Composite of Zinc Using Graphene Quantum Dot Bath: A Prospective

Research Article pubs.acs.org/journal/ascecg

This paper was retracted on July 28, 2017 (ACS Sustainable Chem. Eng. 2017, DOI: 10.1021/acssuschemeng.7b02509).

Composite of Zinc Using Graphene Quantum Dot Bath: A Prospective Material For Energy Storage Z. Protich,† P. Wong,†,‡ and K. S. V. Santhanam*,† †

School of Chemistry and Materials Science, Rochester Institute of Technology, Rochester, New York 14623, United States Department of Chemical Engineering, Rochester Institute of Technology, Rochester, New York 14623, United States

et ra ct ed

‡

ACS Sustainable Chem. Eng. 2017, 5, DOI: 10.1021/acssuschemeng.7b02509

ABSTRACT: The electrodeposition of a zinc−graphene composite has been achieved for the first time using a graphene quantum dot (GQD) electrode. At the GQD electrode, the electrochemical reduction of zinc ion is shifted to a lesser negative potential with the complementary anodic peak due to the oxidation of the composite shifted toward a positive potential as compared to zinc ion reduction in the GQD bath. The charge ratio of anodic to cathodic peaks is one that represents a gain of nearly ten percent over the conventional Zn/Zn2+ in the energy storage systems. In galvanostatic electrolysis, the deposition of zinc−graphene composite is carried out under neutral and acidic conditions. The X-ray diffraction of the electrolytically prepared composite shows distinct features of 2 theta reflection at 8° due to (001) plane of graphene, in addition to the characteristic reflections at 38.9°,43.2°, 54.3°, 70.1° and 90° arising from Zn at (002), (100), (101), (102) and (110). A large scale preparation of the zinc−graphene composite has been achieved with a zinc plate as the working electrode in the GQD bath. The thermogravimetric analysis (TGA) of the composite shows a distinct weight loss that is due to breakdown of the composite. The interaction of GQD with zinc ion has been examined by fluorescence spectroscopy that shows quenching of the fluorescence of GQD. Scanning electron microscopy and energy dispersion X-ray analysis (EDAX) shows a string-like structure with peaks for carbon and zinc in EDAX. The electrochemical data on zinc/zinc−graphene composite reveals that it functions ideally as a charge storage material. Contact angle measurements reveal it to have hydrophilicity. KEYWORDS: Zinc−graphene composite, Graphene quantum dots, XRD, EDAX, Contact angle, Cyclic voltammetry, Galvanostatic electrolysis, Fluorescence

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INTRODUCTION

utilizes zinc with its ions and copper with its ions as the redox couples. This triggered the development of a large number of energy storage batteries such as zinc−air, zinc−chlorine, zinc− bromine and zinc−nickel batteries having theoretical specific energy in the range of 54−1370 Wh/kg. In the process of the operation of the energy storage devices involving zinc, the operational cycle involves its dissolution and redeposition except in the Daniel cell where only dissolution of zinc occurs. Recently, a modified Daniel cell has been proposed where

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Among the elements largely abundant on earth, zinc occupies the twenty-third position1 and is widely used in a large number of applications such as drainage, architectural roofing, galvanic protection and devices. It is environmentally suitable and hence is the most preferred material by designers and architects. The composites of zinc have been very useful in several technological applications such as durability, bending strength, flatness and workability and antifouling.2 Several energy storage devices have been developed using zinc electrodes, and hence its electrochemical deposition is of great interest3−16 as zinc has a large negative potential that permits the reach of high voltage and high energy density in batteries when it is coupled with other redox couples. The earliest successful primary battery is the Daniel cell, which © 2016 American Chemical Society

Special Issue: Building on 25 Years of Green Chemistry and Engineering for a Sustainable Future Received: July 29, 2016 Revised: September 25, 2016 Published: October 17, 2016 6177

DOI: 10.1021/acssuschemeng.6b01794 ACS Sustainable Chem. Eng. 2016, 4, 6177−6185

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ACS Sustainable Chemistry & Engineering

Figure 1. (A) Cyclic voltammetry of 4.37 mM zinc ion in GQD bath. (B) Bob’s cell used in the experiment. WE, working electrode; CE, graphite rod; REF, reference electrode. Sweep rate: 20 mV/s.

