Harnessing Chemical Imperfections to Drive Sustainable Energy Technologies

good evening everyone thank you so much for uh for joining us this evening uh my name is uh is kenicia scholdra and i am the senior development officer with the advancement team with the faculty of science uh and on behalf of our advancement team i am i am thrilled to be the one who gets to uh to welcome you all this evening thank you so much for uh for taking the time out of your evening to uh to join us tonight uh we are going to get things started here shortly we are extremely excited to have uh professor rodney smith from the department of chemistry joining us um he is going to make the presentation but we actually have another special guest uh who will be hosting this evening the chair of chemistry bill power who i am going to uh to turn things over here shortly to uh but before i do um we'll give people a couple more minutes just to uh to join us and to uh to get logged in um perhaps we can uh can kick things off with uh with you know throw throw in the uh the chat where uh where you're joining us from tonight we'd love to uh to hear where everybody is from if you'd like to uh to let us know if you're brave enough to let us know your your graduation year uh graduating class that would be awesome as well perhaps we can reunite some long-lost classmates this evening as well so feel free to uh to use the chat to uh to let us know like i said where you're joining us from i know we've got uh we've got a big crowd tonight um alumni from across the uh the globe we've got people from mexico malaysia the us so like i said let us know where you're joining us from and if you live near campus we would love to know how the geese are doing so i know that there's probably a lot of people who are missing them i'm not one of those people but there's uh there's probably a lot who are i know it's nesting season here that uh that's upon us so let us know are the geese taking over the place if you're uh if you're in waterloo and uh and near campus let us know um a couple of quick little outside uh housekeeping items i should say to uh to go over um you uh you are on mute all of our participants are on mute um and you will not have access to video however our speakers and uh and our advancement team love seeing your questions uh we love seeing your comments so make sure to uh to engage in the uh in the chat uh there's also a uh a question and answer uh box at the bottom of your screen um so please feel free to uh to enter any questions throughout the presentation um we will have a q a session at the at the end of the uh the presentation we also recommend speaker view for uh for viewing the presentation uh that way the presentation is the focal point front and center and you can find that just at the top of your uh your screen so if you go to the view mode and change it over to uh to speaker mode like i said that's what we uh we recommend um the presentation is being recorded and will be made available to uh to those who registered for the event in the coming days um and like i said please uh to use that chat and to uh to ask any questions that you feel feel fit uh throughout the presentation and we'll make sure that the professor rodney smith gets to them at the end of the presentation um so we are at uh at 704 here so i think what i will uh will do is i will pass things over to the chair of chemistry bill power thanks canisia and uh good evening to everyone i want to start tonight with a a statement about the where the university of waterloo lies the university of waterloo acknowledges that much of our work takes place on the traditional territory of the neutral anishinaabeg our main campus is situated on the haldeman tract the land promised to the six nations that include six miles on each side of the grand river our active work toward reconciliation takes place across our campuses through research learning teaching and community building and is centralized within our indigenous initiatives office my name is bill power as kinesia has said i am the chair of chemistry and it's my great pleasure to be here tonight with all of you and also to introduce one of my young colleagues within the department this last year has been a challenging year for all of us at the university of waterloo and indeed around the world but i wanted to assure you that in the department of chemistry our mission continues we have been working hard to continue our academic programs we've continued to teach people both in person and remotely throughout these challenging few months and our research continues our labs continue to turn out great new discoveries and our people within the department of chemistry continue to be recognized a couple of notable achievements over the past year that for uh some of the alumni that may know them rick marta a lecturer in materials nanoscience in the department of chemistry and the department of physics and astronomy received a distinguished teacher award about this time last year we've had two of our other colleagues jean duamel a polymer chemist received the macromolecular science and engineering award and an organic chemist dr graham murphy received the the keith uh ladnow award from the both of them from the canadian society for chemistry but i'm most excited about our speaker tonight uh professor rodney smith joined us in 2017 and has had an immediate impact as an innovative new researcher within our department he just learned earlier this year that he received the early researcher award from the province of ontario a very well-deserved recognition and you'll see that for yourselves momentarily when rodney gives his talk looking at the influence of defects on electrocatalytic materials so without further ado i'll turn it over to rodney rod the floor is yours thank you for the introduction bill so let me just take one second to situate my screen properly here all right hello ladies and gentlemen thank you for tuning in tonight i would like to demonstrate to you tonight exactly how my research group combines chemical synthesis structural characterization and correlational analysis to learn about the specific influence that chemical imperfections also known as defects have on electron transfer reactions on solid-state materials the images on this slide capture our strategy we learn to synthesize a series of materials with systematically varied types and concentrations of defects and then we use those series of materials to extract chemical insights into the behavior of these materials through structural property correlations we then attempt to apply those insights in the design of a next generation of materials so around the world societies are making the decision to transition towards more sustainable options for energy generation and distribution we're seeing this in the improvement and proliferation of infrastructure for harvesting energy from sources such as wind and solar we are unfortunately also feeling this as the growing pains and so this infrastructure yields electricity which is a form of kinetic energy this means that the energy must either be used on the spot or stored in some form of potential energy in the realm of hydroelectricity which everybody's probably more familiar with this is done by building a dam we essentially store the potential energy in gravity and we release that energy on will by opening the tap so to speak and so we allow the water to run through and we harvest the energy that's stored in it we cannot do this with wind and solar however and there's no infrastructure that's currently capable of storing all of the excess energy that we are generating today in ontario alone it has been estimated that we lose over a billion dollars worth of electricity every single year due to curtailment and so this is just a dumping of