In recent years, the performance, reliability and cost of fuel cells have been greatly improved, making fuel cells shine in many fields. Fuel cells can be found in many fields from mobile phone power to large power stations. Compared with traditional heat engine, fuel cell is not limited by Carnot cycle and has high energy conversion efficiency. In contrast to a secondary battery, a fuel cell does not need to be recharged, but it generates electricity consistently as long as it is continuously supplied with fuel. Reducing the activation energy of reaction through the development of advanced catalysts is an effective means of accelerating the electrode reaction rate. The catalytic activity of catalysts is closely related to the surface composition, electronic structure and geometric structure of catalysts. At present, the modification methods to improve catalyst activity include coating, alloying and others.
- Coating: The catalytic efficiency of catalyst can be improved effectively by coating with coating agent. For example, for Pt/C catalyst, the exposed nano-Pt particles have strong adsorption with oxygen-containing intermediates in the oxygen reduction reaction. As a result, the kinetics of catalytic oxygen reduction of Pt is slow due to the lag of desorption of oxygen-bearing species. In addition, the chemical environment during fuel cell operation can easily lead to corrosion of carbon carriers and migration, agglomeration and growth of Pt nanoparticles. Therefore, the fixation and protection of Pt nanoparticles by the method of nitrogen-containing polymer coating can not only improve the activity of catalyst after heat treatment, but also greatly improve the stability.
- Alloying: Alloying can effectively control the surface composition, atomic arrangement and coordination environment of active atoms of the catalyst, resulting in geometric effects, electronic effects and anchoring effects. Therefore, the activation energy of the reaction can be reduced. Due to the different atomic size and band structure of alloying elements, the alloying effect varies with the element, alloy type and alloying degree. Moreover, Pt-Ni alloy catalyst, Pt-Co alloy catalyst and Pt-W alloy catalyst are all successful cases of catalyst alloying.
Figure 1. Pt-Co alloy catalyst used in fuel cell.
- Others: Other modification methods of catalysts include doping, core-shell structure, geometric structure modulation, crystal structure adjustment and others. For example, when nitrogen, sulfur, phosphorus and boron are doped into graphene, it shows catalytic activity for redox reactions, so it is also considered as a kind of non-platinum catalyst with relatively promising application. Doped graphene has higher oxygen reduction catalytic activity, stability and toxicity resistance than traditional Pt/C catalysts in fuel cell. It is widely believed that doped agents can increase the presence of graphene edges by promoting the formation of defects in the hexagonal arrangement of carbon atoms, causing structural rupture of graphene. This can introduce irregular curvature into the graphene stack, leading to enhanced oxygen reduction activity.
Figure 2. A catalyst with core core-shell structure used in fuel cell.
- Kobayashi M, Hidai S, Niwa H, et al. Co oxidation accompanied by degradation of Pt–Co alloy cathode catalysts in polymer electrolyte fuel cells[J]. Physical Chemistry Chemical Physics, 2009.
- Zhixiu Liang et al, Direct 12-Electron Oxidation of Ethanol on a Ternary Au(core)-PtIr(Shell) Electrocatalyst, Journal of the American Chemical Society, 2019.