Fast and Energy Efficient Synthesis of ZnO@RGO ... - ACS Publications

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May 31, 2016 - Yaoming Wu,. †. Zhanyi Cao,*,‡ and Limin Wang*,†. †. State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied ...
Article pubs.acs.org/JPCC

Fast and Energy Efficient Synthesis of ZnO@RGO and its Application in Ni−Zn Secondary Battery Lianshan Sun,†,‡ Zheng Yi,†,‡ Jing Lin,†,‡ Fei Liang,† Yaoming Wu,† Zhanyi Cao,*,‡ and Limin Wang*,† †

State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, CAS, Changchun 130022, China ‡ College of Materials Science and Engineering, Jilin University, Changchun 130022, China S Supporting Information *

ABSTRACT: ZnO with a uniform and complete coating of reduced graphene oxide (RGO) was prepared by a facile and rapid reaction using a solution synthesis scheme. A synthetic route that ZnO was first grown in situ on the GO surface and then reduced by NaBH4 was proposed, and an optimum coating structure was obtained by investigating the effect of precipitators, surfactant and additive amount of GO. After the reduction of NaBH4, 26.84% oxygen-containing groups were reduced and the weight ratio of RGO in the composite was 10.42 wt %, it was very close to the theoretical content of 11.8 wt %. The electrochemical measurements of ZnO@RGO negative electrode in the Zn−Ni secondary battery exhibited a stable cycle performance with a discharge capacity of 510 mAh g−1 after 300 cycles that it was 120 mAh g−1 higher than the capacity of bare ZnO. RGO layer effectively inhibited the direct contact between ZnO with the electrolyte and provided enough active area for the deposition of Zn which plays key roles to enhance the cycle discharge ability for Zn−Ni secondary battery.

1. INTRODUCTION ZnO is the main active material in Ni−Zn secondary battery which has been developed over a century. Tremendous efforts have been done to improve its cycling stability which is mainly attributed to the dissolution of ZnO in alkaline solution.1−3 Though the cycle life limits the application of Ni−Zn battery, the battery is still attracting people to explore for its nontoxic, resourceful, eco-friendly, safe and especially the excellent highrate discharge performance.4 To inhibit the dissolution of ZnO in the alkaline electrolyte, usually, three kinds of measures are carried out (i) to change the composition of the electrolyte which has as less OH− as possible and still reserves enough conductive ionic, (ii) to form a mixture with ZnO which is undissolved in alkaline of adding additive, and (iii) to coat ZnO with a stable layer in alkaline.5−10 Metallic oxides coating with SnO2 or In2O3, which possesses a higher hydrogen evolution overpotential in alkaline solution, will enhance the cycle stability of ZnO more effectively than other improvements.11−14 However, the treatment introduces additional processes and costs, which are not suitable for large-scale application. Thus, to explore a simple synthesis method of ZnO with befitting layer is imperative. Reduced graphene oxide (RGO) is an optional coating material for its good conductivity and anticorrosion ability.15−17 Different sizes from nanoscale to micrometer scale of ZnO@ RGO have been widely synthesized by hydrothermal synthesis, chemical solution synthesis, or chemical vapor deposition (CVD) and the nanoscale ZnO@RGO exhibits excellent properties in many research field.18−25 Among these methods, © 2016 American Chemical Society

