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>
...

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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 ...

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