Lecture 8: Ions, Ionization, and Valence

Lecture 8: Ions, Ionization, and Valence Summary This lecture we continued our discussion of ionization energy. We saw that, similar to Li and Be in the last lecture, the ionization energy for the elements along the periods (from left to right on the periodic table) in the p and d block increases due to a higher number of protons leading to an increased nuclear pull. It makes sense that more energy is necessary to free them. On the other hand, going down the groups (from the top to bottom on the periodic table) the ionization energy decreases because the increased shielding and distance from the nucleus is more significant than the increased nuclear pull due to added protons. The reason for increasing ionization energies for electrons closer to the nucleus is that these electrons are more tightly bound. For example, Li’s first ionization energy is 5.4 eV, while its second ionization energy is 122 eV. This second electron experiences less shielding than the first. This ionization energy information can be viewed on a Photo-electron Spectroscopy (PES) plot. This is a plot showing ionization energy versus relative intensity for a given element. PES involves shining enough energy on an atom to ionize all of its electrons, measuring their kinetic energy, with the difference being equal to how much energy the electron was bound by. PES is a simple, powerful technique to directly measure electron occupation. Another periodic trend discussed was electron affinity, or the change in energy that accompanies an additional electron. The halogens (group 17 on the periodic table) have the highest electron affinity, because an additional electron would result in a full octet and complete filling of the subshell. Electron affinity increases along the periods of the periodic table up until the halogens. This lecture also laid out the idea of valence electrons. These are the electrons that occupy orbitals beyond the highest filled noble gas core. For example, carbon and silFigure removed due to copyright icon both have 4 valence electrons. Carbon’s highest filled restrictions. Lewis Dot Diagrams of noble gas core is [He], while silicon’s is [Ne]. Valence elecSelected Elements on Hyperphysics website. trons are what drive chemical bonds. Valence electrons on atoms are depicted using Lewis Dot Diagrams, which are the chemical symbols of the elements with dots surrounding them that represent the valence electrons. Lewis Dot Structures are helpful in visualizing the octet rule, which states that atoms (ignoring the transition metals for now) are at their lowest and most stable energy state with 8 electrons in their valence. In this lecture we began applying Lewis Dot Structures to ionic bonds. Cations are depicted with plus signs on their top right corner, and anions with negative signs, as shown below. Why this matters In this lecture, we’ve been focusing on atoms that lose or gain electrons, also called ions. Now we’ll discuss how this topic relates to renewable energy solutions. Take a look at this plot of the demand of electricity (in blue) vs. supply of electricity from a wind farm (in red), recorded over a two and a half month period in 2013 in Denmark. Notice that on the demand side, there’s some variability, but there’s also a lot of regularity. The 5-day work week draws more energy than the weekends, and the amount of energy needed doesn’t really vary much from week to week. Now contrast that with the supply of electricity from a wind turbine farm. Even if you put the turbines in a known windy area, there’s still a huge amount of variability. Wind energy is not that expensive to produce, but the biggest challenge isn’t cost, it’s steadiness of supply. If you think about how to build enough of this type of unreliable supply to meet the consistent demand, you’d need to overbuild capacity by a factor of at least 10 (and even that wouldn’t be enough on some days)! That of course isn’t going to be economically feasible, so the real solution, if we want to expand how much energy we get from intermittent renewables like wind and solar, is to be able to store the energy for when it’s needed. If we have reliable storage, then the supply can be quite unreliable and we’d still be able to meet a steady demand, as long as on average the supply can meet it. One of the easiest ways to store energy is to push something up a hill and then let it roll back down. Push a ball up a hill and then hold it there, and when you want the stored potential energy back as kinetic energy, simply let it go. We pump water up hills, called “pumped hydro,” which is currently one of the only viable technologies that can scale up to the size (meaning, power) of actual electricity grids. A massive amount of water is pumped up a hill when there’s excess energy supply, and then rolled back down to turn a turbine to generate electricity when there’s excess demand. Although this is a relatively cheap storage solution, there are all sorts of problems with scaling up any further than it is today, primarily because of the low energy we get from gravitational energy and therefore how much land is required. There’s a great post that explains why it will be hard to increase pumped hydro as an energy storage technology at dothemath, which is a cool site in general if you haven’t seen it: https://dothemath.ucsd.edu/2011/11/pump-up-the-storage/. So what else can we pump up a hill to store energy? This is where our ions come into the picture! Ions are the key ingredient in batteries. Think of a battery as an electronic hill, and when the battery is charged the ions are getting pumped up. Cations in particular, like Li+, get pushed up the energy hill, and the lower energy state (down at the bottom of the hill) would be one in which it can get that electron back and become neutral again. When the circuit is closed, the cation falls down the energy hill to meet up with the electron, which goes around the circuit doing work along the way. In this case instead of mechanical work like the water, the electron is doing electrical work directly. This is one of the great advantages of electrochemical storage (a nice way of saying a battery), namely that we’re storing directly the kind of energy we mostly need, electrical energy. Stored Figure of wind supply and demand shown above © source unknown. mechanical or thermal energy later often needs to be converted