Explaining energy conversion efficiency and combustion
You probably already know that I write a lot about physics experiments. But I also love science toys and today I want to talk about something more chemical than physical. Obviously, the electrochemical fuel cell cars I am talking about can (and in my opinion should) be included in physics education, because talking about energy conversion efficiency in real life and authentic settings is so valuable. The big advantage of fuel cell car toys is that they combine fun with science with real interdisciplinary science learning. See for yourselves!
I’m a sucker for fun science toys. And I really loved getting into the chemistry behind the electrochemical fuel cell car I want to talk about today. Even though I don’t really understand a lot of chemistry and I apologize in advance for any wrong namings. After all, I’m a physicist at heart. So I will go briefly over the chemistry behind the fuel cell car before talking about what we can learn about energy conversion efficiency.
I have already talked about the use of science toys in science education. The drinking bird for example, or the everturning spinning top. They’re fun to watch and with a little effort, behind all the games and giggles is lots of physics and science to learn about. With the fuel cell car it’s a little different. In order to understand how the car works, we need to take a look at the theory behind it first.
Electrolysis of water, a form of water splitting
In order for the hydrogen fuel car toy to work, we need to fuel it. As an everyday science toy, all you need is water and a battery. And then it’s fairly simple and the process is similar to the lemonade clock I presented in one of my earlier posts.
First, we connect a battery to the wiring of the toy and fill the provided container with water.
The reduction at the cathode (2H2O + 2e− → H2 + 2OH−) and the oxidation at the anode (2OH− → 1/2 O2 + H2O + 2e−) have the overall effect of splitting the water into its compounds: 2 H2O → 2 H2 + O2
We have twice the number of hydrogen molecules than we have of oxygen molecules and we can also see that in the tanks. We now have the fuel it takes to make the car work.
Combustion: the car drives!
Removing the battery and plugging the cables into the right places in the car should then ultimately lead to a driving toy car. Basically, it’s the reverse effect of the electrolysis. There is an exothermic redox chemical reaction between the hydrogen and the oxygen, the freed energy gets converted into electrical energy and can now be used for the movement of the car. And this is also where the physics comes into place. Let’s talk about the energy conversion efficiency.
Energy conversion efficiency
In order to fuel the toy car, we needed to plug in a battery to split the water with electrolysis. This is where electrical energy gets converted into chemical energy. When removing the battery and jumpstarting the toy car, this chemical energy then get’s transformed back into electrical energy and ultimately into kinetic energy because the car moves around. And all these conversion processes take their toll. Output energy is always lower than the input energy and this is something very important for students to understand.
Especially with toys like the car we used, energy conversion efficiency is really low. Efficiencies for electrochemical fuel cars can range from 30% up to 65% which is already really good. These tested fuel cars are still extremely expensive and cannot be compared to our simple toy model.
But it’s already an important lesson, as students see that they have to fuel the car with electrical energy and do not get the same amount of electrical energy back when the car moves. The battery had to come from somewhere and the electrochemical fuel car is not the magic solution to all of our energy problems with cars and trucks. I believe, this makes for an extremely valuable lesson regarding content learning but also discussion of real life contexts and socio scientific issues like the sustainability issues we currently discuss in world politics.