 The next two lectures will be by Professor Jack Swett. He is Professor in the Department of Chemistry, Biochemistry and Physics at the University of Quebec Tri-Reverse. This is in Quebec, Canada. Professor Jack, he is well known for his work. He is a renowned scientist working in the field of metal hydrides. Hello everybody. My name is Jacques Uyotte. I'm a professor at the University of Quebec à Trois-Reverre and also a member of the Art de Genre Research Institute. Today, we will talk about tailoring metal hydride for practical application. This is in two parts. The first part will be nanostructure in general and the second part will be on the effect of nanocrystalline structure on the capacity of metal hydride and also some other method to get nanostructure materials. So just a quick recap on what our metal hydride, you could consider metal hydride to be like a kind of hydrogen sponge. So in a metal hydride, you have a chemical bond with the metal atom and the absorption is an exothermic reaction and the desorption is an endothermic reaction. So to give you some idea of the difference but with metal hydride with respect to the other ways to store hydrogen, so if you store hydrogen as a gaseous form at one bar at room temperature, you'll see that the volumetric capacity is quite low. In order to increase that, you have to increase the pressure so if you go to 150 bar, you have a much better volumetric capacity and if you are using liquid hydrogen, so usually if you want to transport a huge quantity of hydrogen, liquid hydrogen in my opinion liquid hydrogen is the best way. Of course it's at low temperature at 20 K but you see that the volumetric capacity is quite good but you see for metal hydride here you have high temperature metal hydride and you have a low temperature metal hydride, so this metal hydride is working at room temperature, this one is working at least 300 degrees, 350 degrees C and you see that actually it's a little bit counterintuitive but you see that the volumetric capacity of hydrogen is higher in a metal hydride than in a liquid. So the hydrogen atoms are more compact in a metal hydride than in a liquid. It's counterintuitive but this is one of the best advantages of metal hydride is that hydrogen is compact so your tank will be very small but this advantage as you see here is that the gravimetric density is quite low so here this means that you only have 7.7 weight percent of hydrogen in a metal hydride tank made of magnesium hydride and the rest is just the weight of magnesium and for Lantanumna K5 it's even worse it's only 1.4 weight percent so that's one of the big disadvantage of metal hydride. So the advantage and the disadvantage well you see that for metal hydride the advantage as we said it's high volumetric density it could work at low pressure but the reaction of this option is endothermic so why I list it as an advantage is because if you have hydrogen metal hydride tank in your car and you have an accident and the tank is rupture and you have hydrogen getting out and hydrogen is taking fire so then it will eat up the tank but as the tank is eating up it will desorb hydrogen but as a desorb hydrogen because the reaction is endothermic it will have a tendency to cool itself so it will kill the reaction by itself so this is inherently safe the disadvantage is usually you have a high temperature of operation if you want to have a good capacity for example with magnesium hydride you have to work at high temperature more than 300 degrees C usually the hydrogen sorption kinetic hydrogenation the hydrogenation is relatively slow the cost depending on the material it could be expensive and again you have to consider not only the cost of the raw material the cost of the alloy but also you have to consider the cost of loading the alloy with hydrogen and putting that in the tank and so on so the process cost could be quite expensive so you have to take that into account and pyrophoricity because metal hydride after many cycles of hydrogenation they will turn into very very small powder and any powder that is very very small could be pyrophoric and also for metal hydride because they react with hydrogen so it means that they could react with oxygen also so you could have problem of pyrophoricity so very quickly just the step for forming metal hydride so first you have hydrogen in gas phase and this hydrogen has to be split into proton to hydrogen before entering because on a metal hydride what is stored is not a molecule H2 but it's a proton it's the hydrogen atom so first you have to break this molecule after that the hydrogen has to break usually you have a small oxide layer on the surface of your alloy so it has to break this oxide layer and for hydrogen first you have to be in adsorption so you adsorb on the surface and then it's split into and then the proton will enter the hydrogen, the metal and first it will form in solid solution so it will just go into the metal structure but as the pressure of the gas is increasing then the concentration of hydrogen in your metal will increase up until you nucleate the metal hydride phase and in that phase then the hydrogen will go on specific site depending on the crystal structure it could be on an octahedral or tetrahedral site and you have a lattice expansion it could be up to 30% of expansion of your lattice of your metal so that's the thermodynamic so you have your metal and this is the pressure of hydrogen this is the capacity of hydrogen so it's the