 1-methylimidazole were added. The resulting mixture was stirred at 30 o C for 24 h. The resulting AGO was obtained by centrifugation and dried in vacuum oven at 60 o C overnight. 1 Preparation of brominated poly(phenylene oxide) (Br-PPO) To a stirred solution of 2.4 g PPO (20 mmol) in chlorobenzene (20 ml) was added N- bromosuccinimide (NBS) (2.314 g, 13 mmol), and 2,2 -azobis-isobutyronitrile (0.1 g, 0.6 mmol). The mixture was heated at reflux conditions (135 o C) for 3 h. After cooling, the reaction mixture was poured into a 10-fold excess of ethanol to precipitate the product. The polymer was filtered and washed with ethanol. The polymer was collected as a light yellow powder and dried under vacuum for overnight 2

3 to get Br-PPO with a bromination ratio: ~55 % (DS=0.55). Preparation of PPO-TMA membrane PPO-TMA membrane was prepared by: dissolved Br-PPO (1.5 g) into N- Methylpyrrolidone (10 ml) at 50 o C and added trimethylamine (1.5 ml, 30% in ethanol by weight). The reaction mixture was stirred at room temperature for 48 h. The resulting solution was cast onto clean glass plate and dried at 80 o C for 24 h. Preparation of PPO-AGO membrane For preparation of PPO-AGO membrane, 10 wt % AGO to solid Br-PPO were added into a 10 wt % Br-PPO solution in NMP. The resulting mixture was stirred at 50 o C for 48 h, cast onto clean glass plate and dried at 80 o C for 24 h to obtain a membrane. Further, the membrane was dipped into 30 wt % solution of trimethyl amine to functionalize the remaining unreacted bromo-methyl groups. AFM measurement Ionic conductivity measurements were performed by the PeakForce TUNA. For current measurements platinum coated OMCL AC240TM probes (Olympus) and for structure measurements SHR 150 (Budget Sensors) were used. For ensuring sufficient humidification of the samples during conductivity measurements, a hydrophilic gas diffusion layer (GDL) that was connected to a water reservoir was placed beneath the membrane adjacent to the measurement spot. For current measurements a voltage between sample and tip of V was applied in which the sample was negatively poled. Brighter colors of the AFM images are always related to higher values of the respective properties. Therefore, the current images have been inverted for display. All error bars show the mean error of the average for p(one sided) =

4 WU equilibrium Park model equation expressed as: C = K H a + A Lb L a 1+b L a + K aa n where C - water concentration, expressed in cm 3 (STP) of absorbed water per gram of dry polymer; K H - chain segment mobility; a - activity of water; A L - free volume and solubility; b L - constant for the same polymer matrix; K a - the equilibrium constant for the clustering formation; n - mean size of water aggregate. The first term describes Henry type sorption; the second is Langmuir type sorption; the third term corresponds to a power function describing the aggregation phenomenon. The table below shows each fitting parameter of AEMs at 30 C according to the Park model equation above. Table S1. Park model fitting parameters at 30 C. K H R 2 (Henry) A L b L K a n R 2 (clustering) HMT-PMBI PPO-AGO PPO-TMA ETFE-TMA ETFE-TEA Figure S1 shows WU contributions from each type of absorption for AEMs according to the fitting parameters in Table S1. 4

5 Figure S1. Contributions of (a) Langmuir, (b) Henry and (c) clustering type of absorption for different AEMs. 5

6 AFM characterization Cross-sections of HMT-PMBI showed small, highly deformable, spherically shaped particles in the membrane, marked by the red arrow in the deformation mappings (Figure S2(a), (b)). These spots increased in size with increasing RH from 40 to 80 % (Figure S2(d)), which indicates water uptake and a hydrophilic nature of this polymer phase. Ionic current measurements (Figure S2(c)) revealed a low conductivity over the entire surface with current values of 1 to 2 pa. Furthermore, highly conductive spots and lamellae, marked by the red arrow in Figure S2(c) (I > 30 pa), with a size in the order of magnitude of the highly deformable spots were apparent. Figure S2. AFM deformation image of cross-sectioned HMT-PMBI membrane at (a) 6

