Environment Modulated Crystallization of Cu2O and CuO Nanowires

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Environment Modulated Crystallization of Cu2O and CuO Nanowires by Electrospinning and their Charge Storage Properties Midhun Harilal, Syam G.Krishnan, Bhupender Pal, Mogalahalli Venkatashamy Reddy, Mohd Hasbi Ab. Rahim, Mashitah Mohd Yusoff, and Rajan Jose Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03576 • Publication Date (Web): 18 Jan 2018 Downloaded from http://pubs.acs.org on January 18, 2018

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Environment Modulated Crystallization of Cu2O and CuO Nanowires by Electrospinning and their Charge Storage Properties Midhun Harilal,1 Syam G. Krishnan,1 Bhupender Pal,1 M. Venkatashamy Reddy,2 Mohd Hasbi Ab Rahim,1 Mashitah Mohd Yusoff,1 Rajan Jose1* 1

Nanostructured Renewable Energy Materials Laboratory, Faculty of Industrial Science &Technology, Universiti Malaysia Pahang, Kuantan, 26300 Pahang, Malaysia. 2 Department of Materials Science and Engineering, National University of Singapore, Singapore 117575, Singapore. *Corresponding author: [email protected] (R. Jose)

Abstract This article reports synthesis of cuprous oxide (Cu2O) and cupric oxide (CuO) nanowires by controlling calcination environment of electrospun polymeric nanowires and their charge storage properties. The Cu2O nanowires showed higher surface area (86 m2 g-1) and pore size than the CuO nanowires (36 m2 g-1). Electrochemical analysis was carried out in 6 M KOH and both the electrodes showed battery-type charge storage mechanism. The electrospun Cu2O electrodes delivered high discharge capacity (126 mA h g-1) than CuO (72 mA h g-1) at a current density of 2.4 mA cm-2. Electrochemical impedance spectroscopy measurements show almost similar charge transfer resistance in Cu2O (1.2 Ω) and CuO (1.6 Ω); however, Cu2O showed an order of magnitude higher ion diffusion. The difference in charge storage between these electrodes is attributed to the difference in surface properties and charge kinetics at the electrode. The electrode also shows superior cyclic stability (98%) and coulombic efficiency (98%) after 5000 cycles.

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Introduction Supercapacitors (SCs) represents a class of energy storage device using two charge storage mechanisms; viz. (i) charge separation at an electrode-electrolyte interface (electric double layer capacitance, EDLC) and (ii) intercalation of solvated ions deeper into electrode surface (pseudocapacitor, PC, or battery type), are under intensive research currently owing to their higher specific power (PS) than most of the batteries.1, 2 In EDLC, charges are stored electrostatically in a double layer at the electrode-electrolyte interface. However, energy storage in PC involves a faraidic reaction which results in the intercalation of solvated electrolyte ions onto the surface of the electrode material. Therefore, PC offers improved specific capacitance (CS) than EDLC owing to the involvement of surface charges along with the double layer formation. This results in improved energy density of PC materials than EDLC materials. However, this intercalation results in higher response time for PC materials lowering its power density than EDLC materials. Usually, carbons (activated carbon, carbon nanotubes, and graphene) are electrochemically characterized as EDLC materials whereas RuO2 and MnO2 are considered to be pseudocapacitive materials.3 The average capacitance of both EDLC and PC material remain constant at equal intervals of applied potential demonstrating its reversibility in charge-discharge process.4 There are reports of characterizing transition metal oxides (TMOs) such as Fe2O3,5 Co3O4,6 CuO,7 NiO8, TiO29 and V2O510 as PC materials owing to its multiple oxidation states and its improved reaction with solvated ions. Recently, these materials are identified as battery-type charge storage materials owing to its inability of obtaining equal capacitance at equal intervals of potentials (nonlinear currents in cyclic voltammogram (CV)) thereby showing inferior reversibility than PC and EDLC materials.4 This inability of battery-type materials can be attributed to the deeper solvated ion intercalation to the electrode surface than PC materials do. The latter deeper intercalation is attributed to the improved charge 2 ACS Paragon Plus Environment

