Bio-Surfactant Assisted Infiltration of SOFC Electrodes
Authors: Ozmen Ozcan , Zondlo John W. , Lee Shiwoo , Hackett Gregory A , Abernathy Harry , Sabolsky Edward M. 
Published: February 2020

Wet nano-catalyst impregnation of porous Solid Oxide Fuel Cell (SOFC) electrodes is a recognized process to enhance electrochemical performance. This impregnation method typically includes multiple pipet deposition steps of aqueous salt solutions onto the SOFC electrodes in order to achieve a desired catalyst loading level. In the current work, it was found that this labor-intensive and time-consuming process could be simplified through the use of a bio-adhesive (or bio-derived surfactant). The surfactant-assisted impregnation protocol allowed for the homogenous incorporation of a nano-catalyst within the electrode in a single salt solution deposition step. The process results in a lower electrode overpotential with enhanced long-term stability.

In the current study, bio-inspired catechol-based surfactants were used to act as a wetting and local chelating agent which adhered within the 3-D electrode architectures. The work initially assessed the use of polymerized dopamine (PDA) and/or nor-epinephrine (PNE) neurotransmitters as the bio-template layer for metal oxide or hydroxide deposition. The adhered bio-template on the pore walls was shown to provide higher nano-catalyst loading by avoiding segregation of the precipitate within the porous structure, which typically results in the migration and distribution of the solute near the surface during the drying process. Another advantage of using the protocol developed in this work was that both electrodes could be infiltrated simultaneously by utilizing a simple dip-coating method within a single thermal step.

Additional studies in this work focussed on the deposition and aggregate (and/or film) formation kinetics of the dopamine and epinephrine surfactants in order to better understand the nano-catalyst deposition morphology. The work investigated the deposition of these layers on YSZ substrates, which would act as an ideal surface to characterize (over the 3-D electrode layers). This work then led to the investigation of other catechol molecules, such as caffeic acid, cinnamic acid, and gallic acid onto YSZ planar substrates. In order to complete this study, commercial YSZ single crystals were submersed into surfactant solutions for 1 to 24 hours. The surface was dried and gently inscribed by a fine needle tip to measure the surfactant thickness by Atomic Force Microscopy (AFM). The result showed that the polymerization method and surfactant type had a significant effect on the adhered layer thickness, which varied from 30 to 500 nm within 24 hours of immersion time. In addition, the effect of these surfactants on the chelation and precipitation of metal oxide nano-catalyst from precursor solutions were also studied The optimal conditions were resolved from the surfactant coating studies which were used to impregnate nano-ceria (CeO2) based nano-catalyst into the electrodes of anode-supported SOFC button cells.

The final studies of this work investigated the effect of the critical nano-catalyst infiltrant concentration on the fuel cell performance. Commercial button cells were infiltrated with norepinephrine (PNE) and then impregnated with different molarities of cerium salt solution for both electrodes by dip-coating method. After deposition, the samples were only exposed to a singular drying/firing step. The process achieved loadings ranging between 0.4 to 8.3 mg of nano-ceria after the firing (calcination) step at 750oC. Infiltrated cells were operated at 750oC using humidified hydrogen fuel for ~300 hours. The SOFC performance was evaluated by electrochemical impedance spectroscopy (EIS) in the 48th, 150th and 300th hours. EIS analysis showed that the infiltrated cells did not show a significantly lower resistance than the baseline cell in the first 48th hour. However, in the 150th to 300th hour, the resistance of the infiltrated cells reduced in both the high-frequency and mid-frequency regimes (associated with the anode and cathode, respectively). Furthermore, the resistance of the low-frequency regime (associate with mass diffusion) decreased between the 48th to 300th hour. The resistance relaxation in this region was probably due to increased gas diffusion related to nano-ceria sintering (and opening of the microstructure porosity). In the study, a gradual decrease in the total electrode overpotential was observed up to the 5 mg nano-ceria loading level. The cell displayed a 35% reduction in polarization over the baseline cell after 300 hours of testing. The overpotential of 8.5 mg nano-ceria infiltrated cell displayed a similar electrode overpotential value as the baseline cell after 300 h of testing (i.e., with no nano-catalyst loading).

The correlation between the cell performance (as a function of loading time) to the nano-catalyst distribution/morphology is currently being investigated. In addition, methods for controlling the grain boundary migration rate using various solid-solution or secondary-phase stabilization strategies are also being investigated in order to achieve long-term stability of the infiltrated cells.