Hydrogen Sulfide Tolerance in High Temperature Pemfcs

Abstract

The effect of hydrogen sulphide (H2S) poisoning on the anode of H3PO4-polybenzimidazole based fuel cells has been investigated. High temperature PEM fuel cells (HT-PEMFC) offer a number of advantages that allow for a simple electricity generating system. Of the main importance is the ability to operate on reformate hydrogen without extensive purification [1]. Most of the research is concentrated on CO poisoning mitigation, but as the feedstock often contains sulphur contamination or sulphur-containing odorant additives, the poisoning effect of sulphur on Pt electrodes at elevated temperatures should not be overlooked. Little literature is available on the topic [2, 3]. Present work shows the effect of anode H2S poisoning on 25 cm² HT-PEMFCs operated at 160 °C, evaluated by polarisation curves and electrochemical impedance spectroscopy (EIS).Cells exposed to 0.2 – 50 ppm H2S in the fuel supply show no significant signs of voltage degradation when operated at 160 °C and 200 mA/cm². The main effect is seen at higher current densities with 10 – 50 ppm levels of contamination, where concentration overpotentials cause the voltage to drop off. The effect is partly reversible, as the voltage at high current densities recovers upon switch back to pure hydrogen operation. This is contrary to the low temperature PEM fuel cells, where the voltage degradation with only trace amounts of H2S is rapid even at low currents and irreversible [4, 5]. Prolonged exposure to 1 – 50 ppm H2S in the fuel supply show a time-dependant nature of the sulphur poisoning with the major voltage degradation occurring within the first 24 hours. Again, the effect is only visible at intermediate and high currents and is partly reversible when the fuel is changed to pure hydrogen after several days of operation with H2S contaminated fuel. EIS measurements at 200 mA/cm² showed no change in the series resistance. Occurrence of a semicircle at low frequencies, growing throughout the prolonged exposure measurements could indicate increased levels of adsorption of sulphur on the Pt sites of the electrode, thus explaining the concentration overpotentials recorded at high current densities. [1] Q. Li et al., Progress in Polymer Science, 2009, 34 (5), p. 449-477. [2] T.J. Schmidt and J. Baurmeister, ECS Transactions, 2006, 3 (1), p. 861-869. [3] G. Qian and B.C. Benicewicz, Ecs Transactions, 2011, 41 (1), p. 1441-1448. [4] V.A. Sethuraman and J.W. Weidner, Electrochimica Acta, 2010, 55 (20), p. 5683-5694. [5] I. Urdampilleta et al., ECS Transactions, 2007, 11 (1), p. 831-842.