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

Summary

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.

Abstract

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) ǁ yttria stabilized zirconia (YSZ) ǁ Pt) was measured at 850 °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.

Introduction

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 seals^3,4. 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 elsewhere^8,9,10.

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 cathodes^11,12. 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 components^14,15. 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.

Protocol

1. Synthesis of chromium getter

1. Synthesize precursor powder using alkaline earth and transition metal oxide salts via conventional coprecipitation synthesis route as depicted in Figure 1¹⁶.
   1. 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. 
   2. Use 9 mL of 2.4 M Sr(NO₃)₂ and add with 7 mL of 2.4 M solution of Ni(NO₃)₂.6H₂O, followed by stirring the mixed solution and heating up to 80 °C.
   3. Add 30 mL of 5 M NH₄OH to increase the pH to 8.5 for precipitation, then, dry the solution in a dry oven and ensure that all the water evaporates until a blue waxy compound is observed. Rinse the powder in DI water to ensure that residual ammonium nitrate is removed by filtration. Finally, dry the powder at 120 °C for 2 h. 
      Note: This will produce precursor powder for strontium nickel oxide (SNO) getter. 
2. Dissolve the powder in water to prepare a slurry. 
3. Immerse the cordierite substrate in the slurry for dip-coating, followed by drying in air at ~120 °C for at least 2 h with a ramp rate of 5 °C.
4. Calcine the substrate in air at the temperature of 650 °C for 12 h with a ramp rate of 5 °C to produce SNO getter. 

2. Screening of chromium getter using Cr transpiration test

1. Set up experiment following the illustration of Figure 2a for the validation of Cr getters.
   1. Place 2 gram of sintered chromium oxide pellet (1,200 °C, 2 h) as a chromium source in a quartz tube. 
      Note: the quartz tube is specifically designed with a diffuser inside (shown in Figure 2) to prevent any back diffusion of chromium vapor during operation. The dimensions of the fabricated getter cartridge match the inside diameter of the quartz tube. Getter cartridge is placed between the chromium source and the outlet elbow (shown in Figure 2). 
   2. Flow the compressed air at a flow rate of 300 sccm through a mass flow controller (MFC). Bubble the air at room temperature water to ensure that the humidity is 3% H₂O. This humidified air passes through the chromia pellets, evaporate chromium vapor and flow through the getter. 
      Note: The chiller and condenser are placed at the outlet of the transpiration setup to enable condensation of chromium-containing vapor which deposits at the outlet elbow (at the low temperature zone).
   3. Place additional wash bottles before venting the gas at the outlet to ensure that the evaporated chromium is captured. 
   4. Having completed the setup, purge the tube with air for at least 1 h to ensure that there is no leaks or contaminants. 
   5. Start the furnace to heat up to the desired temperature (for example, 850 °C in this case) and hold it there for 500 h. 
   6. Monitor the color change of outlet elbow and record for any discoloration due to deposited chromium compounds. 
   7. Lower the furnace temperature back to room temperature (RT) after the completion of the test. Turn off the air flow until the furnace temperature reaches RT. 
   8. Remove the getter sample for post-test analyses and characterization. 
2. Quantitative analysis of chromium species by inductively coupled plasma mass spectroscopy (ICP-MS) 
   Note: ICP-MS sample preparation of post Cr transpiration test¹⁷.
   1. Wash the deposited chromium from glass elbow, condenser, wash bottles and quartz tubes using 20% nitric acid to extract the chromium after conducting transpiration test for 500 h. 
   2. Extract the deposited chromium by dissolving it in 20% nitric acid (HNO₃) for 12 h. 
   3. Further remove any undissolved chromium species from the glass wall by dissolution in alkaline potassium permanganate solution upon heating at 80 °C.
      Note: Convert any partial unreacted Cr³⁺ species to Cr⁶⁺ species in this step. 
   4. Analyze the DI water and nitric acid blank sample by ICP-MS.
   5. Divide each sample into three parts for ICP-MS analysis and report the average value.

