Anode contamination temperature effect

In applications where Nafion is not suitable, at temperatures above 200 °C with feed gas heavily contaminated with CO and sulfur species, a phosphoric acid fuel cell (PAFC)-based concentrator has been effective [15]. Treating the gas shown in Table 1, a H2 product containing 0.2% CO, 0.5%CO2 and only 6 ppm H2S was produced. The anode electrode was formed from a catalyst consisting basically of Pt-alloy mixed with 50% PTFE on a support of Vulcan XC-72 carbon. The cathode was... 

Ammonia, produced due to the coexistence of H2 and N2 at high temperatures in the presence of catalyst, was estimated to be in the concentration range of 30 to 90 ppm [37, 38], Uribe et al. [39] examined the effects of ammonia trace on PEM fuel cell anode performance and reported that a trace in the order of tens of parts per million could lead to considerable performance loss. They also used EIS in their work. By measuring the high-frequency resistance (HFR, mainly contributed by membrane resistance) with an operation mode of H2 + NH3/air (feeding the anode with hydrogen and ammonia), they obtained some information related to membrane conductivity, and found that conductivity reduction due to ammonia contamination is the major cause of fuel cell degradation. 

The vulnerability of Pt and Pt alloy catalysts to poisoning by trace contaminants at operation temperatures typical for a PEFC is well documented and is of clear concern in the design of a power system based on a PEFC stack. Sources of contaminants include both fuel and air feed streams as well as processes derived from chemical instability of cell component(s). As to the feed streams, polishing of anode feed streams generated by fuel processing upstream the cell should leave very low levels of CO to be dealt with effectively within the cell (see Sect. 8.3.7.1), whereas any traces of sulfur or ammonia have to be perfectly eliminated upstream the anode... 

Figure 6.10. The constant current discharging curve of the PEMFC during running with different cathode gas for 100 hours. Current density 500 mA/cm cell temperature 70 °C RH anode 65 °C, and cathode 68 °C dew points, ambient pressure [34]. (Reprinted from Journal of Power Sources, 166(1), Jing F, Hou M, Shi W, Fu J, Yu H, Ming P, et al.. The effect of ambient contamination on PEMFC performance, 172-6, 2007, with permission from Elsevier.)...

Ionic contaminants, which dissolve in the moisture on the surface, increase the conductance (lower the resistance) of the insulating layer between the anode and cathode. Their presence enhances electrochemical migration. How much the migration is enhanced depends on several factors the solubility of the ion, the mobility of the ion, the effect of pH on solu-bihty, the reactivity of the ion, temperature, and relative humidity. 

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