Warning Potassium hydroxide is corrosive. Wear gloves when handling it. Greetings fellow nerds. In a previous video i showed that silver could be restored by electrochemistry. Of particular note is this experiment where i restored silver using zinc metal but did it by connecting it through a nickel strip. The zinc metal oxidized, releasing electrons that travelled through the nickel conductor where they reduced the tarnished silver back into silver metal. But what if we break the conductor and forced the electrons to travel through a load? We could harness the energy of the reaction to perform useful work. What we have just described is a battery. But let’s take it a step further than that. In another experiment i showed you could artificially tarnish silver by driving an electric current through it. And in yet another video i showed that zinc metal could be plated out of a solution by driving a current as well. So if we can both generate the silver tarnish and zinc metal by inputting electric power, and then get that power back by running the silver restoration experiment, we can get a rechargeable battery. In this video we’re going to make the classic rechargeable silver zinc battery. So let’s get started, first let’s make our electrolyte. We get 80ml of water and add in 24g of potassium hydroxide. Stir it until dissolves. You can also use cheaper sodium hydroxide but potassium hydroxide is preferred for batteries since it gives higher conductivity and thus better current. Now once it’s all dissolved we add in 30g of zinc oxide. The amount of zinc oxide you use should match the weight of the silver metal you use for the positive electrode. Now we’re using so much zinc oxide that most of it won’t dissolve, this is acceptable since the zinc is plated out as the battery recharges so the excess zinc oxide will dissolve into solution as needed. Now that we have our electrolyte let’s make our electrodes. First we get a couple of nickel strips. These will be our supporting conductors, also known as current collectors, and I selected them because under alkaline conditions they won’t corrode. So i thought they make very good collectors since they would always maintain good connection to the silver and the zinc and never break down. Unfortunately I forgot they have another property which makes them very bad for this experiment, they have low overpotentials. I only realized this when i got around to charging the battery which you’ll see in a few minutes. So now we need a source of silver. The best thing to use is silver powder but i don’t have any so i’m going to use my silver coin that you’ve seen in my silver restoration and toning videos. Anyway i’ve folded over the bottom of the nickel strip so it holds the coin. Now we wrap the silver in a filter paper separator to hold it in as well as any silver oxide that forms and to prevent the negative electrode from touching it. All we do is insert the electrodes into our electrolyte and there is our silver zinc battery. It’s very easy to build. Now in this state it’s actually fully discharged, it won’t give us any power if we try to use it. So we have to charge it up first. To make this more interesting to watch, i’m going to remake my electrolyte with just enough zinc to dissolve, but not enough to precipitate out. This way we have a nice clear electrolyte so we can see what’s happening. The battery will still work it just won’t work very well. If you want to actually make your battery useful, use the recommended quantity of zinc oxide. And here we are, a fully reassembled battery with zinc depleted electrolyte so we can see what’s going on. I’ve also removed the filter paper separator and attached charging clips. Running without the seperator is fine as long as I make sure the electrodes don’t touch. The silver electrode is connected to the positive lead of the power supply and the negative is connected to the other electrode. Now to charge it we apply two volts across the cell. And there we go. You can see the silver metal darkening as it charges. So what’s happening. At the cathode or negative terminal, electrons are coming in and reducing zinc ions to zinc metal that get deposited. This is exactly what happened in my video on making zinc powder by electrochemistry. At the anode or positive terminal we’re removing electrons from the silver and creating silver oxide. This is essentially a massive artificial tarnishing reaction as i showed in my previous video on toning silver coins. Overall we’re moving oxygen from the zinc oxide to the silver. I’m going to let this run for about twenty minutes to get nice thick layers of silver oxide and zinc metal. I’m not going to run longer because eventually the metals will get thick enough that they start to fall off. This is not a problem for a properly built cell that has porous separator to physically hold the metals in. But for my demonstration cell that has no separator, the loss of metals means they won’t contribute to any power we may want to get back out during discharge. Anyway here we are. And let me take out the electrodes to examine them. Here is the positive electrode with the silver. And you can see the thick black coating of silver oxide. We basically did our silver tarnishing experiment but on a massive scale. Interestingly enough we should also note the fine