Development of new membranes based on aromatic polymers and heterocycles for fuel cells
Proton exchange membrane fuel cells (PEMFC) and direct methanol fuel cells (DMFC) have drawn much attention as alternative power sources for transportation, stationary, and portable applications. Nafion membrane is currently used in PEMFC and DMFC as electrolyte, but is confronted with a few difficulties: (i) high cost, (ii) limited operating temperature (< 100 o C), and (iii) high methanol permeability. With an aim to overcome some of the problems encountered with the Nafion membrane, this dissertation focuses on the design and development of new polymeric materials systems for use in PEMFC and/or DMFC. Sulfonated polysulfone (SPSf) membranes with various degrees of sulfonation were prepared and investigated in DMFC. With a degree of sulfonation of 50 - 70 %, the SPSf membranes exhibit low methanol permeability and electrochemical performance comparable to that of Nafion 115, making it an attractive low-cost alternative to Nafion. However, lower performance at higher current densities due to their low proton conductivities compared to Nafion is a disadvantage. It is found that the low methanol crossover is due to narrower hydrophilic channels, resulting in water/methanol confinement as in sulfonated poly(ether ether ketone) (SPEEK) membranes. Replacement of water by imidazole in Nafion helps to keep high proton conductivity at higher temperatures (> 100 o C) due to Grotthuss-type mechanism, but imidazole poisons the Pt catalyst. Interestingly, doping the Nafion-Imidazole composite membrane with H3PO4 partly suppresses the imidazole poisoning of the Pt catalyst. Employment of Pd-Co-Mo catalyst instead of Pt improves the fuel cell performance at 100 o C further due to a higher tolerance of the non-platinum Pd-Co-Mo catalyst to imidazole. Encouraged by this, benzimidazole group was then selected to promote proton conduction in the environment of sulfonic acid groups instead of imidazole (pKa = 7.0) due to its lower pKa value (5.5). Accordingly, 1,3-1H-dibenzimidazole-benzene containing two benzimidazole groups was synthesized and blended with SPSf. The blend exhibits higher proton conductivity under anhydrous conditions than plain SPSf and offers improved fuel cell performance and lower methanol crossover in DMFC. Polysulfones containing pendant N-heterocycles like benzimidazole, 2-amino-benzimidazole, or 3-amino-1,2,4-1H-triazole units were designed and synthesized. Blend membranes containing these polymers and SPEEK exhibit higher proton conductivities under anhydrous conditions as well as higher fuel cell performance due to acid-base interactions involving Grotthuss-type mechanism. They also lower methanol crossover further due to the insertion of the pendant N-heterocycles into the hydrophilic channels of SPEEK, improving the long-term stability in DMFC and reducing the Pt loading at the cathode side.