Ordered supported electrocatalysts

Electrocatalysts One of the positive features of the supported electrocatalyst is that stable particle sizes in PAFCs and PEMFCs of the order of 2-3 nm can be achieved. These particles are in contact with the electrolyte, and since mass transport of the reactants occurs by spherical diffusion of low concentrations of the fuel-cell reactants (hydrogen and oxygen) through the electrolyte to the ultrafine electrocatalyst particles, the problems connected with diffusional limiting currents are minimized. There has to be good contact between the electrocatalyst particles and the carbon support to minimize ohmic losses and between the supported electrocatalysts and the electrolyte for the proton transport to the electrocatalyst particles and for the subsequent oxygen reduction reaction. This electrolyte network, in contact with the supported electrocatalyst in the active layer of the electrodes, has to be continuous up to the interface of the active layer with the electrolyte layer to minimize ohmic losses. 

The effects of dispersion of the electrocatalyst and of particle size on the kinetics of electrooxidation of methanol have been the subject of numerous studies because of the utilization of carbon support in DMFC anodes. The main objective is to determine the optimum size of the platinum anode particles in order to increase the effectiveness factor of platinum. Such a size effect, which is widely recognized in the case of the reduction of oxygen, is still a subject of discussion for the oxidation of methanol. According to some investigators, an optimum of 2 nm for the platinum particle size exists, but studying particle sizes up to 1.4 nm, other authors observed no size effect. According to a recent study, the rate of oxidation of methanol remains constant for particles greater than 4.5 nm, but decreases with size for smaller particles (up to 2.2 nm). 

The direct electrochemical deposition methods for the preparation of electrocatalysts allow to localize the catalyst particles on the top surface of the carbon support, as close as possible to the solid polymer electrolyte and does not need heat (oxidative and/or reducing) treatment, as most of the chemical methods do, in order to clean the catalytic particles from surfactant contamination [27,28], This will prevent catalyst sintering due to the agglomeration of nanoparticles under thermal treatment. 

Once a carbon support has been prepared, it is desirable to post-treat it to modify the surface structure in order to confer certain properties. Since the electrocatalyst is to be platinum on the carbon, even dispersion of the platinum crystallites over the carbon surface and minimal loss of surface area during fuel cell operation are important concerns. 

In the present work, CO2 electrochemical reduction was examined on higji area metal electrocatalysts supported on activated carbon fibers (ACF), which contain slit-shaped pores with widths on the order of nanometers. Such electrocatalysts were used in the form of gas difiusion electrodes (GDE), which are used in the fuel-cell field. The structure of this type of electrode is shown in Figure 1. The reaction takes places at the gas phase / electrolyte (liquid phase) / electrode interface, the so-called three-phase boimdary. 

Carbon supported Pt and Pt-alloy electrocatalysts form the cornerstone of the current state-of-the-art electrocatalysts for medium and low temperature fuel cells such as phosphoric and proton exchange membrane fuel cells (PEMECs). Electrocatalysis on these nanophase clusters are very different from bulk materials due to unique short-range atomic order and the electronic environment of these cluster interfaces. Studies of these fundamental properties, especially in the context of alloy formation and particle size are, therefore, of great interest. This chapter provides an overview of the structure and electronic nature of these supported... 

As discussed before the amount of platinum catalyst used in electrocatalysts of a PEM fuel cell is supported on carbon in the form of nanoparticles, in order to... 

Oxygen Electrocatalytic Properties Oxygen Reduction. Figure 8 compares steady-state polarization curves for the electroreduction of Op on a typical pyrochlore catalyst, Pb2(Rui.42Pbo.53)06.5 15 w/o platinum on carbon. The latter was considered representative of conventional supported noble metal electrocatalysts. The activities of both catalysts are quite comparable. While the electrodes were not further optimized, their performance was close to the state of the art, considering that currents of 1000 ma/cm could be recorded, at a relatively moderate temperature (75 C) and alkali concentration (3M KOH). Also, the voltages were not corrected for electrolyte resistance. The particle size of the platinum on the carbon support was of the order of 2 nanometers, as measured by transmission electron microscopy. 

