Fuel Cells

Nano-Catalyst Infiltration Enhancement of Porous Solid Oxide Fuel Cell Electrodes Using Catechol Surfactants

Wet nano-catalyst infiltration of porous Solid Oxide Fuel Cell (SOFC) electrodes is a recognized process to enhance electrochemical performance. Simple infiltration protocols includes multiple pipet deposition steps of aqueous salt solutions onto the SOFC electrodes. 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, however, could allow homogenous incorporation of a nano-catalyst within the electrode in a single salt solution deposition step with lower electrode overpotential and enhanced long-term stability.

In this project, 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.

Additionally, we aimed to monitor the growth kinetics of the catechol surfactants in order to understand the nano-catalyst deposition morphology. The work investigated the deposition of these layers on YSZ substrates by Atomic Force Microscopy (AFM), which would act as an ideal surface to characterize (over the 3-D electrode layers). 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 infiltrate nano-ceria (CeO ₂) 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 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. The infiltrated cells displayed up to 35% reduction in polarization over the baseline cell with high electrochemical stability during 300 hours of testing at 750°C.

Aerosol Deposition for Fabrication of Solid Oxide Fuel Cells

A current challenge in SOFC fabrication is manufacturing time and cost. This project seeks to improve both aspects of the process while delivering SOFCs that perform comparably to cells found in literature an available in the market. Using aerosol deposition (AD) technology, layers as thin as 5 micrometers are able to be deposited in as little as 30 seconds. This work seeks to enhance customizability in all three dimensions of a cell's architecturethrough fine layers. This can include particle size, porosity, and material ratio gradients. This research has aimed to achieve high control over printing parameters to eliminate defects found SOFCs fabrication such as warpage, cracking, large porosity, and surface roughness.This research focuses on the use of aerosol deposition as an additive manufacturing method to facilitate the rapid fabrication of SOFCs/SOECs. The aerosol deposition method provides the capability of size control in all three dimensions, with the ability to deposit thickness below 10 µm and widths below 1 mm. These resolution capabilities show the potential to be able to print SOFC/SOEC electrodes without the use of masks, as is found in methods such as screen printing. This also allows for fine-tuning of compositions through the creation of functional gradients based on pore size, particle size, and material ratios. A multi-syringe pump system allows for the in-situ compositional mixing of materials for quick changes to make a desired suspension without the need for preparing inks of every needed compositional ratio. Experiments are conducted to investigate the impact of 3D printing parameters on defect formation in both the 3D-printed layers and the cell supports such as NiO/YSZ anodes. Defects and features being studied both macroscopic and microscopic in scale and include substrate warpage, stress build-up, crack formation, delamination, porosity, and surface roughness. With the relationships gathered from this research, the goal of 3D printing all non-support layers of a cell using a single additive manufacturing method is being pursued.

Anodes

One major issue limiting the application of SOFCs to clean coal technologies is the degradation of the anode upon exposure to trace amounts of impurities that exist within coal-derived syngas. Specifically, H ₂S and PH ₃have been shown to have an immediate effect on cell performance for cells with Ni-based anodes. The nickel reacts to form various nickel-phosphide and -sulfide phases that inhibit catalytic activity. Thus, our work focuses on developing mixed-conducting oxides as alternate anodes that show stable long-term performance. Our studies not only exam the effect of the impurities in the new anodes and traditional cermet anodes, but the role that fuel delivery configuration has on cell performance and degradation.

An important component in anode research is the anode functional layer (AFL). This group has performed investigations of the impact of aerosol deposited AFLs on the roughness of anode supports. This is to ensure that a smooth surface is present for the deposition of a thin electrolyte layer.

Typical Ni-based cermet anodes suffer from anode deactivation due to carbon build upunder hydrocarbon flows. Carbon builds up and covers the triple-phase-boundary (TPB) area which results in poor electrochemical performance. Larger carbon deposits canblock pores within the anode microstructure which leads to gas diffusion issues. Carbonbuildup also causes mechanical stresses to the electrode due to volumetric changeswhich can lead to fracture and complete failure of the cell.Incorporation of reforming catalysts into commercial SOFCs was investigated to makeNi-YSZ anodes more tolerable to hydrocarbon containing fuels by promoting internal-reforming and reducing coking. Uniform incorporation of metal composite nano-catalystsinto the anode microstructure was achieved through a surfactant-enhanced liquid phaseinfiltration process. Nano-catalyst depositions were initially characterized on flatsurfaces to define the morphology of the coatings and to study nano-catalystmicrostructure/chemistry evolution over operation time using atomic force microscopy(AFM).Impregnated SOFCs were evaluated using electrochemical impedance spectroscopywhile operating under high methane fuel streams. Fuel compositions were selected tohave high hydrocarbon content to accelerate degradation. Deposition techniques wereoptimized for multi-component internal reforming catalysts to obtain sustainedperformance enhancement. Post-mortem microstructure and chemistrycharacterizations were used in analysis.

Microwave-Assisted Sintering of SOEC/SOFC To Enhance ThermalProcessing

This research focuses on the production of SOFC’s and SOEC’s using microwavesintering of the cells instead of conventional heating. The cells are fabricated usingscreen printing or tape casting techniques, after which they are sintered usingmicrowave heating that employs the use of a SiC susceptor to rapidly ramp to hightemperatures. This process is favored over conventional furnaces since microwaveheating is an intrinsic method which also produces rapid heating due to its technique.
Current research involves sintering industry standard YSZ button cells on which layersof cathode and anode material are printed on and sintering of tape casted YSZ cells onwhich layers are printed on. These are then compared using analysis software such as XRD, SEM and Optical Microscopy with conventionally sintered cells to identifyvariances in curvature, warping and densification of materials and layers.