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Hello. In this class we are going to look at batteries, they are very ubiquitous devices they are all over the place, and at any given point in time, you know all of us to seem to be using batteries. So, in this class, we will look at some of the basic concepts associated with batteries, the signs of the batteries and how it functions and so on. I don’t know if you realize that, but in any given you know the social setting, that you may be in chances are there are more batteries than there are people, you will typically find this to be true. Because at any point in time most of us seem to be carrying our mobile phone with us, we also typically have a wristwatch of some kind that we are wearing. So, right there, there are two batteries. So, already you know if you see 10 people there are 20 batteries there. So, that’s the extent to which this technology is now you know a commonplace in our life; we don’t even realize it, we don’t realize that we are carrying batteries in on top of it if you have a laptop or a computer that has a battery, and if you have a calculator your calculate scientific calculator whatever you are carrying has a battery. So, all lots of things have batteries which we don’t even realize that it is having a battery and over and above this we carry you know a power bank or something to extend the range of our mobile phones. So, that’s only a battery more or less. So, this kind of situation exists. So, therefore, it is a technology that is interesting to look at. In the context of non-conventional sources of energy, batteries play a very critical role very important role, because a lot of those technologies you know we spoke about solar energy, we spoke about wind energy, we spoke a lot of detail about how those technologies function. But, one common aspect with all of those technologies is that they don’t consistently produce power throughout the day and the year. So, there is a significant impact of the time of the day on you know power being produced, the conditions at the time of day determining the power being produced. If it is solar technology, it could be just you know a passing cloud could make a difference rain could make a difference or you know the completely cloudy day could make a difference and of course, there is a big huge difference between day and night so that difference is there. If it is wind technology again you know you may have a gust of wind, you may have steady wind flowing, you may have fast wind going, so again as the day progresses you may have different conditions for power generation. And then also there are seasonal variations. So, all of these impact those two major technologies that are being pursued and activities around the world. So, you also always need to have some energy storage process or you know technology to go along with these non-conventional sources of energy, to utilize that non-conventional source of energy. And that is the context in which we will study let’s say batteries and later we will look at fuel cells. Both of these are when they don’t; I mean fuel cell is not storing energy, but they give you the flexibility of you know distributing the power that you generate across the day. So, you generate power. So, you may at times be generating more power than you need. So, you need a way in which you can, but your utility. So, your utility pattern has a different profile, it doesn’t necessarily match with the profile of the power that you are generating. (Refer Slide Time: 03:37) So, for example, let’s say if I mark the time of the day right and this is said power and let’s say how are we utilizing it. So, let me say this is 6 am ok. So, this is noon this is 6 pm. So, this is noon and this is a little further down am. So, if you do this you will find that usually in most houses if they are you know let’s assume that on average everybody is up around 6 o clock. So, your power you say this kind of high. So, this stays high because a lot of things are on, you know your kitchen is on, you are a fan is on maybe grinder is on may things might be on. So, this is kind of high everybody is getting ready to go to school or office and then everybody leaves for school office whatever and then suddenly there is a huge drop in power consumption. So, it will stay low kind of stay low maybe at lunchtime it goes up a little bit then stays low then comes back, then again sometime in the evening everybody comes home. So, everybody comes home and then you are back you know you are entertaining yourself watching TV or you know cooking something else heating some food a lot of things you do so, your power consumption goes up and then stays up. So, let’s say this is about. So, this is around here. So, half your mark would be 9 pm. So, little past 9 pm again everybody goes off to bed and then its power decreases. So, you just have some basic fan running and then it stays low and that is how you get 12 12 am, and then if you continue this till 6 am which is the 24-hour mark. So, it will sort of stay low relatively speaking some basic thing will be going on maybe a few fans may be running etcetera all your lights will be off now even if you were running AC, you won’t be running it in several rooms you will be done anything, but. So, you can see this is the profile of power requirement as the day progresses in most households this is kind of likely to be the case and 6 am again as I said this is starting climbing back up to where you saw out here right. So, this is what you have now if you look at the power generation ok. So, let’s say it is the solar panel. So, if it is a solar panel, then you will have some fairly let’s say from 6 am the power goes up then by noon I am just drawing some schematic. So, let’s say this is how the power stays and then this continues of you have bright sunlight till about 6 pm then it starts dropping in intensity by 9 pm you have nothing and then it drops to 0 and then