AP® Environmental Science review sheet from Aim for Five (aimforfive.com/enviro/units/6/6-11)
Unit 6 · Topic 6.11
6.11 Hydrogen Fuel Cell
A hydrogen fuel cell combines hydrogen and oxygen to make electricity, with water as its only exhaust. Hydrogen is an energy carrier rather than an energy source, though: it must be produced first, often from natural gas, and storing and moving it is hard.
Key terms
- fuel cell
- hydrogen
- electrolysis
- energy carrier
How a fuel cell works
A fuel cell is a device that turns the chemical energy of a fuel directly into electricity, without burning it. Hydrogen gas flows to one side of the cell (the anode), where a catalyst splits each hydrogen atom into a proton and an electron. The protons pass through a membrane in the middle. The electrons can't, so they travel through an outside wire to the other side, and that flow of electrons is the electric current that powers a motor. On the other side (the cathode), the electrons and protons combine with oxygen from the air to form water.
The overall reaction is 2H₂ + O₂ → 2H₂O + electricity + heat. The only exhaust is water vapor, so a fuel cell car produces no tailpipe CO₂ or smog-forming pollutants. Fuel cells are also more efficient than gasoline engines.
Energy carrier, not energy source
Pure hydrogen gas does not exist in useful amounts on Earth; it is bound up in water and in hydrocarbons such as methane. You have to spend energy to pull it out. That makes hydrogen an energy carrier, like a battery: it stores and moves energy that came from somewhere else.
Most hydrogen today is made from natural gas by steam reforming, which releases CO₂. Hydrogen can also be made by electrolysis, which uses electricity to split water (2H₂O → 2H₂ + O₂). Electrolysis is only clean if the electricity comes from low-carbon sources like solar, wind, hydro or nuclear. So whether a fuel cell is truly clean depends on where its hydrogen came from.
Where fuel cells are used today
Fuel cells already power some city buses, warehouse forklifts and a small number of passenger cars, and they supply backup electricity for hospitals and cell towers. NASA used them on the Apollo missions, where the water they made was useful aboard the spacecraft. Their wider use for cars has been held back mostly by the cost of making clean hydrogen and the lack of fueling stations, not by the fuel cell itself.
Pros and cons
| Advantages | Disadvantages |
|---|---|
| Only exhaust is water; no CO₂ or air pollutants at the point of use | Most hydrogen is made from natural gas, releasing CO₂ |
| More efficient than burning fuel in an engine | Making hydrogen takes more energy than you get back from it |
| Quiet, and can be refueled quickly | Hydrogen is a low-density gas that must be compressed or chilled to store, and it leaks easily |
| Can store surplus solar or wind energy as hydrogen | Very few fueling stations or pipelines exist; it is flammable |
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Is a fuel cell car really 'zero emission'?
An ad says a hydrogen fuel cell car is 'zero emission.' Evaluate the claim for two cases: (a) the hydrogen comes from natural gas by steam reforming; (b) it comes from electrolysis powered by a wind farm.
Show the solutionHide the solution
- Step 1: At the tailpipe, both cars emit only water vapor, so the claim is true for what comes out of the car itself.
- Step 2: (a) Steam reforming natural gas releases CO₂ at the hydrogen plant, and drilling and piping the gas can leak methane. The emissions moved upstream instead of disappearing.
- Step 3: (b) Wind-powered electrolysis releases almost no CO₂ while running, so this hydrogen is close to zero-emission over its life (apart from building the turbines and equipment).
Answer: Zero emission at the tailpipe in both cases, but only the wind-powered hydrogen is close to zero emission overall; natural gas hydrogen shifts CO₂ emissions to the production plant.
Common mistakes
- Calling hydrogen an energy source. It is an energy carrier that must be produced using another energy source.
- Saying fuel cells burn hydrogen. They combine hydrogen and oxygen chemically without combustion.
- Forgetting that most hydrogen today comes from natural gas, which releases CO₂.
On the exam
- Expect questions asking for one advantage (water is the only exhaust) and one disadvantage (production from fossil fuels, or storage and infrastructure problems) of hydrogen fuel cells.
- If asked to write the reaction, use 2H₂ + O₂ → 2H₂O and note that it makes electricity.
Connected topics
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Check yourself
4 questions on 6.11 Hydrogen Fuel Cell. Pick an answer to see if you got it, and why.
A hydrogen fuel cell car emits only water from its tailpipe. Which statement best explains why driving it may still lead to CO₂ emissions?
A city replaces 40 diesel buses with hydrogen fuel cell buses. The only exhaust from the buses is water vapor.
The city makes the hydrogen at a bus depot by electrolysis, using electricity from the regional grid. About 60% of that grid's electricity comes from coal-fired power plants. The hydrogen is compressed and stored in high-pressure tanks.
Described scenario
Which equation shows the overall reaction that powers the buses?
Which change would most reduce the total CO₂ emissions linked to running the buses?
Hydrogen is often called an energy carrier rather than an energy source. Which statement best explains why?
0 of 4 answered