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.