stays 0 then by 6 am it starts climbing back up when the sunlight starts coming ok. So, they are not very accurately done. So, this is solar energy and this is your household demand; and let’s say this is wind power. So, wind power it doesn’t even have a particular pattern I mean through the day and the night you may have all sorts of you know variation and power. So, just to give you some I mean I am just drawing a schematic which has no you know no basis, I am just except that to show you that it is kind of random. So, you may have fairly good wind power being generated and then it might just drop down and then it may just go up and will go up even higher it might do this. So, some random thing it might do and then it will do something like that. So so, there is no you cannot very accurately predict what wind power is going to do. So, this is how wind power is going. So, you can see that if you look at what your house ultimately let’s say you are only powering your house right. So, let’s say you have set up a small plant either a windmill or a solar panel plant, which powers your house and say neighbouring some 4 or 5 houses are being powered by this. So, the demand for the house is essentially whatever is the red curve that you see here. And the red curve does not match either the green curve or the blue curve. So, it just totally does not match I mean there is no comparison they are off. So, therefore, you need something to balance this outright. So, when you need power if you are just directly connected to it, then you need power at 6 am the solar energy is just barely started producing power you are asking like three times the power. So, you are this is what it is generating this is what you are asking. So, solar energy is not going to do it for you. So, at some other point in time, it is delivering much much higher power and you are demanding much lower power. So, then what is it going to do with that excess power; so all that is getting wasted. So, therefore, you need something in the middle that you know keeps collecting up this energy that is being generated, and then makes it available to you when you need it based on your usage pattern so that you don’t feel any difference. In the house, you are just going to come and switch on lights and switch on fans and tube lights and do not know led lights to watch your TV etcetera. You don’t want that to be disrupted based on what the solar panel is seeing out there right. So, therefore, it is in this context that the batteries make a big difference. So, an energy storage device like a battery makes a huge difference because it helps you manage the supply variation with the demand variation it evens it out for you ok. So, that is why we are and batteries and we will look today at the basic ideas associated with how batteries to function, what are the important parts etcetera. (Refer Slide Time: 09:04) So, our learning objectives for this class are to state the various parts of the battery and their functions. So, to look at that what are the parts of the battery critical important parts of the battery and what are the functions, to indicate the use of the electrochemical series. So, what is that electrochemical series I am sure you have heard about it? So, as a part of our you know complete package of the understanding of batteries, we will briefly visit this electrochemical series and at least highlight the important aspects associated with it and to distinguish between primary batteries and secondary batteries. So, these are two kinds of batteries that are common there and we will distinguish between them. And also in the context of this discussion we also look at the meaning of some terms used in the context of battery technology. In fact, in the few classes that we will deal with this technology, some more terms should be there I am not introducing all the terms here because I don’t want them to be known completely in isolation when you listen to the term in the context of its usage you will better understand what it is that we are referring to. (Refer Slide Time: 10:16) So, these are our learning objectives parts and functions of the battery use of the electrochemical series distinguishing between primary and secondary batteries and some of the terms that are used in this context. So, this is what we are going to look at. So, what is the battery? A battery, first of all, is an electrochemical device it’s an electrochemical device. So, what do we mean by an electrochemical device? So, an electrochemical device, of course, I mean a very I mean suggests as suggested by the name supports an electrochemical reaction. So, it supports an electrochemical reaction. So, for that the parts and whatever the process that is involved is that there is something called an electrode phase, there is an electrolyte phase. So, there are two things in the electrode in the electrochemical device or a battery one is referred to as the electrode and the other is called the electrolyte and the phrase simply refers to the fact that it is an and you know physically distinct, mechanically separable, chemically homogeneous part of the system. So, that is basically what we are looking at and across that I mean it is a region in that system where the properties are uniformly constant. So, if you take a piece of metal that would be a phase, inside that also you may have multiple phases, but generally let’s say we have got a pure metal billet we have not got w are not got an alloy we have got a pure metal then you have a and it is all in one crystal structure then it is one phase. Then you have a liquid that is another phase you can take another piece of metal that’s a different phase. So, that is how you would look at it. So, there is an electrode phase normally we think of it as a solid, and there is an electrolyte phase normally we think of it as a liquid, but these can change, but this is normal you know the perception that we have. And importantly the reaction