Cathodes sintered nickel

The electrodes are flat. The anode is composed of porous sintered nickel along with additives, which inhibit the loss of surface area during operation. The anode is in direct contact with the electrolyte matrix. The cathode is a porous nickel oxide, which is initially fabricated in the form of a porous sintered nickel and is subsequently oxidized during the cell operation. 

The MCFC anodes are made from a porous sintered nickel with a thickness of 0.8-1.0 mm and a porosity of 55-70% with a mean pore diameter of 5pm. This porosity range provides adequate interconnected pores for mass transport of gaseous reactants and adequate surface area for the anodic electrocatalytic reactions. Because the anode kinetics is faster than that of the cathode, less active surface area is sufficient for the anodic process. Partial flooding of the comparatively thick anode is therefore acceptable at the anode interface. 

Alkaline FCs (AFCs) use KOH as electrolyte and work at 70-90 C they are fully developed and very reliable (they powered on-board instrumentation of the Apollo spacecrafts and they power on-board instrumentation of the space Shuttles). Electrodes are mostly sintered nickel (anode) and sintered, lithiated nickel-oxide (cathode). 

The sintered nickel substrate for the cathode is similar to that used for Ni-Cd and Ni-MH cell constructions. The nickel active materials are loaded into the sinter plate using either an aqueous (Bellcore) or alcoholic-based (Air Force or Pickett) electrochemical impregnation process. A 5-10% cobalt additive is deposited with nickel hydroxide to improve charge acceptance. These electrodes have a significantly longer cycle life over the standard vacuum impregnated or pasted nickel electrodes used in commercial Ni-Cd cells. 

Early work on the use of foams and mats has been reviewed [9j. Nickel fiber, nickel-plated steel fiber, or nickel-plated graphite fiber mats are preferred because they have smaller pores ( 50(im) [14]. The most recently developed mats can have porosities as high as 95% [13] and are much lighter than the sintered nickel plaques, which typically have porosities between 80 and 90%. Initially, standard cathodic impregnation methods were used to load the active material into the foam [9]. More recently, the preferred method is to incorporate the Ni(OH)2 in the form of a slurry into the mat [13, 14]. This has been called the suspension impregnation method [14]. Considerable improvement in the Ni(OH)2 has been achieved by the addition of divalent Co compounds to the slurry. The best results... 

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. 

Electrodes The electrocatalytic material of an MCFC is nickel. The cathode becomes oxidized and lithiated during the first hours of the operation. Nickel oxide is soluble in molten carbonates thus in the course of the operation two undesirable effects may occur (1) metallic nickel particles are formed in the electrolyte which can lead to an electronic short circuit of the electrodes, (2) the size of the cathode diminishes. Two approaches have been proposed for solving these problems the use of less corrosive molten carbonate mixtures and more stable cathodes containing iron and cobalt. The nickel anodes usually contain chromium, which promotes the sintering process. 

The MCFC is a promising power generating source because of its unique characteristics such as high fuel efficiency and ability to use various carbonaceous fuels. Although Ni-10wt% Cr is used in the state-of-the-art MCFC as anode, it needs to be improved in terms of better creep and sintering resistance. In spite of the development in the alternate cathode material research, lithiated nickel oxide has been the choice of cathode material in the kilowatt-level MCFC stacks developed by many companies. Continuous research in the development of stable electrolyte retention matrix, identification of suitable molten carbonate electrolyte composition, and additives to the electrolyte will be a significant milestone. Also, research in the area of current collector/bipolar plate to overcome... 

Several investigators have used combined approaches, particularly in the in situ precipitation of active material in the pores of sintered substrates, using cathodic polarization and caustic precipitation in simultaneous or nearly simultaneous steps. A considerable amount of the reported information on the chemistry, electrochemistry, and crystal structure of the nickel electrode has been obtained on thin films (qv) made by the anodic corrosion of nickel surfaces. However, such films do not necessarily duplicate the chemical and/or crystallographic condition of active material in practical electrodes. In particular, the high surface area, space charge region, and lattice defect structure are different. Some of the higher (3.5+) valence state electrochemical behavior seen in thin films has rarely been reproduced in practical electrodes. 

All three layers, namely porous anode, electrolyte, and cathode, were manufactured from agglomerated ceramic powder ZxOi + 8 mol.% Y2O3 with a crystallite size from 10 to 20 nm. The anode and cathode materials were doped with 50 wt.% nickel oxide and 50 wt.% lanthanum manganate, respectively. Taking into account that the sintered electrolyte material should be gas-tight... 

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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