- 6일 전
카테고리
📚
학습트랜스크립트
00:00Greetings everyone. I am Jiwon Son from KUKIS Green School.
00:04Today I will talk about the introduction to the thin film materials for high temperature operating solid oxide cell.
00:13First, let me talk about what is the solid oxide cell.
00:17A solid oxide cell is a cell based on the metal oxide.
00:23And then before I talk about it, let me explain what is a pure cell.
00:28Fuel cell is a device that converts the chemical energy of the fuel directly into the electrical energy by electrochemical
00:37process.
00:39Meaning that if we supply fuel and air into the fuel cell, then the electrochemical reaction happening in the fuel
00:48cell will generate electricity.
00:49And because the reaction itself is the oxidation process of the fuel, so there will be some heat happening here.
00:59And then also because the fuel, when it oxidizes the fuel, especially the hydrogen will have the water in there.
01:10So if we think about the fuel cell and compare it with the battery, then the fuel and air is
01:20supplied into the fuel cell.
01:21And then it generates the electricity, then the H2O comes out.
01:27In battery, the active materials work and only the electricity comes out, so there's no in or no out other
01:35than the electricity.
01:37So it means that this is the open system and this is the closed system.
01:42So fuel cell is like some continuous battery.
01:46And then it seems like that it is really similar or they consider it as the battery that never needs
01:54the charging.
01:57Actually, there's a lot of fuel cell types.
02:00So if you see the fuel cell, then you may think that there's one type of the fuel cell if
02:06you are not familiar with the fuel cell.
02:08But there are various types of fuel cells and the most popular is the polymer electrolyte membrane fuel cell.
02:15And this is based on the polymer electrolyte and that is used for the car.
02:20So if you think, if you heard about the hydrogen car, hydrogen electricity car, then that means that that car
02:27uses this PEMFC system.
02:29And the solid oxide fuel cell, which operates at the highest temperature here, is the one that I'm talking about,
02:36which is based on the solid oxide, which is the same as the ceramic materials.
02:42The most significant difference between these two is because of the operating temperature,
02:47solid oxide fuel cell can use carbon monoxide as a fuel as well,
02:52which means solid oxide fuel cell can operate over methane or the natural gases or sometimes diesel that contains carbon
03:01here.
03:02But the PEMFC operates at a lower temperature, so it should use pure hydrogen.
03:08That is called the fuel flexibility.
03:10So that's one of the most significant advantage of the SOFC.
03:16And other than that, actually the SOFC has the highest efficiency among other types of the fuel cell.
03:21And then it has the highest specific power.
03:24So it means that we can make the smaller, lighter and more powerful system.
03:30The SOFC unit set structure looks like this.
03:33So you have some electrolyte that can conduct the oxygen ion.
03:38And then there's an air electrode, which is cathode for the fuel cell operation.
03:43And the anode, which is the oxidizing fuel electrode.
03:50And then because the gas phase of the fuel and the air is supplied, this part here is porous.
03:58But the electrolyte should be very dense.
04:00So the role of each component is that the cathode is having the role of the reduction of the oxygen.
04:08Anode has the role of the oxidation of fuel.
04:11With this reaction, you can see that this electron is generated by this reduction.
04:18And then consumed in this reduction.
04:22And then by this oxidation reaction, you can see the generation of the electron.
04:27So when the circuit is connected externally, then the electron will flow like this.
04:36So you will have electricity.
04:40And then the each role of the component is listed here.
04:46And then you can see that, as I mentioned, it has porous dense and porous alternating structure.
04:52The real microstructure of the solid oxide fuel cell looks like this.
04:57So you have dense electrolyte and the porous electrode here.
05:01The unit cell of the planar SOFC seems like this.
05:05So it has flat structure.
05:06You have also some different types of the SOFC.
05:08But this is the flat types of flat type structure.
05:12And then in this unit cell, yield about like 1 volt voltage.
05:17You see that for the battery, like AAA battery or some like 2A batteries, it has 1.5 volt.
05:25So same comes here.
