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A Robust Nanocomposite of Nitrogen Doped Reduced graphene Oxide and MnO2 Nanorods for High Performance Supercapacitors and Non-enzymatic Peroxide Sensors Mohit Saraf, Kaushik Natarajan, and Shaikh M. Mobin ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01845 • Publication Date (Web): 27 Jun 2018 Downloaded from http://pubs.acs.org on June 27, 2018

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ACS Sustainable Chemistry & Engineering

A Robust Nanocomposite of Nitrogen Doped Reduced graphene Oxide and MnO2 Nanorods for High Performance Supercapacitors and Nonenzymatic Peroxide Sensors Mohit Sarafa, Kaushik Natarajana and Shaikh M. Mobin*a,b,c

a

Discipline of Metallurgy Engineering and Materials Science, bDiscipline of Chemistry,

c

Discipline of Biosciences and Biomedical Engineering, Indian Institute of Technology

Indore, Simrol, Khandwa Road, Indore 453552, India

*E-mail: [email protected] Tel: +91 731 2438 762

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ABSTRACT The urgent demand of sustainable energy systems and reliable sensing devices has fostered the development of cost-effective, multi-functional electrode material based platforms. In this work, we have demonstrated the bi-functionality of nitrogen doped reduced graphene oxide-MnO2 nanocomposite (NRGO-MnO2), towards two most diverse and challenging applications: (i) supercapacitor and (ii) peroxide sensor, which was synthesized by a facile one-pot hydrothermal method. The electrochemical investigations revealed its high specific capacitance (648 F g-1 at 1.5 A g-1) with remarkable rate performance (retains 80.20 % up to 10 A g-1) and long term cyclic efficiency. Additionally, it can detect peroxide rapidly (2 s), with high sensitivity (2081 µAmM-1 cm-2) and noteworthy detection limit (24 nM) in a wide dynamic range (0.4-121.2 µM). Fascinating features such as the distinguished selectivity, repeatability and operational stability suggests its potency to be an ideal electrode for peroxide sensors. Finally, the charge transfer kinetics and capacitive components, probed by electrochemical impedance spectroscopy (EIS), are found to be in correlation with other investigations. The positive synergism between MnO2 nanorods and NRGO induces higher conductivity and surface area, which eventually promotes superior supercapacitor and sensor performances. The results highlight NRGO-MnO2 nanocomposite as a multi-functional, cutting edge and sustainable material for next-generation energy storage and sensing applications. Keywords: One-pot; hydrothermal; nanocomposite; supercapacitor; peroxide sensor.


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INTRODUCTION The development of multitasking electrode materials for energy storage devices and sensors has recently clinched tremendous research interest.1-14 In particular, design of such suitable platforms relying upon electrodes functionalized with various nanomaterials has drawn wide attention.15-27 In this regard, transition metal oxides (TMOs) have shown key potential for supercapacitors and sensor devices. Among TMOs, MnO2 has been proven to be a potential candidate for supercapacitors owing to its high theoretical capacity, low-cost, natural abundance, excellent electrochemical redox activity, good reversibility and environmental compatibility.28-30 Moreover, it exhibits excellent catalytic activity towards the decomposition of H2O2, and therefore can be used for H2O2 sensing.31-34 However, the intrinsically poor electrical conductivity (10-5 to 10-6 Scm-1) and unstable solid electrolyte interface of MnO2 is a hindrance to obtain high electrochemical performance.31,33 Moreover, for non-enzymatic sensors, TMOs alone usually don’t stand well for electrode attachment, which leads to poor stability.35,36 In order to upgrade the conductivity of MnO2 based electrodes, immensely conductive additives such as carbon nanotubes, graphene/rGO, porous and activated carbons (ACs) have been proposed. Among these materials, graphene/rGO has drawn immense interest owing to its noteworthy properties such as high surface area, high thermal as well electrical conductivity and robustness.7-10,22,37-43 Though, tremendous achievements have been made by introducing rGO as a carbon additive, however major challenges such as the tedious synthesis, poor cycle and rate performance due to the poor association between rGO and the MnO2 still need to be resolved.43 Addressing this issue, functionalization of rGO has recently shown potential to improve its electrochemical properties. In particular, nitrogen (N) doping has shown remarkable potential and become a compelling approach to boost-up the properties of rGO by increasing surface reactive sites.43 So far, most of the research efforts have been devoted for investigations on single phase of N-doped graphene/rGO. Very few attempts have been made to combine N-doped rGO with well dispersed MnO2 nanostructures.28,41,43 Different methods such as in-situ growth under hydrothermal conditions.28,43,44,45 in-situ growth followed by calcination at 800 °C46, or mechanical mixing47, have been employed to form such composites. Additionally, various reducing agents have been incorporated in the synthesis process. Among them, urea has emerged as a promising reducing agent for the synthesis in comparison to hydrazine or NaBH4, due to enhanced capacitive properties and improved electronic conductivity.38 The combination of N-doped rGO with MnO2 3 ACS Paragon Plus Environment


synergistically improves energy storage capability and rate performance, while maintaining long cycle life of the supercapacitor.28-30,48,49Additionally, the nanoscaled MnO2 structures facilitate electrolyte transport and electron transportation by shortening the diffusion pathway, which results into enhanced sensor performance.31-34,43 Herein, we present a facile yet elegant one-pot hydrothermal strategy to fabricate N doped rGO-MnO2 nanocomposite using urea as a reducing as well as doping agent under optimized conditions. The fabricated nanocomposite was explored for two urgent applications: (i) supercapacitors and (ii) peroxide sensors. So far, graphene-MnO2 or similar composites have been electrochemically implied in a unidirectional way either for supercapacitors or sensors. To the best of our knowledge, the present work is the first report, in which the multitasking behavior of same electrode material (N-doped rGO-MnO2 composite) has been explored for two different and significant applications. Moreover, the facile composite and electrode fabrication with notable results for both applications show the immense future potential of N-doped rGO-MnO2 composites.


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EXPERIMENTAL Chemicals Throughout the synthesis, Merck chemicals and Deionized water (DI water, 18.2 MΩ cm) was used. Instrumentations XRD spectra were recorded on a X-ray diffractometer (Rigaku SmartLab) equipped with monochromated CuKα radiation (λ = 1.54 Å). SEM images were obtained through Supra55 Zeiss Field-Emission Scanning Electron Microscope. TEM imaging was performed on a JEOL (JEM-2100) system. The N2 adsorption-desorption isotherm and pore size distribution spectrum was obtained on Autosorb iQ, version 1.11 (Quantachrome Instruments). XPS analysis was performed on Multi-technique X-ray Photoelectron Spectroscopy with XPS-mapping capability (AXIS ULTRA system). Electrochemical Measurements Autolab PGSTAT 204N controlled with NOVA software (version 1.10) was utilized for all electrochemical experiments. The electrochemical testing was performed by using Ag/AgCl as reference electrode; Pt wire to be the counter electrode and 3 mm diameter glassy carbon electrodes (GCE) in the three electrodes configuration. Cyclic voltammetry (CV) experiments for H2O2 sensor and supercapacitor test were performed in 0.1 M PBS solution, and 1 M Na2SO4, respectively, at room temperature (RT). Synthesis of Materials Graphite oxide (GO) and reduced graphene oxide (rGO) were utilized as demonstrated in our previous reports.7 The NRGO-MnO2 nanocomposite was fabricated by a simple yet effective one-pot hydrothermal reaction reported elsewhere.43 In our synthesis, KMnO4 (158 mg) was added into homogeneous solution of GO (20 mL; 1.2 mg mL-1), and the mixture was stirred for half an hour at RT. Thereafter, urea (500 mg) was gradually introduced to the suspension for the reduction of GO to form nitrogen doped rGO (NRGO). The resultant solution was stirred for another half an hour and then subjected to heating for 12 h at 120 °C using stainless steel autoclave (Teflon-lined), under optimized conditions. The optimization was achieved by varying the hydrothermal reaction time and temperature as presented in Figure S1,S2, respectively. Afterwards, the formed product was washed 5 ACS Paragon Plus Environment


thoroughly with DI water and vacuum dried, which was referred to as NRGO-MnO2 nanocomposite.43 N-doped rGO (NRGO) was synthesized by hydrothermal method under similar conditions using urea in absence of KMnO4. Similarly, MnO2 and RGO-MnO2 composite were also synthesized by the same method in absence of GO and urea, respectively. Fabrication of Working Electrodes Initially, glassy carbon electrodes (GCEs) were cleaned with alumina slurries and rinsed several times with DI water and subsequently dried. Subsequently, a stable suspension was prepared by dispersing 10 mg of as-synthesized powder in 5 mL ethanol by ultrasonication for 15 min. Thereafter, 5 µL of this suspension (NRGO-MnO2 nanocomposite in ethanol) was dropcast on the GCE under optimized conditions, and dried in air followed by deposition of a thin layer of Nafion (5 µL, 1%). Subsequently, the modified electrode was rinsed and dried before using in experiments. This modified electrode is now referred to as NRGO-MnO2/GCE. Similarly, other electrodes of rGO, NRGO, MnO2, RGO-MnO2 and physical mixture of NRGO and MnO2 were fabricated and referred to as rGO/GCE, NRGO/GCE, MnO2/GCE, RGO-MnO2/GCE, PM-NRGO-MnO2/GCE, respectively.


