Structure and Interactions in Polybenzimidazole Composite

Article pubs.acs.org/JPCC

Structure and Interactions in Polybenzimidazole Composite Membranes for High-Temperature Polymer Fuel Cells: A Full Multinuclear Solid-State NMR Study Alice S. Cattaneo, Davide C. Villa, Simone Angioni, Chiara Ferrara, Eliana Quartarone, and Piercarlo Mustarelli* Department of Chemistry, University of Pavia, Via Taramelli 16, I-27000 Pavia, Italy S Supporting Information *

ABSTRACT: Fabrication of composite membranes including inorganic or hybrid fillers into polybenzimidazole (PBI) membranes is a promising strategy for improving the performance of a membraneelectrode assembly (MEA) for high-temperature fuel cells. To this aim, a full understanding of the structure and interactions in such a complex system, which includes polymer, filler, and phosphoric acid, is mandatory. In this paper, we used multinuclear magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy to investigate the inclusion of inorganic and hybrid mesoporous fillers into a pyridinebased polybenzimidazole (PBI_5N) matrix and the effects of the subsequent H3PO4 doping. Composite membranes were prepared through an innovative spray deposition method, with 30 wt % of filler loading and SBA-15 or propylsulfonicfunctionalized SBA-15-type silicas as filler. 13C CP MAS NMR experiments showed the structural changes induced by the acid doping on the polymer backbone. 31P(1H) 2D heteronuclear correlation experiments confirmed strong interactions between H3PO4 molecules and imidazole protons through a hydrogen-bonding network. 29Si(1H) CP MAS experiments revealed different interactions between acid and filler particles, depending on the filler type. Here, for the first time, a complete understanding of the interactions among polymer, filler, and phosphoric acid is provided. We demonstrate that a fully inorganic mesoporous filler seems to be more suitable with respect to the hybrid homologues for increasing the proton conductivity of PBI, because of its higher affinity with H3PO4. We gave evidence of a possible proton conductivity inside the mesoporous structure of SBA-15, which became hindered in the propylsulfonic-functionalized silicas due to the presence of −SO3H moieties. than 10−2 S·cm−1 at a high doping level (4−6 mol of PA) at 160 °C. As a consequence, good cell performances can be obtained only with high doping levels.5 The progressive leaching out of the acid during cell operation causes both a dramatic drop in conductivity and membrane degradation, which constitute serious drawbacks of PBI-based fuel cells. In order to overcome this problem, many strategies were discussed in the literature, among which are the chemical modification of the polymer backbone1,6−8 and the addition of inorganic fillers.1−5 Among the potential fillers, pristine and functionalized silica as well as mesoporous silicas were successfully employed for realizing composite membranes, leading to better proton conductivity performances with respect to neat PBI membranes, as well as to improved thermal and mechanical stability.7,9−13 If many papers were published describing the changes of proton conductivity in composite PBI-based membranes depending on PA doping level and filler loading, relatively little effort has been to date devoted to investigate the structural changes induced by the doping with H3PO4 and by the

1. INTRODUCTION In the last years, high-temperature polymer electrolyte fuel cells (HT-PEMFCs) have been attracting a lot of interest due to their possible applications for CO2-free mobility. In fact, despite of their low-temperature homologues, mainly based on perfluorinated polymers such as Nafion, HT-PEMFCs work in the temperature range of 120−180 °C, which well fits the usual operating conditions of vehicle engines. Moreover, they do not require careful water management, because the membrane proton conductivity is only partially influenced by the cell humidification. Phosphoric acid-doped polybenzimidazoles (PA−PBIs) seem to be the most promising candidates for realizing low-cost membranes for HT-PEMFCs, thanks to their high proton conductivity with no or low humidification, good cell performances, and high thermal and mechanical stability.1−4 In polybenzimidazole (PBI) membranes, the proton conductivity is based on a dopant, typically a mineral acid, such as phosphoric acid, which alone is a very good proton conductor at high temperature (0.8 S·cm−1 at 200 °C).5 In the membranes the conductivity dramatically depends on the doping degree: about 10−5 S·cm−1 at a low doping level [2 mol of phosphoric acid (PA) per polymer repeat unit] versus more © 2015 American Chemical Society

Received: May 26, 2015 Revised: July 31, 2015 Published: August 3, 2015 18935

DOI: 10.1021/acs.jpcc.5b05009 J. Phys. Chem. C 2015, 119, 18935−18944

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The Journal of Physical Chemistry C inclusion of raw or functionalized fillers. Solid-state nuclear magnetic resonance (NMR) spectroscopy has been already proven to be a useful tool for exploring the structure and the interface interactions in polymer−filler nanocomposites.14−18 Therefore, it constitutes a very suitable tool for investigating both the polymer backbone and the inorganic/hybrid filler of H3PO4-doped composite membranes. Furthermore, also the evolution of the phosphoric acid dopant and its interaction with the polymer matrix can be monitored through 31P−1H 2D heteronuclear correlation experiments (HETCOR).19 In this paper we report a multinuclear (1H, 13C, 31P, and 29Si) solidstate NMR investigation of several pyridine-based polybenzimidazole nanocomposite membranes, where the filler is a raw mesoporous silica, SBA-15, or a propylsulfonic-functionalized homologue realized with a one pot-synthesis.20 The filler loading was maintained as a constant (30 wt %), whereas the functionalization degree of the filler was 10 or 20 wt %, in order to explore possible changes in the PA−PBI interactions due to the different amount of −SO3H moieties. The membranes were prepared with an innovative spray deposition method, which allows a homogeneous filler distribution, even at high filler loading (at least up to 50 wt %), leading to a proton conductivity enhancement of more than 1 order of magnitude with respect to cast composite membranes.21

