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.
Understanding How Fuel Cells Work
The membrane is the control point inside a PEM fuel cell. It must conduct protons easily, block electrons completely, and keep the hydrogen and oxygen streams apart. If gases cross through the membrane, they react in the wrong place and waste fuel.
The membrane needs enough water to conduct protons well. If it dries out, electrical resistance rises. If too much water collects, gas passages can flood and prevent oxygen from reaching catalyst sites.
This balance is called water management. Pumps, humidifiers, channels, and careful temperature control help maintain it.
Catalysts make the reactions happen quickly enough to deliver useful current. PEM cells commonly use platinum because it is very effective, especially at the oxygen electrode where the reaction is relatively slow. Platinum is expensive and can be damaged by contaminants.
Carbon monoxide in impure hydrogen can stick to catalyst surfaces and reduce performance. Sulfur compounds in air cause problems too.
Engineers therefore need clean fuel, filters, and materials that can survive thousands of operating hours. Research aims to reduce the amount of platinum or replace it with cheaper catalysts.
A fuel cell does not turn all fuel energy into electrical output. Some energy becomes heat because of unavoidable reaction losses and electrical resistance. More power demand usually means more current, but the cell voltage falls as current rises.
At low current, chemical effects reduce voltage. At higher current, resistance in the membrane and connections causes further losses.
At very high current, reactant gases cannot reach every active site fast enough. This is why a stack is designed for a particular operating range rather than simply being pushed to its maximum current all the time.
Students can connect fuel cells to energy choices beyond the vehicle itself. Hydrogen must be produced, compressed or stored, transported, then supplied safely. If electricity from wind, solar, or another low carbon source makes the hydrogen, the whole system can have lower climate impact.
Hydrogen made from fossil fuels without carbon capture has a different impact. Fuel cells are useful where long running power, quiet operation, or fast refuelling matter. They appear in some buses, forklifts, remote equipment, spacecraft, and backup power units.
When studying diagrams, follow the separate paths for gases, protons, electrons, water, and heat. Confusing these paths is the most common reason the process seems harder than it is.
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.