Probing and Mapping Electrode Surfaces in Solid Oxide Fuel Cells

Kevin S. Blinn; Xiaxi Li; Mingfei Liu; Lawrence A. Bottomley; Meilin Liu

doi:10.3791/50161

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Summary

We present a unique platform for characterizing electrode surfaces in solid oxide fuel cells (SOFCs) that allows simultaneous performance of multiple characterization techniques (e.g. in situ Raman spectroscopy and scanning probe microscopy alongside electrochemical measurements). Complementary information from these analyses may help to advance toward a more profound understanding of electrode reaction and degradation mechanisms, providing insights into rational design of better materials for SOFCs.

Abstract

Solid oxide fuel cells (SOFCs) are potentially the most efficient and cost-effective solution to utilization of a wide variety of fuels beyond hydrogen ^1-7. The performance of SOFCs and the rates of many chemical and energy transformation processes in energy storage and conversion devices in general are limited primarily by charge and mass transfer along electrode surfaces and across interfaces. Unfortunately, the mechanistic understanding of these processes is still lacking, due largely to the difficulty of characterizing these processes under in situ conditions. This knowledge gap is a chief obstacle to SOFC commercialization. The development of tools for probing and mapping surface chemistries relevant to electrode reactions is vital to unraveling the mechanisms of surface processes and to achieving rational design of new electrode materials for more efficient energy storage and conversion². Among the relatively few in situ surface analysis methods, Raman spectroscopy can be performed even with high temperatures and harsh atmospheres, making it ideal for characterizing chemical processes relevant to SOFC anode performance and degradation^8-12. It can also be used alongside electrochemical measurements, potentially allowing direct correlation of electrochemistry to surface chemistry in an operating cell. Proper in situ Raman mapping measurements would be useful for pin-pointing important anode reaction mechanisms because of its sensitivity to the relevant species, including anode performance degradation through carbon deposition^{8, 10, 13, 14} ("coking") and sulfur poisoning^{11, 15} and the manner in which surface modifications stave off this degradation¹⁶. The current work demonstrates significant progress towards this capability. In addition, the family of scanning probe microscopy (SPM) techniques provides a special approach to interrogate the electrode surface with nanoscale resolution. Besides the surface topography that is routinely collected by AFM and STM, other properties such as local electronic states, ion diffusion coefficient and surface potential can also be investigated^17-22. In this work, electrochemical measurements, Raman spectroscopy, and SPM were used in conjunction with a novel test electrode platform that consists of a Ni mesh electrode embedded in an yttria-stabilized zirconia (YSZ) electrolyte. Cell performance testing and impedance spectroscopy under fuel containing H₂S was characterized, and Raman mapping was used to further elucidate the nature of sulfur poisoning. In situ Raman monitoring was used to investigate coking behavior. Finally, atomic force microscopy (AFM) and electrostatic force microscopy (EFM) were used to further visualize carbon deposition on the nanoscale. From this research, we desire to produce a more complete picture of the SOFC anode.

Protocol

1. Fabrication of a YSZ-embedded Mesh Anode Cell

Weigh out two batches of 0.2 g of YSZ powder.
Compress one batch YSZ powder in a cylindrical stainless steel mold (13 mm in diameter) with a uniaxial dry press at a pressure of 50 MPa for 30 sec.
Cut a <1-cm piece of Ni mesh and place it onto the surface of YSZ disc inside the mold.
Add the other 0.2 g of YSZ powder on top of the Ni-mesh inside the mold and flatten the surface of the powder using a ram.
Uniaxially press the Ni mesh sandwiched between packs of YSZ powder at a pressure of 300 MPa for 30 sec.
Extract the pressed Ni/YSZ pellet from the mold.
Fire the pellet at 1440 °C for 5 hr in a zirconia crucible using a horizontal tube furnace with a flowing reducing gas atmosphere (4% H₂/bal. Ar).

