Self-Recharging Reduced Graphene Oxide-Prussian Blue Electrodes

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Self-Recharging Reduced Graphene OxidePrussian Blue Electrodes for Transparent Batteries Pedro Henrique Trindade Soares, and Edson Nossol ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b02122 • Publication Date (Web): 28 Mar 2019 Downloaded from http://pubs.acs.org on March 30, 2019

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ACS Applied Nano Materials

Self-Recharging Reduced Graphene Oxide-Prussian Blue Electrodes for Transparent Batteries

Pedro Henrique Trindade Soaresa and Edson Nossola* aFederal

University of Uberlândia, Chemistry Institute, 38400-902, Uberlândia, MG, Brazil.

*[email protected]

Abstract Numerous applications are dependent on the preparation of transparent thin films, including electrochromic windows, touch screens, and solar cells; however, transparent batteries, an essential component in fully integrated transparent devices, have not yet been properly explored in the literature. Here, we report the preparation of a transparent rGO/Prussian blue selfrechargeable cathode battery. Through the combination of rGO, iron hexacyanoferrate electrode exhibits higher crystallinity, better electrical conductivity, and excellent electrochemical stability. Due to the singular interfacial method used for the film preparation, an intimate contact between PB nanoparticles and the carbonaceous material was provided, with superior electrochemical performance in potassium-ion batteries. As a cathode, the rGO/PB film yields an initial discharge capacity of 120 mA h g-1 at a current of 173.7 mA g-1, and an excellent specific capacity stability, with the maintenance of 95.5% of initial value after the application of 50 charge/discharge cycles. Moreover, the rGO/PB film can be used as a self-recharging electrode, that is, recovering 88% of the total specific capacity of PB cathode by reacting with oxygen in the air. Our results represent a step forward and make this material very attractive for use in self-recharging electronic devices made from transparent potassium-ion batteries.

Keywords: carbon, hexacyanoferrate, energy storage, film, nanomaterial

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Introduction

The development of new materials for application on energy storage is a key challenge for the scientific community. Taking into account that energy application domain relies on the design of modified electrode materials with specifically engineered composition,1 structure, and properties, it becomes necessary the development of new materials for application in energy storage devices,2-4 especially combining fast, reversible and stable redox properties.5 One of the candidates that match these characteristics is the metal hexacyanoferrates, specifically the iron hexacyanoferrate, called Prussian blue (PB).6-9 PB presents a face-centered structure in which Fe2+ and Fe3+ are alternating on the lattice. This type of structure confers a zeolite character to hexacyanoferrates, allowing them insertion/extraction of different ions in solution, such as Na+, K+, Mg2+, Ca2+, Sr2+, Ba2+, and Al3+.10-11. This process is coupled with oxidation/reduction of iron species in the structure. Among the various electrode materials, hexacyanoferrates received great attention from the electrochemist community because they offer attractive properties (e.g., rich surface chemistry, templating and hosting capabilities, catalytic or redox activity, selective recognition or permselective properties), being promising notably in the field of electrochemical energy devices.12-16 However, PB is characterized by poor electrochemical stability at neutral media.17-18 One strategy to overcome this issue is the preparation of carbon/Prussian blue composites,19-21 such as reduced graphene oxide and carbon nanotubes. Wang et al reported a synergistic effect between reduced graphene oxide (rGO) and PB, with the resulting nanocomposite material presenting high electrochemical properties as a cathode for sodium ion battery.22 Furthermore, the use of carbon materials can provide a porous arrangement structure, which is very important in electrochemical performance.13, 23-27 Graphene-based materials can be used in storage devices especially because of their impressive electrical conductivity, so acting as a conductive agent as well as an encapsulating carbon matrix. Due to its superior thermal conductivity, graphene also can be important for dissipating the heat generated in the case of high current loads during the charge/discharge process.28


