Utilization of waste polyethylene terephthalate bottles to develop metal-organic frameworks for energy applications: A clean and feasible approach

Journal Pre-proof Utilization of waste polyethylene terephthalate bottles to develop metal-organic frameworks for energy applications: A clean and feasible approach Abdullah M. Al-Enizi, Jahangeer Ahmed, Mohd Ubaidullah, Shoyebmohamad F. Shaikh, Tansir Ahamad, Mu Naushad, Gengfeng Zheng PII:

S0959-6526(19)34121-6

DOI:

https://doi.org/10.1016/j.jclepro.2019.119251

Reference:

JCLP 119251

To appear in:

Journal of Cleaner Production

Received Date: 25 April 2019 Revised Date:

21 October 2019

Accepted Date: 9 November 2019

Please cite this article as: Al-Enizi AM, Ahmed J, Ubaidullah M, Shaikh SF, Ahamad T, Naushad M, Zheng G, Utilization of waste polyethylene terephthalate bottles to develop metal-organic frameworks for energy applications: A clean and feasible approach, Journal of Cleaner Production (2019), doi: https:// doi.org/10.1016/j.jclepro.2019.119251. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Graphical Abstract

Total word count: 6159 Utilization of waste polyethylene terephthalate bottles to develop metal-organic frameworks for energy applications: A clean and feasible approach Abdullah M. Al-Enizia, Jahangeer Ahmeda, Mohd Ubaidullaha*, Shoyebmohamad F. Shaikha, Tansir Ahamada, Mu. Naushada*, Gengfeng Zhengb a b

Department of Chemistry, College of Science, King Saud University, Riyadh 11451, Saudi Arabia

Laboratory of Advanced Materials, Department of Chemistry, Fudan University, Shanghai, 200433 China

Abstract Currently, there is a great environmental and economic need to convert polyethylene terephthalate bottles into high-value metal-organic framework materials. Metal-organic frameworks demonstrate great dominance over other materials owing to high surface area and exceptional porosities. Herein, a simple chemical approach was employed to produce metaloxide nanoparticles (ZnO and Co3O4) embedded mesoporous carbon (ZnO@MC and Co3O4@MC composites) via carbonization of polyethylene terephthalate bottles derived metal-organic frameworks for supercapacitor application. Very high specific surface areas of ~2183 and ~2503 m2g-1were achieved of ZnO@MC and Co3O4@MC composites for the first time. The mesoporous nature of ZnO@MC and Co3O4@MC composites was confirmed through BJH (32 Å and 42 Å) and DA (38 Å and 39 Å) pore size studies. The specific capacitances of ZnO@MC and Co3O4@MC composites were estimated to ~97 and ~180 F/g, respectively, at 5 mV/s using 6 molar KOH in a two-electrode system from cyclic voltammetry analysis. The energy density of 68 Wh/kg at a power density of 149.1 W/kg was estimated for the Co3O4@MC composite. The galvanostatic charge discharge showed an excellent stability for Co3O4@MC composite (~5.20 % loss after 5,000 segments). The Co3O4@MC composite demonstrated extraordinary supercapacitor performance because Co3O4 having good Faradic process and high theoretical capacitance. The symmetric supercapacitive performance of Co3O4@MC composite show good capacitance, stability and rate capability. Keywords: Clean approach; Mesoporous; Composites; Carbonization; Charge-discharge; Supercapacitor Corresponding authors’ email: [email protected] (M. Ubaidullah); [email protected] (M. Naushad) Declarations of interest: none.

1

1. Introduction Polyethylbenzene 1,4-dicarboxylate (PETE or PET) plastic is a condensed polymer made of terephthalic acid and dimethyl terephthalate or ethylene glycol. PET materials play a vital role in human society, such as in food packaging (especially bottles), audio/video tapes, and X-ray films (Genta et al., 2005). A recent study demonstrated that every year more than 24 million tons of PET plastic used globally and this number will continue go up in the future (Huang et al., 2011). The landfilling by PET waste generates severe environmental issues owing to its slow biodegradability, and the long chemical retrieval of their monomers (Siracusa et al., 2008). The monomers of PET i.e. benzene-1, 4-dicarboxylic acid (BDC), can be utilized to produce highly efficient energy storage materials. Hence, to recover BDC from waste PET would be a smart approach for recycling waste, as well as gaining economic benefit to save the environment. In the present study, for supercapacitor application, PET bottles were consumed to produce BDC as a starting material for the production of metal organic frameworks (MOFs) rather than other polymers (Xu et al., 2019; Zhu et al., 2019). It is well known that the metal oxide based materials (Ahmad et al., 2017; Ahmed et al., 2019) also play a vital role in energy applications (Yang Liu et al., 2019; Yi et al., 2019). MOFs have been utilized as sacrificial precursors because of having highly porous conductive network of carbon. The carbon matrix architecture not only improved the electrical conductivity but also provide a good channel for insertion and deinsertion of ions (Hou et al., 2019, 2018). Generally, MOFs are made of metal clusters and organic linkers coordinated with each other. Shape and surface chemistry MOFs can be tune by the selection of metal ions and organic ligands, which have attracted enormous attention to use in advance energy materials (Kim et al., 2012). The unique and pristine structure of MOFs and their tailorable features make them an ultimate sacrificial template to produce highly porous architecture for several applications. The metal clusters in MOFs are covalently bonded with 2

organic linker, which construct more effective functional hetero-structure (Yang et al., 2014). The surface morphology, porosity, and composition of MOFs can be altered by changing the annealing temperature and time. Moreover, carbon and transition-metal oxides (Le et al., 2019) containing hybrid structures (Li et al., 2018, 2017) show better conductivity and stability (H. Pang et al., 2016; M. J. Pang et al., 2016) analogous (J. Yang et al., 2019; L. Yang et al., 2019) to other energy materials (Du et al., 2019; Kirubasankar et al., 2018). In the same way, the carbonization of MOFs produce metal oxide containing carbon network, which promotes the enhancements in surface area, electrical conductivity, and stability. In the last decade, the unusual properties of MOFs attracted considerable attention of researchers to use in energy storage applications, owing to high energy/power density, good reversibility, excellent intrinsic safety and long life (C. Chen et al., 2018). Very recently, a series of MOF materials were reported by (Wang et al., 2014) with various organic linkers and central metal atoms; for supercapacitors applications (Choi et al., 2014) among them, zirconium-based MOFs showed high areal specific capacitance (Y. Chen et al., 2018). (Zhu et al., 2018) reported Co-MOF electrode based supercapacitor and achieved 1.7 mWh/cm2 energy density at a power density of 4 mW/cm2. (Deng et al., 2017) described an MOF-derived honeycomb-like structure of Co3O4/3D graphene and reported the 7.5 Wh/kg energy density and 794 W/kg power density. (Dai et al., 2017) reported Co3O4@Carbon materials which showed 8.8 Wh/kg energy density and 375 W/kg power density. A dual Ni/Co-MOF-rGO composite was reported by (Rahmanifar et al., 2018) which demonstrate an energy density of 72.8 Wh/kg at 850 W/kg power density. (Yi et al., 2015) fabricated CNT@NiO/PCPs composite based ASCs which showed 25.4 Wh/kg energy density and 400 W/kg power density. Herein, PET-derived MOFs were synthesized via a solvothermal route to construct metaloxide nanoparticles (ZnO and Co3O4) embedded in mesoporous carbon to foster the

