Synthesis of hierarchical Mn3O4 nanowires on reduced graphene oxide nanoarchitecture as effective pseudocapacitive electrodes for capacitive desalination application

Journal Pre-proof Synthesis of hierarchical Mn3O4 nanowires on reduced graphene oxide nanoarchitecture as effective pseudocapacitive electrodes for capacitive desalination application G. Bharath, Naman Arora, Abdul Hai, Fawzi Banat, Dennyson Savariraj, Hanifa Taher, R.V. Mangalaraja PII:

S0013-4686(20)30059-1

DOI:

https://doi.org/10.1016/j.electacta.2020.135668

Reference:

EA 135668

To appear in:

Electrochimica Acta

Received Date: 26 November 2019 Revised Date:

5 January 2020

Accepted Date: 7 January 2020

Please cite this article as: G. Bharath, N. Arora, A. Hai, F. Banat, D. Savariraj, H. Taher, R.V. Mangalaraja, Synthesis of hierarchical Mn3O4 nanowires on reduced graphene oxide nanoarchitecture as effective pseudocapacitive electrodes for capacitive desalination application, Electrochimica Acta (2020), doi: https://doi.org/10.1016/j.electacta.2020.135668. 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. © 2020 Published by Elsevier Ltd.

Credit Author Statement

G. Bharath: Conceptualization, Methodology, Validation, Formal analysis, Writing – original draft. Naman Arora: Visualization, Data curation. Abdul Hai: Data curation, Investigation. Fawzi Banat: Validation, Supervision, Writing – review and editing, Project administration. Dennyson Savariraj: Data curation, Investigation. Hanifa Taher: Resources, Writing – review and editing. R.V. Mangalaraja: Writing – review and editing.

Graphical abstract GA text: Hydrothermal method was used for the preparation of high crystalline wire-like Mn3O4 and reduced graphene oxide (Mn3O4/RGO) nanohybrids toward development of effective pseudocapacitive electrodes for desalination application.

Synthesis of hierarchical Mn3O4 nanowires on reduced graphene oxide nanoarchitecture as effective pseudocapacitive electrodes for capacitive desalination application G. Bharath,a* Naman Arora,a Abdul Hai,a Fawzi Banat, a* Dennyson Savariraj,a,b Hanifa Taher,a R.V. Mangalaraja,b a

Department of Chemical Engineering, Khalifa University, P.O. Box 127788, Abu Dhabi, United

Arab Emirates. b

Department of Materials Engineering, Faculty of Engineering, University of Concepción,

Concepción, Chile Corresponding Authors: E-Mail: Prof. Fawzi Banat, E-mail: [email protected] and [email protected]

Abstract We present a hierarchical network architecture fabrication of pseudocapacitive Mn3O4 nanowires immobilized on reduced graphene oxide (Mn3O4/RGO) hybrids for effective capacitive deionization (CDI). A hydrothermal synthesis process was employed for constructing Mn3O4/RGO nanoarchitecture with hierarchical pores to ease the interaction with salt ions. Physico-chemical analysis verified the Mn3O4 nanowires with few microns sized length and diameter of 20-30 nm were evenly immobilized on the surfaces of the RGO. With this tailored nanoarchitecture, the Mn3O4/RGO based electrode shows ideal pseudocapacitive behavior with a higher capacitance of (Cs) of 437 F g-1, the energy density of 41.12 Wh kg−1 and power density of 400 W kg−1 at 1 A g-1 in 1 M NaCl solution. Benefiting from the fascinating electrochemical 1

features, the Mn3O4/RGO nanoarchitecture constructed CDI electrode exhibited high electrosorption capacity (SAC) of 34.5 mg g-1 at 1.2 V with a high salt adsorption rate (ASAR) of 1.15 mg g-1 min-1 in 1000 mg L-1 NaCl solution. The much improved SAC, ASAR, and recyclability could be attributed to the distinctive pseudocapacitive nanoarchitecture, which improves the sodiation/desodiation. The present investigation indicates that the Mn3O4/RGO nanoarchitecture is a capable CDI electrode material for desalination application. Keywords: Pseudocapacitance, Nanowires, Capacitive deionization, Faradic reactions, Cycling stability

1. Introduction Capacitive deionization (CDI) is a capable technique for deionization of the saltwater to produce fresh potable drinking water.[1, 2] In CDI, an electrical field is supplied to the negatively and positively charged electrodes, which leads to effective separation of salt ions including Na+ and Cl- ions. Conversely, the adsorbed salt ions are released into the water stream by changing the potential of the electrodes. In principle, an electrical double-layer (EDL) capacitance or pseudocapacitance (Faradaic reactions) mechanism is involved in the electrochemical storage of ions during the CDI process.[2-5] Recently, different allotropes of the carbon and carbon-based nanocomposites are the general EDL electrode materials applied in the CDI device.[1, 6] However, the current CDI electrode materials displayed a maximum SAC of 10-15 mg g-1, poor cycling stability, and low charge efficiency, which are not sufficient for the seawater desalination.[6-8] Recently, pseudocapacitive redox metal oxides coupled with EDLC carbon materials could potentially enhance the electronic conductivity and salt adsorption capacity.[2, 9] Pseudocapacitance is the storage of the charges due to redox reactions (oxidation-

2

reduction) arising at the electrolyte-electrode interface than electrosorption of salt ions at the EDLC.[10-13] Accordingly, several innovative efforts have been expended to designing the nanostructured hybrid materials and constructing the CDI electrodes for high-performance desalination.[14, 15] In this way, Faradaic nanohybrids materials of MXenes,[16] Mo1.33C-MXene,[17] Na2FeP2O7,[18]

MnO2/Ag,[19]

MnO2/activated

carbon,[20]

MnO2/CNT−chitosan,[21]

