Resolving Deactivation Pathways of Co Porphyrin-Based Electrocatalysts for CO₂ Reduction in Aqueous Medium

Abstract

Carbon-supported first-row transition metal complexes drive electroreduction of CO 2 to CO in aqueous medium with remarkable activity and selectivity. However, their durability under negative potentials is quite low and the deactivation mechanisms are still not clear. Herein, we present an in-depth mechanistic study on the stability of Co porphyrin-based catalysts during CO 2 reduction in an aqueous electrolyte. The mechanisms of the degradation reactions were evaluated for Co tetraphenylporphyrin (CoTPP) using a combination of spectral, electrochemical, and theoretical methods. Our evidence shows that two major pathways contribute to the gradual activity loss. The first route is oxidative and yields the catalytically inactive complex [Co III TPP]OH. The second pathway is based on reductive carboxylation of the porphyrin ring via transient formation of [Co 0 TPP] 2− and [Co 0 TPP-CO] 2− . The latter reaction disrupts the π-system of the porphyrin structure and leads to the complete disintegration of the macrocyclic core. In contrast to the earlier reports, we found that the direct poisoning by CO, demetallation, and reduction to chlorins play no significant role in the deactivation process. It was further determined that the bulky donating functional groups disfavor the formation of dianionic species and restrict access of CO 2 to the vulnerable meso-position of the porphyrin ligand, thus improving the catalyst stability. The effect was found to be especially strong for the −OMe-substituted complex CoTPP-(OMe) 8 that shows excellent reusability under overpotentials below 500 mV. In turn, electronegative substituents such as fluorine suppress the activity of the catalyst and provide no advantages in terms of durability.

Tuning Electronic Structures of Covalent Co Porphyrin Polymers for Electrocatalytic CO 2 Reduction in Aqueous Solutions

“…This redox event was assigned to the Co II/I couple. 36 This result strongly supports the maintenance of the CoP structure in the resulting polymer. The Co II/I redox couple of CoP-Ph@CNT (E 1/2 = −1.27 V versus ferrocene) and CoP-F@CNT (E 1/2 = −1.12 V versus ferrocene), when loaded on GC electrodes for CV measurements in DMF, is also identical to that of corresponding monomer, CoP-Ph Page 6 of 20 CCS Chemistry This article presented here has been accepted for publication in CCS Chemistry and is posted at the © 2021 Chinese Chemical Society.…”

“…Molecular catalysts have clear and controllable structures and thus have benefits to study structure-function relationships for catalysis. 24,25 A variety of metal complexes of polypyridines, [26][27][28] cyclams, [29][30][31] porphyrins, [32][33][34][35][36][37][38] and corroles, 39,40 have been identified as efficient CO 2 RR electrocatalysts in non-aqueous solutions. By studying these molecular catalysts, knowledge of improving catalyst performance has been gained, including tuning electronic structures, 41,42 installing proton relays, 43,44 and introducing hydrogen-bonding and electrostatic interactions.…”

Molecularly Dispersed Cobalt Phthalocyanine Mediates Selective and Durable CO2 Reduction in a Membrane Flow Cell

“…The particles mainly consist of Co and O, and are obviously not the aggregation of CoPc molecules, which may be the result of the demetallation of CoPc aggregates accompanied with the formation of metal oxide or hydroxide. [ 9 ] FT‐EXAFS analyses and density function theory (DFT) methods suggest the different van der Waals interaction surrounding the CoPc molecules. The isolated CoPc molecules may be protected by the interaction with CNT from leaching and agglomeration, as well as demetallation.…”

“…[ 8 ] Very recently, great breakthroughs have been achieved by C. P. Berlinguette and coworkers to mediate fast and selective CO 2 reduction reaction (CO 2 RR) using molecular electrocatalysts in a membrane flow cell reactor. With the addition of phenol, the cobalt phthalocyanine (CoPc) was reported to efficiently catalyze CO 2 ‐to‐CO conversion at the j of 200 mA cm −2 (Faradaic efficiencies of CO [FE CO ] = 88%), and can sustain the current of 50 mA (FE CO > 80%, E cell > 4.2 V) in a microfluidic reactor for >100 h. [ 6 ] However, the non‐conductive nature and the deactivation problem make CoPc aggregates difficult to further improve its performance, [ 9 ] both of intrinsic activity and stability. Thus, isolating metal phthalocyanine onto conductive substrate provides a solution to maximize the intrinsic activity and improve the conductivity.…”

CO2 to CO Electroreduction, Electrocatalytic H2 Evolution, and Catalytic Degradation of Organic Dyes Using a Co(II) meso-Tetraarylporphyrin

“…Moreover, there are one or two axial positions available for binding small molecules [ 10 , 11 , 12 , 26 , 27 , 28 , 29 ]. Both properties make Co porphyrins very suitable for redox catalysis [ 22 , 23 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 ].…”