Pyrolyzed Fe/N/C catalyst is a promising non-precious-metal catalyst (NPMCs) for oxygen reduction reaction (ORR) in fuel cells, and substantial progresses have been made recently in this field. However, to meet the requirement of practical applications, it still needs to improve significantly the ORR activity and stability of Fe/N/C catalyst, and to reveal active site structure for rational design of NPMCs.

Herein, we report our recent progresses in preparation of high performance Fe/N/C catalyst and investigation of the active sites. The Fe/N/C catalyst was prepared by using precursor of poly-m-phenylenediamine (PmPDA), and exhibits an ORR performance of 12.8 A/g@0.80V in acid solution with H₂O₂ yield less than 1%.^[1] When it was applied in proton exchange membrane cell (PEMFC), a peak power density could reach 0.81 W cm^-2. We further improved the catalytic activity of PmPDA-Fe/N/C by co-doping of S element. The peak power density has exceed 1.0 W cm^-2. ^[2] The active sites of Fe/N/C catalyst are mostly located in micropores, so they are susceptible to water flooding. To this point, surface fluorination was applied to the Fe/N/C catalysts (such as the modification of Ar-CF₃). The fluorinated Fe/N/C could perform stably over 120 h at 0.5 V with a current density of 0.56 A cm^-2 in a H₂-O₂ PEMFC.^[3] Alternatively, hydrophobic dimethyl silicon oil (DMS) was introduced into Fe/N/C catalyst layer, which leads to form triple-phase interface in micropores. As a result, Fe/N/C-based direct methanol fuel cell (DMFC) could yield a performance close to that of Pt-based DMFC.^[4]

We developed a molecule/ion probe method to study the active sites. It has revealed that the ORR activity of PmPDA-Fe/N/C is not sensitive to CO and NO_x, but can be suppressed significantly by halide ions (e.g., Cl⁻, F⁻, and Br⁻) and low valence state sulfur-containing species (e.g., SCN⁻, SO₂ , and H₂S).^[1] This indicates that the active sites of the Fe/N/C catalyst contains Fe element in acid medium. We further designed a single-atomic-layer Fe/N/C model catalyst based on monolayer graphene (FeNMLG), and systematically investigated the effects of defect density, the number of graphene layers, and the doped nitrogen species on the ORR activity.^[5] It has demonstrated also that such Fe/N/C model catalyst is also fit for in situ vibration spectroscopic studies.

Acknowledgement: This study was supported by grants from National Key Research and Development Program of China (2016YFB0101202), and NSFC (91645121 and 21621091).

References:

1. Wang, et al, J. Am. Chem. Soc. 2014, 136, 10882−10885.
2. C. Wang, et al, Angew Chem Int Ed. 2015, 54, 9907-9910.
3. C. Wang, et al, ChemElectroChem, 2017, in revision.
4. C. Wang, et al ACS Energy Lett. 2017, 2, 645-650.
5. D. Yang, et al ACS Catal, 2017, 7, 139−145.