Development of Cathode Air Filters for PEM Fuel Cells Using Microfibrous Entrapped Sorbents

Abstract

The performance of Proton Exchange Membrane Fuel Cell (PEMFC) is degraded significantly as a result of poisoning of the cathode catalyst by trace levels of contaminants present in the cathode air. Three of the most common air contaminants present in the operating environments of PEMFC, and which have major negative effects on the performance of PEMFC, are SO2 and NOx (both NO and NO2). Most of these contaminants get chemisorbed on the surface of the precious platinum catalyst layer in PEM fuel cell and this leads to decrease in the active sites available for oxygen reduction reaction on the surface of the platinum. One option to mitigate this problem is to develop contaminant tolerant electrocatalysts for PEMFC. However, developing new catalysts seems to be difficult, time consuming and possibly expensive because of the presence of a variety of contaminants in the air. Another route to solve this problem is the removal of gaseous impurities from the cathode air by adsorptive filtration. This second option appears to be more effective and cheaper. The main parameters for an adsorptive air filter are saturation capacity of the sorbents, single pass removal efficiency and pressure drop across the filter. The research work in this dissertation thus deals with the design and construction of different configurations of adsorptive air filters that will have high adsorption capacity and high contacting efficiency for the effective removal of SO2 and NOx at ambient conditions. This work also involves the comparative study of the pressure drop performances of three possible configurations for the cathode air filter to select the optimal configuration of the cathode air filter for a given application. Several sorbents impregnated with oxides of different transition metals (Zn, Cu, Mn, Ni, Fe) and caustic chemicals (like KOH) were tested for their SO2 adsorption capacity to find out a high capacity sorbent for SO2 removal. MnOx(SO4 route)/Al2O3 prepared by a deposition precipitation route with MnSO4 as a precursor was found to have the highest breakthrough capacity as well as the highest saturation capacity for SO2 removal over wide range of relative humidity (RH) and SO2 concentrations. Effects of RH, SO2 concentration in the challenge gas and particle size of the sorbent on the SO¬2 adsorption performance of MnOx(SO4 route)/Al2O3 were discussed in detail. Adsorption capacity of MnOx(SO4 route)/Al2O3 was found to increase significantly at higher relative humidity. A working hypothesis was proposed to explain the positive effect of the moisture content on the breakthrough performance of MnOx(SO4 route)/Al2O3. (Chapter II) Several sorbents supported on activated carbon as well as γ- Al2O3 were tested for their NO2 adsorption performance. PICA activated carbon derived from a coconut shell showed the highest saturation capacity for NO2 removal amongst all the sorbents tested. However generation of considerable amount of NO was also observed during NO2 adsorption on PICA activated carbon. It was found that the surface of activated carbon partially acts as a reducing agent and converts part of NO2 from challenge gas to NO. Addition of strong oxidizing agent like KMnO4/Al2O3 to PICA activated carbon in the sorbent bed was found to delay the appearance of NO in the exit stream of challenge gas because of oxidation of NO to NO2 by KMnO¬4/Al2O3. Therefore, it was concluded that the mixture of three different sorbents is required for the simultaneous removal of SO2 and NOx from the cathode air. (Chapter III) Microfibrous media (MFM) prepared by entrapping micron size particles in the sinter-locked network of bicomponent polymer fibers have higher contacting efficiency for removal of contaminants because of their high void fractions (> 70%) and use of smaller size particles. This microfibrous media can be pleated to accommodate many times more filter media in the same volume of filter unit. Thus microfibrous media makes it possible to construct two different configurations for the cathode air filters, namely composite bed and pleated filters. These different configurations for cathode air filters were tested for their SO2 ¬breakthrough capacity as well as pressure drop across them. It was found that the composite bed design increased the breakthrough capacity for SO2 by almost 3.4 times as compared to packed bed, with little increase in the volume and pressure drop of the filter system. (Chapter IV) Pressure drops across each of the three configurations (packed bed, composite bed and pleated filter) of cathode air filter were either found experimentally or were predicted by using model equations. Parasitic power losses were then calculated for all the three configurations of cathode air filter and all of these power losses were found to be negligible (< 0.2 W). So it was determined that the pressure drop across cathode air filter is not an important criterion for the final selection of the optimal configuration of the cathode air filter. (Chapter V)