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Highly phosphonated polypentafluorostyrene: Characterization and blends with polybenzimidazole
European Polymer Journal 49 (2013) 3977–3985
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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Highly phosphonated polypentafluorostyrene: Characterization and blends with polybenzimidazole Vladimir Atanasov a,⇑, Dietrich Gudat b, Bastian Ruffmann c, Jochen Kerres a,d a
Institute of Chemical Process Engineering, University of Stuttgart, Germany Institute of Inorganic Chemistry, University of Stuttgart, Germany c Hydrogen and Informatics Institute for Applied Technology, Schwerin, Germany d Focus Area: Chemical Resource Beneficiation, North-West University, Potchefstroom 2520, South Africa b
a r t i c l e
i n f o
Article history: Received 14 June 2013 Received in revised form 5 September 2013 Accepted 6 September 2013 Available online 15 September 2013 Keywords: Phosphonated polymer Polyelectrolyte Fuel cell Conductivity Blend membrane Doping with phosphoric acid
a b s t r a c t In this study we present results of the conductivity and resistance to thermooxidative and condensation reactions of a highly phosphonated poly(pentafluorostyrene) (PWN2010) and of its blends with poly(benzimidazole)s (PBI). This polymer, which combines both: (i) a high degree of phosphonation (above 90%) and (ii) a relatively high acidity (pKa (–PO3H2 M –PO3H) 0.5) due to the fluorine neighbors, is designed for low humidity operating fuel cell. This was confirmed by the conductivity measurements for PWN2010 reaching r = 5 104 S cm1 at 150 °C in dry N2 and r = 1 103 S cm1 at 150 °C (k = 0.75). Furthermore, this polymer showed only 48% of anhydride formation when annealing it at T = 250 °C for 5 h and only 2% weight loss during a 96 h Fenton test. These properties combined with the ability of the PWN2010 to form homogeneous blends with polybenzimidazoles resulting in stable and flexible polymer films, makes PWN2010 a very promising candidate as a polymer electrolyte for intermediate- and high-temperature fuel cell applications. Ó 2013 Published by Elsevier Ltd.
1. Introduction The growing need of devices delivering electricity in different locations is essential in the modern world. A proton – exchange membrane fuel cell (PEM FC) is one example of such a device using a polymer electrolyte as ionconductor. The most commonly used polyelectrolytes are based on sulfonated polymers, e.g. NafionÒ. Their proton conductivity is principally based on water bridging the sulfonic acid function, where the water molecule’s diffusion serves as a vehicle for the proton transport, either as H3O+, H5 Oþ 2 ions, etc. (vehicular proton-transport mechanism), or as a medium for proton tunneling via hydrogen bridges (Grotthus proton transport mechanism) [1]. Therefore, at temperatures above 100 °C, most of these polyelectrolytes show a dramatic decrease of ion-conductivity due ⇑ Corresponding author. Tel.: +49 711 68585163; fax: +49 711 68585242. E-mail address: [email protected] (V. Atanasov). 0014-3057/$ - see front matter Ó 2013 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.eurpolymj.2013.09.002
to water evaporation. The increase in FC operating temperature is, however, required for attenuating the activation enthalpy and overcoming the poisoning effect of CO on the Pt-catalyst by temperature-driven acceleration of the electrode kinetics. Thus, polyelectrolytes based on phosphoric acid (PA) doped polybenzimidazole (PBI) currently attract considerable attention [2].The conductivity in this case is based on the excellent proton-carrier properties of PA [3]. Moreover, high doping levels of PA are required (commonly above 200%) to induce sufficient proton conductivity [4], which can lead to unsustainability and leakage of PA from the polymer membrane especially at FC operation temperatures below 100 °C. One approach to overcome these drawbacks lies in the utilization of highly phosphonated polymers where the phosphonic acid group serves as an amphoteric proton conductor substituting the role of PA. In comparison to the sulfonated ionomers, there are currently only few examples of phosphonated polymers being used as proper electrolytes for FCs [5–7], which
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can be ascribed to their moderate conductivity and the greater synthetic efforts required for phosphonation compared to sulfonation. While the advantage of the phosphonated polymers lies in their amphiphilicity, which allows them to conduct protons in a reduced water content environment, the disadvantage of the phosphonated polymers lies in their moderate conductivity due to their lower acidity, compared to sulfonic acids [8,9]. In order to increase proton conductivity, the polymer must be at least locally highly phosphonated, which will positively influence the percolation of the protons through the membranes. Therefore, in the past few years considerable attention was paid to the preparation and properties of highly phosphonated polymers [3,5,10]. Both concepts of polymerization of phosphonated monomers and post-phosphonation of polymers have been introduced. While post-phosphonation is the easier synthetic approach, in most cases phosphonated polymers with a relatively low functionalization degree, due to the relatively low reactivity of the educts and/or side reactions, have been obtained by this synthetic route. A novel highly phosphonated polymer (PWN2010) based on poly(pentafluorostyrene) (PFS) was recently synthesized in our group [11]. Due to its perfluorinated phenyl rings, PFS easily undergoes nucleophilic substitution reactions (Michaelis–Arbuzov reaction) with phosphites such as tris(trimethylsilyl)phosphite, resulting in a polymer with a degree of phosphonation above 90%. In this paper we present PWN2010 in terms of its most critical properties for FC, such as proton conductivity in reduced water content, resistance to radical attack and the possible occurrence of condensation reactions between adjacent phosphonic acid groups at high temperatures. Blends of PWN2010 with Poly-[(1-(4,40 -diphenylether)-5oxybenzimidazole)-benzimidazole] (PBIOO) and the FC performance of these new class of acid-base blend membranes is discussed. Blending with PBIOO was necessary due to the inability of PWN2010 to form stable polymer films suitable for FC applications. This is due to its water solubility as a consequence of the high phosphonation degree. The blend-membranes comprising ionical cross-linking sites by the protonation of the basic benzimidazole units, have been already verified as a suitable platform for the preparation of highly proton-conductive membranes with reduced water swelling for the application in electro-membrane processes such as electrodialysis [12], FCs [13,14] and water electrolysis [15]. 2. Experimental section 2.1. Materials PWN2010 was prepared as reported earlier [11]. PBIOO was purchased from FuMA-Tech GmbH. All the other chemicals were used in HPLC grade quality. 2.2. PWN2010/PBIOO blends preparation PWN2010 and PBIOO were separately dissolved in DMSO (10 and 5 wt.% respectively). PWN2010/DMSO solution was neutralized with triethylamine (TEA) (4/1
equiv. = TEA/–PO3H2). PWN2010 and PBIOO solutions were mixed (imidazole/–PO3H2 = 3/7 molar) resulting in a viscous mixture. The mixture was cast on a pre-heated (100 °C) glass-plate using a doctor-blade (0.6 mm). The polymer film was dried at 100 °C for 18 h, followed by 120 °C, p = 1 103 mbar (vacuum) for 4 h. Subsequently, the polymer film was detached from the glass-plate and conditioned by immersion in 30% HCl at 90 °C for 3 h. The polymer films were rinsed with water and dried in vacuum at 120 °C for 2 h. Doping of PWN2010/PBIOO blends with PA: Dried polymer films were weighed and immersed into 85% PA for a given temperature and time (see Fig. 3 in Supporting materials). Then, the films were pressed between paper tissues to remove the residue PA from their surface. Before measuring the PA doping degree, blends were dried at 120 °C under vacuum for 2 h. 2.3. Instruments Solution NMR spectra were recorded on a Bruker Avance 400 spectrometer at a resonance frequency of 250 MHz for 1H, 62.9 MHz for 13C, 235 MHz for 19F and 101.2 MHz for 31P NMR at RT. Solid state NMR spectra were recorded at RT on a Bruker Avance 400 spectrometer equipped with a 4 mm magic angle spinning (MAS) probe at a resonance frequency of 162.9 MHz for 31P. All experiments were performed under MAS with spinning speeds between 8 and 9 kHz. High-power 1H decoupling was applied during data acquisition, and cross polarization with a ramp-shaped contact pulse and mixing times of 5 ms was used for signal enhancement. Deconvolution of observed spectra into a sum of individual spinning sideband manifolds was performed with the spectrometer software. Molecular weights