Article pubs.acs.org/Macromolecules
Highly Phase Separated Aromatic Ionomers Bearing Perfluorosulfonic Acids by Bottom-up Synthesis: Effect of Cation on Membrane Morphology and Functional Properties Olesia Danyliv,† Cristina Iojoiu,*,†,‡ Sandrine Lyonnard,§ Nicolas Sergent,†,‡ Emilie Planes,‡,∥ and Jean-Yves Sanchez†,‡,⊥ †
Université Grenoble Alpes, LEPMI, Grenoble, F-38000, France CNRS, LEPMI, Grenoble, F-38000, France § CEA, INAC, SPrAM, Grenoble, F-38000, France ∥ Université Savoie Mont Blanc, LEPMI, Chambéry, F-73000, France ‡
S Supporting Information *
ABSTRACT: Proton-conducting aromatic-based ionomers bearing superacid side chains are usually synthesized by polymer postmodification, which does not allow controlling ion exchange capacity and ionic group distribution along the ionomer and, thus, its chemical structure and functional properties. Bottom-up approach overcomes this problem. Here, we report the preparation of a novel ionic monomer and its polycondensation with commercial monomers. The obtained random ionomers are the first to show high phase separated organization at macro-, micro-, and nanoscale, common to the reference protonconducting material Nafion. Additionally, membranes were cast from the solutions of ionomers in their Li+ and K+ forms in order to study the cation’s influence on both morphology and performance of the materials. The difference in ionic domain organization, depending on the initial cationic form of the ionomers, was reported for the first time. The proposed materials show superior proton conductivity than Nafion, especially at low relative humidity, which makes them potential substitute of the benchmarked Nafion for fuel cell application.
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INTRODUCTION
scattering and electron microscopy revealed a highly organized structure with a distinct separation at nanoscale between the hydrophobic and the hydrophilic domains. These domains, which are well interconnected and wide, yield in highly conductive structures at both high and reduced humidity below 90 °C. However, it is known that Nafion presents some drawbacks, such as (i) high oxygen permeability that is mainly responsible for the formation of aggressive radicals at the cathode, (ii) its nonenvironmentally friendly synthesis, and (iii) significant loss of mechanical properties and conductivity at temperature exceeding 90 °C.13,14
Proton exchange membrane (PEM) is the core part of a PEM fuel cell (PEMFC), responsible for both proton conduction and barrier properties toward electrons and gases. Additionally, it must fulfill the requirements for high thermo-mechanical stability, stability in oxidative environment and high performance in a wide temperature range (0−120 °C) at low relative humidity ( 30. Therefore, clusters of type “am” have the ability to absorb more water molecules than the “ord” ones, which is consistent with our assumption of distinct local environments characterized by more ordered or amorphous polymer phases. In the amorphous zones, it may be expected that water incorporation is favored by the more disordered organization of flexible chains. The trend upon hydration of “am” clusters expansion in P1−1.4 KH is similar to that of the ionic domains in P1−1.4 LiH. Additionally, values of the ionic expansion in P1−1.4 LiH are intermediate between the “am” and “ord” ones, which indicates that P1−1.4 LiH is characterized by an averaged structure with intermediate local order. The evolution of the normalized widths of the peaks (full width at half-maximum (fwhm) over qmax), displayed in Figure 8b, helps to further rationalize the ionomers’ microstructures. The fwhm/qmax values are indicative of the mean size of regions, where long-range order is observed (Scherrer’s law). Therefore, if we consider that the ionomers have a random distribution of locally well-organized regions of mean dimension lord, embedded in a more disorganized matrix with average mean dimension lam, then it is observed that the “am” zones do not show a significant variation (fwhm/qmax being roughly constant, ∼ 0.65) while the “ord” zones get more disordered and reduce in size by a factor 2 over the investigated hydration range (increase of fwhm/qmax from 0.20 to 0.38). Interestingly, for the P1−1.4 LiH membrane, the normalized
well-defined Gaussian-shaped scattering maximum is found at qmax 0.14 Å−1. The presence of such a peak (also called “ionomer peak”) indicates the existence of two distinct and well-separated phases, characterized by two different neutron length densities producing a significant contrast term. Generally, the position of a scattering maximum qmax is related to a characteristic distance d, interpreted as the mean average distance between scattering objects, e.g., ionic domains and/or hydrophobic aggregates.51 Here, a d-value of 45 Å is found for P1−1.4 LiH. The spectrum of P1−1.4 KH shows the presence of a much narrower and sharper ionomer peak, than that of P1−1.4 LiH, located at similar wave vector qmax. This could indicate a better structuration of P1−1.4 KH at nanoscale. Similar results were obtained for P1−1.2 (Figure SI6 in Supporting Information). Yet, in contrast with the LiH-membrane, the asymmetric coherent signal of P1−1.4 KH is clearly not a single Gaussianshaped correlation peak and contains several contributions. The I(q) profile can be reproduced using a two-Gaussian component fit including a linear background, as shown in Figure 7a. The narrow, intense correlation peak is found at qmax
Figure 7. SANS spectra of the ionomers: (a) P1−1.4 LiH and (b) P1− 1.4 KH after equilibration in water at 25−60 °C. λ-values of adsorbed water molecules per acidic site are attributed to each spectrum.
