Enhanced Electrochemical Reduction of CO₂ to CO upon Immobilization onto Carbon Nanotubes Using an Iron‐Porphyrin Dimer

Abstract

Electrochemical reduction of carbon dioxide (CO 2 ) is a viable solution for conversion of atmospheric CO 2 to value-added materials such as carbon monoxide (CO). In this project, a new urea iron-tetraphenylporphyrin-dimer (Fe-TPP-Dimer) was synthesized and applied for electrocatalytic CO 2 reduction under both homogeneous and heterogeneous conditions to selectively reduce CO 2 to CO. Immobilization of the catalyst onto carbon nanotubes (CNTs) in aqueous solution resulted in remarkable enhancement of its electrocatalytic abilities, with exceptional turnover frequencies (10 s À 1 ), high faradic efficiency (FE) of ∼ 90%, and a current density of 16 mA/cm 2 at À 0.88 V vs. RHE. This project exhibits the importance of molecular design in accessing heterogeneous applications with CNTs.Consumption of fossil fuels for energy production in recent decades has dispensed alarming amounts of carbon dioxide (CO 2 ), one of the leading contributors to climate change, into the atmosphere. [1][2][3][4][5] Although the capture and conversion of CO 2 to value-added synthons has become a desirable solution to this problem, challenges in regard to the species' general lack of reactivity and the costs associated with deployment and stability of such strategies remain prevalent. [6][7][8][9][10][11] In this respect, electrocatalytic CO 2 reduction reactions (CO 2 RRs) pose a promising alternative to this end. [12][13][14][15][16][17][18][19][20] Extensive research has been conducted on both homogeneous and heterogeneous molecular catalysts for CO 2 RR. [21][22][23][24] Although homogeneous catalysts are popular for CO 2 RR application, applying heterogeneous catalysts is essential for emerging of CO 2 to CO conversion in large scale. Additionally, most molecular catalysts favor non-aqueous solvents such as N,N-dimethylformamide (DMF) and acetonitrile, making them more costly to implement. [25][26][27] Immobilization of molecular catalysts onto electrode surfaces can occur via several methods which include covalent bonding, [28,29] non-covalent attachment, [30,31] and surface polymerization. , [32,33] Among them, non-covalent surface binding methods that capitalize on strong Van der Waals π-π interactions between carbon surfaces and polyaromatic hydrocarbon moieties is easily accomplished and displays great surface stability, [31,34] making it a favorable technique for immobilization. [30,[35][36][37][38] Carbon nanotubes (CNTs) are of particular interest for CO 2 electroreduction owing to their high stability, conductivity and large surface area. [19,31,[39][40][41][42] Among transition metal complexes, iron, [24,25,[43][44][45] cobalt [46][47][48] and nickel, [49][50][51] are prevailing and more environmentally friendly than other metals used for CO 2 RR application. Metalloporphyrin catalysts are attractive as molecular catalysts in this field because of their robust structure and their stability to harsh conditions such as high temperatures. [52] Demonstrative work by Savéant showed that iron tetraphenylporphyrin (Fe-TPP) complexe...

Homogeneous and heterogeneous molecular catalysts for electrochemical reduction of carbon dioxide

“…This is due to their ability to form a strong noncovalent p-p interactions with aromatic ligands such as pyrene 150 and porphyrin. 41,151 Hu 56 This proved to be twice as efficient as when the same catalyst was applied in a homogenous medium. Similar enhancements to the reduction of CO 2 to methane (CH 4 ) and CO with both metalled and non-metallated ironporphyrin-pyridine (Fe-TPPy) catalysts were seen when noncovalently immobilized onto CNTs.…”

“…118 Due to its simple preparations, one of the most popular immobilization techniques involves depositing conjugated organic ligands onto carbon surfaces which are stabilized by the non-covalent p-p interactions between the catalyst and solid surface. 17,56,119 The molecular catalysts can be also deposited on electrode surfaces through covalent bond. 120,121 Previous reports on CO 2 RR selectivity involved either the use of a metal electrode surface, where the electron-transfer efficiency was largely dependent on the material's conductivity, or the incorporation of small inactive molecules 39 on the surface of the metal electrode to maximize interaction between the electrode and the molecular catalysts.…”

“…120,121 Previous reports on CO 2 RR selectivity involved either the use of a metal electrode surface, where the electron-transfer efficiency was largely dependent on the material's conductivity, or the incorporation of small inactive molecules 39 on the surface of the metal electrode to maximize interaction between the electrode and the molecular catalysts. 56,61,[122][123][124][125] An example of this are electrograing techniques which produce a direct chemical bond between the catalyst and a solid substrate. 98,126 The direct connections that arise from these methods are believed to be the primary factor in increasing the reaction rate of CO 2 RR relative to hydrogen evolution reactions (HERs) and lowering overpotentials.…”

“…This ability is especially pronounced in primary or secondary amines such as monoethanolamine (MEA), diethanolamine (DEA) and decylamine (DCA). [52][53][54][55][56][57] In these reactions, the amino groups initially capture CO 2 to form a zwitterionic species that can react with another amino equivalent to form a carbamate salt (Scheme 1). 58,59 This review will start with a general introduction to the electrochemical reduction of CO 2 and the metrics that are used to quantify catalyst efficiency and continue with a summary of the recent developments of amine molecular catalysts in both homogeneous and heterogeneous environments.…”

Electrochemical Reduction of CO2 at Coinage Metal Nanodendrites in Aqueous Ethanolamine

“…Electrochemical reduction of CO 2 , associated with the development of catalysts, is a crucial area of research and development because it is environmentally friendly, clean, modular, tunable, controllable and scalable compared to other existing technologies. However, the challenge is finding a suitable catalyst for CO 2 reduction to achieve a cost‐effective process with high efficiency and selectivity [7–11] …”