7 40 % RH, and (b) at 80 % RH, (c) inverted current mapping with an applied bias of V, (d) dependence of mean diameter of highly deformable features on increasing RH. AFM-investigation of PPO-AGO at 40 % RH showed three different phases in the deformation mapping (Figure S3(a)): a highly deformable phase in the shape of small spherical particles that are homogeneously spread over the surface, a lowly deformable lamellar phase, and a phase showing medium deformation. Rising the humidification level to 80 % RH led to an increasing area fraction of the highly deformable phase (see Figure S3(b) and (c)). The mean diameter of the highly deformable phase increased slightly (Figure S3(d)), whereas the lowly deformable lamellae did not change significantly (Figure S3(e)). This suggests that the highly deformable phase is water-rich and hydrophilic, whereas the lowly adhesive phase can be assigned to the graphene oxide sheets. By increasing RH, the deformation of the medium deformable phase increased and also indicates a water uptake. Additionally, a few features with highest deformation became apparent at 80 % RH. 7

8 Figure S3. AFM deformation images of PPO-AGO cross-section at (a) 40 % RH, and (b) 80 % RH. (c) Fraction of highly deformable area, (d) dependence of diameter of the highly deformable phase on relative humidity. (e) Diameter of lowly adhesive phase over relative humidity level. WU kinetics from vapor 8

9 Figure S4. Normalized mass gain of HMT-PMBI, PPO-TMA, PPO-AGO, ETFE- TMA and ETFE-TEA membranes versus time at three RH intervals: (a) and (d) 5%- 25%; (b) and (e) 25%-45%; (c) and (f) 45%-65% at 30 C [(a)-(c)] and 60 C [(d)- (f)], t = 0~50 min. Figure S4 shows the same results as Figure 11 and presents them in a larger scale of x-axis (from 0~10 min to 0~50 min). The maximum time needed for equilibrium WU in each step in Figure S4((a)-(f)) gets shorter with temperature increase, as can be noticed from the length of the curves: 50 min vs. 35 min at low RH and 50 min vs. 38 min at medium RH. The high RH step (45%-65%) shows comparable maximal times at different temperatures. 9

10 τ(sec) τ(sec) τ(sec) HMT-PMBI Small RH increment Large RH increment Relative humidity (%) PPO-TMA Small RH increment Large RH increment PPO-AGO Small RH increment 2000 Relative humidity (%) Large RH increment Relative humidity (%) 10

11 τ(sec) τ(sec) 1200 ETFE-TMA Small RH increment Large RH increment Relative humidity (%) 800 ETFE-TEA Small RH increment Large RH increment Relative humidity (%) Figure S5. Time constants versus RH in small RH increment (5-10%) and large RH increment (20%, measured at 5, 25 and 45% RH) for HMT-PMBI, PPO-TMA, PPO- AGO, ETFE-TMA and ETFE-TEA membranes at 30 o C. See from Figure S5, all AEMs with different RH incensements has slight difference in kinetics. Similar magnitude of τ values are obtained at RH= 25 and 45%. At 5% RH, AEMs experienced large RH increase show the kinetics behaviors match the trend of τ at the beginning. 11

12 Water vapor absorption mechanism In Figure S6, HMT-PMBI membrane at RH = 45%, absorption and desorption curves coincide, suggesting Fickian mechanism. The Fickian diffusion is valid for all temperatures. While at lower or higher RH ranges, its sigmoidal shape implies the sorption process is controlled by both diffusion and interfacial mass transport (non- Fickian). 2,3 Figure S6. The absorption mechanisms of HMT-PMBI membrane at 30, 40, 50 and 60 o C. PPO-TMA (Figure S7) and PPO-AGO (Figure S8) present similar trends along RH as HMT-PMBI membrane at corresponding temperatures, though not as clearly. 12

13 Figure S7. The absorption mechanisms of PPO-TMA membrane at 30 and 60 o C. Figure S8. The absorption mechanisms of PPO-AGO membrane at 30 and 60 o C. In Figure S9, ETFE-TMA has similar mechanism as ETFE-TEA membrane at the 13

14 same temperature shown as Figure 13. Figure S9. The absorption mechanisms of ETFE-TMA membrane at 60 o C. References (1) Gopi, K. H.; Peera, S. G.; Bhat, S. D.; Sridhar, P.; Pitchumani, S. Preparation and Characterization of Quaternary Ammonium Functionalized poly(2,6- Dimethyl-1,4-Phenylene Oxide) as Anion Exchange Membrane for Alkaline Polymer Electrolyte Fuel Cells. Int. J. Hydrogen Energy 2014, 39 (6), DOI: /j.ijhydene (2) Krtil, P. Kinetics of Water Sorption in NafionThin Films - Quartz Crystal Microbalance Study. J. Phys. Chem. B 2001, No. 105, DOI: /jp004162t. (3) Rivin, D.; Kendrick, C. E.; Gibson, P. W.; Schneider, N. S. Solubility and Transport Behavior of Water and Alcohols in Nafion TM. Polymer (Guildf). 2001, 42 (2), DOI: /S (00)