# The Development of Current Collection in Micro-Tubular Solid Oxide Fuel Cells--A Review
## 2 Tubular Solid Oxide Fuel Cells  
Solid oxide fuel cells operate at high temperatures between 650 and 850 \({}^{\circ}\)C and exhibit typical electrical efficiencies between 50 and 60%, which is high when compared to other common fuel cells. If the heat generated by the SOFC is deemed useful, a combined heat and power (CHP) efficiency can be claimed at around and above 80-85%. The \"high quality\" heat lends itself to integration into highly efficient steam and gas turbine power production systems [31]. Figure 3: Key areas of development in micro-tubular solid oxide fuel cell (SOFC) technology and breakdown of current collection/interconnection topics covered in this review. References * ([20; 21; 22; 23; 24; 25]), ** ([26; 27]), *** ([28; 29; 30]). Figure 2: Tubular solid oxide fuel cell schematic with cross-sectional view, operating principle and current direction. Cells can either be self-supporting or externally supported. In the former, the electrode or electrolyte provides mechanical support for the cell [32]. In the latter, an external inert support may be used such as a porous ceramic material or the interconnect itself. Early tubular and planar cells were either cathode or electrolyte-supported but now the standard is anode-supported which allows for lower operating temperatures and ohmic polarisation, coupled with improved mechanical stability. Relying on the relatively more conductive anode as a support also comes with issues surrounding mass transport losses through the porous structure, the potential for reoxidation and loss of mechanical integrity and performance during operation. Externally supported cells are increasingly common in planar SOFCs, relying on the interconnect, typically a metal, or an inert ceramic for mechanical support. A schematic of tubular SOFC cell support configurations (which is also applicable to planar geometries) and the progression of the state of the art are seen in Figure 4.  
The trend of SOFCs towards a lower operating temperature has widened the material selection for SOFC components [33]. Metal-supported planar cells have lower thermal masses and are less brittle than a ceramic-supported cell and so can handle faster start-up times [34]. However, the operation is typically limited to lower temperatures (550 to 650 \({}^{\circ}\)C) to avoid rapid degradation of the metallic components which restricts the use of internal reforming of hydrocarbons, typically occurring above 650 \({}^{\circ}\)C. In tubular devices, porous inert supports such as ceramics and metallic supports have been used. This allows for a segmented tubular cell design, essentially stacking cells along a support similar to integrated planar designs [35; 36]. The externally supported SOFCs allow thinner electrode layers and electrolytes, incurring lower ohmic and mass transport losses. Similarly to planar interconnects, tubular supports can contribute to controlling gas flow to improve fuel utilisation. If electrically conductive, the support can also contribute to the current collection. Regular tubular were the first tubular SOFC, developed by Siemens Westinghouse, having a typical diameter of 1\" and 2200 mm length [37]. Micro-tubular SOFCs developed by Kendall in the early 1990s had diameters in the order of a few millimetres and a length one or two orders of magnitude shorter than their larger predecessor [38]. The inverse proportionality of power density with tube radius means smaller cells have higher power densities [39]. More recently, tubular SOFC cells have been reported with larger diameters in the order of a few centimetres giving a higher total power per cell. This reduces the number of cells required in a stack for a given power output which again reduces complexity in manufacturing and assembling. Nevertheless, the larger size brings less mechanical stability.