number of hydrogen atom over the number of metallic atom and as you start you increase the pressure the hydrogen will enter in solid solution in your metal up until you reach that point at that point you start to nucleate the metal hydride phase so it usually has a different crystal structure and now that you have a new phase in your system by the law of Gibbs so if you have a new phase it means that you are losing a degree of liberty so if you are losing a degree of liberty it means that the capacity could increase even if the pressure doesn't increase because as the pressure tries to go up you have more hydrogen that will go into the metal hydride that will form the metal hydride so on this plateau you go from 100% metal and 0% hydride up to here where you have 100% metal hydride and 0% metal and here it's 50-50 so you have the transformation along that plateau you have the transformation from the metallic state to the metal hydride state and once all your alloy is on a metal hydride state then you are losing one phase you are losing the metal phase so you are gaining one degree of liberty and then to increase the capacity the pressure has to increase and here hydrogen will enter in solid solution and the metal hydride and of course this is a standard phase transition so depending on the temperature the plateau pressure will change so you could plot the logarithm of the plateau pressure as a function of 1 over T and the slope will give you the enthalpy of the reaction and the ordinate at the origin will give you the entropy of your reaction so one advantage of the metal hydride it could spend a large range of applications from portable and mobile up to something as big as the fuel cell submarine so I don't think if you have like a micro fuel cell with a smart phone you could not store hydrogen in a liquid phase it doesn't make sense you could not have a reservoir of 20K in your pocket and the same thing with pressure you could not have a small tank at high pressure because the amount of hydrogen that you need for this is very very small so the tank will be almost as big and as heavy as the device itself so it doesn't make sense so the same thing up to the submarine for the submarine well what is important for them is space they don't have a lot of space so they need something that will store hydrogen that will not take space and for them weight is not a problem it could be heavy it doesn't matter for a submarine so this is why the choice of storing hydrogen for a fuel cell submarine is metal hydride so metal hydride for practical application so presently all known metal hydride suffers some limitation for practical application either they are too expensive or the capacity is too low or they lose capacity during cycling or the temperature and the pressure of operation is not right for your application so most of the time the metal hydride are not ideally suited for practical application right now so the goal is to modify this and to try to find metal hydride that will really meet the specification for practical application so how to solve this limitation well we are trying to find new alloy or we could modify existing alloy and nano sizing is a way to obtain new alloy and also to modify the existing alloy so you could the existing alloy as we will see if you have a conventional alloy but you make it in the nano cross-aligned dimension then the characteristic will change a little bit so it could make your alloy attractive for practical application so how to do a nano structure in metal hydride well there is two ways as for a name metal so it's either bottom up or top down so bottom up is that you construct a nano cross-aligned alloy directly so you could do that by cold vapor deposition thin film and so on so you start and you more or less you assemble atom per atom until you get your nano crystal but in my opinion this is not so suitable for scaling up you could not produce ton of material with this but these are techniques that are very interesting from a fundamental point of view because you could really control the size of your material and also the chemistry and so on so you have more control on your sample and it makes the understanding and it makes the study much easier so these are interesting techniques but we will not discuss them in this course so what we will discuss is the top down you start from a polycrystalline material and you reduce its crystallite size so this is what we will study in this course so there are many ways to do that you could do that by bow milling, cold rolling and cold channel angular pressing pressure torsion and so on so we will discuss some of them in this course so nano structure metal hydride so getting a nano structure metal hydride will solve some of the practical problems for example the kinetics or the first hydrogenation so we could make the first hydrogenation much faster and you could make the alloy much easier to cycle so after many cycles of hydrogenation the hydrogenation you will not lose so much capacity so what is the definition of a nano crystalline material well the classical definition is that your crystallite size should be less than 100 nanometer so the crystallite size is the coherent domain of your crystal and in a nano crystalline material you have a lot of defect so these defect could act as a activation nucleation site so they will lower the activation barrier so if you have more nucleation site your kinetic will be faster and also depending on the size of your grain