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We've been studying the applications of voltaic cells. So far, we've examined disposable and renewable batteries. Currently, it seems like rechargeable batteries are the way of the future. Car companies like Tesla produce electric cars that can travel more than 400 miles. But, there are a few caveats. One, we need to charge them with renewable energy sources. If we're getting our electricity from coal, we're still producing greenhouse gases. Two, most renewable batteries use the alkali metal lithium. How do we get lithium? We have to mine it. The environmental impact of lithium mining is under criticism. Also, over time, the battery loses capacity. Currently, most electric vehicles need their battery replaced around 150,000 miles. But, there is an alternative to renewable batteries. It's called a fuel cell. They take advantage of a fundamental chemical reaction. What reaction? The reaction between hydrogen and oxygen gas.

What are Fuel Cells?

Fuel cells are different from other electrochemical cells. They still take advantage of redox reactions. But, they have a unique configuration. The most common fuel cell is the hydrogen fuel cell. It takes advantage of the reaction between hydrogen and oxygen. What's the product? Water! Can you imagine if our vehicles produced water instead of carbon dioxide?

So, how do you build a hydrogen fuel cell? First, we need to keep hydrogen and oxygen separate. That's the same as a normal voltaic cell, where the anode and cathode cells are independent. So, what's the electrolyte in a fuel cell? The most common is a polymer membrane. The polymer allows hydrogen ions to pass through it. But, it blocks hydrogen or oxygen gas. We call it a proton exchange membrane.

So, we have a way to keep our gases distinct. How do we start the reaction? On the hydrogen side, we use a platinum catalyst. Remember, a catalyst lowers a reaction's activation energy. So, we convert hydrogen gas to two hydrogen ions. What happens to the electrons? They travel through the circuit onto the oxygen side. What about the hydrogen ions? They diffuse across the polymer membrane. So, they also end up on the oxygen side. Now we have all three components on one side. What happens? The hydrogen ion, electrons, and oxygen react and yield water!

Where do we get hydrogen and oxygen? We store the hydrogen in a tank. That's the fuel in the name fuel cell. The partial pressure of atmospheric oxygen is enough to react. So, for oxygen, we use air. For example, a car's air intake would provide enough oxygen.

Electrochemistry of Fuel Cells

Let's look at the half-reactions for a fuel cell. Remember, both hydrogen and oxygen are diatomic gases.

Anode: $H_{2(g)}\rightarrow 2H_{(aq)}^{+} + 2e^{-}$

Cathode: $2H_{(aq)}^{+} + 2e^{-} + \frac{1}{2}O_{2(g)} \rightarrow H_{2}O_{(g)}$

Overall: $2H_{2(g)} + O_{2(g)} \rightarrow 2H_{2}O_{(g)}$

The oxidation of hydrogen takes advantage of the standard hydrogen electrode. We even use the same platinum metal catalyst! The standard reduction potential of oxygen has a voltage of $1.21V$ .

Pros and Cons of Fuel Cells

Benefits:

The product of the reaction is water! So it doesn't produce greenhouse gases.
Redox reactions are more efficient than combustion. In internal combustion engines, a lot of energy transfers out of the system via heat. All we do is increase the surrounding air temperature! None of that energy gets used for mechanical work.
Hydrogen fuel cells may be safer than standard gas tanks. The safety standards for hydrogen tanks are significant. We make the tanks from carbon fiber. Even if the tank cracks, it doesn't spark. So, the hydrogen gas diffuses. Hydrogen has a tiny molar mass. So, we know from Graham's law, that it will diffuse quickly. Humorously, Toyota shot their carbon fiber tank with a 50 caliber bullet. The tank punctured. But, the hydrogen gas leaked out. In contrast, gasoline is a liquid. So, if it combusts outside the vehicle, it can cause massive fires.
Hydrogen fuel cells may be safer than batteries too. In theory, batteries are safe. But sometimes, manufacturing errors produce corrupted batteries. The failed batteries allow the cathode and anode to touch. In lithium batteries, the result is a violent reaction. You can find videos of electric cars spontaneously bursting into flames.
One of hydrogen's benefits is its similarity to our current fuels. Let's say you had a hydrogen-powered car. How would you fill it up? You'd still pull up to a gas station and insert the pump. It takes about three minutes to fill the tank. Meanwhile, you can still wash your windows. What's the benefit compared to battery-powered vehicles. Battery-powered cars have much longer charging times. So, if you're traveling a long distance, you need to plan around the charging times.

Challenges:

The biggest challenge is the lack of infrastructure. In 2020, there were only 43 public hydrogen fueling stations. Almost all are in California. Do you live in California? If not a hydrogen fuel cell vehicle is useless.
Another major challenge is that hydrogen is a gas. We know that gases are compressible and expandable. So, we can pressurize the gas tank. Yet, even with pressurization, the amount of fuel is limited. We could make larger tanks. In industrial energy production, this is not an issue. But, for vehicles, larger tanks equal increases in weight and size.
The final challenge is how we produce hydrogen gas. One option is the electrolysis of water. We use electricity to split water into hydrogen and oxygen gas. What's the problem? We still use fossil fuels to produce a lot of our electricity. The goal of the hydrogen fuel is to be a net-zero form of travel. So, we need to use renewable energy to make hydrogen. Unfortunately, we still have a lot of work to do if we want to only use renewable energy sources.

Are Fuel Cells the Future?

So, are fuel cells the future? Possibly. For the foreseeable future, batteries will reign supreme. Like we said in the pros and cons section, hydrogen doesn't have the infrastructure. It's true that public battery charging stations aren't readily available. But, you can charge at home. Also, currently, hydrogen is expensive. As gasoline and natural gas prices rise, hydrogen may become more cost-competitive. But that's not going to happen any time soon. So, why did I say possibly? The benefit of the fuel cell is the accessibility of hydrogen. Every country in the world could make its own. You only need water and electricity. What if we had completely renewable energy? We could cleanly produce hydrogen. We could avoid the expense and destruction of lithium mining. If that were the case, fuel cells could replace batteries.