Highlights

Three Dimensional Reconstruction of Solid Oxide Fuel Cell Electrodes

Scientific Achievement

Utilizing a dual-beam focused ion beam scanning electron microscope (FIB-SEM), we have developed a 3D reconstruction method for solid oxide fuel cell (SOFC) electrodes.  This technique has enabled us to probe many microstructural properties such as Triple Phase Boundary length (TPB), phase connectivity, and tortuosity, and to establish that these have a significant impact on electrochemical performance.

We have analyzed the effects of compositional variations in Ni-YSZ anodes (YSZ = Y-stabilized Zirconia).  It was found that substantial isolated Ni and porosity existed in the sample with 40 wt% NiO, such that the electrochemically active TPB density was much lower than the total TPB density, and hence, the polarization resistance was greatly increased.  Additionally, high YSZ tortuosity in samples with low amounts of YSZ also contributed to reduced electrode performance.  

We have also quantitatively observed the evolution of Ni-YSZ electrodes due to Ni-coarsening at high temperatures.  Experimentally, an 11% decrease in Ni specific surface area was observed after 400 hours as well as a 9% decrease in TPB length.  The original structure was also input for a phase-field simulation of the microstructure evolution based on Ni surface diffusion.  Comparing experimental coarsening structures with simulated ones has allowed us to better understand this degradation mechanism that over time contributes to the unstable operation of SOFCs.

Significance

State-of-the-art fuel cell electrodes typically have a complex micro/nano-structure involving interconnected electronically and ionically conducting phases, gas-phase porosity, and catalytic surfaces. Understanding this microstructure is a critical and typically missing link to understanding the electrode performance, given a set of processing conditions.  This work was the first to three-dimensionally reconstruct a solid oxide fuel cell (SOFC) electrode and continues to lead the field in correlating performance to processing.  Additionally, measurements of electrochemically-active TPB density and phase tortuosity in anodes were the first published results of this kind, and were used to better understand microstructure – performance relationships between anodes of different compositions.  These methods also allow, for the first time, a way to study the underlying fundamental properties of Ni coarsening by comparing experimental coarsened structures to those achieved through computer simulations.

Performers