# The Development of Current Collection in Micro-Tubular Solid Oxide Fuel Cells--A Review
###### Abstract  
Micro-tubular solid oxide fuel cells (uT-SOFCs) are suited to a broad range of applications with power demands ranging from a few watts to several hundred watts. uT-SOFCs possess inherently favourable characteristics over alternate configurations such as high thermo-mechanical stability, high volumetric power density and rapid start-up times, lending them particular value for use in portable applications. Efficient current collection and interconnection constitute a bottleneck in the progression of the technology. The development of current collector designs and configuration reported in the literature since the inception of the technology are the focus of this study. micro-tubular SOFC; current collection; interconnection; ohmic polarisation; conduction pathway length 2021  
The Development of Current Collection in Micro-Tubular Solid Oxide Fuel  
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8% were for portable applications. Out of the units shipped in 2018, 75% were in Asia. Japan contributes strongly to the fuel cell market--particularly for the stationary sector, where SOFCs are increasingly being adopted for domestic and commercial CHP and more than 100,000 units have been installed in Japan within the EneFarm programme [13]. The next largest markets with respect to units shipped are North America, followed by Europe. Polymer electrolyte fuel cell (PEFC) technology is increasingly being adopted in the transport sector in fuel cell/hybridised vehicles including forklifts, buses, boats, trams, trains, drones, motorcycles, submarines and automobiles such as the Toyota Mirai or Hyundai ix35 [14; 15; 16; 17; 18; 19], and even aircraft. PEFCs are the most widely adopted fuel cell technology in terms of units shipped over the last five years as seen in Figure 1b [12]. Of the 68,500 units shipped in 2018, 58% were PEFCs, 36% were SOFCs, 5.4% were DMFCs and the rest PAFCs. SOFC technology is less mature than that of PEFCs. The high SOFC operating temperature poses a significant difficulty, particularly surrounding material selection and design. SOFCs are available in several geometries, planar, tubular, and mixed types. Tubular cells are subdivided by size, regular tubular SOFCs and \(\upmu\)T-SOFCs. \(\upmu\)T-SOFC is a rather broad definition and typically means cells with external diameters of <10 mm. Generally speaking, tubular SOFCs are favoured for portable applications due to their mechanically durable natures and rapid start-up capabilities, which are particularly prevalent at the micro-tubular scale [1,2]. Figure 2 shows a schematic of a tubular cell with the operating principle and an indication of the current direction in the axial and radial dimensions. Planar SOFCs are typically restricted to stationary applications due to their superior power densities and bulky/heavy system designs. Tubular SOFC technology is less mature than planar geometry as the higher demand for stationary power seen in Figure 1a has driven the development. However, the ever-increasing demand for power-hungry portable devices presents an opportunity for \(\upmu\)T-SOFC technology. As such, research and development into SOFCs, and specifically \(\upmu\)T-SOFC technology, must address key bottlenecks to be competitive. The key areas for development in SOFCs are material development [20; 21; 22; 23; 24; 25], current collection/interconnection, sealing [26; 27] and stack/system design [28; 29; 30]. Tubular SOFC current collection/interconnection development is somewhat more geometry-specific than the other aforementioned bottlenecks versus planar designs and thus is the focus of this study. The current collector design can have a significant effect on the performance and durability of a tubular cell [5]. Loose connections lead to unstable cell performance and ultimately cell failure. Current collector designs must be secure enough to withstand impacts incurred during portable applications.