Electrocatalysts for MOR

The complex oxides of transition metals belonging to perovskite and spinel families have also been investigated for electrocatalysis of AOR. Raghuveer et al. [113] have tested a series of rare earth cuprates with compositional formulae Ln2 xM Cui yMy 04 ii (where Ln=La and Nd M=Sr, Ca, and Ba M =Ru and Sb 0.0 < x < 0.4 and 1/=0.1) as anode electrocatalysts for MOR... 

However, PtsCo/C, despite showing the good performance for the ORR, has been reported as a less tolerant electrocatalyst for MOR [130]. This has been attributed to the enhanced metallic character of Ft in the PtaCo due to intra-alloy electron transfer from Co to Ft and to the adsorption of oxygen species on the more electropositive element (Co) that promotes MOR according to the bifunctional theory. 

Much of the work done thus far has involved the use of Pt electrocatalysts and it should be clear from our discussion that surface adsorbates play crucial roles in a number of electrocatalytic reactions. Adsorbate (such as surface-oxide) effects in the traditional field of electrocatalysis in aqueous media have usually been tackled by developing Pt-alloy electrocatalysts (for the HOR, ORR, MOR, and COOR) and it will be interesting to see how ft-aUoy (as well as non-Pt) electrocatalysts perform during electrocatalysis in RTlLs. Compton has already made a step in this direction, comparing the activity of Pt electrocatalysts for the HER in RTlLs to that of other metals. In the case of the ORR in RTlLs, it will be interesting to explore whether Pt alloys are more active than Pt and whether a volcano relationship between the electrocatalyst composition and activity can be identified, as it has for the ORR in aqueous media. In addition, given that the COOR and MOR coincide with oxidation of ft surfaces, it may be natural to assume that inclusion of a readily oxidisable metal into the Pt electrocatalyst can aid in lowering the reaction overpotential but such work is yet to be done. 

Due to the faeile poisoning effect of CO on Pt, many Pt-based binary alloys, such as Pt-Ru, Pt-Os, Pt-Sn, Pt-W, Pt-Mo, and so on, have been investigated as electrocatalysts for the methanol oxidation reaction (MOR) on flic DMFC anode. Among them, the Pt-Ru alloy has been found to be the most active binary alloy catalyst, and is commonly used in state-of-the-art DMFCs [32]. 

The work presented shows that an increase of the electrocatalytic activity can be obtained, if a suitable method for the catalyst synthesis is employed. In this sense, the Alcohol Reduction Method showed a positive effect, probably due to the good particle dispersion at the carbon surface and the suitable particle size distribution that this method produces. For the methanol oxidation results, an increase in the cell potential by PtRu/C electrocatalyst on Vulcan XC72 system was observed compared to the PtRu/C E-TEK formulation. This can be explained due to the better conductivity of this Carbon Suport, enhancing the speed of the electron transference in the Methanol Oxidation Reaction (MOR).These results can also be attributed to the good particle distribution at... 

The electrochemical rate constants for hydrogen peroxide reduction have been found to be dependent on the amount of Prussian blue deposited, confirming that H2O2 penetrates the films, and the inner layers of the polycrystal take part in the catalysis. For 4-6 nmol cm of Prussian blue the electrochemical rate constant exceeds 0.01cm s [12], which corresponds to the bi-molecular rate constant of kcat = 3 X 10 L mor s [114]. The rate constant of hydrogen peroxide reduction by ferrocyanide catalyzed by enzyme peroxidase was 2 x 10 L mol s [116]. Thus, the activity of the natural enzyme peroxidase is of a similar order of magnitude as the catalytic activity of our Prussian blue-based electrocatalyst. Due to the high catalytic activity and selectivity, which are comparable with biocatalysis, we were able to denote the specially deposited Prussian blue as an artificial peroxidase [114, 117]. 

