Microtubular electrodes

The decreased contribution of film resistance for the microtubular electrode makes sense because the effective film thickness for the microtubular system is less than for the thin film control electrode. This is because the surface area of the microtubular current collector is eight times higher than the surface area of the planar current collector. (This factor is calculated from the membrane thickness and the density and diameter of the pores in the membrane.) Since the control and microtubular electrodes contain the same amount of TiS2, the eight times higher underlying surface area of the microtubular electrode means that the TiS2 film is effectively a factor of 8 thinner, relative to the control electrode. 

The decreased contribution due to slow electron transfer kinetics for the microtubular electrode is also attributable to the higher underlying surface area of the tubular current collector. Because the surface area is higher, the effective current density for the microtubular TiS2 is less than for the thin film TiS2, which has a conventional planar current collector. The decreased contributions of film resistance and slow electron transfer kinetics also account for the higher peak current density of the microtubular electrodes (Fig. 27). 

The reason for this loss in capacity with increasing scan can be clearly seen in the voltammograms in Fig. 30A and 30A. The peak separation, discussed in detail above, becomes larger as the scan rate is increased. The result of this enhanced distortion of the voltammetric wave is the inability to utilize the capacity of the electrode over the useful potential window of the electrode (3.0 to 1.5 V). As would be expected (see above), this distortion is less for the microtubular electrode, and this should result in higher capacities for this electrode. 

At the lowest scan rate employed, the microtubular electrode delivers an experimental capacity (256 21 mA hr g ) that is identical to the theoretical capacity. As scan rate is increased, capacity does fall off however, at any scan rate, the experimental capacity obtained from the microtubular electrode is greater than the capacity obtained at the control electrode. At the highest scan rate employed, the microtubular electrode delivers almost seven times the experimental capacity of the control electrode, even though both electrodes contain the same amount of TiS2. 

Finally, Fig. 33 shows the results of constant current discharge experiments at a microtubular electrode and a control electrode containing the same amount of TiS 2. Note that at this discharge current density, the microtubular electrode delivers 90% of its theoretical capacity. In contrast, as would be expected, the control electrode delivers significantly less capacity. 

FIG. 32. Discharge capacity versus scan rate for (A) TiS2 microtubular electrode (0.86 mg TiS2 cm ) and (B) TiS2 film control electrode (0.60 mg TiS2 cm ). 

The microtubular electrode concept described here also offers another possible advantage. In these concentric tubular electrodes, each particle of the Li intercalation material (the outer tube) has its own current collector (the inner metal microtubule). This could be an important advantage for Li+ intercalation materials with low electrical conductivity. This advantage was not demonstrated here because TiS2 has relatively high electronic conductivity. We have recently shown that electrochemical synthesis can be used to coat the gold microtubular current collector with outer mbes of a... 

Vielstich and co-workers [99] modified the analysis of Blaedel and Klatt [66] to derive the steady-state current-potential relationship at the microtubular electrode under turbulent flow conditions. Their analysis assumed that the electrode/cell/flow rate parameters were such that mass transport to the electrode could be considered to be controlled by the laminar sublayer. In terms of the parameter... 

Microelectrodes can be classified into point electrodes as in die case of microdisk electrodes and line electrodes, as in microtubular electrodes, based on structure, size, mass transport, and application. The point electrode means that the electrode appears as a point at a distance. On the other hand, when it looks like a line, it is a line electrode. Although it... 

Li+ intercalation material (V. M. Cepak and C. R. Martin, unpublished). These results, which will be the subject of a future paper, show that other synthetic methodologies, in addition to CVD, can be used to make micro-structured battery electrodes like those described here. In addition, the underlying microtubular current collector does not have to be Au. Microtubules composed of graphite [35] or other metals [1,3] (e.g., Ni) could be used. Finally, for the advantages noted above to be realized in practical cells, large-scale template-fabrication methods would have to be developed. 

The most usual configuration is a thin film of electrolyte on an anode support tube and an outer thick-film cathode. However, a major problem with the microtubular concept is with current collection from the electrodes and interconnection into... 

Other cell designs, such as radial configurations and more recently microtubular designs, have also been developed and demonstrated [21]. The microtubular designs can achieve lower operating temperature, since it has long been recognized that thinner electrolyte and electrode layers promote operation at lower temperature [22]. 

Another approach is to adopt a design similar to the lead acid battery and to use wires brought out from the electrodes and connected externally. This is the approach adopted by Adelan in their microtubular design. Clearly, the design of the cells and how they fit into the overall stack is vitally important in deciding such issues. 

A typical design of a microtubular SOFC is shown in Figure 8.25. A YSZ electrolyte tube (typically 2 mm in diameter and about 150 pm wall thickness), is used as a support for the electrodes, as a gas inlet tube, and also as a combustor tube at its outlet. The overall length of the tube is between 100 and 200 mm, whereas the cell region only occupies a length of about 30 mm towards the outlet end of the tube. The Ni + YSZ anode, 30 mm long, is coated on the inner wall of... 

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