Design of electrocatalysts

Thus, in potential sweep work, when the electrode potential is changed too quickly, the various intermediate radicals of the reaction will not correspond to the 0 s of the steady-state functioning of the reaction and the information obtained, and hence the mechanism determined from it may be irrelevant in providing the information needed for, say, the design of electrocatalysts to last for 3-6 months of continuous production with the reaction in the steady state.4 Thus, there is not much point in carrying out an academic mechanism investigation (mostly done with fast sweeps) because the time domains may be too short for radical buildup to the steady state. 

Lambert reviews the role of alkali additives on metal films and nanoparticles in electrochemical and chemical behavior modihcations. Metal-support interactions is the subject of the chapter by Arico and coauthors for applications in low temperature fuel cell electrocatalysts, and Haruta and Tsubota look at the structure and size effect of supported noble metal catalysts in low temperature CO oxidation. Promotion of catalytic activity and the importance of spillover are discussed by Vayenas and coworkers in a very interesting chapter, followed by Verykios s examination of support effects and catalytic performance of nanoparticles. In situ infrared spectroscopy studies of platinum group metals at the electrode-electrolyte interface are reviewed by Sun. Watanabe discusses the design of electrocatalysts for fuel cells, and Coq and Figueras address the question of particle size and support effects on catalytic properties of metallic and bimetallic catalysts. 

Design of Electrocatalysts for Fuel Cells Masahiro Watanabe... 

The parallels between Schemes 1 and 2 illustrates that a knowledge of stoichiometric reactions can be utilized in the design of electrocatalysts. In particular, the use of electrochemical reductions to generate metal hydride complexes could result in a number of different types of catalytic reductions depending on the nature of the substrate. 

Among the computational approaches, study of adsorption abilities (reactant, intermediate, and product) is the most widely employed approach in understanding catalyst activity and the design of electrocatalysts. The interaction between catalyst and reaction species governs the reaction, and adsorption energy is relatively easier to compute than reaction energy and activation energy, especially the latter, which is computationally expensive. 

With this in mind, this chapter focuses on the most recent advances on the designing of electrocatalysts for the complete electrooxidation of ethanol in add medium. The base catalyst comprises carbon supported nanosized Pt partides which, as inferred from the results given in the previous sections, lead to the preferential formation of acetic acid and acetaldehyde in acid medium. Comprehensive details about the synthesis and characterization of the electrocatalyst described in this section will be omitted and instead we shall focus on the performance of the catalyst for the ethanol electrooxidation reaction. 

In the equation 25 n is the number of electrons and F, k and C(02) are Faraday constant, rate constant and bulk O2 concentration, respectively. In addition, P and y are symmetry factors, while E is the electrode potential. The term 0ad relates to total surface coverage by OHads and adsorbed anions. The effect of surface oxides on the metal electrode surface was clearly demonstrated for series of Pt-based electrocatalysts [53, 54], leading to a general recipe for design of electrocatalysts with improved ORR activity. In specific, if oxide formation is hindered onset potential for ORR is shifted to higher anodic potentials. Underlying principles of this route have been set by combining electrochemical measurement... 

Appreciable interest was stirred by the sucessful use of nonmetallic catalysts such as oxides and organic metal complexes in electrochemical reactions. From 1968 on, work on the development of electrocatalysts on the basis of the mixed oxides of titanium and ruthenium led to the fabrication of active, low-wear electrodes for anodic chlorine evolution which under the designation dimensionally stable anodes (DSA) became a workhorse of the chlorine industry. 

Significant (and even spectacular) results were contributed by the group of Norskov to the field of electrocatalysis [102-105]. Theoretical calculations led to the design of novel nanoparticulate anode catalysts for proton exchange membrane fuel cells (PEMFC) which are composed of trimetallic systems where which PtRu is alloyed with a third, non-noble metal such as Co, Ni, or W. Remarkably, the activity trends observed experimentally when using Pt-, PtRu-, PtRuNi-, and PtRuCo electrocatalysts corresponded exactly with the theoretical predictions (cf. Figure 5(a) and (b)) [102]. 

Only a limited number of true metal complex electrocatalysts have been proposed for proton reduction due to the difficulty inherent in the bielectronic nature of this reaction. It is obvious that the design of such electrocatalysts must take into account the lowering of the overpotential for proton reduction, the stability of the catalytic system, and the regeneration of the starting complex. 

Synchotron based techniques, such as surface X-ray scattering (SXS) and X-ray absorption spectroscopy (XAS), have found increased use in characterization of electrocatalysts during electrochemical reactions.37 These techniques, which can be used for characterization of surface structures, require intricate cell designs that can provide realistic electrochemical conditions while acquiring spectra. Several examples of the use of XAS and EXAFS in non-precious metal cathode catalysts can be found in the literature.38 2... 

Y. D. Jin, Y. Shen, and S. J. Dong, Electrochemical design of ultrathin platinum-coated gold nanoparticle monolayer films as a novel nanostructured electrocatalyst for oxygen reduction, J. Phys. Chem. B 108, 8142-8147 (2004). 

The electrolyte in the AFC is concentrated (85 wt%) KOH in cells designed for operation at high temperature ( 260°C), or less concentrated (35-50 wt%) KOH for lower temperature (<120°C) operation. The electrolyte is retained in a matrix (usually asbestos), and a wide range of electrocatalysts can be used (e.g., Ni, Ag, metal oxides, spinels and noble metals). 

The latter usually are electronic factors, whereas the former often result in geometric effects. It is therefore necessary to be able to distinguish between the two effects to establish with certainty some predictive basis for the design and optimization of electrocatalysts. 

The search for new electrode materials is expected to be guided by the fundamental understanding of the factors governing the activity. In electrochemistry, this branch of the discipline is known by the name of electrocatalysis . Strictly speaking, electrocatalysis is the science devoted to the relationship between the properties of materials and the electrode reaction rate. The scope of electrocatalysis as a science is to establish a predictive basis for the design and the optimization of electrocatalysts. 

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...

The results of primary screening data are used to determine the design of a more focused electrocatalyst library, thereby beginning the next cycle of the primary workflow. In each subsequent cycle, the examined experimental variables become more and more limited as the experimental results move toward an optimized set of process parameters or an optimized catalyst composition. There are several strategies as to how best design the next cycle of catalyst libraries to maximize the... 

Fig. 11.8 Schematic of the automated primary high-throughput electrochemical workflow employed at Symyx Technologies for the combinatorial development of new fuel cell catalysts. Individual steps of the workflow include choice of catalyst concept, design of appropriate materials library using Library Studio [31], synthesis of electrocatalyst library on electrode array wafer, XRD and EDX characterization of individual electrocatalysts before screening, high-throughput parallel electrochemical screening of library, XRD and EDX characterization of catalysts after screening, data processing and evaluation.

Fig. 11.15 Design of an electrocatalyst library of Pt-Ru-Co alloys for a more focused examination of the ternary com position space. The pie-chart character of each catalyst represents its chemical composition, with pure Pt in the upper left corner. The design was created using Library Studio [31],...

The rational design of improved electrocatalyst materials requires first and foremost a fundamental understanding of the origin of the kinetic overpotential. Over... 

It is necessary to discuss four scientific topics for phosphoric acid fuel cells. Those interconnected topics are the design of the precious metal electrocatalyst properties of the phosphoric acid properties of the matrix, and those of the carbon catalyst support. 

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