Three-dimensional porphyrinic covalent organic frameworks for highly efficient electroreduction of carbon dioxide

Abstract

The electrochemical conversion of CO₂ into valuable chemicals would be an effective way to realize the carbon-neutral energy cycle and alleviate the energy crisis. Due to their porous crystalline structures and ordered single active sites, covalent organic frameworks (COFs) are one class of promising candidates for the carbon dioxide reduction reaction (CO₂RR). However, the active sites are usually hidden in the layers of the two-dimensional (2D) COF materials and cannot be accessible for electrolytes and CO₂, thus leading to low activity. In order to increase the available active sites and enhance the current density, herein, a porous three-dimensional (3D) cobalt porphyrinic COF, denoted as 3D-Por(Co/H)-COF, was synthesized via a solvothermal Schiff-base condensation reaction of tetra(4-formylphenyl)methane (TFPM) and a mixture of 5,10,15,20-tetrakis(4-aminophenyl)porphinatocobalt (Co-TAPP) and 5,10,15,20-tetrakis(4-aminophenyl)porphyrin (TAPP). The 3D-Por(Co/H)-COF exhibited high activity for the CO₂RR with a CO faradaic efficiency of 92.4% at −0.6 V versus the reversible hydrogen electrode (RHE), a turnover frequency (TOF) for CO production of 4610 h⁻¹ at an applied potential of −1.1 V, which exceeded those of all reported Co porphyrin-based two-dimensional COFs. The porous 3D framework could maximize active electrocatalytic sites by reducing the aggregation of molecular building blocks, which provides a new way to improve the electrocatalytic activity by changing the dimensions of the catalyst.