and molecular weight distributions were determined by GPC in water on Waters pump model 515, eluent: 0.1 M NaNO3, detectors: RI ERC-101 and UV–VIS Soma S-3702 (270 nm), temperature: 30 °C, standard: PSSNa, concentration: 1.000 g/l, flow rate: 1.0 ml/min, columns: MCX, MCX 10 E7 and MCX 1000. FTIR spectra were recorded as KBr-pellet on a Nicolet 6700 FTIR instrument. The thermal stability of the polymers and membranes was determined by thermogravimetry (TGA, Netzsch, model STA 449C) with a heating rate of 20 °C min1 under an atmosphere enriched with oxygen (65–70% O2, 35–30% N2). The outlet with the released volatile-products was continuously analyzed by a FTIR spectrometer (Nicolet Nexus FTIR spectrometer) to determine the water and the onset temperature of the splitting-off of the phosphonic acid function by the stretching vibration at 1050 cm1. Proton conductivities were measured by ac-impedance spectroscopy (HP-ac-impedance analyzer 4192A LF) using a two-electrode arrangement. The measurements were performed in a closed cell with pre-dried (50 °C, vacuum, 1 week) samples (pressed pellets with diameter 4 mm, thickness 2–5 mm) with gold electrodes. Conductivity measurements in 100% water vapor (p(H2O) = 105 Pa) were carried out in a double-wall temperature-controlled glass chamber with an open outlet at temperatures T = 110– 160 °C. Liquid water was continuously evaporated by a heater and injected into the chamber with a constant flow rate using a digital peristaltic pump (Ismatec). Inside the
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chamber pressed pellets of PWN2010 polymer powder (diameter 6 mm and total thickness of 2–3 mm) were placed in a notched cylindrical glass-tube with a charcoal-coated electrode at the bottom. The second electrode was pressed from the top onto the pellet by a screw in order to ensure optimal contact. The specific conductivity was derived by r = l/(A R), where l is the distance between the electrodes (in cm), A the area of the pellet (in cm2), and R the resistance (in Ohm) derived from the high-frequency intercept of the complex impedance with the real axis. The proton conductivities of PA-doped PWN2010/PBIOO blend membranes under fully or partially hydrated conditions were recorded on a FumaTech MK3 setup with a 4-electrode in-plane AC arrangement, in the frequency range from 1 to 106 Hz. The temperature of the water reservoir was fixed at 100 °C, while the sample temperature was above 100 °C to prevent water condensation in the sample holder. The relative humidity values in the test cell at T = 110–160 °C corresponds to 69.5–15.7%, respectively. To attain equilibrium, all membranes were conditioned for 2 h at each temperature-point. Water content was determined using a Karl Fischer coulometer (Metrohm KF-Coulometer 831 with 774 Oven Sample Processor with HydranalÒ Coulomat AG oven titration reagents) by the method described elsewhere [16]. Scanning Electron Microscopy (SEM) images were recorded on a CamScan CS44 instrument equipped with a wolfram cathode with a working voltage of 15 kV. The surface of the membranes was covered via vacuum deposition with an ultra-thin Au or C layer prior to use. Oxidation Stability was tested by immersing the sample in a Fenton solution of 3 wt.% H2O2 containing 4 ppm Fe2+ at 68 °C. The membrane samples were then washed with water and dried at 120 °C, vacuum for 2 h. Fresh Fenton solution was prepared for successive measurements. In case of a pure PWN2010 after the incubation time in a Fenton solution, the mixture was dialyzed in water (MWCO: 12 kDa) and the PWN2010 was dried at 120 °C under vacuum for 2 h. Fuel cell tests were performed in a 25 cm2 test cell (quick Connect Fixture, Baltic FuelCells GmbH) with a contact pressure of 1 N/mm2. Dry hydrogen and dry air with k (anode) = 1.2 and k (cathode) = 2.5 were used with ambient pressure. The inner resistance Ri of the cell was measured with a DC mX-m (HIOKI) at a fixed frequency of 1 kHz. The membrane was investigated as MEA with GDE’s based on Freudenberg GDL H2315 I3 XC 190. GDE’s were produced by screen printing (EKRA 01) on the GDL, the noble metal load on the anode and the cathode was about 0.9 mg Pt/cm2 each.
3. Results and discussion The most critical issue for polyelectrolyte membranes (PEMs) is their performance (e.g. proton conductivity) at reduced water content conditions. Therefore, we firstly describe the proton conductivity of PWN2010 under nominally dry conditions (‘‘nominally dry’’ is used to emphasize the inability for complete removal of water in the case of PWN2010) (Fig. 1). PWN2010 was pre-dried at 120 °C, p = 1 103 mbar (vacuum) for 18 h in order to
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evaporate the removable water quantitatively. Subsequently, the conductivity as a function of temperature of a pellet of PWN2010 was measured under dried nitrogen flux. As expected, in absence of water susceptible to evaporation, the conductivity increased with temperature due to the decrease of the activation enthalpy. When comparing the conductivity of PWN2010 in a dry state with the conductivity obtained in a saturated water atmosphere (pwater = 105 Pa), a significant decrease of about 2–3 order of magnitudes at T = 110–150 °C was observed. This clearly illustrated the role of the water in the proton-conduction process in view of the same polymer being used for both measurements. However, when comparing the conductivity of PWN2010 to similarly pre-dried polymers such as NafionÒ and poly(vinylphosphonic acid) (PVPA), its conductivity appears to be an order of magnitude higher. This makes PWN2010 one of the best conducting polymers in dry conditions. In order to compare the proton conductivity of PWN2010 with the one of PVPA, we pre-dried PWN2010 in the same manner (50 °C, p = 1 103 mbar, one week) as PVPA [17] and measured the conductivity in a closed cell, so that the water content remained constant during the measurement (Fig. 2). In this case, the conductivity of PVPA was closer to the one of the PWN2010 from about an order of magnitude at T = 60 °C to nearly the same (r = 1 mS cm1) at T = 155 °C. This may be attributed to the higher concentration of phosphonic acid function in PVPA in comparison to PWN2010 (IEC (PVPA) = 9.2 mmol g1, IEC (PWN2010) = 3.5 mmol g1). Additionally, the drying conditions in this case are milder than in the previous one (see drying conditions in Fig. 1). This reduces the risk of phosphonic acid condensation reaction, which is well established in case of PVPA [6,19]. It is noteworthy that no hysteresis occurred during the heating and cooling cycles of PWN2010 (numbers beside the data-points of both Figs. 1 and 2 represent order of the measurements). This means that PWN2010 does not undergo any chemical changes in this temperature range (T = 40–160 °C). This result correlates well with the substantial resistance to thermal degradation of PWN2010, which was not observed for other highly phosphonated polymers such as PVPA (the issue is further discussed in Fig. 3). The conductivity of PWN2010 was also compared to a very recently reported highly phosphonated polymer (PAA-2) [18]. This polymer has shown a higher conductivity, however, under less defined drying and hence water content of the samples (without pre-drying and open to air chamber) than performed in this study. Therefore, activation enthalpy could not be calculated for PAA-2. In comparison to PVPA (Ea = 78 kJ mol1), PWN2010 had a lower activation enthalpy (Ea = 58 kJ mol1), which might be attributed to the lower pKa values being pKa1 (PWN2010) = 0.47 ± 0.36 compared to the pKa1 (PVA) = 2.00 ± 0.28 both calculated by ACD/pKa DB software. The lower pKa value of PWN2010 means a higher dissociation constant, which makes the proton more dissolved by the water as hydronium, Zundel- and Eigen-ion than as a proton on the phosphonic acid. This increased water retention with increasing acidity has already been reported in the literature [19].
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Fig. 1. Conductivity of pre-dried (120 °C, vacuum, 18 h) PWN2010 in dry N2 (d) (numbers represent the measurements sequences). For comparison: (s) PWN2010 in 1 atm. water pressure [11], (h) pre-dried (50 °C, vacuum + 1 h 100 °C + 1 h 140 °C) PVPA [24] and (D) Nafion 117 in dry N2 [25] are given.
Fig. 2. Conductivity of pre-dried (50 °C, vacuum, 1 week) PWN2010 in closed cell (d) (numbers represent the measurements sequences). For comparison: pre-dried (50 °C, vacuum, 1 week) PVPA [6] (h) and PAA-2 [20] in air (s). Activation enthalpies for PWN2010 and PVPA are given in brackets.
Fig. 3. TGA profile (heating rate 1 °C min1) of PWN2010 (pre-dried at 50 °C, vacuum, 1 week) at T = 30–345 °C. Arrows mark the temperature at which weight loss (water loss) is read. Table (inset) with water content of PWN2010 obtained by Karl-Fischer, TGA-FTIR and 1H NMR.
