Anode contaminants major effects

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 choice of anodic material depends on a number of factors but principally cost and the required life. Consumable electrodes in general, are less costly but must be replaced as they are consumed, and they have the limitation that they cannot sustain high current loadings. A major difficulty may be the contaminating effects of the dissolution products. Non consumable alloy electrodes such as... 

Although there is little evidence for auto-catalysis in dechlorination by Fe , it is still possible that localized corrosion contributes to the remediation of contaminants in environmental applications. Various investigators have postulated that localized corrosion contributes through increased surface area (44) and creation of corrosion cell domains (49-51). The corrosion cell model works on the same principle as the electrochemical model described above (Figure 3), but invokes additional effects such as the reduction of protons as the major cathodic reaction, and the creation of an electrical double layer between the anode and cathode that permits transport due to electrical migration as well as diffusion. Although many aspects of these models are plausible, there are not yet any data that specifically support them, and a study that systematically addresses the role of localized corrosion in remediation applications of Fe remains to be done. 

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 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. 

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 6.3 Anode major contaminant effects key sources, levels of concern, impacts and recovery behavior...

The impact of CO on the PEMFC anode performance has been widely studied experimentally and by modelling, and maity mitigation methods have been proposed. In recent years, the impacts of H2S and NHj have become an important subject of research. It is well accepted that the major impact of CO and H2S contaminants on the hydrogen-fed PEMFC anode is a kinetic effect due to poisoning of the electrocatalyst, while NHj mainly affects the ionomeric membrane by reducing ionomer conductivity.In both cases, significant performance degradation can be induced. In order to enhance CO tolerance, bimetallic catalysts such as Pt-Ru, Pt-Mo and Pt-Sn have been proposed however, the... 

---