---
title: "Proton exchange membrane fuel cell"
subject: "Energy Environment and Society"
chapter: 3
key: "semester_7/energy_environment_and_society:pemfc-working"
source: https://www.bctengineer.com/q/semester_7/energy_environment_and_society/pemfc-working
site: BCT Engineering
---

# Proton exchange membrane fuel cell

Energy Environment and Society · Chapter 3: Renewable Energy Sources

- Asked in: year not given
- Video: [How Do Hydrogen Fuel Cells Work?](https://www.youtube.com/watch?v=R6AdX-bdDaw) — Reactions
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/energy_environment_and_society/pemfc-working
- Whole chapter: https://www.bctengineer.com/md/semester_7/energy_environment_and_society/chapter-3

## Question

How does a Proton Exchange Membrane Fuel Cell (PEMFC) work?

## Short answer

In a PEMFC, hydrogen at the anode is split (H2 -> 2H+ + 2e-) by platinum; protons migrate through the polymer membrane, electrons flow through external load. At the cathode, 0.5 O2 + 2H+ + 2e- -> H2O. Operates at 60–80°C, ~0.6–0.7 V per cell, requires pure hydrogen and platinum catalyst.

## Answer

A **Proton Exchange Membrane Fuel Cell (PEMFC)**, also called polymer electrolyte membrane fuel cell, is a low-temperature (60–80 °C) fuel cell that converts the chemical energy of hydrogen and oxygen into electricity, heat and water. Its electrolyte is a solid polymer membrane (for example Nafion) that conducts protons.

#### Construction
- **Membrane (PEM):** thin polymer sheet, conducts H⁺ ions only, and must be kept moist.
- **Catalyst layers:** platinum on carbon on each side.
- **Gas diffusion layers** and **bipolar plates** with flow channels to supply gases and collect current.
- Many cells are stacked in series.

```text
 H2 -> [Anode|  PEM  |Cathode] <- O2/air
        Pt      H+ ->      Pt
          \__ e- -> load __/
 Exhaust: H2O + heat
```

#### Working
1. Hydrogen enters the anode side; platinum splits it:
   **Anode: H₂ → 2H⁺ + 2e⁻**
2. Protons pass through the membrane to the cathode; electrons cannot, and take the external circuit, producing current.
3. At the cathode, oxygen, protons and electrons combine to form water:
   **Cathode: ½O₂ + 2H⁺ + 2e⁻ → H₂O**
4. Overall: **H₂ + ½O₂ → H₂O** (+ electricity and heat). Cell voltage is about 0.6–0.7 V in use (1.23 V theoretical).

#### Features
- Efficiency 40–60%, fast start-up, compact and quiet.
- Used in cars, buses, backup power and portable power.
- Needs pure hydrogen (CO poisons platinum) and costly platinum, with water management of the membrane.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What does the membrane of a PEMFC conduct?
   A. Electrons and protons
   B. Oxide ions
   C. Hydrogen molecules and oxygen
   D. Protons (H+) only, while electrons take the external circuit
   Answer: D. The PEM passes H+ but blocks electrons, which forces them through the external load to give current. (Key point: Membrane role)

2. What happens at the anode?
   A. H2 is split into 2H+ and 2e- on platinum
   B. O2 combines with H+ to form water
   C. H2O is split into H2 and O2
   D. O2 is reduced to oxide ions
   Answer: A. Hydrogen oxidation at the anode: H2 -> 2H+ + 2e-. (Key point: Anode reaction)

3. Where is water produced in a PEMFC?
   A. At the anode
   B. At the cathode
   C. In the membrane
   D. In the bipolar plate only
   Answer: B. At the cathode, 1/2 O2 + 2H+ + 2e- -> H2O. (Key point: Water formation)

4. Why does a PEMFC need pure hydrogen?
   A. CO dissolves the membrane
   B. Nitrogen blocks the bipolar plates
   C. CO poisons the platinum catalyst
   D. Pure hydrogen raises the operating temperature to 1000 °C
   Answer: C. Platinum is poisoned by CO, so the fuel must be clean; platinum is also costly. (Key point: Limitations)

## Related in Energy Environment and Society

- [Fuel cell and its working](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Afuel-cell-working) — Energy Environment and Society, chapter 3, asked 2×
- [Solid oxide fuel cell](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Asofc-construction-working) — Energy Environment and Society, chapter 3, asked 2×
- [Synthetic fuel from biomass, bio fuel cells](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Asynthetic-fuel-biofuel-cell) — Energy Environment and Society, chapter 3

## Related in other subjects

- [Fuel cell voltage, efficiency and hydrogen use](https://www.bctengineer.com/question.md?key=semester_6%2Fenergy_resources%3Afuel-cell-stack-numerical) — Energy Resources, chapter 6
- [Working principle and types of fuel cells](https://www.bctengineer.com/question.md?key=semester_6%2Fenergy_resources%3Apem-fuel-cell-working-types) — Energy Resources, chapter 6
- [RAM vs ROM; EEPROM cell](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Aram-rom-eeprom) — Digital Logic, chapter 4