Effect of Carbon Monoxide on the Performance of Polymer Electrolyte Fuel Cell with Hydrogen Circulation System

Abstract

  Introduction Carbon monoxide (CO) in hydrogen fuel is known to degrade a polymer electrolyte fuel cell (PEFC) performance [1-5]. The maximum allowable concentration of CO is 0.2 ppm, according to the quality standards for hydrogen fuel (ISO14687-2) in fuel cell vehicles (FCVs) [6]. Normally, a single-cell evaluation system is a hydrogen one-way pass system, which allows fuel unused by the cell to be released into the atmosphere. However, FCVs usually have a hydrogen circulation system, which returns the fuel from the anode outlet to the inlet. A comparison between the hydrogen one-way pass system and hydrogen circulation system has not been reported with respect to the effect of CO on hydrogen fuel. Our previous study showed that CO is not accumulated in hydrogen fuel when using a hydrogen circulation system [2]. However, the effect of CO on the actual FCV condition has not yet been fully understood because the experimental conditions in our previous study (CO concentration was 4.8 ppm and anode platinum loading was 0.4 mg cm−2) were far from the actual FCV conditions. In this study, using a hydrogen circulation system, the effect of CO on the PEFC’s performance degradation is investigated and is compared to that using a hydrogen one-way pass system under the conditions of low platinum loading at the anode and a low CO concentration.   Experimental A JARI standard single cell (25 cm2 electrode area), whose temperature was controlled by a heating medium, and commercially available membrane electrode assemblies (MEA) were used for the single-cell tests. The platinum loading on the anode and the cathode were 0.1 and 0.4 mg cm−2, respectively. The electrolyte membrane thickness was 15 μm. The cell temperature was 60 ºC, the anode was non-humidified, and the cathode dew point was 40 ºC. The single-cell operation tests were conducted using both a conventional hydrogen one-way pass system and the hydrogen circulation system (Fig. 1). The cell was operated at a constant current density of 1000 mA cm−2. The anode gas was hydrogen mixed with CO (up to 1.0 ppm), and the cathode gas was a mixture of N2 (79%) and O2 (21%). The stoichiometry of the fuel and the oxidant was 2 and 2.5, respectively. The CO and CO2 in the purge gas in the hydrogen circulation system were sampled at a 1% purge rate and measured by gas chromatography coupled with pulsed discharge helium ionization detector.  Results and Discussion Figure 2 shows, for comparison, the voltage change by adding CO at a steady state in the hydrogen one-way pass system and in the hydrogen circulation system and the CO concentration in the hydrogen circulation system. The voltage change in the hydrogen circulation system is smaller than that in the hydrogen one-way pass system. At 0.2 ppm CO concentration in the hydrogen circulation system, the voltage change induced by adding CO is about 1/10 of that incurred when the hydrogen one-way pass system is used. The CO concentration in the purge gas in the hydrogen circulation system is lower than that in the supplied CO/H2 mixture. Most of the CO in the hydrogen fuel is oxidized to CO2 and gets accumulated in the hydrogen circulation system when a low concentration of CO is supplied to the cell. When the hydrogen circulation system is used, the exhaust gas is sent to the anode inlet again. Consequently, the CO concentration at the anode inlet decreases because the gas in the cylinder, which contains CO, is diluted by the gas from the hydrogen circulation system. In addition, our previous study of the hydrogen one-way pass system revealed that a large excess of O2, compared with CO, existed in the anode [3]. O2 in the hydrogen circulation system is probably circulated and consumed by CO oxidation in the anode electrocatalyst via a non-electrochemical reaction [4]. Consequently, the voltage change in the hydrogen circulation system is smaller than that in the hydrogen one-way pass system.  Acknowledgements This work was supported by the New Energy and Industrial Technology Development Organization (NEDO).