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Although Prussian blue and its analogs have pointed out as a new class of materials for the construction of sodium and potassium batteries,15, 29-34 the cathodic component has been little exploited as a transparent thin film and/or flexible electrode.35-36 The preparation of a transparent electrode could open up the possibility to construct a clear battery and, as a consequence, to make whole devices (cell phones, tablets, cameras, watches) transparent.37 On the other hand, charging of batteries request the application of electrical potential, consuming electricity. So, it would be of great interest in the development of self-recharged batteries based on spontaneous chemical reactions. Wang et al. reported the preparation of a self-recharged battery using PB as a cathode, and although the mentioned device did not present high values of specific capacity and cycling performance, the as-prepared device showed promising applications in different fields.36 Since the main objective of this work is to prepare a transparent rGO/PB thin film used as a cathode for potassium ions battery, the colloidal synthesis will present some drawbacks, especially regarding the post-processing steps for film deposition. Generally, the available methods such as filtration transfer, spin coating, dip coating, electrophoresis and drop casting need a good solubility of the material (which is not the case of nanoparticles/carbonaceous nanocomposites) to guarantee the homogeneity and optical quality of the film. Moreover, to maintain the stability of PB nanoparticles prepared through typical colloidal synthesis, many types of anti-aggregation agents such as polymers,19 surfactants,38 and ionic liquids,39 are commonly used, which is not the case of liquid-liquid approach, since the interfacial region provides a confined ambient that inhibits nanoparticles aggregation. In this work, we demonstrate the preparation of rGO/PB films through a liquid-liquid interface approach (interfacial method) for application as a cathode for transparent self-rechargeable potassium ion battery. The interfacial method consists of an immiscible aqueous/organic biphasic system, with the rGO dispersed in the organic phase and PB precursors presented in the aqueous phase. Over time, and under stirring, a transparent and freestanding film is obtained spontaneously at the organic−water interface. This method enables the film thickness and morphological control by means of varying the concentration of carbon nanotubes dispersion and aqueous solution, which is amenable to optimizing the optoelectrical properties of final devices.40-41



Experimental Section Reagents and sample Potassium ferricyanide K3[Fe(CN)6] (Proquimios, 99.0%), FeCl3 (Synth, 97%), KCl (Synth, 99.0%), toluene (Synth), HCl (Quimibrás, 37% m/v) and H2O2 (Synth, 29%) were used as received. Graphite oxide was synthesized using a modified Hummers method with posterior sonication and reduction with NaBH4 for the attainment of rGO.

RGO/PB nanocomposite film preparation The synthesis of rGO/PB nanocomposite film was carried out through the interfacial method.42 A dispersion containing 2.0 mg of rGO in 20 ml of toluene as organic phase was added to an aqueous phase containing 20 mL of a solution of potassium ferricyanide and ferric chloride (1 x 10-4 mol L-1 pH 2). After one hour of magnetic stirring 10 μL of a 0.5 mol L-1 solution of H2O2 was added. The system remained on stirring for 4 hours. After this time, a freestanding film of rGO/PB was spontaneously formed at the organic−water interface. A representation of the organic-water interface preparation and an image of the transparent rGO/PB film obtained is shown in Scheme 1. The film was transferred to a beaker containing distilled water and removed using a copper haste. Control experiments using only the rGO dispersion or the aqueous phase precursors were also carried out using the same conditions.

Scheme 1. Representation of the organic/water interfacial synthesis approach and the resulted transparent rGO/PB nanocomposite film. Adapted under Creative Commons 3.0 Attribution from James Hedberg Collection.


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Electrochemistry and charge-discharge tests Electrochemical and charge/discharge measurements were performed using a Metrohm Autolab PGSTAT12 on three-electrode cells containing a 0.1 mol L-1 KCl solution (pH = 2). Films of rGO, PB or rGO/PB deposited on ITO substrates were used as working electrode, an Ag/AgCl as a reference electrode,43 and a platinum wire as counter electrode. For insertion of oxygen in the electrolyte during self-recharging tests it was used an Acqua Flux A01 air compressor. Electrochemical impedance spectroscopy measurements were carried out in a Metrohm Autolab PGSTAT 128N potentiostat/galvanostatic equipped with a FRA2 module by applying an AC voltage with 5 mV amplitude in a frequency range of 0.05 Hz–3.5 kHz in the three-electrode configuration.

Other Instrumentation X-ray diffraction patterns were obtained with a Shimadzu XRD-6000 diffractometer using CuKα radiation at 40 kV and 30 mA. UV-Vis measurements of the films deposited on quartz substrates were carried out in Shimadzu UVPC 2501 equipment. Raman spectra were collected in a WITec Alpha 300R spectrometer using an Ar+ laser (λ= 532 nm). Infrared spectra were obtained using a PerkinElmer equipment in the 4000 to 600 cm-1 range. Scanning electron microscopy (SEM) images were collected from films samples deposited on silicon using a Tescan Mira FEG-SEM instrument at 10 kV. Transmission electron microscopy (TEM) imagens were obtained in high resolution mode using a Hitachi HT7700 microscope at 120 kV.

