Enhanced local density of states at the Fermi level of the surface platinum in carbon-supported platinum particles by Naﬁon ionomer Kee Sung Han a,1 , Seong-Soo Kim b,2 , Ki Hyeok Kang a , Byoung Jean Mean a , Heon Hwa Choi a , Oc Hee Han b, * , Moohee Lee a, * a Department of Physics, Konkuk University, 1 Hwayangdong, Gwangjingu, Seoul 143-701, Republic of Korea b Analysis Research Division, Daegu Center, Korea Basic Science Institute, 1370 Sankyuckdong, Bookgu, Daegu 702-701, Republic of Korea article info Article history: Received 12 November 2008 Received in revised form 5 December 2008 Accepted 8 December 2008 Available online 16 December 2008 Keywords: E F -LDOS Fuel cell catalysts Naﬁon ionomer Pt particle 195 Pt NMR abstract 195 Pt nuclear magnetic resonance (NMR) was employed to study the inﬂuence of Naﬁon ionomers (NIs), a common ingredient used as a proton conducting mediator and binder in a membrane-electrode-assembly (MEA) for proton exchange membrane fuel cells, on the local density of states at the Fermi surface (E F - LDOS) of Pt/C catalysts. NIs were observed to increase the surface E F -LDOS of Pt particles, thereby altering of the catalytic properties of metal particles. This correlates well with the reduction of fuel oxidation rates. Thus, ionomers decreasing the surface E F -LDOS of Pt particles would be desirable. In addition, a new concept to interpret the surface E F -LDOS was introduced and validated. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction The major hurdles hindering the commercialization of the direct methanol fuel cell (DMFC), one of the optimum candidates for alternative power sources for portable devices and small vehi- cles [1,2], are the development of a proton exchange membrane (PEM) for higher proton conductivity without methanol crossover [1–4] and of less expensive but highly active electrocatalysts [1–3,5–7]. Naﬁon (DuPont, USA) has become widely used as a com- mercial PEM [1–6] and platinum and platinum-based metal parti- cles supported on carbon with high surface areas, such as Pt/C and PtRu/C, have been the major DMFC electrocatalysts [1–3,5–7]. The morphological and electronic properties of the catalysts, which are strongly related to their electrocatalytic activities, are easily inﬂu- enced by the preparation procedures, pre-treatments, chemical processes, and supporting materials [8,9]. Many nuclear magnetic resonance (NMR) studies have examined the electrocatalysts in conjunction with electrochemistry [10–16]. These previous NMR results demonstrate that 195 Pt and/or 13 C NMR of 13 CO adsorbed on the surface of Pt particles can be used to probe the electronic properties, including the local density of states at the Fermi surface (E F -LDOS), and the inﬂuence on the Pt particle catalysts exerted by various factors such as electrode potentials, adsorbates, alloying materials, supporting materials, and pre-treatments [8,9,11,12]. Here we perform 195 Pt NMR experiments to study the inﬂuence of Naﬁon ionomers (NIs), a common ingredient used as a proton conducting mediator and binder in a membrane-electrode-assem- bly (MEA), on the E F -LDOS of Pt/C catalysts. In addition, a new con- cept to interpret the surface E F -LDOS is introduced and conﬁrmed. The MEA consists of a cathode, an anode and a PEM for oxygen reduction, fuel oxidation, and proton conduction, respectively [3]. To fabricate an MEA, the electrocatalyst ink for each electrode is prepared and NIs are added to the ink to enhance proton conduc- tion and binding among MEA components [3]. However, NIs were reported to reduce CO oxidation in an MEA [17] and to produce H 2 O 2 in the fuel cell reactions [10]. In addition, the performance of DMFCs was strongly inﬂuenced by the mixing states of cata- lyst/ionomer and the MEA fabrication methods [1,3]. Therefore, NIs may modify the properties related to the surface electronic states of Pt particles [11] and may behave as a physical barrier to the diffusion of reagents and products [1,3]. 2. Experimental An MEA was made by hot pressing the sandwich of an anode, a pre-treated Naﬁon 117 (Dupont, USA) as a PEM, and a cathode un- der a pressure of 100 kg cm 2 at 408 K for 3 min. The electrodes 1388-2481/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2008.12.023 * Corresponding authors. Tel.: +82 53 950 7912; fax: +82 53 959 3405 (O.H. Han), tel.: +82 2 450 3084; fax: +82 2 455 4863 (M. Lee). E-mail addresses: ohhan@kbsi.re.kr (O.H. Han), mhlee@konkuk.ac.kr (M. Lee). 1 Present address: Analysis Research Division, Daegu Center, Korea Basic Science Institute, Daegu 702-701, Republic of Korea. 2 Present address: Department of Chemistry, Korea University, Republic of Korea. Electrochemistry Communications 11 (2009) 466–468 Contents lists available at ScienceDirect Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom