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14644 | Phys. Chem. Chem. Phys., 2014, 16, 14644--14650 This journal is © the Owner Societies 2014 Cite this: Phys. Chem. Chem. Phys., 2014, 16, 14644 Iron–nitrogen-doped mesoporous tungsten carbide nanostructures as oxygen reduction electrocatalysts† Je-Suk Moon, a Young-Woo Lee, a Sang-Beom Han, a Da-Hee Kwak, a Kyung-Hoon Lee, a Ah-Reum Park, a Jung Inn Sohn, b Seung Nam Cha b and Kyung-Won Park* a Since Pt-based catalysts have the disadvantages of high cost, large overpotential loss, and limited long-term stability, there have been various promising alternatives to Pt-based catalysts to improve the catalytic activity towards the oxygen reduction reaction (ORR). We have synthesized iron–nitrogen-doped mesoporous tungsten carbide catalysts (WC-m-FT) by pyrolysis of well-ordered mesoporous tungsten carbides with iron porphyrin. WC-m-FT exhibits excellent ORR catalytic activity in an alkaline medium, i.e. a high electron-transfer number as well as superior stability and methanol tolerance. The improved activity and stability of WC-m-FT are ascribed to iron-containing catalytic active sites surrounded by nitrogen species and the well-defined mesoporous tungsten carbide structure. Introduction Since the oxygen reduction reaction (ORR) exhibits the sluggish kinetics of molecular oxygen in low-temperature fuel cells, platinum-based materials are known to be utilized as the most active ORR catalysts in the cathode. 1–3 However, Pt-based catalysts have the disadvantages of high cost, large overpotential loss, and limited long-term stability. 4–6 Currently, there are a variety of promising candidates as alternatives to Pt catalysts, such as nitrogen-doped carbon nanotubes or graphene, and various metal–N 4 chelate macrocycles, to enhance the ORR activity. 7–20 In particular, carbon supported transition metal–N 4 chelate macrocycles such as metal phthalocyanines and metal porphyrins have been intensively investigated as promising non- Pt catalysts since the work of Jasinski in 1964. 21–25 Li et al. reported that an iron and nitrogen doped carbon nanotube– graphene complex showed a high ORR activity and superior stability in both acid and alkaline solutions. 14 Wu et al. presented polyaniline (PANI)–metal–C catalysts with promising ORR activity. 26 PANI–Fe–C and PANI–FeCo catalysts exhibited high ORR activity in an acid medium. Jiang et al. found that the graphene–iron phthalocyanine composite catalyst exhibits comparable activity, superior stability, and methanol tolerance in alkaline solution. 25 Fu et al. reported transition metal nitrogen containing moieties (M–N x , M = Fe and/or Co, x = 2 or 4) embedded in graphene sheets that exhibit high ORR activity in an alkaline medium. 27 The active sites of these compounds are the four nitrogen rings coupled with transition metals, commonly Fe or Co, representing a four-electron transfer process and highly selective catalytic activity for the ORR. 28–33 Furthermore, there have been many eﬀorts to enhance the ORR catalytic activity and stability using particular supporting materials such as transition metal oxides and carbides, because of the stability of oxides and the catalytic properties of carbides. 34–37 In particular, with regard to transition metal carbides mimicking the catalytic properties of Pt-group metals, Levy and Boudart proposed that tungsten carbide (WC), which has a similar electronic density to Pt, displays Pt-like behavior in catalytic reactions and exhibits a comparable catalytic activity in the ORR. 38 Although the catalytic activity of WC is still lower than that of noble metals, it has received considerable interest for fuel cells for the development of inexpensive systems. Recently, to solve the electrochemical activity limitation of tungsten carbide in the ORR, there has been a lot of work on controlling the specific surface area, the deposition of noble metals on tungsten carbides, and tungsten carbide-based composite nanostructures. 39–43 Here, we prepared iron–nitrogen-doped well-defined mesoporous tungsten carbides as cathode catalysts by means of a template-free synthesis. The iron–nitrogen-doped well-defined mesoporous tungsten carbides were characterized by field-emission scanning electron microscopy (FE-SEM), field-emission transmission electron microscopy (FE-TEM), X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. The surface area and porosity of the synthesized a Department of Chemical Engineering, Soongsil University, Seoul 156743, Republic of Korea. E-mail: [email protected]; Fax: +82-2-812-5378; Tel: +82-2-820-0613 b Department of Engineering Science, University of Oxford, Oxford OX1 3PJ, UK † Electronic supplementary information (ESI) available: Additional figures. See DOI: 10.1039/c4cp01157k Received 18th March 2014, Accepted 28th April 2014 DOI: 10.1039/c4cp01157k www.rsc.org/pccp PCCP COMMUNICATION Published on 29 April 2014. Downloaded by University of Waikato on 11/07/2014 09:54:45. View Article Online View Journal | View Issue

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