0:00:01   Hello. My name is Noemi  Leick and I work at the National renewable energy laboratory today. I will
0:00:09   be talking to you about A. L. D. For material based hydrogen storage and we'll use magnesium boron
0:00:16   hydride as a case study.
0:00:20   This cartoon depicts the hydrogen economy and how it ties in with energy generation and demand. And
0:00:28   you can see from main applications such as transportation, agriculture industry and buildings that
0:00:37   energy storage will play a critical role in deploying hydrogen from intermittent renewable energy
0:00:43   sources. The hydrogen material advanced Research consortium, also called High Mark is focusing on
0:00:54   hydrogen storage for light and heavy duty vehicles but recently also for another class of materials
0:01:02   called carriers. And an example of that is for example to Halloween in methyl cyclo hacks ain. And
0:01:11   the idea of those is to be able to transport excess renewable energy created for example in a place
0:01:20   where there's a lot of solar to um another place where there is not that much renewable energy
0:01:28   sources um that is possible. So transporting at this energy in the form of hydrogen is very
0:01:36   appealing and those forms can also be used in long term storage applications. As we saw earlier in
0:01:43   the cartoon. When we talk about hydrogen storage technologies, we focus mainly On two types physical
0:01:53   based and material based. The physical hydrogen storage technology that is currently already applied
0:02:02   in um light duty vehicles is The 700 bar compressed gas technologies, Which has about 40 g per liter
0:02:12   of volumetric capacity. And from the Spider chart, you can see that this technology reaches almost
0:02:21   all of the D. O. E. Targets. However, we see that for the low volumetric density and the high system
0:02:29   costs, there is a true um potential for material.
0:02:36   While there are all kinds of different material used to store hydrogen, we're going to divide those
0:02:43   into two classes. The first have weak interactions with hydrogen. Here's an example of moth five and
0:02:53   high mark um published on a nickel based moth that has the world record Room temperature capacity of
0:03:03   11 g per lir. In this particular talk, we will focus on the second class of material which is
0:03:12   chemise or hydrogen to the material. So it has a strong interaction and one of this example are
0:03:20   complex hydride with sodium aluminium hydride, but we will focus on magnesium boron hydride powders.
0:03:29   For this talk that is porous and nano structure to increase the hydrogen diffusion in and out of the
0:03:36   material as well as increase the reaction rates.
0:03:42   Magnesium world hydrated has been a little bit the rock and roll star of the last five to 10 years
0:03:50   and that's because of its high hydrogen capacity that completely exceeds the GOP targets, The
0:03:56   volumetric capacity is 82 g per liter. In gravimetric is about almost 15 weight percent. Um to
0:04:05   remind you the deal we target is 40 g per liter. The idea on how this would work is um to cycle
0:04:15   magnesium borough hydrate from its hydrogenated form to magnesium bow ride and then re hydrogenated
0:04:23   this back to magnesium boron hydride by inserting hydrogen to the system. It's d Hydrogenation
0:04:31   process occurs around 300°C,, which is too high for Dont targets. The other thing is along its d
0:04:40   hydrogenation pathway and not only releases hydrogen but also some um die Brain B two H 6 which is
0:04:50   not only toxic to people but also to the fuel cell. The other thing that is detrimental is the
0:04:57   formation of B 12 H 12 that is so stable that it's viewed as a thermodynamic energy Well, um when
0:05:09   you look at the absorption side of things, 950, bar 400°C and 108 hours or energetically absolutely
0:05:20   unfeasible for applications, so there is a lot of room for improvement of the system. The approach
0:05:27   we are taking to address these challenges in this project is to use atomic layer deposition in order
0:05:35   to retain the nano structure for cycle ability um while not decreasing the gravimetric capacity of
0:05:42   magnesium boron hydride too much. But we also hope that the encapsulation will help us manipulate
0:05:49   the thermodynamic pathway of the hydrogen release, as well as mitigate material loss. Like we just
0:05:55   talked about in the form of boron depending on the material that we choose to deposit. Um If it's a
0:06:03   catalyst, we might also be able to enhance the reaction rates. In this particular talk, we're going
0:06:09   to focus in on aluminum oxide, L. D. And the process that we used was room temperature L. D. And we
0:06:18   increase the precursor exposure times to eat seconds in order to um account for the um porosity and
0:06:27   high surface area of magnesium boron hydride.
