Towards the Design-Led Optimization of Solid Oxide Fuel Cell Electrodes

Electrode microstructure plays an important role in determining the performance and durability of solid oxide fuel cells (SOFCs). It needs to be tailored towards a variety of requirement for the electrodes, such as transport properties and electrochemical activity. Long-term stability of the microstructure is also important to guarantee the durability of SOFC systems. In order to satisfy the growing demand for their performance and durability, an improved understanding of the microstructure-performance relationships is desired, along with optimization of the microstructure, through a design-led approach.

The focus of this study is on electrodes fabricated with the nano-particle infiltration techniques. Porous framework structures (scaffolds) were first fabricated with gadolinium-doped ceria (GDC), and then nickel metal particles were introduced into the structures in the form of nano particles. 3D microstructural analysis using focused ion beam and scanning electron microscope (FIB-SEM) was carried out and revealed that the infiltration technique has a greater potential, compared with the conventional approach based on powder mixing and sintering, to control the electrode microstructure, helping satisfy multiple requirements for the electrodes. Most notably, the triple-phase boundary (TPB) density in the infiltrated electrodes was found to be one order of magnitude larger than that typically found in the conventional electrodes.

A 1D numerical model for Ni-GDC electrodes was also developed assuming that the GDC phase has mixed ionic and electronic conductivity and hence that the electrochemical reaction takes place on the GDC-pore double-phase boundary (DPB), and successfully reproduced the overpotential characteristics obtained from the experiment. Sensitivity analysis was also conducted with the developed model at 700 °C to investigate the effect of electrode microstructure on electrode performance. This revealed that the electrochemical reaction on the DPB is the rate-determining process within the electrodes; therefore increasing the DPB density is recommended as the most effective route to improving performance of ceria-based electrodes, rather than improving species transport rate.

Using the insights from the experiment, microstructural analysis and numerical simulation, guidelines for further optimizing the electrode microstructure are proposed