Nickel anodes composite

Cells operating at low (2,80,81) and high (79,82) temperatures were developed first, but discontinued because of corrosion and other problems. The first medium temperature cell had an electrolyte composition corresponding to KF 3HF, and operated at 65—75°C using a copper cathode and nickel anodes. A later cell operated at 75°C and used KF 2.2HF or KF 2HF as electrolyte (83,84), and nickel and graphite as anode materials. 

It is well established that sulfur compounds even in low parts per million concentrations in fuel gas are detrimental to MCFCs. The principal sulfur compound that has an adverse effect on cell performance is H2S. A nickel anode at anodic potentials reacts with H2S to form nickel sulfide. Chemisorption on Ni surfaces occurs, which can block active electrochemical sites. The tolerance of MCFCs to sulfur compounds is strongly dependent on temperature, pressure, gas composition, cell components, and system operation (i.e., recycle, venting, and gas cleanup). Nickel anode at anodic potentials reacts with H2S to form nickel sulfide. Moreover, oxidation of H2S in a combustion reaction, when recycling system is used, causes subsequent reaction with carbonate ions in the electrolyte [1]. Some researchers have tried to overcome this problem with additional device such as sulfur removal reactor. If the anode itself has a high tolerance to sulfur, the additional device is not required, hence, cutting the capital cost for MCFC plant. To enhance the anode performance on sulfur tolerance, ceria coating on anode is proposed. The main reason is that ceria can react with H2S [2,3] to protect Ni anode. 

A further advantageous modification of electrodes is claimed by Watanabe [154] for the Central Glass Co. Ltd. which uses anodes coated with a composite layer of nickel containing a dispersed eutectoid of PTFE particles or fluorinated graphite particles. The low surface energy nickel based composite is said to improve the yield of perfluorooctanesulphonyl fluoride to 40.5 % in ECF. 

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. 

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. 

Tasaka A (2007) Anodic behavior and anode performance of nickel, nickel-based composite and carbon electrodes for electrochemical fluorination in a few molten fluorides. Electrochemistry 75(12) 934—944... 

The best yields of NF3 are obtained if the electrolyte has the composition NH4F-1.1HF to NH4F-1.8HF and the electrolysis is carried out between 100 and 130°C [2 to 5]. Pierce and Pace [6] were the first to substitute the original carbon anode with a nickel anode. By using a nickel anode, the contamination of NF3 with CF4 and other fluorinated carbon compounds was avoided. A disadvantage of nickel anodes is that they degrade by forming nickel fluorides [2]. 

One of the great benefits of the SOFC is that it can utilise a wide range of fuels, as described in Chapter 12. The fastest reaction at the nickel anode is that of hydrogen. But other fuels can also react directly on the anode, depending on catalyst composition. For example, carbon monoxide can react on Ni/YSZ, but has a higher overpotential than hydrogen [3 5]. Also, methane can react on the... 

The anode must be an electronic conductor, but unlike the cathode, the operating atmosphere is reducing. It is therefore possible to consider using a metal like nickel. A composite Ni-YSZlO has the advantage of minimizing the mismatch of the thermal expansion coefficients and increasing the adhesion of the metal on the sohd electrolyte. 

Fluorine cannot be prepared directly by chemical methods. It is prepared in the laboratory and on an industrial scale by electrolysis. Two methods are employed (a) using fused potassium hydrogen-fluoride, KHFj, ill a cell heated electrically to 520-570 K or (b) using fused electrolyte, of composition KF HF = 1 2, in a cell at 340-370 K which can be electrically or steam heated. Moissan, who first isolated fluorine in 1886, used a method very similar to (b) and it is this process which is commonly used in the laboratory and on an industrial scale today. There have been many cell designs but the cell is usually made from steel, or a copper-nickel alloy ( Monel metal). Steel or copper cathodes and specially made amorphous carbon anodes (to minimise attack by fluorine) are used. Hydrogen is formed at the cathode and fluorine at the anode, and the hydrogen fluoride content of the fused electrolyte is maintained by passing in... 

Fig. 1. Schematic representation of a battery system also known as an electrochemical transducer where the anode, also known as electron state 1, may be comprised of lithium, magnesium, zinc, cadmium, lead, or hydrogen, and the cathode, or electron state 11, depending on the composition of the anode, may be lead dioxide, manganese dioxide, nickel oxide, iron disulfide, oxygen, silver oxide, or iodine.

The anode material in SOF(7s is a cermet (rnetal/cerarnic composite material) of 30 to 40 percent nickel in zirconia, and the cathode is lanthanum rnanganite doped with calcium oxide or strontium oxide. Both of these materials are porous and mixed ionic/electronic conductors. The bipolar separator typically is doped lanthanum chromite, but a metal can be used in cells operating below 1073 K (1472°F). The bipolar plate materials are dense and electronically conductive. 

As with most other metals, the anodic behaviour of nickel is influenced by the composition of the solution in which measurements are made, particularly if the solution is acidic. Acidic solutions containing d ions or certain sulphur compounds in particular have a pronounced influence both in increasing the rate of anodic dissolution in the active range and in preventing passivation, and in stimulating localised corrosion . Thiourea and some of its derivatives have a complex effect, acting either as anodic stimulators or inhibitors, depending on their concentration . 

Sintered and sprayed ceramic anodes have been developed for cathodic protection applications. The ceramic anodes are composed of a group of materials classified as ferrites with iron oxide as the principal component. The electrochemical properties of divalent metal oxide ferrites in the composition range 0- lA/O-0-9Fe2O3 where M represents a divalent metal, e.g. Mg, Zn, Mn, Co or Ni, have been examined by Wakabayashi and Akoi" . They found that nickel ferrite exhibited the lowest consumption rate in 3% NaCl (of 1 56 g A y at 500 Am and that an increase in the NiO content to 40mol 7o, i.e. O NiO-O-bFejO, reduced the dissolution rate to 0-4gA y at the expense of an increase in the material resistivity from 0-02 to 0-3 ohm cm. 

Commercial processes Commercial electroless nickel plating stems from an accidental discovery by Brenner and Riddell made in 1944 during the electroplating of a tube, with sodium hypophosphite added to the solution to reduce anodic oxidation of other bath constituents. This led to a process available under licence from the National Bureau of Standards in the USA. Their solutions contain a nickel salt, sodium hypophosphite, a buffer and sometimes accelerators, inhibitors to limit random deposition and brighteners. The solutions are used as acid baths (pH 4-6) or, less commonly, as alkaline baths (pH 8-10). Some compositions and operating conditions are given in Table 13.17 . 

Fig. 6. Variations of the open-circuit voltage (OCV) with nickel content of Ni-SDC and Ni-YSZ anodes. Operating conditions 600 °C, 1 atm feed composition CH4 Ar = 10 90 total flow rate 100 ml/min [10].

The raw material for the synthesis was methane. Powder of Nickel carbonyl (NC) or powder of nano-diamond (ND) was the catalyst. Attempts to synthesize pyro-carbon on copper powder were not successful. Powder with the composition 70%PC, 30%NC, and also the set of powders with various ratios of PC and ND were tested. Anodes made of the powder 70PC30NC showed satisfactory cycle behavior and had specific capacity 180 mAh/(g of powder) (260 mA-h/(g 0f carbon)) (Fig. 3a). The anodes made of powder xPCyND, irrespective of the components ratio, had specific capacity... 

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