Hello, in today’s class we are going to look at a topic called supercapacitors and this is a form of you know energy storage device, that people work on and speak about in more recent times. And there are specific applications where this is the kind of device that makes a lot of sense and in some places, it may not be a device that can be used separately many times it may be used to augment a battery so to speak. So, it’s an interesting kind of device. So we will go over it and there is a lot of research that goes on in it and a lot of applications that people are increasingly finding for it. (Refer Slide Time: 00:57) So in fact; if you look at the popular literature, if you read general articles on energy and so on you will see a few different terms that are being used you will here see a term of supercapacitor you also see this term electric double layer capacitor, electric double layer capacitor and you also see this term ultracapacitor. So, these are the terms over and above just a capacitor. So, the capacitor is different, but I mean in some sense this is these 3 terms, that you see here so this is EDLC something like that. So, supercapacitors, electric double-layer capacitor, ultracapacitor they all refer to the same thing okay. So, this is what you will see in the literature, but this is basically what it is that they are essentially the same. So, we will call it supercapacitor for the course of this class and then, we will I mean this is just something that you have to keep it your mind. (Refer Slide Time: 01:57) So, our learning objectives for today’s class are, of course, to first understand; what is a supercapacitor? What is it that is you know since you are already aware of a term called capacitor? What is a supercapacitor? What is different about it I mean, why should it be? When considered as something you know as separate from a capacitor so to speak. so yeah, how does it differ from a capacitor? And what are the applications it is typically suited for what kind of applications it gets used for and what are typical materials used for the supercapacitor? So, these are some of the learning objectives for our class and within this context of these learning objectives, we will look at the content of the class. (Refer Slide Time: 02:40) So, a supercapacitor is nothing, but a capacitor that has very high capacitance. So, I mean you can make you know from different manufacturers you can get capacitors of different capacitance, but just because something has slightly higher capacitance doesn’t make it a supercapacitor we will see that you know them; you are looking at several orders of magnitude difference in capacitance. So, see when a capacitor is simply 2, parallel plates 2 metal plates right and there is some dielectric material in the middle could even be air, but you typically have at some dielectric material. So, you charge this up a supplier you attach this to some battery and you charge it up so you will get positive charges here. And you get negative charges here and corresponding, you will get the opposite charges here lining up with this positive charge you will get negative charges from the dielectric you will get positive charges here. So, this is how it stores up the charge in the capacitor and then you can later release it and then you can get the; you know chargeback to you. So, the charge that is usually there is given by C V, where V is the voltage you applied and C is the capacitance and this is a charge. So, this is the voltage in volts capacitance in Farads and charge in coulombs so this is what you will get so this is the relationship. So, we are talking of capacitors where the; you know capacitance value is very high. So, therefore, for the same voltage, you get a very high amount of charge that is being held there so this is something that we look at it has high energy density. Generally, relative to you know and other options that; you have the voltage, that it can give you is a little low, cycle life is very high. We will talk about this as we go through the other option; I mean the details of the supercapacitor. Generally cycle life is high, high in the sense we are looking at you know again several orders of magnitude that, that’s the point that you have to remember as we look at numbers you will see it is not just high by a relative sense of you know a factor of 20 per cent, or 30 per cent, or even 100 per cent. We are looking at difference both in capacitance in cycle life etcetera of several orders of magnitude so to speak. It charges as well as discharges much faster than batteries. So, that’s the key that you have to understand that it charges and discharges much faster than batteries. And, therefore there are applications where the battery where the change in circumstance is kind of fast and you want something to store that energy quickly. So, there a battery will not be able to do the job although a battery can store energy, it needs some time to pick up that energy, but supposing you are delivering that energy is so fast that you know you don’t have that time you have only a few seconds in during which you delivered a whole amount of energy and you want the battery to hold it the
