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Superior Additive of Exfoliated RuO Nanosheet for Optimizing the Electrode Performance of Metal Oxide over Graphene Seul Lee, Xiaoyan Jin, In Young Kim, Tae Ha Gu, Ji-Won Choi, Sahn Nahm, and Seong-Ju Hwang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b02257 • Publication Date (Web): 23 May 2016 Downloaded from http://pubs.acs.org on May 24, 2016

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The Journal of Physical Chemistry

Superior Additive of Exfoliated RuO2 Nanosheet for Optimizing the Electrode Performance of Metal Oxide over Graphene Seul Lee,a, † Xiaoyan Jin,a, † In Young Kim,a Tae-Ha Gu,a Ji-Won Choi,b Sahn Nahm,c and SeongJu Hwang*,a a

Department of Chemistry and Nanoscience, College of Natural Sciences, Ewha Womans

University, Seoul 03760, Korea b

Electronic Materials Research Center, Korea Institute of Science and Technology, Seoul 02792,

Korea c

Departments of Materials Science and Engineering, Korea University, Seoul 02841, Korea

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ABSTRACT An effective way to optimize the electrode performance of metal oxide was developed by employing exfoliated 2D RuO2 nanosheet as a conducting additive.

The exfoliated RuO2

nanosheet was easily incorporated into the Li−MnO2 nanocomposite via a simple mixing of exfoliated RuO2 and MnO2 nanosheets, followed by the restacking with Li+ ions.

The

incorporation of RuO2 nanosheet was found to be quite effective in increasing the surface area of the restacked Li−MnO2 nanocomposite.

The obtained heterolayered Li−MnO2−RuO2

nanocomposites delivered much greater specific capacitances than do the pristine Li−MnO2 and Li−RuO2 nanocomposites. Considering the fact that the RuO2 nanosheet has higher electrode activity than the MnO2 nanosheet, the greater specific capacitance of Li−MnO2−RuO2 nanocomposite than that of Li−RuO2 strongly suggests that the incorporation of a small amount of RuO2 nanosheet into the restacked Li−MnO2 nanocomposite induces a synergistic improvement in its electrode activity.

Of prime importance is that the Li−MnO2−RuO2

nanocomposites showed somewhat better electrode performances than the reduced graphene oxide (rG-O)-incorporated Li−MnO2−rG-O homologs, attributable to more efficient charge transport and pore structure upon RuO2 incorporation. The hydrophilic RuO2 nanosheet is more effective in making a stronger chemical interaction with hydrophilic MnO2 and also in depressing the self-aggregation of nanosheets compared to hydrophobic rG-O nanosheet. The present study clearly demonstrates that the RuO2 nanosheet can be used as a better additive for improving the electrode performance of metal oxides compared with widely-used rG-O.


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1. Introduction There has been a great deal of research interest devoted to the synthesis and application of the exfoliated two-dimensional (2D) nanosheets of layered compounds.1−5 The wide 2D surface and very thin thickness of the exfoliated nanosheet render this material a useful hybridization matrix for immobilizing diverse chemical species such as inorganic nanostructures, organic molecules, polymers, and biomolecules.5−9

In particular, the hybridization of nanostructured inorganic

solids with reduced graphene oxide (rG-O) nanosheet is widely studied as a powerful way of optimizing their functionalities through the enhancement of electrical conductivity and porosity.10−14 The anchoring of electrochemically active metal oxide on the rG-O nanosheet is highly effective in exploring new 2D hybrid materials having excellent functionality for energy storage, energy harvesting, and electrocatalysis.15−20

However, a strong self-aggregating

tendency of rG-O nanosheets prevents from synthesizing homogeneously mixed hybrid structure of graphene−metal oxide and optimizing their pore structure.21 Moreover, there is a significant dissimilarity in the chemical nature of hydrophobic rG-O and hydrophilic metal oxide, which is not favorable for achieving efficient chemical interaction between these materials.

As an

alternative to the hydrophobic rG-O nanosheet, hydrophilic conductive metal oxide nanosheet can be a good candidate for synthesizing homogeneous hybrid structures with diverse metal oxides. One of the most conductive metal oxide nanosheets is layered RuO2 2D nanosheet having metal-type electronic structure composed of partially filled d orbitals.22,23

The

hydrophilic surface nature of the RuO2 nanosheet makes it possible to achieve strong chemical and electronic interactions with hydrophilic metal oxides. Moreover, the absence of π electron clouds in the RuO2 nanosheet is advantageous in forming highly porous open stacking structure owing to its weak self-aggregating tendency. Of prime importance is that the well-dispersed



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colloidal nature of the exfoliated RuO2 nanosheet facilitates the synthesis of homogeneous nanocomposites with diverse nanostructured metal oxides. The incorporation of a small amount of conductive RuO2 nanosheet is expected to remarkably improve the electrode performance of metal oxide via the optimization of electrical conductivity and pore structure. Despite a great deal of researches regarding the synthesis and electrode applications of graphene−metal oxide nanocomposites,24−30 there is no report about the application of RuO2 2D nanosheet as a conducting additive for exploring high performance nanocomposite electrodes. Herein, we report the beneficial role of exfoliated RuO2 2D nanosheet as a conducting additive for optimizing the electrode functionality of metal oxide-based nanocomposite.