3

electrochemical performance and stability. The high surface area of the as-prepared ZnO@MC and Co3O4@MC composites produce high specific capacitance and effective multi-electron transfer Faradaic processes. The highly porous ZnO@MC and Co3O4@MC composites are utilized to fabricate high-performance supercapacitors on Ni-foam. The Co3O4@MC composite exhibited excellent energy density of 68 Wh/kg at power density of 149.1 W/kg with superior cyclic as well as charge-discharge stabilities (~5.20% loss after 5,000 segments). 2. Experimental 2.1 Materials used AR grade chemicals were utilized as obtained from commercial sources without any additional refinement. Zn(NO3)2.6H2O (purity 98%), Co(NO₃)₂.6H₂O (purity 99.9%), ethylene glycol (purity 99.8%) potassium hydroxide (purity ≥85%) from Sigma Aldrich. N, N-dimethyl formamide (purity 99%) from Acros Organics, and benzene-1,4-dicarboxylic acid (waste PET bottle derived) were used. 2.2 De-polymerization of PET bottles PET bottles were collected from the garbage and washed thoroughly using distilled water, and then cut into small chips. Briefly, 2 g of PET chips were kept in a 300 ml Teflon autoclave with 40 ml EG and 60 ml de-ionized water, and then treated at 180℃ for 4 hrs. Then, the autoclave left for cooling to temperature (27°C). Furthermore, the resulting products were centrifuged and washed twice with DMF, then dried at 100°C for 24 hrs. The white powder was collected (i.e., BDC), and used as a starting material for MOFs synthesis. Here, we used Fourier-transform infrared (FT-IR) spectroscopy to compare the as synthesized PET-bottle-derived BDC with the purchased BDC (98%, Sigma Aldrich) and found nearly similar IR bands (Fig. S1- Supplementary section). 2.3 Synthesis of MOFs from waste PET derived BDC

4

All reagents were used as obtained, without further purification. The experiments were performed in Teflon reactor autoclave as defined in depolymerisation of waste PET bottles. In a simple synthesis procedure, 6 mmol of zinc nitrate and 2 mmol of BDC were homogenously mixed in 60 mL of DMF in a beaker at constant stirring for 10 minutes, this solution was then transferred to high-pressure Teflon reactor for heat treatment at 100°C for 24 hrs under autogenous pressure. Similarly, 6 mmol of cobalt nitrate and 2 mmol of BDC were homogenously mixed in 60 mL of DMF at constant stirring and treated at 100°C for 24 hrs in Teflon reactor. The crystals of MOFs were collected via centrifugation at ten thousand rpm for 10 minutes and followed by twice washing with DMF and then vacuum dried at 100°C (<10-7 bar) for 12 hrs. The pristine structures of MOF-5 (clear crystals) and Co-MOF (pink crystals) were obtained. The formation of as synthesized MOF-5 and Co-MOF was confirmed through X-ray diffraction and FTIR measurement (Fig.S2- Supplementary section). The crystals of as synthesized MOFs were carbonized at 600°C for 3 hrs with a heating and cooling rate of 2°C per minute under the constant flow of argon. The resultant black masses/composites were referred to as ZnO@MC and Co3O4@MC (MC = mesoporous carbon). 2.4 Characterization details The surface texture and morphologies of the crystals and carbonized materials were recorded using field emission scanning electron microscopy (JEOL FE-SEM). The high resolution transmission electron microscopic investigation of ZnO@MC and Co3O4@MC composites was performed on JEOL, HR-TEM, JEM 1400 machine at an acceleration voltage of 200 kV a. Selected area electron diffraction (SAED) was also recorded on the same machine. For this purpose, the composites (ZnO@MC and Co3O4@MC) powder was dispersed in ethanol and a drop of the suspension was placed onto a carbon film coated copper grids for TEM measurements. The elemental analysis was performed by CHN and inductively coupled

5

plasma optical emission spectrometry (VISTA MPX,). The crystallinity and phase of the produced composites powders of ZnO@MC and Co3O4@MC were examined through powder X-ray diffraction (PXRD) studies, which was carried out with Rigaku diffractometer (PXRD, D-Max 2550 VB-PC). The furrier transform infrared spectroscopic (FT-IR) studies of both the (ZnO@MC and Co3O4@MC) composites were performed using a Bruker Tensor 27 FT-IR instrument in the frequency range of 4,000–500 cm-1. Brunauer-Emmett-Teller (BET) surface area, Barrett-Joyner-Halenda (BJH) and Dubinin-Astakhov (DA) pore size distribution of the materials were checked using BET physisorption analyser (Micromeritics, ASAP-2020, USA) in liquid nitrogen at 77 K. The electrochemical properties of fabricated symmetric supercapacitor of ZnO@MC and Co3O4@MC composites were analysed on a two electrode system by utilizing Nickel foam as current collector in 6 M KOH at room temperature (CHI-660E electrochemical work station). For this purpose, working electrodes of ZnO@MC and Co3O4@MC composites were fabricated on Ni-foam by taking the active material and poly vinylidene difluoride in a mass ratio of 95:5 with few drops of N, methyl-2pyrrolidinone (NMP) solution. The produced slurry was coated on1 cm2 Ni-foam and then dried in vacuum at 70°C for 10 hrs. Weight of the active material was calculated by weighing the Ni-foam before and after the coating. Two sheets (1 cm2) of the active material’s coated Ni-foam were combined like a sandwich type model and separated via a porous filter to construct the supercapacitor device. The prepared electrode works as both the positive and negative electrode. The specific capacitance of the material was calculated by using the standard formulas which are given in the supplementary section (Text S1). 3. Results and Discussion 3.1 Surface architecture FE-SEM investigation demonstrates the well-defined crystalline pristine cubic structures of the prepared MOFs before carbonization, i.e., MOF-5 and Co-MOF (Fig. 1 a, b). After carbonization (i.e., ZnO@MC and Co3O4@MC composites), the pristine cubical structure is 6

