Ammonia cathode contamination

Ammonia in the hydrogen fuel originates from the hydrogen production process. The impact on fuel cell performance is as described for the cathode contamination, and is similar whether it is introduced in the cathode or anode. 

Ammonia (NHj) Hydrogen production process <1 ppm Same mechanism as for cathode contamination of ammonia Same as for cathode contamination of ammonia... 

With TAA salts of small alkyl groups (e.g., ethyl, methyl), cation reduction is usually the limiting cathodic reaction. The anodic limiting reaction for ammonium ions is their oxidation to nitrogen and protons. It should be emphasized that atmospheric contaminants are supposed to influence the above cathodic and anodic limits of liquid ammonia, as they do for the other nonaqueous systems discussed in the previous sections. 

The contaminants may be deposited on the surfaces of the materials in the form of anhydrous or hydrated species. Some pollutants, like CO2, SO, NO, and HCl, are typical of urban and industrial areas, give rise to acid rains, and might contribute to the cathodic processes, while others, such as chlorides, are typical but not exclusive of marine and coastal areas and give rise to hygroscopic salts that increase the duration of wetting of surfaces, increase the conductivity of solutions, and make less protective the corrosion products. Some others, such as the sulfides, which can result from microbiological activity, alter the composition of the corrosion products, their protective capability, and the nobility of the metal often they are semiconductors, depolarize the cathodic process of hydrogen evolution, and may be oxidized to sulfuric acid by bacteria. Ammonia alters the composition of corrosion products and the solubility of metal ions it has particularly drastic effects on copper alloys and their corrosion forms. In the transport of these contaminants toward the surfaces, an important role is exerted by the wind and by the orientation of the surfaces, which can promote or hinder the washout by the rains. 

While cathode reactions tend to be quite efficient, low concentrations of mercury and oxygen may be objectionable. Sections 9.2.5.1 and 9.2.5.2 deal with their removal. Volatile impurities in the catholyte may also contaminate the hydrogen. This is most likely in diaphragm cells, and Section 7.5.8.5 gave an example in which the removal of ammonia from brine reduced the concentrations of chloramines and other nitrogen compounds in the hydrogen. 

Except for ammonia, all air contaminants are believed to lead to poisoning of the catalytic sites. Ammonia is thought to lead to a loss of ioific conductivity in the ionomer in the cathode layer, as a result of an acid-base reaction between the basic ammonia and the acidic sulfonic acid group of the ionomer. 

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. 

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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