Lesson Video: Batteries and Fuel Cells Chemistry

In this video, we will learn how to describe the production of electrical energy from batteries and fuel cells.

15:44

Video Transcript

In this video, we will learn how batteries and fuel cells produce the electrical energy that we use to power all kinds of devices. We’ll also learn about the pros and cons of these two kinds of technologies. The first battery was created in 1799 by Alessandro Volta. His battery was made of alternating plates of zinc and copper metal, with an electrolyte or a substance that contains ions between them. Batteries like this work by pairing up two different substances, in this case, zinc and copper, so that we have a reduction oxidation or redox reaction occurring.

Since all redox reactions involve the transfer of electrons, we can use the electrons that are produced in a redox reaction to do electrical work for us and power devices. Recall that different substances have different reactivities, which is described by the reactivity series. This difference in reactivity between the materials that make up the battery creates what’s called a voltage. This voltage is what’s responsible for pushing electrons through the battery, so we can use them to power devices. To see how all of this works, let’s see how we might construct a very simple electricity-producing device called an electrochemical cell.

The type of electrochemical cell that we’ll be looking at is called a galvanic cell, or sometimes called a voltaic cell in honor of Alessandro Volta. In an electrochemical cell, we’re converting chemical energy that’s produced as a result of a chemical reaction into electrical energy. To construct our electrochemical cell, the first thing we’ll need is two different substances to act as what’s called the electrodes of the cell. These will typically be metals since they need to conduct electricity. These electrodes will both be in an electrolyte, something that contains ions. Then we need a wire to connect our two electrodes so that electrons can flow.

But how will the electrons flow in this cell? As we mentioned previously, reactivity is going to play a big role here. Zinc is more reactive than copper. And since more reactive substances tend to lose electrons more easily, the zinc electrode will lose electrons forming the zinc two plus ion and two electrons. And of course, since oxidation corresponds to the loss of electrons, zinc here is being oxidized, which means that copper must be being reduced, Cu2+ plus two electrons forming solid copper.

Since electrons are being produced at the zinc electrode, we call that electrode the negative electrode. And since electrons are being used up at the copper electrode, we call this electrode the positive electrode. So the electrons are produced at the zinc electrode; then they travel through the wire to the copper electrode. At the copper electrode, they react with copper two plus ions, which are likely in the electrolyte solution to form solid copper. Of course, we don’t just have to have a wire connecting the two electrodes. We could hook up our cell to something like a light bulb and use the electrons generated in the cell to power it. We could also hook up our cell to a voltmeter, which could measure the voltage or the difference in reactivity between the two electrodes.

However, we wanna make sure that we’re connecting our voltmeter the right way. If we swap the leads or the wires that are coming out of the voltmeter, the difference in reactivity will be measured the opposite way, so the sign will be flipped. So if we get a negative reading on a voltmeter when we connect it to a cell, we simply need to switch which lead is connected to which electrode to get a positive reading. Since the voltmeter is measuring a difference in reactivity between the electrodes and the electrodes have different reactivities, the voltage reading that we get will be different, depending on what materials our electrodes are made of.

So let’s see how the voltage would change if we swapped out the zinc electrode for an iron electrode. Well, iron is less reactive than zinc; it’s still more reactive than copper, meaning that iron will be oxidized and lose electrons and copper will still be reduced and gain electrons. When we connect our iron copper cell to a voltmeter, we would see that the voltage that we measure is less than the voltage that we got for the zinc and copper cell. So the greater difference and reactivity we have between our two electrodes, the greater the voltage reading will be. Of course, the electrolyte solution can also alter the voltage reading. So this is only true when we have two cells that we’re comparing that are in the same electrolyte.

This also implies that if we had the same material for both electrodes, for example, if we had two copper electrodes, there wouldn’t be any difference in reactivity. So the voltage reading would be zero, and there would be no electrons that would flow through the cell. So we wouldn’t be able to use it to power anything. Let’s go back to our zinc and copper cell. We’ll notice that as the cell produces two electrons, that is, where the zinc reacts to form zinc two plus ions and two electrons, the zinc will be used up. As this happens, we’ll notice that the voltage reading that we get will start to decrease. And it will continue decreasing until the zinc is used up, at which point the voltage reading will be zero. And there will be no more electrons flowing in the cell.

Since batteries are simply multiple cells connected together, which is why we saw the alternating plates of zinc and copper metal in Alessandro Volta’s battery, this explains why batteries go dead over time. Eventually, the reactants are all used up, and we can no longer use them to power electrical devices. But some batteries can be recharged. How does this work? When a rechargeable battery is being used to power something, it works exactly like the cells that we were just looking at. Electrons flow from the negative end of the battery to the positive end of the battery until the reactants are used up, at which point the battery stops working.

But when this happens, we don’t throw the battery away. We connect it to an external power supply, like plugging it into a wall to recharge it. When we supply electricity to a battery or a cell, it essentially causes the electrons to flow in the opposite direction that the electrons were flowing when the cell or battery was being used to power something. To see how this works, let’s go back to our zinc and copper cell. When we connect this cell to an external power supply, the electricity would flow into the zinc electrode. Since electrons are flowing to the zinc electrode, that means that zinc will be reduced. Zinc two plus ions would react with those electrons to form solid zinc. And since zinc is being reduced, that means that copper must be oxidized.

