Surface engineering of MnO2/nitrogen-doped porous carbon with a protic ionic liquid toward efficient electrochemical oxygen reduction in alkaline solution

Abstract

Surface engineering of electrocatalysts with ionic liquids (ILs) has emerged as an effective strategy for regulating electrochemical interfaces and improving oxygen reduction reaction (ORR) performance. Herein, this concept is extended from noble-metal catalysts to a transition-metal-oxide system by employing a protic ionic liquid (PIL), 2-isopropylimidazolium trifluoroacetate ([iPrIm]CF₃CO₂), as a molecular surface modifier for the MnO₂/NPC-400 electrocatalyst, a composite prepared from KMnO₄-derived MnO₂ and banana pseudostem-derived nitrogen-doped porous carbon (NPC) calcined at 400 °C. The catalysts were systematically characterized using scanning electron microscopy (SEM), thermogravimetric analysis (TGA), Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and contact-angle measurements to elucidate their structural, chemical, and interfacial properties. The results confirm the formation of α-MnO₂ nanorods integrated within NPC and the successful surface functionalization by [iPrIm]CF₃CO₂ without disrupting the composite morphology. The electrocatalytic activity, selectivity, and short-term operational stability of PIL-modified MnO₂/NPC-400 were systematically investigated as a function of PIL loading. An optimized ratio of 1 : 2 (MnO₂/NPC-400 + PIL) delivers a positive shift of approximately 100 mV in the half-wave potential of ORR, from 0.62 to 0.72 V vs. RHE, relative to the unmodified catalyst. Rotating ring-disk electrode (RRDE) analysis indicates a mixed ORR pathway with a dominant four-electron contribution (n ≈ 3.5–3.6) and a lower peroxide yield than MnO₂/NPC-400. The optimized catalyst retains approximately 85% of its initial current after 18 000 s of continuous operation and shows resistance to methanol. RRDE kinetic analysis shows that PIL loading ratios control the balance between peroxide formation and conversion during ORR.