# The Development of Current Collection in Micro-Tubular Solid Oxide Fuel Cells--A Review
## 6 Current Collector Shape and Structure
### Structurally Integrated/Embedded  
A novel interconnection methodology was first reported by [PERSON] et al. [133] in 2011 and later in 2013 [134]. The design involved the integration of current collector wires into the anode support during the manufacturing process as seen in Figure 14b. The wire is coiled around a sacrificial central support pin (pencil led/carbon fibre) and then \"dip-coated\" with layers of the anode. The wire passes through the entire length of the anode and protrudes from either end of the tube.  
Electrochemical performance of two cells with the internal and embedded designs seen in Figure 14a,b, respectively, were reported. At 750 \({}^{\circ}\)C, operating on dry H\({}_{2}\), the embedded wire achieved a current density of 0.15 A\(\cdot\)cm\({}^{-2}\) at 0.7 V. This was a 3.8-fold increase in performance when compared to the internal current collector wire as seen in Figure 14a, which achieved 0.04 A\(\cdot\)cm\({}^{-2}\). Under the same conditions, the peak power  
Figure 14: (**a**) Internal current collector and (**b**) embedded current collector, where A, C, E and O refer to the anode, electrolyte, cathode and outer current collector, respectively [134].  
Figure 13: Brush current collector: (**a**) AMI design front view, (**b**) AMI design side view, and (**c**) schematic in a tubular cell with cross-sectional view adapted from AMI [129].  
densities were 0.035 and 0.142 W\(\cdot\)cm\({}^{-2}\) for designs a) and b), respectively. The ASR of the embedded cell as estimated from the gradient of the polarisation curve was 1.74 \(\Omega\cdot\)cm\({}^{2}\). The configuration allowed for current collection along the entirety of the anode length, ensuring a well-distributed anode electron conduction profile. The option to collect the current from either end of the cell will approximately halve the lateral current conduction pathway of electrons in the wire compared to a single end connection, also reducing the axial path length. Integrating the wire into the anode ensured sound mechanical contacting and might have somewhat shielded the wire from the harsh gas stream environments. However, some concern surrounded the difference in CTE of the nickel wire and the Ni-YSZ anode which could lead to bending and cracking of the anode, the wire and the surrounding fuel cell components, leading to failure [135].  
In 2015, [PERSON] et al. [136] looked into the effect of coiling anode-embedded, nickel and palladium wires, versus an embedded straight wire. They changed the number of coil rotations (coil density) in a given length to increase the surface area and amount of wire available for collecting current. They found that increasing the nickel wire coil density (by 83%) from six turns.cm\({}^{-1}\) to 11 turns.cm\({}^{-1}\) significantly increased the performance. They observed a 3.5-fold increase in current density at 0.7 V and 800 \({}^{\circ}\)C from 0.055 to 0.19 A\(\cdot\)cm\({}^{-2}\), citing a more homogenous current distribution and an increased contact area resulting in lower ohmic polarisation as the root causes. The ASR of the 11 turns.cm\({}^{-1}\) nickel wire as estimated from the gradient of the polarisation curve was 1.6 \(\Omega\cdot\)cm\({}^{-2}\) at 800 \({}^{\circ}\)C. The palladium wires broke, which did not allow a direct comparison between materials and number of coils. However, the authors tested a (nonembedded) palladium wire which achieved a similar current density of 0.2 A\(\cdot\)cm\({}^{-2}\) at 0.7 V and 800 \({}^{\circ}\)C operating temperature when compared to the 11 turns.cm\({}^{-1}\) nickel wire. The authors claimed that the palladium enhanced the catalytic activity of the anode, resulting in a higher performance. They did not attribute any difference in performance to the different conductivity of materials even though palladium is 35% more conductive than nickel at 20 \({}^{\circ}\)C and 6.8% more conductive at 750 \({}^{\circ}\)C.