Supporting Information
Improved Electrochemical Performance of Alkaline Batteries Using Quaternary Ammonia PolysulfoneFunctionalized Separators Hooman Hosseini*, Donald A Dornbusch, Galen J. Suppes Department of Chemical Engineering, University of Missouri-Columbia, W2033 Lafferre Hall, Columbia, Missouri 65211
*
[email protected]
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Figure S1. Schematic of general batteries: illustration of the general types of batteries with four components (1anode, 2-cathode, 3- electron transport, 4-ion transport) and their transformation into diffusive cells or flow convective cells 1 Reprinted with permission from 1 Copyright 2011 AIChE.
Experimental Procedures
Figure S2. Schematic of coating procedure and functionalized substrate preparation and testing: (a) QAPSF synthesis, chloromethylation and ammonization; (b) polyelectrolyte (PE) dilution; (c) soaking the filter papers in PE; (d) vacuum oven extraction – solvent depletion; (e) ion exchange process with electrolyte – electrolyte uptake; (f) testing the functionalized separators in batteries (either sandwich diffusion or convective-flow batteries (CFBs)).
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Cell Preparation Cathode and anode materials were applied by spatula to a 0.4-mm depression on the end of the thicker current-collecting piston, and then scraped flat to ensure a uniform thickness. The slurry-coated pistons were allowed to dry for approximately 40 min, followed by 5 min under vacuum (90 kPa), to completely remove any volatile binder solvent from the system. Typically, the anode: cathode mass ratio after drying was approximately 2.5:1 (slightly greater than the actual stoichiometric ratio of 1.86:1, according to the experiments). The electrodes were assembled facing each other, and the coated separator was in between them.
Figure S3. Schematic of the convection cell setup (a), outside view of an expanded cell (b), and cross-sectional view of an assembled cell (c).2
Polyelectrolyte (PE) Synthesis Chloromethylated polysulfone (CMPSF) was synthesized by the Friedel-Crafts-type reaction reported by Avram and co-workers. A total of 2 g of polysulfone (PSF, Sigma Aldrich, Mv ~ 75,000 g/mol) were dissolved in 0.1 L of chloroform (CHCl3, ACS grade, Fisher). After the PSF was dissolved, the mixture was transferred to a bulb flask with a stir bar equipped with a reflux condenser. Then, 1.36 g of
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paraformaldehyde (Sigma Aldrich, Reagent Grade) were added to the bulb flask, and the solution was mixed while raising the temperature to 55°C. At 55°C, 6 mL of chlorotrimethylsilane (Sigma Aldrich) and 105 μL of SnCl4 (Sigma Aldrich) catalyst were added. The headspace of the bulb flask was blanketed with nitrogen and sealed. The reaction was carried out for various amounts of time to control the degree of functionalization (DF) of the chloromethylated sites (DF=0.98 for 38 h of reaction time in this case). The CMPSF reaction mixture was filtered, and the filtrate was precipitated in MeOH (Fisher, ACS Grade) (5:1 volume ratio of MeOH to CMPSF solution). The solution was then vacuum filtered, and the solid was collected. CMPSF was further purified by dissolving it in 80 mL of CHCl3. The dissolved CMPSF was filtered, and the filtrate was precipitated in MeOH. The collected solid was dried in a vacuum oven at 50°C for 12 h, and dried CMPSF powder was obtained. CMPSF was dissolved in NMP. For PSF–TMA+ PE, a 3:1 molar ratio of base reagent to chloromethylated sites was used to determine the amount of TMA to add to the dissolved CMPSF 3. The reaction was continued at 30°C for 48 h. The reaction solutions were used in both the filter paper coatings and the AEM separator casting. The degree of functionalization of PSF by chloromethylation was derived from the experimental table presented by Arges et al.3, and the corresponding DF was 0.98 for a reaction time of 38 h. This information was also confirmed by the 1HNMR results based on the following equation:
DF=
3 AreaCH2 Cl substituent AreaPSF substituent
The theoretical DF is 0.98 DF; the reaction time may promote cross-linking, which should be controlled to avoid gelation. Based on the characteristic chart presented by Arges et al.3 (Figure S4), the DF for this process is 0.98.
