Platinum Alloy Oxygen-Reduction Electrocatalysts

DEVELOPMENT OF PLATINUM ALLOY OXYGEN-REDUCTION ELECTROCATALYSTS 

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  

The next advance in development of PAFC binary-alloy cathode electrocatalysts was the use of Pt-Cr alloys.27 In this patent, it was disclosed that with the platinum-vanadium alloy in 99 % phosphoric acid at 194 °C and at an electrode potential 0.9 volts, over 67 % by weight of the vanadium had dissolved in the first 36 hours. In the case of Pt-Cr, only 37 % had dissolved under the same conditions. It is not clear from the descriptions in these patents whether or not there is any unreacted vanadium or chromium present in the catalyst because it is not identified that all of the vanadium or all of the chromium was initially alloyed with the platinum. It is conceivable that significant amounts of the non-noble metal components are not fully reacted. 

It was proposed by Jalan and Taylor28 that the improvement in oxygen reduction activity for platinum by producing binary-alloys of 

Although interesting, the answer for the electrocataly tic enhancement on alloys may lay elsewhere, since X-ray analyses are bulk analytical 

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VI. DEVELOPMENT OF PLATINUM ALLOY OXYGEN-REDUCTION ELECTROCATALYSTS... 

Greeley J, Stephens IEL, Bondarenko AS, et al. Alloys of platinum and early transition metals as oxygen reduction electrocatalysts. Nat Chem. 2009 1 552-6. 

Beard B, Ross PN Jr (1990) The structure and activity of platinum-cobalt alloys as oxygen reduction electrocatalysts. J Electrochem Soc 137(11) 3368—3374... 

Alloys of noble metals have been tested for O2 reduction [31] and some should even be more efficient than Pt [63]. These metals are expensive so that other materials have been tested, like bronze, some being even slightly better electrocatalysts than platinum [45] however, they do not seem to be stable enough. The factors governing oxygen reduction on oxides used as cathodes in fuel cells have been reviewed recently [92]. 

For fuel-cell technology development, it has been important to understand the characteristics and operation of highly dispersed platinum and platinum alloy electrocatalysts. A series of papers on platinum crystallite size determinations in acid environments for oxygen reduction and hydrogen oxidation was published together by Bert, Stonehart, Kinoshita and co-workers.5 The conclusion from these studies was that the specific activity for oxygen reduction on the platinum surface was independent of the size of the platinum crystallite and that there were no crystallite size effects. 

E25.17 Electrocatalysts are compounds that are capable of reducing the kinetic barrier for electrochemical reactions (barrier known as overpotential). While platinum is the most efficient electrocatalyst for accelerating oxygen reduction at the fuel cell cathode, it is expensive (recall Section 25.18 Electrocatalysis). Current research is focused on the efficiency of a platinum monolayer by placing it on a stable metal or alloy clusters your book mentions the use of the alloy PtsN. An example would be a platinum monolayer fuel-cell anode electrocatalyst, which consists of ruthenium nanoparticles with a sub-monolayer of platinum. Other areas of research include using tethered metalloporphyrin complexes for oxygen activation and subsequent reduction. 

Other approaches have focused upon using non-precious metals and their oxides as alternatives to the platinum catalysts. For example, the mixed oxide catalysts of the binary and ternary alloys of noble metals and transition metals have been investigated for the oxygen evolution reaction in solid polymer electrolyte water electrolyzers. Binary, ternary, and quaternary platinum alloys with base metals of Cu, Ni, and Co have been used as electrocatalysts in liquid acid electrolyte cells. It was also reported that a R-Cu-Cr alloy displayed better activity to oxygen reduction than R and Pt-Cr in liquid electrolyte.The enhanced electrocatalytic activity of these types of alloys has been attributed to various factors, including the decrease of the nearest neighbor distance of platinum,the formation of Raney type... 

Wang C, Markovic NM, Stamenkovic VR. Advanced platinum alloy electrocatalysts for the oxygen reduction reaction. ACS Catal 2012 2(5) 891-8. 

Gong K, Chen WF, Sasaki K, Su D, Vukmirovic MB, Zhou WP, Izzo EL, Perez-Acosta C, Hirunsit P, Balbuena PB, Adzic RR (2010) Platinum-mraiolayer electrocatalysts palladium interlayer on IrCo alloy core improves activity in oxygen-reduction reaction. J Electroanal Chem 649 232-237... 

Although platinum and platinum alloys are the state-of-the-art electrocatalysts for PEM fuel cell applications, the platinum loading required to reduce the effective overpotential of the oxygen reduction reaction at high current densities (1-2 A/cm )... 

Platinum electrocatalysts are dispersed as small particles on high surface area conductive supports for effective use of costly Pt. The size of platinum particles, therefore, plays an important role in the oxygen reduction kinetics for fuel cell applications, in terms of both electrocatalytic activity and practical application of catalysts. Carbon-supported platinum shows a large surface area and increased catalytic activity. Alloy catalysts with various transition metals have been employed to increase the catalytic activity and reduce the cost. Various Pt-based alloy catalysts (binary, ternary, and quaternary alloy) have been tested over the last two decades. Many researchers have reported that Pt-based alloy catalysts show not only higher activity than Pt alone, but also exhibit good performance in the ORRs in PEFCs and DMFCs [100-108]. 

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