dissolution and redeposition is carried out.14 In this cycle, dendritic growth of zinc and the shorting of the battery occurs in a large number of zinc batteries leading to their restricted usage. This problem is encountered with other metals such as copper and has been researched to control the dendrite formation.16 The zinc redox couple has also found usage in lightweight conducting polymer batteries.12,13 Hubbard’s11 pioneering contribution into surface attachment and interfacial electrochemistry has been used for understanding the electron transfer reactions in the batteries, resulting in the prosperous growth of this area. The electrodeposition of zinc is carried out in acid solutions containing chloride or sulfate on a variety of substrates such as mild steel, copper, etc.16−22 Among other deposition baths, cyanide and fluoroborate baths have been widely used successfully.18,19 Because of the toxicity of cyanide, the bath disposal requires additional treatment procedures. This has resulted in the development of noncyanide baths,17,19,20 as cyanides are toxic and enforce environmental pollution control. Graphene and its metal composites are of importance in a number of applications such as energy storage devices, sensors, flexible electronics, LED lighting, solar cell and battery super capacitors, flexible display touch panels, high speed transistors, conductive ink and chemical sensors. The metal composites have potential applications in hydrogen storage and automobile and airplane components. A literature survey shows that graphene has been combined with zinc oxide by plasma enhanced chemical vapor deposition,23 thermal decomposition24 and electrohydrodynamic atomization25 and a solovo thermal method.26 This results in the formation of a graphene− zinc oxide composite. Although this type of composite is useful for solar energy storage, such composites have very limited applications in other energy storage systems. For several other energy storage systems such as batteries, the zinc composites have provided greater stability and performance.3 We report here a novel method of preparing a nanosized string-like zinc− graphene composite using a GQD bath that shows potentiality for near 100% charge storage and recovery. In addition, we compare the characteristics of the zinc ion reduction on an immobilized GQD electrode with its reduction in the GQD

bath through current−time transients for recognizing the type of nucleation process in forming the composite. The GQD bath studied here makes it conducive for the deposition of the composite on metal plates such as zinc.

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MATERIALS AND METHODS

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Chemicals. Zinc sulfate, potassium ferrocyanide and sodium sulfate were procured from Aldrich Chemical Co. Graphene quantum dot (GQD) was prepared as per the procedure reported27 and stored in an amber colored bottle. A TTK-4 graphite plate was used in the preparation of graphene and was supplied by Ohio Carbon Black. An argon gas (99.99% pure) cylinder was obtained from Linde, Fulton, NY. Instruments. A Gamry electrochemical instrument was used in the measurements. Bob’s cell was used for the electrochemical measurements. It consists of a five necked “V” shaped glass container to fit the working, counter and reference electrodes as shown in Figure 1. A saturated calomel electrode (SCE) was used as the reference electrode. Electrodes: Pt disc (A = 0.0766 cm2), graphite rod (diam. 0.5 cm, length 6 in.), glassy carbon electrode (Gamy Instrument) (A = 0.0750 cm2) were used as appropriate for the measurements. A glassy carbon electrode was deposited with GQD (5% w/v; 5−100 nm) following the method described in a previous publication.28 The electrode is characterized for the area by recording the cyclic voltammetry of 10.47 mM K4Fe(CN)6. Electrodeposition of Zinc−Graphene Composite. All the studies were carried out either in (A) aqueous sulfate bath or (B) GQD bath. The aqueous sulfate bath was made using 0.1 M sodium sulfate. The bath may have sulfuric acid added when electrodeposition is carried out at low pH in some experiments. The GQD bath contains 0.1 M sodium sulfate along with graphene quantum dot. For large scale deposition for scanning electron microscopy (SEM) recordings, the solution composition used was a mixture of zinc sulfate (0.6 M), sodium sulfate (0.1 M) and GQD for 90 or 900 s deposition. The solution pH was adjusted to 1.6. In galvanostatic electrolysis, the current density was maintained at 300 mA/cm2. All experiments were carried out under argon gas atmosphere after degassing the solution. Thermogravimetric Measurements. TGA measurements were carried out using a TA-Q500. A platinum pan is used for the experiments. The temperature was ramped at 5 °C/min from ambient to 800 °C. The experiments were carried out by passing purified air from a cylinder. Quantum Dots. Scanning electron microscopy and transmission electron microscopy (TEM) were used for characterizing samples.

