(Keynote) Electrocatalysts Design for Carbon Dioxide Reduction and Valorization
Authors: Atanassov Plamen 
Published: December 2024

There are several materials design strategies that have been utilized with various levels of success for CO2 reduction reaction (CO2RR) to C1, C2 and occasionally, C3 products (Ci denotes the number of carbon atoms in the product of CO2RR). Among those Cu based materials take leading role as this class of catalysts is relatively simple to synthesize and implement.1 Adding co-catalyst in the composite materials allow for regulating of selectivity towards a specific product while sustaining some simplicity of the synthesis and deployment strategies.2

Alternatively, CO2RR can be carried on carbon-based electrocatalysts with atomically dispersed transition metals, stabilized by nitrogen (known as M-N-C). We synthesized a library of nitrogen-doped carbonaceous materials with atomically dispersed 3d transition metals and corresponding metal-free electrocatalysts. The sacrificial support method (SSM) was used yielding catalyst materials of high dispersity and high graphitic content. The resulting electrocatalysts were impurity free, hence allowing a better understanding of the mechanism of CO2 reduction. By combining the electrochemical results with density functional theory, we were able to separate the electrocatalysts into several categories, based on their CO2 → COOHads free energy and their COads binding strength.3 The ‘strong-CO binder’ electrocatalysts (e.g. Cr, Mn and Fe – N – C) achieved a Faradaic efficiency up to 50% at – 0.35 V vs. RHE (at pH = 7.5, in 0.1 M phosphate buffer). Such Faradaic efficiency was also achieved for a metal-free electrocatalyst, therefore showing the high activity of the metal-free, N-containing, moieties toward the CO2 reduction reaction. A separate study showed materials hydrophobicity as one of the major factors of metal-free catalysts selectivity.4

Among the many practical products of CO2RR syngas (an H2/CO mixture) attracts special attention. Appropriate electrocatalysts, such as the metal–nitrogen–carbon (M-N-C) materials, allow for the production of CO alongside H2(from the hydrogen evolution reaction), and thus leads to syngas generation.5 Selectivity of mono- and bi-metallic (M-N-C, M = Fe, Mo or FeMo) electrocatalysts towards syngas production have been extensively studied.6 The ratio of the CO:H2 in the syngas was tuned by modifying the ratio of metallic precursors in the bi-metallic FeMo-N-C catalysts, tailoring the catalysts’ selectivity towards the CO2RR or the hydrogen evolution reaction (HER).

Further development of CO2RR towards valorization of its products may lay through its integration with bioprocesses.7 Success of that strategy will rely on the ability of such systems to result in highly selective synthesis of formate, acetate, or propionate as major products.8,9 Those examples show the path to designer catalyst materials for a desired scalable CO2RR-based electrosynthesis technology.

References

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