Coplanar electrode width

Sub-millimeter inter-electrode gaps (in the case of plate and charmel reactors) or electrode widths (in the case of coplanar interdigitated band electrodes) lead to thin concentration boundary layers with any flow rate [14,23] resulting in enhanced mass transfer rates and thus increasing the attainable space-time-yield [Equation (17.17)]. 

Based on equivalent circuit models of the cell component resistances [66], maximum ceU performance of SC-SOFCs with coplanar electrodes is predicted for very small electrode widths (6-10 pm) and gap sizes (2-12 pm). Performance comparisons of macro-, miUi-, and microcells [71] revealed a 10 times higher power density for the micro SC-SOFC which had smaller inter-electrode gaps and electrode widths. As closely spaced small-scale electrodes lower the ohmic resistance and the inter-digitated electrode pattern maximizes the electrode surface area, miniaturization of SC-SOFCs with coplanar, interdigitated electrodes is expected to yield suitable cell performance for small- and microscale power applications. The fabrication of microcells (Figure 2.3) presents many challenges and requires the manufacturing of coplanar microscale electrode patterns from multicomponent ceramic materials. 

The first SC-SOFCs with coplanar electrodes were fabricated by painting electrodes with a brush onto the electrolyte, yielding inter-electrode gaps and electrode widths larger than 0.5 mm [9]. Interdigitated electrode patterns were not fabricated by this method. Cells with side-by-side anode and cathode and cells with interdigitated electrode structures were also prepared by screen printing... 

Minimum gap sizes and electrode widths were in the order of 0.2 and 0.5 mm, respectively. Using photo-masks, coplanar microscale Ptand Au electrode structures were fabricated by sputtering [77]. Although the cells delivered very low OCVs, the smallest inter-electrode gaps (5 am) and electrode widths (15 am) reported so far were realized by this fabrication technique. 

In 2009, Wei et al. reported a star-shaped stack comprised of four Ni-YSZIYSZILSM cells [21], This stack generated an OCV of approximately 3.5 V and a peak power output of 421 mW in a methane-air mixture feed at 700 °C. They concluded that the symmetric design can ensure the identical operation of each cell. On the other hand, as described in the previous section, SC-SOFCs allows for a coplanar electrode arrangement, where the two electrodes reside on the same surface of the electrolyte. Also in 2009, Kuhn et al. fabricated cells with one, two, three, four, five and ten pairs of electrodes with a width of 260 pm, thickness of 17 pm, and interelectrode gap of 114 pm [22]. The OCV roughly increased with increasing number of the electrode lines, although the maximum OCV value was as low as 0.8 V. 

Figure 17.2 (a) Top-gate, staggered contact TFT architecture and (b) bottom-gate, coplanar contact TFT architecture, where (1) is the gate electrode, (2) is the dielectric (typical thickness = 100-500 nm), (3) is the organic semiconductor (typical thickness = 20-100 nm), (4) are the source and drain electrodes (channel length = 1-100 fj,m, channel width = 10-500 nm) and (5) is the substrate... 

Fig. 10. Magnified view of the interdigitated microarray band. (A) b is the length of the band, Wj, the width of a single band (B) the cross-section of the coplanar type of the IDA w and Wg, respectively, are widths of the electrode band and the gaps the current traces are indicated by dashed lines (C) the cross-section of a vertically separated IDA. 1, is the silicon wafer, 2, the Si02 wafer. Adapted according to [39-42].

The electrodes in coplanar cells are mostly either a pair of parallel straight lines (strips) or multiple such strips (Figure 18.1—hence the often-used term strip cells). Sometimes more complicated electrode patterns are used [e.g., comblike interdigitated structures (Figure 18.2)]. In strip cells, typically, the widths of the electrodes and the gaps range from tens of a micrometer to a few millimeters. 

The main advantage of the coplanar fuel cell design is the possibility of considerably reducing the internal ohmic resistance of the cell, which is due primarily to the ohmic resistauce of the electrolyte. To decrease the ohmic resistance of conventional MEAs it is necessary to reduce the thickness of the electrolyte. An excessive reduction of this thickness can have detrimental consequences the formation of cracks and pinholes and the loss of stability and gas-tighmess. In strip cells the ohmic resistance is determined by the width of the gap between cathodes and anodes and can be decreased simply by narrowing this gap. If the electrodes are micropattemed to be close together, the cell s power density can be increased considerably. 

Figure 9.27 shows the transient photocurrent, measured at a photon energy of hv = 2.0 eV and at room temperature, in a C60 film for different levels of oxygen contamination [43]. Measurements are performed in a 0.5 pm thick film, with top coplanar Au electrodes, at an applied electric field of 10 V/cm, using laser pulses having a width of 20-30 ps and an intensity of 2—3 pJ. The overall temporal resolution of the measurements is about 50 ps. Each decay in Figure 9.27 consists of a short and a long-lived component. 

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