Catalyst Synthesis Techniques

Christina Bock, Helga Halvorsen and Barry MacDougall 

This chapter deals with aspects of the synthesis of fuel cell catalysts. Practical catalysts for low-temperature fuel cells are typically in the nano-size range and are frequently formed or deposited on high-surface-area supports. Pt is the most eommonly used eatalyst for both cathode and anode in proton exchange membrane fuel eells (PEMFCs). In the case of the cathode, combined catalyst systems such as Pt nanoparticles supported on Au or Pt alloy catalysts, as well as Pt-skin catalysts formed in combination with the iron group metals have also attracted attention. Much work has been carried out on the development of non-noble metal (Pt-ffee) catalysts, the synthesis of which will be discussed in Section 9.5. In the case of the anode, bi-metallie eatalysts are typically employed unless the fuel is neat H2. Pt-Ru is the state-of-the-art catalyst for both methanol and reformate fuel cells. For the latter, other anode catalysts such as Pt/MoOx and Pt/Sn are also considered promising. 

In this chapter we present an overview of methods used to synthesize fuel cell catalysts, with a focus on catalysts for PEMFCs and direct methanol fuel cells (DMFCs). Examples for the synthesis of catalysts for other low-temperature fuel cells, such as formic acid, are not included in this ehapter. The synthesis of both Pt-based and Pt-free catalysts is discussed, and dedieated sections describe methods that allow control of catalyst size and composition. First, general catalyst synthesis methods are introduced. Subsequently, the reader is introduced to particle size and stracture control of fuel cell catalysts. 

The purpose of this sub-section is to present to the reader with a general overview of the most common synthetic methods for fabricating Pt-based catalysts. Although no single method is superior to the others, depending on the end application of the catalyst and the instrumentation available, one method may be advantageous over 

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C. Bock, H. Halvorsen, B. MacDougall, 2008, Catalyst Synthesis Techniques in PEM Fuel Cell Electrocatalysts and Catalyst Layers, J. Zhang (Ed.), Springer, p. 471. 

Some apphcations require PE with a very high molecular weight nearly 10 times that of common PE materials. These resins are essentially nonbranched and require special catalysts, synthesis, and fabrication techniques. 

An important breakthrough in that respect was the use of soHd-phase organic synthesis (SPOS) where the attachment of the substrate to an insoluble support allowed for easy workup (filtration) and for rapid generation of products via split-mix procedures [1,2]. An important subsequent development consisted of the immobihzation of reagents, scavengers and catalysts. This technique, coined polymer-assisted solution phase chemistry (PASP), allowed solution phase synthesis of compoimds, yet still enjoying the bene-... 

As vitally important as the capabilities for experimental planning, screening, and data analysis are the procedures for preparation of inorganic catalysts. In contrast to the procedures usually applied in conventional catalyst synthesis, the synthetic techniques have to be adapted to the number of catalysts required in the screening process. Catalyst production can become a bottleneck and it is therefore necessary to ensure that HTE- and CombiChem-capable synthesis technologies are applied to ensure a seamless workflow. 

Depending on the synthesis technique, CNTs may contain various impurities, including fullerenes and irregular carbon structures on the surface (i.e. amorphous) as well as residual salts and metal catalysts often encapsulated within a carbon shell. 

The use of precursor synthesis techniques as described above is driven by the fact that decomposition of the precursor material into the catalyst often results in catalysts that have activity or selectivity superior to that of preferred products. The amine thiomolybdate decomposition described above results in MoS2 catalysts with surface areas exceeding several hundred square meters per gram (27). The HDS activity increases correspondingly and unpromoted catalysts have activities approaching those of promoted systems. 

Cobalt-silicon bonds, in hydridocobalt complexes, 7, 5 Cobalt—tin bonds, in hydridocobalt complexes, 7, 5 Co-catalyst effects, in olefin polymerization, 4, 1111 (—)-Goccinine, via Alder-ene reactions, 10, 593 Co-condensation sites, in metal vapor synthesis technique,... 

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