Enhancement of oxygen reduction activity of iron phthalocyanine electrocatalyst supported on carbon nanotubes through molecular encapsulation

Abstract

Encapsulation of dibenzo-18-crown-6 complexed with potassium ions (K⁺–DB18C6) inside single-walled carbon nanotubes (SWCNTs) enables electron doping while preserving the tubular structure. Thermoelectric measurements demonstrated that K⁺–DB18C6@SWCNTs switched from p-type (pristine SWCNTs) to n-type. Raman spectroscopy further confirmed electron doping through an upshift of the G-band and a decrease in radial breathing mode (RBM) intensity, while ultraviolet photoelectron spectroscopy (UPS) showed a decrease in work function from Φ = 4.41 eV to Φ = 4.21 eV. When iron(II) phthalocyanine (FePc) was supported on K⁺–DB18C6@SWCNTs (FePc/K⁺–DB18C6@SWCNT), the composite exhibited excellent oxygen reduction reaction (ORR) catalytic activity. Linear sweep voltammetry with a rotating ring–disk electrode (RRDE) revealed an onset potential (E_onset = 0.624 V vs. RHE) nearly identical to that of Pt/C electrodes. The enhanced ORR performance is attributed to perturbation of the FePc electronic state by the electron-doped SWCNT support, as evidenced by recovery of RBM intensity upon FePc loading. RRDE analysis further showed that the ORR followed a nearly complete four-electron pathway (n = 3.97). Durability tests by chronoamperometry at 0.4 V (vs. RHE) indicated that FePc/K⁺–DB18C6@SWCNT retained 31% of its initial current after 3 h, outperforming untreated SWCNT electrodes. This study demonstrates a novel strategy for ORR catalyst design, where molecular encapsulation within SWCNTs modulates the electronic states of supported metal complexes, offering a new route to high-performance and stable ORR electrodes.