0.15 Å−1 (solid line) and corresponds to a typical d-spacing of 42 Å, while the broader component found at qmax 0.13 Å−1 (dashed line) corresponds to a higher typical separation distance, 49 Å. The presence of two ionomer peaks suggests the coexistence of two distinct nanoscale structuration of ionic clusters, which might be related to the presence of highly organized and fully amorphous polymer phases, as evidenced by the DSC results (Thermomechanical Characterization). It can be hypothesized that the sharper (solid line) peak arises from long-range ordering of the ionic domains within well-structured domains (named further as “ord”, i.e., P1−1.4 KH ord), while the broader (dashed line) peak characterized organization of the J
DOI: 10.1021/acs.macromol.6b00629 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
phase exhibiting a medium level of ordering, less organized than the P1−1.4 KH “ord” zones, but better than the P1−1.4 KH am ones. Accordingly, d-spacings and normalized fwhm values were found to be an average between the “ord” and “am” phases, with trends dictated either by homogeneous swelling (as “am”) either by continuous disordering (as “ord”). The SANS results are consistent with the AFM measurements, which evidenced that P1−1.4 LiH exhibits a more homogeneous microstructure at the microscopic scale. Better connectivity and higher ionic interface of P1−1.4 LiH, as deduced from the AFM analysis, can relate to the presence of ionic nanodomains more homogeneous in size and randomly connected within a polymer matrix, defined by the averaged distribution of single-type hydrophobic aggregates. At this point, one could expect a beneficial impact of such morphology on the proton conductivity, as will be further investigated and discussed in the next section. Conductivity and Water Uptake. The PEM’s conductivity and the PEM’s morphological stability are very dependent on water uptake. High water uptake generally leads to high conductivity, but it can also cause important dimensional changes, which often reduce the mechanical and morphological stabilities of the membranes.52 SANS studies show a continuous evolution of ionic cluster organization in the case of both P1−1.4 KH and P1−1.4 LiH membranes, yet higher mean ionic expansion within the “am” phase is found for P1−1.4 KH. The water uptake was performed in water at different temperatures (Figure 9a). The P1 of the same IEC cast from Li+- and K+-forms show similar increase in hydration number. Upon the increase of IEC, the λ-values increase dramatically: the P1 membranes of 1.0 mequiv/g accommodate slightly lower amount of water Figure 8. Evolution of (a) the characteristic distance, (b) the normalized width (fwhm over qmax), and (c) the ratio between the surfaces of the Gaussian decomposition peaks upon hydration for (□) P1−1.4 LiH, (●) P1−1.4 KH ord, and (○) P1−1.4 KH am. The graph in part c describes the ratio between the surfaces of the peaks P1−1.4 KH ord and P1−1.4 KH am.
