Thick Film Solid Electrolytes

Thick-Film Solid Electrolytes 19.5.2.1 Screen Printing... 

It can be shown that for typical thicknesses of solid electrolyte (say, 100 ym) and porous metal catalyst film (say, lOym), the major source of ohmic resistance (>99%) is the solid electrolyte. The consequence is that the actual dimensionless operating voltage X] is constant along each channel and also constant for all channels electrically connected in parallel (as shown in Figure 1), i.e., constant within each unit battery. This observation significantly simplifies the development of the electron balance equations. 

The tape-casting method makes possible the fabrication of films in the region of several hundred micrometers thick. The mechanical strength allows the use of such a solid electrolyte as the structural element for devices such as the high-temperature solid oxide fuel cell in which zirconia-based solid electrolytes are employed both as electrolyte and as mechanical separator of the electrodes. 

Most of the electrochemical promotion studies surveyed in this book have been carried out with active catalyst films deposited on solid electrolytes. These films, typically 1 to 10 pm in thickness, consist of catalyst grains (crystallites) typically 0.1 to 1 pm in diameter. Even a diameter of 0.1 pm corresponds to many (-300) atom diameters, assuming an atomic diameter of 3-10 10 m. This means that the active phase dispersion, Dc, as already discussed in Chapter 11, which expresses the fraction of the active phase atoms which are on the surface, and which for spherical particles can be approximated by ... 

The conductivity ofthe film was calculated for 30 monolayers. The film was deposited onto a Ag microelectrode array with a 1-mm distance between fingers. The thickness ofthe monolayer was taken to be 2 x 10"7 cm. For an air humidity value of 60% the conductivity equals 1.3 x 10"6 (Q/cm)-1 The current through the film has an ionic character, and there is apparently layered solid electrolyte... 

While the amount of electricity that can be conducted by polymer films and wires is limited, on a weight basis the conductivity is comparable with that of copper. These polymeric conductors are lighter, some are more flexible, and they can be laid down in wires that approach being one-atom thick. They are being used as cathodes and solid electrolytes in batteries, and potential uses include in fuel cells, smart windows, nonlinear optical materials, LEDs, conductive coatings, sensors, electronic displays, and in electromagnetic shielding. 

Other developments in the area of solid state lithium batteries include prototype production and testing of thin-film microbatteries at Oak Ridge National Laboratory in the USA. The fabrication involves electrode and electrolyte film deposition to form compact layers of thickness of the order of few microns. The cell uses a lithium anode, an amorphous Li3 3PO3.9N0.17 solid electrolyte and an amorphous V205 cathode ... 

O2 gas is generated electrolytically at the interface between the reference electrode and the solid electrolyte layer. The mass of the O2 gas is equal to the mass of O2 gas which diffuses through the porous thick film zirconia, plus the mass of O2 gas which reacts with CO gas at the reference electrode. 

At the reference electrode/solid electrolyte layer interface, the mass of the CO gas which diffuses from the porous thick film zirconia is equal to the mass of CO2 gas which diffuses into the porous thick film zirconia. 

The structure of films of zirconia-based solid electrolytes was analyzed in a scanning electron microscope. It was found that both one- and multilayer films (Fig. la, lb) of zirconia, which was stabilized to its cubic modification, up to 10 pm thick had a columnar structure, that is, consisted of mutually adjoining crystallites, which generally were oriented perpendicularly to the film surface. The observed deviation of the crystallites from the normal direction to the film plane was not over 15° and was explained by the mutual misorientation of the target and the substrate. 

Figure 18, taken from Ref. 77, describes several models proposed for the Li electrodes in solutions, their equivalent circuit analogs, and the expected impedance spectra (presented as Nyquist plots). Assuming parallel plate geometry for the solid electrolyte interface, as well as knowledge of the surface species involved from spectroscopy (and thus their dielectric constant, which is around 5 for many surface species formed on Li, including R0C02Li, Li2C03, LiF, ROLi, etc. [186]), it is possible to estimate the surface film s thickness from the electrode s capacitance (calculated from the model fitted to the spectra) ...

Instead of the system silica/silicate also other systems such as titania/titanate, zirconia/zirconate can be used as a reference system [xiv]. The response time of freshly fabricated thick-film sensors based on thin-film /3-alumina is very short (about 15 ms at 650 °C). After several weeks of operating this time increases 10 times (150 ms) [xv]. Solid electrolyte C02 sensors using Ni/carbonate composite as measuring electrode are suited for measuring of C02 in equilibrated water gases [xiv]. Using semiconducting oxides and carbonates like ITO (indium tin oxide) Nasicon-based C02 sensors are able to measure at room temperature [xvi]. 

Fig. 9 Schematic cross-section of a R thick film electrode on a Na+-p-alumina solid electrolyte of a type III potentiometric COg sensor.

A commercially available fully dense Na "-(3"-alumina disc was used as the solid electrolyte. The working electrode (sink) of the system is a screen-printed platinum thick film of 7 pm in thickness. The porosity of the Pt thick film is in the range of 65 to 75 %, and the average pore size is about 2 pm and ranges from 0.5-5 pm according to microstructure studies. A source Na COj disc was made by pressing Na COg powder. The green body was then cosintered onto a Pt mesh with a spot-welded Pt lead. 

Fig. 14 Cross section SE image from a cleaved sample. Area (A) is the Na+-p"-alumina solid electrolyte area (B) is the Pt thick film electrode and area (C) is the PEVD product...

For sample 1, a stable EMF response from the sensor was obtained after passing about 0.468 C of Na ions. According to the previous discussion, this corresponds to the point where the PEVD auxiliary phase just covers the entire Pt thick film surface. The response times and recovery times of the sensor after each PEVD process step are recorded. Both response and recovery times are plotted against the Na ion flux through the solid electrolyte during the PEVD process in Figure 23. Curve (1) is the response time and curve (2) is the recovery time. 

The PEVD sample utilized in this investigation is a solid electrochemical cell with a ytterbia and yttria stabilized zirconia pellet (8%Yb303-6%Y303-Zr03) as the solid electrolyte to conduct oxygen anions from the source to the sink side. A commercially available Pt thick film paste was screen printed on the center of both surfaces of the solid electrolyte disk. Two Pt meshes, with spot welded Pt leads,... 

The PEVD system used in this investigation is schematically shown in Eigure 36. A Na -p/ P -alumina disc, 16 mm in diameter and 5 mm in thickness, was used as the solid electrolyte with a working electrode on one side and both counter and reference electrodes on the other. To simplify data interpretation, the same electrode material, a Pt thick film, was used for all three electrodes, so the measured potential difference could be directly related to the average inner potential difference between the working and reference electrode. In order to make good electrical and mechanical contact, Pt meshes, with spot-welded Pt wires, were sintered on the Pt thick films as electron collectors and suppliers. 

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