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In order to determine this relationship in the case of PWN2010, the water content was determined by three different methods using PWN2010 dried under well defined conditions (50 °C, vacuum for 1 week): TGA-FTIR, KarlFischer titration and 1H NMR (Fig. 3). The direct volumetric Karl-Fischer titration failed due to insufficient solubility of PWN2010 in the commonly used solvent formamide. Therefore, the sample was heated to T = 160 °C in a closed chamber and the released (evaporated) water was collected and titrated with Karl-Fischer reagents, which resulted in a water content of 2.46 wt.%. This correlated very well with the value obtained from the thermo-gravimetric profile of PWN2010 which was 2.55 wt.% for the same temperature interval (T = 30–160 °C). This water is most probably free water and not a product of intra-/intermolecular condensation reactions of the phosphonic acid functional groups. This is supported by both (i) the double step-like form of the TGA profile (see Fig. 3), where the first step (up to T = 160 °C) is commonly attributed to the evaporation of the free water and (ii) the absence of hysteresis in conductivity measured up to T = 150 °C (see Fig. 1). Furthermore, the TGA coupled with FTIR showed a drop in weight of 5.0 wt.% in the temperature range T = 30– 330 °C, which was detected as pure water evaporation according to the FTIR data. In order to crosscheck this value by 1H NMR, the integral of the hydronium signal was compared to those of the methane (CH) and the methylene (CH2) groups in the polymer backbone. Besides the well known inaccuracy (an error of about 10%) by rating the integrals at 1H NMR, the ratio of [H2O]/[–PO3H2] (known also as the hydration number k) was found to be k = 0.75, which corresponds to a water content of 5.2 wt.%. This hydration number of PWN2010 pre-dried at 50 °C, vacuum for a week is comparable to k = 0.8 found for PVPA conditioned at RH 40% [19].This contributes well to our initial hypothesis for a relatively high ability of PWN2010 to retain water. The enhanced water retention of PWN2010 probably contributes to the better conductivity of PWN2010 in comparison to PVPA. A further contribution to the high conductivity is the suppression of the condensation reaction between the phosphonic acids of the PWN2010, which has been already suggested in our previous paper [11]. This reaction reduces the number of protonogenic functions and therefore leads to a drop in ionconductivity. A further indication for the suppression of the phosphonic acid condensation reaction in the case of PWN2010 is the absence of hysteresis in the conductivity measurements (see Figs. 1 and 2 and related discussions). In order to further support this statement, we compared the 31P solid-phase MAS NMR spectrum of PWN2010 dried in vacuum at 50 °C for a week with the spectrum of a sample annealed at 250 °C for 5 h (see Fig. 4A). The 31P MAS NMR spectrum of a sample of PWN2010 that had been dried at 50 °C for a week showed a symmetric signal at 4.2 ppm which indicates that the polymer contains a single phosphorus species with an uniform chemical structure. However, after annealing at 250 °C for 5 h, the spectrum displayed an additional shoulder which indicates that a new phosphorus species had formed. Spectral simulation allowed to deconvolute the experimental line-shape into
A
PWN2010 after 5 hrs at 250 °C
PWN2010 after 1 week at 50 °C, vacuum
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100
50
0
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50
0
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B
(i) exp.
(ii) sim.: sum
(iii) sim.: acid
(iv) sim.: anhydride
(v) difference
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Fig. 4. (A) P solid-state MAS NMR spectra of PWN2010 (MAS spinning rate 9000 Hz) dried at 50 °C in vacuum for a week (gray trace) and after annealing at 250 °C for 5 h (black trace). (B) Deconvolution of the experimental line-shape of a sample of annealed PWN2010 into a sum of two overlapping spinning sideband manifolds; (i) experimental spectrum, (ii) sum of fitted lines, (iii) and (iv) individual fit components, (v) difference between experiment and simulation. The relative intensities were determined by integration of the simulated line shapes as (iii):(iv) = 58:42.
two overlapping spinning sideband patterns with relative intensities of 58:42 (see Fig. 4B). The chemical shift of the more intense signal (3.5 ppm) coincides with that of the phosphonic acid function of PWN2010. The second signal, with a chemical shift at 4.5 ppm, displays a similar upfield shift and increased shielding anisotropy as had been observed for pyrophosphoric as compared to phosphoric acid, and is thus assigned to phosphonic anhydride functions. Based on this assignment, we concluded that about 58 mol% of the phosphonic acid functions remained intact, whereas the 42 mol% had been converted to corresponding anhydrides via an inter- or intra-molecular selfcondensation reaction during annealing. For comparison, PVPA annealed under the same conditions (250 °C, 5 h) has shown a degree of condensation above 90 mol% [19].This result supports our hypothesis of a suppressed condensation reaction as has been inferred from the absence of hysteresis at the conductivity measurements (see Figs. 1 and 2) and TGA data at Fig. 3. In order to better understand the effect of heating on PWN2010, 31P- and 1H NMR spectra of the sample annealed at 250 °C for 5 h were recorded (Fig. 5A and B).
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The 31P NMR spectrum of annealed PWN2010 showed, in addition to the broad signals of polymeric species, two sharp signals at 0.34 and 11.9 ppm which are assigned to phosphoric and pyrophosphoric acid, respectively. The formation of these acids, which were absent in the spectrum of solid, annealed PWN2010 (see Fig. 4), might be attributed to solvent-induced cleavage of phosphonic acid functions, or to, as a speculation, a mechanical splitting-off of the phosphonic acid group from a phosphonic anhydride bridge by the swelling of the PWN2010 in DMSO. The process of releasing the phosphoric acid involves most probably a water molecule that had previously been formed by the condensation reaction of the phosphonic acid resulting in a para-hydrogen substituted 2,3,5,6-tetrafluorophenyl ring. This might be considered as the origin of a hump at about 7.3 ppm in the 1H NMR spectrum of the dissolved sample (see Fig. 5B). A cross-check with the simulated 1H NMR spectrum (ACD/C + H NMR predictors and DB software) of 2,3,5,6-tetrafluorophenyl ring revealed a chemical shift of a para-hydrogen atom at 7.05 ppm, which is in good agreement with the experimental value (see above). Further characterization of the annealed PWN2010 by 19F NMR and FTIR did not reveal any insights into the structural changes of the polymer (see Figs. 1 and 2 in Supporting materials).
Fig. 5.
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Fig. 6. Weight loss in dependence of time for PWN2010 (d), Blend membrane (PWN2010/PBIOO) (h) and PBIOO (s) in Fenton solution (3 wt.% H2O2, 4 ppm Fe2+, T = 68 °C).
It is worth to note that all the attempts to fully dissolve the annealed PWN2010 in DMSO or water failed. This allows us to conclude that besides the strong retardation of the phosphonic acid condensation reaction, the formed anhydride possesses relatively high resistance to hydrolysis (reverse) reaction.
P NMR (A) and 1H NMR (B) spectra of a DMSO-d6 solution of PWN2010 annealed at 250 °C for 5 h and at 50 °C in vacuum for a week.
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Fig. 7. Molecular weight profiles obtained by GPC of PWN2010 before (solid line) and after FT (dashed line (24 h FT), dotted line 48 h FT, gray line 96 h FT). Table (inset) with Mn, Mw and PDI of PWN2010 before and after FT.
Fig. 8. SEM images of PWN2010/PBIOO blend before FT: surface (A1) and cross-section (A2); after 96 h FT: surface (B1) and cross-section (B2); before (A3) and after (B3) extraction with water.
In order to obtain polymer membranes suitable for a FC application, PWN2010 was blended with PBIOO resulting in homogeneous, stable and flexible polymer films. The pure PWN2010 is a hard and brittle material with poor film-forming properties. This may be attributed to its high degree of phosphonation. In terms of FC applications, a critical issue for both the blends and the pure PWN2010 is their resistance to radical attack. Generally, polystyrene based polymers degrade rapidly in the presence of radicals, due to the apparently very labile a-proton in the structure where the so formed radical is stabilized by the conjugation with the phenyl-aromatic system [20].Therefore, we subjected both the blend and the pure PWN2010 to a Fenton test (FT), where free hydroxyl and superoxide radicals
are continuously generated and exposed to the polymer material. The obtained results are presented in Fig. 6 in terms of the weight loss as a function of immersion time in a Fenton solution. The pure PWN2010 showed excellent resistance to radical attack losing only 2% of its weight after 96 h, whereas the PBIOO weight loss was about 17% after 96 h. Moreover, PBIOO showed significant weight loss within the first 24 h followed by a slower, relative timeindependent, further decomposition. In the case of PWN2010 the first weight loss was observed only after 48 h. In comparison, the blend of PWN2010/PBIOO showed, similarly to the pure PWN2010, no weight loss for the first 20 h followed, however, by a fast increase of the weight loss to the level of the pristine PBIOO. In the
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Fig. 9. Conductivity (pwater = 1 105 Pa, in-plane) of PWN2010/PBIOO blends doped with phosphoric acid (doping degree is given next to each set of points).
