 In the previous class we have seen the different adsorption based materials where the hydrogen it gets adsorbed onto the surface of sorbent and the bond formed was weak Vendables bond. Now in this class we will look at another class of materials which are absorption based materials wherein the bond formed is a stronger bond compared to the materials we have seen earlier. Now this is just a repetition to mention that different among the different hydrogen storage materials that we have we will be looking at we have already seen where we can store hydrogen as a molecule without dissociating it into hydrogen atoms and that can be done on the surface of high surface area materials or the porous materials with high porosity and pore volume this we have already studied. Now in this class we will start with the other category of materials used for solid state hydrogen storage which are wherein the hydrogen molecule it disintegrates or dissociates to form hydrogen atom and that hydrogen atom that forms a bond with the material. So the class of materials that we will look in today's class is metal hydrates. Now we have already seen that the different solid state storage methods wherein these could be either physics option based or adsorption based where the molecule of hydrogen that sits onto the surface of these materials and it forms a weak bond that we have already studied. Now today we are going to look at the absorption based material and here in the molecule it disintegrate it dissociates into hydrogen atom gets into the interstitial sites and this is a bulk phenomena compared to the adsorption based phenomena which was a surface phenomena and here the bond formed comparatively is a stronger bond. A paper which is reported in nature and this is a very good comparison which shows that if we have to store hydrogen in different ways like whether it is liquid state or solid state or gaseous state, how the volume of the tank, how the size of the tank will vary. Now let us say if we want to store 4 kg of hydrogen for a vehicle which has to be driven for a 400 kilometers and then we can store hydrogen either in a compressed form then the tank size at when it is storing at 200 bar will look like something this. If however we liquefy hydrogen then this will be the tank size however if we store in the form of metal hydride that will even reduce. So on a volume basis this is reduced further if we store in one of the metal hydride form that we are going to learn which is LA Ni5 or it can further reduce depending upon what is the capacity of these materials, metal hydrides which we are using. Let us say if we use MG2 Ni then the size further reduces. So this is a very good pictorial representation of how the size of the tank it reduces as we move from compressed to liquid to solid state storage. Thus like if we have for example compared to gasoline which is having an energy density 47.3 mega joule per kg and an energy density 35.5 mega joule per liter here with solid state storage if we compare this is volumetric energy density is about 12.63 mega joule per liter. So this is how the comparison goes. Now if we plot together the volumetric and gravimetric storage capacity for different ways of storing hydrogen we can see that the capacities for a pressurized container at a metal tank or type 1 tank this is lower. If it is pressurized with a composite tank then the capacity of storage it increases but compared to the method of either liquid state storage here in this shows 70.8 is the typical volumetric capacity of hydrogen being stored in the liquid form. So all the materials based storage or the solid state storage they have a higher capacity that is volumetric capacity. But then we will also see that they have some of the materials they have good gravimetric capacity also. But at the same time the materials which we are going to learn like the metal hydrides they have good volumetric capacity but their own weight because of their own weight the gravimetric capacity is relatively lower. So the solid state materials we can see definitely they have a advantage when it comes to volumetric hydrogen storage capacity but then depending on what materials we are using their gravimetric capacity is definitely vary over a wider range. Now hydrogen in the very first class we have seen that it has one electron and that one electron in the 1s orbital that is reactive and can react with many elements so almost all elements of the periodic table. Now depending upon what sort of bond it is forming these can be the hydrides which are formed they can be classified. Like for example the elements highlighted in red form ionic hydride, elements highlighted in pink they form covalent hydride, those in blue they form metallic hydride, in teal they form covalent polymeric hydride and those which are in yellow they form metastable hydride. So depending upon what is the interaction, what is the metal to hydrogen or the element to hydrogen interaction the bond formed we can classify these hydrides into different classes. Like the ionic hydrides these are formed by hydrogen being reacting with the alkali or alkaline earth metals. These are usually formed when the electronegativities of the two atoms these are very much different. The hydrogen in ionic hydrides it is found usually found as negatively charged ion and these materials the ionic hydrides where the element to the hydrogen bond or the bonding is ionic in nature they have high conductivities either below or at the melting point. Now typical examples the binary hydrides with which are the ionic hydrides are sodium hydride or calcium hydride or there can be complex hydrides like lithium aluminium hydride or sodium aluminium hydride. Now the other class of such hydrides is the covalent hydride. These hydrides are formed when the electronegativities are almost similar. So in that case the sharing of electron takes place and then the formation of hydride occurs. So usually these hydrides are formed with non-metals and the interaction is involved is a weak van der Waals force as like these covalent