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Figure S4: The degree of functionalization (DF) for the chloromethylation of polysulfone (PSF) at different reaction times for
multiple batches (red data points) and for a single batch (black data points with line). Reproduced with permission from3
Copyright 2012 RSC
The PSF structural unit has a molecular weight of 442 g/mol, and, after functionalization, the molecular weight (MW) of the CMPSF structural unit is 489.432 g/mol (MW of the halogenated group added as a pendant group to the main chain is 49.4 for CH2Cl). Dividing the mass of the halogenated polymer to be used in functionalization by the MW of the halogenated polymer (489.432 g/mol) gives the moles of reactive halogenated sites ready for reaction with trimethyl ammine (TMA). According to the original procedure developed by Avram and Arges, the moles of base reagents (TMA) should be three times greater than the moles of halogenated sites to ensure complete substitution of halogenated groups with trimethyl ammonia in coordination with negative chloride (quaternary ammonia)3-4. The molar proportion of 3:1 for TMA to CH2Cl substituents has been verified by 1HNMR measurements in the literature. This process typically results in a 4 to 5% concentration of QAPSF in the corresponding solvent (DMF, NMP or DMSO).
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Figure S5. Functionalization of polysulfone by the Avram method modified by Pan & Arges to obtain QAPSF in both chlorinated and hydrated form.
Another way to obtain the QAPSF 1HNMR measurement is to precipitate the QAPSF solution (DMF solvent) in diethyl ether at a 1:5 proportion and wash the precipitate several times with diethyl ether. The filtrate is then placed in a vacuum oven for 15 h. The obtained solid filtrates can then be dissolved in dDMSO for 1HNMR measurements. The conversion of chloromethylated sites to cation sites is calculated as follows: Conversion=
Areacation substituent (δ) Ratio.DF.AreaPSF substituent
assuming that the conversion is almost 1. Determining the theoretical IEC for the AEMs and polyelectrolytes obtained from this process is performed as follows: Theor. IEC [
𝑚𝑚𝑜𝑙 𝑔
]=
DF∗1000 (MWPSF, monomer +DF.MWCation )
. conversion
Where MWCation = (MWcation free base conjugete + MWcounterion + MWCH2 − 1) S6
So, MWCation = (59.11 + 14 + 35.4 − 1) = 107.51 MW 0.98∗1000
Theor. IEC = (442+0.98∗107.51)= 1.79 [
𝑚𝑚𝑜𝑙 𝑔
]
Equation 3-5 Equation 3-6
The ion exchange capacity can be calculated theoretically or based on the titration results (tabulated in (65)), but here, only the theoretical work is presented. Figure S6 depicts each step of the chloromethylation reaction. 1HNMR was used to examine the molecular structure variation of functionalized PSF. The aryl ring between the isopropyl and ether bond is the site of the chloromethylation reaction. The concentration of the attached chloromethyl group can be estimated based on the integrated intensity of peak 5 (δ= 3.0 (s, 9H, CH3)). According to this finding and to other studies, there is never more than one chloromethyl group attached to one PSF unit.3 Conversion from a chloromethyl group into a quaternary ammonium group can be observed in Figure S6 (δ= 4.6, (br s, 2H, CH2)). The newly formed peak 6 is attributed to this conversion.
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Figure S6. NMR spectra of a) polysulfone (PSF), b) chloromethylated PSF and c) quaternary ammonia polysulfone (QAPSF or PSF-TMA+).
Table S1. Summary of the Chemical Components found in PSF, CMPSF and PSF-TMA+ and their Chemical Shifts in 1HNMR Spectra Chemical Component
Aromatic Hydrogen on Phenyl
CH3 on PSF Backbone
-CH2Cl
-N+(CH3)3
Phenyl-CH2N+(CH3)2
8
1.7
4.5
3.1
4.4
Chemical Formula Chemical Shift (δ ppm)
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Figure S7. (a) Schematic of the open pores available for ion transfer in functionalized substrates/separators in batteries at high drainage. (b) Using concentrated polyelectrolyte blocks the way of ion movement in the pores (c) Using lower concentrations paves the way for mass transfer but weakens the ion exchange enhancements – there should be an optimal polyelectrolyte (PE) concentration used to impregnate the substrate (10-ohm constant resistant load – equivalent to 150 mA discharge current- 92 mA/cm2).