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RESULTS AND DISCUSSION The NRGO-MnO2 nanocomposite was synthesized by a one-pot hydrothermal reaction of KMnO4 with graphite oxide solution in the presence of urea as a nitrogen source at 120 °C (Scheme 1). The homogenous dispersion of MnO2 within NRGO sheets provide mechanical robustness, thermal stability and high surface area to the resulting composite. The synthesized composite was characterized by various advanced physicochemical techniques such as XRD, SEM/TEM, XPS, and BET.

Scheme 1. Schematic synthesis of NRGO-MnO2 nanocomposite.



Characterizations The crystallinity and phase purity of the synthesized materials were investigated through PXRD. Figure S3a shows the representative XRD pattern of NRGO-MnO2 composite, where the emerged peaks can be indexed to the reflection planes of tetragonal αMnO2 (JCPDS No. 44-0141).43 An, additional diffraction peak at ~25°, corresponds to the lattice plane (002) was observed, which verifies the presence of reduced graphene oxide in the composite.7,22 The XRD spectrum of composite clearly show the poor crystallinity of MnO2 due to the presence of NRGO.50-53 The peak of NRGO in the composite suggests that urea is not only acting as N-source, but also play important role in reducing GO into rGO. The XRD spectra of GO, rGO and NRGO have also been presented in Figure S3b-d, which are consistent with the literature.7,22,48,49,54-58 The XRD spectrum of RGO-MnO2 has also been provided in Figure S4, which clearly shows that the sample prepared in absence of urea exhibit much more aggregation and verifies that urea plays an important role in alleviating the aggregation of MnO2 in the composite. The structures and morphologies of the prepared materials were examined by SEM and TEM techniques. Surface morphological images obtained through SEM clearly show two different types of structures: one is the rod shaped MnO2 with dimensions in nanorange and another is NRGO aggregates (Figure 1a,b). To get further insights into the composite structure, TEM analysis was performed. TEM images verify that the prepared composite is composed of MnO2 nanorods and nearly transparent NRGO thin sheets (Figure 1c,d). It was also observed that the nanoscaled MnO2 with rod like morphology are homogeneously dispersed within NRGO matrix. During TEM measurements, it was found out that even after sonication for half an hour, MnO2 nanorods are properly attached to the NRGO, from which it can be concluded that sonication doesn’t affect the integrity of composite and such type of mechanically robust materials can provide high cyclic stability during charge-discharge measurements. TEM images also reveal that MnO2 nanorods have diameter ranging from 5-15 nm and length from 0.3-1.5 µm and are properly distributed within NRGO sheets. The selected area electron diffraction (SAED) pattern of composite reveals the poor crystallinity of MnO2 nanorods (inset of Figure 1d), which is also consistent with the XRD results. It is believed that such amorphous structure improves the electrochemical performance.43,54-56 Moreover, it has been observed previously that MnO2 nanostructures tend to agglomerate with graphene/rGO. However, in the present work nitrogen doping plays crucial role in preventing agglomeration by homogenously dispersing MnO2 structures within doped rGO matrix as can be seen from the TEM images. 8 ACS Paragon Plus Environment

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During composite preparation, a considerable portion of O containing functional groups were replaced by N atom provided by urea and very few sites remains available to anchor MnO2. Hence, highly dispersed nanoscaled MnO2 rods are formed in NRGO without any possible aggregation.38 The high magnification TEM image of NRGO-MnO2 is shown in Figure 1e, and the marked area has been zoomed to show the interplanar distance (Figure 1f). The presence of blurred, indistinct lattice fringes in the images are implying higher amorphous nature39, but also infers low defect density in the structure.40 The interplanar spacing was determined to be 0.205 nm. The SEM images of rGO, NRGO and RGO-MnO2 composite at different magnifications are also shown in Figure S5-S7, respectively. The as-prepared NRGO-MnO2 nanocomposite was further investigated by XPS to probe elemental composition and electronic structure. As shown in Figure 2a, the survey level spectrum reveals the Mn, C, O and N elements in the composite, authenticating the N doping in rGO matrix. The C (1s) spectra of composite (Figure 2b) exhibit four main peaks at 284.3, 285, 286.02, and 288.26 eV attributing to C-C, C-N, C-O and C=O, respectively.30 The high-resolution N (1s) spectrum of composite is presented in Figure 2c, which can be deconvoluted into two peaks, pyridinic (399.48 eV) and pyrrolic (400.81 eV). The presence of pyridinic N can provide an e- pair for conjugation with the π-conjugated rings of rGO and hence improve the electrochemical performance of composite by introducing e- donor properties. Due to better e- donor characteristics, the pyrrolic N in rGO have higher charge mobility, which also plays crucial role in improving capacitances.43 The Mn (2p) spectrum unveils two peaks centred at 642.1 and 653.9 eV, corresponding to Mn (2p3/2) and Mn (2p1/2) peaks, respectively (Figure 2d), verifying the presence of MnO2 in the composites with an energy separation of ~11.8 eV. In addition, the spectrum of O (1s) exhibits two peaks centred at 529.58 and 531.30 eV (Figure 2e), attributing to the O−Mn and C−O bonding configurations, respectively.43,44 The doping of N element in the composite was determined to be 1.39 atom % (Figure 2f). It is to be noted that in the high-rate electrochemical process, N doping enhances the electrochemical activity of rGO, which leads to high rate performance in supercapacitors. In addition, the enhanced conductivity improves sensor performance. In order to probe the surface area and porosity of NRGO-MnO2 nanocomposite, N2 adsorption-desorption was performed. Figure 3 shows the N2 isotherm and respective pore size distribution curve (BJH) of NRGO-MnO2 nanocomposite (inset of Figure 3), suggesting that the isotherm of the NRGO-MnO2 nanocomposite possibly corresponds to a type IV 9 ACS Paragon Plus Environment


isotherm, which is generally indicative of mesoporous nature of the material.43 The presence of hysteresis loop at high relative pressure, indicates the existence of ample mesopores revealing its mesoporous nature. It can also be deduced that MnO2 nanorods inserted between the NRGO matrixes effectively prevents the restacking among the sheets and generates inter-connected pores in the composite. The NRGO-MnO2 nanocomposite has a specific surface area of 196.76 m2 g-1, which is larger than most of the earlier reports based on composites of same materials (Table S1). Moreover, the composite presents high porosity with an average pore radius of 11.6 nm, and total pore volume of 0.63 cm3 g-1, confirming the mesoporous structure of the composite. Such type of structures with large surface area is favorable for supercapacitors.30 This may also facilitates the electrolyte ion diffusion into the activated sites of electrode surface with less resistance to induce large specific capacitance and high rate ability during continuous charge-discharge process. Moreover, high surface area and porosity are also favorable to achieve high sensor performance.30,43 The N2 isotherm was performed for NRGO also, which exhibited a much less surface area of 68.195 m2 g-1 with an average pore radius and total pore volume of 3.73 nm and 0.127 cm3 g-1, respectively (Figure S8). Furthermore, the NRGO-MnO2 nanocomposite was used as a modifier for the GCE and thus fabricated electrode NRGO-MnO2/GCE was employed as working electrode to demonstrate its potential capability towards high performance supercapacitors and peroxide sensors (Scheme 2). The SEM image of glassy carbon electrode modified with NRGOMnO2 composite was also recorded (Figure S9) and a clear appearance of MnO2 nanorods along with NRGO sheets was observed.


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Scheme 2. Schematic of supercapacitor and peroxide sensor based on NRGO-MnO2 nanocomposite. NRGO-MnO2/GCE as a Supercapacitor The electrochemical supercapacitor behavior of NRGO-MnO2/GCE has been investigated by cyclic voltammetry (CV) and Galvanostatic charge-discharge (GCD) techniques by employing 1 M Na2SO4 solution (Figure 4). The CV and GCD results of NRGO-MnO2/GCE

were

simultaneously

compared

with

bare

GCE,

rGO/GCE,

NRGO/GCE. The supercapacitive performance of NRGO-MnO2/GCE are firstly analysed by CV technique. The CV curves at various scan rates (5-500 mVs-1), demonstrate excellent charge propagation and enhanced CV integrated area under the curve for NRGO-MnO2/GCE, due to synergistic contribution of electronically conductive NRGO sheets and MnO2 nanorods (Figure 4a). The healthy coalition of pseudocapacitive MnO2 nanorods and double layer capacitive NRGO significantly boosts the process of charge propagation. Hence, the resultant charge-storage performance of NRGO-MnO2 nanocomposite is the result of synergy between NRGO sheets and MnO2 nanorods. Here, NRGO assists MnO2 being physical support and also allocate additional ion transportation channels. Moreover, the high 11 ACS Paragon Plus Environment


