In situ encapsulated Co3O4 nanoparticles into self-catalyzed grown CNTs for efficient CO2 conversion

Co3O4-based catalysts emerged as promising candidates for electrocatalytic CO2 reduction reaction (eCO2RR), while the unclear active sites and poor stability are challenging for the application of Co3O4 as an electrocatalyst. Here, we developed a feasible method to in situ encapsulate Co3O4 nanoparticles (NPs) into self-grown CNTs. Co3O4 NPs were elaborately controlled to terminate with the occupation Co2+Th/Co3+Oh sites (with Co2+ and Co3+ in the center of tetrahedral/octahedral cells). The well-dispersed Co atoms derived from ZIF-67 catalyzed the in situ assembly of CNTs, allowing controlled oxidation and growth into Co3O4 nanoparticles spatially confined by CNTs, resulting in Co3O4 highly-dispersed into hollow CNTs and well-protected. The enhanced electrocatalytic performance of Co2+Th was revealed with an onset potential of −0.24 V, and the CO production rate (131 μmol/s·g·cm2, −0.85 V vs. RHE) on Th-Co3O4/CNTs (exposed with Co2+Th sites) is 3.1 times of that on Oh-Co3O4/CNTs (exposed with Co3+Oh sites). DFT calculations demonstrated a significant reduction in the rate-limiting activation barrier of CO2, and Co2+Th was proved to be more active than Co3+Oh with elongated *CO-OH bonds. In situ ATR-SEIRAS confirmed that the enhanced CO2 adsorption and rapid formation of *COOH on Th-Co3O4/CNTs were key factors leading to efficient CO production. This work introduced a clear understanding of the eCO2RR performance of Co2+Th/Co3+Oh sites, along with a feasible approach for catalyst construction and protection.