into electrical at great efficiency loss. But how good are batteries today, really? Could they ever be used to store the massive amounts of energy needed at “grid scales”? For reference, the U.S. uses 3 TW on average, or 3x1012 Watts = 3x1012 Joules/second of energy, so you get a sense of what it means to be at the scale of the grid. Let’s make sure we’re straight on energy vs. power: a Watt is a Joule/second which is energy per time, which is power. That’s an important metric since we need to be able to access energy at a given pace. The total amount of energy is power multiplied by time, so for the U.S. if our average power draw as a nation is 3 TW, then our daily energy use is 72 TW-hours (TWh) and our yearly energy use is 26,280 TWh. Just to give you a sense of how big this number is, a typical AAA battery stores about a single Wh of energy. That means if we wanted to store the U.S. energy needs all we would need to do is build a storage facility containing 26,280,000,000,000,000 AAA batteries! In case you’re wondering, that’s about a million times the number of AAA batteries sold in the U.S. each year. And therein lies the problem with batteries, namely their low energy density. But as I hope you’re seeing in this class, such problems are often opportunities for chemistry to come in and make a game-changing difference. This figure shows the energy per weight vs. energy per volume for a few different storage materials. Note that batteries are in the lower left, which isn’t so good. You can see why gasoline is so unique: it’s got a very high energy density both by weight and by volume, and on top of that it’s safe and easy to transport and extremely cheap. The competition is tough. One liter of gasoline stores 33 megajoules (MJ) of energy, or about 9 kWh if you prefer those units. That liter costs about a dollar. Now take one MIT Professor, who can output about 60 W or 60 J/s on a good day. This means that the professor can generate the same amount of energy as in a liter of gasoline, 33 MJ, in roughly 153 hours. Given the average pay of an MIT professor of $10/hr, it would cost $1530 to get the energy from the professor. Compared to about $1 to get the same amount of stored energy from the gasoline. Batteries need to start looking a lot more like gasoline and a lot less like an MIT Professor if they’re going to be competitive grid-scale energy storage technologies. The good news is that there has already been tremendous progress in just the last 10-20 years. Check out this plot of energy density vs. year for batteries. Notice the very slow progress for about 150 years, with a doubling of energy density occurring roughly every 60 years. Then the slope of progress changes, and the reason is simple: chemistry. Specifically, the rapid improvements are happening because we’ve reached an era wherein we can make energy hills for ions with many more types of materials than ever before. I’ve added to the plot the number of materials cathodes are made of: historically only about 10 different cathode materials were used during those slow-paced first 150 years, but now we can and do make cathodes out of over 80 different materials, and the number is growing by Figure energy vs. year © source unknown. the week. This explosion of materials possibility is due to a deeper understanding of ions and new ability to design materials with more efficient energy landscapes for them to traverse. So yes, there’s a lot of work to do to make batteries competitive in grid-scale energy storage, but the time is absolutely ripe to get it done. Why this employs Knowledge of ions can get you all sorts of jobs. But since I focused on the battery field in Why This Matters, let’s focus on that same topic here. The global battery market is already enormous and its projected growth is stunning. Here’s a chart from a report from Deutsche Bank from 2016 showing the projected near-term growth in battery production by some of the top companies that make batteries. All of these companies are hiring thousands and thousands of scientists and engineers, techno-economists (predicting tech trends is not easy!), packaging and mechanical design experts, and much more, all to work on various aspects of one thing: the ion. And there are many more jobs than that. Big battery companies like those listed in the chart are investing heavily in hiring people to work on batteries, but so are little companies. And so are many research teams at universities, who are spinning out companies based on new concepts. Not only are there many different battery chemistries that have been around for a while and are being greatly improved, from lithium-ion to nickel-metal-hydride to lead-acid, but there are also so many exciting completely new directions for batteries. All of these future battery technologies involve jobs, and while it’s hard to list them all because of how much the field has grown, here are a few cool directions companies are pursuing: • Silicon-based anodes (replacing the currently used graphite) which would increase energy density by a factor of 3 • Using metal nanowires in the electrolyte to prevent breakdown, making the battery last 10 or 100x longer New Battery Capacity figure removed due to copyright restrictions. • Making the electrolyte a solid instead of the current liquid, which would make the battery more stable and allow much faster charge times • Throwing graphene into the battery since, anyway, what problem can graphene not solve? (ok, but in all seriousness, graphene has been shown to allow for much faster charge/discharge) • Making batteries out of foam metals • Making batteries foldable (paper-thin but that won’t break) and also stretchable • Aluminum air batteries that have demonstrated a 1,100 mile range in an electric car • Powering the charging of batteries with variou means, from wi-fi signals (not inductive wireless, which is something else), to ultrasound, to amino acids, to urine. • Batteries made from sodium, zinc, sulfur, magnesium, and so many other materials! • Making batteries that can never catch fire 44 Example problems 1. Choose the larger atom from the following pairs: a) Rb, Rb+ b) Rb+ , Kr 2. Draw the Lewis dot structures of FeCl3 and MgF2 . Solutions 1. a) Rb is larger, because it has an additional shell (Rb+ loses its 5s1 electron) b) Kr is larger, because it has one less proton than Rb+ , so the nucleus pulls less on the electrons 2.