Abstract
The performance and durability of anode supported solid oxide fuel cells is heavily dependent on the porosity of their anode layer. An optimal amount of porosity is needed to achieve high performance without compromising the flatness, and by extension, the hermetic sealing of the cell. This presents challenges for commercialization, as anode porosity heavily influences the densification of the cells during sintering. In this work, the effect of pore former loading was evaluated on both cell performance and curvature. It was also found that anodes with an intermediate amount of pore former had the best performance but also the most curvature. The mechanism behind this was evaluated with an optical dilatometer. As expected, when cells were sintered with thicker anode support layers, they resulted in significantly flatter cells. Improvements to performance but not flatness were achieved with the use of graded porosity in the anode, with high porosity at the fuel side that decreased closer to the electrolyte. This effect was more pronounced at lower temperatures and fuel ratios, further demonstrating that this is a more efficient microstructure for the anode than a typical anode with uniform porosity. The use of graded porosity was then evaluated as a means of improving the performance of thicker anodes, to take advantage of their greater flatness and mechanical strength. Unfortunately, thicker anodes exhibited a significant drop in performance. It was found that the greater distance for gas diffusion in thicker anodes means that a different microstructure is needed for efficient gas flow and charge transfer. In the case of thicker anodes, it was found that having two anode regions, one thinner region with moderate porosity adjacent to the anode functional layer and one with high porosity making up the bulk of the anode resulted in the best performance. This allows sufficient fuel flow in a thicker anode, without significantly reducing the activity of the anode. In this way, this anode microstructure behaves similarly to an anode functional layer, which is well known to improve fuel cell performance. While graded anode microstructures have been reported before, there have been few, if any, studies that evaluated both their performance and flatness simultaneously. Optimization of anode porosity in anode supported cells is essential to improving both their performance and reliability, which are necessary for widespread adoption of SOFCs.