Solid Oxide Fuel Cell Materials

Solid oxide fuel cell (referred to as SOFC) belongs to the third generation fuel cell. It is a kind of all-solid-state chemical power generation device that can directly convert the chemical energy stored in fuel and oxidant into high-efficiency and environmentally friendly electric energy at medium and high temperatures. It is widely regarded as a kind of fuel cell that will be widely used as a proton exchange membrane fuel cell (PEMFC) in the future. The solid oxide fuel cell (SOFC) is mainly composed of electrolyte, anode or fuel electrode, cathode or air electrode, and connector or bipolar plate (bipolar separator).The working principle of solid oxide fuel cell is the same as that of other fuel cells, which is equivalent to the "inverse" device of water electrolysis. The single cell is composed of anode, cathode and solid oxide electrolyte, in which the anode fuel is oxidized, the cathode oxidizer is reduced, and both poles contain catalysts to accelerate the electrochemical reaction of the electrode. When working, it is equivalent to a constant current power supply, the anode is the negative pole of the power supply, and the cathode is the positive pole of the power supply.

Figure 1. Flat SOFCs structure

Components of solid oxide fuel cells:

The main components of solid oxide fuel cell usually include solid electrolyte, the anode or fuel electrode, the cathode or air electrode.

• Solid electrolyte: Solid electrolyte is the core component of SOFC. Its performance not only directly affects the battery's operating temperature and electrical energy conversion efficiency, but also determines the choice of matching electrode materials and their corresponding preparation techniques. The currently discovered oxygen ion conductors that may be used in SOFC mainly include fluorite phase structure ZrO₂ group, CeO₂ group, Bi₂O₃ group materials and perovskite-type LaGaO₃ group materials.

Figure 1. Operation principle of solid oxide fuel cell

• Cathode material: As the cathode material used in SOFCs system, it generally needs to meet the following three requirements. First, it must have high ionic and electronic conductivity. The higher the conductivity of the cathode material, the less its ohmic loss. And the high oxygen ion conductivity will improve the reaction and diffusion capacity of the three-phase catalytic interface. Second, it must have high catalytic activity. The high catalytic activity of the cathode material can promote the covalent bond of oxygen molecules to open, so that the three-phase interface catalytic reaction can proceed smoothly. Third, the cathode material must have a matching thermal expansion coefficient with other components in the SOFCs system. At present, most cathode materials are composite oxides of the perovskite structure.
• Anode material: The anode materials used in SOFCs should meet the following conditions: (1) Good conductivity; (2) Good gas permeability; (3) Stable under reducing atmosphere; (4) Good thermal compatibility with electrolyte. The preferred materials that satisfy these conditions are Ni-ZrO₂ cermets. Porous (NiO)_0.7-(YSZ)_0.3 is often used as the anode material of SOFCs. Other candidate anode materials are cobalt, iron, titanium, etc., but their performance and economic practicality are not as good as nickel.

References:

1. Qingxin Zhang, Azam Sayadi, Peter G.(2018) "Separation of kinetic and mass transport effects in the electrolysis of formic acid in a flow-through cell." Electrochimica Acta 35, 795-807.
2. Yifeng Li, Wenqiang Zhang, Tong Wu, Yun Zheng (2019) "Segregation Induced Self-Assembly of Highly Active Perovskite for Rapid Oxygen Reduction Reaction." Advanced Energy Materials .48, 694-697.