(Invited) Novel Nitrogen and Sulfur Doped Colloid Imprinted Carbons (CICs) As Catalysts for Electrochemical CO2 Reduction
Authors: Mousavizadeh Mojarad Fatemeh Sadat , Singh Anand , Li Jialang , Birss Viola Ingrid 
Published: December 2024

Electrochemical carbon dioxide (CO2) reduction represents a promising strategy for transforming this ubiquitous greenhouse gas into valuable commodities. Moreover, it offers a solution for storing intermittent renewable electricity by converting CO2 to chemical fuels. Unlike the formation of hydrocarbons, the reduction of CO2 to carbon monoxide (CO) involves only two electron/proton transfers, making it a less complex process. Also, CO is a very useful product as it can be used as a crucial feedstock for the production of fuels via the catalytic Fischer-Tropsch process. Nevertheless, the electrocatalytic conversion of CO2 to CO, carried out typically at metals, experiences significant challenges due to changing morphology with time, poor CO2RR product selectivity, and unavoidable competition with the hydrogen evolution reaction in aqueous environments.
Therefore, attention has also been extended to CO2RR at carbon-based catalysts, as carbon possesses a plethora of inherent advantages, including its customizable and stable porous structures, high surface area, low cost, and environmental friendliness. While these properties make carbon highly favorable as a catalyst, it is inactive towards CO2 reduction in its pure state and must be doped or surface modified to achieve reasonable CO2RR kinetics. As an example, nitrogen-doped carbons (N-C) have shown acceptable activity towards the CO2RR,[1] attributed to the electronic modulation of conjugated sp2 carbon atoms by adjacent nitrogen dopants, disrupting electroneutrality by delocalizing π-orbital electrons within the carbon framework and creating active sites for CO2 activation. Even so, as N-C catalysts require significant overpotentials to attain satisfactory reaction rates and as the Faradaic efficiencies can be low and unstable, there is a pressing need to devise novel strategies to enhance the catalytic efficacy of N-C materials and achieve highly efficient CO2-to-CO conversion.
To overcome these limitations, a dual-doping approach has been suggested, with the observed enhancement in activity of co-doped catalysts (e.g., N and sulfur) generally attributed to synergistic effects between nitrogen and the secondary atoms.[2] Computational simulations have predicted that the inclusion of sulfur into N-graphene could increase the asymmetrical spin density of the carbon system due to the higher polarizability of sulfur atoms compared to nitrogen and carbon atoms, thereby leading to improved catalytic performance. Moreover, sulfur modification can offer a potentially effective avenue for enhancing CO2RR performance over N-C materials by leveraging the electronic contribution from sulfur.[3] However, to the best of our knowledge, there are limited reports on the implementation of sulfur decoration strategies for electrocatalytic CO2RR.
Our team has been working on a family of fully tunable monodisperse mesoporous carbon sheets, designated as nanoporous carbon scaffolds (NCS), along with colloid imprinted carbon (CIC) powders as carbon support materials. The CIC and NCS materials possess identical ordered monodisperse pore diameters, selected to be anywhere between 10 and 100 nm. These mesoporous carbons present numerous advantages compared to microporous carbons, notably their internal accessibility to solutions and gases, as well as their highly defective surfaces, making them easily modified. Specifically, the NCS offers additional benefits due to its self-supported nature and suitability for conducting fundamental CO2RR membrane electrode assembly (MEA) studies. To date, we have successfully N-doped both the CICs and the NCS and are now moving towards co-doping with S. N-doping has been achieved by subjecting the carbons to an NH3 atmosphere at 800 °C and S-doping is carried out by heat treatment of a mixture of the carbons and benzyl disulfide in Ar. We are also investigating a carbon precursor having intrinsic N and S content to simplify the preparation steps. In this case, the N,S-C catalysts were prepared by ball milling of dry silica particles (85 nm) and Alberta-sourced mesophase pitch. Subsequently, the mixture underwent carbonization at 900 °C for 2 hours, followed by removal of silica using a NaOH solution to yield the final product. CO2RR testing of these first generation of N,S doped catalysts has been carried out primarily in CO2-saturated bicarbonate solutions in an H-cell, giving an impressive CO Faradaic efficiency of 80% at overpotentials of only 390 mV and maintaining their stability for many hours of polarization. XPS studies are underway to establish a correlation between S, N content and speciation with the electrocatalytic activity and durability of these materials, with CO2RR testing also being carried out under flow conditions for comparison.

Acknowledgements

We would like to thank Momentum Materials Solution (Calgary) for providing the mesophase pitch. This work was supported by NSERC and CANSTOREnergy.

References

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Pan, F., et al., Applied Catalysis B: Environmental, 2019. 252.