Nanotechnology - The New Science of Small || 20 - Nanotechnology for Storing Energy
[Music] thank you we've spoken about capturing energy from the Sun and capturing is the right word for this sometimes we talk of energy production but really we're just harvesting what's being given to us from the Sun but there's another key part of managing energy especially renewable energy and that's storing it in this lecture we'll talk about how nanotechnology is enabling us to advance and enhance the storage of energy we'll speak about using nanomaterials to improve the density measured either by weight or volume or both by which we can store energy and we'll also talk about novel Concepts ways of using viruses for example to assemble new styles of batteries for us but first I'd like to speak just a bit about the balance between harvesting energy and storing it we anticipate in the future a much larger fraction of our energy budget coming from renewable energies solar energy is an example wind is another good one and so many attractive things about these abundant a clean pollution free but one of the challenges that renewable energy forms present is that we don't turn them on and off they're not what we call dispatchable sources the way say a coal Generating Station would be or we can turn up and turn down the amount of energy production at our will in the case of Renewables solar for example a cloud comes by and the amount of solar energy available to us at that moment changes now that doesn't necessarily map perfectly onto our energy needs in fact if you look at the time of day cycles if you look at the way solar the sun rises in the morning and then right at midday we get a great deal of energy and towards the end of the day less so and overnight none this also mandates the need to buffer kind of hedge these temporal variations in the available amounts of energy that's kind of a time of day variation perspective we have some Cycles throughout the day where our energy needs vary kind of along lines with the Sun so for example an air conditioner is typically needed more at the peak of the day when the Sun is up and so that's a good alignment but there are many other reasons why it would be attractive to be able to play back energy at a different time from when it's made available in fact A New Concept has emerged recently in the management of energy known as the smart grid the smart grid recognizes that the traditional power distribution network is based kind of on new unidirectional flow of energy it's based on the idea that we have a small number of large centralized power generating stations that spread energy out towards the customer but with Renewables now the customer for energy URI is also producing energy say on our own Rooftop in fact we may produce sometimes more energy than we need we may wish to sell our energy to our neighbor or to somebody in another state another part of the continent and so now the grid needs to become two-dimensional we need to be able to buy and sell energy from many many local points and one of the key elements in building a better smart grid that's two-dimensional two-directional and that allows the flow of energy in a managed fashion without losing control without leading to blackouts or brownouts one of the key elements is energy storage so what that means is that our challenges on the energy storage front occur on in many different scales there's a familiar one say the battery inside your mobile device or inside your laptop these are obviously crucially important for a variety of reasons they enable us to go anywhere go everywhere with energy computation and Communications we've marveled in our discussions of nanoelectronics at the rapidity with which mobile Computing has become available we've marveled at how nanostructured materials nanoscale transistors have allowed us to communicate at remarkable rates anywhere but until now we haven't marveled at the fact that we're able to take the power with us that we need anywhere in fact people often remarket how the computing power that you have in your laptop is vastly vastly greater than the power in some of the computers that used to fill a room well the same could be said from the power standpoint we're able to take this power with us anywhere we go have our battery packs last for 10 hours enable continued use and enjoyment so the mobility that batteries and related devices enabled is tremendous but this can also be scaled up imagine in the context of the overall Energy System our desire to take large quantities of Renewables say solar energy for a community or on a solar farm and store that uh and that's where batteries can play a role but fuel cells which we'll speak about a little bit today provide another strategy for storing energy and what they do is they decouple the playback of the energy from the storage of the energy in a battery which we'll talk about in a second the means of energy is storage and replay are kind of integrated with one another but in a fuel cell we have the fuel such as hydrogen or ethanol and separate from that we have the cell which takes in that fuel and which spits out energy and so depending on the scale we're interested in depending on the cost depending on whether portability is a requirement or whether just looking for a a stationary and very cost effective method of energy storage and Playback we're interested in different systems in fact there's another exciting technology that we'll also speak about today and that's benefiting from nanotechnology called super capacitors and what supercapacitors do is they function kind of like batteries they're closed systems they're also self-contained but they allow us to charge them up much more rapidly and then also play back their energy in much bigger spikes and more rapid spikes so one of the example areas in which Super capacities super capacitors are expected to have a use is in automobiles where when we start it up we need a real burst of energy typically that's challenging for a battery to supply and when we use the brakes and we want to store the energy from in uh in a some kind of electrical device a storage device inside our electric vehicle super capacitors are able to receive that energy very rapidly and very efficiently so let's dive into batteries now these of course are familiar and they've been around for a while and the building blocks of batteries go back a couple a couple of centuries uh and so these have not always been consciously devices in which we're using nanomaterials but we'll talk today about how