2. Exposure, Polishing, and Modification of Ni Mesh Electrode

Mechanically grind away one face of the sintered YSZ sample using 6 μm diamond grit until the Ni mesh surface is revealed.
Further polish the exposed Ni mesh surface using 3 μm, 1 μm, and 0.1 μm diamond media in a water / ethylene glycol suspension for approximately 1 min at each polishing step.
Ultrasonically clean the polished sample in acetone, ethanol, and DI water for 10 min each.
Dry the sample under a clean compressed air stream.
For Ni mesh with increased coking resistance, fire the sample at 1,200 °C for 2 hr in reducing atmosphere in the presence of, but not in contact with, BaO powder.

3. Preparation and Electrochemical Testing of Full Cells

Brush-paint Ag paste on the opposite surface of the YSZ sample from the Ni-mesh to act as a counter-electrode.
Attach a coiled Ag wire to the counter-electrode using Ag paste.
After drying the Ag paste on the sample at 120 °C in an oven for 30 min, connect a 0.2-mm diameter Ag wire to the Ni-mesh using Ag paste on the tip.
Dry the sample again at 120 °C in an oven for 30 min.
Seal the cell (Ni mesh down) on top of a 3/8 inch ceramic cell fixture tube using Aremco Seal 552 (Ceramabond).
Allow the sealant to dry in air for 2-4 hr.
Connect two insulated silver wires to each of the two electrode wires.
Mount the cell fixture in a tubular furnace, connect the fixture to a gas line, and attach the wires to proper electrochemical testing equipment.
Begin flowing ultra-high purity grade (99.999%) H₂ gas through the cell fixture at a rate of 50 sccm; the gas should be bubbled through room-temperature water to humidify the gas to 3% vol. H₂O prior to entering the cell fixture.
Heat the furnace with the mounted cell to 100 °C for 2 hr, followed by 260 °C for 1 hour, and then finally 800 °C at a ramping rate of 1 °C with continued flowing of H₂ during all heating to avoid oxidation of the Ni electrode. The first two heating steps are for curing the Ceramabond.
Hold the cell in the furnace at 800 °C for 2 hr to allow the Ag counter electrode to sinter.
Cool the cell slightly to 767 °C for electrochemical performance testing.
After testing, carefully remove the cell fixture from the furnace for quenching at room temperature while continuing to flow humidified H₂. (CAUTION: Use proper PPE for handling extremely hot ceramics, such as thermal gloves and mats!)
Detach the cell from the fixture for post-characterization by detaching the electrode wires and carefully separating the cell from the Ceramabond sealant.

*Figure 1 presents a schematic of the YSZ-embedded Ni mesh cell, along with a typical photograph and optical micrograph of the embedded mesh.

*For our investigations, cells were electrochemically characterized with an EG&G PAR potentiostat (model 273A) coupled with a Solartron 1255 HF frequency response analyzer using CorrWare and ZPlot softwares (Scribner and Associates). Linear sweep voltammetry and constant-voltage amperometry were used to characterize cell performance, and impedance spectra were acquired in the frequency range of 100 kHz to 0.1 Hz with an amplitude of 10 mV. For the sulfur poisoning study, a certified gas mixture of 100 ppm H₂S in H₂ was mixed into the fuel gas stream with pure H₂ to obtain a 20 ppm H₂S/H₂ mixture.

4. Post-test Raman Spectromicroscopic Mapping

Affix the cell sample with the mesh anode facing upward onto the Raman microscope stage plate with tape or adhesive to prevent sample movement during Raman analysis.
Use the microscope and XYZ stage to locate an interface boundary between the Ni mesh and YSZ substrate.
Bring the laser into focus by switching the microscope filters and finely adjusting the Z coordinate of the stage.
Set the Raman spectrometer to obtain spectra at the nodes of a rectangular mesh overlaying the area of the interface with 2 μm intervals separating the nodes. The spectra should be centered around the wavenumber(s) corresponding to the Raman mode(s) of the species or phase(s) of interest. In this case, 980 cm^-1 is chosen for SO_x.
For each spectra, integrate the intensity across the Raman mode(s) of interest and divide the intensity by a flat baseline with the same spectrum. The relative intensity can then be plotted in a contour / color map with respect to its coordinates.