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storage; therefore, the CS of these materials is to be reported either in mA h g-1 or in C g-1. There are reports of testing the charge storage properties of these TMOs in various aqueous electrolytes such as Na2SO4, NaOH, K2SO4, H2SO4, KOH, and LiOH. Out of these electrolytes, typically 6 M KOH is preferred owing to its higher conductivity which increases the intercalation process thereby enhancing the storable charge with dominant redox peaks in the CV curves.11 However, other TMOs such as RuO2,4 MnO2,12 and few composites (for example, recently reported metal oxide hybrid CoO-MnO2-MnCo2O413) show pseudocapacitive charge storage properties. Properties of these battery-type and pseudocapacitive TMOs can be tailored through synthesis mechanisms by developing them into nanostructures as one-dimensional nanowires,14 flat two-dimensional nanosheets,15 and three-dimensional hierarchical nanostructures16 or nanoparticles.17 Besides, studies were conducted in synthesizing composite materials with reduced graphene oxides or composite of two or more transition metal oxides in order to achieve synergistic properties thereby enhancing the charge storage capability.18-20 The relevance of CuO as a battery-type electrode for ASCs is researched extensively by synthesizing its various morphologies as well as composites.21-23 While Cu2O (cuprous oxide), a p-type semiconductor with a band gap of ~2.2 eV, is widely researched for its applications in solar cell, catalysts, sensors, and energy storage devices.24-26 As Cu2O consist of Cu (+1) oxidation state, its reactivity to solvated ions shall be higher than CuO (+2) oxidation state thereby enabling the access of more active sites to store more charges. However, fewer attempts were made to synthesis Cu2O using a chemical route and most of the research results for the supercapacitive performance of Cu2O are not promising (Table 1). These researches focused on synthesizing micro27 and nanoparticles28 which resulted in the lower active surface area for the solvated ions resulted in poor capacitance and cycling stability. Table 1 compares the recent research development in Cu2O based supercapacitors. 3 ACS Paragon Plus Environment

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Most of the research focused on developing Cu2O on copper thin sheets and synthesizing its composites with CuO, graphene and reduced graphene oxides. The theoretical capacitance of Cu2O is ~1800 F g-1, and as seen from the table the achieved capacitance is much lower. For making a meaningful comparison and as has been recommended recently,4 the stored charge has been re-calculated from CV or galvanostatic charge discharge (GCD) measurements in the units of mA h g-1 and shown in Table 1. One could observe that the cycling stability and CS of Cu2O are improved in hierarchical nanotubes29 owing to the directional charge transport properties. Therefore, it is justified to hypothesize that synthesis of Cu2O in one-dimensional morphology, such as continuous nanowires, can improve the transport of solvated ions to increase its charge storage.18 Electrospinning is a simple and scalable technique to produce nanowires30 and to the best of our knowledge, there are no reports on the synthesis of Cu2O nanowires and its electrochemical characterization as an electrode for supercapacitor applications using this technique. This paper reports a simple and novel method for developing continuous Cu2O nanowires for energy storage applications in supercapacitors. By regulating calcination atmosphere, Cu2O and CuO nanowires were obtained from similar electrospun polymeric nanowires. The electrochemical characterization of Cu2O nanowires shows superior charge storage properties compared to the CuO nanowires thereby synthesized. Experimental details The electrospinning technique was used to synthesize CuO and Cu2O nanowires. Copper acetate tetrahydrate [Cu(CH3COO)2.4H2O; 99%; R&M Chemicals] and polyvinyl alcohol (PVA; Mw– 95000, Merck) were used as precursors. The spinning solutions consisted of either 1.6 g of copper acetate (CuAc) dissolved in 15g aqueous PVA at a concentration of 7wt% PVA in deionized water. Electrospinning of the above homogeneous solution was carried out using Electroris (NanoLab, Progenelink Sdn Bhd, Malaysia) with 4 ACS Paragon Plus Environment