0:06:33   We're gonna talk about three things. The first is how aluminum oxide lt. So the use of both try
0:06:43   metal, aluminum and water alter the performance of magnesium boron hydride. And then we'll focus in
0:06:52   on what water alone does and what tm a alone does. We've actually screened a lot more materials than
0:07:01   just aluminum oxide here. You can see the hydrogen signal released during the heating of the
0:07:06   material. After the LT process
0:07:12   we tried syria um titania mixtures there of, we incorporated also catalyst metals like platinum and
0:07:21   palladium and you can see that aluminum oxide still has the highest low temperature hydrogen release
0:07:32   on this side. You can see the signals for just palladium and routinely um compared to the blue curve,
0:07:39   which is the uncoated and except for aluminum oxide, they don't perform much better than that.
0:07:50   One thing that will be important to highlight is aluminum oxide was the only L. D. Process that we
0:07:56   performed at room temperature. All the other ones necessitated higher temperatures than that. And
0:08:01   we'll come back to that in a little bit.
0:08:06   I'm going to start by showing you a thickness series that we did on aluminum oxide. This is the
0:08:13   temperature program description. So the hydrogen signal as a function of heating of the sample after
0:08:19   A. L. D. The encoding material um is shown in blue and we increase the number of cycles up to the
0:08:30   red signal, which is 100 cycles A. L. D. When we quantify the hydrogen signal from that, we could
0:08:38   see a big improvement from 0.6 week percent to 1.6% in the um 200 degrees Celsius range. So that's
0:08:48   very promising. Another very encouraging step was to see the dye brian signal. So this time it's di
0:08:56   brain from the uncoated material. We could see that it went down to pretty much zero when it was
0:09:03   coated with 100 cycles aluminum oxide. So all of those things were extremely encouraging. And we
0:09:10   couldn't wait to do even more cycles with that hydrogen cycles. So we did Pressure composition
0:09:19   temperature measurement with a 100 cycles L. D. Coded material. And here I am showing you deception
0:09:27   kinetics for the uncoated versus the coating materials. Where we could increase the kinetics by a
0:09:35   factor of around threat. And down here is the absorption kinetic plot where um u unfortunately you
0:09:44   can't see it that well because I'm on this. But the green plot is again the coded material and the
0:09:53   kinetics are not that much better in this case of the hydrogen absorption step. So now there are
0:10:01   more cycles. And this plot is getting a little bit messy. But on Top here, you can see the uncoated
0:10:08   materials and you can see three um D hydrogenation re hydrogenation cycles for this material and the
0:10:17   same for the coating material in green at the bottom here. And except for the first day
0:10:24   hydrogenation um cycle, the encoded encoding material performed very similarly. So that is a little
0:10:36   disappointed.
0:10:39   So the first description steps performs better with aluminum oxide. Not only from a point of view of
0:10:46   quantity of hydrogen diz orbed, but also with the kinetics. However, absorption of hydrogen still
0:10:54   remains challenging with or without aluminum oxide. LD
0:11:01   moving away a little bit from the performance of the sample. We started looking into what is
0:11:08   actually occurring with the sample um during the L. D. Process itself here you can see a porous
0:11:16   cemetery plot of the thickness series performed and um the very narrowly defined poor of eight
0:11:25   Angstrom and the need material kind of goes away as you increase the number of cycles. We can't
0:11:31   really speak of airport size distribution anymore as we increase the number of cycles. And as for
0:11:37   the specific surface area, it also decreases as a function of cycles. So that still doesn't really
0:11:44   tell us whether or not we have a coding or if it's mainly infiltration. So we did some small angle X
0:11:53   ray spectroscopy on the coding materials and focusing in on this region here, Which is the sensitive
0:12:01   one to the around 10 inches from poor region. We can see two things. Not only that this shoulder is
0:12:10   increasing in intensity but there's also a shift occurring toward lower cure numbers and all of that
0:12:18   is suggesting that um T. M. A. And water are infiltrating into the pores. And in a way that even at
0:12:28   100 cycles A. L. D. Um the pores are accommodating TME and water coming in and there is some kind of
0:12:38   swelling mechanism that is occurring. So this picture is trying to depict this in an exaggerated way.
0:12:47   So if this is magnesium bore hydrate that is encoded whether it's poor structure inside. As we look
0:12:55   at 100 cycles aluminum oxide particle, we would have some kind of coating but also some pores that
0:13:03   are completely um trapped. And we've had some swelling because even at 100 cycles um team and water
0:13:12   are still able to get into the material. So that was very interesting to us and has some um
0:13:21   consequences as to how to design um L. D. Processes. Another finding came from differential scanning
0:13:33   calorie symmetry of the neat material that exhibits to very characteristic and a thermic phase
0:13:41   transitions with comparing this with the aluminum coded sample. Which didn't really clearly show any
0:13:50   of those to enter therms and had a little bit of an extra thermic um bump over here and coupled that
0:13:58   with X ray diffraction that showed it's a loss of crystalline itty as a function of temperature. We
0:14:06   started wondering if maybe there is some in situ hydrologists reaction that is occurring as we heat
0:14:12   the samples the coded samples. So I'm now going to start um diving into the hydraulic sis reactions
0:14:22   that could be occurring. We are suggesting that there are two different steps of hydraulics
0:14:28   reactions, one occurring in A. L. D. And um another one that would release water and hydrogen during
0:14:38   the heating at the samples. So we're kind of done right now talking about the dual process using T.