conductivity of NRGO improves the rate performance and power density of the resulting composite at a larger charge-discharge current. In contrast, MnO2 is the main source of charge storage, whose electro-activity immensely contributes to generate high charge storage ability and energy density of the composite. As a result, after formation of composite, the expected synergy between NRGO and MnO2 nanorods guides an amplification in the charge storage capacity of the NRGO-MnO2 nanocomposite. The CV curves appear to be nearly rectangular shaped at low scan rates exhibiting nearly capacitive behavior. However, the distortion from ideal rectangular shape was observed at high scan rates, which may be attributed to increasing over-potentials from ion transport between the electrolyte and the composite59 or incomplete adsorption of alkali cations onto the sites on the electrode material.60 The CV profile of NRGO-MnO2/GCE at a scan rate of 25 mV s-1 is individually shown in Figure 4b, which shows nearly rectangular shaped curve. The CV curves of rGO/GCE and NRGO/GCE at various scan rates (10-500 mV s-1) are also presented in Figure S10a and S11a, respectively. The CV comparison of all four electrodes i.e. bare GCE, rGO/GCE, NRGO/GCE and NRGO-MnO2/GCE has been shown in Figure 5a, which clearly highlights the enhanced and superior charge propagation and integrated area of NRGO-MnO2/GCE over other electrodes. The NRGO-MnO2/GCE was further electrochemically probed by investigating it galvanostatic charge-discharge (GCD) properties. The GCD curves of NRGO-MnO2/GCE at different current densities (1.5-10 Ag-1) are all-together shown in Figure 4c. The nearly symmetrical GCD curves with almost constant slopes suggest the superior electrochemical reversibility during the charging-discharging process. Furthermore, no obvious potential drop (iR drop) was observed further indicating the minimized internal resistance and good electrical conductivity. It also suggests that NRGO-MnO2/GCE primarily follows double layer capacitive mechanism. The GCD curve for NRGO-MnO2/GCE at 1.5 A g-1 is individually shown in Figure 4d, which clearly shows the nearly symmetrical chargedischarge curve. The specific capacitance of electrodes was calculated by the following equation: C = I dt /m ∆V ---------------------------------- (1) Where dt, I, ∆V and m are the discharge time, discharge current, potential window, and mass of active material, respectively. The specific capacitances calculated at current density of 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 7.5 and 10 A g-1 are 624, 605, 591, 577.5, 575, 553.5, 540, 532.5 and 520 F g-1, respectively. The GCD curves of NRGO-MnO2/GCE at different current densities (2-10 Ag-1) are separately shown in Figure 6. It can be observed that 12 ACS Paragon Plus Environment

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discharge time kept on decreasing on increasing current density due to occupancy of extremely less number of electrolyte ions in the electrode’s inner spaces at higher current densities, as a result of which specific capacitance decreases with increasing current density.22 However, NRGO-MnO2/GCE shows excellent rate ability as can be seen in the plot of the generated specific capacitance versus the applied current density (Figure 5b). Roughly, it retains 80% (520 F g-1) of the initial specific capacitance (648 F g-1) on increasing the current density increases from 1.5 to 10 A g-1. The remarkable capacitance retention ability arises due to positive synergism between highly conductive NRGO sheets and well-dispersed MnO2 nanorods as mentioned earlier. It also describes that double layer capacitance mechanism predominates in the NRGO-MnO2/GCE, since nearly symmetrical GCD curves were obtained at all current densities and moreover no considerable iR drop was observed. The GCD curves of rGO/GCE and NRGO/GCE at different current densitites (1.5 to 10 A g-1) are also recorded (Figure S10b and Figure S11b, respectively), exhibiting nearly symmetrical charge-discharge curves at all current densities for both electrodes. However, the discharge time and therefore specific capacitance (198 and 240 F g-1 at 1.5 A g-1 for rGO/GCE and NRGO/GCE, respectively) were found to be considerably lower than that of NRGO-MnO2/GCE. The plotted graphs of specific capacitance w.r.t. applied current densities for rGO/GCE and NRGO/GCE have been demonstrated in Figure S10c and Figure S11c, respectively. The supercapacitor properties of bare MnO2, RGO-MnO2 and a physical mixture of NRGO and MnO2 based electrodes (MnO2/GCE, RGO-MnO2/GCE and PMNRGO-MnO2/GCE, respectively) were also evaluated as shown in Figure S12. The calculated specific capacitances of these electrodes at 1.5 A g-1 have been listed in Table S2, which are clearly inferior than NRGO-MnO2/GCE. The comparison of GCDs of all seven electrodes i.e. bare GCE, rGO/GCE, MnO2/GCE, RGO-MnO2/GCE, PM-NRGOMnO2/GCE, NRGO/GCE and NRGO-MnO2/GCE clearly shows that the NRGOMnO2/GCE has longest discharge time than rest electrodes (Figure 5c). The probable mechanism of charge-discharge of NRGO-MnO2/GCE can be understood from Scheme 3. In the NRGO-MnO2 nanocomposite, NRGO nanosheets are primarily responsible to provide conductive support and also inducing a large surface area. The good interfacial attachment between NRGO and MnO2 results into increased contact area, which can synergistically foster the conductivity of the NRGO-MnO2/GCE. Additionally, the large surface area and small dimensions of the MnO2 rods actuate the easy 13 ACS Paragon Plus Environment


surface accessibility to the electrolyte, which could engender high pseudocapacitance and high capacitance retention at higher rates. The Faradic redox reaction between MnO2 and Na2SO4 delivers high specific capacitance.61-75 The reaction mechanism for charge-storage of NRGO-MnO2/GCE can be attributed to the expeditious intercalation and de-intercalation of Na+ during reduction and oxidation, respectively.73 Na+ + MnO2 + e- ↔ MnO2Na ---------------------------- (2) It should be noted that the Mn participates in the redox reaction in both III and IV oxidation states.73 The CV curve of NRGO-MnO2 composite at a scan rate of 25 mV s-1 in 1 M Na2SO4 aqueous solution is nearly rectangular shaped without obvious redox peaks, indicating ideal capacitive behavior due to the presence of NRGO. However the CV curves of NRGO-MnO2 composite exhibits a big distortion at higher scan rates, which is consistent with previous reports of carbon-MnO2 composites.61-75 The excellent supercapacitance of NRGO-MnO2 composite may be assigned to the following reasons: (i) improved diffusion rate of Na+ due to generation of the pores within the bulk of the composite for ion-buffering reservoirs, (ii) shortening of the diffusion channels for Na+ during the charging-discharging and boosting the electrochemical utilization of MnO2, (iii) NRGO being the conductive support matrix for the deposition of MnO2, and (iv) enhanced interfacial area between MnO2 and NRGO for the fast transportation of electrons throughout the process.43,50-75

Scheme 3. The probable mechanism of charge-discharge based on NRGO-MnO2 composite. The cycling stability performance of NRGO-MnO2/GCE at a current density of 10 A g-1 in Na2SO4 solution between potential window (-0.3 and +0.7 V) has been shown in 14 ACS Paragon Plus Environment

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Figure 5d, which shows that ~93.5 % retention of initial specific capacitance was achieved for the NRGO-MnO2/GCE after 2000 cycles. It suggests that the morphology of NRGOMnO2 is retained after complete electrochemical analysis, which is advantageous for any supercapacitor device. The SEM image was recorded after electrochemical testing also and negligible variations were observed in the morphology (Figure S13), further confirms the structural integrity of the composite. EIS Analysis of NRGO-MnO2/GCE for Supercapacitor The charge transport mechanism and impedance elements of NRGO-MnO2 composite were analyzed by electrochemical impedance spectroscopy (EIS), performed by applying an AC signal in the frequency range of 1 Hz to 10000 Hz under open circuit potential condition. In EIS, the data is usually represented by Nyquist plots, which can be fitted to an ideal circuit model using appropriate algorithms.21,26 In Nyquist plots, the measurements of impedance provide information on real (Z’) and imaginary parts (-Z”) of complex impedance. Figure 6a presents the Nyquist plot of NRGO-MnO2/GCE before and after cycling. Due to the shape of the semicircles obtained in impedance spectroscopic measurement, with a depression indicating a center below the real axis, an empirical model of constant phase element is utilised in modelling the equivalent circuit.61 The complex impedance is represented by the equation mentioned below:

∗ = − " = + --------------------------- (3) According to this model, a simple series of resistor-capacitor (RC) elements are utilised, in which each combination represents a contributing component such as double layer, charge transfer etc. The total complex impedance is obtained according to the following equation:

∗ =

+

+

+ ----------------------- (4)

Wherein, the two terms denote the bulk (depletion layer) and grain boundary (Helmholtz region/interfacial) properties of the material, after excluding the components



from the underlying substrate (in this case, the GCE electrode).62-71 The impedance of the CPE is expressed by the following equation:

!"

