Probing and Mapping Electrode Surfaces in Solid Oxide Fuel Cells

The aim of the following experiment is to promote understanding of solid oxide, fuel cell or SOFC electrode reaction and degradation mechanisms for cells operating on hydrocarbon and sulfur contaminated fuels. Thus providing insights into rational design of SOFC materials. This is achieved by fabricating cells with a working electrode consisting of a nickel mesh electrode embedded in irea, stabilized zirconia or YSC and testing the cells electrochemical performance and stability in hydrocarbon and hydrogen sulfide containing fuels.

As a second step, Raman spectroscopy is performed on the mesh electrode under both in situ U and XE two conditions, which provides information on the chemical species and phases involved in electrode degradation, where the degradation happens and how it might be prevented. Next, scanning probe microscopy techniques are used to characterize the electrode in order to visualize electrode degradation and surface morphology on the nanometer scale. The main advantage of these techniques over other surface analysis methods, such as electron SPECT prospies, is that they do not require high vacuum for operation.

These Methods can help gather useful information in the field of solid oxide fuel cells, such as the nature of mechanisms that determine the electrode performance and degradation. The implications of those techniques extend towards microscopic oscopy and energy transformation processes. Therefore, they would be of interest to mature scientists, electro chemists, special copies, and other interest.

Those topics, Though, these techniques have been developed for probing and mapping electrode surface processes in salt oxide fuel cells. They can also be applied to electrochemical systems such as batteries and supers. To begin fabricating the anode cell weigh out two batches of two tenths of a gram of YSZ powder using a uni axial dry press and a cylindrical stainless steel mold.

13 millimeters in diameter compress one batch of YSZ powder at a pressure of 50 megapascals for 30 seconds. Then cut a less than one centimeter piece of size, 50 nickel mesh woven from 50 micrometer wire, and place it onto the surface of the wire Z disc inside the mold. Next, add the other two tenths of a gram of YSC powder on top of the nickel mesh inside the mold and flatten the surface of the powder using a ram.

Press the nickel mesh sandwich between packs of YSC powder uni axially, and a pressure of 300 MegaPath scales for 30 seconds. Now extract the pressed nickel YSC pellet from the mold. Place the pellet in a zirconia crucible and place the crucible into a horizontal tube furnace with a flowing reducing gas atmosphere of 4%molecular hydrogen and 96%argon.

Fire the pellet at 1, 440 degrees SIUs for five hours after the electrode has been fabricated and cooled. Mechanically grind away one face of the centered wire C from the sample, using six micrometer diamond grit until the nickel mesh surface is revealed. Continue polishing the exposed nickel mesh surface using three micrometer, one micrometer and 0.1 micrometer Diamond media in a water ethylene glycol suspension for approximately one minute at each polishing stage to produce a mesh with increased coing resistance.

After cleaning and drying, arrange the sample and bury amide powder in the crucible so that they're not touching. Then fire the sample at 1, 200 degrees Celsius for two hours in a reducing atmosphere. Prepare the sample for electrochemical testing by brush painting silver paste on the wire C surface opposite the nickel mesh to act as a counter electrode.

After drying, attach a coiled silver wire to the counter electrode using silver paste and connect a 0.2 millimeter diameter silver wire to the nickel mesh using silver paste on the tip. After drying the sample, again, seal the cell with the nickel mesh down on top of a three eighths of an inch ceramic cell fixture tube using a remco seal 5 5 2, also known as cera bond. After the sealant dries, connect the insulated silver wires to the electrode leading wires.

Next, mount the cell fixture in a tubular furnace. Connect the fixture to a gas line and attach the wires to electrochemical testing equipment. Next bubble ultrahigh purity molecular hydrogen gas through room temperature water to humidify it to 3%volume of water.

Direct this humidified gas through the cell fixture at a rate of 50 standard cubic centimeters per minute. After properly heating the cell and centering the silver counter electrode, call the cell to 767 degrees Celsius for electrochemical performance testing. Once the testing is done and the sample is prepared for characterization, affix the cell using tape or adhesive to the ramen microscope stage plate with the mesh anode facing upward.

With the laser focused on an interface of nickel and YSC substrate, set the ram and spectrometer to obtain spectra from the intersections of a rectangular grid. Overlying the area of the interface with intersections separated by two micrometers, the spectra should be centered around the wave number corresponding to the Raman mode of interest. In this case, 980 inverse centimeters is chosen for oxides of sulfur.

The next step is to integrate the intensity over the Raman mode of interest for each spectra and divide by a flat baseline with the same spectrum, the relative intensity can then be plotted in a contour and color map with respect to its coordinates. To monitor the coing of the anode, use silver paste to seal the wire C embedded nickel mesh to the rim of a harrick scientific ramen high temperature reaction chamber stage facing upward. Then cap the ramen stage chamber.

