Anode catalysts bimetallic

This survey focuses on recent developments in catalysts for phosphoric acid fuel cells (PAFC), proton-exchange membrane fuel cells (PEMFC), and the direct methanol fuel cell (DMFC). In PAFC, operating at 160-220°C, orthophosphoric acid is used as the electrolyte, the anode catalyst is Pt and the cathode can be a bimetallic system like Pt/Cr/Co. For this purpose, a bimetallic colloidal precursor of the composition Pt50Co30Cr20 (size 3.8 nm) was prepared by the co-reduction of the corresponding metal salts [184-186], From XRD analysis, the bimetallic particles were found alloyed in an ordered fct-structure. The elecbocatalytic performance in a standard half-cell was compared with an industrial standard catalyst (bimetallic crystallites of 5.7 nm size) manufactured by co-precipitation and subsequent annealing to 900°C. The advantage of the bimetallic colloid catalysts lies in its improved durability, which is essential for PAFC applicabons. After 22 h it was found that the potential had decayed by less than 10 mV [187],... 

Figure 1.13 Fuel cell characteristics of a 5 cm DEFC recorded at llO C. Influence of the nature of the bimetallic catalysts (80 20 atomic ratio with 30% metal loading). Anode catalyst,...

Lobato J, Canizares P, Rodrigo MA, Linares JJ (2009) Study of different bimetallic anodic catalysts supported on carbon for a high temperature polybenzimidazole-based direct ethanol fuel cell. Appl Catal B-Environ 91 269-274... 

Abstract Direct formic acid fuel cells offer an alternative power source for portable power devices. They are currently limited by unsustainable anode catalyst activity, due to accumulation of reaction intermediate surface poisons. Advanced electrocatalysts are sought to exclusively promote the direct dehydrogenation pathway. Combination and structure of bimetallic catalysts have been found to enhance the direct pathway by either an electronic or steric mechanism that promotes formic acid adsorption to the catalyst surface in the CH-down orientation. Catalyst supports have been shown to favorably impact activity through either enhanced dispersion, electronic, or atomic structure effects. 

At present, the Pt-Ru bimetallic system is recognized as the most promising CO-tolerant anode catalyst for the DMFCs. A large body of hterature exist demonstrating improvement of the CO oxidation on the Pt-Ru alloy and Ru-modified Pt catalysts. The superior CO tolerance of the Pt-Ru bimetallic catalysts compared with the monometallic Pt catalyst is frequently explained with concepts of bifunctional mechanism [17] and ligand effect [22, 23]. The former mechanism proposed by Watanabe and Motoo is widely accepted. They claimed that the Ru has higher reactivity with water than Pt and that formation of Ru-OH at a lower potential promotes the electrooxidation of the chemisorbed CO on the Pt (formulas (4) and (5)). 

Diemant T, Hager T, Hoster HE, Rauscher H, Behm RJ. Hydrogen adsorption and coadsorption with CO on weU-defined bimetallic PtRu surfaces—a model study on the CO tolerance of bimetaUic PtRn anode catalysts in low temperature potymer electrolyte fuel cells. Surf Sci 2003 541 137-46. 

Studies of this type, eoupled with different surface characterization methods, enabled development of several elasses of CO tolerant anode catalysts [77], Markovic and Ross [77] provided eomprehensive description of the strategy of development of CO-tolerant catalyst based on extrapolation of fundamental electrochemisty of massive bimetallic surfaces to real-life supported eleetroeatalysts. 

OH adsorption on Ru is a key factor that makes this metal the major component of various bimetallic catalysts for anode reactions. Ru-OH causes a signihcant inhibition of the ORR [Inoue et al., 2002]. In situ SXS data for the oxidation of Ru(OOOl) in acid... 

PEM fuel cells use a solid proton-conducting polymer as the electrolyte at 50-125 °C. The cathode catalysts are based on Pt alone, but because of the required tolerance to CO a combination of Pt and Ru is preferred for the anode [8]. For low-temperature (80 °C) polymer membrane fuel cells (PEMFC) colloidal Pt/Ru catalysts are currently under broad investigation. These have also been proposed for use in the direct methanol fuel cells (DMFC) or in PEMFC, which are fed with CO-contaminated hydrogen produced in on-board methanol reformers. The ultimate dispersion state of the metals is essential for CO-tolerant PEMFC, and truly alloyed Pt/Ru colloid particles of less than 2-nm size seem to fulfill these requirements [4a,b,d,8a,c,66j. Alternatively, bimetallic Pt/Ru PEM catalysts have been developed for the same purpose, where nonalloyed Pt nanoparticles <2nm and Ru particles <1 nm are dispersed on the carbon support [8c]. From the results it can be concluded that a Pt/Ru interface is essential for the CO tolerance of the catalyst regardless of whether the precious metals are alloyed. For the manufacture of DMFC catalysts, in... 

Zhong and co-workers [530] described recent results of an investigation of the electrocatal3dic oxidation of methanol using carbon-supported An and Au-Pt nanoparticle catalysts. The exploration of the bimetallic composition on carbon black support was aimed at modifying the catalytic properties for the methanol oxidation reaction at the anode in direct methanol fuel cells (DMFCs). An and Au-Pt nanoparticles of 2-3 nm sizes encapsulated in an organic monolayer were prepared, assembled on carbon black materials and treated thermally. The results have revealed that these Au-Pt nanoparticles catalysts are potentially viable candidates for use in fuel cells under a number of conditions [530],... 

By far the most widely explored approach to decrease the anodic potential has been the formation of bimetallic, trimetallic or multimetallic platinum based catalysts. In a bimetallic platinum based catalyst the second metal will be chosen... 

The introduction of NMonNM deposition method has initiated applications as possible route to produce catalysts for fuel cells with improved peifoimance [37, 38, 42]. Different experimental methods were used to characterize electrosorptiOTi characteristics and activity of these modified bimetallic noble metal surfaces for different reactions [38,43,44]. These efforts have led to the design and characterization of one of the most efficient catalysts known today for polymer electrolyse membrane fuel cell anodes [4,45 7]. 

Nitrate electroreduction has been extensively studied over the last few decades. This reactitMi is a multi-electron transfer process showing different mechanisms as a function of pH, nitrate and supporting electrolyte concentration, chemical composition and structure of the catalyst. In recent years, nitrate electroreduction has been widely studied over diamond and many monometallic electrodes such as Pb, Ni, Zn or Rh, Ru, Ir, Pd, Cu, Ag and Au. Because none of the common pure metals is able to provide high selectivities for nitrogen, bimetallic alloys or monometals modified with foreign metal adatoms were prepared and evaluated for the reduction of nitrate. More recently, an electrochemical process in which nitrate ions are reduced to ammonia at the cathode, and where the produced ammonia is oxidized at the anode to nitrogen with the contribution of hypochlorite ions, has been evaluated. 

---