 In the previous class, we have studied the adsorption based material, we started with the fundamentals of such materials, how hydrogen is taken up, how are the isotherms being considered for hydrogen uptake in such materials. And in this class, we will look at what are the different materials which can be used for hydrogen storage, which operate on species option. Now the important thing is that designing such optimized adsorbent materials is a challenging task. Now, there are two requirements that can be met and that has to be taken care of, this is one is like we have to increase the number of adsorption sites per unit weight and per unit volume of the adsorbent. Now this can be done by either using materials, selecting materials which have high specific surface area and they have a high pore volume. So the hydrogen uptake will depend upon the pore dimensions here and the specific surface area. Another thing that we can do is we can increase the heat of adsorption of the adsorbent. This can be done through different ways, this can be increased by either introducing hydrogen spillover effect or we can have the unsaturated adsorption sites or we can even see, look at the adsorption when it is in a constructing these materials such that they have narrow pores, very small sized pores where the adsorption potential can be increased. Now if we see the first effect that is the hydrogen spillover effect, in that case transition metals like platinum or palladium have been introduced inside the host matrix, but this has not seen shown much of promising results. The another thing that can be done is by including unsaturated adsorption sites and this can be done by changing the chemical composition of the porous host material by introducing certain other species and these species can be even metal, but the problem that was found was that was still not partially that was helpful. The reason being the interaction of the hydrogen molecule with the functionalised adsorption sites was not very strong and as such this not held much at the same time the metals which we introduced they were heavy and that in fact reduced the gravimetric capacity. So the other choice could be adsorption of hydrogen into very small or very narrow sized pores and in that case the hydrogen molecule can interact with multiple pore walls and that helps in improving onto the adsorption potential. Now what are the different materials that can be used for adsorption of hydrogen and thereby can be used for solid state hydrogen storage. Let us see those. The first one being carbon based materials that we are going to look at. Now these carbon we know that carbon exist in various allotropic forms and they differ these allotropic forms differ in the chemical structures. Now this allotropic form one of the allotropic form is diamond another like being graphite, both of these forms they are non porous but there are several other structures which are highly porous and they have high specific surface areas like the various carbon nanostructures. Now these carbon nanostructures these have structure which is similar to that of benzene like hexagons with carbon being sp2 hybridised and these carbon nanostructures they differ in the way these hexagons are being arranged. So we can divide broadly the different carbon based materials into two classes. One is which have a long range order like these can be long range order in terms of arranging these hexagons and these are like example being carbon nanotubes or carbon nanofibers and the another one could be which have irregular structure like the activated carbon or porous carbon so amorphous carbon. So they have highly irregular structure and these activated carbon porous carbon they have been found to be very good for hydrogen storage they have a higher hydrogen uptake capacity. Now these activated carbons in fact they have very high specific surface area like the regular activated carbon can have specific surface area of about 800 to 1700 meter square per gram area. But the highly porous forms of activated carbon can even have like AX21 that can even have specific surface area that could be 3000 meter square per gram. Now there are different nanostructures of carbon which are ordered one like the single walled carbon nanotubes. Now these carbon nanotubes are in fact considered to be like rolled graphene sheets and this is having a single wall then there can be multiple such walls so multi walled carbon nanotubes. Then there are other carbon structures like onion like carbon or carbon dots, full rins, graphene dots then it could be multi layered graphitic sheets or graphene carbon nano ribbons, carbon nano haunts, carbon nanofibers. So there are large number of such carbon allotropes which are existing. Now the about two decades back a lot of interest was developed in these materials for hydrogen storage and this was basically driven by very large capacities being reported in such materials. However for some of the results these were found to be non-repetitive. This was because of the there were certain impurities present, there were certain erroneous results. However still these carbon based materials they have a higher storage capacity and in these micro porous carbon based structures actually the capacity depends upon the specific surface area