should occur in a manner that there is a transfer of charge between the electrode phase and the electrolyte phase. When that happens, so let me say that this is an electrode and it is sitting in contact with an electrolyte ok. So, and then it goes off to some external circuit some wire is that let’s say. So, now, when this electrode is in contact with this electrolyte, you should have a transfer of charge. So, let me just change the colour here. So, you should have a transfer of charge this way or this way, some charge should come from the electrolyte to the electrode or from the electrode to the electrolyte someone of the two should happen. If you have this happening as part of the reaction, then this transfer of charge between the electrode phase and the electrolyte phase is what creates this situation that this reaction gets referred to as the as an electrochemical reaction. So, an electrochemical reaction and that is a transfer of charge between the electrode phase and the electrode-electrolyte phase. So, that’s the point and the battery device itself that we use is an energy storage device. So, I will highlight this aspect a little more a couple of slides down the through this class little later in this class, but that’s just something that you need to keep in mind. So, a battery is an electrochemical device, and it also happens to be an electrochemical storage device. So, these are two characteristics of a battery that it is an electrochemical device. So, that is the reactions are occurring there are of electrochemical reactions and it happens to be an energy storage device. (Refer Slide Time: 13:43) So, what are the major parts? As I mentioned there should be an electrode and an electrolyte, but for the battery to be complete you need two electrodes and an electrolyte ok. So, one electrode we refer to as the anode, and the other electrode we refer to as the cathode and in the middle, there is the electrolyte. So that is what we have. So, the electrochemical reaction, two electrochemical reactions are happening, there is one reaction that is happening in this region between the anode and the electrolyte, there is another electrochemical reaction that is happening between the cathode and the electrolyte. So, in each interface the cathode electrolyte interface which is this region here this border between these two. So, this region here in this border as well as in this border, let’s assume these are electrodes are plain electrodes, flat electrodes, flat metallics electrodes or whatever. So, in these two borders, there is a reaction that is occurring. So, there is an electrochemical reaction that is occurring between the cathode and the electrolyte, there is a different electrode electrochemical reaction that is happening between the anode and the electrolyte and these two reactions are allowed to interact with each other using two different pathways. One is the electrolyte that is one pathway, and the other is the external circuit that is the other pathway okay. So, that is the other pathway. So, the external circuit could be anything, I mean you could have you know maybe you have a bulb there or a fan there or whatever or you just have some measuring device that measures what is going on, but basically, this is what is. So, there are two pathways through which these two reactions are interacting with each other, one is the external circuit one is the electrolyte. And the way the system works is that through the electrolytes there is the movement of ions okay movement of ions. So, ions are moving through the electrolyte, and in the external circuit, you have movement of electrons. So, that is what we have at the external circuit electrons are moving in the electrolyte ions are moving. And so, if you look in terms of property what property is being expected of these various parts especially concerning the electrolyte. The major property that is required is that the ions should be able to move quickly ok. So, the electrolyte should have good ionic conductivity and that simply implies that the ions can move quickly from one side to the other side, whichever side they are moving they may be moving from cathode to anode or anode to cathode depending on the reaction, but whichever way they are moving they should be able to move quickly. If they move slowly that is slowing down the overall reaction and that impacts the current coming out of this reaction ok. So, this has to have good ionic connectivity. Importantly it should have poor electronic conductivity. So, you should have good ionic conductivity and poor electronic conductivity why should it have poor electronic conductivity? This is only for the electrolyte that I am talking about. So, this is for the electrolyte ok. So, for the electrolyte, it should have good ionic conductivity and poor electronic conductivity. So, why should it have poor electronic conductivity? It needs to have poor electronic conductivity because we don’t we want these electrons to go through the external circuit only then it does some work for you right. So, if you have a bulb which is sitting in the external circuit, that bulb is select up that led or whatever that you have put an external circuit is say lit up only of electrons pass through it. Now, if you on the other hand if you gave some you know some internal path for the electrons to pass, then the electrons will not come to the led they will go in within the cell itself and that is a waste of the reaction. So, this reaction on the anode and the reaction the cathode are generating these electrons, these electrons you want in the external circuit only then something happens and so, if you accidentally provide an internal path for the electrons to go, then you have lost those electrons or if you know 10 20 per cent of the electrons go that way or 50 per cent of the electrons go that way half the current that you have generated has been wasted. And therefore, the electrolyte should have this twin property of