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Tuning the Double Layer of Graphene Oxide through Phosphorus Doping for Enhanced Supercapacitance Weixin Song, Johannes Lischner, Victoria Garcia Rocha, Heng Qin, Jiahui Qi, Joseph H.L. Hadden, Cecilia Mattevi, Fang Xie, and D. Jason Riley ACS Energy Lett., Just Accepted Manuscript • Publication Date (Web): 24 Apr 2017 Downloaded from http://pubs.acs.org on April 24, 2017
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ACS Energy Letters
Tuning the Double Layer of Graphene Oxide through Phosphorus Doping for Enhanced Supercapacitance Weixin Song,1,3 Johannes Lischner,1,2,3 Victoria Garcia Rocha,1,4 Heng Qin,1,3 Jiahui Qi,1,3 Joseph H.L. Hadden,1,3 Cecilia Mattevi,1,3 Fang Xie,1,3 D. Jason Riley1,3* 1 Department of Materials, Imperial College London, London SW7 2AZ, UK. 2 Thomas Young Centre at Imperial College London, London SW7 2AZ, UK. 3 London Centre for Nanotechnology, London SW7 2AZ, UK. 4 School of Engineering, Cardiff University, Cardiff CF243AA, UK. ABSTRACT: The electrochemical double layer plays a fundamental role in energy storage applications. Control of the distribution of ions in the double layer at the atomistic scale offers routes to enhanced material functionality and device performance. Here we demonstrate how the addition of an element from the third row of the periodic table, phosphorus, to graphene oxide increases the measured capacitance and present density functional theory calculations that relate the enhanced charge storage to structural changes of the electrochemical double layer. Our results point to how rational design of materials at the atomistic scale can lead to improvements in their performance for energy storage.
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In an Electrochemical Double Layer Capacitor (EDLC) charge is stored in the ionic double layer formed between the solid substrate and the liquid electrolyte. This mechanism of interfacial charge storage leads to high discharge rates and excellent recyclability. To maximise the capacitance of EDLC devices conductive materials of high specific area are employed as electrodes. Graphene with its high surface area to volume ratio, low density and remarkable electronic, mechanical and thermal properties has been shown to be a suitable electrode material in EDLCs1, 2. Heteroatomic doping into graphene, which is also called surface functionalization, is a promising method of tuning the physicochemical and electronic properties of the material. Heteroatoms such as nitrogen (N), oxygen (O), phosphorus (P), boron (B) and sulphur (S) yield functional groups on the surface of graphene and introduce defects into the lattice3. Heteroatomic doping not only helps maintain the layer separation but also alters the electron distribution in the graphene layer, provides sites for specific adsorption and may yield redox active surface entities that can enhance the specific capacitance of the electrodes. Graphene oxide (GO), oxygen-functionalized graphene, is formed when graphene is exfoliated from graphite in strong acids. A range of oxygen functional groups, including hydroxyl, epoxy, carboxyl, phenol, carbonyl and quinone, have been identified on the surface of GO4. The enriched oxygen functional groups play important roles in the electrochemical capacitive performance of chemically functionalized graphene in electrolytes of different pH and provide abundant covalent bonding sites for further functionalization with other atoms. Specific capacitances as high as 189 F g-1 and 348 F g-1 have been reported for GO5 and reduced graphene oxide (rGO)6, 7, respectively. rGO is typically formed by reducing GO with hydrazine or a hydrogen plasma or thermal treatment during processing. Electron-rich nitrogen (N) doping has been widely used to improve the electronic conductivity and catalytic activity of graphene8. Pyridinic N (N-6), pyrrolic N (N-5) and graphitic sp2 N (N-Q) are formed when nitrogen replaces carbon atoms in the graphene9. Capacitances as high as 287 F g-1 have been reported for nitrogen doped graphene10.
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Phosphorus (P) is located in the same main group (15) as N and has the same number of valence electrons. The electronegativity of P atoms (2.19) is lower than that of C atoms (2.55), so P will be partially positively charged in the C-P bond and the polarity of the C-P bond is opposite to that of C-N bond. Moreover, the larger radius of a P atom relative to a C atom will lead to structural distortions of the hexagonal carbon framework with P protruding out of the graphene plane due to the longer P-C bond as compared with the C-C11 bond and could result in increased surface area and improved separation of the graphene layers. Recently, P-doped carbon materials have been proposed for use in supercapacitors, batteries, fieldeffect transistors and catalysts for oxygen reduction reaction (ORR)12, 13,14. To date the amount of P incorporated on graphene oxide has been low, typically