not retained so far and turns to rods and leaflets like agglomerated structures exhibited in Fig. 1 c, d. FE-SEM micrographs of the composites reveal that in both the samples, small nanoparticles of ZnO and Co3O4 were embedded in the carbon matrix. Such types of architecture would enable to access the interior surface of the electrode with an easy infiltration of electrolyte ions, and also fast charge passage with least diffusion lengths. The HR-TEM images of ZnO@MC and Co3O4@MC composites (Fig. 2 (a, b)) demonstrate that the ZnO and Co3O4 metal oxide nanoparticles embedded uniformly in the carbon matrix with slight agglomeration. The high resolution TEM image of Co3O4@MC composite (Fig. 2 d) reveals the well-arranged fringes correlated to 0.242 and 0.293 nm d-spacing, confirming to (311) and (220) planes of cubic Co3O4. The SAED patterns of the ZnO@MC composite can be indexed to the (100), (002), (101), (102), (110), and (103) planes from the inward to the outward directions (Fig. 2 (e)). Moreover, the SAED patterns of the Co3O4@MC composite showed the (220), (311), (400), and (422) planes (Fig. 2 (f)). The SAED patterns show good agreement with the XRD patterns indexed above. The elemental compositions of ZnO@MC and Co3O4@MC composites were checked via CHN (elemental) and ICP-OES analysis to identify the metal compositions. The ICP-OES results revealed that the Zn and Co contents in the ZnO@MC and Co3O4@MC composites are 34.0 and 36.0 wt. %, respectively. Moreover, elemental analysis results show 39.0 and 39.1% carbon content, present in ZnO@MC and Co3O4@MC composites respectively, summarized in Table 1. 3.2 Structure elucidation The conversion of PET waste into high value MOFs through BDC was confirmed by the FTIR and XRD pattern given in the supplementary information Fig (S1 and S2). The PXRD patterns of ZnO@MC and Co3O4@MC composites (Fig. 3 (a, b)) indexed well with JCPDS: 36-1451 and 43-1003, respectively. The sharp intense peaks of the diffraction patterns corroborated the highly crystalline nature of ZnO@MC and Co3O4@MC composites. The

7

diffraction at 2-theta = 31.82, 34.68, 36.42, 47.71, 56.73, 63.02, and 68.12° indexed to the 100, 002, 101, 102, 110, 103, and 112 hkl planes of the cubic phase of ZnO (Kuriakose et al., 2014). The characteristic 002 hkl plane in the ZnO@MC composite at 28.36° belongs to the carbon matrix. Moreover, for the Co3O4@MC composite, the peaks at 32.45, 34.55, 45.29, 52.3 and 56.89° correspond to the Co3O4 cubic crystal planes of 220, 311, 400, 101 and 422 (hkl) (Al-Tuwirqi et al., 2011). The prominent peak of carbon (002) indexed at 20.43 and 24.44° 2-theta confirm the formation of the carbon matrix (Miao et al., 2015). The absence of impurity peaks confirms of the formation of highly pure ZnO@MC and Co3O4@MC composites. FT-IR spectrum of ZnO@MC and Co3O4@MC composites was illustrated in Fig. 3 (c). The strong vibrations at higher wavelengths of 3435-3441 cm−1 and 1390-1392 cm−1 were attributed to stretching vibrations and deformation of O-H (Naushad et al., 2019) and C-O, and the 1595-1599 cm−1 band was assigned to the C=O vibrations (Guchhait et al., 2000; Xu et al., 2008). The metal-oxide bands shown at lower wavelength, i.e., 752-881 cm−1 in both composites with a low transmittance value support the formation of ZnO and Co3O4 nanoparticles (Handore et al., 2014; Liu et al., 2012). The Raman analysis was performed to check the grade of graphitization as well as crystallinity of the ZnO@MC and Co3O4@MC composites. Fig. 3 (d) demonstrate two bands at 1341-1336 and 1619-1587 cm-1belongs to graphitic defects (D and G bands) for ZnO@MC and Co3O4@MC composites (Claramunt et al., 2015). The ratio of the D and G bands intensities (ID/IG) was estimated approximately 0.82 and 0.84 for ZnO@MC and Co3O4@MC composites supporting the presence of rich defects in graphite. The Raman band 583 cm-1 belongs to E1 mode of ZnO and band at 692 cm-1links to A1g mode of Co3O4 (Cuscó et al., 2007; Deng et al., 2015). 3.3 X-ray photoelectron spectroscopic (XPS) studies XPS studies was performed to characterize the surface composition and oxidation state of ZnO@MC and Co3O4@MC composites, as indicated in Fig. 4 (a-d). Fig. 4 (a) illustrate the

8

XPS spectra of the ZnO@MC composite confirmed the presence of C, O, Zn elements, while in case of Co3O4@MC composite, Co, C and O was found. Fig. 4 (b) represent two stronger peaks located at 1017 and 1040 eV correspond to 2P3/2, 2P1/2 of Zn respectively, which are consistent with the reported values for Zn2+ (Ganesh et al., 2016).The Co2p peak in Fig. 4 (c) consists of two main spin-orbital lines. The Co2p3/2 peak is centered at 789.2 eV and the Co2p1/2 peak at 795.3 eV, are in good agreement with the literature reported values (Qiu et al., 2017). A symmetric one-band was observed in the O1s spectrum centred at 530.2 eV is characteristic of the lattice oxygen of ZnO and Co3O4 (Fig. 4 (d) (Koo et al., 2017). 3.4 BET Surface area and pore size analysis BET surface area of the composites was calculated by using the multipoint BET equation (Text S2). Before analysis, to remove the contaminants (water vapour and adsorbed gases) the samples were degassed at 200°C for 12 hrs. After degassing, to estimate the surface area and porosity the samples were subjected to analysis in broad range of relative pressure to produce Nitrogen adsorption-desorption isotherms. The well-known BET method is widely used standard procedure to determine the surface area of fine powders and porous materials (Tan et al., 2012). The specific surface area of Co3O4@MC composite was found ~2503 m2g−1 which was higher than analogous ZnO@MC composite (2183 m2g−1). It is noticeable that the N2 adsorption–desorption analysis revealed the type-IV isotherms of ZnO@MC and Co3O4@MC composites, which clearly indicate the capillary condensation in the meso- and micropore structure (Fig. 5 (b)). The initial parts of both isotherms show a rounded knee, which is attributed to the initial few-monolayer adsorptions (Cychosz et al., 2017). Moreover, the low-slope region of the isotherms resemble the multilayer adsorption (Naushad et al., 2016) shown by ZnO@MC and Co3O4@MC composites. The exceptional porosity shown by the ZnO@MC and Co3O4@MC composites is due to the MOF-5 structure (Fig. 5 (c, d)). The BJH pore sizes of 32 and 43 Å for the ZnO@MC and Co3O4@MC composites also verify the