DL (%) =

nMPTMS nMPTMS + n TEOS

Wd

× 100

(2)

where Wp and Wd are the weights of the doped and undoped membranes, respectively. In a similar way also neat PBI_5N undoped and H3PO4-doped membranes were prepared. A detailed list of the prepared samples and their labeling is reported in Table 1, together with the estimated doping level. Table 1. List of the Analyzed Samples, Including SO3H Functionalization Amount and Estimated Acid Uptake sample 5N 5Nd 5N-S 5N-Sd 5N-S10 5N-S10d 5N-S20 5N-S20d

filler loading (wt %) SO3H units (wt %) doping level (wt %) 0 0 30 30 30 30 30 30

100 0 0 10 10 20 20

125 130 140

In order to facilitate the 13C NMR peak assignments, a neat mPBI (poly[2,2′-m-(phenylene)-5,5′-bibenzimidazole]) doped membrane was prepared starting from the condensation reaction of isophtalic acid and 3,3′-diaminobenzidine.22 Solid-State NMR. Spectra were recorded on a Bruker Avance III 400 MHz spectrometer at room temperature. The samples were prepared by cutting the membranes into small pieces, which were then placed inside the rotors. The doped membranes were cut just after the drying at 110 °C. 13C, 1H, and 31P MAS NMR spectra were acquired in a 4 mm MAS probe operating at the spinning speed of 10−11 kHz. 1H onepulse experiments were recorded using a 90° pulse of 4.3−4.6 μs and a recycle delay of 4 s. 13C(1H) CPMAS experiments were acquired using a 1H 90° pulse of 4.6 μs, contact time of 3 ms, spinal.64 decoupling scheme, and recycle delay of 4 s. 13C single-pulse experiments were obtained using a 90° pulse of 4.6 μs, a recycle delay of 15 s, and spinal.64 decoupling. For both nuclei the chemical shifts (δCS) are reported relative to TMS, using adamantane as secondary standard. 31P single-pulse experiments were acquired using a 90° pulse of 4.2−4.4 μs and a recycle delay of 4−10 s. The quantitative recycle delay was estimated after T1 inversion recovery measurements. 31P(1H) CP MAS spectra were acquired with a 1H 90° pulse length of 4.6 μs, ramp CP, variable contact time, and recycle delay 3 s. The most suitable contact time was found to be 3 ms. Hartmann−Hahn matching conditions and the decoupling scheme (spinal.64) were adjusted on NH4H2PO4. 31P(1H) HETCOR experiments were acquired under similar conditions using 3 ms as the contact time. Chemical shifts are reported relative to an 85% H3PO4 solution. 29Si MAS NMR spectra were acquired in a 7 mm MAS probe operating at a spinning speed of 5 kHz. For single-pulse experiments, a 30° pulse of 4 μs and a recycle delay of 40 s were used. 29Si(1H) CP MAS experiments were obtained using a 1H 90° pulse of 5.5 μs, contact time of 3 ms, spinal.64 decoupling scheme, and recycle delay of 6 s. Chemical shifts are reported relative to TMS, using tetrakis(trimethylsilyl)silane as a secondary standard, with δCS = −9.8 ppm for the trimethylsilyl groups. Some of the spectra obtained were fitted to Gauss/Lorentz curves, using the DMFIT simulation routine.24

2. EXPERIMENTAL SECTION Polymer and Fillers Preparation. The poly-2,2′-(2,6pyridine-5,5′-bibenzimidazole) polymer matrix (in the following labeled as 5N) was prepared as a powder, starting from a condensation reaction of 2,6-pyridindicarboxylic acid and 3,3′diaminobenzidine (the complete procedure is reported elsewhere).22 The mesoporous SBA-15 silica (labeled as S) was prepared by means of a template-based sol−gel procedure, following the method developed by Zhao et al. in 1998.23 The hybrid propylsulfonic SBA-15-type silicas were synthesized by a one-step process, involving a co-condensation of tetraethylorthosilicate (TEOS) and 3-mercaptopropyltriethoxysilane (MPTMS) in the presence of Pluronic 123, a templating agent, following the procedure optimized by Margolese et al.20 The nominal fraction of −SO3H units was varied between 10 and 20 mol %, obtaining the samples S10 and S20. The −SO3H molar ratio, X, is defined as the molar fraction of the silane precursor with respect to the total amount of condensation coreagents through the following: X (%) =

(Wp − Wd)

(1)