= # $ -------------------------- (5)

Here, A is a constant, ω is the angular frequency and φ (0 ≤ φ ≤ 1) is a distributing factor. If φ=0, the impedance is an ideal resistor and φ=1 implies the impedance as an ideal capacitor. The Figure 6b shows the equivalent circuit used for fitting the EIS data. A comparison between the obtained EIS parameters of the pristine NRGO-MnO2/GCE and the spent NRGO-MnO2/GCE after cycling has been made in Table 1. Herein, Rs is nearly constant due to being a function of the experimental setup and wiring. The contributions of the constant phase elements from the bulk (or charge transfer-impedance, CPEbulk), and the grain boundary (or Helmholtz layer, CPEgb) are notably reduced for the NRGO-MnO2/GCE electrode after cycling. An increase in the bulk (or depletion layer) resistance after cycling is attributed to the saturation of active sites on the semiconductor surface and bulk, while the increase in double-layer or grain boundary resistance implies the decomposition of salt species and subsequent deposition of products on the surface and the near vicinity of the electrode. A drop in constant phase element and hence capacitance is seen for both bulk and grain boundary components, which is attributed to the pore blockage and production of saltbased derivatives in the vicinity of the electrode. These observations are in line with the study by Moss et. al.61 The values of φ imply a mixed resistive-capacitive behaviour, as is clearly seen from the EIS, CV and GCD tests shown earlier. The values of φ indicate surface inhomogeneity.68,69 However, this is not indicative of inhomogeneity at the bulk of the composite electrode.70 The Ragone plot, further summarizes the superior rate performance of NRGO-MnO2/GCE (Figure 7). The maximum values of energy and power density were determined to be 90 Wh kg-1 and 5000 W kg-1, respectively. A detailed comparative study of supercapacitor performance of NRGO-MnO2 nanocomposite as presented in this work with graphene/MnO2 based or similar composites reported earlier has been summarized (Table 2).43,44,72,76-85 After careful evaluation, it can be concluded that the high surface area, porous and amorphous NRGO-MnO2 nanocomposite is a promising next generation supercapacitive electrode material. Moreover, the remarkable performance in absence of any conductive additives such as acetylene black etc. highlights its edge over traditional materials. 16 ACS Paragon Plus Environment

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NRGO-MnO2/GCE as a Peroxide Sensor The fast and reliable detection of hydrogen peroxide (H2O2) is very urgent owing to its extensive applications in paper, food and textiles industry, and chemical synthesis.86,87 Moreover, H2O2 is associated with many biological processes. Its improver level may cause severe diseases such as cancer, aging, diabetes.86-90 Though, various enzyme based electrodes have been incorporated to rapidly detect H2O2, however, they suffer from critical operating conditions such as pH and temperature, stability, high cost of enzymes, and limited activity. Therefore, non-enzymatic approach of H2O2 electrocatalysis with presently synthesized NRGO-MnO2 composite was used in the present work. The fabricated NRGO-MnO2/GCE was further employed as a peroxide (H2O2) sensor using 0.1 M PBS solution (Figure 8). The electrochemical properties of NRGOMnO2/GCE towards H2O2 were firstly investigated by CV technique. As observed in the absence of H2O2, NRGO-MnO2/GCE induces higher current than bare GCE and exhibits effective charge transfer capability (Figure 8a). Interestingly, NRGO-MnO2/GCE shows an oxidation peak around +0.65 V in the presence of H2O2, suggesting the oxidation of H2O2 with subsequent generation of an e-, which can be seen in the CV profiles both in the absence and presence of H2O2 (Figure 8b). It also indicates that the synergistic effects between NRGO-MnO2/GCE and H2O2 might have accelerated the electron transfer rate of electro-oxidation of H2O2. The DPV spectrum was also recorded and a clear oxidation peak at around +0.65 V was observed confirming the value of H2O2 oxidation potential (Figure 8c). To CV profile of NRGO-MnO2/GCE at a scan rate of 10 mV s-1 is separately shown in Figure 8d, which further assures that H2O2 oxidizes at around +0.65 V and transforms into water molecule with enhanced CV current indicating the generation of an electron. The probable mechanism of peroxide sensing at NRGO-MnO2/GCE can be demonstrated in terms of equations as follows:91,92 NRGO-MnO2 + H2O2 → NRGO-Mn (OH)2 + O2 (reduction) --------(6) NRGO-Mn(OH)2 + 2 OH- → NRGO-MnO2 + H2O2 + 2 e- (oxidation of Mn2+ to Mn4+) --- (7) Herein, H2O2 participates in reduction and oxidation of MnO2, as a result of which the reduction and oxidation current increases. H2O2 reduces MnO2 into bivalent species, and then Mn(II) oxidizes into Mn (IV). The NRGO acts as a catalyst to boost up this



process.91,92A schematic representation of the peroxide sensing mechanism has also been provided (Scheme 4).

Scheme 4. Schematic illustration of peroxide sensing mechanism. The effect of scan rate (50-500 mV s-1) in the presence of H2O2 on the oxidation peak currents of NRGO-MnO2/GCE was also recorded (Figure 8e), which shows a linear enhancement in CV current w.r.t. square root of scan rate. This observation advocates a diffusion-controlled process occurring at the surface of the electrode (Figure 8f).3 On the injections of different H2O2 concentrations also, NRGO-MnO2/GCE displays sensitive and linear current response (Figure 8g) with a straight line (R2=0.9924), underlining the potential of NRGO-MnO2 nanocomposite as H2O2 sensing material (Figure 8h). EIS Analysis of NRGO-MnO2/GCE for H2O2 Sensor The impedance spectroscopy graphs of NRGO-MnO2/GCE (in the absence and presence of H2O2) were recorded by applying an AC signal (1 Hz to 100 KHz) under open circuit potential and compared with bare GCE (Figure 9a). The electric circuit model was developed by modification of previous work by Jacobsen and West93 which accounts for the cylindrical symmetry as displayed by a GCE dipped in a large quantity of electrolyte. This assumption of the followed experimental setup closely correlates with the wire-like cylindrical shape of electrodes. The equivalent circuit diagram is shown in (Figure 9b). The distributed element “Zd” (representing diffusion impedance) is used to represent a model of an infinite transmission line that assumes a positive proportionality factor for resistance and a negative proportionality factor for capacitances, w.r.t. the rise in flow surface with distance from the


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electrode. The resultant perturbation in concentration near the interface of the electrode, is represented by ∆c(%& . In this case, a mix of empirical models and differential equations have been used to model the equivalent circuit parameters.3 Zd is represented by the complex components of the below equation:93 *ℰ /- 01 ∆,- 4,01 / 4,0 2 1 -

' ( = ∑ *,

----------------------- (8)

Where s is used to represent the Laplace transformation (to frequency domain), 6ℰ/ 682 is the derivative of the applied electric potential w.r.t. the concentration (of the chemical compound i). J is used to represent flux density of a given species.3 The values obtained from the fit are shown in (Table 3). Rct indicates charge-transfer resistance of the material for a given electrochemical process, while Cdl is the double-layer capacitance and Rd represents diffusion resistance. No significant changes are observed for the bare GCE in the presence or absence of H2O2, as it is not sensitive towards H2O2. However, interesting changes are observed when the NRGO-MnO2/GCE is analysed in the same experimental setup. Firstly, the values of Rct and Rd are reduced noticeably. The diffusion resistance decreases, indicating that the detection of H2O2 in this case is based on diffusion-limited processes, which is also confirmed by the cyclic voltammetry. Upon addition of analyte, the Rd decreases but the Rct increases. As the detection of analyte in this case is a diffusion-controlled process, this is consistent with the cyclic voltammetry result. The increase in Rct can be explained by the adsorption of oxygen and other ionic species, probably intermediate products of the catalysis process between H2O2 and MnO294 on the surface sites of the electrode.95 Amperometric Detection of H2O2 by NRGO-MnO2/GCE The electrochemical determination of H2O2 by NRGO-MnO2/GCE was further examined by chrono-amperometry in 0.1 M PBS solution by giving a fixed potential of +0.65 V under constant stirring conditions. However, prior to this the amperometric response were checked at three different potentials (+0.6 V, +0.65 V and +0.7 V). It was observed that highly sensitive responses towards H2O2 injections were observed at +0.65 V with very less signal variation (Figure S14). Therefore, in all further amperometric experiments +0.65 V was used as input potential. Figure 10a demonstrates the amperometric responses upon successive additions of different H2O2 concentrations at +0.65 V. Highly 19 ACS Paragon Plus Environment


sensitive and swift responses were noted for the NRGO-MnO2/GCE, which achieve a steady state within 2 s. The blow up of amperometric responses is shown in the inset of Figure 10a. The overall current response was found to be linearly proportional (R2=0.99888) to the H2O2 concentrations as indicated in the calibration plot (Figure 10b). The sensor response was linear in a wide dynamic range (0.4-121.2 µM) of H2O2 concentrations with a prominent detection limit of 24 nm and satisfactory sensitivity (2081 µAmM-1 cm-2). The distinguished sensing ability is due to the accelerated charge transfer and shorten ion diffusion path as a result of large surface area, high electronic conductivity and efficient porosity of NRGOMnO2 nanocomposite. Selectivity, Repeatability, Operational Stability and Storage Stability Test Selectivity is among the inconsequential features of the electrochemical H2O2 sensor. The selectivity of NRGO-MnO2/GCE towards H2O2 was judged in the presence of common agents such as citric acid, ascorbic acid, KCl, NaNO3, glucose, and uric acid. The amperometric responses were recorded towards the successive additions of H2O2 and 3-fold excessive addition of each of interfering agents, under similar conditions as described earlier. The fabricated sensor shows sensitive and stable responses towards successive H2O2 additions and doesn’t respond significantly towards any of interferents (Figure 11a). The anti-interference ability can be attributed to the positive synergism between NRGOMnO2/GCE and H2O2, which provides enhanced charge transfer efficiency. Furthermore, the Nafion layer present over material is a negatively charged membrane, which repels interfering agents such as uric acid and ascorbic acid also helps in achieving distinguished selectivity.3 Hence, the NRGO-MnO2/GCE shows selective response towards H2O2. A histogram has also been shown demonstrating the highly selective and sensitive response towards different H2O2 injections and negligible response towards other interfering agents (Figure 11b). The repeatability of the NRGO-MnO2/GCE has also been probed by observing the amperometric response towards 10 µM H2O2 for the successive 15 injections. The induced current response from NRGO-MnO2/GCE remains similar and stable displaying the good repeatability (Figure 11c). In the operation stability test, continuous amperometric (i-t curve) analysis was performed up to 1000 s and ~96 % retention of its initial current value was observed, highlighting the stability (Figure 11d). The storage stability tests were also successful as the 3 modified GCEs, which were kept in air-tight desiccators, didn’t display any obvious variation in the amperometric responses after a period of 15 days, suggesting their robustness. The electrodes. Moreover, this test demonstrates that a large arrays of 20 ACS Paragon Plus Environment

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similar electrode can be fabricated with same amperometric response and hence they can be revised (Figure S15). The comparison of H2O2 sensing performance of NRGO-MnO2/GCE with previously reported sensors concludes that the NRGO-MnO2/GCE has an edge in terms of LOD and sensitivity (Table S3). Also, the facile preparation of composite and subsequent electrode fabrication make the present work fascinating.