9

mesoporous nature of the materials (Fig. 5 (c)). The Dubinin-Astakhov pore size studies shows 38 and 39 Å pores for ZnO@MC and Co3O4@MC composites, respectively (Fig. 5 (d)) which revealed a good agreement with the BJH pore size distribution. It is assumed that the mesoporous structure plays critical role to facilitate higher electron and ion transfer between the material and electrolyte interface and provide various active sites to promote the fast electrochemical reactions. These active sites may lead to a great improvement in the electrochemical properties (Thi et al., 2015). Table 2 summarize the specific surface area and porosity of the ZnO@MC and Co3O4@MC composites. 3.5 Electrochemical studies The electrochemical measurements of ZnO@MC and Co3O4@MC composites were performed in two-electrode system. The CV of ZnO@MC composite were examined in5-200 mVs-1 scan rate range with a potential window ranging from -0.5 V to +0.5 V shown in Fig. 6 (a). No redox peak was detected, which identify the pseudocapacitive behaviour of the electrode materials, exhibiting spectacular electrochemical performance (Augustyn et al., 2014). It was observed form CV curves investigation on increasing the scan rate from 5 to 200 mVs−1 the value of specific capacitance steadily decreased from 97 to 57 F/g for ZnO@MC composite electrode (Fig. 6 (b)).Notably, high specific capacitance 180 to 51 F/g at 5 to 200 mVs-1 scan rate was found of Co3O4@MC composite electrode (Fig. 7 (a, b)) which was higher than the corresponding ZnO@MC composite electrode. The decrease of specific capacitance with an increase of scan rate could be due the quick and easy penetration of the electrolyte ions into the materials architecture with an enormous and maximum accessibility to materials area by the electrolyte ions during the CV measurement at lower scan rates, and vice-versa (Shinde et al., 2019). The porosity among the leaflet-like structures of the Co3O4@MC composite could offer high surface area and easy infiltration of the electrolyte ions. At difference current densities, the GCD curves demonstrated the

10

symmetrical nature within the potential range between -0.2 to 0.4, without internal voltage drops which suggest the highly reversible charge-discharge behaviour of the materials shown in Fig. 7 (c), On the other hand at lower potential values it is showing the asymmetric nature. The Co3O4@MC composite electrode showed a high specific capacitance of 136 F/g at low current density of 6 A/g estimated through GCD curves. Furthermore, the enhancements in the specific capacitance of both the ZnO@MC and Co3O4@MC composites electrodes might be due to the conductive network formed between the mesoporous carbon and metal-oxide (ZnO, Co3O4) nanoparticle framework and the pores in the carbon matrix acting as a reservoir for electrolyte ions (Cao et al., 2015). The rapid ionic transport and stable supply of electrolyte ions can be ensured by these reservoirs. The mesoporosity and high surface area of both the ZnO@MC and Co3O4@MC composites proclaim that the synthesized materials can be utilized as efficient electrode materials for supercapacitor applications (M. J. Pang et al., 2016). Various parameters for instance energy and power density, stability, and long life play vital roles in energy storage devices (Mai et al., 2014). Table 3 represents the specific capacitance of ZnO@MC and Co3O4@MC composites calculated by CV (at different scan rates) and GCD (at and current densities). Cyclic stability of Co3O4@MC composite electrode was examined by running 100 segments of CV in -1 V to +1 V potential range at 50 mVs−1scan rate Fig. 8 (a). GCD stability of the Co3O4@MC composite was tested by running 5,000 segments and only ~5.20 % loss was observed. It shows the stable nature of the Co3O4@MC composite. The ion transfer phenomenon of the electrode/electrolyte interface was checked through electrochemical impedance study (de Bruin and Franklin, 1981). For Co3O4@MC composite electrode the Nyquist plot show a small semi-circle of charge transfer resistance (Rct) with a low value of (~2 Ω) presented in Fig. 8 (c). The energy and power density of Co3O4@MC composite were estimated and compared well with the established Ragone plot presented in

11

Fig. 9 (a, b). The energy density and power density (Ragone plot) of Co3O4@MC composite were shown in Fig. 9 (a). The energy density of the Co3O4@MC composite can reach 68 Wh/kg at a power density of 149.1 W/kg. The obtained results are much better than those of supercapacitors made of metal/metal oxides (Yiyang Liu et al., 2019). Table 4 summarizes this work and previously reported work on supercapacitors. 4. Conclusions In conclusion, value added MOFs of Zn and Co were synthesized from waste PET bottles to construct ZnO and Co3O4 nanoparticles embedded mesoporous carbon (ZnO@MC and Co3O4@MC composites) via one-step solvothermal route. Very high specific surface areas of 2183 and 2503 m2g-1wereattained for ZnO@MC and Co3O4@MC composites respectively for the first time. The symmetric supercapacitors of ZnO@MC and Co3O4@MC composites exhibit excellent capacitive performance in 6M KOH electrolyte. From the CV analysis the highest specific capacitances of ~97 and ~180 F/g at a scan rate of 5 mVs−1 were found for ZnO@MC and Co3O4@MC composites respectively. The energy density of 68 Wh/kg at a power density of 149.1 W/kg was estimated for the Co3O4@MC composite. The GCD cycles show an excellent stability of Co3O4@MC composite electrode materials (~5.20 % loss after 5,000 cycles). The excellent electrochemical performance of the prepared composites endorse the following key points (i) the high surface area (2503 m2/g) of Co3O4@MC composite provides more active sites and accessibility to electrolyte ions for an easy infiltration, (ii) the mesoporous carbon-incorporated metal-oxide nanoparticles reduce the resistance (iii) a favourable Faradaic process takes place in Co3O4@MC composite, and (iv) the coating of Co3O4@MC composite slurry onto current collector (Ni-foam)offers a better mechanical adhesion and electrical connection. Hence; these results are a testament to the novelty of the as-prepared ZnO@MC and Co3O4@MC composites, which would not only provide us high-

12

performance electrode materials for symmetric supercapacitors, but also open up an interesting new avenue for next-generation energy storage devices. Acknowledgement The authors extend their sincere appreciation to researchers supporting project number (RSP2019/55), King Saud University, Riyadh, Saudi Arabia for funding this research. References: Ahmad, T., Ubaidullah, M., Lone, I.H., Kumar, D., Al-Hartomy, O.A., 2017. Microemulsion synthesis, structural characterization and dielectric properties of Ba1-xPbxZrO3 (0.05≤x≤0.20)

nanoparticles.

Mater.

Res.

Bull.

89,

185–192.

doi:https://doi.org/10.1016/j.materresbull.2017.01.044 Ahmed, J., Ubaidullah, M., Ahmad, T., Alhokbany, N., Alshehri, S.M., 2019. Synthesis of Graphite Oxide/Cobalt Molybdenum Oxide Hybrid Nanosheets for Enhanced Electrochemical Performance in Supercapacitors and the Oxygen Evolution Reaction. ChemElectroChem 6, 2524–2530. doi:10.1002/celc.201900055 Al-Tuwirqi, R., Al-Ghamdi, A.A., Aal, N.A., Umar, A., Mahmoud, W.E., 2011. Facile synthesis and optical properties of Co3O4 nanostructures by the microwave route. Superlattices Microstruct. 49, 416–421. doi:https://doi.org/10.1016/j.spmi.2010.12.010 Augustyn, V., Simon, P., Dunn, B., 2014. Pseudocapacitive oxide materials for high-rate electrochemical

energy

storage.