Composite Membranes Preparation. PBI_5N/filler composite films were produced using a spray deposition method.21 A proper amount of polymer powder was dissolved in a sealed flask at 120 °C in a dimethylacetamide/secbutylamine (8:2) solution, 30% w/w with respect to PBI_5N of filler was added, and the mixture was sonicated for 30 min. The suspension was then sprayed under N2 flux onto a hot plate (150 °C) in a ventilated homemade chamber. The 50-μm-thick film was peeled off of the cooled plate and washed in deionized water overnight. Part of the so obtained membranes were doped in a phosphoric acid (PA) solution (70% w/w) for 1 day and then dried at 110 °C for 2 h. The doping level (DL) was calculated using the following 18936


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3. RESULTS AND DISCUSSION Membranes having a poly-2,2′-(2,6-pyridine-5,5′-bibenzimidazole) polymer matrix (5N, see Scheme 1) and SBA-15 or

other units and/or H2O molecules. It is known that protons involved in hydrogen-bonding networks have a higher chemical shift, due to deshielding effects. Furthermore, the δCS value is proportional to the H···N distance, which could be estimated using fast MAS NMR experiment.26,27 The broadening of the signal is a sum of several factors: the broad distribution of possible geometries and the very large 1H−1H and 1H−15N dipolar couplings, which cannot be averaged out at 10 kHz spinning speed. The composite membranes show additional signals, which originate from the included fillers. 5N-S (Figure 1b) presents additional signals in the 4−0 ppm range, due to a residual amount of Pluronic 123,28 the templating agent using in the synthesis of the mesoporous silica. The surface silanols of the inorganic filler mainly interact with water and resonate at about 5 ppm, i.e., are included in the broad peak at about 5 ppm, whereas the minor amount of isolated silanols, due to small structural defects or remoteness from water molecules, was reported to resonate at about 2 ppm.29 The membranes doped with SO3H-functionalized SBA-15 type silicas [5N-S10 (Figure 1c) and 5N-S20 (Figure 1d)] show signals in the 4−0 ppm range due to the methylene units of the propylsulfonic moieties.30 The sulfonic acid protons are under chemical exchange with protons from water molecules, and their chemical shift is an average of δCS of the −SO3H units and δCS of water (3−5 ppm), resonating presumably at about 6.5 ppm.30 This signal partially overlaps with the signal due to adsorbed water. On the right side of Figure 1, the 1H MAS spectra recorded for the H3PO4-doped samples are reported. The spectra are dominated by two relatively narrow signals at about 9.2 and 8 ppm attributable to H3PO4 and H2O molecules, respectively.

Scheme 1. Chemical Structure of Poly-2,2′-(2,6-pyridine5,5′bibenzimidazole) (PBI_5N)

propylsulfonic-functionalized SBA-15-type mesoporous silica as filler were prepared. In Table 1 a detailed list of the analyzed samples is reported. For all samples, both undoped and H3PO4doped membranes were considered. The filler loading is fixed to 30 wt %. This loading was proven to lead to improved proton conductivity and mechanical stability with respect to neat 5N membranes. However, the electrochemical analysis of cells assembled with these membranes also reveled that the best performances could be obtained only using pure SBA-15 as filler, whereas a propylsulfonic functionalization greater than 10 wt % is detrimental for the cell performances.25 1 H NMR. Figure 1 reports the 1H MAS NMR spectra recorded for all the samples. Neat undoped 5N (Figure 1a, left) presents a very broad line shape, which can be simulated with three signals at 12.3, 8.3, and 5.5 ppm, attributed to protons in NH units, aromatic protons, and adsorbed water protons, respectively. The signal at 12.3 ppm is very broad, about 2200 Hz, and include all NH units, mostly involved in H-bonds with

Figure 1. 1H MAS NMR spectra of undoped (left) and PA-doped (right) membranes: (a) 5N, (b) 5N-S, (c) 5N-S10, and (d) 5N-S20. The colored lines are line shape deconvolution components. In the doped samples, δCS and line width of the peak related to aromatics protons were considered as constant parameters in the best-fit procedure throughout the series (they were estimated in the undoped 5N sample). 18937


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Figure 2. 13C(1H) CP MAS NMR spectra recorded for undoped (left) and PA-doped (right) membranes: (a) 5N, (b) 5N-S, (c) 5N-S10, and (d) 5N-S20. The contact time was 3 ms.