details are almost completely destroyed. This is not a coin anymore, just a disk of metal. This also illustrates that for coin preservation efforts, it’s actually the tarnishing of the coin that does the most damage. The cleaning or restoration processes are the lesser contributors to coin decay. For some reason there is a misconception among some collectors that the opposite is true. Anyway. Let’s take a look at the negative electrode. And we have a nice loosely adherent layer of zinc metal. This is just like our previous video where we made zinc powder by electrolyzing a solution of sodium zincate. So now we have a small but useable charge on our battery. Let’s take some measurements. Let me first measure the voltage. And it looks like we’re getting an open circuit voltage of about 1.8. In practice the working voltage is around 1.55 for silver zinc batteries. Now let’s check the current. Looks like we went off the scale. This is a surprise since i wasn’t expecting so much current from such a small demonstration battery. I’m going to have to switch over to high current mode. On a side note, measuring the current directly by shorting the ammeter across the battery terminals is usually a very bad thing to do with proper commercial batteries, especially the larger ones. The current surge will at the very least blow a fuse in your meter. Anyway, let’s see what we have now. And there we go. I briefly saw two amps of current before it ran out of charge. This is quite strong for such a tiny and poorly built battery. Let me rollback the video and i want you to pay special attention to the battery itself. The silver coin is actually being restored as the battery runs. The black silver oxide is being converted into silver metal. This is essentially the silver restoration reaction i showed in my previous video except now we’re running the current through our meter. The overall chemical reactions during discharge are just like the charging reactions except now we’re running them in reverse. Let met get the silver electrode out to show you. As you can see the silver oxide has been converted back. Unfortunately the restored silver does not restore the previously destroyed features of the coin. Okay, so now that we have a working rechargeable battery, what can we do with it? Originally i thought it wasn’t going to be too powerful but if it can put out a couple of amps at 1.55 volts then that’s enough to charge a cellphone. Now cellphone actually needs 5 volts, so we would need about three or four of these batteries to reach the right voltage. But there is a workaround if you just have one. There are these boost converter modules that can increase the voltage of a low voltage power source up to the 5 volts needed to power a cellphone. But in order to do so they sacrifice current so you need a high current to use them. The overall power usage is the same so we’re not getting free energy. The battery just drains faster because of the higher current draw. Luckily our battery is high current so we can get the power we need. Anyway let me first recharge my battery again for another twenty minutes. And here we are along with my cellphone with the power converter. Now we just hook it up. And the cellphone responds to charging. It’ll only work for a minute though before the battery runs out of power. Let me recharge it. And we can try again. And the battery does work again. Proof that we have a working rechargeable battery even though it’s very weak and just lasts a minute. You’re probably asking how we can improve this and it’s actually pretty straight forward. First we use the full amount of zinc oxide we specified earlier. And for the silver electrode rather than use a solid piece of silver like a coin, we instead use silver powder. Technically we can use solid silver but we’d have to condition it by repeatedly charging and discharging the battery to destroy the silver and make it crumble into a powder. Now at this point i should address what i meant earlier about nickel being a bad choice of support metal. If you watch closely during the charging cycle you can see bubbling of the electrodes. In addition to the desired reactions of depositing zinc and oxidizing silver, we’re also getting electrolysis of water. Now zinc has a very large negative overpotential for hydrogen so hydrogen does not form on it. But nickel has a comparatively small overpotential so hydrogen forms on the nickel and bubbles out. This is a big problem early in the charging cycle since it wastes power better spent on plating out the zinc. But as it runs the zinc does get deposited at a slow rate and eventually the deposit is thick enough that it covers up all the nickel sites and the hydrogen formation slows. The more problematic electrode is the positive electrode with the silver. This time oxygen is the gas being formed, and while silver itself has a decently high overpotential for oxygen formation, the nickel does not. So it generates oxygen gas, wasting power better spent on oxidizing the silver. It’s more problematic than the negative electrode as the silver is not plating in or out of solution and remains solid as either metal or oxide throughout the charge and discharge cycles. So the nickel current collector is never covered up and continues to bubble oxygen, always wasting power. I’ll admit