Solid oxide fuel cell

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.

 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.