0:14:47   M. A. And water. And we're really going to focus in on hydra Alice is now this data is a little bit
0:14:54   um overwhelming. So I'm going to try to walk you through as slowly as I can. This is um still mass
0:15:04   normalized mass spectrometer signals as we heat the samples after it's been exposed to Ditto and T.
0:15:15   M. A. N. D. 20 And we did different numbers of cycles. So here you see three cycles versus 100
0:15:22   cycles. And then here 10 cycles of T M. A. N. D. 20 versus 100 cycles TME and decorated water. So
0:15:33   look at the intensity as well. Um they're not all at the same scale. One thing that we see is that
0:15:42   the signal for HD is much higher after 100 cycles of D. Two exposure compared to the three cycles
0:15:51   due to exposure that is lowered when we intercollegiate T. M. A. In between the water pulses. So in
0:16:01   a more traditional aluminum oxide process, you can see that we still have um measurable amounts of
0:16:09   HD. The hydrogen coming from the sample and the deuterium coming from the deuterium exposure. Either
0:16:16   in the water pulse or only detail. So that already shows us already right here that the water paul's
0:16:27   during L. D. Is not self limiting. When we use magnesium boron hydride as it seems. When we now take
0:16:35   a look at the signals from water and die brain released during the heating of the same sample, we
0:16:43   can see that um a lot of die Broin is released With low numbers of cycles, three cycles of T. two
0:16:54   and 10 cycles of TM. And deuterium here. And when we increase those numbers of cycles, we can
0:17:00   measure a lot of water also in the Form of D. 2 0. itself. Which clearly means that hydroxyl roots
0:17:08   are left behind on the surface after L. D. And also we can see that that during these high numbers
0:17:17   of cycles almost no diaper rain is formed when we heat up the sample. So taking all of those
0:17:27   findings together and looking at hydrologists reactions that that can occur. We are suggesting that
0:17:36   during A. L. D. We already have one of the hydraulic Asus reactions that occurs which um comes along
0:17:44   with the formation of hydroxyl groups. Die Brain meaning die brain is already released during the
0:17:51   field. Which means that with the increasing number of cycles of L. D. We lose die brian. Which is
0:18:00   probably why when we heat them after, we barely see any dye brian left. And we also lose some
0:18:08   hydrogen during this L. D. Process. So this is during a L. D. At room temperature when we now go
0:18:17   over after L. D. So when we do temperature program description or any kind of other heating which is
0:18:23   necessary to D hydrogen eight our material. Um we think that this um water which comes from the
0:18:32   combination of these hydroxyl groups, is taking place this reaction we are forming boring species
0:18:40   which kind of kills are usable species. So we have material laws occur. We are releasing water which
0:18:50   is consistent with the data and we're releasing also way more hydrogen and in this reaction. So
0:19:00   unfortunately the discouraging results that we were seeing as we were cycling hydrogen in and out of
0:19:08   the material probably comes from These two reactions here.
0:19:16   And so that also explains the fact that when we did the other processes with other metal oxides that
0:19:24   required higher temperatures than room temperature. And the use of water, we actually, it looks like
0:19:32   there is less hydrogen because we lost probably way more hydrogen um during the L. D. Cycles.
0:19:39   Because in these cases we have both hydrologists reactions that are occurring during the L. D.
0:19:48   So the fact that water was involved and that we were doing L. D. At um higher temperatures is
0:19:57   explaining why we didn't see as good of a hydrogen gee hydrogenation performance in the other
0:20:05   materials. We looked at.
0:20:11   Now we're gonna be looking at T. M. A. Pulsing. Only if water is doing something maybe T. M. A. Is
0:20:20   also already reacting with magnesium for hydrated.
0:20:25   Looking at 100 cycles of T. M. A. Or I should say 100 times pulsing T. M. A. N. Magnesium boron
0:20:33   hydride. We um did TPD and DSC which you can see here at identical rates.
