Preparation of Fuel Cell Catalysts

Recently it was proposed that PEMLC electrocatalysts may also be prepared by water-in-oil microemulsions. These are optically transparent, isotropic, and thermodynamically stable dispersions of two nonmiscible liquids. The method of particle preparation consists of mixing two microemulsions carrying appropriate reactants (metal salt + reducing agent), to obtain the desired particles. The reaction takes place during the collision of water droplets, and the size of the particles is controlled by the size of the droplets. Readers are referred to the early work of Boutonnet et al. [149], the review paper of Capek [150] and refs. [128,151], and 152 for fuel cell apphcations. The carbonyl route has the ability to control the stoichiometry between bimetallic nanoparticles, but also the particle size. The reader is referred to review papers for more details [106,107]. Other methods, including sonochemical and radiation-chemical, have been used successfully for the preparation of fuel cell catalysts (see, e.g., review articles 100 and 153). 

Colloidal Pt/RuO c- (C5 0.4nm) stabilized by a surfactant was prepared by co-hydrolysis of PtCU and RuCls under basic conditions. The Pt Ru ratio in the colloids can be between 1 4 and 4 1 by variation of the stoichiometry of the transition metal salts. The corresponding zerovalent metal colloids are obtained by the subsequent application of H2 to the colloidal Pt/Ru oxides (optionally in the immobilized form). Additional metals have been included in the metal oxide concept [Eq. (10)] in order to prepare binary and ternary mixed metal oxides in the colloidal form. Pt/Ru/WO c is regarded as a good precatalyst especially for the application in DMECs. Main-group elements such as A1 have been included in multimetallic alloy systems in order to improve the durability of fuel-cell catalysts. PtsAlCo.s alloyed with Cr, Mo, or W particles of 4—7-nm size has been prepared by sequential precipitation on conductant carbon supports such as highly disperse Vulcan XC72 [70]. Alternatively, colloidal precursors composed of Pt/Ru/Al allow... 

Preparation and control of the size and structure of fuel cell catalysts in the nanosize range using templates synthesis methods has attracted some attention. The best known synthesis method is probably the deposition of metal films from lyotropic liquid crystalline mixtures developed to by Phil Bartlett s group [71-73]. They have used the method to deposit films of Pt, Pt/Ru alloys, and other metals. 

Physical characterization of low-temperature fuel cell catalysts is very important for the exploration and preparation of novel catalysts. The emergence of some new techniques, like electrochemical mass spectroscopy (EMS), and continuing progress in some traditional techniques, like SEM and TEM, will effectively improve the characterization of fuel cell catalysts. 

The catalytic applications of Moiseev s giant cationic palladium clusters have extensively been reviewed by Finke et al. [167], In a recent review chapter we have outlined the potential of surfactant-stabilized nanocolloids in the different fields of catalysis [53]. Our three-step precursor concept for the manufacture of heterogeneous egg-shell - nanocatalysts catalysts based on surfactant-stabilized organosols or hydrosols was developed in the 1990s [173-177] and has been fully elaborated in recent time as a standard procedure for the manufacture of egg-shell - nanometal catalysts, namely for the preparation of high-performance fuel cell catalysts. For details consult the following Refs. [53,181,387]. 

From the above experimental results, it can be seen that the both PtSn catalysts have a similar particle size leading to the same physical surface area. However, the ESAs of these catalysts are significantly different, as indicated by the CV curves. The large difference between ESA values for the two catalysts could only be explained by differences in detailed nanostructure as a consequence of differences in the preparation of the respective catalyst. On the basis of the preparation process and the CV measurement results, a model has been developed for the structures of these PtSn catalysts as shown in Fig. 15.10. The PtSn-1 catalyst is believed to have a Sn core/Pt shell nanostructure while PtSn-2 is believed to have a Pt core/Sn shell structure. Both electrochemical results and fuel cell performance indicate that PtSn-1 catalyst significantly enhances ethanol electrooxidation. Our previous research found that an important difference between PtRu and PtSn catalysts is that the addition of Ru reduces the lattice parameter of Pt, while Sn dilates the lattice parameter. The reduced Pt lattice parameter resulting from Ru addition seems to be unfavorable for ethanol adsorption and degrades the DEFC performance. In this new work on PtSn catalysts with more... 

