Cathode Catalyst Layer Contamination

The slow kinetics of the cathode oxygen reduction reaction (ORR) plays the key role in limiting PEMFC performance when pristine hydrogen is used as the fuel. Therefore, improving the catalytic activity for the ORR has drawn most of the research attention in catalysis studies. Cathode contamination has attracted less attention compared with anode contamination, and only a limited number of papers have been published. Pollutants in air include NOx (NO2 and NO), SOx (SO2 and 

and some volatile organie speeies (VOCs) sueh as toluene. The major sourees of these contaminants are vehiele exhaust, industrial emissions, and agrieultural activities. In speeial eases sueh as military applieations, battlefield eontaminants released from either ehemieal warfare or normal warfare, or both, should also be considered. These eontaminants inelude Sarin, sulfur mustard, eyanogen ehloride, hydrogen eyanide, and so on, whieh eould eause signifieant irreversible performance drops. Sinee NOx and SOx are the main air pollutants, in this seetion we will mainly diseuss NOx and SOx contamination. Contamination caused by other pollutants will also be touehed upon. 

The SOj (mainly SO2) in the air stream can cause degradation in fuel cell performance by poisoning the catalyst sites, changing MEA properties such as hydrophilicity/hydrophobicity, and then affecting water management. The major effect is believed to be poisoning of the catalyst sites, thereby decreasing catalytic activities. 

Sulfur coverage Mass activity % Mass Overpotential fW 

Cycle voltammetry (CV) studies [34, 35] revealed that there were some sulfur species on the surface of the Pt catalyst during the SO2 poisoning process. The SO2 adsorbed on the Pt surface was reduced electrochemically to sulfur through the formation of an SO intermediate, as shown in Reactions 6.17 and 6.18 [35, 38]. The end products of SO2 adsorption on a Pt electrode are linearly and bridge adsorbed sulfur [39]. 

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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-... 

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. 

The membrane electrode assembly (MEA) in a proton exchange membrane (PEM) fuel cell has been identified as the key component that is probably most affected by the contamination process [1]. An MEA consists of anode and cathode catalyst layers (CLs), gas diffusion layers (GDLs), as well as a proton exchange membrane, among which the CLs present the most important challenges due to their complexity and heterogeneity. The CL is several micrometers thick and either covers the surface of the carbon base layer of the GDL or is coated on the surface of the membrane. The CL consists of (1) an ionic conductor (ionomer) to provide a passage for proton transport ... 

Hui et al. [29] conducted toluene contamination tests at different toluene concentrations. They also studied the effects of different operational conditions on toluene contamination, including the effects of fuel cell relative humidity (RH), of Pt loading in the cathode catalyst layer, of back pressure, and of air stoichiometry [30]. Figure 3.8 shows a set of representative results of contamination tests at 1.0 A cm with various levels of toluene concentration in the air. It can be seen that the cell voltage starts to decline immediately after the introduction of toluene, and then reaches a plateau (steady state). These plateau voltages indicate the saturated nature of the toluene contamination. For example, the cell voltage drops from 0.645 V to 0.522 V at 1.0 A cm-2 within 30 min of the cathode... 

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... 

This chapter has examined a variety of EIS applications in PEMFCs, including optimization of MEA structure, ionic conductivity studies of the catalyst layer, fuel cell contamination, fuel cell stacks, localized impedance, and EIS at high temperatures, and in DMFCs, including ex situ methanol oxidation, and in situ anode and cathode reactions. These materials therefore cover most aspects of PEMFCs and DMFCs. It is hoped that this chapter will provide a fundamental understanding of EIS applications in PEMFC and DMFC research, and will help fuel cell researchers to further understand PEMFC and DMFC processes. 

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. 

Thus, NO2 would compete with O2 for electrons on the cathode and the NHJ thus formed would further contaminate the fuel cell, causing performance degradation by poisoning the Nation membrane and ionomer in the catalyst layer, as discussed in section 2.2.3. 

When contaminant presents in the fuel cell, its concentration at the catalyst layer varies with both the inlet contaminant concentration and the current density, as discussed in Shi et al. [18]. Furthermore, the contaminant adsorption (desorption) rate constant is also related to the electrode potential. This variation of the contaminant concentration can be obtained by introducing the CGDL and cathode flow field into the model, which definitely increases its complexity. For simplicity here, we considered the product of the contaminant adsorption (desorption) rate constant and the contaminant concentration at the CCL, as a fimction of current density and contaminant inlet concentration (kCp-- f Cpr J))/ where Cp is the contaminant inlet concentration in the cathode charmel. Based on the experimental data at current densities of 0.2, 0.5, 0.75, and 1 A/cnP, and contaminant inlet concentrations of... 

Catalyst layers based on pure platinum require comparatively clean reactants pure platinum works with CO contaminations below 1 ppm and CO2 less than 10%. Appropriate fuels are therefore either pure hydrogen or purified reformate. Higher concentrations of CO or CO2 lead to a steady decrease of the performance due to catalyst poisoning. For reformate use without a purification step, platinum-ruthenium alloys are used leading to a typical loading of 0.3 gp,Ru/kWg, at the anode and 0.3 gpj/kW, at the cathode. 

St-Pierre (2009) developed a zero-dimensional model that considers competitive adsorption for a contaminant with O2 or H2 at the cathode or anode side, respectively. This model assumes that contaminant transport through the gas flow channels, GDLs and ionomer in the catalyst layers is much faster compared to surface kinetics. The rate determining step is considered to be due to contaminant reaction or desorption of reaction product from the platinum surface. Other model assumptions include the absence of lateral interaction between adsorbates, first-order reaction kinetics, constant pressure, and constant temperature at the cathode/anode sides. Using a set of parameters, St-Pierre (2009) successfully used his model in order to describe experimental transient data obtained in the presence of SOj, NOj, and HjS in the cathode airstreams. 

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