Electrocatalysts synthesis

The recent study by Lakshmi et al. [63] is just one example of the very extensive research efforts devoted to the improvement of performance of fuel cells by increasing the dispersion of the electrocatalyst on the carbon support by virtue of carbon surface functionalization [64], Without acknowledging their familiarity with the most relevant prior studies, they did note that the point of neutral charge evaluation helps in identifying the platinum complex to be used for electrocatalyst synthesis based on their charge, in the sense that, for example, if the carbon surface is positively charged an anionic platinum complex is needed (see Section 5.2.1). 

The choice of the most appropriate preparation procedure relies on the following considerations. It is well known that the preparation procedure for electrocatalysts influences their physico-chemical properties and thus their activity. The performance characteristics of an electrocatalyst depend on its chemical composition (surface and bulk), structure and morphology. Accordingly, the selected methodology of electrocatalyst synthesis should allow one to address the process for the attainment of a proper structure (crystalline or amorphous) and with a chemical composition on the surface as close as possible to the nominal or bulk composition. 

Figure 20.4. Scauniag electron micrographs of a) aggregates obtained in a conventional electrocatalyst synthesis and b) aggregate size distribution of the electrocatalyst powders obtamed by spray conversion. (Images courtesy of NRC-IFCI.)...

Thus indeed CH4 oxidation in a SOFC with a Ni/YSZ anode results into partial oxidation and the production of synthesis gas, instead of generation of C02 and H20 as originally believed. The latter happens only at near-complete CH4 conversion. However the partial oxidation overall reaction (3.12) is not the result of a partial oxidation electrocatalyst but rather the result of the catalytic reactions (3.9) to (3.11) coupled with the electrocatalytic reaction (3.8). From a thermodynamic viewpoint the partial oxidation reaction (3.12) is at least as attractive as complete oxidation to C02 and H20. 

We have already referred to the Mo/Ru/S Chevrel phases and related catalysts which have long been under investigation for their oxygen reduction properties. Reeve et al. [19] evaluated the methanol tolerance, along with oxygen reduction activity, of a range of transition metal sulfide electrocatalysts, in a liquid-feed solid-polymer-electrolyte DMFC. The catalysts were prepared in high surface area by direct synthesis onto various surface-functionalized carbon blacks. The intrinsic... 

Solorza-Eeria O, EUmer K, Giersig M, Alonso-Vante N (1994) Novel low-temperature synthesis of semiconducting transition metal chalcogenide electrocatalyst for multielectron charge transfer Molecular oxygen reduction. Electrochim Acta 39 1647-1653... 

Gochi-Ponce Y, Alonso-Nunez G, Alonso-Vante N (2006) Synthesis and electrochemical characterization of a novel platinum chalcogenide electrocatalyst with an enhanced tolerance to methanol in the oxygen reduction reaction. Electrochem Commun 8 1487-1491... 

Nickel oxide anodes are another example for a relatively simple oxide electrocatalyst used rather widely in the oxidation of organic substances (alcohols, amines, etc.) in alkaline solutions at relatively low anodic potentials (about +0.6 V RHE). These processes, which occur at an oxidized nickel surface, are rather highly selective. As an example, we mention the industrial oxidation of diacetone-L-sorbose to the corresponding acid in vitamin C synthesis. This reaction occurs at nickel oxide electrodes with chemical yields close to 100%. 

The small metal particle size, large available surface area and homogeneous dispersion of the metal nanoclusters on the supports are key factors in improving the electrocatalytic activity and the anti-polarization ability of the Pt-based catalysts for fuel cells. The alkaline EG synthesis method proved to be of universal significance for preparing different electrocatalysts of supported metal and alloy nanoparticles with high metal loadings and excellent cell performances. 

Ni(cyclam)]2+ was shown to be an efficient electrocatalyst for the intramolecular cyclization-carboxylation of allyl or propargyl-2-haloaryl ethers,200 and for the synthesis of cyclic carbonates from epoxides and carbon dioxide.201... 

