With the development of social civilization and the rapid growth of world population, the demand of energy for human life is also increasing continuously. As the main source of energy, fossil fuels provide a lot of resources for our social life. However, the excessive consumption of fossil fuels has produced large amounts of carbon dioxide and nitrogen-containing toxic gases, raising concerns about environmental pollution and climate change and posing a threat to the health of our civilization. Therefore, in order to achieve a sustainable future, renewable energy and green energy conversion technologies are urgently needed. Fuel cells have the advantages of high fuel flexibility, low cost, high safety and stability, which has great development prospects in energy conversion, storage and other fields. Cathode material is an important part of fuel cell, which is the site of oxygen reduction reaction. After adsorption and dissociation, the oxygen molecule gains electrons and is reduced to oxygen ions.
- Perovskite structure oxide material: The standard perovskite structure is cubic crystal system and can be expressed by ABO3 molecular formula. The A position is rare earth element and alkali earth metal element, which is located at eight vertices of the cube and the coordination number is 12. Common rare earth elements and alkaline earth metal elements include La, Pr, Sm, Gd, Nd, Sr, Ca and Ba. B sites are mostly transition metal elements with small ionic radius, such as Mn, Fe and Co, which are located in the center of the cube with coordination number of 6. LaMnO3, LaCoO3 and LaFeO3 are three main cathode materials with perovskite structure. For example, LaMnO3 has good electronic conductivity and high catalytic activity for reduction reaction, and its structure is stable. In addition, the electrical conductivity of LaCoO3 and LaFeO3 is not high, so Sr element doping is needed to improve the porosity and increase the electrical conductivity.
Figure 1. Structure of perovskite structure oxide material.
- Spinel type oxide material: The general formula of the structure of spinel type oxide material is AB2O4, and its performance is mainly caused by the distribution of divalent and trivalent ions. In the structure of this material, oxygen ions are packed tightly in cubic order, with divalent cations in tetrahedral spaces and trivalent cations in octahedral spaces. Spinel type oxide materials have been widely concerned as cathode materials in the field of fuel cells due to their strong chemical stability, high corrosion resistance and good thermal stability. For example, Mn1.5Co1.5O4 can be used as a cathode material for solid fuel cells, and it has good thermal stability and high electronic conductivity.
Figure 2. Structure of spinel type oxide material.
- Ruddlesden-popper type material: Ruddlesden-popper type material can be expressed by the chemical formula (ABO3)nAO. A represents rare earth or alkali metal elements, and B represents transition metal elements. The structure can be regarded as n-layer tetrotetic perovskite (ABO3)n and AO interactively stacked. This material has high oxygen diffusion coefficient and surface exchange coefficient, which can significantly improve the activity of the cathode material.
- Others: In addition to the materials introduced above, many other materials can be used as cathode for fuel cell including carbon cloth, carbon paper, carbon brush, graphite rod, graphite brush and others.
- Trivedi S, Prasad R, Mishra A, et al. Current scenario of CNG vehicular pollution and their possible abatement technologies: an overview[J]. Environmental Science and Pollution Research, 2020, 27(8).
- Walsh A, Ahn K S, Shet S, et al. Ternary cobalt spinel oxides for solar driven hydrogen production: Theory and experiment[J]. Energy & Environmental Science, 2009, 2.