Figure 3. 13C(1H) CP MAS NMR spectra recorded with a contact time of 3 ms for (a) PBI and (b) PBI_5N powders and (c) doped PBI and (d) PBI_5N membranes (5Nd). (e) 13C one-pulse MAS experiment recorded with 30 s recycle delay for the doped PBI_5N sample (5Nd). The colored lines are guidelines for the peaks assignment (see also Table 2).

signal of residual adsorbed water undergoes a downfield shift, because of its multiple interactions with both phosphoric and sulfonic acid moieties. The broad peak at 13−14 ppm has to be attributed to both H···NH and NH···OP units. The higher δCS with respect to the undoped samples likely reflects the increased strength of the hydrogen bonding and the presence of hydrogen bonds between imidazole protons and H3PO4. This interaction will be more deeply explored with 2D 31P(1H) HETCOR MAS NMR spectra (see the section on 31P NMR). The broad and less intense signal at 8.3 ppm is due to the aromatic protons of the PBI matrix. 13 C NMR. Using 13C(1H) CP MAS NMR experiments we were able to characterize the 5N polymer backbone in the

The chemical shift of pure phosphoric acid was reported to be 10.2 ppm, but its signal was observed at 9.3 ppm in H3PO4equilibrated Nafion, due to interactions with −SO3H moieties and water molecules.31 In our samples the presence of propylsulfonic pendants on the SBA-15-type silica did not influence the δCS value, which remained the same as in the 5N and 5N-S samples. The final drying step at 120 °C of the membrane production has removed the water coming from the PA-doping process; however, PA is highly hygroscopic and a partial readsorption of water during NMR sample preparation occurs, as demonstrated by the peak at 8 ppm. This can happen also when a fuel cells is assembled, so the presence of residual water should not be ignored in structural investigation. The 18938


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probably due to differences in the chemical environment arising from the twisting of two adjacent benzimidazoles.33 The signals related to the other carbons of the benzimidazole group do resonate at 142, 135, 122, and 112 ppm. The signals are relatively broad, suggesting both consistent 1H−13C dipolar couplings and distributions of slightly different chemical environments, due to the packing of the polymer chains. The peak at 129 ppm is attributed to aromatic carbons bearing a proton (Ci, Cj, and Ck), while on the basis of liquid NMR studies,34 the signal related to the CIV in the h position is located at 135 ppm. In the spectrum of 5N (Figure 3b), we can find a similar pattern for the benzimidazole carbons, which are only slightly influenced by the presence of the pyridine group. Taking into account the 13C liquid spectra of pyridine,36 the introduction of a nitrogen atom changes the deshielding effects on the aromatic carbons, bringing about a downfield shift for Ch and Cj and a upfield shift for Ci. As a consequence, we assigned the peaks at 147, 138, and 120 ppm to Ch, Cj, and Ci, respectively. The assignment process for the doped samples started by taking into account the high-resolution liquid NMR work of Pugmire and Grant,37 where the chemical shift values for both benzimidazole and its protonated derivative are analyzed. In the following, the primes will indicate the carbon atoms of the single benzimidazole molecule. The following was observed for the protonated benzimidazole: (i) Ca′ and Cb′,g′ carbons show an upfield shift of the δCS; Ca′ changes from 141 to 139 ppm and Cb′,g′ from 138 to 130 ppm; (ii) Cd′,e′ carbons resonate at about 127 ppm, downfield with respect to the pure benzimidazole (123 ppm); and (iii) Cc′,f′ carbons resonate at 114 ppm (it was at 115 ppm in benzimidazole). Starting from these results, we were able to progressively assign most of the signals of the PBI PA-doped membrane (Figure 3c). The main signal relative to Ca is clearly shifted to 147 ppm (−3 ppm shift). In the PBI units, Cc and Ce carbons explore an environment similar to Cd′,e′, resonating at 122 ppm, so we expect for a downfield shift of about 5 ppm in the protonated polymer, and we assign to these carbons the signal at about 127 ppm. The signals at 116 and 112 ppm can be assigned to Cf carbons. The splitting of the signal related to these carbons into two peaks probably arises from a conformational heterogeneity, already present (but not resolved) in undoped PBI. Here it can be observed due to the reduced 1H−13C dipolar coupling, which leads to an improved resolution. Excluding the signal related to the aromatic carbons Ci,j,k and Ch resonating at 131 and 134 ppm, respectively, which are presumably less affected by the imidazole protonation, we have still to assign the peaks at 138, 135, and 123 ppm. Following the liquid NMR study previously cited,37 we expect that Cb,g carbons undergo to a marked upfield shift in benzimidazolium, so we assign the peak at 123 ppm to these carbon species (−12 ppm shift). The peak at 138 and 135 ppm should be then attributed to carbons in the d position, also exploring slightly different environments as Ca and Cf. The 13C(1H) CP MAS and DD one-pulse spectra recorded for the doped 5N membrane are reported in parts d and e of Figure 3, respectively. According with the previous assignment of the PBI-doped sample, we can assign the peak at 146 ppm to Ca, the peak at 137 ppm to Cd, the peak at 128 ppm to Cc,e, and the couple of peaks at 116 and 112 ppm to Cf carbons. If we consider also CP MAS experiments acquired with a very short contact time of 0.1 ms (Figure 4, undoped (a) and doped (b) 5N-S20), we can observe that the peaks in the 150−130 ppm

composite membranes. An overview of all CP MAS spectra is reported in Figure 2 (part a reports the spectra of pristine (left) and doped (right) 5N membrane, while spectra of composite membranes 5N-S, 5N-S10, and 5N-S20 are reported in parts b, c, and d, respectively). The presence of the filler, either fully inorganic or sulfonated, has no significant influence on the spectra of both undoped and doped samples. As expected, the major changes in the spectra are introduced by the PA doping. When the polymer is immerged in the PA solution, the imidazole groups are protonated by two PA molecules: H3PO4 + [−CN−] ⇆ H 2PO4 − + [−CNH+−]