The

heterolayered Li−MnO2−RuO2 nanocomposites can be easily synthesized by the restacking of the colloidal mixture of MnO2 and RuO2 nanosheets with Li+ cations. The effect of the RuO2 nanosheet on the crystal structure and chemical bonding nature of the present nanocomposites together with the accompanying variation in their electrode performances was investigated. To understand the effect of RuO2 content on the performance of the Li−MnO2−RuO2 nanocomposites, several compositions, LMR0, LMR1, LMR2.5, and LMR4, with the RuO2 content of 0, 1, 2.5, and 4wt%, respectively, were employed.

As references, the rG-O-

incorporated Li−MnO2−rG-O nanocomposites with several rG-O contents were also synthesized and their electrode performances were compared to those of the LMR nanocomposites to investigate the relative efficiency of the incorporation of the RuO2 and rG-O nanosheets for improving the electrode performance of metal oxides.

2. Experimental


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2.1 Synthesis. The suspension of layered RuO2 nanosheet was synthesized by the exfoliation of layered Na0.2RuO2 material.23 The layered NaRuO2 was prepared by the solid state reaction, followed by the treatment with Na2S2O8 solution for the removal of excess Na ions, yielding Na0.2RuO2. The protonation for the Na0.2RuO2 material was carried out by the reaction with aqueous 1 M HCl solution. The exfoliation of RuO2 nanosheet was achieved by the reaction of the protonated derivatives with tetrabutylammonium (TBA+) ions for more than 10 days. The colloidal suspension of exfoliated MnO2 nanosheet was prepared by the one-pot solution-based synthesis, as reported previously.31 The oxidation reaction of MnCl2 by H2O2 in the presence of tetramethylammonium hydroxide (TMA·OH) yielded the colloidal suspension of MnO2 nanosheet. The colloidal mixtures of the MnO2 and RuO2 nanosheets were obtained by the simple mixing of pure colloidal suspensions of these nanosheets.

The Li−MnO2−RuO2

nanocomposites with the RuO2 contents of 0, 1, 2.5, and 4wt% were synthesized by the stoichiometric colloidal mixture with Li+ ions at room temperature for 24 h. LiOH was used as the lithium source. The addition of lithium cations led to the immediate flocculation of colloidal nanosheets, yielding black-colored precipitates. The obtained powders were thoroughly washed with distilled water and then freeze-dried.

The rG-O-incorporated Li−MnO2−rG-O

nanocomposites with several rG-O contents were synthesized using the same synthetic process except the use of rG-O suspension. The colloidal suspension of rG-O precursor was prepared by the reduction of graphene oxide (G-O) synthesized by the modified Hummers’ method.32 For another reference, the pristine Li−RuO2 nanocomposite was prepared by the restacking of the exfoliated RuO2 nanosheets with Li+ ions using the same synthetic process. 2.2 Characterization. The crystal structures of the synthesized materials were studied by powder X-ray diffraction (XRD) analysis (Rigaku, Ni-filtered Cu Kα radiation). The optical



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properties of the colloidal suspensions were characterized by UV−vis spectroscopic analysis using a SINCO S-4100 UV−vis spectrometer. The zeta potentials of the colloidal suspensions of exfoliated MnO2 and RuO2 nanosheets and their mixtures were measured by a Malvern Zetasizer Nano ZS (Malvern, UK) instrument. The crystal shapes and spatial elemental compositions of the present nanocomposites were investigated by field emission-scanning electron microscopy (FE-SEM) analysis using a Jeol JSM-6700F microscope equipped with an energy dispersive Xray spectrometer.

The crystal morphologies and composite structures of the present

nanocomposites were examined by transmission electron microscopy (TEM), selected area electron diffraction (SAED), and energy dispersive X-ray spectroscopy (EDS)−elemental mapping analysis using a Jeol JEM-2100F microscope at an accelerating voltage of 200 kV. The N2 adsorption−desorption isotherms of the restacked nanocomposites were measured to determine their surface areas and pore structures. The X-ray photoelectron spectroscopy (XPS) data were measured by XPS machine (Thermo VG, UK, Al Kα). The X-ray absorption nearedge structure (XANES) spectroscopic experiment was carried out at Mn K-edge and Ru K-edge at the beam line 10C of Pohang Accelerator Laboratory (PAL) in Korea. XANES spectra were collected at room temperature in the transmission mode using gas-ionization detectors. The energies of the measured XANES data were calibrated by measuring simultaneously the spectrum of Mn or Ru metal foil. The standard procedure reported previously was applied for data analysis.33

The electrical conductivities of present electrode materials were tested by

standard four-point probe measurements (DASOLENG, FPP-40K). The contact angles of water droplet on the freestanding membranes of the exfoliated RuO2, MnO2, and rG-O nanosheets were measured using a DSA 100 (KRÜSS) instrument to characterize the surface properties of these