CONCLUSION In summary, NRGO-MnO2 nanocomposite was prepared by a simple yet effective one-pot hydrothermal method, in which MnO2 nanorods were found to be properly dispersed into the NRGO matrix. The fabricated composite based working electrode NRGOMnO2/GCE was used to investigate its supercapacitor and H2O2 sensing properties. The results reveal its ability to deliver noticeable specific capacitance (648 F g-1 at 1.5 A g-1) with remarkable rate ability (retains 80.20 % up to 10 A g-1) and excellent cycle life (96 % up to 800 cycles). Additionally, NRGO-MnO2/GCE can also be employed as peroxide sensor, which show remarkable sensing performance, as it can selectivity detect peroxide within a short response time (2 s) and wide dynamic range (0.4-121.2 µM) with a high sensitivity (2081 µAmM-1 cm-2) and notable LOD (24 nM). The comparison of supercapacitor and sensor performance with several related electrodes highlights its edge over others and demonstrates the multi-functionality of NRGO-MnO2 nanocomposite. The results presented herein pave the way for future investigations into engineering of sustainable materials at nanoscale for achieving desired results for varied applications.


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ACKNOWLEDGEMENTS Authors thank Sophisticated Instrumentation Centre (SIC), IIT Indore for providing the instrumentation facility. Authors thank Prof. G. Hundal and Dr. Sanyog Sharma, GNDU, Amritsar, for helping in TEM analysis. Authors gratefully acknowledge MNCF, CeNSE, IISc Bengaluru for providing XPS facility. M. S. and K. N. thank MHRD, New Delhi, India for providing fellowship. S. M. M. thanks SERB-DST (Project No. EMR/2016/001113), New Delhi and IIT Indore for financial support. Supporting Information All other data including, SEM images and XRD spectra of materials, Table comparing the surface area of previous reports, BET and BJH of NRGO, calculations of supercapacitor parameters, graphs related to supercapacitor performance of rGO, NRGO, MnO2, RGO-MnO2, PM-NRGO-MnO2, Table listing out the specific capacitances of different electrodes, SEM images of electrode, Amperometric results corresponding to peroxide sensing tests and Table comparing the peroxide sensing activity of different works, are present in Supporting Information file.



REFERENCES (1) Liu, S.; Hui, K. S.; Hui, K. N. Flower-like Copper Cobaltite Nanosheets on Graphite Paper as High-Performance Supercapacitor Electrodes and Enzymeless Glucose Sensors. ACS Appl. Mater. Interfaces, 2016, 8, 3258-3267. (2) Rajak, R.; Saraf, M.; Mohammad, A.; Mobin, S. M. Design and construction of a ferrocene based inclined polycatenated Co-MOF for supercapacitor and dye adsorption application. J. Mater. Chem. A 2017, 5, 17998-18011. (3) Saraf, M.; Natarajan, K.; Mobin, S. M. Multifunctional porous NiCo2O4 nanorods: sensitive enzymeless glucose detection and supercapacitor properties with impedance spectroscopic investigations. New J. Chem. 2017, 41, 9299-9313. (4) Gueon, D.; Moon, J. H. MnO2 Nanoflake-Shelled Carbon Nanotube Particles for HighPerformance Supercapacitors. ACS Sustainable Chem. Eng. 2017, 5, 2445–2453. (5) Bindu, K.; Sridharan, K.; Ma, A. K.; Lim, H. N.; Nagaraja, H. S. Microwave assisted growth of stannous ferrite microcubes as electrodes for potentiometric nonenzymatic H2O2 sensor and supercapacitor applications. Electrochim. Acta 2016, 217, 139-149. (6) Zhao, B.; Wang, T.; Jiang, L.; Zhang, K.; Yuen, M. M. F.; Xu, J.-B.; Fu, X.-Z.; Sun, R.; Wong, C.-P. NiO mesoporous nanowalls grown on RGO coated nickel foam as high performance electrodes for supercapacitors and biosensors. Electrochim. Acta 2016, 192, 205-215. (7) Saraf, M.; Rajak, R.; Mobin, S. M. A fascinating multitasking Cu-MOF/rGO hybrid for high performance supercapacitors and highly sensitive and selective electrochemical nitrite sensors. J. Mater. Chem. A 2016, 4, 16432-16445. (8) Hui, N.; Chai, F.; Lin, P.; Song, Z.; Sun, X.; Li, Y.; Niu, S.; Luo, X. Electrodeposited Conducting Polyaniline Nanowire Arrays Aligned on Carbon Nanotubes Network for High Performance Supercapacitors and Sensors. Electrochim. Acta 2016, 199, 234-241. (9) Saraf, M.; Natarajan, K.; Mobin, S. M. Emerging Robust Heterostructure of MoS2–rGO for High-Performance Supercapacitors. ACS Appl. Mater. Interfaces 2018, 10, 16588-16595. (10) Salunkhe, R. R.; Kaneti, Y. V.; Yamauchi, Y. Metal–Organic Framework-Derived Nanoporous Metal Oxides toward Supercapacitor Applications: Progress and Prospects. ACS Nano 2017, 11, 5293-5308. (11) Spanopoulos, I.; Tsangarakis, C.; Klontzas, E.; Tylianakis, E.; Froudakis, G.; Adil, K.; Belmabkhout, Y.; Eddaoudi, M.; Trikalitis, P. N. Reticular synthesis of HKUST-like tboMOFs with enhanced CH4 storage. J. Am. Chem. Soc. 2016, 138, 1568-1574. (12) Yassine, O.; Shekhah, O.; Assen, A. H.; Belmabkhout, Y.; Salama, K. N.; Eddaoudi, M. H2S Sensors: Fumarate‐Based fcu‐MOF Thin Film Grown on a Capacitive Interdigitated Electrode. Angew. Chem. Int. Ed. 2016, 55, 15879-15883. (13) Yao, L.; Yan, Y.; Lee, J.-M. Synthesis of Porous Pd Nanostructure and Its Application in Enzyme-Free Sensor of Hydrogen Peroxide. ACS Sustainable Chem. Eng., 2017, 5, 1248– 1252. (14) Hao, C.; Shen, Y.; Wang, Z.; Wang, X.; Feng, F.; Ge, C.; Zhao, Y.; Wang, K. Preparation and Characterization of Fe2O3 Nanoparticles by Solid-Phase Method and Its Hydrogen Peroxide Sensing Properties. ACS Sustainable Chem. Eng. 2016, 4, 1069–1077. (15) Saraf, M.; Natarajan, K.; Mobin, S. M. Non-enzymatic amperometric sensing of glucose by employing sucrose templated microspheres of copper oxide (CuO). Dalton Trans. 2016, 45, 5833-5840. (16) Lee, F.; Tsai, M.-C.; Lin, M.-H.; Ni'mah, Y. L.; Hy, S.; Kuo, C.-Y.; Cheng, J.-H.; Rick, J.; Su, W.-N.; Hwang, B.-J. Capacity retention of lithium sulfur batteries enhanced with nano-sized TiO2-embedded polyethylene oxide. J. Mater. Chem. A 2017, 5, 6708-6715. 24 ACS Paragon Plus Environment