Nano-electrode arrays can be formed through nano-structuring of the electrocatalyst on an inert electrode support. Indeed, if the current of the analyte reduction (oxidation) on a blank electrode is negligible compared to the activity of the electrocatalyst, the former can be considered as an insulator surface. Hence, for the synthesis of nanoelectrode arrays one has to carry out material nano-structuring. Recently, an elegant approach [140] for the electrosynthesis of mesoporous nano-structured surfaces by depositioning different metals (Pt, Pd, Co, Sn) through lyotropic liquid crystalline phases has been proposed [141-143],... 

C.G. Tsiafoulis, P.N. Trikalitis, and M.I. Prodromidis, Synthesis, characterization and performance of vanadium hexacyanoferrate as electrocatalyst of H202. Electrochem. Commun. 7, 1398 (2005). 

Chromium (I I) salts are widely used in organic synthesis. However, the high sensitivity to air and the low solubility of Cr(III) salts in anhydrous solvents prevent their synthetic use as electrocatalysts. The electroreduction of both CO2 and CO to methanol has been achieved by the... 

Lee, K., Zhang, J., Wang, H., and Wilkinson, D. P. Progress in the synthesis of carbon nanotube- and nanofiber-supported Pt electrocatalysts for PEM fuel cell catalysis. Journal of Applied Electrochemistry 2006 36 507-522. 

PB and its derivatives are of interest for a variety of reasons, the most important of which is its electrochromism [93]. In addition, it is an electrocatalyst for several different types of substrates, notably hydrogen peroxide, as will be seen below. Synthesis of nanopartides of Prussian Blue is relatively straightforward. It relies on many of the prindples of colloid chemistry, and produces ionically stabilized colloidal solutions (Figure 4.7). As a consequence, the electrochemical behavior of PB N Ps has been examined by several groups. In this section, we discuss the behavior of P B N Ps immobilized at electrodes. 

Fig. 3. Schematic illustration of the synthesis of metal nanoparticles within dendrimer templates. The composites are prepared by mixing of the dendrimer and metal ion, and subsequent chemical reduction. These materials can be immobilized on electrode surfaces where they serve as electrocatalysts or dissolved in essentially any solvent (after appropriate end-group functionalization) as homogeneous catalysts for hydrogenation and other reactions...

Investigations of enzyme-catalyzed direct electron transfer introduce the basis for a future generation of electrocatalysts based on enzyme mimics. This avenue could offer new methods of synthesis for nonprecious metal electrocatalysts, based on nano-structured (for example, sol—gel-derived) molecular imprints from a biological catalyst (enzyme) with pronounced and, in some cases, unique electrocatalytic properties. Computational approaches to the study of transition state stabilization by biocatalysts has led to the concept of theozymes . " ... 

Combinatorial Synthesis and High-Throughput Screening of Fuel Cell Electrocatalysts... 

Fig. 11.2 A 64-element, individually addressable, Ti electrode array on a 3" diameter quartz wafer for the synthesis of electrocatalyst libraries.

Structural and compositional characterization of individual elements of a combinatorial library can be important for the initial validation of a particular combinatorial synthesis method. Many earlier reports on combinatorial synthesis and screening of electrocatalysts fall short of reporting the complete structural and compositional characterization of individual library elements of interest. The workflow described here includes catalyst characterization before and after screening, thereby establishing an activity-composition-structure-stability relationship for electrocatalysts. This can be relevant in light of the extreme conditions present in a conventional fuel cell environment. 

Fig. 11.4 Library design of a 64-element electrocatalyst library of Pt-Fe binary alloys. The square (Plate 1) and the 64 round spots represent the wafer substrate and the location of individual electrocatalyst alloys, respectively. The pie-chart character of each catalyst represents its chemi cal composition, ranging, from left to right, from 100% Pt to 100% Fe. Each row, A—H, is identical. During synthesis, this library design will be deposited onto the electrode array. The design was created using...

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