Electrocatalysts alloys

Table 3.1 lists some of the anodic reactions which have been studied so far in small cogenerative solid oxide fuel cells. A more detailed recent review has been written by Stoukides46 One simple and interesting rule which has emerged from these studies is that the selection of the anodic electrocatalyst for a selective electrocatalytic oxidation can be based on the heterogeneous catalytic literature for the corresponding selective catalytic oxidation. Thus the selectivity of Pt and Pt-Rh alloy electrocatalysts for the anodic NH3 oxidation to NO turns out to be comparable (>95%) with the... 

Quantitative analysis can be carried out by chromatography (in gas or liquid phase) during prolonged electrolysis of methanol. The main product is carbon dioxide,which is the only desirable oxidation product in the DMFC. However, small amounts of formic acid and formaldehyde have been detected, mainly on pure platinum electrodes. The concentrations of partially oxidized products can be lowered by using platinum-based alloy electrocatalysts for instance, the concentration of carbon dioxide increases significantly with R-Ru and Pt-Ru-Sn electrodes, which thus shows a more complete reaction with alloy electrocatalysts. 

The above results demonstrate that computational screening is promising technique for use in electrocatalyst searches. The screening procedure can be viewed as a general, systematic, DFT-based method of incorporating both activity and stability criteria into the search for new metal alloy electrocatalysts. By suggesting plausible candidates for further experimental study, the method can, ultimately, result in faster and less expensive discovery of new catalysts for electrochemical processes. 

Igarashi H, Fujino T, Zhu Y, Uchida H, Watanabe M. 2001. CO tolerance of Pt alloy electrocatalysts for polymer electrolyte fuel cells and the detoxification mechanism. Phys Chem Chem Phys 3 306-314. 

Murthi VS, Urian RC, Mukeijee S. 2004. Oxygen reduction kinetics in low and medium temperature acid environment Correlation of water activation and surface properties in supported Pt and Pt alloy electrocatalysts. J Phys Chem B 108 11011-11023. 

Shao MH, Huang T, Liu P, Zhang J, Sasaki K, Vukmirovic MB, Adzic RR. 2006a. Palladium monolayer and palladium alloy electrocatalysts for oxygen reduction. Langmuir 22 10409-10415. 

Wan L-J, Moriyama T, Ito M, Uchida H, Watanabe M. 2002. In situ STM imaging of surface dissolution and rearrangement of a Pt-Fe alloy electrocatalyst in electrol3de solution. Chem Commun 1 58 59. 

Zhang L, Lee K, Zhang JJ. 2007. The effect of heat treatment on nanoparticle size and ORR activity for carbon-supported Pd-Co alloy electrocatalysts. Electrochim Acta 52 3088-3094. 

Numerous studies have shown that Pt-based binary alloy electrocatalysts such as Pt-Fe, Pt-Co, Pt-Ni, and Pt-Cr exhibit a higher catalytic activity for the ORR in an... 

Yang H, Alonso-Vante N, Leger JM, Lamy C. 2004. Tailoring, structure, and activity of carbon-supported nanosized Pt-Cr alloy electrocatalysts for oxygen reduction in pure and methanol-containing electrolytes. J Phys Chem 108 1938-1947. 

Sun YP, Buck H, Mallouk TE. 2001. Combinatorial discovery of alloy electrocatalysts for amperometric glucose sensors. Anal Chem 73 1599-1604. 

Palladium electrocatalysts, 183 Palladium-alloy electrocatalysts, 298-300 Pareto-optimal plot, 85 Platinum-alloy electrocatalysts, 6, 70-71, 284-288, 317-337 Platinum-bismuth, 86-87, 224 Platinum chromium, 361 362 Platinum-cobalt, 71, 257-260, 319, 321-330, 334-335 Platinum-iron, 319, 321, 334-335 Platinum-molybdenum, 253, 319-320... 

The behavior of Pt and other alloy electrocatalyst crystallites used as the electrode materials for phosphoric acid electrolyte fuel-cells. 

Figure 30. Correlation of the oxygen reduction performance (log igoo mv) of Pt and Pt alloy electrocatalysts in a PEM fuel cell with Pt—Pt bond distance (filled circles) and the d band vacancy per atom (open circles) obtained from in situ XAS at the Pt L3 and L2 edges.(Reproduced with permission from ref 34. Copyright 1995 The Electrochemical Society, Inc.)...

