Recently, research group led by Prof. Dehui Deng in the State Key Laboratory of Catalysis (SKLC) of Dalian Institute of Chemical Physics (DICP), Chinese Academy of Sciences (CAS) made a new progress in understanding the active site of metal-free nitrogen-doped (N-doped) carbon in electrocatalytic CO2 reduction (CO2R) to CO. Via tuning the type and doping content of N in the controllable synthesis of N-doped carbon catalysts, they found that the carbon atoms adjacent to the graphitic N (GN) are triggered as the active sites for the CO2R to CO. This work provides a guidance for the deep understanding of the reaction mechanism in the electrocatalytic CO2 reduction as well as the rational design of catalysts.
Converting CO2 to value-added chemicals driven by renewable energies is a strategy of reducing carbon emission and alleviating the pressure from the depleting fossil resources. Electrocatalytic CO2R to CO is an important way of CO2 utilization since CO is an important feedstock for chemical synthesis. N-doped carbon material possesses excellent conductivity and tunable electronic structure. It exhibits an excellent catalytic activity in the electrocatalytic CO2R to CO. However, the active site for the reaction is still under debate due to the complexity in the N species as well as the lack of methods for the controllable doping of N.
Based on the previous studies of Prof Deng’s research team on graphene-confined single atom catalysts (Sci. Adv. 2015, 1, e1500462; Nat. Nanotechnol. 2016, 11, 218; Angew. Chem. Int. Ed. 2016, 55, 6708; Nano Energy 2017, 32, 353; Chem 2018, 4, 1902; Angew. Chem. Int. Ed. 2018, 57, 16339; Chem. Rev. 2019, 119, 1806; Adv. Mater. 2019, 31, 1901996), they prepared a series of N-doped carbon foam catalysts with precisely controlled N types and doping contents through an innovated approach of silicon dioxide sphere template-assisted pyrolysis of phthalocyanine. Electrochemical performance tests show that the catalyst dominated by GN dopants presents the highest CO selectivity with CO Faradaic efficiency reaching 95% at -0.5 V versus the reversible hydrogen electrode (RHE), compared with those with higher contents of pyridinic N (PN) or pyrrolic N (ProN) dopants. Theoretical calculations indicate that the limiting potentials for the CO2R to CO over the carbon atoms adjacent to the GN are significantly lower than those for the hydrogen evolution reaction (HER), which favors the electrocatalytic CO2R to CO. In comparison, the PN site tends to be blocked by the strongly adsorbed H* and the HER is more favorable with lower limiting potentials at the carbon atoms neighbouring to the PN, while the ProN disfavors both reactions. Thus, compared with the PN and ProN, the GN significantly improves the activity of the adjacent carbon atoms for the electrocatalytic CO2 to CO.
This work has been published as an article in Cell Reports Physical Science. This work was supported by the National Key R&D Program of China, the major program of the National Natural Science Foundation of China, the Strategic Priority Research Program of Chinese Academy of Sciences, the DNL Cooperation Fund, CAS, and Collaborative Innovation Center of Chemistry for Energy Materials (2011. iChEM). (Text and image by Zheng Zhang, Liang Yu and Hehua Gao)