Pyrolysis-free approach towards synthesis of oxygen reduction electrocatalysts

Abstract

The accelerated consumption of traditional fossil energy has aggravated the energy crisis and environmental pressure worldwide, so clean and renewable energy technologies have been attracting increasing interest. The development of fuel cell technology, metal air batteries, etc., is an important way to deal with the crisis and relieve pressure. As an important part in the operation of battery system, oxygen reduction reaction (ORR) electrochemical catalysts have become the focus of more and more researchers. At present, the commercial catalysts are metal base, especially the electrocatalysts based on precious metals (platinum, ruthenium, etc.) show excellent electrochemical catalytic performance and good stability in ORR. However, the introduction of precious metals increases the cost of battery systems, and the scarcity of raw materials greatly limits the large-scale commercial application of such catalysts. Therefore, the development of high efficiency, stability and low cost non-noble metal based electrocatalysts is the current research target. Recently, a large number of non-precious metal carbon-based catalysts (Fe, Co and other transition metals) have been developed. As early as 1964, scientists found that cobalt phthalocyanine with M-N4 structure in alkaline electrolyte showed ORR properties, but the stability and catalytic activity of the material were poor. On the basis of this research, M-N x /C electrocatalysts prepared by high-temperature pyrolysis with materials with non-noble metals, N and C sources as precursors have been widely studied in the field of new energy materials. Non-noble metal carbon oxygen reduction catalyst is the current popular fuel cell catalysts, the traditional preparation process, most of the carbon catalyst with high temperature carbonization decomposition (usually in 800−1100°C) carbonization process to improve the conductivity and catalytic activity of material, but because of the high temperature in the process of sheet metal atoms to reunite, pyrolysis carbonization of artificial is high, the preparation conditions demanding, not guaranteed, reproducibility and subsequent brings environmental pollution problems such as acid treatment process, is not conducive to practical production applications. In the process of high-temperature carbonization, the material structure may change unpredictably or even be reconstructed, and the catalytic activity potential is unclear and difficult to control, which brings great challenges to understand the reaction and deactivation mechanism and subsequently develop large-scale preparation approach. The research and development of non-carbonization method can avoid the change of material structure caused by high temperature from the source of production. This method can not only effectively reduce energy consumption and improve the repeatability of material preparation, but also realize the controllable construction of high-density, high-activity center and multi-function, and even realize the centralized appearance of different catalytic active centers in the same catalyst, providing a model catalyst for the study of catalytic mechanism. More and more researchers have been working on the preparation of carbon-based catalysts by non-carbonized methods. This paper systematically introduces the preparation and application of non-carbonization strategy to construct new oxygen reduction catalytic materials. Although the non-carbonization strategy is still in its infancy, it is playing an important role in promoting the research of active sites and catalytic mechanism.