excess electricity that we can't do anything with on the spot if we wish to scale towards 100 sustainable energy and away from fossil fuels the magnitude of this problem clearly grows this is not a problem with energy harvesting infrastructure it is a problem with energy storage technologies and so there's a desperate need to design build and deploy a whole suite of technologies that are capable of converting electricity into some form of potential energy as a chemist i clearly prefer that the energy be stored in chemical bonds and so we can do this directly using electricity via electrochemistry where we split an overall reaction into what we refer to as two half reactions one half of this reaction involves the removal of electrons and protons from one chemical species had an electrode which we refer to as an anode the other half reaction uses the electrons and protons in the secondary reaction at the cathode this overall reaction is fully reversible we can invest all of our kinetic energy from the electricity to synthesize a fuel we can then subsequently harvest that energy by simply running this reaction in reverse what would be referred to as a fuel cell once such a sustainable fuel cycle one such sustainable fuel cycle which we colloquially refer to as water splitting as shown here where protons and electrons are removed from water at our anode and then recombined with each other at the electrode to generate hydrogen gas and so the overall products of this reaction are then hydrogen and oxygen gases notice that the protons in this slide are highlighted in the half reactions but they become invisible in the overall reaction the electrochemical synthesis of fuels is critically dependent on the transfer of protons and electrons the specific fuel that we choose can be tailored but really water is the only source of protons and electrons that can capably scale to a global level and so i focus most much of my attention on this individual half reaction because it is really the linchpin of the entire concept of sustainable fuel cycles via electrochemistry it is important to understand that electrochemical fuel cycles have been with us ever since the inception of electrochemistry and so shinban first published the world's first fuel cell way back in 1839 it ultimately lost out to the internal combustion engine but the dream of using water splitting to power society has remained so there's a few examples highlighted here from popular literature and so jules verne in 1874 and john haldane in 1923 there was a bit of a revitalization in the 70s moving towards modern times and so in 1970 john bachrus down in texas drew attention uh by coining the term the hydrogen economy and so it takes this concept and says that we can power the entire world using hydrogen as our fuel this concept has been kind of batted around over the years and george ola uh subsequently modified this idea to the methanol economy uh where it would be based on co2 recycling and so we could take co2 convert it into methanol and then use that as a reversible cycle they actually developed the plant in iceland where they use the geothermal energy to actually drive that specific reaction modern research has a substantial focus on a range of different cycles very common these days are carbon dioxide and nitrogen but i'll draw your attention again that water is always involved and so we see down in the bottom lower half with the different cycles that there's always water and oxygen involved in any of these cycles now one cannot hope to improve technology without first understanding what has been previously attempted so this slide tries to put some of the historic perspective in so way back in 1972 sergio trussadi published the world's first volcano plot for the hydrogen evolution reaction shown in this bottom left corner um and here essentially what we're showing is the sabote principle um in action and so changing the comp composition of the electrode alters the metal to hydrogen bond strength so this is one of our reaction intermediates and the reaction rate can be tuned by optimizing this thermodynamic value in essence we the bond can either be too strong or too weak there's a sweet spot where we get optimal reaction rates for hydrogen evolution platinum electrodes end up being at the peak of this volcano in 1983 this particular publication in the top left from john baucus provided a rigorous analysis of reaction mechanisms for the oxygen evolution reaction where we're oxidizing water to oxygen using a series of solids in a perovskite crystal structure i'll highlight that those based on nickel yielded the best performance in 1985 yoshiohori was the first to show that metal electrodes that were being used for electrochemical reduction of co2 to what would be used as fuels in aqueous conditions could be very clearly classified and grouped based on the dominant product that forms and so the options are hydrogen formic acid carbon monoxide or hydrocarbons hydrogen of course being that it's better at hydrogen evolution than co2 reduction copper in this case was identified as the only metal capable of yielding any appreciable amount of hydrocarbons if we fast forward 35 to 40 years we see that the state-of-the-art catalysts are all generally the same we still use platinum for her we still use nickel-based catalysts for oxygen evolution and we still use copper for co2 reduction that's not to say that we haven't made any advances in this field because we have rather the identification of fundamentally new catalysts the advances have been in establishing a detailed understanding of materials in the reactivity so that we can better synthesize and deploy these types of materials some of my personal favorite examples are highlighted on the right side of the slide and so in 2011 shown in the top center there yangshuo horn down at mit showed a modern volcano plot for water oxidation catalysts where she based on an experimentally measured value specifically the occupancy of molecular orbitals and she looked at these eg anti-bonding orbitals and showed that we could generate the same sort of descriptor as what sergio trisadi had originally shown in 2012 this particular article in the top right published by jens norskov and collaborators does provides a beautiful illustration of how the savage a principle uh can be related and their related concepts of scaling relations uh can be applied to much more complex reactions and so sergio trisani showed this for the simple two-step hydrogen evolution reaction jens norskov and all of his collaborators are trying to expand this outwards into multi-step reactions such as co2 reduction and so this is a framework that has been developed and continues to be developed over many many years in the bottom right corner we see an example from 2014 originally uh through matt cannon's group down at stanford uh where he shows that copper electrodes can be significantly better for co2 reduction if you prepare them from copper oxides and so instead of starting from the metal phase you start from a copper oxide and convert it to the metal phase the subsequent work on this has highlighted the underlying reasons for this the microstructure and the grain boundaries and so the grain boundary is a type of defect and the identity of those grain boundaries turns out to be very critical in the products and the performance of those catalysts each of these three examples arose from the resulted in an explosion of interest in each of the respective fields and the key to each of these successful papers was the pursuit of an idea through systematic experimentation using really state-of-the-art scientific infrastructure to study defects and materials so my team at the university of waterloo is using such a systematic approach to try to advance fundamental understanding