As a new efficient and clean way of energy utilization, fuel cell has attracted great attention from all over the world for its wide application prospect, and is hailed as one of the new energy sources in the 21st century. Fuel cell is a device that converts the chemical energy of chemical reaction into electrical energy. Its most prominent feature is high energy conversion efficiency. Fuel cells are electrically more efficient than the electricity produced by turbo-generators and have few harmful emissions other than a small amount of carbon oxide. Therefore, from the perspective of energy saving and ecological environment protection, fuel cell is the most promising power generation technology. Electrolyte is one of the most important components in various fuel cells. Electrolyte can be divided into liquid electrolyte and solid electrolyte according to the state, and solid electrolyte is favored by researchers because of its higher safety. In general, solid electrolytes are made from oxygen-deprived oxides that allow oxygen ions to be transported through oxygen vacancies.
- ZrO2 electrolyte: ZrO2 electrolyte is one of the earliest studied solid electrolytes, which is widely used in high temperature solid oxide fuel cells. The thermal stability of ZrO2 material is improved by partially substituting Zr4+ with appropriate bivalent or trivalent cations. Cationic doping can not only introduce oxygen vacancy, but also stabilize crystal structure in a wide temperature range. Y2O3 is the most commonly used dopant of ZrO2 electrolyte. Yttrium stabilized zirconia (YSZ) exhibits excellent mechanical properties, good chemical stability and high ionic conductivity in both reduction and oxidation atmospheres. In addition, when YSZ materials are used as electrolytes, YSZ must be thin enough to maintain low ohmic resistance.
Figure 1. An example of the SEM of ZrO2 electrolyte used in fuel cell.
- CeO2 electrolyte: CeO2 is a fluorite ceramic material that can work stably in the temperature range of 500-700°C. CeO2 electrolytes have higher ionic conductivity than ZrO2 based electrolytes. Pure CeO2 does not have enough oxygen vacancies, so the ionic conductivity is low. The most common way to solve this problem is to partially replace Ce4+ with doped metal cations to appropriately raise oxygen vacancies and thus improve ionic conductivity. Commonly used doped metal cations include Gd3+, Sm3+, Y3+, La3+, Ca2+ and others.
Figure 2. SEM images of the CeO2 powder (a) before and (b) after fuel cell measurement.
- Bi2O3 electrolyte: Bi2O3 belongs to polycrystalline materials, which can present different crystal structures at different temperatures and conditions, mainly consisting of α, β, γ and δ phases. Furthermore, δ-Bi2O3 is a cubic fluorite structure, which is stable in the temperature range of 730~825°C. Stable δ-Bi2O3 has higher oxygen ionic conductivity and excellent oxygen surface exchange kinetics than doped CeO2, which promotes the migration and diffusion of oxygen between cathode and electrolyte interface.
- Others: In addition to electrolytes with the above structure, common electrolytes include perovskite oxide (ABO3), which is a good electrolyte material with mixed ionic and electron conductivity. For example, LaGaO3 electrolyte is typical of this class. The B site of LaGaO3 can produce oxygen vacancies and improves ionic conductivity. At the same time, the doping limit of A site is also increased by the doping of B site.
- Hong, Soon, Wook, et al. Properties of nanostructured undoped ZrO2 thin film electrolytes by plasma enhanced atomic layer deposition for thin film solid oxide fuel cells[J]. Journal of Vacuum Science & Technology A Vacuum Surfaces & Films, 2016.
- Wang B, Zhu B, Yun S, et al. Fast ionic conduction in semiconductor CeO2-δ electrolyte fuel cells[J]. NPG Asia Materials, 2019.