0:00:02 Hello and thank you for tuning in. I'm Katie hurst and I'm excited to tell you about the work that
0:00:08 we are doing at the National Renewable Energy Laboratory. I'll start with some brief comments about
0:00:15 L. D. And how we're using it for catalyst development and then talk specifically about two different
0:00:22 applications. The first one in gas storage and the second one in photo electrolysis where we're able
0:00:29 to change the functionality of the catalyst by using L. D.
0:00:37 When we think of the properties that make a good catalyst. These include high activity, the
0:00:42 appropriate reaction selectivity, good stability as well as being inexpensive in terms of the
0:00:50 material, the materials themselves and the process to make the catalyst.
0:00:59 What determines the catalyst characteristics are really the material properties and these material
0:01:05 properties include morphology, uniformity, surface area and chemistry, particle size density and and
0:01:12 scalability. The old growth technique provides very good control over creating films and particles
0:01:21 with these defined properties. And we can change the material properties by changing the temperature,
0:01:29 pressure, precursor chemistry and exposure times as well as the type of reactor.
0:01:38 So L. D. Is a route to synthesize materials with defined properties, which in turn alter the
0:01:45 functionality of the catalyst performance. And this approach is used for many different applications
0:01:52 of materials.
0:01:56 The first example I would like to talk about is in hydrogen storage and as industries economies move
0:02:03 to use hydrogen as a fuel, we need methods and materials for transportation and storage and we are
0:02:10 working to create materials to do this ideally, the materials would have a high capacity. They would
0:02:18 be easily discharged and charged with hydrogen at near ambient temperatures and pressures and also
0:02:27 be inexpensive. Current technologies including high pressure compression and liquefaction, are
0:02:34 expensive and non scalable. The graph shown here on the right shows some of the materials of
0:02:42 interest plotted with volumetric capacity on the left versus gravimetric capacity on the right. The
0:02:50 color code indicates the amount the temperature that's required to do is orb the hydrogen down in
0:02:58 this regime. We have carbon materials that have a low capacity but also require low temperatures to
0:03:05 dissolve the hydrogen for mental hydrates and chemical hydrates. They have a higher capacity.
0:03:11 However, they also require a higher temperature, sometimes up to temperatures of 500°C.. One of the
0:03:22 approaches that we're looking at at a general is to reduce the overall bulk heat and temperature
0:03:28 required to deserve the hydrogen by incorporating plas Monica nanoparticles with metal hydrates. So
0:03:37 a plasma on is a collective oscillation of electrons that is excited by a specific frequency of
0:03:43 light in a metal or semi metal. And this frequency is a function of the particle size and material
0:03:50 composition. The absorption data and the lower right um indicates that the titanium nitride
0:03:58 nanoparticles used in Our work have a plasma at a wavelength of 680 nm. By exposing the titanium
0:04:09 nitrate particles to light. At this frequency or wavelength, we can excite the localized surface
0:04:15 plasma of these particles
0:04:19 when the titanium nitride nanoparticles are mixed with a metal hydride, similar to this image in the
0:04:26 center. In the lower part of the slide, we have found that hydrogen is released from the metal
0:04:34 hydride at the ambient temperatures without directly heating the material. And so this provides a
0:04:40 really exciting route to avoid energy intensive bulk heating of materials that is costly and slow.
0:04:48 So oftentimes people talk about gold or silver plasmas and this is similar to the titanium nitrate
0:04:55 plasma. However, titanium nitrate is a more stable material and can retain its nano structure up up
0:05:03 to much higher temperatures and plus it is lighter and less expensive than gold. So we will get to
0:05:10 the L. D. In a minute. But first I'd like to show you the work with the particles. Okay, so in these
0:05:17 experiments were looking at a mixed physical mixture of titanium nitride with ball milled magnesium
0:05:26 hydrated. So that's just in order to make it smaller particles. And then we excited with a 625
0:05:34 nanometer led and that gives off hydrogen and then the titanium nitride and magnesium is left behind.
