The Influence of Reduction Conditions on a Ni-YSZ SOFC Anode Microstructure and Evolution

Polarization resistance on SOFC electrodes is principally due to resistances associated to the process taking place in the electrocatalytically active region. This region is known as triple phase boundary (TPB) in common, non-MIEC (Mixed Ionic -Electronic Conductors) based electrodes. For this reason the electrode microstructural configuration, as determining for the extent of TPB accessible for reaction, is decisive regarding the electrode polarization resistance. It is also critical in order to develop a high performing device to minimize ohmic resistances through it. A possible strategy towards this goal is to maximize the conducting phase connectivity. Although it is generally assumed that the cathode accounts for the major contribution to the global polarization resistance in the Ni-YSZ/YSZ/LSM-YSZ system, the anode contribution is susceptible to be optimized to some extent by maximizing the TPB length and phase connectivity present in said electrode.

A general practice when manufacturing Ni-YSZ porous electrodes is to start from nickel oxide powders, and have them reduced after the last processing step and before the first operation of the electrode. This nickel oxide to metallic nickel reduction features a volumetric shrinkage close to 40%, forming a new porous network through the volume previously filled by the nickel oxide phase. As the ion conducting phase, in this case YSZ, generally shows negligible mobility at common reduction temperatures, it will act as a backbone during nickel reduction preventing the whole electrode to shrink.

The final metallic nickel microstructure, will depend on the initial nickel oxide phase microstructure, but also on the reduction conditions. This final structure is on one hand determined by the thermodynamically most stable configuration, mainly defined by the minimization of the surface energy, but on the other hand it has been observed that kinetics also play a major role on this final configuration.  It has been observed that nickel phases tend to coarsen at high temperatures during long term operations [1].  This is due to the fact that Ni atoms present a significant amount of diffusive mobility through the nickel phase in the most common operation conditions, and as the thermodynamic constraint of the minimization of surface energy will always be present, some degree of evolution is expected during long term operation.

On this work several samples of Ni:YSZ:Pore anode cermet (proportions 1:1:2 in volume approx.) was characterized after reduction at different temperatures and gas compositions. Two initial nickel oxide particle sizes where also tested.  An indirect measurement of the Ni phase connectivity can be made from electrical conductivity measurements [2]. Selected samples were observed using a field emission scanning electron microscope (FESEM). On the range of reducing conditions explored, a trend towards better performance was observed on samples reduced at higher rates and with finer initial oxide microstructures. The most extreme conditions tested lead to the formation of a singular sponge-like nickel structure.

REFERENCES

[1] Journal of The Electrochemical Society, 160 (11) F1293-F1304 (2013)

[2] Solid State Ionics 189 (2011) 82–90