So, the best way you can do it is to introduce one more layer referred to as the gas diffusion layer on either side of the cell, ok. So, we introduced something called the gas diffusion layer and these are simply porous materials could be made out of you know carbon fibres which have good conductivity and also good porosity. So, if you now, look at this assembled view it looks something like what you see here. So, you suddenly have now, more parts here you have flow channels on either side then you have the gas diffusion layers on either side then the two-electron electrodes and right in the middle, you have the electrolyte. So, we have now, added more parts to create this assembled fuel cell which helps us deal with a wide range of requirements ok. So, that is what we have done here. So, I will again magnify the same region just to show you what we have accomplished by adding these gas diffusion layers. (Refer Slide Time: 35:29) So, if you now, go back to the same region I have whereas, previously this electrode was directly in contact with this gas flow channel we now, have in the middle a gas diffusion layer. The same is true on this side you have the catalyst layer you have the gas diffusion layer that is marked here and then you have the flow channel ok. So, this is what we have accomplished. So, now, look at the same two regions A and B. So, although you have gas here this gas is in a position to diffuse to all regions, right. So, the gas is now, able to diffuse to all regions along the surface of the electrode and therefore, not just B, but also locations adjacent to A get sufficient access to gas. So, gas access is not an issue we then look at, for example, the region B if you have electricity generated in region B it can find its way back to this flow channel through this mesh which is a conducting mesh ok. So, you have an electronically conducting mesh which can transfer electrons from the electrode to the gas flow channel and the same mesh because it is porous can transfer gas from the flow channel to the electrode. So, since it can serve both these purposes suddenly both region A as well as a region can comfortably participate in the process of generation of electricity. Therefore this structure is now, much better suited for generating electricity as a standalone unit. Indeed today's a fuel cell technology the extent of fuel cell technology is broadly based on this structure, at least the main the version of the fuel cell which is used for low-temperature applications which is the proton exchange membrane fuel cell PEM fuel cell version of the fuel cell essentially uses this structure. And it is this structure that is you know takes into account all the issues that this kind of a fuel cell design requires. (Refer Slide Time: 37:30) So, we are now, closer to technology. So, we are just a few steps away from you know having the complete cell put together so to speak, and you can see here the only other point that was missing there was the gasket. So, you do need a gasket and I am just showing you the gaskets here. These are gaskets that ensure that there is sealing and there is no you know leak of gas on either the fuel side of it or the oxygen side of the fuel cell. So, that is something that you will have. I also want to you know to draw your attention to something that maybe this figure does not completely convey to you, and that is the dimension associated with this system I have drawn these as large components. So, that they are visually easy for you to look at, but actually in a real fuel cell you are looking at this membrane electrode combination from here to here which has the 2 GDLs, 2 electrodes and membrane that entire unit is likely to be just about you know 2 or 3 millimetres thick if at all maybe even less than that you know. So, just a couple of millimetres thick is what you are looking at this entire setup. So, it will look very thin it is some it’s like a membrane that you can hold in your hand and it will flutter in the air. So, to speak it’s a very thin membrane on either side of which you put this catalyst the catalyst layers as well as the GDS. The flow channels themselves are typically only about you know say 4 or 5 millimetres thick. So, you are looking at an entire or even less may be. So, you are looking at an entire set up here where this whole structure that you see here from here till here is less than about a centimetre, may be less than a centimetre 1 centimetre, less than 1 centimetre thick. So, even if you put in 100 such cells together that would just be 1 meter long ok. So, so that’s the point that I wanted to highlight which is perhaps not immediately evident to you from this image because I have drawn things on a very large scale ok. (Refer Slide Time: 39:30) So, I just want to show how you know this single cell in in in many ways the fuel cell has this basic idea which is similar to what you would see in you know batteries which is that you know you use a single you know double-A battery or a triple-A battery which would then be referred to as a single cell for certain applications. But if you want to run a larger you know activity with it you would put several such cells in series or parallel right. So, that’s a similar concept like that exists concerning fuel cells you would need to put several of these in series or parallel for you to handle a much larger you know output to generate a much larger output for some application which requires a larger output ok. So, although the only thing I have added in this figure which was not there in the previous figure are these two things on either side which is the current