# The Development of Current Collection in Micro-Tubular Solid Oxide Fuel Cells--A Review
## 5 Materials for Current Collection
### Metallic Interconnects  
Metallic interconnects come with their own problems, namely the increase in ASR and decrease in mechanical properties resulting from reactivity in redox environments through high-temperature corrosion mechanisms, as seen in Figure 8 [69; 70; 71]. It should be noted that the ASR values of a current collector material are not the same as the ASR values of a cell using this material as a current collector. The ASR of a cell will be significantly higher given the additional contribution of the electrodes and electrolyte. The ASR of a material has a contribution from the bulk conductivity and the surface contact resistance, the resistance at the interface of that material with the measuring device/adjacent materials. Chromium is typically used as an alloying element in metal interconnects to adjust the coefficient of thermal expansion (CTE) to fuel cell electrodes and the electrolyte. The chromium content is also important as it serves as a reservoir for the formation of a protective coating to prevent further oxidation. For an SOFC interconnect the ideal chromium range is between 18 and 22 wt%, above which it contributes considerably to the ohmic resistance and below which it will not form the protective coating. Volatile chromium species migrate to the cathode which causes poisoning [72]. Nevertheless, protective barrier layers can be applied to the exterior surface of the interconnect to minimise these effects. Studies to mitigate such degradation have focused on optimisation of the composition and application techniques of the protective coatings such as (Mn,Co)\({}_{3}\)O\({}_{4}\) and the control of naturally formed oxide layer growth, particularly in stainless steels [58; 73; 74; 75].  
A key advantage of metallic current collectors is the ability to be joined to the electrodes, with other current collectors (interconnection), and to stack components with standard welding or brazing techniques [59; 76]. A compressive load is applied to a planar stack to ensure intimate contact between fuel cell and interconnect components which is not possible in a tubular configuration, further highlighting the need for secure interconnect-electrode and interconnect-interconnect joints able to withstand the thermally induced stresses and mechanical impacts incurred during operation [77]. Welding and brazing both use heat to melt a filler material that joins parent materials together. Welding is limited to metals and requires a higher temperature than brazing as the parent metal must also melt. Welding is suitable for exterior joints or where localised hot spots caused by the welding flame are not an issue. Welded joints often have very high strengths and can be fabricated at scale at a low cost. Brazing can be achieved at far lower temperatures than welding and both metals and nonmetals, such as ceramics, can be brazed as only the filler material melts. Brazing can be performed by induction, whereby a current passes through a magnetic metal brage causing it to flow and form a joint once cooled. Brazing can also be performed in a furnace with a flame or electrical heating. The furnace can be used to carefully control the heating/cooling rate of the parent and brage materials which can alleviate CTE mismatches between components and avoid the formation of mechanical stress and failure in fuel cell components such as the electrolyte.  
Figure 8: Area-Specific Resistance (ASR) evolution of typical interconnect materials over exposure time [6].  
Gas tightness is an imperative interconnect property when acting as a barrier between opposing chemical environments as any leakage would lead to fuel combustion, fuel which will not be converted to electrons, leading to thermal hot spots which accelerate local degradation [78].