and overall performance of solid-state electrocatalysts and so our approach is to study heterogeneous electrocatal catalysts by combining synthesis with advanced structural characterization and behavior and performance monitoring through a range of spectroscopies and so our research intentionally spans the continuum from amorphous solids to solids to very well crystallized solids because defects are notoriously difficult and expensive to identify and quantify and so we attempt to facilitate this process by developing the ability to synthesize a series of materials that vary in the concentration of a single type of defect and so if we can systematically vary the degree of disorder within the solids in this way we can equip ourselves to apply structure property correlation analysis to determine the specific influence that any individual defect has on material behavior and so properties of interest are generally measured as i mentioned through spectroscopy and electrochemistry and so in terms of electrochemistry in the center here i show an example of a taffle plot these are very common in electrochemistry and they simply relate the electrical current flow or the reaction rate to the voltage applied or the energy invested into driving that reaction and so what we expect is a semi-log relationship with a logarithm of current density is linearly related uh to the current or the voltage that we applied to the electrode and so i would like to highlight here that uh although it's presented linearly our correlation process in in the lab is not at all linear and so we sometimes get very lucky and manage to synthesize intended materials with seemingly very little effort more often however the students work very difficult i work very hard and rigorously in the lab to apply it kind of a two-way cyclic process and so we synthesize series of materials characterize them and analyze them for correlations we take what we've learned and we work backwards using those correlations to redesign the synthetic approach and again apply this correlation approach again and again until we have uh confidence that we're really controlling these materials and that these trends that we're observing are consistent and meaningful so as a first demonstration of our process i would like to discuss amorphous metal hydroxides as electrocatalysts for the oxygen evolution reaction and so i'll take using several examples here to highlight how correlation analysis was used to apply it was applied to multiple types of reaction sites in materials based on iron and cobalt as illustrated on the left and so we identify unique reactivity based on different coordination environments i'll then show how substantially different the mechanism is when we switch over to an iron nickel catalyst and so the example provided on the right shows how we're applying this systematically across different compositions as stated earlier state of the art infrastructure is really required to carry out much of this research and so the portion that we'll cover first here relies extensively on x-ray absorption spectroscopy which is typically carried out at synchrotron facilities such as the canadian light source in saskatoon and so the top left here shows the sample x-ray absorption spectrum that was acquired in fluorescence mode on nickel hydroxide and so for those not familiar with the technique in essence what we're doing is directing a beam of x-rays onto the sample we can scan the energy of these incident x-rays while we monitor the intensity of new x-rays with a different energy that are emitted from the material this fluorescence increases when the energy of the instant x-rays is sufficient to excite electrons within the sample and so the spectrum here is specific to the nickel k edge which means that we're selectively exciting the 1s core electrons from nickel ions within the material the information we extract from it is therefore element specific which makes it very powerful we can probe the local bonding environment exclusively for the nickel ions within the material or any other ion that we choose to focus our energy on and so the x-ray absorption spectrum can be broken into two distinct regions the first oh jumped ahead too much the first is shown on the bottom here which is the referred to as the near edge region or zanes this region contains information about both the electronic and bonding structure for the element of interest for today's purposes we're simply going to highlight that the location of the edge is linearly correlated to oxidation state of the element and so the edge moving to higher energies means that the oxidation state of the element of interest is increasing which of course when we're doing water oxidation we should see it being oxidized and so we should see a nickel 2 being oxidized to a nickel 3 or 4 state the second region is referred to as the extended fine structure region and we refer to it as exafs shown up at the top and so what we're looking at here is really a series of squiggles that arise due to quantum interference in the materials so the incident x-ray ejects an electron from one of the ions as shown in this image in the center now there's a probability that this electron will hit the electron density of one of the neighboring atoms and reflect back towards the initial atom there is then another probability that this electron will end up back exactly where it started if this happens we develop an interference pattern where the probability of that initial absorption process is actually dependent on the energy of the incident photoelectron and so the key outcome of this is that each individual coordination shell highlighted by these circles surrounding that dark blue species in the middle will appear as an individual sine wave in this data and so the x apps equation shown at the top provides access uh to some very useful structural properties and so the key ones are the three shown on the right uh the n the sigma and the r and so what we're looking at is coordination number how many species are held at a given distance the sigma factor is the variance in bond length which represents how much disorder there is in that distance and the r is the interatomic distance or the bond length and so we use each of these parameters as our structural properties for correlational analysis and so what we do is take this these squiggles shown in the top left corner extract them and do some light processing to generate this interference pattern shown by the blue curve human brains aren't equipped to readily process data presented in a frequency domain like this and so it's common to apply a fourier transform and convert the frequency into distance each of the peaks in the lower fourier transform version of it represent a coordination shell and so the location of each of those peaks is related to bond distance and the intensity is related to the number of species in that shell as well as the degree of disorder inside of it each different colored curve refers to a different perturbation to the system we're doing electrochemistry the perturbations are the application of a voltage and so what we do is watch the change in these coordination environments as a function of voltage so some sample data is shown here what we did was acquire all of this data for 10 different amorphous cobalt iron hydroxide samples where the ratio of the metals was buried we mathematically fitted each of these with as many coordination shells were necessary to develop a valid structural model across this sample series five coordination shells are necessary two being cobalt to oxygen distances and three being cobalt to metal distances uh the distance for the seventy-five percent cobalt sample are summarized in the table on the left uh to give you an idea of what kind of lengths and dis