Table 7.3 Catalysts prepared from different precursors and used in present study of fuel cells...

The influence of the applied potential on the XAS of PtRu fuel cell catalysts is also apparent in data collected under fuel cell conditions. Viswanathan et al. reported XANES data obtained at both the Pt L3 and Ru K edges for a 1 1 PtRu/C catalyst prepared as a Nafion bound MEA. They found that both the Pt and Ru were metallic in both the freshly prepared ME As and ME As under operating conditions. 

Preparation and Characterization of CNFs and Fuel Cell Catalysts 73 3.2... 

The preparation of Cu/ZnO catalysts and precursors for the methanol synthesis reaction have been described [87, 88], while others [89] used a mixture of Pt, Ru and a leachable metal such as A1 to prepare catalysts for CO-tolerant catalysts for fuel cells. 

Pt superfine clusters on conductive supports are effective catalysts of redox reactions proceeding in fuel cells. High specific surface, support conductivity, high dispersity (nanosizes of Pt clusters) and their strong fixation on a surface are necessary criterions of preparation of the effective catalyst. From these points of view CNM for example single- (SWNT) and multi-walled (MWNT) nanotubes, nanofibers (CNF) and x-ray amorphous carbon (AC) can be a successful supports of Pt clusters. 

Preparation of the Working Electrodes for Catalyst/Catalyst Layer Studies In fuel cell catalyst/catalyst layer down-selection, the process of preparing the working electrodes includes several steps ... 

Yoshitake, T. et al.. Preparation of fine platinum catalyst supported on single-wall carbon nanohorns for fuel cell application, Physica B, 323, 124, 2002. 

Several authors [298-300] reported on the preparation of platimun and palladium nanoparticles having a preferential diameter from about 1 to 2 nm stabihzed by polymeric carbo-, phospho-, and sulfobetaines for fuel-cell catalysts and vinyl acetate production. 

Recent work by Lukehart et al. has demonstrated the applicability of this technique to fuel-cell catalyst preparation [44g,h]. Through the use of microwave heating of an organometallic precursor that contains both Pt and Ru, PtRu/Vulcan carbon nanocomposites have been prepared that consist of PtRu alloy nanoparticles highly dispersed on a powdered carbon support [44g]. Two types of these nanocomposites containing 16 and 50 wt.% metal with alloy nanoparticles of 3.4 and 5.4 nm, respectively, are formed with only 100 or 300 s of microwave heating time. The 50 wt.% supported nanocomposite has demonstrated direct methanol fuel-cell anode activity superior to that of a 60 wt.% commercial catalyst in preliminary measurements. 

Using the simple hydrolysis/condensation of metal salts under basic aqueous conditions in the presence of carbo- or sulfobetaines, respectively, colloidal Pt02 was obtained, the analytical data of which correspond to pure a-Pt02 and to commercial samples of Adams catalyst. Bi- and trimetallic colloidal metal oxides as precursors for fuel-cell catalysts, e.g., colloidal Pt/RuO and Pt/Ru/WO, have also been prepared this way [5If]. 

A preparation and characterization of new PtRu alloy colloids that are suitable as precursors for fuel-cell catalysts have been reported [43cj. This new method uses an organometallic compound both for reduction and as colloid stabilizer leading to a Pt/Ru colloid with lipophilic surfactant stabilizers that can easily be modified to demonstrate hydrophilic properties. The surfactant shell is removed prior to electrochemical measurements by reactive annealing in O2 and H2. This colloid was found to have nearly identical electrocatalytic activity to several other recently developed Pt/Ru colloids as well as commercially available Pt/Ru catalysts. This demonstrates the potential for the development of colloid precursors for bimetallic catalysts especially when considering the ease of manipulating the alloy composition when using these methods. 

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