Solution Plasma Synthesis of Nitrogen-Doped Carbon Nanoballs as Effective Metal-Free Electrocatalysts for Oxygen Reducti
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Solution Plasma Synthesis of Nitrogen-Doped Carbon Nanoballs as Effective MetalFree Electrocatalysts for Oxygen Reduction Reaction Gasidit Panomsuwan,1 Satoshi Chiba,1 Nagahiro Saito,2,3 and Takahiro Ishizaki1,3 1 Department of Material Science and Engineering, Faculty of Engineering, Shibaura Institute of Technology, 3-7-5 Toyosu, Koto-ku, Tokyo 135-8548, Japan 2 Department of Materials, Physics and Energy Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan 3 JST CREST, 4-1-8 Honcho, Kawaguchi, Saitama 333-001, Japan ABSTRACT Nitrogen-doped carbons nanoballs were synthesized from an organic liquid precursor (a mixture of benzene and pyrazine) by solution plasma process. After synthesis, they were further annealed at 700 and 900 °C under N2 atmosphere. The nitrogen-doped carbon nanoballs before and after thermal annealing process exhibit a similar morphological feature, and their diameters are in the range between 20 and 40 nm. With higher annealing temperature, the graphitization of the nitrogen-doped carbon nanoballs increases. For the electrocatalytic activity in an alkaline solution, the limiting current density and onset potential for the ORR activity can be significantly improved for the samples after thermal annealing at 900 °C. We anticipate that solution plasma process will be a viable alternative way for synthesizing heteroatom-doped carbon electrocatalysts for broad application in the field of fuel cells, metal-air batteries, and supercapacitors. INTRODUCTION Fuel cells have been of great interest as clean and sustainable energy conversion technology for advanced technological devices in the upcoming decade. The fabrication of cathode electrocatalysts with highly efficient oxygen reduction reaction (ORR) plays a key role to develop and enhance the performance of the fuel cells [1]. To date, Pt-based electrocatalysts have been commonly used as cathode material owing to highly active ORR activity. However, large-scale implementation has been hindered by the high cost, scarcity, and poor long-term operation stability [2]. Therefore, it has been attractive to search and develop non-precious metal and/or metal-free catalyts in order to substitute the Pt-based electrocatalysts. Recently, nitrogendoped carbon nanomaterials (e.g. carbon nanotubes, carbon nanoballs, hollow carbons, graphene, etc.) have rapidly gained interest in material and electrochemical research as a promising metalfree ORR electrocatalyst. Doping nitrogen atom into graphitic structure can break the electroneutrality of the carbon materials owing to the difference in electronegativity between carbon (2.55) and nitrogen (3.04). This thus results in the change in atomic charge distribution as well as spin density, leading to the creation of active sites favorable for the ORR activity [3−5]. In addition, in comparison with the Pt-based electrocatalysts, nitrogen-doped carbon nanomaterials have prominent advantages, such as relatively low cost, natural abundance, and long-term stability unde