uh distances and coordination environments we're looking at being amorphous i chose to represent the material with the cluster on the top where we're simply highlighting the key distances in each coordination shell in reality the material is of course some form of an extended structure and so there's numerous iterations of each of these types of structural motif in the material but they're not related by any translational symmetry and so conventional structural analysis is really not applicable our confidence nonetheless in the existence and assignment of each of these five coordination shells arises due to the correlations in the data shown at the bottom and so what i'm highlighting in this data in the center is the cobalt the metal shells and so i've left out the cobalt oxygen shells the assignment of each of these coordination shells is supported by the lower panel which shows the relative contribution of each of these environments as a function of iron content and so presented this way an increasing contribution with an increase in iron content as seen for the red dots would signify that iron is responsible for that shell we can therefore confidently assign that red process as a cobalt iron vector within the solid the upper panel shows the consistency of the bond distances and so we can see from 30 to 100 percent it follows quite closely that's seen in pure cobalt and so this signifies that we get clean incorporation we get a solid solution if you're familiar with the term of iron ions into the cobalt hydroxide lattice at higher ox iron contents what we do is transition into an iron oxide lattice instead with cobalt dissolved into that the panel shown on the right shows the oxidation state of the metal from the zanes data where we measured it using quasi-institute's experiments and so these experiments very clearly identify that what we're oxidizing is the cobalt in the material and so the iron remains in a plus three oxidation state whereas the cobalt transitions from somewhere between an average of uh two and three so starting at about 2.3 up to three during a catalytically active state from a behavioral perspective we extracted a very large number of different descriptors for catalytic performance and i'll show a subset of them here and so traditionally we would analyze using taffle slope which is simply the slope of the linear portion of the semi-log plot up in the top left and in this case we also observed a point where the behavior transitioned from one linear slope into a second linear slope both the initial taffle slope and the current density at which this transition into a new type of behavior uh happened we found to correlate very well with the relative contribution of the cobalt cobalt and cobalt iron vectors for the coordination shells in the sample shown in the bottom corner so i would uh highlight here that the addition of iron improves the catalyst but the trends that we see in these two parameters are the reverse of what we would expect and so adding iron into the material improves the catalytic performance by making the stop slope steeper so we're decreasing our taffle slope it's an inverse thing and so we're improving the catalyst by decreasing this toefl slope however what we see is that it forces this transition to occur at lower current densities and so the iron is therefore involved in improving the catalysis but not directly right the more iron that we have the lower the the payback for that iron in the catalyst is and so in operando uh using zane's measurements confirmed that the oxidation in all of these uh was still the oxidation of divalent cobalt the trivalent cobalt during initial stages of catalysis as shown in the middle and so we're taking a derivative of the edge position as a function of voltage which gives us a rate of change which is the same as what we're measuring in current so what we see in that data is that there's a linear shift in the location of the cobalt based oxidations as a function of iron content and so this pairs with the voltage at which we see this transition in the steady state catalytic behavior happening which is very consistent and coincides at a voltage where we uh from the literature we can refer that there's a transition involving the oxidation of trivalent cobalt into quadrovalene cobalt and so we therefore suspected that the interaction between these two reaction steps a cobalt two to three oxidation and three to four oxidation was critical in the observed behavior rather than relying exclusively on the steady-state electrochemical uh measurements uh that the taffle data if you will as is common in the literature uh we also inspected the transient behavior that was leading up to the steady state and so this paid off uh in spades here and so what we saw what we expect is a single exponential decay in measured currents towards a steady state catalytic current and we see that in the red curves for pure cobalt up at the top however when we introduce iron into the material we see a secondary exponential growth in this case occurring and so we get an exponential decay followed by an exponential growth and so two different processes therefore coexist in this material again applying inoperandozines this time and so monitoring oxidation state of cobalt as a function of of time during these experiments uh we observed that the change in cobalt perfectly matched um the change in current density during the second process and so once again we're confirming that iron is indirectly involved in each of these steps and so it helps improve it splits into two reaction sites but iron itself does not appear to be doing anything directly by itself so we tied all the results together uh with a big branching mechanism based on all of the correlations that we saw and so in the absence of iron the amorphous catalyst follows the topmost branch and so cobalt undergoes two sequential electrochemical oxidations it goes from a divalent to a trivalent state and then a rate limiting oxidation of trivalent to quadrivalent happens this step initiates oer and everything behind that step becomes invisible when we introduce iron into the system the mechanism becomes dependent on voltage as we saw in that transition in the taffle slopes and so in the initial lower voltage region which is where a lot of that improvement comes from the next step uh the initial step is the oxidation of divalent cobalt the trivalent cobalt but the second step becomes disproportionation a chemical reaction as opposed to an electrochemical reaction and so this reaction we believe is activated because iron causes the potential at which the divalent to trivalent process happens to shift to higher voltages such that it begins to overlap with the trivalent quadrivalent step and so at this transition point it becomes possible to begin directly oxidizing cobalt three to four and so we're doing the trivalent quadrivalent reaction which results in the red mechanism shown at the bottom and so we're oxidizing cobalt two to cobalt three and then an oxidation of football three to cobalt four triggers catalytic catalytic water oxidation so in the end our correlational analysis enabled us to advance the fundamental understanding of a very complex reaction cycle that honestly i don't believe would be possible through any other means we applied a related analysis to amorphous nickel iron hydroxide films and iron in this case induces a similar shift in the pre-catalytic oxidation process as we saw in the iron cobalt but this is a sample that becomes dramatically better at catalyzing oers significantly better catalysts in the iron cobalt ones unfortunately the changes in catalyst behavior are far too abrupt to parameterize and apply any correlation analysis and so this lack of gradients really introduces a challenge into analyzing