Fig. 10. FC-test: CV- and CP-curves of PWN2010/PBIOO blend doped with phosphoric acid (270 wt.%).
case of the polymer blend (PWN2010/PBIOO), an extraction with pure water was done to eliminate the effect of weight loss due to dissolution of the PWN2010 in water. The pure water extraction led to a weight loss of approximately 7%. The data presented in Fig. 6 has been corrected in terms of the weight loss due to the pure water extraction. In order to investigate the polymer degradation caused by the attack of the hydroxyl radicals, we additionally used GPC measurements to determine the change of the molecular weight of PWN2010 (see Fig. 7). Despite the fact that the mean molecular weights after the FT were higher in comparison to the molecular weight before the test (see the table intercepted into Fig. 7), which may be due to the loss of low molecular weight fractions during the dialysis after the Fentons test, the maximum in the molecular weight diagram of PWN2010 treated with Fentons solution was slightly shifted to lower molecular weights. Based on the shift of the maximum of the molecular weight diagram, the decrease of the molecular weight of PWN2010 due to the FT was found to be only 18% [21,22]. These weight and molecular weight losses are negligible compared to the one of poly(styrene sulfonic acid) (PSSA), for instance [23].
In the case of the blend, SEM images were recorded before and after FT as well as after water extraction without a Fenton test (Fig. 8). As can be seen, the only visual damage is observed on the surface of the blend membrane after 96 h in a Fenton solution (Fig. 8B1). This surface irregularity is not a result of the extraction with water (see Fig. 8B3). Morphological irregularities are not observed inside the polymer membrane (see membrane cross-sections in Fig. 8B2), which might indicate that the radical attack is taking place preferably on the membrane surface and not in its inside. Prior to the application of the blend in the FC test, their doping behavior with phosphoric acid (PA) was investigated. The doping degree as a function of temperature, using 85% PA for 3 h doping time, is shown in Fig. 3 of the supporting materials. As expected, the doping degree increased with the temperature and the concentration of the PA. The significant increase above 100 °C, indicating an acceleration of the doping process, may be attributed to the overcoming effect of the PA solvation of the polymer scaffold over the ion-ion interactions in the polymer matrix. The blend membranes were doped with 77–300 wt.% PA and further subjected to conductivity measurements at pwater = 1 105 Pa water pressure (see Fig. 9). Accordingly the conductivity increased with increasing doping level reaching r = 1 S cm1 at 110 °C, 300 wt.% PA. This unusually high conductivity at high doping degree is in discrepancy with the data obtained by fuel cell tests (see below). This can be attributed to the design of the impedance cell where conductivity was measured in-plane by pressing the electrodes towards membrane surface. This could lead to a local leaching of PA at the areas pressed by the electrodes and thus partially covering the membrane surface by the liquid PA. The decrease in conductivity with temperature is due to the decrease of the RH and subsequently to the water content of the sample. Hysteresis, especially at high doping level, was observed when the sample was applied to heating-cooling cycles. This can be attributed to the condensation of the PA to pyrophosphoric acid at elevated temperatures, as supported by the increase of the hystere-
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sis with the doping level, indicating the PA contents dependence of the condensation process. A membrane with a thickness of 35–40 lm and a doping level of 270% PA was further subjected to FC tests. The obtained power density increased with temperature from 90 mW/cm2 at 80 °C up to 230 mW/cm2 at 150 °C (Fig. 10). This is in good agreement with the inner resistance Ri of the cell measured with a DC mX-m at a fixed frequency of 1 kHz. The inner resistance decreased from Ri = 40 mX (at 80 °C), 25 mX (at 100 °C) and 17 mX (at 130 °C) to 14 mX (at 150 °C). Although the power density is not explicitly high, the overall performance of this blend membrane increased with temperature, which gives indications for the suitability of this type of membrane in FCs operating at intermediate to high temperature regimes. 4. Conclusions and outlook In this paper we have shown that the PWN2010 polymer can be subjected to intermediate and high temperature FC-applications. In this context, PWN2010 showed: (i) enhanced conductivity in a nominally dry state (r = 2 104 S cm1 at 150 °C, dry N2) and at low water content (r = 1 103 S cm1 at 150 °C, k = 0.75); (ii) retarded anhydride formation (only 48% anhydride at 250 °C, 5 h); (iii) high resistance to radical attack (only 2% loss in weight after 4 days FT) and (iv) the ability of the PWN2010 to make stable polymer films by blending it with PBIOO, which after doping with PA reached a peak power density of about 230 mW cm2 at 150 °C. All this confirms the potential of PWN2010 as a polyelectrolyte building-block for FC applications. The synthesis of poly(pentafluorostyrene) (PFS) possessing higher molecular weights (above 100 kDa), which may improve the film-forming properties of the PWN2010, as well as blends of PWN2010 with other PBI derivatives, is in progress. Modifying PFS with other functional groups (e.g. thiol and sulfonic acid) is currently in the characterization phase and will be presented soon. Acknowledgements The authors gratefully acknowledge K. Toeroek (Univ. Stuttgart) for NMR analysis, analytic group at MPI for Polymer Research (Mainz) for the GPC measurements, I. Kharitonova and G. Schumski (Univ. Stuttgart) for the TGA, FT and SEM work, R. Kohlus and E. Denzel (Univ. Hohenheim) for the Karl-Fischer titration and K.-D. Kreuer (MPI-FKF,
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Stuttgart) for the conductivity measurements. Financial support was obtained from the Deutsche Forschungsgemeinschaft (KE 673/10-1). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.eurpolymj.2013.09.002. References [1] Agmon N. The Grotthus mechanism. Chem Phys Lett 1995;244:456–62. [2] Li Q, He R, Jensen JO, Bjerrum NJ. Fuel Cells 2004;4(3):147–59. [3] Vilcˇiauskas L, Tuckerman ME, Bester G, Paddison SJ, Kreuer K-D. Nat Chem 2012;4:461–6. [4] Asensio JA, Sánchez EM, Gómez-Romero P. Chem Soc Rev 2010;39:3210–39. [5] Rusanov AL, Kostoglodov PV, Abadie MJM, Voytekunas VYu, Likhachev DYu. Adv Polym Sci 2008;216:125–55. [6] Steininger H, Schuster M, Kreuer K-D, Kaltbeitzel A, Bingoel B, Meyer WH, et al. Phys Chem Chem Phys 2007;9:1764–73. [7] Lafitte B, Jannasch P. In: Zhao TS, Kreuer K-D, Van Nguyen T, editors. On the Prospects for Phosphonated Polymers as Proton-Exchange Fuel Cell Membranes, Advances in Fuel Cells. Oxford: Elsevier Ltd.; 2007. p. 119–79. [8] Herath MB, Creager SE, Kitaygorodskiy A, DesMarteau DD. Chem Phys Chem 2010;11:2871–8. [9] Schuster M, Rager T, Noda A, Kreuer K-D, Maier J. Fuel cell 2005;5(3):355–65. [10] Bock T, Moehwald H, Muelhaupt R. Macromol Chem Phys 2007;208:1324–40. [11] Atanasov V, Kerres J. Macromolecules 2011;44:6416–23. [12] Cui W, Kerres J, Eigenberger G. Sep Purif Technol 1998;14:145–54. [13] Kerres J. J Membr Sci 2001;185:3–27. [14] Kerres J. Fuel Cells 2005;5(2):230–47. [15] Schoeman H, Krieg HM, Kruger AJ, Chromik A, Krajinovic K, Kerres J. Int J Hydr En 2012;3:603–14. [16] Fischer K. Angew Chem 1935;48:394–6. [17] Kaltbeitzel A, Schauff S, Steininger H, Bingöl B, Brunklaus G, Meyer WH, et al. Solid State Ionics 2007;178:469–74. [18] Abouzari-Lotf E, Ghassemi H, Shockravi A, Zawodzinski T, Schiraldi D. Polymer 2011;52:4709–17. [19] Chang Y, Brunello GF, Fuller J, Hawley M, Kim YS, Disabb-Miller M, et al. Macromolecules 2011;44(21):8458–69. [20] Assink RA, Arnold Jr Ch, Hollandsworth RP. J Membr Sci 1991;56:143–51. [21] Hübner G, Roduner E. J Mater Chem 1999;9:409–18. [22] Lui H, Coms FD, Zhang J, Gasteiger HA, LaConti AB. In: Büchi FN, Inaba M, Schmidt TJ, (Eds.), Polymer Electrolyte Fuel Cell Durability, part I Stack Components, 3 Membranes, Chemical Degradation: Correlations Between Electrolyzer and Fuel Cell Findings. New York: Springer; 2009. p. 71–118. [23] Gubler L, Guersel SA, Scherer GG. Fuel Cells 2005;5(3):317–35. [24] Bingöl B. Dissertation. J. Gutenberg University, Mainz; 2007. [25] Montoneri E, Boffa V, Bottigliengo S, Casciola M, Sganappa M, Marigo A, et al. Solid State Ionics 2010;181:578–85.