as well as the pore volume. So there are different forms of carbon which can be used for hydrogen storage like the activated carbons they give the very good hydrogen storage capacity other than that there can be carbon nanotubes or activated carbon nanofibers or it could be nano haunts, graphene and these are all commercially available. And they have a very porous microstructure, these microstructures can be micro porous or meso porous or macro porous and they have a very high specific surface area that could range from 10 meter square per gram to 3000 meter square per gram and at the same time they possess low mass density that also results into a higher gravimetric capacities. So this is one of the adsorbent that can be used for hydrogen storage. Now the other possible sorbent could be zeolites. Now these zeolites are in fact 3D crystalline, alumino silicate structures these are built up of TO4 type of tetrahedral structures with corner sharing. Now this T here stands for Si4 plus or Al3 plus. Now in them the general chemical formula for these type of zeolites can be written as where the capital M is the non-framework metal cation or exchange metal cation. Now in this type of zeolite structures the bond which is TOT bond this is highly flexible and this flexibility of this bond that allows these tetrahedrons to get linked in a variety of ways to form a large number of network topologies. So there are large number of such zeolite structures which are existing. At the same time this flexibility of TOT bond that also makes the framework flexible and the framework can adopt for variety of various changes in the temperature or pressure or chemical surroundings. Now the electroneutrality of such structures because there is a if let us say Si4 plus is being replaced by Al3 plus then the electroneutrality of this structure will be maintained by introducing a non-framework exchange metal cation. Now this non-framework exchange metal cation this choice will depend upon what is the accommodating pore volume. At the same time it will also depend upon what is the charge on this metal cation. Now when we introduce this charge metal cation into the framework structure it introduces an electrostatic force and that electrostatic force that increases the polarization effect of the various sites its option sites and in fact the channels or the pores which are there in such frameworks their polarization increases and that electrostatic force that depends upon what is the charge on the cation. So that electrostatic force increases when the charge on the cation increases and it increases and it decreases with the pore volume. So these type of zeolites in fact these are some of these zeolites are naturally occurring however most of these zeolites are synthetically synthesized in laboratory. In this type of materials the hydrogen uptake primarily depends upon what is the specific surface area, what is the type of the metal cation which is being used, what is the concentration of these metal cations. So there are different classes of such zeolites like these can be classified on the basis of what is the silica to alumina ratio. Like this can be with low silica to alumina ratio where the ratio Si to Al could be between 1 to 1.5 these zeolites can be with intermediate Si to Al ratio wherein this ratio is 2 to 5 or it can be with high Si to Al ratio so from 10 to several thousands. So there are different types of zeolite structures like the three X type of zeolites wherein the cation could be different it could be sodium or it could be potassium then there are like sodium A type of another type of like these sodalite type of cages then you are getting. So there are lot many zeolite structures which are being synthesized and these have been studied for hydrogen uptake. The another class of material which could be used for hydrogen storage these are the adsorbent based materials these are metal organic frameworks. Now these metal organic frameworks these are in fact the class of compounds which consist of a metal ion it could be metal ion or metal oxide cluster or inorganic cluster which is having high dimensionality and this can be coordinated with organic ligand and that forms either 1, 2 or 3 dimensional structures. Now we can change this metal at the same time we can change this organic ligand. So this combination of this metal and the organic linker that makes the framework. Now we can change this metal we can change this organic linker and that can give us a wide variety of possibilities so a large number of metal organic frameworks they exist and they vary depending upon what is the composition what is the structure like there are the well-known metal organic frameworks like MOF 5 or MOF 177 then there are ZIF ZIF 100 or ZIF 21 mil 53 mil 101 and there are many other metal organic frameworks which are having high specific surface areas. So they can have surface areas as high as even predicted ones like more than 10,000 meter square per gram they can have. So they have specific surface areas like 1000 ranging to 10,000 meter square per gram they have ultra high porosity at the same time the pore sizes can be tuned depending upon the choices they have high thermal and chemical stability and we can modify their internal structure they have very good adsorption capacity