Cathode Catalyst Layer Degradation

The performanee and durability of a membrane electrode assembly (MEA) is affected signifieantly by the eathode eleetrode eomposition and structure, due to the poor kineties of oxygen reduetion and reaetant transport limitations. Utilization and stability of platinum or its alloys in the PEMFC play important roles in fuel cell efficiency, durability, and the drive for eost reduction through reduced Pt loadings. Cathode catalyst layer degradation is a critical issue for fuel cell durability to meet the requirement of 5000 hours for automotive applications and 40,000 for stationary applications. 

1 Platinum Dissolution During Fuei Ceil Operation 

1 Thermodynamic Stability of Platinum Under Fuel Cell Operating Conditions 

Although platinum is regarded as a very stable metal in aeidie enviromnents, its long-term stability under fuel cell operating conditions has attracted mueh attention reeently due to loss of platinum surface area during fuel cell operation. Thermodynamically stable phases of platinum metal and its oxides have been discussed in the literature, and the main electroehemieal reaetions are summarized in the following equations [9, 33, 68, 69]  

The platinum eleetroehemical dissolution rate in fuel cell operation is a critical durability issue due to loss of catalyst surface area vs. time. The dissolution behavior and solubility of platinum are governed by the chemical state of the platinum surfaee and the equilibrium platinum species in the solution. Temperature, pH, eleetrolyte composition, potential, and particle size are all major factors influencing the solubility and the dissolution rate. Potential cycling of Pt electrodes has been extensively investigated, and the platinum electrode dissolution rates from the literature are summarized in Table 23.5 [9, 33, 70-74], Both Pt(II) and Pt(IV) speeies were detected in sulfuric solution after potential cycling from 

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One of the critical issues with regard to low temperamre fuel cells is the gradual loss of performance due to the degradation of the cathode catalyst layer under the harsh operating conditions, which mainly consist of two aspects electrochemical surface area (ECA) loss of the carbon-supported Pt nanoparticles and corrosion of the carbon support itself. Extensive studies of cathode catalyst layer degradation in phosphoric acid fuel cells (PAECs) have shown that ECA loss is mainly caused by three mechanisms ... 

MEA performance is mainly limited by ORR kinetics, as well as oxygen transport to the cathode catalyst. Another major loss is due to proton conduction, in both the membrane and the cathode catalyst layer (CL). Characterization of the ionic resistance of fuel cell electrodes helps provide important information on electrode structure optimization, and quantification of the ionomer degradation in the electrodes [23],... 

Two major degradation processes are affecting the cathode catalyst layer when carbon supported catalysts are used ... 

The described experiments and simulations support Reiser et al. s theory of the RCD mechanism. This mechanism occurs during start-up and shutd-own of the cell provided that the anode is not completely filled with hydrogen or air. Carbon and water are oxidized in the cathode catalyst layer, resulting in a reverse current and cell degradation. 

Young, A.P., Stumper, )., and Gyenge, E. (2009) Characterizing the stmctural degradation in a PEMFC cathode catalyst layer carhon corrosion. J. Electrochem. Soc., 156 (8), B913-B922. 

Young AP, Stumper J, Gyenge E (2009) Characterizing the structural degradation in aPEMFC cathode catalyst layer carbon corrosion. J Electrochem Soc 156 B913-B922... 

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 8.12 Cyclic voltammograms (CVs) of the cathode catalyst layer recorded after the different poisoning tests. CVs performed at room temperature and a potential scan rate of 5 mV s h Anode gas Hj and cathode gas Nj. (Reprinted from Journal of Power Sources, 179, Matsuoka, K. et al. Degradation of polymer electrolyte fuel cells under the existence of anion species, 560-565, Copyright (2008), with permission from Elsevier.)... 

Nara, H., Tominaka, S., Momma, T., and Osaka, T. 2011. Impedance analysis counting reaction distribution on degradation of cathode catalyst layer in PEFCs. 158,... 

One critical issue facing the commercialization of low-temperature fuel cells is the gradual decline in performance during operation, mainly caused by the loss of the electrochemical surface area (EGA) of carbon-supported platinum nanoparticles at the cathode. The major reasons for the degradation of the cathodic catalyst layer are the dissolution of platinum and the corrosion of carbon under certain operating conditions, especially those of potential cycling. Cycling places various loads on... 

Figure 10 shows TEM images of an MEA following an open-circuit endurance test in which was supplied to the anode and to the cathode. The test conditions were a cell temperature of 90 C, 30% relative humidity, anode atmosphere of H, and cathode atmosphere of O. Similar to the results of the load-cycling test, it was found that platinum from the cathode catalyst layer dissolved and was redeposited in the electrolyte membrane. Under these test conditions, redeposited platinum particles were observed near the center of the electrolyte membrane. The position of redeposited platinum particles is determined by a balance between the mixed potential of the electrolyte membrane and the partial pressures of the anode and cathode O. It was estimated that platinum particles would be redeposited near the center of the electrolyte membrane under the conditions used in this test (Fig. 11). Chemical degradation of the electrolyte membrane was observed centered on the band of redeposited platinum particles. An analysis was made of the drain water discharged from the MEA during the test and fluoride ions were detected, which suggests that the electrolyte manbrane was partially decomposed (Ohma et al. 2007).

Mitsubishi Electric Corporation investigated alloyed catalysts, processes to produce thinner electrolytes, and increases in utilization of the catalyst layer (20). These improvements resulted in an initial atmospheric performance of 0.65 mV at 300 mA/cm or 0.195 W/cm, which is higher than the IFC performance mentioned above (presented in Table 5-2 for comparison). Note that this performance was obtained on small 100 cm cells and may not yet have been demonstrated with full-scale cells in stacks. Approaches to increase life are to use series fuel gas flow in the stack to alleviate corrosion, provide well-balanced micro-pore size reservoirs to avoid electrolyte flooding, and use a high corrosion resistant carbon support for the cathode catalyst. These improvements have resulted in the lowest PAFC degradation rate publicly acknowledged, 2 mV/1000 hours for 10,000 hours at 200 to 250 mA/cm in a short stack with 3600 cm area cells. 

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