Recently, a research team led by Prof. CHEN Qianwang from Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China (USTC), turned main-group element magnesium (Mg) into a highly active electrocatalyst experimentally for oxygen reduction reaction (ORR) with the assist of density function theory calculations. The article was published on Nature communication on 18 Feb.

For modern environmentally-friendly society, energy conversion and storage largely rely on electrocatalytic ORR. In the past decades, design and manufacture of electrocatalysts have transitioned from noble metals like platinum and gold to transition metals like nickel to decrease the cost with the famous d-band center model. 

In the d-band center model, the center position of d-band is linearly related to the adsorption free energies of adsorbates on transition metal surface, which has been wildly applied in designing transition metal-based electrocatalysts. 

Due to lacking the combination of empty and filled host-orbitals, the cheaper main-group element metals, like Mg and Al have not shown to be any electrocatalysts in the reported experiments so far. The delocalized s/p-band of main-group metals result in too strong or too weak adsorption in the interaction of reaction intermediates. It means they are not good catalysts.

However, Mg cofactors existing in enzymes, such as DNA polymerase and hexokinase, are extremely active in biochemical reactions. Then the research team caught the flash of light that the Mg cofactor with suitable adsorption strength to oxygenated species could be favorable for ORR if its p-electronic state is tuned to a reasonable level. 

Calculation of the catalytic activity of metal cofactor models. a) Models of single metal atoms (Mg, Al and Ca) coordinated with different nitrogen atom structures. b-c) Linear relationships between reactants. d) The volcano map of metal cofactors. e) the performance comparation of screened main group metal cofactors and transition metal cofactors. Copyright from Nature Research 2020.

Their theoretical calculation suggested it possible that altering the number of nitrogen coordination atoms would ameliorate the band energy level of the center atom to even match platinum. In the experiment, they synthesized new Mg-based metal-organic-framework (MOF). After pyrolysis, a few Mg atoms were bonded with surrounding atoms in graphene carbon matrix to form Mg-N cofactor catalysts. The catalysts performed stably in over 5000 cycles, surpassed the performance of commercial Pt/C in alkaline conditions and far exceeded that of most transition metal-based catalysts reported so far.

Through controlling electron structure, the catalysts based on the main-group metal can also be comparable to the transition metal catalysts, which provides a great idea for the coming catalyst design. 

The first author of the work is LIU Shuang, a PhD student of USTC, and Prof. CHEN Qianwang is the corresponding author. This study was supported by the National Natural Science Foundation, the National Key R&D Program of China and Fundamental Research Funds for the Central Universities.

Reference

1、https://www.nature.com/articles/s41467-020-14565-w (Open Access)

(Written by ZHAO Xiaona, edited by YE Zhenzhen, USTC News Center)