Cathode contamination effects

In neutral and alkaline environments, the magnesium hydroxide product can form a surface film which offers considerable protection to the pure metal or its common alloys. Electron diffraction studies of the film formed ia humid air iadicate that it is amorphous, with the oxidation rate reported to be less than 0.01 /rni/yr. If the humidity level is sufficiently high, so that condensation occurs on the surface of the sample, the amorphous film is found to contain at least some crystalline magnesium hydroxide (bmcite). The crystalline magnesium hydroxide is also protective ia deionized water at room temperature. The aeration of the water has Httie or no measurable effect on the corrosion resistance. However, as the water temperature is iacreased to 100°C, the protective capacity of the film begias to erode, particularly ia the presence of certain cathodic contaminants ia either the metal or the water (121,122). 

As the anode is dissolved and the sUme layer increases, it is noted above that the voltage increases for a given ceU current and there is a tendency for increased dissolution of unwanted impurities and contamination of the lead cathode. This effect is shown by the fact that the impurity level, exemplified by the bismuth content of refined lead, increases as the anode scrap ratio falls, as shown in Figure 13.2. This clearly illustrates that there are limits to the extent of recovery of lead from each anode and the life of the anode. 

Degradation of the cathode catalyst is the main contributor to performance degradation in the fuel cell in the absence of contamination effects. This will be discussed in terms of degradation of the catalyst metal itself as well the corrosion of the carbon catalyst support. 

Contamination modeling is an important aspect of fuel cell development. It is required to interpolate and extrapolate experimental results to expected conditions in real-world operation, as it is impractical to test all combinations of reactant concentrations and fuel cell operating conditions. Modeling also assists in the development and validation of hypothesized contamination mechanisms. Model development for the anode is more extensive than that for the cathode contamination. The majority of the modeling deals with the kinetic effects associated with adsorption of contaminant species on the cathode and anode catalysts. 

In practical applications, a PEM fuel cell cathode will be exposed to air, and all of the impurities in that air may enter the fuel cell system if it has no effective filtration. The major air contaminants are CO (CO2, CO), SO (SO2, SO3), NO (NO2, NO), and NH3. In this section, we will discuss each of these cathodic contaminants in detail. 

Li, H., et al. 2009. PEM fuel cell contamination Effects of operating conditions on toluene-induced cathode degradation. /. Electrochem. Soc. 156 B252-B257. 

In general, PEM fuel cell contamination effects are classified into three major categories (1) kinetic effect (poisoning of the catalyst sites or decreased catalyst activity) (2) ohmic effect (increases in the membrane and ionomer resistances, caused by alteration of the proton transport path) and (3) mass transfer effect (mass transport problems caused by changes in the structure of CLs and GDLs, and in the ratio between their hydrophilicity and hydro-phobicity). Of these, the kinetic effect of the electrocatalysts on both anode and cathode sides is the most significant. 

Penning et al. [10] studied the contamination effect by feeding the fuel cell cathode with four types of gases for comparison (1) pure air, (2) 1 ppm N02/air, (3) 1 ppm S02/air, and (4) a mixture of 1 ppm NO2 and 1 ppm SO2 balanced with air. Figure 3.10 shows the four performance curves of the fuel cell running for 100 h. It can be seen that the contamination effect of the gas mixture on fuel cell performance was between that of 1 ppm N02/air and that of 1 ppm S02/air, indicating that the effect of the mixed... 

Contamination in a PEM fuel cell directly affects the kinetics, conductivity, and mass transport properties of the cell. In particular, the blocking of electrocatalysts by impurity adsorption can drastically reduce the effective surface area of the catalysts and, thus, slow down the kinetics and hinder cell performance. This chapter is devoted to cathode contamination modeling. 

In future work, more contamination modeling is needed, especially to account for the effect of different types of contaminants, such as anion and cation contaminants, on cell performance. In reality, of course, multiple contaminants are often the case, so a multicontaminant model needs to be developed that will take into accoimt both anode and cathode contaminants, as well as membrane contamination. 

Contamination effects of impurities on a PEMFC may be classified into three categories (1) kinetic effects, caused by adsorbing onto the catalyst surface and poisoning active sites on both the anode and cathode catalyst layers (2) mass transfer effects, due to changes in the structure, pore size, pore size distribution, and hydrophobicity/hydrophilicity of the catalyst layers or gas... 

Ammonia (NH3) or ammonium (NH4+) can exist in both the fuel and air streams. The diffusion of ammonium is fast, therefore, the ammonium entering the fuel cell from either side can quickly diffuse to the other side causing the contamination effect on both sides. For instance, for a typical membrane with a thickness of 10 to 100 jim, the estimated characteristic time constant for diffusion is 1 to 100 sec [149]. Ammonia may affect the PEMFC performance in different ways (1) by the reduction of the ionic conductivity of the membrane, which in its ammonium form is a factor of 4 lower than in the protonated form [149-151] (2) by poisoning the cathode catalyst [151] and (3) by poisoning the anode catalyst [149]. Recently, fuel cell tests have shown that the reduced membrane conductivity is not the major reason for performance losses induced by ammonia [149,150]. The effect of ammonia on the HOR was found to be minor at current densities below 0.5 A cm", but would increase with increasing current densities. The current density did not exceed 1 A cm in the presence of ammonia [149]. 

Table 6.2 Cathode major contaminant effects key sources, levels of concern, impacts and recovery behavior...

PEMFC dynamic behaviour in the presence of both anode and cathode contaminants appears to be unexplored in the literature. Again, there is no reason to not consider possible synergetic or cancellation effects between individual contaminants impacts. 

---