Perovskite or nanocomposite materials-synthesis for ASOFC
In this section different synthesis routes e.g. solid state reaction, co-precipitation, wet chemical, sol gel etc., have been described, how they have been applied for the preparation of perovskite-type or nanocomposite materials for LTSOFCs or ASOFCs.
Synthesis of Perovskite-type or Nanocomposite electrodes
Different nanocomposite electrode materials were prepared by performing the experiments on different platforms (laboratories) provided by Fuel Cell Group at KTH. Some were used, as-prepared cathode, in fuel cell calibration.
Preparation of electrode Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF)-perovskite
BSCF, an electrode material was synthesized by co-precipitation method. 0.5M of Barium Nitrate, Ba(NO3)2.H2O (Sigma-Aldrich, USA) named as solution A was mixed with Strontium Nitrate, Sr(NO3)2.H2O (Sigma-Aldrich, USA) in a bath, using 0.5:0.5 molar ratio of precursors of both compounds. This solution was then stirred on a magnetic stirrer for 1 hour at 70°C, meanwhile 0.5M of Cobalt Nitrate hexa-hydrate, Co (NO3)2.6H2O (Sigma-Aldrich) named as solution B was prepared with Iron Nitrate nona-hydrate, Fe (NO3)3.9H2O (Sigma-Aldrich) in a separate bath with molar ratio of 0.8:0.2 for the two compounds, this solution was also stirred for half an hour at 70°C. Solution B was mixed drop wise in solution A. Adding both of the solutions, resultant solution was named as solution C. The complete scheme for preparation of local BSCF is given in the flow chart in figure 2.1. Almost same scheme was adopted for most of the samples preparation.
An appropriate percentage of Ammonia, NH3 (Sigma-Aldrich) was added to the composite solution C (mixture of solution A and B) for maintaining the pH value between 2-4 according to the literature and the solution was named as solution D, it was well stirred for 4 hours at 70°C. Then 0.047M of oxalic acid dehydrated solution was prepared and added drop wise in well stirred solution D. It was again stirred for half an hour till clear precipitates of the BSCF were observed in the beaker. Precipitates were washed in water and filtered properly. These resultant precipitates were then dried at 120 °C overnight. The dried precipitates were ground well homogeneously into powder form in mortar with pestle. This ground powder was then sintered at 1000 °C for 5 hours to get the perovskite structure. The sintered material was nicely ground again to get BSCF powder to use for different requirements. It is observed that BSCF shows perovskite structure with ABO3 structure formula.
To prepare the nanocomposite cathode material, as-prepared BSCF powder and samarium doped Ceria, SDC (Sigma-Aldrich) were mixed with 1:1 volume ratio using standard solid state reaction method for synthesizing a mixed conductor. After ensuring a homogeneous ground material, its calcination was done at 700 °C for 4 hours. In this way BSCF-SDC a nanocomposite cathode with mixed conductivity was achieved for fuel cell fabrication.
Preparation of cathode Ba0.3Ca0.7Co0.8Fe0.2O3-δ (BCCF)
BCCF, a cathode material was prepared by wet-chemical method. Specified amounts of Barium Nitrate, Ba(NO3)2.H2O (Sigma-Aldrich, USA), Calcium Nitrate tetra-hydrate, Ca(NO3)2.4H2O (Sigma-Aldrich, USA), Cobalt Nitrate hexa-hydrate, Co(NO3)2.6H2O (Sigma-Aldrich) and Iron Nitrate nona-hydrate, Fe(NO3)3.9H2O (Sigma-Aldrich) were mixed all-together in a separate beaker with molar ratios 0.3:0.7: 0.8:0.2 respectively to make a 100 ml aqueous solution. This scheme was used to replace Strontium with Calcium at A-site in ABO3 structure. It was aimed that resultant material should have the perovskite-type structure with ABO3 form. The total mixture was continuously stirred for 30 minutes. An appropriate amount of citric acid was also added in the precursor’s solution for auto-combustion of the material, and then the net solution was stirred again for 2 hours at 80 °C. After getting auto-combustion, the ash was ground well in mortar with pestle (conventional method) so that fine and homogeneous powder was obtained and sintered at 950 °C to get the proper structure of the material. It was ground again to get the fine powder form of the sintered BCCF.