More recently, Stamenkovic et al. [95,107] reported on the formation of Pt skins on Pt alloy electrocatalysts after high-temperature annealing. Pt skins were reported to exhibit strongly enhanced ORR activity. It was argued that the electronic properties of the thin Pt layer on top of the alloy alter its adsorption properties in such a way as to reduce the adsorption of OH from water and therefore to provide more surface sites for the ORR process (see Section 5.2 in Chapter 4 for a detailed discussion of skin catalysts, compare also Section 4.1.5 in the present Chapter). 

Figure 6.19. Experimental cyclic voltammograms of carbon-supported high surface area nanoparticle electrocatalysts in deaerated perchloric acid electrolyte. Solid curve pure Pt dashed curve Pt5oCo5o alloy electrocatalyst. Inset blow up of the peak potential region of Pt—OH and Pt— formation. Scan rate 100 mV/s. Potentials are referenced with respect to the reversible hydrogen electrode potential (RHE).

Figure 6.20. Experimental linear sweep voltammogram of carbon-supported high surface area nanoparticle electrocatalyst in oxygen-saturated perchloric acid electrolyte (room temperature). Solid curve pure Pt dashed curve Pt50Co50 alloy electrocatalyst. Inset a blow up of the kinetically controlled ORR regime. Inset b comparison of the specific (Pt surface area normalized) current density of the Pt and the Pt alloy catalyst for ORR at 0.9 V.

Figure 6.22. Trends in the ORR activity of Pt skin alloy electrocatalysts as function of oxygen binding...

Figure 6.25. Schematic of the mechanism of the CO electrooxidation on a Pt or Pt alloy electrocatalyst.

The dependence of the Gibbs free energy pathway on electrode potential (Figure 3.3.10A) manifests itself directly in the experimental current potential characteristic illustrated in Figure 3.3.10B. At 1.23 V, no ORR current is measureable, while with decreasing electrode potentials the ORR current increases exponentially until at +0.81 V, processes other than surface kinetics (e.g. mass transport) begin to limit the overall reaction rate. Figure 3.3.10B represents a typical performance characteristic of a Pt or Pt-alloy electrocatalyst for the ORR. 

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. 

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 ... 

It would appear from this patent literature that the development trends for PAFC cathode alloy electrocatalysts are from platinum, to Pt-refractory metal (vanadium) to Pt-Cr, to Pt-Cr-Co through to Pt-Fe-Co and then the various gallium additions to these combinations. It would seem, particularly in the case of gallium additions, that the gallium should induce porosity into the platinum alloys, since it would be expected to leach out easily. 

Previously, the first reviews on alloy electrocatalysts for oxygen reduction in phosphoric acid fuel-cells44"46 concentrated on those patents that had been issued in the United States, since that was where most of the early work had been done. Subsequendy, similar alloy work has been done in Japan, and that work is reflected in the Japanese patent literature shown in Table 3, whence corresponding alloy-combination atom ratios and the air/oxygen performance values are given in Table 3a. 

S62-163746 Takashi Itoh, Sigemilsu Matsuzawa, Katsuaki Katoh Pt Alloy Electrocatalyst and Acid Fuel Cell Electrode (Pt-Fe-Co) 13 Jan 1986 20 July 1987 Nippon Englehard... 

H2-61961 Takashi Itoh, Sigemitsu Matsuzawa Supported Platinum Alloy Electrocatalyst (Pt-Fe-Cu) 26 Aug 1989 1 Mar 1990 N.E. Chemcat... 

H4-87260 Takashi Itoh, Katsuaki Katoh, Shinji Kamitomai Supported Platinum Quartemary Alloy Electrocatalyst (Pt-Co-Ni-Cu) 31 July 1990 19 Mar 1992 N.E. Chemcat... 

It became obvious that long-term stability of high surface area electrocatalysts was as important, or even more important than short-term activity. Luczak36 and Landsman pioneered the heat treatment of ternary alloy electrocatalysts in order to provide an ordered crystallite structure. This work was followed in Japan by Itoh and Katoh, and subsequently by... 

Takashi Itoh Katsuaki Katoh Platinum Alloy Electrocatalyst (Pt-Fe-Co-Cu) 9 Mar 1990 18 June 1991 N.R. Chemcat (Japan)... 

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