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nanosheets. The freestanding membranes of these nanosheets were obtained by vacuum-assisted filtration of the corresponding colloidal suspension.5 2.3 Electrochemical measurements. The electrode performances of the nanocomposites were studied by performing cyclic voltammetry (CV) and galvanostatic charge−discharge (CD) cycling. A conventional three-electrodes cell with a potentiostat/galvanostat (Won A Tech) was utilized for all the electrochemical studies. The slurry for fabricating working electrode material was composed of the active material, acetylene black, and polyvinylidenefluoride in a mass ratio of 75:20:5, mixed by stirring in N-methyl-2-pyrrolidone for 1 h. The mass of the electrode material of ~1.2 mg cm−2 was loaded. The resulting slurry was pressed on a stainless steel substrate (see Figure S1 of Supporting Information), followed by drying in vacuum at 80 °C for 1 h. A Pt mesh and saturated calomel electrode (SCE) were used as the counter and reference electrode, respectively; 0.2 M aqueous solution of Na2SO4 was used as electrolyte. The potential window of −0.1~0.9 V at a scan rate of 20 mV s−1 was employed for the CV experiment. The specific capacitance of the electrodes was calculated from the CV curves using the following equation: Cs= ∫IdV/νm∆v, where Cs is the specific capacitance (F g−1), I is the response current (A), ∆v is the potential window (V), ν is the scan rate (mV s−1), and m is the mass of the active material (g). The galvanostatic CD cycling was carried out at a current density of 1 A g−1. The specific capacitance was calculated from the CD curves using the following equation: Cs= It/[(∆v) m], where Cs is the specific capacitance (F g−1), I is the constant current (A), t is the discharge time (s), ∆v is the potential window (V), and m is the mass of active material (g). The areal capacitance was calculated from the CV curves according to the following equation: Ca= ∫ IdV/νS∆v, where Ca is the areal capacitance (F cm−2), I is the response current (A), ∆v is the potential window (V), ν is the scan rate (mV s−1), and S is the area of the coated electrode (cm2). 7 Environment ACS Paragon Plus


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Electrochemical impedance spectroscopy (EIS) data were collected using an IVIUM impedance analyzer in the frequency region 0.01−105 Hz.

3. Results and Discussion 3.1. Mixture colloidal suspensions of exfoliated MnO2 and RuO2 nanosheets. As demonstrated in Figure 1a, the formation of the colloidal suspensions of the exfoliated MnO2 and RuO2 2D nanosheets was confirmed by the observation of Tyndall phenomenon. Both the exfoliated MnO2 and RuO2 colloids formed stable colloidal mixtures with variable MnO2/RuO2 ratios, as shown in Figure 1b. The evolution of the optical property of the colloidal suspension upon the increase of RuO2 content was probed with UV−vis spectroscopic analysis. As shown in Figure 1c, the colloidal suspension of the exfoliated MnO2 nanosheet exhibits a strong absorption peak at ~380 nm, corresponding to the d−d transition of manganese ion.34 The observation of this peak clearly demonstrates the formation of molecule-like semiconducting MnO2 nanosheet. Similarly, the exfoliated RuO2 nanosheet also displays a distinct absorption peak related to the electronic transition of Ru ion at 350 nm.23 However, a higher electrical conductivity of RuO2 nanosheet makes this absorption peak much weaker, as compared to the absorption peak of MnO2 nanosheet. As shown in Figure 1c, the addition of RuO2 nanosheet into the MnO2 colloidal suspension significantly decreased the spectral weight of absorption peak, a result of the improvement in the electrical conductivity caused by the addition of metallic RuO2 nanosheet. Figure 1d shows the zeta potential data of the pure colloidal suspensions of the MnO2 and RuO2 nanosheets, and their colloidal mixtures. All the present colloidal suspensions including the pure suspensions of the MnO2 and RuO2 nanosheets commonly possess negative surface charge like many other metal oxide nanosheets.35,36 It is worthwhile to note that the


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obtained colloidal mixtures show a single peak without any notable broadening, verifying the homogeneous mixing of the exfoliated MnO2 and RuO2 nanosheets in the present colloidal suspensions.

Figure 1. (a) Photoimages of the precursor suspensions of exfoliated MnO2 and RuO2 nanosheets. (b) Photoimages of the mixture colloids of exfoliated MnO2 and RuO2 nanosheets with the RuO2 content of 0, 1, 2.5, and 4wt%. (c) UV−vis absorption spectra and (d) zeta potential data of the mixed colloids of exfoliated MnO2 and RuO2 nanosheets with the RuO2 content of 0 (solid lines, black), 1 (dashed lines, red), 2.5 (dotted lines, blue), and 4wt% (dotdashed lines, green), and RuO2 nanosheet (dot-dot-dashed lines, pink).

3.2. XRD, FE-SEM, and TEM analyses for restacked Li− −MnO2−RuO2 nanocomposites. As depicted in the left panel of Figure 2, the heterolayered LMR nanocomposites were synthesized by the electrostatically-derived restacking of the anionic MnO2/RuO2 nanosheets with Li+ cations. Because the exfoliated MnO2 and RuO2 are negatively-charged, the Li+ ions were easily adsorbed on the surface of nanosheets and then restacked with these nanosheets. The



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powder XRD patterns of the obtained LMR nanocomposites are plotted in the right panel of Figure 2.

Figure 2. (Left) Schematic diagram for the synthetic route to the heterolayered LMR nanocomposite. (Right) Powder XRD patterns of the Li−MnO2−RuO2 nanocomposites of (a) LMR0, (b) LMR1, (c) LMR2.5, and (d) LMR4.

