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Hello, in this class we will look at types of fuel cells. (Refer Slide Time: 00:21) The learning objectives for this class are threefold first we look at what are the different types of fuel cells then we will also see what differentiates them from each other. Because as a technology it seems like it’s the same thing, but there are specific aspects by which one fuel cell or a type of fuel cell differs from the other and so we will try to look at that in some level of detail. And we will close by also looking at what are there you know relative advantages and disadvantages. Of course, as we go through the material in this class we will see a lot more detail about each type of fuel cell. But this is the broad set of objectives that I would like to you know accomplish as we go through this class. (Refer Slide Time: 01:06) So, on the screen you see a sketch of a fuel cell or a schematic of a fuel cell and so as you broadly know there is you know at the end of the day there is an external load that you are trying to drive. So, that could be anything that could be a fan or a light or you know powering a house or it could be powering an automobile. So, all of that you know whatever is your end use is all clubbed into this you know the terminology that we call here as the external load and so that is something that we will is something that we are already using. The unit that you see here is the simplified this entire unit that you see here, is the simplified you know a schematic of the fuel cell. So, that is the primary piece in this entire you know circuit that you see here which is what we are discussing. So, you see that the fuel cell consists of 3 parts here, one is the anode that you see here, then there is a cathode which is this unit here which is marked here, and then in the centre, you have the electrolyte. So, if you look at the 3 major parts of a fuel cell which is the anode, cathode and electrolyte in principle is the same as a battery. So, therefore, you know in terms of technology, in terms of science at least the fuel cell is fundamentally the same as that of a battery the science that you know governs the behaviour of a fuel cell many of you know parameters that are of interest in a fuel cell are essentially the same as what you will see in a battery. The main difference is in the fact that the reactants that are being used here or either gases or liquids whereas, in a typical battery they are solids. So, that’s something and we will see that in, so that’s something that is the distinction between these two and can also be looked at in greater detail. But that’s not the focus of our current discussion, but that is something that we should be aware of. So, in any case, an anode, an electrolyte and a cathode and typical use of a fuel cell would involve having a several of these fuel cells connected in series or parallel and so that would be considered as a stack. So, that’s no different from what you do with the battery. So, if you take the remote of your you know television or you know let’s say a calculator or any such you know or even a toy electronic toy you see multiple cells which are present inside that toy. If you watch it carefully if you follow the circuit carefully you will find that those cells are connected in series in some cases they are connected in parallel and you can have a combination of these. So, that combination simply decides what is the current you can have in the circuit and also what is the voltage you can have in the circuit. So, that combination is then decided by how you have arranged these cells in series and parallel and therefore, that also decides the power that is available in the circuit. And the power is usually a critical parameter that decides whether or not your external load is going to be able to function given that you have attached this power supply to it ok. So, that’s the basic idea and typically an anode is where the oxidation reaction takes place. So, some species are getting oxidized, typically that means, in the most fundamental way in which you define the oxidation process is that an electron is released from the species ok. So, an electron is released that electron is what you see here which is headed off into this external circuit, right. So, that’s what you see here that the electron has been released by some species at the anode, at this region somewhere in the anode, throughout this anode. I am just giving you a location there so that you know it lines up with the wire. So, that you just see the relationship general relationship, but it could be anywhere on that anode these species is getting oxidized it releases an electron. And that electron is actually as we will see in just in a moment it is put under circumstances where it can only go through the external circuit. And this you know this wire that you see here which is headed out is called is referred to as the external circuit because it leads to this external load bank here. So, it leads to this external load bank here and then comes off the external load not load bank the external load and comes off the external load and returns to this fuel cell unit at some region with at the cathode. So, this is the general you know process that is happening. Now, at the cathode you have a reduction reaction going on. So, the reduction reaction is typically again an electron. So, oxidation is the loss of electrons and reduction would be a gain of electrons ok. So, the reduction occurs at the. So, this is for some species that are present in the circuit. So, that species happens to be at the cathode and that incident that picks up the electrons and is getting reduced. So, this is the general process that is happening in this circuit so to speak. And, one key parameter here in all this discussion that I just went through I spoke about the anode. So, I spoke about this region, I spoke about the circuit, I spoke about the external load, I