(3)

The pyridine nitrogen can also be protonated at a higher degree of doping.32 The samples here analyzed are doped at 70% w/w concentration of the acid solution, so protonation of the pyridine nitrogen cannot be excluded. Pristine PBI shows π−π stacking interactions between the aromatic backbones in X-ray diffraction analysis, due to hydrogen bonding between the amine and the imidazole groups of two different chains.22 MD simulations also showed that, in m-PBI molecules, the phenylene group and the neighboring benzimidazole group are in an almost coplanar conformation, while two adiacent benzimidazoles were twisted with respect to each other.33 In the doped samples, in contrast, such order is lost. The protonation of the polymer modifies the chemical environments of many carbons with respect to the pristine PBI. Furthermore, the changes in the hydrogen-bonding network and the π−π stacking do alter the 1H−13C dipolar couplings for some carbon nuclei, leading to a general narrowing of the line shapes. The effectivness of the cross-polarization is markedly reduced, due to the consistent amount of highly mobile PA molecules dispersed among the polymer chains. Due to all these modifications, the assignment of 13C peaks for the doped samples is difficult. In order to make such an assignment more easily, we used two complementary strategies: (i) we prepared a doped m-PBI membrane and (ii) we recorded CP MAS NMR spectra with very short contact time for discriminating CIV carbons. In Figure 3 the 13C CP MAS spectra obtained for m-PBI (a) and 5N (b) powders were compared with the spectra recorded for their corresponding doped membranes [doped m-PBI (c), doped 5N (d)]. The 13C peak assignment for m-PBI (see Table 2) was performed starting from previous literature.34,35 The carbon atom between the amino and the imidazole groups (Ca) resonates at about 150 ppm and splits into two signals, Table 2. Assignments of the 13C MAS NMR Spectra Reported in Figure 3 δCS (ppm, ±0.2) C site

PBI

doped PBI

PBI-5N

doped PBI-5N

Ca Cb Cc Cd Ce Cf Cg Ch Ci Cj Ck

150, 153 135 122 142 122 112 135 135 129 129 129

147, 150 123 127 138, 135 127 116, 112 123 134 131 131 131

151 134 120 143 120 111 134 147 122 138

146 132 128 137 128 116, 112 132 142 125 132

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Figure 4. 13C(1H) CP MAS NMR spectra recorded with a similar number of scans (about 20K) and contact time of 0.1 or 3 ms for (a) 5N-S20 and (b) doped 5N-S20. Figure 5. 31P MAS NMR spectra recorded for 5Nd (a), 5N-Sd (b), 5N-S10d (c), and 5N-S20d (d).

range are largely underestimated, in accordance with their attribution to CIV carbons. In Figure 4b only the peak at 142 ppm presents a significant, although small, intensity. In our opinion it should be assigned to Ch, which is surrounded by more protons than in the neat 5N, due to the protonation of the pyridine nitrogen. To assign the two last peaks remaining at 132 and 125 ppm, we took also into account the peak integration in the one-pulse MAS spectrum (Figure 3e). The peak at 132 ppm accounts for three carbons, while the peak at 125 ppm only for one, so we could assign the first one to Cj and Cb,g carbons, while the latter were related with Ci. However, the different shielding effect on the signal related to the Cb,g carbons due to the PA doping in PBI and PBI_5N is not easy explainable, suggesting that the two polymer matrices differ in some conformational details. This question may be solved with a multitechnique approach, which goes beyond the scope of this paper. 31 P NMR. Figure 5 shows the 31P MAS NMR spectra recorded for 5Nd (a), 5N-Sd (b), 5N-S10d (c), and 5N-S20d (d). In all spectra two signals can be observed: the main signal at about 0 ppm, due to H3PO4, and a relatively small signal at about −11 ppm attributable to pyrophosphoric acid, H4P2O7.38,39 In no one sample does the amount of this species exceed 8% of the total phosphorus species. The peak related to H3PO4 presents a couple of small asymmetric sidebands, indicating that its mobility is at least partially hindered. T2 relaxation time estimated for a H3PO4 85 wt % solution is about 74 ms, while in the doped 5N membranes it is about 2.3 ms. The 31P(1H) CP MAS NMR spectra recorded for all samples are very similar, and a comparison with the one-pulse MAS experiment (see the Supporting Information) shows for the peak at 0 ppm a substantial intensity decrease. This can be explained by considering that a part of the H3PO4 molecules could be so highly mobile that cross-polarization from the rigid protons of the polymer backbone is not efficient. However, due to the large quantity of adsorbed acid, also the relative spatial remoteness of some acid molecules from the polymer backbone could prevent these from receiving sufficient magnetization to

