dc.description.abstract | Numerous advantages are identified in operating fuel cell at elevated temperature, which include better tolerance towards CO poisoning, higher electro-chemical kinetics, and promising of higher output power. However, the output power of PEMFC using state of art proton exchange membrane is dramatically reduced due to increase of internal resistance from the loss of water from these membranes as the temperature increases to above 80C. Currently the proton conduction of the perfluorosulfonic acid ( PFSA , such as Nafion ) membranes rely on water for proton transport. Water evaporation means a dramatic decrease of proton conductivity and, consequently loss of fuel cell performance. Developing new proton exchange membrane suitable for PEM fuel cells (PEMFC) to operate at temperatures above 150 °C becomes the new challenge in this field which is critical for the future of fuel cell technology.
Present study examines the effect of sulfonated poly aryl ether ketones (sPAEK)s composited with sulfonated inorganic nanoparticle for high temperature purpose. Sulfonated titanium oxide nanotube (sTNT) displayed excellent water retention capability which preserved certain amount of water in temperature higher than the normal boiling temperature. As a result, composite membrane delivered impressive conductivity surpassing sPAEK membranes without using the nano-composite and other membranes operating at low humidity conditions or at elevated temperature above 110 C. These nanocomposite proton exchange membranes were prepared from sulfonated poly(ether ether ketone) (sPEEK) and physically blended with various amounts of sulfonated titanium oxide nanotube (sTNT) using common solvent. Fucntions of the inorganic moiety are two folds, first it improves thermal stability and provided tougher strength at elevated temperature. Secondly, it shows residual water content even above 140C, which facilitate proton transport. From low moisture content (R.H.=20%) to saturated moisture (RH=100%) conditions, proton conductivity of the membrane containing sTNT is higher than pristine sPEEK. This is especially the case with 5wt% sTNT composite, where the most homogeneous morphology (best inorganic dispersion) is observed. For 5wt% sTNT/sPEEK composite membrane the porton conductity reached above 10-2 S/cm at 100oC, 60%RH. These membranes display excellent proton conductivity when temperature is higher than 100oC, and continue to increase with increasing temperature while other membranes either lost the proton conductivity or yield to severe swelling and loss of membrane strength under elevated temperature. A unique proton conducting behavior is also identified in these composite membranes where proton migration along the tubular inorganic surface is assisted with the presence of a small amount of water retained in the tube.
Since the IEC ( ion exchange capacity ) content of these membranes depend on the degree of sulfonation on both the sPEEK and on sTNT, it is important to find out if increasing the degree of sulfonation on TNT surface would improved the conductivity further. This is easy to do, since the degree of sulfonation on the TNT surface can be controlled by varying the amount of sultone. The results shows, in spite of the higher IEC value, the membrane using TNT with higher degree of sulfonation (0.8sTNT. IEC = 1.68mmol/g) delivered worse proton conducting behavior than that from lower degree of sulfonation (0.5sTNT, IEC = 1.4mmol/g). These results suggested the proton conductivity does not depend on IEC alone, but the membrane morphology with well connected conducting channel and suitable distribution of sulfunated group within the membrane are also important factors in contribution towards proton conductivity.
These membranes are promising as high temperature PEM fuel cell materials. The feasibility is demonstrated by fuel cell performance operating under temperature above 120C. | en_US |