MnFe2O4/Porous rGO electrodes,[22] GA/TiO2,[23] and RuO2-activated carbon[24] based CDI cells possesses a significant salt removal capacity of 13, 15, 30.2, 17.8, 9.3, 6.01, 8.9, 24.2, and 11.26 mg g−1, respectively. Despite these promising electrodes, the various oxidation states of manganese including α,β,γ-MnO2, Mn2O3, and Mn3O4 based redox electrodes could be potentially used in lithium (LIBs) and sodium ions (SIBs) batteries.[25-29] The high theoretical capacities (936 mAh g−1), low voltage potential with a charge voltage of 1.25 V vs Li/Li+ and discharge voltage of ∼0.5 V vs Li/Li+, and long-term cyclic stability of Mn3O4 used as an effective electrode in LIBs and supercapacitors.[26, 30-32] Thus, it is reasonable to believe that the Mn3O4 redox electrode material is a suitable and potential candidate for CDI application with enhanced desalination performances. Recent advances have focused on the preparation of carbons with Mn3O4 nanohybrids, which improve the specific capacitance and their available redox sites to obtain high desalination performances.[30, 31, 33] Fortunately, graphene and Mn3O4 framework possesses a good flexibility, high SSA, and chemical stability with higher electronic conductivity.[27, 34] Enthused by this strategy, various morphology of Mn3O4 including nanoparticles, hierarchical network, nanorods and nanowires on graphene hybrids have been assembled to increase their electrochemical performances.[35-37] Good advancement has been made in the past decade, specifically wire-like Mn3O4 on graphene and CNT based

3

nanocomposites to improve the electrochemical kinetics behavior leading to fabricate greater electrode materials for oxygen reduction reaction (ORR) and improved the performances of Mg/Air batteries.[37-39] On the basis of these characteristics, wire-like Mn3O4 on graphene based nanohybrids possesses a fast Faradaic reactions and double-layer (EDL) charging capacity which is beneficial for the fabrication of cost effective CDI device for desalting seawater. In this context, we propose new Mn3O4 nanowires fabricated on reduced graphene oxide (wire-like Mn3O4/RGO nanoarchitecture) for desalting saltwater by CDI process. The wire-like Mn3O4 and Mn3O4/RGO nanocomposites were obtained via a hydrothermal method. Electrochemical studies (EIS and CV) confirmed the suitability of the as-obtained wire-like Mn3O4/RGO based electrodes in CDI application. The SAC, ASAR, and energy consumption were determined through batch-mode CDI experiments and theoretical investigations.

2. Experimental 2.1 Materials The reagents used in this study included manganese(II) nitrate hydrate (98%), Sodium hydroxide (NaOH, 98%), graphite flakes, polyvinylpyrrolidone (PVDF), N-Methyl-3pyridinamine (NMP), sodium chloride (NaCl, 99.5%), ethanol (CH3CH2OH), and acetone (CH3COCH3) received as without purification from Merck (Sigma-Aldrich). 2.2 Preparation of the wire-like Mn3O4 and Mn3O4/RGO nanocomposites In a typical wire-like Mn3O4 nanostructure preparation, a 0.5 M of manganese(II) nitrate was dissolved into 35 mL of double distilled water under magnetic stirring for 10 min. A 0.1 M NaOH solution was dropwise added into the above solution and stirred for 30 min. The obtained suspension was transferred into 75 mL capacity of the hydrothermal vessel and placed in an oven at 180 oC for 12 h. Finally, a brown color precipitate was separated and washed with double 4

distilled water and dried in an oven for overnight. Meanwhile, a 0.5 M of manganese(II) nitrate was dissolved into 40 mL of GO solution with GO concentration of 2g mL-1. Afterward, 0.1 M aqueous solution of NaOH dropwise added into Mn-GO suspension with magnetically stirring for 30 min. The homogeneous suspension was shifted into the hydrothermal vessel and applied temperature up to 180 oC for 12 h. The final residue was collected by centrifugation (rotating speed 6000 rpm) and washed with deionized water. Finally, the brownish wire-like Mn3O4/RGO sample was dried at 80 °C in a hot air oven for 12h. 2.3 Characterizations The elemental composition and surface morphology of the Mn3O4 nanowires and Mn3O4/RGO nanohybrids were analyzed using scanning electron microscopy with EDS (JEOL JSM-7610F FEG-SEM) and transmission electron microscopy (TEM, TITAN TEM 300KV). XRD patterns of the samples were obtained from the powder X‐ray diffractometer (XRD PANalytical Empyrean) at 40 kV and 40 mA. The FTIR spectra of the samples were obtained from the transmission module of a Bruker Vertex 80v FT-IR (wavenumber range-450 to 4500 cm-1) and the Raman spectrum of the samples was obtained on the Witec Alpha 300 RAS with a laser wavelength of 532 nm. The X‐ray photoelectron spectroscopy (XPS, ESCALab MKII) using excitation source of non‐monochromatized AlKα radiation was used to estimate the chemical composition of the samples. The N2 adsorption isotherms were obtained on a Quantachrome Autosorb 06 surface area analyzer at –196 °C with the samples degassed before the adsorption measurements at 200 °C for 4 h. A NETZSCH high-temperature thermogravimetric analysis (TGA) was used to analysis the thermal properties and composition ration of the samples. 2.4 Electrochemical studies 5

Electrochemical

measurements

of

the

obtained

wire-like

Mn3O4 and

Mn3O4/RGO

nanocomposites were performed in a three‐electrode VMP-300 (BioLogic Instruments, US) at ambient conditions. A platinum sheet (1×1 cm2) and a saturated calomel electrode (SCE, 3 M KCl) used as counter and reference electrodes, respectively. The working electrode was prepared by mixing 10 % of carbon black, 10 wt % PVDF and 80 wt % active materials were in NMP. A portion of the slurry was drop cast onto a carbon-coated sheet and dried for 100 oC for 6 h under ambient conditions. The CV performed at a scan rate of 10 to 100 mV s-1 with a potential window of 0 to 0.8 in 1.0 M aqueous NaCl used as the electrolyte. Charge transport and internal resistance of the obtained RGO, wire-like Mn3O4 and Mn3O4/RGO were determined via EIS analysis with the frequency range 100 kHz−0.01 Hz in1.0 M aqueous NaCl solution. 2.5 CDI studies In typical experiments, the CDI electrodes of RGO, wire-like Mn3O4 and Mn3O4/RGO nanocomposites were fabricated via the same procedure stated in the electrode preparation. The total size and mass of the fabricated RGO, wire-like Mn3O4 and Mn3O4/RGO nanocomposites based electrodes were 8 cm × 4 cm × 100 µm, and 250 mg, respectively. A nylon spacer with a thickness of 2 mm was used for the separation of the positive and negative electrodes in the CDI cell. A peristaltic pump was used for the continuous supply of the 70 mL of NaCl solution (concentration-1000 mg L-1) to the CDI cell at a flow rate of 5 mL min−1. The CDI cell was functioned by the applied voltage of 1.2 V. A online conductivity meter (Suntex SC-2300) used to monitor the changes in the solution concentrations. Moreover, the CDI experiments were performed with various applied voltages of 0.8 to 2.0 V and different concentrations of NaCl from 100 to 1500 mg L-1 for optimization purposes. The maximum electrosorption capacity

6

(SAC) of the obtained electrodes was estimated by the amount of salt adsorbed from initial concentration per unit mass of the active electrode materials and the Eqn as follows SAC  =

    

(1)

Where V is the solution volume, Ci and Cf are the initial and final equilibrated NaCl concentrations, respectively, and m is the total mass of the active electrode materials. Further, the electrosorption rate (ASAR) can be evaluated through the Ragone plot  =



(2)



where SAC (qe, mg g-1) is the NaCl removal capacity at t min and the charge efficiency (Λ) can be described as follows: Λ =

 × ƹ



(3)

where ƹ (C g−1) is the total charge estimated from the integrated value of total current and F is the (96 485 C mol−1) Faraday constant.