The least squares fitting analysis demonstrates that the incorporation of RuO2 nanosheet slightly expands the c-axis lattice parameter of the restacked nanocomposite (7.171, 7.225, 7.237, and 7.282 Å for LMR0, LMR1, LMR2.5, and LMR4, respectively, error estimation: 2.5−5.4× 10−5), indicating the gradual expansion of basal spacing caused by the replacement of the thinner MnO2 layer with a thicker RuO2 layer. The obtained basal increase upon the incorporation of RuO2 nanosheet provides a strong evidence for the substitution of the MnO2 layers with RuO2 layers. This was further confirmed by the distinct increase in the XRD peak intensity with increasing the content of RuO2 incorporated nanosheet; because the RuO2 layer has a greater electron density than does the MnO2 layer, the replacement of the MnO2 layers with RuO2 layers enhances the (00l) reflections, confirming the formation of heterolayered Li−MnO2−RuO2 nanocomposite. The composite formation is confirmed with XPS analysis (see Figure S2 of 10 Environment ACS Paragon Plus

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Supporting Information). While the RuO2-free LMR0 nanocomposite shows spectral features in the regions of Li 1s, Mn 3s, C 1s, O 1s, Mn 2p, Mn 2s, Mn LM, and O KL, the additional peaks of Ru 3d and Ru 3p are discernible in all the RuO2-incorporated LMR nanocomposites, confirming the incorporation of RuO2 nanosheet in these materials.

Figure 3. (Left) FE-SEM images, (middle) TEM images, and (right) SAED patterns of the Li−MnO2−RuO2 nanocomposites of (a) LMR0, (b) LMR1, (c) LMR2.5, and (d) LMR4. The FE-SEM, TEM images and SAED patterns of the LMR nanocomposites are illustrated in the Figure 3. As clearly shown from the FE-SEM results, the highly porous stacking structure of very thin sheet-like crystallites occurs commonly for all the restacked LMR nanocomposites. This result indicates the creation of significant amount of pores during the restacking of exfoliated nanosheets. The observation of uniform stacking morphology strongly suggests that both MnO2 and RuO2 nanosheets are homogeneously mixed with each other in the present nanocomposite materials. The formation of the porous stacking structures of very thin exfoliated nanosheets was further confirmed by TEM analysis. The maintenance of very thin 2D nanosheet morphology is clearly discernible for all the TEM images of the LMR nanocomposites. Since both the exfoliated MnO2 and RuO2 nanosheets commonly exhibit 2D sheet-like morphology (see Figure S3 of Supporting Information), the LMR materials with different RuO2 contents



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show similar 2D crystal shapes. The co-existence of exfoliated MnO2 and RuO2 nanosheets in the present LMR2.5 nanocomposite is confirmed by the SAED result presented here. The SAED pattern of the LMR2.5 nanocomposite clearly shows the diffraction spots of MnO2 and RuO2 lattice. The present result provides clear evidence for the homogeneous hybridization of layered MnO2 and layered RuO2 in the LMR material. The nanoscale hybridization of two types of nanosheets is further evidenced by EDS−elemental mapping analysis (Figure S4 of Supporting Information). A uniform distribution of Mn, Ru, and O elements is observed for the LMR nanocomposites, verifying the homogeneous substitution of MnO2 layer with RuO2 layer in the entire part of the present materials.

3.3.

N2

adsorption− −desorption

isotherm

analysis

for

restacked

Li− −MnO2−RuO2

nanocomposites. The left panel of Figure 4 represents the N2 adsorption−desorption isotherms of the restacked LMR nanocomposites. Regardless of the RuO2 content, all the nanocomposites showed similar isotherm features, classified as Brunauer−Deming−Deming−Teller (BDDT) type IV isotherm with H3-type hysteresis loop.37,38 This finding can be interpreted as evidence for the open slitshaped capillaries formed by the stacking of plate like particles.37,38 The surface areas of the present nanocomposites were calculated on the basis of Brunauer−Emmett−Teller (BET) equation. According to the fitting analysis based on the BET equation, the surface areas of LMR nanocomposites were estimated to be 127, 130, 156, and 146 m2 g−1 for LMR0, LMR1, LMR2.5, and LMR4, respectively. In addition to the surface area, the total pore volume was derived from the amount of N2 molecules adsorbed at a relative pressure close to unity (p p0−1 = ~1). The total pore volumes of the LMR nanocomposites were determined to be 0.62, 0.71, 0.84,


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and 0.77 cm3 g−1 for LMR0, LMR1, LMR2.5, and LMR4, respectively. This result underscores the effective formation of porous structure through the restacking of exfoliated nanosheets. The incorporation of RuO2 nanosheet increases the surface areas and pore volumes of the restacked nanocomposites, and could be related to the weakening of the chemical interaction between identical MnO2 nanosheets by the intervention of heterogeneous RuO2 nanosheets. A slightly smaller surface area and pore volume of LMR4 nanocomposite than LMR2.5 are attributable to an increase in the molar weight of the nanocomposite caused by the replacement of the MnO2 layer with a heavier RuO2 layer.

Figure 4. (Left) N2 adsorption−desorption isotherms and (right) pore size distribution curves of the Li−MnO2−RuO2 nanocomposites of (a) LMR0, (b) LMR1, (c) LMR2.5, and (d) LMR4. The open and close symbols represent the adsorption and desorption data, respectively.

The sizes of mesopores in the nanocomposites were calculated by Barrett−Joyner−Halenda (BJH) equation, as shown in the right panel of Figure 4. All the synthesized materials have mesopores with the average size of ~3.6−3.7 nm, confirming the formation of mesopores through the house-of-cards-type stacking structure of nanosheets.



