Enhancing ORR/OER active sites through lattice distortion of Fe-enriched FeNi3 intermetallic nanoparticles doped N-doped carbon for high-performance rechargeable Zn-air battery

Figure. Schematic illustration of the controllable fabrication procedure of pristine FeNi3/NC, Fe-FeNi3/NC and Fe-enriched-FeNi3/NC.

Abstract

Low-cost, high-activity, non-precious metal electrocatalysts are needed to enhance the bifunctional oxy- gen activities of rechargeable Zn-Air batteries. In this study, a Fe-enriched FeNi₃ inter-metallic nanoparticle/nitrogen-doped carbon (Fe-enriched-FeNi₃/NC) electrocatalyst was designed and prepared using a facile method based on plasma engineering. The excess Fe-ions in the Fe-enriched FeNi₃ nanopar- ticles led to a high degree of lattice distortion that produced abundant oxygen-active sites. The electro- catalyst exhibited excellent oxygen evolution reaction (OER) activity as well as favorable oxygen reduction reaction (ORR) activity in an alkaline electrolyte. In addition, the electrocatalyst revealed a lower potential difference (ΔE = 0.80 V vs. RHE) in a bifunctional oxygen reaction compared to that of the benchmark 20 wt% Pt/C + Ir/C (ΔE = 0.84 V vs. RHE), and most of the reported FeNi₃ alloy-doped car- bon catalysts. Based on DFT calculations, the lattice distortion in Fe-enriched-FeNi₃/NC promoted a higher density of active electrons around the Fermi level. Owing to its great bifunctional oxygen activities, Fe-enriched FeNi₃/NC was applied as an ORR/OER catalyst in the air cathode in a homemade zinc-air battery and exhibited an excellent discharge-charge voltage gap (0.89 V), peak power density (89 mW/cm²), and high specific capacity of 734 mAh/g at 20 mA/cm², which outperformed the benchmark 20 wt% Pt/C + Ir/C electrocatalyst. In summary, this research provides a novel strategy to enhance the OER/ORR.

Conclusions

A novel synthesis route based on plasma engineering was used to fabricate an excess of Fe-doped FeNi₃ alloy nanoparticles in situ in a nitrogen-doped carbon matrix. Despite some studies applying lattice distortion to tune the electronic structure in bimetallic elec- trocatalysts, the lattice distortion and enhanced catalytic activity of excess Fe-doped FeNi3-based electrocatalysts are not completely understood. This study tuned the excess Fe into FeNi3 alloy pre- cisely with various ratios (Fe:Ni = 1:3, 1:1, and 3:1) and adjusted the lattice structure and electronic interaction through lattice dis- tortion. Based on theoretical calculations and XRD, the disorder of the atom arrangement and the volume of the primitive cell increased with increasing Fe ratio, leading to a higher degree of lat- tice distortion. DFT calculations showed that the valence electrons near the Fermi-level increased significantly when the pristine FeNi3 crystal structure was doped with excess Fe atoms, generating more active sites and activated electrons. Based on electrochemical studies, the ORR/OER activity improved with a higher excess of Fe doped into the FeNi3 alloy. In particular, Fe-enriched-FeNi₃/NC (with the maximum excess ratio of Fe: Ni = 3:1) exhibited superior bi-functional ORR/OER activity (ΔE²_{1/2-10 mA/cm} = 0.80 V vs. RHE) compared to that of pristine FeNi₃ /NC (ΔE²_{1/2-10 mA/cm} = 0.90 V vs. RHE) and the benchmark 20 wt% Pt/C + Ir/C electrocatalysts (ΔE²_{1/2-10 mA/cm} = 0.84 V vs. RHE) in an alkaline environment. In a rechargeable Zn-air battery test, Fe-enriched-FeNi3/NC as an air cathode catalyst showed a high open-circuit voltage, large specific capacity, and high peak power density of 1.43 V, 734 mAh/g and 89 mW cm^-2, respectively, which outperformed the benchmark 20 wt % Pt/C + Ir/C electrocatalysts. Overall, this study provides a simple strategy to enhance the bifunctional ORR/OER activities of FeNi₃ alloys through the formation of lattice distortion defects and pro- vides a new pathway for achieving noble metal-free air cathode materials for the next generation Zn-air battery.

Author name

Jun Kang, Associate Professor