what's happening in these materials so my approach to solving this problem was to utilize a secondary electrochemical property specifically electrochromism this is a fancy way of saying that the color of the material change when we oxidize it so when the reduced state nickel is a pale green color when we oxidize these thin films they turn dark black and so we can see this in the uv visible absorption spectrum in the bottom left corner electrocatalytic experiments were therefore paired with visible light spectroscopy where we directed a 430 nanometer light on the sample and watch the absorbance with as a function of time and voltage and so this approach revealed a single electrochromic process in the pure nickel samples that was aligned with the pre-catalytic peaks which is exactly what we expect no color changes at all were observed within the catalytic region when when oxygen was being evolved a secondary process however emerges uh when we introduce iron into the films and so we can see this in the central panel labeled 88 nickel where we have 12 iron in the sample and the two panels at the bottom and so the first process remains aligned with the pre-catalytic peak and so that peak before before we drive catalysis is assigned to a nickel oxidation process the second process however is very broadly assorted with associated with electrocatalytic oxygen evolution and so what we have here is a secondary reaction oxidizing nickel within the material once again we applied hexafs analysis to extract some structural parameters uh by except this time we did it in a much more uh rigorous thorough approach and so we use many more samples held at different voltages and so a total of two oxygen nickel shells were required and two nickel to metal shells across the whole sample series these shells once again could be assigned based on the correlational analysis shown in the bottom left corner and so the shells that increase in the in prevalence as iron content increases within the materials can be assigned to a nickel to iron distance and vice versa and so the next step was that we realized that we had all of the infinite information necessary to apply some high school math to this problem and so the we can apply the pythagorean theorem to start analyzing bond angles and so now what we're measuring directly is bond distances but we can combine all of that information together to start looking at angles and so this type of an approach highlighted the coordination shells that we assigned as nickel bound to an iron neighbor were severely distorted under catalytic conditions and so the bonds were being pinched such that the neighboring oxygen atoms were coming much closer to each other and so based on this we concluded that there are therefore uh the electrochemistry in these materials is really dictated by two distinct types of nickel ions inside of the lattice as shown here and so the first which is responsible for the pre-catalytic peaks and the elec initial electrochromic process that we see is the initial oxidation of divalent nickel ions within the material with these divalent nickel ions being completely surrounded by other nickel ions the second site which is responsible for electrocatalytic oxygen evolution are nickel ions that have at least one iron neighbor in its immediate vicinity and so the geometric distortions that these iron ions force upon the nickel ions we believe activates them for catalysis this raises a very clear question if these distortions can improve catalysis can we control them to intentionally tune performance and behavior and so my team set out to test this by replicating the behavior that we see for nickel iron hydroxide materials in materials that do not contain iron and so specifically we set our eyes on testing looking for two of the distinctive properties this linear shift in the location of pre-catalytic redox processes and the step change that we see in catalytic performance we challenged ourselves to do this without having to rely on any synchrotron facilities and the associated challenges in obtaining sufficient uh beam time access to perform those experiments and so the key here is that nickel ions in this material are bound to six hydroxide ions and we know that well crystallized nickel hydroxide shows a trigonal distortion what this means is that rather having rather than having perfect 90 degree bond angles between all of the oxygen nickel oxygen lengths to form a perfect octahedron the bonding axes are tilted and so what we're doing is pinching together some of the angles this introduces two distinct angles inside of this coordination shells in the crystalline material these are 82 and 98 degrees from a molecular orbital perspective this breaks the equivalence in bonding orbitals uh and so we label in the bottom center here these t2g orbitals we have three orbitals that have uh the same symmetry properties and the same energy they're all equivalent to each other this splits into what we would label as an a1g and an eg pair of orbitals and so we have two orbitals that are identical and one that is different and so um the thought process was that if we can change the magnitude of this trigonal distortion we should be able to observe this as a change in the spacing of these new orbitals right and so we should be able to exploit this uh breaking of degeneracy of the orbitals in order to study structure directly and so this information can be attracted through near-infrared spectroscopy which likely works out for me because we have one in our labs and so the measurement itself is only half of the problem of course the second problem which ends up being the more challenging one is uh our desire to completely replace all of the iron ions with something else and so the composition of nickel hydroxide materials has been studied and tuned in over decades of research and so there's a couple of examples shown in the top left exam corner here of the different types of elements that have been introduced into nickel hydroxide none of these were able to reproduce uh what we see in the iron nickel hydroxide materials and so the behavior that we're chasing is really unique to nickel hydroxide and so we started thinking critically about the unique properties of this material and the one that really jumps out is it experiences a very large change in bond lengths when oxidized and an expansion uh when it's reduced and so we get this kind of breathing motion in the lattice this volume change we surmised must be associated with the behavior that we're observing and so we set some guidelines in selecting suitable elements to try to do this in a more systematic fashion and so our guidelines are shown in the bottom left the first is that the ionic radius has to reside between the nickel 2 and the nickel 4 species because that's what the iron 3 does the element that we use has to be compatible with the structure that we're synthesizing it has to be be able to be actually installed into this layered hydroxide like material the element has to be redox inert we don't want it changing oxidation state and therefore size while we're doing our experiments and finally we wanted to stick to trivalent cations so we didn't have to worry about electric fields and charging aspects in the material now the periodic table is quite large but unfortunately it's not large enough after eliminating all of the elements that were previously employed employed in studies and applying our requirements what we end up with is really two candidates aluminum and gallium and so both are not ideal as they are both main group p-block elements but they would have to suffice and so we synthesize these i won't go into too much detail in the data here but we synthesize these three different series of materials using iron gallium and aluminum and we