High-Quality Electrolyte Fabrication Processes

One of the main issues in slowing down the advances in SOFCs has been the difficulty of making good cells. The electrolyte has to meet several criteria for success  

It has to have the correct composition to give good ionic conduction at the operating temperatures. 

It must be extended in area to maximise the current capacity. 

In the late 1970s, electrochemical vapour deposition began to be used to make tubular cells at Westinghouse [9.10]. A porous tubular substrate, around 15-20 mm in diameter, made originally from calcia-stabilised zirconia but later from the cathode material, doped lanthanum manganite, was placed in a low-pressure furnace chamber, and zirconium chloride plus yttrium chloride 

To prevent the thermal shock problem, smaller diameter tubes have been produced by extrusion as described in Chapter 8 [18]. Again, these compositions were prepared by mixing zirconia powder with water and polymer, for example polyvinyl alcohol. Extrusion through a die gave tubes which could be as little as 2 mm in diameter and 100-200 pm in wall thickness, sinterable at 1450°C. 

---

The growth process takes place close to thermodynamic equilibrium and can be well controlled. MBE is usually applied to produce single crystals. The deposits obtained by these methods are of a high quality, but such processes are cost-intensive and need vacuum conditions for preparation thus making the semiconductors quite expensive. An alternative process to the above mentioned techniques is electrochemical deposition, which allows an easy control of the process parameters and which is comparably cheap. In general, the electrochemically deposited films do not possess crystalline perfection. However the structure and the size of the deposit can be adjusted by variation of the parameters (e.g., composition of the electrolyte, electrode potential, current density, and temperature) and thus the semiconductors with the required properties can be fabricated. 

Electrochemistry of LB films of fullerenes has been widely studied and remains the subject of much research effort from both theoretical and experimental approaches. Bard etal. have studied basic electrochemistry of Ceo fullerene LB films on an electrode in acetonitrile solutions [23]. The study indicated that reduction of the fullerene films could form insoluble films with incorporated electrolyte cations or lead to dissolution. The study on Cgo LB films has become a focus of considerable interest however, it is difficult to fabricate high-quality LB films of pure Cgo due to its intrinsic hydropho-bicity. Kajiyama et al. applied a multistep creep method as an LB technique for constructing a fairly homogeneous Ceo monolayer, which is regularly packed in a hexagonal array [44]. Kunitake etal. developed the electrochemical replacement method to form epitaxial adlayers of fullerenes on Au(lll) surfaces [45]. The wet process method consists of the transfer of Langmuir films of fullerene onto iodine-modified Au(lll) surfaces at an air-water interface followed by the electrochemical removal and replacement of iodine adlayers with fullerene adlayers in solution. The fullerene adlayers prepared by this method showed excellent quality and uniformity. A visuahzing... 

---