Results and discussion

To investigate the structure and composition of rGO, PB and rGO/PB, the materials were characterized by X-ray diffraction (XRD), infrared (IR), ultraviolet-visible (UVis) and Raman spectroscopies. The structure of the nanocomposite films was investigated through XRD (Fig. 1a). The pattern for rGO presents its characteristic (002) peak at 26.5°. It is interesting to note that this peak is narrower than those observed in literature,44 which can be related to the high two-



dimensional order of rGO layers in the film.45 The diffractogram for PB shows peaks at 2θ= 17.4, 26.6, 28.4, 35.3, 39.5, 43.6, 50.9, 54.0 and 57.3, corresponding to the indicated Miller indices and indexed to the face-centered cubic structure (JCPDS 52-1907).46 The rGO/PB profile presents all indexed peaks for both rGO and PB materials. It is important to highlight that PB peaks in the nanocomposite are narrower compared with the individual PB material. This result indicates that the presence of carbonaceous material induces a higher crystallinity in the hexacyanoferrate nanoparticles. Fig. 1b illustrates the UV-Vis spectra for the different nanocomposite films. It is noted for PB, bands in the 200-420 nm region, corresponding to ligant-metal charge transfer. The intense color observed for PB is related to intervalence transitions (metal-ligant-metal) between Fe2+ and Fe3+, centered at 700 nm.47 The rGO film presents a characteristic band at 267 nm attributed to π→ π* and related to restoration of π C=C bond due to the reduction of GO precursor and consequent rGO formation. It is interesting to note that for rGO/PB, besides the presence of the previously discussed bands, the spectrum exhibits a significant increase in absorption values compared with the individual components. This fact is related to the stabilization of the film in the interface occurred when the two components are present since the carbonaceous material acts as a building block for PB growing. From UV-Vis spectra, the transmittance of the films at 550 nm was calculated (Fig. S1), showing values from 79 to 94% for rGO and PB, respectively, to 63% for rGO/PB nanocomposite. Raman spectroscopy data (Fig. 1c) for rGO present two characteristic bands of carbonaceous materials. The D band, centered at 1350 cm-1, associated with sp3 C-C atoms, structural defects, and heteroatoms and the presence of G band (1576 cm-1), assigned to in-plane sp2 C=C stretching. It is also noted the so-called 2D band (2675 cm-1) related with the degree of structural organization in the two-dimensional plane, and due to the reduction process during rGO synthesis, it presents low intensity. The spectrum of PB shows the bands attributed to the bending mode of Prussian blue at 286 cm-1 (Fe-C-N-Fe), and stretching modes of hexacyanoferrate, centered at 550 (Fe-CN) and 625 cm-1 (Fe-C), besides the intense band at 2146 cm-1, attributed to C≡N vibration mode.48 In addition, it is noticed a band at 1095 cm-1, attributed to the glass substrate. The appearance of this signal only for hexacyanoferrate material is related to the smaller


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thickness of PB film, in accordance with UV-Vis spectra (Fig 1b). The rGO/PB spectrum reveals all bands of individual nanomaterials, attesting the success of the synthesis. The ID/IG bands ratio can act as a structural organization parameter for carbonaceous material. It is noted an increase in this value (2.51) for rGO/PB compared with rGO film (1.80). This fact can be associated with the presence of defects/edge planes in graphene layers when the nanocomposite preparation.

Fig. 1 (a) XRD patterns; (b) UV-Vis data; (c) Raman spectra (λ= 514 nm); and infrared data (d) of rGO, PB and rGO/PB.