CVD can produce a complete graphene coating with controllable thickness,26 while this method is not suitable for mass production. Zhang et al. reported a facile one-pot route of preparing well-dispersed ZnO on GO at 423 K for 8 h.18 Kavitha et al. synthesized a RGO-ZnO nano composite with a complete coating layer by hydrothermal method at 373 K for 7 h.19 Luo et al. obtained RGO-ZnO hollow sphere composites just using a ultrasonic apparatus in solution,23 and Bu et al. prepared a graphene-ZnO quasi-shell−core composite with stirring for 5 h at 368 K in DMF.24 Hydrothermal synthesis is the most conventional method but there is a higher energy consumption than chemical solution synthesis. By now, the reported ZnO-RGO composites by one-pot route chemical solution synthesis is partial coated or only attached to the RGO, and this method should be improved. To restricting the dissolution of ZnO in the maximal degree, an entire coating is necessary. Herein, we put forward an in situ growth of ZnO on GO sheets and then the composite is reduced by NaBH4. Meantime, as the active material in battery, the ratio of ZnO with RGO is taken into account. The nanoscale ZnO with a higher specific surface free energy is easier to assemble with GO than the micrometer scale particle.27 To get a uniform and complete coating, ZnO in situ attached to GO is the best option.28,29 Fortunately, both the synthesis of ZnO and the homodisperse of GO need an alkaline Received: January 29, 2016 Revised: May 26, 2016 Published: May 31, 2016 12337

DOI: 10.1021/acs.jpcc.6b01025 J. Phys. Chem. C 2016, 120, 12337−12343

Article

The Journal of Physical Chemistry C environment.30,31 And as we know, the activity of NaBH4 can be controlled by adjusting the value of pH in alkaline solution and it has a fast reaction speed and a good reducing property which could be achieved without high temperature and pressure.32 Nevertheless, the reduction process of NaBH4 is too severe to control. In this paper, a series of contrast tests have been done, a facile way to synthesize complete and even coated ZnO with RGO is given, and the factors that have effects on the final product are discussed. The electrochemical performance of the ZnO@RGO as the negative materials in Zn−Ni secondary battery shows a much enhanced stable cycle life than the commercial ZnO.

to gain the maximum discharge capacity under different currents. The electrochemical impedance spectroscopy (EIS) analysis was taken at 50% depth of discharge (DOD) with a conventional three electrode system, which was measured with an Auto lab PGSTAT 302. Hg/HgO electrode was selected as the reference electrode, a commercial sintered Ni(OH)2 electrode as the counter and the ZnO electrode was the working electrode, which were immersed in 6 M KOH saturating with ZnO and measured at 25 °C.

3. RESULTS AND DISCUSSION 3.1. Synthesis of the Uniform Coated ZnO@RGO. As shown in eqs 1,32 2, and 3,3 NaBH4 + H 2O → BH3 + H 2 + NaO (1)

2. EXPERIMENTAL SECTION 2.1. Preparation of ZnO@RGO Particle. Zinc nitrate hexahydrate, polyvinylpyrrolidone (PVP), ammonium hydroxide, and sodium borohydride of analytical grade were used without further purification. Graphene oxide was prepared by Hummer method33,34 and subsequent freeze-drying. In a typical experiment, 360 mg Zn(NO)3·6H2O and 50 mg PVP were dissolved in 40 mL deionized water in turn, then 15 mL of GO aqueous solution (0.6 mg mL−1) was added into the solution under stirring. Subsequently, the beaker containing the above solution was transferred to a water bath and heated to 80 °C. When the temperature was stable, a pH value around 8−9 of the solution was adjusted by NH3·H2O (10 wt %) under stirring and kept for an hour. Finally, 10 mL of NaBH4 aqueous solution of 20 mg mL−1 was added dropwise cautiously, and then the final solution was heated for 2 h at 80 °C. After the solution cooling down, the product was washed with deionized water by centrifugation under 4000 rmp and separated in 10 mL deionized water for Freeze-drying. 2.2. Characterization. X-ray diffraction (XRD) patterns were recorded by a Bruker D8 Focus and D/max 2500pc power X-ray diffractometer using Cu Kα radiation at a scan rate of 5° min−1. Transmission electron microscopy (TEM) image was obtained utilizing a FEI Tecnai G2 S-Twin instrument. Scanning electron microscopy (SEM) images were produced using a Hitachi S-4800 field emission scanning electron microscope at an accelerating voltage of 10 kV. X-ray Photoelectron Spectroscopy (XPS) was performed with a VG ESCALAB 250 spectrometer with Al Kα. Thermogravimetric (TGA) curves were characterized on a STA 449 °C Jupiter (NETZSCH) thermogravimetry analyzer from room temperature to 900 °C under air atmosphere with a heating rate of 10 °C min−1. 2.3. Electrochemistry Test of ZnO@RGO. The active materials of ZnO@RGO were mixed with PTFE emulsion (65 wt %) and acetylene black at a weight ratio of 8:1:1, after wellmixed with moderate amounts of water, the mixture was coated evenly on 1 cm2 copper mesh which was integrated with a copper lead. And after drying in an air-dry oven at 60 °C for 12 h, the prepared electrode was pressed under 20 MPa. A simulated battery was constructed with the prepared negative and wrapped with a nonwoven Nylon separator and a microporous polypropylene membrane, which were clamped using two Ni(OH)2 anodes of a much larger capacity than the negative, then the sandwich simulated cell was immersed in 6 M KOH which was saturated with ZnO. In the cycle performance test, a charge capacity of 600 mAh g−1 was adopted rather than the theoretical value of 659 mAh g−1 for inhibiting the hydrogen evolution.35 And for the high rate discharge test, a charge capacity of 659 mAh g−1 was employed

