Regulation of Fe/Co–N–C catalysts by hydrogen-assisted calcination for enhanced alkaline oxygen reduction

Abstract

Energy demand and environmental pollution have driven the growth in fuel cells; however, the cathode oxygen reduction reaction remains the key bottleneck limiting efficiency improvements. Herein, an electrocatalyst was synthesized using a straightforward, efficient approach in which 5 vol% H₂ was introduced into the calcination atmosphere to regulate the zeolitic imidazolate framework-8 (ZIF-8)-derived FeCo–NC. SEM observations confirmed that both the Ar-treated FeCo–NC and the H₂/Ar-treated FeCo–NC–V retained the rhombic dodecahedral morphology inherited from ZIF-8. Compared with FeCo–NC, FeCo–NC–V exhibited a reduced average particle size together with H₂/Ar-induced modulation of the local coordination environment, defect structure, and short-range ordering within carbon microcrystallites. This was supported by sharper selected-area electron diffraction (SAED) rings and discernible (002) fringes with an expanded interlayer spacing of 0.36 nm along with a higher Raman defect ratio (A_D/A_G = 3.13) than that of FeCo–NC (2.89). In 0.1 M KOH, FeCo–NC–V achieved an oxygen reduction reaction (ORR) half-wave potential (E_1/2) of 0.89 V (vs. RHE) and limiting current density of 6.02 mA cm⁻², exceeding that of Pt/C (0.86 V; 5.32 mA cm⁻²). The kinetics were also improved, as reflected by a smaller Tafel slope of 123.26 mV dec⁻¹ compared with 132.61 mV dec⁻¹ for FeCo–NC. Moreover, the intrinsic activity was enhanced, with a relative turnover frequency (TOF) of 1.48 at 0.80 V (vs. RHE). Durability is further demonstrated by modest E_1/2 losses of 20.4 mV after 5000 cycles and 31.4 mV after 10 000 cycles along with 84.6% current retention at 60 000 s, which exceeds that of Pt/C (62.9%). Overall, hydrogen-assisted calcination offers a practical strategy to regulate defect formation and the local coordination environment, thereby enabling the construction of high-performance, non-precious-metal ORR cathode catalysts. This approach balances ORR activity and operational durability in alkaline electrolyte and may guide the design of advanced ORR electrocatalysts derived from metal–organic frameworks (MOFs).