# The Development of Current Collection in Micro-Tubular Solid Oxide Fuel Cells--A Review
## 7 Current Collector Sizing, Spacing and Positioning
They found that two current collectors, one at either end, were the optimal configuration, incurring a penalty of loss of active area and increasing ohmic resistance if they deviated, as seen in Figure 16c. Contrary to this finding, [PERSON] et al. [141] found that for their cell geometry, a single current collector node, at the centre of the cell, was superior to that of a cell with multiple anode current collectors, notably two at either end of the active area. The single central current collector incurred a 6 to 9% (estimated) loss in active area, whereas the two current collectors did not. The geometry of the cell was longer than that of [PERSON] et al. at 152 mm in length but had a similar diameter of 6.7 mm. Evidently, the cell geometry has a considerable effect when it comes to finding the optimal current collector configuration to maximise the total power from a cell. The experimental data presented in the literature are limited on this subject and more research needs to be conducted to get a better understanding. The data produced must be representative of the typical cell geometries and materials used in tubular SOPCs. Models built from this data could reliably predict the optimum sizing of current collectors for a wide range of cell designs and would be a cost- and time-effective tool. [PERSON] et al. [142] conducted a similar experiment with an anode-supported segmented tubular cell 48 mm in length and 8 mm OD. They positioned three 2.5 cm\({}^{2}\) cathode segments 2 mm apart on their in-house fabricated cell. They performed an initial comparison of the cell with all three cathodes connected in parallel to that of a cell with the same active area but with no segmentation and determined that the performance was the same and that the segmentation had no noticeable impact. They compared the performance from each cell segment with 120, 80 and 40 mL.min\({}^{-1}\) total flow rates of equimolar H\({}_{2}\) and N\({}_{2}\). For the highest fuel flow rate, they observed a similar performance from all three cell configurations which indicated a homogenous current distribution in the cell and sufficient volumetric flow rate to minimise the effect of concentration polarisation. The current density at 0.7 V, in this case, was around 0.125 A\(\cdot\)cm\({}^{-2}\) for the upstream, 0.105 A\(\cdot\)cm\({}^{-2}\) for the midstream, and 0.1 A\(\cdot\)cm\({}^{-2}\) for the downstream segments, respectively--i.e., values were within 23% (or less) of each other. Reducing the flow rate resulted in a larger difference of 50% (or less) between current density values at 0.7 V. The current density of the upstream cell remained at 0.125 A\(\cdot\)cm\({}^{-2}\) while the midstream and downstream segments dropped to 0.075 and 0.095 A\(\cdot\)cm\({}^{-2}\), respectively. The largest performance decrease of 29% occurred in the midstream segment. [PERSON] et al. ascribed this to a relatively low activation potential at the downstream cell versus midstream cells, arising from improved anode kinetics due to increased partial pressure of water, the reaction product. For the lowest flow rate, the current distribution was highly inhomogeneous, with a difference of 58% in current density between segments at 0.7 V. The upstream cell was reduced to 0.11 A\(\cdot\)cm\({}^{-2}\) whilst the midstream and downstream segments were down to 0.06 A\(\cdot\)cm\({}^{-2}\). Increasing the polarisation of the downstream beyond 0.7 V gave a somewhat unusual result--the peak current density was reached at 0.45 V. Increasing the polarisation beyond this did not increase the current density--in fact, the converse. Meanwhile, the current density of the upstream segment increased to a value higher than for higher flow rates. The starvation of the downstream segment came from the low inlet flow rate, compounded with the consumption of fuel and the increase in reaction product (water) from the upstream and midstream cell segments. The authors also measured the temperature at each segment for the lowest flow rate.