Name: development and validation of chromium getters for solid oxide fuel cell power systems
Uploaded: 2019-05-26T14:00:00
Description: Watch this Scientific Journal Video about Development and Validation of Chromium Getters for Solid Oxide Fuel Cell Power Systems at JoVE.com

Authors acknowledge financial support from U.S. Department of Energy (US DOE) under the federal grant DE-FE-0023385. Technical discussion with Drs. Rin Burke and Shailesh Vora (National Energy Technology Laboratory) is gratefully acknowledged. Drs. Amit Pandey (LG Fuel Cells, Canton OH), Jeff Stevenson and Matt Chou (Pacific Northwest National Laboratory, Richland WA) are acknowledged for their help with long term test validation of the performance of the getters. Authors acknowledge the University of Connecticut for providing laboratory support. Dr. Lichun Zhang and Ms. Chiying Liang is acknowledged for technical discussion and help with the experiments.

1. Synthesis of chromium getter
<ol>
	<li>Synthesize precursor powder using alkaline earth and transition metal oxide salts via conventional coprecipitation synthesis route as depicted in Figure 116.
	<ol>
		<li>Prepare a stock solution using 50.33 g of strontium nitrate Sr(NO3)2 and 43.97 g of nickel nitrate hexahydrate Ni(NO3)2.6H2O in order to prepare 2.4 M solutions in 100 mL of de-ionized water.&#160;</li>
...

<ol>
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K., Keane, M., Zhang, H., Singh, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Effect+of+CO2+on+the+stability+of+strontium+doped+lanthanum+manganite+cathode.">Effect of CO2 on the stability of strontium doped lanthanum manganite cathode.</a> Journal of Power Sources. 268, 404-413 (2014).</li><li>Hu, B., Keane, M., Mahapatra, M. K., Singh, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Stability+of+strontium-doped+lanthanum+manganite+cathode+in+humidified+air.">Stability of strontium-doped lanthanum manganite cathode in humidified air.</a> Journal of Power Sources. 248, 196-204 (2014).</li><li>Li, C., Habler, G., Baldwin, L. 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The experimental results clearly demonstrate the effectiveness of chromium getters during long-term chromium transpiration tests and electrochemical tests. Presence of getters successfully mitigates the contamination of the electrode which otherwise would lead to rapid increase in polarization resistance and electrochemical performance degradation.
The formation of gas phase chromium species from chromia is favored and enhanced with an increase of water vapor concentration (humidity level)<sup...

Solid oxide fuel cell (SOFC) power system, a high temperature direct electrochemical energy conversion device, offers an environmentally friendly pathway to generate electricity from a wide variety of fossil and renewable fuels. SOFC technology finds its applications in centralized as well as distributed power generation areas1. This technology relies on electrochemical conversion of chemical energy stored in the fuels into electricity. Numerous advantages are offered by SOFCs in terms of high energy efficiency, high quality heat, ease of modularity, and no or negligible carbon footprints2. Several individual SOFC cells are connected in series or parallel fashion (namely SOFC stacks) to obtain desired output voltage. SOFC stacks consist of components such as dense electrolyte, porous electrodes, interconnection (IC) and seals3,4. Anode and cathode of adjacent cells are connected using IC, which not only serves as a separator to prevent any mixing of oxidant with fuel but also provides electrical connection between the adjacent anode and cathode5.
Improvements over decades of research and development in materials engineering have led to reduction in operating temperature for SOFCs, enabling replacements of ceramics materials with inexpensive stainless-steel alloys for the fabrication of electrochemically active cell and stack components and balance-of-plant (BOP) sub-systems. Commercially available ferritic and austenitic stainless steels are utilized for the fabrication of system components due to their low cost, matched coefficient of thermal expansion (CTE) and resistance to oxidation and corrosion at high operating temperatures6. Formation of Cr2O3 type passivating oxide scale on the alloy surface acts as a barrier layer against inward diffusion of oxygen from air or outward diffusion of cations from bulk alloy7.
In the presence of humidified air, Cr2O3 undergoes significant chemical transformation leading to hydrated chromium vapor species formation at SOFC operating temperatures. The gaseous chromium vapor is subsequently carried through the air stream into the cathode leading to surface and interface reactions with the cathode materials. Such cathode experiences both ohmic and non-ohmic increases in the polarization and electrical performance degradation. Details of the cathode degradation mechanisms have been illustrated elsewhere8,9,10.
The state-of-the-art methods to reduce or eliminate the above cathode degradation processes commonly consist of modifications of the alloy chemistry, application of surface coating and the use of chromium tolerant cathodes11,12. Although these techniques have demonstrated reduction of the cathode degradation due to Cr vapor interactions (namely Cr poisoning) for short-term, long-term efficacy for performance stability remains a concern, mainly due to cracking and spallation within the coating and interdiffusion of cations.
We have demonstrated a novel method to mitigate the problem of chromium poisoning by capturing the incoming chromium vapor before it reacts with the cathode materials13. The getters have been synthesized from low-cost alkaline earth and transition metal oxides using conventional ceramic processing techniques. The cost advantage of this approach is use of non-noble and non-strategic materials as well as conventional processing methods to fabricate getters for the mitigation of cathode degradation arising from airborne contaminants. The placement of the getter can be tailored to capture chromium vapor arising from BOP components or it can also be tailored to be placed within the electrochemically active stack components14,15. Here, we present methods to validate the chromium getters using transpiration and electrochemical tests. Experimental setup and characterization results will also be demonstrated to show the getter effectiveness and the mechanisms of Cr capture on the getter under typical SOFC operating conditions.

