 In the earlier classes we have studied in very much detail the compressed state hydrogen storage and liquid state hydrogen storage where the goal has been towards making the hydrogen molecules as close as possible to store it compactly increasing the volumetric density and at the same time using as little material additional material as possible. Like for this purpose we have to provide certain amount of work to compress the hydrogen gas or to liquefy it or maybe we have to reduce the repulsions between the various molecules by using different materials. Now there is another method of hydrogen storage which is a hybrid of both these compressed as well as liquid state that is known as cryo compressed method of hydrogen storage. Now this is having advantages of both the methods compressed and liquid state storage and the tanks are designed in such a way that they can hold compressed cryogenic fluids. Now since we have already studied in very much detail the compressed and liquid state storage we will just touch upon this particular method. Now in this method of storage cryo compressed storage there are several prototypes that have been also been demonstrated like BMW demonstrated a prototype where they could achieve using a cryo compressed tank gravimetric capacity of 5.4 weight percent and volumetric energy density of 4 mega joules per liter and a boil off of 4 to 7 grams per hour. There was another prototype which was demonstrated wherein hydrogen was stored at 40 to 80 Kelvin and 300 bar. Now the method of storage of cryo compression has advantage in the sense that we can curb the boil off losses. Now the tank inner tank which is a type III tank which could hold pressure say of 300 or 350 bar is having an outer vacuum and then multi-layer insulation so as to prevent the heat losses as well. Now in this method of storage since we are not going to very high pressures like 700 bar in case of compressed hydrogen tank we have already seen as such the strength of material or the there is a saving in terms of the material. While we are also not considering storage at liquid hydrogen temperatures 20 Kelvin rather the temperatures are above that temperature as such the super insulation requirement that also is relaxed and we can save in terms of that. At the same time as we have seen in the liquid state storage that because of the heat loss because of evaporation there is a pressure buildup and liquid hydrogen tanks these are not meant they do not have as much of strength to bear very high pressures. So at that time boil off or loss of hydrogen is inevitable but in case of cryo compressed tank since the inner tank can hold a higher pressure as such the dormancy period can be extended and boil off can be curved. So as such there are several advantage of cryo compressed storage and that is another way of storing hydrogen. Now in this class we are going to start with the solid state hydrogen storage methods broadly hydrogen storage can be categorized into physical storage and chemical storage. Now in physical storage hydrogen is constrained by the boundaries of a storage media or storage tank or storage vessel. The difference is that the hydrogen in this method of storage do not interact with the storage media. So this method of storage we have seen like compressed state storage, liquid state storage, cryo compressed method of storage and then if you further cool it down below the boiling point like the liquid state storage temperature was 20 Kelvin if we further cool it down to less than 14 Kelvin then we can achieve solid hydrogen. So here in this method hydrogen is stored in the molecular form and it is confined within the boundaries of a storage vessel or container with which it does not interact or it is not desired that it should interact. In the other method of storage which is chemical storage in this particular method of storage hydrogen atoms or molecules they in fact interact with the storage media or they react or they form a bond with the storage material which is being used for storing hydrogen as such forming certain compounds or a different material. So in the chemical storage method comes the different absorption based materials that we are going to start to learn today. Metal hydrides, complex hydrides and chemical hydrides, liquid organic hydrogen carriers including C6S6, C6H12, C10H8, C10H18 then there are reformed fuels different reformed fuels can also be used like converting it into form of ammonia or different frissure drops liquids and then can be stored. Now we will start with the absorption based material today. Now when it comes to solid state hydrogen storage there are certain characteristics which are required for the materials if they are to be used for hydrogen storage. So the basic requirements for material to be used to be hydrogen storage is that the material should have high hydrogen capacity and that capacity should be both per unit mass and per unit volume and that can be expressed in the form of either gravimetric energy density or gravimetric hydrogen storage capacity or volumetric storage capacity. So the gravimetric capacity can be defined as the mass of hydrogen being stored divided by the sum of mass of hydrogen and the storage media. And same way volumetric storage capacity can be defined as mass of hydrogen per unit volume of the storage media. It should be able to store hydrogen under moderate conditions of temperature and pressures unlike the compressed state storage where the pressures were very high 700 bar or 350 bar or rather it should be very low temperatures like in case of liquid hydrogen which was 20 Kelvin. So the operational temperatures and pressure should be moderate. There should be moderate heat of formation and desorption desorption desorption should be associated with such materials by moderate heat of formation we mean the thermal management should not be very very strictive or very strict. This is because if undergoing a reversible hydrogen storage if certain amount of heat is also released during the cycling process in that case that should not