Catalyst Contamination in PEM Fuel Cells

The dominant type of PEMFC is the hydrogen/oxygen fuel cell, in which the anode is fed by hydrogen and the cathode is fed by oxygen or air. In this section, we will focus our attention on anode catalyst contamination caused by impurities in the hydrogen stream. 

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Li, H., Song, C., Zhang, J., et al. 2008. Catalyst contamination in PEM fuel cells. In PEM fuel cell electrocatalysts and catalyst layers, ed. Zhang J., 331-54. London Springer-Verlag. 

The catalysts are required to promote the desired reactions to occur at an appreciable rate. The catalysts used in PEM fuel cells are typically Pt-based due to the high stability and reactivity of Pt. Pt alloys may also be introduced to further increase kinetic activity, improve stability, and improve tolerance to contaminants on the anode for use in reformed fuel. The high cost of Pt necessitates a maximum utilization of the Pt. For this reason, Pt is typically in the form of very small particles of approximately 2 to 8 nm diameters, supported on larger carbon particles. The carbon particles provide a high surface area support structure to enhance the dispersion of the catalyst particles as well as providing an electrical and thermal pathway from the reaction sites... 

In a H2/air PEM fuel cell, the HOR and the ORR take place in their respective CLs. Thus, the anode and cathode electrocatalysts both play critical roles in fuel cell performance. To date, the most active and widely employed catalysts in PEM fuel cells are highly dispersed Pt-based catalysts. Although they pose several challenges, such as costliness, sensitivity to impurities/contaminants, and insufficient stability/durability under fuel cell operating conditions [14], Pt-based catalysts are recognized as the most practical choice in current PEM fuel cell technology. 

As an application of Pt nanowires in heterogeneous catalysis, we performed preferential oxidation (PROX) of CO as a test reaction [32]. The PROX reaction is useful for PEM fuel cells for the selective removal of contaminating CO from hydrogen gas, because CO works as a strong catalyst poison for Pt electrode catalysts (Figure 15.24). H2 produced in steam-reforming and the water-gas shift reaction needs further to be purified in the PROX reaction to selectively oxidize a few% CO towards inert CO2 in a H 2-rich atmosphere, to reduce the CO content to <10ppm. Under the PROX conditions, the facile oxidation of H2 to H2O may also occur, thus the catalyst selectivity for CO oxidation over H2 oxidation is an... 

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. 

For use in proton exchange membrane (PEM) fuel cells (see section 3.6), the CO contamination in the hydrogen produced must be below 50 ppm (parts per million). This is due to the poisoning limit of typical platinum catalysts used in PEM cells. The implication is the need for a final CO cleaning treatment, unless the main reaction steps (2.1) and (2.2) can be controlled so accurately that all reactants are accounted for. This CO cleaning stage may involve one of the following three techniques preferential oxidation. 

PEM fuel cells use a solid proton-conducting polymer as the electrolyte at 50-125 °C. The cathode catalysts are based on Pt alone, but because of the required tolerance to CO a combination of Pt and Ru is preferred for the anode [8]. For low-temperature (80 °C) polymer membrane fuel cells (PEMFC) colloidal Pt/Ru catalysts are currently under broad investigation. These have also been proposed for use in the direct methanol fuel cells (DMFC) or in PEMFC, which are fed with CO-contaminated hydrogen produced in on-board methanol reformers. The ultimate dispersion state of the metals is essential for CO-tolerant PEMFC, and truly alloyed Pt/Ru colloid particles of less than 2-nm size seem to fulfill these requirements [4a,b,d,8a,c,66j. Alternatively, bimetallic Pt/Ru PEM catalysts have been developed for the same purpose, where nonalloyed Pt nanoparticles <2nm and Ru particles <1 nm are dispersed on the carbon support [8c]. From the results it can be concluded that a Pt/Ru interface is essential for the CO tolerance of the catalyst regardless of whether the precious metals are alloyed. For the manufacture of DMFC catalysts, in... 

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 ... 

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. 

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. 

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