Advanced science: mysterious ligand: revealing the true features of fe-n-c fuel cell catalyst

The key to the large-scale commercial application of

is to find a cheap efficient non noble metal oxygen reduction reaction (ORR) catalyst to replace the platinum based noble metal catalyst with high price rare reserves. Among many non noble metal catalysts fe-n-c is considered to be the most potential substitute for platinum based catalysts due to its low price excellent orr catalytic activity. However the active site structure of fe-n-c materials is still controversial. Some previous literatures suggested that the catalytic active structures of fe-n-c catalysts are fe-n4-c10 fe-n4-c12 etc. However theoretical calculations show that the orr catalytic activities corresponding to these planar configurations are very low the order of relative orr catalytic activities is not consistent with the experimental results. Besides the in-plane fe-n4 coordination structure there is an axial lig in the Fe center. However it is still unclear what the lig is its formation mechanism its role in orr catalysis.

in view of this Li Jia Gan Lin of Shenzhen International Graduate School of Tsinghua University worked together to analyze the axial ligs of Fe metal sites in fe-n-c materials found that the Fe metal sites in fe-n-c catalysts spontaneously form - Oh axial ligs under the oxygen reduction reaction potential. -The existence of Oh ligs can explain not only the relative catalytic activity of orr with different fe-n-c configurations but also the great difference of oxygen reduction activity of fe-n-c catalysts in acid-base solutions. In addition it was found for the first time that fe-n-c catalyst exhibited a pH dependent KSCN poisoning kinetics due to - Oh lig.

in view of the various possible configurations of fe-n4-c proposed in fe-n-c materials the research team found that - Oh axial ligs can significantly reduce the adsorption strength of Fe metal sites in fe-n4-c structure to the intermediates of orr reaction significantly improve the theoretical orr activity of fe-n4-c give the order of Orr catalytic activity of different fe-n4-c configurations consistent with the experiment. Because the axial lig of - Oh is easy to form in alkaline solution the orr activity of fe-n-c catalyst in alkaline solution is much better than that in acidic solution. At the same time the presence of - Oh axial ligs also directly affects the toxicity of SCN - to Fe metal active sites. The addition of KSCN in acidic solution can significantly inhibit orr catalytic activity but the addition of KSCN in alkaline solution does not significantly reduce orr catalytic activity. X-ray photoelectron spectroscopy (XPS) was used to characterize the samples. It was found that the oxygen content of fe-n-c catalyst increased significantly after orr reaction in alkaline environment while the sulfur content of fe-n-c catalyst poisoned by KSCN was lower than that in acidic environment. It is proved that the - Oh axial lig is easier to form under alkaline conditions it reduces the toxic degree of Fe metal sites by KSCN. The above results are of great significance to underst the active site structure of fe-n-c materials help to further improve the orr catalytic activity of fe-n-c materials.

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