Comparative and Comprehensive Studies of Tolerance to Airborne Contaminants of PEMFC with Pt and Non-Pt Cathodes Using Segmented Cell Approach and Spatial EIS
A segmented fuel cell system was operated (4) with a commercially available Pt-containing (0.4 mgPt cm-2 for anode and cathode) membrane/electrode assembly (MEA) and home-made MEA consisting of Fe/N/C (3 mg cm-2) cathode gas diffusion electrodes (GDEs), Pt anode GDE (0.04 mgPt cm-2, Alfa Aesar) and Nafion XL. The MEA was operated under galvanostatic control of the whole cell current, 80°C, 48.3 kPag or 2 atmg back pressure and 100/50-100% relative humidity for the anode and cathode, respectively. SO2/NO2/CO concentrations varied from 2 to 20 ppm. The dry contaminant was injected into the humidified cathode air stream. The exposure proceeded until the cell voltage reached a steady value. Subsequently, the contaminant injection was stopped to evaluate the cell recovery ability in air.
Fig. 1 a) shows the voltage response and normalized current density for each segment vs. experiment time for the Pt cathode PEMFC at 1.0 A cm-2. For the first 16 h, the cell was operated with pure air resulting in a cell voltage of 0.665 V. The injection of 2 ppm SO2 significantly decreased the voltage during a transition period that lasted ~6-7 h eventually reaching a steady state of 0.370 V. The voltage decrease was accompanied by a significant change in the current density distribution which undergoes several steps. Recovery was only partial and took ~4 h, the cell voltage reached 0.475 V instead of initial performance of 0.665 V. Chemisorption of SO2 at intermediate potentials (0.4-0.8 V) most likely results in formation of zero-valent sulphur and leads to a decrease of the ECA, a shift of the oxygen reduction from a 4-electron to a 2-electron mechanism and an increase in H2O2 production, which negatively impact the PEMFC performance (5).
Fig. 1 b) presents profiles of the segment voltages and normalized current densities for the Fe/N/C cathode PEMFC at 0.2 A cm-2. The injection of 10 ppm SO2 for 7 hours did not cause any noticeable changes in segments’ cell voltage and current distribution. Only segment 1 showed a slight variation in performance. Exposure of the Fe/N/C cathode to 2 and 10 ppm NO2 resulted in a performance loss of 30 and 70-75 mV, respectively, which is less than in a Pt cathode fuel cell with a performance drop of ~80 mV. Detailed analyses of the current density distribution, its correlation with spatial EIS results obtained during SO2, NO2 and CO exposure and density functional theory calculations will be presented and discussed.
We gratefully acknowledge ONR (N00014-12-1-0496), DOE EERE (DE-EE0000459) and Hawaiian Electric Company.
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