To confirm the formation of the nanocomposite, the IR-ATR spectra (Fig. 1d) from rGO, PB and rGO/PB were compared. The spectrum of rGO indicates the presence of residual functional groups in the carbon material, showing bands corresponding to C=O (1710 cm-1), C=C (1565 cm1),

C-O (1415 cm-1), epoxy groups (1220 and 1068 cm-1) , C-H stretching modes (630, 2840 e

2910) and hydroxyl groups related with alcohol (3370 cm-1) and carboxylic acid (3780 cm-1).49 The PB film presents an intense band at 2060 cm-1 assigned to C≡N stretching. It is also noted



bands at 3370 and 1610 cm-1, attributed to O-H stretching and H-O-H angular deformation modes, respectively. These bands are associated with the presence of interstitial water in the structure of PB.50 The rGO/PB spectrum shows all bands related to the individual components, attesting the formation of the nanocomposite film. The SEM image of rGO (Fig. 2a) shows uniformly distributed graphene sheets deposited on the substrate. For PB film (Fig. 2b) it is observed irregular particles distributed within a wide range of 100-500 nm in size. For rGO/PB nanocomposite (Fig. 2c-d) SEM images show PB nanoparticles wrapped in graphene layers. HRTEM images of rGO/PB film (Fig. 2e-f) reveals that PB nanoparticles had uniform and well anchored growth on rGO sheets. The histogram (Figure 2-g) obtained from HRTEM images also reveal that such PB nanoparticles are well distributed in size and have diameters of about 20 nm. The difference in size of PB nanoparticles indicates that graphene sheets make the PB more resistive toward coalescence during the precipitation process. The formation of Prussian blue was carried out using potassium hexacyanoferrate(III) and FeCl3 as precursors, with H2O2 acting as a reductant agent, according to the following reactions:51

2[FeIII(CN)6]3- + H2O2 → 2[FeII(CN)6]4- + 2H+ + O2 (1) 4Fe3+ + 3[FeII(CN)6]4- → FeIII4[FeII(CN)6]3 (2)

During the synthesis, Fe3+ ions are confined on the surface of the carbon material via electrostatic and coordination forces, especially due to strong interaction with residual oxygenated groups on rGO. These interactions allow PB nanoparticles to attach on rGO, and as consequence, the formation of the transparent film. Additional FEG-SEM images confirming that graphene layers are coating PB nanoparticles are displayed in Fig. S2.


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Fig. 2 SEM images of the different films: (a) rGO; (b) PB; (c) and (d) rGO/PB. HRTEM images of rGO/PB nanocomposite film (e) and (f). Histogram showing size distribution for rGO/PB film.



The cyclic voltammetric (CV) response of the rGO/PB film is shown in Fig. 3a. One pair of reduction/oxidation peaks noticed at the potentials of 0.27 and 0.12 V can be assigned to the oxidation of PW to PB and the respective reduction of PB to PW. A second redox pair is observed at 0.99 and 0.82 V, corresponding to oxidation of PB to Berlin green (FeIII[FeIII(CN)6]) and its respective reduction. CV curve reveals the high stability of the nanocomposite film, with no substantial decrease (less than 15%) in current after the application of 50 cycles. Fig. 3b presents the CV curves of the rGO/PB nanocomposite at different scan rate in the potential region from −0.3 to 0.6 V.

Fig. 3 (a) Continuous cyclic voltammograms (50 cycles) of the rGO/PB film at 50 mV s-1; (b) Cyclic voltammograms of rGO/PB electrode at different scan rates (10-100 mV s-1) and (c) corresponding dependence of peak current on the square root of scan rate; and (d) Nyquist plots of rGO, PB and rGO/PB films.


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Fig. 3c shows the respective linear dependence of the current intensity upon the square root of scan rate. A linear relationship between the peak currents of rGO/PB and the square root of the scan rates (v1/2) from 10 to 50 mV s−1 was observed, indicating that electrochemical behavior is a solution diffusion-limited process. At higher scan rates (50 to 100 mV s−1), the peak currents were proportional to the scan rate. This indicates that reaction kinetics change from a solution diffusionlimited process to a surface process. The porosity and high surface area, along with the presence of residual oxygenated groups in the carbonaceous material could be responsible for the change in mass transport regime in the film.52-53 To understand the superior electrochemical performance of rGO/PB nanocomposite, EIS was performed in 0,1 mol L-1 KCl. Fig 3d shows the Nyquist plots of the different films. Generally, the data for rGO exhibit a characteristic plot of electrical double layer capacitors, with nearly vertical shape at lower frequencies.54 However, this behavior is not so obvious for the curve of rGO film prepared in this work, which is attributed to the excellent electronic properties of rGO, providing a faster electron conduction pathway between the electrode and the electrochemical probe.55 It is observed for PB film a high charge transfer resistance value (Rct= 2260 Ω), which was calculated using the semicircle portion at high frequencies in Nyquist diagrams (fitting details in Fig.S3). This result is related to the semiconductor characteristic of PB and its low crystallinity, as attested by XRD (Fig. 1a).56 When PB was modified by the presence of rGO, the Rct value dramatically decreased to 17.9 Ω. Therefore, the rGO can substantially improve the electrochemical performance of PB. This fact is mainly ascribed to an increase of electron transfer in PB and efficient access of electrolyte ions proportioned by the presence of the carbonaceous material. It is also noted for rGO/PB film a slight semicircle in the high-frequency region, indicating that the mass transfer process is mainly controlled by diffusion, confirming the results obtained by scan rate versus current analyses (Fig. 1c). It was also calculated the n factor, which is known as a roughness coefficient. Its value generally decreases with the increase in surface roughness, and a smooth surface often has n > 0.8. RGO/PB material revealed a value of n=0.531, indicating a high roughness for the film, which can be the cause of changing in mass transport process at higher scan rate values.