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3.4. Mn K-edge and Ru K-edge XANES analyses. The Mn K-edge XANES spectra of the LMR nanocomposites are plotted in Figure 5a, as compared to the reference spectra of layered LiMn0.9Cr0.1O2, β-MnO2, and exfoliated MnO2 nanosheet. Regardless of the RuO2 content, all the LMR nanocomposites showed similar edge energies, which are higher than that of LiMn3+0.9Cr0.1O2 but lower than that of β-Mn4+O2, indicating the mixed Mn3+/Mn4+ oxidation state in these materials. Negligible dependence of the edge energy on the RuO2 content clearly demonstrates no significant effect of the RuO2 incorporation on the electronic structure of the hybridized MnO2 nanosheet. The pre-edge peaks P and/or P’ are observed for all the materials under investigation, corresponding to the transitions from the core 1s level to the unoccupied 3d states.39,40 All the materials demonstrate weak intensities for these pre-edge features, indicating the octahedral local symmetry of Mn ions in these materials.41 The intensity ratio of both the peaks reflects the average oxidation state of Mn ions.41 As shown in Figure 5b, the reference LiMn3+0.9Cr0.1O2 shows only one pre-edge peak P, whereas a rather intense peak P’ appears for the reference β-Mn4+O2. Conversely, two pre-edge peaks P and P’ are discernible for the LMR nanocomposites, confirming the mixed oxidation states of Mn3+/Mn4+.41 There is no notable variation in the intensity ratio of the peak P’/P with increasing content of RuO2 nanosheet, confirming a negligible change in the Mn oxidation state upon the incorporation of RuO2 nanosheet.

In the main-edge region, two peaks A and B

corresponding to the dipole-allowed 1s → 4p transitions were observed for all the spectra.40,42 The intensity and shape of the feature B provide a sensitive measure for the ratio of edge-sharing of MnO6 octahedra over corner-sharing.40 All the LMR nanocomposites displayed an intense


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and sharp peak B, confirming the maintenance of a layered MnO2 lattice composed of edgesharing MnO6 octahedra after the restacking with Li+ cations.

Figure 5. (a) Mn K-edge XANES spectra and (b) expanded view for pre-edge spectra of (i) LiMn0.9Cr0.1O2 (circles), (ii) β-MnO2 (triangles), (iii) exfoliated MnO2 nanosheet (squares), and the restacked Li−MnO2−RuO2 nanocomposites of (iv) LMR0 (solid lines), (v) LMR1 (dashed lines), (vi) LMR2.5 (dotted lines), and (vii) LMR4 (dotted-dashed lines). (c) Ru K-edge XANES spectra and (d) expanded view for edge jump region of (i) RuCl3 (circles) (ii) RuO2 (triangles), (iii) exfoliated RuO2 nanosheet (solid lines), and the Li−MnO2−RuO2 nanocomposites of (iv) LMR1 (dashed lines), (v) LMR2.5 (dotted lines), and (vi) LMR4 (dotdashed lines).

Figure 5c represents the Ru K-edge XANES spectra of the LMR nanocomposites and several references of the exfoliated RuO2 nanosheet, RuCl3, and RuO2. As shown in Figure 5d, all the nanocomposites show similar edge energies to that of the exfoliated RuO2 nanosheet, which are



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higher than that of Ru3+Cl3 but nearly similar to that of Ru4+O2. This result indicates the tetravalent oxidation state of Ru ions in the LMR nanocomposites and negligible effect of RuO2 content on the oxidation state of Ru ions.

The overall spectral features of the LMR

nanocomposites are fairly similar to that of the exfoliated RuO2 nanosheet, clearly demonstrating the maintenance of layered RuO2 structure upon the restacking process.

3.5. Electrochemical measurements. The effect of RuO2 incorporation on the electrochemical activity of the restacked Li−MnO2 nanocomposite was investigated by CV measurement, at a scan rate of 20 mV s−1 in 0.2 M Na2SO4 solution as the electrolyte. The CV data of the present LMR nanocomposites are plotted in Figures 6a−d. The pseudocapacitance-type CV data are commonly observed for all the materials. The partial incorporation of conductive RuO2 nanosheet for the restacked Li−MnO2 nanocomposite remarkably increased the area of CV curve, reflecting the improvement in the electrochemical activity. The pseudocapacitance behaviors of the LMR nanocomposites are further confirmed by the galvanostatic CD cycling measurement (Figure S5 of Supporting Information). The CD data of all the materials clearly demonstrate the linear dependence of the working potential with time.43

The beneficial effect of the RuO2 addition on the specific

capacitance of the Li−MnO2 nanocomposite is also evidenced from the obtained CD data. As plotted in Figure 6e, regardless of the RuO2 content, all the restacked LMR nanocomposites show larger specific capacitances than does the layered K0.45MnO2 (~50 F g−1),44 suggesting the usefulness of the restacking of exfoliated nanosheets in improving the electrode performance of metal oxide.

Moreover, the RuO2-incorporated LMR materials exhibit greater specific

capacitances than does the RuO2-free LMR0 (i.e. Li−MnO2) nanocomposite for the entire cycles,


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highlighting the positive effect of RuO2 addition on the electrode activity of the restacked Li− −MnO2 nanocomposite.

Figure 6. CV curves of the Li−MnO2−RuO2 nanocomposites of (a) LMR0, (b) LMR1, (c) LMR2.5, and (d) LMR4, (e) capacitance retention plots, and (f) comparison of the variations of the specific capacitance of samples at various scan rate plots of the Li−MnO2−RuO2 nanocomposites.

To confirm the beneficial role of RuO2 nanosheet as an additive in improving the electrode performance of the Li− −MnO2 nanocomposite, the pristine Li−RuO2 nanocomposite was also prepared as a reference by the restacking of the exfoliated RuO2 nanosheets with Li+ cations and its specific capacitance is measured (Figure S6 of Supporting Information).

