Covalent triazine framework modified with coordinatively-unsaturated Co or Ni atoms for CO₂ electrochemical reduction

The electrochemical reduction of carbon dioxide (CO₂) has attracted considerable attention as a means of maintaining the carbon cycle. This process still suffers from poor performance, including low faradaic efficiencies and high overpotential. Herein, we attempted to use coordination number as a control parameter to improve the electrocatalytic performance of metal species that have previously been thought to have no CO₂ reduction activity. Covalent triazine frameworks (CTF) modified with coordinatively-unsaturated 3d metal atoms (Co, Ni or Cu) were developed for efficient electroreduction of CO₂. Co-CTF and Ni-CTF materials effectively reduced CO₂ to CO from -0.5 V versus RHE. The faradaic efficiency of the Ni-CTF during CO formation reached 90% at -0.8 V versus RHE. The performance of Ni-CTF is much higher than that of the corresponding metal-porphyrin (using tetraphenylporphyrin; TPP). First principles calculations demonstrated that the intermediate species (adsorbed COOH) was stabilized on the metal atoms in the CTF due to the low-coordination structure of this support. Thus, the free energy barriers for the formation of adsorbed COOH on the metal atoms in the CTF supports were lower than those on the TPP supports.