Fig. 4a presents the specific charge and discharge capacity of the original and selfrecovered rGO/PB electrode. The calculated discharge capacity at 173.7 mA g-1 was 120 mA h g-1, which is two-fold higher compared with self-rechargeable PB/Al36 cell and copper hexacyanoferrate battery,16 and superior to LiFeO2 transparent electrode.57 This result can be related to the following aspects: 1) Interfacial method provides the obtaining of a thin film with improved contact between Prussian blue and rGO, maximizing the synergistic effect and improving the electrochemical performance; 2) improved the electrical conductivity of the film through the combination of PB with rGO, as demonstrated by EIS studies. Since the presence of the carbonaceous material seems to be crucial for the improved charge/discharge performance of the nanocomposite film, we carried out the same test using only a PB film (synthesized using the identical conditions of rGO/PB material). It is observed in Fig 4a that specific capacity decays significantly for the single component film (38 mA h g-1). This result confirms the key role of graphene derivate material for PB performance as a cathode. The spontaneous self-recharging process of the rGO/PB film was studied with 11 hours of recovery time in deionized water bubbled with atmospheric air. After initial cycling, the recovered rGO/PB nanocomposite can still provide a discharge capacity of 105.6 mA h g-1 (Fig. 4a), with the maintenance of 88% of the total capacity compared with the first discharge process (120 mA h g-1). In addition, we have found that rGO/PB has an excellent stability during galvanostatic cycling between -0.2 and 0.8 V at a current density of 173.7 mA g-1 (Fig. 4b), since 95.5 % of the initial discharge capacity of 120 mA h g-1 was retained after 50 cycles. These results are very superior to the previous results presented for hexacyanoferrate self-rechargeable battery,36 and may be attributed to the following advantages. First, the smaller and well distributed PB nanoparticles on the rGO film provide much shorter diffusion lengths for the potassium ions and reduce the strain–stress induced by volume changes, thus allowing the material to maintain its structural stability. Second, the rGO film can retain the oxygen on the surface of the film,58 facilitating the oxidation of PB and consequent recharging process according to the equations (3) and (4):36


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KFeIII[FeII(CN)6] + K+ + e- ⇄ K2FeII[FeII(CN)6] (3) 4K2FeII[FeII(CN)6] + O2 + 2H2O ⇄ 4KFeIII[FeII(CN)6] + 4K+ + 4OH- (4)

Although the nanocomposite film does not present an excellent rate capability (Fig. 4c), the as-prepared rGO/PB material is potentially capable of providing a reasonably large total capacity in a limited volume, applying a lower current density of 173.7 mA g-1, since the electrode is constituted by a thin film structure. Stabilization of Coulombic efficiency in the first cycles is very important for commercialization of these materials as batteries.

Fig. 4 (a) Galvanostatic charge and discharge curves and self-discharging profile for PB and rGO/PB films after recovery process in deionized water bubbled with O2. (b) Cycling performance of rGO/PB nanocomposite film applying 33 A. (c) Discharge curves of the rGO/PB electrode at different current values (27, 30, 33, 35 and 37 A). (d) Energy density and Coulombic efficiency upon the application of 50 charge/discharge cycles applying 33 A.