be very high and if that is very high there are lot of thermal management issues will be there. The material should be reversible by reversible we means once the hydrogen is used from such materials then putting back hydrogen to get the initial state of material should be relatively possible. By reversibility we mean there are ways of doing making the material reversible or there are it could be either onboard reversible it could be off-board reversible. By onboard reversible we mean that at a certain temperature and pressure if we put hydrogen into the material which has already desorbed hydrogen then it should take back that hydrogen and get back to its initial state that is we can do onboard. However there are certain materials that we will see that that take up hydrogen again at very high temperatures or pressures or very strict conditions. They can also undergo certain set of chemical reactions to so that the used material gets back to its initial state that means they can be off-board reversible. But reversibility is a must for sustainable hydrogen storage. The next characteristic required is fast kinetics. Now fast kinetics plays an important role when we are getting hydrogen from the such materials so that is when the hydrogen is being released or when hydrogen is being charged into such materials. So that becomes important during the refuelling of such materials or during the withdrawal of hydrogen. These materials should be highly stable against the condition environmental conditions they should be stable against the oxygen they should not form oxides they should be stable when in the presence of even moisture and that means that it should not degrade or the performance should not degrade when exposed to such conditions that means they should have a long cycle life. So could be there should not be any performance degradation with cycling. They should be definitely low cost and high safety although this is a long list for any material to satisfy. However there are different materials which can be used for hydrogen storage. Now in this class we are going to learn in more detail the fundamentals of hydrogen storage in physics option based materials. Now before we go on to looking at those materials and the concepts let us understand some of the basic things that we have already studied in the earlier classes like what is absorption what is absorption. Now the first figure which shows absorption when a substance when the atoms or molecules of a substance here in our case it is hydrogen when it sticks onto another material surface and either it gets sticks, adheres or it gets accumulated on the surface of another substance in that case the process is known as absorption. So the molecules which adhere onto the surface are known as adsorbate and on the surface of which the substance on the surface of which these molecules adhere is known as adsorbate. Similarly if the molecules or atoms of one substance they enter into another substance. So the blue colored is another substance if they enter into another substance they permeate into this substance then they get assimilated into the internal volume or they get incorporated into completely incorporated diffuse into the volume or internal structure or dissolved into this material in that case the process is known as absorption. So the material which gets into is known as adsorbate and the material into which it this this molecules get inside is known as absorbent. In our case these red colored molecules these are representing hydrogen gas however the blue colored matrix is the material or the hydrogen storage material into which the hydrogen gas dissociates and enter in the atomic form. So this is what is absorption. So we can see that absorption is a surface phenomena while absorption is a bulk phenomena. Now in order to look at the describe the adsorption of different gases there are different isotherms that can be plotted. Now these isotherms in fact represents the amount of adsorbate on the adsorbent at different as a function of pressure at a constant temperature. Now this amount of adsorbate that has to be normalized. This quantity needs to be normalized with respect to the mass of adsorbent and this is required if we want to compare the different materials. Now there are different isotherms which are available where which we can quantify the amount of adsorbate which is stored in a particular adsorbent material. Now the fraudulite was the first one. So fraudulite with coaster they came up with the isotherm fraudulite isotherm and that was in fact a mathematical fit that they provided to the adsorption isotherm and that was fitted using an empirical relationship. So this empirical relationship is x by m with where x is the mass of the adsorbate, m is the mass of the adsorbent and that is equal to k p to the power 1 upon n where p is the gas pressure, k and n these are respective coefficients constants which are associated with a pair of adsorbate and adsorbent. The another isotherm set of isotherm is being provided by Irving Langmuir and Irving Langmuir was the first one to derive scientifically these adsorption isotherms and in fact he modelled the adsorption of gases onto the solid surface and this was in fact kinetically based and this was derived using statistical thermodynamics. However there were several assumptions which were there in Langmuir isotherm and these assumptions are like the adsorption sites these are all equivalent and each adsorption site is occupied by a single molecule, there are no phase transitions, the surface is energetically homogeneous. At the same time the adsorbents they do not interact, the adsorption takes place in the form of a mono layer. So these are several assumptions, all these to be followed are very difficult. The reason being the surfaces may have their own imperfections, the adsorbent may not be always inert, it may not be possible that always a mono layer is formed, there could be multi-layer formation which is actually taken care of in the next set of adsorbents which is by means of BET theory. At the