Page 24 of 44

Page 25 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(17) Haregewoin, A. M.; Wotango, A. S.; Hwang, B.-J. Electrolyte additives for lithium ion battery electrodes: progress and perspectives. Energy Environ. Sci., 2016, 9, 1955-1988. (18) Zhang, L. L.; Zhao, X. S. Carbon-based materials as supercapacitor electrodes. Chem. Soc. Rev. 2009, 38, 2520-2531. (19) Chen, K.-J.; Lee, C.-F.; Rick, J.; Wang, S.-H.; Liu, C.-C.; Hwang, B.-J. Fabrication and application of amperometric glucose biosensor based on a novel PtPd bimetallic nanoparticle decorated multi-walled carbon nanotube catalyst. Biosens. Bioelectron. 2012, 33, 75-81. (20) Rakhi, R. B.; Nayak, P.; Xia, C.; Alshareef, H. N. Novel amperometric glucose biosensor based on MXene nanocomposite. Sci. Rep. 2016, 6, 36422. (21) Deep, A.; Saraf, M.; Neha; Bharadwaj, S. K.; Sharma, A. L. Styrene Sulphonic Acid Doped Polyaniline Based Immunosensor for Highly Sensitive Impedimetric Sensing of Atrazine. Electrochim. Acta 2014, 146, 301-306. (22) Saraf, M.; Dar, R. A.; Natarajan, K.; Srivastava, A. K.; Mobin, S. M. A Binder-Free Hybrid of CuO-Microspheres and rGO Nanosheets as an Alternative Material for Next Generation Energy Storage Application. ChemistrySelect 2016, 1, 2826-2833. (23) Miller, J. R.; Simon, P. Electrochemical Capacitors for Energy Management. Science 2008, 321, 651-652. (24) Simon, P.; Gogotsi, Y. Materials for electrochemical capacitors. Nat. Mater. 2008, 7, 845-854. (25) Rakhi, R. B.; Cha, D.; Chen, W.; Alshareef, H. N. Electrochemical Energy Storage Devices Using Electrodes Incorporating Carbon Nanocoils and Metal Oxides Nanoparticles. J. Phys. Chem. C 2011, 115, 14392-14399. (26) Saraf, M.; Natarajan, K.; Mobin, S. M. Microwave assisted fabrication of nanostructured reduced graphene oxide (rGO)/Fe2O3 composite as a promising next generation energy storage material. RSC Adv. 2017, 7, 309-317. (27) Wang, G.; Zhang, L.; Zhang, J. A review of electrode materials for electrochemical supercapacitors. Chem. Soc. Rev. 2012, 41, 797-828. (28) Mei, J.; Zhang, L. Anchoring High-dispersed MnO2 Nanowires on Nitrogen Doped Graphene as Electrode Materials for Supercapacitors. Electrochim. Acta 2015, 173, 338-344. (29) Gao, H.; Xiao, F.; Ching, C. B.; Duan, H. High-Performance Asymmetric Supercapacitor Based on Graphene Hydrogel and Nanostructured MnO2. ACS Appl. Mater. Interfaces 2012, 4, 2801-2810. (30) Ma, W.; Chen, S.; Yang, S.; Chen, W.; Cheng, Y.; Guo, Y.; Peng, S.; Ramakrishna, S.; Zhu, M. Hierarchical MnO2 nanowire/graphene hybrid fibers with excellent electrochemical performance for flexible solid-state supercapacitors. J. Power Sources 2016, 306, 481-488. (31) Wang, Z.; Ma, Y.; Hao, X.; Huang, W.; Guan, G.; Abudula A.; Zhang, H. A novel electroactive hybrid film electrode with proton buffer effect for detecting hydrogen peroxide and uric acid. J. Mater. Chem. A 2014, 2, 15035-15043. (32) Yan, S.; Wang, B.; Wang, Z.; Hu, D.; Xu, X.; Wang, J.; Shi, Y. Supercritical carbon dioxide-assisted rapid synthesis of few-layer black phosphorus for hydrogen peroxide sensing. Biosens. Bioelectron. 2016, 80, 34-38. (33) Kuo, C.-C.; Lan, W.-J.; Chen, C.-H. Redox preparation of mixed-valence cobalt manganese oxide nanostructured materials: highly efficient noble metal-free electrocatalysts for sensing hydrogen peroxide. Nanoscale 2014, 6, 334-341. (34) Mahmoudian, M. R.; Alias, Y.; Basirun, W. J.; Woi, P. M.; Sookhakian, M. Facile preparation of MnO2 nanotubes/reduced graphene oxide nanocomposite for electrochemical sensing of hydrogen peroxide. Sens. Actuator B-Chem. 2014, 201, 526-534. 25 ACS Paragon Plus Environment


(35) Xiang, C.; Li, M.; Zhi, M.; Manivannan, A.; Wu, N. A reduced graphene oxide/Co3O4 composite for supercapacitor electrode. J. Power Sources 2013, 226, 65-70. (36) Jia, W.; Guo, M.; Zheng, Z.; Yu, T.; Rodriguez, E. G.; Wang, Y.; Lei, Y. Electrocatalytic oxidation and reduction of H2O2 on vertically aligned Co3O4 nanowalls electrode: toward H2O2 detection. J. Electroanal. Chem. 2009, 625, 27-32. (37) Mai, Y.; Tu, J.; Gu, C.; Wang, X. Graphene anchored with nickel nanoparticles as a high-performance anode material for lithium ion batteries. J. Power Sources 2012, 209, 1-6. (38) Lei, Z.; Lu, L.; Zhao, X. S. The electrocapacitive properties of graphene oxide reduced by urea. Energy Environ. Sci. 2012, 5, 6391-6399. (39) Li, W.; Bose, A. B.; Rusakova, I. A. An approach for restoring the proton conductivity of sintered tin pyrophosphate membranes for intermediate temperature fuel cells. J. Power Sources 2016, 307, 146-151. (40) Fei, L.; Lei, S.; Zhang, W.-B.; Lu, W.; Lin, Z.; Lam, C. H.; Chai, Y.; Wang, Y. Direct TEM observations of growth mechanisms of two-dimensional MoS2 flakes. Nat. Commun. 2016, 7, 12206. (41) Wang, H.; Maiyalagan, T.; Wang, X. Review on recent progress in nitrogen-doped graphene: synthesis, characterization, and its potential applications. ACS Catal. 2012, 2, 781794. (42) Yu, G.; Hu, L.; Vosgueritchian, M.; Wang, H.; Xie, X.; McDonough, J. R.; Cui, X.; Cui, Y.; Bao, Z. Solution-Processed Graphene/MnO2 Nanostructured Textiles for HighPerformance Electrochemical Capacitors. Nano Lett. 2011, 11, 2905-2911. (43) Yang, S.; Song, X.; Zhang, P.; Gao, L. Facile Synthesis of Nitrogen-Doped GrapheneUltrathin MnO2 Sheet Composites and Their Electrochemical Performances. ACS Appl. Mater. Interfaces 2013, 5, 3317-3322. (44) Lei, Z.; Shi, F.; Lu, L. Incorporation of MnO2-Coated Carbon Nanotubes between Graphene Sheets as Supercapacitor Electrode. ACS Appl. Mater. Interfaces 2012, 4, 10581064. (45) Storck, S.; Bretinger, H.; Maier, W. F. Characterization of micro-and mesoporous solids by physisorption methods and pore-size analysis. Appl. Catal. A 1998, 174, 137-146. (46) Zhang, K.; Han, P.; Gu, L.; Zhang, L.; Liu, Z.; Kong, Q.; Zhang, C.; Dong, S.; Zhang, Z.; Yao, J.; Xu, H.; Cui, G.; Chen, L. Synthesis of nitrogen-doped MnO/graphene nanosheets hybrid material for lithium ion batteries. ACS Appl. Mater. Interfaces 2012, 4, 658-664. (47) Park, H. W.; Lee, D. U.; Nazar, L. F.; Chen, Z. Oxygen reduction reaction using MnO2 nanotubes/nitrogen-doped exfoliated graphene hybrid catalyst for Li-O2 battery applications. J. Electrochem. Soc. 2013, 160, A344-A350. (48) Indrawirawan, S.; Sun, H.; Duan, X.; Wang, S. Low temperature combustion synthesis of nitrogen-doped graphene for metal-free catalytic oxidation. J. Mater. Chem. A 2015, 3, 3432-3440. (49) Moghaddam, S. Z.; Sabury, S.; Sharif, F. Dispersion of rGO in polymeric matrices by thermodynamically favorable self-assembly of GO at oil–water interfaces. RSC Adv. 2014, 4, 8711-8719. (50) Hu, P.; Yan, M.; Wang, X.; Han, C.; He, L.; Wei, X.; Niu, C.; Zhao, K.; Tian, X.; Wei, Q.; Li, Z.; Mai, L. Single-nanowire electrochemical probe detection for internally optimized mechanism of porous graphene in electrochemical devices. Nano Lett. 2016, 16, 1523-1529. (51) Hassan, S.; Suzuki, M.; El-Moneim, A. A. Facile synthesis of MnO2/graphene electrode by two-steps electrodeposition for energy storage application. Int. J. Electrochem. Sci. 2014, 9, 8340-8354.


