Cathode contamination cell performance

Certain performance losses of fuel cells during steady-state operation can be fully or partially recovered by stopping and then restarting the life test. These recoverable losses are associated to reversible phenomena, such as cathode catalyst surface oxidation, cell dehydration or incomplete water removal from the catalyst or diffusion layers [85]. Other changes are irreversible and lead to unrecoverable performance losses, such as the decrease in the ECSA of catalysts, cathode contamination with ruthenium, membrane degradation, and delamination of the catalyst layers. 

Since fuel cell performance cannot be totally recovered after SO poisoning, it is critical to prevent the cathode from SOx contamination in the air stream. Unfortunately, to date few studies have addressed the mitigation of SOx contamination at the cathode side. Two methods deal with air contamination of fuel cells increasing catalyst tolerance to impurities [45] and/or filtering contaminants from the air [46—48]. A Pt-Fe phosphate catalyst (Pt-FePO) was... 

At the cathode catalyst layer, the common contaminants include SO, NO, H2S, NH3, VOCs, and ozone. A trace amount of SQc in air can cause a significant performance decrease. Increases in SO concentration accelerate the degradation. This degradation is due to the adsorbed sulfur on the Pt surface produced from SO reduction, which not only poisons the catalyst but also changes the ORR mechanism. The fuel cell performance is only partly recovered after SO contamination. contamination of the cathode catalyst is also concentration-... 

Figure 23.8. Effect of CO contamination time on fuel cell performance in H2/IOO ppm CO during the poisoning period anode and cathode Pt on Vulcan XC72 T = 80 °C P(h2) = 0.22 MPa, P(02) = 0.24 MPa [51]. (Reprinted by permission of ECS— The Electrochemical Society, from Oetjen H-F, Schmidt VM, Stimming U, Trila F. Performance data of a proton exchange membrane fuel cell using H2/CO as fuel gas.)...

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. 

At the fuel cell anode side, the main contaminants are CO, HjS, and NH3. These can originate from the reformate gas and hydrogen production processes. Both CO and H2S reduce fuel cell performance by strong adsorption on the Pt catalyst surface, thus poisoning the catalyst and retarding H2 oxidation. NH3 causes a decrease in membrane conductivity by forming NH and then replacing H+ in the Nafion membrane and ionomer in both anode and cathode catalyst layers. 

Ft (platinum) catalysts supported on a conductive matrix, such as carbon, to provide electron conduction and (3) a hydrophilic agent, such as polytet-rafluoroethylene (PTFE) to provide sufficient porosity and adjust the hydro-phobicity/hydrophilicity of the CL for gaseous reactants to be transferred to active sites [2,3]. With each of those elements optimized to provide the best overall performance, the CL functions as a place for electrochemical reactions. The processes occurring in a CL include mass transport of the gaseous reactants, interfacial reactions of the reactants (e.g., H2 at anode and O2 at cathode) at the electrochemically active sites, proton transport in the electrolyte phase, and electron conduction in the electronic phase. When contaminants are present in the reactant streams, one or more of the above processes can be adversely affected, causing degradation in fuel cell performance or even fuel cell failure. 

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. 

Figure 6.5 shows the transient cell performance behaviors at different current densities and with different toluene inlet concentrations. On the one hand, the effect of toluene contamination becomes more severe with a higher toluene concentration at the same cell current density for example. Figure 6.5(d) indicates that at the same current density of 1.0 Acm, the cell voltage drops due to toluene concentrations of 250, 500, and 750 ppb are 37, 42, and 48 mV, respectively. On the other hand, the toluene contamination increases steadily with increasing cell current density for example, the voltage drops in response to 750 ppb toluene in the cathode flow channel are 9,16, 27, and 48 mV, corresponding to cell current densities of 0.5, 0.75, and 1.0 AcmV respectively, as shown in Figure 6.5. Furthermore, the time required for the cell voltage to reach steady state is also affected by both toluene concentration and current density, i.e., a larger toluene concentration and a lower current density result in a longer time before cell performance reaches steady state.

Besides the kinetic models we discussed above, the other cathode contamination models available in the literature are the empirical model and the competitive adsorption model [4,57]. Empirical models have been successfully used to describe FEM fuel cell performance at different temperatures and pressures [55,56]. Equation (6.73) was proposed by Kim et al. [56] ... 

Currently, several air-side contamination models have been published in the literature, ranging from simple empirical and adsorption models to general kinetic models. These models have been applied to simulate and predict SO2, NO2, NH3, and toluene contamination. The kinetic model is a very general one based on the associative oxygen reduction mechanism. It takes into account contaminant reactions, such as surface adsorption, competitive adsorption, and electrochemical oxidation, and has the capability of simulating and predicting both transient and steady state cell performance. The model can be applied to other cathode contaminants, e.g., SO2 and NO2. 

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. 

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