Zn 2 + + 4OH− → Zn(OH)24 −

(2)

Zn(OH)24 − → ZnO + 2OH− + H 2O

(3)



the hydrolysis of NaBH4 provides OH for the formation of ZnO and BH3 has a strong reducing property for the reduction of GO, so ZnO with RGO composite can be obtained by one step reaction. While the product of the one step reaction is partially combined together (Figure S1), and it is possible that the simultaneously reaction is unsuitable for coating. In subsequent trial, ZnO is at first obtained in a GO mixture solution by adding NaOH solution to a final pH = 8−9, then NaBH4 solution is added. However, the final product is still partially composited (Figure S2a), and the reason may be there is not enough binding force between the RGO and ZnO particle. PVP as the surfactant is selected in the further attempt, and the coating effect is improved by contrasting with the product without PVP (Figure S2b). Nevertheless, the complete ionization of NaOH or LiOH (Figure S3) in aqueous solution leads to a fast nucleation of ZnO which is uncontrollable for an even coating. By contrast experimentations, NH3·H2O is a superior alternative donor of OH− than NaOH or LiOH for it is partial ionization which offeres OH− for the nucleation and growth of ZnO continuously and moderately. A final synthetic route is shown in Scheme 1: Scheme 1. Illustration of the Growth Strategy of ZnO@RGO

3.2. Characterization of ZnO@RGO. The prepared GO is measured by XRD and the obvious characteristic peak appears at 11°,22 the vanishment of the peak of GO is an evidence of the reduction of GO by NaBH4 and the characteristic peak of RGO is presented at around 22−26°, which coexists with ZnO (Figure 1). The SEM images of the final ZnO@RGO are shown with two kinds of scales, and a uniform coating with a lower amplification factor proving a totally coating for each ZnO particle (Figure 2b) and a close-knit coating of RGO with ZnO is observed at a higher amplification factor (Figure 2c). TEM image of the composite offers a further proof of the entire coating of RGO (Figure 2d). After reduced by NaBH4, the flat GO sheets are crimped and gathered together, and obvious 12338

DOI: 10.1021/acs.jpcc.6b01025 J. Phys. Chem. C 2016, 120, 12337−12343

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The Journal of Physical Chemistry C

