Carbon-supported electrocatalysts

Fig. 11.9 Photograph of a 16-channel parallel rotating disk electrode (PRDE) test station for high-throughput screening of high surface-area carbon-supported electrocatalysts. The PRDE station is part of the secondary screening workflow for electrocatalysts.

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 right choice of a carbon support greatly affects cell performance and durability. The purpose of this chapter is to analyze how structure and properties of carbon materials influence the performance of supported noble metal catalysts in the CLs of the PEMFCs. The review chapter is organized as follows. In Section 12.2 we give an overview of carbon materials utilized for the preparation of the catalytic layers of PEMFC. We describe traditional as well as novel carbon materials, in particular carbon nanotubes and nanofibers and mesoporous carbons. In Section 12.3 we analyze properties of carbon materials essential for fuel cell performance and how these are related to the structural and substructural characteristics of carbon materials. Sections 12.4 and 12.5 are devoted to the preparation and characterization of carbon-supported electrocatalysts and CLs. In Section 12.6 we analyze how carbon supports may influence fuel cell performance. Section 12.7 is devoted to the corrosion and stability of carbon materials and carbon-supported catalysts. In Section 12.8 we provide conclusions and an outlook. Due to obvious space constraints, it was not possible to give a comprehensive treatment of all published data, so rather, we present a selective review and provide references as to where an interested reader may find more detailed information. 

The current state of the art carbon supported electrocatalyst is made using variants of the colloidal approach. A common approach is to dissolve the metal salt solution in an appropriate solvent followed by reduction to form a colloid. A wide variety of recipes using reducing agents, organic stabilizers, or shell-removing approaches have also been developed in recent years. The patents most frequently referred to in this field are from United Technologies by Petrow and Allen.Sols of the metal are obtained for instance by an initial formation of a metastable platinum-sulfito complex, which is inert at ambient temperature but decomposes and produces the small Pt-crystallites at temperatures in excess of 60°C. Thus a relatively well-defined crystal size between 2 to 6 nm can be obtained. 

The impregnation-reduction method has been frequently used for the synthesis of PtSn supported on inorganic carriers such as SiOg, AlgOs, or SAPO, but this approach has rarely been employed for the synthesis of carbon supported electrocatalysts. ° In general, the metal content in those samples is ca. 1-2 wt%, well below the demands of a state of the art fuel-cell electrocatalyst. A number of routes have been explored for the synthesis of carbon supported bimetallic PtSn samples. In general, they lead to materials composed of a wide range of phases, such as metallic and/or oxide Pt, Sn oxides, or PtSn solid solutions of different stoichiometry. 

Rodriguez Varela FJ, Savadogo O (2008) Catalytic activity of carbon-supported electrocatalysts for direct ethanol fuel cell applications. J Electrochem Soc 155 B618-B624... 

The Cr(2p), Co(2p), and Ni(2p) X-ray photoelectron spectra for the samples were also studied, and the oxidation states of Cr, Co, and Ni as well as their relative intensities were obtained. From these data it was found that the Pt-Co/C sample had the lowest overall oxidizing components among the binary- and ternary-alloy electrocatalysts. Surface atomic ratios for Cr Pt, Co Pt, and Ni Pt of the carbon supported electrocatalysts, obtained from their respective X-ray photoelectron spectra, are summarized in Table 10.5. The results indicate some surface enrichment of platinum metal in all the binary-alloy electrocatalysts, namely Pt-Cr/C, Pt-Co/C, and Pt-Ni/ C. However, a surface enrichment of base metals was found in the ternary-alloy electrocatalysts, as can be seen from Table 10.5. The results suggest a higher electrocatalytic activity towards the oxygen... 

Papageorgopoulos DC, Keijzer M, de Bruijn FA. The inclusion of Mo, Nb and Ta in Pt and PtRu carbon supported electrocatalysts in the quest for improved CO tolerant PEMFC anodes. Electrochim Acta 2002 48 197-204. 

Spray-based methods can produce carbon-supported electrocatalyst powders with a controlled aggregate size and aggregate size distribution. The hierarchical structure of an electrocatalyst powder batch is illustrated in Figure 20.2. 

Papageorgopoulos, D.C., M. Keijzer, J.B.J. Veldhuis, and F.A. de Bruijn. 2002. CO tolerance of Pd-rich platinum palladium carbon-supported electrocatalysts. J. Electrochem. Soc. 149 A1400-A1404. 

Ahluwalia RK, Arisetty S, Wang X et al (2013) Thermodynamics and kinetics of platinum dissolution from carbon-supported electrocatalysts in aqueous media under potentiostatic and potentiodynamic conditions. J Eletrcochem Soc 160 F447-F455... 

On bulk Pt-M alloys, where M is a 3d-transition metal, the specific activity for the ORR is enhanced by two to four times with respect to pure Pt [121,136-143]. Interestingly, the enhancement factor is maintained on nanometer size carbon-supported electrocatalysts [121,140,141,144-146]. Nevertheless, the long-term stability of Pt-alloy electrocatalysts remains questionable in the harsh operating conditions of a PEMFC. Dubau et al. [147-149] showed that PtsCo/C electrocatalysts suffer compositional changes at the nanoscale in real PEMFC... 

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