collector which is the which will be are the two ends of the cell, simply to create you know to connect a which would be a connection to the external circuit. And so that’s all that these two units are doing here you just you know connect attach leads to those two points and then you would reach the external circuit. If you take several of these units and put them in series so that you know you can now, generate you know each of them to let’s say how generates half a volt and you put know 100 of these together you can get 50 volts right. So, if you want to do something like your arrangement would look something as you see in your screen here. (Refer Slide Time: 40:44) You will have right only are the very extremes of this setup you would have the two current collectors in between you will have a whole bunch of these fuel cells stacked one against the other. And hence this is referred to as a PEM fuel cell stack proton exchange membrane fuel cell stack okay. So, PEM fuel cell stack is what you will have here. And while it may not be very evident immediately you have for every cell there are several cells here each is a cell here, this is a cell, this is a cell, this is a cell and this is a cell. So, there are 4 cells that you see on your screen and they are all touching each other. So, some aspects of the common you know region between the cells has been marginally modified, but just to compare against something that you previously saw if you see here what you see between these 2 green lines is what you were previously looking at as a single cell ok. So, you have the anode let’s say this is the anode, so this is the anode of the flow of flow field list. So, this is the if you look at this image here this is the cathode flow field this side you have the anode flow field this side and you have the membrane in the middle and the two electrodes and the 2 GDLs. What you have in the central region here, is a coolant channel which helps you control the temperature of the stack because as the stack runs it can generate a lot of heat and you need to have some control on it and you can even use that heat for some purpose. And so, you have a coolant running through the channel usually it is water, but they may also try other coolants for various applications. So, this is a fuel cell stack. And this is how the complete system builds from you know the demonstration that I first showed you in the laboratory. So, we are now, very close to a product and in fact, this is the primary unit that sits in your product as a fuel cell stack. (Refer Slide Time: 42:38) So, for example, if you have some current density from the cell let’s say it is 0.4 amps per centimetre square and it’s it has an operating voltage of 0.5 volts, this is something that is referred to as a polarization curve from the fuel cell and we will discuss polarization curves in greater detail. But for the moment you please see that this is the performance characteristic of the fuel cell it shows you what kind of Voltages the fuel cell will demonstrate when you draw different amounts of current from it or different amounts of current density from it because it normalizes for the area. So, for example, at 0.5 volts I am saying that approximately it’s generating about 0.4 at or at 0.5 volts here is generating roughly about 0.4 amps per square centimetre. Let’s just say that that is the operating point for the fuel cell. So, the charge is clear that here. So, that’s 0.5 volts and the 0.4 amps per square centimetre. So, supposing you have 200 of these cells in series and each of the cells has a 100-centimetre square area because you have 0.4 amps per square centimetre that will generate 40 amps for you, each cell will generate 40 amperes and then since they are 100, 200 of those cells in series that comes to about 100 volts. So, this will generate 4 kilowatts ok. So, 4 kilowatts of power is going to be generated by this cell by the stack and that is more than adequate to power a household. So, that is the kind of a power density that you are looking at, power that you are looking at. (Refer Slide Time: 44:12) So, a few important design issues associated with this technology. The first is the hazard from the use of pure hydrogen and oxygen. I think a lot of people recognize the hazard associated with the hydrogen. But in in in reality what is true is that any fuel has hazard with it whether it’s petroleum, it’s gasoline, it’s diesel, it’s hydrogen, compressed natural gas, all of them have a hazard with them because fundamentally they can burn, fundamentally they can be oxidized and you can get oxidized and fundamentally there is a lot of energy that they can release. So, you have to handle them carefully. Also if you take pure oxygen that also has some hazards associated with it because much of what we use is stable at atmospheric condition. And under atmospheric condition, under 1 atmosphere of whatever it is that we breathe that is only 21 per cent oxygen it is not 100 per cent oxygen. So, when you shift from 21 per cent oxygen to 100 per cent oxygen and still keep the pressure at say 1 atmosphere you have increased you know partial pressure of oxygen by a factor of 5. And that can cause certain things which were stable at you know one atmosphere of you to know atmospheric air to suddenly be a little bit more reactive because they are seeing one atmosphere of pure oxygen. So, you have to be a little bit careful about it. And therefore, we look at things like an oxygen replacement, the air is often used as a replacement for oxygen and then natural gas or other fuels can often be used as a replacement for hydrogen and