spoke about this part of the circuit and I spoke about the cathode. So, in all this discussion there is one, one important part of this circuit that I did not talk about at all for the most part and that is the electrolyte. And it is very interesting to note that actually in many of these technologies the electrolyte is a very deciding factor, it’s a very crucial factor is very critical factor although it’s not the part that is generating the current. So, if you see all the description I just gave you some reaction happens at the anode electrons are appearing in the external circuit and through that electron, some work is being done and that is what you are trying to accomplish you know to run some machine or run something at your house. So, all that is happening in the external circuit and then finally, the electron comes back to the cathode and then another reaction happens. So, in all this process it appears that the electrode is doing nothing I am sorry it appears if the electrolyte is doing nothing. But actually, it’s a very critical part of the circuit because it decides in which direction the electrons will flow and in which direction and how the reaction occurs and how the reaction is completed ok. After all, you have a reaction where some electron is released, another reaction where an electron is gained and a pathway for the electron. Now, if you didn’t have the electrolyte you will have this oxidation and reduction happening in an uncontrolled manner and no energy will come to the external circuit. So, no energy will arrive at this you know the external load that you have and therefore, nothing will happen. So, it is just like you know burning fuel in a useless manner ok. So, you want to utilize that fuel to do something and to enable you to utilize that fuel to do something you need this electrolyte because this electrolyte then splits the reaction into two parts. One part is this electron going into the external circuit and getting some job done for you and another part is this some species go through the electrolyte, arrives at the cathode and regains this electron in some manner and then gets you to know reduced and some species gets reduced and the reaction is completed. So, in the as we progress through this class, we will look at different types of fuel cells, in particular, you will find that one important manner in which these fuel cells differ is in the choice of the electrolyte ok. So, this part of this electrolyte part is very crucial in differentiating between different types of fuel cells. And it also then creates some other aspects of the fuel cells are that of the fuel cell is then controlled by this choice of the electrolyte and so that is something that we will also look at. (Refer Slide Time: 09:05) So, in this context, I want to talk about a little bit about ionic conductivity versus electronic conductivity. So, for example, in our last in our previous slide which we will go back to, I spoke about the, I will just clear this a little bit. So, we spoke about the flow of electrons. So, now, in the external circuit, you have a flow of electrons. So, in this path, you have a flow of electrons and in this path also you have a flow of electrons. So, in other words in this entire circuit which is other than anything other than this central part which is the fuel cell, anything other than the central path which is the fuel cell is getting referred to as the external circuit and in the external circuit, the electron is travelling. So, in other words, there is a path for electronic conductivity in the external circuit Now, clearly, the way I have drawn this and the way I am explaining this to you it also means that the electron cannot go across like this. So, this pathway is not permitted for the electron, all right. So, the electron simply goes through the external circuit and arrives at the cathode, but cannot go through the go from the anode to the cathode, through the electrolyte, right. So, if you simply have electrons continuously going from anode to cathode you will have a continuous build-up of negative charge and the cathode and continuous build-up of positive charge at the anode. So, that it doesn’t happen. If it continues to happen and then it will simply build up enough of a reverse potential on the circuit that it will stop the flow of electrons ok. So, it will so basically or it is like piling up things in the in one direction and once you have piled it up to some point you are unable to push things up they will start sliding down faster as fast as you push it up. So, then you will halt. So, that is basically what will happen, if you don’t complete the reaction and make it neutral again. If you complete the reaction and make it neutral again you can keep continuing to push electrons in one direction. So, to complete the reaction from in two in the process the reaction process to complete the reaction process you have to have some species that goes across and completes this reaction the species that go across through the electrolyte is typically an ion, ok. So, the ion goes through the electrolyte electron goes to the external circuit. So, the electrolyte has ionic conductivity whereas, the wires which are there in the external circuit have electronic conductivity ok. So, you have electronic connectivity in the external circuit, but you have ionic conductivity in the electrolyte. It is very important to know that you know when you measure conductivity there are various instruments which are used to measure conductivity, sometimes you will get a value of a conductivity which is the mix of both these conductivities okay because conductivity fundamentally means there has been a transfer of charge ok. So, you conducted some charge. So, that is what it means. It is just at in common parlance, in common usage, we assume that it is electrons. So, we say something has some high conductivity metals have high conductivity is a common statement that we have. Typically