The development of cost-effective and Pt group metal (PGM)-free catalysts for kinetic sluggish oxygen reduction is the key to large-scale application of proton exchange membrane fuel cells (PEMFCs). At present, the best PGM-free catalysts are Fe-N-C catalysts. However, the fuel cell stability of Fe-N-C is still poor partially due to Fenton reaction catalyzed by Fe ions to produce oxidative free radicals that attack the carbon support, which limits their practical application. Therefore, despite the inferior performance, Co-N-C catalysts hold the potential to be stable PGM-free catalysts by virtue of their immunity to Fenton reaction. Improving the performance of Co-N-C fuel cell requires an in-depth understanding of the performance enhancement mechanism.

In this regard, Prof. Jianglan Shui’s research team at Beihang University synthesized a series of Co-N-C single-atom catalysts with different densities of Co-N₄ active sites for a quantitative study of the structure-property relationship. Related research was published in Applied Catalysis B: Environmental on June 13, 2019. The researchers found that the fuel cell power density increased slowly and linearly with the Co-N₄ active site density in the low density region; whereas it showed an accelerated increase trend in the high density region. This indicated that we could effectively improve the fuel cell performance of a catalyst with low intrinsic activity by increasing the active site density. The optimized Co-N-C single atom catalyst achieved an ultra-high peak power density of 826 mW cm^-2. In addition, DFT calculations together with peroxide reduction reaction experiments clarified a direct 4e^- pathway instead of a 2e^- + 2e^- pathway on a Co-N₄ site, which explained the relatively higher stability of Co-N-C than Fe-N-C in fuel cells.

Figure: The atomic structure of the active sites identified by the Beijing Synchrotron Radiation Facility (BSRF). (a) XANES spectra. (b) Fourier transformed EXAFS spectra. (c) EXAFS fitting result.

In this work, the researchers relied on the Beijing synchrotron radiation facility 1W1B-XAFS station to study the atomic structure of the active sites of Co-N-C catalysts. The extended X-ray absorption fine structure (EXAFS) analyses and fitting showed that the synthesized Co-N-C catalysts had atomically dispersed active sites with a structure of Co-N₄ adsorbed by one O₂ molecule. The unambiguous identification of active sites provided a strong support for theoretical calculations.

The disclosed performance enhancement mechanism demonstrated a viable approach to increase the power density of non-precious metal catalysts. Meanwhile, the excellent comprehensive performances of Co-N-C catalyst indicated its promising future for low cost PEMFCs.

Article:

Linyun Chen, Xiaofang Liu, Lirong Zheng, Yongcheng Li, Xu Guo, Xin Wan, Qingtao Liu, Jiaxiang Shang, Jianglan Shui* Insights into the role of active site density in the fuel cell performance of Co-N-C catalysts. Appl. Catal. B-Environ. 256 (2019) 117849.