Adsorption Behavior of Carbon Monoxide at Low Concentration on the Polymer Electrolyte Fuel Cell

Introduction

                   The carbon monoxide (CO) in hydrogen fuel is known to degrade the fuel cell performance [1-4]. The allowable concentration of CO is 0.2 ppm in the quality standards for hydrogen fuel of fuel cell vehicles [5]. The allowable concentration of impurities would be revised in accordance with the development of the fuel cell components, such as reduction of platinum loadings and thinning of the membrane. In order to discuss the allowable concentration of CO, the understanding of the performance degradation mechanism by CO and the evaluation in conditions close to the actual environment are required. In our previous work, we showed that the low anode platinum loadings would decrease the fuel cell performance significantly at the CO concentration as low as 1 ppm in the hydrogen [4]. But the CO adsorption behavior of near the allowable concentration remain incompletely understood. In this study, the adsorption behavior of low concentration CO on the polymer electrolyte fuel cell was investigated by gas analysis at the anode outlet.

Experimental

                   Commercial Pt/C catalyst (TEC10E50E, Tanaka Kikinzoku Kogyo) and electrolyte membrane (Nafion NR211, DuPont) were used for the single cell tests. The platinum loading on the anode / cathode was set at 0.1/ 0.3 mg cm^-2, respectively. The MEAs were assembled into a JARI standard cell (25 cm² of electrode area). The cell temperature was 60ºC, and the dew point temperatures of the anode and the cathode were 47ºC and 40ºC , respectively. The anode gas was hydrogen mixed with CO (0.2 - 1.0 ppm), and the cathode gas was purified air. The stoichiometry of fuel and air was 1.4 and 2.5, respectively. The CO and carbon dioxide (CO₂) at the anode outlet concentrations were analyzed by a gas chromatograph with a pulse discharged helium ion detector.

Results and Discussion

                   The cell voltage at 1000 mA cm^-2 and the CO and CO₂ exhaust rate at the anode outlet (R_{CO, out} and R_{CO2, out}, respectively) during the 0.4 ppm of CO exposure test were shown in Fig. 1. The cell voltage started to drop after 5 hours and became stable after 25 hours. The exhaust rate of CO and CO₂ rose when the cell voltage dropped, and became stable after 25 hours. The sum of CO and CO₂ exhaust rate after 30 hours was nearly equal to the CO supply rate (R_{CO, in}).

                   The effect of CO concentration was investigated at the ranges from 0.2 to 1.0 ppm. The amount of CO adsorption was calculated from the results of the CO and CO₂ measurement. Firstly, the CO adsorption rate (q_CO) can be determined from the molar balance of carbon in the gas phase at the inlet and the outlet, that is  q_CO = R_{CO, in} - (R_{CO, out} + R_{CO2, out}). Then, the amount of CO adsorption was obtained by time integration of q_CO. The amount of CO adsorption during the CO exposure tests were shown in Fig. 2. At the beginning, the amount of CO adsorption increased proportional to time, and the slope correspond approximately to the CO supply rate. This indicates that most of the CO species were adsorbed on the anode. Then, the amount of CO adsorption were come to steady regardless of the cumulative CO supply amount. The saturated adsorption of CO was increased with the increase of CO concentration at cell inlet. The increase of saturated adsorption of CO will cause the voltage degradation of the cell.

                   From these results, the adsorption behavior of CO near the allowable concentration was analyzed. The relationship between the voltage drop and the amount of CO adsorption will be discussed at the meeting.

Acknowledgements

                   This work was partly supported by the New Energy and Industrial Technology Development Organization (NEDO).

References

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[2] Mahesh Murthy, Manuel Esayian, Alex Hobson, Steve MacKenzie, Woo-kum Lee, J. W. Van Zee, J. Electrochem. Soc., 148(10), A1141-A1147 (2001).

[3] Michael S. Angelo, Jean St-Pierre, Keith P. Bethune, Richard E. Rocheleau, ECS Trans., 35(32), 167-178 (2011).

[4] Yoshiyuki Hashimasa, Yoshiyuki Matsuda, Motoaki Akai, ECS Trans., 26(1), 131-142 (2010).

[5] ISO 14687-2 (2012).