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anions in solutions32−34 and temperature35,36 have been discussed in the literature. These factors also influence the morphology of the zinc deposits. The cyclic voltammetric peak potential difference of 30 mV between anodic and cathodic peaks for zinc ion reduction in the GQD bath (Figure 1) is indicative of a reversible 2e− process. In comparison, the electron transfer reaction in methanesulfonic acid bath is quasi reversible37 with peak potential difference of 160 mV and 100− 150 mV in chloride and sulfate electrolytes, respectively. The charge ratio of the two peaks runs in the range of 79−81%.37 Measurements of Diffusion Coefficient of Zn2+ in GQD Bath. Using the diffusion model and the boundary conditions appropriate for cyclic voltammetry,29 the diffusion coefficient of zinc ion is related to the peak current as shown below

Images and selected-area diffraction patterns were obtained in either a JEOL 2000FX or a 100CX TEM, both with tungsten filaments. Image magnification was calibrated using phase-contrast images of asbestos fibers. High resolution (HRTEM) images were obtained on a JEOL JSM-6460 LV. Energy dispersive spectrum is made by Thermo Fisher Scientific. It is equipped with an UltraDry detector and NSS software. The images were recorded on a Kodak 4489 electron microscope film and digitized with a Nikon 9000 film scanner.

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RESULTS AND DISCUSSION To arrive at the conditions required for the electrochemical deposition of the zinc−graphene composite, experiments were carried out in aqueous sulfate and acid electrolytes containing GQD, using cyclic voltammetry, chrono amperometry and differential pulse voltammetry. Figure 1 shows the cyclic voltammetric curve of zinc ion reduction in GQD bath that results in the formation of the zinc−graphene (Zn-GQD) composite on a glassy carbon electrode by the reaction shown in eq 1: Zn

2+

−

+ GQD + 2e = Zn‐GQD

D1/2 = [i p/(2.65 × 105)n3/2AC*v1/2]

where D is the diffusion coefficient of zinc ion, ip is the cathodic peak current, n is the number of electrons, A is the area of the electrode, C* is the zinc ion concentration in solution and v is the sweep rate (V/s). The equation is also called the Randle− Sevcik equation.29 Taking the average value of six measurements, the diffusion coefficient of zinc ion is calculated as Dzn2+ = 4.19 × 10−6 cm2/s at 23 °C. It is interesting to compare the diffusion coefficient of zinc ion that has been reported in the literature by Cathro.38 At 30 °C, the diffusion coefficient in sulfate bath having a kinematic viscosity of 1.53 m2/s is reported as 4.7 × 10−10 m2/s (equivalent to 4.7 × 10−6 cm2/s). There are other reports of diffusion coefficient measurements of zinc ion at 25 °C. Using a nonelectrochemical method such as Rayleigh optics, Albright and Miller39 obtained a diffusion coefficient that is dependent on the electrolyte (ZnSO4 concentration); at 0.004 M, the value is 8.48 × 10−6 cm2/s and in 3.33 M concentration it is 2.81 × 10−6 cm2/s. Using a mercury electrode as the working electrode, El-Hallag40 reported in chloride medium a diffusion coefficient value of 2.44 × 10−6 cm2/s at 25°. In another report,41 the diffusion coefficient is measured as 7.03 × 10−6 cm2/s in potassium nitrate solution at 25 °C. In 10% ethanol containing potassium nitrate, the D value changes to 6.19 × 10−6 cm2/s. Using a radioactive label under the electric field created by LiCl,42 the diffusion coefficients for Zn salts are listed in the range of 6.54− 6.77 × 10 −6 cm 2 /s. Interestingly, the electrochemical techniques have always yielded lower values ranging from 1.51 to 3.35 × 10−6 cm2/s.43 Considering all the reports in the literature, it is apparent that the diffusion coefficient of the zinc ion is in the range of 1.51−8.48 × 10−6 cm2/s due to the different conditions and techniques used for the measurement. It is significant to note that the diffusion coefficient value obtained by electrochemical techniques are in the range of 1.51−4.7 × 10−6 cm2/s; whereas at the mercury electrode, the value obtained is lower than the glassy carbon electrode. This difference shows the processes occurring are different: metal amalgam formation with Hg electrode and metal deposition at glassy carbon electrode here. In addition, there is a spherical diffusion that occurs at the mercury drop electrode to semiinfinite linear diffusion at the glassy carbon electrode. To get further insight into the diffusion process that occurs at the glassy carbon electrode, we have examined the electrochemical reduction of zinc by potential step electrolysis. Based on the slope of the plot i vs t−1/2, the diffusion coefficient is calculated as 4.20 × 10−6 cm2/s (Figure 2A,B). The value obtained here agrees very closely with the cyclic voltammetric data. In a GQD