After reduced by NaBH4, the intensity of the oxygencontaining groups decreases obviously (Figure 3d) and the contents of the fitting peak area corresponding to different bonds are shown in Table 1. By a rough addition operation, 26.84% oxygen-containing groups are reduced from 51.29% of the GO to a final 27.35% of ZnO@RGO. The addition of GO is an important factor which will affect the uniformity of coating and the content of active substances. After heating in steps under an air atmosphere, 10.42% thermal weight loss is measured by TG analysis (Figure 4). Meantime, an exothermic peak at 338.2 °C is observed on the curve, which is attributed to the oxidation of RGO. The initial additive amount of GO is 9 mg, and the output of ZnO from 360 mg Zn(NO3)2·6H2O is 66.9 mg in theory, that is, there is 11.8 wt % GO in the final product. Take account of the weight loss in the transformation process from GO to RGO and the existence of the measurement errors, the result of TG analysis is a relative approach with the calculated result. Scilicet, a devisible ratio of ZnO with GO can be gained by the complete reaction as this synthesis scheme. To get a higher content ratio of ZnO, the attempt of decreasing the amount of GO is performed, while an imperfect coating of 6 mg GO is obtained (Figure S4). Thus, the befitting additive of GO for an even coating on ZnO is around 10 wt %. 3.3. Electrochemical Performances of ZnO@RGO. To prove the aspiring role of RGO, the discharge cycle performances of the ZnO@RGO and commercial ZnO as negative electrode in the Zn−Ni secondary battery are performed at 1 C current (Figure 5). And as contrasts, the cycle performances of ZnO prepared with NH3·H2O and ZnO coating with different amounts of RGO are measured. The commercial ZnO, which has smaller size and better crystallinity (Figure S6) shows a higher discharge capacity and a more stable ability (Figure S7) than the ZnO (Figure 2a) prepared in our work. More addition of RGO shows more stable discharge ability, but the maxium discharge capacity of the composite will decrease as well (Figure S7). After running for 300 cycles, the discharge capacity of commercial ZnO decreases from the initial 420 mAh g−1 to a final 390 mAh g−1 and there is a stable period at first 130 cycles. The commercial ZnO reaches its maximum discharge capacity after cycling for two cycles, because commercial ZnO is easy to react in alkaline solution. The capacity fading of the commercial ZnO is chiefly caused by the dissolution of the active material and the irreversible formation of Zn crystallization. As the contrast, ZnO@RGO has experienced 25 cycles with a stable discharge capacity of 510 mAh g−1 and sustains after 300 cycles which has a similar performance with the carbon coated ZnO in the reported work.35 Clearly, the RGO layer inhibits the activation efficiency as a dielectric film but, it offers enough conducting medium on the particle surface to ensure a high utilization coefficient of active materials. In addition, the variations of the charge/ discharge curves reflect the polarization of the voltage, which is another key parameter of the battery (Figure 5b,c). According to eqs 4−7,3

Figure 1. XRD pattern of GO and ZnO@RGO.

Figure 2. SEM images of the (a) bare ZnO prepared with NH3·H2O, (b) and (c) ZnO@RGO, and the TEM image of (d) ZnO@RGO.

defects in the RGO sheets show a strong reduction (Figure S5).To get a better insight of the surface composition and the redox condition of RGO, XPS studies are performed on the GO and ZnO@RGO samples. As shown in the survey spectrum of Figure 3a, ZnO@RGO composite shows emissions of Na, Zn, O, and C elements, and the different peaks observed are assigned to Na 1s, Zn 2p1, Zn 2p3, O 1s, and C 1s core levels and to O Auger and Zn Auger features. The high resolution XPS spectra of Zn 2p (Figure 3b) reveals the Zn 2p3/2 at 1024 eV and Zn 2p1/2 at 1047 eV, which correspond to the standard spectrum of ZnO in the literature.36 The high resolution XPS of C 1s spectral region for the GO is shown in Figure 3c, after a Tougaard-background subtraction, curve fitting is performed using Lorentzian−Gaussian functions. The intense subpeak at 284.5 eV can be assigned to CC bond, and the peak at 285.2 eV can be attributed to C−C/H bond, such as carbon atoms bound to subsurface hydrogen to form polyhydride carbon species (CHx).37,38 The intense binding energy peak at 286.65 eV is attributed to C−O binding, the lower peak at 288.2 eV is assigned to CO binding and a weak peak at 289.2 eV is attributed to HO-CO binding which have been proved in the reported works.22

( +)Positive: NiOOH + H 2O + e− → Ni(OH)2 + OH− (4)

( −)Negative: Zn + 4OH− → Zn(OH)24 − + 2e−

(5)

Zn(OH)24 − → ZnO + 2OH− + H 2O

(6)

Total reaction: 12339

DOI: 10.1021/acs.jpcc.6b01025 J. Phys. Chem. C 2016, 120, 12337−12343

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Figure 3. XPS patterns of (a) survey spectrum of ZnO@RGO, (b) Zn 2p region of ZnO@RGO, (c) C 1s region of GO, and (d) C 1s region of ZnO@RGO.

