---
title: "Solid oxide fuel cell"
subject: "Energy Environment and Society"
chapter: 3
key: "semester_7/energy_environment_and_society:sofc-construction-working"
source: https://www.bctengineer.com/q/semester_7/energy_environment_and_society/sofc-construction-working
site: BCT Engineering
---

# Solid oxide fuel cell

Energy Environment and Society · Chapter 3: Renewable Energy Sources

- Asked in: 2072 Kartik, 4 marks; 2070 Chaitra, 4 marks; year not given (asked 2 times)
- 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/sofc-construction-working
- Whole chapter: https://www.bctengineer.com/md/semester_7/energy_environment_and_society/chapter-3

## Question

What is a fuel cell? Describe the basic construction and working of a solid oxide fuel cell (SOFC), with chemical equations.

## Short answer

An SOFC has a YSZ electrolyte, Ni-YSZ anode, and LSM cathode. At the cathode, O2 + 4e- → 2O2-; O2- migrates to the anode to oxidize H2 (H2 + O2- → H2O + 2e-) or CO. Overall: 2H2 + O2 → 2H2O. It operates at high temperature and is fuel flexible.

## Answer

A **fuel cell** is an electrochemical device that converts the chemical energy of a fuel (such as hydrogen) and an oxidant directly into electrical energy, heat and water, without combustion. It keeps producing electricity as long as fuel and oxidant are supplied.

#### Solid oxide fuel cell (SOFC)
A high-temperature fuel cell (800–1000 °C, newer designs 500–700 °C) that uses a solid ceramic electrolyte which conducts oxide ions (O²⁻).

#### Construction
- **Electrolyte:** dense yttria-stabilised zirconia (YSZ), a ceramic that conducts O²⁻ ions at high temperature.
- **Anode (fuel electrode):** nickel–YSZ cermet, porous.
- **Cathode (air electrode):** lanthanum strontium manganite (LSM), porous.
- **Interconnect:** conducts current between cells in a stack (ceramic or steel).
- Designs: tubular or planar.

```text
 Fuel (H2, CO, CH4)       Air (O2)
        |                    |
   +---------+-----------+---------+
   | ANODE   | ELECTROLYTE| CATHODE |
   | (Ni-YSZ)|   (YSZ)   |  (LSM)  |
   +---------+-----------+---------+
        H2O <---- O2- <---- O2
          e- ---> external load ---> e-
```

#### Working and reactions
1. Oxygen from air is reduced at the cathode:
   **Cathode: ½O₂ + 2e⁻ → O²⁻**
2. Oxide ions pass through the solid electrolyte to the anode.
3. At the anode, hydrogen is oxidised by the oxide ions to form water and release electrons:
   **Anode: H₂ + O²⁻ → H₂O + 2e⁻**
4. The electrons flow through the external circuit, giving electric power.
   **Overall: H₂ + ½O₂ → H₂O**
   With carbon monoxide as fuel: CO + O²⁻ → CO₂ + 2e⁻.

#### Features
- Fuel flexible: can use hydrogen, natural gas, biogas or syngas (internal reforming).
- Efficiency 50–60% (up to 80–85% in combined heat and power).
- No expensive platinum catalyst.
- Drawbacks: high temperature, slow start-up, material stress and high cost; used for stationary power plants.

## Check yourself

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

1. What ion does the SOFC electrolyte conduct?
   A. Protons (H+)
   B. Hydroxide ions (OH-)
   C. Oxide ions (O2-)
   D. Electrons
   Answer: C. The YSZ ceramic electrolyte conducts O2- at high temperature. (Key point: SOFC electrolyte)

2. What happens at the SOFC cathode?
   A. H2 + O2- gives H2O + 2e-
   B. 1/2 O2 + 2e- gives O2-
   C. O2 + 4H+ + 4e- gives 2H2O
   D. CO + O2- gives CO2
   Answer: B. Oxygen from air is reduced at the cathode to oxide ions that migrate through the electrolyte. (Key point: Cathode reaction)

3. Which anode reaction is correct for hydrogen?
   A. H2 + O2- gives H2O + 2e-
   B. H2 gives 2H+ + 2e-
   C. 1/2 O2 + 2e- gives O2-
   D. H2O gives H2 + O2-
   Answer: A. Hydrogen is oxidised by oxide ions to water, releasing electrons to the external circuit. (Key point: Anode reaction)

4. Which is an advantage of SOFC?
   A. Operates at room temperature
   B. Fast start-up
   C. Uses only pure hydrogen
   D. Fuel flexibility and no platinum catalyst needed
   Answer: D. SOFC can use H2, natural gas, biogas or syngas and uses no platinum, but runs at high temperature with slow start-up. (Key point: SOFC features)

## 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×
- [Proton exchange membrane fuel cell](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Apemfc-working) — Energy Environment and Society, chapter 3
- [Working of a solar cell](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Asolar-cell-working) — Energy Environment and Society, chapter 3

## Related in other subjects

- [Physical and chemical characteristics of solid waste](https://www.bctengineer.com/question.md?key=semester_7%2Fenvironment_and_pollution_control%3Asolid-waste-characteristics) — Environment and Pollution Control, chapter 5
- [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
- [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
- [Solid breakdown due to actual working conditions](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Asolid-breakdown-working-conditions) — High Voltage Engineering, chapter 6