High surface area tungsten carbide microspheres as effective Pt catalyst support for oxygen reduction reaction Yi Wang a , Shuqin Song a , Vasiliki Maragou b , Pei Kang Shen a, **, Panagiotis Tsiakaras b, * a State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics and Engineering, Sun Yat-Sen University, Guangzhou 510275, China b Department of Mechanical Engineering, School of Engineering, University of Thessaly, Pedion Areos 38334, Volos, Greece 1. Introduction The most widely used cathode catalyst in proton exchange membrane fuel cells (PEMFCs) is nano-structured Pt particles dispersed on carbon materials. However, due to the sluggish process of oxygen reduction reaction (ORR), its corresponding overpotential is more than 0.2 V, even under open circuit conditions [1]. Recently, many efforts have been oriented towards the development of new ORR electrocatalysts, either with lower Pt loading or non-Pt electrocatalysts [2–6], focusing also on the synergistic effect between catalyst and support [7–9]. Tungsten carbide has been intensively studied since Levy and Boudart discovered that it possesses catalytic properties similar to those of platinum group metals [10]. In electrocatalysis, tungsten carbides have been mainly used as the electrocatalyst supports for methanol oxidation [11–13], oxygen reduction [7,14], nitrophenol oxidation [15,16] and hydrogen evolution [17,18]. Although the stability of tungsten carbides is still controversial [11,19–21], they are highly tolerant to both carbon monoxide and bisulfide [22,23]. However, the low surface area of tungsten carbides limits their application as the support materials in the electrocatalysts for PEMFCs and the related fields. In the present work, the preparation of high surface area tungsten carbide microspheres (TCMSs) by the aid of a simple hydrothermal method is realized and the performance of the Pt electrocatalyst supported on the as-prepared TCMSs towards oxygen reduction reaction is investigated. The corresponding structural characteristics were determined by the techniques of X-ray diffraction (XRD), scanning electron micro- scopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX) and Brunauer–Emmet–Teller (BET) method. Finally, the activity towards the oxygen reduction reaction was investigated with the electrochemical methods of cyclic voltammetry (CV), rotating disk electrode (RDE) voltam- metry and electrochemical impedance spectroscopy (EIS). 2. Experimental 2.1. Preparation of the materials The high surface area TCMSs were synthesized by using a simple hydrothermal method. The desired quantity of glucose and ammonium metatungstate (AMT) salt was dissolved in de-ionized water to form a transparent solution. The glucose/AMT molar ratio was kept equal to 100:1. The solution was then heated up to 170 8C at a rate of 5 8C min 1 and kept for 12 h in a sealed 50 ml Teflon- lined stainless autoclave. After left to be cooled down to the room Applied Catalysis B: Environmental 89 (2009) 223–228 ARTICLE INFO Article history: Received 30 July 2008 Received in revised form 14 November 2008 Accepted 22 November 2008 Available online 6 December 2008 Keywords: Tungsten carbides Proton exchange membrane fuel cells Oxygen reduction reaction ABSTRACT In the present work, the preparation of high surface area (256 m 2 g 1 ) tungsten carbide microspheres (TCMSs) by the aid of a simple hydrothermal method is realized and the performance of the Pt electrocatalyst supported on the as-prepared TCMSs towards the oxygen reduction reaction (ORR) is investigated. The SEM micrographs indicated that both the synthesized carbon microspheres (CMSs) and TCMSs showed perfect microsphere structure and uniform size. The EDX measurements confirmed that when the C/W mass ratio is 2.5/1, tungsten and carbon coexist in the microspheres. Moreover, from the XRD results, it can be found that both W 2 C and WC are detected and W 2 C exists as the main phase. It was found that the Pt particles are uniformly dispersed on the supports, while the corresponding average particle size is 3.7, 4.1 and 4.3 nm for Pt/C, Pt/CMSs and Pt/TCMSs, respectively. It was also found that in terms of ORR onset potential and mass activity, the Pt/TCMSs catalyst exhibits superior performance to that of Pt/CMSs and Pt/C, enhancing the ORR catalytic activity by more than 200%. The above behavior could be attributed to its higher electrochemical surface area (ESA), as well as to the synergistic effect between Pt and tungsten carbides. ß 2008 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +30 24210 74065; fax: +30 24210 74050. ** Co-corresponding author. Tel.: +86 20 84036736; fax: +86 20 84113369. E-mail addresses: stsspk@mail.sysu.edu.cn (P.K. Shen), tsiak@mie.uth.gr (P. Tsiakaras). Contents lists available at ScienceDirect Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb 0926-3373/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2008.11.032