Hierarchically porous N-doped carbon confined single-atom Fe catalyst for efficient electrochemical CO2 reduction

Due to good catalytic stability and low potential, the Fe-based single-atom catalyst has become a promising catalyst for efficient electrocatalytic CO₂ reduction. However, Fe-N_x sites are strongly bound to CO, and they are inhibited by CO desorption, thus resulting in poor activity. Therefore, it is necessary to further improve the catalytic performance of Fe-based catalysts. Significant research has been devoted to improving the catalytic performance of Fe-N-C. For example, a team of material scientists led by Rui Wu from the University of Electronic Science and Technology of China in Sichuan recently reported an axially coordinated Fe-N₅ structure for CO₂RR. The optimized catalyst reduced the energy barrier of CO desorption and inhibited the generation of H₂. The team published their work in Small on August 21, 2022. A team of chemical scientists led by Chuan Zhao from The University of New South Wales in Sydney, Australia optimized the carbon carrier with the assistance of polystyrene microspheres, which reduced mass transfer resistance while accelerating the diffusion of the gas. The team published their research in Advanced Energy Materials on August 10, 2023. However, previous studies involving N-C supports anchoring single atoms mainly show three-dimensional bulk features, resulting in some active sites buried and insufficient mass transfer process. The shape effect of N-doped carbon substrates on single-atom metal centers and catalytic performance needs to be further elucidated.

In this research, the team successfully anchored Fe single atoms to three different morphologies of carbon carriers through controllable synthesis, including hierarchically porous carbon (Fe-HP), carbon nanosheets (Fe-NS), and carbon nanotubes (Fe-NT). The Fe-HP consisting of micropores, mesopores, and macropores exhibited high activity and selectivity for CO₂ reduction to CO with a maximum of 80 % of FE_CO at -0.5 V vs. RHE, which was about 2.5 times than that of Fe-NS and nearly 12 times than that of Fe-NT. In addition, the Fe-HP achieved a high partial current density of CO of -9.1 mA cm^-2 and a high TOF value of 1372 h^-1 at -0.8 V vs. RHE even with a low metal loading (0.35 wt.%), which was comparable to the current state-of-the-art Fe SACs. Detailed characterization and kinetic analysis showed that the hierarchically porous structure facilitated the exposure of active sites and reduced the valence state of Fe centers. The relatively low valence state of Fe and its porous structure benefited CO₂ activation and promoted the desorption of CO, thereby boosting CO₂ reduction performance. This work highlights the crucial role of carbon carrier morphology and provides a new approach to enhanced catalysis by creating porous structures.

Other contributors include Hui Li from the Centre for Atomaterials and Nanomanufacturing (CAN), School of Science, RMIT University, Melbourne, VIC 3000, Australia, Hua Fan from the Aqualux AU Pty Ltd, 12 Kanangra Cres, Clontarf, NSW 2093, Australia and Ying Sun, Shuoshuo Jiang, Xin Cui from the Institute of Clean Energy Chemistry, Key Laboratory for Green Synthesis and Preparative Chemistry of Advanced Materials of Liaoning Province, College of Chemistry, Liaoning University, Shenyang, 110036, China.

This work was supported by the Australian Research Council (ARC) through Future Fellowship (FT210100298), Discovery Project (DP220100603), Linkage Project (LP210200504, LP220100088, LP230200897) and Industrial Transformation Research Hub (IH240100009) schemes, the China Postdoctoral Science Foundation (2023M743430), the Australian Government through the Cooperative Research Centres Projects (CRCPXIII000077), the Australian Renewable Energy Agency (ARENA) as part of ARENA's Transformative Research Accelerating Commercialisation Program (TM021), and European Commission's Australia-Spain Network for Innovation and Research Excellence (AuSpire).

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