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(52) Samdani, J.; Samdani, K.; Kim, N. H.; Lee, J. H. A new protocol for the distribution of MnO2 nanoparticles on rGO sheets and the resulting electrochemical performance. Appl. Surf. Sci. 2017, 399, 95-105. (53) Yan, D.; Li, Y.; Liu, Y.; Zhuo, R.; Geng, B.; Wu, Z.; Wang, J.; Ren, P.; Yan, P. Design and influence of mass ratio on supercapacitive properties of ternary electrode material reduced graphene oxide@MnO2@poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate). Electrochim. Acta 2015, 169, 317-325. (54) Toupin, M.; Brousse, T.; Bélanger, D. Charge storage mechanism of MnO2 electrode used in aqueous electrochemical capacitor. Chem. Mater. 2004, 16, 3184-3190. (55) Wei, C.; Xu, C.; Li, B.; Du, H.; Nan, D.; Kang, F. Anomalous effect of K ion on crystallinity and capacitance of the manganese dioxide. J. Power Sources 2013, 225, 226230. (56) Brousse, T.; Toupin, M.; Dugas, R.; Athouël, L.; Crosnier, O.; Bélanger, D. Crystalline MnO2 as possible alternatives to amorphous compounds in electrochemical supercapacitors. J. Electrochem. Soc. 2006, 153, A2171-A2180. (57) Wu, T.; Wang, X.; Qiu, H.; Gao, J.; Wang, W.; Liu, Y. Graphene oxide reduced and modified by soft nanoparticles and its catalysis of the Knoevenagel condensation. J. Mater. Chem. 2012, 22, 4772-4779. (58) Kim, M.; Hwang, Y.; Min, K.; Kim, J. Concentration dependence of graphene oxidenanoneedle manganese oxide composites reduced by hydrazine hydrate for an electrochemical supercapacitor. Phys. Chem. Chem. Phys. 2013, 15, 15602-15611. (59) Sun, Y.; Zhang, W.; Li, D.; Gao, L.; Hou, C.; Zhang, Y.; Liu, Y. Facile synthesis of MnO2/rGO/Ni composite foam with excellent pseudocapacitive behavior for supercapacitors. J. Alloys Compd. 2015, 649, 579-584. (60) Wan, C.; Yuan, L.; Shen, H. Effects of electrode mass-loading on the electrochemical properties of porous MnO2 for electrochemical supercapacitor. Int. J. Electrochem. Sci. 2014, 9, 4024-4038. (61) Moss, P. L.; Au, G.; Plichta, E. J.; Zheng, J. P. Study of Capacity Fade of Lithium-Ion Polymer Rechargeable Batteries with Continuous Cycling. J. Electrochem. Soc. 2010, 157, A1-A7. (62) Zhang, H.; Wang, X.; Chen, C.; An, C.; Xu, Y.; Dong, Y.; Zhang, Q.; Wang, Y.; Jiao, L.; Yuan, H. Facile synthesis of diverse transition metal oxide nanoparticles and electrochemical properties. Inorg. Chem. Front. 2016, 3, 1048-1057. (63) Abram, E. J.; Sinclair, D. C.; West, A. R. A strategy for analysis and modelling of impedance spectroscopy data of electroceramics: doped lanthanum gallate. J. Electroceram. 2003, 10, 165-177. (64) Irvine, J. T. S.; Sinclair, D. C.; West, A. R. Electroceramics: characterization by impedance spectroscopy. Adv. Mater. 1990, 2, 132-138. (65) West, A. R.; Sinclair, D. C.; Hirose, N. Characterization of electrical materials, especially ferroelectrics, by impedance spectroscopy. J. Electroceram. 1997, 1, 65-71. (66) Lopes, T.; Andrade, L.; Ribeiro, H. A.; Mendes, A. Characterization of photoelectrochemical cells for water splitting by electrochemical impedance spectroscopy. Int. J. Hydrogen Energy 2010, 35, 11601-11608. (67) Nikolic, M. V.; Slankamenac, M. P.; Nikolic, N.; Sekulic, D. L.; Aleksic, O. S.; Mitric, M.; Ivetic, T.; Pavlovic, V. B.; Nikolic, P. M. Study of dielectric behavior and electrical properties of hematite α-Fe2O3 doped with Zn. Sci. Sinter. 2012, 44, 307-321. (68) Brug, Eeden, G. J.; A. L. G. V. D.; Sluyters-Rehbach, M.; Sluyters, J. H. The analysis of electrode impedances complicated by the presence of a constant phase element. J. Electroanal. Chem. 1984, 176, 275-295. 27 ACS Paragon Plus Environment


(69) Kerner, Z.; Pajkossy, T. On the origin of capacitance dispersion of rough electrodes. Electrochim. Acta 2000, 46, 207-211. (70) Duval, S. B. C.; Holtappels, P.; Vogt, U. F.; Pomjakushina, E.; Conder, K.; Stimming, U.; Graule, T. Electrical conductivity of the proton conductor BaZr0.9Y0.1O3− δ obtained by high temperature annealing. Solid State Ionics 2007, 178, 1437-1441. (71) Sandifer, J. R.; Buck, R. P. Impedance characteristics of ion selective glass electrodes. J. Electroanal. Chem. Interfacial Electrochem. 1974, 56, 385-398. (72) Peng, L.; Peng, X.; Liu, B.; Wu, C.; Xie, Y.; Yu, G. Ultrathin Two-Dimensional MnO2/Graphene Hybrid Nanostructures for High-Performance, Flexible Planar Supercapacitors. Nano Lett. 2013, 13, 2151-2157. (73) Yan, J.; Fan, Z.; Wei, T.; Qian, W.; Zhang, M.; Wei, F. Fast and reversible surface redox reaction of graphene–MnO2 composites as supercapacitor electrodes. Carbon 2010, 48, 38253833. (74) An, G.; Yu, P.; Xiao, M.; Liu, Z.; Miao, Z.; Ding, K. Low-temperature synthesis of Mn3O4 nanoparticles loaded on multi-walled carbon nanotubes and their application in electrochemical capacitors. Nanotechnology 2008, 19, 275709. (75) Xie, X. F.; Gao, L. Characterization of a manganese dioxide/carbon nanotube composite fabricated using an in situ coating method. Carbon 2007, 45, 2365-2373. (76) Zhao, B.; Lu, M.; Wang, Z.; Jiao, Z.; Hu, P.; Gao, Q.; Jiang, Y. Self-assembly of ultrathin MnO2/graphene with three-dimension hierarchical structure by ultrasonic-assisted co-precipitation method. J. Alloys Compd. 2016, 663, 180-186. (77) Zhang, L.; Li, T.; Ji, X.; Zhang, Z.; Yang, W.; Gao, J.; Li, H.; Xiong, C.; Dang A. Freestanding three-dimensional reduced graphene oxide/MnO2 on porous carbon/nickel foam as a designed hierarchical multihole supercapacitor electrode. Electrochim. Acta 2017, 252, 306-314. (78) Guo, W.-H.; Liu, T.-J.; Jiang, P.; Zhang, Z.-J. Free-standing porous Manganese dioxide/graphene composite films for high performance supercapacitors. J. Colloid Interface Sci. 2015, 437, 304-310. (79) Wang, P.; Zhou, C.; Wang, S.; Kong, H.; Li, Y.; Li, S.; Sun, S. Facial synthesis of MnO2/three dimensional graphene composite and its application in supercapacitors. J. Mater. Sci: Mater. Electron. 2017, 28, 12514–12522. (80) Ge, J.; Yao, H.-B.; Hu, W.; Yu, X.-F.; Yan, Y.-X.; Mao, L.-B.; Li, H.-H.; Li, S.-S.; Yu, S.-H. Facile dip coating processed graphene/MnO2 nanostructured sponges as high performance supercapacitor electrodes. Nano Energy 2013, 2, 505-513. (81) Liu, Y.; Miao, X.; Fang, J.; Zhang, X.; Chen, S.; Li, W.; Feng, W.; Chen, Y.; Wang, W.; Zhang, Y. Layered-MnO2 nanosheet grown on nitrogen-doped graphene template as a composite cathode for flexible solid-state asymmetric supercapacitor. ACS Appl. Mater. Interfaces 2016, 8, 5251-5260. (82) Veeramani, V.; Dinesh, B.; Chen, S.-M.; Saraswathi, R. Electrochemical synthesis of Au–MnO2 on electrophoretically prepared graphene nanocomposite for high performance supercapacitor and biosensor applications. J. Mater. Chem. A 2016, 4, 3304-3315. (83) Zhang, J.; Yang, X.; He, Y.; Bai, Y.; Kang, L.; Xu, H.; Shi, F.; Lei, Z.; Liu, Z.-H. δMnO2/holey graphene hybrid fiber for all-solid-state supercapacitor. J. Mater. Chem. A 2016, 4, 9088-9096. (84) Singu, B. S.; Yoon, K. R. Synthesis and characterization of MnO2-decorated graphene for supercapacitors. Electrochim. Acta 2017, 231, 749-758. 28 ACS Paragon Plus Environment

Page 28 of 44

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(85) Xiong, C.; Li, T.; Dang, A.; Zhao, T.; Li, H.; Lv, H. Two-step approach of fabrication of three-dimensional MnO2-graphene-carbon nanotube hybrid as a binder-free supercapacitor electrode. J. Power Sources 2016, 306, 602-610. (86) Yang, J.; Ye, H.; Zhao, F.; Zeng, B. A Novel CuxO Nanoparticles@ZIF‐8 Composite Derived from Core-Shell Metal-Organic Frameworks for Highly Selective Electrochemical Sensing of Hydrogen Peroxide. ACS Appl. Mater. Interfaces 2016, 8, 20407-20414. (87) Chen, K.-J.; Pillai, K. C.; Rick, J.; Pan, C.-J.; Wang, S.-H.; Liu, C.-C.; Hwang, B.-J. Bimetallic PtM (M = Pd, Ir) nanoparticle decorated multi-walled carbon nanotube enzymefree, mediator-less amperometric sensor for H2O2. Biosens. Bioelectron. 2012, 33, 120-127. (88) Zhang, X.; Liu, D.; Yua, B.; You, T. A novel nonenzymatic hydrogen peroxide sensor based on electrospun nitrogen-doped carbon nanoparticles-embedded carbon nanofibers film. Sens. Actuator B-Chem. 2016, 224, 103-109. (89) Wu, W.; Li, Y.; Jin, J.; Wu, H.; Wang, S.; Xia, Q. A novel nonenzymatic electrochemical sensor based on 3D flower-like Ni7S6 for hydrogen peroxide and H2O2. Sens. Actuator B-Chem. 2016, 232, 633-641. (90) Ray, C.; Dutta, S.; Roy, A.; Sahoo, R.; Pal, T. Redox mediated synthesis of hierarchical Bi2O3/MnO2 nanoflowers: a non-enzymatic hydrogen peroxide electrochemical sensor. Dalton Trans. 2016, 45, 4780-4790. (91) Li, L.; Du, Z.; Liu, S.; Hao, Q.; Wang, Y.; Li, Q.; Wang, T. A novel nonenzymatic hydrogen peroxide sensor based on MnO2/graphene oxide nanocomposite Talanta 2010, 82, 1637-1641. (92) Zhang, L.; Fang, Z.; Ni, Y.; Zhao, G. Direct electrocatalytic oxidation of hydrogen peroxide based on nafion and microspheres MnO2 modified glass carbon electrode. Int. J. Electrochem. Sci. 2009, 4, 407-413. (93) Jacobsen, T.; West, K. Diffusion impedance in planar, cylindrical and spherical symmetry. Electrochim. Acta 1995, 40, 255-262. (94) Do, S.-H.; Batchelor, B.; Lee, H.-K.; Kong, S.-H. Hydrogen peroxide decomposition on manganese oxide (pyrolusite): kinetics, intermediates, and mechanism. Chemosphere 2009, 75, 8-12. (95) Chen, H.; Jiang, J.-H.; Huang, Y.; Deng, T.; Li, J.-S.; Shen, G.-L.; Yu, R.-Q. An electrochemical impedance immunosensor with signal amplification based on Au-colloid labeled antibody complex. Sens. Actuator B-Chem. 2006, 117, 211-218.