boundary and the size of your nano crystalline you could have a grain boundary could make up to 30% of your material and this grain boundary could serve as an hydrogen pathway so hydrogen could go through this grain boundary to reach your nano crystalline material and all of this will make that the kinetic will be much faster in a nano crystalline material compared to a polycrystalline material so today we will look mainly at ball milling to prepare metal hydride this is the technique that is used by most of the researcher in the field of metal hydride so almost all of them are using ball milling to get a nano crystalline structure and as we saw by ball milling you could get defect so that will increase the nucleation site you could have formation of the metastable phase this is a common feature of ball milling so not only in metal hydride but also in other type of material usually you could get a metastable phase a high pressure, high temperature phase just by ball milling at room temperature of course you could synthesize nano composites so you could mix two alloys and you could get a nano composite usually when you do ball milling the particles are of the order of the micron but the crystallite are order of nanometer so you have to be careful when we talk about nano crystalline material it doesn't necessarily means that the particle are nano crystalline size it's the crystallite that are nano size and sometime in the literature you see people are talking about grain size but I don't like to use this word because for some people grain may mean particle and for some other people grain may mean crystallite so I prefer to talk about particle and crystallite and then it's clear for everybody so if you talk about particle that's a particle so that particle could be of the order of micron but it's made of many many smaller crystallite of the order of nanometer and of course by ball milling you could increase the specific surface area so ball milling very schematically so in ball milling you have a crucible and you put your raw material in form of powder and you have balls that are made of stainless steel hardened stainless steel or tungsten carbide or some other material you select the material depending on what you want to ball mill and you shake this crucible and then you will have a collision between two particles or between a particle and the wall of your crucible and of course if you have powder in your crucible then you will have powder that is trapped between two balls or between a ball and a wall so when the two ball collide you have your powder that is here will be broken so you will have or they could be cold work the cold well together the two particle could be cold well with the force of the impact of the two balls so it's a repetition of fracture, cold welding fracture, cold welding and so on and after you repeat that many many times you will have a nanostructure material so today I will show you milling of magnesium hydride magnesium hydride well it's a well known hydride it has a high capacity 7.6% and it's relatively low cost because it's abundant in the earth crust the disadvantage is that it has a high temperature of operation more than 300 degrees C usually the first hydrogenation is very difficult and also the kinetic is relatively slow so what I will show you today is that the ball milling of commercial magnesium hydride that we ball mill in a high energy milling machine so this is the powder before milling and after milling 20 hours so you see and the scale is not the same here this is 30 micron here it's a 7.5 micron so you see that before milling the particle are like a smooth okay the surface is smooth but you see here after milling the particle are smaller because this is only 7.5 but these are the big one I have smaller one here so you have a strong reduction of particle size and also you see that for example this particle is an agglomeration of smaller particles so sometimes by ball milling you will break particles but also you will tend to fuse particles together okay and you see that the specific surface area here was only 1.2 square meter per gram and by ball milling we increase it almost by a factor of 10 but it's still not a very high specific surface area so you increase a specific surface area by an order of magnitude but it's still not very very high like you see in carbon nanotube or graphite or so on when you have 100,000 square meter per gram so this is the polar diffraction the on mill is here and after two hours you see that the break peak are getting broad so that means that it's a nanocrystalline structure and you see that these peak are appearing okay that we did not have these peak in the on mill and the mill one you have these peak well these peak actually are this phase so this is a metastable phase of manismine right so the normal phase is the beta phase that the room temperature phase but the gamma phase is the high temperature high pressure phase and you see that you could synthesize it just by ball milling at room temperature so if you look at if you do a ridval refinement if you analyze this pattern then you see that after milling you have 74% of manismar drive and 18% of the metastable phase of manismar drive and you see a reduction of the crystallite size so now it's only of the order of 10 nanometer so while before it was a polycrystalline so it was more than 100 nanometer so if we look at the hydrogenation characteristic so you see so this is on mill is the fill mark and mill