Elsevier

Polymer

Volume 53, Issue 16, 19 July 2012, Pages 3587-3593
Polymer

A facile crosslinking method of polybenzimidazole with sulfonyl azide groups for proton conducting membranes

https://doi.org/10.1016/j.polymer.2012.05.052Get rights and content

Abstract

A facile crosslinking method of polybenzimidazole (PBI) with sulfonyl azide groups (sPBI-SA) for proton conducting membranes is proposed. Thermally crosslinkable sPBI-SA is synthesized from sulfonated PBI (sPBI) and sodium azide. The structures of sPBI, sPBI-SA, and crosslinked sPBI are confirmed by FTIR and 1H NMR. Upon heating, sPBI-SA loses nitrogen and form nitrene, which reacts with CH-bond of the backbone of another chain of PBI via the reactions of hydrogen abstraction, recombination, or CH-bond insertion. The crosslinking structure of PBI membranes is thus formed. Compared with the uncrosslinked membranes, the crosslinked sPBI membranes exhibit improved tensile strength, migration stability of phosphoric acid (PA), dimensional stability, and chemical oxidative stability. Whereas, the doping ability of PA and the proton conductivity of the crosslinked membranes decrease a little.

Introduction

Proton exchange membrane (PEM) is one of the key components of polymer electrolyte membrane fuel cells (PEMFCs). Perfluorosulfonic acid polymer as PEM is widely used at present, such as Nafion®. However, these membranes are limited by their high cost, water dependent proton conductivity, and subsequent humidification requirement for medium to high temperature (>80 °C) operation [1], [2], [3]. Polybenzimidazole (PBI), which possesses good anhydrous proton conductivity after doped with acid, as well as excellent chemical and thermal stability, shows great potential in the application of high temperature (100–200 °C) PEMFCs [4], [5], [6]. However, in practical application, acid doped PBI also has some problems. Firstly, after doped with a large number of H3PO4, PBI suffers from obvious sacrifice of mechanical properties. Secondly, the water-soluble phosphoric acid may be brought out slowly from the membrane when the vapor is produced at the cathode in the operation process of PEMFCs, which results in the degradation of the proton conductivity. Up till now, some methods have been proposed to solve these two problems, respectively. For example, silica was employed to improve the mechanical properties of PBI/H3PO4 composite membranes [7], [8] and Jiang et al. [9] used tridecyl phosphate (TP) instead of H3PO4 in PBI membranes to prevent the leaching of acidic components.

Crosslinking modification is a useful method to improve the mechanical properties of PBI. PBI with appropriate crosslinking network can exhibit better mechanical and dimensional stabilities than the homologous linear polymer, and phosphoric acid can be blocked in the crosslinked membrane [10]. Generally, there are two methods to achieve the crosslinking for PBI. One is ionic crosslinking, which is often mentioned in polymeric acid-base blend membranes [11], [12], [13], [14]; the other is the covalent crosslinking, which can be achieved by free radical polymerization of vinyl functionalized PBI or an amide-type linkage via imidazole groups of the polymers [15], [16], [17], [18]. Obviously, the stability of the covalent crosslinking is higher than that of the ionic crosslinking. Crosslinked PBI (13% crosslinking degree) doped with 8.5 PA per repeat unit reported by Li et al. [17] showed 21–23 MPa of tensile strength, which equals to linear PBI doped with 6.8 PA per repeat unit, and the swelling ratio of crosslinked PBI is also smaller than that of linear PBI. The crosslinked membranes of vinyl grafted PBI and vinyl imidazole doped with PA showed higher mechanical strength than PA doped linear PBI [16]. However, the methods for the covalent crosslinking of PBI with high efficiency are limited.

Breslow et al. [19] found that sulfonyl azide groups can cleave off nitrogen and form nitrene when heated to a sufficiently high temperature. These nitrene species, which show electron deficiency and high reactivity, can extract hydrogen atoms or insert into any saturated C–H bonds [19], [20], [21], [22], [23]. Incorporating the sulfonyl azide group into the linear polymer may give the polymer a reactive group, which can react with any C–H containing polymers when heated. Therefore, the crosslinking structure is formed. In present study, a new crosslinkable polybenzimidazole with sulfonyl azide group (sPBI-SA) is synthesized. A facile strategy for the crosslinking of PBI is proposed. Accordingly, the effects of crosslinking on the mechanical properties, the dimensional stability, the migration stability of phosphate, proton conductivity of PBI will be discussed.

Section snippets

Materials

3,3-Diaminobenzidine (DAB, 99%) was purchased from Shanghai Bangcheng Chemical Co. and recrystallized before used. Phosphorus pentachloride (99%), polyphosphoric acid (PPA, 115%), N,N-dimethylacetamide (DMAc, 98%), sulfuric acid (96%), phosphoric acid (85%), 1-methyl-2-pyrrolidone (NMP, 98%), N,N-dimethyl formamide (DMF, 98%), toluene (98%), H2O2 (30%, w/v), and ferrous sulfonate (FeSO4·7H2O, 98%) were purchased from Shanghai Chemical Reagent Co. and used as received. 2-sulfoterephthalic acid

Synthesis of polybenzimidazole with sulfonyl azide groups

Synthetic procedure of polybenzimidazole with sulfonyl azide groups is shown in Scheme 1. sPBI is synthesized first from DAB, SASS, and IPA, and then reacts with PCl5 to form PBI containing sulfonyl chloride, which can further react with NaN3 to form sPBI-SA. FTIR spectra of sPBI-67 and sPBI-SA are shown in Fig. 1. For sPBI-67, the peak at 3060 cm−1 is attributed to the stretching modes of aromatic C–H groups, and the peaks at 1443 cm−1 and 1228 cm−1 are attributed to the in-plane ring

Conclusions

A facile crosslinking method of polybenzimidazole with sulfonyl azide groups for proton conducting membranes is proposed. Thermally crosslinkable sPBI-SA is synthesized from sulfonated PBI and sodium azide. The structure of sPBI, sPBI-SA, and crosslinked sPBI are confirmed by FTIR and 1H NMR. Upon heating, sPBI-SA loses nitrogen and form nitrene. This nitrene reacts with CH bond of the backbone of another chain of PBI and leads to the efficient crosslinking of PBI membranes. Compared with the

Acknowledgments

The project is sponsored by China High-Tech Development 863 Program (SS2012AA110501), Natural Science Foundation of Shanghai (11ZR1439600), and Program for Young Excellent Talents in Tongji University (2009KJ002).

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