Cathode electrocatalysts stabilization

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-... 

The activity, stability, and tolerance of supported platinum-based anode and cathode electrocatalysts in PEM fuel cells clearly depend on a large number of parameters including particle-size distribution, morphology, composition, operating potential, and temperature. Combining what is known of the surface chemical reactivity of reactants, products, and intermediates at well-characterized surfaces with studies correlating electrochemical behavior of simple and modified platinum and platinum alloy surfaces can lead to a better understanding of the electrocatalysis. Steps, defects, and alloyed components clearly influence reactivity at both gas-solid and gas-liquid interfaces and will understandably influence the electrocatalytic activity. 

In a recent review on the durability and degradation of PEM fuel cells, electrocatalyst stability was primarily ascribed to Pt dissolution under nonequilibrium conditions as well as Pt particle growth (sintering) during fuel cell operation [24], These issues also remain problematic for Pt-based alloy catalysts, whereas the stability of the alloying elements is an extra concern in alloy systems. The impact of alloying on the stability of cathode alloy catalysts in a PEM fuel cell environment can be considered from two perspectives negative effects and positive effects. 

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. 

In this review, we briefly discuss the dissolution and solubility of platinum (Sect. 2), the degradation of platinum nanoparticles in fuel cells (Sect. 3), and carbon corrosion (Sect. 4). We then describe new cathode electrocatalysts wherein the platinum content can be dramatically reduced, while offering possibiUties for enhancing catalytic activity and stability (Sect. 5). 

Fig. 5 The long-term stability tests of the Pt/Pd cathode electrocatalyst in an operating fuel cell at 80°C. (a) Commercial Pt/C anode catalyst, (b) Brookhaven National Laboratory s PtRUj, /C anode catalyst...

At present there are no alternative cathode electrocatalysts to platinum. Some platinum alloy electrocatalysts prepared on traditional carbon black supports offer a 25 mV performance gain compared with Pt electrocatalysts. However, only the more stable Pt-based metal alloys, such as PtCr, PtZr, or PtTi, can be used in PEMFC, due to dissolution of the base metal by the perfluorinated sulfonic acid in the electrocatalyst layer and membrane [26]. The focus of the continued search for the elusive electrocatalyst for oxygen reduction in acid environment should be on development of materials with required stability and greater activity than Pt. 

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. 

In a fuel cell, the electrocatalysts generate electrical power by reducing the oxygen at the cathode and oxidizing the fuel at the anode [1], Pt and Pt alloys are the most commonly used electrocatalysts in PEFCs due to their high catalytic activity and chemical stability [99-103]. 

Phosphoric Acid Fuel Cell (PAFC) Phosphoric acid concentrated to 100% is used for the electrolyte in this fuel cell, which operates at 150 to 220°C. At lower temperatures, phosphoric acid is a poor ionic conductor, and CO poisoning of the Pt electrocatalyst in the anode becomes severe. The relative stability of concentrated phosphoric acid is high compared to other common acids consequently the PAFC is capable of operating at the high end of the acid temperature range (100 to 220°C). In addition, the use of concentrated acid (100%) minimizes the water vapor pressure so water management in the cell is not difficult. The matrix universally used to retain the acid is silicon carbide (1), and the electrocatalyst in both the anode and cathode is Pt. 

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. 

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. 

Figure 21. Long-term test of the performance stability of the PtRu2o electrocatalyst in an operating fuel cell. The fuel cell voltage at constant current of 0.4 A cm is given as a function of time for the electrode of 50 cm with an anode containing to 0.18 mg Ru cm and 0.018 mg Pt cm / (approximately 1/10 of the standard Pt loading) and a standard air cathode with a Pt/C electrocatalyst. The fuel was clean H2 or H2 with 50 ppm of CO and 3% air temperature 80 C.

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]. 

A typical example includes the yttria-stabilized-zirconia-based high-temperature potentiometric oxygen sensor which is widely used in automotive applications. Platinum thick films are applied, forming both the cathode and anode of the sensor. The thick electrode has a porous structure which provides a larger electrode surface area compared to non-porous structures. For current measurement, a porous electrode is desirable since it leads to a larger current output. If the metallic film serves as the electrocatalyst, a porous structure is also desirable, for it provides more catalytic active sites. On the other hand, electrodes formed by the thick-film technique do not have an exact, identical... 

Noble metals applied as electrocatalysts for the oxygen reduction have been largely utilized because of their high electrocatalytic activity and stability. Investigations are concentrated on platinum, palladium, silver and gold. The application of noble metal catalysts is limited by two fundamental disadvantages high cost and low availability. Thus, it is important to construct cathodes with small amounts of the noble metal which are obtained, for example, by dispersed platinum on an appropriate support. 

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