This explains the higher methanol tolerance of the alloy material in relation to that of pure Pt/C. For Pt-free electrocatalysts, PCI4C01/C showed to be very active for the ORR even at a high concentration of methanol. The addition of noble metal such as Au, Ag and Pt onto the PdCo material, in order to increase their stability in acid electrolyte, conducts to a lowered MOR activity and high ORR kinetics. For the RuSe/C and RhS/C materials, the former presents a considerable tolerance to the presence of methanol. However, the observed loss of selenium from the surface, observed upon exposure to potentials greater than 0.85 V, indicates a detrimental effect on the implementation of RuSe/C as a cathode material in fuel cell applications. The commercially available rhodium sulphide underperforms and exhibits higher susceptibility to methanol compared to RuSe/C, but it is more stable under similar testing conditions. 

Pt-Ru electrocatalysts are generally considered to be the most active binary catalysts for the MOR. Several commercial Pt-Ru alloy nanoparticles supported on carbon black have been available for applications in DAFCs. The catalytic effect has been observed using different kinds of Pt-Ru materials, such as adsorbed Ru on bulk Pt [46, 47], UHV-evaporated Ru on bulk Pt [46, 47], Pt-Ru electrodeposits [48, 49], Pt-Ru alloys [50-60], and Pt-RuOj [63-66]. 

Pt-Sn electrocatalysts only show modest improvement in catalyzing MOR compared to pure Pt catalysts, despite the superior performance of Sn as a cocatalyst for enhancing CO oxidation [86]. Generally, comparisons between ft-Ru and Pt- n catalysts indicated that the former are more active for the MOR, and DMFCs with Pt-Ru/C anode catalysts demonstrated substantially better performance compared to one with Pt-Sn/C catalysts under similar operating conditions [62, 87-89]. 

One should note that poisoning of PEMFC anode catalysts by CO is also a severe problem as CO is found to some extent in most H2 gas supplies, as H2 is usually produced by steam reforming of CH4 (and CO is a by-product). It has been reported that a CO content as low as 10 ppm in H2 fuel will result in the poisoning of Pt electrocatalysts [74], As shown in Eqs. 17.8 and 17.9, the formation of OHads by water oxidation at the Pt surface is necessary for the oxidative removal of adsorbed CO. However, the formation of Pt-OH only occurs appreciably above 0.8 V vs. RHE [75]. This factor is considered to be the origin of the high overpotentials for the MOR and COOR and, often, a second metal that can provide oxide species at low potentials is added to Pt electrocatalysts to reduce such overpotentials. For example, Pt-based alloys containing elements such as Ru, Mo, W, and Sn have been used in attempts to speed up the electrocatalysis of methanol [70,76,77]. The Pt-Ru alloy (1 1 atomic ratio) is the most active binary catalyst and is most frequently used as the anode catalyst in DMFCs [78]. Ru is more easily oxidised than Pt and is able to form Ru-oxide adsorbates at 0.2 V vs. RHE, thereby promoting the oxidation of CO to CO2, as summarised in Eqs. 17.11-17.13 ... 

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... 

Electrocatalysts advocated for methanol oxidation at the anode and oxygen reduction at the cathode in DMFCs are required to possess well-controlled structure, dispersion, and compositional homogeneity [46 9]. The electrocatalytic activities of both anode and cathode catalysts are generally dependent on numerous factors such as particle size and particle size distribution [50-54], morphology of the catalyst, catalyst composition and in particular its surface composition [55,56], oxidation state of Pt and second metal, and microstructure of the electrocatalysts [49,57,58]. With the frequently attempted surface manipulation strategies for nanosized electrocatalysts to increase their catalytic efficiencies toward MOR and ORR, rigorous characterization techniques which can provide information about nanoscale properties are critically required. For example, parameters such as particle size and variation in surface composition have strong influence on catalytic efficiency. Further, if the nanoparticles are comprised of two or more metals, both the composition and the actual distribution will... 

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