hydrides are usually not usable for hydrogen storage. The reason being most of these hydrides these are having low melting point these are low melting point solids or low boiling point liquids or these are gases and they have toxicity they can have even like low thermal stability. So as such they are usually not used for hydrogen storage purpose. Some of the typical well known examples like ammonia water or H2S. The next category of such hydrides is the metallic hydride and this metallic hydrides are formed when hydrogen reacts with either transition metals or rare earth metals or lanthanide actinide series. So the bond formed based on the nature of the bond formed these are known as metallic hydrides. So the bond formed is metallic and because of the very easy uptake and good reversible capacity of these materials the taking up of hydrogen release of hydrogen is relatively easier as such these are most widely used for hydrogen storage application. Some of the examples like vanadium hydride or titanium hydride or titanium iron hydride of this class of hydrides. Now the important category of absorption based materials is the metal hydrides and they are specifically of interest because of their reversible capacities. These are simple in the sense they can combine chemically directly with hydrogen when these materials are exposed to hydrogen at certain pressure temperature conditions they form hydrides. So the uptake as well as release of hydrogen is simple with these metal hydrides. So it is a simple method subjected to temperature and pressure changes we can have hydrogen being stored and released from these metal hydrides. They also have good storage capacity and that is high on the volume basis for such metal hydrides. In fact this is higher than the liquid state hydrogen storage. This is about 60% higher than the liquid storage in case of metal hydrides. At the same time with these metal hydrides we can store hydrogen for any amount of time. So for long durations we can store and that too at ordinary temperature and pressure conditions. By ordinary temperature and pressure conditions we mean we have a variety of metal hydrides they can even store we have some of the metal hydrides which can even store at room temperature or ambient pressure conditions. Now this becomes important when we compare this method of storage as against the compressed state storage where we have seen that the storage of hydrogen occurs at very high pressure 700 bar. However in metal hydrides we do not require very high temperatures at the same time the pressure the temperature required are also close to room temperature or it could be higher depending upon which metal hydride we are selecting. But unlike the liquid state storage when the storage was carried out at 20 Kelvin here the temperatures involved are not that low. At the same time the metal hydrides are very much stable they can be stored for longer much longer durations and they are quite stable below their dissociation temperature. Important thing here is that until the temperature is raised or let us say if there has been a leak in a metal hydride based storage system the reaction of release of hydrogen is endothermic. Because of that a self cooling effect may arise and that will further reduce the temperature of the metal hydride because of the endothermicity of the reaction and that will stall the reaction or the release of hydrogen. So it is very safe to handle and store hydrogen and since we are not using very high pressures or very low temperatures for storing hydrogen as such that tank design are not that sophisticated as we have used in case of compressed hydrogen tank where the pressure requires were high requirement was very high or like the liquid hydrogen tanks where super insulations were required so as to reduce the heat flow. But since the temperatures of operation and the pressure of operations are moderate here we do not require such sophisticated tank design. Now let us look at the mechanism of hydrogen uptake in these this class of material which is metal hydrides. Now the first figure it shows hydrogen molecule far away from the surface and as it approaches the near to the metal surface. In that case in the second figure this metal these hydrogen molecule it interacts with the surface atoms and initially that interaction is by means of weak van der Waals forces there after the hydrogen molecule it dissociates dissociates into hydrogen atoms and these hydrogen atoms get chemisorbed after dissociation in the third step. Remember that hydrogen enters into the matrix of the material it sits into the it occupies the subsurface sites further it diffuses deep into the bulk and then it occupies the bulk lattice sites. So this is how the hydrogen uptake in these metal hydrides take place. Now if we see the mechanism of hydrogenation or dehydrogenation in these type of materials a simple reaction can be written the metal. Now this M is either a metal or it could be it could form the metal it could be a metal hydride it could be a solid solution or it could be inter metallic compounds that can be used. So metal on reacting with hydrogen molecule like this is x by 2 H2 forward reaction forming metal hydride plus certain amount of heat is being released. Now since the metal hydride which is being formed that has a lower energy compared to the metal and the hydrogen molecules so a heat is released in the forward reaction. So the formation of metal hydride from metal and hydrogen is a exothermic reaction heat is released however the reverse of it that is getting the hydrogen or release of hydrogen from these metal hydrides is an endothermic reaction. That means we have to supply the heat to the metal hydrides in order to get this hydrogen back. So the discharging process of the reverse reaction is a endothermic reaction. Now the conditions of absorption and desorption or the uptake of hydrogen is absorption here and the release of hydrogen is desorption. So the conditions are like for absorption when the supply pressure it is higher than the equilibrium pressure