battery is unable to pick it up. Supercapacitors are in a very good position to pick it up so that is why the super the capacitor is interesting. So, in a sense, it bridges the gap between regular capacitors and rechargeable batteries ok. So, this; what I have shown you here is a typical regular capacitor. So, we will see that some more again anyway, but this is a regular capacitor. So, this is not what we are doing here we are talking about supercapacitor and it the supercapacitor bridges the gap between these kinds of regular capacitors and rechargeable batteries where you have you know an anode a cathode and an electrolyte so you will have. So, this is a regular capacitor the rechargeable battery will have anode, electrolyte, and cathode ok. So, this is how we have the rechargeable battery and then you go again you charge it discharges it etcetera. So, between these two is where the supercapacitors end upcoming so we will look at that. (Refer Slide Time: 06:59) So, some, for example, I just mentioned to you that you know it is ideally suited for conditions, where the circumstance is changing fast and you need the energy to be handled rather quickly. So, for example, if you look at regenerative braking okay so regenerative braking is the braking that is these days being done in electric vehicles? You could do it any in any other vehicle also, but generally since the electric vehicle is set up to run everything operationally using electricity the scheme of you know any energy that is any energy transaction that is happening people try to convert that to electricity. So, normally in a regular car you press the brakes, the brakes just become hot okay so they become very hot and essentially that’s what they are doing they are converting your kinetic energy into heat and releasing it. So, there is a lot of friction that is put there it converts the kinetic energy of the car into heat and then throws it out into the atmosphere; so that’s, really what is going on? So that is why you know when you are going on hilly terrain and you are climbing up and climbing down and so on your brakes can heat up very extensively; if you are continuously using the brakes. So, that’s why we keep telling you to use a gear lower gear and enable the gear to help you slow down your vehicle rather than just keep on pressing your brake you have to press the brake if you need it, but this is what is happening. So, when you keep the when you keep pressing the brake you know continuously; you know you know say one hour downhill drive that heats the brake to the point that they may fail. So, you have to be careful with your brakes, but that’s what is happening in a conventional vehicle in a conventional car that is exactly what is happening. Now, in electric cars the idea that you know when you try to generate electricity back from something that is; moving it slows down whatever is moving. So, you are using that energy that is there in the rotational process to generate electricity by just the way a dynamo would work in an in you know cycle that you had except that there also the dynamo is pressing to slow down the cycle, but you don’t want it to slow down the cycle. Here, you are consciously using this electricity generation process to brake the car to enable braking for the car so that that comes to home. So, regenerative braking as you can imagine is something that’s happening in fractions of second’s right. So, fractions of seconds to a second is, what you see the traffic you are driving and traffic you need to hit the brake you hit the brake immediately you don’t hit the brake over several minutes you hit the brake instantaneously. So, the breaking process occurs over a fraction of a second so that’s; what I meant by saying that? The time scale is very small in that time scale in that fraction of a second the entire energy that was there in the car the kinetic let’s say a car was travelling at say 30 kilometres an hour ok. And half a second later not even half a second maybe 0.2 seconds later, you have the car at 0 kilometres in hour ok. So, whatever was the weight of the car so, if you look at half m V square. This much kinetic energy was available.
So, if a car is about so this is half into let us say a 1000 kilogram car. So 1000 kgs you have and V is 30 kilometres an hour, so that is we have what we have 3000 you know 30000 meters in so you will have 30000 in 60 minutes. So you have this goes 500. So, you have 500 meters per second per minute squared and you divide this by 60 so you will have whatever 50 by 6 meters per second so, that’s the speed. So, roughly about what is it 8 8.4 8.3 meters per second that’s the velocity it is travelling at. So, you have half into 1000 into 8.3 square so this is the amount of energy that the car has in joules so that is quite significant. So, this is about 64 so about 30, 35. So, you are looking at 35000, about 35000 joules so 35000 joules you have. So, this is 64, 64 by 2 is about little more than 64, so I will I am just assuming say it’s 70, 70 divided by 2 is about 35 and 35 into 1000 so 35000. So, you have to 35000 joules of energy in a car that is; travelling at 30 kilometres an hour it travels about 8 meters every second and you are essentially in 0.2 seconds you are bringing it to a complete halt. So, in 0.2 seconds this much energy has to be stored somewhere a battery will not be able to pick up this much even though it is maybe a battery pack inside it will not be able to generally it takes time, it takes a lot of time for all the reactions to happen for the electricity to be picked up by the battery. So, a battery typically is unable to pick up 35000 joules in 0.2 seconds ok. So, you need something else which can pick up this 35000 joules in 0.2 seconds, if you want to do regenerative braking. So that’s the kind of you know quick transition that which a battery cannot handle that a supercapacitor handles okay so, that is the point that we want to understand so that is one application. Similarly, loading and unloading so you load something on some you know loading process of adding weight to something and then removing the weight from something. So, you need suddenly a fair amount of energy to lift something to put it somewhere and then release right. So, again you need a huge amount of energy being delivered shortly in a short duration of time, you have to pick this object up