peak width evolves similarly to the “ord” component of the P1−1.4 KH membrane, with values increasing from 0.4 to 0.57. To evaluate the relative proportion of the “am” and “ord” components of P1−1.4 KH, plotting of their peak area ratio upon hydration is presented in Figure 8c. The “ord” regions gradually disappear at the profit of the “am” ones, which can be related to the progressive disappearance of well-ordered grains under the plasticizing effect of water. This can be associated with a “dissolution-like” effect, as polymer chains belonging to the organized regions get incorporated in the amorphous zones. To summarize, the SANS results can be consistently understood by considering that the P1−1.4 KH membrane is composed of ordered domains and regions, surrounded by poorly ordered ones. Upon dilution, both regions swell and get more disordered. The degree of swelling is limited within ordered regions (d saturates at λ = 21.5), but the order-todisorder transformation is obviously more pronounced (fwhm of “ord” increases by factor 2). Water preferentially swells the amorphous zones and dissolves the organized ones, yielding a more homogeneous structure at high hydration with the presence of few, small remaining highly ordered nodules. The P1−1.4 LiH membrane behaves as a homogeneous polymer
Figure 9. (a) Water and (b) water vapor uptake of the ionomers P1. (□) Membranes acidified from Li+ form, (○) membranes acidified from K+ form, (×) Nafion 117. Solid line represents P1−1.0, dashed line represents P1−1.2, and dot-and-dash line represents P1−1.4. Error of the WU measurement is ±10%. Data of Nafion 117 in (b) is extracted from ref 52. K
DOI: 10.1021/acs.macromol.6b00629 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
of membrane to take and keep water at this low relative humidity. Similar results of high conductivity at low RH for the aromatic ionomers were reported by Chen et al.53 for the annealed membranes based on a multiblock copolymer with improved morphology. The similar or lower conductivity of P1 membranes at high RH, compared to that of Nafion 117, might be provoked by (i) excessive water uptake (in case of P1−1.4 and P1−1.2), leading to the dilution of ionic sites, and/or (ii) higher tortuosity of the aromatic PEM, especially of low IEC.
molecules per acidic site, compared to Nafion 117; while the P1 membranes of 1.4 mequiv/g adsorb twice as much water as Nafion 117 already at 40 °C. Moreover, after 60 °C P1 1.4 membranes become soluble in water. By consequence, these membranes are not recommended to be used in water or at high RH. Nevertheless, at 80 °C and low RH (Figure 9 (b)) the λ-values of P1−1.4 are lower than those of Nafion 117; however, they start to increase faster than λ of Nafion after 70% RH. To summarize, due to the higher IEC of the aromatic ionomers P1−1.4 (1.4 vs 0.9 mequiv/g in Nafion 117) their absolute water uptake is higher, but only at high humidification. The conductivity measurements at high RH, i.e., 95% RH at different temperatures, and at 80 °C and different RH were performed. First of all, it is evidenced that despite the similar λvalues of the P1 membranes acidified from different cations (K+ or Li+), the conductivities of P1 membranes cast in Li+ form are higher, than those of P1 membranes cast in K+ form, in the whole range of temperatures and relative humidity (Figure 10,
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CONCLUSIONS In this work, we reported the successful synthesis of (i) an ionic monomer functionalized with perfluorosulfonic function of high purity level and (ii) a series of random ionomers of the IEC ranging between 1.0 and 1.4 mequiv/g of high molecular weight. The theoretical IEC-values were confirmed by NMR and acid−base titration. The acidified ionomers P1 show good thermal stability for PEMFC application; they exhibit a Tg range of 107−114 °C, depending on the IEC, which is close to that of Nafion. P1 acidic ionomers also exhibit good mechanical strengths, i.e., their storage moduli are higher than 500 MPa at 80 °C. The membranes were initially cast in two cationic forms: lithium and potassium. The morphology of these samples was broadly studied at macro-, micro-, and nanoscale, and it was concluded that all the membranes showed a highly developed phase separation between the hydrophilic and the hydrophobic domains. However, the hydrophilic domains in the membranes obtained from Li+ form are more homogeneous and better percolated, while those obtained from the ionomers in K+ form are composed of two types of the ionic clusters: the one of a very high organization and the other of worse organization. Upon hydration the ionic clusters of higher organization tend to dissolve in the phase of the hydrophilic phase of the other type. Such morphology dictates the higher proton conductivity of the P1 membranes cast initially in Li+ form, compared to those obtained from ionomers in K+ form. An important feature of the newly synthesized P1−1.4 and P1−1.2 membranes is that their proton conductivities are higher than those of Nafion 117 at low RH (