Recently, taking advantage of the very narrow size distribution of the metal particles obtained, microemulsion has been used to prepare electrocatalysts for polymer electrolyte membrane fuel cells (PEMFCs) Catalysts containing 40 % Pt Ru (1 1) and 40% Pt Pd (1 1) on charcoal were prepared by mixing aqueous solutions of chloroplatinic acid, ruthenium chloride and palladium chloride with Berol 050 as surfactant in iso-octane. Reduction of the metal salts was complete after addition of hydrazine. In order to support the particles, the microemulsion was destabilised with tetrahydrofurane in the presence of charcoal. Both isolated particles in the range of 2-5 nm and aggregates of about 20 nm were detected by transmission electron microscopy. The electrochemical performance of membrane electrode assemblies, MEAs, prepared using this catalyst was comparable to that of the MEAs prepared with a commercial catalyst. 

Many ORR experiments have been made on electrocatalysts composed by Pt and Pd with the addition of non-noble metals, such as Co, Fe, and Ni. However, under electrochemical conditions these non-noble metals might leach out from the electrocatalyst, as demonstrated in previous investigations [25]. In order to avoid this problem, Yang and co-authors [26] have investigated PdPt-based electrocatalysts for the ORR in absence and in the presence of methanol, because the long-term stability of Pd in acidic solution is comparable to that of Pt (but this depends on the potential - additionally, Pt may stabilize Pd atoms in the alloy). It was report a novel strategy for surface and structure-controlled synthesis of carbon-supported Pd3Pti nanoparticles for the ORR as well as for methanol-tolerant ORR electrocatalyst. The influence of the surface composition and structure of the PdsPti/C on the ORR activity in the absence and presence of methanol was also reported. 

It has been reported in the course of this review that a recent study of the targets of a costless electrocatalyst to replace Pt in automotive applications requires that such non-noble metal catalysts have an activity no less than 1/lOth of the current industrial Pt activity under equivalent conditions. This requires mainly a sizeable increase in the site density (defined as catalytic sites/cm in the electro-catalytic layer) of the non-noble metal catalysts. A knowledge of the molecular structure of the catalytic site for the electrochemical reduction of oxygen in acid medium is, therefore, essential in order to increase the site density on the carbon support for those catalysts. The long-term stabilities of the same catalysts under current industrial conditions are yet to be demonstrated, as weU. 

On most of the electrocatalysts, oxygen reduction takes place by the formation of high-energy intermediate, peroxide, which is then further reduced to H2O. This is probably a consequence of the high stability of the 0—0 bond, which has a dissociation energy of 494 kj mol. In contrast, the dissociation energy of the 0—0 bond in H2O2 is only 146 kJ mor In order to obtain maximum efficiency and to avoid corrosion of carbon supports and other materials by peroxide, it is desired to achieve a... 

One of the most practical applications of RDE/RRDE techniques is the evaluation of ORR electrocatalysts in terms of both their activity and stability for fuel cells or metal-air batteries. In order to increase the catalyst surface area and the catalyst utilization, all catalysts developed are normally supported on conductive support particles such as active carbon. In the... 

In order to get answers to these questions, the ability to better characterize catalysts and electrocatalysts in situ under actual reactor or cell operating conditions (i.e., operando conditions) with element specificity and surface sensitivity is crucial. However, there are very few techniques that lend themselves to the rigorous requirements in electrochemical and in particular fuel cell studies (Fig. 1). With respect to structure, in-situ X-ray diffraction (XRD) could be the method of choice, but it has severe limitations for very small particles. Fourier transform infra red (FTTR), " and optical sum frequency generation (SFG) directly reveal the adsorption sites of such probe molecules as CO," but cannot provide much information on the adsorption of 0 and OH. To follow both structure and adsorbates at once (i.e., with extended X-ray absorption fine stmcture (EXAFS) and X-ray absorption near edge stmc-ture (XANES), respectively), only X-ray absorption spectroscopy (XAS) has proven to be an appropriate technique. This statement is supported by the comparatively large number of in situ XAS studies that have been published during the last decade. 16,17,18,19,20,21,22,23,24,25 highly Versatile, since in situ measme-... 

---