# The Development of Current Collection in Micro-Tubular Solid Oxide Fuel Cells--A Review
## 5 Materials for Current Collection
### Micro-Tubular SOFC Geometry-Specific Current Collector Material Trend  
In tubular configurations, solely the electrolyte provides the barrier between the fuel and oxidant atmospheres, giving an additional degree of freedom to interconnect material choice, potentially tailoring to suit either the anode or cathode side. This, combined with the lack of requirement for load-bearing/compressive strain, has led to the use of highly conductive but costly precious metals, the most common being silver, followed by platinum, gold and palladium. These precious metals are known for their inertness and stability in high-temperature redox environments and do not form nonconductive oxide layers in air at elevated temperatures. Nickel, a nonprecious, favourably low-cost but relatively less conductive metal, is also frequently used as seen in Figure 9.  
While most frequently used, silver is not without fault. [PERSON] et al. [79] suggested using silver as an SOFC interconnect at temperatures below 650 \({}^{\circ}\)C to avoid loss of silver via sublimation, evaporation and diffusion. Degradation is particularly prevalent in silver under a dual atmosphere (redox) which may occur if silver is being used as a conductive paste on an externally accessed anode current collector or around fuel manifolds. Oxygen and hydrogen permeate the structure and form steam bubbles, leading to cavity nucleation and growth, and henceforth a loss of hermeticity [80]. [PERSON] et al. [81] observed the degradation of silver in a redox environment within 8 h of operation when used as a \(\upmu\)T-SOFC current collector at 700 \({}^{\circ}\)C. They observed temperatures 100 \({}^{\circ}\)C above the furnace temperature at the interface, indicating fuel crossover and resulting (spontaneous) combustion due to porosification of the originally gas-tight/dense silver sealant. Any silver migration could also lead to short-circuiting and reduced performance. While the melting temperature of silver is 961 \({}^{\circ}\)C, higher than typical SOFC operating temperatures, softening (annealing) of the wire and hence the loss of mechanical integrity occurs at far lower temperatures, a key factor for loss of interconnect-anode contact of silver-based internal current collectors [82]. [PERSON] et al. [83] attributed the failure of their four-cell stack to severed silver wires after only 4 h of operation at 800 \({}^{\circ}\)C and stated an urgency in finding an alternative current collector material for their tubular cells. While nickel is somewhat stable in reducing environments, it undergoes high-temperature corrosion in oxidising environments, causing the formation of nonconductive oxide layers, hence its use on the anode side only (unless coated). Stable as it may be in reducing envi  
Figure 9: Current collector materials used in \(\upmu\)T-SOFC tests reported in literature between 2015 and 2020.  
ronments, nickel is a well-known reforming catalyst, which is employed in Ni-YSZ anodes for the electrochemical oxidation of hydrogen and in direct internal reforming. When used as an anode side interconnect with hydrocarbon fuels, there will likely be a reduction in performance of the interconnect over time from carbon formation [84]. The metallic structure, high melting point and thermo-mechanical integrity of nickel across SOFC operating conditions favour it as an affinitive base metal to support well-adhered protective coatings. Coating with a more inert/corrosion resistant, more electrically conductive and hard/hard wearing natured materials such as gold, silver, nickel alloys and even perovskites can enhance the electrical performance and durability of nickel as a current collector for both the anode and cathode. Coatings can be achieved by simple painting/dipping techniques, by electroplating, a current-driven process, or by electroless plating, a chemically driven process, of which the latter two can be produced at scale.