# The Development of Current Collection in Micro-Tubular Solid Oxide Fuel Cells--A Review
## 3 Current Collection in \(\upmu\)T-SOFCs  
To extract power from a tubular SOFC cell, current collectors must be used at each electrode. The low voltage characteristic of a fuel cell means cells are typically stacked to achieve a useful power output by interconnecting current collectors of adjacent cells in series. Current collection in tubular SOFCs is complex and is particularly tricky at the \(\upmu\)T-SOFC scale. Minimising current conduction pathways and thus minimising the resistance to electron flow is critical in reducing the ohmic resistance of interconnections [4; 5]. In planar configurations, the interconnect is in-plane with the electrodes of repeating cell units, contacting a large area of the electrochemically active region, resulting in short, mostly out-of-plane electron conduction pathways [6]. In tubular configurations, however, cells are spaced apart. The minimum limit for cell spacing which is controlled by the manifold design is determined as a function of assembly logistics, wiring logistics and by the thermal management and gas supply design. These restrictions limit compactness, resulting in an interconnection network with a considerably higher pathway length, as compared to the planar configuration. The cylindrical geometry plays a factor in the complexity, as current flows in the axial and radial dimensions (seen in Figure 2), increasing current conduction pathway length and leading to complexity in the interconnect design [46]. The direction of the current flow within each cell and throughout a stack can vary greatly with the current collector/interconnect configuration. The early micro-tubular cells of the 1990s, developed by [PERSON] and [PERSON] from Keele University (UK), had \"internal\" anode current collection, typically nickel or silver, either as a spring-loaded coil or as a rolled mesh inserted within the cell (on the bore side in the fuel channel). Cathode current collection was typically achieved via silver wire wrapped/wound around the cell \"exterior\" [47; 48; 49; 50]. Anode current collection changed direction somewhat in the 2000s when research from AIST (Japan), such as [PERSON] [51; 52] and [PERSON] [PERSON] (GB) [53; 54; 55], developed a technique to contact the anode from the cell exterior (external anode current collection) by either partially covering the anode support with electrolyte during fabrication, or by moving a portion of electrolyte to expose the anode interior (of an anode-supported cell). A schematic of internal and external current collections of an anode-supported tubular SOFC is shown in Figure 6.  
Results from the latter researchers showed that loss of contact between internal current collectors and the electrode due to thermal mismatch/shrinking and improper contacting led so severe degradation, indicating that R&D into a method for interconnect-electrode contacting for internal current collector designs was needed. The externally contacted anode showed enhanced electrochemical stability when tested and a reduced degradation rate. However, the new style necessitated sealing of the porous, exposed anode with high-temperature gas-tight sealants, another key area of development in SOFC technology [56]. Accessing the anode from the cell exterior leads to a loss of external cell surface which could  
Figure 6: Cross-sectional schematic of internal and external current collection in an anode-supported \(\upmu\)T-SOFC with available internal anode current collector locations (not to scale). be otherwise used for current production. However, using an internal current collector does not incur such penalties and thus the entire cell exterior can be utilised as indicated by the available internal anode current collector locations seen in Figure 6. Therefore, for a given cell length, using an internal current collector leads to a higher total power output potential from a cell.