Top-down synthesis of S-doped graphene nanosheets by electrochemical exfoliation of graphite: Metal-free bifunctional catalysts for oxygen reduction and evolution reactions Jinheui Lee a, 1 , Sunguk Noh a, 1 , Nhan Duy Pham a , Jun Ho Shim a, b, * a Department of Chemistry, Daegu University, Gyeongsan, 38453, Republic of Korea b Institute of Basic Science, Daegu University, Gyeongsan, 38453, Republic of Korea article info Article history: Received 22 March 2019 Received in revised form 3 May 2019 Accepted 4 May 2019 Available online 6 May 2019 Keywords: Top-down synthesis S-doped graphene Electrochemical exfoliation Oxygen reduction reaction Oxygen evolution reaction abstract In this study, top-down synthesis of sulfur-doped graphene nanosheets (SDGNs) by simple electro- chemical exfoliation was explored as a means of producing metal-free electrocatalysts for oxygen reduction and oxygen evolution reactions (ORR and OER, respectively). In a typical procedure, graphite foils were used to obtain bulk quantities of SDGN catalysts in the presence of thiosulfate as a sulfur source. Highly stable colloidal dispersions of SDGNs were obtained by applying a voltage of 15 V at an optimized Na 2 S 2 O 3 :H 2 SO 4 molar ratio of 5 : 1 (denoted SDGN(5)). Physicochemical characterizations by Raman spectroscopy, transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy confirmed the existence of sulfur and its electronic/structural properties in graphene nanosheets. In alkaline media, SDGN(5)-modified electrodes were comparable or superior to pristine graphene and a benchmark commercial platinum-based electrodes in terms of stability, methanol tolerance, n values, and onset potential for ORR and OER. The specific capacitance (149.9 F g 1 ) of SDGN(5) supported its excellent ORR/OER performance and enhanced surface area. © 2019 Elsevier Ltd. All rights reserved. 1. Introduction Carbon nanostructures, such as graphene [1 ,2], carbon nano- tubes [3,4], and carbon quantum dots [5,6] have attracted consid- erable attention in the scientific and technological fields due to their unique chemical, physical, and electronic properties, possi- bilities of mass production, and controllable structural properties. Graphene, which is composed of sp 2 bonded carbon atoms ar- ranged in a one atom thick 2D honeycomb-like lattice, has been shown to be suitable for a wide spectrum of applications in the fields of energy [7e9], electronics [10, 11], medical [12] and biotechnology [13, 14]. Up to now, graphene can be synthesized using the bottom-up or top-down approaches. Some of the appropriate techniques for mass production of graphene have been found in top-down methods including ball milling [15e17], ultra- sonic treatment [18, 19], electrochemical exfoliation [20e22], and high shear mixing [23]. The main advantages of these top-down methods are that they produce graphenes composed of few layers with low defect ratios and use inexpensive graphite as a raw material. Therefore, the development of efficient top-down methods for producing graphene nanosheets has continued to attract the attention of researchers, since Hummers’ method was first used to isolate solution-dispersible graphene [24]. Over the past decade, the inclusion of other atoms, such as bo- ron, nitrogen, oxygen, phosphorous, and sulfur, into the sp 2 -hy- bridized framework of pristine graphene has been demonstrated theoretically and experimentally to provide an effective means of altering chemical activities and electrical properties [25e29]. When used for renewable electrochemical energy conversion, doped carbon nanostructures act as effective electrocatalysts. For example, oxygen reduction and oxygen evolution reactions (ORR and OER) are the major cathode and anode reactions, respectively, during fuel cell and water splitting operations. Dai et al. proposed that the high electrocatalytic activity for oxygen was due to nitro- gen doping in carbon nanotubes and the different electronegativ- ities of carbon and nitrogen [30]. Carbon atoms around nitrogen dopant may carry a substantial positive charge, which can lead to good oxygen adsorption. Qu et al. also synthesized metal-free N- * Corresponding author. Department of Chemistry, Daegu University, Gyeongsan, 38453, Republic of Korea. E-mail address: junhoshim@daegu.ac.kr (J.H. Shim). 1 Authors are equally contributed to this work. Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta https://doi.org/10.1016/j.electacta.2019.05.015 0013-4686/© 2019 Elsevier Ltd. All rights reserved. Electrochimica Acta 313 (2019) 1e9