1831 Korean J. Chem. Eng., 33(6), 1831-1836 (2016) DOI: 10.1007/s11814-016-0030-9 pISSN: 0256-1115 eISSN: 1975-7220 INVITED REVIEW PAPER † To whom correspondence should be addressed. E-mail: shulyg@yonsei.ac.kr Copyright by The Korean Institute of Chemical Engineers. The particle size effect of N-doped mesoporous carbons as oxygen reduction reaction catalysts for PEMFC Ulziidelger Byambasuren, Yukwon Jeon, Dorjgotov Altansukh, Yunseong Ji, and Yong-Gun Shul † Department of Chemical Engineering, Yonsei University, Yonsei-ro 50, Seodaemun-gu, Seoul 03722, Korea (Received 9 November 2015 • accepted 25 January 2016) Abstract-The particle size effect of N-doped mesoporous carbon was investigated for ORR activity in acid condition and for issue of a mass transfer and gas diffusion in PEMFCs. As for a non-Pt ORR catalyst, nitrogen (N)-doped ordered mesoporous carbons (OMCs) with a various particle sizes with the range of the average 20, 45 and 75 μm were synthesized by the precursor of polyaniline for the N/C species, and a mesoporous silica template was used for the physical structure for preparation of nitrogen doped OMCs. The N-doped mesoporous carbons are promoted by a transition metal (Fe) to improve catalytic activity for ORR in PEMFCs. All the prepared carbons were characterized by via scanning electron microscopy (SEM), and to evaluate the activities of synthesized doped carbons, linear sweep was recorded in an acidic solution to compare the ORR catalytic activities values for the use in the PEMFC system. The surface area and pore volume were increased as the particles decreased, which was effective for the mass transfer of the reactant for higher activity at the limiting current regions. Keywords: N-doped Carbons, Ordered Mesoporous Carbons (OMCs), Particle Size, Oxygen Reduction Reaction (ORR), Polymer Electrolyte Membrane Fuel Cell (PEMFC) INTRODUCTION Developing metal-free, carbon-based catalysts to replace plati- num-based catalysts for oxygen reduction reactions (ORRs) is an emerging area of research in proton exchange membrane fuel cells (PEMFCs) [1]. Since Pt is a precious metal of low abundance, it is thus of great interest to develop Pt-free cathode catalyst for PEMFC. In addition to the need to improve PEMFC commercialization, the cost of catalytic materials also needs to be reduced. Therefore, the search for non-precious-metal as well as metal-free catalysts has become one of the most active and competitive endeavors in the field of fuel cells [2-9]. Recently, incorporation of heteroatoms (e.g., N, B, and S) on to the carbon supports so as to modify their surface and physico- chemical properties has been investigated. Among them, N-doped carbons have received considerable attention because the strong electron donor nature of N should promote enhancement in π bonding, leading to improved stability, electron transfer rate, and hence durability of the carbon supports during electrocatalytic pro- cesses [10-12]. Therefore, it is of great importance to explore non- precious ORR electrocatalysts with excellent performance by elab- orately designing their porous structure, especially the durability and resistance to the fuel crossover effect. Besides, nitrogen-doped carbon materials possess advantages of excellent electrocatalytic activity, long durability, environmental friendliness and low costs, and have been studied intensively [13]. Various kinds of N-dop- ing structures such as N-doped carbon nanotube [14], N-doped carbon nanofiber [15], N-doped graphene [16], N-doped ordered mesoporous carbon [17], N-doped hierarchically macro/meso- porous carbon [18], and so on, have been reported. Among them, the newly developed N-doped hierarchically macro/mesoporous carbon has been thought to be the more promising and commer- cially valuable ORR catalyst owing to the integrated macro- and mesoporosities in the same catalyst [19]. Macropores can serve as an electrolyte buffering reservoir to shorten the electrolyte diffu- sion distances to the interior surfaces, while mesopores can pro- vide a large electro-active surface area of the catalyst to facilitate the rapid transport of ions and/or charges [20,21]. However, either the electrocatalytic activity [19,22] or the durability [18,23] of the available N-doped hierarchically macro/mesoporous carbon mate- rials is still unsatisfactory, probably due to their relatively low spe- cific surface areas and/or less extensive hierarchically porous struc- ture, which need to be improved substantially. Over the past few decades, tremendous research efforts have been exerted in developing metallic catalysts for ORRs, including M-Nx/C materials (M=Fe, Co, Ni or Mn) [24-28]. Of many dif- ferent catalysts developed so far, heat-treated nitrogen-coordinated iron on a carbon matrix (Fe/N/C) has been recognized as a prom- ising Pt-free catalyst with potential to match the performance of Pt-based catalysts, although the stability of these novel nonprecious metal catalysts under acidic conditions requires to be improved [25,29]. However, some nitrogen-containing carbon nanostructures also retain a significant ORR activity even after removal of metal components [14,27,30]. These results suggest that metal species (e.g., iron) may not act as an active site but rather facilitate the for- mation of stable nitrogen sites. One of the main advantages of the carbon materials is the pos- sibility to control a well-established pore size and develop surface