now what is this equilibrium pressure we will see little later. So provided that the supply pressure it gets it is higher than the equilibrium pressure absorption of hydrogen into such metal hydrides take place and this process as I mentioned it is an exothermic process however this equilibrium pressure that also changes if there is a change in the temperature. So this equilibrium pressure also depends upon temperature. Similarly the desorption is a endothermic process wherein heat has to be supplied for releasing these hydrogen being stored from the matrix to get hydrogen molecules. So whenever the equilibrium pressure, equilibrium pressure is higher than the pressure outlet pressure we can get the desired hydrogen and this process since we know that it is an endothermic process and all at the same time the equilibrium pressure we know that it is also temperature dependent. Now the changing both the pressure and temperature that will changing the pressure and temperature will result in to either absorption or desorption of hydrogen in these materials. Now let us look at the formation of metal hydrides in little more detail how the process takes place. Now this can be represented on a potential energy diagram. Now this is how the distance is increasing from the metal. Now far away from the metal surface where there is hydrogen molecule now if we see the potential energy difference between the hydrogen molecule and when it dissociates into 2 hydrogen atoms this is 435.99 kilojoule per mole that is the dissociation energy for hydrogen. Now the first attractive interaction that the hydrogen molecule when it approaches on to the metal surface that it observes is the weak van der Waals interaction. So that is the weak van der Waals force it observes and it gets physics of onto the surface. Now the first step is physics option that occurs on to the surface of the metal and at that time like the distance is about 1 hydrogen molecule distance and the heat of absorption for this physics option is about 10 kilojoule per mole. Now there is a difference or there is an activation barrier between the hydrogen molecule and its dissociation into hydrogen atoms. Also there is a barrier towards hydrogen being chemisobbed onto the surface of the metal. So chemisoption of the hydrogen. So now these potential energy activation energy barriers these needs to be overcome and that then the process occurs. So that requires sufficient energy to be supplied for the for overcoming this activation energy barrier. Now what happens is onto the surface of the metal the hydrogen once it gets dissociated into hydrogen atoms that hydrogen atom forms a bond with the surface atoms and this the electron transfer or the electron sharing there occurs electron sharing occurs with the surface atoms and then it gets chemisobbed onto the surface. So this dissociation and chemisoption occurs at the surface and usually this energy involved is about 50 kilojoule per mole in the process this is roughly around 50 kilojoule per mole. Now these hydrogen atoms which gets chemisobbed onto the surface these have high surface mobility and they interact with the other surface atoms they form different phases and thus that gives rise to a very high surface coverage. Now after they get chemisobbed they enter into the subsurface they penetrate inside the metal surface and they enter into the matrix or the interstitial positions that we have seen in the previous slide. They enter into the metal matrix so they penetrate into the metal surface and they start to form hydride phase by means of diffusion. And finally they further diffuse into the undergoes bulk diffusion and then occurs the nucleation and growth of the hydride phase. So this is how the entire process of formation of metal hydride is. So starting from physisoption onto the surface of the metal atom it enters it undergoes dissociation, chemisoption, penetration into the metal surface finally forming a hydride phase and that hydride phase or the hydrogen diffuses inside forming hydride phase and further the nucleation and growth of the hydride phase occurs. Now this hydrogen which enters into the metal matrix or the material and sits into the interstitial position that goes into the different voids of the different lattice. Like they can go into either tetrahedral or octahedral voids of the different structures like BCC or HCP or FCC type of lattice. Now when it enters into the lattice then like when the hydrogen when the concentration of hydrogen is lower into the metal matrix it exothermically it enters into the lattice and that leads to expansion that entering into the lattice that leads to expansion of the lattice. That also leads to reduction in the symmetry like typically that expansion which is considered is like 2 to 3 angstrom cube per hydrogen atom. Now thereafter the problem is that the stability of these hydrides that also depends upon the size of these interstitials. Now in the BCC lattice like tetrahedral and octahedral voids they got distorted largely and in FCC hydrogen concentration in the octahedral sites is comparatively lower. So in alpha phase where it forms a sort of solid solution it starts to form hydride that it enters into the tetrahedral sites these are half filled here in another phase these are fully foiled voids and that leads to significant distortion of the lattice. Now if we want to know how much amount of hydrogen has been taken up by a material then there are different ways of finding that. So there are different measurement techniques which exist. Now these measurement techniques are like can be divided into either volumetric technique or gravimetric technique or temperature programmed desorption. Now we usually draw like the in these measurement techniques like the two volumetric and the gravimetric technique we draw isotherms. Now these isotherms they are plotted showing the hydrogen uptake for different pressures at constant temperature. Now in the first method which is the volumetric technique usually what we do is we have a we introduce known aliquots of hydrogen into