Modern industrialization is accompanied with the extensive usage of fossil fuels for energy demands and consequently, an excessive emission of CO₂ into the atmosphere. To combat the injurious greenhouse effects, the CO₂ electroreduction (CER) to feedstocks and fuels becomes an appealing approach to reducing CO₂ emission and simultaneously producing useful products. It is generally recognized that the solid-catalyst/liquid-electrolyte/gaseous-CO₂ triple-phase boundary is the key microstructure feature of CER, where CO₂ molecules react with protons (H⁺) and e⁻ and are reduced. Besides the intrinsic activity of catalyst itself and the accessibility of active sites, CER performance also strongly depends on the transport of H⁺ and CO₂ molecules through electrolyte to catalyst surface. Apparently, the availability of H⁺ in aqueous solution can be readily achieved via H₂O ionization, while the low solubility of CO₂ limits the supply of CO₂ to the catalyst surface. Moreover, previous study has demonstrated that complete depletion of CO₂ on the catalyst surface can even occur when high overpotential is applied.¹ In this regard, rationally designing the structure of catalyst to increase the concentration of CO₂ at the triple-phase boundary is immensely significant for CER since this could overcome the limited diffusion of CO₂ in aqueous medium.

Recently, surface hydrophobicity engineering has been proved to be a wise tactic to increase the local CO₂ concentration by trapping CO₂ near the catalyst surface, thus improving CO₂ electrolysis. For example, modification of the hydrophobic organics on catalyst surface could create triple-phase boundary and increase CO₂ concentration on catalyst surface, these improved CER performance and simultaneously inhibited HER.² Unfortunately, the insulative organics coated on the catalyst surface will sacrifice its activity, while some small pieces of organics may desorb from the surface or in the case of flow cell, these pieces can be flushed away by the fluid. Since most catalysts are in situ grown on carbon support with the advantages of rapid electron transfer and seamless contact,^{3, 4} it is possible to tailor the microenvironment around the catalyst through chemical modification of carbon support. For example, platinum-based catalysts supported on hydrophobic carbon with a desirable microenvironment display a state-of-the-art catalytic activity for oxygen reduction reaction.⁵ However, few studies have been conducted to investigate how the carbon support can be modified to create a favourable triple-phase boundary for CER.

In this study, we have in situ grown Bi₂O₃ nanosheets (NSs) on two types of carbon materials, hydrophobic carbon nanofiber (Bi₂O₃@C/HB) and hydrophilic carbon nanofiber (Bi₂O₃@C/HL), respectively, and used them as the cathode catalysts for CER. Compared to Bi₂O₃@C/HL, the as-obtained Bi₂O₃@C/HB exhibits significantly boosted CER performances for formate formation with the high FE_formate of ˃ 93% over an extremely wide potential window of 1000 mV, high formate partial current density (j_formate) of 102.1 mA cm⁻² and high formate formation rate of 1905 μmol h⁻¹ cm⁻². Molecular dynamics (MD) simulations together with electrochemical measurements reveal that the hydrophobic carbon support can create a hydrophobic microenvironment by avoiding the formation of hydrogen bond. This increases the local CO₂ concentration and pH, both contributing to the enhancement of the overall CER. We believe that the findings from this work can provide significant guidelines for designing highly active CER catalysts and showcase a promising approach to improving other types of electrolysis involving gas phase.

1. Raciti, D.; Mao, M.; Park, J. H.; Wang, C., Mass transfer effects in CO₂ reduction on Cu nanowire electrocatalysts. Catal. Sci. Technol. 2018, 8, 2364-2369.
2. Wang, J.; Cheng, T.; Fenwick, A. Q.; Baroud, T. N.; Rosas-Hernández, A.; Ko, J. H.; Gan, Q.; Goddard Iii, W. A.; Grubbs, R. H., Selective CO₂ Electrochemical Reduction Enabled by a Tricomponent Copolymer Modifier on a Copper Surface. J. Am. Chem. Soc. 2021, 143, 2857-2865.
3. Liu, S.; Lu, X. F.; Xiao, J.; Wang, X.; Lou, X. W., Bi₂O₃ nanosheets grown on multi‐channel carbon matrix to catalyze efficient CO₂ electroreduction to HCOOH. Angew. Chem., Int. Ed. 2019, 58, 13828-13833.
4. Li, F.; Chen, L.; Knowles, G. P.; MacFarlane, D. R.; Zhang, J., Hierarchical mesoporous SnO₂ nanosheets on carbon cloth: a robust and flexible electrocatalyst for CO₂ reduction with high efficiency and selectivity. Angew. Chem., Int. Ed. 2017, 56, 505-509.
5. Zhao, Z.; Hossain, M. D.; Xu, C.; Lu, Z.; Liu, Y.-S.; Hsieh, S.-H.; Lee, I.; Gao, W.; Yang, J.; Merinov, B. V., Tailoring a Three-Phase Microenvironment for High-Performance Oxygen Reduction Reaction in Proton Exchange Membrane Fuel Cells. Matter 2020, 3, 1774-1790.