Effect of nitrogen doping on the catalytic activity of carbon nano-onions for the oxygen reduction reaction in microbial fuel cells Thi Hiep Han a,b,c, 1 , Debananda Mohapatra a, 1 , Neelima Mahato a , Smrutiranjan Parida d , Jun Ho Shim e , Anh Thi Nguyet Nguyen e , Van Quang Nguyen a , Moo Hwan Cho a, *, Jae-Jin Shim a, * a School of Chemical Engineering, Yeungnam University, Gyeongsan, Gyeongbuk 38541, Republic of Korea b Department for Management of Science and Technology Development, Ton Duc Thang University, Ho Chi Minh City, Vietnam c Faculty of Applied Sciences, Ton Duc Thang University, Ho Chi Minh City, Vietnam d Department of Metallurgical Engineering and Materials Science, IIT Bombay, Powai, Mumbai, Maharashtra, 400076, India e Department of Chemistry, Daegu University, Gyeongsan-si, Gyeongsangbuk-do, 38453, Republic of Korea A R T I C L E I N F O Article history: Received 17 June 2019 Received in revised form 5 September 2019 Accepted 11 September 2019 Available online xxx Keywords: Carbon nano-onion Exohedral In situ nitrogen doping Graphitization Oxygen reduction reaction Microbial fuel cells A B S T R A C T In this study, highly graphitic nitrogen-doped carbon nano-onions (N-CNOs) were prepared by a one- step, direct, in situ flame synthesis technique and their potential applications as catalysts for oxygen reduction reaction (ORR) in a microbial fuel cell (MFC) were evaluated for the first time. The ORR activity of the CNO, N-CNO, and the commercial Pt/C were measured using a rotating ring-disk electrode (RRDE). The reaction mechanism for the N-CNO was found to follow a four-electron transfer pathway and possess a higher onset potential in RRDE measurement than CNOs. The ORR activity of N-CNO was 5.4 times better than that of CNO, which was attributed to the introduction of nitrogen to the carbon faramework. The MFC fabricated with the N-CNO cathode produced a maximum power density of 49.6 mW m –2 , which was approximately double the performance of the CNO-based MFC. The performance of N-CNO was low compared to Pt/C but the cost per power was only 1/310th. These results confirmed that N-CNOs could be used as a low-cost alternative and an energy-efficient metal-free ORR catalyst for practical MFC applications. © 2019 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry. Introduction Microbial fuel cell (MFC) is an electrochemical device that can convert organic/inorganic matter into electricity by bacteria as a biocatalyst. MFCs have attracted considerable interest because of their ability to generate sustainable energy coupled with wastewater treatment [1–4]. The main challenges limiting the practical applications of MFCs are the high cost of materials and limited power production. The oxygen reduction reaction (ORR) catalyst in the cathode is one of the most critical factors limiting MFC applications. Therefore, many studies have focused on developing efficient and low-cost ORR catalysts for enhancing the MFC performance and replacing the expensive Pt catalyst. Many types of carbon-based catalysts, such as carbon nanotubes, activated carbon, and graphene, have been studied extensively [5]. Among them, the most efficient reported ORR catalysts are heteroatom-doped nanomaterials [6] because of their excellent electrocatalytic activity, long-term stability, which is superior to the benchmark Pt catalyst. Recently, carbon nano-onions (CNOs) and heteroatom-doped CNOs have shown potential as ORR catalysts [7–9]. CNO is a new addition to nanostructured carbon materials and is a member of the fullerene family. This allotrope of carbon consists of spherical, semispherical, or faceted nanoparticles with concentric graphitic rings [10–12]. Because of their unique shell-shaped physical form, they demonstrate a high specific surface area, high electrical conductivity, and excellent tribological behavior. The unique multi-cage shaped microstructure, i.e., concentric graphitic rings with exposed graphitic edges of CNOs, enhances the efficient charge-transfer in electro-catalyst research, due to their high electrical conductivity and the strong interaction of electrolyte ions to their entire surface because of their exohedral positive curvature. The deliberate introduction of controlled graphitic defects could increase the overall chemical reactivity of CNO particles. The presence of nitrogen heteroatoms in CNOs can * Corresponding authors. E-mail addresses: mhcho@ynu.ac.kr (M.H. Cho), jjshim@yu.ac.kr (J.-J. Shim). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.jiec.2019.09.014 1226-086X/© 2019 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry. Journal of Industrial and Engineering Chemistry xxx (2019) xxx–xxx G Model JIEC 4773 No. of Pages 9 Please cite this article in press as: T.H. Han, et al., Effect of nitrogen doping on the catalytic activity of carbon nano-onions for the oxygen reduction reaction in microbial fuel cells, J. Ind. Eng. Chem. (2019), https://doi.org/10.1016/j.jiec.2019.09.014 Contents lists available at ScienceDirect Journal of Industrial and Engineering Chemistry journal homepa ge: www.elsev ier.com/locate/jie c