The specific

capacitance of the pristine Li−RuO2 nanocomposite measured in a neutral electrolyte is notably smaller than those of the Li−MnO2−RuO2 nanocomposites with smaller RuO2 contents, even though the RuO2 nanosheet has a higher electrode activity than the MnO2 nanosheet. The



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present finding strongly suggests that the incorporation of a small amount of RuO2 nanosheet as a conducting additive synergistic enhanced the specific capacitance of the restacked nanocomposite.

Among the RuO2-incorporated nanocomposites, LMR2.5 nanocomposite

delivers the largest capacitances of 281 F g−1 at the maximum point and 256 F g−1 for the 5000th cycle, which are much greater than those of the RuO2-free LMR0 (i.e. Li−MnO2) homologue (187 F g−1 for the maximum point and 179 F g−1 for the 5000th cycle) and pristine Li−RuO2 nanocomposite (213 F g−1 for the maximum point and 176 F g−1 for the 5000th cycle). Also, the areal capacitances are calculated, as summarized in Table 1. The obtained areal capacitances of the LMR nanocomposites are not sufficiently large because of very low mass loading per area of active material employed in this study. The larger capacitance of LMR2.5 than those of pristine Li−MnO2 and Li−RuO2 nanocomposites is a result of the synergistic combination of these two types of nanosheets. A further increase in the RuO2 content beyond an optimal value of 2.5wt% slightly decreased the specific capacitance of the nanocomposite, attributed to the increase in the molar weight of nanocomposites caused by increasing content of heavy RuO2 component and also to the accompanying decrease in the surface area. As can be seen clearly from Figure 6f, the beneficial effect of the incorporation of RuO2 nanosheet on the electrode performance of the Li−MnO2 nanocomposites is more prominent for higher scan rates, clearly demonstrating the improvement of rate performance upon the incorporation of RuO2 nanosheet. There are two major factors contributing to the specific capacitance of layered metal oxide; the intercalation/deintercalation and the surface adsorption/desorption.45 Taking into account the similar basal spacings for the RuO2-incorporated LMR1, LMR2.5, and LMR4 nanocomposites and the RuO2-free LMR0 homologue, the improvement in the specific capacitance upon the RuO2 incorporation cannot be ascribed to the variation in the intercalation kinetics of Na+ ions.


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Instead, the increase in the surface area upon the incorporation of RuO2 nanosheet is responsible for the greater capacitance of the RuO2-substituted LMR1, LMR2.5, and LMR4 nanocomposites than the LMR0. In addition, the enhancement of the rate characteristics of interstratified Li−MnO2 nanocomposite upon the RuO2 substitution strongly suggests the increase in the electrical conductivity.

This factor also contributes to the improvement of

electrode performance upon the substitution of RuO2 nanosheet via the effective polarization of electron density. The incorporation of RuO2 nanosheet is more effective in enhancing the electrode activity of restacked Li−MnO2 nanocomposite than the Ru ion substitution for the MnO2 nanosheet;46 the Ru-substituted Li−Mn1−xRuxO2 nanocomposite with a Ru content of 7.1wt% can deliver a specific capacitance of ~250 F g−1 for the 5000th cycle at a scan rate of 20 mV s−1, which is smaller than the present capacitance of the LMR2.5 nanocomposite with a lower Ru content (2.5wt%). This result can be interpreted as a result of the optimization of the pore structure of restacked MnO2 nanosheets upon the intervention of heterogeneous RuO2 nanosheets. This mixing effect of two different nanosheets is surely absent in the case of Ru-substitution for MnO2 nanosheets yielding only identical Mn1−xRuxO2 nanosheets.

3.6. Comparative study for rG-O- and RuO2-incorporated nanocomposites. To study the relative efficiency of RuO2 nanosheet as a conductive additive over rG-O nanosheet, the rG-O-incorporated Li−MnO2−rG-O nanocomposites with the rG-O content of 2.5, 5, and 10wt% were also synthesized by the restacking of the colloidal mixtures of MnO2 and rGO nanosheets with Li+ cations (The resulting materials are denoted as LMG2.5, LMG5, and LMG10, respectively). According to the powder XRD, FE-SEM, and TEM analyses, all the rG-



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O-incorporated

LMG

nanocomposites

possess

layer-by-layer-ordered

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structure

with

mesoporously restacked morphology, which is nearly identical to those of the RuO2-incorporated LMR nanocomposites (Figure S7 of Supporting Information).

Table 1. The surface areas and electrochemical data of the LMR and LMG nanocomposites.

Sample

Surface area (m2 g−1)

Maximum capacitance (F g−1)

5000th capacitance (F g−1)

LMR0

127±7

187±14

179±10

LMR1

130±10

263±13

LMR2.5

156±13

LMR4

Areal capacitance (F cm−2)

σω

Rct (Ω)

(Ω Ω·s−0.5)

0.194±0.014

69.12

28.26

250±12

0.271±0.013

51.94

22.37

281±16

256±13

0.326±0.019

19.74

13.18

145±11

265±16

213±12

0.282±0.017

11.73

14.26

LMG2.5

136±11

240±14

210±10

0.264±0.015

35.66

26.39

LMG5

118±7

258±15

251±12

0.273±0.016

8.90

25.22

LMG10

111±7

231±13

211±10

0.233±0.013

8.33

36.73

The N2 adsorption−desorption isotherm measurement demonstrates that all the LMG nanocomposites have expanded surface areas (Figure S8 of Supporting Information). As listed in Table 1, even with the same content of rG-O and RuO2 additives, LMG2.5 nanocomposite has somewhat smaller surface area than that of LMR2.5 nanocomposite, underscoring that the incorporation of RuO2 nanosheet is slightly more effective in increasing the porosity of restacked nanocomposite than that of rG-O nanosheet. This observation can be interpreted as a result of stronger self-aggregation of rG-O nanosheets caused by the strong π−π interactions compared to the π electron-free RuO2 nanosheets. This interpretation is confirmed by the decrease in the surface area of LMG nanocomposites with the further increase in the rG-O contents, as listed in