Table 1. Relative Fractions of C 1s Components of GO and ZnO@RGO by XPS Measurements position (eV)

attribution

GO (%)

ZnO@RGO (%)

284.5 285.2 286.65 288.2 289.2

CC C−C/H C−O CO HO-CO

39.22 9.479 35.84 13.23 2.217

37.36 35.28 19.02 5.461 2.872

Zn + 2NiOOH + H 2O → ZnO + 2Ni(OH)2

(7)

the theory value of open circuit voltage of Zn−Ni battery is 1.73 V, the potential of positive is 0.49 V, and the negative potential is −1.24 V. The midpoint charge voltage of ZnO at 50th cycle is 1.75 V, which is close to the theory voltage, and the midvalue charge voltage of ZnO@RGO is 1.83 V. The RGO coating layer causes a 0.08 V polarization potential owing to the lower hydrogen evolution overpotential of carbon materials, despite its good conductivity. In the subsequent cycle test, the charge voltages of ZnO and ZnO@RGO gradually go up, while the increase amplitude of ZnO@RGO is smaller than ZnO. Meantime, the discharge voltages of ZnO and ZnO@ RGO increase synchronously and there are 0.06−0.08 V voltage differences. Usually, the polarization of the electrode

Figure 4. TG and DSC analysis of ZnO@RGO at air atmosphere from room temperature to 900 °C of 10 °C min−1.

leads to a rise of the charge voltage and a simultaneous fall of the discharge voltage. As H2O splits and the generated O2 and H2 escape out of the open test system, the abnormal changes of the discharge voltages are possibly caused by the changes of the electrolyte concentrations. For all these, ZnO@RGO exhibits a weak voltage fluctuation and a stable discharge capacity during 12340

DOI: 10.1021/acs.jpcc.6b01025 J. Phys. Chem. C 2016, 120, 12337−12343

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The Journal of Physical Chemistry C

Figure 5. Discharge performances of (a) 300 cycles at 1C, the charge and discharge curves at different cycles of (b) ZnO and (c) ZnO@ RGO, 600 mA g−1.

the cycle test. To further understand the electrochemical behaviors of RGO, the electrochemical impedance spectroscopy measure-ments are utilized. As a typical model curve of Zn− Ni battery, the semicycle in the high frequency region is attributed to the charge transfer resistance and the slope in the low frequency region is caused by the diffusion of Zn2+ ions into the electrode material.22 At the initial period, ZnO@RGO has a higher charge transfer resistance (Rct = 17.82 Ω) than the bare ZnO (Rct = 12.98 Ω; Figure 6a,b). After 100 cycles, the Rct of ZnO@RGO (Rct = 12.23 Ω) becomes lower than the initial condition. And as a contrast, the Rct of bare ZnO (Rct = 22.29 Ω) is obviously higher than the composite one after running for 100 cycles. Owing to the mass deposition of Zn on the electrode, a third small semicycle occurs before the semicycle of Rct, which is attributed to the intrinsic resistance of the active materials (Figure 6b).39 A distinct change of the slope comes up depending on the diffusion of the ions. For the slope of ZnO, the strength of ions spread is enhanced and for ZnO@ RGO, the strength of ions spread is inhibited. So a conclusion can be deduced that RGO coating inhibits the dissolution of ZnO and offers enough contact area to the even deposition of Zn, which is corresponding to the analysis results of the cycle life. The microscopic surface shape of the test electrodes after 100 cycles and 300 cycles are captured by SEM (Figure 7). In a typical charge process of ZnO electrode, Zn as the reduction product of ZnO is formed attaching to the surface of ZnO particle. For a complete reduction of ZnO, as the conversion reaction repeats for times, the ZnO particle gradually diminishes, while Zn is inclined to nucleating at the interface and growing by the way of gathering.8 As shown in Figure 7a, the surface structure of the ZnO electrode cycling for 100 cycles is constructed with plenty of tiny Zn particles and the particles have begun to gather together. And for the ZnO@ RGO electrode, the morphology of RGO coating is still reserved although the initial shape likes a petal has been etched into a small size round ball (Figure 7b). That is a direct evidence to confirm the anticorrosion effect of RGO. Big size of Zn crystal with a smooth surface has been taken shape after 300 cycles (Figure 7c), unlike the coarse surface at the 100th cycle. In addition, an interesting phenomenon is observed in Figure

