Synthesis and Characterization of Polymeric Materials Containing Benzimidazole Groups and Their Application to Fuel Cell Membranes at Elevated Temperature

Abstract

Novel polybenzimidazole-based materials were synthesized and analyzed for use in polymer electrolyte membrane fuel cells at elevated temperature. Firstly, a polybenzimidazole containing bulky basic benzimidazole side groups, poly[2,2-(2-benzimidazole-p-phenylene)-5,5-bibenzimidazole] (BIpPBI), was prepared via the condensation polymerization of 3,3-diaminobenzidine (DABI) tetrahydrochloride dihydrate with 2-benzimidazole terephthalic acid in poly(phosphoric acid) (PPA). BIpPBI was found to be soluble in aprotic polar solvents without the addition of inorganic salts, such as lithium chloride, and the BIpPBI film also showed very good phosphoric acid (PA) retention capability as well as very high proton conductivity. The maximum PA content of the BIpPBI film was approximately 81 wt% and the proton conductivity value of the PA-doped BIpPBI membrane was 0.16 S cm-1 at 180 oC and a 0% relative humidity. For comparison, the maximum proton conductivity of the most commonly used polymer for the elevated-temperature fuel cell membrane, poly[2,2-(m-phenylene)-5,5-bibenzimidazole] (PBI) membrane, is approximately 0.06 S cm-1 at 180 oC under anhydrous conditions at a 65 wt% PA content, which is the maximum PA content that a PBI membrane can have. Secondly, cross-linked poly(2,5-benzimidazole) (C-ABPBI) membranes consisting of wholly aromatic groups were prepared by a reaction between three-arm poly(2,5-benzimidazole)s with three carboxylic acid end groups (T-ABPBI) and DABI in PPA i during the casting process. T-ABPBI was prepared by the polymerization of 3,4-diaminobenzoic acid (DABA) and 1,3,5-benzenetricarboxylic acid in PPA, and the polymerization solution was directly used for the casting process. The cross-linking process improved the chemical stability of the host polymer, poly(2,5-benzimidazole) (ABPBI). The C-ABPBI membranes could contain a larger PA content than the membranes of linear polybezimidazoles, such as ABPBI and PBI. Consequently, the C-ABPBI membranes showed higher proton conductivity values than the linear polymer membranes. For example, the C-ABPBI-50 membrane could contain a PA content >84 wt% with a proton conductivity of 0.12 S cm-1 at 150 ◦C under anhydrous conditions. Finally, we report new phosphoric acid–doped cross–linked benzoxazine–benzimidazole copolymer membranes showing high proton conductivity and long–term durability for use in proton exchange membrane fuel cells at elevated temperatures (> 100 °C). The cross–linked copolymer membranes were prepared by mixing of polybenzimidazole (PBI) with either mono-functional benzoxazines such as 3–phenyl–3,4–dihydro–6–tert–butyl–2H–1,3–benzoxazine (pBu) or 6-fluoro-3-(pyridine-2-yl)-3,4-dihydro-2H-benzoxazine (pF) or di-functional benoxazine such as 6,6'-(hexafluoroisopropylidene)bis(3-phenyl-3,4-dihydro-2H-benzoxazine) (HFa) in N,N–dimethylacetamide, with subsequent step–wise heating to 220 °C. The membranes showed high proton conductivities at 120 and 150 °C under anhydrous conditions. Membrane–electrode assemblies (MEAs) employing the membranes showed operating voltages of > 0.70 V @ 0.2 A cm–2. Furthermore, the MEAs displayed long–term ii durability up to ca. 2000 cycles of mono-functional benzoxazine and ca. 3000 cycles of di-functional benzoxazine, with much slower performance decay than those prepared using PBI membranes in in situ accelerated lifetime mode (load cycling testing). In addition, cross-linked benzoxazine-benzimidazole copolymer membranes were also prepared by using direct casting of PPA polymerization solution of ABPBI with pF, followed by step-wise heating. Once the PPA in the membranes has been hydrolyzed to PA, the membranes can contain large amounts of PA and showed high proton conductivity, ca. 0.14 S cm-1, at 150 oC under anhydrous conditions. MEAs prepared using the copolymers showed high operating voltages of 0.69 V at 0.2 A cm-2, long-term durability for up to 1584 cycles, with much slower performance decay than those prepared using poly(benzimidazole) homopolymers, such as PBI and ABPBI, from the in situ accelerated lifetime test.