Catalyst degradation cells

Virkar AV, Zhou YK. 2007. Mechanism of catalyst degradation in proton exchange membrane fuel cells. J Electrochem Soc 154 B540-B547. 

It appears that the heme/imidazole motif can be realized in metaUoporphyrins that are available in just two steps (Fig. 18.21b). However, it is not yet known how to accomplish objectives 2 and 3. It is also important to understand the mechanism of catalyst degradation during the ORR and to identify alternative functional groups that may increase catalyst stability to be useful in fuel cells, a metaUoporphyrin catalyst would probably have to retain its catalytic properties over at least 4 x 10 turnovers (about 1000 hours of operation at a turnover frequency of 1 s ), i.e., more than a hundred times longer than the most stable metaUoporphyrin catalysts reported to date. 

The series of degradation steps comprising mineralization is similar, whether the carbon source is a simple sugar (e.g., glucose), a plant polymer (e.g., cellulose), or a pollutant molecule [49,50,62 - 64,72,73]. Each degradation step in the pathway is facilitated by a specific catalyst (i. e., an enzyme) made by the degrading cell. Enzymes are found mostly within a cell (i. e., internal enzymes),... 

As part of the early work to find alloys ofplatinum with higher reactivity for oxygen reduction than platinum alone, International Fuel Cells (now UTC Fuel Cells, LLC.) developed some platinum-refractory-metal binary-alloy electrocatalysts. The preferred alloy was a platinum-vanadium combination that had higher specific activity than platinum alone.25 The mechanism for this catalytic enhancement was not understood, and posttest analyses26 at Los Alamos National Laboratory showed that for this binary-alloy, the vanadium component was rapidly leached out, leaving behind only the platinum. The fuel- cell also manifested this catalyst degradation as a loss of performance with time. In this instance, as the vanadium was lost from the alloy, so the performance of the catalyst reverted to that of the platinum catalyst in the absence of vanadium. This process occurs fairly rapidly in terms of the fuel-cell lifetime, i.e., within 1-2000 hours. Such a performance loss means that this Pt-V alloy combination may not be important commercially but it does pose the question, why does the electrocatalytic enhancement for oxygen reduction occur ... 

Much of the knowledge in Pt/C durability derives from the experience with phosphoric acid fuel cells (PAFCs) at operating temperatures of about 200°C. Catalyst degradation is witnessed as an apparent loss of platinum electrochemical surface area over time, " associated with platinum crystal growth. These changes are ascribed to different processes which include... 

Bi W and Fuller T (2008), Modeling of PEM fuel cell Pt/C catalyst degradation. Journal of Power Sources, 178,188-196. 

Yousfi-Steiner N, Mocoteguy P, Candusso D and Hissel D (2009), A review on polymer electrolyte membrane fuel cell catalyst degradation and starvation issues Causes, consequences and diagnostic for mitigation,/oMrnaZ of Power Sources, 194,130-145. 

Mayrhofer KJJ, Meier JC, Ashton SJ, Wiberg GKH, Kraus F, Hanzlikn M, Arenz M (2008) Fuel cell catalyst degradation on the nanoscale. Electrochem Commun 10(8) 1144—1147... 

Hayakawa, K., Tada, T., and Shao-Horn, Y. (2010) Platinum-alloy cathode catalyst degradation in proton exchange membrane fuel cells nanometer-scale compositional and morphological changes. J. Electrochem. Soc., 157, A82 A97. 

Fuel cell catalyst degradation on the nanoscale. Electrochem. Commun., 10, 1144-1147. 

Fuel Cleaning - Removal of sulfur, halides, and ammonia to prevent fuel processor and fuel cell catalyst degradation. 

Research on the degradation of catalysts has mainly focused on low-temperature PEMFCs (< 90 °C) [28-42]. For high-temperature operation, studies on eatalyst degradation have been in the areas of phosphoric acid fuel cells (PAFCs) and PBI-based MEAs [41-43]. Since the catalysts used in PAFCs are the same as those in PEMFCs, the degradation mechanisms should be applicable to high-temperature PEMFCs. Normally, catalyst degradation includes two parts Pt catalyst degradation and carbon support oxidation. 

Figure 18.3. A schematic showing the mechanism of particle growth by dissolution/precipitation. The chemical potential of smaller particles is higher than that of larger particles [32]. (Reproduced by permission of ECS— The Electrochemical Society, from Virkar AV, Zhou Y. Mechanism of catalyst degradation in proton exchange membrane fuel cells.)...

Bi W, Fuller TF. Temperature effects on PEM fuel cells Pt/C catalyst degradation. J Electrochem Soc 2008 155 B215-21. 

Merzougui B, Swathirajan S. Rotating disk electrode investigation of fuel cell catalyst degradation due to potential cycling in acid electrolyte. J Electrochem Soc 2006 153 A2220-6. 

Carbon support corrosion in a fuel cell is another cause of catalyst degradation. Noble metals are usually supported on a high surface area carbon support to increase utilization of the metals. Carbon corrosion can take place according to Equation 21.46 ... 

Diagnostic Tools to Identify Catalyst Degradation During Fuel Cell Operation Electrochemical Methods... 

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