same time it is also the first molecule when it gets adsorbed and the last molecule when it gets absorbed the conditions not be similar. So it is not possible always that all these assumptions may hold true. Now the relationship between k and theta here is that k is equal to theta upon 1 minus theta p. So p is the pressure, theta is the surface coverage, so theta surface coverage is given by k p upon 1 plus k p. Now for very low pressures theta is approximately equal to k p and for high pressures theta is approximately equal to 1 in the Langmuir isotherm. The another set of adsorption isotherm was being given by Stephen Bronner, Paul Emmett and Edward Teller and this is the BET theory. Now this BET theory is in fact an extension of Langmuir theory and it is applicable for multi-layer adsorption. The basic assumption that goes in the BET theory is that the gas molecules they are physically adsorbed in the form of infinite number of layers. So as such it is applicable for multi-layer adsorption. These layers are non-interacting and individual layers for them Langmuir model is applicable, is valid for each of these independent layers. Now when we analyze these the surface coverage to form the isotherms the gas which is used usually is nitrogen. This is because it is inert, pure and then cost effective. However at the ambient conditions at the room temperature pressure conditions since the adsorption involves very weak forces as such very small amount of gas gets adsorbed. So as such to get a detectable amount of gas being adsorbed liquid nitrogen temperatures are being used. Now the expression that relates is 1 upon p to p by p0 minus 1 where p upon p0 is the relative pressure, x is the amount getting adsorbed is equal to 1 upon c plus c minus 1 upon xc p by p0. So this is the relative pressure where c is a coefficient which is related to the heat of adsorption. Now from the BET theory there are five different types of isotherms which are possible. Now if we look these one by one then type 1 isotherm where we can see at lower pressure there is a steep rise in the mass of gas adsorbed and then it flattens to form a platu. Now this type 1 is a sort of pseudo Langmuir isotherm and this holds true for microscopic materials. However it predicts the formation of a monolayer adsorption. The second isotherm is we can see a sharp rise, steep rise at the very low pressures then a sort of linear rise and then it peaks. So this type of type 2 isotherm this is at low pressure there is sort of micro pores filling that is taking place with the gas being adsorbed then a slight platu that is arising because of the monolayer being formed and at higher pressures their multi-layer adsorption occurs and that multi-layer adsorption continues until there is a condensation which takes place due to capillary forces. So this is the type 2 type of BET isotherm type 3 we can see that there is no platu being observed there is no asymptote also. So in this there is a multi-layer adsorption only that takes place there is no monolayer formation or we can say that from the beginning the multi-layer formation takes place adsorption takes place it begins before even the monolayer adsorption gets over. Type 4 type of isotherms this represent here in in fact the capillary condensation occurs a monolayer adsorption takes place and that is at a lower pressure and that is followed by a multi-layer adsorption and this is usually obtained in case of mesoporous material. The last one which is type 5 this is almost similar to type 4 and here also capillary condensation can be seen. Now these are the different isotherms if we want to measure the hydrogen uptake in case of physics option based material then these isotherms are much helpful. Now when it comes to hydrogen storage in the sorbent materials physics option is the basic process in physics option hydrogen is stored in the molecular form and this is stored in the molecular form on the surface of the different sorbent materials and these are interacting with the surface of the sorbent by means of weak vendor walls forces. Now these weak vendor walls forces these arise because of the intermolecular the dipoles which are created because of the random fluctuations in the charge distribution that results into the weak vendor walls forces or we can say the absorption and the attraction between the different gas molecules and the surface or repulsion between the gas molecules can be represented by the potential energy curve which is given by the Lienard Jones potential. So the potential is given by 4 epsilon this is the depth of the potential well sigma upon R. So R is the distance between the atom and the sorbent sigma is that finite length distance at which L R becomes 0 sigma upon R to the power 12 minus sigma upon R to the power 6 this is the potential energy relationship. Now if we draw this potential energy relationship its minima occurs at a distance from the surface which is approximately equal to the sum of the radii of both adsorbate and the adsorbent. So it is minima occurs at a distance from the surface which is sum of vendor walls radii of the adsorbent atom and the adsorbent molecule. Now when it is a microporous material in that case the pore size is very small and we know that it is say less than 2 nanometer in that case the pore size is almost equivalent to that of the adsorbent dia. Now in that case the interaction gets very much affected by the pore size and pore dimension. Physics option is we have already seen that it is a surface phenomena. Now in surface phenomena what is the requirement is that a high specific surface area should be there it is dependent upon what is the pore geometry what is the pore dimension what is the pore volume what is the porosity of the material all these determine the adsorption of the adsorbate onto the adsorbent. Also this adsorption process is an exothermic process and this is because of the low polarizability of the hydrogen molecule the interaction which is taking place by means of weak vendor walls forces is weak in the case of physics option or