is the hollow mark so this is the on mill this one this one and this one and so you see for example at 573 degree k the on mill one is really not dissolving it doesn't want to be solved but at the same temperature for the mill one you see it started to dissolve and if we look at 623 k so 350 degrees C so this is the desorption of the on mill one and you see the mill one is here it's much much faster it's totally desorbing 800 seconds for about 12 minutes while here after half an hour it's still not fully desorbed and you see even the on mill one at 648 it's still slower than the mill one at 623 so you see that ball milling just getting a nanocrystalline structure drastically improve the kinetic of the desorption kinetic and also the adrogenation kinetic so you have the same thing here so for example at 573 so this is the absorption and after half an hour it's not fully absorbed and you see that this one is fully absorbed about maybe 12 minutes 800 seconds fully filled so you see again that nanocrystalline makes the adrogenation and the adrogenation much faster but this is the thermodynamics so this is the pressure composition isotherm as we saw in one of the first slide so this is the on mill one so the on mill one you have your plateau in the first slide and then you have so this is the absorption and after that you have the desorption and you have a plateau again so the plateau on absorption and desorption are usually not at the same position this is what we call hysteresis but this hysteresis depends on manufacturer but also on the kinetic so you see that this is not a flat plateau here in principle at this point should be a stable point so you should have equilibrium at this point but how do you define equilibrium? you define equilibrium when the pressure doesn't change so you expose your material to that pressure and then you see that the material doesn't absorb adrogen anymore so you say well this is on equilibrium but if the kinetic is very very slow then the kinetic is so slow that your upper just think that you reach equilibrium when actually it's not at equilibrium so this is due to the very slow kinetic but as we saw when you do ball milling and you get a nanocrystalline structure the kinetic is much faster and you see that your plateau now is flat because the kinetic is faster and also this plateau move up to here so the hysteresis is much less because you could see here for the on mill one you see that it starts to bend here but the kinetic is so slow that it doesn't reach the equilibrium point so this is why this plateau is lower so this is not due to thermodynamics this is due only to kinetic but you see that you basically do not change the thermodynamics and you just change the kinetic and you see a small reduction of capacity after ball milling so this is confirmed by a calorimetry so this is under hydrogen pressure so that was under 2 bar of hydrogen so if you put your mechanism on the right and you start to increase the temperature at one point it will dissolve and that gives you that peak and that's the temperature of the peak and if you ball mill then you reduce that temperature because the kinetic is faster and you notice that here I have only one peak and here I have two peaks but you remember that in the ball mill material we had two phase we had the gamma phase and the beta phase so that's why I have two peaks here now one of them is the desorption of the gamma phase and the other one is the desorption of the beta phase here I just have the beta phase but you see a reduction of that temperature peak so it means that I have a reduction of the activation energy so we can measure that and what we measure for the on mill for this one the activation energy was 156 kilojoule per mole while for the mill one it was 120 so that's one of the reasons why the kinetic is faster because the activation energy is lower so the conclusion for the magnesium hydride so by ball milling we found that you could have nano crystalline material with a metastable phase and after ball milling the particles were still of the order of micron but much smaller than before milling and you have a 10 fold increase in the specific surface area you have a faster hydrogenation and dehydrogenation and a reduction of the activation energy so this improved the kinetics and also because we have defect and the small particle size and the increase of the specific surface area so these are all featured that will make your kinetic faster so reduction of activation energy smaller size defect and so on but we also saw that intensive milling does not alter the thermodynamic properties of magnesium hydride and this is a common feature in nano crystalline material usually you do not change the thermodynamic unless you're going to very very low very very small crystallite size so for example for magnesium hydride you have to go to the thermodynamic starts to change at about 7 nanometer and just a little bit in order to have a good change of thermodynamics you have to reach like 2 or 3 nanometer and this is not the type of size that we get by ball milling usually so it's much much too small so in principle by ball milling usually you do not change the thermodynamics so with this this will conclude this part one and in part two we will see so here we mainly see the effect on kinetics but in part two we will see the effect on capacity and also we will see some other ways to make nano crystalline material metal hydride so thank you very much for your attention