Strategies for impure hydrogen use in polymer electrolyte fuel cell systems

Availability of high purity H2 for low temperature polymer electrolyte fuel cell (PEFC) technology is a necessity for optimum performance and durability. H2 purity depends on the production process as well as post purification measures, which may increase the final pump outlet cost of H2 fuel and operating cost of PEFC applications. Low cost H2 is produced in refineries and other reforming processes. Presence of certain contaminants such as methane, carbon dioxide and carbon monoxide in this H2 may however cause performance degradation in fuel cells. The aim of this thesis is to investigate strategies to mitigate and overcome the fuel cell performance losses resulting from the use of impure H2. Firstly, stack level testing on H2 with isolated contaminants of known concentration are conducted to assess performance losses. These studies are expanded in a single cell for comparison with stack results as well as in devising strategic mitigation techniques. CO, being a major contaminant, is extensively studied with variation of operating current density and CO concentration. Performance losses are partially rectified through application of a CO tolerant electrocatalyst, Pt-Ru/C instead of Pt/C. Two more techniques of air-bleeding and pulsed oxidation are also investigated alongside Pt-Ru/C electrocatalyst for performance recovery through CO oxidation. Parametric studies of pulsed oxidation are undertaken with 80 ppm CO containing H2 fuel for performance recovery and energy efficiency comparison with respect to pure H2 efficiencies. Up to 95% recovery in performance is observed at 0.5 A cm-2 with strategic application of pulsed oxidation when using a threshold cell potential for activation. These studies are further extended to long term pulsing operation of up to 4000 cycles to gauge the robustness and effectiveness of the pulsing process. These studies demonstrate cell potential recovery over extended pulse cycles without any significant decay of fuel cell performance through monitoring of droptime and peak potential values. Lastly, a zero-dimensional model is developed to study the transient surface coverage of different species present at the anode during CO poisoning and predict cell potential losses. It is extended to cover the pulsed oxidation effect and provide overall efficiency of the fuel cell with change of anodic flow parameters. The cost effectiveness of pure and impure H2 fuel used with mitigation techniques are compared and discussed for the interest of commercialization of such processes for the practical use of impure H2 in PEFC systems.