Electrooxidations discussion

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). 

A third way to increase both the active surface area and the number of oxygenated species at the electrode surface is to prepare alloy particles or deposits and then to dissolve the non-noble metal component. This technique, which is similar to that used to prepare Raney-type catalysts, yields very high surface area electrodes and hence some improvements in the electrocatalytic activities compared with those of pure platinum. However, it is always difficult to be sure whether the mechanism of enhancment of the activities is due to this effect or the possible presence of remaining traces of the dissolved metal. Results with PtyCr and PtSFe were encouraging, although the effect of iron is still under discussion. From studies in a recent work on the behavior of R-Fe particles for methanol electrooxidation, it was concluded that the electrocatalytic effect is due to the Fe alloyed to platinum. ... 

The electrosynthesis of metalloporphyrins which contain a metal-carbon a-bond is reviewed in this paper. The electron transfer mechanisms of a-bonded rhodium, cobalt, germanium, and silicon porphyrin complexes were also determined on the basis of voltammetric measurements and controlled-potential electrooxidation/reduction. The four described electrochemical systems demonstrate the versatility and selectivity of electrochemical methods for the synthesis and characterization of metal-carbon o-bonded metalloporphyrins. The reactions between rhodium and cobalt metalloporphyrins and the commonly used CH2CI2 is also discussed. 

The second type of porphyrin electrosynthesis discussed in this paper is controlled potential electrooxidation of a-bonded bis-alkyl or bis-aryl porphyrins of Ge(lV) and Si(IV). This electrooxidation results in formation of a-bonded mono-alkyl or mono-aryl complexes which can be isolated and characterized in situ. Again, cyclic voltammetry can be coupled with this method and will lead to an understanding of the various reaction pathways involved in the electrosynthesis. 

The electrodes in the direct methanol fuel cell (DMFC) (i.e. the anode for oxidising the fuel and the cathode for the reduction of oxygen) are based on finely divided Pt dispersed onto a porous carbon support, and the electrooxidation of methanol at a polycrystalline Pt electrode as a model for the DMFC has been the subject of numerous electrochemical studies dating back to the early years ot the 20th century. In this particular section, the discussion is restricted to the identity of the species that result from the chemisorption of methanol at Pt in acid electrolyte. This is principally because (i) the identity of the catalytic poison formed during the chemisorption of methanol has been a source of controversy for many years, and (ii) the advent of in situ IR culminated in this controversy being resolved. 

The chemisorption of sulfur from mixtures of H,S and H2 has been widely studied we have discussed some of the results. Nevertheless, introduction of irreversible and reversible adsorbed sulfur, which is in line with adsorption stoichiometries varying from more than 1 to 0.4 sulfur atom by accessible platinum atom, shows that different adsorbed species are involved in sulfur chemisorption. In fact, electrooxidation of adsorbed sulfur on platinum catalysts occurs at two different electrochemical potentials (42) in the same way, two different species of adsorbed sulfur were identified on gold by electrochemical techniques and XPS measurements (43,44). By use of 35S (45) it was pointed out that, according to the experimental conditions, reducible PtS2 or nonreducible PtS mono-layers can be created. 

Ralph etal. (2003) give, at Figure 2, a complex methanol electrooxidation process at a platinum particle, which highlights the extent of development problems. The same topic was discussed at Palm Springs by University of Washington authors. 

Different electron-conducting polymers (polyaniline, polypyrrole, polythiophene) are considered as convenient substrates for the electrodeposition of highly dispersed metal electrocatalysts. The preparation and the characterization of electronconducting polymers modified by noble metal nanoparticles are first discussed. Then, their catalytic activities are presented for many important electrochemical reactions related to fuel cells oxygen reduction, hydrogen oxidation, oxidation of Cl molecules (formic acid, formaldehyde, methanol, carbon monoxide), and electrooxidation of alcohols and polyols. 

Considering these conclusions, it was assumed that the reaction pathways for the electrooxidation of propargyl alcohol and allyl alcohol should comprise adsorbates with appreciably different structure and metal interactions. In order to get a better insight in these structures, the role of various reaction centers (acetylenic hydrogen, the atom containing the OH group the c donor property of the 7t-bond, etc.) in the formation of adsorbates are discussed. 

In this section we will discuss the role of surface modification to enhance electrocatalytic oxidation of methanol, one of the interesting components for fuel cell technology. Perhaps the most successful promoter of methanol electrooxidation is ruthenium. Pt/Ru catalysts appear to exhibit classical bifunctional behavior, whereas the Pt atoms dissociate methanol and the ruthenium atoms adsorb oxygen-containing species. Both platinmn and ruthenimn atoms are necessary for eomplete oxidation to occur at a significant rate. The bifunctional mechanism can account for a decrease in poisoning from methanol, as observed for Pt/Ru alloys. Indeed, CO oxidation has been attributed to a bifimctional mechanism that reduces the overpotential of this reaction by 0.1 V on the Pt/Ru surface. 

The redox reactions of dopamine, norepinephrine, and 6-hydroxy-dopamine and related compounds discussed above have been elucidated primarily by electrochemical techniques. Such studies clearly provide some real insights into the ways in which these compounds and intermediate species formed upon electrooxidation might be involved in certain types of neurochemical behavior. 

Gas-phase quantum chemistry (QC) calculations of CO and OH adsorption on Pt-based anodes provide valuable information on structure and energetics of adsorbates (for e.g. see [15, 19, 21]). A detailed review of CO adsorption calculation was presented by Fiebelman and co-workers [15]. Detailed CO and OH adsorption calculations on Pt-based electrodes have also been reported (for e.g. see [19] and references therein). Potential effects on CO binding energy and frequency have been discussed in detail by Koper and coworkers [20]. However, these calculations do not attempt to investigate the mechanism of the CO electrooxidation. Anderson and co-workers have used first-principle QC chemistry and semi-empirical calculations to understand the effect of potential on fuel cell electrochemistry in general, and CO oxidation electrochemistry in particular [16, 32, 33]. 

