Principle, types and characteristics of solid oxide fuel cells (SOFC)

I.Principle of SOFC

Solid oxide fuel cells (SOFC) are an advanced energy technology that directly converts chemical energy in hydrocarbon fuels into electrical energy and thermal energy through electrochemical reactions. They have the advantages of wide fuel sources, high power generation efficiency, high waste heat quality, quiet operation, low emissions, and modular installation.

The SOFC system can be divided into a stack and a peripheral BOP auxiliary unit. The stack is the core of the fuel cell and is a device that directly converts chemical energy into electrical energy. The peripheral auxiliary units surrounding the stack include air supply preheating unit, fuel supply (reforming) unit, tail gas recovery unit, electrical management unit, and control unit.

The working principle of SOFC is based on electrochemical processes and usually operates at high temperatures of 650~950℃. SOFC uses solid oxide ceramics (usually zirconium oxide) as electrolytes, which not only transfer O²⁻ ions but also separate air and fuel.

The fuel (usually hydrogen reformed from hydrocarbons) diffuses through the anode to the electrolyte interface, while the air absorbs oxygen through the cathode surface and is converted into O²⁻ ions by the catalytic action of the cathode itself. After entering the electrolyte, it diffuses to the anode and reacts with the fuel. The lost electrons return to the cathode through the external circuit, thereby generating electricity.

SOFC schematic diagram

II. Types of SOFC

According to the different design forms of unit components, SOFC is mainly divided into two structures: tubular and flat. The tubular design has good sealing performance and has the characteristics of long-term stable operation; the flat design has a high power density due to the short current path.

1.Tubular structure

The tubular structure SOFC is the earliest and more mature form. Its power generation unit consists of a tube with one end closed and the other end open, which is composed of a porous support tube, anode, electrolyte and cathode film from the inside to the outside.

In this structural design, the fuel is input from the tube core and the air is supplied through the outer wall of the tube. Its battery stack has a large degree of freedom and is not easy to crack; porous ceramics are used as the support body, and the structure is strong; it has good sealing performance and long-term operation stability.

Its battery assembly is also relatively simple, and it is easy to combine high-power battery packs by connecting battery cells in parallel and in series. However, the spacing between electrodes is large, the path of current passing through the battery is long, and the internal resistance loss is large, so the corresponding power density is low.

Tubular SOFC power generation unit and system integration diagram

2. Flat plate structure

The flat plate structure SOFC has a simple geometric shape and low manufacturing cost. It consists of a single cell composed of anode, electrolyte, and cathode film. The connector with grooves on both sides connects the adjacent cathode and anode, and provides gas channels on both sides to separate the two gases at the same time.

Its battery assembly requires high-temperature resistant sealing at the edge to isolate oxygen and fuel gas, and has high requirements for the bipolar connecting plate material, which needs to be thermally matched with the electrode material and have good high-temperature oxidation resistance and conductivity.

Flat-plate SOFC power generation unit and system integration diagram

III. Characteristics of SOFC

1. High efficiency

SOFC converts fuel chemical energy through electrochemical means, avoiding the energy loss of traditional combustion work, and the power generation efficiency is as high as 50%~60%, and is not limited by scale, and small-scale distributed power generation can also maintain high efficiency.

2. Rich fuel sources

Due to the high operating temperature of SOFC, a variety of fuels can be used, including natural gas, biogas, ethanol, methanol, etc., and the fuel sources are flexible and diverse.

3. Quiet operation

In addition to the BOP auxiliary system, there is no high-power rotating equipment in the SOFC power generation process, and the operation is quiet, which is suitable for installation in living and working areas.

4. Solid electrolyte stack

Using solid oxide as the electrolyte, no precious metal catalyst is required, long operating life, simple system structure, and low large-scale production cost.

5. Low emissions

There is almost no pollutant emission during the electrochemical reaction, and due to the high power generation efficiency, less fuel is required for the same power generation, thereby reducing CO2 emissions.

6. Modular structure

SOFC adopts modular design, which is flexible to install, occupies a small area, has a short construction period, and is easy to promote and apply.