Chemistry
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Development and Validation of Chromium Getters for Solid Oxide Fuel Cell Power Systems
Chapters
Summary May 26th, 2019
Cathode poisoning from airborne contaminants in trace levels remains a major concern for long-term stability of high-temperature electrochemical systems. We provide a novel method to mitigate the cathode degradations using getters, which capture airborne contaminants at high temperature before entering electrochemically active stack area.
Transcript
Chromium containing alloys are used in SOFC's as metallic interconnects to form chromia scale for corrosion protection. However, chromium vaporization at high temperatures produces gaseous chromium species resulting in SOFC degradation. This method provides a solution for chromium poisoning in solid oxide fuel cell power systems.
The main advantages are the use of low cost materials and effective capture of contaminants at both low and high temperatures. Other high temperature industrial systems using chromium containing alloys, such as steam electrolysis systems, oxygen transport membrane systems and petrochemical systems could use this method for quality and emission control. This video demonstration can have interested researchers quickly learn these technicals, some steps are very simple for beginners.
These technicals can have researchers develop the skills for an advance to electrochemical technology research. To begin, combine nine milliliters of 2.4 molar aqueous strontium nitrate, with seven milliliters of 2.4 molar aqueous nickel nitrate. Stir the mixture for 30 minutes at 300 RPM while heating it to 80 degrees celsius to dissolve the solids.
Then, add 30 milliliters of 5 molar aqueous ammonia to increase the solution pH to 8.5. Continue stirring the mixture at 80 degree celsius for 24 hours, to precipitate the precursor powder. Dry the solution in a dry oven at 120 degrees Celsius, until the water evaporates completely, which usually takes about 24 hours, to leave a blue waxy compound.
We suspend the compound in 50 milliliters of deionized water using both manual and magnetic stirring. Centrifuge the suspension at 5000 RPM for 5 minutes. And remove the liquid which contains residual ammonium nitrate.
At 200 to 380 degrees Celsius, ammonium nitrate will decompose and produce ammonia nitrate acid, nitrogen oxide gases. Proper washing with distilled water will reduce or eliminate the emission of these gases. Dry the rinsed precursor powder at 120 degrees Celsius for two hours.
Next, add the ionized water to the powder and mix it for at least five minutes to make a thick slurry. De-gas the slurry in a vacuum chamber to remove air bubbles. Then, place a cordierite honeycomb substrate in the slurry and perform vacuum infiltration for five minutes to fill the pores with slurry.
Afterwards, flow air through the dip coated substrate to remove excess slurry from the channels. Place the sample in an air filled furnace and heat it to about 120 degrees Celsius at five degrees per minute. Dry the sample in air for at least two hours.
Then, ramp the furnace to 650 degrees Celsius at five degrees per minute and calsign the sample in air for 12 hours to finish producing the chromium getter. To begin the validation test, place two grams of centered chromia pellets in a quartz tube furnace equipped with a diffuser. Place a chromium getter on the other side of the diffuser.
Connect the chromium side of the furnace to a compressed air source via a room temperature water bubbler. Connect the getter side to a vent via a glass elbow and a chromium vapor trapping assembly. Purge the system with humidified air at 300 SCCM for 15 minutes to an hour.
Then ramp the furnace to 850 degrees Celsius at three degrees per minute and maintain that temperature for 500 hours. Check the outlet elbow for discoloration indicating the deposition of chromium compounds every 100 hours. Once the test is finished, cool the furnace to room temperature before turning off the air flow and retrieving the getter sample.
Collect the water from the chromium trapping assembly then, soak the quartz tube, glass elbow, condenser and wash bottles with 20%by weight nitric acid to extract deposited chromium and collect the rinses. Soak the glassware in 20%nitric acid for 12 hours to extract additional deposited chromium and collect the rinse. If any glassware is still discolored, soak it in alkaline potassium permanganate for 12 hours at 80 degrees Celsius.
Then collect and mix chromium extract from all the components to analyze the chromium content with ICPMS. Then, slice the getter sample in half with a knife and coat the exposed surfaces with gold. Coat the chromium getter sample with gold and assess the elemental distribution with energy dispersive x-ray spectroscopy.
Perform another EDS analysis and plot the quantity of chromium with respect to the distance from the chromium source. To begin SOFC fabrication, screen print lanthanum strontium manganate paste on the surface of three yttria-stabilized zirconia electrodes and center the assemblies. Then, attach a platinum electrode to each YSZ disk as the anode using platinum ink.
Attach platinum gauze to both the anode and cathode and attach short platinum wires to the cathode, anode and YSZ disk. Place the SOFC's in a furnace, ramp them to 850 degrees Celsius at three degrees per minute and cure them in air for two hours. Then, connect silver conductive wires to a cured SOFC and mount it in the constant heating zone of a cylinder tube furnace.
Seal the SOFC in the furnace with ceramic paste and connect the electrodes to a potentiostat. Follow standard procedures to set up the experiment. Make sure that these are good cylinder cell and that all three electrodes are properly connected to the potentiostat.
Then, ramp the furnace to 850 degrees Celsius at five degrees per minute. While the furnace heats, configure the potentiostats to record the cell current every minute with a 0.5 volt bias between the cathode and the reference electrode. Set the potentiostats to perform electrochemical impedance spectroscopy between the cathode and the reference electrode every hour.
When the furnace reaches the test temperature, flow humidified air towards the cathode at 300 SCCM and dry air towards the anode at 150 SCCM. Start the measurements and let the test run for 100 hours. After the test, cool the furnace to room temperature and retrieve the cell for characterization.
For the next test, place two grams of chromia pellets in a perforated alumina tube in the constant heating zone. Fix a new SOFC above the chromium source and repeat the test end measurements in the exact same way. For the third test, load two grams of chromia pellets into the tube and mount a chromium getter above the chromium source.
Fix a new SOFC over the getter and perform the test end measurements under the same conditions. In the transpiration test, the chromium profile indicated that most of the chromium was trapped within the first four millimeters of the getter. Analysis of the chromium getter material deposited on an alumina fiber substrate showed large chromium and strontium rich particles near the vapor inlet.
Elemental maps of fiber cross-sections confirmed that chromium and strontium occurred on the surface of the fiber. Electrochemical tests of LSM-YSZ SOFC's in the presence and absence of chromium showed that chromium vapor rapidly poisoned the cell. This was attributed to chromium oxide deposits on the LSM-YSZ interface, hindering the oxygen reduction reaction at that interface.
Placing an SNO chromium getter between the chromium source and the SOFC resulted in SOFC performance comparable to the performance in the absence of chromium. This performance was maintained over a wide range of chromium vapor flow rates. The fabrication protocol produces a stable efficient getter for airborne chromium impurities.
Using different chemicals we can develop getters to capture other gaseous contaminants such as boron and silicon vapors. The transmission protocol measures the evaporation of chromium containing alloy materials and validates the performance of getters capturing hexaamminechromium vapor in air under typical SOFC operating conditions. The electrochemical validation protocol demonstrates getter efficience at a nominal SOFC operating conditions.
Since the information is essential for scaling up getter and SOFC technologies for industry and their commercial uses. This method uses small amounts of chemicals and reasons that can be managed and handled according to existing laboratory health and safety policies.
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