Development of highly active and stable non-precious oxygen reduction catalysts for PEM fuel cells using polypyrrole and a chelating agent Hyung-Suk Oh a , Jong-Gil Oh a , Bumwook Roh b , Inchul Hwang b , Hansung Kim a, ⁎ a Dept. of Chemical and Biomolecular Engineering, Yonsei University, 262 Seongsanno, Seodaemun-gu, 120-749, Seoul, Republic of Korea b Hyundai Motor Company, Mabuk-Ri, Gyeonggi-Do, Republic of Korea abstract article info Article history: Received 1 May 2011 Received in revised form 24 May 2011 Accepted 25 May 2011 Available online 1 June 2011 Keywords: Non-precious catalysts Polypyrrole Chelate Ethylenediamine Oxygen reduction reaction PEM fuel cell Nitrogen-modified carbon nanofibers to be used as catalysts in the oxygen reduction reaction (ORR) were synthesized by pyrolysis in the presence of cobalt with PPy and a chelating agent, such as ethylenediamine (ED). It has been found that the type of nitrogen source has a remarkable impact on the catalytic activity and stability of ORR catalysts. XPS and elemental analysis indicated that the combination of ED and PPy as nitrogen sources provided a synergistic effect toward increasing the total nitrogen content of the catalysts and the content of useful nitrogen functional groups such as pyridinic-N and graphitic-N that are responsible for high ORR activity and stability in acidic environments. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Platinum is the most commonly used cathode electrocatalyst for the oxygen reduction reaction (ORR) in proton exchange membrane (PEM) fuel cells. However, to become commercially feasible, PEM fuel cells must overcome the barrier of high catalyst cost caused by the exclusive use of platinum catalysts [1]. Therefore, it is of exceeding interest to develop Non-precious catalysts for the ORR. Several types of non-precious ORR catalysts such as Ru based chalcogenide, transition metal oxide, carbide and nitride catalysts have been proposed as potential substitutes for Pt in the ORR [2–4]. Compared to other non precious ORR catalysts, transition metal macrocycles such as porphyrins and phthalocyanines, have been researched extensively due to their reasonable activity toward the ORR. However, they also suffer from significant limiting factors, including their costs and their lack of stability under the operating environments of PEM fuel cells [5]. To solve this problem, N-based ligands in transition metal macrocycles, which are considered to be an active site in the ORR, have been synthesized by the pyrolysis of carbon, transition metals and nitrogen-containing precursors. These compounds have been found to possess superior stability and high ORR activity. Following this approach, many attempts have been made to explore new nitrogen-containing organic compounds such as ammonia and acetonitrile [6,7]. Recently, Popov et al. suggested the use of chelating agents as nitrogen sources [8,9]. Non-precious catalysts of this type were synthesized by depositing cobalt or an iron-chelating complex using ethylenediamine on carbon surfaces, followed by heat treatment. These prepared catalysts exhibited high activity and selectivity for the ORR. In an effort to improve catalyst stability, Zelenary and Bashyam have described using polypyrrole (PPy) as a nitrogen source [10]. In their study, PPy was first deposited on carbon black via an oxidative polymerization process using hydrogen peroxide. After impregnating with cobalt ions, a carbon-supported cobalt catalyst (Co-PPy-C) was generated. Although the performance of this Co-PPy-C composite cathode was not particularly strong, it did exhibit very promising stability in the fuel cell operating conditions. In this study, novel nitrogen-modified carbon nanofibers were synthesized by pyrolysis of cobalt, PPy and ED to obtain highly active and stable ORR catalysts. The synergistic effects of ED and PPy on the catalytic activity and stability durability towards ORR conditions were explored and will be explained based on results of the nitrogen contents of the catalysts and the type of nitrogen functional groups as determined by XPS and elemental analyses. 2. Experimental Polypyrrole-coated carbon nanofiber (PPy-CNF) composites were synthesized by in situ chemical oxidative polymerization of pyrrole monomer on CNF supports. CNF powder (0.2 g; Suntel Co. Ltd., Korea) was ultrasonically dispersed in 20 ml ethanol over 3 h. Pyrrole monomer (0.1 g) dissolved in ethanol was added to the above Electrochemistry Communications 13 (2011) 879–881 ⁎ Corresponding author. Tel.: +82 2 2123 5753; fax: +82 2 312 6401. E-mail address: elchem@yonsei.ac.kr (H. Kim). 1388-2481/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2011.05.027 Contents lists available at ScienceDirect Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom