Highly sulfonated polyphenylsulfone nanocomposite membranes for improving proton exchange membrane fuel cell performance

Abstract

Fuel cell has become a rising technology that has widely been explored due to its promising efficient energy conversions. Currently, proton exchange membrane fuel cell (PEMFC) is the most studied fuel cell systems because of the simple structure and wide application range. Recent research has been devoted to develop a proton exchange membrane (PEM) using sulfonated aromatic polymers with high proton conductivity and good durability. The state-of-the-art of PEM-based polyphenylsufone (PPSU), which has excellent thermal stability and appropriate mechanical strength with high proton conductivity and increases along with the degree of sulfonation have been widely explored. Unfortunately, increasing the degree of sulfonation always results in swelling and physical expansion of the materials leading to mechanical failures. To keep the benefit of having high proton conductivity of high sulfonation degree, hybrid/blend membranes and applying crosslinking step become alternatives to solve the critical issue of the mechanical failure. Therefore, this research aimed to develop PEM using highly sulfonated polyphenylsulfone (SPPSU) membrane, incorporating different types and structures of inorganic fillers for PEMFC applications. PPSU was directly sulfonated using sulphuric acid and had achieved the desired degree of sulfonation, which was DS~2. This SPPSU was highly soluble in water upon heating at 80 ºC. The thermal crosslinking process was applied up to 180 ºC to improve the mechanical properties of the SPPSU membrane, and it is suggested that heat promoted the crosslinking between the SPPSU polymer matrix. The proton conductivity achieved about 1.12x10-2 S/cm, which is still lower than the requirements for the desired proton conductivity values for PEMFC applications (10x10-2 S/cm). Consequently, in this study, the different types and structures of fillers which were carbon nanodots (CND), sulfonated polyhedral silsesquioxane (SPOSS), and imogolite (Im) were incorporated into the SPPSU polymer matrix. All three fillers exhibited proton conductivity of about 3 to 4 fold higher compared to the SPPSU membrane. SPPSU-2% CND, SPPSU-2% SPOSS, and SPPSU-1% Im nanocomposite membrane were chosen for further performance testing and membrane durability. The SPPSU and composite SPPSU membrane exhibited excellent dimensional stability after prolonged exposure to water for 720 h. At 80 °C under fully hydrated conditions, the maximum power density of the SPPSU membrane was 81.05 mW/cm2. SPPSU with fillers showed a significantly improved performance compared to the SPPSU membrane. The highest maximum power density belongs to the SPPSU-2% SPOSS nanocomposite membrane that reached up to 131.53 mW/cm2. This is followed by SPPSU-2% CND, Nafion 117 and SPPSU-1% Im each with 118.75 mW/cm2, 111.76 mW/cm2, and 89.76 mW/cm2, respectively. Besides, SPPSU and composite SPPSU showed stable potential voltage during the 8 h operation. It is interesting to state that the membrane electrode assembly using the SPPSU-2% SPOSS nanocomposite membrane showed excellent electrochemical properties under operating conditions of 80 °C and 100% relative humidity, which is comparable to commercial Nafion 117. It can be deduced that the incorporation of CND, SPOSS, and Im into SPPSU has improved not only the membrane properties but also the PEMFC performance and membrane durability.