Through the calculation of density for QAPSF based on molecular weight (1.5), volume measurement and estimation of volume expansion (thickness) after coating (Typically 20%), amount of mass gain can be calculated. Our calculations show mass gains less than 0.3% is enough to coat the fibers with more than 10 nm coated thickness and this fact has been verified by experimental data.
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Figure S8. Voltage performance versus specific capacity for three separate experiments via SDBs containing EMD paste materials, 6 M aqueous KOH electrolytes and two PallFlex filter papers as separators (10-ohm constant resistant load – equivalent to 150 mA discharge current- 92 mA/cm2).
Figure S9. Normalized voltage performance for different mass increases on the substrate, 6 M aqueous KOH electrolytes and two PallFlex filter papers as separators (10-ohm constant resistant load – equivalent to 150 mA discharge current- 92 mA/cm2). Comparison with an inert substrate.
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Figure S10. Voltage performance versus specific capacity for SDBs containing EMD paste materials, 6 M aqueous KOH electrolytes and three Whatman GF-A filter papers as separators (68-ohm constant resistant load); inert filter paper versus QAPSF-functionalized substrate.
Figure S11. Normalized voltage performance for SDBs containing EMD paste materials, 6 M aqueous KOH electrolytes and three Whatman GF-A filter papers as separators (15-ohm constant resistant load); inert filter paper versus QAPSF-functionalized substrate.
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Figure S12. Voltage performance versus specific capacity for SDBs containing EMD paste materials, 6 M aqueous KOH electrolytes and three P8 filter papers as separators (15-ohm constant resistant load).
Figure S13. Voltage performance versus specific capacity for SDBs containing EMD paste materials, 6 M aqueous KOH electrolytes and three P8 filter papers as separators (15-ohm constant resistant load); inert filter paper versus QAPSF-functionalized substrate.
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Figure S14. Normalized voltage performance for SDBs containing EMD paste materials, 6 M aqueous KOH electrolytes and three Whatman GF-A filter papers as separators (15-ohm constant resistant load); inert filter paper versus QAPSF-functionalized substrate.
Figure S15. Voltage performance versus specific capacity for SDBs containing EMD paste materials, 6 M aqueous KOH electrolytes and three Whatman GF-A filter papers as separators (15-ohm constant resistant load); inert filter paper versus QAPSF-functionalized substrate.
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Figure S16. Whatman GF-D filter papers soaked in 6 M KOH and DI water for 24 h. That soaked in 6 M KOH exhibits degradation and extensive hydration due to exposure to the harsh alkaline media.
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Figure S17. Filter papers soaked in 6 M KOH aqueous solution (left) and DI water (right) for 24 h. The 6 M KOH does appear to have a negative impact on the chemical stability of the Fisher P8 filter paper, but degrades the cellulose coatings.
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Electrochemical Impedance Spectroscopy (EIS)
Figure S18: Conductivity measurement assembly using sandwich electrodes connected to a Gamry Interface 1000 under sinusoidal voltage applied to measure the current response.
For initial evaluation, the conductivity of the dried filters was measured for comparison, and the filter paper was then soaked in 1 M and 6 M aqueous KOH solutions alongside filter paper coated with QAPSF that had previously been soaked in 6 M KOH for 24 h prior to examination by EIS. The configuration is presented in Figure S18 for sandwich diffusion batteries (SDBs). Separators were placed between stainless steel electrodes connected to the Gamry Interface 1000.
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Figure S19. Nyquist plots of dried P8 filter paper (sandwich diffusion-based systems).
