Electrocatalyst deactivation

Other technical hurdles must be overcome to make fuel cells more appealing to automakers and consumers. Durability is a key issue and performance degradation is usually traceable to the proton exchange membrane component of the device. Depending on the application, 5,000 40,000 h of fuel cell lifetime is needed. Chemical attack of the membrane and electrocatalyst deactivation (due to gradual poisoning by impurities such as CO in the feed gases) are critical roadblocks that must be over come. 

With both reactions occurring at the same rate, a surface composition of CjHjOj would correspond to these strongly adsorbed species. However, other compositions with noninteger i, p, and q are likely, depending on the reactivity of each site. The oxidation of these intermediates is considered difficult, resulting in electrocatalyst deactivation 195). Formation of [CHO] and [COOH] has also been postulated for the reverse reaction, that is, the electroreduction of COj 202, and for formic acid electrooxidation 201, 203,204. One should remember that species [CHO] was also proposed as a surface intermediate of alkane oxidations (type I), reacting only with difficulty (Section IV,E,1). 

Although Reactions (41)—(44) are usually viewed as electrocatalyst deactivation steps 195, 199, surface reactions of these intermediates with adsorbed [HjO] or [OH] have also been postulated for the overall oxidation process of CH4, CHjOH, HCHO, and HCOOH 81, 206, 207. The presence of certain proposed intermediates such as [COOH], [CjOjH], and... 

The activation observed in titania-supported Au electrocatalysts is unlikely to arise from electronic effects in monolayer or bilayer Au [Valden et al., 1998 Chen and Goodman, 2004], since the electrocatalytic activity was correlated with the size of three-dimensional titania-supported Au particles [Guerin et al., 2006b Hayden et al., 2007a, c]. The possibility that titania-induced electronic modification of three-dimensional particles below 6.5 nm is responsible for the induced activity, however, could not be excluded. It was pointed out, though, that such electronic effects should dominate for the smaller particle regime (<3 nm), where deactivation of the Au is observed on all supports. 

The kinetics of electrochemical reactions are often modified by the nature of the electrode material, and by the presence of atomic and molecular species either adsorbed on the surface or in the bulk solution [14]. Electrocatalysis is primarily concerned with the study of this phenomenon and, particularly, with the factors that govern enhancements in the rates of redox processes. Implicit in this general statement is the ability of the species responsible for these effects, or electrocatalyst, or the electrode itself, to carry out the reaction numerous times before undergoing possible deactivation. Electrocatalytic processes in which the electrode simply serves as a source or sink of electrons to generate solution phase species that... 

The operation of electrocatalysts in an electrolyte and in the presence of an electric field imposes stringent conditions on maintaining activity. Avoiding catalyst aging or deactivation is of great economic importance for the successful application of electrochemical processes. Deactivation can arise because of ... 

Reduction of unsaturated hydrocarbons and alcohols is similarly retarded by Cl , Br , and I ions 34) to the extent that reaction may become diffusion controlled 246). The anions SO4 and Cl affect hydrocarbon and alcohol oxidations by shifting the formation of C HpOq intermediate to more positive potentials 199, 247). Of course, the Type II oxygenated intermediate does itself deactivate the electrocatalyst due to its low reactivity. 

The time-dependent decline of reaction rate (and potential) of a porous electrocatalyst could be modeled in analogy to gas phase deactivation (267-268. Figure 19 shows a partly poisoned pore of a gas diffusion electrode. If Wp is the surface concentration of the poison in the poisoned part of the pore (moles per unit area of the catalyst) and Cp(x) is the local poison concentration in the pore, the one-dimensional continuity equation for the poison yields... 

Figure 20 shows the decline of the rate in a catalytic pore with time. Such behavior is often observed in gas phase catalytic reactions 264). Similar behavior is anticipated for constant potential operation of electrocatalysts. However, if the reaction is diffusion controlled h > 0), deactivation proceeds... 

For surface structure studies, perhaps the most popular technique has been LEED (373). Elastically diffracted electrons from a monoenergetic beam directed to a single-crystal surface reveal structural properties of the surface that may differ from those of the bulk. Some applications of LEED to electrocatalyst characterization were cited in Section IV (106,148,386). Other, less specific, but valuable surface examination techniques, such as scanning electron microscopy (SEM) and X-ray microprobe analysis, have not been used in electrocatalytic studies. They could provide information on surface changes caused by reaction, some of which may lead to catalyst deactivation (256,257). Since these techniques use an electron beam, they can be coupled with previously discussed methods (e.g. AES or XPS) to obtain a qualitative mapping of the structure and composition of a catalytic surface. 

