Poly(2,6-dimethyl-1,4-phenylene oxide) (PPO)—A versatile starting polymer for proton conductive membranes (PCMs)

Abstract

Polymer electrolyte membrane fuel cells (PEMFCs) have been recognized as the important alternative to current power suppliers due to their high power density and environmental compatibility; however, one of their key components—proton conductive polymer electrolyte membranes (most are Nafion ® series membranes) are not favorable for practical application. To prepare a membrane in a more cost-effective manner, hydrocarbon polymers can be used as the backbone; but it requires that the polymer possess specific properties, such as excellent thermal and hydrolytic stability. Poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) can be listed as one of such polymers because it has high glass transition temperature ( T g = 210 °C), high mechanical strength, and excellent hydrolytic stability. In addition, its distinctive but simple structure allows a variety of modifications in both aryl and benzyl positions: (1) electrophilic substitution on the benzene ring of PPO, (2) radical substitution of the hydrogen from the methyl groups of PPO, (3) nucleophilic substitution of the bromomethylated PPO (BPPO), (4) capping and coupling of the terminal hydroxyl groups in PPO chains, and (5) metalation of PPO with organometallic compounds. These modifications can tune PPO for preparation of proton conductive membranes (PCMs). Therefore, the application of PPO and its derivatives to PCM preparation will constitute this review. Instead of a summary on the literature concerning PCMs, this review aims to extract the principles for preparation of PCMs by using PPO as an example material.