nanomaterials are enabling better energy densities inside batteries to do that to talk about how nanomaterials are helping us we need to talk just a little bit about the basic principles of a battery in a battery there are two electrodes such as in any electrical device or any electrical circuit needs two electrodes to complete the circuit and uh in a charged up battery we have chemical energy stored typically in the form of some kind of anion a lithium is a famous example and of course we're very familiar with Lithium-ion batteries we have energy stored in the form of lithium all being kind of piled up we use the word intercalated to mean this anion this ion is interpenetrated into this electrode and graphite is an example of a typical electrode that's traditionally been used to store this energy the form of playing back that energy and turning it into current involves the ionic flow through what's called an electrolyte of these ions towards the opposite electrode and then in the case of non-rechargeable batteries that's it we've expended the available energy that was you can think of it as kind of having been uphill and the energy the anions flow downhill to release their energy provide a voltage to a circuit to a device in the case of rechargeable batteries we're able to reverse that reaction and drive them back to be stored at their original electrode and we can do this back and forth and back and forth many times now as you know even rechargeable batteries have finite lifetimes and the various electrodes that we use to build batteries can be degraded over time after we discharge and recharge them many times and so it's very important to look at new electrode Technologies especially that can enable batteries that have longer usable lifetimes more charges and discharges just thinking back to our periodic table why is lithium a particularly logical metal a particularly logical element to be using well lithium is what we call the most electropositive metal available to us in the periodic table that means that it gives us the opportunity to develop the largest potential the largest voltage which is going to translate directly into energy now it's also the lightest metal the least massive per anion and as a consequence of that those two things combine this large amount of potential that every ion can store combined with the low mass of each of the ions means that the density in terms of energy per gram or energy per kilogram can be very high with lithium so it's an extremely good choice from this point of view now where do the nanomaterials come in well I mentioned that the graphite electrode is where we store the lithium in the traditional Lithium-ion batteries and graphite has a reasonable capacity for storing these ions uh it turns out though that silicon so familiar from our discussions uh of nanoelectronics and also of light guiding that silicon has another remarkable property on top of all of its attractive electrical properties and that's that it's extremely good at storing lithium ions and by good I mean that it can store a huge density of these ions in fact if you look at a picture of the Silicon lattice it's got kind of a crucible right inside the lattice where the silicon crystal is ready to receive a very high density of lithium ions so this is very appealing but if you look at this lattice you'll also realize that this lithium ion doesn't penetrate into the lattice without disrupting that lattice without changing it in fact if you were to exploit the full energy capacity for storage of lithium within silicon which is about 10 times the capacity per unit mass that you can store within graphite if you were to exploit that full capacity you'd also expand the volume of the Silicon electrode by about 400 percent and as a result if you just do this straightforwardly if you take a bulk piece of silicon or a non-crystalline piece of silicon like amorphous silicon what you'll find is you can stuff it full of lithium atoms you can get an incredible energy density in there and then after a couple of Cycles it will crumble it will collapse because of all this expansion and contraction and expansion and contraction it's like the thermal wear on parts or on you know parts of your house where the it gets cold and hot and cold and hot and the joints start to wear out the joints also start to wear out with thin silicon if we change its volume back and forth too many times but this is where the opportunity for Nano comes in there's been some really exciting work going on at Stanford that takes advantage of nanomaterials to overcome these limitations to allow us to exploit the incredible energy density of silicon for the storage of lithium without suffering these challenges related to the volume expansion the way this works is that the researchers have built these long very very skinny nanowires and so their surfaces are now accessible for lithium ion intercalation for the penetration of lithium ions into the Silicon but now the nanowires can expand and contract and expand and contract rather than a single bulk solid which would suffer these catastrophic fractures these small nanowire diameters enable these silicon-based materials to accommodate without fracture these significant changes in volume another great thing about the Silicon nanowires is they're functioning now as wires as electrical wires and so they're able to directly electrically connect to the metallic collector of current as a result we don't have isolated materials which can't be collected electrically or where their electrons aren't efficiently collected in contrast we're able to make very good use of every one of the nanowires all of the Silicon is active there's no islanding effects and finally it's a direct path it's a straight line path we get a direct connection between the silicon nanowire and the electrodes that we seek to connect it to now looking a little bit further out into the distance into some conceptually disruptive or transformative techniques to try to build batteries there's been some very exciting very creative work that's gone on at MIT in which viruses have been utilized to try to assemble new classes of batteries and one of the stars of this show it's been actually an amazing example of teamwork part of the team is Angela Belcher who's distinguished herself over the last decades as somebody who's able to work with viruses and use genetic engineering to create families of viruses that have particular properties that dovetail with Material Science that allow us to use bio entities such as viruses