The high-frequency real impedance in the negative region illustrates the fact that the system cannot determine any real positive value for the bulk resistance of dried filter paper because there are no ion species nor ion exchanger groups on the porous media. Therefore, the σ value for inert and dried P8 likely equals to zero based on the generation of meaningless results. For conductive membranes or coated surfaces with meaningful results, the original curves are in the form of whole frequency lines, and only the high-frequency intercept of these curves is the purpose of this study investigating the kinetically controlled regions for charge transfer analysis.
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Figure S20. General format of curves obtained showing the whole frequency range.
The next step includes the ionic conductivity measurement of P8 filter paper soaked in aqueous KOH solutions with different molarity (1 and 6). We expect is to see a meaningful trend between a different number of separators versus the detected charge resistance, showing the sensibility of ion conduction measurements and differentiating the different molarities in aqueous solutions. In 1 M KOH, the results of using one, three and six P8 filter paper separators soaked in solution for more than 2 h and sandwiched between the electrodes are presented in Figure S21. The real impedance increases at larger separation distances, in addition to good repeatability that is observable among the multiple experiments.
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Figure S21. Nyquist plot of filter paper soaked in 1 M aqueous KOH solutions for different separation thicknesses (0.17 mm, 0.51 mm and 0.106 mm) between stainless steel electrodes.
The average of these trends considering the standard deviations is presented in Figure S22.
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Figure S22. Nyquist plot of filter paper soaked in 1 M aqueous KOH solutions for different separation thicknesses (0.17 mm, 0.51 mm and 0.106 mm) between stainless steel electrodes.
The same trend can be observed in separators soaked in 6 M KOH in the difference that corresponding high impedances shift to lower impedance regions, representing the higher ionic conductivity due to the greater hydroxide concentration between the sandwich layers (Figures S23 and S24).
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Figure S23. Nyquist plot of filter paper soaked in 6 M aqueous KOH solutions for different separation thickness (0.17 mm, 0.51 mm and 0.106 mm) between stainless steel electrodes.
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Figure S24. Nyquist plot of filter papers soaked in 6 M aqueous KOH solutions for different separation thicknesses (0.17 mm, 0.51 mm and 0.106 mm) between stainless steel electrodes.
All of the ionic conductivities (resistivity) can be calculated based on these curves, and they have been included in Table S2. Plotting the charge transfer against the number of filter paper pieces will represent a trend independent of filter paper numbers.
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Table S2. Charge Resistance correlation
Soaking Media KOH 1 KOH 1 KOH 1 KOH 6 KOH 6 KOH 6
FP Number 1 3 6 1 3 6
Total Thickness (mm) 0.17 0.51 0.106 0.17 0.51 0.106
Rct (Ave) 0.306 0.704 1.298 0.1744 0.32 0.5173
The conductivity of 1 M and 6 M KOH hydroxide ions through porous media can be calculated regardless of the number of separators from the slope of the lines in Figure S25. The y-intercept is believed to be the electrode resistance and ohmic overpotentials, and the intercepts are matched together at one point (0.1114).
Figure S25. Charge resistance versus number of filter paper for porous separators soaked in 1 M and 6 M aqueous KOH solutions.
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The conductivity increases more than three times by shifting to a higher ion containing system. Reaching a conductivity of approximately 200 mS/cm calls into question the actual conductivity of the 6 M aqueous KOH solutions tabulated in literature (Figure S26). The conductivity of the 6 M aqueous KOH solution is approximately 600 mS/cm, which is more than three times than obtained with our proposed experimental system. The ionic movement in the sandwich diffusive system was measured in the porous separator media, not the free liquid bulk media; therefore, the percentage of empty space in the filter paper (soaked in the aforementioned electrolyte) is the confinement media for allowing the ions to move from electrodes to carry the charge species.
