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Abstract
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Proton transfer is the most important task of proton exchange membranes (PEMs) for application in fuel cells. One vital disadvantage of currently used commercial Nafion membranes is the low proton conductivity at high temperatures. Therefore, the objective of this research was to increase the proton conductivity of PEMs based on sulfonated poly (ether ether ketone) (SPEEK). Herein, modification of SPEEK-based PEM was carried out using polydopamine-coated halloysite nanotubes (HNT) alone and in combination with sulfonated polystyrene (SPS) and phosphotungstic acid (PWA). In this method, poly (ether ether ketone) sulfonation process was performed under optimum operating conditions to create more sulfonic acid groups on its chains. Here, polydopamine was doped on the outer surface of HNT (DHNT) and employed as the additive to create additional proton transferring pathways in the membrane. The hydrophilicity of the modified nanotube was enhanced through silanization (named as DHNTS). Moreover, SPS and PWA were applied to improve the ability of protons to transfer through the proton barrier channels in the membrane. Performing the sulfonation of polystyrene in the solution phase was a novel approach in this study, which led to significant increase in the degree of sulfonation. The results showed that the SPEEK/DHNTS∣SPS and SPEEK/DHNTS∣PWA membranes in the presence of 15% weight ratio additives and 100% relative humidity exhibited 109% and 90% higher proton conductivity than the neat SPEEK membrane, respectively. Furthermore, 20% and 10% higher proton conductivity was observed for the aforementioned membranes compared to the commercial Nafion117 membrane. Because of the strong acid-base bonding between DHNTS and SPEEK and the sticky nature of polydopamine, the chemical stability of the modified PEMs was higher than the neat membrane. In terms of fuel cell performance, there was little difference between Nafion117 membrane and DHNTS-modified PEM. These modified membranes
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