Cathode material catalytic activity

It is so universally applied that it may be found in combination with metal oxide cathodes (e.g., HgO, AgO, NiOOH, Mn02), with catalytically active oxygen electrodes, and with inert cathodes using aqueous halide or ferricyanide solutions as active materials ("zinc-flow" or "redox" batteries). The cell (battery) sizes vary from small button cells for hearing aids or watches up to kilowatt-hour modules for electric vehicles (electrotraction). Primary and storage batteries exist in all categories except that of flow-batteries, where only storage types are found. Acidic, neutral, and alkaline electrolytes are used as well. The (simplified) half-cell reaction for the zinc electrode is the same in all electrolytes ... 

Solid alkaline membrane fuel cells (SAMECs) can be a good alternative to PEMFCs. The activation of the oxidation of alcohols and reduction of oxygen occurring in fuel cells is easier in alkaline media than in acid media [Wang et al., 2003 Yang, 2004]. Therefore, less Pt or even non-noble metals can be used owing to the improved electrode kinetics. Eor example, Ag/C catalytic powder can be used as an efficient cathode material [Demarconnay et al., 2004 Lamy et al., 2006]. It has also... 

It is shown that the first step in the electrochemical reduction of cathodic material CoxOy(OH)z in power sources is controlled by kinetics. This process at the interface depends on the catalytic activity of compound, Figure 7 (a) the reduction of the bulk of cathodic materials is controlled by diffusion Figure 7 (b). 

Catalysis Issues in SOFC Catalytic processes play in important role in the operation of in particular, the SOFC anode and cathode, and the major challenges for the further development of SOFC towards a commercial project are largely related to improving catalytic activity and robustness. The critical technical issues for SOFC are reliability and lifetime, while economic requirements impose low manufacturing and materials costs. 

Hence, catalysis related challenges for SOFC cathode are the development of cathode specifications, i.e., material and microstructure, having high catalytic activity for oxygen reduction at 600 °C, high electron and ion conductivity, and a low sensitivity for poisoning by volatile Cr species. Again, as for the anode, cost and compatibility related requirements have to be considered. 

A further argument for the choice of the cathode material may be the catalytic activity for hydrogenation reactions. Vice versa, this is also important if the cathode is the counter electrode - usually evolving hydrogen - where hydrogenation reactions are undesired. 

A solid oxide fuel cell (SOFC) consists of two electrodes anode and cathode, with a ceramic electrolyte between that transfers oxygen ions. A SOFC typically operates at a temperature between 700 and 1000 °C. at which temperature the ceramic electrolyte begins to exhibit sufficient ionic conductivity. This high operating temperature also accelerates electrochemical reactions therefore, a SOFC does not require precious metal catalysts to promote the reactions. More abundant materials such as nickel have sufficient catalytic activity to be used as SOFC electrodes. In addition, the SOFC is more fuel-flexible than other types of fuel cells, and reforming of hydrocarbon fuels can be performed inside the cell. This allows use of conventional hydrocarbon fuels in a SOFC without an external reformer. 

Fuel cells operate in a manner reverse to that of electrolysis, discussed in Chapter 2, combining fuel to make electricity. The basic design consists of two electrodes separated by an electrolyte. The oldest type of fuel cell is the alkaline fuel cell where an alkaline electrolyte like potassium hydroxide is used. The hydrogen enters through the anode compartment and oxygen through the cathode compartment. The hydrogen is ionized by the catalytic activity of the anode material and electrons are released into the external circuit. The protons react with the hydroxyl ions in the electrolyte to form water. The reaction can be written as ... 

Catalytic Activity of Cathodic Materials for MCFCs. This project, supported by the Programme for Scientific and Technical Bilateral Cooperation between Portugal and Spain), addressed stability of materials and the electrolyte under operating conditions and consequent loss of catalytic activity. Given the complexity of the problem and the time context for this type of project, the research was limited to the cathode Dissolution of LiNiO cathodes has lead to the search for novel alternative materials that, exhibiting high electronic conductivity and... 

Partial oxidation of cyclohexane at the cathode of an 02/H2 fuel cell takes place at ambient temperature.197 Catalytic oxidation with 100% selectivity of the formation of cyclohexanol and cyclohexanone is achieved. Of the cathode materials comprising a mixture of alkaline-earth or rear-earth metal chlorides and graphite, the one which contains SmCl3 exhibits the highest activity. 

A third catalytic route consists of using cathode materials with catalytic properties. The electrocatalytic activity of electrode materials towards the reduction of organic halides has been the object of many studies during the past few years [82], with silver having been shown to possess powerful electrocatalytic activities... 

Ni can be taken as the reference material against which all other materials should be evaluated. On the average, the operating overpotential of untreated Ni electrodes is about 0.4 V at 0.2 A cm-2 [5], Beyond Ni, we deal with activated cathodes , which in fact derive from the idea of activated anodes such as the DSA . By activated electrodes we mean that the surface has been subjected to some treatments aimed at increasing its catalytic activity. This can be a treatment which modifies the surface structure and the morphology of the base metal, but more often the treatment is aimed at coating the base metal with a more active material [31]. 

The most outstanding source of instability, which from an academic point of view may be difficult to realize, is the corrosion that can arise at open circuit when a cell is shut down for maintenance or other reasons [108]. The cathode can simply corrode, or can be oxidized. In the latter case, the residual oxide can be deleterious for the catalytic activity since it may remain unreduced even under cathodic load. Therefore, cathode materials usually contain additives whose function is to reduce the consequences of shut-downs [109]. Laboratory experiments should thus include also this kind of test if a complete analysis of the material is to be done [105, 108, 110]. If the cathode corrodes, some cathodic protection may need to be maintained during shut-downs [7], which is of course to be accounted for in the evaluation of the economic efficiency. 

The study of inactive adatoms on noble (precious) metals has little impact on the practical problems of cathode activation for two reasons (i) deactivation is the more common occurrence (ii) adatoms are not stable in the absence of ions in solution where a finite level of precursors must be maintained, which in fact corresponds to the approach of in situ activation. The presence of ionic impurities in solution may pose serious technical problems. Studies of adatoms activation of Raney Ni, a material of current use in technology, can have a greater practical impact. It is interesting that the adsorption of Cd or Pb normally results in a sizable enhancement of the catalytic activity of Raney Ni [307-312]. The Tafel slope of the Raney Ni used by these authors is reported to decrease to ca. 30 mV as the catalyst is first soaked in a solution of the nitrates of the above metals [307, 308] (Fig. 15). The electrocatalytic activity is observed to increase slowly with time of adsorption as well as of polarization. 

Anode materials are required to have a high electrocatalytic activity for the partial oxidation of the fuel in order to facilitate Reaction (1). Consequently, several anode materials have been tested including various compositions of Ni-cermets such as 70 wt.% Ni/30 wt.% YSZ (Ni + YSZ), 70 wt.% Ni/30 wt.% Sm0.2Ceo.8Ox (Ni + CSO), and 60 wt.% Ni/40 wt.% Gdo iCeo gOx (Ni + CGO). Because cathode materials exhibit a high electro-catalytic activity for the reduction of the oxygen in order to facilitate reaction (4), several cathode materials have also been tested, including various compositions of (La, Sr) (Co, Fe)C>3 (LSCF), and CSO-LSCF. 

A typical cell employs a sheet of the polymeric material approximately 10 mils thick. A thin catalyst film is pressed on each face of this sheet to form the anode and cathode electrodes. Since the electrolyte is solid, the electrodes do not have to perform any structural or containment functions. Consequently, they are very simply designed for the sole function of providing sufficient catalytic activity to achieve desired performance levels. 

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