(1)

The reversal oxidation process (anodic peak) removes the deposit from the electrode. The reduction of zinc ion in this bath is contrastingly different from the aqueous bath with no GQD; the cathodic peak potential is shifted anodically at all sweep rates and the cathodic peak currents are higher in the GQD bath. Table 1 shows the magnitude of the differences Table 1. Zinc Ion Reduction in GQD Batha sweep rate (V/s)

ΔEIpc (mV)

R1

ΔEIpa (mV)

R2

0.02

40

1.25

30

1.04

0.05

30

1.32

30

1.32

0.10

30

1.25

40

1.25

0.20

31

1.20

30

1.20

ΔEp/2

0.0261 0.0612 0.0251 0.0612 0.0261 0.0622

(2)

R3

0.812 0.792

ΔEpc = (EpcZn2+ − EpcZn2+‑GQD) ; ΔEpa = (EpaZn − EpaZn‑GQD); R1 = ratio of cathodic peak currents for zinc ion reduction in GQD to aqueous baths. ΔEp/2 = (Epc − Ep/2c); 1, aqueous bath; 2, GQD bath; R3, ratio of cyclic voltammogram integrated anodic peak to the cathodic peak areas. a

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observed at different sweep rates. The cyclic voltammetric peaks are characterized by different ways;29 one way to characterize the peak is by examining the magnitude of the difference between peak and half peak potentials. For a truly reversible one electron reduction, the expected difference is 59 mV.29 When the zinc ion reduction is carried out in a GQD bath, this difference is 61 mV (Table 1). The shift in peak potential from the aqueous bath, taken along with the above difference, suggests there is binding of GQD to the deposited zinc (see XRD discussion). The cyclic voltammetric feature shows no evolution of hydrogen is observed at the cathodic potential of −1.2 V as at this potential when the glassy carbon electrode is deposited with zinc, one would expect hydrogen ion reduction to follow on this surface. Zinc deposition potential based on its standard potential is less negative than hydrogen ion reduction in the GQD bath. Zinc nucleation interestingly occurs depending on the medium at different potentials on different electrodes. At stainless steel electrode zinc nucleation is observed at −0.80 V vs SCE30 and the nature of the deposits as a function of the exchange current density,31

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Figure 2. (A) Current−time curve obtained in potential step electrolysis at −1.45 V at glassy carbon electrode. (B) Cottrell’s plot. GQD bath. (C) Current−time curve obtained in potential step electrolysis at −1.45 V at GQD electrode. (D) Cotrell’s plot.

Figure 3. (A) Cyclic voltammetric curve of 10.47 mM K4Fe(CN)6 in 0.1 M Na2SO4. (B) Plot of peak current vs sweep rate at GQD coated glassy carbon electrode.

bath, the diffusion coefficient value obtained is 9.67 × 10−6 cm2/s, which is higher than that obtained in the aqueous bath. With GQD bath, the graphene particles are in constant motion in the solution that produces the accelerated diffusion. Bard and co-workers44 have shown that nanoparticles tend to be in constant motion in solution, which results in collisions with an ultra micro electrode. A similar situation exists with GQD bath where the motion of these particles results in the collision with the zinc ions in solution. This movement is removed at the GQD electrode described in the next section. Measurements on GQD Electrode. Cyclic voltammetry of potassium ferrocyanide is used as a benchmark for the calibration of the electrochemical area of the GQD coated electrode. In Figure 3A, the cyclic voltammetric curves of K4Fe(CN)6 are shown at different sweep rates that exhibit the peak current increasing with sweep rate. Figure 3B depicts the plot of peak current with sweep rate. The effective area was calculated using the diffusion coefficient of the ferrocyanide ion reported in the literature29 using the peak current value in the calculation. The cyclic voltammetric curve for the ferrocyanide ion at a GQD working electrode showed higher currents, which resulted in a 1.71× increase in the effective area of the GQD

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electrode over the glassy carbon electrode. At the GQD electrode, the peak current increased with sweep rate as shown in Figure 3B, as expected from the cyclic voltammetric peak current equation.29 At the GQD electrode, the zinc ion reduction occurs as shown in Figure 4. The peak potential differences between the anodic and cathodic peaks, the peak current ratio and the current function values of anodic and cathodic peaks are similar to the one obtained in GQD bath.

