Coplanar cell performance

A further application of the coplanar cell configuration showed in Fig. 3.1(c) concerns the study of the time dependence of the photocurrent following carrier excitation by means of a short pnlse of illnmination. This transient photodecay technique enables the examination of the interaction of initially free carriers with varions localized states. In principle, the decay of photocnrrent measured in this manner should (in the absence of recombination effects and phenomena associated with drift close to the surface of a thin film) correspond to the behavior in the initial pre-transit regime of a TOF pnlse. Becanse it allows measurements to be performed on very thin films under conditions appropriate to their nse in many device applications, and because the photocurrent may be examined over several decades of time withont the complications associated with carrier extraction, the techniqne has become rather popular over recent years. 

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. 

Table 2.1 presents the coplanar SC-SOFC with the smallest electrode dimensions and highest cell performance for each fabrication techrtique. 

In combination with a LSM-based cathode, a Ni-YSZ anode and YSZ electrolyte appeared blackened after sintering whereas cells with a LSCF cathode did not exhibit such a color change [73]. The black coloration was attributed to the diffusion of manganese from the cathode to the anode through the electrolyte. Chemical interaction between closely spaced coplanar anodes and cathodes could affect the cell performance and material compatibility studies could facilitate the selection of suitable ceU component materials. 

The VEH band structure calculations have been carried out using the geometries depicted in Fig. 2 as unit cells, and assuming a fully coplanar conformation,with all the vinyl groups in trans position with respect to adjacent rings (see Fig. 3a). This assumption has been taken on the basis of the accurate ab initio calculations performed to study the rotation potential of the vinyl group in 2-VT, 2-VF, and 2-VPy. For all of these monomers, the trans rotamer is predicted as the most stable conformation. The VEH band structures calculated for the all-cis conformations displayed in Fig 3b show no significant difference with respect to those obtained for trans conformations and they will not be discussed here. 

Kuhn et al. (2008) tested coplanar SC-SOFCs with different curvilinear microelectrode configurations of arbitrarily complex geometry. It was shown that the performance of the cells depends only on electrode and interelectrode dimensions, not on the electrode s shape. 

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