to build materials for us when she's been collaborating with a researcher yet Ming Chang who's distinguished himself for a long period in the field of energy storage in the field of batteries and they've brought their work together to form a very creative collaboration that's completely uh Breaking All the Rules of traditional disciplinary boundaries you don't traditionally see genetic engineering and viruses combined with the kind of electrical mechanical chemical engineering of energy storage and Angela Belcher herself has this fascinating story she really distinguished herself a little over a decade ago in understanding how living beings build hard inorganic materials so the abalone shell is a great example of this and of course we've been familiar with the fact that you can get these uh organic entities you know animals and vegetables that as well as forming soft or uh malleable materials which is more what we're familiar with in the biological world that they can also form hard shells like the shells on seashells and Dr Belcher looked at understanding exactly at a molecular at a nano level how this occurred and what she discovered was the mechanisms by which proteins were being expressed by these organisms that were able to capture out of the sea particular elements such as metals that were used to build up say calcium carbonate so capture the calcium component to build these shells up and then another class of proteins were able to promote the formation of the complementary let's say the oxide or the carbonate and so they were able to build up layer by layer by layer by layer these very hard shells essentially they were utilizing organic materials biomaterials proteins to build hard inorganic materials and she saw this as a real inspiration for her own work after she did she kind of did the science of it she understood how these biological entities were building these hard very structured inorganic materials using their proteins she thought well maybe we can exploit this maybe we can engineer with this now in order to try to engineer with these things she need to have some kind of capability to select she needed to be able to tailor viruses in a direction that she desired and the approach that she took here which is taken from the world of phage display and the idea is that a virus can have various genetic options injected into it these these various families or classes of viruses can grow up and then we can use selection approaches to only preserve the ones and then amplify the ones that we desire well what would be a selection algorithm say you wanted to build a material that was based on say Cobalt well you could coat some kind of sieve effectively with Cobalt introduce your various viruses into it and only those viruses that happen to have a gene that led a protein to be expressed that would strongly bind Cobalt would stick to the walls of your sieve and everything else would fall through in fact people refer to this as panning or bio panning an analogy with panning for gold where what you're looking for the viruses that you desire to select in favor of remain inside your container and everything else goes out you then have these viruses stuck to the wall of your container you elute them which means you bring them back out into the solution and then you can amplify them you can have them reproduce and have families and as a consequence you can get more and more of the ones that you desire and so you run this essentially biological selection and a rapid Evolution within the laboratory where you prefer the evolution of certain viruses that have certain functions people had been using this concept in genetics for a long time but using it in the context of building up inorganic materials selecting viruses that could do the work of building say a battery for you this was a very novel concept so Dr Belcher and Dr Chang working together we're particularly interested in building Cobalt oxide Cobalt oxide is a very interesting electrode for new battery technologies it's been found to have particular promise also enabling the dense and reliable storage of a great deal of energy and there were really two stages to building a virus that was coated with Cobalt oxide nanoparticles and that could then be useful in conduction so the first was to put these Cobalt oxide particles on the surface and the second was to make the material more conductive so for the first one the researchers searched for used their selection techniques to find viruses that were particularly prone to attract Cobalt and to grow Cobalt oxide on their surfaces and they were able to show using transmission electron microscopy that they could generate viruses that had a very rich coating of cobalt oxide nanoparticles on their surface and that these particles were very similar to one another so they were quite consistent in their size in fact some of the power of transmission electron microscopy is that it goes beyond just our ability to look in what we call real space to actually look at the image itself and we can also Focus the Electron Beam onto individual particles and see how the crystals that the electron beams impinge upon spread the energy out cause it to diffract the way in which these diffract gives us a clue as to the composition to the structure of our crystals and these researchers were able to show that they had indeed built viruses that were able specifically to grow Cobalt oxide single crystals on the surface of their viruses now this is one of the building blocks that they needed the other building block that they needed was something to make these more conductive and so the researchers found a way to put an additional peptide so an additional protein on the surfaces of their viruses to make a kind of they called it a bi-functional virus something that could enable them both to grow the Cobalt oxide nanoparticles which were crucial to the battery function but also be able to grow gold nanoparticles that led to a dramatic Improvement in the conductivity of these devices and so the viruses as a result had these codes that consisted of two classes of particles one the Cobalt oxide another the gold and the researchers were able to show that they were able to generate these a hybrid or bifunctional viruses that allow them to achieve their purposes in making a battery in this manner of course making a battery is more than just making one wire which these viruses effectively had become they needed to put them down onto a substrate they needed to make a practical device out of them and they found the