As-prepared BCCF powder was mixed with SDC with a volume ratio of 1:1 by using solid state reaction method to use it as cathode material in 2, 3-layer SOFCs. The mixed conductor BCCF-SDC was calcined at 700 °C for 4 hours for its further use in cell performance. In this way, nanocomposite mixed-electronic and ionic conductor was prepared. This powder was further used for cell operation in the Fuel Cell Rig.
Preparation of cathode Ba0.9Ca0.1Co0.8Fe0.2O3-δ (BCCF)
Another perovskite cathode material BCCF was also prepared by wet-chemical method. The stoichiometric amounts of Ba(NO3)2.H2O, Ca(NO3)2.4H2O, Co(NO3)2.6H2O and Fe(NO3)3.9H2O were mixed as it is, were obtained from the supplier without any further purification all-together in a separate beaker with molar ratios 0.9: 0.1: 0.8: 0.2 respectively in order to get 100 ml aqueous solution. The mixture was stirred on magnetic stirrer accompanied with a heating system for 30 minutes. A prescribed amount of citric acid was also added in the solution for auto-combustion, and was stirred again for 2 hours at 70-80 °C. Magnetic bead is responsible for homogeneous mixing during the stirring process. As a result of auto-combustion, the ash was obtained which was ground well in mortar with pestle (a conventional method) to get a fine and homogeneous powder. Powder was sintered at 950 °C in the furnace to get the expected structure of the cathode material. It was ground again to get the fine powder form of the BCCF structure. It was expected according to theoretical background that BCCF would be perovskite material with ABO3 general formula.
As-prepared BCCF powder was mixed with SDC with a volume ratio of 1:1 by using conventional method by mortar and pestle for its future use as cathode material in the form of mixed conductor to use in 2 and 3-layer SOFCs. The mixed conductor, BCCF-SDC was calcined at 700 °C for 4 hours for its further use in cell performance. So nanocomposite mixed-electronic and ionic conductor was prepared. This powder was then used for cell fabrication and tested in the Fuel Cell testing Rig. These (BCCF) prepared materials were used in Fuel cell Laboratory at Aalto University Helsinki, Finland.
Preparation of cathode Ba0.5Sr0.5Cu0.7Fe0.3O3-δ (BSCF)
Another electrode perovskite material BSCF was prepared by wet-chemical method with the same strategy. Pure precursors of Ba (NO3)2.H2O, Sr (NO3)2.H2O, Copper Nitrate tri-hydrate Cu (NO3)2.3H2O (Sigma-Aldrich, USA) and Fe (NO3)3.9H2O was mixed as it is, they used as obtained from the supplier without any further purification in a beaker with molar ratios 0.5: 0.5: 0.7: 0.3 respectively in a 100 ml aqueous solution. The mixture was stirred by electronic magnetic stirrer accompanied with a heating system for 30 minutes. Temperature of the stirring solution of the mixed precursors was controlled by an internal temperature sensor system.
A supporting amount of citric acid was then added in the solution for auto-combustion, and was stirred again for 1.5 hours at 80 °C. Material obtained from auto-combustion, was ground well in mortar with pestle using conventional method to get a uniform powder. The powder was sintered at 950 °C in the furnace to get the expected structure of the cathode material. It was ground again to get the fine powder of the copper supported BSCF structure. This attempt was made to replace Cobalt with Copper at B-site in ABO3 structure. And it was assumed that obtained material would have the perovskite structure.
As-prepared Copper supported BSCF powder was simply mixed with SDC with a standard volume ratio of 1:1 by using conventional method of mixing by mortar and pestle for its future use as cathode material as a mixed conductor in 2 and 3-layer devices. The mixed conductor, BSCF-SDC was calcined at 700 °C for 4 hours for its further use in fuel cell testing. So in this way nanocomposite mixed (electronic, ionic) conductor was obtained. The NANOCOFC approach was applied in order to obtain mix-conductor powder. Conductivity was measured by the tubular furnace at the Fuel Cell Rig at KTH.
Nanocomposite materials-synthesis for new energy conversion device
Zhu. Bin., et al revealed a breakthrough when they invented single component fuel cell or new energy conversion device by applying NANOCOFC approach and they introduced nanocomposite (two phase) materials with mix conductivities . This part contains synthesis of nanocomposite materials for new energy conversion device. These materials are also referred as single component materials. These materials are very important for fuel cell power generation.