give a CP signal. The signal at −11 ppm also remains. In order to explore the acid−polymer interactions and to observe differences due to the presence of the two filler types, we performed also bidimensional 31P(1H) HETCOR experiments for 5Nd, 5N-Sd, and 5N-S20d, which are reported in Figure 6. In the HETCOR spectrum recorded for the 5Nd sample (Figure 6a), the 31P peak at 0 ppm strongly correlates with the 1 H signals at 9.2 and 14 ppm. The first one was attributed to phosphoric acid, indicating a strong hydrogen-bonding network between acid molecules, while the second one is due to the NH units of the benzimidazolium, suggesting also a strong N−H··· O−P weak bond network, as already observed by Hughes et al.19 The residual small tail of the 1H signal in the 8−5 ppm range could suggest an interaction with the large signal due to aromatic protons. The signal at −11 ppm due to the pyrophosphoric acid correlates only with the proton peak at 9 ppm due to H3PO4, which reveals a low level of interactions with the polymer backbone. A similar situation can be observed also in the 2D spectrum recorded for the 5N-Sd composite membrane (Figure 6b). However, in this case, the main 31P signal possesses a shoulder at −2.3 ppm, which correlates preferentially with the proton species resonating at about 8 ppm, which we identify as partially protonated phosphoric acid molecules. Therefore, this correlation presumably describes the following equilibrium: 2H3PO4 ⇆ H4PO4 + + H 2PO4 −

(4)

The peak at −11 ppm due to H4P2O7 shows a correlation peak similar to that observed for the neat 5Nd sample, but looking to the 3D picture of the HETCOR experiment (Figure 6b, bottom), it is more intense and broader, suggesting that the peak is an overlapping of slightly different chemical environments, which correlates with both H3PO4 and acid water molecules. In the 31P dimension of the HETCOR experiment recorded for the composite sample 5N-S20d (Figure 6c), only 18940


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Figure 6. 31P(1H) HETCOR experiments recorded using 3 ms as contact time for (a) 5Nd, (b) 5N-Sd, and (c) 5N-S20d. The 2D spectra are plotted as a contour plot with projection on both 1H and 31P dimensions (top) and as a 3D picture (bottom), in which the z-axes represent the intensity of the 2D peaks.

Figure 7. (Left) 29Si(1H) CPMAS spectra recorded for SBA-15 (a), 5N-S (b), and 5N-Sd (c) using 3 ms as the contact time; spectrum a was acquired with 12 288 scans, while spectra b and c were acquired with 20 480 scans. (Right) 29Si one-pulse spectra acquired for pristine SBA-15 (d), 5N-S (e), and 5N-Sd (f) samples.

reactions like silylation41 or when the silica particles are dispersed in a polymeric matrix.14 In fact, the 1H → 29Si magnetization transfer buildup markedly changes, depending on which proton reservoir does transfer the magnetization to the silica surface and where it is spatially located. Therefore, the CP experiments give complementary information with respect to the quantitative one-pulse experiments, where the surface species often cannot be properly observed due to their low abundance. We focused first our attention on the 5N-Sd composite, which showed the better electrochemical performances.25 In Figure 7 the 29Si(1H) CP MAS NMR spectrum of the pristine SBA-15 filler (a) is reported. It can be interpreted in terms of Qn species {Qn refers to [Si(OSi)n(OH)4−n] units, with n = 0, 1, 2, 3, 4}. The signal at −92 ppm is attributed to

the signal at 0 ppm can be observed. There is still a small shoulder at about −2 ppm. In all samples this shoulder never interacts with the NH units, taking no part in the hydrogenbonding network of the polymeric backbone. A comparison of the three 2D experiments allows one to note that the main correlation peak is more intense for the neat 5N doped membrane with respect to the composite membranes, in particular with respect to the 5N-S sample. This suggests that the acid−polymer interactions in the composite membrane are weaker, likely because a portion of the acid molecules have strong interactions with the filler particles. 29 Si NMR. The 29Si(1H) CP MAS experiment has been already proven to be a suitable tool for exploring the silica surface,40 particularly when it is modified by functionalization 18941


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Figure 8. (Left) 29Si(1H) CPMAS spectra recorded using 3 ms as the contact time for pristine S20 filler (a), 5N-S20 (b), and 5N-S20d (c) samples. Spectrum a was acquired with 12 288 scans, while spectra b and c were acquired with 20 480 scans. (Right) 29Si one-pulse spectra acquired for pristine S20 (d) and 5N-S20d (e) sample.