0:05:43 This tp this light induced distortion spectra here shows the hydrogen signal comes on instantly as
0:05:52 the light is turned on and then it starts to decay. And then when we agitate it, we're exposing a
0:05:58 new material and that causes these spikes here and then again when we turn the light off, you can
0:06:06 see that the signal goes down. We also did the blank experiments and demonstrated that the hydrogen
0:06:13 was not released by just showing just showing the magnesium hydride and the titanium nitride
0:06:20 separately to the light. So it's a synergistic effect that is that occurs. This is a tm image
0:06:28 showing how the titanium nitride nail particles are dispersed. And you can see they're hearing these
0:06:35 black dots here and there throughout. It's very it's pretty well dispersed. And then these are these
0:06:41 big glob, gray glob is the magnesium hydrate. And now the amount of hydrogen that has is coming off
0:06:51 of this material is less than one weight percent. And so in order to be useful, we need a lot higher
0:06:59 um weight percent coming off. For example, the amount of hydrogen that is on this magnesium hydrate
0:07:05 itself is 7.68%. So we're only getting a small amount off um And in order to increase that, we need
0:07:14 to understand well, what is the real mechanism that's occurring here? And how can we increase the
0:07:19 interaction between the titanium nitride and the magnesium hydrate?
0:07:27 This is where L'd comes in. So a child can be used to help us investigate what the mechanism is. So,
0:07:34 in general, there are two main ways that plasmas can control the reaction first is through plasma
0:07:41 ionic heating. So the photons are absorbed and excite the electrons and then they decay down and
0:07:48 emitting phone ins and this releases. This causes localized heating. Alternatively, the Excited
0:07:57 electrons can be transferred to participate in the reaction. So they can transfer from one from,
0:08:03 from the titanium nitrate into the metal hydrate. Um Now there was a lot of different mechanisms
0:08:11 that actually occur when you have electron transfer, whether it's hot electron transfer or direct or
0:08:17 indirect. But the main idea here is that the excited electron is leaving the titanium nitrate. And
0:08:25 so we're going to use, we used an aluminum oxide coating that's surrounding this titanium, not
0:08:33 titanium nitrate to block any electron transfer. And so we've we've grown four and 8 nm of alumina
0:08:45 onto these, directly onto the titanium nitride particles in order to block that. And then we
0:08:54 measured again the hydrogen signal as a function of time. And when we expose it to the light and you
0:09:01 can see there's a quick turn on again for each of them. And the spectrum looked very, very similar.
0:09:09 And this is showing that um the Ox fied the oxide is really not affecting the capacity or the
0:09:19 kinetics of the hydrogen deserving. So if the electron transfer was affected by the coding, we would
0:09:26 expect to see this peak much smaller or a different shape if there was some kind of kinetic
0:09:33 alteration from this this coding. And so this kind of leads us to believe that the dominating
0:09:39 mechanism for this process is plasma ionic heating secondly, to increase the amount of hydrogen diz
0:09:47 orbed, we want to increase the interaction of the thai nitrate in the middle hydrate. So in order to
0:09:54 do this, we grew an A. An L. D. Film of thai nitrate around the magnesium hydride particles. And
0:10:02 this thin film geometry at the nanoscale can create a similar plasma tonic effect as as to that in
0:10:09 the titanium nitrate nanoparticle um in this configuration, when the time nitride material is
0:10:16 exposed to the light, the plasma sonic excitation will cause the metal hydride to diz orb hydrogen
0:10:23 and then transport through the titanium nitride film. In fact, titanium nitrate films have been
0:10:30 demonstrated to have good hydrogen purification behavior similar to that of palladium membranes for
0:10:37 the L. D. We Used tickle and hydrazine at 200 c. in our Vinick reactor. And we and this reactor
0:10:46 allows for airless transfer. The TM on the upper right shows that we were able to deposit titanium
0:10:54 nitride uniformly on the magnesium hydride particles and the high resolution image shows that the
0:11:01 thickness Was around 19 nm. This material was tested for hydrogen description using the six 25
0:11:10 nmeter led. However, unfortunately there was a lot of water that deserved with the hydrogen and we
0:11:18 believe that there could have been some contamination from the hydrogen from the hydrazine and so we
0:11:23 tried to dry it and then redid the deposition. However, once again, water was still evolved with,
0:11:31 with the hydrogen
0:11:35 analysis of the X ray absorption. Spectroscopy for the nitrogen K edge showed the differences in
0:11:42 nitrogen bonding between the titanium nitride nano materials. The black curve shows the titanium
0:11:48 nitride as um the nanoparticles. While the titanium nitride with water. What the water contaminated
0:11:57 hydrazine is labeled the wet curve in blue. And then our attempt to dry the hydrazine is shown in in
0:12:07 red and we can see that even when after our attempt to dry the hydrazine, the hybridization of the
0:12:14 nitrogen atoms is still different than that of the than commercial grade nano thai nitride particles.
