Nickel-nitrogen-modified graphene (Ni-N-Gr) is fabricated and Ni-N coordination sites on Ni-N-Gr as active centers effectively reduce CO to CO. The faradaic efficiency for CO formation reaches 90% at -0.7 to -0.9 V versus RHE, and the turnover frequency for CO production comes up to ≈2700 h at -0.7 V versus RHE.

The electrochemical oxygen reduction reaction (ORR) is an important cathode reaction of various types of fuel cells. The development of electrocatalysts composed only of abundant elements is a key goal because currently only platinum is a suitable catalyst for ORR. Herein, we synthesized copper-modified covalent triazine frameworks (CTF) hybridized with carbon nanoparticles (Cu-CTF/CPs) as efficient electrocatalysts for the ORR in neutral solutions. The ORR onset potential of the synthesized Cu-CTF/CP was 810 mV versus the reversible hydrogen electrode (RHE; pH 7), the highest reported value at neutral pH for synthetic Cu-based electrocatalysts. Cu-CTF/CP also displayed higher stability than a Cu-based molecular complex at neutral pH during the ORR, a property that was likely as a result of the covalently cross-linked structure of CTF. This work may provide a new platform for the synthesis of durable non-noble-metal electrocatalysts for various target reactions.

Nickel-modified covalent triazine frameworks effectively reduced CO2 to CO because adsorbed COOH was stabilized on the coordinatively-unsaturated Ni atoms in CTF.

It was found that copper-modified covalent triazine frameworks (Cu-CTF) efficiently catalyze the electrochemical reduction of nitrate and promote N−N bond formation of nitrous oxide (N 2 O), a key intermediate for N 2 formation (denitrification). A Cu-CTF electrode exhibited an onset potential of −50 mV versus RHE for the electrochemical nitrate reduction reaction (NRR). The faradaic efficiency for N 2 O formation by Cu-CTF reached 18% at −200 mV versus RHE, whereas that for Cu metal was negligible. On the basis of density functional calculations for Cu-CTF, both solvated and surface-bound nitric oxide (NO) were generated by the NRR due to the moderate adsorption strength of Cu atoms for NO, a property that facilitated the effective dimerization of NO through an Eley−Rideal-type mechanism.

So-called local cells resulting from the coupling of oxidation and reduction reactions on the same conductive substrate represent a well-known cause of metallic corrosion. In the present study, we attempted to demonstrate that catalytic systems based on the principle of local cell reactions can be successfully fabricated using metal-doped covalent triazine frameworks as catalytic units. A conductive substrate carrying platinum- and copper-doped covalent triazine frameworks as catalysts for the oxidation and reduction processes, respectively, was developed to fabricate a local cell catalytic unit for the concurrent reduction of nitrate to nitrous oxide and oxidation of hydrogen.

The development of oxygen reduction reaction (ORR) electrocatalysts comprising abundant elements is highly desirable for achieving widespread use of fuel cells. Optimal ORR catalysts should have moderate binding strength (ΔE ads) with O2-derived intermediates, where the metal species and its coordination numbers are the essential determining factors for ΔE ads. However, in conventional non-noble-metal-based ORR catalysts, such as metal–nitrogen-doped carbons, the metal species and its coordination structure cannot freely be chosen. In contrast, covalent organic frameworks (COFs), which are cross-linked microporous polymers, have high design flexibility; as such, they can be purposefully designed by using a wide range of monomers. The present work investigated the adsorption strength of ORR intermediates on single 3d metal atoms (Mn, Fe, Co, Ni, and Cu) doped in COFs with different coordination structures using first-principles calculations toward the development of efficient non-noble-metal ORR catalysts. The adsorption strength of the intermediates was found to monotonically increase as either the number of d-electrons or coordination number of metal centers decreased, and a volcano-type relationship was observed between the adsorption energies of the intermediates and the theoretical ORR activities. Therefore, to develop efficient non-noble-metal-based ORR electrocatalysts, the adsorption strength should be tuned close to the volcano peak by an appropriate choice of metal species and/or coordination number as the control parameters. Considering the high designability of metal species and of its coordination numbers in COFs, COFs are expected to become the next-generation platform of supports of single-atom catalysts using the design direction provided by the present work.

The electrochemical oxygen reduction reaction (ORR) is an important cathode reaction of various types of fuel cells.T he development of electrocatalysts composed only of abundant elements is ak ey goal because currently only platinum is asuitable catalyst for ORR. Herein, we synthesized copper-modified covalent triazine frameworks (CTF) hybridized with carbon nanoparticles (Cu-CTF/CPs) as efficient electrocatalysts for the ORR in neutral solutions.T he ORR onset potential of the synthesized Cu-CTF/CP was 810 mV versus the reversible hydrogen electrode (RHE;p H7), the highest reported value at neutral pH for synthetic Cu-based electrocatalysts.C u-CTF/CP also displayed higher stability than aC u-based molecular complex at neutral pH during the ORR, ap roperty that was likely as ar esult of the covalently cross-linked structure of CTF.T his work may provide an ew platform for the synthesis of durable non-noble-metal electrocatalysts for various target reactions.

The electrochemical reduction of carbon dioxide (CO2) to chemical feedstocks is an attractive method for the removal of CO2 from the environment. Although copper (Cu)-based catalysts produce hydrocarbons with relatively high selectivity during CO2 electroreduction, such catalysts evolve a certain amount of H2 via proton reduction reactions. Because low-coordinated Cu sites are likely active for the competing hydrogen evolution reaction (HER), hindering such low-coordinated Cu sites by decoration with inert metal atoms is a promising approach to increasing the selectivity of the CO2 reduction reaction (CO2RR) over the HER. In the present study, we synthesized tin (Sn)-modified Cu nanoparticles with varied Sn ratios via a simple wet-chemical method. Physical and theoretical characterizations revealed that Sn atoms preferentially locate at the low-coordinated sites when Sn is present at low contents (less than 1.5%). Compared with the bare Cu catalyst, the Sn-modified Cu electrocatalyst shows suppression of the HER and acceleration of the carbon monoxide (CO) evolution reaction. The first-principles calculations about the adsorption strength of reaction intermediates revealed that low-coordinated Cu sites with the modification of Sn atoms exhibited lower activity for both HER and CO2RR than that without modification. As a result, the activity of coordinatively saturated Cu atoms, where the CO2RR is more favorable than the HER, was emphasized. The modification of trace foreign metals in metal nanoparticles may provide an avenue for the synthesis of selective electrocatalysts for various target reactions.