3. Results and Discussion 3.1 Formation mechanism and morphological analysis of the wire-like Mn3O4 and Mn3O4/RGO nanostructures The schematic of the synthesis wire-like Mn3O4 nanostructure is illustrated in Fig.1(a). Aqueous manganese nitrate releases the Mn2+ ions in the suspension which can further be transferred into manganese hydroxide (MnOOH) precipitate under alkaline conditions. During the hydrothermal step, the manganese hydroxide was converted into manganese oxide (MnO) nuclei toward the formation of Mn3O4 nanostructures. Specifically, under higher temperatures, the several Mn3O4 nuclei are attached together and prominent to the formation of wire-like Mn3O4 nanostructures.[40, 41] The following reaction mechanism can be described as 7

  + "#  + " → ""# ∆

""# → & "' + # " + " ↑

(4)

(5)

The surface morphology and crystallinity of the obtained wire-like Mn3O4 nanostructure was studied via TEM with SAED pattern and SEM and as depicted in Fig.1(b-g). The SEM with different magnification images in Fig.1(b-d) showed a large quantity of wire-like Mn3O4 nanostructure formed under hydrothermal conditions. Fig.1(d) showed a magnified SEM image of the formation of agglomeration free Mn3O4 nanowires under hydrothermal conditions. Further, TEM and SAED pattern was used to recognize the complete microstructures and crystalline structure of the wire-like Mn3O4 nanostructure as depicted in Fig.1(e-g). A lower magnification TEM image in Fig.1(e) clearly establishes the well-dispersed individual Mn3O4 nanowires. The obtained Mn3O4 nanowires have few micrometers in length with the diameter in the ranges of 20-30 nm, respectively which estimated from high magnification TEM image as shown in Fig.1(c). The SAED image in Fig.1(f) illustrates the crystalline nature of the Mn3O4 (Hausmannite) phase which exhibits the diffraction rings of (112), (211), (220), (105), and (321) planes corresponding to the crystalline planes of Mn3O4.

8

Fig.1 (a) schematic illustration for the formation mechanism of wire-like Mn3O4 nanostructures, (b-d) FE-SEM with different magnification images, (e and f) TEM with different magnification images and (g) SAED pattern of wire-like Mn3O4 nanostructures

Further, a high crystalline wire-like Mn3O4 nanostructures were regularly grown on the surfaces of RGO as schematically depicted in Fig.2(a). The oxygen moieties on the GO play a major role in offering the anchoring sites for the manganese nuclei. Scientifically, the atomic distance of 0.2480 nm spacing of the (211) planes for Mn3O4 is very close to the 0.2464 nm

9

atomic distance of C-C for graphene resulting a well crystalline Mn3O4 formed on the surfaces of RGO nanosheets.[12] In a typical formation process, the Mn2+ ions electrostatically interact with the GO sheets due to electrostatic interaction of the oxygen moieties on the GO nanosheets. In the alkaline medium, the absorbed Mn2+ ions are converted into MnOOH as appearing on the surfaces and edges of the GO nanosheets. During the hydrothermal treatment, the intermediate phases of the MnOOH were completely transformed into Mn3O4 nuclei while GO was converted into RGO causing the Mn3O4 nanowires to be homogeneously immobilized on the RGO sheets.[42] The following Eqns (6 and 7) can be described for the formation mechanism of Mn3O4 nanowires on RGO nanosheets.   /*" + "#  + " → ""#/*" ∆

""#/*" → & "' /*" + # " + " ↑

(6)

(7)

The morphology and elemental composition of the GO and wire-like Mn3O4/RGO nanostructures are displayed in Fig.2. The microstructure of the prepared GO exhibits a smooth and transparent thin layer as observed in the various magnification SEM images (Fig.2(b and c)). In the case of the nanocomposites, wire-like Mn3O4 nanostructures tended to homogeneously assemble on the RGO sheets surfaces due to negatively charged oxygen moieties on the RGO sheets as clearly presented in Fig.2(d and e). In the enlarged SEM images of wire-like Mn3O4/RGO nanostructures, a well crystalline and uniform sized Mn3O4 nanowires were formed on the RGO nanosheets as presented in Fig.2(f and g). As perceived, the Mn3O4 nanowires are few micron-sized lengths and the diameter of the specific nanowires is 20-30 nm, respectively. The coexistence of the dual phases of Mn3O4 and RGO and their elemental compositions in the nanocomposites were estimated via EDS analysis of the nanocomposites. Fig.2 (g) clearly demonstrates the elements of Carbon (C), Oxygen (O) and manganese (Mn) are present in the 10

Mn3O4/RGO nanocomposites with no other impurities testifying the purity of the prepared sample. This unique wire-like Mn3O4/RGO nanoarchitecture can provide abundant sites for the salt ions transport, permitting the effective contact between the Mn3O4/RGO based electrodes and electrolyte.

Fig.2 (a) schematic diagram of the formation of wire-like Mn3O4/RGO nanocomposites, (b and c) FE-SEM images of GO, (d-f) FE-SEM images of Mn3O4/RGO nanocomposites and (g) EDS of Mn3O4/RGO nanocomposites.

