Nickel anodes carbon formation

Nickel compounds are of great importance industrially and a review is available on the use of nickel in heterogeneous catalysis, electroplating, batteries, pigments, ceramics and hydrogen storage.76 This concerns simple aqua complexes of nickel(II) with anions such as carbonate, halide, hydroxide, nitrate and sulfate. Nickel acetate and formate find similar use, and the acetate is employed in the sealing of anodized aluminum.77 [Ni(NH3)6]Cl2 has been shown to be potentially applicable in heat pumps.78... 

There are parallel achievements at the University of Pennsylvania, (Park etai, 1999 2000 2001 Gorte etai, 2000), using anodes with copper substituted for nickel to avoid carbon formation. The last two papers include the electrochemical oxidation of dry fuels other than methane, for example gasoline and diesel, the chemical exergy of which is difficult to calculate, since they are mixtures requiring separative work. 

The activity of the nickel anode decreases due to sintering and coke formation when carbon-containing fuels are used. The ceramic parts can easily break if vibrational forces are present. This is one reason why SOFCs are best suited for stationary applications rather than mobile applications. 

C. M. Finnerty, N. J. Coe, R. H. Cunningham, and R. M. Ormerod. Carbon formation on and deactivation of nickel-based/zirconia anodes in solid oxide fuel cells running on mehtane. Catalysis Today 46, (1998) 137-145. 

Direct oxidation (or direct utilization) The fuel is oxidized directly in the SOFC without external reformation. The SOFC has been shown to have the capabihty for direct oxidation of different types of fuels [4, 36-38]. To address the carbon deposition issue associated with nickel commonly used in the anode composition, other metals such as copper have been tested. The abihty of copper to resist carbon formation leads to the development of a composite anode composed of a ceria support and a copper phase [38]. The key technical challenges in the development of direct-oxidation SOFCs relate to the anode, especially the electrode s performance, stability, and direct-oxidation capabihty. 

Various dopants have been incorporated into nickel/zirconia and nickel/ceria anodes, in an attempt to modify their behaviour. Dopants that have been studied include molybdenum, gold, ruthenium and lithium [16,26-29]. The effect of gold in dramatically increasing resistance to carbon formation and build-up was previously shown in Table 12.1 and Figure 12.16. 

Further improvements in anode performance have been achieved through the inclusion of certain metal salts in the electrolyte, and more recently by dkect incorporation into the anode (92,96,97). Good anode performance has been shown to depend on the formation of carbon—fluorine intercalation compounds at the electrode surface (98). These intercalation compounds resist further oxidation by fluorine to form (CF ), have good electrical conductivity, and are wet by the electrolyte. The presence of certain metals enhance the formation of the intercalation compounds. Lithium, aluminum, or nickel fluoride appear to be the best salts for this purpose (92,98). 

The anodic oxidation of catechol in the presence of 1,3-dimethylbarbituric acid was carried out in aqueous solution containing sodium acetate in an undivided cell at graphite and nickel hydroxide electrodes [114]. The results did not fit with the expected structure (Scheme 47, path A) but a dis-piropyrimidine was isolated in 35% yield (Scheme 47, path B). It seems that the initial attack of 1,3-dimethylbarbituric acid on the anodically formed o-quinone does not occur through the carbon-oxygen bond formation but rather through the carbon-carbon bond formation, giving rise to the final product via several consecutive reaction steps. 

These results indicate that zinc ions formed by oxidation of the anode do not play a part or only have side effects in the direct electroreductive carbon—carbon bond formation carried out with a zinc anode and a nickel catalyst. In these reactions, a nickel organometallic is involved. 

Fluorine cells use carbon anodes, steel cathodes, and nickel or monel wine mesh diaphragms contained in a monel tank with a water jacket for cooling. Cells operate at about 6 kA, 10—12 V, and 95—105°C. Energy consumption for fluorine production is about 22 kWh/1. Figure 6 is a diagram of a fluorine cell. Fluorine cell voltages ate iacteased by the formation of fluorinated carbon. Conditions for the formation of (CF) and on anode... 

Nickel is used as the anode because it is economical and exhibits high performance, although due to reasons of adherence and different expansion coefficients, it flakes off easily from the electrolyte unless it is mixed with zirconia, creating a cermet. Ni-YSZ anodes allow a rapid and clean connection to the fuel and are good electronic conductors although Ni is susceptible to become coated with a carbon layer when it reacts with carbon-based fuels. Coke formation usually impedes further reaction from... 

The MCFC anode operates under reducing atmosphere, at a potential typically 700-1000 mV more negative than that of the cathode. Many metals are stable in molten carbonates under these conditions, and several transition metals have electrocatalytic activity for hydrogen oxidation. Nickel, cobalt, copper and alloys in the form of powder or composites with oxides are usually used as anode materials. Ceramic materials are included into the anode composition to stabilize the anode structure (pore growth, shrinkage, loss of surface area) at the time of sintering. An alloy powder of Ni + 2-10 wt% Cr can be used. The initial formation of CrjOs, followed by surface formation of LiCr02, can stabilize the anode structure. 

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