Understanding mass transport mechanisms in oxygen transport membrane porous support layers: correlating 3D image based modelling with diffusion measurements

The rate limiting step of an oxygen transport membrane at high fuel conversation ratios is governed by mass transport limitations of the gaseous reactants through the porous support layer of the device. Such transport limitations are directly linked to the microstructural characteristics of the porous support layer including porosity, tortuosity and pore size distribution. Among these parameters, tortuosity is the most crucial for diffusion calculation processes but notoriously difficult to quantify. The porous support layer is an indispensable part of the overall membrane assembly as it provides mechanical stability during operation as well as providing facile routes for delivery of reactants. By combining different imaging techniques, diffusion cell experiments and simulations, the connection between the microstructure and mass transport of tubular, yttria stabilized zirconia porous support membranes is explored. Lab-based X-ray nano computed tomography and focused ion beam scanning electron microscope tomography are used to reconstruct the microstructure of the porous support layers in 3D and extract the tortuosity. In addition, diffusion cell experiments at temperatures of up to 600 °C are carried out on the same samples. It is shown that image based algorithms provide lower tortuosity values in comparison to diffusion cell experiments. The reason for this is found in the lack of considering Knudsen diffusion effects, which are often neglected in diffusion simulation models. Moreover, it is found that tortuosity alone is insufficient to provide conclusive insights when evaluating the mass transport resistance of a microstructure. A holistic approach, where additional parameters, such as porosity and sample thickness, are taken into account, is recommended. The experiments have shown that to ensure high mechanical stability and high mass transport performance at steady state, the porous support layer should feature high porosity and high thickness. The obtained insights are used to optimise future support designs in collaboration with industrial partners.