Conventional heterogeneous catalysis and empiricism could provide a starting point in the selection of electrocatalysts for new unexplored processes for chemical production, energy generation or conservation, and environmental control. However, a fundamental understanding of adsorption characteristics, electrode kinetics, mechanisms, adsorbate-support interactions, and deactivation processes are needed for improved electrocatalyst... 

Hydrogen produced via the decomposition of methanol would be suitable for use in fuel cells, except for the fact that carbon monoxide impurities in the hydrogen would poison the electrocatalysts. The addition of a purification unit would of course add further complexity to the overall fuel-cell system, to such an extent that the adoption of methanol as a hydrogen carrier is no longer considered to be viable for vehicular applications. Serious attempts are being made to develop fuel cells that run on methanol directly see Section 6.4, Chapter 6. A similar poisoning problem has to be tackled which, in this case, arises from deactivation of the electrocatalyst by intermediate species formed during methanol electro-oxidation. 

Recently, there has been an increasing interest in the development of fuel cells. However, the major problems of electrocatalyst in fuel cells are the high loading of Pt and deactivation of Pt electrocatalyst [1,2], On the other hand, since the discovery of carbon nanotubes (CNTs), extensive research in the fields of applied physics, chemistry, materials science and engineering has rapidly emerged. Formaldehyde, as one of the intermediate products of methanol oxidation, can be activated to decompose to smaller fragments, protons, electrons and CO2 at high efficiency. 

Pd nanoparticles supported on PANI-NFs are efficient semi-heterogeneous catalysts for Suzuki coupling between aryl chlorides and phenylboronic acid, the homocoupling of deactivated aryl chlorides, and for phenol formation from aryl halides and potassium hydroxide in water and air [493], PANl-NF-supported FeCl3 as an efficient and reusable heterogeneous catalyst for the acylation of alcohols and amines with acetic acid has been presented [494]. Vanadate-doped PANI-NFs and PANI-NTs have proven to be excellent catalysts for selective oxidation of arylalkylsulfides to sulfoxides under nuld conditions [412]. Heterogeneous Mo catalysts for the efficient epoxidation of olefins with ferf-butylhydroperoxide were successfully synthesized using sea urchin-Uke PANI hollow microspheres, constructed with oriented PANI-NF arrays, as support [495]. Pt- and Ru-based electrocatalyst PANI-NFs—PSSA—Ru—Pt, synthesized by the electrodeposition of Pt and Ru particles into the nanofibrous network of PANI-PSSA, exhibited an excellent electrocatalytic performance for methanol oxidation [496]. A Pt electrode modified by PANI-NFs made the electrocatalytic oxidation reaction of methanol more complete [497]. Synthesis of a nanoelectrocatalyst based on PANI-NF-supported... 

Among the oxygen reduction catalysts, Pt, Pt-based catalyst, or Ag dispersed in carbon is the most powerful catalyst. However, there are some problems such as the cost for the material, cohesion of the metals, and deactivation from dropping out of the catalyst from the base electrode. Nano-sized perovskite-type oxide electrocatalyst, which could be synthesized by a wet chemical route, is one of the most... 

Depending on cell design and construction, the sealing material can come into contact with the catalyst layer of the membrane electrode assembly in these cases care must be taken that the material does not contain any substances which in the presence of the catalyst lead to unwanted side reactions. The sealing materials must neither contain any components which can act as catalyst poisons which will contaminate and thus deactivate the electrocatalyst, leading to reduced cell efficiency. 

During the experiments, the An electrode is at the open circuit potential which is established at a macroscopic blank area of Au located several millimeters away from the enzyme-modified regions. From thermodynamic considerations, one might expect H2O2 to be oxidized at the Au surface. However, this reaction occurs only at high overpotentials or requires an electrocatalyst. The electrocatalytic properties of noble metal surfaces are deactivated rapidly by the chemisorption of thiolates or other organic molecules therefore, H2O2 diffuses into the solution and can be detected by the microelectrode. 

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