Can hydrogen be a clean fuel, and where does the hydrogen come from?
Topic 6.11 Hydrogen Fuel Cell: explain how a hydrogen fuel cell works and evaluate its benefits and drawbacks.
A focused answer to APES Topic 6.11, covering how a hydrogen fuel cell generates electricity from hydrogen and oxygen, the only direct emission (water), the benefits and the key drawback that producing hydrogen often uses fossil fuels, with a worked fuel-cell energy calculation.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this topic is asking
The College Board (Topic 6.11) wants you to explain how a hydrogen fuel cell works and evaluate its benefits and drawbacks, including where the hydrogen comes from.
How a fuel cell works
Benefits
The catch: where the hydrogen comes from
Why this matters
The hydrogen fuel cell is the AP exam's classic test of upstream thinking: a technology that is clean at the tailpipe but only as clean as the energy used to make its fuel. This links Unit 6 to fossil fuels, to renewables (which can provide clean hydrogen), and to the climate concerns of Unit 9. Examiners often present hydrogen as a "clean" car fuel and ask you to evaluate the claim; the points-winning move is to trace the energy back to its source and note that hydrogen made from natural gas or from a fossil-heavy grid still carries carbon dioxide emissions, so the technology is only as green as its hydrogen. The same logic applies to electric vehicles and to any energy carrier, which is why this topic is a useful template for the science practice of proposing and justifying solutions.
Try this
Q1. Identify the only direct emission from a hydrogen fuel cell. [1 point]
- Cue. Water (water vapor).
Q2. Explain why hydrogen fuel is not automatically a clean energy source. [2 points]
- Cue. Hydrogen must be produced, usually from natural gas or by electrolysis using grid electricity; if that energy comes from fossil fuels, the overall process still emits carbon dioxide, so hydrogen is only clean if made with renewable energy.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2021 (style)4 marksSection II (FRQ). (a) Describe how a hydrogen fuel cell produces electricity. (b) Identify the only direct emission from a hydrogen fuel cell. (c) Explain why hydrogen fuel is not automatically a clean energy source. (d) Identify one advantage of hydrogen fuel cells over conventional batteries or fossil fuels.Show worked answer →
A 4-point FRQ on hydrogen fuel cells.
(a) Describe (1 point): hydrogen and oxygen react in the fuel cell; the reaction drives electrons through an external circuit, producing electricity, with water as the product.
(b) Identify (1 point): water (water vapor), the only direct emission.
(c) Explain (1 point): most hydrogen is produced from natural gas or by electrolysis using grid electricity, so if that energy comes from fossil fuels the overall process still emits carbon dioxide.
(d) Identify (1 point): any one of producing only water at the point of use, high efficiency, quick refuelling, or serving as a way to store energy from intermittent renewables.
Markers reward the hydrogen-plus-oxygen reaction driving electrons, water as the emission, the upstream fossil-fuel point, and a valid advantage.
AP 2019 (style)1 marksSection I (multiple choice). The only direct emission produced when a hydrogen fuel cell generates electricity is: (A) carbon dioxide (B) water (C) sulfur dioxide (D) methane. Justify your choice.Show worked answer →
A 1-point MCQ on fuel cells. The answer is (B).
A hydrogen fuel cell combines hydrogen and oxygen to produce electricity, and the only direct product is water. It emits no carbon dioxide (A), sulfur dioxide (C) or methane (D) at the point of use. The trap is forgetting that, although the cell itself is clean, the upstream production of the hydrogen may emit carbon dioxide if it relies on fossil fuels.
Related dot points
- Topic 6.1 Renewable and Nonrenewable Resources: distinguish renewable from nonrenewable energy resources and explain why the distinction matters for sustainability.
A focused answer to APES Topic 6.1, covering the difference between renewable and nonrenewable energy resources, examples of each, the idea of potentially renewable resources, and why the distinction matters for sustainability, with a worked depletion calculation.
- Topic 6.8 Solar Energy: describe how solar energy is captured using photovoltaic, active and passive systems and evaluate its benefits and drawbacks.
A focused answer to APES Topic 6.8, covering photovoltaic cells, active and passive solar heating, the benefits (renewable, low emissions) and drawbacks (intermittency, land, cost) of solar energy, and a worked photovoltaic output calculation.
- Topic 6.12 Wind Energy: describe how wind turbines generate electricity and evaluate the benefits and drawbacks of wind power.
A focused answer to APES Topic 6.12, covering how wind turbines convert wind into electricity, onshore and offshore wind, the benefits (renewable, low emissions, low operating cost) and drawbacks (intermittency, location, wildlife, noise) of wind power, and a worked wind farm output calculation.
- Topic 6.13 Energy Conservation: describe strategies for energy conservation and efficiency and explain how they reduce environmental impact.
A focused answer to APES Topic 6.13, covering energy conservation and efficiency strategies (efficient vehicles, appliances, lighting, insulation, public transport, CAFE standards), the difference between conservation and efficiency, and how they reduce impact, with a worked energy-saving calculation.
- Topic 6.5 Fossil Fuels: explain how fossil fuels form and are used to generate electricity, and describe their environmental impacts, including cogeneration.
A focused answer to APES Topic 6.5, covering how fossil fuels form, how a fossil-fuel power plant generates electricity, fracking, cogeneration, and the environmental impacts of coal, oil and gas, with a worked power plant efficiency calculation.
Sources & how we know this
- AP Environmental Science Course and Exam Description — College Board (2020)