A review of alternative polymer electrolyte membrane for fuel cell application based on sulfonated poly(ether ether ketone)

A proton exchange membrane (PEM) is a crucial component in a fuel cell application as a proton carrier and separator for the anode and cathode. Sulfonated poly(ether ketone ether) (SPEEK) has been recognized as one of the promising alternatives PEM in fuel cell application due to the advantagious properties of SPEEK, including the high thermal stability, proton conductivity, mechanical strength, low fuel crossover, and easy to operate. The proton conductivity properties of SPEEK are influenced by the degree of sulfonation (DS). In practical, high DS of SPEEK-based membrane will produce high proton conductivity and a high density of sulfonic acid functional groups grafted into the polymer backbone. However, the excessive production of DS will have side effects on the stability of membrane dimensional and chemical properties. Meanwhile, the low DS of SPEEK has reduced the ability to diffuse the proton within the polymeric matrix. Therefore, getting the optimal DS SPEEK is very important to fabricate the alternative of PEM. But this process is hard. Thus, the researchers have modified the SPEEK membrane with various types of nanoparticles as a filler. This is one of the promising efforts to improve the SPEEK membrane properties and fuel cell application performance. This review aimed to discuss eight conductive materials such as silica, clay, metal oxide, heteropolyacids, carbon, graphene, metal organic framework, and zeolite used as fillers to develop the PEM component based on SPEEK for fuel cell applications. This review also comprehensively discusses the characterization of SPEEK-based membrane modification, including mechanical properties, water intake, proton conductivity, thermal properties, fuel permeability, crystal and structural analysis, chemical oxidation stability, and single-cell performance. Thus, this article can serve as a reference for researchers to planning the best strategies to optimize the properties and performance of the SPEEK-based membrane for fuel cell application.