11

3.3 Physico-chemical properties of wire-like Mn3O4 and Mn3O4/RGO nanocomposites The crystalline structure of the wire-like Mn3O4 and Mn3O4/RGO nanocomposites was characterized using XRD as displayed in Fig.3(a). The RGO exhibit a broad with a low intense peak at 2θ around 24° originates from the (002) plane of the graphene sheets. The XRD spectrum of the wire-like Mn3O4 and Mn3O4/RGO nanocomposites showed the diffraction peaks at 18.1°, 29.2°, 33.9°, 36.2°, 38.1°, 44.4°, 51.2°, 58.4°, 60.2°, and 64.7° positions can be well-assigned to (101), (112), (103), (211), (004), (220), (105), (321), (224), and (400) planes of the hausmannite Mn3O4 with tetragonal structure.[12] Specifically, a broadened and sharp diffraction peak attributed at 36.2° for (211) diffraction, thus suggesting nano-sized grain sizes of Mn3O4 formed on the surfaces of RGO sheets. Other diffraction peaks are also exhibited only hausmannite Mn3O4 with tetragonal structure with lattice parameters of a = 5.75 Å and c = 9.42 Å and space group I41/amd (No. 141) corresponding to that of ICDD-JCPDS Card No.080-0382.[12, 43] Moreover, the average crystalline sizes (D, nm) of the wire-like Mn3O4 and Mn3O4/RGO nanocomposites can be estimated to be 18 and 14 nm, respectively using Sherrer's formula. The smaller crystalline size of the Mn3O4/RGO nanocomposite is mostly due to the regular and controlled crystalline growth of the Mn3O4 on the RGO sheets. The FT-IR spectrum of the RGO, wire-like Mn3O4 and Mn3O4/RGO nanocomposites is shown in Fig.3(b). The FT-IR spectrum of the RGO, wire-like Mn3O4 and Mn3O4/RGO show the existence of the oxygen-containing groups at 1060, 1732 and 3280 cm−1 corresponds to the C−O in COH/COC (epoxy), C=O, and −OH functional groups, respectively.[42] The absorption bands at 1630 cm−1 are attributable to the stretching mode of C=C (sp2) carbon structure.[44] While, wirelike Mn3O4 and Mn3O4/RGO nanocomposites exhibit additional bands in the range of 507 to 671 12

cm-1 corresponds to Mn3O4 octahedral sites (MnO−Mn2O3 with Mn2+, Mn3+). Specifically, the attributed intense bands of 671 and 507 cm-1 are associated with the Mn−O in an octahedral and Mn−O stretching mode in the tetrahedral site, respectively.[42, 44] Additionally, wire-like Mn3O4 and Mn3O4/RGO nanocomposites exhibit the intense band at 3605 cm-1 due to the O−H vibrations.[44] Raman analysis was observed for the structural information of the RGO, wire-like Mn3O4 and Mn3O4/RGO nanocomposites as displayed in Fig.3(c). The spectra corresponding to RGO, as observed, exhibits two intense peaks of A1g vibration mode of the disordered carbon (D bond) at 1336 cm-1 and the E2g vibration mode of the sharp graphitic carbon C=C (G bond) at 1585 cm-1 with small ID/IG intensity ratio of 1.2. The smaller ID/IG specifies the high graphitization degree of RGO, favorable in enhancing the electrical conductivity of the electrode.[45] Raman spectrum of the wire-like Mn3O4 shows a dominant intense peak at 648 cm−1 and the minor intense peaks at 360 and 310 cm-1 corresponds to the typical phonon lines of spinel Mn3O4 phase. [25, 42, 44] Further, the Raman spectrum coupled with the AFM image of the wire-like Mn3O4 sample display a homogeneous distribution of the Mn3O4 nanowires which inferring the purity of the asobtained sample. The Raman spectrum coupled with AFM image of the Mn3O4/RGO nanocomposites clearly displayed three intense peaks attributed at 648, 1336 and 1585 cm-1 are characteristic of the spinel phase of Mn3O4, sp3-hybridized carbon (D band) and sp2-hybridized carbon (G band), respectively. The Mn3O4/RGO nanocomposite shows a small ID/IG intensity ratio of 1.25 reflecting the graphitization degree of RGO, suggesting the effective reduction of GO during the hydrothermal process.[25] Moreover, the inset Raman with AFM image displays two distinguish colors with homogenously assembled of wire-like nanostructures confirming the successful decoration of Mn3O4 nanowires on the surfaces of RGO nanosheets. Besides, analysis

13

of N2 adsorption-desorption isotherms was conducted to further evaluate the surface area and pore size distribution of the Mn3O4 nanowires and Mn3O4/RGO nanocomposite as shown in Fig.3(d). The isotherm behavior of the Mn3O4 nanowires and Mn3O4/RGO nanocomposite can be classified as a type-IV isotherm with H3 type weak hysteresis loop in the IUPAC classification.[46] Specifically, the complete immobilization of the Mn3O4 nanowires on surfaces of RGO tends to obtain the type-IV isotherm with the existence of micropores in the pore structure.[46] The Brunauer−Emmett−Teller (BET) equation is used to estimate the specific surface area of the Mn3O4 nanowires and Mn3O4/RGO nanocomposite. The specific surface area (SSA) of the Mn3O4/RGO sample is estimated to be a higher surface area of 160 m2 g-1 with pore volume 0.268−0.385 cm3·g−1 while the surface area of the wire-like Mn3O4 exhibited 138 m2 g-1 with a pore volume of 0.135−0.218 cm3·g−1. The obtained high specific surface area of the Mn3O4/RGO sample is relatively higher than the recently reported manganese oxide and carbonbased nanocomposites. Moreover, the week N2 isotherms and inset Fig.3(d) indicate the presence of the significant amount of micropores and the absence of the mesoporous and macroporous structure in the as-obtained wire-like Mn3O4 and Mn3O4/RGO nanocomposite due to high aspect ratio of the Mn3O4 nanowires were completely covered on the surfaces of RGO nanosheets. Additionally, the XPS analysis was used to investigate the chemical composition and oxidation states of the Mn3O4 and Mn3O4 /RGO nanocomposite. Fig.4 (a) shows an XPS survey scan analysis of wire-like Mn3O4 and Mn3O4 /RGO composite in the binding energy range of 01200 eV. The survey spectrum is consistent with composition Mn3O4 and RGO with the possession of peaks corresponding to only Mn3O4 and RGO proving the absence of any impurity. The peaks obtained from XPS spectra corresponding to Mn and O confirms the formation of Mn3O4. Fig.4 (b) and (c) show the deconvoluted doublets of Mn 2p3/2 and Mn 2p1/2 peaks from

14

Mn3O4 and Mn3O4 /RGO respectively at high-resolution XPS spectrum, which is the key factor to elucidate the oxidation state of Mn. In Mn3O4, the pair of peaks centered at 642.5 and 654.0 eV and other pair located at 644.0 and 655.2 eV correspond to Mn3+ and Mn2+, respectively.

Fig.3 (a) XRD analysis, (b) FT-IR analysis and (c) Raman analysis of RGO, wire-like Mn3O4 and Mn3O4/RGO nanocomposite and (d) N2 adsorption-desorption isotherms with inset BJH pore size distribution of wire-like Mn3O4 and Mn3O4/RGO nanocomposite. On the contrary, when Mn3O4 forms composite with RGO the peaks corresponding to Mn3+ (641.7 and 653.2 eV) and Mn2+ (643.3 and 654.4 eV) undergo a negative shift almost 0.8 eV. The ratio of peak areas corresponding to Mn2+ and Mn3+ is almost 1:2 indicating the successful formation of Mn3O4. Fig.4(d) and (e) represent the high-resolution spectrum of O1s deconvoluted into three components. The three components represent the three types of oxygencontaining species including O-Mn3+, O-Mn2+ and H-OH attributed to the binding energies of 529.5, 531.08, and 532.8 eV, respectively. In the case of Mn3O4, the peak corresponding to O 1s 15

centered at 530.1 eV represents the lattice oxygen of O-Mn3+ interaction in Mn3O4, while in the case of Mn3O4 /RGO composite the peak is shifted by -0.6 eV (529.5 eV). The O 1s peak corresponding to the oxygen from O-Mn2+ interaction has a negative shift of -0.8 eV between Mn3O4 ( 532.1 eV ) and Mn3O4 /RGO composite ( 531.3 eV).[47] The negative shift of Mn 2p and O 1s spectra of Mn3O4/RGO is due to the oxygen-containing functional group present on the surface of RGO. The functional groups present on RGO strengthen the chemical interaction with Mn which results in the shift of Mn 2p and O 1s spectra of Mn3O4/RGO. The high-resolution spectra of RGO from C1s show four major components namely C-C, C-O, C=O and -COOH present at the binding energies of 284.4, 286.5, 287.6 and 288.3 eV, respectively as depicted in Fig.4(f). [37-39] The basal-plane of the chemistry of RGO in Mn3O4/RGO composite is dominant with C=O and carboxyl groups. The functional groups present in the basal-plane are the sites are the nucleation and growth points of Mn3O4 nanowires. The divalent Mn cations (Mn2+) interact with the oxygen groups available in basal plane and are cross-linked with the graphene oxide sheets and in the solution, the manganese ions react the epoxide groups forming Mn3O4 nanowires. [27] The presence of C-C, C=O, C-O and -COOH functional groups act as reactive sites whereby weak interactions between them and Mn3O4 nanowires make the nanocomposite. The result obtained from XPS analysis matches well with the XRD and EDS analysis confirming the formation of Mn3O4 /RGO with multiple valences of Mn2+ and Mn3+.[27]

16

Fig.4 (a) XPS survey scan of wire-like Mn3O4, and Mn3O4/RGO nanocomposite, (b and c) highresolution spectrum of O1s and Mn 2p for wire-like Mn3O4 and (d-f) high-resolution spectrum of Mn 2p, O1s, and C1s for Mn3O4/RGO nanocomposite The thermal properties of the wire-like Mn3O4 and Mn3O4/RGO nanocomposites were studied by TGA in a range of 30-550 °C with a heating rate of 10 °C min−1. Fig.5 shows the TGA curves of wire-like Mn3O4 and Mn3O4/RGO nanocomposites. The wire-like Mn3O4 has the final weight loss of 4.5 wt.% in the temperature range of 30-550 °C which related to the elimination of adsorbed water or OH groups adsorbed on the surface of the Mn3O4 nanowires. On the other hand, Mn3O4/RGO nanocomposites shows two stages of weight loss: (i) a slight weight loss of 5.2 wt.% at 30 to 300 oC temperature range corresponds to the evaporation of OH groups on the surface of the Mn3O4 and oxygen moieties (epoxy, carbonyl, and carboxyl groups) on the RGO sheets, and (ii) a sharp weight loss of 20 wt. % at around 300 to 550 °C is related to the oxidation or combustion of carbon atoms in the RGO, respectively. Additionally, the RGO content in the Mn3O4/RGO nanocomposites is determined to be approximately 20 wt %.[48, 49]

17

100 20 wt.%

Weight loss (wt.%)

80 Mn3O4/RGO

60

Mn3O4

40

20

0 100

200

300

400

500

600

Temperature (oC)

Fig.5 TGA curves of wire-like Mn3O4 and Mn3O4/RGO nanocomposites 3.4 Electrochemical properties The CV and EIS analysis are used to analyze the electrochemical properties of the RGO, wirelike Mn3O4 and Mn3O4/RGO nanocomposite based electrode was determined in a 1 M NaCl aqueous solution. The large surface area, high graphitization degree of RGO, and redox sites of the Mn3O4 are supposed to be advantageous for the adsorption and desorption of Na+ ions. The Mn3O4 with its structural versatility, multiple oxidation states (+2, +3 and +4) and coexistence of two oxidation states (+2 and +3), stands out to be a suitable candidate for capacitive deionization application.[27] Fig. 6(a) displays the CVs of the wire-like Mn3O4, Mn3O4/RGO nanocomposite, and RGO at scan rates of 10 mV s-1 in 1M NaCl electrolyte. The CVs of all electrodes showed perfect reversibility and symmetrically rectangular shape, while the Mn3O4/RGO nanocomposite exhibited a larger area with small redox peaks. The electrochemical reduction of Mn3O4 to MnO and MnO to Mn (0) are locating at the redox peaks of 0.65 and 0.4 V, respectively. It is perceived the CV of the Mn3O4/RGO nanocomposite exhibited good cycling performance and

18

electrochemical reversibility. During the CV cycle, the Na+ ions are intercalated into the manganese redox sites and then the Na+ ions are de-intercalated from the manganese redox sites via Faradaic reaction to accomplish charge neutrality in the electrolyte solution. Hence, the electrochemical reversible reaction can be represented between Mn3O4 and Na+ as follows:[50] "+ "# , + -./ + -0  → ./1 "+ "# ,

(8)

