Re(bipyridine)-Polypyrrole Monolayer Deposition on Nanoporous Carbon Scaffolds for Electrochemical CO2 Reduction
Authors: Mousavizadeh Mojarad Fatemeh Sadat , OV Manila , Carr Cody , Li Jialang , Piers Warren Edward , Birss Viola Ingrid 
Published: February 2024

The CO2 concentration in the atmosphere has increased from 280 ppm in 1750 to nearly 420 ppm in 2022, leading to multiple environmental issues. Thus, developing an efficient approach for the conversion and use of CO2 is one of our most pressing objectives today. Among different strategies to remove CO2, the electrochemical conversion of CO2 to carbon-based fuels is actively being pursued by many researchers. However, CO2 is thermodynamically quite stable (the dissociation energy of a C=O bond is approximately 750 kJ mol-1) and thus the activation of the bonds in CO2 requires a considerable overpotential, especially when carried out at low temperatures in aqueous solutions, resulting in low efficiency. Although a range of gaseous and liquid products, including formic acid, carbon monoxide, methane, ethylene, and ethanol, can be produced by transferring multiple protons and electrons, the formation of CO is simpler and more straightforward for industrial adoption. In this process, renewable energy would be used to operate the CO2 electrolyzer to convert CO2 to CO, which can then be utilized as a chemical feedstock to produce a variety of fuels [1].

Molecular catalysts have been widely studied for CO2 reduction, with research on the development of solid hybrid systems that comprise an immobilized catalyst on a support well underway. This method offers further advantages, as it can further facilitate catalyst recyclability and product separation. Carbon-based materials are the most extensively employed supports discussed in the literature, due to their low cost, high electrical conductivity, and flexibility. [Re(2,2′-bipyridine)(CO)3Cl] ([Re(bpy)]), commonly known as Lehn's catalyst, is among the most extensively studied molecular electrocatalysts for the selective CO2 reduction to CO. While various studies have focused on its optimization, showing excellent performance in organic solutions, CO2 reduction should ultimately be accomplished in aqueous media. Despite the fact that hybrid systems eliminate the requirement for aqueous solubility of the catalyst, examples of immobilized [Re(bpy)] for the stable and effective CO2-to-CO conversion in water remain uncommon.

In our previous work, the [Re(4-(4-aminophenethyl)-4′-methyl-2,2′-bipyridine)(CO)3Cl] ([Re-(NH2-bpy)]) complex was synthesized and covalently anchored to a colloid-imprinted carbon (CIC) powder by the oxidation of its amino group. The CICs have a large surface area, homogeneous and controllable pore sizes, and ultra-low tortuosity. While the Re/CIC catalyst converted CO2 to CO with a high selectivity of 93% and a Re-based TON of around 900, its stability was poor and the coverage of the CIC by the Re catalyst was quite low [2]. In later work, catalyst anchoring on the CIC surface was achieved by electrochemical polymerization of the pyrrole group attached to the complex, producing uniform films that exhibited Faradaic efficiencies from 90-100% as well as very good long-term stability under catalytic conditions, producing CO with a selectivity of over 70% for at least 24 hours [3].

Herein, we report a similar [Re(bpy)] hybrid electrode constructed by covalent surface tethering of the Re catalyst via pyrrole polymerization but using a self-supported nanoporous carbon scaffold. The advantages of adopting the NCS as opposed to powdered supports include the lack of needed binder, which can cover active sites, the ease of imaging the catalyst before and after testing, and the ability to carry out flow through experiments to overcome mass transport issues and also local pH effects. The CO2RR tests were performed in an H-cell using a KHCO3 solution saturated with CO2, while the catalyst was loaded onto the NCS either by immersing the NCS in a 5 mM Re solution followed by electrodeposition in a Re-free solution (Method A), or by electrodeposition at slow sweep rates in the Re-containing solution (Method B). The bare NCS was studied first, showing the exclusive production of hydrogen, similar to the CIC powders, whereas our hybrid electrode generated 100% CO between -0.4 and -0.7 V vs. RHE. Method A showed that the number of immersion steps used for Re deposition correlated with the coverage of the NCS by the catalyst, in turn giving higher selectivity, higher activity, and a significantly higher TOF (0.13 s-1 vs. 0.03 s-1, found with the CICs). This is due to better [Re] utilisation within the NCS pores (more electroactive [Re]) and a better morphology, retaining the open pores after catalyst deposition. However, the durability was compromised at higher CO2RR rates. In comparison, Method B showed similar excellent performance metrics, while also exhibiting better reproducibility for samples prepared identically.

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