0:12:25 In our future work, we're excited to develop tie nitride films using ammonia instead of hydrazine.
0:12:32 And this was just installed into our lab and there are a number of papers in the literature that
0:12:38 show that um tonight tried L. D. With tickle or td matt paired with ammonia has is able to be grown
0:12:46 and and demonstrated in the literature. Um one tricky thing is we need to keep the deposition
0:12:53 Temperature lower than 200 in order to keep the hydrogen in the middle hydrated. And so that would
0:12:59 be a little bit of a challenge. But in this example of a hydrogen storage material, L. D. Has
0:13:07 demonstrated the change to change the functionality of the catalyst by first blocking the reaction
0:13:13 pathway and then second by increasing the surface interaction of the plasma sonic material and the
0:13:20 metal hydrate.
0:13:24 Okay, so now I'd like to talk about another example where L. D. Films can act to change the mass
0:13:31 transport of species to the catalyst surface. This project is working with the esposito group at
0:13:38 Columbia University as part of a larger project led by Shane ardo at UC Irvine. So this image here
0:13:47 shows how we might think of mass transport of reactions to in an electrochemical reaction such as
0:13:56 the hydrogen evolution reaction or h er might occur during photo electrolysis. So the reactant are
0:14:04 here in this bulk electrolyte. They transport to the diffusion layer through the oxide layer and
0:14:12 interact with the catalyst layer and then the products that need to defuse outward. Oftentimes this,
0:14:19 this oxide layer is used to enhance the stability of the catalyst layer.
0:14:28 Okay, so now think of two chemical reactions occurring in the same electrolyte and we want the
0:14:34 coding to enable the hydrogen evolution reaction but not the second reaction. This is our challenge
0:14:41 now in this project to determine the material properties of this oxide film. To engineer this
0:14:50 functionality
0:14:54 and such a challenge with two reactions in the same electrolyte is a situation for a Z scheme photo
0:15:00 catalytic water spilling reaction. So two particles in the same solution are doing different
0:15:08 reactions. And the key to the transport is the mediator species here, if we focus on the catalyst
0:15:17 particle for the hydrogen evolution reaction, we don't want the occident mediator to be reduced with
0:15:25 this particle
0:15:28 here, we can simplify The two D particle situation by focusing on the HDR reaction using a thin film
0:15:36 with a flat platinum electrode and a T. I. O. To coding. So this flat plate model provides an easier
0:15:45 growth surface to create a continuous T. I. 0. 2 or T. I. O. X. Over layer with uniform thickness,
0:15:52 it provides a well defined electric catalyst surface area, which is easier to characterize and
0:15:57 determine the oxide film properties. And it's also amendable to modeling the 1D transport problems
0:16:06 that may arise. So this is the system that we will use to optimize and understand the optimal
0:16:13 conditions for growing and oxide coating. We have grown these films by L. D. Using tickle and water
0:16:21 for a range of different temperatures. The thicknesses of the film here are measured by a lip
0:16:29 symmetry um using a silicon witness sample. And here are the synthesis details. Here we're using a
0:16:39 custom made reactor and our growth rates seem to be reasonable as a function of temperature. The
0:16:49 density of the film will play a large role in determining the mass transport characteristics of the
0:16:54 film. This graph on the left here shows the X ray reflectivity of The T. 0. 2 films that are grown
0:17:04 as witness samples onto silicon substrates. And this is done with an understanding that the density
0:17:11 may vary somewhat when the films are grown directly onto the platinum electrode surface. We're
0:17:17 currently working on determining the density of these films. Um using X. R. R. So in the graph on
0:17:24 the right, um the red data here shows the density determined by X. R. R. For the different films
0:17:32 that are all Approximately five nm in thickness but grown at different temperatures. And then also
0:17:41 plotted in blue. Our values from the literature, from Percy at all, who also use tickle and water.