everyone is sitting quite. Suddenly you pick up this object which could be you know 500 kilograms, 1000 kilograms, whatever pick it up to put it up into some shelf. So, this may be happening in some storage warehouse or something, so that kind of a thing where you have a machine which goes and does this it goes and picks up a package lifts it off the ground puts it into shelf releases comes down. So, that has to you know suddenly give you that energy in this matter of a fraction of a second again, that is; something that a supercapacitor can deliver to you. Also, we spoke about regenerative braking and that that comes to the stop part of an electric vehicle, similarly start part of the electric vehicles, suddenly you want to start the car right. So, if you look at what all people are doing to you know to optimize the energy usage for a vehicle which is you know a major aspect associated with you know running clean energy, environmentally friendly energy, and so on the usage of energy. right now they have they are implementing and you know the operational characteristic of a car in such a way that when you come to a halt for any reason even let’s say traffic has halted and you just come to a halt because the vehicle in front of you is halted you just stop your car at that instant it will switch off the engine automatically you don’t have to do any such thing. So, for example, in Indian conditions, we are often told that if you come to a traffic light and you see that it is going to take some time you switch off the engine and that way you can save a lot of energy because a massive amount of energy is wasted just waiting for the traffic light to change traffic signal to change right. So, most of the time we are advised to switch off the car and many of us we do it sometimes we do not do it and so on so a lot of energy is wasted. So, the modern-day electric vehicles do it automatically. So, the moment it senses that the velocity is 0, it instantaneously switches off the engine it’s completely it goes to sleep for several seconds. The moment you start it will; immediately start that’s the beauty; of it, you don’t have to you know to give it any you don’t have to give it even a second to start almost instantaneously it starts. For it to almost instantaneously start again that much amount of energy has to be delivered suddenly all of a sudden you have to deliver this massive amount of energy to get this you know maybe 1 ton or 1 and a half-ton car to start moving again almost immediately so, that is again done by the supercapacitor. So, both starting as well as stopping when you stop picking up the energy in a fraction of a second is done by the supercapacitor. When you start again delivering a lot of energy in a fraction of a second is done by a supercapacitor. So, like this, you can think of a variety of interesting end-uses where the supercapacitor is unique in it is the ability to handle that task. So, the battery can take care of the general operation, okay the general operation of the device can be handled by the battery, but the transitions when you go from one operating point to as a distinctly different operating point could be from start to an accelerated starting point or from an accelerated, I mean a high velocity to certain stop those kinds of transitions which are drastic transitions occurring over a very short period, that is; where the supercapacitor makes a difference which the battery cannot do. So, like it’s a good combination to have a; some set of supercapacitors, and some set of batteries which are appropriately matched for particular end-use. So, that the user experience as a user you buy a vehicle and you use it your user experience is very enjoyable you don’t see problems you don’t go back and complaining saying that you know I used to drive a petrol-powered vehicle. Now I got this electric vehicle every time I try to start it struggles for a fraction of a second and then starts or a lot of energy is getting wasted I am not seeing enough benefit from it and so on. So, all those things are not that you will get all those benefits by having this combination right so that is what a supercapacitor is capable of doing ok. (Refer Slide Time: 16:32) So we have 3 terms here we have a supercapacitor, we have a battery, and we have a capacitor. So, we will spend a little bit of time trying to understand; what is the difference between these 3? So, primarily if you look at battery we are; we know that it is an electrochemical device ok. So, the energy that is stored there is stored in the form of chemical energy, chemical energy meaning you have a reactant sitting on one electrode and another reactant sitting in another electrode. So, a chemical reaction happens it’s an electrochemical reaction sure this so there is a charge transfer between electrolyte phase and the electrode phase. In the end, there is a chemical change that has happened to some constituent right so some compound has formed. So, we saw a lead, lead could become lead sulfate or lead oxide could become lead sulfate cobalt oxide could become lithium cobalt oxide, so many chemistries we saw for batteries. So, but the bottom line is there is a chemical change and that chemical change cost you to this electrochemical reaction is how the energy is stored. So, in a way you are storing it in the form of chemical energy the energy that is being stored is to being stored in a form of chemical energy right and to do this process you are using ions. So, you have so for example, in a battery, you will have the let me just put it here. So, you will have as we just saw anode, cathode and this is the electrolyte and you have some ions going across right. So, that is the process by which the activity happens. So, there is chemical energy that is stored here.


















