High temperature-proton exchange membrane fuel cells (HT-PEMFCs) (operating over 100°C) have several advantages: higher electro-catalyst tolerance toward CO, faster electrode reaction kinetics, simplified water and thermal managements, easier manipulation of combined heat and power (CHP) systems and minimization of expensive auxiliary units (i.e., external humidifier).
One method to operate PEMFCs at high temperature is to exploit phosphoric acid (PA) doped polybenzimidazoles (PBIs) polymer electrolyte, because of their high proton conductivity without additional humidification and outstanding fuel cell performance at elevated temperature.
Although poly[2,2''-(m-phenylen)-5,5''bibenzimidazol] (m-PBI) is being widely used as high temperature-proton exchange membrane (HT-PEM), it has two major problems such as low solubility and brittle character, due to less molecular flexibility in the main chain and the strong inter & intra-chain hydrogen bondings. Hence, in this study, benzophenone moiety and ether groups were introduced in main chain of polybenzimidazole in order to enhance flexibility and solubility. Initially, dicarboxylic acid monomer 4,4’-(benzophenone-4,4’-diylbis(oxy))dibenzoic acid (BDA) was synthesized from 4―hydroxybenzoic acid and 4,4’-difluorobenzophenone via nucleophilic aromatic substitution reaction. Next, benzophenone-containing polybenzimidazole (BDA-PBI) was synthesized from BDA and 3,3’-diaminobenzidine via polycondensation reaction. The m-PBI (reference polybenzimidazole) was synthesized in the same manner, isophthalic acid was used instead of BDA. Finally, polymer electrolyte membranes were prepared using afore-synthesized polymer powders by solution casting method.
The synthesize of dicarboxylic acid monomer and the polymers were confirmed by using proton nuclear magnetic resonance spectroscopy (1H-NMR) and Fourier transform-infrared spectroscopy (FT-IR). The thermogravimetric analysis (TGA) curve of BDA-PBI indicates that the residual weight of corresponding membrane is more than 60% at a high temperature of 800°C. Further investigations on thermal stability were done using differential scanning calorimetry (DSC). BDA-PBI shows a glass transition temperature (Tg) value of around 264°C. Overall, the BDA-PBI had sufficient thermal stability to sustain operating temperature of HT-PEMFCs. The proton conductivity of membranes were measured with a 4-probe cell by AC impedance spectroscopy. The resistance (Ω) of membranes were measured as a function of temperature without applying any external humidity. The conductivity of membranes were found to be increased as increase the temperature, due to rapid proton movement at high temperature. At 90°C, proton conductivity of PA doped m-PBI (7.9 mol PA/repeating unit) was 52.7 mS/cm, while that of PA doped BDA-PBI (7.6 mol PA/repeating unit) was 43.7 mS/cm. It is noteworthy that the acid-base interactions exerted between －NH group of benzimidazole and －OH group of PA create extended architecture of proton conducting channels, which leaded to rapid proton conduction via Grotthuss mechanism. Still, the proton conductivity of BDA-PBI is lower by 1.2 fold with respect to that of m-PBI. This is due to the lower number of repeating units in the synthesized BDA-PBI.
BDA-PBI exhibits excellent thermal and oxidative stability, and mechanical strength. While the membrane shows reasonable PA uptake and proton conductivity. The afore-mentioned results demonstrate that BDA-PBI membrane able to generate appropriate power density in HT-PEMFC under anhydrous condition.
achieve suitable PEM for the application of PEMFC.