Solution-plasma-derived Fe-N-C/NiFe-LDH composite for bifunctional oxygen electrocatalysis: effects of surface...

Herein, a noble-metal-free bifunctional electrocatalyst was synthesized by growing NiFe layered double hydroxide (NiFe-LDH) nanosheets via a hydrothermal method on a solution-plasma-derived Fe-N-C substrate for rechargeable air-battery applications. Nitric acid modification increased the amount of oxygen-containing surface groups, including O-C[double bond, length as m-dash]O species, which can serve as anchoring sites for subsequent LDH nucleation. However, acid treatment also decreased the surface N content and the N 1s component assigned to Fe-N coordination, resulting in a trade-off between surface functionalization and preservation of ORR-active structures. SEM/EDS and XRD supported the formation of a NiFe-LDH phase on the Fe-N-C-based support, while the approximately +0.3 eV Ni 2p binding-energy shift observed by XPS indicated a change in the local chemical/electronic environment upon composite formation. The present data, however, do not directly establish an atomically intimate heterointerface or the direction and rate of interfacial charge transfer. Under the measurement conditions used in this study, Fe-N-C@NiFe-LDH exhibited an ORR half-wave potential (E _1/2) of 0.849 V vs. RHE and an OER potential at 10 mA cm^-2 (E _j=10) of 1.61 V, corresponding to an overall potential gap (ΔE) of 0.76 V. In a short-term ORR durability test, the composite retained 81.7% of its current after 10 000 s and showed a 23 mV negative shift in E _1/2. In contrast, during OER durability testing, E _j=10 shifted positively by 95 mV after 1000 potential cycles, indicating that the initial OER enhancement was not fully maintained. These results highlight the need to balance surface functionalization, preservation of ORR-active structures, NiFe-LDH loading and coverage, and structural durability in the design of Fe-N-C/NiFe-LDH bifunctional oxygen electrocatalysts.