Atomically Dispersed NiN3 Sites on Highly Defective Micro-Mesoporous Carbon for Superior CO2 Electroreduction.

Direct electrochemical conversion of CO2 to CO product powered by renewable electricity is widely advocated as an emerging strategy for alleviating CO2 emissions while addressing global energy issues. However, the development of low-cost and efficient electrocatalysts with high Faradaic efficiency for CO production (FECO ) and high current density remains a grand challenge. Herein, a robust single nickel atomic site electrocatalyst, which features isolated and dense single atomic NiN3 sites anchored on highly defective hierarchically micro-mesoporous carbon (Ni-SAs/HMMNC-800), to enable enhanced charge transport and more exposed active sites for catalyzing electrochemical CO2 -to-CO conversion, is reported. The Ni-SAs/HMMNC-800 catalyst achieves excellent activity and selectivity with high FECO values of >90% throughout a wide potential range (the FECO reaches 99.5% at -0.7 V vs reversible hydrogen electrode) and a CO partial current density as high as 13.0 mA cm-2 at -0.7 V versus reversible hydrogen electrode, as well as a far outstanding durability during long-term continuous operation, indicating a superior CO2 electroreduction performance than that of other reference samples and most of previously reported carbon-based single atom electrocatalysts. Experimental and density functional theory calculations reveal that atomic NiN3 coordination sites coupled adjacent defects are favorable to significantly enhancing the formation of COOH* reaction intermediates while suppressing the competing hydrogen evolution reaction, thereby enhancing the electrocatalytic activity for CO2 -to-CO reduction. Notably, this work provides a valuable new prospect for designing and synthesizing efficient and cost-effective single atom CO2 electroreduction catalysts for practical applications.

Enhanced Photocatalytic CO2 Reduction through Hydrophobic Microenvironment and Binuclear Cobalt Synergistic Effect in Metallogels.

Assemblies that mimic natural lipid bilayers are theoretically efficient for photocatalytic CO2 reduction; however, such an approach has not been yet explored. Herein, metallogels (LG/Nico-Co) based on the co-assembly of L-glutamic acid lipid (LG/Nico) and cobalt ions exhibited excellent photocatalytic CO2 reduction, with 208 724 µmol g-1 CO production within 24 h and 90 % CO/H2 selectivity, or 166 826 µmol g-1 CO production within 12 h and 46 % CO/H2 selectivity, depending on the water content of the solvent. The alkyl chains of LG/Nico provide a hydrophobic microenvironment for efficient gas transfer, and the assembled bilayers induce a synergistic effect between two adjacent Co ions for catalyzing the CO2 reduction reaction. These architectures present new alternatives for the development of highly efficient photocatalytic soft matter based on the assembly of small amphiphilic molecules.

Maximizing Electroactive Sites in a Three-Dimensional Covalent Organic Framework for Significantly Improved Carbon Dioxide Reduction Electrocatalysis.

Synthesis of functional 3D COFs with irreversible bond is challenging. Herein, 3D imide-bonded COFs were constructed via the imidization reaction between phthalocyanine-based tetraanhydride and 1,3,5,7-tetra(4-aminophenyl)adamantine. These two 3D COFs are made up of interpenetrated pts networks according to powder X-ray diffraction and gas adsorption analyses. CoPc-PI-COF-3 doped with carbon black has been employed to fabricate the electrocatalytic cathode towards CO2 reduction reaction within KHCO3 aqueous solution, displaying the Faradaic efficiency of 88-96 % for the CO2 -to-CO conversion at the voltage range of ca. -0.60 to -1.00 V (vs. RHE). In particular, the 3D porous structure of CoPc-PI-COF-3 enables the active electrocatalytic centers occupying 32.7 % of total cobalt-phthalocyanine subunits, thus giving a large current density (jCO ) of -31.7 mA cm-2 at -0.90 V. These two parameters are significantly improved than the excellent 2D COF analogue (CoPc-PI-COF-1, 5.1 % and -21.2 mA cm-2 ).

Hydrogen-Bonded Organic Framework Ultrathin Nanosheets for Efficient Visible-Light Photocatalytic CO2 Reduction.

Post-modification of robust guanine-quadruplex-linked 2,2'-pyridine-containing HOF-25 with Ni(ClO4 )2 ⋅ 6 H2 O followed by exfoliation using sonication method affords hydrogen-bonded organic framework (HOF) nanosheets (NSs) of HOF-25-Ni in the yield of 56 %. TEM and AFM technologies disclose the ultrathin nature of HOF-25-Ni NSs with thickness of 4.4 nm. STM observation determines the presence of sql segments assembled from HOF-25-Ni building blocks at the heptanoic acid/highly oriented pyrolytic graphite interface, supporting the simulated 2D supramolecular framework. ICP-MS, XAS, and XPS data prove the successful immobilization of atomic nickel sites on the 20 % total 2,2'-pyridine moieties in crystalline HOF-25-Ni. With the aid of [Ru(bpy)3 ]2+ and triisopropanolamine, 10 wt% HOF-25-Ni NSs dispersed on graphene oxide efficiently promotes visible-light-driven CO2 reduction, showing a 96.3 % CO selectivity with a prominent conversion rate up to 24 323 µmol g-1 h-1 .

Two-Dimensional Covalent Organic Frameworks with Cobalt(II)-Phthalocyanine Sites for Efficient Electrocatalytic Carbon Dioxide Reduction.