Composite material (LNCZ) synthesis for new energy conversion device
A nanocomposite material (LNCZ) was prepared by standard solid state reaction method. Lithium carbonate, Li2CO3, Nickel carbonate basic, NiCO3.2Ni(OH)2.4H2O, Copper carbonate basic, CuCO3.Cu(OH)2 and Zinc carbonate basic, 2ZnCO3.3Zn(OH)2 were obtained from Sigma Aldrich and were used without further purification. Precursors of all compounds were ground with a molar ratio of 1.5:7:2.5:7. Well mixed homogeneous mixture of the precursors was sintered at 800 °C for 4 hours in a furnace. As-prepared composite LNCZ oxide was ground for 10 minutes to get homogeneous fine powder. This electrode material was then mixed and ground with electrolyte powder of SDC in an appropriate volume ratio of 1:1 to prepare single component material. Finally, this composition was sintered at 700 °C for 4 hours. The sintered material was well ground for more than 15 minutes and was converted into fine powder form to use as single component fuel cell or new energy conversion device.
Preparation of composite (LiNiO) for new energy conversion device
A nanocomposite material for multipurpose functionalities was prepared by solid state reaction method. The stoichiometric amounts of Li2CO3 (Sigma Aldrich), NiCO3.2Ni(OH)2.4H2O (Sigma Aldrich) were mixed with molar ratio 1:1 respectively and were ground in mortar with pestle for better homogeneity of the mixture. Once the uniform mixture was prepared then it was sintered in a furnace at 800 °C for 4 hours. After cooling down at room temperature, as-prepared material was ground for 10 minutes and uniform grains of nano-powder were achieved in this process applying NANOCOFC approach. Afterward, as-prepared LiNiO was mixed with electrolyte powder of SDC with a volume ratio of 1:1 to prepare a composite material to be used as single component material. For this purpose, mixture of both powders was ground for more than 15 minutes so that uniform mixture could be achieved. This composite powder was sintered again at 700 °C for 3 hours to get well-structured single component material. The resultant material was ground again for 10 minutes and used in new energy conversion device.
Dry press method for conductivity measurement
Homogeneous ground powder was compacted in the form of pellets for transport purposes to measure the conductivity and other properties evaluation, respective powders were pressed by dry press method. Powder of electrode materials was pressed in a die of 13 mm diameter to shape the thick pellets with diameter of size of the die. It was observed that the active area of the prepared pellet was 0.64 cm2 when used in sample holder for conductivity measurements. The thickness of the pellets was about 2-3 mm for its testing and in some cases up to 5 mm. Powder for manufacturing the pellet, in the die was provided a stress under the pressure of 50 MPa for 5 minutes. When the pellets were made, they were put in the furnace at 650 °C for heat treatment for 1 hour. The pellets were pasted with silver paste on both sides for better electrical contact before testing them at Fuel Cell Testing Rig. For testing, tubular furnace was used, where sample holder was put and shielded with heat blocking material.
Fuel Cell Fabrication
A number of SOFCs were fabricated for testing the performance. For this purpose, fuel cells were prepared with different configurations.
Conventional fuel cell or 3-layers device (anode/electrolyte/cathode)
Conventional fuel cells consist of two different electrode configurations as given below.
a. Asymmetrical fuel cell (with different materials for anode and cathode)
b. Symmetrical fuel cell (anode and cathode of same materials)
Asymmetrical fuel cell
An asymmetric fuel cell of standard 13 mm diameter for testing was compacted. Fine powder of anode material was laid on nickel foam substrate as a first layer in a stainless steel die; a thin layer of electrolyte was spread over the anode as second layer and finally cathode powder (different from anode) was layered next to the electrolyte forming a complete assembly which can work as conventional energy conversion device. Mostly, NiO was used as anode, SDC as electrolyte and synthesized material (BSCF, BCCF, copper supported BSCF or LiNiO) as cathode. In some cases, LiNiO was also used as anode and NiO or BSCF as cathode. This fundamental configuration was pressed in a dry molding press. Stress was applied by exerting a pressure of 200 MPa.
The designed sample was put under the same load for 5 minutes. After this, fuel cell was extracted safely from the die and saved for later usage. Samples of different thickness were prepared and then sintered at 650 °C for one hour only. In few cases, sintering was done to make the bared device active and mechanically strong but most of the time, devices were used as un-sintered. In every case of the sample of all pellets, the active area of the fuel cell was measured as 0.64 cm2. For fuel cell’s performance measurements, both of the electrode surfaces were painted with silver paste for efficient current collectors. The cells were tested at different temperatures.
