# The Development of Current Collection in Micro-Tubular Solid Oxide Fuel Cells--A Review
## 6 Current Collector Shape and Structure
### Current Collecting Layers  
Somewhat similar to conductive pastes, current collecting layers can be used to improve the overall conductivity of an electrochemical cell. They can be electrode or nonelectrode materials, with properties biased towards current conduction rather than electrochemical/catalytic activity. The layers are typically formed in situ during cell manufacture and can also provide mechanical support for the cell. [PERSON] et al. employed a nickel anode current collector layer in their porous zirconia-supported tubular cells, as seen in Figure 11[110; 111]. In their 2017 study, they achieved a current density of 0.8 A\(\cdot\)cm\({}^{-2}\) at 0.7 V and a peak current density of 2 A\(\cdot\)cm\({}^{-2}\) operating in H\({}_{2}\) at 750 \({}^{\circ}\)C. The impedance at OCV under the same conditions was 0.44 \(\Omega\cdot\)cm\({}^{2}\).  
Figure 10: Cross-sections of a 6-channel \(\upmu\)T-SOFC cell with optimised microstructure [103] (**a**) overall cross-section, (**b**) zoom of micro-structure.  
[PERSON] et al. [112] also adopted a nickel current conduction layer in their \"inert-substrate-supported\" cells. They studied the effect of increasing the current conducting layer thickness, finding that thickening the layer was favourable for the electrochemical performance but not for the redox stability of the cell. For their optimal design, they achieved a current density at 0.7 V of 0.63 A\(\cdot\)cm\({}^{-2}\) with a peak power density of 2.5 A\(\cdot\)cm\({}^{-2}\) in dry H\({}_{2}\) at 800 \({}^{\circ}\)C. The ASR recorded at OCV at the same operating conditions was 0.75 \(\Omega\cdot\)cm\({}^{2}\). The degradation rate was 4 mVh\({}^{-1}\) over 11 redox cycles.  
[PERSON] et al. [113] looked into varying the ratio of thickness between a porous nickel anode current collecting layer and the anode, fixing the total thickness to 250 \(\upmu\)m. The cell was a closed-end tube, single-chamber design, meaning anode current collection was only possible from one end of the tube. They achieved a significant improvement with even the thinnest, 20 \(\upmu\)m of current collecting layer, increasing the peak power density over four-fold from 0.12 to 0.52 W\(\cdot\)cm\({}^{-2}\). This allowed them to reduce the thickness of their anode and current collector layer to 50 and 100 \(\upmu\)m, achieving 0.6 W\(\cdot\)cm\({}^{-2}\) from this design.  
[PERSON] et al. [114] developed a dual-layered, hollow fibre cell fabricated by a phase-inversion assisted coextusion methodology. They were able to achieve great control over the morphology of the nickel current conduction and Ni-YSZ anode microstructure. The objective was to improve the electrical conductivity and reduce the resistance to mass transfer throughout the cell; the former was quantified, observing a two-fold increase in electrical conductivity over a cell with no current collector layer as seen in Figure 12a. They formed finger-like protrusions in the axial (seen in Figure 12b) and radial dimensions of the anode and current collector to establish a mesh-like 3D structure.