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Highly phosphonated polypentafluorostyrene: Characterization and blends with polybenzimidazole Vladimir Atanasov a,⇑, Dietrich Gudat b, Bastian Ruffmann c, Jochen Kerres a,d a
Institute of Chemical Process Engineering, University of Stuttgart, Germany Institute of Inorganic Chemistry, University of Stuttgart, Germany c Hydrogen and Informatics Institute for Applied Technology, Schwerin, Germany d Focus Area: Chemical Resource Beneficiation, North-West University, Potchefstroom 2520, South Africa b
a r t i c l e
i n f o
Article history: Received 14 June 2013 Received in revised form 5 September 2013 Accepted 6 September 2013 Available online 15 September 2013 Keywords: Phosphonated polymer Polyelectrolyte Fuel cell Conductivity Blend membrane Doping with phosphoric acid
a b s t r a c t In this study we present results of the conductivity and resistance to thermooxidative and condensation reactions of a highly phosphonated poly(pentafluorostyrene) (PWN2010) and of its blends with poly(benzimidazole)s (PBI). This polymer, which combines both: (i) a high degree of phosphonation (above 90%) and (ii) a relatively high acidity (pKa (–PO3H2 M –PO3H) 0.5) due to the fluorine neighbors, is designed for low humidity operating fuel cell. This was confirmed by the conductivity measurements for PWN2010 reaching r = 5 104 S cm1 at 150 °C in dry N2 and r = 1 103 S cm1 at 150 °C (k = 0.75). Furthermore, this polymer showed only 48% of anhydride formation when annealing it at T = 250 °C for 5 h and only 2% weight loss during a 96 h Fenton test. These properties combined with the ability of the PWN2010 to form homogeneous blends with polybenzimidazoles resulting in stable and flexible polymer films, makes PWN2010 a very promising candidate as a polymer electrolyte for intermediate- and high-temperature fuel cell applications. Ó 2013 Published by Elsevier Ltd.
1. Introduction The growing need of devices delivering electricity in different locations is essential in the modern world. A proton – exchange membrane fuel cell (PEM FC) is one example of such a device using a polymer electrolyte as ionconductor. The most commonly used polyelectrolytes are based on sulfonated polymers, e.g. NafionÒ. Their proton conductivity is principally based on water bridging the sulfonic acid function, where the water molecule’s diffusion serves as a vehicle for the proton transport, either as H3O+, H5 Oþ 2 ions, etc. (vehicular proton-transport mechanism), or as a medium for proton tunneling via hydrogen bridges (Grotthus proton transport mechanism) [1]. Therefore, at temperatures above 100 °C, most of these polyelectrolytes show a dramatic decrease of ion-conductivity due ⇑ Corresponding author. Tel.: +49 711 68585163; fax: +49 711 68585242. E-mail address: [email protected] (V. Atanasov). 0014-3057/$ - see front matter Ó 2013 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.eurpolymj.2013.09.002
to water evaporation. The increase in FC operating temperature is, however, required for attenuating the activation enthalpy and overcoming the poisoning effect of CO on the Pt-catalyst by temperature-driven acceleration of the electrode kinetics. Thus, polyelectrolytes based on phosphoric acid (PA) doped polybenzimidazole (PBI) currently attract considerable attention [2].The conductivity in this case is based on the excellent proton-carrier properties of PA [3]. Moreover, high doping levels of PA are required (commonly above 200%) to induce sufficient proton conductivity [4], which can lead to unsustainability and leakage of PA from the polymer membrane especially at FC operation temperatures below 100 °C. One approach to overcome these drawbacks lies in the utilization of highly phosphonated polymers where the phosphonic acid group serves as an amphoteric proton conductor substituting the role of PA. In comparison to the sulfonated ionomers, there are currently only few examples of phosphonated polymers being used as proper electrolytes for FCs [5–7], which
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can be ascribed to their moderate conductivity and the greater synthetic efforts required for phosphonation compared to sulfonation. While the advantage of the phosphonated polymers lies in their amphiphilicity, which allows them to conduct protons in a reduced water content environment, the disadvantage of the phosphonated polymers lies in their moderate conductivity due to their lower acidity, compared to sulfonic acids [8,9]. In order to increase proton conductivity, the polymer must be at least locally highly phosphonated, which will positively influence the percolation of the protons through the membranes. Therefore, in the past few years considerable attention was paid to the preparation and properties of highly phosphonated polymers [3,5,10]. Both concepts of polymerization of phosphonated monomers and post-phosphonation of polymers have been introduced. While post-phosphonation is the easier synthetic approach, in most cases phosphonated polymers with a relatively low functionalization degree, due to the relatively low reactivity of the educts and/or side reactions, have been obtained by this synthetic route. A novel highly phosphonated polymer (PWN2010) based on poly(pentafluorostyrene) (PFS) was recently synthesized in our group [11]. Due to its perfluorinated phenyl rings, PFS easily undergoes nucleophilic substitution reactions (Michaelis–Arbuzov reaction) with phosphites such as tris(trimethylsilyl)phosphite, resulting in a polymer with a degree of phosphonation above 90%. In this paper we present PWN2010 in terms of its most critical properties for FC, such as proton conductivity in reduced water content, resistance to radical attack and the possible occurrence of condensation reactions between adjacent phosphonic acid groups at high temperatures. Blends of PWN2010 with Poly-[(1-(4,40 -diphenylether)-5oxybenzimidazole)-benzimidazole] (PBIOO) and the FC performance of these new class of acid-base blend membranes is discussed. Blending with PBIOO was necessary due to the inability of PWN2010 to form stable polymer films suitable for FC applications. This is due to its water solubility as a consequence of the high phosphonation degree. The blend-membranes comprising ionical cross-linking sites by the protonation of the basic benzimidazole units, have been already verified as a suitable platform for the preparation of highly proton-conductive membranes with reduced water swelling for the application in electro-membrane processes such as electrodialysis [12], FCs [13,14] and water electrolysis [15]. 2. Experimental section 2.1. Materials PWN2010 was prepared as reported earlier [11]. PBIOO was purchased from FuMA-Tech GmbH. All the other chemicals were used in HPLC grade quality. 2.2. PWN2010/PBIOO blends preparation PWN2010 and PBIOO were separately dissolved in DMSO (10 and 5 wt.% respectively). PWN2010/DMSO solution was neutralized with triethylamine (TEA) (4/1
equiv. = TEA/–PO3H2). PWN2010 and PBIOO solutions were mixed (imidazole/–PO3H2 = 3/7 molar) resulting in a viscous mixture. The mixture was cast on a pre-heated (100 °C) glass-plate using a doctor-blade (0.6 mm). The polymer film was dried at 100 °C for 18 h, followed by 120 °C, p = 1 103 mbar (vacuum) for 4 h. Subsequently, the polymer film was detached from the glass-plate and conditioned by immersion in 30% HCl at 90 °C for 3 h. The polymer films were rinsed with water and dried in vacuum at 120 °C for 2 h. Doping of PWN2010/PBIOO blends with PA: Dried polymer films were weighed and immersed into 85% PA for a given temperature and time (see Fig. 3 in Supporting materials). Then, the films were pressed between paper tissues to remove the residue PA from their surface. Before measuring the PA doping degree, blends were dried at 120 °C under vacuum for 2 h. 2.3. Instruments Solution NMR spectra were recorded on a Bruker Avance 400 spectrometer at a resonance frequency of 250 MHz for 1H, 62.9 MHz for 13C, 235 MHz for 19F and 101.2 MHz for 31P NMR at RT. Solid state NMR spectra were recorded at RT on a Bruker Avance 400 spectrometer equipped with a 4 mm magic angle spinning (MAS) probe at a resonance frequency of 162.9 MHz for 31P. All experiments were performed under MAS with spinning speeds between 8 and 9 kHz. High-power 1H decoupling was applied during data acquisition, and cross polarization with a ramp-shaped contact pulse and mixing times of 5 ms was used for signal enhancement. Deconvolution of observed spectra into a sum of individual spinning sideband manifolds was performed with the spectrometer software. Molecular weights and molecular weight distributions were determined by GPC in water on Waters pump model 515, eluent: 0.1 M NaNO3, detectors: RI ERC-101 and UV–VIS Soma S-3702 (270 nm), temperature: 30 °C, standard: PSSNa, concentration: 1.000 g/l, flow rate: 1.0 ml/min, columns: MCX, MCX 10 E7 and MCX 1000. FTIR spectra were recorded as KBr-pellet on a Nicolet 6700 FTIR instrument. The thermal stability of the polymers and membranes was determined by thermogravimetry (TGA, Netzsch, model STA 449C) with a heating rate of 20 °C min1 under an atmosphere enriched with oxygen (65–70% O2, 35–30% N2). The outlet with the released volatile-products was continuously analyzed by a FTIR spectrometer (Nicolet Nexus FTIR spectrometer) to determine the water and the onset temperature of the splitting-off of the phosphonic acid function by the stretching vibration at 1050 cm1. Proton conductivities were measured by ac-impedance spectroscopy (HP-ac-impedance analyzer 4192A LF) using a two-electrode arrangement. The measurements were performed in a closed cell with pre-dried (50 °C, vacuum, 1 week) samples (pressed pellets with diameter 4 mm, thickness 2–5 mm) with gold electrodes. Conductivity measurements in 100% water vapor (p(H2O) = 105 Pa) were carried out in a double-wall temperature-controlled glass chamber with an open outlet at temperatures T = 110– 160 °C. Liquid water was continuously evaporated by a heater and injected into the chamber with a constant flow rate using a digital peristaltic pump (Ismatec). Inside the
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chamber pressed pellets of PWN2010 polymer powder (diameter 6 mm and total thickness of 2–3 mm) were placed in a notched cylindrical glass-tube with a charcoal-coated electrode at the bottom. The second electrode was pressed from the top onto the pellet by a screw in order to ensure optimal contact. The specific conductivity was derived by r = l/(A R), where l is the distance between the electrodes (in cm), A the area of the pellet (in cm2), and R the resistance (in Ohm) derived from the high-frequency intercept of the complex impedance with the real axis. The proton conductivities of PA-doped PWN2010/PBIOO blend membranes under fully or partially hydrated conditions were recorded on a FumaTech MK3 setup with a 4-electrode in-plane AC arrangement, in the frequency range from 1 to 106 Hz. The temperature of the water reservoir was fixed at 100 °C, while the sample temperature was above 100 °C to prevent water condensation in the sample holder. The relative humidity values in the test cell at T = 110–160 °C corresponds to 69.5–15.7%, respectively. To attain equilibrium, all membranes were conditioned for 2 h at each temperature-point. Water content was determined using a Karl Fischer coulometer (Metrohm KF-Coulometer 831 with 774 Oven Sample Processor with HydranalÒ Coulomat AG oven titration reagents) by the method described elsewhere [16]. Scanning Electron Microscopy (SEM) images were recorded on a CamScan CS44 instrument equipped with a wolfram cathode with a working voltage of 15 kV. The surface of the membranes was covered via vacuum deposition with an ultra-thin Au or C layer prior to use. Oxidation Stability was tested by immersing the sample in a Fenton solution of 3 wt.% H2O2 containing 4 ppm Fe2+ at 68 °C. The membrane samples were then washed with water and dried at 120 °C, vacuum for 2 h. Fresh Fenton solution was prepared for successive measurements. In case of a pure PWN2010 after the incubation time in a Fenton solution, the mixture was dialyzed in water (MWCO: 12 kDa) and the PWN2010 was dried at 120 °C under vacuum for 2 h. Fuel cell tests were performed in a 25 cm2 test cell (quick Connect Fixture, Baltic FuelCells GmbH) with a contact pressure of 1 N/mm2. Dry hydrogen and dry air with k (anode) = 1.2 and k (cathode) = 2.5 were used with ambient pressure. The inner resistance Ri of the cell was measured with a DC mX-m (HIOKI) at a fixed frequency of 1 kHz. The membrane was investigated as MEA with GDE’s based on Freudenberg GDL H2315 I3 XC 190. GDE’s were produced by screen printing (EKRA 01) on the GDL, the noble metal load on the anode and the cathode was about 0.9 mg Pt/cm2 each.
3. Results and discussion The most critical issue for polyelectrolyte membranes (PEMs) is their performance (e.g. proton conductivity) at reduced water content conditions. Therefore, we firstly describe the proton conductivity of PWN2010 under nominally dry conditions (‘‘nominally dry’’ is used to emphasize the inability for complete removal of water in the case of PWN2010) (Fig. 1). PWN2010 was pre-dried at 120 °C, p = 1 103 mbar (vacuum) for 18 h in order to
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evaporate the removable water quantitatively. Subsequently, the conductivity as a function of temperature of a pellet of PWN2010 was measured under dried nitrogen flux. As expected, in absence of water susceptible to evaporation, the conductivity increased with temperature due to the decrease of the activation enthalpy. When comparing the conductivity of PWN2010 in a dry state with the conductivity obtained in a saturated water atmosphere (pwater = 105 Pa), a significant decrease of about 2–3 order of magnitudes at T = 110–150 °C was observed. This clearly illustrated the role of the water in the proton-conduction process in view of the same polymer being used for both measurements. However, when comparing the conductivity of PWN2010 to similarly pre-dried polymers such as NafionÒ and poly(vinylphosphonic acid) (PVPA), its conductivity appears to be an order of magnitude higher. This makes PWN2010 one of the best conducting polymers in dry conditions. In order to compare the proton conductivity of PWN2010 with the one of PVPA, we pre-dried PWN2010 in the same manner (50 °C, p = 1 103 mbar, one week) as PVPA [17] and measured the conductivity in a closed cell, so that the water content remained constant during the measurement (Fig. 2). In this case, the conductivity of PVPA was closer to the one of the PWN2010 from about an order of magnitude at T = 60 °C to nearly the same (r = 1 mS cm1) at T = 155 °C. This may be attributed to the higher concentration of phosphonic acid function in PVPA in comparison to PWN2010 (IEC (PVPA) = 9.2 mmol g1, IEC (PWN2010) = 3.5 mmol g1). Additionally, the drying conditions in this case are milder than in the previous one (see drying conditions in Fig. 1). This reduces the risk of phosphonic acid condensation reaction, which is well established in case of PVPA [6,19]. It is noteworthy that no hysteresis occurred during the heating and cooling cycles of PWN2010 (numbers beside the data-points of both Figs. 1 and 2 represent order of the measurements). This means that PWN2010 does not undergo any chemical changes in this temperature range (T = 40–160 °C). This result correlates well with the substantial resistance to thermal degradation of PWN2010, which was not observed for other highly phosphonated polymers such as PVPA (the issue is further discussed in Fig. 3). The conductivity of PWN2010 was also compared to a very recently reported highly phosphonated polymer (PAA-2) [18]. This polymer has shown a higher conductivity, however, under less defined drying and hence water content of the samples (without pre-drying and open to air chamber) than performed in this study. Therefore, activation enthalpy could not be calculated for PAA-2. In comparison to PVPA (Ea = 78 kJ mol1), PWN2010 had a lower activation enthalpy (Ea = 58 kJ mol1), which might be attributed to the lower pKa values being pKa1 (PWN2010) = 0.47 ± 0.36 compared to the pKa1 (PVA) = 2.00 ± 0.28 both calculated by ACD/pKa DB software. The lower pKa value of PWN2010 means a higher dissociation constant, which makes the proton more dissolved by the water as hydronium, Zundel- and Eigen-ion than as a proton on the phosphonic acid. This increased water retention with increasing acidity has already been reported in the literature [19].
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Fig. 1. Conductivity of pre-dried (120 °C, vacuum, 18 h) PWN2010 in dry N2 (d) (numbers represent the measurements sequences). For comparison: (s) PWN2010 in 1 atm. water pressure [11], (h) pre-dried (50 °C, vacuum + 1 h 100 °C + 1 h 140 °C) PVPA [24] and (D) Nafion 117 in dry N2 [25] are given.
Fig. 2. Conductivity of pre-dried (50 °C, vacuum, 1 week) PWN2010 in closed cell (d) (numbers represent the measurements sequences). For comparison: pre-dried (50 °C, vacuum, 1 week) PVPA [6] (h) and PAA-2 [20] in air (s). Activation enthalpies for PWN2010 and PVPA are given in brackets.
Fig. 3. TGA profile (heating rate 1 °C min1) of PWN2010 (pre-dried at 50 °C, vacuum, 1 week) at T = 30–345 °C. Arrows mark the temperature at which weight loss (water loss) is read. Table (inset) with water content of PWN2010 obtained by Karl-Fischer, TGA-FTIR and 1H NMR.