having good ionic conductivity, and poor electronic conductivity this combination is very important. It should be an from the perspective of electron movement it should be an insulator, it should not be a conductor of electrons ok. So, that forces the electrons to go through the external circuit and that is how we end up using the battery. So, that is the idea here. (Refer Slide Time: 18:39) Now, we mentioned these terms anode and cathode and as I said you know maybe many of this at a different differing level is levels of detail, you may have even seen it in high school days and some part of your college days I would still like to highlight some specific points associated with it because I think often people have confusion on these points and therefore, I will highlight some points and then that makes our discussion more complete and cogent. So, we will do that. So, we use these terms anode and cathode. Now a lot of people then associate it with you know some something like you know negative electrode, positive electrode they get a lot of these terms are used and sometimes it is confusing. Now the way you should you need to understand it is that, there is only one strong definition of what is an anode and what is a cathode and then once you understand that definition everything else is related to that definition. So, the definition is that anode is the electrode, where oxidation is occurring ok. So, the anode is the electrode where oxidation is occurring and oxidation means loss of electrons anode is the electrode where oxidation is occurring if oxidation occurs there is a loss of electrons. The cathode is the electrode where the opposite is happening which is reduction and reduction means a gain of electrons. So, what is gaining electrons? Some species are gaining electrons some species is gaining electrons, what is losing electrons? Some other species are losing electrons ok. So, that is the way you have to think about it; so anode oxidation, cathode reduction that is the main definition. So, you can look at any electrochemical circuit, and try to understand where oxidation is happening where reduction is happening. Wherever oxidation is happening that is the anode wherever reduction is happening that is the cathode. You can even have electrochemical cells, were on the same metal one region of the metal is undergoing oxidation; another region of the same metal is undergoing reduction. So, that location where the anode oxidation is happening, that location is called the anode the other location where reduction is happening is called the cathode. And so, you may even have like 10 such locations; you may have 10 locations where reduction is happening 10 other locations where or say 7 locations where oxidation is happening, but at a higher rate than those seven locations are anodes these 10 locations are cathodes. So, that’s the way you should remember it. So, now, if you look at this definition anode oxidation loss of electrons and you go back to this figure. So, at the anode, we have oxidation occurring at this anode and then there is a loss of electrons. So, what is happening? Those electrons are the ones that are coming out into your external circuit. So, whatever material is there in the anode is undergoing that oxidation, and because of that oxidation electrons have been released and they are not in a position to go through the electrolyte because the electrolyte does not have electronic conductivity, and they are forced to go through the external circuit. So, that is why when you are using the battery, the location from which the electrons are coming out is an anode ok. And as you continue using the battery the cathode is the place where reduction is happening there. There’s a gain of electrons; the only place where it can gain electrons from is the external circuit because that is where the electrons are coming. So, that is how these electrons are going in here. So, the reduction is happening here and this is where oxidation is happening ok. So, oxidation is happening at the anode, reduction is happening at the cathode. So, now, we understand the terminology. So, anytime you are confused you try to figure out where the oxidation is happening, where the reduction is happening I will touch upon this once more in another context and then I think it will become really clear to you. So, at that point we; so you don’t have to just memorize that you know zinc is the anode you know say silver is the cathode, you don’t have to memorize like that in that cell you figure out where the oxidation and reduction are happening. So, importantly you have to understand that those circumstances can change, we can make the what one electrode is when one electrode is undergoing oxidation, you can create other circumstances where that same electrode now undergoes reduction ok. So, during the time that it was undergoing oxidation, it was behaving like an anode, during the time that it is undergoing reduction it is behaving like a cathode. So, that’s the point you have to remember that is why this definition is important. Because the same metal in the same solution can be forced to do one reaction in preference to the other reaction. When it is forced to do one reaction in preference to the other reaction based on the reaction that is happening you decide whether its anode or cathode is ok. So, that’s the point. (Refer Slide Time: 23:07) So, now, the electrochemical cell what we do as we saw here is that you have this anode and cathode and so, you can have a whole bunch of anodes and cathodes. So, you want to create a situation where you understand what will happen when you take one anode, and put it in or one material and connect it to another material with some electrolyte in the middle, you want to understand what will happen right. So, to do that we have to try and create some standard versions of these combinations, and once you understand how the standard versions behave you can you know couple two