Coplanar microscale electrodes

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 small electrode size not only imposes fabrication challenges and hmits the feasibility of SC-SOFCs with coplanar microscale electrodes, it also limits detailed analyses of these cells. SC-SOFCs with coplanar microscale electrodes yield very low conversion of the reactant gases, so that differences between input and output gases cannot be easily detected by mass spectrometry and information about the reactions that occur cannot be obtained. Similarly, the small electrode size makes impedance analysis difficult. The lack of fundamental studies and appropriate characterization and fabrication techniques leaves the working principles of SC-SOFCs with coplanar electrodes to a great extent unexplored. 

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. 

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