The rapid development in synthesis methodology and applications for covalent organic frameworks (COFs) has been witnessed in recent years. However, the synthesis of highly stable functional COFs still remains a great challenge. Herein two-dimensional polyimide-linked phthalocyanine COFs (denoted as CoPc-PI-COF-1 and CoPc-PI-COF-2) have been devised and prepared through the solvothermal reaction of the tetraanhydrides of 2,3,9,10,16,17,23,24-octacarboxyphthalocyaninato cobalt(II) with 1,4-phenylenediamine and 4,4'-biphenyldiamine, respectively. The resultant CoPc-PI-COFs with a four-connected sql net exhibit AA stacking configurations according to powder X-ray diffraction studies, showing permanent porosity, thermal stability above 300 °C, and excellent resistance to a 12 M HCl aqueous solution for 20 days. Current-voltage curves reveal the conductivity of CoPc-PI-COF-1 and CoPc-PI-COF-2 with the value of 3.7 × 10-3 and 1.6 × 10-3 S m-1, respectively. Due to the same Co(II) electroactive sites together with similar permanent porosity and CO2 adsorption capacity for CoPc-PI-COFs, the cathodes made up of COFs and carbon black display a similar CO2-to-CO Faradaic efficiency of 87-97% at applied potentials between -0.60 and -0.90 V (vs RHE) in 0.5 M KHCO3 solution. However, in comparison with the CoPc-PI-COF-2&carbon black electrode, the CoPc-PI-COF-1 counterpart provides a larger current density (jCO) of -21.2 mA cm-2 at -0.90 V associated with its higher conductivity. This cathode also has a high turnover number and turnover frequency, amounting to 277 000 and 2.2 s-1 at -0.70 V during 40 h of measurement. The present result clearly discloses the great potential of 2D porous crystalline solids in electrocatalysis.

Tailored Local Electronic Environment of Co-N4 Sites in Cobalt Phthalocyanines for Enhanced CO2 Reduction Reaction.

Atomically dispersed Co-N4-based catalysts have been recently emerging as one of the most promising candidates for facilitating CO2 reduction reaction (CO2RR). The local electronic environment of Co-N4 sites in these catalysts is considered to play a critical role in adjusting the catalytic performance, the effort of which however is not yet clearly verified. Herein, a series of cobalt phthalocyanines with different peripheral substituents including unsubstituted phthalocyanine Co(II) (CoPc), 2,9,16,23-tetramethoxyphthalocyaninato Co(II) (CoPc-4OCH3), and 2,9,16,23-tetranitrophthalocyaninato Co(II) (CoPc-4NO2) are supported onto the surface of the multi-walled carbon nanotubes (CNTs), affording CoPc@CNTs, CoPc-4OCH3@CNTs, and CoPc-4NO2@CNTs. X-ray photoelectron spectroscopy and X-ray absorption near-edge structure measurements disclose the influence of the peripheral substituents on the local electronic structure of Co atoms in these three catalysts. Electrochemical tests indicate the higher CO2RR performance of CoPc-4OCH3@CNTs compared to CoPc@CNTs and CoPc-4NO2@CNTs as exemplified by the higher Faraday efficiency of CO, larger part current densities, and better stability displayed by CoPc-4OCH3@CNTs at the applied voltage range from -0.6 to -1.0 V versus RHE in both H-cell and flow cell. These results highlight the effect of the electron-donating -OCH3 substituent on the enhanced catalytic activity of CoPc-4OCH3@CNTs, which will help develop Co-N4-based catalysts with promising catalytic performance toward CO2RR.

Porphyrin Coordination Polymer with Dual Photocatalytic Sites for Efficient Carbon Dioxide Reduction.

The resurgence of visible light photocatalysis for carbon dioxide reduction reaction (CO2RR) has resulted in the generation of various homogeneous and heterogeneous paradigms. Herein, a new system has been established by incorporating dual catalytic sites into porous coordination polymer toward the photocatalysis of CO2RR. A functional ligand, 5,10,15,20-tetrakis[4'-(terpyridinyl)phenyl]porphyrin (TTPP), has been used to assemble discrete divalent nickel ions into the coordination polymer (TTPP-Ni) through metal bis(terpyridine) nodes. Both the porphyrin and terpyridine moieties prefer to bind with nickel ions, giving rise to TTPP-Ni with dual active catalytic sites. By controlling different molar ratios of ligand and metal and the reaction temperature, four samples including TTPP-Ni-n (n = 1, 2, 3, and 4) with different molar ratios of nickel porphyrin and nickel bis(terpyridine) subunits have been fabricated. The predesigned two-dimensional chemical structures of TTPP-Ni samples have been fully characterized using powder X-ray diffraction, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and IR and UV-vis spectroscopies. The photocatalytic activities of these coordination polymers have been screened using [Ru(bpy)3]Cl2·6H2O as a photosensitizer together with triisopropanolamine as the sacrificial electron donor in CH3CN and H2O. Among these photocatalysts, TTPP-Ni-3 and TTPP-Ni-4 with almost saturated metal sites are able to display extraordinary photocatalytic performance including a CO generation rate of ca. 3900 µmol g-1 h-1 and 98% selectivity. The mechanism associated with dual active sites has been rationalized on the basis of theoretical simulations.