Ammonia membrane contamination

Other contaminants of concern include ammonia (membrane deterioration), alkali metals (catalyst poisoning, membrane degradation), particles, and heavy hydrocarbons (catalyst poisoning and plugging). Both the anode and cathode flows must be carefully filtered for these contaminants, as even ppb-level concentration can lead to premature cell and stack failure. 

Of the most common air and fuel gaseous contaminants, ammonia is the most significant membrane contaminant, as the cationic species Nff + will interfere with the proton conduction mechanism and result in decreased membrane conductivity. Other gaseous contaminants may also affect the membrane and ionomer functionality through pH effects and decomposition products these are discussed in chapter 5. 

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. 

Cationic contaminants may emanate from many sources. Metals, such as iron and copper, in system components may ionize due to corrosion exchange with protons in the membrane. Metallic salts, such as sodium and calcium, may enter the fuel cell from coastal water or from deicing agents. The most likely source of cationic contaminants is from the fuel line. Hydrogen from reformed hydrocarbons usually contain parts per million (ppm) of ammonia. This ammonia can be oxidized to ammonium ions and enter the polymer electrolyte. 

Cationic contaminants tend to build up in the polymer electrolyte. This is because the sulfonate sites have a higher affinity for most other cations than protons and because most other cations do not partake in a suitable reaction to exit the polymer electrolyte phase [2,3]. In the case of ammonia, there is a suitable reaction at the cathode to remove ammonium ions from the system, but this reaction is likely slower than proton reduction. Some other metal ions, such as copper and cobalt, are electrochemically active in the fuel cell potential window and tend to "plate out" of the system. In general, once a cationic contaminant is in the polymer electrolyte phase it tends to stay there until the membrane has an acid treatment. 

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]. 

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