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an injection rate of 0.6 mLh-1 at a potential of 22 kV, maintaining relative humidity of chamber at ~35%. The electrospun wires were collected on a rotating drum placed at a distance of ~18 cm away from the spinneret. The annealing temperature was determined using thermogravimetric analysis (TGA) using Toledo STAR-1(Mettler, Switzerland). For TGA analysis, decomposition behaviour of ~2 mg as-spun sample was analysed in the 0 – 500 °C range. The weight stabilization temperature was determined from the TGA data (Supporting Information Figure S1). Accordingly, to remove polymeric components and to allow nucleation and growth of CuO nanowires, the as-spun wires were annealed at 500 °C for 1h in the air. Annealing the as-spun wires in an argon atmosphere at 500 °C for 1h resulted in the formation of Cu2O nanowires. The crystal structures of the material were studied by X-ray diffraction (XRD) using a Rigaku Miniflex II X-ray diffractometer employing CuKα radiation (λ = 0.15406 nm). XRD analysis was carried out by putting the powder sample into the holder followed by pressing it lightly using a glass slide to obtain smooth flat surface and was scanned in the range 2θ = 20 to 70° with step size 0.02° and scan speed 1°/min. The scanning electron microscopy (7800F, FESEM, JEOL, USA) was used to analyse the morphology and microstructure of the materials. For this analysis, the metal oxide samples were coated with gold (Au) using BIO-RAD Polaron Division SEM Coating System machine. This coating process was conducted under 0.1 mbar pressure and 30 mA for 75 seconds. The samples were then placed in the FESEM holder and were evacuated at a pressure of 10-5 Torr. Samples for transmission electron microscopy (TEM) analysis were prepared by ultrasonically dispersing the metal oxides in ethanol for 3 h. A drop of this solution was then allowed to dry on a carbon coated copper grid. Morphology of the materials and high resolution lattice images of the samples were obtained using TEM operating at 300 kV (FEI, Titan 80–300 kV). X-ray photoelectron spectroscopy (XPS) was performed using a PHI 5 ACS Paragon Plus Environment

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Quantera II (Physical Electronics) operating with an X-ray source of Al-Kα radiation at 100 W for the chemical analysis of the hybrid material. The XPS data were analysed in the range 0 – 1327 eV at pass energy 120 eV with a resolution of 0.5 eV maintaining a low pressure of 10-10 Torr; high resolution spectra were analysed with lower constant pass energy of 20 eV with a resolution of 0.1 eV. Adventitious carbon, with binding energy at 284.8 eV, was used for charge referencing. The spectra were analyzed using Origin 9.0 by fitting the high resolution spectra into multiple Gaussian curves; the baseline was modeled by adjacent averaging. Micrometrics (Tristar, 3000) instrument, in a nitrogen atmosphere, was used for analysing gas adsorption behavior and BET surface area of the materials. The electrode fabrication for electrochemical analysis were carried out by coating slurry of the active material on pre-cleaned nickel foam substrates using acetone, HCl, water, and ethanol. The slurry, in a typical experiment, was prepared by mixing the active material with polyvinylidenefluoride (PVDF) (Sigma Aldrich, USA) and carbon black (Super P conductive, Alfa Aesar, UK) in the ratio 75:10:15. N-methyl-2-pyrrolidinone (NMP), which works as a homogenizer, was added to the above mixture and stirred well for 24 h. The prepared slurry was pasted on an area of 1 cm2 over the pre-cleaned nickel substrate and dried at 60 oC. Further, using a hydraulic press a pressure of 5 ton was applied to the electrode. The active material loading on the electrodes was ~2.4 mg cm-2. The cyclic voltammetry (CV), charge-discharge cycles (CDC), electrochemical impedance spectroscopy (EIS) measurements of the electrodes were studied using potentiostat-galvanostat (PGSTAT M101, Metrohm Autolab B.V., Netherlands) employing NOVA 1.9 software. The EIS measurements were recorded in the frequency range 100 kHz–0.01 Hz at respective open circuit potential. A platinum rod and a saturated Ag/AgCl electrode were used as the counter and the reference electrodes, respectively. The electrolyte used was 6 M KOH because of its high ionic conductivity (~627 mS cm-1). 6 ACS Paragon Plus Environment

