Platinum-ruthenium electrocatalyst

Investigations at Siemens in Erlangen, Germany, have used unsupported platinum-ruthenium anodes (4 mg/cm ) and platinum black cathodes (4 mg/cm ). Their best performances were 0.52 V at 400 mA/cml At Los Alamos National Laboratory in New MexicoJ the electrocatalyst was unsupported R-RuOx at the anode and unsupported R black at the cathode (R loading about 2 mg/cm ). In a subsequent study, the thinner Nafion 112 membrane was used to reduce the ohmic drop. Under pressure at 400 mA/cm cell potentials of 0.57 V with Oj and 0.52... 

Improvements in solid polymer electrolyte materials have extended the operating temperatures of direct methanol PEFCs from 60 C to almost 100 C. Electrocatalyst developments have focused on materials that have higher intrinsic activity. Researchers at the University of Newcastle upon Tyne have reported over 200 mA/cm at 0.3 V at 80 C with platinum/ruthenium electrodes having platinum loading of 3.0 mg/cm. The Jet Propulsion Laboratory in the U.S. has reported over 100 mA/cm at 0.4 V at 60 C with platinum loading of 0.5 mg/cm. Recent work at Johnson Matthey has clearly shown that platinum/ruthenium materials possess substantially higher intrinsic activity than platinum alone (45). 

Since on pure platinum, methanol oxidation is strongly inhibited by poison formation, bimetallic catalysts such as PtRu or PtSn, which partially overcome this problem, have received renewed attention as interesting electrocatalysts for low-temperature fuel cell applications, and consequently much research into the structure, composition, and mechanism of their catalytic activity is now being undertaken at both a fundamental and applied level [62,77]. Presently, binary PtRu catalysts for methanol oxidation are researched in diverse forms PtRu alloys [55,63,95], Ru electrodeposits on Pt [96,97], PtRu codeposits [62,98], and Ru adsorbed on Pt [99]. The emphasis has recently been placed on producing high-activity surfaces made of platinum/ruthenium composites as a catalyst for methanol oxidation [100]. 

Electrocatalyst selection and design are the key aspects of PEM fuel cells. The most popular catalyst is platinum for the anode and the cathode in pure hydrogen cells. For direct methanol fuel cells and for hydrogen cells with carbon monoxide present, a platinum/ruthenium alloy is used. 

Three decades ago, Bockris et al. reported enhancement of the efficiency of methanol oxidation with a platinum-ruftienium alloy electrocatalyst. Two decades ago, another promising approach to electrocatalysis of methanol oxidation was presented. That was the platinum-ruthenium oxide electrocatalyst proposed by Watanabe and Motoo [24]. 

These reactions occur at a very slow rate on pure platinum and this has resulted in a large research effort to discover more active electrocatalysts. At present, platinum—ruthenium offers the best performance. Ruthenium adsorbs water more readily than platinum and the resulting species, Ru—OHads, assists the removal of carbon monoxide from neighbouring platinum sites. Despite this beneficial effect of ruthenium, still more efficient electrocatalysts are required to enhance the power delivered by DMFCs, especially if the system is to compete favourably with hydrogen—air PEMFCs. 

Vigier et al. [13] used three types of catalysts (platinum, ruthenium, and tin) in a DMFC. The atomic compositions of the catalysts were varied to get the best performance from each of the combinations. Under different voltammetric conditions Pt-Sn, electrocatalysts are the most active combination toward the methanol oxidation and production of CO, compared to Pt and Pt-Ru electrodes [14-16]. 

Most of the electrocatalysts used in H2 technical electrodes for HOR in low temperature fuel eells are noble metals sueh as platinum, ruthenium, and palladium. Some non-noble metals with aeeeptable catalytie aetivity, alloyed or mixed with noble metals, are eobalt, iron, molybdenum, niekel, tin, and tungsten [31]. Some organic materials, like metal phtaloeianines have also been satisfactory for some reaetions [32]. 

At temperatures below about 125°C, CO adsorption on platinum is very strong. Even few ppms in the H2 stream cause substantial performance losses on the anode. Therefore, the use of platinum alone is not viable for HOR in the presence of CO in low temperature fuel cells. Thus, platinum-ruthenium, platinum-molybdenum and platinum-tin are being used as anode electrocatalysts for hydrogen oxidation in the presence of CO because they tolerate low ppms of CO without excessive polarization losses. Timgsten carbide (WC) also shows high CO-tolerance [38,44]. 

Green CL, Kucemak A (2002) Determination of the platinum and ruthenium surface areas in platinum-ruthenium alloy electrocatalysts by underpotential deposition of copper. 1. Unsupported catalysts. J Phys Chem B 106 1036-1047... 