The capacity of a battery is directly related to the extent that Prussian blue consumes K+ ions, so if the Coulombic efficiency is not sufficiently high, the Prussian blue continuously consumes potassium ions, causing a decrease in the discharge time.59 For rGO/PB electrode was observed a stabilized Coulombic efficiency of 98,3% in the first 5 cycles (Fig. 4d). Another important parameter is the energy density, which is related to the ability of a material to be applied as a battery, so the higher the energy density the greater its potential in battery application.60 It was obtained an energy density of 20.29 W h kg-1 for rGO/PB film (Figure 4d), that remained constant after 50 cycles, which proved to be superior to lead-acid batteries.61 Rate performance is another important parameter for practical applications in energy storage devices. So, we evaluated the performance of rGO/PB electrode at different currents, as shown in Fig. 5. It is observed that the nanocomposite film can retain 97, 87, 63, and 48% specific capacity at 805, 833, 861, and 972 mA g-1, respectively. Furthermore, a capacity of 129.1 mA h g-1 (99.1%) can be recovered once the current is restored to the initial value at 750 mA g-1, demonstrating an excellent reversibility.

Fig. 5 Rate performance for rGO/PB nanocomposite film obtained in different current range.

Moreover, it is possible to clear observe a huge amount of spherical PB nanoparticles decorating rGO before (Figure S4-a) and after (Figure S4-b) the application of 50 cycles of charge/discharge. This result confirm the stability and morpholy preservation of rGO/PB film after the above mentioned process.


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Conclusions In summary, we have prepared a rGO/PB nanocomposite film through the interfacial method, providing an intimate contact between the components, which provided the control of PB morphology, increasing material crystallinity and conductivity. As a cathode material for potassium ion battery, the as-prepared rGO/PB film exhibits good specific capacity (120 mA h g-1) and cycle life (95.5% stability). In addition, the rGO/PB electrode can be used as a selfrecharging battery, that is, recovering 88% of the total specific capacity of PB cathode by reacting with oxygen in the air. The low-cost synthesis makes this material very attractive for use in energy devices for grid-scale storage. Moreover, rGO/PB as a film structure can be operated in inexpensive, light, safe and a highly conductive potassium chloride aqueous electrolyte, compared with current organic electrolyte dependent lithium-ion cells.

Acknowledgments The authors would like to acknowledge the support from FAPEMIG (APQ-01207-17), CNPq (406529/2016-7), FINEP, CAPES and GMIT research group supported by FAPEMIG (APQ00330-14). The authors would also like to thank Materials Chemistry Group at Federal University of Paraná for assistance in obtaining SEM images, NUPE – Federal University of Uberlândia for EIS data, Multiuser Laboratory of Chemistry Institute at the University of Uberlândia for support involving XRD experiments, and Center of Advanced Microscopy from the Institute of Biomedical Sciences for all support in the acquisition of HRTEM images. We also would like to thank Mackgraphe – Mackenzie Presbyterian University for rGO sample and Raman experiments.

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Table of contents



Scheme 1. Representation of the organic/water interfacial synthesis approach and the resulted transparent rGO/PB nanocomposite film. Adapted under Creative Commons 3.0 Attribution from James Hedberg Collection. 290x71mm (150 x 150 DPI)


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Fig. 1 (a) XRD patterns; (b) UV-Vis data; (c) Raman spectra (λ= 514 nm); and infrared data (d) of rGO, PB and rGO/PB. 296x209mm (96 x 96 DPI)



Fig. 2 SEM images of the different films: (a) rGO; (b) PB; (c) and (d) rGO/PB. HRTEM images of rGO/PB nanocomposite film (e) and (f). Histogram showing size distribution for rGO/PB film. 135x232mm (150 x 150 DPI)


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Fig. 3 (a) Continuous cyclic voltammograms (50 cycles) of the rGO/PB film at 50 mV s-1; (b) Cyclic voltammograms of rGO/PB electrode at different scan rates (10-100 mV s-1) and (c) corresponding dependence of peak current on the square root of scan rate; and (d) Nyquist plots of rGO, PB and rGO/PB films. 296x209mm (96 x 96 DPI)



Fig. 4 (a) Galvanostatic charge and discharge curves and self-discharging profile for PB and rGO/PB films after recovery process in deionized water bubbled with O2. (b) Cycling performance of rGO/PB nanocomposite film applying 33 A. (c) Discharge curves of the rGO/PB 296x209mm (96 x 96 DPI)


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Fig. 5 Rate performance for rGO/PB nanocomposite film obtained in different current range. 296x209mm (96 x 96 DPI)


Self-Recharging Reduced Graphene Oxide-Prussian Blue Electrodes

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