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Results and discussion To analyze the difference in crystal and chemical structures of Cu2O and CuO XRD and XPS techniques were employed. Figure 1a shows the XRD patterns of Cu2O compared with CuO. The peak positions of Cu2O corresponds to the planes (1 1 0), (111), (200), (220) and (311) indicating a Pn3m space group (JCPDS 05-0667) fitted to a cubic crystal structure. For CuO, the peaks fit a monoclinic unit cell with space group C 12/c 1. Characteristics peaks of CuO were absent in the XRD patterns of Cu2O indicating a pure phase formation. The XRD peaks of Cu2O were broader than the CuO peaks indicating a lower crystallite size of the former. This comparatively higher amorphous nature of Cu2O adds to its advantage as an electrode material for supercapacitors.31 To examine the chemical composition, the materials were further characterized by XPS. The survey spectrum in Figure 1b shows the peak positions of copper and oxygen in Cu2O. Figure 1c shows the detailed XPS of Cu 2p in Cu2O displaying two distinct peaks at binding energies of 932.6 and 953.1 eV, corresponding to the Cu 2p3/2 and Cu 2p1/2 core levels, respectively.32 Both the peaks could be deconvoluted into two, attributing to the +1 and +2 oxidation states of copper. No satellite peaks were observed in the Cu 2p detailed spectrum further confirming the chemical purity of Cu2O. However, annealing Ar atmosphere to get Cu2O likely to remain carbonaceous species in the samples. Figure S3 shows XPS of carbonaceous species present in Cu2O due to calcination under Ar atmosphere. The deconvolution of the C 1s spectra yielded four peaks: peak 1 (282.3 eV), carbide; peak 2 (284.6 eV), adventitious carbon; peak 3 (287.1–287.6 eV), carbon in carbonyl or quinine groups; peak 4 (288.3–288.9 eV), carbon in carboxyl or ester groups.3336

The detailed XPS of Cu 2p in CuO (Figure 1d) also displayed two peaks at binding

energies 934.7 and 955.1 eV along with two strong satellite peaks at 943.4 and 962.5 eV, which is characteristic of CuO phase.37 Thus XRD and XPS analysis unambiguously 7 ACS Paragon Plus Environment

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confirm the chemical purity of the Cu2O synthesized. One of the obvious concerns in the results present here is the formation of an inorganic oxide compound while heating the precursor in an inert gas (argon); the source of oxygen for the formation of Cu2O in this case is expected during the decomposition of PVA used for electrospinning.38-41 The difference in formation mechanism of Cu2O and CuO are schematically shown in Figure 2. FESEM and TEM analysis (Figure 3&4) show the nanowire morphology of electrospun Cu2O and CuO samples. Diameter and diameter distribution of the wires were determined from randomly selecting nearly 30 wires; a histogram showing the diameter distribution is shown in Supporting Information (Figure S2). Average diameter of the Cu2O and CuO nanowires were ~50 and ~60 nm, respectively. The bright field TEM image (Figure 4a) further confirm the diameter of the Cu2O nanowire and show that they were composed of densely packed particles of around 5 – 10 nm in size. The selected area electron diffraction (SAED) images of Cu2O (Figure 4b) were acquired, which consists of diffraction spots aligned in patterns of polycrystalline ring pointing to the higher degree of particle orientation in the nanowires. Two of those spots were indexed to (220) and (111) planes of Cu2O with a lattice spacing of 0.15 and 0.24 nm, respectively. Similarly, Figure 4c shows the bright field TEM image of CuO nanowire with 20 nm particle size and Figure 4d the SAED image indexed to (111) and (202̅) planes with lattice spacing of 0.23 and 0.18 nm, respectively. The materials were further characterized by the nitrogen adsorption and desorption isotherms to probe the difference in surface area and the porosity of Cu2O and CuO. Figure S4 (See Supporting Information) shows the adsorption isotherms of the samples. The isotherm of both materials, classified as type IV, indicates mesoporous nature. According to BJH pore size distribution data (inset Figure S4), the average pore size of Cu2O nanowires is ~22 nm and that of CuO nanowires is ~16 nm. Further the BET surface area plots (Figure 8 ACS Paragon Plus Environment

