A Vacuum Vapor Deposition Strategy to Fe Single-Atom Catalysts with Densely Active Sites for High-Performance Zn-Air Battery

Restricting Two-Electron Oxygen Reduction via Secondary Coordinated Sulfur Enabling Ultralong-Lifespan Zn-Air Batteries

The direct four-electron oxygen reduction reaction (4e- ORR) critically governs efficiency and lifespan in metal-air batteries and fuel cells, yet selectively suppressing competitive 2e- and stepwise 2e-pathways that generate corrosive hydrogen peroxide remains a major challenge. Herein, we demonstrate the strategic incorporation of secondary coordinated sulfur atoms into transition metal-N-C electrocatalysts to effectively promote direct 4e- ORR and simultaneously suppress undesirable 2e- pathways. Density functional theory (DFT) calculations and operando spectroscopy reveal that enhanced adsorption of key intermediate *OOH facilitates efficient O─O bond cleavage, underpinning altered catalytic selectivity. Importantly, this approach is universally applicable to various carbon-based catalysts, including Co─N@C, Ni─N@C, Mn─N@C, and N@C. Specifically, a sulfur-mediated Co─N/Co@C catalyst, comprising Co─N4 sites and Co nanoparticles, dramatically lowers the 2e- O2-to-H2O2 rate constant to merely 0.05-fold of its original value at 0.78 V. Consequently, Zn-air batteries using Co─N/Co@C-S as cathode exhibits an outstanding peak power density of 220 mW cm-2, remarkable lifespan over 2500 h, and outstanding rate performance from 5 to 50 mA cm-2. This work paves a generalizable route for designing highly active and selective electrocatalysts suitable for advanced long-life energy storage devices.

Improved performances toward electrochemical carbon dioxide and oxygen reductions by iron-doped stannum nanoparticles

The CO2 reduction reaction (CO2RR) and oxygen reduction reaction (ORR) show great promise for expanding the use of renewable energy sources and fostering carbon neutrality. Sn-based catalysts show CO2RR activity; however, they have been rarely reported in the ORR. Herein, we prepared a nitrogen-carbon structure loaded with Fe-doped Sn nanoparticles (Fe-Sn/NC), which has good ORR and CO2RR activity. The results reveal that the Fe-Sn/NC catalysts deliver a high FECO of 99.0% at a low overpotential of -0.47 V in an H-type cell for over 100 h. Notably, a peak power density of 1.36 mW cm-2 is achieved in the Zn-CO2 battery with the Fe-Sn/NC cathode at discharge current densities varying from 2.0 to 4.0 mA cm-2, and the FECO remains above 99.0%. Due to efficient oxygen reduction reaction (ORR) performance and Zn-air battery (ZAB) characteristics, the ZAB-driven CO2RR has strong catalytic stability. This work proves that Fe-Sn/NC enhances the performance of the CO2RR and ORR, and the study of Zn-based batteries provides a new research direction for energy conversion.

Zinc Assisted Thermal Etching for Rich Edge-Located Fe-N4 Active Sites in Defective Carbon Nanofiber for Activity Enhancement of Oxygen Electroreduction

Single-atom catalysts (SACs) with edge-located metal active sites exhibit superior oxygen reduction reaction (ORR) performance due to their narrower energy gap and higher electron density. However, controllably designing such active sites to fully reveal their advantages remains challenging. Herein, rich edge-located Fe-N4 active sites anchored in hierarchically porous carbon nanofibers (denoted as e1-Fe-N-C) are fabricated via an in situ zinc-assisted thermal etching strategy. The e1-Fe-N-C catalyst demonstrates superior alkaline ORR activity compared to counterparts with fewer edge-located Fe-N4 sites and commercial Pt/C. Density functional theory calculations show that the accumulation of more negative charges near the Fe-N and the formation of partially reduced Fe state in the edge-located Fe-N4 sites reduce the energy barrier for the ORR process. Additionally, the unique hierarchically porous structures with mesopores and macropores facilitate full utilization of the active sites and enhance long-range mass transfer. The zinc-air battery (ZAB) assembled with e1-Fe-N-C has a peak power density of 198.9 mW cm-2, superior to commercial Pt/C (152.3 mW cm-2). The present strategy by facile controlling the amount of the zinc acetate template systematically demonstrates the superiority of edge-located Fe-N4 sites, providing a new design avenue for rational defect engineering to achieve high-performance ORR.

Diatomic Iron with a Pseudo-Phthalocyanine Coordination Environment for Highly Efficient Oxygen Reduction over 150,000 Cycles

Atomically dispersed Fe-N-C catalysts emerged as promising alternatives to commercial Pt/C for the oxygen reduction reaction. However, the majority of Fe-N-C catalysts showed unsatisfactory activity and durability due to their inferior O-O bond-breaking capability and rapid Fe demetallization. Herein, we create a pseudo-phthalocyanine environment coordinated diatomic iron (Fe2-pPc) catalyst by grafting the core domain of iron phthalocyanine (Fe-Nα-Cα-Nβ) onto defective carbon. In situ characterizations and theoretical calculation confirm that Fe2-pPc follows the fast-kinetic dissociative pathway, whereby Fe2-pPc triggers bridge-mode oxygen adsorption and catalyzes direct O-O radical cleavage. Compared to traditional Fe-N-C and FePc-based catalysts exhibiting superoxo-like oxygen adsorption and an *OOH-involved pathway, Fe2-pPc delivers a superior half-wave potential of 0.92 V. Furthermore, the ultrastrong Nα-Cα bonds in the pPc environment endow the diatomic iron active center with high tolerance for reaction-induced geometric stress, leading to significantly promoted resistance to demetallization. Upon an unprecedented harsh accelerated degradation test of 150,000 cycles, Fe2-pPc experiences negligible Fe loss and an extremely small activity decay of 17 mV, being the most robust candidate among previously reported Fe-N-C catalysts. Zinc-air batteries employing Fe2-pPc exhibit a power density of 255 mW cm-2 and excellent operation stability beyond 440 h. This work brings new insights into the design of atomically precise metallic catalysts.