with these metal organic frameworks. However the challenge remains is to have homogenous material and that to synthesized on a larger scale. So having large scale MOFs being synthesized that is the major bottleneck. Now if we see some of the representative examples of these materials that we have studied then like for the MOFs the materials like MOF 210 that has a specific surface area of 6000 to 40 can store about 176 milligrams of hydrogen per gram of the material at a pressure of 80 bar and 77 Kelvin. So we have studied in the earlier class that since the bond in these materials adsorption based materials is a weak winter walls bond as such to have appreciable amount of hydrogen being stored we have to cool these down to 77 Kelvin at room temperature the capacities are very low. It could be MOF 177 with a specific surface area of 4746 and can give has shown to give even 7.5 weight percent at a pressure of 70 bar and 77 Kelvin temperature. Anyo 106143 meter square per gram of specific surface area hydrogen storage 164 milligrams per gram of the material at 70 bar pressure and 77 Kelvin. IROMF 20 that is having a specific surface area of 4024 meter square per gram can give a capacity of 6.7 weight percent and this is at 80 bar and 77 Kelvin temperature. Similarly, there are many more MOFs which can which have been studied and reported in literature. Similarly, there are covalent organic frameworks cough 102 cough 103 cough 10 with surface areas of 3620 or 3530 1760 respectively and they have been reported to store 72.4 or 70.53 milligrams per gram of the material at a pressure of 1 bar and 77 Kelvin. Similarly, the different carbon based nanostructures like the activated carbon a typical example that has been considered here is AXK5 with a surface area of 3190 and it has been reported to store 7.08 weight percent at 60 bar pressure and 77 Kelvin temperature. Single volt carbon nanotubes 4.2 weight percent at a pressure of 120 bar and 298 Kelvin, multi volt carbon nanotubes 6.3 weight percent at 148 bar and 298 Kelvin, carbon nanofibers with a very small specific surface area it has been reported 6.54 weight percent 120 bar and 298 Kelvin. In case of like the zeolites like sodium AX or sodium Y they have reported to have a smaller capacity 1.5 to 1.8 weight percent at 50 bar pressure and 77 Kelvin. The reason for the low capacity being observed in zeolite is many of the cages they are not accessible to the hydrogen molecule and as such they are not considered as they are not available for hydrogen uptake. Like the other materials are ZSM5 with a storage capacity it has been reported in different literature the amounts 1.97 or 0.65, 0.38 weight percent at 70 bar pressure and at different temperatures like 77 Kelvin or 195 Kelvin it was 0.65 at 293 Kelvin it was 0.38 because we know that these store hydrogen at a lower temperature and at higher temperature or close to room temperature the capacity of such materials decreases. MCM 41 at 35 bar and 77 Kelvin has reported to have a capacity of 2.01 weight percent while at 10 bar pressure and 298 Kelvin it had a capacity of 0.69 weight percent. Silica or alumina they also have been reported to have certain smaller capacities at 1 bar and 77 Kelvin. So, to summarize this part we have seen the different adsorption based materials which can be used for hydrogen storage and we know that in them the hydrogen is stored by forming weak bonds and there is a weak interaction that exists between the adsorbate which is hydrogen molecule and the adsorbent and that results into storage appreciable storage capacity only at low temperature like 77 Kelvin at room temperatures the capacities are lower. However, the biggest advantage of such materials which are based on adsorption process is that they have a rapid kinetics because the taking since the bond which is formed a weak van der Waals bond and not a strong chemical bond unlike in case of chemisorption. So, the kinetics is very by slight change in temperature we can get back that hydrogen or we can store that hydrogen. At the same time there is no electronic structure change there is no diffusion of the hydrogen that takes place into the interstitial sites as such there is no distortion structural imperfections or distortions or electronic distribution change. So, as such the reversibility is ensured because the bond formed is very weak at the same time it is not resulting into severe distortions into the structure it is not diffusing into the bulk of the material. At the same time because of the same reasons the cycle life of such materials is higher and since the bond formed is weak. So, the heat of adsorption is lower and that is important when we design a hydrogen storage system. So, the heat released which will heat that will be released will be lower and as such the thermal management will be less severe in such storing such materials. There is a correlation that we have seen is with the with the specific surface area of the hydrogen uptake at 77 Kelvin it also depends upon the amount of hydrogen being taken up by these materials that also depends upon the pore volume and also like we have seen in case of zeolite metal organic frameworks and zeolite if there is a polarizing effect then that also affects the hydrogen uptake in such materials. Thank you.