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5a, b) were drawn to measure their surface areas. The Cu2O nanowires show a BET surface area of ~86 m2 g-1, whereas that of CuO nanowires is ~36 m2 g-1. Because the surface area and pore size of Cu2O nanowires are considerably higher than that of CuO nanowires, electrochemical properties of former are expected to be better than that of latter.42 Galvanostatic charge-discharge cycling (CDC) studies were employed to analyze the practically achievable discharge capacity, internal resistance, and long-term cyclability of the electrodes. The discharge curves at 2.4 mA cm-2 of Cu2O and CuO can be seen in Figure 6a. The discharge curves of battery-type electrodes are usually as a result of three processes (Figure 6b, discharge curves of Cu2O at different current densities): (i) an abrupt initial potential drop due to the surface contribution of the PC (ii) a slow potential decay through deep intercalation, and (iii) relatively faster voltage drop indicating EDLC mechanism. Interestingly, CuO electrodes showed a maximum potential of ~0.4 V while Cu2O showed ~0.5 V. The nonlinear shape of discharge curve suggests that faradaic reactions are predominant in the electrodes. Evidently, the discharge time increased considerably for Cu2O when compared to CuO. The variation of discharge capacity (Q) with a current density of all materials studied can be observed in Figure 6c. The Q of the electrodes at 2.4 mA cm-2 are 126 and 72 mA h g-1 for Cu2O and CuO, respectively. The electrospun Cu2O nanowires showed higher Q with fairly high rate capability, which could be ascribed to their enhanced surface area than CuO. Evidently, the Q decreased only 15% with current density increasing from 2.4 mA cm-2 to 36 mA cm-2 in the case of Cu2O nanowires, in comparison to 25% decrease for CuO. Figure 6d shows the charge-discharge cycle of Cu2O from which equivalent series resistance (ESR) could be reliably calculated. The potential drop (VIR) between the charge and discharge curves is a measure of the ESR of the electrodes. The factors contributing the ESR are (i) intrinsic resistance of the electro-active material, (ii) electrolyte resistance, and (iii) the contact resistance at the active material–current collector 9 ACS Paragon Plus Environment

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interface. The ratio of potential drop (VIR) to the discharge current (ID) of the electrode gave ESR values as Cu2O – 1.1 Ω, CuO – 1.5 Ω. Thus, the ESR of the Cu2O is lower than that of CuO thereby offering improved capacitive performance. The reversibility and extended cyclability of the electrode can be determined from the CD curves by calculating the coulombic efficiency (η), which is defined as the ratio of discharging to the charging time. The η of Cu2O nanowires was 98% which is superior to that observed for CuO (94%). The operational stability of the Cu2O nanowires was evaluated by galvanostatic charge-discharge testing at a current rating of 24 mA cm-2 (Figure 7). Retention in capacity and η of ~98% was shown by Cu2O nanowire electrodes at the end of the 5000 cycle test program. Cyclic voltammetry has been employed to determine the nature of the oxidationreduction reactions occurs in the electrodes giving rise to the above discharge capacitance. Figure 8a displays the cyclic voltammetry (CV) curves of Cu2O and CuO at a scan rate of 2 mVs-1. The cathodic and anodic peaks, arising due to oxidation and reduction reactions, illustrate the battery-type behavior of materials which is also supported by the non-linear variation of current with scan rate in the charge-discharge cycling. Moreover, the voltammetric current of the Cu2O electrode was much higher than that of the CuO electrode, indicating its superior electrochemical performance. The electrochemical reactions of the Cu2O electrodes can be routinely expressed as 43, 44 1 1 Cu O + OH ⇋ CuO + H O + e

2 2 1 1 Cu O + H O + OH ⇋ Cu(OH) + e

2 2 Comparing the oxidation peaks of Cu2O and CuO at 2 mV s-1, one would observe that the area under the CV curve of Cu2O is higher suggesting increased discharge capacity. High surface area and improved conductivity could be the reason behind the enhanced Q 10 ACS Paragon Plus Environment

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value. From the scan rate dependent CV analysis (Figure 8b) oxidation/reduction peak positions were seen slightly move towards the higher potentials indicating the charge polarization at the electrode-electrolyte surface. The variation of Q with scan rate is analyzed (Figure 8c) and Cu2O showed the highest value, in accordance to that observed from CDC. The variation of scan rate (v) with voltammetric current (i) was studied to analyze the charge storage mechanism involved in both the materials. If the current arises from bulk intercalation process, it follows diffusion kinetics and the current i varies as v1/2 (battery-type). On the contrary, if the current arises from surface charge storage processes (pseudocapacitance), a linear relation with v can be observed. It was observed that (Figure 8d) both the materials show ∝ / relationship, suggesting dominance of bulk intercalation charge storage processes. Moreover, the slope of that graph is proportional to the diffusion coefficient according to the Randles-Sevcik equation 45, = 2.69 × 10 × / × / × × × √

