One of the basic challenges in advancing polymer electrolyte membrane fuel cell technology (PEMFC) is the development of new durable polymeric membranes that will allow fuel cell operation at high temperatures without extensive humidification requirements. Access to these membranes promises important benefits concerning the complexity, cost and performance of the fuel cell system. Problems of the common PEM based on the aliphatic and/or aromatic backbone with soulfonic acid groups (Nafions are commercially ones) in higher temperatures, encouraged scientist to find the appropriate acidic. In this context, membranes functionalized with covalently linked phosphonic acid may potentially show some crucial advantages in relation to the commonly employed sulfonated membranes. Because of the hydrogen bonding and amphoteric properties of the phosphonic acids, the former membranes may transport protons through structure diffusion under low-humidity conditions. Furthermore, phosphonated polymers generally show a high thermal stability due to the strength of the carbon–phosphorus bond, which is especially critical under high-temperature operation. With regards to these benefits, there are many researches on developing phosphonated PEM in the recent years with considering structural and synthetic problems. But there are a few examples from aromatic phosphonated PEM mainly because of synthetic difficulties.
In this research project our aim is to prepare new thermally stable aromatic backbone with phosphonic acid groups directly linked to the backbone. We used different synthetic routes with considering their limitation. Finally we prepared novel thermally stable phosphonated polyimides based on new diamines. The high molecular weight polymers in phosphonic ester form were obtained in high yields and inherent viscosities up to 0.83 dL g-1. The hydrolysis of the phosphonated ester into phosphonic acid groups was carried out quantitatively in acidic condition. All polymers after hydrolysis were studied by TGA and showed 10% weight loss above 427oC. Membranes with total ion-exchange capacities of 4-6 meq/g showed proton conductivities around 73 mS/cm at 25oC and 100% relative humidity and it reach to almost 110 mS/cm at 100oC. Upon doping with phosphoric acid, the membrane conductivity without external humidification reached 150 mS/cm at 100oC.