applied uh near-infrared spectroscopy to analyze this and so what we see very clearly is that we can actually use the splitting uh so the the break in degeneracy and the extent of the distortion we can see it through this new infrared spectroscopy and so if we look at the structure property correlation shown in the middle we can see that we have a systematic trend that emerges as we tune the composition of each of these materials and they all reflect each other quite nicely and so we we do seem to be able to tune the distortion to a large degree we backed this up with some dft plus u calculations looking at the electronic structure the predicted electronic structure of the materials and they all follow very similar trends these calculations also enabled us to generate uh potential energy surfaces as shown in the top corner and so our approach here was to systematically vary the dimensions of our unit cell um for just a pure nickel hydroxide lattice at first and so what we're doing is basically introducing strain rather compressing or expanding our lattice and so what we do then is generate these parabolic curves which map out a reaction coordinate and so if we were to convert from an oxidized to a reduced phase we have to follow these lines the key thing is the height of that intersect right the height of that intersect gives us the activation energy it tells us how quickly the reaction is going to occur when we apply a voltage we shift the relative location between those curves as shown in the red and the blue and so what we do is tune the activation energy and force the reaction to go one way or the other what we see is as we chain as we start introducing different elements into it shown here are the gallium and the iron that activation energy the height of that intersect decreases so our catalysis should increase and so we get these nice clean trends shown in the bottom now when we look at the electrochemical behavior what we see is that we do replicate the pre-catalytic processes and so we get exactly the same behavior and gallium aluminum as we do for iron which is promising however from the catalytic perspective we don't see that same uh payback in improvements in performance and so um we don't have complete success here but we're not ready to completely rule it out because we did confirm that gallium is actually leaching from the surface aluminum and gallium are both highly soluble in the conditions used here so what we see is a significant loss of gallium from the surface there's also the possibility that we have bonding effects present because of the d electrons involved in the iron or some threshold in distortion that's necessary um again it comes back in our group what we want to do is combine synthesis structure and kind of go back and forth between these with correlation analysis and so what we've looked at is trying to resolve some of the discrepancies that we see in the literature some examples of which are shown on the left by looking at the technique that we use to synthesize these materials and so this example published in mrs advances last or two years ago i guess now there's shows an example where we used two slightly different synthetic protocols and x-ray diffraction to to look at the structure of the lattice what we see is by tuning the protocols by adding a small organic into the solution uh we have a significantly improved uh trend it's a much steeper trend in changes in bond lengths inside of the material as a function of iron content this suggests that iron is more effectively being incorporated into the material and so that excess iron from the red dots must be going somewhere in the material and so we have ongoing research so an example shown here using the most part spectroscopy with a collaborator in germany we're actually identifying that we do indeed have two unique iron coordination environments so we're looking at nuclear environments here for the iron species one synthesized under conventional techniques we can back this up with raman spectra acquired in our own labs here that we do have uh two distinct reaction environments in the conventional way of approaching this i'll switch gears really quickly and provide a survey of what we're doing in photoelectrocatalysis and so in this situation what we're doing is asking photons for help and so we're applying some of the energy requirements to drive catalytic reactions using solar energy and so the key here is that we can run at uh over 100 electrical efficiency because we're providing a lot of that energy from photons and so what i'm particularly interested in here is that photoelectric catalysis requires semiconductors and so we need well-defined crystalline materials where defects are generally harmful and so this provides us a useful measure to start looking at defects in the materials and so the bottom left kind of shows the energy diagram for semiconductors and so the very bottom portion of it would be our filled valence band we transition through an empty band gap and up at the top we have an empty series of orbitals that can accept electrons and so when we shine visible light on this what happens is this bolded black arrow occurs and so we excite electrons from our valence band into our conduction band there are a number of things that can happen here and that's really the thing of interest what we're interested in is the effect of defects on those subsequent processes that happen now when we're designing these the specific material that we choose has an impact on what we observe and so the band gap the spacing between the empty and the filled orbitals sets the minimum energy required to do this which is related to the wavelength and it also sets the total current that we can extract from this and so the integrated areas in the central curve show the turtle the total amount of photons that can be absorbed which translates into the total electron density that we can float and so for our experiments we chose hematite with this 2.1 electron volt band gap now hematite's been studied over decades and one of the things that really stood out to us is that it is very very um sporadic and so the the data is shows a high degree of variance all across the literature it's synthesized by a very large number of different techniques and we literally span four orders of magnitude in performance parameters that are reported this has to relate down to defects and so we know a lot about defects through semiconductor doping in these and so an example here if we if we introduce quadrivalent titanium into iron 3 sites we change the charge in the lattice and we force the lattice to accommodate that to neutralize that charge through either an ionic or an electronic mechanism and so we either remove iron ions in this example or we remove oxygen and subsequently transfer those electrons into the iron to generate iron two sites there's a lot of information known about that semiconductor doping but the pure hematite phases there's not a lot known and so there's a lot of emerging research on oxygen vacancies and undoped hematite but we're interested in expanding the scope of that and so to start studying this we started looking uh at using different iron phases as precursors and then um systematically applying different uh synthetic conditions and so shown here is our first example where we use lepidochrotite a layered iron hydroxide sample and we heat it at various temperatures to generate hematite which occurs through an intermediate phase shown in the middle this megamind and so our structural analysis here is performed with raman spectroscopy and so what we see here at the bottom is that the vibrations that we see and so raman spectroscopy is probing the movement of ions within the lattice what we see is the the symmetry of the lattice dictates the number