Electrochemical Carbon Dioxide Reduction to Ethylene: From Mechanistic Understanding to Catalyst Surface Engineering.

Electrochemical carbon dioxide reduction reaction (CO2RR) provides a promising way to convert CO2 to chemicals. The multicarbon (C2+) products, especially ethylene, are of great interest due to their versatile industrial applications. However, selectively reducing CO2 to ethylene is still challenging as the additional energy required for the C-C coupling step results in large overpotential and many competing products. Nonetheless, mechanistic understanding of the key steps and preferred reaction pathways/conditions, as well as rational design of novel catalysts for ethylene production have been regarded as promising approaches to achieving the highly efficient and selective CO2RR. In this review, we first illustrate the key steps for CO2RR to ethylene (e.g., CO2 adsorption/activation, formation of *CO intermediate, C-C coupling step), offering mechanistic understanding of CO2RR conversion to ethylene. Then the alternative reaction pathways and conditions for the formation of ethylene and competitive products (C1 and other C2+ products) are investigated, guiding the further design and development of preferred conditions for ethylene generation. Engineering strategies of Cu-based catalysts for CO2RR-ethylene are further summarized, and the correlations of reaction mechanism/pathways, engineering strategies and selectivity are elaborated. Finally, major challenges and perspectives in the research area of CO2RR are proposed for future development and practical applications.

Epoxy-rich Fe Single Atom Sites Boost Oxygen Reduction Electrocatalysis.

Electrocatalysts for highly efficient oxygen reduction reaction (ORR) are crucial for energy conversion and storage devices. Single-atom catalysts with maximized metal utilization and altered electronic structure are the most promising alternatives to replace current benchmark precious metals. However, the atomic level understanding of the functional role for each species at the anchoring sites is still unclear and poorly elucidated. Herein, we report Fe single atom catalysts with the sulfur and oxygen functional groups near the atomically dispersed metal centers (Fe1/NSOC) for highly efficient ORR. The Fe1/NSOC delivers a half-wave potential of 0.92 V vs. RHE, which is much better than those of commercial Pt/C (0.88 V), Fe single atoms on N-doped carbon (Fe1/NC, 0.89 V) and most reported nonprecious metal catalysts. The spectroscopic measurements reveal that the presence of sulfur group induces the formation of epoxy groups near the FeN4S2 centers, which not only modulate the electronic structure of Fe single atoms but also participate the catalytic process to improve the kinetics. The density functional theory calculations demonstrate the existence of sulfur and epoxy group engineer the charges of Fe reactive center and facilitate the reductive release of OH* (rate-limiting step), thus boosting the overall oxygen reduction efficiency.

Modulating Electronic Structure of PtCo-Ptrich Nanowires with Ru atoms for Boosted Hydrogen Evolution Catalysis.

Rational design and development of highly efficient hydrogen evolution reaction (HER) electrocatalysts is of great significance for the development of green water electrolysis hydrogen production technology. Ru-engineered 1D PtCo-Ptrich nanowires (Ru-Ptrich Co NWs) are fabricated by a facile electrodeposition method. The rich Pt surface on 1D Pt3 Co contributes to the fully exposed active sites and enhanced intrinsic catalytic activity (co-engineered by Ru and Co atoms) for HER. The incorporation of Ru atoms can not only accelerate the water dissociation in alkaline condition to provide sufficient H* but also modulate the electronic structure of Pt to achieve optimized H* adsorption energy. As a result, Ru-Ptrich Co NWs have exhibited ultralow HER overpotentials (η) of 8 and 112 mV to achieve current densities of 10 and 100 mA cm-2 in 1 m KOH, respectively, which far exceed those of commercial Pt/C catalyst (η10  = 29 mV, η100  = 206 mV). Density functional theory (DFT) calculations further demonstrate that the incorporated Ru atoms possess strong water adsorption capacity (-0.52 vs -0.12 eV for Pt), facilitating water dissociation. The Pt atoms in the outermost Pt-rich skin of Ru-Ptrich Co NWs achieve optimized H* adsorption free energy (ΔGH* ) of -0.08 eV, boosting hydrogen generation.

Atomically dispersed Mn atoms coordinated with N and O within an N-doped porous carbon framework for boosted oxygen reduction catalysis.

Developing efficient and robust catalysts to replace Pt group metals for the oxygen reduction reaction (ORR) is conducive to achieving highly efficient energy conversion. Here, we develop a general ion exchange strategy to construct highly efficient ORR catalysts consisting of various atomically dispersed metal atoms anchored on N-doped porous carbon (M-SAs/NC) to investigate the atomic structure-catalytic activity relationship. The structure characterization results demonstrated that the achieved atomic structure varied due to the presence of different metal centers. Mn-SAs/NC consists of MnN3O1 centers, in which the Mn single atoms are stabilized by 3 N and 1 O. In contrast, the center metals in Fe-/Co-/Cu single-atom catalysts are coordinated by merely N atoms. Mn-SAs/NC delivers superior performance for the ORR with a half-wave potential (E1/2) of 0.91 V vs. RHE in 0.1 M KOH solution, outperforming that of the commercial Pt/C catalyst and the control Fe-/Co-/Cu single-atom catalysts. Furthermore, Mn-SAs/NC also shows excellent methanol tolerance and stability up to 5000 cycles. Density functional theory (DFT) calculations reveal that Mn single atom catalysts with MnN3O1 centers contributed to the superior ORR performance with lower energy barriers and optimized adsorption capacity of intermediates. These findings provide insights into the design and development of specific coordinated structures of atomically dispersed catalysts to facilitate the practical applications of energy conversion.