# The Development of Current Collection in Micro-Tubular Solid Oxide Fuel Cells--A Review
## 6 Current Collector Shape and Structure  
Once the designer has chosen the optimal interconnect material, the thickness, shape and structure must be determined. For a planar device, this will be some sort of flat plate with an optimised corrugation/patterning for fuel and exhaust flow. In a tubular device, the decision is less clear and depends on whether current is collected on the bore side of the tube or the outer surface. The material will need to be formed to achieve intimate contact with the electrode and so a format that can be easily worked/shaped is preferable. The need to minimise the electron pathway and maximise the contact area for current collection  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Material** & **CTE** & **Conductivity** & **Melting Point** & **Young’s Modulus** & **Ref(s).** \\  & \(\times\)**10\({}^{-6}\) [K]** & **[S.cm\({}^{-1}\)]** & **[\({}^{\circ}\)]** & **[GPa]** & **Ref(s).** \\ \hline Ni-YSZ & 11–13 & \(3\times 10^{2}\) (800 \({}^{\circ}\)C) & - & 57–58 (800 \({}^{\circ}\)C) & [6; 20; 88] \\ LaCrO\({}_{3}\) & 9.5 & 0.34 (700 \({}^{\circ}\)C) & 2510 & - & [20; 24; 77] \\ Crofer 22-APU (ferritic) & 11.5–12.5 & \(8.3\times 10^{4}\) (1000 \({}^{\circ}\)C) & 1510–1530 & 216 (750 \({}^{\circ}\)C) & [88; 90; 91] \\  & 8.7 \(\times\) 10\({}^{3}\) (800 \({}^{\circ}\)C) & 1510–1371 & 159 (800 \({}^{\circ}\)C) & [89; 92] \\ [PERSON] 23 (nickel-based) & 15.2 (800 \({}^{\circ}\)C) & \(7.7\times 10^{3}\) (800 \({}^{\circ}\)C) & 1301–1371 & 159 (800 \({}^{\circ}\)C) & [67; 92] \\ Ducrolloy & 11.8–12 (800 \({}^{\circ}\)C) & \(1\times 10^{4}\) (1000 \({}^{\circ}\)C) & 1700 & - & [93; 94] \\ (Cr-based) & 11.8–12 (800 \({}^{\circ}\)C) & \(1\times 10^{5}\) (800 \({}^{\circ}\)C) & 961 & 69–74 (800 \({}^{\circ}\)C) & [24; 95; 96] \\ Silver (99.9\%) & 18.9, 22 (800 \({}^{\circ}\)C) & \(2.5\times 10^{4}\) (800 \({}^{\circ}\)C) & 1455 & 190–220 (800 \({}^{\circ}\)C) & [97] \\ Nickel & 12–13.5 (800 \({}^{\circ}\)C) & \(2.3\times 10^{4}\) (800 \({}^{\circ}\)C) & 1769 & 127 (800 \({}^{\circ}\)C) & [24; 98] \\ Platinum & 10 (800 \({}^{\circ}\)C) & \(1.1\times 10^{5}\) (800 \({}^{\circ}\)C) & 1064 & 76–81 (800 \({}^{\circ}\)C) & [24; 99] \\ Gold & 16.6 (800 \({}^{\circ}\)C) & \(2.55\times 10^{4}\) (800 \({}^{\circ}\)C) & 1552 & 118–124 (800 \({}^{\circ}\)C) & [24; 100] \\ Palladium & 12.3(800 \({}^{\circ}\)C) & \(2.55\times 10^{4}\) (800 \({}^{\circ}\)C) & 1552 & 118–124 (800 \({}^{\circ}\)C) & [24; 100] \\ \hline \hline \end{tabular}
\end{table}
Table 2: Material properties of commonly used SOFC interconnect materials. Typical anode material, Ni-YSZ included for reference.  
means designs can vary greatly. A study in planar SOFCs by [PERSON] et al. (1998) showed that when current collector-electrode contact area was increased from 4.6% coverage to 27.2% the resistance of the cell decreased by 87% from 1.43 \(\Omega\cdot\)cm\({}^{2}\) to 0.19 \(\Omega\cdot\)cm\({}^{2}\) at 800 \({}^{\circ}\)C. Consideration to preventing the current collector from impeding gas flow to and from the electrode must also be made remembering that an impermeable/nonporous current collector covering the entire electrode surface would render the fuel cell useless. The tube dimension is a key factor in the design and problems can be common or specific to small/large bore and short/long variants. For example, in small-bore tubes (less than 2 to 3 mm ID), internal access is particularly difficult and the current collector size is limited, limiting the size and wire thickness and thus its current-carrying ability (with respect to voltage loss), which is exacerbated if the length of the cell and contact wires are long. For larger bore tubes, achieving a suitable spring force (arising from the elasticity of the contacting material), which is often relied on for smaller bores, becomes difficult due to the softening of the material at an operating temperature which has a more significant impact on larger structures than smaller.