of peaks that we see while the identity of the ions in the lattice dictates the energies at which we see them and so we see the different phases being formed so the black being our starting material the red being our intermediate and then the blue and the green being our final hematite phase our final hematite phase has seven vibrations that we expect but there's also an eighth vibration that we don't expect and so what we have here uh is a vibration that is formally forbidden and so symmetry selection rules would tell us that this should be visible in an infrared spectrum not a raman spectrum this tells us us that there's a distortion in the lattice which provides very valuable structural information that we can exploit so what we did was uh use some state-of-the-art instrumentation that we purchased with the cfi grant to perform roman microscopic mapping and ensure that we have a good spectrum that's representative of the materials some of the capabilities of this instrument are shown on the left here where we have a sample heated at 350 degrees and if we focus the laser on the blue and the red dots we can very clearly see that we have kind of an homogeneous heterogeneous very mixed composition and so we have a mixture of this intermediate megamite phase blended in with some hematite this is of course not what we want if we go up to 800 degrees celsius we can now see something that's much more desirable and so what we see here is a series of 600 spectra acquired over a 23 by 24 micron area and if we look at the histogram of this we see a nice tight distribution of spectra inside of this where we have our structural parameter being the ratio of two intensities of raman vibrations and so if we take the maximum of that plus or minus 15 percent what we're looking at is over 85 percent of the total spectra in the sample and so we have an approximation for error bars this takes some time and so we sped things up and instead we started just kind of surveying the surface and these little three by three maps and so total of 27 spectra what we see is a spectrum that's perfectly uh reproduced we then systematically curve fit all of the different samples and when we remove all of the samples that show signs of magnemite contamination which will negatively impact our results what we can do is correlational analysis and so what we see are specifically the peak at 500 wave numbers and 610 wave numbers if we take their ratio to this symmetry forbidden process at 660 wave numbers we get these nice clean uh correlations and so what we're doing is systematically changing the band structure of these materials shown in the top portion of the panel and this in turn affects the catalytic performance shown in the bottom panel and so what we have here appears to be a very nice single defect what it doesn't tell us is necessarily the identity of the defect and so if we look at the structure of hematite what we see are really columns of these kind of face sharing polyhedral motifs which are linked together through edge-sharing motifs the thing that links these three important vibrations for these correlations is the movement of iron ions within those polyhedral faces that are face sharing in nature and so that face sharing motif becomes uh very important and so scanning the literature we deduce that what must be happening is a defect in those sites and so we propose that what we're likely forming is proto-hematite and so in this case what we do is trap residual protons in the lattice and the charge balance uh the material expels iron from uh some of these sites so we have iron vacancies distributed throughout the lattice we've been following this up with some direct structural evidence and so yutung liu who's the one working on this has done some x-ray absorption spectroscopy i won't go too much detail on this because it's not quite published yet but key here is that we have again these interatomic distances which we can directly measure and correlate and so our correlations confirm that we do have these magnumite under oxygen conditions and so in this case we changed the synthetic protocols a little bit to accommodate the beam time that were allocated and so when we use these ones under nitrogen what we see is uh as clear secondary form of defect which we believe to be oxygen vacancies and reduction of iron sites to iron too we followed this up as well with studies of the mechanism and so here we do variable temperature raman spectroscopy using a fun new attachment that we bought for our instrument and so we acquire acquire spectra as a function of temperature and so what we see here uh is very clearly the spectra of our initial magmite shown in the black bar leopard occupied converts into megamite as the red bar and the blue bar being hematite at the top we pair this with thermal analysis and so we can watch the mass change we should be removing oxygen as we do the reaction as well as flow and energy and so we're looking at a mixture of endothermic and exothermic processes and so we can assign that the expected processes labeled one two and three what we see is this fourth process in the calorimetry data we see evidence of this fourth process as well in the spectroscopy data when we look at it in more detail so what we believe is happening is that that fourth process is the formation of protohematite so this is this defective material which is inhibiting our catalytic performance in these materials also expanding this outwards and so we've started looking at different iron oxide precursors um i'm running out of time and so i'll go over this very quickly but but here's an example looking over layered double hydroxides where they're converting away from our layered structure into a channel structure and so here we're looking at a phase known as a kegenite and so what we see are instead of a clean single segment correlations that depend on the synthetic conditions and so a thorough analysis of the trends that we see suggest that what we're seeing are three distinct types of defects which are uh distinguishable from three distinct types of trends in the data and so again our end goal here is to do complete defect analysis in this semiconducting material and so what we want to do is tune the structure and synthesis and correlation analysis such that we can use these raman vibrations to fingerprint the identity and concentration of defects inside the semiconductor and then apply this to new systems i don't have time to talk about all the research that we're doing obviously there's a number of different directions that we're doing that i have that i'll just briefly introduce and so we're looking at electrocatalysis and so uh some examples of some inter-stratified materials for co2 reduction uh we're looking at tuning the geometry so again here we're going from orthorhombic to trigonal structures looking at distortions in a lattice in situ spectroscopy to study reaction mechanisms as shown in the center again looking at co2 reduction and of course one of the problems that we often run into is overlapping signals and so overlapping signals present a real problem to confidently analyzing peaks and so we're looking at expanding our toolbox and so here's an example of just mathematical deconvolution of processes using correlational analysis and so of course i don't do any of this work myself anymore or very little of it instead i rely on on a team of of experts here that are in the lab and so my phd candidates are the three lists in the top right corner and so pinar alsac alex whittingham and utanglu has been a postdoctoral fellow in my group for the last year ji shi zhang has been working on the new hematite data that i showed she's currently a bsc candidate she'll be joining as a graduate student in september and sarah stepan was involved in some