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In order to determine this relationship in the case of PWN2010, the water content was determined by three different methods using PWN2010 dried under well defined conditions (50 °C, vacuum for 1 week): TGA-FTIR, KarlFischer titration and 1H NMR (Fig. 3). The direct volumetric Karl-Fischer titration failed due to insufficient solubility of PWN2010 in the commonly used solvent formamide. Therefore, the sample was heated to T = 160 °C in a closed chamber and the released (evaporated) water was collected and titrated with Karl-Fischer reagents, which resulted in a water content of 2.46 wt.%. This correlated very well with the value obtained from the thermo-gravimetric profile of PWN2010 which was 2.55 wt.% for the same temperature interval (T = 30–160 °C). This water is most probably free water and not a product of intra-/intermolecular condensation reactions of the phosphonic acid functional groups. This is supported by both (i) the double step-like form of the TGA profile (see Fig. 3), where the first step (up to T = 160 °C) is commonly attributed to the evaporation of the free water and (ii) the absence of hysteresis in conductivity measured up to T = 150 °C (see Fig. 1). Furthermore, the TGA coupled with FTIR showed a drop in weight of 5.0 wt.% in the temperature range T = 30– 330 °C, which was detected as pure water evaporation according to the FTIR data. In order to crosscheck this value by 1H NMR, the integral of the hydronium signal was compared to those of the methane (CH) and the methylene (CH2) groups in the polymer backbone. Besides the well known inaccuracy (an error of about 10%) by rating the integrals at 1H NMR, the ratio of [H2O]/[–PO3H2] (known also as the hydration number k) was found to be k = 0.75, which corresponds to a water content of 5.2 wt.%. This hydration number of PWN2010 pre-dried at 50 °C, vacuum for a week is comparable to k = 0.8 found for PVPA conditioned at RH 40% [19].This contributes well to our initial hypothesis for a relatively high ability of PWN2010 to retain water. The enhanced water retention of PWN2010 probably contributes to the better conductivity of PWN2010 in comparison to PVPA. A further contribution to the high conductivity is the suppression of the condensation reaction between the phosphonic acids of the PWN2010, which has been already suggested in our previous paper [11]. This reaction reduces the number of protonogenic functions and therefore leads to a drop in ionconductivity. A further indication for the suppression of the phosphonic acid condensation reaction in the case of PWN2010 is the absence of hysteresis in the conductivity measurements (see Figs. 1 and 2 and related discussions). In order to further support this statement, we compared the 31P solid-phase MAS NMR spectrum of PWN2010 dried in vacuum at 50 °C for a week with the spectrum of a sample annealed at 250 °C for 5 h (see Fig. 4A). The 31P MAS NMR spectrum of a sample of PWN2010 that had been dried at 50 °C for a week showed a symmetric signal at 4.2 ppm which indicates that the polymer contains a single phosphorus species with an uniform chemical structure. However, after annealing at 250 °C for 5 h, the spectrum displayed an additional shoulder which indicates that a new phosphorus species had formed. Spectral simulation allowed to deconvolute the experimental line-shape into
A
PWN2010 after 5 hrs at 250 °C
PWN2010 after 1 week at 50 °C, vacuum
150
100
50
0
-50
-100
ppm
50
0
-50
-100
ppm
B
(i) exp.
(ii) sim.: sum
(iii) sim.: acid
(iv) sim.: anhydride
(v) difference
150
100
31
Fig. 4. (A) P solid-state MAS NMR spectra of PWN2010 (MAS spinning rate 9000 Hz) dried at 50 °C in vacuum for a week (gray trace) and after annealing at 250 °C for 5 h (black trace). (B) Deconvolution of the experimental line-shape of a sample of annealed PWN2010 into a sum of two overlapping spinning sideband manifolds; (i) experimental spectrum, (ii) sum of fitted lines, (iii) and (iv) individual fit components, (v) difference between experiment and simulation. The relative intensities were determined by integration of the simulated line shapes as (iii):(iv) = 58:42.
two overlapping spinning sideband patterns with relative intensities of 58:42 (see Fig. 4B). The chemical shift of the more intense signal (3.5 ppm) coincides with that of the phosphonic acid function of PWN2010. The second signal, with a chemical shift at 4.5 ppm, displays a similar upfield shift and increased shielding anisotropy as had been observed for pyrophosphoric as compared to phosphoric acid, and is thus assigned to phosphonic anhydride functions. Based on this assignment, we concluded that about 58 mol% of the phosphonic acid functions remained intact, whereas the 42 mol% had been converted to corresponding anhydrides via an inter- or intra-molecular selfcondensation reaction during annealing. For comparison, PVPA annealed under the same conditions (250 °C, 5 h) has shown a degree of condensation above 90 mol% [19].This result supports our hypothesis of a suppressed condensation reaction as has been inferred from the absence of hysteresis at the conductivity measurements (see Figs. 1 and 2) and TGA data at Fig. 3. In order to better understand the effect of heating on PWN2010, 31P- and 1H NMR spectra of the sample annealed at 250 °C for 5 h were recorded (Fig. 5A and B).
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The 31P NMR spectrum of annealed PWN2010 showed, in addition to the broad signals of polymeric species, two sharp signals at 0.34 and 11.9 ppm which are assigned to phosphoric and pyrophosphoric acid, respectively. The formation of these acids, which were absent in the spectrum of solid, annealed PWN2010 (see Fig. 4), might be attributed to solvent-induced cleavage of phosphonic acid functions, or to, as a speculation, a mechanical splitting-off of the phosphonic acid group from a phosphonic anhydride bridge by the swelling of the PWN2010 in DMSO. The process of releasing the phosphoric acid involves most probably a water molecule that had previously been formed by the condensation reaction of the phosphonic acid resulting in a para-hydrogen substituted 2,3,5,6-tetrafluorophenyl ring. This might be considered as the origin of a hump at about 7.3 ppm in the 1H NMR spectrum of the dissolved sample (see Fig. 5B). A cross-check with the simulated 1H NMR spectrum (ACD/C + H NMR predictors and DB software) of 2,3,5,6-tetrafluorophenyl ring revealed a chemical shift of a para-hydrogen atom at 7.05 ppm, which is in good agreement with the experimental value (see above). Further characterization of the annealed PWN2010 by 19F NMR and FTIR did not reveal any insights into the structural changes of the polymer (see Figs. 1 and 2 in Supporting materials).
Fig. 5.
31
Fig. 6. Weight loss in dependence of time for PWN2010 (d), Blend membrane (PWN2010/PBIOO) (h) and PBIOO (s) in Fenton solution (3 wt.% H2O2, 4 ppm Fe2+, T = 68 °C).
It is worth to note that all the attempts to fully dissolve the annealed PWN2010 in DMSO or water failed. This allows us to conclude that besides the strong retardation of the phosphonic acid condensation reaction, the formed anhydride possesses relatively high resistance to hydrolysis (reverse) reaction.
P NMR (A) and 1H NMR (B) spectra of a DMSO-d6 solution of PWN2010 annealed at 250 °C for 5 h and at 50 °C in vacuum for a week.
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Fig. 7. Molecular weight profiles obtained by GPC of PWN2010 before (solid line) and after FT (dashed line (24 h FT), dotted line 48 h FT, gray line 96 h FT). Table (inset) with Mn, Mw and PDI of PWN2010 before and after FT.
Fig. 8. SEM images of PWN2010/PBIOO blend before FT: surface (A1) and cross-section (A2); after 96 h FT: surface (B1) and cross-section (B2); before (A3) and after (B3) extraction with water.
In order to obtain polymer membranes suitable for a FC application, PWN2010 was blended with PBIOO resulting in homogeneous, stable and flexible polymer films. The pure PWN2010 is a hard and brittle material with poor film-forming properties. This may be attributed to its high degree of phosphonation. In terms of FC applications, a critical issue for both the blends and the pure PWN2010 is their resistance to radical attack. Generally, polystyrene based polymers degrade rapidly in the presence of radicals, due to the apparently very labile a-proton in the structure where the so formed radical is stabilized by the conjugation with the phenyl-aromatic system [20].Therefore, we subjected both the blend and the pure PWN2010 to a Fenton test (FT), where free hydroxyl and superoxide radicals
are continuously generated and exposed to the polymer material. The obtained results are presented in Fig. 6 in terms of the weight loss as a function of immersion time in a Fenton solution. The pure PWN2010 showed excellent resistance to radical attack losing only 2% of its weight after 96 h, whereas the PBIOO weight loss was about 17% after 96 h. Moreover, PBIOO showed significant weight loss within the first 24 h followed by a slower, relative timeindependent, further decomposition. In the case of PWN2010 the first weight loss was observed only after 48 h. In comparison, the blend of PWN2010/PBIOO showed, similarly to the pure PWN2010, no weight loss for the first 20 h followed, however, by a fast increase of the weight loss to the level of the pristine PBIOO. In the
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Fig. 9. Conductivity (pwater = 1 105 Pa, in-plane) of PWN2010/PBIOO blends doped with phosphoric acid (doping degree is given next to each set of points).