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Table 1. The present result clearly demonstrates a better role of the RuO2 nanosheet in forming porous stacking structure compared to the rG-O nanosheet. The electrode performances of the rG-O-incorporated LMG nanocomposites are compared to that of the LMR2.5 nanocomposite, as presented in Figure S9 of Supporting Information. Similar to the LMR nanocomposites, the LMG nanocomposites display the pseudocapacitancetype CV data. As listed in Table 1, LMG2.5 exhibits slightly smaller specific capacitances than that of the LMR2.5 nanocomposite with the same Ru content of 2.5wt%. This result clearly demonstrates a lower efficiency of rG-O addition in improving the electrode performance of MnO2 nanosheet compared to that of RuO2 incorporation. Notably, the specific capacitance of LMR2.5 nanocomposite is distinctly greater than those of LMG5 and LMG10 with higher rG-O contents (Table 1). This result clearly demonstrates a better role of RuO2 nanosheet as an additive for improving the electrochemical activity of metal oxide compared to widely-used rGO nanosheet. Based on the N2 adsorption−desorption isotherm results, the observed superior role of the RuO2 nanosheet over the rG-O one is attributable to somewhat greater surface expansion upon the RuO2 incorporation than that upon the rG-O addition. Besides, a distinct dissimilarity in the surface natures of the RuO2 and rG-O nanosheets makes additional contribution to the different electrode performances of the LMR and LMG nanocomposites.

To verify this

supposition, the surface natures of the MnO2, RuO2, and rG-O nanosheets were examined by measuring the contact angle of the restacked freestanding membranes of these nanosheets (Figure S10 of Supporting Information). A small contact angle of 19.9° was observed for the freestanding membrane of the MnO2 nanosheet, which is similar to the contact angle of the freestanding RuO2 membrane. These contact angles are much smaller than that of the rG-O nanosheet (83.3°). The result of contact angle measurement clearly demonstrates the hydrophilic



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surface nature of the MnO2 and RuO2 nanosheet, and the hydrophobic surface nature of the rG-O nanosheet. Thus, hydrophilic MnO2 nanosheets can interact more strongly with the hydrophilic RuO2 nanosheet than with hydrophobic rG-O nanosheet. To verify the superior role of RuO2 nanosheet over the rG-O nanosheet, the electrical conductivities of these materials were measured using a four-point probe method (Table S1 of Supporting Information). The resistivity of exfoliated RuO2 nanosheet is smaller than that of rG-O nanosheet, confirming a promising role of the exfoliated RuO2 nanosheet as efficient conductive additive in enhancing the electrical conductivity of MnO2 material. The resulting improvement in the electrical conduction upon the RuO2 incorporation also contributes to the better electrode performances of the LMR nanocomposites than those of the LMG nanocomposites.

3.7. EIS analysis. The evolution of the charge transfer behavior of the restacked Li−MnO2 nanocomposite upon the incorporation of the RuO2 and rG-O nanosheets was examined with EIS technique. The EIS data of the present LMR nanocomposites are presented in Figures 7a and 7b, as compared to that of LMG2.5 nanocomposite. Moreover, the evolution of the EIS features of the reference LMG nanocomposites upon the change of rG-O content is plotted in Figures 7c and 7d. All the LMR and LMG nanocomposites commonly demonstrate the semicircle in the high-medium frequency region and the inclined line in the low frequency region. This semicircle is attributed to the charge transfer resistance and represents electronic impedance of the electrode material.47,48 A line with a slope close to 90° along imaginary axis in the low frequency region indicates the ideally polarizable behavior of an electrode.47,48 The observed deviation from the ideal behavior is attributable to Warburg impedance, which is related to the ionic diffusion in redox reactions


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across the electrode−electrolyte interface. As illustrated in Figure 7e, the fitting analysis using the equivalent circuit was carried out to further analyze the evolution of charge-transfer kinetics upon the RuO2 and rG-O incorporation. The Re, Cdl, Rct, and Zw represent the resistance of electrolyte, double-layer capacitance, charge-transfer resistance, and Warburg impedance, respectively.47,48 The validity of the present fitting analysis is evidenced by the small values of χ2 function (χ2 = 0.0017−0.0063). The incorporation of RuO2 nanosheet in the nanocomposites significantly affects the diameters of the semicircle which gradually decreased with increasing RuO2 nanosheet.

Figure 7. (a) Nyquist plots and (b) enlarged view at high-medium frequency region of the Li−MnO2−RuO2 nanocomposites of LMR0 (circles), LMR1 (triangles), LMR2.5 (squares), LMR4 (diamonds). (c) Nyquist plots and (d) enlarged view at high-medium frequency region of the Li−MnO2−rG-O nanocomposites of LMG2.5 (circles), LMG5 (triangles), and LMG10 (squares). (e) The equivalent circuit used to analyze the electrode−electrolyte interface for all the materials.



