Anode electrocatalysis

Electrocatalysis Again by definition, an electrocatalyst is a solid, in fact an electrode, which can accelerate a process involving a net charge transfer, such as e.g. the anodic oxidation of H2 or the cathodic reduction of 02 in solid electrolyte cells utilizing YSZ ... 

I.V. Yentekakis, Y. Jiang, S. Neophytides, S. Bebelis, and C.G. Vayenas, Catalysis, Electrocatalysis and Electrochemical Promotion of the Steam Reforming of Methane over Ni Film and Ni-YSZ cermet Anodes, Ionics 1, 491-498 (1995). 

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]. 

Dinh FIN, Ren X, Garzon FTF, Zelenay P, Gottesfeld S. 2000. Electrocatalysis in direct methanol fuel cells in-situ probing of FTRu anode catalyst surfaces. J Electroanal Chem 491 ... 

Enzymes are efficient catalysts for cathodic and anodic reactions relevant to fuel cell electrocatalysis in terms of overpotential, active site activity, and substrate/reaction specificity. This means that design constraints (e.g., fuel containment and anode-cathode separation) are relaxed, and very simple devices that may take up ambient fuel or oxidant from their environment are possible. While operation is generally confined to conditions close to ambient temperature, pressure, and pH, and power densities over about 10 mW cm are rarely achieved, enzyme fuel cells may be particularly useM in niche environments, for example scavenging trace H2 released into air, or sugar and O2 from blood. Thus, trace or unusual fuels become viable for energy production. 

The electrode reaction of an organic substance that does not occur through electrocatalysis begins with the acceptance of a single electron (for reduction) or the loss of an electron (for oxidation). However, the substance need not react in the form predominating in solution, but, for example, in a protonated form. The radical formed can further accept or lose another electron or can react with the solvent, with the base electrolyte (this term is used here rather than the term indifferent electrolyte) or with another molecule of the electroactive substance or a radical product. These processes include substitution, addition, elimination, or dimerization reactions. In the reactions of the intermediates in an anodic process, the reaction partner is usually nucleophilic in nature, while the intermediate in a cathodic process reacts with an electrophilic partner. 

Electrocatalysis with nickel-bpy complexes has been shown useful for synthetic applications,202,211 especially when used in combination with the sacrificial anode process in an undivided cell (Equation (45)).207,211 Under these very simple experimental conditions, efficient nickel catalysts can be also generated in the presence of the cheap pyridine ligand.212... 

Figure 6.7 illustrates the voltammetric response of the third-generation SOD-based 02 biosensors with Cu, Zn-SOD confined onto cystein-modified Au electrode as an example. The presence of 02" in solution essentially increases both the cathodic and anodic peak currents of the SOD compared with its absence [150], Such a redox response was not observed at the bare Au or cysteine-modified Au electrodes in the presence of 02". The observed increase in the anodic and cathodic current response of the Cu, Zn-SOD/cysteine-modified Au electrode in the presence of 02 can be considered to result from the oxidation and reduction of 02, respectively, which are effectively mediated by the SOD confined on the electrode as shown in Scheme 3. Such a bi-directional electromediation (electrocatalysis) by the SOD/cysteine-modified Au electrode is essentially based on the inherent specificity of SOD for the dismutation of 02", i.e. SOD catalyzes both the reduction of 02 to H202 and the oxidation to 02 via a redox cycle of its Cu (II/I) complex moiety as well as the direct electron transfer of SOD realized at the cysteine-modified Au electrode. Thus, this coupling between the electrode and enzyme reactions of SOD could facilitate the development of the third-generation biosensor for 02". ...

At IREQ, besides the participation in the field tests run by the engineers of Hydro-Quebec (12), the main effort has been to tackle fundamental problems in the field of electrocatalysis (18-22) and of anodic oxidation of different potential fuels (23-26). A careful and extensive study of the electrochemical properties of the tungsten bronze has been carried out (18-20) the reported activity of these materials in acid media for the oxygen reduction could not be reproduced and this claim by other workers has been traced back to some platinum impurities in the electrodes. Some novel techniques in the area of electrode preparation are also under study (21,22) the metallic deposition of certain metals on oriented graphite show some interesting catalytic features for the oxygen reduction and also for the oxygen evolution reaction. 

