Symmetric pseudocapacitors based on molybdenum disulfide (MoS 2 )-modified carbon nanospheres: correlating physicochemistry and synergistic interaction on energy storage† Tobile N. Y. Khawula, a Kumar Raju, b Paul J. Franklyn, c Iakovos Sigalas a and Kenneth I. Ozoemena * bc Molybdenum disulfide-modified carbon nanospheres (MoS 2 /CNS) with two different morphologies (spherical and flower-like) have been synthesized using hydrothermal techniques and investigated as symmetric pseudocapacitors in an aqueous electrolyte. The physicochemical properties of these MoS 2 / CNS layered materials have been investigated using surface area analysis (BET), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman, Fourier transform infrared (FTIR) spectroscopy, and advanced electrochemistry, including cyclic voltammetry (CV), galvanostatic cycling with potential limitation (GCPL), long-hour voltage-holding tests, and electrochemical impedance spectroscopy (EIS). The two different MoS 2 /CNS layered materials exhibit unique differences in morphology, surface area, and structural parameters, which have been correlated with their electrochemical capacitive properties. The flower-like morphology (f-MoS 2 /CNS) shows lattice expansion (XRD), large surface area (BET analysis), and small-sized nanostructures (corroborated by the larger FWHM of the Raman and XRD data). In contrast to the f-MoS 2 /CNS, the spherical morphology (s-MoS 2 /CNS) shows lattice contraction and small surface area with relatively large-sized nanostructures. The presence of CNS on the MoS 2 structure leads to slight softening of the characteristic Raman bands (E 1 2g and A 1g modes) with larger FWHM. MoS 2 and its CNS-based composites have been tested in symmetric electrochemical capacitors in an aqueous 1 M Na 2 SO 4 solution. CNS improves the conductivity of the MoS 2 and synergistically enhances the electrochemical capacitive properties of the materials, especially the f-MoS 2 /CNS-based symmetric cells (most notably, in terms of capacitance retention). The f-MoS 2 /CNS-based pseudocapacitor shows a maximum capacitance of 231 F g 1 , with high energy density 26 W h kg 1 and power density 6443 W kg 1 . For the s-MoS 2 /CNS-based pseudocapacitor, the equivalent values are 108 F g 1 , 7.4 W h kg 1 and 3700 W kg 1 . The high- performance of the f-MoS 2 /CNS is consistent with its physicochemical properties as determined by the spectroscopy and microscopy data. These findings have opened doors for further exploration of the synergistic effects between MoS 2 graphene-like sheets and CNS for energy storage. Introduction Pseudocapacitors are redox-based electrochemical capacitors (ECs). Unlike their counterparts, the electrical double layer capacitors (EDLC) that only use carbon materials as electrode materials, pseudocapacitors employ redox-active materials such as conducting polymers, metal oxides and metal sulphides. 1–4 Unlike batteries with high energy densities, ECs are character- ized by their high power characteristics, which make them very attractive for several technologies and devices that require high- power applications (i.e., the ability to release energy pulses in a very short time, in a few seconds) such as in regenerative braking energy systems in vehicles and metro-rails, “stop–start” applications in modern cars, uninterrupted power supply (UPS), emergency doors in aircras, and escalators in buildings. 5,6 One of the emerging high-power supercapacitor electrode materials is molybdenum disulde (MoS 2 ), a member of the transition-metal dichalcogenides (TMDs). MoS 2 has found applications in electrochemical devices, hydrogen storage, catalysis, capacitors, solid lubricants, and intercalation hosts. 7,8 a School of Chemical and Metallurgical Engineering, University of the Witwatersrand, PO Wits 2050, Johannesburg, South Africa b Energy Materials, Materials Science and Manufacturing, Council for Scientic and Industrial Research (CSIR), Pretoria 0001, South Africa. E-mail: kozoemena@csir.co.za c Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand, PO Wits 2050, Johannesburg, South Africa † Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ta00114a Cite this: J. Mater. Chem. A, 2016, 4, 6411 Received 5th January 2016 Accepted 23rd March 2016 DOI: 10.1039/c6ta00114a www.rsc.org/MaterialsA This journal is © The Royal Society of Chemistry 2016 J. Mater. Chem. A, 2016, 4, 6411–6425 | 6411 Journal of Materials Chemistry A PAPER Open Access Article. Published on 24 March 2016. Downloaded on 5/30/2020 12:01:40 PM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online View Journal | View Issue