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.