Spray-based and CVD processes for synthesis of fuel cell catalysts and thin catalyst layers

Abstract

In spite of many efforts and improvements by thousands of scientists worldwide, PEMFCs and DMFCs have not been commercially used. State-of-the-art electrocatalysts for PEMFCs rely on large quantities of platinum to achieve acceptable performance levels. This presents a significant hurdle to market acceptance of FC-powered vehicles; a commercially viable electrocatalyst will require nearly an order-of-magnitude reduction in Pt usage to meet both cost and Pt availability constraints. Pt fine particles are dispersed on carbon blacks with a large surface area to reduce the total Pt used and to enhance catalytic activity. This activity depends not only on the primary structure of the catalyst Pt/carbon composites (i.e., carbon surface area, Pt particle size, Pt surface area, etc.), but also on the secondary structure (i.e., aggregation and agglomeration of carbon grains). Catalyst production methods must meet commercial standards for mass manufacturing such as scalability, reproducibility, and quality. Conventional MEA manufacturing practices involve screen printing, flexographic printing, gravure printing, spraying or rolling, and calendering [1]. Electrocatalyst processing falls into two distinct manufacturing methods: electrocatalyst powder formation and in situ electrocatalyst formation. Powder formation is achieved either by more conventional solution precipitation/impregnation techniques or by newer spraybased methods such as spray pyrolysis and combustion chemical vapor deposition (CCVD). In situ electrocatalyst formation involves forming the composite platinum/carbon (Pt/C) catalyst directly on the proton exchange membrane (PEM) or gas diffusion layer (GDL) and includes techniques such as physical vapor deposition (PVD), chemical vapor deposition (CVD), combustion chemical vapor deposition, reactive spray deposition technology (RSDT) and electrochemical deposition. The drivers for each process are cost, performance, and reliability, which cannot be separated from the MEA because the electrode structure (porosity, thickness, catalyst type, ionomer content, kinetic losses, ohmic losses, and transport losses) is not only a function of the electrocatalyst itself but also of the way in which it is formed or deposited. Raw material cost reduction takes the form of improved catalyst utilization and alternative formulations, while manufacturing costs are driven down by reducing the number of processing steps, using continuous processing, and avoiding energy-or time-intensive steps such as vacuum environments. The electrocatalytic activity of Pt/C catalysts is influenced by many factors, such as average particle size, relative crystallinity, surface groups, surface morphology, and the Pt-C interface [2-6]. Therefore, when Pt/C catalysts are prepared using different methods or different carbon supports, many factors can influence their electrocatalytic activity, leading to different conclusions. Using Pt/C catalysts prepared by different methods, Attwood et al. [7] found that the best average size of Pt particles in a Pt/C catalyst for methanol oxidation is about 3 nm. Frelink et al. [8] indicated that the electrocatalytic activity of Pt/C catalysts for methanol oxidation decreases as the Pt particle size decreases in the range 4.5-1.2 nm. Takasu et al. [9] came to a similar conclusion using a Pt catalyst supported on glassy carbon electrodes with Pt particle sizes from 7 to 2 nm. Watanabe et al. [10] reported that no Pt particle size effect was observed when Pt/C catalysts with different sizes of Pt particles on different carbon supports were prepared by the same method. Direct deposition techniques can have many advantages over bulk ink processing techniques. For instance, a thin catalyst layer applied between the electrode and electrolyte is in the immediate neighborhood with respect to both the proton-conducting membrane and the electrode, and since at high cell current densities and gas permeability limitations thick catalyst layers are only active closest to the gas supply a thin layer eliminates platinum underutilization [11]. Multi-objective optimization techniques are necessary that can uncover Pareto optimal fronts and generate several acceptable choices in catalyst layer design or processing conditions. In terms of MEA cost reduction, novel electrocatalyst production technologies might enable minimization of the production steps typically followed in conventional "ink-based" deposition techniques. In situ electrocatalyst formation (e.g., by PVD, CVD, RSDT) offers the opportunity to reduce the number of steps and therefore the potential to reduce cost (provided the new processing costs are not higher) by avoiding ink-based deposition processes altogether [12-14]. In most CVD processes the CVD material is formed on the surface of a substrate, but in some cases the substrate can react with a deposit to form a compound. In cases where powder is desirable, high reaction concentrations and temperatures are deliberately used so that homogeneous nucleation will take place in the gas phase. The reaction gases in a CVD process are introduced separately, or premixed and passed into the reaction area, depending on whether the gases will react before they reach the substrate, which is to be avoided unless powder formation is desired [15, 16]. In this chapter, we review the spray-based powder production method, flamebased process, and CVD process for electrocatalyst powder production or direct film deposition, and describe the attributes of this process in terms of the nature of the catalyst produced, the particle size, and the influences on MEA layer structure, performance, and cost. © 2008 Springer-Verlag.