Energy

Environ.

Sci.

7,

1597–1614.

doi:10.1039/C3EE44164D Cao, X., Zheng, B., Shi, W., Yang, J., Fan, Z., Luo, Z., Rui, X., Chen, B., Yan, Q., Zhang, H., 2015. Reduced Graphene Oxide-Wrapped MoO3 Composites Prepared by Using Metal– Organic Frameworks as Precursor for All-Solid-State Flexible Supercapacitors. Adv. Mater. 27, 4695–4701. doi:10.1002/adma.201501310 Chen, C., Wu, M.-K., Tao, K., Zhou, J.-J., Li, Y.-L., Han, X., Han, L., 2018. Formation of

13

bimetallic metal–organic framework nanosheets and their derived porous nickel–cobalt sulfides for supercapacitors. Dalt. Trans. 47, 5639–5645. doi:10.1039/C8DT00464A Chen, Y., Ni, D., Yang, X., Liu, C., Yin, J., Cai, K., 2018. Microwave-assisted synthesis of honeycomblike hierarchical spherical Zn-doped Ni-MOF as a high-performance batterytype

supercapacitor

electrode

material.

Electrochim.

Acta

278,

114–123.

doi:https://doi.org/10.1016/j.electacta.2018.05.024 Choi, K.M., Jeong, H.M., Park, J.H., Zhang, Y.-B., Kang, J.K., Yaghi, O.M., 2014. Supercapacitors of Nanocrystalline Metal–Organic Frameworks. ACS Nano 8, 7451– 7457. doi:10.1021/nn5027092 Claramunt, S., Varea, A., López-Díaz, D., Velázquez, M.M., Cornet, A., Cirera, A., 2015. The Importance of Interbands on the Interpretation of the Raman Spectrum of Graphene Oxide. J. Phys. Chem. C 119, 10123–10129. doi:10.1021/acs.jpcc.5b01590 Cuscó, R., Alarcón-Lladó, E., Ibáñez, J., Artús, L., Jiménez, J., Wang, B., Callahan, M.J., 2007. Temperature dependence of Raman scattering in $\mathrm{ZnO}$. Phys. Rev. B 75, 165202. doi:10.1103/PhysRevB.75.165202 Cychosz, K.A., Guillet-Nicolas, R., García-Martínez, J., Thommes, M., 2017. Recent advances in the textural characterization of hierarchically structured nanoporous materials. Chem. Soc. Rev. 46, 389–414. doi:10.1039/C6CS00391E Dai, E., Xu, J., Qiu, J., Liu, S., Chen, P., Liu, Y., 2017. Co@Carbon and Co3O4@Carbon nanocomposites derived from a single MOF for supercapacitors. Sci. Rep. 7, 12588. doi:10.1038/s41598-017-12733-5 de Bruin, H.J., Franklin, A.D., 1981. An impedance spectroscopy model for electron transfer reactions at an electrode/solid electrolyte interface. J. Electroanal. Chem. Interfacial Electrochem. 118, 405–418. doi:https://doi.org/10.1016/S0022-0728(81)80557-8 Deng, S., Chen, N., Deng, D., Li, Y., Xing, X., Wang, Y., 2015. Meso- and macroporous

14

coral-like

Co3O4

for

VOCs

gas

sensor.

Ceram.

Int.

41,

11004–11012.

doi:https://doi.org/10.1016/j.ceramint.2015.05.045 Deng, X., Li, J., Zhu, S., He, F., He, C., Liu, E., Shi, C., Li, Q., Zhao, N., 2017. Metal– organic frameworks-derived honeycomb-like Co3O4/three-dimensional graphene networks/Ni foam hybrid as a binder-free electrode for supercapacitors. J. Alloys Compd. 693, 16–24. doi:https://doi.org/10.1016/j.jallcom.2016.09.096 Du, W., Wang, X., Zhan, J., Sun, X., Kang, L., Jiang, F., Zhang, X., Shao, Q., Dong, M., Liu, H., Murugadoss, V., Guo, Z., 2019. Biological cell template synthesis of nitrogen-doped porous hollow carbon spheres/MnO2 composites for high-performance asymmetric supercapacitors.

Electrochim.

Acta

296,

907–915.

doi:https://doi.org/10.1016/j.electacta.2018.11.074 Ganesh, R.S., M, N., Mani, D.G.K., Ponnusamy, S., Tsuchiya, K., Muthamizhchelvan, C., Kawasaki, S., Hayakawa, Y., 2016. Influence of Al doping on the structural, morphological, optical, and gas sensing properties of ZnO nanorods. J. Alloys Compd. 698. doi:10.1016/j.jallcom.2016.12.187 Genta, M., Iwaya, T., Sasaki, M., Goto, M., Hirose, T., 2005. Depolymerization Mechanism of Poly(ethylene terephthalate) in Supercritical Methanol. Ind. Eng. Chem. Res. 44, 3894–3900. doi:10.1021/ie0488187 Guchhait, N., Ebata, T., Mikami, N., 2000. Vibrational Spectroscopy for Size-Selected Fluorene−(H2O)n=1,2 Clusters in Supersonic Jets. J. Phys. Chem. A 104, 11891–11896. doi:10.1021/jp002958m Handore, K., Bhavsar, S., Horne, A., Chhattise, P., Mohite, K., Ambekar, J., Pande, N., Chabukswar, V., 2014. Novel Green Route of Synthesis of ZnO Nanoparticles by Using Natural Biodegradable Polymer and Its Application as a Catalyst for Oxidation of Aldehydes. J. Macromol. Sci. Part A 51, 941–947. doi:10.1080/10601325.2014.967078