Figure 4. Cyclic voltammetry of 5 mM zinc sulfate in aqueous bath. Inset shows the GDQ coated electrode used in the experiment. 6180


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ACS Sustainable Chemistry & Engineering im = 0.6382nFDC(KNo)1/2

The cathodic peak is observed at the same potential as seen with glassy carbon electrode, except the peak current is higher than at a glassy carbon electrode. The anodic peak is shifted toward the potential observed in the GQD bath (Epa = −1.03 V). An interesting feature of this bath is that the integrated areas of anodic and cathodic peak areas in three measurements (Figure 4) are Q1c = 1.401 mC, Q2c = 1.399 mC, Q3c = 1.403 mC, Q1a = 1.43 mC, Q2a = 1.401 mC and Q3a = 1.405 mC where Qc is charge passed for cathodic peak and Qa is the charge passed for the anodic peak, and the corresponding area ratios of are 1.00. The charge ratio of near 100% is achieved at the GQD electrode, suggesting that the composite stability is higher than in the GQD bath. The performance of the GQD electrode opens up the prospects of its usage in redox flow batteries. Currently, with these batteries the maximum reported charge recovery with Zn/Zn2+ is 90%, which in comparison with the composite studied here is less by 10%. The flow batteries have been widely considered for power grid applications45 involving the use of two redox couples having high solubilities.46 Among the flow batteries, Zn/Zn2+ couple in combination with other redox couples plays an important role. However, the redox process associated with zinc metal has a problem that is similar to what occurs in batteries: the redeposition of zinc ion from the medium results in dendrite growth that results in a reduced performance with time. The dendritic growth in the redisposition of zinc has been observed at low current densities of 15 mA/cm2;37 and it increased with increasing current densities. This resulted in the poor performance of the flow battery. With Br2/Br− couple in combination with Zn/Zn2+, the flow battery is estimated to give 1.8 V with a theoretical specific energy of 429 Wh/kg that is seldom realized. In practice, the flow battery gives 65 Wh/kg45 and gives a round trip energy efficiency of 65−75%. To overcome the dendritic formation, the additives such as methanesulfonic acid have been used. Banik and Akolkar47 used polyethylene glycol as an additive to suppress the dendritic growth and examined the kinetic parameters operating in this suppression. Leung et al.37 claimed charge efficiency of 91% by using methanesulfonic acid as an additive. In the present work, a higher efficiency of near 100% has been obtained with a zinc−graphene composite using the GQD electrode with no dendrite formation (see SEM section). Modeling Nucleation of Zinc−Graphene Composite. The 3D growth of zinc during electrodeposition has been previously investigated by a number of studies in acid and alkaline medium using theoretical models.48,49 The theoretical models show divergent behaviors for instantaneous and progressive nucleations, which are described by nondimensional eqs 3 and 4:

(7)

The corresponding equations for progressive nucleation are tm = {4.6733/πANoDK }1/2

(8)

Ns = {ANo /2KD}1/2

(9)

and im = 0.4615nFD3/4C(KANo)1/4

(10)

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where n is the number of electrons in the reduction of zinc ion, F is the Faraday constant, D is the diffusion coefficient of zinc ion, t is the time of electrolysis in seconds, No and Ns are the number density of nucleation sites, A is the steady state nucleation rate and K is growth rate constant of nucleus given by K = [8λCM /ρ]1/2

where C is the zinc ion concentration, M is molar mass and ρ is the density. With the data obtained in potential step electrolysis at −1.45 V; tm = 0.04s, im = 2.3 × 10−4 A, D = 9.67 × 10−6 cm2/ s, M = 65.38 g/mol, ρ = 7.14 g/cm3 and C = 4.54 mM in the GQD bath, Ns = 3.0 × 107 (instantaneous) and No= 0.78 × 105 (progressive) have been obtained. Figure 5 gives the observed

Figure 5. Plot of (i/im)2 vs (t/tm) in GQD bath and GQD coated electrode.