a known calibrated volume and after that the hydrogen absorption that will cause a decrease in the pressure. So because we know the pressure all the volumes in the systems they are calibrated and we introduce hydrogen at a certain pressure and that reduction in the pressure because of absorption or taking up of the hydrogen by the material will result into a drop in pressure and that change in pressure is measured at a known volume and then we can use the different state equations to find out how much amount of hydrogen has been taken in. Similarly, we can do for desorption. So there was a decrease in pressure during absorption where when we supply the required heat so that the hydrogen comes out in the reverse process that is desorption the increase in the pressure can be measured and then we can find out the hydrogen released amount. So this is in the volumetric technique. Now the another method is the gravimetric method wherein instead of recording the change in pressure we see the change in mass. So a very well calibrated micro balance and a sensitive micro balance is used for measuring the change in mass subjected to different hydrogen like gas pressures and then the major problem lies is that of buoyancy. And the third method is instead of keeping here the samples at constant temperature here we can change we can see the desorption of the material subjected to different temperatures. So these are the different methods in which using which we can measure how much is the hydrogen uptake. Now when we measure how much is the hydrogen uptake what we do is we plot like the isotherms which are like this is one of the isotherm and ideal isotherm shown here as a function of concentration of hydrogen. So the amount of hydrogen being taken up by the material H by M hydrogen by metal ratio and as a function of pressure. Now what happens is when we have low amount of hydrogen in the beginning when the we have smaller amount of hydrogen to metal ratio for say less than 0.1 the hydrogen it gets dissolved into the metal and this is a sort of exothermic process and then it forms a solid solution which is known as the alpha phase. In this process the metal expands as I mentioned like the expansion is about 2 to 3 angstrom cube per hydrogen atom however what happens is when the hydrogen concentration it increases it goes beyond say 0.1. So the strong hydrogen-hydrogen interaction occurs hydride phase starts forming that starts to nucleate that starts to grow and that leads to distortion in the metal distortion in the metal matrix. Now usually the hydrogen concentration in the hydride phase usually is considered as H by M equal to 1. Now it is observed that when this higher concentration the volume expansion is in the of the lattice is significant and that is approximately even 10 to 20 percent of the metal lattice which introduces variety large number of large amount of stresses on to the phase boundaries and because of these stresses the decrepitation of the material can result and the material can become brittle and so particle size can also reduce. Now if we see the typical and ideal pressure temperature isotherm then there are 3 regions we can see the first region which is here shown as alpha phase, second where the hydrogen is formed as a this is a solid solution phase. The second region where both the alpha and beta phase are coexisting beta is the hydride phase. So alpha and beta phase both coexist and the third phase where it is only the beta phase. Now if we apply the Gibbs phase rule which is the number of degrees of freedom that is equal to the number of components here the components are 2 there are 2 components the metal and the hydrogen so C is equal to 2 in this case. Now if we apply the Gibbs phase rule for the first region which is the alpha region. Now here the here if we see then this Gibbs phase rule if we apply there are 2 phases there is hydrogen and alpha. So for the region 1 for the region 1 we have 2 components and then we have 2 phases. If we put in this equation F is equal to C plus 2 minus P then we get the number of degrees of freedom equal to 2 that means our concentration varies with the pressure this is in the first region. Now if we see the second region where the 2 phases are coexisting region 2 again the number of components is same but the number of phases are 3. So there is hydrogen and alpha phase and a beta phase. So as such the number of degrees of freedom for region 2 is 1 and that is why we get a plattue. So the concentration changes here at a constant pressure and this we will see we will define in the next class that equilibrium pressure is given by this plattue pressure. So in the second region the number of degrees of freedom is only 1 however if we now look at the third region again in the region third we have 2 components and 2 phases hydrogen and beta phase. So as such the number of degrees of freedom is again 2 and thus there is a rise in the pressure as the hydrogen gas pressure rises and here is also a change in the concentration. So this is a typical and ideal isotherm now there are irreversibilities involved there are several changes are involved in these isotherms that we will see in the next class. To summarize what we have learned in this class we have seen the metal hydrides special class of material which can store hydrogen reversibly and that is the biggest advantage of these materials. They show a high storage capacity on the volume basis and hydrogen can absorb on these metal hydrides and we have also learned what are the different steps involved in absorption of hydrogen in these materials. We have seen that there are different ways in which we can measure the uptake of hydrogen like the volumetric method or the gravimetric method or it could be TPD and thereafter we can get the different PCT curves. So the series of PCT curves we will see in the next class these PCT isotherms or curves these are used to study the behavior of hydrogen absorption and desorption and what more information we can get from these PCT that we will see in the next class. Thank you.