15

Hou, C., Tai, Z., Zhao, L., Zhai, Y., Hou, Y., Fan, Y., Dang, F., Wang, J., Liu, H., 2018. High performance MnO@C microcages with a hierarchical structure and tunable carbon shell for efficient and durable lithium storage. J. Mater. Chem. A 6, 9723–9736. doi:10.1039/C8TA02863J Hou, C., Wang, Jun, Du, W., Wang, Jianchuan, Du, Y., Liu, C., Zhang, J., Hou, H., Dang, F., Zhao, L., Guo, Z., 2019. One-pot synthesized molybdenum dioxide–molybdenum carbide heterostructures coupled with 3D holey carbon nanosheets for highly efficient and ultrastable cycling lithium-ion storage. J. Mater. Chem. A 7, 13460–13472. doi:10.1039/C9TA03551F Huang, Y.-T., Lai, Y.-L., Lin, C.-H., Wang, S.-L., 2011. Direct use of waste PET as unfailing source of organic reagents in the synthesis of intrinsic white/yellow luminescent nanoporous zincophosphates. Green Chem. 13, 2000–2003. doi:10.1039/C1GC15427C Kim, M., Cahill, J.F., Fei, H., Prather, K.A., Cohen, S.M., 2012. Postsynthetic Ligand and Cation Exchange in Robust Metal–Organic Frameworks. J. Am. Chem. Soc. 134, 18082–18088. doi:10.1021/ja3079219 Kirubasankar, B., Murugadoss, V., Lin, J., Ding, T., Dong, M., Liu, H., Zhang, J., Li, T., Wang, N., Guo, Z., Angaiah, S., 2018. In situ grown nickel selenide on graphene nanohybrid electrodes for high energy density asymmetric supercapacitors. Nanoscale 10, 20414–20425. doi:10.1039/C8NR06345A Koo, W.-T., Choi, S.-J., Jang, J.-S., Kim, I.-D., 2017. Metal-Organic Framework Templated Synthesis of Ultrasmall Catalyst Loaded ZnO/ZnCo2O4 Hollow Spheres for Enhanced Gas Sensing Properties. Sci. Rep. 7. doi:10.1038/srep45074 Kuriakose, S., Satpati, B., Mohapatra, S., 2014. Enhanced photocatalytic activity of Co doped ZnO nanodisks and nanorods prepared by a facile wet chemical method. Phys. Chem. Chem. Phys. 16, 12741–12749. doi:10.1039/C4CP01315H

16

Le, K., Wang, Z., Wang, F., Wang, Q., Shao, Q., Murugadoss, V., Wu, S., Liu, W., Liu, J., Gao, Q., Guo, Z., 2019. Sandwich-like NiCo layered double hydroxide/reduced graphene oxide nanocomposite cathodes for high energy density asymmetric supercapacitors. Dalt. Trans. 48, 5193–5202. doi:10.1039/C9DT00615J Li, B., Gu, P., Feng, Y., Zhang, G., Huang, K., Xue, H., Pang, H., 2017. Ultrathin Nickel– Cobalt Phosphate 2D Nanosheets for Electrochemical Energy Storage under Aqueous/Solid-State

Electrolyte.

Adv.

Funct.

Mater.

27,

1605784.

doi:10.1002/adfm.201605784 Li, B., Shi, Y., Huang, K., Zhao, M., Qiu, J., Xue, H., Pang, H., 2018. Cobalt-Doped Nickel Phosphite for High Performance of Electrochemical Energy Storage. Small 14, 1703811. doi:10.1002/smll.201703811 Liu, Yang, Liu, Z., Huang, D., Cheng, M., Zeng, G., Lai, C., Zhang, C., Zhou, C., Wang, W., Jiang, D., Wang, H., Shao, B., 2019. Metal or metal-containing nanoparticle@MOF nanocomposites as a promising type of photocatalyst. Coord. Chem. Rev. 388, 63–78. doi:https://doi.org/10.1016/j.ccr.2019.02.031 Liu, Yiyang, Zeng, Z., Sharma, R.K., Gbewonyo, S., Allado, K., Zhang, L., Wei, J., 2019. A bi-functional configuration for a metal-oxide film supercapacitor. J. Power Sources 409, 1–5. doi:https://doi.org/10.1016/j.jpowsour.2018.10.084 Liu, Y., Zhu, G., Ge, B., Zhou, H., Yuan, A., Shen, X., 2012. Concave Co3O4 octahedral mesocrystal: polymer-mediated synthesis and sensing properties. CrystEngComm 14, 6264–6270. doi:10.1039/C2CE25788B Mai, L., Tian, X., Xu, X., Chang, L., Xu, L., 2014. Nanowire Electrodes for Electrochemical Energy Storage Devices. Chem. Rev. 114, 11828–11862. doi:10.1021/cr500177a Miao, J., Xiao, F.-X., Yang, H. Bin, Khoo, S.Y., Chen, J., Fan, Z., Hsu, Y.-Y., Chen, H.M., Zhang, H., Liu, B., 2015. Hierarchical Ni-Mo-S nanosheets on carbon fiber cloth: A

17

flexible electrode for efficient hydrogen generation in neutral electrolyte. Sci. Adv. 1. doi:10.1126/sciadv.1500259 Naushad, M., Ahamad, T., Sharma, G., Ala’a, H., Albadarin, A.B., Alam, M.M., ALOthman, Z.A., Alshehri, S.M., Ghfar, A.A., 2016. Synthesis and characterization of a new starch/SnO 2 nanocomposite for efficient adsorption of toxic Hg 2+ metal ion. Chem. Eng. J. 300, 306–316. doi:http://dx.doi.org/10.1016/j.cej.2016.04.084 Naushad, M., Sharma, G., Alothman, Z.A., 2019. Photodegradation of toxic dye using Gum Arabic-crosslinked-poly(acrylamide)/Ni(OH)2/FeOOH nanocomposites hydrogel. J. Clean. Prod. 241, 118263. doi:https://doi.org/10.1016/j.jclepro.2019.118263 Pang, H., Li, B., Zhao, Q., Lai, W.-Y., Huang, W., 2016. Cu superstructures hydrothermally reduced by leaves and derived Cu–Co3O4 hybrids for flexible solid-state electrochemical energy storage devices. J. Mater. Chem. A 4, 4840–4847. doi:10.1039/C6TA00123H Pang, M.J., Jiang, S., Long, G.H., Ji, Y., Han, W., Wang, B., Liu, X.L., Xi, Y.L., Xu, F.Z., Wei, G.D., 2016. Mesoporous NiCo2O4 nanospheres with a high specific surface area as electrode materials for high-performance supercapacitors. RSC Adv. 6, 67839–67848. doi:10.1039/C6RA14099H Qiu, Y., Li, X., Bai, M., Wang, H., Xue, D., Wang, W., Cheng, J., 2017. Pseudocapacitive behaviors of mesoporous nickel–cobalt oxide nanoplate electrodes in different electrolyte systems. New J. Chem. 41, 2124–2130. doi:10.1039/C6NJ03688K Rahmanifar, M.S., Hesari, H., Noori, A., Masoomi, M.Y., Morsali, A., Mousavi, M.F., 2018. A dual Ni/Co-MOF-reduced graphene oxide nanocomposite as a high performance supercapacitor

electrode

material.

Electrochim.

Acta

275,

76–86.

doi:https://doi.org/10.1016/j.electacta.2018.04.130 Shinde, N.M., Xia, Q.X., Yun, J.M., Shinde, P. V, Shaikh, S.M., Sahoo, R.K., Mathur, S.,

18

Mane, R.S., Kim, K.H., 2019. Ultra-rapid chemical synthesis of mesoporous Bi2O3 micro-sponge-balls for supercapattery applications. Electrochim. Acta 296, 308–316. doi:https://doi.org/10.1016/j.electacta.2018.11.044 Siracusa, V., Rocculi, P., Romani, S., Rosa, M.D., 2008. Biodegradable polymers for food packaging:

a

review.