geminal silanols, i.e., Q2 units, and the signal at −101 ppm to Q3 units, whereas the peak at −110 ppm arises from the [Si(OSi)4] units (Q4), the fundamental building block of the SBA-15 thick walls. In contrast, Q2 and Q3 species, presenting one and two OH groups respectively, are mainly located on the silica surface and at the edges of the pores.20,23 These two species, which together account for about 40% of the silicon nuclei of the SBA-15 silica (see the one-pulse MAS experiment in Figure 7d), were markedly emphasized with respect to Q4 units in the SBA-15 spectrum, because they directly bear protons capable of magnetization transfer. In the spectrum recorded for the composite 5N-S membranes (Figure 7b), the signal due to Q4 units was enhanced with respect to spectrum a). This is possible because the magnetization source is now mainly constituted by the protons of the polymer backbone, which is close to the surface of the silica particles (at a scale of nanometers).14 The 29Si(1H) CP spectrum recorded with the same experimental conditions as spectrum b for the doped composite (5N-Sd, Figure 7c) gives no signal, implying that the chemical environment surrounding the silica surface is deeply changed. This evidence suggests that the surface of the SBA-15 is now completely surrounded by the mobile PA molecules, which hinder the magnetization transfer. We have already observed a detrimental effect of H3PO4 on the crosspolarization efficiency in the 13C NMR section, especially for this sample (see Figure 2). Because Q2 and Q3 sites are both on the external surface of the particles and on the inner surface of the pores, a complete disappearance of the signal suggests that H3PO4 is diffused also into the mesoporous structure. In order to clarify if the phosphoric acid is also able to destructure the silica particles, a comparison of the 29Si one-pulse MAS experiments recorded for undoped and doped 5N-S samples is reported (Figure 7, spectra e and f). The spectrum of the doped sample shows a broader line shape, suggesting an increased structural disorder; however, the main signals of the SBA-15 could still be recognized, indicating that most of the mesoporous structure is preserved. We also analyzed the 5N-S20d sample, which contains the propylsulfonic functionalized filler, in order to evaluate if an analogous behavior can be observed. In Figure 8 the 29Si(1H) CP MAS spectra of the pristine filler S20 (a) and of both undoped (b) and doped composite membranes (c) are reported, together with the 29Si one-pulse spectra recorded for S20 (e) and doped 5N-S20d (f). In this case, we were still able to record a signal using the cross-polarization transfer. The Qn region of the spectrum seems to be only partially affected by

a signal loss, while the Tn region seems to be unaffected. From this evidence, it can be presumed that in the 5N-S20d sample the chemical environment surrounding the silica particles did not change so much after the PA doping of the membrane. Previous works in literature were able to demonstrate using N2adsorption isotherms20 and TEM images13 that, in a SBA-15type mesoporous silica prepared with the one-pot synthesis proposed by Margolese et al.,20 a homogeneous distribution of the propylsulfonic pendants also into the inner surface of the silica can be obtained; i.e., −SO3H groups are also inside the mesopores. We could then presume that the presence of the organic pendants hinders the access of the H3PO4 molecules into the silica pores. Therefore, PA molecules can interact only with the OH groups on the external surface of the silica particles. By this way the chemical environment, which transferred the magnetization in the CP experiment of the undoped sample, is mainly preserved in the doped sample.

4. CONCLUSIONS A deep characterization of the structure and interactions in pyridine-based PBI composite membranes, having inorganic or propylsulfonic mesoporous silica as filler, was performed by means of solid-state NMR spectroscopy. 13C(1H) CP MAS spectra monitored the changes induced on the packing of the PBI chains by the PA doping. The H3PO4 molecules diffuse between the polymer chains and reduce their intermolecular interactions, as proved by the visible reduction of the 1H−13C dipolar coupling, which is instead marked into the pristine heteroaromatic structure. The 31P(1H) correlation experiments allowed one to observe that acid molecules constitute a strong hydrogen-bonding network with NH groups, especially in the neat PBI_5N membranes and that, if present, residual pyrophosphoric acid molecules are far from the polymer backbone. The acid−PBI correlation is partially weakened by the presence of filler particles, and the 29Si NMR analyses were able to give an explanation to this, adding some new insights regarding the role of the filler into the composite membranes. The interactions of the acid−filler particles are different for the two types of filler we considered. A fully inorganic mesoporous structure seems to allow H3PO4 molecules to diffuse into the pores, where they interacts with the OH groups of the silica on the CP time scale. In contrast, when the SBA15-type structure is modified by the homogeneous introduction of propylsulfonic pendants, the latter hinder the acid diffusion into the pores, and the acid−filler particles interactions are presumably limited only to the external silica surface. Due to 18942