Figure 6. EIS curves of (a) ZnO and ZnO@RGO at 5th discharge cycle and (b) ZnO and ZnO@RGO at 100th discharge cycle.

Figure 7. SEM images of the surface morphology of ZnO electrode cycling after (a) 100 and (c) 300 cycles and ZnO@RGO electrode at (b) 100th and (d) 300th cycle.

7d, in spite of the coating form of RGO is gone that RGO remains covering on the surface of the active materials as a protective and a conductive layer. RGO can effectively compact with the active materials for its good hydrophilicity and interfacial compatibility, the advantage is beneficial to the integration of the electrode and inhibiting the loss of active materials caused by volume expansion or gravity settling. 12341

DOI: 10.1021/acs.jpcc.6b01025 J. Phys. Chem. C 2016, 120, 12337−12343

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Figure 8 shows the high rate discharge performances of the ZnO and ZnO@RGO at different currents. In order to obtain

Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: +86-0431-85262447. *E-mail: [email protected]. Tel.: +86-431-8509-5878. Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (21373198, 21221061) and the Science and Technology Support Project of Jilin Province (20140306003GX, 20130305009GX), Jiangsu province natural science foundation of China (BK20141174), and Changzhou natural science fund (cj20140016).



Figure 8. High rate discharge performances of ZnO@RGO and ZnO at different discharge currents of 1C (659 mA g−1), 2C, 3C, and 5C.

the maximum discharge capacity with the corresponding discharge current, a theory charge capacity of 659 mAh g−1 is utilized and both the electrodes have been activated at 0.5C for 10 cycles. At 1C discharge current, the capacity of ZnO@RGO gains a maximum value of 570 mAh g−1, which is 130 mAh higher than the commercial ZnO of 440 mAh g−1. Through the calculation of net ZnO of 89.58 wt % in the composite by TGA, the theoretical discharge value of ZnO is 636 mAh g−1. In other words, an active material utilization of 96.56% is obtained by RGO coating. At the other discharge currents, ZnO@RGO is always in an advantaged state and keeps a high level of discharge capacity than the commercial ZnO.

4. CONCLUSIONS The ZnO@RGO has been successfully synthesized using a facile and efficient route in water solution with a controllable ratio of RGO, and the in situ growth of ZnO attached to the RGO causes an even and complete coating on the particle. The composite exhibits a stable and enhanced discharge performance in the application of Ni−Zn secondary battery. A series of test results indicate that the RGO layer as a protective film could effectively inhibit the dissolution of ZnO into the alkaline electrolyte. Meanwhile, owing to the good conductivity and surface adsorption properties, Zn could be uniformly formed on the RGO surface with a high utilization in the charge process. With good electrode morphology stability and enough energy density of the composite, this method is a promising way to manufacture the raw material for the next generation Zn−Ni secondary batteries. This synthesis scheme is also a chance to prepare RGO coating composite for other metallic oxides or hydroxides which is stable in the alkaline solution.



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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b01025. Additional information on SEM figures of ZnO@RGO (PDF). 12342

DOI: 10.1021/acs.jpcc.6b01025 J. Phys. Chem. C 2016, 120, 12337−12343

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DOI: 10.1021/acs.jpcc.6b01025 J. Phys. Chem. C 2016, 120, 12337−12343