features of the plot of (i/im)2 vs (t/tm) for zinc−graphene composite at a glassy carbon electrode in GQD bath and the composite formation at the GQD electrode. The observed features at the GQD electrode fit well with progressive nucleation48,50−52 by examining the very early stages of electrodeposition from the start until [t/tm = 2], the region where the theoretical model46−48,50−52 predicts a faster decay of current ratio than in the instantaneous nucleation. It appears that in the GQD bath the zinc−graphene composite follows an instantaneous nucleation. Interaction of GQD with Zinc Ion. The absorption and fluorescence spectra of GQD are given in Figure 6. The GQD fluorescence has been studied previously51−55 in detail: when the GQD solution is excited at λ = 360 nm, the fluorescence spectrum shows a maximum at λ = 440 nm. The lifetime of the excited state of GQD has been reported as 6.77 ns.51 The fluorescence of GQD is quenched by metal ions like cupric ion.15 With a view to understand the zinc ion interaction with GQD, the fluorescence intensity of GQD is measured at different concentrations of zinc ion in the solution. The decrease in intensity is attributed to collisional quenching of GQD fluorescence by zinc ion. This result

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(i/im)2 = [1.9542/(t /tm)]{1 − exp(− 1.2564(t /tm)}2

(3)

and

(i/im)2 = [1.2254/(t /tm)]{1 − exp(− 2.3367(t /tm)2 }2 (4)

The theoretical model defines the expected maximum current and the time at which it is reached by the following expressions for an instantaneous case: tm = {1.2564/NoπKD}

(5)

Ns = {1.2564/tmπKD}

(6)

(11)

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content was obtained. If we continue the TGA experiment beyond 450 °C, there is weight gain observed due to oxidation of zinc that results in ZnO.56−58 XRD Features of Zinc−Graphene Composite. The electrochemically deposited zinc−graphene composite is characterized by its XRD features (Figure 8). It showed 2θ

Figure 8. XRD of electrolytically prepared zinc−graphene composite.

demonstrates that there is interaction between GQD and zinc ion in solution. Thermogravimetric Analysis (TGA) of Zinc−Graphene Composite. A large scale preparation of the composite is carried out using a zinc plate working electrode and a large zinc foil rolled into a cylinder as the counter electrode. By carrying out a galvanostatic electrolysis, the material deposited on the working electrode is collected, washed and dried at 80 °C for 2 h. The composite is examined by TGA at a 5 °C ramp from a temperature range of 25 to 400 °C. Figure 7 gives the TGA of the composite that shows a small initial weight loss up to 120 °C due to moisture that is followed by a sharp loss of weight followed by leveling. This transition is attributed to the break down of the composite to Zn, presumably by air oxidation of the composite, because the experiments are carried out in oxygen atmosphere. The mass of zinc in the composite has been determined by this method. In a typical experiment, 4.728 mg of the composite gave 4.288 mg of zinc (after removal of the moisture content in the sample). By subtracting the mass of zinc from the net dry weight of the composite, the GQD

reflections at 8°, 38.9°,43.2°, 54.3°, 70.1° and 90°, which are attributed to graphene and zinc (100), (101), (102), (103) and (110) phases (JCPDS Card Number 87-0713). An interesting feature of the zinc−graphene composite is that the most intense peak is located at 70.1 corresponding to the (110) plane, whereas in zinc the most intense peak is at 43.2° that represents the (101) plane. Note the absence of 2θ reflections at 48°, 57° and 63°, which are characteristic reflections for zinc oxide demonstrating its absence in the composite. Scanning Electron Microscopy and Energy Dispersion Analysis. The electrolytically prepared zinc−graphene composite was mounted on Al stubs using silver paint to make electrical contact and secure the sample to the stub. The individual stubs were attached to a large sample holder using silver paint. The use of silver paint prevents any contamination coming from other sources. The surfaces were analyzed and imaged for understanding the growth of the composite. The SEM of the zinc-GQD composite is shown in Figure 9. The composite shows a string-like structure with no dendritic growth. The dendritic growth generally shows needle shaped

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Figure 6. Fluorescence of GQD as a function of Zinc ion concentration in the range of 0 to 1000 μM.

Figure 7. TGA of Zinc-GQD composite made electrolytically. 6182


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Figure 9. (A) SEM of electrolytically prepared Zn−graphene sample. (B) EDAX of the same sample.