0:20:43   And this event a That occurs around 90°C actually corresponds to the structural phase
0:20:50   transformation of the gamma phase to the epsilon phase. But this release is very rapid, which is
0:21:01   interesting. So, already event A is telling us that T. M. A. Is reacting with magnesium boron
0:21:08   hydride. And a second indication of that is of N. C. Which corresponds to Magnesium hydride to
0:21:17   magnesium transition, which usually occurs 120 five degrees higher than what we observed here.
0:21:26   While the overall hydrogen capacity was low about 2.5 hydrogen weight percent. The rapid release was
0:21:34   really intriguing to us and we decided to test it with less TME exposure and see if we could retain
0:21:42   that.
0:21:47   So here you see uh the T. P. D. Of uncoated magnesium borough hide right here in black. Then the 10
0:21:56   cycles T. M. A. In red. And then the data I just previously showed you is now in blue. So this is
0:22:03   the um rapid release Occurring around 100°C.. And um this blue curve really has almost no
0:22:15   similarity with the uncoated material in the black curve. And that was also the case with the
0:22:23   structural information we got from X. R. D. However, the 10 cycle Tm a simple shows a lot of
0:22:32   similarities. The shape is the same and it actually decreased the onset Description temperature by
0:22:39   around 100%, 100 degrees Celsius as well as retaining its um Full hydrogen capacity. If you look up
0:22:51   to 500°C,, it still was able to deliver around 14 weight percent. And this difference here is
0:22:58   inherent to the air bars we get from TPD. So that is very encouraging. Very low numbers of pulsing
0:23:08   of T. M. A. Was able to trigger that kind of structural changes. However, it retained its um native
0:23:17   capacity and most of its structural um characteristics. This information, along with NMR and
0:23:29   infrared spectroscopy, helped us see that there is no reaction with boron and almost no
0:23:35   incorporation of aluminum containing species. Which means that um The reaction is mainly a ligand
0:23:44   exchange reaction between the b. h44 and the method group of T. M. A, which is in line with their
0:23:51   very similar venables, radi I. So overall the complete reaction that we are suggesting which is a
0:23:59   ligand exchange reaction is that the method groups go over to the magnesium and the bh we'll go over
0:24:06   to the aluminum, a traditional ligand exchange reaction. And um so basically we usually see that in
0:24:15   a traditional L. D. Process between the two precursors itself, it looks like in this particular case
0:24:24   when we use a very reactive substrate, the substrate is taking on the role as of the second
0:24:32   precursor. So now we see like an exchange reaction Just between one precursor and the substrate. And
0:24:41   we think that um as we increase these numbers of cycles, well we are close to completion of this um
0:24:49   reaction by the time we reached 100 cycles or 100 pulses. However using only 10 of those cycles, we
0:25:01   are probably only partially reacting, meaning that the magnesium still has one Bh four at least and
0:25:09   S. C. H. Three. And if we think about our infiltration model um the which could explain why there Is
0:25:17   such a drastic change in temperature between the encoded and then only 10 pulses of T. M. A. is that
0:25:25   the team they can infiltrate the entire um structure of magnesium bora hydrates. So it's a very
0:25:32   surface modification And most of the magnesium for hydra it still retains its to be H4 functional
0:25:43   groups. So all of this to say that the team a half cycle is also not self limiting,
0:25:54   wrapping up all of the data presentation. We conclude that none of the precursor pulses were self
0:26:02   limiting in the case of tri metal, aluminum and water on magnesium boron hydride. So if we transfer
0:26:12   that to more reactive substrates we can say that L. G. Does not retain its unique attributes and
0:26:20   actually becomes just in a more general term a pulsed fiber face technique which still offers a lot
0:26:29   of um promising properties.
0:26:34   So transferring what we just showed to more complex hydrates in general we recommend to use a short
0:26:42   exposures to pre closures, either in short exposure times or low number of cycles. Um one exposure
0:26:52   to one precursor can be sufficient and triggering what kind of reaction you want. You can take into
0:26:59   account. The van evolves radi i of the functional groups of your substrate and of your precursor.
0:27:07   And by matching the pore sizes of your material with the size of your leggings, you can control if
0:27:15   you want a more infiltrated or coding um dominated growth mechanism of your phone and last. We need
0:27:27   more chemistries that at the same time they're room temperature and avoid the use of water and
0:27:32   oxygen and other oxygen precursors. So I'll definitely be on the lookout for some of those more
0:27:38   exquisite chemistries. With that. I would like to thank the hydrogen storage group at enroll the
0:27:47   Department of Energy for funding high Mark and among others this research. And finally, I'd like to
0:27:53   thank you for your attention