Comparative and Comprehensive Studies of Tolerance to Airborne Contaminants of PEMFC with Pt and Non-Pt Cathodes Using Segmented Cell Approach and Spatial EIS

As fuel cell technology moves toward a mass-production stage, research has to address issues relating to cost production, durability and stability of proton exchange membrane fuel cell (PEMFC) performance. Most fuel cells use ambient air as an oxidant, which contains a variety of contaminants originating from vehicle and industrial exhausts as well as from naturally occurring processes. It was shown that in addition to being expensive conventional Pt cathodes do not have a sufficient tolerance to major air pollutants (SO₂, H₂S, NH₃, NO_x) (1, 2). So attempts to replace Pt with non-platinum group metals have been pursued in order to reduce cost production and gain stability of PEMFC performance in harsh environments (3). The evaluation of fuel cell performance with a single, lumped cell does not reveal its spatial behavior. In contrast, a segmented cell system provides locally resolved voltage, current and impedance, which is a powerful tool for understanding the environmental adaptability, durability and poisoning mechanisms. In this work, the spatial performance of Pt and Fe/N/C cathode fuel cells exposed to 2-20 ppm SO₂, NO₂ and CO in air was comprehensively studied and compared using a segmented cell system and spatial electrochemical impedance spectroscopy (EIS) for the first time.

A segmented fuel cell system was operated (4) with a commercially available Pt-containing (0.4 mg_Pt 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 mg_Pt cm^-2, Alfa Aesar) and Nafion XL. The MEA was operated under galvanostatic control of the whole cell current, 80°C, 48.3 kPa_g or 2 atm_g 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.

ACKNOWLEDGEMENTS

We gratefully acknowledge ONR (N00014-12-1-0496), DOE EERE (DE-EE0000459) and Hawaiian Electric Company.

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