So, chemical energy is stored here, chemical energy is stored here in the form of whatever reactants are present and then the ions move across and enable the process to happen. So, ions are moving and chemical energy is being stored. So, this is the idea concerning a battery. Now concerning a capacitor, we are storing electrical energy so meaning you have a flat plate another flat plate and then in the middle, you have a dielectric ok. So, you store electrical energy so well as I said you have all these charges here you have minus charges here you have plus, plus, plus, minus, minus, minus. So, you don’t have ions you have electrons basically and the energy is being stored and as in the form of electrical energy there is no chemical change that’s happening we will discuss that some more, but there is no chemical change that is happening the supercapacitor uses this aspect of it that there is no that it’s electrical energy that is being stored that is the aspect that in which it resembles a capacitor, but it uses ions and in this process, it resembles a battery. So, it picks these two concepts so that’s what you have here it uses ions and it stores electrical energy ok. So, you have the similar thing you have some electrode structure some something in the middle and another electrode structure and then you have ions here okay and that’s how the supercapacitor is functioning. So, we will see that functioning a little bit in more detail in just a moment, but this is how the three of them differ. So, the supercapacitor uses ions the just away battery does and it stores electrical energy just the way capacitor does okay so this is the idea. (Refer Slide Time: 20:17) So, as I said if you look at the capacitor this is how a capacitor schematic of a capacitor would look you would have two parallel plates. So, one plate here, and one plate here which would be your electrodes to which you connect an external battery. So, you connect this to a battery. So, this is the positive and then I connect it here negative comes in ok. So, then in the middle, you have a dielectric material which is what which is shown here and then as you have supply electrons to this side and you take away electrons from here. What happens is this becomes positively charged, this becomes negatively charged, and then correspondingly the dielectric material the side of the dielectric material that faces the negative electrode becomes positively charged, and the side of the dielectric material that is facing the positive electrode becomes negatively charged. So, overall there is charge neutrality there’s just the same amount of positive charge as there is negative charge etcetera. So, you have charge neutrality and also you should remember that this charge that you see here in the dielectric material is more or less in one region it’s a flat region. So, you are looking at flat regions here. So, in a very flat region, you have a negative charge collecting on this surface of the electrode, similarly, a flat region here both the electrodes are flat smooth surfaces here you again you have a positive charge that is being collected. In the middle in the middle, you have the dielectric material there also on the surface of the dialect material on one side you have a positive charge on the other side you have a negative charge. So, this is how the capacitor is functioning okay and in this process, the charge is stored energy stored you can reduce the energy depending on your requirement. (Refer Slide Time: 22:19) Now, if you now look at a supercapacitor some of these ideas are being carried over, but and there’s a lot of change so that is what we are trying to understand here so this is the supercapacitor. So, again you can see here there is something like an electrode, but here the electrode simply serves as a current collector so this is like a current collector and so this side also you have a current collector okay and important difference that you have here is you have a porous electrode. On both sides, you have a porous electrode and we will talk about that in just a moment in the middle you have a separator ok. So, you have something called a separator. And, then in this entire region that you see here this entire region; that you see here from here to here you have electrolyte this electrolyte ok. So, this is what we are looking at in the course context of a supercapacitor, so the point being that the current collector essentially enables electrical contact for the external circuit with these particles. In this case, it is particles but we will we can look at other options available, but this is particles. So, we have a structure where you have a lot of particles that are present and therefore because it is particles and not a solid object like this is not a this is a solid flat object okay, solid flat surface because it is a solid flat object here you have particles. So, you have a very high surface area that is divided and distributed ok. So, you have a very high surface area that is now distributed through the electrode. So, it is not a flat surface it is not a single flat location where the charges represent when you talk about the charge, but anyway, these particles are present and there’s an electrolyte that is present the electrolyte that is present over the extent entire extent of this region that I just marked here between this current collector and that current collector you have electrolyte. So, this electrolyte penetrates these particles. So, all these gaps that you see here all the gaps that you see here are all filled by the electrolyte everywhere ok. All the gaps that you see here so even here whatever gaps you see between the particles are all filled by the electrolyte so the electrolyte permeates into these porous into this porous structure. So, the electrode must be a porous structure the way we have shown here in a schematic, but the specific purpose of it being a porous structure is to ensure that this electrolyte that we are talking about has penetrated that structure and it is present throughout that structure. So, now whereas previously; you just had this flat surface. So, therefore, whatever was the geometric area was the area of the electrode