Two‐dimensional Metal‐organic Frameworks for Electrochemical CO 2 Reduction Reaction

Combining Nickel- and Zinc-Porphyrin Sites via Covalent Organic Frameworks for Electrochemical CO2 Reduction

Covalent organic frameworks (COFs) are ideal platforms to spatially control the integration of multiple molecular motifs throughout a single nanoporous framework. Despite this design flexibility, COFs are typically synthesized using only two monomers. One bears the functional motif for the envisioned application, while the other is used as an inert connecting building block. Integrating more than one functional motif extends the functionality of COFs immensely, which is particularly useful for multistep reactions such as electrochemical reduction of CO2. In this systematic study, we synthesized five Ni(II)- and Zn(II)-porphyrin-based COFs, including two pure component COFs (Ni100 and Zn100) and three mixed Ni/Zn-COFs (Ni75/Zn25, Ni50/Zn50, and Ni25/Zn75). Among these, the Ni50/Zn50-COF exhibited the highest catalytic performance for the electroreduction of CO2 to CO and formate at -0.6 V vs RHE, as was observed in an H-cell. The catalytic performance of the COF catalysts was further extended to a zero-gap membrane electrode assembly (MEA) operation where, utilizing Ni50/Zn50, CH4 was detected along with CO and formate at a high current density of 150 mA cm-2. In contrast, under these conditions predominantly H2 and CO were detected at Ni100 and Zn100 respectively, indicating a clear synergistic effect between the Ni- and Zn-porphyrin units.

Dual-Anion Regulation for Reversible and Energetic Aqueous Zn-CO2 Batteries

Aqueous Zn-CO2 batteries can not only convert CO2 into high-value chemicals but also store/output electric energy for external use. However, their performance is limited by sluggish and complicated CO2 electroreduction at the cathode. Herein, a dual-anion regulated Bi electrocatalyst is developed to selectively reduce CO2 to formate with a Faradaic efficiency of up to 97% at a large current density of 250 mA cm-2. With O and/or F, the rate-determine step of CO2 electroreduction has been manipulated (from the first hydrogenation to *HCOOH desorption step) with a reduced energy barrier. Significantly, the fabricated Zn-CO2 battery exhibits a high discharge voltage of 1.2 V, optimal power density of 4.51 mW cm-2, remarkable energy density of 802 Wh kg-1, and energy-conversion efficiency of 70.74%, stability up to 200 cycles and 68 h. This study provides possible strategies to fabricate reversible and energetic aqueous Zn-CO2 batteries by addressing cathodic problems.

Quantitative Structure–Electrochemistry Relationship (QSER) Studies on Metal–Amino–Porphyrins for the Rational Design of CO2 Reduction Catalysts

The quantitative structure–electrochemistry relationship (QSER) method was applied to a series of transition-metal-coordinated porphyrins to relate their structural properties to their electrochemical CO2 reduction activity. Since the reactions mainly occur within the core of the metalloporphyrin catalysts, the cluster model was used to calculate their structural and electronic properties using density functional theory with the M06L exchange–correlation functional. Three dependent variables were employed in this work: the Gibbs free energies of H*, C*OOH, and O*CHO. QSER, with the genetic algorithm combined with multiple linear regression (GA–MLR), was used to manipulate the mathematical models of all three Gibbs free energies. The obtained statistical values resulted in a good predictive ability (R2 value) greater than 0.945. Based on our QSER models, both the electronic properties (charges of the metal and porphyrin) and the structural properties (bond lengths between the metal center and the nitrogen atoms of the porphyrin) play a significant role in the three Gibbs free energies. This finding was further applied to estimate the CO2 reduction activities of the metal–monoamino–porphyrins, which will prove beneficial in further experimental developments.