Fig.6 (a) CVs of the RGO, wire-like Mn3O4, and Mn3O4/RGO at 10 mV s-1, (b) CVs of the RGO at different applied potential window (0 to 0.8 and 0 to -0.8 V vs Ag/AgCl), (c and d) CV curves of the wire-like Mn3O4 and Mn3O4/RGO nanocomposites at 10 to 100 mV/s scan rates, (e) different scan rate vs. specific capacitance of the electrodes and (f) EIS for the obtained electrodes of the wire-like Mn3O4, Mn3O4/RGO, and RGO. The electrochemical studies were conducted by using 1 M NaCl as the electrolyte. The CVs showed that the Mn3O4/RGO nanocomposite holds a higher specific capacitance (Cs) of 424 F/g than the wire-like Mn3O4 (348 F/g) and RGO (98 F/g) based 19

electrodes. The great aspect ratio of Mn3O4 nanowires is uniformly dispersed on the RGO, resulting in increases of effective redox sites of the Mn3O4 thus enhancing the utilization ratio of Mn3O4.[27, 42] The asymmetrical CV curve exhibited by Mn3O4/RGO nanocomposite is evidence of combined pseudo and double-layer capacitance. The CV behavior of RGO is presented with a different applied CV window of 0 to 0.8 and 0 to -0.8 V as presented in Fig.6(b). This observation reveals that the pseudocapacitance of RGO in the negative window with Cs of 167 F/g is higher than the Cs (98 F/g) of the positive potential window. It is observed that the RGO based negative electrode is a suitable candidate for the utilization of anode material for cations (e.g., Cl+). Fig.6(c) and Fig.6(d) show the CVs obtained at different scan rates from 10 to 100 mV s-1 for wire-like Mn3O4 and Mn3O4/RGO nanocomposite. All the CV response of wire-like Mn3O4 and Mn3O4/RGO nanocomposite originates from EDL capacitance with Faradaic charge-transfer of redox-active behavior.[27, 30, 42, 43] Intently, Mn3O4/RGO nanocomposite exhibits a broader area and higher current density over RGO than pristine wirelike Mn3O4 owing to higher surface area (SSA) that offers better conductive pathways for the easy transport of ions. Fig.6(e) appearances the scan rate dependence of the Cs of the wire-like Mn3O4 and Mn3O4/RGO nanocomposite. It is observed that the Cs of the wire-like Mn3O4 and Mn3O4/RGO nanocomposite based electrodes decreases with the increasing the scan rates of 10 to100 mV s-1. A 60% loss in the Cs of the electrodes attributed at a higher scan rate of 100 mV s1

, indicating that the salt ions were intercalated probably at the surfaces of the electrode.

However, Mn3O4/RGO nanocomposite based electrode can retain a Cs of 160 F g-1 at 100 mV s-1 than that of wire-like Mn3O4, thus enabling the higher electrochemical interactions of Na+ ions. EIS is used to estimate the charge transport property of RGO, wire-like Mn3O4 and Mn3O4/RGO nanocomposite carried out at an open circuit potential with a frequency range of 0.1 Hz to 100

20

kHz. The impedance plot has an inclined line at low frequency and a semicircle at high frequency.[51] The diameter of the semicircle is used to estimate the charge transfer resistance (RCT) of the electrodes and sheet resistance (Rs) is estimated from the intercept at the real axis in the Nyquist plot and the vertical inclination gives Warburg diffusion element.[52, 53] The sheet resistance (RS) of RGO (1.71 Ω/cm2), wire-like Mn3O4 (4.32 Ω/cm2) and Mn3O4/RGO nanocomposite (1.78 Ω/cm2) have very low values indicative of perfect adhesion of the materials on to the surface of the carbon sheet. The lowest RCT of 1.78 Ω/cm2 for Mn3O4/RGO nanocomposite exhibits much better charge transfer property than wire-like Mn3O4 with the charge transfer resistance of 4.32 Ω/cm2 respectively. The RS and RCT values of the Mn3O4/RGO nanocomposite are much lower than wire-like Mn3O4 which is reflected in the charge-transport property in terms of its better capacitive deionization ability. The high charge transfer resistance by wire-like Mn3O4 restricts the transfer of ions which is attributed to high charge density and eventual low capacitance.[54] Based on the CV and EIS studies, the immobilization of Mn3O4 nanowires on the surfaces of RGO significantly enhances the charge storage capability with rapid Faradaic charge transfer reactions, thus enabling the electrochemical adsorption and desorption of the Na+ ions. To further evaluate the electrochemical properties including cyclic stability and reversibility of the as-prepared RGO, wire-like Mn3O4 and Mn3O4/RGO nanocomposite based electrodes, galvanostatic charge-discharge (GCD) tests were performed. Fig.7a shows the GCD curves of RGO, wire-like Mn3O4, and Mn3O4/RGO electrodes at a current density of 1 A g-1 in 1 M NaCl as an electrolyte. The GCD curve of RGO showed a perfect triangle with symmetry, representing that the as-prepared RGO electrode has good electrochemical reversibility. The GCD curves of wire-like Mn3O4 and Mn3O4/RGO electrodes exhibited triangle and linear charge-discharge with redox reactions, revealing good reversibility

21

of pseudocapacitive behavior. Notably, the Mn3O4/RGO electrode showed a higher specific capacitance of 437 F g-1 than wire-like Mn3O4 (340 F g-1) and RGO (108 F g-1) based electrodes at a current density of 1 A g-1 which is a good agreement with the capacitance calculated from CV curves (Fig.5). Especially, the Mn3O4/RGO electrode possesses an excellent electrical conductivity due to the presence of RGO with a typical pseudocapacitive site of Mn3O4, which beneficial for the higher storage of electric energy. Fig.7b shows the GCD curves vs different current density (1, 2, 3 and 4 A g-1) of the Mn3O4/RGO electrode. The GCD curve at 1 A g-1 showed the rate of the Na+ ions intercalation/deintercalation redox reaction is longer than the GCD curves at a current density of 2, 3 and 4 A g-1, which is attributed to the maximum utilization of electro-active redox sites of Mn3O4. The inset Fig.7b showed the cyclic stability of the Mn3O4/RGO electrode at a current density of 2 A g-1 in 1 M NaCl as an electrolyte. The obtained Mn3O4/RGO electrode shows good cyclic stability with 100% of capacitance is retained over 10 consecutive GCD cycles at 2 A g-1. On the basis of the GCD curves of the samples, the specific capacitance (Cs) can be estimated by using the equation: Cs=i∆t/m∆E, where ∆E (in volts) is the voltage range of discharge after IR drop, m (mg) is the mass of the active electrode material, ∆t (sec) is the discharge time and i (A) is the discharge current. [55-57] The Cs of 437, 425, 330 and 265 F g-1 corresponding to the various current densities of 1, 2, 3 and 4 A g-1 are estimated for the Mn3O4/RGO electrode as shown in Fig.7c. This result indicated that 60 % of capacitance retained its initial specific capacitance as the current density increased from 1 to 4 A g-1.