of our correlational analysis she has graduated and since moved to industry and with that i will close out and and my contact information is shown in the bottom right corner i'm always willing to talk science and so if you have any questions do feel free to reach out that's great thank you very much rodney yeah excellent talk it makes me uh think with all the your focus on defects and and their importance of any of the reactions you've considered how important are defects and many things that we think are due to ordered materials um it turns out that a lot of the properties in solid state materials that we exploit and make use of are really driven by defects and so many things don't work well unless we have some form of defect in them so in situations such as silicon for example in photovoltaics those those defects in the structure has been really clearly worked out over several decades in other materials such as hematite and other photoelectric catalyst materials we're really just beginning to scratch the surface and understand some of it well that brings me to one of the questions that came up in the q a from megan uh do we know where the defect is in hematite or is just that there is a defect uh so there are several possibilities um and so i had introduced here the possibility of excuse me of proto-hematite and so in proto-hematite the idea is that extra protons from the water or the hydroxide in the material are trapped in the lattice and to compensate iron gets ejected from uh lattice sites this is only one possibility uh it's also possible that we have uh iron vacancies and so i showed an example of that right at the beginning that are not associated with protons there's the possibility of oxygen vacancies there's a possibility of interstitial sites and so iron or oxygen or or foreign materials such as silicon or tin can reside in sites that should formally be empty all right and so there's a very large number of different types of defects that can form um again the dopant based ones are fairly well characterized because it's easy to control them because we can control the composition the native ones are a lot more difficult to analyze um and so in this case what we're looking at is really a balance of multiple site multiple defects that are all coexist and so our strategy here is really trying to force ourselves into a situation where we have more of one and less of another so that we can analyze one at a time and some of your tailored work where you've been introducing different levels of iron i mean how how many defects can you introduce into a material before they take over the structure and are no longer really a defect uh this is for the amorphous hydroxide materials yes uh yeah so this this um so i very broadly introduced it as there's this continuum between crystalline and amorphous uh realistically it's far more complex than that um and so our formal definition of crystallinity is really based on translational symmetry um it's it's of course much more complex than that in the real world and so we can have materials we can introduce different types of defects at different concentrations and still maintain a relative degree of that translational symmetry and still use x-ray diffraction for example to measure the structure and so you know we're going towards a disordered material we would formally define amorphous as having no peaks in an x-ray diffraction pattern but it's also conceivable that there are different ways in which we can have that amorphous structure right and so we might have for example tetrahedral sites inside of the material or purely octahedral sites and so that's that's really um kind of at the forefront of where research is now is trying to understand those defects because our instrumental techniques are really getting to the stage where we can start asking those questions you showed uh some really nice work on the expansion of of the nickel hydroxides between the reduced and the oxidized state does that put a constraint on your use of those catalysts in different environments um so it's not entirely clear exactly how far it applies yet i i personally believe that that expansion and contraction is critical to why nickel is the preeminent catalyst for water oxidation i think there's a special combination of distortion and prevention of that expansion contraction process and bonding effects that occur when iron is brought into the material um this of course has implications beyond catalysis and so nickel hydroxide or nickel oxide layered nickel oxide like materials similar to this are used in batteries and so this for example this ability to to shift the nickel based oxidation process in a predictable way can be used to extend the voltage of a battery material for example and so it can be systematically applied to different technologies as well as just a fundamental concept i invite the audience to put any questions you have into the q a and we can ask professor smith to answer them for you certainly give us a lot of stimulating thoughts to process um where do you see uh some of the application of electrocatalysis going are these going to be very small scale devices that occur in every uh in every application that you need or these gonna be very centralized facilities like the oil refineries of old where uh the technological requirements are such that they need to be quite centralized um my personal belief is that we need to diversify um and so i don't think there's going to be any magic bullet that solves the energy problem i think what we're going to do is have to develop and deploy a range of scales and so obviously we're going to have to have some kind of grid scale energy storage um i think it probably has to be combined with some local storage right and so you know batteries are obviously gaining in the local storage front but there's no reason that that fuels and synthetic fuels can't gain foothold in that area as well and so i was previously engaged in my postdoc in a startup company that was initially focused on trying to bring out small-scale reactors to use some of these catalysts and on on a more local small scale um for example that you can put in your house or your garage um i'm not entirely sure that the market is ready for such a thing uh but there's no reason that we shouldn't be developing towards that i think again that we really need a variety of of solutions some some catalysis some batteries and some non-electrochemical approaches excellent are there any questions from the audience that we can ask professor smith well since you have the slide up i'll point out uh to everyone here that we this is one of the a series of lectures that we're giving from the faculty of science uh the next one is coming up on may 26th from the department of earth and environmental science professor angie brookfield will be providing a talk on supporting sustainable water management decisions with science one of the things this new faculty lecture series is really giving us is a view of the extremely broad re reach of the research that goes on within the faculty science at the university of waterloo i hope you take advantage of these opportunities to see the full breadth of of the challenges that faculty within uh the faculty of science are taking on these days all right so if there are not any other questions from the audience perhaps we can bring things to a close there i do want to thank first professor smith for an excellent lecture uh he's given us a lot of novel ideas to ponder and i think a lot of break promise for the future when it comes to electric catalysis and understanding some of these materials that may give us real advantages in the energy requirements of the future i also want to thank everyone who ca

2021-04-14 04:39

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