3. Electrochemical validation of chromium getter using SOFC cells with and without getter 

1. Cell fabrication and in-operando electrochemical testing of Cr getters^18,19
   1. Fabricate SOFCs by screen printing LSM paste on the surface of YSZ electrolyte (Figure 3a). 
   2. Sinter the applied LSM ink at 1,200 °C for 2 h, heated with a ramp rate of 3 °C/min. 
   3. Use Pt electrode as the anode. Attach a Pt on the YSZ disc (anode side) as a reference electrode, and attach Pt gauze and Pt wires to YSZ electrolyte disc using Pt ink and then cure the SOFC at 850 °C for 2 h at a ramp rate of 3 °C/min. 
   4. Conduct three distinct experiments using three identical SOFCs (namely Cell a, b, and c) to validate the efficacy of getters and demonstrate chromium poisoning without a getter. 
      Note: make sure to use the identical test conditions to simulate nominal SOFC operating conditions of 850 °C and maintain the anode air (dry) for all the tests at 150 sccm. 
   5. Assembly the Cell-a in the tube reactor in the absence of chromium source using paste for sealing. Heat the furnace with a ramp rate of 5 °C/min up to a designed temperature (for example: 850 °C in this study). Then, flow the 3% H₂O/air (for example 300-500 sccm) to the LSM cathode.
   6. Measure the electrochemical performance of the SOFC using a multi-channel potentiostat⁹. 
   7. Record the cell current every minute with a bias of 0.5 V applied between the cathode and the reference electrode. 
   8. Conduct electrochemical impedance spectroscopy (EIS) analyses between cathode and reference electrode using three electrode mode in the frequency range of 0.5 Hz to 200 KHz with 10 mV sinus amplitude at an interval of 1 h. After a 100-hour test, cool down the furnace to room temperature and take the Cell-a for characterization.
   9. Place 2 gram chromium oxide (Cr₂O₃) pellets (source of chromium vapor) in a porous container at the constant heating zone of the alumina tube. Assembly the Cell-b in the tube reactor using paste for sealing.  Heat the furnace with a ramp rate of 5 °C/min up to 850 °C. Then, flow the humidified air (for example 300-500 sccm) through the chromia pellets and ensure a constant generation of the chromium vapor species⁹. 
   10. Repeat Steps 3.1.6 – 3.1.8. After a 100-hour test, cool down the furnace to room temperature and take the Cell-b for characterization.
   11. Place 2 gram chromium oxide (Cr₂O₃) pellets (source of chromium vapor) in a porous container at the constant heating zone of the alumina tube. Place a chromium getter above the chromium source. Assembly the Cell-c on the top of the tube reactor using paste for sealing. Heat the furnace with a ramp rate of 5 °C/min up to a designed temperature (for example: 850°C in this study). Then, flow the 3% H₂O/air (for example 300-500 sccm) to the LSM cathode.
   12. Repeat Steps 3.1.6 – 3.1.8. After a 100-hour test, cool down the furnace to room temperature and take the Cell-c for characterization.
2. Posttest getter morphological and chemical characterization
   Note: Posttest characterization is conducted using scanning electron microscopy coupled with energy dispersive spectroscopy and scanning transmission electron microscopy (STEM) coupled with EDS analyses. Focused electron and ion-beam technologies (FIB) have utilized for the preparation of nanoscale samples. 
   1. Analyze the microstructures of the cell component by fracturing after the electrochemical test.  Use SEM instrument for morphological analysis. Ensure that both the morphologies and chemical compositions of LSM cathode surface and LSM/YSZ interface are analyzed^13,14
      1. Prior to conducting SEM analysis, prepare samples by sputter coating of gold (Au) films to make sure the sample surface is conductive (avoiding of charge on the sample surface). The coating chamber was under a vacuum (< 50 mm Torr). The applied current was at 40 mA and the coating time was 1 min.  
      2. Conduct quantitative elemental distribution using energy dispersive X-ray spectroscopy (EDS) technique. The distance between the specimen and the lower pole piece in SEM system was set at 10 mm. A voltage of 20 KV was applied for the SEM and EDS analysis. 
   2. Conduct the chemical, structural and morphological analyses of the chromium getter using the SEM-EDS technique to obtain the chromium capture profile across the getter channels. 
      1. Prepare the posttest getter sample by dissecting the getter sample into half using a knife. 
      2. Repeat Step 3.1.1.1 to coat conductive gold films on the getter surface.
      3. Repeat Step 3.2.1.2. Ensure that detailed EDS analyses were conducted from the inlet of the getter towards the outlet along the central channel as shown in Figure 2b. Use weight (wt.) % of total chromium measured against the channel length to plot the chromium profile.
   3. Conduct in-depth chemical, structural and morphological analyses of the chromium getter using the FIB-STEM-EDS technique^17,20.
      1. Repeat Step 3.1.1.1 to coat conductive gold films on the getter surface.
      2. Load the sample in the FIB-STEM instrument, select the region of interest (ROI) for sample extraction, deposit four layers of Pt to mark and protect the sample (a typical area of 30 µm length × 15 µm width).
      3. Mill the channels around the above ROI using the FIB beam until a “bridge-like” strip is left. Then, make wedges at both sides of the strip to make sure the depth is enough for your analysis (a typical depth is 10-20 µm).
      4. Mount to the micromanipulator needle and cut the FIB sample by milling using an ion beam with 15 nA current. Then lift the FIB sample from the bulk getter sample to the FIB-STEM grid holder, which is perpendicular to the electron beam. After the specimen touch the grid at right position, Pt is deposited using an ion beam current of 0.5 nA to connect the specimen to the grid.    
      5. Make the specimen thinner using a FIB current of about 20 pA at 2 kV to obtain a 50-60 nm sample thickness. Final clean-up of the specimen is also performed using argon-milling at an extra low current (0.5 pA at 1 kV). 
      6. Conduct STEM-EDS mapping of the above getter specimen. The scanning transmission electron microscope was operated at 200 kV. A High Angle Annular Dark Field (HAADF) image of the selected area on the getter specimen was obtained and elemental maps of relevant elements (such as Cr and Sr) were taken.  

Results

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 °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, plac...

Discussion

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)

Materials

List of materials used in this article

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