22

Fig.7 (a) Galvanostatic charge/discharge curves of RGO, wire-like Mn3O4 and Mn3O4/RGO nanocomposites at a current density of 1 A g-1, (b) Galvanostatic charge/discharge curves vs different current density (1, 2, 3 and 4 A g-1) of Mn3O4/RGO nanocomposites and inset of 10 consecutive galvanostatic charge/discharge cycle curves obtained at 2 A g-1, (c) specific capacitance (F g-1) of Mn3O4/RGO nanocomposites at different current density (A g-1), and (d) Ragone plot (power density vs energy density) of Mn3O4/RGO nanocomposites and comparison with

several

reported

graphene-MnO2-based

supercapacitors.

All

the

Galvanostatic

charge/discharge studies were carried out in 1 M NaCl solution.

Further, the energy density (E, Wh kg−1) and power density (P, W kg−1) of the Mn3O4/RGO supercapacitor was calculated using the following equation: E=(Cs∆V2)/7.2 and P= E×3600/∆t respectively, where Cs is capacitance (F g-1) of the Mn3O4/RGO electrode, ∆V is the potential

23

window (V), and ∆t is the discharge time (S) of the GCD curve for Mn3O4/RGO electrode.[5557] Fig.7d shows the Ragone plot of energy density (E, Wh kg−1) vs power density (P, W kg−1) at different current densities for the Mn3O4/RGO supercapacitor. It shows that the Mn3O4/RGO supercapacitor exhibited an energy density of 41.12, 38.22, 30.0, and 24.44 Wh kg−1 and power density of 400, 800, 1200, and 1600 W kg−1 under a current density of about 1, 2, 3, and 4 A g−1, respectively. The obtained results are relatively prominent compared to the earlier reports on manganese and graphene-based electrodes as depicted in Fig.7d.[55, 58-61] This good capacitive performance including cyclic stability and reversibility can be ascribed to the design of a highperformance Mn3O4/RGO based CDI electrode for desalination application. 3.5 CDI performance of the fabricated electrodes To estimate the CDI performance, an asymmetric pseudocapacitive CDI cell (RGO// Mn3O4 and RGO// Mn3O4/RGO) fabricated with RGO as anode and Mn3O4 or Mn3O4/RGO as a cathode. Fig.8(a) shows the SAC performances of the RGO//Mn3O4 and RGO// Mn3O4/RGO CDI electrodes in a 1000 mg L−1 of NaCl solution with an initial conductivity of 2010 µS cm-1 at 1.2 V. It is stated that the solution conductivity decrease rapidly at the early stage and then plateaued within 30 min which indicating the equilibrium of the electrosorption. Obviously, the RGO//Mn3O4/RGO based CDI system reaches the lowest conductivity of 204 µS cm−1, whereas 150 µS cm−1 for RGO//Mn3O4. Especially, anions of Cl− ions are migrated to the anode side and cations of Na+ ions are intercalated with the cathode. Notably, the pH of the treated solution monitored to be 7.5 to 8.0 during the CDI experiments demonstrating the negligible variation of the total concentration of H+ and OH− ions were present in the treated water. The electrosorption capacity (SAC, mg g-1) of the RGO//Mn3O4/RGO estimated to be 34.2 mg g-1 from the CDI experimental result which is higher than the RGO//Mn3O4 based CDI electrode of 23.7 mg g-1 as

24

displayed in Fig.8(b). As known, Mn3O4/RGO pertains good capacitance, higher electronic conductivity, and a larger surface area, leading to obtaining higher SAC.

Fig.8 (a) plots of desalination time (min) vs. NaCl solution conductivity (µS cm-1) with inset plots of desalination time (min) vs. solution concentration (mg L-1) , (b) plots of SAC (mg g-1) vs. deionization time (min) at applied voltage of 1.2 V, (c) plots of SAC (mg g-1) vs. various concentration of NaCl solution (100, 250, 500, 750, 1000, 1250 and 1500 mg L-1) at different applied voltages of 0.8, 1.2 and 1.6 V and (d) Ragone plots of ASAR (mg/g/min) vs. SAC (mg/g) for the wire-like RGO//Mn3O4 and RGO//Mn3O4/RGO electrodes at 1.2 V in a 1000 mg L-1 NaCl solution.

25

Fig.8 (c) shows the further CDI experiments performed with various applied voltages of 0.8, 1.2 and 1.6 V in a different initial concentration of NaCl solution from 100 to 1500 mg L-1. The maximum SAC of the RGO//Mn3O4/RGO estimated to be 34.2 mg g-1 at 1.2 V in a 1000 mg L-1 NaCl. The pH of the solution monitored to be 7.5, 7.56 and 8.6 corresponds to 0.8, 1.2 and 1.6 V, respectively. The higher pH value of 8.6 is obtained at an applied cell voltage of 1.6 V due to the production of H+ and OH− ions via electrolysis of water, resulting in a decrease in SAC of the RGO//Mn3O4/RGO electrodes. Further, the SAC is increased with increasing the initial concentration from 100 to 1000 mg g-1 at 1.2 V due to compression of the EDL with pseudocapacitive which reduced the overlapping effect. While the slight decreases in SAC obtained at the higher initial concentration of 1500 mg g-1 NaCl due to the limited availability of the redox sites in the electrodes. The ASAR is another parameter to determine the kinetics rate of the present electrodes. The Ragone plots of the RGO//Mn3O4 and RGO//Mn3O4/RGO electrodes are revealed in Fig.8(d). Mostly, the plots with an upper and right side shift related to A higher SAC and faster ASAR. Obviously, the RGO//Mn3O4/RGO electrode displays the most upper and right Ragone plot, revealing of the highest SAC and fastest ASAR than RGO//Mn3O4 based CDI electrodes. Specifically, the RGO//Mn3O4/RGO electrode can adsorb more salt ions at a fast adsorption rate. Moreover, the charge efficiency of the RGO//Mn3O4 and RGO//Mn3O4/RGO electrodes are about 0.56 and 0.67, respectively indicating the charge utilization by the electrodes during the CDI.