in some cases, they would need to be reformed before they are used. So, this reforming process and you know fuel processing is a topic that we discussed in another class, but this is how you know the technology comes together. (Refer Slide Time: 45:47) So, this is the overall schematic of the fuel cell and you can see where you can have either hydrogen or other fuel or something that goes through a reformer gets converted to a hydrogen-rich stream. And then on the other side you have oxygen or air and both of these are piped into a fuel cell, and the output from the fuel cell is dc power which is what you see here, but in many cases, DC power is not what we use in most of our homes are set up to run on AC, so alternating current. So, therefore, we also need an electrical unit which does the conversion from DC to AC. If that is what is necessary if you have some other set of applications where DC power can directly be used you can directly use the output from the fuel cell with some minor modifications for the Voltage for example. But if you wanted it to be used for any alternating current application you have to do this process of power conditioning which would then get you your AC power. (Refer Slide Time: 46:43) So, I will close with just a couple of comments one on the idea of how this can be used for residential application and another on how this can be used for automotive application. Now, what we have seen so far is the sequence of you know steps that are involved in moving from concept to product for a fuel cell, and I am hoping that in this in this class you have learnt what is that concept that is there is a fuel cell and what are those steps that have led us from the concept to a product that can be deployed. So, in the case of residential use, the couple other points that you have to keep in mind is that generally speaking sizing is not a very critical issue from a residential application. So, many of the companies that look at you know creating this for a residential application are essentially okay with a unit that says the size of a refrigerator, the size of a refrigerator or the size of a washing machine these are units that are already there in many of our homes and the assumption is that such a sizing of the product will be completely acceptable to most of the users. The target lifetime that people are looking for is about 4,0000 hours of a lifetime and if you generally look at you know the number of hours present in a year which is a little over 8000 hours, this is roughly 5 years of operation. So, people would like a fuel cell system to be set up with the sizing of say a refrigerator or a washing machine or some combination thereof that can last for 5 years and generate current for a household for 5 years. So, that’s the kind of target and you know ideas that are present when people look at developing fuel cells for residential application. (Refer Slide Time: 48:10) If you look at the automotive application, size is a critical issue, it’s a very very critical issue because you have a very compact automobile that people are already used to. You cannot put a refrigerator and a washing machine inside an automobile, you don’t have that freedom, we don’t have that flexibility, you have you know basically the hood of the vehicle and maybe some space in the trunk. You have to leave some space in the trunk for the occupants to also use for other purposes, but between the hood, the trunk and some region under the car that’s all the space that you have. So, your entire fuel system your gas supply you are a storage tank, your gas supply the fuel cell stack any reforming that you are doing any electrical you know modifications that you are doing everything has to sit compactly within this region and still generate enough power for that vehicle to operate very comparable to a modern automobile. So, sizing is a critical issue for automotive applications, and in this case of automotive applications, the lifetime target is about 4,000 hours as opposed to the 4,0000 hours. If you see 4,000 hours you may think that it’s distinctly less than that of a residential application, but that’s not the critical issue. If you take a normal automobile to let’s say it is travelling at on average only let’s say between 40 and 50 kilometres per hour, then in 4,000 hours it has you know covered a distance of 160,000 kilometres to 200,000 kilometres which are roughly what we expect as a lifetime for most automobiles. There are, of course, automobiles which do you know a million kilometres and so on, but generally, you know the mass market automobiles that you are looking at are typically doing between 150,000 to 200, 250,000 kilometres in their lifetime, during which the engine is actually on essentially for about 4,000 hours. And therefore, if you can show a fuel cell system that can comfortably operate for 4,000 hours meeting all it’s you know operational parameters then you have a fuel cell system that is applicable for automotive applications. So, in summary in this class, we have looked at the journey of fuel cells from concept to product. We have looked at all these steps involved, how the ideas have come together, how they have been incorporated into this fuel cell design such that in the end, you have something that can deliver power to a specific application. And we have also finished the class by looking at what those kinds of constraints would be from the perspective of you know residential or stationary application versus that for an automotive or you know the mobile application. So, with this, we will conclude this class and look at other topics in another class.