Cathode Catalyst Stability

The stability of electrocatalysts for PEMFCs is increasingly a key topic as commercial applications become nearer. The DoE has set challenging near-term durability targets for fuel cell technology (automotive 5,000 h by 2010 stationary 40,000 h by 2011) and has detailed the contribution of the (cathode) catalyst to these. In particular, for automotive systems as well as steady-state stability, activity after simulated drive cycles and start-stop transients has been considered. In practice, both these treatments have been found to lead to severe degradation of the standard state-of-the-art Pt/C catalyst, as detailed next. 

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Recently, rhodium and ruthenium-based carbon-supported sulfide electrocatalysts were synthesized by different established methods and evaluated as ODP cathodic catalysts in a chlorine-saturated hydrochloric acid environment with respect to both economic and industrial considerations [46]. In particular, patented E-TEK methods as well as a non-aqueous method were used to produce binary RhjcSy and Ru Sy in addition, some of the more popular Mo, Co, Rh, and Redoped RuxSy catalysts for acid electrolyte fuel cell ORR applications were also prepared. The roles of both crystallinity and morphology of the electrocatalysts were investigated. Their activity for ORR was compared to state-of-the-art Pt/C and Rh/C systems. The Rh Sy/C, CojcRuyS /C, and Ru Sy/C materials synthesized by the E-TEK methods exhibited appreciable stability and activity for ORR under these conditions. The Ru-based materials showed good depolarizing behavior. Considering that ruthenium is about seven times less expensive than rhodium, these Ru-based electrocatalysts may prove to be a viable low-cost alternative to Rh Sy systems for the ODC HCl electrolysis industry. 

It is very important to develop a high performance cathode catalyst, because a sluggish ORR causes a large overpotential at low temperatures. With respect to the total performance of activity and stability, the cathode catalyst material is limited to Pt or its alloys at present. In acidic media such as Nation electrolyte or aqueous acid solutions, four-electron reduction is dominant at Pt-based electrodes ... 

Effect of High Cathode Voltages on Catalyst Stability... 

In addition, slacks are also expected to sif af idle ( 0.9 V) for much of the time. Mathias et al. studied the effect of fhese volfages on cathode carbon stability. Holding a standard 50% Pt/C catalyst at 1.2 V caused 15% loss of ifs carbon in 20 h and if was predicted nof to survive the required 100 h. At 0.9 V, the catalyst was expected to lose 5% over a few thousand hours, which may be acceptable for long-ferm use (see Figure 1.16). The effect on MEA performance was also studied. After 20 h at 1.2 V, a 30 mV loss in performance was observed and it became progressively worse at longer times. The loss in... 

The possible complete replacement of Pt or Pt alloy catalysts employed in PEFC cathodes by alternatives, which do not require any precious metal, is an appropriate final topic for this section. Some nonprecious metal ORR electrocatalysts, for example, carbon-supported macrocyclics of the type FeTMPP or CoTMPP [92], or even carbon-supported iron complexes derived from iron acetate and ammonia [93], have been examined as alternative cathode catalysts for PEFCs. However, their specific ORR activity in the best cases is significantly lower than that of Pt catalysts in the acidic PFSA medium [93], Their longterm stability also seems to be significantly inferior to that of Pt electrocatalysts in the PFSA electrolyte environment [92], As explained in Sect. 8.3.5.1, the key barrier to compensation of low specific catalytic activity of inexpensive catalysts by a much higher catalyst loading, is the limited mass and/or charge transport rate through composite catalyst layers thicker than 10 pm. 

Charreteur F, Jaouen F, Dodelet JP (2009) Iron porphyrin-based cathode catalysts for PEM fuel cells Influence of pyrolysis gas on activity and stability. Electrochim Acta 54 6622-6630... 

Most of the electrochemical reactors fail due to different attacks on the electrocatalysts, where the anodes are attacked faster than the cathodes (electrochemical corrosion, mechanical fissures due to electrodissolution, or bubble formation and evolutions, etc.) [43]. In new technologies, the use of the anode, membrane, or cathode assemblies solves this problem. In the case of the solid polymer electrolytes, the anode and the cathode catalysts are integrated to the membrane promoting the mechanical and electrochemical stability of the device [44,45]. This new technology replaces the problem of the diaphragm-based electrochemical industry that was established in the beginning of the twentieth century [46]. 

Build and operate single cells and protot5 e DMFC stacks with different anode and cathode catalysts, membrane materials, flow patterns and optimized MEAs to maximize performance and demonstrate stability. 

Colon-Mercado Hector R., Popov Branko N. Stability of platinum based alloy cathode catalysts mPEM fuel cells, Journal of Power Sources , 155,253-263 (2006). 

Figure 3.13. H2/O2 PEM fuel cell short-term stability test performed at 0.5 V on various MEAs using non-noble metal cathode catalysts obtained by heat treating FePc/C at various temperatures. The curve for 2 wt% Pt is given for comparsion (according to Figure 4 in ref. [76] reproduced with permission of Elsevier).

Zelenay et al. explored Co-polypyrrole (CoPPy) material as a PEM fuel-cell cathode catalyst. The composite CoPPy catalyst, even without a heat treatment, could generate a power density of 0.15Wcm in a H2—O2 fuel cell and displayed no signs of performance degradation for more than 100 h. Their results showed that heteroatomic polymers can be used not only to stabilize the non-noble metals in a PEM fuel cell environment but also to generate active sites for the ORR. Study of the interaction between the catalyst and oxygen also demonstrated that CoPPy... 

As a solution to provide a long-term solution to Pt cost and scarcity, a variety of non-noble metal-based catalysts has been explored as promising cathode catalysts for fuel cells. These ORR catalysts include heat-treated metal-nitrogen-carbon complexes (M-Nx/C, M = Fe or Co), carbon-supported chalcogen-ides, and carbon-supported metal oxides. These catalysts have been synthesized and showed considerable ORR activity and stability when compared to those of Pt/C catalyst. In the exploration, RDE/RRDE techniques are the most commonly employed tools in evaluating the catalysts activity and stability toward ORR and its associated mechanism. 

Colon-Mercado HR, Popov BN (2006) Stability of platinum based alloy cathode catalysts in PEM fuel cells. J Power Sources 155(2) 253-263... 

In this chapter, first we summarized the investigations about nitrides and carbonitrides as cathode catalysts. Second, the stability of nitrides, carbonitrides, and oxides in acid electrolyte was discussed and introduced the applicability of oxides to support of platinum particles. Then, the necessity of modifications of oxides was described. As mentioned above, the modifications of oxides were classified into four ways, and development of oxide-based cathodes was summarized. 

Non-precious Metal Oxide-Based Cathode Catalysts 13.4.1 Stability of Group 4 and 5 Metal Oxide-Based Catalysts... 

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