05:26So it was determined by thermodynamic reaction.
05:29So each cell have like about 1 volt of the voltage.
05:32So for the application, we should have more layers to have higher voltage.
05:39That is called stack.
05:40And this is a repeating stack component.
05:43And for the materials of the SOFC, the most popular one is the etria-stabilized zirconia.
05:51This is the zirconia that has etria as a dopant.
05:55And then that will stabilize the crystallographic structure of the zirconia.
06:01And based on the electrolyte, that's the most important part of the SOFC,
06:07the anode and cathode is composed of some catalysts with these electrolyte materials.
06:13Anode is composed of like something like nickel with YSG and LSM with YSG.
06:20And there are some other materials that have higher performance.
06:23In here, the electrical conductivity of the YSG is listed here.
06:29And then you see that there is other candidating materials that have higher conductivity here.
06:35And then with this electrolyte, you will also have some other cathode and anode that has higher performance.
06:43You can make smaller SOFC and then bigger SOFC.
06:47This is the same for the other fuel cell types as well.
06:50You can make the portable if you make it small.
06:52And if you make it larger, then you can have something like power plant.
06:57The most popular large capacity SOFC is produced by Bloom Energy in the US.
07:09This has like this 100 kilowatt system, which is about like parking lot size.
07:14And then if you have more connected, then you have really high capacity.
07:21They can supply electricity and then heat and warm water to the building.
07:26And then there's a lot of companies that actually employ this system, SOFC system.
07:33Other type of SOFC is a small SOFC, which is called micro SOFC.
07:38This is specifically for the military.
07:42As I mentioned previously, the biggest advantage of SOFC is the fuel flexibility.
07:49So in military, this fuel flexibility is really important.
07:54So you have to use fuel that you can obtain easily.
07:59So SOFC in that aspect has really high advantage.
08:04So that there are some research groups and those companies working on these military SOFCs based on a very small
08:11SOFC.
08:15The next topic is why thin film and nanomaterials are needed in SOFC.
08:21There is a need for this micro SOFC, as I mentioned in the military.
08:26And then also the battery has the capacity that is limited by the material.
08:32So there's always a research activity that develop the micro SOFC for the portable and the military sector uses.
08:41For making this large SOFC to this small SOFC, the requirement is like this.
08:47We need to have it small and then this should be integrated.
08:52And then for that, we need lower temperature operation.
08:57And lower temperature operation is important in conventional SOFC as well.
09:01In this high temperature operation, even for the ceramic material, there is always something happening.
09:07So a reaction can happen, breakage happens, and then some corrosion and degradation happens.
09:13So the reliability and cost issues are really a significant problem in the conventional SOFC system.
09:20So the lower temperature operation is expected to have improved reliability and cost effectiveness in conventional SOFC as well.
09:30However, we cannot lower the operating temperature of the conventional SOFC simply.
09:37This is the IVB curve that shows the performance of the SOFC.
09:42This is voltage and this is current density.
09:45So this is the IV curve.
09:47If you multiply voltage and current, then you can have this power curve.
09:52So you read power density here.
09:55At 800 degrees C, it yields really high performance of more than 2 W per square centimeter.
10:01But if you decrease the temperature to 150 degrees Celsius, then the power drop is more than,
10:09you will lose more than two-thirds of your original power.
10:14So the biggest issue is that how we can have the lower operating temperature, lower temperature operating SOFC without compensating
10:28the performance.
10:28The reason why we lose a lot of performance at low temperature is that because the ionic conduction and also
10:36catalytic activity is all thermally activated process,
10:39you will increase the resistance exponentially as you decrease the temperature.
10:46You can see it here.
10:47This is a low scale, so you can see the linearity when you decrease the temperature.
10:53So this is the question, the question, so how we can compensate the performance degrees.
10:59One way is that to differentiate the microstructure.
11:04If you have thin electrolyte, then you will have lower resistance.
11:09And if you have a nanostructure, then you will have more contact point that yield the more the reaction side
11:18density.