Figure S26. Specific conductivity versus molarity at 25°C. Reprinted with permission from 5. Copyright 2007 Elsevier
The percentage of porosity in the P8 filter paper is measurable by the volume absorption test. This is essentially conducted by measuring the difference between the water volume absorbed by 32 pieces of filter S24
paper (or the volume displaced by water when the filter papers are mounted in aqueous media) and actual volume of 32 pieces of filter paper. Therefore, the void space can be easily calculated as follows: (Actual volume of filter paper – Volume displaced by water) / Volume displaced by water = (1.55-1)/ 1.55= 35.50 Therefore, there is 35.5% void space in the P8 filter paper, which is the free space for ion movement through the EIS conductivity measurements. This concept should be included in calculations by adding this factor and dividing the measured conductivities over the free space percentage 197.25⁄0.355= 555.63 mS/cm, which is close to that reported in the literature for the conductivity of 6 M KOH (Figure S26), verifying the validity of our results.
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Figure S27. Nyquist plot of Fisher P8 filter paper soaked in 6 M aqueous KOH solutions as a control, and P8 impregnated with NMP solvent, which does not follow the same trend and indicates a negative impact of NMP on electrical resistance.
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Figure S28. Nyquist plot of Fisher P8 filter paper soaked in 6 M aqueous KOH solutions as a control, and P8 impregnated with all types of polymer systems, which does not exhibit a meaningful decline in electrical resistance.
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Figure S29. Nyquist plot of PallFlex filter paper soaked in 6 M aqueous KOH solutions as the inert material, compared to substrates coated with QAPSF (1 FP).
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Figure S30. Nyquist plot of Whatman GF-A filter paper soaked in 6 M aqueous KOH solutions as the inert material compared to substrates coated with QAPSF (1 FP).
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Figure S31. Nyquist plot of Whatman GF-A filter paper soaked in 6 M aqueous KOH solutions as the inert material compared to substrates coated with QAPSF (3 FPs).
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Figure S32. Nyquist plot of Whatman GF-D filter paper soaked in 6 M aqueous KOH solutions as the inert material compared to substrates coated with QAPSF (1 FP).
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PallFlex Q (Anion Exchange Membrane)
Figure S33. Nyquist plot of filter paper soaked in 6 M aqueous KOH solutions with different thickness of the PallFlex Q membrane.
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PallFlex S (Cation Exchange Membrane)
Figure S34. Nyquist plot of filter paper soaked in 6 M aqueous KOH solutions with different thickness of the PallFlex S membrane.
In CFBs, the lack of hydroxide at the anode and the hydroxide accumulation at the cathode is eliminated by convective forces. In this case, double- and triple-layer movements also occur, and migration is strengthened facing the convective force, which boosts the battery performance as observed in Figure S35. However, examining CFBs is not the concern of this research, and a separate study is recommended to verify their efficiency.
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Figure S35. Mechanism of ion exchange improvement by the PSF-TMA+ coating on filter paper in sandwich diffusion (SD) and convective flow (CF) zinc-alkaline batteries.
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Figure S36. Mechanism of ion exchange by the PSF-TMA+ coating on filter paper in EIS.
Figure S37. Mechanism of ion exchange by CEMs such as Nafion or Pall S membranes in SDBs.
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References. (1) Suppes, G. J.; Sawyer, B. D.; Gordon, M. J., High-energy density flow battery validation. AIChE J. 2011, 57, 1961-1967. (2) Dornbusch, D. A.; Hilton, R.; Lohman, S. D.; Suppes, G. J., Experimental Validation of the Elimination of Dendrite Short-Circuit Failure in Secondary Lithium-Metal Convection Cell Batteries. J. Electrochem. Soc. 2015, 162, A262-A268. (3) Arges, C. G.; Parrondo, J.; Johnson, G.; Nadhan, A.; Ramani, V., Assessing the influence of different cation chemistries on ionic conductivity and alkaline stability of anion exchange membranes. J. Mater. Chem. 2012, 22, 3733-3744. (4) Avram, E.; Butuc, E.; Luca, C.; Druta, I., Polymers with Pendant Functional Group. III. Polysulfones Containing Viologen Group. J. Macromol. Sci., Part A: Pure AppL. Chem. 1997, 34, 1701-1714. (5). Gilliam, R. J.; Graydon, J. W.; Kirk, D. W.; Thorpe, S. J., A review of specific conductivities of potassium hydroxide solutions for various concentrations and temperatures. Int. J. Hydrogen Energy 2007, 32, 359-364.
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