adsorption based materials. Now the that means adsorption energy is very small the value of adsorption energy typically lies in the range of 1 to 10 kilo joules per mole. This is unlike in case of the materials which work on the principle of chemisorption where the bond form is more stronger and there the adsorption energy can be even an order of magnitude higher than this particular adsorption energy. Now if we want to measure the hydrogen uptake there are different ways of measuring hydrogen uptake either it could be gravimetric or volumetric storage capacity could be monitored. So as I mentioned that the gravimetric capacity is mass of hydrogen being stored divided by the mass of hydrogen stored plus the material mass or the mass of the storage media and volumetric storage capacity is the mass of hydrogen stored per unit volume of the storage media. Now in order to find the hydrogen uptake in these porous materials the best way could be to find the adsorption isotherms. Now this adsorption absorbed amount of hydrogen in these porous materials can be mentioned by either finding out the absolute storage capacity or excess storage capacity. So these are the terms that can be used for deciding on to how much is the hydrogen uptake in a particular class of material. Now this term excess storage capacity is in fact what we measure experimentally using the different experiments and this excess capacity if we try to define then it is the difference of the gas which is being stored at a particular temperature and pressure in a volume which already has adsorbent minus the quantity of hydrogen which will be stored in the same condition, same temperature pressure, same volume in the absence of gas solid interaction or we can say that excess capacity is the amount of hydrogen being stored at a certain temperature pressure in a volume wherein adsorbent is there in the presence of gas solid interaction minus in the absence of gas solid interaction. Now but the how to measure this quantity which is stored in the absence of gas solid interaction and that is very difficult to obtain. Now in order to mimic that quantity which will be corresponding to absence of gas solid interaction what is being done is non-absorbing gas is taken which does not interact with the solid surface and the volume of this non-absorbing gas is being measured to find the second term in the excess capacity. The another term is absolute adsorption. Now this absolute adsorption as such is difficult to find out this is actually predicted by means of different theoretical calculations and what it tells is the amount of hydrogen which is adsorbed when we are not considering the hydrogen in the gas phase. So this adsorbed amount of gas n adsorbed can be given by Ni minus Vg rho g. Ni is the amount of gas which is introduced into the sample cell the total hydrogen which is introduced into the sample cell minus the free gas volume and that free gas volume minus the free gas which is there in the sample cell. So this is given by the product of free gas volume occupied by the gas inside the pore volume of the sample times the gas density. So that gives the absolute adsorption. Now it is very difficult to find out what will be the real free volume. It is not known. What experimentally we can find out is excess adsorption. Now to find out the free volume we usually do the helium expansion and find out. Now what we get is the excess adsorption that excess adsorption is given by Ni minus V0 rho g where Ni is the hydrogen which is introduced into the sample cell or the measuring cell minus V0 is the helium volume which is there which is being taken up in the absence of gas solid interaction because that is a non-absorbing gas and the gas density rho g. So this is the excess adsorption and the earlier one was the absolute adsorption. So there is a relationship between the experimentally measured excess adsorption and the absolute adsorption and that is given by the excess adsorption is given by the absolute adsorption N adsorbed minus rho g times V0 minus Vg where V0 we have as we have seen this is the helium volume in the absence of gas solid interaction while Vg is the gas volume in the presence of gas solid interaction. So the difference of 2 can be represented by Va and this is the difference between the real free volume and the free volume which is measured by the helium gas in the absence of adsorbed phase. So this is the volume occupied by the adsorbed phase and this is how we can calculate the hydrogen uptake. Now if we see the absolute hydrogen uptake at a temperature of 77 Kelvin that is actually given by a Langmuir isotherm or type 1 type of isotherm and that shows the formation of monolayer. So in fact if we plot the amount of gas adsorbed with as a function of pressure at a particular temperature then we will see there is a sharp rise and then it becomes constant. So that is the absolute adsorption at 77 K. So it follows a sort of Langmuir type of isotherm or type 1 type of isotherm and that shows the formation of monolayers. However the other term which we have studied is the axis adsorption. This axis adsorption we know that can be measured experimentally. However the variation does not show a plateau. So there is no plateau if we plot the axis adsorption initially it remains consistent with the absolute adsorption however it peaks and then it decreases. So there is a peak and thereafter there is a decrease in the adsorption. So a maxima is reached and this decrease in the with the increase in pressure and then thereafter it is the decrease in with the pressure increase in pressure. Now it can also become 0 or it can also become negative. Now as such it is not corresponding to the real physical decrease. So this is the gas this is because the gas is adsorbed in pores. So this nature is because the gas is adsorbed in the pores and that it gets saturated but the gas density it keeps on increasing. So actually it does not corresponds to this decrease does not corresponds to real physical decrease of the adsorption capacity rather it is the