conditions under which they were able to coat onto just a flexible piece of plastic effectively their viruses and by managing the charge on the viral particles they were able to ensure that the virus is assembled into nice smooth what are called lamellar structures so are these kind of stripy patterns of viruses with their Coatings of nanoparticles of cobalt oxide and of gold on top of them uh that's a discussion of one very important storage technology and how nanotechnology is enabling us to make better batteries that allow for greater capacity or in the case of this very intriguing work on viruses allow us to do the synthesis under very environmentally benign conditions we consume very small amounts of energy in building the batteries this way it's a very friendly approach to building storage devices but another area where there's a real need to build devices with greater performance and also with the kind of flexibility that you can sometimes get by using Nano materials to make devices is in the area of super capacitors and I was mentioning earlier as I introduced energy storage generally that super capacitors are another Paradigm in storing energy their advantage is that they allow you to store accept energy very quickly charge up very quickly and play back very quickly as well and it's worth defining just the idea of a capacitor a capacitors are actually traditionally the domain of electrical engineers capacitors are one of the building block circuit elements that allow us to store charge we usually think in electrical engineering of having two plates separated by an insulator and on one of the plates we can put a positive charge and by putting that positive charge there a negative charge is induced on the opposite plate as a result there's an electric field and we have the potential to then release this charge in the form of a current with the presentation of a voltage in order to then drive a device so their storage devices but not typically in the electrochemical sense that's used in batteries where we're actually moving lithium anions uphill for storage and subsequent replay instead these are typically based simply on the storage of electrical charges themselves of electrons now in super capacitors the name of the gain is to try to achieve the greatest possible energy density while also achieving a very good lifetime this is one of the great things about super capacitors is that a result of not moving more massive ions around and imparting true chemical changes but instead of principally moving electrons around we're able to achieve devices that can go through many more Cycles so if you think of something say an electric vehicle that needs to happen many many times how many times do you apply the brakes for example in your car if it needs to go through a hundred thousand or more Cycles then often super capacitors can supply the solution some of the exciting work that happened also at Stanford was um figuring out how to make a wearable supercapacitor in fact the field of wearable Electronics has attracted a lot of people's attention and excitement the idea would be that as well as perhaps making a flexible solar cell that you could go around wearing you could also store that energy you could later use it to charge up a mobile device in fact people even think of the devices themselves as being wearable of course there's the emerging wearable electronic glasses that can have cameras and processors embedded in them any number of things that you can imagine doing but what we'll need to do this is to have a flexible Fabrics that are also electronic materials and uh the Stanford group under each way figured out how to build Fabrics really right on top of traditional cotton in such a way that they were able to store a great deal of charge in them the way they did this is they started with a millimeter or two thickness of just a fluffy cotton sheet and then they dipped it into single wall nanotube ink so basically a solution containing many nanotubes and the nanotubes instead of being of the multi-walled site sort which are contain rings and Rings kind of a coaxial set of nanotubes instead they focused on single wall nanotubes which have a better defined and more straightforward and controllable properties they dipped their sheet of cotton in a couple of times dried it at 120 degrees C to remove the water from it and essentially it had a foldable textile that was now a conductor as well as being a conductor they were able to synthesize and modify the properties of their single walled carbon nanotubes where there's a lot of control over the surface properties to build two properties into it first of all they were able to make it what we call hydrophilic hydrophilic means that it's friendly to water and water likes it and so water was able to absorb on these electrodes that was important because the electrolyte that they were using to make their super capacitor was water-based they wanted to keep everything simple environmentally friendly not use solvents that people might wonder about whether they should be wearing on their clothes and so they used a water-based electrolyte but that meant they needed to convert their carbon nanotubes over to being hydrophilic they succeeded in doing that and they also used the structure the nanostructure the area of these carbon nanotubes as their means to enhance the capacity of their devices to store charge they proved the idea that they were able to enhance both the density of storage but also the lifetime of their devices their ability to play back charge play back their devices many many tens of thousands of Cycles even though they were making an electronic device that was on a piece of cotton that was on a piece of textile fabric we've talked in today's lecture about a few strategies for energy storage and how nanotechnology through the large area of nanostructures through the ability to process nanostructures from the solution phase and through the ability to use biological organisms like viruses now to build inorganic materials how nanotechnology is giving us new ways to make storage elements that are denser lighter in weight or cheaper in cost or integrated with flexible Fabrics we're going to continue this discussion in the next lecture and talk about additional energy storage strategies and then more broadly about how we can use nanomaterials to control the flow the playback of energy and also how we can use nanotechnology to implement desirable chemical reactions with high efficiency through a process known as catalysis
2023-02-13 02:02
