Exploration of solid oxide fuel cell principle

Working principle of solid oxide fuel cell

Solid oxide fuel cell structures typically consist of a dense solid-state oxygen ion conductive electrolyte sandwiched between two porous electrodes, namely the anode and cathode. The hydrogen fuel enters the anode side and binds to oxygen ions from the cathode side.

The air is reduced on the cathode side to form oxygen ions and passes through the lattice of the ceramic electrolyte (e.g., yttria stabilized zirconia, YSZ), constantly chemically reacting with the fuel on the anode side, resulting in electrons.

The resulting electrons form an electrical current through an external circuit (from anode to cathode). Water and heat are the only byproducts of this reaction process.

Reaction of solid oxide fuel cell

The hydrogen required can be extracted from hydrocarbons such as natural gas by external or internal reforming techniques. The fuel reforming reaction is an endothermic reaction.

The heat required for the fuel reforming reaction may be provided by the overpotential loss and entropy changing heat of a high temperature fuel cell, such as a solid oxide fuel cell. Unlike other types of fuel cells, solid oxide fuel cell can take advantage of high temperatures for internal reforming. This eliminates the need for external disarmament.

It can be seen that some parts of hydrocarbon fuel are internally reformed for indirect internal reform. The other part of the fuel is directly reformed into the fuel cell. The heat generated by the electrochemical reaction is used in the internal reforming process. Spent fuel containing hydrogen and carbon monoxide is sent to the combustion chamber for oxidation.

Key design of anode material

Fuel oxidation occurs in the solid oxide fuel cell anode, where oxygen ions combine with compound fuels such as hydrogen to produce electrons, which are then transferred to the cathode via external circuits. Understanding its design requirements helps battery maintenance. A suitable anode material should have:

• High oxidative activity
• High stability
• High electronic conductivity (electrical)
• High ionic conductivity (ionic)
• Coefficient thermal expansion (TEC) compatible with electrolytes and connections.

Selection of cathode material

The operating temperature, energy consumption and efficiency of a solid oxide fuel cells are greatly related to the material from which it is made, and the right material can improve the efficiency of the solid oxide fuel cells.

Oxygen binds electrons at the SOFC cathode to form oxygen ions, which diffuse through the electrolyte to the anode. Unlike electrolytes, cathodes should have high conductivity of electrons and oxide ions. In addition, it should be porous to allow rapid oxygen diffusion to the cathode electrolyte interface, where oxygen reduction occurs.

High temperature operation challenge

SOFC belongs to the third generation of fuel cells, which is a new type of power generation device that directly converts chemical energy stored in fuel and oxidizer into electrical energy at high temperatures in an efficient and environmentally friendly manner.

It is generally considered to be a clean fuel cell that will be widely used in the future. SOFC, whether in single-cell or stacked configurations, must provide the required electrochemical performance and sufficient mechanical stability to achieve long-term performance objectives.

Solid oxide fuel cell need to operate at high temperatures (600℃ to 1000℃) to facilitate the migration of oxygen ions through the electrolyte, resulting in high ionic conductivity. Reducing the operating temperature of a solid oxide fuel cell not only results in a decrease in ionic conductivity (i.e., an increase in ohmic losses), but also reduces the catalytic activity of the electrode, which negatively affects battery capacity.

However, there are challenges to operating solid oxide fuel cell at very high temperatures (800℃ to 1000℃). These include:

• Reaching the operating temperature increases fuel burning time.
• Extend startup time
• Tightness issues
• The connection of solid oxide fuel cell stacks requires very expensive materials.

Potential solutions to reduce the temperature

Reducing the operating temperature of the solid oxide fuel cell to 600°C or even lower can solve the above problems. Therefore, recent research has focused on how to reduce the operating temperature of solid oxide fuel cell to solve their potential problems.

Over the past decade, there has been a growing interest in ceramics with proton conductivity. Because they have high proton conductivity and low activation energy at temperatures below 600℃.
● Proton conductive ceramic material
Proton conductive ceramic materials are called high temperature proton conductors (HTPCs) and exhibit proton conductivity in hydrogen or vapor environments. Usually used as electrolytes are BaCeO3 and BaZrO3 doped materials.

HTPC electrolytes must meet a wide range of requirements:

• High ionic conductivity
• Excellent thermodynamic stability
• High ceramic and mechanical properties
• Thermal and chemical compatibility with other functional materials

However, state of the art HTPC still has the disadvantages of a highly conductive silicate (BaCeO3) and low chemical stability in the face of gas components containing carbon and sulfur.

Reactions with carbon gas components result in severe electrolyte degradation and cannot be used as fuel cell applications based on hydrocarbon fuels. The research and development of low-temperature solid oxide fuel cell from 400°C to 650°C has been carried out extensively and is widely regarded as the next generation of technology. And its installed capacity data in top 10 power battery installations in the world is also very impressive.

Low operating temperatures also have potential uses for reversible solid oxide fuel cell, as lowering the temperature can transform the water and carbon dioxide co-electrolyte composition into a product composition with a large amount of methane.

Conclusion

This article describes how solid oxide fuel cell work and how they perform at high temperatures. And provide a solution at low temperature, which plays an important role in its subsequent development.