The type of anode catalyst has a strong effect on the severity of CO poisoning, since the catalyst affects the kinetics of CO adsorption (equation (2.12) and equation (2.13)) and CO oxidation (equation (2.18) and equation (2.19)). Based on these mechanisms, many CO-tolerant electrocatalysts have been developed by Pt alloying, such as PtRu (platinum/ruthenium) [24,38], PtSn (platinum/tin) [39-41], and PtMo (platinum/molybdenum) [42-44]. Generally, alloying Pt with a second element can enhance the catalytic activity of the Pt through one or more of the following effects ... 

Examples for the influence of the electrocatalyst on the oxidation of CgHpOq formed [72] at open circuit in 0.5 M H2SO4+1M CH3OH on platinum-black, electrolytically prepared platinum-ruthenium, and platinum-rhenium mixtures are illustrated by curves d, e, and f in Fig. 54. 

Fig. 54. Charging curves in the absence (a, b, c) and presence of chemisorbed carbonaceous species (d, e, f) on platinum (a, d), platinum-ruthenium (b, e), and platinum-rhenium (c, 0 in 0.5 M H2SO4. Current density was 2mA/cm for Pt, and 5mA/cm for the other electrocatalysts...

Over the past 35 years, much has been learned about the electrooxidation of methanol on the surface of noble metals and metal alloys, in particular platinum and ruthenium [2, 4, 6, 7]. Significant overpotential losses occur in the reaction due to poisoning of the alloy catalyst surface by carbon monoxide. Yet, Pt-based metal alloys are still the most popular catalyst materials in the development of new fuel cell electrocatalysts, based on the expectation that a more CO-tolerant methanol catalyst will be developed. The vast ternary composition space beyond Pt-Ru catalysts has not been adequately explored. This section demonstrates how the ternary space can be explored using the high-throughput, electrocatalyst workflow described above. 

A large screening was recently done to identify such a third metal, X, to add to platinum and ruthenium [52]. Figure 5 summarizes the behavior of the nine investigated Pt-Ru-X trimetallic electrocatalysts toward methanol oxidation. At low potentials, the Pt-Ru-Mo ternary catalyst gives the highest current densities compared to other ternary electrocatalysts. This catalyst exhibits a current density 10 times greater than Pt-Ru at a potential of 400 mV versus RHF under steady-state conditions (data taken after 5 minutes). 

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. 

The modification of platinum catalysts by the presence of ad-layers of a less noble metal such as ruthenium has been studied before [15-28]. A cooperative mechanism of the platinurmruthenium bimetallic system that causes the surface catalytic process between the two types of active species has been demonstrated [18], This system has attracted interest because it is regarded as a model for the platinurmruthenium alloy catalysts in fuel cell technology. Numerous studies on the methanol oxidation of ruthenium-decorated single crystals have reported that the Pt(l 11)/Ru surface shows the highest activity among all platinurmruthenium surfaces [21-26]. The development of carbon-supported electrocatalysts for direct methanol fuel cells (DMFC) indicates that the reactivity for methanol oxidation depends on the amount of the noble metal in the carbon-supported catalyst. 

The first example addresses ruthenium-modified platinum electrodes, vsdiich show an enhanced electrochemical activity for the oxidation of H2/CO gas mixtures as compared with pure platinum [4, 5]. This makes them interesting electrocatalysts for low-temperature fuel cell applications. Here we discuss the mesoscopic structure of... 

In the above reactions, the oxidation process takes place in the anode electrode where the methanol is oxidized to carbon dioxide, protons, and electrons. In the reduction process, the protons combine with oxygen to form water and the electrons are transferred to produce the power. Figure 9-1 is a reaction scheme describing the probable methanol electrooxidation process (steps i-viii) within a DMFC anode [1]. Only Pt-based electrocatalysts show the necessary reactivity and stability in the acidic environment of the DMFC to be of practical use [2], This is the complete explanation of the anodic reactions at the anode electrode. The electrodes perform well due to the presence of a ruthenium catalyst added to the platinum anode (electrode). Addition of ruthenium catalyst enhances the reactivity of methanol in fuel cell at lower temperatures [3]. The ruthenium catalyst oxidizes carbon monoxide to carbon dioxide, which in return helps methanol reactivity with platinum at lower temperatures [4]. Because of this conversion, carbon dioxide is present in greater quantity around the anode electrode [5]. 

DMFC performance loss due to catalyst degradation has been attributed to several factors a decrement of the electrochemically active surface area (ECSA) of the platinum electrocatalyst supported on a high-surface-area carbon, a loss of cathode activity towards the ORR by surface oxide formation, and ruthenium crossover [83, 85, 116, 117]. 

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