This journal is © The Royal Society of Chemistry 2016 Chem. Commun. Cite this: DOI: 10.1039/c6cc07231c Proton conduction through oxygen functionalized few-layer graphene† Chanderpratap Singh, a Nikhil S., a Anwesha Jana, a Ashish Kumar Mishra* b and Amit Paul* a The first report of oxygen functionalized few-layer graphene (OFG) having an interlayer distance of 3.6 Å as an excellent proton conductor (8.7 Â 10 À3 S cm À1 at 80 8C, 95% RH) utilizing hydrophilic oxygen functionalities present at sheet edges bypassing the theoretical limita- tion of proton conduction through a basal plane. The synthesized OFG also exhibited excellent supercapacitor performance (296 F g À1 ). In recent years, renewable energy research has received huge momentum because of limited fossil fuel resources and graphene has played a pivotal role in this field because of its superior electrical, mechanical, and thermal properties. 1 These properties made gra- phene a potent contender for a broad range of applications in various electronics and energy storage devices such as field-effect transistors (FETs), batteries, supercapacitors etc. 2,3 On the other hand, interest towards proton conduction has increased because of its applications such as in fuel cell membranes, sensors, chemical filters, biological transport systems etc. Currently, Nafion is being used as a proton conducting membrane; 4 however, due to its high production cost and poor stability at elevated temperature, a low cost robust material is desirable for replacement. Metal–organic frameworks, covalent organic frameworks etc. have been shown as proton conductors having conductivities in the range of 10 À3 to 10 À7 S cm À1 . 5,6 However, it can be unambiguously anticipated that carbon based proton conductors could be the best alternative because of cost effectiveness, mechanical and chemical stability, ease of synthesis and fabrication simplicity. In this regard, graphite oxide (GO)/reduced graphite oxide (RGO), a precursor of graphene, has been shown to be an excellent proton conductor (10 À2 – 10 À5 S cm À1 ) due to its hydrophilic layered structure and the presence of a significant number of oxygen functionalities. 7–9 The presence of epoxide functionalities and adsorbed water molecules between the graphene layers having a large interlayer distance (8–9 Å) was primarily responsible for proton conduction. 10 The low cost synthesis method of GO makes it an attractive material for proton conduction; however, epoxide functionalities degrade slowly under ambient conditions which prohibited its technological application. 7 On the other hand, graphene has been unexplored as a proton conductor due to its hydrophobic nature and delocalized electron density present over the graphene sheet which makes it unassailable for proton conduction due to a high energy barrier of 3.9 eV. 11 Moreover, in contrast to GO/RGO, the interlayer distance in few-layer graphene is only 3.6 Å wherein graphene layers are held together by van der Waals force which prohibits proton conduction through basal planes, and thus the task becomes more challenging. 2 Nonetheless, theoretical investigations suggested that hydroxyl (C–OH) terminated graphene can reduce the energy barrier for proton conduction substantially, revealing that graphene could be a possible potential contender for proton conduction. 11 Interestingly, the low-cost, robust, and easy synthesis method of graphene makes it one of the most attractive materials for proton conduction which possibly can overcome maximum challenges for scientific and technological advancement in near future. 2 Based on these literature precedents, we conceptualized that by modulating the bulk synthesis of few-layer graphene, it may act as a proton conductor. In the next few sections, we discuss the synthesis, characterization and proton conductivity of oxygen functionalized few-layer graphene (OFG). Furthermore, we also demonstrate OFG as an excellent supercapacitor electrode material. All experimental details and capacitor performance are provided in the ESI. † OFG was synthesized in two steps. In the first step, formic acid intercalated GO was synthesized from a graphite precursor following a modified Hummers method. 12,13 In the second step, formic acid intercalated GO was reduced to produce OFG using a thermal exfoliation procedure reported by us recently. 14 The synthesized GO and OFG are abbreviated as GO(150) and OFG(150) throughout this manuscript where the number 150 indicates the particle size of the parent graphite precursor. In order to achieve the goal of proton conduction, modifications in the synthesis protocols have been made from our previous reports in order to significantly enhance a Department of Chemistry, Indian Institute of Science Education and Research (IISER) Bhopal, Madhya Pradesh, 462066, India. E-mail: apaul@iiserb.ac.in b Department of Physics, Indian Institute of Science Education and Research (IISER) Bhopal, Madhya Pradesh, 462066, India. E-mail: akmishra@iiserb.ac.in † Electronic supplementary information (ESI) available: PXRD, TGA, RAMAN, XPS, and water sorption analysis results with additional information of all characterization and synthesis details. See DOI: 10.1039/c6cc07231c Received 5th September 2016, Accepted 3rd October 2016 DOI: 10.1039/c6cc07231c www.rsc.org/chemcomm ChemComm COMMUNICATION Published on 03 October 2016. Downloaded by Indian Institute of Science Education and Research – Bhopal on 08/10/2016 05:02:48. View Article Online View Journal