Electrocatalyst metal-oxide

Alkaline Fuel Cell. The electrolyte ia the alkaline fuel cell is concentrated (85 wt %) KOH ia fuel cells that operate at high (- 250° C) temperature, or less concentrated (35—50 wt %) KOH for lower (<120° C) temperature operation. The electrolyte is retained ia a matrix of asbestos (qv) or other metal oxide, and a wide range of electrocatalysts can be used, eg, Ni, Ag, metal oxides, spiaels, and noble metals. Oxygen reduction kinetics are more rapid ia alkaline electrolytes than ia acid electrolytes, and the use of non-noble metal electrocatalysts ia AFCs is feasible. However, a significant disadvantage of AFCs is that alkaline electrolytes, ie, NaOH, KOH, do not reject CO2. Consequentiy, as of this writing, AFCs are restricted to specialized apphcations where C02-free H2 and O2 are utilized. 

The overpotentials for oxygen reduction and evolution on carbon-based bifunctional air electrodes for rechargeable Zn/air batteries are reduced by utilizing metal oxide electrocatalysts. Besides enhancing the electrochemical kinetics of the oxygen reactions, the electrocatalysts serve to reduce the overpotential to minimize... 

Metal oxides, 31 78-79, 89, 102, 123, 157-158, 191, 32 199-121 see also Amorphous metal oxides Sulfate-supported metal oxides specific oxides adsorbed oxygen on, 27 196-198 binary, surface acidity, 27 136-138 catalytic etching, 41 390-396 coordination number, 27 136 electrocatalysts, 40 127-128 Fe3(CO)i2 reaction with, 38 311-314 Lewis acid-treated, 37 169-170 multiply-valent metals, electrocatalytic oxidations, 40 154-157 superacids by, 37 201-204 surface acidity, methods for determining, 27 121... 

Alkaline Fuel Cell (AFC) The electrolyte in this fuel cell is concentrated (85 wt%) KOH in fuel cells operated at high temperature ( 250°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 fuel supply is limited to non-reactive constituents except for hydrogen. CO is a poison, and CO2 will react with the KOH to form K2CO3, thus altering the electrolyte. Even the small amount of CO2 in air must be considered with the alkaline cell. 

The band structure of oxides is very important for their behavior as electrocatalysts through the role of surface states and chemisorption of intermediates at their surfaces. When a crystalline solid is terminated by a surface, a new set of electronic states appears associated with the surface which are a continuation from the bulk band structure of the solid. These surface states are d-band surface states on transition metal oxides, which play a vital role... 

The use of gas diffusion electrodes is another way to achieve high current densities. Such electrodes are used in the fuel-cell field and are typically made with porous materials. The electrocatalyst particles are highly dispersed inside the porous carbon electrode, and the reaction takes place at the gas/liquid/solid three-phase boundary. COj reduction proceeds on the catalyst particles and the gas produced returns to the gas compartment. We have used activated carbon fibers (ACF) as supports for metal catalysts, as they possess high porosity and additionally provide extremely narrow (several nm) slit-shaped pores, in which nano-space" effects can occur. In the present work, encouraging results have been obtained with these types of electrodes. Based on the nanospace effects, electroreduction under high pressure-like conditions is expected. In the present work, we have used two types of gas diffusion electrodes. In one case, we have used metal oxide-supported Cu electrocatalysts, while in the other case, we have used activated carbon (ACF)-supported Fe and Ni electrocatalysts. In both cases, high current densities were obtained. 

There are several physical and chemical characteristics of these oxide pyrochlores which may contribute to their high electrocatalytic activity. The previously described alkaline solution synthesis technique (6,7) provided these materials with surface areas typically ranging from 50 to 200 m /g. Thus, one of the basic requirements for an effective electrocatalyst has been satisfied the electrocatalytic activity is not limited by the unavailability of catalytically active surface sites, as is so often the case with metal and mixed metal oxides. 

It is known that deposition on substrates where nonconductive metal oxide layers are formed at positive potentials is difficult. This includes electrodes made of stainless steel, tantalum, and aluminum. However, the rate of oxide formation is important because we have shown that stainless steel is an ideal substrate for deposition of some PPy s. The rate of oxide formation depends on the solution and the electrochemical conditions employed. If the polymer can deposit before the onset of metal oxide formation, then consistent films of excellent quality can be deposited. This is aptly demonstrated with the successful electrocoating of aluminum when the electrocatalyst Tiron 4 is used. 

The hydrated Nafion membrane currently used in SPE cells provides a highly acidic environment, equivalent to a 10 wt% H SOif solution (13). Thus, noble metals or noble metal oxides have to be used as electrocatalysts. It is of particular interest to develop an anion exchange membrane which will transport hydroxyl ions under water electrolysis or fuel cell conditions. This implies that the cell environment would be alkaline, which would enable the substitution of these expensive... 

Corrosion of the material used is another factor that limits the selection of the electrocatalyst. The electrochemical corrosion of pure noble metals is not as important as in the case of binary or ternary alloys in strong acid or alkaline solutions, since these catalysts are widely used in electrochemical reactors. In the case of anodic bulk electrolysis, noble metal alloys used in electrocatalysis mainly contain noble metal oxides to make the oxidation mechanism more favorable for complete electron transfer. The corrosion problem that occurs from this type of catalyst is the auto-corrosion of the electrode surface instead of the electrode/electrolyte solution interface degradation. The problem of corrosion is considered in detail in Chapter 22. 

Since the electrocatalytic reaction implies the existence of an adsorbed species as an intermediate, reactant, or product, the direct interaction with the electrode surface has to be considered first. In this sense, the kinetics of the formation and the stability of the adsorbate are of great importance and may be the determining step for the final value of The slow adsorption kinetics in the case of a reactive adsorbate will make the reaction at the electrocatalyst not fast enough to become operative. However, the same situation can occur in the case of an adsorbed product with a slow desorption kinetics. The most problematic situation can arise due to the stability of an adsorbed intermediate on the surface, which is the rate-determining step of the whole process. In the case of an anodic process, the species desorption can be aided by the presence of a metal oxide on the surface. An interesting example of stable and efficient anodes is the dimensionally stable electrodes (DSE) used in brine... 

Jung, S. McCrory, C. Ferrer, I. M. Peters, J. C. Jaramillo, T. F. Benchmarking Nanoparticulate Metal Oxide Electrocatalysts for the Alkaline Water Oxidation Reaction. J. Mater. Chem. A, 2015, DOI 10.1039/C5TA07586F... 

Thus the cell reaction is spontaneous under standard-state conditions. Note that the reaction is the same as the hydrogen combustion reaction, but the oxidation and reduction are carried out separately at the anode and the cathode. Like platinum in the standard hydrogen electrode, the electrodes have a twofold function. They serve as electrical conductors, and they provide the necessary surfaces for the initial decomposition of the molecules into atomic species, prior to electron transfer. They are electrocatalysts. Metals such as platinum, nickel, and rhodium are good electrocatalysts. 

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