Figure 1. (a,b) SEM, and (c,d) TEM images of NRGO-MnO2 nanocomposite at different magnifications, inset of Figure 1d shows the SAED pattern for NRGO-MnO2 nanocomposite, (e) high magnification TEM image, and (f) magnification of marked area showing the interplanar distance.


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Figure 2. (a) XPS survey scan spectra of NRGO-MnO2 nanocomposite, (b) the narrow spectra of C (1s), (c) the narrow spectra of N (1s), (d) the narrow spectra of Mn (2p), (e) the narrow spectra of O (1s) and (f) elemental analysis of NRGO-MnO2 composite.



Figure 3. N2 adsorption-desorption isotherm for NRGO-MnO2 nanocomposite and inset shows the corresponding BJH pore size distribution curve.


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Figure 4. (a) CV profiles of NRGO-MnO2/GCE at different scan rates (10-500 mV s-1) in 1 M Na2SO4, (b) CV curve of NRGO-MnO2/GCE at a scan rate of 25 mV s-1, (c) GCD curves for NRGO-MnO2/GCE at different current densities (1.5-10 A g-1), (d) GCD profiles of NRGO-MnO2/GCE at a current density of 1.5 A g-1.



Figure 5. (a) CV comparison of bare GCE, rGO/GCE, NRGO/GCE MnO2/GCE, RGOMnO2/GCE, PM-NRGO-MnO2/GCE and NRGO-MnO2/GCE at a scan rate of 100 mV s-1, in the potential window (-0.3 to +0.7 V) in 1 M Na2SO4, (b) Specific capacitance as a function of current density for NRGO-MnO2/GCE, (c) GCD comparison of bare GCE, rGO/GCE, NRGO/GCE, MnO2/GCE, RGO-MnO2/GCE, PM-NRGO-MnO2/GCE and NRGOMnO2/GCE a current density of 1.5 A g-1, in the potential window (-0.3 to +0.7 V) in 1 M Na2SO4, and (d) Cyclic stability analysis of NRGO-MnO2/GCE, where inset shows the first 20 cycles.


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Figure 6. (a) Nyquist plots for NRGO-MnO2/GCE before and after cycling vs. Ag/AgCl over the frequency range of 1 Hz to 100 KHz. Here Z’= real impedance and -Z’’= imaginary impedance, and (b) the equivalent circuit used to fit EIS data.



Figure 7. Ragone plot for NRGO-MnO2/GCE.


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Figure 8. (a) CV profiles of bare GCE and NRGO-MnO2/GCE in the absence of H2O2, (b) CVs of NRGO-MnO2/GCE in absence and presence of H2O2, (c) DPV curve for NRGOMnO2/GCE, (d) CV profiles of NRGO-MnO2/GCE at a scan rate of 10 mVs-1, (e) CV profiles of NRGO-MnO2/GCE at different scan rates (50-500 mVs-1), (f) the plot between anodic current and square root of scan rate, (g) CV profile of NRGO-MnO2/GCE at different injected concentration of H2O2 and (h) the calibration plot between current response and concentration, in 0.1 M PBS solution. 37 ACS Paragon Plus Environment


Figure 9. (a) Nyquist plots for bare GCE and NRGO-MnO2/GCE in the absence and presence of H2O2 vs. Ag/AgCl over the frequency range of 1 Hz to 100 KHz. Here Z’= real impedance and -Z’’= imaginary impedance, and (b) the equivalent circuit used to fit EIS data for peroxide sensor.


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Figure 10. (a) Amperometric response for NRGO-MnO2/GCE towards different injected concentrations of H2O2 in 0.1 M PBS, where inset shows the blow up of low concentration region, (b) calibration plot between current response and injected H2O2 concentrations.



Figure 11. (a) Selectivity test for NRGO-MnO2/GCE towards H2O2 in the presence of some interference species such as citric acid (CA), ascorbic acid (AA), KCl, NaNO3, glucose (Glu) and uric acid (UA), (b) amperometric current response ratio of interfering species on that of H2O2 (c) repeatability test for NRGO-MnO2/GCE with successive injections of 10 µM H2O2, (d) long time amperometric response test for NRGO-MnO2/GCE with 10 µM H2O2 injection.


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Table 1. Circuit parameters obtained after fitting EIS data to the presented equivalent circuit for supercapacitor. Parameter

NRGO-MnO2/GCE

Rs (Ω)

16.305

16.81

Rbulk (Ω)

79.58

194

CPEbulk (F)

1.47167 x 10-4

7.198 x 10-5

φbulk

0.5487

0.8105

Rgb (Ω)

1.8846 x 104

2.59 x 104

CPEgb (F)

2.32249 × 10-3

9.227 × 10-4

φgb

0.5752

0.5944

NRGO-MnO2/GCE after cycling



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Table 2. A comparison of present work with some previous reported supercapacitors based on same materials composition. Material

Electrode Used

Electrolyte Medium

Rate Performance

Sp. Cap. (F g-1)

Rate (mV s-1)/(A g-1)

Stability

Reference

NGMC

Nickel foam (1×1 cm2)

1 M Na2SO4

~74.9 % up to 2 A g-1

257.1

-/0.2

94.2 % after 2,000 cycles

43

MnC/RGO

Nickel foam (1×1 cm2)

1 M Na2SO4

54.4 % up to 5 A g-1

193

-/0.2

94-96 % after 1300 cycles

44

2D δMnO2/graphene hybrid

PET Substrate

PVA/H3PO4 gel

77.9 % up to 10 A g-1

267

-/0.2

92 % after 7,000 cycles

73

3D hierarchical MnO2/graphene

Nickel foam (1×1.5 cm2)

1 M Na2SO4

56.52 % up to 10 A g-1

216

-/1

89.3 % after 2000 cycles

76

3D reduced graphene oxide/MnO2

Carbon/nickel foam

1 M Na2SO4

51.12 % up to 40 mV s-1

356.5

10/-


77

Porous MnO2 /rGO films

Swagelok-type cells

1 M Na2SO4

46% up to 40 A g-1

266.3

-/0.2

85.1 % after 2,000 cycles

78

MnO2 /3D graphene composite

Ni mesh (1x1 cm2)

1 M Na2SO4

81.48 % up to 4 A g-1

324

-/0.4


79

Sponge@RGO @MnO2

Pt-sheets

1 M Na2SO4

-

450

2/-

90 % after 10,000 cycles

80

Layered-MnO2 on nitrogendoped graphene

Graphite paper

5 M LiCl

65.6 % up to 100 mV s-1

305

5/-

>90 % after 1500 cycles

81

Au-MnO2 on graphene

Stainless steel (1x1 cm2)

0.5 NaOH

69.56 % up to 10 A g-1

575

-/2.5

74 % after 1000 cycles

82

δ-MnO2/holey graphene hybrid

Ni foam (1x1 cm2)

1 M Na2SO4

64 % from 0.05-0.6 mA cm−2 (device)

245

-/1


83

rGO-MnO2 nanocomposite

GCE

1 M Na2SO4

63 % up to 10 A g-1

290

-/1


84

Ti substrate

1.5 M Li2SO4

56.69 % up to 400 mV s-1

330.75

200/-


85

GCE (3 mm diameter)

1 M Na2SO4

80 % up to 10 A g-1

648

-/1.5

93.5% after 2000 cycles

Present work

3D MnO2graphenecarbon nanotube hybrid NRGO-MnO2 nanocomposite


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Table 3. Circuit parameters obtained after fitting EIS data to the presented equivalent circuit for H2O2 sensor. Parameter

bare GCE

bare GCE

NRGO-MnO2/GCE

NRGO-MnO2/GCE

(without H2O2)

(with H2O2)

(without H2O2)

(with H2O2)

Rs (Ω)

1.1038 × 102

1.3371 × 102

1.5461 × 102

1.5004 × 102

Rct (Ω)

7.11355 × 103

7.6918 × 103

3.75651 × 10-3

1.05081 × 103

Cdl (F)

3.50929 × 10-7

3.454 × 10-7

2.27067 × 10-7

3.599 × 10-7

Rd (Ω)

2.7822 × 108

9.40867 × 108

1.08165 × 106

2.879 × 104

Cd (F)

7.80025 × 10-2

5.64315 × 10-2

7.87342 × 10-5

4.65 × 10-6



A robust NRGO-MnO2 nanocomposite contributes in environment sustainability through effective energy storage and peroxide sensing activity. 122x74mm (96 x 96 DPI)

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Robust Nanocomposite of Nitrogen-Doped ... - ACS Publications

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