Platinum cathode catalyst 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). 

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. 

Many cathode catalyst materials have been used. For noble metal catalysts, platinum was mainly used in fuel cells for space applications. For terrestrial use, one has to use less expensive materials, and non-noble metal catalysts are therefore mainly employed. Bacon used lithium-doped nickel oxide as a cathode catalyst for high-temperature AFCs. Lithium-doped nickel oxide has a sufficient electrical conductivity at temperatures above 150 °C. Currendy, mainly Raney silver and pure silver catalysts are favored. Developments of silver-supported materials containing PTFE are sometimes successful. Silver catalysts are usually prepared from silver oxide, Raney silver, and supported silver. Typically, the catalysts on the cathode are supported by PTFE because it is highly stable under basic and acidic conditions. In contrast, carbon is oxidized at the cathode in contact with oxygen, when carbon is used as an inexpensive support material. In the past, the silver catalysts frequentiy contained mercury as part of an amalgam to increase the stability and the lifetime of the cathode. Because mercury is partially dissolved during the activation procedure (see below) and during the fuel-cell operation, some electrolyte contamination can be observed. Because of the environmental hazard of mercury, this metal is currently not used in silver catalysts. 

As the anion-exchange membrane fuel cell is the alkaline-based system, we can use non-platinum-based catalyst. This is a big advantage to lower the cost of fuel cells. Especially perovskite-type and pyrochlore-type oxides have high performance to oxygen-electrocatalysts which could be applicable to the cathode materials. Some oxides have also bifunctional activities as oxygen electrode catalyst to produce a reversible fuel cell thus, future deployment is expected. While, the big problems are stability of the base... 

Oxide-based cathode catalysts are entirely new non-precious metal cathode catalysts for low-temperature fuel cells such as jxtlymer electrolyte fuel cells (PEFCs). These catalysts were developed from a viewpoint that high chemical stability was essentially required for the cathode for PEFCs. The cathode catalysts for PEFCs are exposed to an acidic and oxidative atmosphere, that is, a strong corrosive environment, therefore, even platinum nanoparticles dissolved during a long-time operation. This instability of electrocatalysts is one of the factors which hindered the wide commercialization of PEFCs. 

In summary, while fuel ceU/batteiy hybrid systems reduce the degradation from platinum surface area loss from voltage-cycling to an acceptable level, novel cathode catalysts with increased stability toward voltage-cycling would bring significant benefits and are therefore a very active field of research. 

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. 

FIGURE 2.10 Non-noble metal dissolution data as a function of time for different Pt-alloy catalysts at a fixed potential of 0.8V vs. NHE. (Reproduced from /. Power Sources, 155, Col6n-Mercado, H. R. and Popov, B. N. Stability of platinum based alloy cathode catalysts in PEM fuel cells, 253-263, Copyright (2006), with permission from Elsevier.)... 

Besides activity, durability of metal electrode nano-catalysts in acid medium has become one of the most important challenges of low-temperature fuel cell technologies. It has been reported that platinum electrode surface area loss significantly shortens the lifetime of fuel cells. In recent years, platinum-based alloys, used as cathode electrocatalysts, have been found to possess enhanced stability compared to pure Pt. The phenomenon is quite unusual, because alloy metals, such as Fe, Co and Ni, generally exhibit greater chemical and electrochemical activities than pure Pt. Some studies have revealed that the surface stmcture of these alloys differs considerably from that in the bulk A pure Pt-skin is formed in the outmost layer of the alloys due to surface segrega-... 

---