we mean metals have high electronic conductivity. So, a typical metal, for example, will not conduct any ion by ion, I mean anything you know like H plus or an O2 minus ion oh. So, typical ion like that it is not going to be conducted by metal, but when we keep saying it has high conductivity, but it is going to conduct electrons. So, when we talk of conductivity that is an aspect that we should be alert to, that there is the conductivity of different species possible and therefore, in a particular circumstance you may have any one of those species being conducted or more than one species being conducted. So, there are materials where you can have a mix of both electronic as well as ionic conductivity, you can also have materials where you have only ionic conductivity you can also have materials we have only electronic conductivity. In a typical circuit which involves a power source of this nature where you are having either a battery or a fuel cell, you want only ionic conductivity in the electrolyte and only the electronic conductivity in the external circuit. So, that is the basic thing that you want to do. You want only electronic, I am sorry only ionic and only electronic; If you have electronic conductivity in the wrong place. So, in the context of this circuit, I would be referring to the possibility that you have electron transfer also occurring through the electrolyte. If you have electron transfer also occurring through the electrolyte then you have what is referred to as an internal short circuit. It means you are providing the electrons are a very easy path to complete the circuit and they do not go through the external circuit instead they simply cut across the electrolyte from anode to cathode, and that completely waste the energy that is available in the fuel ok. So, the summary of what I am trying to describe here is that in a typical circuit that involves a power source of this nature there are parts of the circuit that have to have electronic conductivity and other parts that are supposed to have ionic conductivity. And you have to be careful to ensure that you know the material such as the electrolyte or components such as the electrolyte must not have ionic conductivity. Sometimes as the material deteriorates for various reasons it may develop electronic conductivity and that is considered bad, ok. In various ways, there may be other processes which may occur which may create a pathway for internal short circuit and that is considered bad and it's even considered unsafe. So, you have to be careful about that you have to be aware of that. So, as I mentioned this is a very important distinction between what is ionic conductivity versus what is electronic conductivity. And the fact that when you look at a circuit there are regions that should have one and not the other, and if you have a mix then you are doing then your device is not performing, correctly. (Refer Slide Time: 15:45) As I mentioned the fuel cells are of a variety of types ok. So, fundamentally a fuel cell is going to have a supply of fuel which is typically in most cases the standard fuel that people discuss in the context of a fuel cell is hydrogen. You can have other fuels, but most often we talk of hydrogen as the source of energy or source of as a fuel, is used in a fuel cell and typically the oxidant is just oxygen or air. So, on the anode side of the fuel cell, you will supply hydrogen, on the cathode side will supply air or oxygen and that’s a typical kind of setup in which you are looking at fuel cells. But you have a wide range of you know possibilities concerning how the fuel cell deals with this fuel-air combination and how it operates. As I told you very interesting even though you have the electrolyte not being critical in actually generating any power for you I am going to show you now that the range of fuel cells that are possible. Differ fundamentally in the choice of electrolyte that exists in the fuel cell. So, it is not so much the difference in the anode or the cathode although those also have differences, fundamentally the differences arise due to a difference in the selection of the electrolyte. That makes a lot of other decisions for the fuel cell and that then decides what the fuel cell can do, what are its advantages, what are its disadvantages what are its limitations. So, all of those are decided by the choice of electrolyte. So, in the table that I am going to show you, I am going to look at a few different types of fuel cells and in each case, the primary difference is the electrolyte. So, on the left-hand side of your table that you see here, you see different types of fuel cells, the one that is right at the top is what is referred to as a polymer electrolyte a fuel cell or a proton exchange membrane fuel cell, a PEM fuel cell which is a proton exchange membrane fuel cell. As you go from the top of the table to the bottom of the table the electrolytes keep changing and one important characteristic which changes as a result of this change of the electrolyte is the temperature of operation of the fuel cell which is what you see here ok. So, the temperature of operation of the fuel cell changes as you change the electrolyte and this is one important parameter that is being changed because of the selection of the electrolyte. It also then impacts other parameters of the fuel cell such as the catalysts being used and also what kind of fuel can be used in the fuel cell you will see in this technology. That for different types of fuel cells there are particular chemicals which may be present in your reactant stream the reactant could either be on the airside or the fuel side both are reactants although the fuel is the oxidant and I mean. So, fuel is the fuel and oxygen is the oxidant. So, you have those two combinations of what you are sending into the fuel cell and they are generally being referred to as reactants. So, it will turn out that many of the reactants based on the source from