Phosphoric Acid-Doped Azole-Containing Polymer Electrolyte Membranes for High-Temperature Polymer Electrolyte Membrane Fuel Cells

Abstract

High-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs) have recently been developed due to their advantageous high-temperature operation (120-180 oC) that leads to enhanced electrode kinetics, tolerance to impurities and the possibility of a more compact and simpler system. For the electrolyte materials in HT-PEMFCs, azole-containing polymers and phosphoric acid (PA) have been widely used as PA-doped membranes. Phosphoric acid is an amphoteric acid with the highest intrinsic proton conductivity. As being relatively acidic nature compared to azole groups, phosphoric acid is easily absorbed into azole-containing polymer membranes. However, several challenges exist for the commercialization of HT-PEMFCs such as long-term stability with high performance retention. There have been issues of the plasticization effect of doped acid causing mechanical degradation of membranes and phosphoric acid leaching during fuel cell operation. Here, we report the cross-linked polymer electrolyte membranes for enhanced stability and the investigation of influence of azole structures for HT-PEMFCs performance. Chapter 2 describes the cross-linking approach to enhance the stability of phosphoric acid-doped polymer membranes for HT-PEMFCs. The cross-linked membrane (XTPPO) was fabricated from poly(phenylene oxide) containing triazole groups (TPPO) on the side chains by in situ casting and click reaction with 1,7-octadiyne. PA uptake was controlled with the amount of triazole and the degree of cross-linking. XTPPO exhibited more than 90% of the membrane retained after the gel fraction test. The cross-linked membranes showed high thermal stability with TD5% 325 oC and enhanced oxidative stability in the Fenton test. Cross-linking also improved the mechanical strength to have great tensile strength over than 10 MPa in PA-doped states. Phosphoric acid leaching was effectively controlled by the degree of cross-linking of membranes. The highest proton conductivity measured in the anhydrous condition was 64 mS/cm at 180 oC. Chapter 3 identifies the influence of azole structures in the polymer membranes on HT-PEMFCs performance and stability. Three types of cross-linked polymer electrolyte membranes were fabricated by functionalizing poly(phenylene oxide) with triazole, benzimidazole and imidazole groups, respectively on the side chains. The PEM properties of the membranes were analyzed and compared at sufficient PA-doping levels for practical use. We reveal for the first time that PEM performance is influenced by azole structures (pKa and steric hindrance). The higher pKa of imidazole was advantageous to the formation of protonic defects, enhancing the proton conduction. However, the steric hindrance of bulky benzimidazole interrupted acid-base interaction and the formation of protonic defects. The imidazole-containing membrane (XIMPPO) showed the anhydrous proton conductivity 130 mS/cm at 150 oC and the triazole-containing membrane (XTAPPO) and the benzimidazole-containing membrane (XBIPPO) showed 114 mS/cm and 102 mS/cm, respectively. The fuel cell employing imidazole-containing membrane (XIMPPO) showed a higher peak power density (0.20 W/cm2) than fuel cells employing XTAPPO and XBIPPO which showed peak power density of 0.16 W/cm2 and 0.17 W/cm2, respectively. Chapter 4 investigates the influence of N-methylation of azole rings for performance of HT-PEMFCs. Triazole, benzimidazole and imidazole groups were functionalized on the side chains of poly(phenylene oxide), respectively. Each azole groups are categorized by their N-substituent into two types: unsubstituted and methyl-substituted azoles. N-substituent minimally affected the membrane thermal stabilities showing similar TD5% of 300 oC. The membranes with methyl-substituted azoles showed an average increase of 20% in PA doping levels compared to those with unsubstituted azoles. Methyl-substituted azoles also leaded to a higher toughness of membranes with a 50% increase of tensile strength and longer elongations. However, unsubstituted azoles more effectively improved the proton conduction and the membrane with unsubstituted imidazole (IMPPO-H) showed the highest anhydrous proton conductivity of 153 mS/cm at 150 oC. On the other hand, methyl-substituted azoles leaded to the higher fuel cell performance compared to unsubstituted azoles by a maximum increase of 500% in the power density. N-substituent also affected the phosphoric acid loss and methyl-substituted azoles leaded to a maximum increase of 112% in the phosphoric acid retention compared to unsubstituted azoles.