Next, mount the sample chamber on the rama microscope with a nickel wire C boundary in view and begin flowing humidified, 4%molecular hydrogen and 96%argon gas through the chamber at approximately 100 standard cubic centimeters per minute. Now heat the ramen chamber to 625 degrees Celsius. Bring the laser into focus and collect baseline ramen scans from spots on the nickel mesh and YSC substrate in the 150 to 2000 inverse centimeter range.

Next, introduce three to 5%propane into the gas flow and collect ram and spectra from the nickel at regular intervals while the gas is flowing To observe the deposition of carbon on the surface over time. Pure nickel foil is used for visualization of carbon deposition by atomic force microscopy or a FM and electrostatic force microscopy or EFM using a square 10 milliliter by one milliliter coupon cut from nickel foil polish one face of the coupon down to the grade of 0.1 micrometers. Following the steps used earlier in a quartz tube lined furnace at 550 degrees Celsius, exposed the polished nickel coupon to humidified flowing gas containing 10%propane balanced by Argonne for one minute.

Then remove the sample from the furnace and quartz tube lining. Inspect the surface morphology by optical microscopy and scanning electron microscopy to prepare the sample for study using atomic force microscopy and electrostatic force microscopy. Mount the sample on a metal puck using copper conductive tape.

Following this, install an end type silicon base, a FM tip or a conductive a FM tip onto the tip holder. Now take the sample to the A FM.Then collect a morphology image using a FM in tapping mode for gathering information in lift mode. Start with the lift height at 100 nanometers and gradually decrease it to approximately the value of the surface roughness so that the electrostatic interaction may be strong enough for EFM imaging.

Scan the sample at the appropriate height in lift mode. To collect information on both topography and phase angle. Identify a clear phase or morphological interface and collect a series of EFM line scans across this interface while changing the sample bias.

Then identify the voltage at which the contrast between the two phases flips by comparing the EFM line scans at different sample bias potentials. Then collect a full image using a sample bias that is one to two volts below the switch voltage and collect another image with the sample bias one to two volts above the switch voltage. The area covered by carbon appears bright when the sample is bias negatively and appears dark when it is biased positively by scanning the sample surface.

In this mode, the distribution and surface morphology of carbon deposition can be obtained. The nickel mesh electrode was tested for performance under both a molecular hydrogen and hydrogen sulfide condition. Typical results for current voltage and current power curves at 767 degrees Celsius as shown here, the introduction of just a few parts per million of hydrogen sulfide can cause considerable performance degradation.

Here are nyquist plots for the cell before and after the anode was exposed to fuels containing zero and 20 parts per million hydrogen sulfide at 767 degrees Celsius. The AC impedance spectroscopy of the cell was performed under open circuit voltage conditions. The left curve is the typical poisoning behavior, and the right side is the typical recovery behavior of the cell at 767 degrees Celsius.

Under 20 parts per million hydrogen sulfide and DC bias of negative three tenths volts. Both poisoning and recovery seem to finish and reach steady state within a few minutes, which is different from the long recovery times needed for thicker nickel wire. Odes previously studied an optical micrograph.

The nickel wire C interface is shown on the left. A ramen map of the same area is on the right. The ramen map shows the intensity of a mode associated with various oxides of sulfur.

The species were exclusively observed on the nickel surface and were concentrated away from the triple phase boundaries. This picture is an optical micrograph of the nickel mesh embedded in YSZ. This picture is the same mesh after exposure to propane containing gas at 625 degrees Celsius for 15 hours.

The right figure is a Raman spectra collected in situ from the spots marked on the optical micrograph at left after 15 hours of exposure to propane. Shown on the right is a plus of the change in the carbon ramen signal. Intensity collected over time from the location identified by the green spot on the optical micrograph at left.

These two panels allow comparison of the nickel YSC mesh modified with barrier oxide to give increased coing resistance under the same conditions as the unmodified mesh. Both have been exposed to propane containing gas at 625 degrees Celsius for 15 hours. This scanning electron microscope image shows the in homogeneous dark patches formed on the surface of nickel after it was exposed to gas with some propane content.

On the left are atomic force microscope images of regions with different levels of coing. These same regions are also imaged using electrostatic force microscopy on the right While attempting this procedure. It's important to remember to keep some me surfaces as clear as possible of other contaminants such as polished media.

Don't forget that working with high temperature furnaces and flammable gases can be extremely hazardous and proper precautions should be taken, such as wearing protective equipment and using proper alarms. I hope this video has illustrated how to gain information on electrode surface processes in cell oxide fuel cells that will help you to better design electrode materials.