0:17:48 And you can see that there is a good agreement and the overlapping temperatures. And this provides a
0:17:54 range of synthesis temperatures where we can control the density of these films. And then by
0:18:02 additionally changing the thickness of these films, it can add more variable, more variation to
0:18:10 create an even larger range of densities for the oxide films in order to affect the transport of the
0:18:16 reacting species. Next, we want to test the performance of our coatings and it's important to
0:18:23 determine that the coding has not eliminated the transport to the electrode and the coated electrode
0:18:30 is still active for hydrogen the hydrogen evolution reaction. And I would like to acknowledge that
0:18:37 all of the electrochemical characterization showing this talk was done by robert Stinson at the at
0:18:43 Columbia University. On the left here, we see the current density versus potential for different
0:18:51 oxide thickness is um and we have the bare electrode and then various various thicknesses. And
0:18:58 here's the HDR region. You see that The bearer platinum electrode as well as the thinnest layer at
0:19:06 one five nm has the highest activity. However, when we skate, when we expand that this same data
0:19:13 here on an expanded on the right, um you can see that it it still is active and we do have current
0:19:21 density and this current density actually scales with the thickness of the oxide film. So now we've
0:19:30 established that the buried platinum interfaces electro chemically active. We can now include the
0:19:37 mediator species to the electrolyte to see if the films are selective
0:19:44 In these next experiments, iron is added to the electrolyte and there is a desired hydrogen
0:19:50 evolution reaction and then an undesired iron three plus reduction that can occur and we compare it
0:19:59 to the bear platinum electrode versus the coded the T. I. 02 coated platinum electrode. This shows
0:20:09 the current density versus the potential. And we can see here this is the Bear platinum electrode as
0:20:15 well as the thinnest oxide coating. And we can see in this regime this is evident of The Iron three
0:20:23 reduction, but for the thicker oxide coatings, There is no iron three reduction on the right side.
0:20:34 Here is an expanded view of the data showing this, this large difference in the behavior of
0:20:42 different codings. And this is actually quite remarkable when one considers that the undesired
0:20:49 reaction has a large thermodynamic advantage over the desired hydrogen evolution reaction
0:20:59 for negative potentials where both the H. R. And the Iron three reduction are occurring. We can use
0:21:06 these linear sweep altimetry curves measured to estimate the reaction selectivity of these
0:21:13 electrodes toward the desired er for each thickness of the oxide coating, the bare electrode here
0:21:21 shown in purple along with the thinnest 15 nanometer coating shows these selectivity here. Yet For
0:21:29 films greater than two nm, the selectivity is much greater and it's estimated to have a three and
0:21:38 then a nine times increase compared to the bear platinum electra. Um And so we are still working to
0:21:46 correlate the material properties such as density or porosity. To understand how the transport
0:21:52 properties of these films can be optimized and eventually the sort will be applied to particles. For
0:21:59 this Z. Scheme PC application we will grow our oxide films directly onto the catalyst particles. So
0:22:08 this coding work is really part of emerged larger effort led by Shane ardo with many great
0:22:13 collaborators that involves a catalyst development measuring electron charge transfer and particles
0:22:20 and reactor modeling In summary um L. D. Is a well suited growth technique to control thin film um
0:22:30 and material properties that in turn determine the catalyst performance. In the gas storage example,
0:22:37 an l dioxide coding was used to uniformly block possible electron transfer from a plasma sonic
0:22:43 material. And L. D. Was used to increase the interaction of the catalyst with the metal hydrate and
0:22:51 then finally through good control of material properties that L. D. Offers specifically the density
0:22:58 and the film thickness. The selectivity of reactions can be controlled.
0:23:05 And finally, I would like to acknowledge my colleagues who contributed to this work from an real and
0:23:13 also from Columbia University, as well as my collaborators in both High Mark and hydrogen, as well
0:23:21 as as funding from E. R. E. So thank you for listening to my talk.