Fig. 10. FC-test: CV- and CP-curves of PWN2010/PBIOO blend doped with phosphoric acid (270 wt.%).
case of the polymer blend (PWN2010/PBIOO), an extraction with pure water was done to eliminate the effect of weight loss due to dissolution of the PWN2010 in water. The pure water extraction led to a weight loss of approximately 7%. The data presented in Fig. 6 has been corrected in terms of the weight loss due to the pure water extraction. In order to investigate the polymer degradation caused by the attack of the hydroxyl radicals, we additionally used GPC measurements to determine the change of the molecular weight of PWN2010 (see Fig. 7). Despite the fact that the mean molecular weights after the FT were higher in comparison to the molecular weight before the test (see the table intercepted into Fig. 7), which may be due to the loss of low molecular weight fractions during the dialysis after the Fentons test, the maximum in the molecular weight diagram of PWN2010 treated with Fentons solution was slightly shifted to lower molecular weights. Based on the shift of the maximum of the molecular weight diagram, the decrease of the molecular weight of PWN2010 due to the FT was found to be only 18% [21,22]. These weight and molecular weight losses are negligible compared to the one of poly(styrene sulfonic acid) (PSSA), for instance [23].
In the case of the blend, SEM images were recorded before and after FT as well as after water extraction without a Fenton test (Fig. 8). As can be seen, the only visual damage is observed on the surface of the blend membrane after 96 h in a Fenton solution (Fig. 8B1). This surface irregularity is not a result of the extraction with water (see Fig. 8B3). Morphological irregularities are not observed inside the polymer membrane (see membrane cross-sections in Fig. 8B2), which might indicate that the radical attack is taking place preferably on the membrane surface and not in its inside. Prior to the application of the blend in the FC test, their doping behavior with phosphoric acid (PA) was investigated. The doping degree as a function of temperature, using 85% PA for 3 h doping time, is shown in Fig. 3 of the supporting materials. As expected, the doping degree increased with the temperature and the concentration of the PA. The significant increase above 100 °C, indicating an acceleration of the doping process, may be attributed to the overcoming effect of the PA solvation of the polymer scaffold over the ion-ion interactions in the polymer matrix. The blend membranes were doped with 77–300 wt.% PA and further subjected to conductivity measurements at pwater = 1 105 Pa water pressure (see Fig. 9). Accordingly the conductivity increased with increasing doping level reaching r = 1 S cm1 at 110 °C, 300 wt.% PA. This unusually high conductivity at high doping degree is in discrepancy with the data obtained by fuel cell tests (see below). This can be attributed to the design of the impedance cell where conductivity was measured in-plane by pressing the electrodes towards membrane surface. This could lead to a local leaching of PA at the areas pressed by the electrodes and thus partially covering the membrane surface by the liquid PA. The decrease in conductivity with temperature is due to the decrease of the RH and subsequently to the water content of the sample. Hysteresis, especially at high doping level, was observed when the sample was applied to heating-cooling cycles. This can be attributed to the condensation of the PA to pyrophosphoric acid at elevated temperatures, as supported by the increase of the hystere-
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sis with the doping level, indicating the PA contents dependence of the condensation process. A membrane with a thickness of 35–40 lm and a doping level of 270% PA was further subjected to FC tests. The obtained power density increased with temperature from 90 mW/cm2 at 80 °C up to 230 mW/cm2 at 150 °C (Fig. 10). This is in good agreement with the inner resistance Ri of the cell measured with a DC mX-m at a fixed frequency of 1 kHz. The inner resistance decreased from Ri = 40 mX (at 80 °C), 25 mX (at 100 °C) and 17 mX (at 130 °C) to 14 mX (at 150 °C). Although the power density is not explicitly high, the overall performance of this blend membrane increased with temperature, which gives indications for the suitability of this type of membrane in FCs operating at intermediate to high temperature regimes. 4. Conclusions and outlook In this paper we have shown that the PWN2010 polymer can be subjected to intermediate and high temperature FC-applications. In this context, PWN2010 showed: (i) enhanced conductivity in a nominally dry state (r = 2 104 S cm1 at 150 °C, dry N2) and at low water content (r = 1 103 S cm1 at 150 °C, k = 0.75); (ii) retarded anhydride formation (only 48% anhydride at 250 °C, 5 h); (iii) high resistance to radical attack (only 2% loss in weight after 4 days FT) and (iv) the ability of the PWN2010 to make stable polymer films by blending it with PBIOO, which after doping with PA reached a peak power density of about 230 mW cm2 at 150 °C. All this confirms the potential of PWN2010 as a polyelectrolyte building-block for FC applications. The synthesis of poly(pentafluorostyrene) (PFS) possessing higher molecular weights (above 100 kDa), which may improve the film-forming properties of the PWN2010, as well as blends of PWN2010 with other PBI derivatives, is in progress. Modifying PFS with other functional groups (e.g. thiol and sulfonic acid) is currently in the characterization phase and will be presented soon. Acknowledgements The authors gratefully acknowledge K. Toeroek (Univ. Stuttgart) for NMR analysis, analytic group at MPI for Polymer Research (Mainz) for the GPC measurements, I. Kharitonova and G. Schumski (Univ. Stuttgart) for the TGA, FT and SEM work, R. Kohlus and E. Denzel (Univ. Hohenheim) for the Karl-Fischer titration and K.-D. Kreuer (MPI-FKF,
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Stuttgart) for the conductivity measurements. Financial support was obtained from the Deutsche Forschungsgemeinschaft (KE 673/10-1). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.eurpolymj.2013.09.002. References [1] Agmon N. The Grotthus mechanism. Chem Phys Lett 1995;244:456–62. [2] Li Q, He R, Jensen JO, Bjerrum NJ. Fuel Cells 2004;4(3):147–59. [3] Vilcˇiauskas L, Tuckerman ME, Bester G, Paddison SJ, Kreuer K-D. Nat Chem 2012;4:461–6. [4] Asensio JA, Sánchez EM, Gómez-Romero P. Chem Soc Rev 2010;39:3210–39. [5] Rusanov AL, Kostoglodov PV, Abadie MJM, Voytekunas VYu, Likhachev DYu. Adv Polym Sci 2008;216:125–55. [6] Steininger H, Schuster M, Kreuer K-D, Kaltbeitzel A, Bingoel B, Meyer WH, et al. Phys Chem Chem Phys 2007;9:1764–73. [7] Lafitte B, Jannasch P. In: Zhao TS, Kreuer K-D, Van Nguyen T, editors. On the Prospects for Phosphonated Polymers as Proton-Exchange Fuel Cell Membranes, Advances in Fuel Cells. Oxford: Elsevier Ltd.; 2007. p. 119–79. [8] Herath MB, Creager SE, Kitaygorodskiy A, DesMarteau DD. Chem Phys Chem 2010;11:2871–8. [9] Schuster M, Rager T, Noda A, Kreuer K-D, Maier J. Fuel cell 2005;5(3):355–65. [10] Bock T, Moehwald H, Muelhaupt R. Macromol Chem Phys 2007;208:1324–40. [11] Atanasov V, Kerres J. Macromolecules 2011;44:6416–23. [12] Cui W, Kerres J, Eigenberger G. Sep Purif Technol 1998;14:145–54. [13] Kerres J. J Membr Sci 2001;185:3–27. [14] Kerres J. Fuel Cells 2005;5(2):230–47. [15] Schoeman H, Krieg HM, Kruger AJ, Chromik A, Krajinovic K, Kerres J. Int J Hydr En 2012;3:603–14. [16] Fischer K. Angew Chem 1935;48:394–6. [17] Kaltbeitzel A, Schauff S, Steininger H, Bingöl B, Brunklaus G, Meyer WH, et al. Solid State Ionics 2007;178:469–74. [18] Abouzari-Lotf E, Ghassemi H, Shockravi A, Zawodzinski T, Schiraldi D. Polymer 2011;52:4709–17. [19] Chang Y, Brunello GF, Fuller J, Hawley M, Kim YS, Disabb-Miller M, et al. Macromolecules 2011;44(21):8458–69. [20] Assink RA, Arnold Jr Ch, Hollandsworth RP. J Membr Sci 1991;56:143–51. [21] Hübner G, Roduner E. J Mater Chem 1999;9:409–18. [22] Lui H, Coms FD, Zhang J, Gasteiger HA, LaConti AB. In: Büchi FN, Inaba M, Schmidt TJ, (Eds.), Polymer Electrolyte Fuel Cell Durability, part I Stack Components, 3 Membranes, Chemical Degradation: Correlations Between Electrolyzer and Fuel Cell Findings. New York: Springer; 2009. p. 71–118. [23] Gubler L, Guersel SA, Scherer GG. Fuel Cells 2005;5(3):317–35. [24] Bingöl B. Dissertation. J. Gutenberg University, Mainz; 2007. [25] Montoneri E, Boffa V, Bottigliengo S, Casciola M, Sganappa M, Marigo A, et al. Solid State Ionics 2010;181:578–85.