In this section, we discuss anion and OH coadsorption on Pt(l 11) surface to elucidate the effect of competitive adsorption on CO oxidation electrocatalysis. We first perform coadsorption simulations to understand base voltam-mograms, i.e., in the absence of CO oxidation. Next, we show the effect of anions on CO electrooxidation by performing simulations of CO stripping voltammetry, where a monolayer of CO is oxidized by a potential sweep. 

This chapter discusses a staged multi-scale approach for understanding CO electrooxidation on Pt-based electrodes. In this approach, density functional theory (DFT) is used to obtain an atomistic view of reactions on Pt-based surfaces. Based on results from experiments and quantum chemistry calculations, a consistent coarse-grained lattice model is developed. Kinetic Monte Carlo (KMC) simulations are then used to study complex multi-step reaction kinetics on the electrode surfaces at much larger lengthscales and timescales compared to atomistic dimensions. These simulations are compared to experiments. We review KMC results on Pt and PtRu alloy surfaces. 

As mentioned above, the alcohol crossover from the anode to the cathode is a important problems to be overcome to improve the DAFC performance. This is due to the fact that the commonly used Pt-based cathode electrocatalysts are also active for the adsorption and oxidation of methanol [1]. So, in addition to the resulting mixed potential at the cathode, there is a decrease in the fuel utilization. Therefore, considering the above exposed reactions for the alcohol electrooxidation, and the features that govern the ORR electrocatalytic activity, as discussed in the Sect. 5.2, it is ready to conclude the importance of the modification of the active ORR electrocatalyst surfaces in order to inhibit the methanol or ethanol oxidative adsorption steps. In the next sections, some recent materials being developed to overcome the problems caused by the alcohol crossover will be presented. 

In particular, the electrooxidation of ethanol over Pt in acidic media has two major hmitations which prevent its viability as it was discussed by Koper [109]. The first relates to the fact that reaction predominantly produces acetate and acetic acid intermediates thus resulting in only 2 and 4 electrons respectively which are only very minor contributions to the possible current. Both are thus unwanted side products for fuel cell applications. The second limitation is that the path to CO2 is rather difficult in that it requires the activation of C-C bond as well as the oxidation of both the CHx and CO intermediates that form. Both of these intermediates tend to inhibit or poison metal surfaces at lower potentials [110]. 

The polymers described in Sect. 2.3 can be considered to be copolymers, and in many cases they are actually called copolymers. However, those polymers have been synthesized from monomers with polymerizable groups (e.g., thiophene), and the monomer already contains the redox functionality. The copolymers that will now be discussed have been prepared from two or more difierent monomers, which can also be electropolymerized separately, and the usual strategy is to mix the monomers and execute the electropolymerization of this mixed system. It should be mentioned that the structures of the copolymers have not been clarified unambiguously in many cases. Usually the cyclic voltammetric responses detected show the characteristics of both polymers, and so it is difficult to establish whether the surface layer consists of a copolymer or whether it is a composite material of the two polymers. However, several copolymers exhibit electrochemical behaviors that differ from the polymers prepared from the respective monomers. The properties of the copolymer depends on the molar ratio of the monomers (feed rate), and can be altered by other experimental conditions such as scan rate, pH, etc., since generally the electrooxidation of one of the comonomers is much faster than that of the other one (a typical example is the comonomer aniline, whose rate of electropolymerization is high even at relatively low positive potentials). In many cases the new materials have new and advantageous properties, and it is the aim of these studies to discover and explore these properties. We present a few examples below. 

Catalyst activity towards formic acid electrooxidation is strongly influenced by preparation method and nanoparticle size. As discussed in the previous chapter, the optimal sizes for Pt/C and Pd/C are 4 nm and 5.2-6.5 imi, as determined by Park et al. [14] and Zhou et al. [15], respectively. This chapter is segregated into two sections bimetallic catalysts and catalyst supports. The section on bimetallic catalysts is subdivided into adatoms, alloys, and intermetallics. 

To illustrate the primary effects of adatom addition, single-crystal electrodes are discussed here. Feliu and Herrero have extensively studied formic acid electrooxidation on Pt single-crystal substrates modified with an array of various adatoms. They have established a connection between the electronegativity of the adatoms in relation to Pt and the type of active enhancement mechanism incurred as a function of adatom coverage [42]. Their results support inhibition of the indirect pathway on Pt(lll) terraces and they have demonstrated that COads formation occurs at step and defect sites. For Pt(l 11) substrates decorated with electropositive adatoms, such as Bi, Pb, Sb, and Te, the electronic enhancement is extended to the second or third Pt atom shell from the adatom. While for electronegative adatoms, in respect to Pt, the third-body effect dominates with increased coverages, such as S and Se. 

While Pd is less expensive than Pt, the electrocatalytic activity of bulk polycrystalline Pd for ORR is at least five times lower than that of Pt, which prevents it from being used directly in fuel cells. Great efforts have been dedicated to improve the activity of Pd by surface modification and alloying. This chapter attempts to summarize the recent progress of electrocatalysts containing Pd for ORR. The development of Pd electrocatalysts for electrooxidation of hydrogen and small organic molecules is not discussed in this chapter but has been adequately covered in Chaps. 5 and 6 and recent reviews [5-7]. 

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