# The Development of Current Collection in Micro-Tubular Solid Oxide Fuel Cells--A Review
## 7 Current Collector Sizing, Spacing and Positioning  
It is intuitive to expect that current collection from a larger area of the electrode will decrease the ohmic losses of a cell. Contacting the entirety of the electrode, however, is unfeasible, blocking fuel transport to the electrode as well as increasing weight and cost. Depending on the configuration, maximising the current collecting area and thus minimising ohmic loss could lead to a loss of available active area which will reduce the total amount of current that can be produced. Therefore, there is a trade-off in the current collector design concerning the sizing of the current collector node, positioning on the electrode and, if opting for multiple nodes, their spacing. Collecting current from either end of an anode has been common in the literature for externally accessed anode current collectors. As can be seen from Figure 15, this has the effect of shortening the distance that the electrons travel from where they are produced, as compared to a single collection node [135]. Figure 15: Current conduction pathway for current collection at both ends of the electrode versus one end (blue arrows indicate electron direction). As cells become longer, the distance between current collectors at each end of a cell increases and the need for additional current collectors can be justified. Some empirical research has been conducted to determine just how many current collectors are needed for a particular cell length. To do so, the cell is segmented, with multiple current collector nodes spaced along the electrode in question. Although the anode is typically an order of magnitude more conductive than the cathode, nickel-based anode cermets typically have an electronic conductivity of similar order of magnitude (and below) to the conductivity of pure nickel, which is around 2.7 \(\times\) 10\({}^{6}\) S-m\({}^{-1}\) at 750 \({}^{\circ}\)C. Values of the effective cermet electronic conductivity reported in the literature vary between 1 \(\times\) 10\({}^{6}\) and 1 \(\times\) 10\({}^{5}\) S-m\({}^{-1}\) at typical SOFC operating conditions [137]. Traditional cathodes have relatively lower electronic conductivities compared to the anode at around 1 \(\times\) 10\({}^{3}\) S-m\({}^{-1}\) but more recent mixed ionic-electronic conductors such as LSCF are comparable and can display electronic conductivities up to 1 \(\times\) 10\({}^{4}\) S-m\({}^{-1}\), depending on composition and microstructure [138; 139]. [PERSON] et al. [140] looked into determining the optimal number of anode current collectors of their 4.8 cm long and 7 mm outer diameter (OD) segmented single-chamber fuel cell. They spaced five current collectors along the length of the cell, creating four active areas connected in series. The geometry and configuration of the current collectors are seen in Figure 16a. They found that increasing the number of current collectors from the single inlet and the symmetrically spaced inlet and outlet, inlet and centre and outlet, and then all five positions increased the power density from the cell setup as seen in Figure 16b. They did not, however, look at the effect of the positioning of a single current collector node  
Figure 16: Segmented tubular cell (**a**) anode current collector configurations; number of anode current collectors versus (**b**) maximum power density and (**c**) total power available from the cell [140]. on performance, measuring only the current observed from a single current collector at the inlet. The authors only presented data from 5 of the 31 possible current collector combinations for this setup. They did, however, consider the resulting loss of active area from increasing the number of current collectors, each collector incurring a 5.6% loss of the total active area available. They used the power density data from their segmented cell to estimate the total power that could be derived from a cell of the same geometry but with a different number of current collectors.