Enhanced Nano-Catalyst Infiltration of Anode-Supported SOFCs through Surface Modification of Electrodes By Catechol Surfactants
Authors: Ozmen Ozcan , Lee Shiwoo , Zondlo John W. , Hackett Gregory A , Abernathy Harry , Sabolsky Edward M. 
Published: February 2020

The wet impregnation/infiltration method has been a well-utilized route to incorporate nanoparticles inside the electrode architecture by co-firing the metal salt solutions that are wicked to the internal pore structure. Impregnation/infiltration of catalytically active nanoparticles brings an extension of the phase boundaries in the active area which reduces the polarization resistance. There are three main infiltration parameters that impact on reducing the electrode polarization resistance and over potential: a) nano-catalyst concentration, b) nano-catalyst location after infiltration, and c) nano-catalyst decoration. It should be noted that those factors are all interrelated and can fail or be achieved altogether. The nanocatalyst concentration is usually controlled by performing repetitive infiltration steps. In these experiments, co-firing is needed after each infiltration step, to open up free network path inside the electrode, then the process is repeated until the desired amount is achieved. One major potential risk of repetitive infiltration is the clogging of the porosity. This labor-intensive protocol also suffers from inhomogeneous deposition and gas starvation issues during the cell operation due to the inefficient decoration of the infiltrates at the active area where the deepest region of the electrode is located. Otherwise, the metal nitrate solution dries at the top or middle region of the electrode and the nano-catalysts sits far away from the active layer, hence the impact in polarization resistance reduction can be seen less.

Due to these issues, there is a need to control the deposition rate and achieve a homogenous nano-catalyst distribution and reduce the number of infiltration steps. One solution is adding wetting agents to the precursor mixture to enhance the penetrability. Pressure or vacuum assisted infiltration can also help to wick the solution through the electrode.

In this study, catechol-based molecules which are a specific type of bio-monomers found in the nature of multi-functional end-groups, such as dopamine, poly-norepinephrine (pNE), and derivatives are used as electrode surface modifiers. Apart from the neurotransmitter functions, these end-groups are naturally capable of strong adhesive properties when bio-monomers are linked and polymerized. The polymerized surfactant can bind with the metal cation by chelation. Within the past decades, catechol based molecules are used as bio-derived surfactants in various coating studies. Mostly, studies have been focused on planar, dense and single-phase substrates. However, SOFC electrodes of percolated porous architecture vary tens to hundreds of microns in thickness. The porosity level depends on the SOFC type and manufacturing method, but can vary between 10-50%. In addition, the electrodes consist of multi-constituents. For example, conventional anode electrode comprises nickel oxide (NiO) and yttria stabilized zirconia (YSZ) mixture and the conventional cathode is composed of lanthanum strontium manganite (La1−xSrxMnO3 , LSM) and yttria stabilized zirconia (YSZ).

The motivation of this work is to monitor the long-term stability of the polymerized norepinephrine assisted CeO2, ceria nano-catalyst infiltrated commercial button SOFCs by single step infiltraton. Investigating the optimum nano-catalyst concentration for that particular off-the-shelf SOFC architecture is also a focus area for this work.

The initial part of the study investigated the deposition and polymerization kinetics of the catechol-surfactant film on NiO, YSZ and LSM surfaces. Deposition parameters such as immersion time, solid loading and polymerization technique were all studied. The study aim was to pinpoint the acceptable polymerization degree and adhered film/particle layer thickness range to enhance homogeneous coating of porous SOFC electrode walls by preventing pore clogging. The experiments showed that the pNE coating layer can be tuned from 25 nm to 150 nm. In the second part of the study, these coated substrates were immersed into the cerium nitrate solution to form CeO2, ceria, nanoparticles to observe the ceria layer coarsening as a function of the pNE layer thickness and cerium precursor immersion time. The study assisted in understanding the optimal ceria decoration after the annealing process at the SOFC operating temperature (750oC).

Experiments showed that tuning the immersion time and solid loading induced nano-catalyst decoration from discrete particles to film formation onto the planar substrates. The optimal protocols were applied to impregnate anode-supported SOFC button cells. The electrochemical performance and the stability of the infiltrated anode-supported SOFCs were measured over 300 hours and the performance was correlated to the decoration of the nano-catalyst impregnated electrodes. The developed impregnation protocol resulted in >20% increase in power density at 750° when the same protocol was applied to a commercial anode-supported SOFC.

Acknowledgements:

The authors would like to thank James Poston at NETL-Morgantown for his assistance in SEM/EDS characterization. The WVU Shared Research Facilities are also acknowledged for their assistance in materials characterization.