Elsevier

Solid State Ionics

Volume 252, 1 December 2013, Pages 140-151
Solid State Ionics

Degradation studies on acid–base blends for both LT and intermediate T fuel cells

https://doi.org/10.1016/j.ssi.2013.05.017Get rights and content

Highlights

  • Applicability of the acid-base blend concept to the development of both low-T and intermediate-T fuel cell membranes.

  • Excellent fuel cell stability of the investigated membranes.

  • Excellent fuel cell performance of one of the investigated intermediate-T fuel cell membranes (0.85 A/cm2@0.5 V at 180 °C).

Abstract

In this study the ex-situ and in-situ behavior of acid–base blend membranes from sulfonated polyethersulfone and a partially fluorinated sulfonated polymer (prepared by condensation of decafluorobipenyl with bisphenol AF, followed by sulfonation of the obtained polymer) and two different polybenzmidazoles (F6-PBI and PBIOO®) was investigated. Two types of acid–base blend membranes from the abovementioned polymers were prepared and characterized: acid–base blend membranes with a molar excess of acidic blend component for low-T H2 fuel cells (LT-FC) where the proton conductivity is overtaken by the sulfonic acid groups, and blend membranes comprising a molar excess of basic blend component which were subsequently doped with phosphoric acid for the usage in intermediate-T H2 fuel cells (IT-FC) where the network of phosphoric acid molecules in the membrane provides the proton conduction. For elucidation of the radical stability of the membranes, the membranes were subjected to Fenton's Reagent and were operated in a H2-PEMFC. After these tests, the membranes were investigated via SEC for molecular weight degradation. As a result, correlations could be found between degradation of the blend membranes in the fuel cell and after Fenton's test. Moreover, at IT-FC membranes, a correlation could be found between doping degree and fuel cell performance which are discussed in this paper. One of the membranes, a H3PO4-doped base-excess membrane from sPSU and PBIOO showed an excellent performance in an IT-FC at 180 °C of 0.85 A/cm2@0.5 V without pressurization of the reactant gases.

Introduction

Due to the global CO2 problem and the finiteness of fossil energy sources such as coal, gas and oil, hydrogen is an alternative as energy carrier since the combustion of hydrogen leads to just water. By fuel cells, hydrogen can be transformed into electrical energy with a high efficiency. Therefore, fuel cell research and development have been intensified very strongly during the last decades. Meanwhile some fuel cell applications are in pre-commercialized or even commercialized state: Fuel cells can be applied in different fields: they can provide the electrical energy for electric cars [1], they can be used in outdoor applications as substitute for batteries [2], and they can deliver both electrical energy and heat for housing applications [3].

In fuel cell research, the membrane is the key component. So far, only a few fuel cell membrane types are commercially available: At LT-FC, mainly perfluorinated sulfonic acid ionomers (PFSA) of the Nafion® type [4], and at IT-FCs membranes based onto basic polymers such as polybenzimidazoles [5], polyarylpyridines [6] etc. which contain phosphoric acid [7] or partially hydrolyzed polyphosphoric acid [8] as the proton conductor.

Regarding to LT-FC membranes, due to the disadvantages of PFSAs such as high price, production process involving highly toxic intermediates [9], and, particularly at fuel cells using liquid fuels such as methanol [10], ethanol [11], ethylene glycol [12], dimethylether [13] etc., high fuel permeability leading to poisoning of the cathode catalyst [10], there is an intensive worldwide search for alternative polymeric proton conductors in the scientific community. Due to the fact that the sulfonated arylene main-chain ionomers show chemical stabilities which are more or less close to that of the PFSAs or are even more stable than them, they are an object of more and more increasing research and development efforts. The groups of investigated arylene main-chain polymers, entitled in the following as polyarylenes, comprise polyphenylenoxides [14], polyetherketones [15], polyethersulfones [16], polyphenylphosphineoxides [17], and polymers which contain more than one of these building blocks in one macromolecular chain (e.g. [18]), and, more recently, polysulfones which show extraordinary chemical stabilities [19] and proton conductivities [20]. The major disadvantage of polyarylene ionomers, compared to PFSA, is, that they show too high water uptake degrees when having sufficient proton conductivities [9]. To avoid these disadvantages, e.g. to reduce water uptake/swelling, different measures can be applied to the polyarylene membranes: (i) introduction of an inorganic phase into the membranes for physical cross-linking, increase of thermal stability, improvement of internal water management, and partially even for increase of proton conductivity [21], [22], [23]; (ii) development of microphase-separated block-co-ionomers [24], [25], and (iii) covalent [26] or ionical [27], [28] cross-linking of the ionomer membranes. The concept of ionical cross-linking was developed in the authors' group, including both the development of blend membranes from sulfonated (and phosphonated) polyarylenes with basic polymers, either self-developed [29] or with a commercial basic polymer such as poly(4-vinylpyridine) [30] or different types of polybenzimidazoles [31], and the development of polymers containing both the sulfonic acid and the basic group onto the same backbone [32]. The properties of ionically cross-linked membranes can be fine-tuned by choice of (i) suitable acidic and basic polymers; (ii) choice of polymers which have a suitable content in acidic and basic groups, respectively, and (iii) an appropriate molar relation between acidic and basic groups in the blend to obtain membranes which still have a sufficiently high proton conductivity combined with significantly reduced water uptake. The dependence of the properties of the blend membranes from these variations has been reviewed in [33]. Many studies, for example [34], have been performed to find the best-suited sulfonated polyarylenes for the acid–base blends.