(1)

where n is the number of electrons transferred to the electrode surface, A is the surface area of the electrode, D is the ion diffusion coefficient, v is the scan rate, and C0 is the initial ion concentration. Enhanced charge diffusion in Cu2O electrodes could be observed from the higher slop value. Higher D value (3.2×10-13 cm2 s-1) was observed for Cu2O than D value (1.6×10-14 cm2 s-1) of CuO indicating enhanced ion transfer rate and, consequently, relatively lower electrode polarization during charge-discharge process.46, 47 Pore size and electrical conductivity of the electrodes largely influence the D values.48 The combination of electrical conductivity along with larger electrode pore size of Cu2O than CuO, leads to a higher D value. One of the most important properties of a charge storage material, the electrochemical reversibility or the coulombic efficiency (η), was calculated from the ratio of the area of the anodic to the cathodic peaks of CV curves. The η measured from scan rate (2 mV s-1) of Cu2O electrodes show the highest value (98%) compared to the 95% for CuO 11 ACS Paragon Plus Environment

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electrodes indicating improved electrochemical reversibility and better capacity retention for a long cycle of operation. The CV measurements, thereby, show that the Cu2O could offer improved discharge capacity and coulombic efficiency than the CuO electrodes. Electrochemical impedance spectroscopy (EIS) provides a powerful technique to study the charge kinetics at an electrode-electrolyte interface. Figure 9a compares the Nyquist plot of the two electrodes and electrode parameters such as the charge transfer resistance (RCT) and relaxation time constant (τ) is determined. The RCT values, measured from the diameter of the semicircle at high frequency (100 kHz), corroborate the results obtained from CV and CDC analysis that Cu2O nanowire electrodes have superior electrochemical properties. A low RCT (1.2 Ω) of Cu2O sample, when compared to RCT (1.6 Ω) of CuO sample, indicates enhanced ionic conductivity and electrolyte diffusion through electrode material pores leading to improved rate capability.49 For further analysing the electrical parameters of the Cu2O electrode, an equivalent circuit comprising series and parallel combination of RS, RCT, electric double layer capacitance (Cd), Warburg impedance (ZW) which represent the ionic diffusion of the electrolyte and a constant phase element (CPEPC) representing the supercapacitance dispersion on the nickel foam owing to the surface irregularities, is modeled using NOVA software. This circuit helps to study the capacitive behavior and charge transfer parameters at higher and intermediate frequencies, and the Warburg diffusion of electrolyte ions. The detailed modeled equivalent circuit of Cu2O electrode is placed in the inset of Figure 9a. The corresponding values of the individual parameters are RS = 1.3 Ω, RCT = 1.2 Ω, Cd = 1.6 mF, ZW = 235 mMho and ZCPE = 37.3 (mFs)1/n (n = 0.94). Figure 9b shows the plot of frequency vs phase difference, which gives another important electrode parameter, the charge relaxation time (τ) or RC time constant (the time required dissipating half of the energy stored). The charge relaxation time (τ) ( τ = 1

fo

) is the reciprocal of the characteristic frequency (fo), the point at which the 12 ACS Paragon Plus Environment

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circuit is equally capacitive and resistive.50 Typically, for EDLCs using carbon materials a phase angle of 90o is observed (suggesting an ideal capacitive nature), while for pseudocapacitors deviations from 90o are observed. The fo is 17.92 Hz for Cu2O electrode, which is much higher than the conventional activated carbon (~0.05 Hz)

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, and CuO

electrode (5.52 Hz). As observed from Figure 9b Cu2O nanowires show highest fo, which means that the Cu2O electrode remains capacitive for a wider frequency range. Thus the lower τ value of Cu2O electrode (0.05 s) suggests higher power density when compared to the CuO (0.18 s) nanowire electrode. This indicates a faster frequency response of the Cu2O electrode, which in turn leads to an enhanced rate capability and cycling stability for practical devices.

Conclusions In conclusion, cubic Cu2O and monoclinic CuO nanowires can be synthesized from the same set of electrospun polymeric wire mats containing a copper precursor by appropriately controllong the calcination environment. Annealing the as-spun polymeric wire mats in Argon and air resulted in Cu2O and CuO, respectively. Under the present reaction conditions, Cu2O showed higher BET surface area (~86 m2 g-1) and the average pore size (~22 nm) than those of CuO (surface area ~36 m2 g-1 and average pore size ~16 nm). The Cu2O electrodes showed a discharge capacity of 126 mA h g-1 at 2.4 mA cm-2, which is 75% higher than the CuO electrodes in 6 M KOH electrolyte. The electrodes retained 85% of its initial stored charges with a 15 times increase in current density thus showed high rate capability. The improved charge storability in Cu2O is attributed to its high surface area and an order of magnitude higher ion diffusion coefficient. Improved coulombic efficiency (98%) along with higher cycling stability (98%) after 5000 cycles and its lower characteristic resistances prove the reliability of Cu2O nanowire as an anode for ASC. 13 ACS Paragon Plus Environment