Symmetrical fuel cell
In this type, fuel cell was fabricated in similar way as asymmetrical fuel cell with same components and their structure. The only difference in this type was that anode and cathode were of same nanostructured material. Device was pressed under the applied load of 200 MPa for 10 minutes. In symmetrical case, LNO was used as anode and cathode of the fuel cell while SDC was fixed as an electrolyte. Pellets were fabricated of 13 mm diameter and an active area of 0.64 cm2. On both sides of the cell, silver paste was painted and performance was measured at different temperatures. In either of the type, electrolyte was sandwiched between anode and cathode. In every case, when I measured the performance of the non-sintered cell immediately after fabrication, metal support was given by using nickel foam to the anode side. I have not used nickel foam in all of my sintered cells.
Two layers fuel cell (anode/cathode) (Electrolyte free fuel cell)
Two layers fuel cell device was designed by just removing the electrolyte layer and compacting rest of the two layers (anode and cathode). 2-layers devices were fabricated of 13mm diameter with an active area of 0.64 cm2. Most of the times LNO or NO was used as anode and BSCF or LNO respectively were used as cathode materials. Pellets were pressed under an external load of 200 MPa for 10 minutes. Metal support was provided by nickel foam on anode side of the device. Pellets were coated by silver paste on cathode sides before testing them. 2-layer devices were tested in a tubular furnace between 400-600°C temperatures.
New energy conversion device (Single component fuel cell)
New energy conversion device was made by two kinds of single component materials separately. In this type of fuel cell only single homogeneous nanocomposite layer is pressed to make the system for energy conversion. The single component fuel cell is also known as electrolyte free fuel cell because of the removal of electrolyte layer which is being considered as responsible to avoid short circuit but in case of single layer device, we get the same performance without electrolyte layer as it was with electrolyte layer in conventional 3-layer device. In this fuel cell, homogeneous mixture of electrode and electrolyte is prepared and pressed in the form of pellets.
Single component fuel cell based on LNCZ
To prepare this type of fuel cell, first of all LNCZ was mixed with SDC in a volume ratio of 1:1. A homogeneous mixture was prepared and sintered at 700 °C for 4 hours, after cooling; it was ground and pressed in the form of pellets of size about 1 mm. Nickel foam was provided as a substrate for support as well as a current collector. Single layer devices were fabricated by applying a stress of 250 MPa for 30 minutes. Diameter of the fuel cell was 13 mm and active area of the device was 0.64 cm2. Before testing in the tubular furnace, air providing side of the electrolyte free fuel cell were painted by silver paste. Performance of the cell was measured at 550 °C, hydrogen was provided from nickel foam side and the other layer was exposed to air.
Single component fuel cell based on LiNiO
First of all single component nanocomposite material by homogeneous mixture of LiNiO and SDC was prepared in a volume ratio of 1:1. Then this mixture was sintered at 700 °C, after cooling this sample, was ground for 10 minutes to make it fine powder for new energy conversion device. This powder with mixed conductivity was compacted by dry press method under the load of 250 MPa. To get this fuel cell of thickness about 1 mm, only one layer of single component material was spread over nickel foam and obtained the pellets with uniform density. Pellets of 13 mm diameter and active area 0.64 cm2 were fabricated. The pellets without Ni foam were sintered at 600 °C for testing. Then these pellets were painted with silver paste on both sides for efficient current collectors. Performance was measured without any further heat treatment of the device.
Table of contents :
1.2 Fuel Cell
1.3 Types of Fuel Cell
1.4 Perovskite Materials and ASOFC or LTSOFC
1.5 Thermodynamics of SOFC
1.6 Renewable and Nonrenewable Energy Resources for SOFC
1.7 NANOCOFC science
2. EXPERIMENTAL TECHNIQUES
2.1 Perovskite or nanocomposite materials-synthesis for ASOFC
2.2 Nanocomposite materials-synthesis for new energy conversion device
2.3 Dry press method for conductivity measurement
2.4 Fuel Cell Fabrication
2.5 New energy conversion device (Single component fuel cell)
3. RESULTS AND DISCUSSION
3.1 Electrode Material-Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF)
3.2 Electrode Material- Ba0.3Ca0.7Co0.8Fe0.2O3-δ (BCCF)
3.3 Electrode Material- Ba0.9Ca0.1Co0.8Fe0.2O3-δ (BCCF)
3.4 Electrode Material- Ba0.5Sr0.5Cu0.7Fe0.3O3-δ (Copper supported BSCF)
3.5 Microstructure of As-prepared Materials
3.6 Microstructure of Single Component Material
3.7 Cell Performances