고분자 전해질 연료전지의 전극촉매 성능저하에 대한 연구

Abstract

The activation procedure of membrane electrode assembly (MEA) leads chemical or physical changes and then increases the performance of cell. There are some kinds of activating methods

cyclic-voltage, constant low-voltage (0.4V) and constant high-voltage (0.6V). Activation process by acting at low voltage was better because it saved time as well as improves more performance. And activation course helped that the carbon around Pt catalysts was oxidized to CO which effects on reactions inside electrode. This leaded that the activity of catalysts were increased in the operation of cell.Long-term operation of a polymer electrolyte membrane fuel cell (PEMFC) was carried out in constant-current (CC) and open-circuit-voltage (OCV) modes. The main factors causing electrocatalyst deactivation were found to be Pt sintering and dissolution. In Pt sintering, growth in particle size occurred mostly during the initial stage of operation (40 h). Pt dissolution occurred mostly at the cathode, rather than the anode, due to chemical oxidation of Pt to PtO by residual oxygen present in the cathode layer, resulting in a gradual decrease in cell performance during long-term operation. After the dissolution of PtO in water, Pt2+ was formed, which migrated from the cathode to the membrane phase, and was re-deposited as Pt crystal upon reduction by cross-over hydrogen, as was confirmed by transmission electron microscopy (TEM) after long-term operation. Under normal operating conditions, there exists a balance at the cathode between chemical oxidation by oxygen and electrochemical reduction by input electrons. Therefore, Pt dissolution at the cathode is accelerated by an imbalance of these reactions under OCV conditions or by a high O2 concentration in the feed.There were effects of sintering temperature and transitional metal on catalytic activity when cell operation using alloy catalyst was carried out. The dissolution of alloy catalysts did not occur well at 900℃ of sintering temperature. Comparing the dissolving pattern of catalysts by dissolution experiments with electrochemical analysis like oxygen reduction reaction (ORR) test, we considered effects of electrocatalysts and how to occur cell degradation. And, the transitional metal was dissolved rapidly at initial time, after that time, dissolved very slowly as operating time. Under short-term operations which only include effect of catalyst sintering, the cell performance was higher at using alloy catalysts than single Pt catalyst. This means that alloy catalyst leaded improvements of cell performance. The alloy catalysts also showed improvements in dissolution under long-term operations, especially at the OCV mode (high residual O2 in cathode). When cell made by only single Pt was operated during long time, the Pt size was increased rapidly at the initial stage of cell operation in both the cathode and anode. However, in case of alloy catalyst at the cathode, particle size of alloy catalysts was almost constant in long-term operation. The cause of decreasing in cell performance was not sintering effect of Pt particle but metal dissolution in alloy catalysts.