DMFCs and direct ethanol fuel cells (DEFCs) are based on the proton exchange membrane fuel cell (PEM FC), where hydrogen is replaced by the alcohol, so that both the principles of the PEMFC and the direct alcohol fuel cell (DAFC), in which the alcohol reacts directly at the fuel cell anode without any reforming process, will be discussed in this chapter. Then, because of the low operating temperatures of these fuel cells working in an acidic environment (due to the protonic membrane), the activation of the alcohol oxidation by convenient catalysts (usually containing platinum) is still a severe problem, which will be discussed in the context of electrocatalysis. One way to overcome this problem is to use an alkaline membrane (conducting, e.g., by the hydroxyl anion, OH ), in which medium the kinetics of the electrochemical reactions involved are faster than in an acidic medium, and then to develop the solid alkaline membrane fuel cell (SAMFC). 

Wendt, H. and Plzak, V. (1990) Electrode kinetics and electrocatalysis of hydrogen and oxygen electrode reactions. 2. Electrocatalysis and electrocatalysts for cathodic evolution and anodic oxidation of hydrogen, in Electrochemical Hydrogen Technologies (ed. H. Wendt), Elsevier, Amsterdam, Chapter 1. 2. 

Electrocatalysis in fuel cells requires as well substances capable of catalyzing the anodic oxidation of fuels as catalysts for the cathodic reduction of oxygen. Several dyestuffs that catalyze oxygen reduction are known, but up to now only one has been described as active in anodic reactions. All these dyestuffs are N4-chelates. 

Erdey-Gruz, 1048, 1306 1474 Erschler, 1133, 1134, 1425 Ethylene oxidation, anodic, 1052 1258 Exchange current density, 1049, 1066 correction of, 1069 definition, 1053 electrocatalysis and, 1278 impedance and, 1136 interfacial reaction, 1047 and partly polarizable interface, 1056 Excited states, lifetime, 1478 Exothermic reaction, 1041 Ex situ techniques, 785, 788, 1146... 

Cells can be made in which the cathode-anode distance is only 10-3 cm. Such cells have the advantage that the total impurity present is veiy small and may not be enough to cover more than 0.1% of the electrode surface if they were all adsorbed. Thus, suppose the impurity concentration were 10-6 mol liter-1 or 10-9 mol cc 1 or 10 12 mol in the cell Because an electrode surface can cany (at most) about 10-9 mol cm-2, the maximum fraction of the surface covered with impurity molecules is 0.1%. Does work with thin-layer cells eliminate the inpurity problem in electrode kinetics It improves it. However, active sites on catalysts may occupy less than 0.1% of an electrode and preferentially attract newly arriving impurities, so that even thin-layer cells may not entirely avoid the impurity difficulty,32 particularly if the electrode reaction concerned (as with most) involves adsorbed intermediates and electrocatalysis. 

These results are most informative as far as electrode structures and electrocatalysis are concerned. They show clearly that the two peaks on the polyciystal arise because the crystal consists of a mixture of 111, 110, and 100 planes. Evidently, our earlier crude perception that H is uniformly adsorbed in the electrode must be radically modified. It is adsorbed heterogeneously, and it ionizes from the 111 and 100 planes of the polycrystal sequentially, in relation to the potential when the latter is swept anodically. 

Electrocatalysis of electrochemical chlorine evolution from Ru02-coated titanium anodes rests on the same grounds according to the so-called Krasil shchikov mechanism, which is schematically described by the reactions (8a) and (8b) for anodic chlorine evolution (9-/2) ... 

B. Electrocatalysis and Selectivity of Anodic Chlorine Evolution at Ru02-Anodes... 

Electrocatalysis of the Anodic Oxygen Evolution by Raney-Nickel Coatings... 

V. Electrocatalysis of Cathodic Oxygen Reduction and Anodic Hydrogen Oxidation in Fuel Cells... 

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