 So what you need to be able to do is describe the advantages and disadvantages of using fuel cells compared to regular galvanic cells. One of the advantages is, as long as you're supplying fuel to the anode, you're going to generate electricity. With a typical galvanic cell, as soon as your anode breaks up and is turned into its ions, then you can't use it anymore unless you recharge it. You're not burning fuel, so you're not generating large amounts of heat necessarily from the combustion process. The only product if we're looking at a hydrogen-oxygen fuel cell is water vapor, which is better for the environment than, I don't know if you're using petrol for example, better for the environment than the carbon dioxide, carbon monoxide, particular sorts, and so on that are being produced from a petrol engine. The other way of looking at it is compared to a galvanic cell, you're not throwing out a battery at the end of it, so you can reuse it as long as you can supply a fuel to it. Hydrogen is very abundant, it's one of the most abundant elements, well it is the most abundant element in the universe. And their efficiency compared to burning diesel or burning gas or burning petrol was really, really high. Again, this doesn't really relate to a galvanic cell, but you get more energy out per gram of fuel from a fuel cell than you do from burning it. Disadvantages, since they run on hydrogen, if we're talking about a hydrogen-oxygen fuel cell, there's no really easy and efficient way of generating hydrogen. We have lots of water which contains hydrogen, but in order to get hydrogen out you have to put electricity in to break up the hydrogen away from the oxygen. And that requires energy to break it up and then you don't get that amount of energy back when you use it in the fuel cell, nothing is 100% efficient like that. So just producing the hydrogen at the moment, oftentimes we just strip it off of methane and other natural gases and that's bad for the environment because it generates carbon dioxide anyway. At the moment it's very difficult to transport hydrogen because we don't have the infrastructure to do that. It's a gas, it's a very small gas so it can leak out easily from containing, and it's also quite flammable. So we need to have a distribution method to distribute hydrogen all around so it can be used very easily. Like I said, storage, it's a very small gas so it leaks out from the gaps between whatever storage medium you're using. So in a metallic tank, the hydrogen can leak out of the tank slowly over time. They're very expensive to produce fuel cells at the moment, they're a new technology, there's not really a big scale in terms of their production. So because they're expensive to produce at the moment compared to say a galvanic cell. And it's a technology that's been around for a while but it hasn't really progressed as far as we would like it to. At the moment we just can't use it in a way compared to using petrol to power a car or even using electricity in typical galvanic cells in electric cars. We don't have the distribution system to run it. The other thing we're going to talk about today is the nature of charging and discharging batteries. There is why batteries go flat and this relates to our discussion about fuel cells is because your anode gets corroded, it breaks down, it goes into the electrolyte solution. So here we have our zinc copper danial cell that we made. The zinc is the anode so this is going to break it down from zinc metal to zinc ions that will go into solution. And once that happens you don't have a source of electrons anymore so you're not going to generate electricity. So I've got a wonderful animation here which should show the anode disappearing. There it goes. So once the anode disappears you don't have a source of electrons anymore and the battery won't generate electricity. Now the normal batteries you have in your devices don't look like this. They're not two cells connected like this at all. But the idea is the same. Once the anode has been broken down into its ions it doesn't generate electricity anymore so that's why your battery goes flat. Now depending on the type of battery they can be recharged the way that this is done is by reversing the currents flowing into the cell. So the battery discharges and you have your anode being oxidized. If you put electricity in you can reverse those reactions so your anode which was oxidized can then be the ions from the anode can be reduced back to a new anode and then you can discharge your cell again and again. So if we look at our zinc copper cell here I've got a little bit of anode left over. I'm going to put in energy and that's what this symbol here is that's showing a battery. So I'm going to put in energy and hopefully if my animation works we should see the anode being rebuilt. Look at that. Isn't that amazing? So what I'm doing is I'm turning the zinc ions from the solution back into solid zinc on my anode. By forcing the electrons the other direction we're making what wasn't a oxidation reaction, a reduction reaction. We're reducing the zinc ions to solid zinc to rebuild our anode. And once we've done that we could reuse this again. Now in your typical devices you're using a lithium ion battery in your mobile phone say on your laptop. But the idea is the same. You're putting in electricity when you're charging your battery. You're reversing the typical reaction which would be the oxidation of your anode. You're turning that into a reduction reaction to regenerate your anode. So today on Flipping Science we looked at fuel cells and the charging and discharging of batteries. That's it for Flipping Science today. See you.