Tuning hydrocarbon selectivity in electrochemical CO2 reduction via copper-porphyrin immobilization on carbon nanotubes

Abstract

Electrochemical CO₂ conversion using renewable energy offers a promising pathway for producing value-added chemicals with zero emissions. In this study, copper porphyrin (Cu-TMCPP) molecules are immobilized on multi-wall carbon nanotubes (MWCNTs) to form Cu-TMCPP/CNTs, which serve as a tunable, heterogeneous electrocatalyst for electrocatalytic CO₂ reduction (ECR). A systematic comparison of the synthesized catalyst with stacked Cu-TMCPP and a physical mixture of Cu-TMCPP with MWCNTs (Cu-TMCPP + CNT) showed that the Cu-TMCPP/CNT catalyst suppressed the hydrogen evolution reaction (HER) to a large extent and improved selectivity towards hydrocarbon (total FE: 75.68%), mainly CH₄ (FE: 37.3%), at a potential of −1.08 V versus the reversible hydrogen electrode (RHE) with a partial current density of 91.8 mA cm⁻². The in-depth mechanistic analysis of in situ and ex situ X-ray adsorption spectroscopy (XAS) and electrochemical characterization illustrated the presence of ultrathin copper porphyrin blocks immobilized on CNTs, presumably located at the Helmholtz layer with a high oxidation state and the co-existence of Cu²⁺/Cu⁰ under reaction conditions, which are responsible for improved selectivity towards hydrocarbons. This research provides insights into the immobilization impact of molecular catalysts and the advantage of the consequent high capacitance double layer in suppressing the HER, which enhances electron transfer, thereby improving heterogeneous electrocatalytic CO₂ performance.