Pt-Ru alloy catalyst

Electro-catalysts which have various metal contents have been applied to the polymer electrolyte membrane fuel cell(PEMFC). For the PEMFCs, Pt based noble metals have been widely used. In case the pure hydrogen is supplied as anode fuel, the platinum only electrocatalysts show the best activity in PEMFC. But the severe activity degradation can occur even by ppm level CO containing fuels, i.e. hydrocarbon reformates[l-3]. To enhance the resistivity to the CO poison of electro-catalysts, various kinds of alloy catalysts have been suggested. Among them, Pt-Ru alloy catalyst has been considered one of the best catalyst in the aspect of CO tolerance[l-3]. 

Pt is deposited only at the surface of Ru nanoparticles rather than throughout the Pt-Ru nanoparticles. The method facilitates tuning of the electronic and catalytic properties of Pt-Ru catalysts by controlling the Pt cluster size. In contrast to the Pt-Ru alloy catalysts, this structure has all the Pt atoms available for the catalytic reaction, which decreases the Pt loading. 

The Pt-Ru alloy catalyst with aPt Ru atomic ratio of 1 1 is generally considered as the best MOR catalysts in terms of activity and stability [58]. Lamy et al. [59]... 

These comparisons show that our PEDOT/PSS supported catalysts are as active for methanol oxidation as the best polymer supported catalysts r >orted in the literature. However, a question that must be answered is vdiether polymer supported catalysts can provide superior performance to commercially available carbon supported catalysts. To answer this, the PEDOT/PSS supported catalyst used for the experiments in Fig. 5, was compared with a commercial 0 -TEK) carbon supported Pt-Ru alloy catalyst Fig. 6 shows normal pulse voltammograms at 60° C, while Fig. 7 shows the results of constant potential experiments (at 22 °C) over a much longer time period. In both experiments, and over all timescales studied, the carbon supported catalyst delivers currents that are as much as 10 times higher than for the polymer supported catalyst. The only conditions under which the polymer supported catalyst is superior are at short times and high potentials, which are not relevant to fuel cell operation. 

Pt alloys with first-row transition metals, such as Ni, Co, and Ee, have also been explored for CO tolerance and MOR electrocatalysts [80-82] their activities are generally considered comparable to those of Pt-Ru alloy catalysts. Various... 

Owing to these features of the DMFCs, extensive studies have been conducted on highly CO-tolerant anode catalysts in the MOR. The highly CO-tolerant catalysts were strongly desirable for the MOR however, it was drfticult to realize the character with a monometaUic Pt catalyst under an ambient condition. Fortunately, it has been found that the MOR activity of the Pt can be enhanced with a second component such as Ru [15-19] and Sn [20, 21]. In the 1960s, Adihart and Heuer have shown that the Pt-Ru alloy catalyst shows a lower overpotential toward the MOR in 1 mol/1 H2SO4 aqueous solution and that the MOR polarizations of the Pt-Ru alloy and the Pt catalysts are 0.24 V and 0.44 V versus NHE 20 mA/cm at 373 K, respectively [15]. 

However, the power density of a DMFC using the state-of-the-art Pt-Ru/C anode catalyst is still a factor of 10 lower than that of a hydrogen-fuelled PEM fuel cell if the same noble metal loading is used. The efficiency of DMFCs operating on Pt-Ru alloy catalysts may yet be insufficient for practical application. Thus, much investigation has been done to improve the performance of the Pt-Ru binary catalysts by the incorporation of a third metal, such as W, Mo, Co, Ni, V, Pd, Rh, Sn, Os, or Ir. Some of these ternary Pt-Ru-M alloy catalysts show... 