so if it is you know 2 centimetres by the 2-centimetre electrode, that’s a 4-centimetre square electrode right. So, if it is 2 into 2 so that’s a 4-centimetre square electrode. Here, even if your current collector is 2 into 2 that is; only the current collector area only is the 4-centimetre square right actual surface area of the electrode is the surface area of each of these particles. So, you have to calculate the surface area of each of these particles and the total surface area that you get by a totalling up the surface area of all those particles is the surface area of the electrode. So, therefore, this has an extremely high surface area very high surface area relative to this regular capacitor. The supercapacitor relative to the regular capacitor the supercapacitor has a dramatically different amount of surface area you are looking, at several orders of magnitude higher surface area relative to the regular capacitor and we are utilizing that surface area by enabling this electrolyte to penetrate throughout it and of course, you don’t want you to know positive electrode to directly come in contact with the negative electrode right. So, otherwise, you will short circuit the thing if that short circuit the concept of a short circuit is the same here also. So, you don’t want a short circuit between the positive electrode and the negative electrode that is why we have this separator material. So, the separator material will be sitting in the middle it is a non-conducting material it is also soaked in the electrolyte. So, the electrolyte is continuously present throughout the structure. And separated materials are essentially the same kind of separator materials that you would use even in a battery. So, you have the non-conducting separator materials you know maybe polymer-based materials which are porous so it’s a porous structure. So, you can have electrolyte going in all directions through that structure and it fills that whole structure. So, so though those are the things you have done you have put in a separator material which was not there in the capacitor you have put in a porous electrode structure not there in a regular capacitor and this electrolyte is distributing itself throughout this porous structure ok. So, this is how the structure of a supercapacitor differs from that of a regular capacitor. So, as you can see at least at a structurals level and keeping in mind that there is so much more surface area there is a huge difference between what a capacitor is and what a supercapacitor is ok. So, this is the point we have to remember. (Refer Slide Time: 28:07) So, now, I will also point out that when you put an electrode in contact with an electrolyte. So, this is the electrode, and this is the electrolyte and let’s say; I connect this to the positive, positive of a battery ok. So, when you do that let’s say so you get a bunch of positive charges here ok. So, that part is still the same now the electrolyte contains ions okay so, the ions have you know they will have. So, let’s say I have negatively charged ions so those ions are now affected by the presence of this charge that is there on the electrode. So, naturally, just the way you would expect in all these circumstances the opposite ion gets attracted to this electrode. So, what happens though is that it doesn’t necessarily just form a single layer here ok, initial models used to suggest that it will also form a single layer, but because it’s an electrolyte and because you know there is thermal energy associated with it and there is the movement of the electrolyte simply because of thermal energy that is present there etcetera it does not form a single layer it distributes itself some more. So, much deeper into the electrolyte also you will have this region that is affected by this by the electrode ok. So, this is how the electrode-electrolyte interface operates. So, we even here there is the notion of a double layer capacitor here, but the second part of the layer is not necessarily a flat structure it is a little bit more distributed structure that’s the point we have to keep in mind. And you should remember that you should also notice that on this site you have electrons and on this site you have ions. So, on one side of the electrode-electrolyte interface on the electrode side, you have electrons on the electrolyte side you have ions ok. So, when you look at this structure of the supercapacitor what is different here relative to what I have drawn here is that the in the electrode in this case I have drawn as the flat surface. Whereas, in a supercapacitor, we have one flat surface which acts as a current collector and just for you know to wake for clarity sake I just only draw say two or three particles. So, I have a large particle I have a small particle and I have one more large particle something like this right. So, then the charges so you put a positive charge here, this positive charge because these particles are electronically conducting is also the positive charge that distributes itself here ok. Then the negative charge gets distributed in this intermediate area is all con containing electrolyte. So, all here you have electrolyte all over this place. So, you have negative charges collecting here ok. So, this is how the now you have sort of extended the electrode into the system you have extended the electrode into a larger region as supposed to a flat. So, what was two dimensional has suddenly become a three-dimensional structure right so that is what you have done. So, this is two dimensional, this is three dimensional. So, the electrode-electrolyte interface is three dimensional it is distributed into a larger region and that is how you know electrical contact has been made here, the electrical contact is there, electrical contact is here, electrical contact between these particles is then that’s how the positive charge that you put on the electrode goes to all of those particles and then the electrolyte is you know it has permeated everywhere and that is very important. If the elect so for example, if the electrolyte were not here so supposing this region there is no electrolyte then in that place there is no charge this charge will not have this charging will not happ