Electricity driven CO2 reduction (CO2RR) have been attracted widespread attention as it offers sustainable avenue to produce value-added chemicals and carbon-based fuels, and address the CO2 related environmental issue. Specially, electrochemical CO2RR to CO along with competing HER to H2, generating the mixture products as syngas, is of high relevance for various chemical industry. Unfortunately, current CO2RR electrocatalysts still fail to meet the requirement of electrochemical syngas synthesis with tunable H2/CO ratio at relatively low overpotential due to the high thermodynamic/kinetic stability of CO2 and the competing hydrogen evolution reaction (HER) in aqueous media, respectively. Therefore, we aim to develop effective electrocatalyst that enables tuning the competitive reactivity between the CO2RR and HER at room temperature.
Metal-organic frameworks (MOFs), one class of highly ordered crystalline coordination polymers, emerge as highly attractive catalytic systems due to the uniquely combining homogeneous and heterogeneous features (well-defined structure, readily accessible active sites, and ideal catalytic environment for fundamental reaction mechanism studies). However, it is imperative but challengeable to address the serious problems of MOFs (electrical insulators and blockage of metal centers by organic ligands) in electrocatalysis. Excitingly, inspired by my group leader (Renhao Dong)’s work in two-dimensional (2D) conjugated MOFs (2D c-MOFs), utilizing 2D c-MOFs would benefit for exposing higher density of metal centers and improving electron conductivity, and thus boosting high catalytic activity. Differently, controlling the competitive reactivity between the CO2RR and HER by tailoring the structures and compositions of 2D c-MOFs is more important. Therefore, we rationally proposed layered 2D c-MOFs with bimetallic centers to improve electrocatalytic CO2RR activity toward syngas synthesis; hereby one metal center is used for selective CO2-to-CO conversion while the other metal center will be utilized for H2 generation.
Based on different binding energy of CO and proton generation rate of different transition metal sites, we developed 2D c-MOF (PcCu-O8-Zn) with phthalocyaninato copper as the ligand and zinc-bis(dihydroxy) complex (ZnO4) as the linkage to catalyze CO2RR toward syngas synthesis, which showed highly selective catalytic activity for CO2-to-CO conversion (88%) and high turnover frequency of 0.39 s-1, excellent stability and tunable syngas compositions with molar H2/CO ratio from 1:7 to 4:1. Taking the advantage of 2D c-MOFs in mechanism studies, Operando X-ray absorption spectroscopy (XAS) and surface-enhanced infrared absorption (SEIRA) spectroelectrochemistry combined with contrast experiments and density functional theory (DFT) calculation reveal that ZnO4 complexes in the linkages exhibit high catalytic activity for CO2-to-CO conversion, while CuN4 complexes in the Pc macrocycles act as the synergistic component to promote the protonation process and hydrogen generation along with the CO2RR. Thus, the bimetallic active sites contribute to a synergistic effect on the CO2RR. Our work demonstrates the capability of bimetallic 2D c-MOFs as highly efficient electrocatalysts for promoting CO2RR, which is of importance for conductive MOFs design and their electrocatalysis application, and sheds light on the development of high-performance bimetal-heteroatom doped carbon electrocatalysts.
If you are interested in our work, you may find the full paper here: H. Zhong, M. Ghorbani-Asl, K. H. Ly, J. Zhang, J. Ge, M. Wang, Z. Liao, D. Makarov, E. Zschech, E. Brunner, I. M. Weidinger, J. Zhang, A. V. Krasheninnikov, S. Kaskel, R. Dong, X. Feng, Nat. Commun. 2020, 11, 1409. https://rdcu.be/b20JF