The bi-functional mechanism, although simple, can explain very well the promoted MOR activity of Pt-Ru alloy catalysts. This mechanism is also well adapted by other binary alloys such as Pt-Sn [48]. It has been identified that CO does not bind to the Sn sites, with the result that OH can more easily adsorb on the Sn sites without competition from CO. The synergetic effect on Pt and Sn sites gives rise to Pt-Sn, a very active CO electrooxidation catalyst. However, the strong adsorption of OH species on Sn sites, particularly at high potentials, makes the Pt-Sn catalyst inferior to the Pt-Ru catalyst for the methanol oxidation reaction. 

Pt-Ru alloy catalyst wifli high CO tolerance is used in the anode, especially when a reformate H2-rieh gas (in which CO content is high) is used for fuel. 

The addition of ruthenium (Ru) as a second metal to Ft was originated from the exploration of electrocatalyst for methanol oxidation [50]. Pt-Ru alloy catalysts have proved to be the most active ones at low temperatures ( 80°C) in both the direct methanol fuel cells (DMFC) and H2/O2 fuel cells to oxidize adsorbed CO. Detailed studies on catalysts with defined Pt to Ru surface ratios have shown that Pto Ruos (atomic ratio 1 1) has the lowest overpotential in comparison to pure Pt [15]. The function and mechanism for the addition of Ru have been concluded by Ye [51]. In this work, we just have a brief review of it. 

Further development of Pt-Ru alloy catalysts that minimize contaminant effects... 

K. Han, J. Lee, H. Kim, Preparation and characterization of high metal content Pt-Ru alloy catalysts on various carbon blacks for DMFCs , Electrochim. Acta 52 (2006) 1697. 

Nitrogen-doped carbon materials shows good catalysts support in terms of catalytic activity and stability [18,19]. A graphitic carbon nitride with three-dimensionally extended highly ordered pore arrays has been reported as a support for a Pt-Ru alloy catalyst of a DMFC anode. The nanostructured C3N4 has 73-83% higher power density than Vulcan XC-72, a commercial carbon black. 

Carbon nanocoils, as well as carbon nanotubes, constitute a new class of carbon nanomaterials with properties that differ significantly from other forms of carbon. The structure of a nanocoil is similar to that of MWCNTs, except helical shape. The catalysts supported on carbon nanocoils exhibited better electrocatalytic performance compared with the catalyst supported on Vulcan XC-72 carbon. In particular, the Pt-Ru alloy catalyst supported on the CNC, which has both good crystallinity and a large surface area, showed a superior electrocatalytic performance, compared with other CNC catalysts [43]. A fuller-ene (Cso) film electrode was also suggested as a catalyst support for methanol oxidation after electrodeposition of Pt on these fullerene nanoclusters [44]. 

The most effective CO tolerance is found to be with Pt/Ru alloys. Pt/Ru alloy catalysts, supported on carbon were found to be effective upto 2% CO without a significant drop in performance. However, the other promising metals are Co and Ni. It may be noted that the Ru attracts the CO molecules, thus keeping the Pt free for hydrogen oxidation. Additionally on Ru, the oxidation of CO to CO2 occurs at a lesser potential of about 0.35 w.r.t. (SHE) at 50 atom% Ru and at 0.2V (SHE) for 90 atom% Ru. Thus, at a lesser polarization the electrochemical oxidation of CO starts for Pt/Ru based catalysts. These two effects make Pt/Ru based anode catalyst as the most accepted CO tolerant PAFC catalyst. As Ru percent increases, CO tolerance is enhanced at a cost of activity due to less Pt availability. In view of this, a trade-off is required, and the optimal composition that most of the developers claim is 1 1 atomic ratio of Pt Ru. 

In an effort, to reduce acid loss by absorption in separator plates, better materials with low open porosity are used. Additionally, acid loss can be reduced by low temperature operation of PAFC. Some groups including the author s laboratory is adopting a temperature range that is around 150-170°C. This reduces component corrosion and acid loss. However, for anode catalyst, Pt/Ru alloy catalyst has to be used for better CO tolerance with reduced current density. Additionally, the upstream reformer system has to be tuned to ensure CO level at < 1%. However, this design is found to be quite acceptable for small capacity power plants. 

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