which you are getting the reactant. So, for example, you can get high purity oxygen which will be coming straight of our tank or you could be just taking air which is just you know ambient air. Now, if you take ambient air you have oxygen, you have nitrogen, you will have some tiny amounts of carbon dioxide, maybe some extremely tiny amounts of you know other gases, maybe there is some other vehicle that is nearby. So, it may also be giving out some you know carbon monoxide or nitrous oxides of different kinds. So, various things can enter your fuel cell. Now, some of those ingredients in the gas stream can either affect the anode or affect the cathode or affect the electrolyte. And if it affects them negatively then one of those components will stop functioning and that will impact the overall you know functioning of your fuel cell right. So, therefore, it is very important to understand which parameter which reactant is you know good for a fuel cell which reactant is sort of being described as a poison for the fuel cell because it is destroying the functioning of the fuel cell. So, if you look at the different types of fuel cells as I said you know the one right at the top on the top of your table is a proton exchange membrane fuel cell. Typically the electrolyte there is a polymeric it that is the standard electrolyte that I used and usually these operate at less than 100 degrees C. Now, if you shortly going to look at the reactions that occur in a fuel cell, but one of the main products that you get of a fuel cell is is water. So, hydrogen reacts with oxygen and generates water. So, if you are below 100 C in temperature this water is going to be in liquid form ok. So, its liquid water just the way you would have water in a glass of I mean glass holding water etcetera. So, it is not in vapour form it is not steam then is just sitting in liquid form. So, this may not seem like much,
but basically in this technology, the state of water defines some of the operational difficulties that the fuel cell may face as you operate the fuel cell over some time okay. So, that that is something that you have to be aware often as you understand this technology more and more you will appreciate these nuances associated with the state of water. But I am just alerting it you to it that you know this particular kind of fuel cell operates below 100 degrees C typically and therefore, it means that the water is now in a liquid state. The next type of fuel cell here is the alkaline fuel cell and as you can see here it operates between 100 and 250 degrees C. And from here on forward here on downwards on this table you have fuel cells which are at progressively operating at progressively higher temperatures and therefore, in all of these fuel cells the water is typically not in the liquid state ok. So, it is in vapour form that you are dealing with water unless you pressurize it, but basically, it is sitting in a vapour state. So, the alkaline fuel cell is the next one the at even higher temperatures between 160 and 220 degrees C, you have phosphoric acid fuel cell. So, that’s the even higher temperature of operation. If you go to 600 to 700 degree C there’s something called as the molten carbonate fuel cell or rather. If you select the molten carbonate as your electrolyte the operation range of temperatures that you have to go to operate it is 600 to 700 degree C. And finally, you arrive at the solid oxide fuel cell where you are looking at a temperature of operation of 1000 degrees C or more. Now, I have just mentioned some names and I have mentioned some temperatures. So, it is of interest to understand why these names result in these temperatures. So, the first thing is the names that I have mentioned to you which whether it be proton exchange membrane fuel cell or alkaline fuel cell or a phosphoric acid fuel cell or molten carbonate fuel cell or solid oxide fuel cells all of these fuel cells the name refers to the material that has been selected as the electrolyte ok. So, in in a polymer electrolyte membrane cell fuel cell, it’s a polymer electrolyte, polymer-based electrolyte, an alkaline fuel cell uses OH ions KOH kind of material as the electrolyte. You have phosphoric acid is used in a phosphoric acid fuel cell, carbonate ions in the form of molten carbonates are being used as electrolytes in molten carbonate fuel cells and finally, in solid oxide fuel cells, you have ceramic materials which have oxides. So, Yttrium stabilized zirconia we are going to see that Yttria stabilized zirconia etcetera are used as the electrolyte. So, these 4 5 fuel cells that I have shown you here differ in the material that has been used as the electrolyte and that also defines the name of the fuel cell. Well, that leads you to the next question okay. So, what? So, you have selected a different electrolyte. Why you should that make a difference to the temperature of operation? As I told you to complete the circuit you have to have ions travelling through the electrolyte right. So, you have electrons travelling through the external circuit and you have ions travelling through the electrolyte. Now, when you draw current from the circuit when you draw current that is effectively the rate at which you are drawing electrons from the circuit ok. So, you have some load, you are putting some load there and let’s say it is a 5 amp circuit or something like that you know let’s say we usually do not draw 5 amps even though it is a 5 amp circuit we are drawing much less than that. So, in any case, let’s assume you are drawing half an amp. So, in half an amp you can calculate how many coulombs per second it is and therefore, how many electrons per second it is ok. So, once you figure out how many electrons per second are required to handle that half an amp. So, the reaction has to occur at that rate ok. So, in other words, electrons have to be generated at the anode and introduced into the external circuit at that rate, electrons have to