Solid copper is reacting to form copper two plus ions and two electrons. So this means that connecting the cell to an external power supply allows us to reform the reactants in the cell that we started with. Notice here that since electrons are being used up at the zinc electrode, that makes the zinc electrode the positive electrode, even though zinc is more reactive than copper. And on the other side of the cell, we have electrons being produced at the copper electrode, making that electrode the negative electrode, even though it’s the less reactive material.

When we set up a cell like this and connect it to an external power supply, it’s called an electrolytic cell. Once the battery is recharged, that is, our reactants are reformed, we can once again connect our battery or cell to a device. Then electrons will again flow from the more reactive electrode to the less reactive electrode. And we can use the battery or cell to power things once again. Of course, the rechargeable batteries that are in the devices we use aren’t made of zinc and copper; rather they’re lithium-ion batteries. Lithium-ion batteries are the batteries that you’ll find powering cell phones, laptop computers, and even electric vehicles. Lithium ion batteries have a number of advantages and disadvantages, so let’s go through those.

The primary advantage of lithium-ion batteries is that they’re small and light weight, meaning that they’re perfect for use in a cell phone or a laptop. They also carry a lot of charge, which means that if we use a lithium-ion battery in a car, the car can drive for quite a long time without needing to be recharged. But when they do need to be recharged, it does take time, which means that if we’re using lithium-ion batteries to power vehicles, we might need to wait around for a long time if we’re trying to take the car on a long trip. There’s also a limit to the number of times that lithium-ion batteries can be recharged, which is why cell phones and laptops that have lithium-ion batteries in them don’t quite hold their charges well over time.

When these batteries can no longer be recharged and it’s time to get rid of them, they can leach harmful chemicals into the environment. Of course, lithium-ion batteries can be recycled to minimize this impact on the environment. Finally, lithium-ion batteries can be flammable if they’re damaged or improperly handled.

Another option for generating electricity on the go is fuel cells. Fuel cells work by reacting some kind of fuel, usually hydrogen with oxygen, to create energy. Let’s take a look at how hydrogen fuel cells work to understand them. The fuel, in this case, hydrogen gas, enters the cell on one side. When the hydrogen enters the cell, it reacts to form hydrogen ions and electrons. The part of the cell that the hydrogen enters in is usually made of platinum or some kind of platinum-containing alloy to catalyze the reaction. Once the hydrogen ions are produced, they travel through the electrolyte to the other side of the cell, which is where the oxygen enters. There, the oxygen reacts with the hydrogen ions and electrons to form water. So the overall reaction in a hydrogen fuel cell is 2H2 plus O2 reacting to form 2H2O.

Notice that the electrons are produced at the hydrogen side of the cell, which makes that part of the cell the negative electrode. These electrons then travel through a wire that connects the two sides of the cell, and they’re used up on the oxygen side of the cell, which makes the oxygen side of the cell the positive electrode. And of course, we can use the electrons that flow from the negative to the positive side of the fuel cell to power devices. A big difference between fuel cells and batteries is that in fuel cells, the fuel must be continuously supplied in order for the cell to supply electricity, whereas in a battery, all of the reactants are there at the beginning and they’re used up over time.

Fuel cells have a different list of advantages and disadvantages than rechargeable batteries did. Let’s take a look at what they are. A huge advantage of a hydrogen fuel cell is that the only product is water. So if we were to use hydrogen fuel cells to power our vehicles, the emissions from driving would be much cleaner than using other kinds of fuels. Another advantage of fuel cells is that they don’t need to be recharged like batteries do. If we have a car that’s powered by a battery, we need to wait in order for the battery to be recharged. But if a car is powered by a fuel cell, we can simply refuel the car similar to a car that’s powered by gasoline or diesel. But on the disadvantages side, while a fuel cell itself is quite small because the fuel in a fuel cell is a gas, it takes a lot of space in order to restore the fuel.

If we wanted to have a car that could drive 100 kilometers without stopping that was powered by a hydrogen fuel cell, the storage tank would need to be 11 meters cubed. A volume of 11 meters cubed corresponds to a cube with sides of about 2.2 meters, which would be totally infeasible to have in a car. However, hydrogen storage is a very active area of research in chemistry. Scientists are looking into various techniques to reduce the amount of space that’s needed to store hydrogen, such as pressurizing the gas or using materials to capture the hydrogen. Another disadvantage of hydrogen fuel cells is that our current methods for obtaining hydrogen gas are often energy intensive. This is because we often obtain hydrogen through mining natural gas or splitting water. Finally, H2 gas can be flammable.

Now we’ve discussed everything we need to know about simple electrochemical cells, batteries, and fuel cells. So let’s wrap up this video with the key points. Simple electrochemical cells and batteries are powered by a redox reaction between substances that have different reactivities, which creates a voltage. This voltage pushes electrons in the cell from the more reactive electrode to the less reactive electrode. A greater difference in reactivity between the substances in a battery or cell produces a greater voltage. Some batteries, like lithium-ion batteries, can be recharged if they’re connected to an external power supply, making them useful in all kinds of devices. Hydrogen fuel cells are powered by the reaction of hydrogen with oxygen, the only product of which is water.

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