i blundered big and totally forgot about overpotentials when i assembled this. Nonetheless we still had charge build up and were still be able to demonstrate the use of the battery. But if we wanted to build it better, what would we do? The easiest method is to use solid metals throughout, no current collectors. We can do that for the negative side with the zinc but silver is expensive so finding cheaper solutions would be a good idea. The better solution is to use a better material for the current collector. In this case graphite. Graphite has a very high overpotential for oxygen, actually higher than that of silver. So it makes the perfect current collector to ensure connection to the silver without itself participating in the chemical reactions. I’m using a graphite block clipped to the silver disk. The metal clip itself is separated by plastic so it doesn’t participated in the reactions other than to hold everything together. Let me put it into the battery. I didn’t bother changing the negative zinc electrode because now that the nickel is completely coated it’s no longer going to generate hydrogen. Now we turn on the current to begin charging. And there we go. The charging is working and very little bubbling is happening. We’re getting much more efficient charging this way. Understanding what materials to use to promote the reactions you want and suppress the ones you don’t want is a huge part of battery research. It’s also why a lot of promising batteries that sound great on paper don’t translate into real life applications, they just haven’t been able to solve a lot of the side reactions and problems. Now the next logical step was to make a longer lasting high powered battery by using powdered electrodes with more surface area. But i’ll let you know up front that I failed miserably. What I tried was putting zinc oxide along with a carbon electrode as well as silver powder and a carbon electrode in separate bags made of filter paper. Now i said earlier that i didn’t have silver powder so i had to make some. The process was so interesting that i produced a separate video on it that i linked in the video description. Then I put the two assemblies in a beaker, hooked them up to power, and added the potassium hydroxide electrolyte we made earlier. The battery accepted charging and i left it there overnight. Unfortunately when i tried to use it the battery didn’t work. It was even worse than before and couldn’t charge the cellphone at all. I think this is because the powder is very loosely held together in the filter bag so it doesn’t form a strong high current connection. In commercial batteries, the electrode powders are compressed for better contact and conductive fillers like carbon are added to improve conductivity. The battery isn’t dead though, it powers this small USB monitor device. But it can’t provide enough current to charge a cellphone. To improve the performance I would need to find a better way to make the electrodes. Most likely by electroplating silver and zinc onto seperate graphite electrodes. But that’s another project for another time. I’ll be honest though I probably won’t attempt to make a viable rechargeable battery because of the already well-known problems with this chemistry. One of the big problems of rechargeable silver zinc batteries is dendrite formation. Basically as you recharge the battery it doesn’t deposit evenly. We saw this issue in my video on making zinc powder. As you can see the zinc deposited in a bulge here. After many repeated charge and discharge cycles the zinc would reach over to the other electrode and short circuit. The dendrites could even puncture the separators given enough time and cycles. At that point the battery was inoperable and must be disposed of or recycled. So overall, making a rechargeable silver zinc battery is not high on my priority list. Now you may be asking if these batteries are used anywhere. They actually were used a lot in the past being easy to construct and furnishing tremendous currents when properly built. For spacecraft and military applications their high performance outweighed their high cost. The fact that they were easy to recycle also helped to mitigate costs somewhat. But recent decades saw the advent of modern rechargeable batteries like lithium ion. With their more usable cycle life they replaced rechargeable silver zinc batteries in most large scale applications. Although you can still find them in smaller battery sizes. The non-rechargeable silver oxide battery is actually still used a lot as hearing aid batteries. And they are still occasionally used as reserve or emergency standby batteries owing to their high power output and reliability. Also, being water based batteries they can’t spontaneously explode or catch fire. Anyway, there we have it, a simple but working rechargeable silver zinc battery. And using it we were able to recharge a cellphone for only about a minute, but we still did it. Thanks for watching. Special thank you to all of my supporters on patreon for making these science videos possible with their donations and their direction. If you are not currently a patron, but like to support the continued production of science videos like this one, then check out my patreon page here or in the video description. I really appreciate any and all support.