1. Introduction

N-doped carbon materials (e.g., Fe/N/C and carbon alloy catalysts) with high oxygen reduction reaction (ORR) activity are promising alternatives to conventional Pt-based catalysts for polymer electrolyte fuel cells (PEFCs). However, their durability is insufficient compared with Pt based catalysts. Hence, in this study, a Fe/N/C-type oxygen reduction catalyst was hydrothermally-treated with fluoride ions to improve its durability. The electrochemical properties and the deterioration behavior of the catalysts were examined by a rotating ring disk electrode (RRDE) method and an accelerated durability test (ADT).

2. Experimental

Ketjenblack (KB, ECP600JD), 1,10-phenanthroline (phen) methanol solution and iron acetate aqueous solution were added to zinc acetate methanol/water solution, and the obtained suspension was heated at 75°C for 24 h followed by drying and grounding with a mortar to obtain a catalyst precursor. The precursor was heat-treated in a tube furnace at 1050°C for 60 min under Ar flow and at 950°C for 15 min under ammonia gas to obtain a catalyst (denoted as Fe-phen-Zn/KB). The catalyst was dispersed in 100 mM NH₄F aqueous solution in a sealed PTFE container, and heated from 20°C to 120°C at a heating rate of ca. 3.3°C min^-1 and kept 120°C for 30 min to obtain a catalyst hydrothermally-treated with fluoride ions (denoted as “Fe-phen-Zn/KB(F)”). A catalyst hydrothermally-treated without fluoride ions was also prepared for the comparison (denoted as “Fe-phen-Zn/KB(HyT)”).

The catalysts-modified electrode was prepared by casting 2.0 mg mL^-1 catalyst / 0.1 wt% Nafion^® 2-PrOH ink to a glassy carbon (GC) disk for a Pt ring-GC disk electrode and dried under hot wind. Electrochemical measurements were carried out in 0.1 M HClO₄ with the modified working electrode, a reversible hydrogen reference electrode and a GC rod counter electrode. ADTs were carried out for different periods of “ADTs time (t_ADT)” using a start/stop protocol, consisting of a triangular waveform (1.0 V to 1.5 V vs. RHE at 500 mVs^-1), as proposed for carbon corrosion by the Fuel Cell Commercialization Conference of Japan (FCCJ).

X-ray photoelectron spectroscopy (XPS) experiments were carried out using a PHI X-tool (ULVAC-PHI, Inc.) using Al-Kα X-rays. The binding energies were calibrated based on the peak energy positions of Au_7/2 (84.0 eV).

3. Results and discussion

Fig. 1 shows the dependence of ORR current at 0.7 V vs. RHE on t_ADT / min of each catalyst. Whereas the initial ORR currents of the three catalysts were almost the same, the current values of the catalyst after 6 h of ADT is in the order: Fe-phen-Zn/KB(F) > Fe-phen-Zn/KB(HyT) > Fe-phen-Zn/KB, suggesting that the hydrothermal treatments with NH₄F are effective for the improvement of durability for Fe/N/C catalysts.

The peak energy positions of F_1s XPS for Fe-phen-Zn/KB(F) before and after ADTs were ca. 684 eV (fluoride ion environment) and ca. 689 eV (C-F bonding environment), respectively, suggesting that fluoride ions , which adsorbed on and/or intercalated to the catalyst during the hydrothermal treatment with NH₄F, formed bond with carbons during ADTs. Based on the N_1s XPS spectra, Fe-phen-Zn/KB(F) before ADT involved pyridinic N, Fe-N, graphitic N, NO₃ and NO, whereas the catalyst after ADTs only involved pyridonic N, which is probably derived from pyridinic N.