11:18So this thin film structure and nanostructure can compensate the power loss at the low temperature.
11:28So this is the actual diagram that is required structure for lowering the operating temperature.
11:36At a high temperature, you can have thin electrolyte.
11:40But if you have temperature lower than this temperature, then you should have thin electrolyte.
11:45Below 650 degrees, then you need to have ultradine electrolyte.
11:51And even lower, you will need nanostructure electrode.
11:55So the requirement for making a conventional SOFC to thin film structure is that like this.
12:02So you have electrolyte thickness less than 1 micron.
12:06And you should have also nanostructure of the electrode that has the grain size intense and 100 nanometer scale.
12:13And you should also have dense porous-porous alternating structure to make the furesor structure.
12:21The way to make this is using thin film technology.
12:25Thin film technology is widely used in the semiconductor industry.
12:30And this is the 30 centimeter wafer developed in the Intel for making CPU.
12:36You see a lot of thin layers and then oxidized layers and then so on.
12:41So the thin film process is combining a lot of various processes like deposition, oxidation, diffusion, and so on.
12:54Especially for the thin film deposition means like we have physical vapor deposition, chemical vapor deposition, and other various techniques
13:01are there.
13:02So this is the basic technique for making thin electrolyte.
13:07However, when thin film is deposited, that will follow the structure of the substrate.
13:13So if you have porous substrate like this, as I mentioned in the first slide, you will need porous structure
13:20in the electrode.
13:22If you deposit your thin film over this structure, then you will have this continuous electrolyte because of this structure.
13:29CVD, you have a complementary position, but the precursor or the materials are really limited.
13:38So when you use the PVD-based structure, then this is really significant issue.
13:43So even if you increase the film thickness, you still have some pin holes here.
13:49If you have pin holes here, then you will have fuel and oxidant mixed with each other.
13:55Then that will have lower the cell voltage because the cell voltage is determined by the pO2 difference between this
14:04electrolyte.
14:05And if you have leakage, the pO2 difference will decrease.
14:08Then you will have lower voltage.
14:14Then next, let me explain what is the common thin film SOFC.
14:19Well, as I mentioned, like we need dense substrate to have dense electrolyte.
14:25So usually for making the thin film SOFC, people usually use very dense substrate like silicone,
14:32which is used for the semiconductor devices or some glass or something like that.
14:38First, they deposit silicon nitride as an edge stop and then deposit the thin film and thin and dense electrolyte
14:48by the deposition,
14:50which can be the sputtering or the other physical base for deposition and the chemical vapor deposition, etc.
14:57This is needed for the following sequence.
15:02And then for that, for making the fuel cell structure, the electrolyte should be exposed to the electrode side.
15:10So the backside of this dense substrate is removed by the chemical etching.
15:17If we don't have this etched stop, then this will be also damaged by the chemical etching.
15:22So that's why we need this etched stop.
15:25After chemical etching, this is removed by the reactive ion etching because this part cannot be removed by the chemical
15:33etching.
15:34Then now we have the electrolyte that both sides are exposed toward the free air.
15:41And then the membrane looks like this under the electron microscope.
15:44And then the thickness of this membrane is about like several hundreds of nanometer.
15:50And then this opening is about like 100 micron to 500 micron.
15:56Then the porous electrode, usually platinum, is deposited by the sputtering like that way.
16:04After making that, then you will see the thin film SFC structure is now completed.
16:10Porous, dense porous structure is like this.
16:13And if you see it under the microscopy, then you can see that porous electrode, dense electrolyte.
16:20And then porous electrode is formed like this way.
16:24And this part is something like this.
16:27This is due to the chemical etching nature of the silicon in the KOHC solution.
16:35With this, the effect of the impact of the thin film electrolyte and then also nanostructure electrode is quite significant.
16:43As I mentioned, like the Stanford University have the most developed thin film SFC technology here.
16:50And then they made this freestanding membrane with this corrugated structure.
16:55This corrugated structure is intended to maximize the active area.
17:02Projection area is the same as this membrane area.