Trends

Food

Sci.

Technol.

19,

634–643.

doi:https://doi.org/10.1016/j.tifs.2008.07.003 Tan, Y.H., Davis, J.A., Fujikawa, K., Ganesh, N.V., Demchenko, A. V, Stine, K.J., 2012. Surface area and pore size characteristics of nanoporous gold subjected to thermal, mechanical, or surface modification studied using gas adsorption isotherms, cyclic voltammetry, thermogravimetric analysis, and scanning electron microscopy. J. Mater. Chem. 22, 6733–6745. doi:10.1039/C2JM16633J Thi, T.V., Rai, A.K., Gim, J., Kim, J., 2015. High performance of Co-doped NiO nanoparticle anode material for rechargeable lithium ion batteries. J. Power Sources 292, 23–30. doi:https://doi.org/10.1016/j.jpowsour.2015.05.029 Wang, L.J., Deng, H., Furukawa, H., Gándara, F., Cordova, K.E., Peri, D., Yaghi, O.M., 2014. Synthesis and Characterization of Metal–Organic Framework-74 Containing 2, 4, 6, 8, and 10 Different Metals. Inorg. Chem. 53, 5881–5883. doi:10.1021/ic500434a Xu, J., Li, K., Deng, H., Lv, S., Fang, P., Liu, H., Shao, Q., Guo, Z., 2019. Preparation of MCA-SiO2 and Its Flame Retardant Effects on Glass Fiber Reinforced Polypropylene. Fibers Polym. 20, 120–128. doi:10.1007/s12221-019-8284-6 Xu, Y., Bai, H., Lu, G., Li, C., Shi, G., 2008. Flexible Graphene Films via the Filtration of Water-Soluble Noncovalent Functionalized Graphene Sheets. J. Am. Chem. Soc. 130, 5856–5857. doi:10.1021/ja800745y Yang, J., Yang, W., Wang, X., Dong, M., Liu, H., Wujcik, E.K., Shao, Q., Wu, S., Ding, T., Guo, Z., 2019. Synergistically Toughening Polyoxymethylene by Methyl Methacrylate–

19

Butadiene–Styrene Copolymer and Thermoplastic Polyurethane. Macromol. Chem. Phys. 220, 1800567. doi:10.1002/macp.201800567 Yang, J., Zheng, C., Xiong, P., Li, Y., Wei, M., 2014. Zn-doped Ni-MOF material with a high

supercapacitive

performance.

J.

Mater.

Chem.

A

2,

19005–19010.

doi:10.1039/C4TA04346D Yang, L., Shi, M., Jiang, J., Liu, Y., Yan, C., Liu, H., Guo, Z., 2019. Heterogeneous interface induced formation of balsam pear-like PPy for high performance supercapacitors. Mater. Lett. 244, 27–30. doi:https://doi.org/10.1016/j.matlet.2019.02.064 Yi, H., Wang, H., Jing, Y., Peng, T., Wang, X., 2015. Asymmetric supercapacitors based on carbon nanotubes@NiO ultrathin nanosheets core-shell composites and MOF-derived porous carbon polyhedrons with super-long cycle life. J. Power Sources 285. doi:10.1016/j.jpowsour.2015.03.106 Yi, H., Yan, M., Huang, D., Zeng, G., Lai, C., Li, M., Huo, X., Qin, L., Liu, S., Liu, X., Li, B., Wang, H., Shen, M., Fu, Y., Guo, X., 2019. Synergistic effect of artificial enzyme and 2D nano-structured Bi2WO6 for eco-friendly and efficient biomimetic photocatalysis.

Appl.

Catal.

B

Environ.

250,

52–62.

doi:https://doi.org/10.1016/j.apcatb.2019.03.008 Zhu, G., Cui, X., Zhang, Y., Chen, S., Dong, M., Liu, H., Shao, Q., Ding, T., Wu, S., Guo, Z., 2019. Poly (vinyl butyral)/Graphene oxide/poly (methylhydrosiloxane) nanocomposite coating for improved aluminum alloy anticorrosion. Polymer (Guildf). 172, 415–422. doi:https://doi.org/10.1016/j.polymer.2019.03.056 Zhu, G., Wen, H., Ma, M., Wang, W., Yang, L., Wang, L., Shi, X., Cheng, X., Sun, X., Yao, Y., 2018. A self-supported hierarchical Co-MOF as a supercapacitor electrode with ultrahigh areal capacitance and excellent rate performance. Chem. Commun. 54, 10499– 10502. doi:10.1039/C8CC03669A

20

Figure Caption Fig. 1. FE-SEM micrographs of (a) MOF-5, (b) Co-MOF, and after carbonization, (c) ZnO@MC and (d) Co3O4@MC composites. Fig. 2. HR-TEM micrographs of (a) ZnO@MC and (b) Co3O4@MC; HR-TEM images of (c) ZnO@MC and (d) Co3O4@MC; SAED patterns of (e) ZnO@MC and (f) Co3O4@MC composites. Fig. 3.Powder X-ray diffraction pattern of (a) ZnO@MC and (b) Co3O4@MC composites. (c) FTIR spectra of ZnO@MC and Co3O4@MC composites. The symbols denoted in X-ray diffraction pattern are C=*, Co3O4 =# and ZnO=@ and (d) Raman spectrum of ZnO@MC and Co3O4@MC composites. Fig. 4. (a) XPS Spectrum of ZnO@MC and Co3O4@MC composites, (b) Zn 2p, (c) Co 2p and (d) O1s Fig. 5. Brunauer–Emmett–Teller surface area and pore size distribution of the ZnO@MC and Co3O4@MC: (a) BET plot overlay, (b) N2 adsorption–desorption isotherms overlay, (c) BJH pore size distribution overlay, and (d) DA pore size distribution. Fig. 6. (a) CV of ZnO@MC composite at different scan rates from 5 to 200 mVs−1. (b) Specific capacitance graph of ZnO@MC composite calculated from CV at different scan rates. Fig. 7. (a) CV at different scan rates from 5 to 200 mVs−1, (b) specific capacitance graph calculated from CV at different scan rates, and (c) GCD curves evaluated at different current densities from 6 to 15 Ag−1, of Co3O4@MC composite electrode. Fig. 8. (a) Cyclic stability (100 segments) at 50 mVs−1, (b) GCD stability (5,000 segments) at 6 Ag−1 current density, and (c) Nyquist plots at the high-frequency region in 6 M KOH, of the Co3O4@MC composite electrode.

21

Fig. 9. (a) Ragone plot indicating energy density vs. power density for the Co3O4@MC composite with inset demonstrating pencil-type symmetric supercapacitor-inspired digital image of glowing LED, and (b) Ragone plot for different energy devices.

