N‑,O‑, and S‑Tridoped Nanoporous Carbons as Selective Catalysts for Oxygen Reduction and Alcohol Oxidation Reactions Yuying Meng, †,‡ Damien Voiry, ∥ Anandarup Goswami, †,⊥ Xiaoxin Zou, § Xiaoxi Huang, ⊥ Manish Chhowalla, ∥ Zhongwu Liu,* ,‡ and Tewodros Asefa* ,†,⊥ † Department of Chemical and Biochemical Engineering, ⊥ Department of Chemistry and Chemical Biology, ∥ Department of Materials Science and Engineering, Rutgers, The State University of New Jersey, Piscataway, New Jersey 08854, United States ‡ School of Materials Science and Engineering, South China University of Technology, 381 Wushan Road, Tianhe District, Guangzhou 510640, China § State Key Laboratory of Inorganic Synthesis & Preparative Chemistry, College of Chemistry, Jilin University, 2699 Qianjin Street, Changchun 130012, China * S Supporting Information ABSTRACT: Replacing rare and expensive metal cata- lysts with inexpensive and earth-abundant ones is currently among the major goals of sustainable chemistry. Herein we report the synthesis of N-, O-, and S-tridoped, polypyrrole- derived nanoporous carbons (NOSCs) that can serve as metal-free, selective electrocatalysts and catalysts for oxygen reduction reaction (ORR) and alcohol oxidation reaction (AOR), respectively. The NOSCs are synthesized via polymerization of pyrrole using (NH 4 ) 2 S 2 O 8 as oxidant and colloidal silica nanoparticles as templates, followed by carbonization of the resulting S-containing polypyrrole/ silica composite materials and then removal of the silica templates. The NOSCs exhibit good catalytic activity toward ORR with low onset potential and low Tafel slope, along with different electron-transfer numbers, or in other words, different ratios H 2 O/H 2 O 2 as products, depending on the relative amount of colloidal silica used as templates. The NOSCs also effectively catalyze AOR at relatively low temperature, giving good conversions and high selectivity. T he practical large-scale applications of many useful catalytic and electrocatalytic systems are currently constrained by the lack of sustainable/inexpensive (electro)catalysts composed of earth-abundant elements. Notable examples of such systems include fuel cells, where the most effective catalysts for these important energy conversion systems still remain noble metal- based materials, e.g., Pt/C. 1 Hence inexpensive and sustainable catalytic materials are critically needed to make fuel cells widely applicable. 2 Recently, efforts to tackle these issues have been gaining more traction, with reports of various nonprecious metal or metal-free materials, such as N-doped nanocarbons, that can catalyze reactions that were previously known to be catalyzed only by noble metals. In particular, the revelations that heteroatom-doped carbon materials show catalytic activity toward the oxygen reduction reaction (ORR), the hydrogen evolution reaction (HER), and the oxygen evolution reaction (OER) with comparable efficiency as noble metal-based catalysts have triggered a race among researchers to find other “unconventional” or sustainable materials with similar, or not better, catalytic activity. 3 Moreover, the very fact that a simple doping of carbon materials by heteroatoms (e.g. N, S, B, and P) can make these materials highly effective catalysts 4 currently calls for more research in this area. At this juncture, finding new synthetic methods that can lead to carbon-based materials with better structures, and thereby more favorable catalytic properties, is of utmost interest. As high surface area often makes catalysts more effective, 5 not surprisingly many of the heteroatom-doped carbons reported before are made with high surface area, typically via nanocasting using porous materials such as mesoporous silica as templates. However, other templating synthetic strategies, which could lead to carbon materials with high porosity, good electrochemical contact area, and better diffusion pathways, for substrates/ products should be explored as well. Moreover, methods that can co-introduce different types of heteroatom dopants within carbon nanomaterials and the properties of the resulting materials for electrocatalysis are also worth investigating. Following the same thoughts, herein we report the synthesis of metal-free, N-, O- and S-tridoped polypyrrole-derived nano- porous carbons (NOSCs) that show efficient and selective (electro)catalytic activity in ORR and AOR (Scheme 1). The synthesis involves three steps: (1) polymerization of pyrrole in the presence of colloidal silica templates using (NH 4 ) 2 S 2 O 8 as oxidant, as reported by Maeda and Armes, 6 (2) pyrolysis of the resulting S-containing polypyrrole-silica (S-PPY- silica) composite material, and (3) removal of the silica templates from the carbonized material using aqueous NaOH solution (see Supporting Information, SI, for more details). By varying the synthetic conditions and the relative amount of silica nano- Received: July 22, 2014 Published: September 4, 2014 Scheme 1. Synthetic Procedures Used to Make NOSCs Communication pubs.acs.org/JACS © 2014 American Chemical Society 13554 dx.doi.org/10.1021/ja507463w | J. Am. Chem. Soc. 2014, 136, 13554−13557