17:05But with this corrugated structure, then you will have actually much larger active area.
17:10With this, they achieved 1.3 watt per square centimeter at 450 degrees C.
17:19And then this performance is obtained in the conventional SFC at 700 degrees C,
17:24which means that we can obtain same performance at 250 degrees lower temperature.
17:29So this perfectly fit our original purpose that we can lower the operating temperature of SFC without compensating the performance.
17:40However, these have really huge problems.
17:44As I mentioned, this structure has very large membrane size compared to in comparison with the thickness of the membrane.
17:51So this membrane is quite weak.
17:55And then also the thin film has very large surface area.
18:01And the nanostructure electrode also have very large surface area.
18:06After high temperature operation, because of the frailty of this membrane structure, it ruptures quite well.
18:13And also electrolyte can center and then shrinkage can happen.
18:19And this nanostructure, a metal electrode, especially the metal, is quite weak to the high temperature.
18:27So it tried to reduce the surface area by agglomeration.
18:31So this is like a very significant defect because of the nature of the nanoporous metal structure.
18:39So we need to solve these critical issues in thin film SFC to have reasonable device.
18:47Electrolyte, we need to, this freestanding membrane cannot sustain a very small impact, a very, very low impact.
18:55So we need supporting structure.
18:56And for the, when we use this pore structure, then we cannot have dense electrolyte.
19:03The void can happen.
19:04So in this case, we need some defect suppression.
19:09Electrode, the agglomeration is so, so fast.
19:12So we need to have some suppression methods to, to suppress this kind of agglomeration at elevated temperatures.
19:22These problems in common thin film SFC result in poor stability of the devices.
19:30So you can see that the cell voltage just fluctuate.
19:34And then also the degradation rate is quite fast.
19:37So this is really a decent one in considering thin film SFC.
19:41Common thin film SFC ruptures only after one measurement.
19:46Usually that, that's what happens.
19:47So with this technology, we cannot make any realistic device.
19:53So we need to fabricate thin and very stable thin film electrolyte of the SFC.
20:01First, as I mentioned, electrolyte should be very thin and dense.
20:07And it should have thermomechanical stability for higher temperature operation.
20:12Even if it is 450 or 550 degrees C, which is much lower than the 800 degrees C operating temperature
20:19of the common SFC.
20:21So we wanted to, in case, we wanted to avoid this problem by making supported structure.
20:27But as I mentioned, over the porous structure, we cannot have a thin and dense electrolyte.
20:32So what we have in mind is that two approaches.
20:37One is that we first make this dense support structure, which can convert to the porous structure after the deposition
20:46of the thin film.
20:47Second approach is that deposit the thin film over the porous structure.
20:51Then you will have a lot of defects and thin holes.
20:54Then block these pin holes like some kind of plugs or something like that.
21:01So let me first explain what is this and how it was possible.
21:07This is the as-prepared cell structure.
21:09You see totally dense structure, totally dense electrolyte is formed here.
21:14But after the cell test, you can recognize there is a porous structure formed in here.
21:19This is possible by the nature of the anode of the SFC.
21:26This anode is comprised of the nickel and then YSJ, as I mentioned.
21:31Nickel originally upon the fabrication is nickel oxide.
21:35During the cell test, we need to reduce this anode with hydrogen or some other fuels.
21:42Then nickel oxide will shrink upon the reduction.
21:46So that's how we generate this porous structure.
21:52But while we make the structure, this structure is totally dense.
21:56So we can deposit our dense electrolyte without problem.
22:01With this approach, we realized the cell in 2009.
22:09And then you can see that the OCV is quite high.
22:12So that means that we can deposit our dense electrolyte over that designed structure.
22:18But after the years, we optimized its component.
22:24So we optimized anode, cathode, and also the current collection.
22:28Now we have more than four times of the cell performance with the original design.
22:35At the original design, we were so afraid of making, try to avoid the density.
22:46I mean the pin holes in the electrolyte.
22:47So we made very, very dense support.
22:50That is very detrimental to the gas diffusion.
22:54So that's why we have this low performance.
22:57But after we raised our confidence level, we can optimize its component and have this high performance.
23:08Approach two is blocking the pin holes.
23:11So when we have this porous structure, if we deposit the electrolyte, then you will have this porous structure.
23:19After making this, we use the thin film deposition technique called atomic layer deposition.
23:25Atomic layer deposition can cover the structure quite conformally.
23:32So that's how we make this plugging structures penetrate into this defect pin holes and can plug the pin holes.
23:43So here we can see that this is porous anodized aluminum oxide.
23:49And then you can see that we have this type of the grain structures over it.
23:53And we use the ALD to plug this part of the pin hole.
23:59And then you see that very small plugging.
24:02We use the alumina AL2O3.
24:05So you can see that aluminum can be detected in this plugging structure.
24:09As I mentioned, by using this one, the voltage was quite low before we used this plugging structure.
24:15But after we used this plugging structure, now we can have voltage, which is the indication of the very dense
24:23electrolyte film.
24:25With this, we also have the cell performance.
24:27At the time, it was very high performance.
24:29But considering what we have now, it's quite low performance.
24:36But at that time, this approach was considered quite noble.
24:39So that's why it was published in the advanced functional material.
24:45But the second approach still has some problems related to the stability, long-term stability.
24:50So that's why we focus on the approach one.
24:54And then with this approach one, we have achieved a lot of beneficial property of the thin film SOFC.
25:03Here, I collected some data from Stanford University, Harvard University, and ours to compare 500 degrees to performance.
25:11And then you can see that here, we have a highest peak power density.
25:15But the area of the cell is quite small.
25:19So the power per cell is only like micron-watt level, 134 micron-watt level.
25:26Harvard tried to increase the area of this pre-standing membrane cell.
25:31And this is 5 mm by 5 mm.
25:35But as they increase the area, the power density just decreased.
25:41So if you compare this and this, this is like about one-tenth of the original pre-standing membrane structure.
25:50But still the area is big.
25:51So now the total power is about 38 milliwatt.
25:55But in our cell, we don't need to worry about this.
25:58We can make a bigger cell, several centimeters.
26:02If your deposition technique allows, you can have like 5 by 5 centimeter or much larger cells.
26:09And then 500 degrees C, we have also very high performance.
26:12It's even the peak power density is bigger than, higher than this one.
26:16It's about like 0.588 watt per square centimeter.
26:21And total power is same as this because our activity area is 1 by 1 centimeter.
26:26If you compare it with other techniques, it's more than 4,000 times to the Stanford one.
26:33And then more than 1,000 times than the Harvard case.
26:40Also the stability is quite excellent.
26:43After the thermocycling, as I mentioned, the ceramic is quite brittle.
26:49So thermocycling is the most serious harsh test for the thin film SFC.
26:54But it can sustain its structure more than 50 times.
26:58Compared with the Harvard cell and then our previous works on some other this fluctuation,
27:06this is really impressive improvement.
27:10So if I say, I said like if we decrease the temperature of the conventional SFC from 800 degrees C
27:20to 600 degrees C,
27:22then we will lose two-thirds of our power.
27:24But with our thin film structures and the nano structures,
27:29we can have similar performance at 650 degrees C with this conventional SFC.
27:36So let me close my lecture with comparing our cell performance with other reports.
27:45These are the SFC.
27:47These are the performance of the protein conducting cells.
27:51And then in our thin film,
27:57KISS thin film structure can have this performance, especially temperature below 650 degrees C.
28:04I would say this is one of the highest performance among all the SFC technology.
28:12And also the proton conducting cell itself, it also have very high performance.
28:18So still we are saying that like our approach can make it possible to have both high performance
28:26and then also stability at low temperature with thin film and nanostructure.
28:33Okay, here I close my talk.
28:35Thank you for your attention.
28:36Thank you.
28:36Thank you.
28:36Thank you.
28:36Thank you.
28:36감사합니다.
댓글