Phosphomolybdic Acid and Mixed Phosphotungstic/Phosphomolybdic Acid Chitosan Membranes for H2/O2 Fuel Cells

Abstract

Proton exchange membrane fuel cells (PEMFCs) have received much attention in recent years because of their high power density, efficiency and zero-environmental pollution. As one of the key components in fuel cells, the proton exchange membrane is expected to have high proton conductivity and good electrochemical stability. In the attempt to promote PEMCFs commercialization, high cost of fuel cell systems and short lifecycle are the two main issues that need to be addressed, thus large research effort has been devoted in developing new polymer electrolytes that can replace the usually employed proton conductors, e.g. Nafion®, with other membranes of comparable performances but lower cost. As a low-cost and eco-friendly biopolymer, Chitosan (CS)-based membrane electrolyte is currently studied as alternative candidate for PEMFC application to possibly produce economically viable fuel cells. Several works have shown that Heteropolyacids (HPAs) can be used to prepare Chitosan polyelectrolytes (PECs) to be employed as proton exchange membrane in low temperature fuel cell. HPAs, such as phosphotungstic acid (PTA) or phosphomolybdic acid (PMA), are strong Bronsted acid as well as solid electrolytes, thus being very effective in the fabrication of high proton conductive organic–inorganic nanocomposite membranes for fuel cell. A survey of the already published works on the use of CS/HPA composite membranes shows that they have been tested as proton conductors in direct methanol, in borohydride and hydrogen fed fuel cells [1-3 and refs therein]. In previous works [1-3] we have shown that CS/PTA membranes, prepared using an alumina porous medium for the slow release of H3PW12O40, show good performances when employed as electrolyte in H2 fed fuel cell with proton conductivity of ~ 14 mS cm-1. Notably, in ref. [4] CS/PMA membranes are reported to exhibit proton conductivity higher than that measured with CS/PTA, which is mainly because the proton conductivity of PMA is higher than that of PTA. However, a careful inspection of published papers revealed that CS/PMA membranes haven’t been yet tested in low temperature fuel cell. Starting from these findings, this work is focused on the characterization of CS/PMA and mixed CS/PMA-PTA membranes prepared according to the above described procedure with the aim to evaluate their performance as proton conductors in hydrogen fed fuel cell. X-ray diffraction and Fourier Transform Infrared Spectroscopy analyses were performed to study the structure and composition of the membranes, while Scanning Electron Microscopy was used to get information on the membranes morphology and thickness as a function of the preparation conditions. The membranes were tested in a H2/O2 fuel cell working at low temperature (25°C) and fixed Pt loading (1 mg cm-2). Impedance Spectroscopy was used to get information of the conductivity of the membrane as well as to model the overall electrical behaviour of the cell.