For the reductive CO_2 fixation using solar energy, photo-generated electrons and/or holes must be captured and utilized for chemical reactions that generate products beneficial to humans. To construct the artificial photosynthesis systems, electron transfer catalysts designed to promote electrochemical CO_2 reduction reactions (CO_2RR) are a critical requirement. Previous studies have shown that precise tuning of adsorption energies of key intermediates is critical for obtaining high activity and reaction selectivity. There are two important factors that determine the adsorption energy of the intermediate on electrocatalysts. First, the metal species in the catalyst, i.e., the transition metals at the active sites with a higher number of d-electrons, tend to reduce the adsorption energy. Previous works have investigated the effects of metal species on the CO_2RR using organometallic complexes as platforms to support various metals via coordination bonds. For example, cobalt-modified tetraphenylporphyrin (TPP) is known to exhibit efficient CO_2RR activity due to its proper binding strength to COOH*, whereas nickel (Ni) and copper (Cu) modified TPP are known to have low catalytic activity due to the low binding energy of COOH*. The second critical factor is the coordination structure of the metal. First-principles calculations have revealed that a decrease in the coordination number of Pt and Cu in the bulk increases the bond strength of CO* and affects the activity of the CO_2RR. Thus, it can be expected that the coordination number can be a control parameter to improve the performance of metal species that were previously thought to have no CO_2RR activity. We have investigated the effects of metal species and the coordination structure on CO_2RR using graphene and covalent triazine frameworks (CTF). CTF consisting of microporous conjugated polymers with 1,3,5-triazine linker units, are suitable for investigating the above issue due to the high design flexibility. Through the studies, we have revealed that Ni-modified graphene and CTF can work as efficient electrocatalysts for CO_2RR due to the unsaturation nature of the Ni atoms on the substrate. In this talk, we report our systematic studies on CO_2RR using carbon-based materials with 3d metal atoms, focusing on the effects of metal species and coordination structures.
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