The carbon dioxide reduction reaction (CO2RR) powered by renewable electricity is promising for the sustainable production of carbon-based chemicals. Current electrocatalysts for CO2RR still could not fulfill the requirements for practical applications, and insight mechanism understandings are needed to optimize electrocatalyst design. As molecular catalysts, metal-N4 (M-N4) macrocyclic complexes have well-defined structures and their properties could be easily tuned by structural engineering, which are ideal platforms to study structure-property relationships.
Among M-N4 macrocyclic complexes, metal phthalocyanines have demonstrated competitive performance for CO production. However, other complexes, such as metal porphyrins and metal corroles, have been less studied in heterogeneous systems for practical applications, and showed much inferior performances. Thus, it is important to develop platforms to study the heterogeneous performances of M-N4 macrocyclic complexes.
Recently, a research team led by Prof. Yongye Liang from Southern University of Science and Technology of China investigated three cobalt macrocyclic complexes (including cobalt phthalocyanine, cobalt meso-tetraphenylporphyrin, and cobalt meso-triphenylcorrole (CoPc, CoTPP, and CoTPC)) in heterogeneous electrocatalysis of CO2RR. They revealed weak molecule-substrate interaction as a major reason for the low electrocatalytic performance of CoTPP and CoTPC, and proposed two ways to improve their performances. The results were published in Chinese Journal of Catalysis (DOI: 10.1016/S1872-2067(21)63880-9).
In their work, the carbon nanotube (CNT) hybridization method was first adopted to prepare samples. Although CoPc/CNT hybrid exhibited high electrocatalytic activities, CNT hybridization did not work for CoTPC and CoTPP that held weak interactions with CNTs. Using the drop-dry method, the performance of CoTPC and CoTPP substantially improved as molecular loadings were raised to higher levels due to increasing number of molecules interacting with CNTs. At the high molecular loading of 5.4 × 10-7 mol cm-2, CoTPC+CNT-e and CoTPP+CNT-e exhibit partial current densities of carbon monoxide production (jCOs) of 14.0 mA cm-2 and 7.61 mA cm-2 at ~ -0.67 V versus the reversible hydrogen electrode (RHE), 8.7 and 7.9 times higher than the low-loading counterparts. A facile poly(4-vinylpyridine) (PVP) bridge method was further developed to improve the electrocatalytic performance at low molecular loadings.
PVP was introduced as the bridge for molecules and CNTs: PVP can wrap CNTs by hydrophobic interactions, while pyridine ligand can coordinate to the cobalt macrocyclic complexes to enhance molecule-substrate interactions. At a low molecular loading of 1.08 × 10-8 mol cm-2, jCO of CoTPP+CNT/PVP is 7.84 mA cm-2 at ~ -0.67 V, about 8 times higher than the sample without PVP. These methods can be extended to other molecular systems to study their electrocatalytic performances and construct high-performance molecular electrocatalysts.