Supplementary Information Significant Performance Enhancement in Asymmetric Supercapacitors Based on Metal Oxides, Carbon Nanotubes and Neutral Aqueous Electrolyte

Amongst the materials being investigated for supercapacitor electrodes, carbon based materials are most investigated. However, pure carbon materials suffer from inherent physical processes which limit the maximum specific energy and power that can be achieved in an energy storage device. Therefore, use of carbon-based composites with suitable nano-materials is attaining prominence. The synergistic effect between the pseudocapacitive nanomaterials (high specific energy) and carbon (high specific power) is expected to deliver the desired improvements. We report the fabrication of high capacitance asymmetric supercapacitor based on electrodes of composites of SnO2 and V2O5 with multiwall carbon nanotubes and neutral 0.5 M Li2SO4 aqueous electrolyte. The advantages of the fabricated asymmetric supercapacitors are compared with the results published in the literature. The widened operating voltage window is due to the higher over-potential of electrolyte decomposition and a large difference in the work functions of the used metal oxides. The charge balanced device returns the specific capacitance of ~198 F g−1 with corresponding specific energy of ~89 Wh kg−1 at 1 A g−1. The proposed composite systems have shown great potential in fabricating high performance supercapacitors.

In order to investigate the crystal structure and phase formation, powder XRD patterns were collected for MWCNTs, MWS and MWV composites (Fig. S4). In Fig. S4a, the XRD pattern of MW showed characteristic graphitic peaks at 2θ ~25.4° and ~43.5° corresponding to (002) and (100) planes, respectively. These two characteristics peaks have nearly vanished in the MWS composite whereas newly originated peaks could be readily indexed to tetragonal phase of SnO 2 according to Joint Committee on Power Diffraction Standards (JCPDS) card no 411445.
The disappearance of the characteristics peaks for MW in MWS composite could be assigned to covering of MWCNTs surface with SnO 2 nanoparticles as evident from HRTEM micrographs.
Moreover, In the case of MWV composites, all the peaks excluding those for MWCNTs could be indexed to V 2 O 5 /VO 2 phase according to JCPDS files no. 411426/812392 as shown in Figure   S4b. The vanishing of the characteristics peaks of MWCNTs is due to dispersion of CNTs underneath the layered structures of V 2 O 5 /VO 2 , which is in well agreement with HRTEM results.  (d) and (e) shows de-convoluted V 2p 3/2 peak. 8 To determine the chemical composition and valance state of the present elements, the core level X-ray photoelectron spectra (XPS) were recorded for the MWS and MWV composites.
The XPS spectrum of standard Ag sample was used to remove surface charging effect. The sample's surface was sputtered up to few nanometers before recording of the XPS spectra. Figure S5a-b shows C 1s and Sn 3d core level spectra for MWS composite, respectively. The C 1s peak is appeared at 284.2 eV with several deconvoluted peaks underneath the C 1s peak, which could be assigned to C=C Sp 2 carbons (284.3 eV), C-OH (285.9 eV), C=O (288.2 eV) and -* transition (290.2 eV) in the MW structures, respectively. Moreover, the observed two symmetrical peaks (i.e., Sn 3d 5/2 at ~487.1 eV and Sn 3d 3/2 at ~495.5 eV) with a binding energy separation of 8.4 eV could be assigned to presence of the SnO 2 in the MWS composite (Fig. S5b).
The estimated atomic percentage from XPS spectra for the elements present in MWS composite is: C=41.1 %, O=39.3 % and Sn=19.6 % suggesting single phase of SnO 2 (as O and Sn atomic percentage ratio is ~2), which is in well agreement with XRD results. Figure S5c S5d-e shows de-convolution of the V 2p 3/2 peak into two components; high energy component peak is assigned to V 5+ and low energy component peak (smaller) is attributed to presence of V 4+ in V 2 O 5 structures. The presence of V 4+ state is found to be increased from ~9.4% (before sputtering) to ~16.8% (after sputtering) due to sputtering of the sample surface. This result shows 9 the presence of small fraction of VO 2 phase in assistance with V 2 O 5 phase (before sputtering) and is in well agreement with the XRD results. The atomic percentage of the elements presents in MWV composite is: C=42.3 %, O=41.4 % and V=16.2 %.