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How Fuel Cells Work
Fuel Cells
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A fuel cell is an electrochemical device that converts the chemical energy of a fuel directly into electrical energy. In a hydrogen PEM fuel cell, hydrogen and oxygen react to produce electricity, water, and heat. This matters because fuel cells can power vehicles, buildings, spacecraft, and backup systems with high efficiency and low local emissions. Unlike a battery, a fuel cell can keep operating as long as fuel and oxidizer are continuously supplied.
In a proton exchange membrane fuel cell, hydrogen gas enters the anode side and oxygen from air enters the cathode side. A catalyst splits hydrogen molecules into protons and electrons, while the membrane lets only protons pass through to the cathode. The electrons are forced through an external circuit, creating useful electric current before rejoining protons and oxygen to form water. Engineers stack many individual cells together because one cell produces only a small voltage.
Key Facts
- Anode reaction: H2 -> 2H+ + 2e-
- Cathode reaction: 1/2 O2 + 2H+ + 2e- -> H2O
- Overall reaction: H2 + 1/2 O2 -> H2O + electrical energy + heat
- A single PEM fuel cell typically produces about 0.6 V to 0.8 V under load.
- Stack voltage is approximately Vstack = N x Vcell, where N is the number of cells.
- Electrical power is P = IV, so higher current or higher voltage increases output power.
Vocabulary
- Fuel cell
- A device that converts chemical energy from a fuel and oxidizer directly into electrical energy through electrochemical reactions.
- Anode
- The electrode where hydrogen is oxidized into protons and electrons in a PEM fuel cell.
- Cathode
- The electrode where oxygen combines with protons and electrons to form water.
- Proton exchange membrane
- A thin polymer layer that allows protons to pass through while blocking electrons and gases.
- Catalyst
- A material, often platinum in PEM fuel cells, that speeds up the electrochemical reactions without being consumed.
Common Mistakes to Avoid
- Thinking a fuel cell stores energy like a battery. A fuel cell mainly converts supplied fuel into electricity, so it needs a continuous input of hydrogen and oxygen to keep running.
- Assuming the membrane carries electrons across the cell. In a PEM fuel cell, the membrane carries protons, while electrons must travel through the external circuit to do useful work.
- Forgetting that water and heat are products. The reaction is not just electricity production, and engineers must manage water flow and cooling to keep the stack operating well.
- Multiplying current by the number of cells in a stack incorrectly. Cells in series mainly add voltage, while the current is limited by cell area, reactant supply, and internal resistance.
Practice Questions
- 1 A PEM fuel cell stack has 80 cells, and each cell operates at 0.70 V under load. What is the total stack voltage?
- 2 A fuel cell stack delivers 120 A at 56 V. What electrical power does it produce in watts and kilowatts?
- 3 Explain why electrons in a PEM fuel cell must travel through an external wire instead of passing through the proton exchange membrane.