Article Electrochemistry, 88(5), 407–412 (2020) The 64th special issue "Frontiers of Carbon Materials" Electrocatalytic Activity of Heteroatom-Doped Graphene for Oxidation of Hydroquinones Masanori HARA, a, * Prerna JOSHI, a Rajashekar BADAM, a,b,c Hsin-Hui HUANG, a,d and Masamichi YOSHIMURA a a Toyota Technological Institute, 2-12-1 Hisakata, Tempaku, Nagoya 468-8511, Japan b Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan c Elements Strategy Initiative for Catalysts & Batteries (ESICB), Kyoto University, 1-30 Goryo-Ohara, Nishikyo, Kyoto 615-8245, Japan d Nanostructures Research Laboratory, Japan Fine Ceramics Center, 2-4-1 Mutsuno, Atsuta, Nagoya 456-8587, Japan * Corresponding author: haram@toyota-ti.ac.jp ABSTRACT In the present study, we aim to synthesize heteroatom (nitrogen or boron) doped-reduced graphene oxide (N-rGO or B-rGO) as a catalyst for the electro-oxidation of hydroquinones, used as a candidate of fuel (hydrogen carrier molecule) for direct-type fuel cells (DFCs), and evaluate the doping effect on its catalytic activity. N-rGO and B-rGO were prepared from a mixture of graphene oxide (GO) and urea or boron trioxide by pyrolysis method. We characterized the morphology and crystal structure of the prepared materials by transmission electron microscopy, and X-ray diffraction, respectively. Energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy show the loading amount of the heteroatoms, 10.4 wt% N and 2.9 wt% B, as well as their chemical nature. The electrochemical analysis of the prepared materials by rotating disk electrode system reveals high activity of B-rGO, 15 and 85 mV lower overvoltage compared with rGO at the half-wave potential of diffusion-limited current, for the electro-oxidation of hydroquinone and methyl-hydroquinone, respectively, because of its electron-accepting nature. We demonstrate that thus modified carbons exhibit high activity, B-rGO > N-rGO > rGO, for the oxidation of hydroquinone derivatives as non-metallic anodes of DFCs. © The Author(s) 2020. Published by ECSJ. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium provided the original work is properly cited. [DOI: 10.5796/electrochemistry.20-64070]. Uploading "PDF file created by publishers" to institutional repositories or public websites is not permitted by the copyright license agreement. Keywords : Direct-type Fuel Cell, Reduced Graphene Oxide, Hydroquinone Oxidation, Heteroatom Doping 1. Introduction In recent years, developing energy storage and supply systems combined with renewable energies such as solar cells and wind- powered electricity have attracted attention to utilize renewable energies effectively, 1 since environmental and energy problems, known as global warming and the depletion of fossil fuels, became critical issues. Direct-type fuel cell (DFC), where alcohols 2–6 and hydrogen-containing small molecules 7–17 are used as fuels, is one of the candidates of environmental friendly and efficient energy supply systems. DFC has similar constitution to the polymer electrolyte fuel cell (PEFC), 18,19 which consists of five components, electrolyte membrane, anode for electro-oxidation of fuels, cathode for oxygen reduction reaction (ORR), anode and cathode bipolar plates. The latter two plates are used as current collector and flow channel to supply fuel and oxygen, respectively. As a fuel for DFC, hydrogen carrier molecules hydrogenated by surplus renewable energies have been examined to use in energy storage and supply systems. In previous researches, many molecules such as methylcyclohex- ane, 17,20,21 ammonia, 7,8 sorbitol, 13 hydroquinone derivatives, 22–28 were tested as fuels. Among these molecules, methyl cyclohexane and ammonia have been intensively investigated, because they have high energy density and liquid state at room temperature. 7,8,17,20,21 However, both molecules are toxic and flammable. On the other hand, sorbitol and hydroquinones are biological materials, and aqueous solutions of these organic molecules as fuels are non-toxic, safe, and easy to handle. Though the solubility of hydroquinone derivatives, which is related to the energy density of a fuel, is lower than sorbitol, their reactivity for electro-oxidation is higher than that of sorbitol and the equivalent potential of redox reaction of hydroquinones can be controlled by chemical structures. 25,29 Hydroquinone derivative, adjusted by chemical structure and functional groups to optimize cell power density, is a potential candidate of fuel for DFC. However, activity 24–26 of anode catalysts, where a fuel is electrochemically oxidized, is inadequate for carbonaceous electrodes such as glassy carbon. While high electro-oxidation activity for hydroquinone on a boron-doped diamond electrode was reported, 22,23 cost of the electrode is insufficient for commercial application. To improve the efficiency and cost of DFC for practical applications, development of the catalytic activity of anode is an important topic. As electro-catalysts for PEFC, heteroatom-doped carbons, such as nitrogen, boron, sulfur, and phosphorus, have raised attention because carbon-based materials are abundant, cheap, and have highly tunable electronic structure. 30– 40 A principal application of nitrogen- or other elements-doped carbon is as electrocatalysts for ORR, 30–33,35 a cathodic reaction in PEFC. Recently, nitrogen-doped graphene has been examined as an electrocatalyst to clarify the difference in its properties arising due to nitrogen, pyridinic, pyrrolic, and graphitic structure in the lattice. 41–44 The reaction phenomena and activity of heteroatom-doped graphene for ORR have been empirically 41,42,45–51 and theoretically 43,44,52–60 discussed by several researchers. We have also demonstrated that graphitic- and pyridinic-nitrogen act as reaction sites for ORR on nitrogen- doped high ordered pyrolytic graphite (HOPG) prepared by electron-beam-excited plasma treatment. 61,62 Previous researches suggest that the catalytic activity of doping elements for ORR is dependent on both elemental species and its lattice structure. However, for organic molecules, few studies were reported on the investigation of the catalytic activity using heteroatom-doped Electrochemistry Received: May 29, 2020 Accepted: July 5, 2020 Published online: August 7, 2020 The Electrochemical Society of Japan https://doi.org/10.5796/electrochemistry.20-64070 407