standard versions, and then from that, you will know what those that combination will do and then from there you can create a non-standard version of the same combination. So, that is the process by which the people understand what a battery does right. So, you have to first come up with a standard version for an electrode and an electrolyte, and like that you will have several such standard versions then you pair up them pair them up. So, then you have one electrode one electrolyte, and then the electrolyte and this is a second electrode. So, that becomes a complete pair again they are still under some standard conditions. So, there is some behaviour associated with that. And then you may use them in non-standard conditions, but how it will behave in the non-standard condition is something that you learn from how it behaves in the standard condition. So, this is how you do. So, what we do is that the electrochemical cell will typically consist of two electrodes. So, let me just put some kind of a beaker here and there is some electrolyte here, and some membrane which may be a slight separator which doesn’t allow complete mixing of the two electrolytes because you may have a slightly different electrolyte on one side and different electrolyte on the other side. You put one metal in here and another metal here okay and the let’s say this is metal A and this is metal B ok. So, A and B you have. So, what we will have is you will have the possibility of this reaction here, that is A going to A-plus e minus and similarly, here you will have the possibility of the reaction B going to B plus e minus and these could happen reversibly. Let’s assume that they are happening reversibly. So, in other words, it can do both ways ok. So, this is likely happening and then let’s connects this in an external circuit and here we will put a voltmeter and we will connect it up this way ok. So, what we are doing is we will also say that the electrolyte that is here contains one molar concentration of B plus ions and the electrolyte here contains one molar concentration of A plus ions. So, this combination that you have here of a metal A sitting in contact with a solution that has one molar concentration of its ions under one-atmosphere pressure and you know let 25-degree centigrade, this combination is referred to as a standard electrode. So, this is a standard electrode similarly here also you have a standard electrode. When you combine two standard electrodes like this and that is why you have this membrane in the middle because you have A-plus ions on the left side and B plus icons on the right side and so, you don’t want a massive mixing of them you want to maintain the standard condition that is how it is done this way then you measure a voltage this voltage is the voltage between two standard electrodes, okay and this is consistent with the voltage difference, that you will measure between these two electrodes using the standard electrochemical series which we are going to see in just a moment ok. So, you have various possibilities for metal A, you have various possibilities for metal B you can keep on taking different-different combinations and you can measure these voltages, and typically this voltage is the difference in voltage between these two electrodes. So, now, you have a wide range of different voltages. So, it gets confusing. So, you will have all sorts of different combinations of different pairs of materials you can take, and you will have I mean infinite possibilities you may have of these potentials. So, what we do instead is we try to standardize one part of the cell and then compare all electrodes concerning that standard part. So, what is that standard part? It is simply an electrode which contains a beaker. So, let me just change that here. So, you will now have some let’s say some beaker of this sort with one small opening the side, in which we will put the electrolyte here will be one molar of h plus ions ok. One molar of it is an acidic electrolyte. So, some acid is there, but with one molar concentration, and in this you bubble hydrogen gas H2 gas is bubbled. So, that bubbles up ok. So, at one-atmosphere pressure you are bubbling this gas and it is bubbling up and in this, you dip a piece of platinum; platinum wire and from this, you can connect it to some external circuit. So, if you do this then you create this reaction of H2 going to 2 H plus 2 e minus that is the reaction that is occurring in this electrode, this is referred to as the standard hydrogen electrode this setup that you see here is the standard hydrogen electrode or SHE for short it will be called SHE standard hydrogen electrode SHE. So, this now becomes a standard. So, in the pair that you are seeing on the left-hand side of your screen, in that pair you can always remove let’s say B instead of b you put this electrode SHE instead of B you make put her then you will get the standard potential for A. So, A versus this A a part of this 10 this standard electrode with the A as the metal, if you combine it with SHE you will get its standard electrochemical potential; similarly, you combine B separately with SHE you will get its standard electrochemical potential. So, like this, you can get in instead of simply coupling A with B you couple A you couple B with SHE you couple C with SHE like this you get potentials always concerning SHE and then once you get there are potentials concerning SHE you can get the potential of A versus B by simply adding those potentials or subtracting those potentials based on how your reaction is being written. So, if it is written always as a reduction potential you subtract those potentials. So, so by making these combinations of A versus standard I mean this half of the cell versus the standard hydrogen electrode B half of the cell versus standard hydrogen electrode etcetera by doing this and noting down those potentials we create this standard electrode potent standard electrochemical series ok.