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Conducting Membranes based on PBI5N, SiO2-Im and H3PO4 for High-Temperature Fuel Cells. Phys. Chem. Chem. Phys. 2011, 13, 12146−12154. (10) Ghosh, S.; Maity, S.; Jana, T. Polybenzimidazole/silica Nanocomposites: Organic-Inorganic Hybrid Membranes for PEM Fuel Cell. J. Mater. Chem. 2011, 21, 14897−14906. (11) Chu, F.; Lin, B.; Qiu, B.; Si, Z.; Qiu, L.; Gu, Z.; Ding, J.; Yan, F.; Lu, J. Polybenzimidazole/Zwitterions-coated Silica Nanoparticle Hybrid Proton Conducting Membranes for Anhydrous Proton Exchange Membrane Application. J. Mater. Chem. 2012, 22, 18411− 18417. (12) Singha, S.; Jana, T. Structure and Properties of Polybenzimidazole/Silica Nanocomposite Electrolyte Membrane: Influence of Organic/Inorganic Interface. ACS Appl. Mater. Interfaces 2014, 6, 21286−21296. (13) Tominaga, Y.; Maki, T. Proton-Conducting Composite Membranes based on Polybenzimidazole and Sulfonated Mesoporous Organosilicate. Int. J. Hydrogen Energy 2014, 39, 2724−2730. (14) Simonutti, R.; Comotti, A.; Negroni, F.; Sozzani, P. 13C and 29Si Solid State NMR of Rubber-Silica Composite Materials. Chem. Mater. 1999, 11, 822−828. (15) Hou, S. S.; Beyer, F. L.; Schmidt-Rohr, K. High-Sensitivity Multinuclear NMR Spectroscopy of a Smectite Clay and of ClayIntercalated Polymer. Solid State Nucl. Magn. Reson. 2002, 22, 110− 127. (16) Zhang, L.; Xu, J.; Hou, G.; Tang, H.; Deng, F. Interactions between Nafion Resin and Protonated Dodecylamine modified Montmorillonite: a Solid State NMR Study. J. Colloid Interface Sci. 2007, 311, 38−44. (17) Lee, D.; Balmer, J. A.; Schmid, A.; Tonnar, J.; Armes, S. P.; Titman, J. J. Solid-State Nuclear Magnetic Resonance Studies of Vinyl Polymer/Silica Colloidal Nanocomposite Particles. Langmuir 2010, 26, 15592−15598. (18) Bonhomme, C.; Gervais, C.; Laurencin, D. Recent NMR Developments applied to Organic-Inorganic Materials. Prog. Nucl. Magn. Reson. Spectrosc. 2014, 77, 1−48. (19) Hughes, C. E.; Haufe, S.; Angerstein, B.; Kalim, R.; Mähr, U.; Reiche, A.; Baldus, M. Probing Structure and Dynamics of Poly[2,2‘(m-phenylene)-5,5‘-bibenimidazole] Fuel Cells with Magic-Angle Spinning NMR. J. Phys. Chem. B 2004, 108, 13626−13631. (20) Margolese, D.; Melero, J. A.; Christiansen, S. C.; Chmelka, B. F.; Stucky, G. D. Direct Syntheses of Ordered SBA-15 Mesoporous Silica Containing Sulfonic Acid Group. Chem. Mater. 2000, 12, 2448−2459. (21) Quartarone, E.; Villa, D. C.; Angioni, S.; Mustarelli, P. Facile and Green Assembly of Nanocoposite Membranes for Fuel Cells. Chem. Commun. 2015, 51, 1983−1986. (22) Carollo, A.; Quartarone, E.; Tomasi, C.; Mustarelli, P.; Belotti, F.; Magistris, A.; Maestroni, F.; Parachini, M.; Garlaschelli, L.; Righetti, P. P. Developments of New Proton Conducting Membranes based on Different Polybenzimidazole Structures for Fuel Cells Applications. J. Power Sources 2006, 160, 175−180. (23) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Frederickson, G. H.; Chmelka, B. F.; Stucky, G. D. Triblock Copolymer Syntheses of Mesoporous Silica with Periodic 50 to 300 Angstrom Pores. Science 1998, 279, 548−552. (24) Massiot, D.; Fayon, F.; Capron, M.; King, I.; Le Calvé, S.; Alonso, B.; Durand, J. O.; Bujoli, B.; Gan, Z.; Hoatson, G. Modeling One- and Two-Dimensional Solid State NMR Spectra. Magn. Reson. Chem. 2002, 40, 70−76. (25) Angioni, S.; Villa, D. C.; Cattaneo, A. S.; Mustarelli, P.; Quartarone, E. Influence of Variously functionalized SBA-15 Silicas on the Electrochemical Properties of PBI Composite Membranes for HTPEMFCs. J. Power Sources 2015, 294, 347−353. (26) Chierotti, M. R.; Gobetto, R. Solid-State NMR Studies of Weak Interactions in Supramolecular Systems. Chem. Commun. 2008, 14, 1621−1634. (27) Goward, G. R.; Schnell, I.; Brown, S. P.; Spiess, H. W.; Kim, H. D.; Ishida, H. Investigation of a N···H Hydrogen Bond in a Solid

the increasing acid uptake with the increasing amount of organic functionalization, it could be argued that −SO3H moieties trap acid molecules but hinder their translational motions on the CP time scale. As a consequence, we presume that in the inorganic filler a proton conduction could take place inside the pores, which positively adds to the conduction in the polymer, while in the composite membrane with SO3Hfunctionalized mesoporous silica, the filler has no positive effects on the conductivity, but only on the acid uptake. This evidence could explain the observed increase in proton conductivity in composite membranes having 30 wt % SBA15 filler loading with respect to neat PBI_5N membranes and the progressive decrease of proton conductivity with the increase of functionalization of the mesoporous structure.25

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b05009. 13 C(1H) CP MAS experiments recorded at different contact times for m-PBI and comparison of 31P one-pulse and CP MAS experiments recorded for the 5Nd sample (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +39-0382-987205. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was done in the frame of the project financed by the Italian Ministry of University and Research (MIUR) (PRIN 2010, 2010CYTWAW). We also gratefully thank the Cariplo Foundation (Project 2011-1812).

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REFERENCES

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