Recently, Professor Wang Zhenbo of the School of Chemical Engineering and Chemistry of our school and Professor Wu Gang of the State University of New York at Buffalo cooperated with ZIF-8 high-temperature carbonized nitrogen-doped carbon (ZIF-NC) as a matrix, through Fe3 + adsorption and thermal activation process An Fe-NC catalyst with atomically dispersed FeN4 active centers was prepared, and the model system was used to study and reveal the formation mechanism of high-performance FeN4 active centers during thermal activation, as shown in Figure 1.

Figure 1 Schematic diagram of the model system established by the adsorption of Fe3 + with nitrogen-doped porous carbon (ZIF-NC)

The research and preparation of a high-performance, low-cost non-precious metal oxygen reduction catalyst is the key to the commercial application of proton exchange membrane fuel cells. The Fe-N co-doped carbon material (Fe-NC) is currently the most active non-precious metal catalyst, among which The FeN4 coordination structure is considered to be the main active center. However, the formation mechanism of high-performance FeN4 active centers is not yet clear, because existing catalysts are prepared by pyrolysis of precursors composed of high-temperature pyrolysis transition metal salts, nitrogen sources, and carbon sources. The evolution process of the Fe-Nx structure and the high-temperature carbonization process And the nitrogen doping process occurs simultaneously. Exploring and studying the formation mechanism of FeN4 active center contributes to the rational design and further improvement of Fe-NC catalyst.

The research team first confirmed the effectiveness of the above model system with the help of transmission electron microscopy (TEM), Raman, N2 adsorption and desorption test, X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) and other physical characterizations. The morphology, structure and nitrogen doping of the substrate did not change significantly during thermal activation. Subsequently, with the aid of scanning electron microscopy (STEM) and X-ray absorption spectroscopy (XAS) with atomic resolution, the dispersion state and structural evolution of Fe species during thermal activation were characterized and analyzed, with the help of rotating disk ring electrode (RRDE) ) And the fuel cell to test the performance evolution of the catalyst, establish the structure-performance evolution of the catalyst during the thermal activation process and conduct a first-principle study on it, as shown in Figure 2. The results show that: (1) The FeOx particles formed during the Fe3 + adsorption process will degrade into an atomically dispersed FeN4 coordination structure during thermal activation, thereby increasing the density of the FeN4 active center. This process occurs due to the thermal stability of the FeN4 structure It is higher than FeOx; (2) The coordination number of Fe-Nx coordination structure generated at room temperature increases during thermal activation, the symmetry decreases, the Fe-N bond length becomes shorter, the Fe-N bonding strength increases, and the FeN4 structure The shrinkage of the Fe-N bond will change the charge distribution of the central Fe ion and surrounding C atoms, thereby promoting the adsorption of O2 on the FeN4 site and the subsequent OO bond breaking process, increasing the intrinsic activity and stability of the FeN4 active center; ( 3) Activation at 400 ℃ can generate stable and efficient FeN4 active centers, indicating that the high temperature of 800 ℃ and above in the preparation process of traditional pyrolytic Fe-NC catalyst is only the necessary temperature for precursor carbonization rather than the necessary temperature for active center generation .

Figure 2 The structure-performance evolution and first-principles study of the catalyst during thermal activation

In addition, this model system achieves the first adjustment of the density of FeN4 active centers without changing the pore structure of the carbon matrix and nitrogen doping, providing a good platform for other relevant theoretical studies. After optimizing the carbon matrix structure and Fe3 + adsorption capacity, the half-wave potential of the resulting catalyst catalyzed ORR in 0.5 MH2SO4 was as high as 0.84 V (vs. RHE, 0.6 mg / cm2), and the current density at fuel cell test was 0.9 V at 30 V mA / cm2 (US Department of Energy target 44 mA / cm2) is the highest value reported.

The relevant results were published in Angew. Chem. Int. Ed. The first author of the article was Li Jiazhan, a doctoral student of Harbin Institute of Technology, and Harbin Institute of Technology was the first communication unit of the thesis. (Wang Zhenbo / text)

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