Regarding IT-FC membranes, H3PO4-doped polybenzimidazoles suffer from the following problems: (i) possible dissolution of PBI during phosphoric acid doping [35]; (ii) accelerated radical degradation at increased temperatures [36]; (iii) leaching out of phosphoric acid during fuel cell operation [37]. Several approaches have been reported in literature to decrease degradation of the phosphoric acid-doped polybenzimidazole membranes. One of these approaches is covalent cross-linking of the imidazole N-H of PBI by 3,3′,5,5′-tetramethyl-4,4′-biphenol diepoxide [38] or by reaction of p-xylene dibromide with the amine hydrogen of the imidazole group [39], another one ionical cross-linking of PBI by preparation of PBI-excess acid–base blends where a polymeric sulfonic or phosphonic acid serves as ionical cross-linker. For example, in [40] base-excess blends of PBI Celazol with sPSU sulfonated in the electron-rich part of the PSU repeat unit have been presented in terms of miscibility behavior between the blend components and in terms of suitability for ITFC when doped with phosphoric acid. Studies dealing with the preparation and fuel cell application of such blend membranes revealed that these blend membranes show much less radical degradation when applied to Fenton's Reagent than pure PBI membranes [41], particularly when appropriate base–acid combinations are used [42]. Moreover, the acid–base blend membranes showed good performance in IT-FCs [42], [43].

From the above mentioned it can be concluded that acid–base blend membranes offer the possibility to use them in both LT-FC, or IT-FC, depending on the molar relation between acidic and basic polymer in the blend. The ionical cross-linking of LT-FC and IT-FC membranes is schematically represented in Fig. 1.

Based onto the results of preliminary work [34], [41], [42], [43] two promising sulfonated and basic polymers were chosen for further investigations regarding LT-FC and IT-FC performance and radical stability which are presented in this study. The combinations comprise (i) blends of the partially fluorinated S1 polymer with the electron-deficient PBI polymer B1 (F6PBI) and the electron-rich PBI polymer B2 (PBIOO) (both acid-excess blends S1B1 and S1B2 and base-excess blends B1S1 and B2S1), and (ii) blends of the nonfluorinated S2 polymer with the electron-rich PBI polymer B2 and the electron-deficient PBI polymer B1 (both acid-excess blends S2B1 and S2B2 and base-excess blends B1S2 and B2S2).

The four polymers for the acid-excess and base-excess blends are depicted in Fig. 2.

Section snippets

Preparation of the sulfonated polymers

The preparation of the S1 polymer is described in [34]. The sulfonation degree of the polymer was 1.7 groups per repeat unit which is identical with an ion-exchange capacity (IEC) of 2.2 meqs SO3H/g membrane. The molecular weight of the sulfonated polymer, as determined by SEC, was Mw = 98,000 Da, Mn = 63,000 Da (PDI = 1.56) (refractive index detector). The S2 polymer was synthesized according to reference [44]. A polymer having an IEC = 1.8 meqs SO3H/g was synthesized. The molecular weight of the

MWD of the membranes before and after FT (SEC)

Except for the membrane S1B2, all membranes showed a relatively good chemical stability in FT, with the tendency that the blend membranes containing the partially fluorinated PBI B1 seem to be more stable in Fenton's solution, which was expected from earlier studies due to its strongly stable perfluoroisopropylidene bridges [42], [46]. It was not possible to perform GPC measurements of the S1B2, due to insufficient amounts of nondissolved material after FT.

From the MWD curves (Fig. 3) one may

Conclusions

In this study was shown that the acid–base blending concept can be used for the preparation of both low-T and intermediate-T fuel cell membranes. By ionical cross-linking, the membrane properties can not only be optimized and fine-tailored in terms of proton conductivity, water uptake, swelling, meOH uptake (for DMFC applications), but also in terms of chemical stabilities, provided that suitable and stable acidic and basic polymers are used. It turned out that the nonfluorinated sulfonated

Acknowledgments

We gratefully acknowledge the valuable help of Inna Kharitonova and Galina Schumsky in performing the practical work. The study was financed in-part by a DFG project entitled “Ion-exchange membranes for intermediate T fuel Cells” with the reference number KE 673/10-1.

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