Covalently crosslinked poly(biphenyl dimethylamino benzene) membranes for high temperature proton exchange membrane fuel cells

Abstract

The electrolyte membrane plays a pivotal role in high temperature proton exchange membrane fuel cells (HT-PEMFCs). Herein, we develops a new triarylmethane-backboned polymer, poly (biphenyl dimethylamino benzene) (PBDAB), synthesized through a facile one-step Friedel-Crafts reaction using biphenyl and 4-(dimethylamino) benzaldehyde as monomers. The presence of tertiary amine groups in PBDAB facilitates strong interactions with phosphoric acid (PA) molecules. To enhance the membrane's dimensional and mechanical stabilities, covalent crosslinking is performed via the Menshutkin reaction between the tertiary amine groups in PBDAB and chloromethyl groups in various crosslinkers. In order to study the influence of crosslinker chemical structure on the properties of PBDAB-based membranes, five different crosslinkers are employed, including small-molecule types such as 1,2-bis(2-chloroethoxy) ethane (BCE) and 1,4-bis(chloromethyl) benzene (BCB), siloxane type 3-chloropropyl (trimethoxy) silane (CTS), polymer types such as poly (vinylbenzyl chloride) (PVBC) and polyepichlorohydrin (PECH). Results reveal that the flexible and ether containing polymer crosslinker of PECH crosslinked membrane, denoted as CL-y%PECH, exhibits superior overall performance. When doped in an 85 wt% PA solution, the CL-5%PECH membrane achieves a PA uptake of 201%, resulting in an anhydrous proton conductivity of 0.064 S cm−1 at 180 °C and a mechanical strength of 7.2 MPa at room temperature. Using H2 and O2 as fed gases without humidification and backpressure, a single cell with abovementioned membrane attains a peak power density of 302 mW cm−2 at 160 °C. This work offers a kind of promising PBDAB based membranes for HT-PEMFC applications.