22

Table Caption Table 1. Elemental analysis (CHN) and ICP-OES (wt. %) of ZnO@MC and Co3O4@MC composites. Table 2. Physisorption properties of ZnO@MC and Co3O4@MC composites Table 3. Specific Capacitance at different sweep rates and current densities for ZnO@MC and Co3O4@MC composites in two-electrode system. Table 4. Characteristics of different metal-oxide-based supercapacitors and this work

23

Table 1 Weight Percentage (%) Sample

C

H

O

Co

Zn

ZnO@MC

39.0

2.34

24.5

-

34.0

Co3O4@MC

39.1

2.07

22.7

36.0

-

Table 2 Compound

SBET

BJH Pore Size

DA Pore Size

Pore

m2g−1

Distribution (Å)

Distribution (Å)

Volume cm3/g

ZnO@MC

2183

32

38

0.9042

Co3O4@MC

2503

43

39

1.1344

1

Table 3 Specific Capacitance (F g-1)

Co3O4 @MC

ZnO @MC

Samples

Sweep Rate (mVs−1)

Current Density (Ag−1)

5

10

25

50

100

200

6

9

10

12

15

97

88

79

75

69

57

-

-

-

-

-

18

163

114

89

68

51

136

81

80

77

54

0

Table 4 Material

Cs (F/g)

NiCoFe2O4

50

ED (Wh kg−1) 4.7

PD (W kg−1) 1426.2

Ref. (Bhujun et al., 2017)

CuCoFe2O

76.9

7.9

1711.9

(Bhujun et al., 2016)

Mn0.5Zn0.5Fe2O4

783

15.8

899.7

(Ismail et al., 2018)

MnCo2O4

290

10.0

5000.2

(Sahoo et al., 2015)

NiMoO4·xH2O

-

34.4

165

(Liu et al., 2013)

NF@NCP

958.3

43.7

516.2

(Al-Farraj et al., 2018)

ZnCo2O4

193

49.5

222.7

(Niu et al., 2015)

Co3O4@MC

180

68

2

149.1

This work

Fig. 1

Fig. 2

20

30

40

50

60

70

80

10

20

30

2 Theta (deg.) ZnO@MC Co3O4@MC

(c)

0.4 0.2

C=O

C-O

0.6

Metal Oxide

1.0 0.8

50

60

(440)

(511

(400)

40

70

80

2 Theta (deg.)

3600 3000 2400 1800 1200 -1

Intensity (arb.units)

1.2

O-H

Transmittance (%)

1.4

JCPDS: 43-1003

(220) (311) (222)

(Carbon)

(b)

(111)

(Carbon)

Intensity (arb.units)

(202)

(103) (112) (201)

(002)

(Carbon) (100) (002) (101)

JCPDS: 36-1451

(110)

Intensity (arb.units)

(a)

600

Wavenumber (cm )

ZnO@MC Co3O4@MC ID/IG=0.82

(d)

ID/IG=0.84

E A1g 1

500

1000

Ramman Fig. 3

1500 2000 -1 shift (cm )

Intensity (arb.units)

Intensity (arb.units)

800 600

800

400 0

O1s

C1s

Zn3s Zn3p Zn3d

Co3s Co3p

Intensity (arb.units)

1040

Zn 2p1/2

1030

532

Zn 2p3/2 1010

536 538

ZnO@MC Co3O4@MC

1020

534

Binding Energy (eV)

(b)

1050

(d)

530

Binding energy (eV)

O1s

ZnLMM2 ZnLMM3 O1s ZnKKL ZnMML1 C1s

200

780

528

O1s

Co2p CoLMM

ZnO@MC Co3O4@MC

790

Fig. 4

Intensity (arb. units)

OKLL Co2s

(a)

1000

810

Co2p3/2

Binding Energy (eV)

(c)

820

Binding Energy (eV)

Co2p1/2

Zn 2p1/2 Zn 2p3/2 OKLL

(a)

0.0020

3

ZnO@MC Co3O4@MC

Vol. adsorbed (cm /g)

1/[Q(P0/P-1)]

0.0024

0.0016 0.0012 0.0008 0.0004 0.05

0.10

0.15

0.20

0.25

0.30

1000 800 600 400 200 0.0

0.5

0.020

dV (r)(cc/Å/g)

dV(d) ( cm 3 /Å /g )

ZnO@MC Co3O4@MC

(c)

0.4 0.3 0.2 0.1 0.0 0

30

60

90

120

0.2

0.4

0.6

0.8

1.0

Relative Pressure (P/P0)

Relative Pressure (P/P0) 0.6

ZnO@MC Co3O4@MC

(b)

150

Pore Diameter (Å)

ZnO@MC Co3O4@MC

(d)

0.016 0.012 0.008 0.004 0.000 0

10

20

30

40

50

Pore Size (Å)

Fig. 5

60

70

2.1 1.4 0.7

200 mVs-1 100 mVs-1 50 mVs-1 25 mVs-1 10 mVs-1 5mVs-1

Sp. Capacitance (F g-1)

Current Density (A g-1)

2.8

(a)

0.0 -0.7 -1.4 -0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

Potential (V)

120

(b)

100 80 60 40 20 0

0

30

60

90 120 150 180 210

Scan Rate (mV/s) Fig. 6

2.1 1.4

200 mVs-1 100 mVs-1 50 mVs-1 25 mVs-1 10 mVs-1 5 mVs-1

Sp. Capacitance (F g-1)

2.8

(a)

0.7 0.0 -0.7 -1.4 -2.1 -0.6 -0.4

-0.2

0.0

0.2

0.4

250 200

(b)

150 100 50

0.6

0

0.6

0

50

100

(c)

15 A g-1 12 A g-1 10 A g-1 9 A g-1 6 A g-1

0.4 0.2 0.0 -0.2 -0.4 -0.6

0

150

Scan Rate (mV/s)

Potential (V)

Potential (V)

Current Density (Ag-1)

3.5

150

300

450

Time (s)

Fig. 7

600

750

200

0.8

(a)

(b)

Potential (V)

0.6 0.4 0.2 0.0 -0.2 -0.4 -0.5

0.0

0.5

1.0

-0.6

0

4000

Potential (V) 10

(c)

8 6

1.0

4

0.8 0.6 0.4 0.2

2

0.0 4.0

0

4

8000

Time (s)

Z '' (O hm )

-1.0

Z'' (Ohm)

Current Density (A g-1)

5 4 3 2 1 0 -1 -2 -3 -4

6

8 Z' (Ohm)

Fig. 8

4.5

5.0

Z' (Ohm)

10

5.5

6.0

12

12000

Fig. 9

Highlights •

Highly porous composites were fabricated using waste PET bottles

•

The utilization of PET bottles takes a new route in solid waste management

•

Electrochemical properties of composites were studied in two electrode system

•

ZnO@MC and Co3O4@MC showed high specific surface area of ~2183 and ~2503 m2 g-1

•

Supercapacitor electrodes with high specific energy & specific power were developed

The authors declare that there is no conflict of interest regarding the publication of this article