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As listed in Table 1, LMR4 nanocomposite with the highest RuO2 content displays the smallest Rct of 11.73 Ω in the LMR nanocomposites. Similarly to the RuO2 addition, the incorporation of the rG-O nanosheet also decreased the semicircle diameter, indicating the enhanced electronic conductivity and the improved charge transfer kinetics of the rG-Oincorporated nanocomposites. Similar to the case of LMR nanocomposites, an increase in the rG-O content of the LMG nanocomposites gradually decreased the Rct (Table 1). The slope of Zre vs. ω−0.5 plot in the Warburg region provides the Warburg coefficient (σw) (Figure S11 of Supporting Information). The Warburg coefficient is inversely proportional to the measure of diffusion coefficient of the ions that take part in the redox reactions across the electrode−electrolyte interface.

Among the RuO2-incorporated nanocomposites, LMR2.5

nanocomposite shows the highest Na+ ion diffusivity with the smallest σw of 13.18 Ω·s−0.5, as listed in Table 1. The observed lowering of Warburg coefficient upon the incorporation of the RuO2 nanosheet indicates the improvement of the polarizability of the present electrode materials. However, the positive effect of rG-O nanosheet on the Warburg coefficient is weaker than that of RuO2 nanosheet; the LMG nanocomposites show larger σw values than LMR2.5 nanocomposite with a lower RuO2 content. The observed inferior role of rG-O over RuO2 in optimizing the Warburg coefficient is because of the stronger self-aggregating tendency and greater hydrophobicity of the rG-O nanosheet, leading to the smaller surface area and less extent electronic coupling in the LMG nanocomposites than in the LMR nanocomposites. The present EIS results clearly demonstrate that the conductive RuO2 nanosheet is an effective additive for improving the overall charge-transfer kinetics of the electrodes across the electrode−electrolyte interface. Summarizing all present experimental findings, the excellent electrode performance of LMR2.5 nanocomposite can be interpreted as a result of the increase in the electronic


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conductivity and surface area upon the incorporation of conductive RuO2 nanosheet and the efficient chemical interaction between hydrophilic RuO2 nanosheet and MnO2 electrode.

4. Conclusion In this study, we report a better role of the exfoliated 2D RuO2 nanosheet as a conducting additive for optimizing the electrode performance and porosity of metal oxide than the widelyused rG-O nanosheet. In comparison with the rG-O-incorporated LMG nanocomposites, the RuO2-incorporated LMR nanocomposites showed somewhat higher electrochemical activity, underscoring the better efficiency of the RuO2 addition.

Considering that the specific

capacitances of the present LMR nanocomposites with a small quantity of RuO2 nanosheet are also greater than those of the pristine Li−RuO2 and Li−MnO2 nanocomposites, and the incorporation of RuO2 nanosheet as a conductive additive induces a synergistic improvement in the electrode performance of the restacked nanocomposites. According to the contact angle measurement, the RuO2 nanosheet shows much greater hydrophilicity than does the rG-O nanosheet, and is favorable for enhancing the chemical interaction with hydrophilic MnO2 nanosheet. Thus, the observed better role of RuO2 nanosheet than widely-used rG-O nanosheet in optimizing the electrode performance of metal oxide can be interpreted as a result of a stronger interaction between hydrophilic RuO2 nanosheet and hydrophilic MnO2 species, and a weaker self-aggregating tendency of RuO2 nanosheet, remarkably improving the charge transport behavior and pore structure of the RuO2-incorporated nanocomposite. Taking into account the hydrophilic surface nature and excellent dispersion ability of the exfoliated RuO2 nanosheet, this conducting material can be readily applied as a universal additive for diverse types of nanostructured

polar

inorganic

solids

including


0D

nanoparticles,

1D


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Page 26 of 34

nanowires/nanorods/nanotubes, and 2D nanosheets/nanoplates. The present synthetic strategy can provide a universal methodology to explore high performance nanocomposite electrode materials. Considering the fact that the use of rG-O nanosheet as a conducting additive has evoked a large numbers of researches about the graphene-based composite electrode materials,10−14,16,17,19,25,26,30 the application of this emerging RuO2 nanosheet must induce a great deal of research activity for the development of new efficient composite electrode materials. Exploring novel high performance electrode materials for emerging secondary batteries such as Na ion, Li−sulfur, and Li−air batteries via the immobilization of electrochemically-active inorganic solids on the surface of RuO2 nanosheet is currently undergoing in our laboratory.

ASSOCIATED CONTENT Supporting Information. Photoimage of the coated electrode and TEM images of exfoliated MnO2 and RuO2 nanosheet. XPS data, EDS−elemental mapping data and galvanostatic CD curves of the LMR nanocomposites. Powder XRD, FE-SEM, TEM, BET, and CV curves of the LMG nanocomposites. Contact angles of the exfoliated nanosheets, four point probe data and the Zre vs. ω−1/2 plot in the Warburg region of the restacked LMR and LMG nanocomposites. This material is available free of charge via the Internet at http://pubs.acs.org/.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; phone: +82-2-3277-4370; fax: +82-2-3277-3419 Author Contributions


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†These

authors contributed equally.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea(NRF) grant funded by the Korea government(MSIP) (No. NRF-2014R1A2A1A10052809) and the National Research Foundation of Korea Grant funded by the Korean Government (MEST)" (NRF-2010C1AAA001-2010-0029065). The experiments at PAL were supported in part by MOST and POSTECH.

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Superior Additive of Exfoliated RuO2 Nanosheet ... - ACS Publications

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