Impaired conjugation boosts CO2 electroreduction by Ni(ii) macrocyclic catalysts immobilized on carbon nanotubes

Abstract

Metal complexes hybridized with conductive supports are desirable as high-performance catalysts for CO₂ electroreduction, while the delicate molecular design to improve both the intrinsic activity of complex and the molecule–support interactions still remains challenging. We here employ a conjugation-tuning strategy by comparison between Ni(II) octabutoxyphthalocyanine and Ni(II) octabutoxynaphthalocyanine on multi-walled carbon nanotubes (NiPc-B@CNT and NiNc-B@CNT) in aqueous electrocatalytic CO₂ reduction, respectively. In contrast to the conventional promotive effects from extended conjugation, the impaired conjugation in the Ni(II) macrocycles unusually boosts both activity and molecule–support affinity. These merits can be attributed to the favored electronic effects and the higher flexibility of long alkyl chains both arising from the absent extended benzene ring in NiPc-B. Consequently, NiPc-B@CNT exhibits much higher faradaic efficiencies for CO production (FE_CO ≥ 94% among −0.79 to −1.09 V vs. RHE) than NiNc-B@CNT (FE_CO < 20%) in an H-cell configuration. The use of a gas-diffusion electrode further raises the electrocatalytic performances of NiPc-B@CNT under 1 atm CO₂ (FE_CO ≈ 100% at −0.15 A cm⁻²) or simulated flue gas (10% CO₂, FE_CO ≈ 80% at −0.1 A cm⁻²), respectively.