arrive at the cathode and the same rate and the electrons have to be consumed at the cathode at the same rate only then you do not have a build-up of electrons, only then you have a continuous flow of electrons. Now, I only spoke about a release of from the anode travelling through the external circuit arrival at the cathode. The ions also have to cross from the anode to the cathode or cathode to the anode depending on the type of ion which we will see shortly, at the same rate to ensure that the circuit is always completed. If they don’t cross over at the same rate you will have a build-up of charge it means not enough ions are arriving to complete the reaction. So, the electrons just arrive there and they are waiting for the ions. The few that have a come they have completed the reaction the rest of them are just sitting around waiting for the ions to come. If they sit around and wait they are building up-charge if they build up charge they stop further current in the circuit. So, it is important that the ions also move through the circuit at an acceptable rate. In fact, at the same rate, except you know looking at the charge of the ion. I mean after you factor in the charge of the ion it has two more the same rate. So, if it is a 2 minus ion you can have half as many ions move across as you have electrons moving the external circuit so that the charges being balanced. But the point is they have to move across. Now, it turns out that in most of these materials which have some ion being conducted the rate at which you can move the ion across that material is dependent on temperature. In other words, the conductivity the ionic conductivity of most electrolytes, the ionic conductivity of most electrolytes is dependent on temperature its temperature-dependent. And typically it means, typically it is seen that if you raise the temperature of the electrolyte then the ionic conductivity improves okay so in fact, in this manner in this in this context it is very different from electronic conductivity. If you take any wire, any metallic conductor which is conducting electrons if you raise the temperature of that material it will typically increase in resistance because it increases the number of you knows collisions that the electron will face as it tries to move through their conductor and therefore, it slows down the electron. So, typically this is seen in the form of increased resistance to the flow of electrons in the external circuit. So, this is what you will see when you have any metallic conductor. In all ionic conductors, it is the opposite. In ionic conductors, the pathway of for the ionic conductivity is such that and the process is such that if you raise the temperature it happens faster. So, you have better conductivity at faster for these ions as at higher temperatures. So, that is the reason why the choice of electrolyte affects the temperature at which you are operating. The choice of the electrolyte affects the temperature at which you are operating because only at that temperature this material can conduct ions at an appreciable rate. So, you can take a solid oxide fuel cell and try to operate it at room temperature. So, reactants are the same you're going to send the same reactant at the anode and the corresponding reactor at the cathode. So, the reactants are not different they are the same independent of the temperature of operation for a solid oxide fuel cell. But at the room temperature, the conductivity or the ionic conductivity of the solid oxide electrolyte will be so low it with so many orders of magnitude low it could be like you know 6 7 orders of magnitude lower than what is available in the external circuit that you cannot draw any appreciable current from it; Even though you are sending sufficient reactants on the anode side and sufficient reactants on the cathode side.
And as you raise the temperature of the solid oxide fuel cells gradually correspondingly the ionic conductivity rises and when you arrive at about 1000 degree C of operation you have sufficient ionic conductivity that it matches up with whatever is required in the circuit and therefore, you can draw appreciable current. So, you can see that the choice of the electrolyte directly impacts the temperature at which the fuel cell can operate primarily because it impacts the conductivity the ionic conductivity of that electrolyte as a function of temperature. (Refer Slide Time: 28:49) In the table that you see here for the various fuel cells you at this point, I only put the abbreviations down here, but they correspond directly to the types of fuel cells we saw previously. You can see here what reaction happens at the anode or what reactants arrive at the anode or what are possible reactants that can be used at the anode, what is the ion that goes across in the electrolyte and what are the possible reactants that are available for you at the cathode. So, if you look at the PEM fuel cell or the PAFC fuel cell phosphoric acid fuel cell they are all fuel cells where the ion being transferred across is the proton. H plus ion, H plus ion is a proton and that is the ion that is being transferred across to enable this and so the conductivity of the electrolyte is the electrolyte conducts protons. It’s called a proton conductor and that is why sometimes they call it proton exchange membrane fuel cell ok. So, that is a proton that’s the H plus ion. And the reactant that arrives at the anode is H2 and at the cathode the reactant that arrives is O2, and the reaction once it is complete you have product water which is being ejected from the circuit from the fuel cell on the cathode side. So, this is what happens any in a PEM fuel cell or a proton exchange membrane fuel cell. If you go to the alkaline fuel cell as the name suggests it’s an OH minus ion that is involved in the process of completing the reaction and it is the ion that travels through the electrolyte ok. So, an OH minus ion is involved here. So, you can see h