consequence because we have measured the void volume of the sample using the helium gas expansion. That was in the absence of adsorbed phase that is the reason why we are getting this dip in the adsorbed amount. At moderate pressures both we can see that the absolute and axis they are similar they coincide. There is not much of difference between the two and they can directly be represented by the type 1 type of isotherms. However there is a difference that occurs at a higher pressure. So we can see the deviation that takes place at a higher pressure. For absolute there is a plateau is reached however for excess after peaking that there is a dip in the axis adsorption. Now this was the effect of pressure. Now if we consider these adsorption we are considering at a liquid nitrogen temperature. However if we consider at room temperature then the amount of hydrogen uptake will be very small. So a very small amount of hydrogen will be there being stored in such porous materials and the adsorption isotherm in that case will not show any plateau. There will be in fact a rise, linear rise that will be observed and that will continue to even higher pressures. And if it is room temperature adsorption then the isotherm which is we obtained is a linear isotherm that is a Henry type isotherm that we will be getting. Now the temperature effect is the for most of these type of physics option based material as the temperature increases the adsorption capacity decreases. And the reason is since the molecules of gas, hydrogen gas molecules they are attached onto the surface of sorbent by means of weak van der Waals forces at a higher temperature or at room temperature the thermal motion energy is also equivalent to the van der Waals forces and as such the capacity or the adsorption potential decreases as the temperature increases. However, if we want to store hydrogen in an appreciable amount either we have to reduce the temperature of the gas like 77 K liquid hydrogen temperature is preferably used for storing hydrogen in such adsorption based materials at ambient pressure. So if we see the excess adsorption capacity of many of the materials this peaks at around 77 Kelvin and maximum like 4 megapascal it can we can obtain the highest capacities. The other choice if we want if we do not want to reduce the temperature of we do not want to cool it down or have insulations then the other method could be pressurize the gas so as to increase the adsorption potential. Now if we quickly compare with the other methods like we have already seen the compressed hydrogen storage at 350 bar and 700 bar then the weight percent gravimetric capacity can be 5.2 to 5.5 weight percent volumetric capacity can reach to 23.65 kg per meter cube when compressed hydrogen is stored at 350 bar and for 700 bar this is 40.2 kg per meter cube but these both these are at room temperature condition storage. In case of liquid hydrogen storage when the pressure is held at 1 bar the gravimetric capacity will depend upon size the volumetric density could be 70.8 kg per meter cube and the temperature of storage we have already seen is 20 Kelvin. Same was for cryo compressed that the temperature of storage can be 20 Kelvin or higher the volumetric capacity could even reach 87 kg per meter cube which is even higher than the liquid storage when it is being stored at 240 bars. With the materials which we are going to see adsorption based materials the pressure can be at 100 bar the gravimetric capacity is 2 weight percent typically 2 weight percent there are materials which have higher than 2 weight percent as well the volumetric density is 20 kg per meter cube and the temperature could be 77 to 80 Kelvin. Now there are other class of materials which are based on chemisoption there could be absorption based materials where the pressure could be close to 1 bar or even higher the weight percent that gravimetric capacity that can be reached is 2 weight percent and the volumetric energy density volumetric capacity is 150 kg per meter cube and they can store hydrogen even close to room temperature or higher temperature complex hydride again they can store at 1 bar or higher pressure their gravimetric capacity less than 18 percent volumetric capacity even 150 kg per meter cube can be achieved but the temperature of operation will be greater than 100 degree centigrade. So there are different class of materials in the today's class like we have seen the different fundamentals associated with these materials what are the ways in which we can measure what are the ways in which we can express the hydrogen uptake we have to these type of materials they have certain advantages as against the chemical hydrides that they have a very fast kinetics. The reason for fast kinetics of such materials is since there is no activation energy involved as such the uptake of hydrogen or release of hydrogen is very quick because the bond in these materials is a weak bond. Now the reversibility of these materials is very high the reversibility is an important criteria when selecting a hydrogen storage material the reason of reversibility is there is no strong bond with the storage media. Now this is an advantage for such materials that we can get a reversible hydrogen store at the same time the cycle life of such materials is very good because there is no diffusion of hydrogen into the molecule no permeation of molecule no formation of strong bonds into the molecule not getting up into the internal structure of the adsorption materials. So as such there is no structural changes which take place or no electronic changes that can occur no perturbation of the electronic charge that can occur as such the cycle life of such materials is higher. There is a low heat of adsorption which is associated which is with the weak van der Waals forces and that becomes an advantage when it comes to designing a hydrogen storage tank. So in the next class we will see in more detail what are these materials what are the different classes of such materials which store hydrogen by means of physics option. Thank you.