Raman spectromicroscopy was performed using a Renishaw RM1000 system equipped with a Modu-Laser StellarPro 514 nm Ar-ion laser (5 mW) and a Thorlabs HRP170 633 nm He-Ne laser (17 mW). The system is equipped with an X-Y-Z motorized stage (Prior Scientific H101RNSW) and a 50X objective lens, which together allow for ~2 μm mapping resolution. Renishaw WiRE 2.0 software was used in conjunction with the hardware. Data was processed using MATLAB (MathWorks).

5. In situ Raman Monitoring of Coking⁸

Attach an YSZ-embedded Ni mesh sample to the Raman chamber stage using Ag paste with the mesh facing upward.
Heat the open chamber to 300 °C for 1 hr to dry and eliminate the Ag paste suspension medium.
Seal the Raman chamber's cap and affix it to the Raman microscope stage. Use the microscope to locate a Ni/YSZ interface as described in Protocol 4.2.
Begin flowing 4% H₂ / Ar gas humidified by water bubbler through the chamber at ~100 sccm.
Heat the Raman chamber to 625 °C.
Bring the laser into focus and collect baseline Raman scans from spots on the Ni mesh and YSZ substrate in the 150-2000 cm^-1 range.
Introduce 3-5% C₃H₈ into the gas flow and collect Raman spectra from the Ni at regular intervals while the gas is flowing to observe the deposition of carbon on the surface over time (e.g. 15 hr).
Cool the sample down slowly (5 °C /min) in flowing 4% H₂ / Ar.

*The in situ Raman analysis was performed with a custom-modified Harrick Scientific high-temperature reaction chamber.The chamber is equipped with a quartz window cap, gas connections, and a cooling line. A schematic and photograph is provided in Figure 2.

CAUTION: Cooling water should be used to protect the optical microscope on the Raman system from heating!

6. Nanoscale Visualization of Coking by AFM and EFM

Polish one face of a 1 cm x 1 mm square nickel coupon down to the grade of 0.1 μm as described in Protocol 2.2.
In a quartz tube-lined furnace, expose the polished nickel coupon to flowing gas containing 10% C₃H₈ balanced by Ar at 550 °C for 1 min; the gas should be bubbled through room-temperature water to humidify the gas to 3% vol. H₂O prior to entering the quartz tube.
Remove the sample from the furnace. Inspect the surface morphology by optical microscopy and SEM.
Mount the sample onto a metal puck using copper conductive tape for AFM and EFM study.
Collect a morphology image using AFM in Tapping Mode.
Install an n-type Si based AFM tip (NSC16) or a conductive AFM tip (CSC11/Cr-Au) onto the electrical holder (MMEFCH).
Scan the sample surface in "Lift Mode", in which the tip first collects the topographic information on its first trip across the sample surface and then senses the phase angle on its second trip for electrostatic force information. Set the lift height initially to 100 nm, and gradually decrease it to approximately the same value of the surface roughness (20-30 nm).
Across a clear interface between the coked and clean region of nickel surface, collect a series of EFM linescans while changing the sample bias.
By comparing the EFM linescans at different sample bias potentials, identify the voltage at which the phase angle contrast flips²¹.
Collect an image with a sample bias that is 1-2V negative w.r.t. the switch point, and another image with sample bias that is 1-2V positive w.r.t. the switch point.
By comparing the topography image, and the two sets of EFM images at different sample biases, obtain a distribution map of the carbon and nickel phase on the sample. *For our SPM analyses, a Veeco Nanoscope IIIA system was used. A schematic of the working principle of the EFM analysis^{23, 24} is shown in Figure 3.