Figure 10. (A) Contact angle as measured with Zn-GQD, Zn(t) substrate etched with acid, graphite, GQD and GO. (B) Contact angle of Zn-GQD. (C) Model for hydrophilicity zinc treated with hydrophilic model for Zn-GQD.

deposition that grows to short the electrodes in the batteries. The EDXA spectrum shows that the Zn−graphene composite has Zn, C and O as the elemental composition. By comparing the XRD data described in the earlier section with the EDXA data, it appears that the O is associated with graphene and not with the zinc atom based on the following reasons: (A) The XRD spectrum of the Zn−graphene composite does not show peaks due to ZnO at 31.7° and 62.5° due to the (100) and (103) planes.57,58 (B) The XRD spectrum shows a peak at 10° that is attributed to graphene oxide, suggesting carbon and oxygen are bonded. Efficient Energy Storage System. A large number of reports3−11 on energy storage systems involving Zn/Zn2+ redox couple show that it is a low cost, lightweight, high specific capacity, high energy density solution with additional advantages of free from pollution and high safety. For a zinc−air storage system, the expected theoretical capacity and energy densities are 820 mAh/g and 1312 Wh/kg. For a zinc− bromine system, the corresponding values are 238 mAh/g and 428 Wh/kg with nearly one-seventh of the energy density practically being realized.47 With these systems, a charge ratio of 0.80−0.90 has been accomplished. Because of the high negative potential of this couple, it provides an advantage of achieving a high open circuit voltage. Zinc batteries are used in a number of applications involving communications. Currently, there is demand to improve their performances especially with regard to durability and Coulombic efficiency.59−66 As this kind of energy storage systems is considered for green grid

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applications, any improvement that results in better performance would be a bonus. With zinc redox systems, the cause for the charge ratio of less than unity has been analyzed and is attributed to the zinc deposition process in the charge/ discharge cycles. Because of the very negative potential required for the deposition process, there is a slow concurrent reduction of hydrogen ion as a competing process that causes the Coulombic efficiency to be lower. In the present study, at the GQD electrode, the reduction of zinc ion is shifted anodically (see Figure 4) thereby inhibiting the hydrogen evolution during the charging process. As a result, the charge ratio of unity (see earlier section on GQD electrode) has been achieved at the GQD electrode where the zinc−graphene composite is formed through progressive nucleation process. Contact Angle Measurements. The wettability of graphene is of great technological importance66,68,69 in the construction of energy storage devices, robots, catalysis and sensors. GQDs have a contact angle for water of 127°, whereas graphite has a contact angle of 90−95°. The higher value obtained for graphene has been theoretically interpreted as due to the first two carbon layers having identical interaction energy.70,67 The zinc−graphene deposit has super hydrophilicity with a contact angle for water of 23.5°. Figure 10 shows the contact angles of Zn-GQD relative to graphene and graphite. The results obtained suggest that surface energy of graphene increased by electrochemical polarization, resulting in increased hydrophilicity. The deposition of zinc metal inside the graphene appears to have induced a polarized structure of

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having a partial negative charge on carbon (Figure 10) that binds strongly with water molecules through hydrogen bond, thereby creating a super hydrophilic surface.

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CONCLUSIONS A zinc−graphene composite has been made by using a GQD electrode and also by using a GQD bath. The composite shows high charge storage and recovery that is amenable for use in redox storage batteries. The nucleation process has been analyzed from the current−time transients, which show that there is a difference in the nucleation process on a GQD electrode and in a GQD bath. This is attributed to constant agitation occurring due to kinetic movement of GQD particles. TGA analysis shows that the break shown of the composite occurs at 250 °C with loss of weight, followed by a weight increase beyond 450° due to the formation of zinc oxide. XRD analysis shows the presence of identifiable zinc phases in the composite along with graphene. The fluorescence of GQD is quenched by zinc ions, suggesting a collisional interaction with GQD resulting in the loss of energy. The contact angle measurements of the zinc-GQD composite reveal that it is hydrophilic and opens up several technological opportunities.

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AUTHOR INFORMATION

Corresponding Author

*K. S. V. Santhanam. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful to Mr. Tom Murphy of Hoeganaes Corp. for help with the interpretation of the SEM and EDAX of the zinc−graphene composite and Prof. S. K. Gupta for XRD recordings. We also thank Prof. S. Williams, Mr. A. Jaikumar, and Prof. S. Kandlikar for their interest in this work. The authors gratefully acknowledge the financial support provided by the National Science Foundation under Award No. 1335927.

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