Nano-catalyst infiltration within porous Solid Oxide Fuel Cells (SOFC’s) electrodes is a well-known process to increase electrochemical performance. Conventional dripping methods usually require multiple infiltration steps in order to achieve optimum catalyst loading. In this study, an alternative process was developed to efficiently infiltrate nano-catalyst into the NiO/YSZ anode of an anode-supported commercial SOFC. Mussel inspired catechol-based adhesives were used as a surfactant layer within the 3-D electrode architecture to better control the infiltration kinetics and dispersion of the nano-catalyst. The work assessed the use of adhesive molecules, polymerized dopamine (PDA) and/or nor-epinephrine (PNE), as the bio-template layer for either metal oxide or hydroxide deposition. The bio-template adhesiveness on the pore walls was shown to provide higher nano-catalyst loading by avoiding segregation of the precipitate within the porous structure, which usually results in the migration and preferential deposition of the solute at the surface (during the drying process). The first objective was to better understand the templating kinetics of PDA and PNE with altering processing variables (eg. pH, solid loading, immersion time, etc.), in order to control nano-catalyst deposition amount. The second objective was to control the catalyst grain growth rate by manipulating the level of nano-catalyst percolation and by altering the composition of the catalyst to hinder diffusion and grain boundary mobility. The process started by first immersing the cell into the polymerized template solution up to 24 h and then immersed directly into the catalyst precursor solution, with the same protocol. Cerium oxide was used as a demonstration catalyst composition for this work, but later work will focus on other catalyst compositions. A dip-coating process for the entire cell was used for this work in order to evaluate the possibility of infiltrating both the anode and cathode at the same time. The final step included a singular firing to 750oC to form CeO2 catalyst. In that case, only one thermal firing step was needed to achieve high solids loading levels, which is unlike conventional dripping methods where multiple thermal steps are required. A correlation between ~300 hour cell performance and nano-catalyst characterization was developed by I-V-P testing and impedance spectroscopy at various duration during the constant current loading. In one heat cycle, dopamine assisted dip-coated cells reached 1.1 mg to 1.6 mg deposition of nano-ceria incorporation (for the combined anode and cathode). The most promising of the protocols included a) ex-situ polymerization, (where the biotemplate solution was polymerized in a separate beaker before infiltration) and b) in-situ polymerization (where the biotemplate solution was polymerized when the cell was in the bio-template medium). These methods showed an initial increase of the maximum power density at 750°C using humidifed H2 fuel by ~20.7 and 17.9%, respectively. However, the grain growth of the nano-catalyst within the in-situ polymerized fuel cell caused a decrease in performance from 11.5% to 5.6 % in 300 h. Also, PNE assisted dip-coating protocol resulted in the deposition of 1.8 mg of nano-ceria by the same process. The cell showed ~13.6% higher increase in the maximum power density at ~24 h, but the cell degraded and the maximum power density decreased to ~2.1% lower than the baseline cell performance. Future work is focusing upon methods to control the nano-catalyst growth at the operating temperature. Methods of controlling the grain boundary migration rate using various solid-solution or secondary phase stabilization strategies are currently being investigated in order to achieve long-term stability of the infiltrated cells. Acknowledgements: As part of the National Energy Technology Laboratory’s Regional University Alliance (NETL-RUA), a collaborative initiative of the NETL, this technical effort was performed under the RES contract DE-FE0004000. This project was funded by the Department of Energy, National Energy Technology Laboratory, an agency of the United States Government, through a support contract with URS Energy & Construction, Inc. Neither the United States Government nor any agency thereof, nor any of their employees, nor URS Energy & Construction, Inc., nor any of their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.
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Apr 9, 2018
Philip E Boahene + 1
Open Access