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Acknowledgements This work is supported by the Research and Innovation Department of University Malaysia Pahang (http://ump.edu.my) under the Flagship Leap 3 Program (RDU172201 and GRS150328). Table 1: Summary of charge storage properties of Cu2O based on syntheses methods Material/ Synthesis method

Morphology

CS (F g-1)

Potenti al (V)

Cu2O/ chemical reflux Cu2O/hydrothermal method Cu2O/room temperature chemical reaction CuOx/solution phase cation exchange Cu2O anchored graphene Cu2O/colloidal method Cu2O@MnO2/ solvothermal reduction Cu2O/CuO/Co3O4/ chemical deposition Cu2O-rGO composite/ chemical mixing Cu2OrGOcomposite/ hydrothermal RGO-Cu2O-TiO2 composite/ solution phase Cu2O/ electrospinning

Microspheres

144 @ 0.1 A g-1 660 @ 1 A g-1 88 @ 1 A g-1

0.8

Q (mA h g-1) 32

0.4

73.3

0.45

11

Microparticles Core/shell

Electrol yte

Stability/ cycles

Ref

2M KOH 6M KOH 2M KOH

~99%/ 100 ~55%/ 1000 N.R

27

52

53

Hierarchical nanotubes

554 @ 1 A g-1

0.5

77

1M KOH

85%/ 10000

29

Rose-rock shaped Nanocrystals

416 @ 1 A g-1 495 @ 1 A g-1 371 @ 0.5 A g-1

1.0

116

86%/180

54

0.6

82.5

N.R

28

0.5

52

6M KOH 3M KOH 6M KOH

95.8%/ 2000

55

Octahedral microcrystals Core/shell

318 @ 0.5 A g-1

0.4

35

3M KOH

80%/300 0

56

Nanosheets

195 @ 2 A g-1

1.0

54

1M Na2SO4

92%/ 1000

57

Nanoparticle

98.5 @ 1 A g-1

0.8

22

1M KOH

50%/ 1000

58

Nanoparticle

80 @ 0.2 A g-1

0.7

16

6M KOH

113%/ 1000

59

Nanowires

896 @ 1 A g-1

0.5

126

6M KOH

98%/ 5000

This work


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Figure 1. (a) XRD patterns of Cu2O and CuO; (b) XPS survey scan of Cu2O; (c) peak fitted spectrum of Cu 2p corresponding to Cu2O; (d) peak fitted spectrum of Cu 2p corresponding to CuO.

Figure 2. Schematics showing formation mechanism of Cu2O and CuO nanowires


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Figure 3. (a&b) FESEM images of uniform Cu2O nanowires; (c&d) FESEM images of uniform CuO nanowires.


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Figure 4. (a) Bright-field TEM image of Cu2O showing particle size; (b) SAED indexed pattern of Cu2O nanowires; (c) Bright-field TEM image of CuO showing particle size; (d) SAED indexed pattern of CuO nanowires.

Figure 5. BET surface area plots of (a) Cu2O nanowires; (b) CuO nanowires.


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Figure 6. (a) Discharge curves of Cu2O and CuO at a current density of 2.4 mA cm-2; (b) discharge curves of Cu2O at various current densities; (c) variation of Q with current density for Cu2O and CuO; (d) charge-discharge curves of Cu2O at 2.4 mA cm-2.

Figure 7. Cyclic stability curves for Cu2O showing Q and η retention over 5000 cycles.


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Figure 8. (a) Comparative CV curves of Cu2O and CuO at a scan rate of 2 mV s-1; (b) CV curves of Cu2O at varying scan rates; (c) variation of Q with scan rate for Cu2O and CuO; (d) linear relation of anodic peak current with square root of scan rate for Cu2O and CuO.


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Figure 9. (a) EIS Nyquist spectra of Cu2O compared to CuO (inset show Cu2O equivalent circuit with corresponding electrical parameters); (b) variation of phase difference with frequency for Cu2O and CuO.


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TOC graphic


Environment Modulated Crystallization of Cu2O and CuO Nanowires

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