Hematite-modified MWCNT/ionic liquid carbon paste electrodes for selective four-electron oxygen reduction in acidic media

Abstract

Developing efficient, stable, and cost-effective electrocatalysts for the oxygen reduction reaction (ORR) in acidic media remains a critical challenge for advancing technologies such as proton exchange membrane fuel cells (PEMFCs). Platinum group metals (PGMs) dominate this field, but they are costly and scarce. This work explores transition metal oxides (TMOs) integrated into a carbon paste matrix as PGM-free alternatives. We synthesized α-Fe2O3, Co3O4, and CoFe2O4 nanoparticles and incorporated them into a composite electrode comprising multi-walled carbon nanotubes (MWCNTs) and an N-octylpyridinium hexafluorophosphate ionic liquid (IL) binder. Preliminary electrochemical screening by cyclic voltammetry in O2-saturated 0.1 M HClO4 suggested a qualitative activity trend: MWCNTs/IL/ α-Fe2O3 > MWCNTs/IL/CoFe2O4 > MWCNTs/IL/Co3O4 > MWCNTs/IL. The optimized MWCNTs/IL/α-Fe2O3 composite exhibited enhanced ORR performance, with an onset potential of approximately +350 mV vs. RHE and a current density of 11 mA cm−2. Such activity was attributed to a cooperative effect between the well-dispersed α-Fe2O3 nanoparticles and the conductive MWCNTs/IL matrix. Electrochemical impedance spectroscopy (EIS) confirmed this interfacial effect, showing that the α-Fe2O3 composite had a charge-transfer resistance of 1.05 × 103 Ω, indicating the fastest reaction kinetics among the oxide composites. Furthermore, rotating ring-disk electrode (RRDE) studies demonstrated the catalyst’s selectivity, favoring the direct 4-electron pathway to water with minimal hydrogen peroxide production (<10%) across the relevant potential range. The α-Fe2O3 system also exhibited operational stability in acidic media, with negligible performance degradation after 100 h under chronoamperometric conditions. These findings highlight the potential of the α-Fe2O3-functionalized nanocomposite as a PGM-free cathode material for acidic ORR applications.