<table>
	<tbody>
		<tr>
			<td>Sr(NO3)2</td>
			<td>Sigma-Aldrich</td>
			<td>243426</td>
			<td>Getter precursor material</td>
		</tr>
		<tr>
			<td>Ni(NO3)2-6H2O</td>
			<td>Alfa Aesar</td>
			<td>A15540</td>
			<td>Getter precursor material</td>
		</tr>
		<tr>
			<td>NH4OH</td>
			<td>Alfa Aesar</td>
			<td>L13168</td>
			<td>Getter precursor material</td>
		</tr>
		<tr>
			<td>Pt ink</td>
			<td>ESL ElectroScience</td>
			<td>5051</td>
			<td>Current collector paste</td>
		</tr>
		<tr>
			<td>Pt wire</td>
			<td>Alfa Aesar</td>
			<td>10288</td>
			<td>Current collector wire</td>
		</tr>
		<tr>
			<td>Pt gause</td>
			<td>Alfa Aesar</td>
			<td>40935</td>
			<td>Current collector</td>
		</tr>
		<tr>
			<td>Cr2O3 powder</td>
			<td>Alfa Aesar</td>
			<td>12286</td>
			<td>Chromium source</td>
		</tr>
		<tr>
			<td>Nitric acid (HNO3)</td>
			<td>Sigma-Aldrich</td>
			<td>438073</td>
			<td>Chromium extraction</td>
		</tr>
		<tr>
			<td>Potassium permanganate (KMnO4)</td>
			<td>Alfa Aesar</td>
			<td>A12170</td>
			<td>Chromium extraction</td>
		</tr>
		<tr>
			<td>LSM paste</td>
			<td>Fuelcellmaterials</td>
			<td>18007</td>
			<td>Cathode</td>
		</tr>
		<tr>
			<td>YSZ electrolyte</td>
			<td>Fuelcellmaterials</td>
			<td>211102</td>
			<td>Electrolyte</td>
		</tr>
		<tr>
			<td>Alumina fiber board</td>
			<td>Zircar</td>
			<td>GJ0014</td>
			<td>Getter substrate</td>
		</tr>
		<tr>
			<td>Ceramabond paste</td>
			<td>AREMCO</td>
			<td>552-VFG</td>
			<td>For cell sealing</td>
		</tr>
		<tr>
			<td>ICP-MS (7700s)</td>
			<td>Agilent</td>
			<td>NA</td>
			<td>For Cr analysis</td>
		</tr>
		<tr>
			<td>Potentiostat (VMP3)</td>
			<td>Biologic</td>
			<td>NA</td>
			<td>For EIS/I-t measurement</td>
		</tr>
		<tr>
			<td>FIB (Helios Nanolab 460F1)</td>
			<td>FEI</td>
			<td>NA</td>
			<td>For Nano-sample preparation</td>
		</tr>
		<tr>
			<td>TEM (Talos F200X S/TEM)</td>
			<td>FEI</td>
			<td>NA</td>
			<td>For composition analysis</td>
		</tr>
	</tbody>
</table>

development and validation of chromium getters for solid oxide fuel cell power systems

Degradation of cathode in solid oxide fuel cells (SOFC) remains a major concern for the long-term performance stability and operational reliability. The presence of gas phase chromium species in air has demonstrated significant cathode performance degradation during long-term exposure due to unwanted compound formation at the cathode and electrolyte interface which retards the oxygen reduction reaction (ORR). We have demonstrated a novel method to mitigate the cathode degradation using chromium getters which capture the gas phase chromium species before it is ingested in the cathode chamber. Low-cost getter materials, synthesized from alkaline earth and transition metal oxides, are coated on the cordierite honeycomb substrate for application in the SOFC power systems. As-fabricated getters have been screened by chromium transpiration tests for 500 h in humidified air atmosphere in presence of chromium vapor. Selected getters have been further validated utilizing electrochemical tests. Typically, electrochemical performance of SOFCs (lanthanum strontium manganite (LSM) &#449; yttria stabilized zirconia (YSZ) &#449; Pt) was measured at 850 &#176;C in the presence and absence of Cr getter. For the 100 h cell tests containing getters, stable electrochemical performance was maintained, whereas the cell performance in the absence of Cr getters rapidly decreased in 10 h. Analyses of Nyquist plots indicated significant increase in the polarization resistance within the first 10 h of the cell operation. Characterization results from posttest SOFCs and getters have demonstrated the high efficiency of chromium capture for the mitigation of cell degradation.

A Cr transpiration experiment is a screening test for the selection of Cr getters. Cr transpiration setup was utilized to validate the performance of chromium getter under the SOFC operating conditions. Experiments were conducted in the presence of a chromium getter operated at 850 &#176;C in humidified (3% H2O) air for 500 h. Visual observations during Cr transpiration tests indicated significant discoloration of the outlet elbow during 500 h in the absence of getter. However,...

Watch this Scientific Journal Video about Development and Validation of Chromium Getters for Solid Oxide Fuel Cell Power Systems at JoVE.com

Development and Validation of Chromium Getters for Solid Oxide Fuel Cell Power Systems

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.

Degradation of cathode in solid oxide fuel cells (SOFC) remains a major concern for the long-term performance stability and operational reliability. The ...

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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