26

Table 1. Comparison of the SAC vs. different CDI electrodes reported in the literature

S.No

Electrode

Specific

Initial

salt Applied SAC

materials

capacitance concentration voltage

27

mg g-1

ASAR ref

1

MXenes

132

5 mM

1.2

13

1

[16]

2

Mo1.33C-

155

600 mM

0.8

15

-

[17]

200

100 mM

1.2

30.2

0.081

[18]

100 mg L-1

1.2

0.490

-

[62]

MXene 3

Na2FeP2O7

4

CNT-MnOx for 215 selective

Na+

adsorption 5

MnO2/ Ag

300

20 mM

1.2

17.8

0.025

[19]

6

MnO2/activate

77.6

0.01 M

1.0

9.3

-

[20]

237

50 mg L-1

1.6

8.9

-

[22]

5 mM

1.2

11.26

-

[24]

40 mg L-1

1.6

32.3

0.56

[63]

1000 mg L−1

1.4

140.7

1.60

348

1000 mg L−1

1.2

23.7

0.58

437

−1

d carbon 7

MnFe2O4/Poro us rGO

8

RuO2-activated 56 carbon

11

N,S-Codoped

427

Carbon Materials 12

Mn3O4 nanowires

13

Mn3O4/RGO

Present work

1000 mg L

1.2

34.2

1.2

Present work

28

Fig.9 (a) Regeneration curve of the wire-like Mn3O4/RGO nanocomposites electrodes in a 1000 mg L−1 NaCl solution at 1.2 V, (b) 20 consecutive cycles of CDI, (c) schematic diagram of CDI set-up, and (d) schematic diagram of CDI mechanism using wire-like Mn3O4/RGO nanocomposites. Fig.9 (a) displays the consecutive adsorption and desorption cycles of the RGO//Mn3O4/RGO nanocomposite based electrodes in a 1000 mg L−1 concentration of NaCl solution. The adsorption-desorption experiment was performed for 5 consecutive cycles: 1.2 V for 30 min in the adsorption step and 0 V for 30 min in the desorption step. The solution conductivity changes were recorded accordingly. The conductivity retentions attain over 99.1% for the five cycles, indicating the RGO//Mn3O4/RGO can be reproducible for CDI application. Further, the cycling 29

stability of the RGO//Mn3O4/RGO was measured by monitoring the SAC changes over the 20 consecutive cycles as depicted in Fig.9(b). The 99.5 % of the SAC retentions obtained over the first ten consecutive cycles and then dropped into 98.9% due to the detachment of active materials from the electrodes after long-term operations. After several cycles, the SAC retention of the RGO//Mn3O4/RGO based electrodes retained over 98.9 % presenting better cyclic stability due to the well-dispersed Mn3O4 nanowires on RGO nanosheets. The adsorption and desorption of the salt ions to the electrodes was better than the recently reported metal oxides based electrodes. These results prove that the RGO//Mn3O4/RGO based CDI electrodes are good for treating seawater. Fig.9(c and d) demonstrates the schematic diagram and functioning of the CDI device using the RGO//Mn3O4/RGO electrode. Generally, CDI cell is composed of a pair of electrodes such as positive and negative, which serve as host frameworks for the electrosorption of the Na+ and Cl- ions in NaCl solution. In this asymmetric CDI system, RGO was used as an anode and Mn3O4/RGO was used as a cathode at 1.2 V as shown in Fig.9(d). During the adsorption, the Na+ ions are intercalated into Mn3O4/RGO, while Cl- ions are migrated into the RGO. The high SAC and ASAR confirm that the wire-like Mn3O4/RGO is capable of rapid attraction of Na+ ions via surface redox reactions as demonstrated in Eqn.8. The higher SAC of the present electrode can be attributed due to the synergetic effect of EDL with pseudocapacitive properties of Mn3O4/RGO and higher electronic conductivity of the RGO. Importantly, RGO prevents the agglomeration of the Mn3O4 nanowires and also acts as an electron shuttling mediator to the outer circuit. The incorporation of Mn3O4 nanowires onto an RGO has potentially enhanced the SAC and ASAR. Table.1 summarizes the SAC and ASAR performances of the recently developed various electrodes.

30

4. Conclusion In summary, the wire-like Mn3O4 and Mn3O4/RGO nanocomposite were successfully prepared via hydrothermal at 180 oC for 12h. The physicochemical and electrochemical properties of the fabricated wire-like Mn3O4 and Mn3O4/RGO nanocomposites were characterized in detail. FESEM and TEM revealed that the well-dispersed Mn3O4 nanowires with few microns in length and diameter in the range of 20-30 nm were regularly immobilized on the RGO sheets. The obtained Mn3O4/RGO nanocomposite possesses a high SSA of 160 m2 g-1 with a pore volume of 0.268−0.385 cm3 g−1. CV analysis revealed that the Mn3O4/RGO nanocomposite exhibited in pseudocapacitive performance, and resulted in Cs of 437 F g-1 at 1 A g-1 in 1 M NaCl as an electrolyte. EIS analysis revealed the fabricated Mn3O4/RGO nanocomposite possesses a good charge transferability and the lowest charge transfer resistance of (RCT) 1.70 Ω/cm2 than wirelike Mn3O4 nanostructure. In the CDI experiment, the Mn3O4/RGO nanocomposite exhibited SAC of 34.5 mg g-1 in 1000 mg L-1 NaCl solution at 1.2 V applied cell voltage with a high ASAR performance of 1.15 mg g-1 min-1. The Mn3O4/RGO electrode shows admirable cycling stability up to 20 consecutive cycles with 98.5 % regeneration. The performance of the CDI, Mn3O4/RGO nanocomposite was better than that of the recently reported electrodes. Such promising results of pseudocapacitive Mn3O4/RGO nanocomposite based electrode represent the promising prospect for CDI application.

5. Acknowledgment This work is supported by the Khalifa University of Science and Technology through Award No CIRA-2018-27. 31

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Highlights
A hierarchical Mn3O4/RGO nanoarchitecture have been prepared via hydrothermal method
Capacitive deionization cell consists of Mn3O4/RGO cathode with RGO anode
The Mn3O4/RGO possessed pseudocapacitive behavior with capacitance of 437 F g-1
RGO//Mn3O4/RGO CDI cell exhibited SAC of 34.5 mg g-1 and ASAR of 1.1 mg g-1 min-1
Pseudocapacitive Mn3O4/RGO represent the promising prospect for CDI application

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Bharath Govindan