Carbon as Support Material in Fuel Cell Electrocatalysts

Other methods which can also be apphed include electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR). Information on the charge carrier concentration and their mobihty can be obtained by EPR thus rendering a deeper insight into the conduction behavior of carbons [14]. NMR furthermore provides structural information, such as quantitative sp /sp ratio comparable to Raman and XPS. 

Even more valuable than the ex situ characterization of carbon materials with the aforementioned methods, is their in situ characterization during operation or at least under conditions very similar to their application. However, not all measurements can be applied in situ, as for instance in XPS, the mean free wavelength of the electrons in not-vacuum conditions is too low and in TEM ultrahigh vacuum (UHV) is required to detect the electrons. Consequently, electron probe techniques relying on UHV are not (or only with immense effort) applicable in situ [26]. Novak et al [27] give an overview of advanced in situ characterization applied to carbonaceous materials for Li-ion batteries. 

Depending on the final application of carbon materials, very different properties and structures on different length scales need to be analyzed in detail using the aforementioned methods and, most favorably, a combination of them. A systematic Raman, TEM, SEM, NEXAFS, and EPR study of various carbon materials for impregnation into carbon felts used as positive electrode in all-vanadium redox flow batteries is given by Melke et al. [14]. 

Of importance to fuel cell applications is the electron conductivity of carbons, when used as electron pathway in the porous electrode. Moreover, their functional groups with respect to hydrophobicity and water transport phenomena are also important. Also their defect density plays a role, when these defects function as nucleation sites during the synthesis of nanoparticulate catalysts and also when anchoring the particles to hinder Ostwald ripening during degradation. This chapter is further subdivided into three parts corresponding to the carbon s three main functions in polymer electrolyte membrane fuel cell (PEMFC)  

3) Carbon as structure-forming element in porous fuel cell electrodes. 

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Carbon as Support Material in Fuel Cell Electrocatalysts 251 7.1.2.4 Other Methods and In situ Studies... 

Carbon as Support Material in Fuel Cell Electrocatalysts... 

Fig. 15.25 Pathways for future electrocatalyst development for automotive PEMFCs. (a) Thick films or bulk single crystal and polycrystalline catalysts that are ideal for fundamental studies on surface structure and mechanisms these materials need to be modified into (c) and (d) to be applicable to fuel cells, (b) Typical commercial nanoparticles (2-4 nm) on a high-surface-area carbon support used in fuel cells at this time (c) Thin continuous films of catalyst on a support such as carbon nanotubes that may provide a physical porous structure for mass transport in a fuel cell (d) Core-shell catalysts where only the shell eonsists of precious metals and are supported on a typical high-surface-area support [72, 77, 89]...

Electronic interaction and synergistic effects between catalysts and the support material have been investigated in the context of fuel-cell electrocatalysts. Electron spin resonance (ESR) has been used to demonstrate the electron donation by Pt to carbon [11] support. This has been further supported by XPS studies [12], which show that the metal acts as an electron donor to the support, their interaction depending on their respective Eermi levels. Bogotsky and Snudkin [13] have shown that the characteristics of the electrical double layer formed between the microdeposit (Pt) and the support depends to a certain extent on the difference in the work function of Pt (5.4 eV) and carbon support (pyrolytic support 4.7 eV), thereby resulting in an increase of the electron density of Pt. However, the rise in the electron density can be significant only when the particle size of the microdeposit is comparable to the thickness of the double layer. 

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. 

Carbon supports strongly affect fuel cell performance. They may influence the intrinsic catalytic activity and catalyst utilization, but also affect mass transport and ohmic losses. This makes analyses of the role of carbon materials rather complicated. Although numerous studies have been devoted to the carbon support improvement, only a few have attempted to establish relationships between the substructural characteristics of carbon support materials and cell performance. The influence of carbon supports on the intrinsic catalytic activity is the subject of Section 12.6.1. In Section 12.6.2 we consider the influence of support on macrokinetic parameters such as the catalyst utilization, mass transport, and ohmic losses. In Section 12.6.3 we review briefly recent data obtained upon utilization of novel carbon materials as supports for fuel cell electrocatalysts. 

For polymer electrolyte membrane fuel cell (PEMFC) applications, platinum and platinum-based alloy materials have been the most extensively investigated as catalysts for the electrocatalytic reduction of oxygen. A number of factors can influence the performance of Pt-based cathodic electrocatalysts in fuel cell applications, including (i) the method of Pt/C electrocatalyst preparation, (ii) R particle size, (iii) activation process, (iv) wetting of electrode structure, (v) PTFE content in the electrode, and the (vi) surface properties of the carbon support, among others. ... 

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. 

F. MaUlard, P. Simonov, E.R. Savinova, Carbon materials as support for fuel cells electrocatalysts, in P. Serp,... 

Recently, rhodium and ruthenium-based carbon-supported sulfide electrocatalysts were synthesized by different established methods and evaluated as ODP cathodic catalysts in a chlorine-saturated hydrochloric acid environment with respect to both economic and industrial considerations [46]. In particular, patented E-TEK methods as well as a non-aqueous method were used to produce binary RhjcSy and Ru Sy in addition, some of the more popular Mo, Co, Rh, and Redoped RuxSy catalysts for acid electrolyte fuel cell ORR applications were also prepared. The roles of both crystallinity and morphology of the electrocatalysts were investigated. Their activity for ORR was compared to state-of-the-art Pt/C and Rh/C systems. The Rh Sy/C, CojcRuyS /C, and Ru Sy/C materials synthesized by the E-TEK methods exhibited appreciable stability and activity for ORR under these conditions. The Ru-based materials showed good depolarizing behavior. Considering that ruthenium is about seven times less expensive than rhodium, these Ru-based electrocatalysts may prove to be a viable low-cost alternative to Rh Sy systems for the ODC HCl electrolysis industry. 

Carbon is unique among chemical elements since it exists in different forms and microtextures transforming it into a very attractive material that is widely used in a broad range of electrochemical applications. Carbon exists in various allotropic forms due to its valency, with the most well-known being carbon black, diamond, fullerenes, graphene and carbon nanotubes. This review is divided into four sections. In the first two sections the structure, electronic and electrochemical properties of carbon are presented along with their applications. The last two sections deal with the use of carbon in polymer electrolyte fuel cells (PEFCs) as catalyst support and oxygen reduction reaction (ORR) electrocatalyst. 

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