Mechanism-Driven Design of Heterogeneous Molecular Electrocatalysts for CO2 Reduction

The electrochemical reduction of CO2 (CO2ERR) in aqueous electrolytes is one of the most promising routes for commercial CO2 utilization. A switch from the currently used noble-metal-based catalysts to carbon-supported macrocyclic complexes could bring about a much-needed cost reduction, thus making the technology economically viable. However, the inherently low conductivity and the tendency of molecular catalysts to degrade during the long-term operation present a significant challenge to the material design which we endeavored to resolve using a mechanism-driven approach. This Account puts into perspective a series of our recent studies on the phenomena taking place during CO2ERR and the employment of these insights in the catalyst development. It is demonstrated that in a heterogeneous system CO2ERR relies on the sequential rather than simultaneous transfer of two electrons. This feature causes heterogeneous CO2ERR to be extremely dependent on the ability of the catalyst to support the interfacial charge migration. Using recently developed variable-frequency square wave voltammetry (VF-SWV), which allows one to map the interfacial charge transfer for the heterogeneous electrochemical systems, we also found that the fast heterogeneous charge transfer is confined to a thin layer of the complex at the interface between the supporting electrode and the bulk of the porphyrin complex. Hence, it is desirable to construct an electrode with a uniform dense layer of the complex strongly bound to the surface. These requirements are conveniently addressed using the covalent immobilization of the catalysts on the surface of carbonaceous electrodes. In this regard, electrografting of the cobalt tetraphenylporphyrin (CoTPP) onto the surface of carbon cloth yields a material exhibiting a TOF of 8.3 +/- 0.9 s(-1) at a 500 mV overpotential and a Faradaic efficiency to CO evolution (FE(CO)) of 62 +/- 7%. The application of the chemical covalent immobilization to CNT-based supports further improves the FE(CO) to similar to 100% and allows the complex to reach an impressive intrinsic TOF0 of 36.6 s(-1). We also applied mechanistic investigations to tackle the challenging problem of catalyst durability in CO2ERR. It was determined that the loss of activity takes place because of the reductive carboxylation and co-occurring formation of the [(CoTPP)-T-III]OH complex. Thus, quite unexpectedly, the stability could be enhanced via the introduction of bulky donating substituents around the macrocyclic core. This insight allowed us to design and synthesize catalyst CoTPP-(OMe)(8) bearing eight -OMe groups around the lateral aromatic moieties which demonstrates no noticeable degradation during the repetitive long-run electrolyses. These design principles, combined with the recent advances in the development of gas diffusion electrodes (GDE) and heterogeneous molecular catalysts, could provide a low-cost and very stable catalytic system for industrial-scale CO2ERR.