Multi-functional energy conversion and storage electrodes using flower-like Zinc oxide nanostructures

Energy 65 (2014) 639e646

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Energy journal homepage: www.elsevier.com/locate/energy

Multi-functional energy conversion and storage electrodes using flower-like Zinc oxide nanostructures Valentina Cauda a, *, Diego Pugliese a, b, Nadia Garino a, Adriano Sacco a, Stefano Bianco a, Federico Bella a, b, Andrea Lamberti a, b, Claudio Gerbaldi a, b a b

Center for Space Human Robotics @Polito, Istituto Italiano di Tecnologia, Corso Trento 21, 10129 Torino, Italy Department of Applied Science and Technology e DISAT, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy

a r t i c l e i n f o

a b s t r a c t

Article history: Received 23 September 2013 Received in revised form 4 December 2013 Accepted 13 December 2013 Available online 8 January 2014

In the present paper we demonstrate the efficient use of shape controlled flower-like ZnO (Zinc oxide) nanostructured particles as multifunctional electrode for both energy conversion and storage applications, i.e. Dye-sensitized Solar Cells (DSCs) and lithium-ion batteries. As regards DSC (Dye-sensitized Solar Cell) device, ZnO flower-like particles, prepared by a simple, lowcost and reliable hydrothermal method under mild reaction temperature, are efficiently used as photoanode in a microfluidic architectured cell in combination with NMBI (N-methylbenzimidazole), employed as additive of the electrolytic solution for the first time in a ZnO-based DSC. We obtain a remarkable sunlight conversion efficiency of 3.6%. As regards storage applications, a stable long-term ambient temperature cycling behavior in lithium cell is demonstrated, even at increasingly higher currents. Remarkable charge-discharge efficiency and specific capacity are obtained up to 200 cycles, which is the highest number of cycles reported so far for similar systems. Noteworthy, such results are achieved without the addition of foreign additives, nor during the synthesis process neither during the electrode preparation, and also no carbon coating on ZnO surface is used. The originality of the present paper consists not only in showing for the first time the efficient operation of such ZnO particles as anode in Li-ion batteries for prolonged cycling, but also in demonstrating the versatile and multifunctional use of the same material for two different energy related applications. The reported results enlighten indeed the promising prospects of the flower-like ZnO nanostructured material for the successful implementation as stable and long-term performing anodic material in the next generation of both energy conversion and storage devices. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: Zinc oxide Nanostructured petal Dye-sensitized Solar Cell Energy conversion efficiency Lithium battery Cycling behavior

1. Introduction ZnO (Zinc oxide) is a well-known inorganic material and, due to its peculiar properties, was proposed for plenty of different applications. It is a wide band-gap semiconductor (3.37 eV), characterized by a high electronic mobility [1] and a low electronehole recombination probability [2]. In addition, it is characterized by high transparency across the visible spectrum with an ultra-violet luminescence [3], tunable super-hydrophobicity or hydrophilicity [4,5], and piezoelectric behavior [6,7]. All these properties make ZnO suitable for being effectively used in the fabrication of different kinds of devices, such as Dye-sensitized Solar Cells (DSCs) [8], gas sensors [9], optical devices (e.g., light-emitting diodes) [10],

* Corresponding author. Tel.: þ39 (0)110903436; fax: þ39 (0)110903401. E-mail address: [email protected] (V. Cauda). 0360-5442/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.energy.2013.12.025

piezoelectric nanogenerators [11], thermo-voltage generators [12], Li-based batteries [13], and thermoelectric power generators [14]. In particular, regarding the field of energy conversion and storage [15,16], ZnO has recently raised noticeable attention as an alternative material with respect to TiO2 for DSC (Dye-sensitized Solar Cell) photoanodes [17]. Indeed, despite showing a similar band gap, ZnO theoretically presents higher electron mobility, significantly longer carrier lifetime and similar electron injection process from the light-excited dye molecules [18]. However, the photocurrent and the efficiency of the ZnO-based cells are still lower than those of TiO2ebased DSCs [19]. Long incubation times in the sensitizing solution have proven to induce the formation of aggregates between the dye molecules and the dissolved Zn2þ ions originating from the ZnO surface [20,21]. This phenomenon leads to a strong reduction of the electron injection in the semiconductor conduction band, thus resulting in a lower overall device efficiency. Sensitization procedure needs to be optimized in order to avoid

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ZnO surface damaging and molecular aggregate formation. In the framework of optimizing the photoanode architecture (i.e. porosity, specific surface area) to improve the DSCs performances [22], ZnO can be exploited to investigate the effect of the morphology on the charge collection efficiency [23], thanks to its capability of growing with a great variety of different shapes. For instance, ZnO nanowires are characterized by a very high electron transfer [24], but more complex 3-D architectures like branched nanowires [25] or coral-shaped nanostructures [26] can combine the effect of improved charge transfer properties with a high surface area available for the dye chemisorption. In addition, like other transition metal (e.g., Cu, Fe, or Co) oxides [27], ZnO is now attracting considerable interest as anode material for the next generation of Li-ion batteries [28,29], conceived for both portable and/or automotive/aerospace applications [30e32]. It can offer indeed higher capacity and improved safety with respect to conventional graphite [13]. However, it is challenging at present to ensure the electrode integrity upon prolonged cycling [33,34], due to the typical conversion reaction leading to the metaloxide structure dissolution and LieZn alloys formation [35,36]. Moreover, a passivation layer can be formed during cycling, preventing the fully reversible insertion of Li-ions into the structure [37]. It is therefore of paramount importance to optimize the material properties to improve the surface electrochemical reactivity, thus achieving higher performances. Many papers report on the synthesis of ZnO micro and nanostructures by different techniques, exploiting both physical and chemical growth methods, such as sputtering [38,39], pulsed laser [40], atomic layer [41], electrochemical [42], chemical vapor [43,44] and metal-organic chemical vapor [45] depositions, or templateassisted solegel [46] and hydrothermal [7,44] growths. In the present paper, we report on a simple synthesis route to form shapecontrolled ZnO particles, leading to nanostructured flower-like morphologies with nanosized petals. The advantages of using the hydrothermal method for the synthesis are numerous: low temperatures (usually below 90  C) for obtaining a final single phase crystalline wurtzite structure, absence of catalysts, use of simple equipment, strict morphology control, low cost of the reagents which can be selected as environmentally friendly and less hazardous, high reproducibility and scale-up prospects [47]. The aim of this work is to provide a simple and fast way of synthesizing nanostructured high surface area ZnO, demonstrating here for the first time its efficient electrochemical characteristics to be effectively used as multifunctional anode material in both DSCs and Li-ion batteries. In particular, the originality of the present paper consists not only in showing for the first time the efficient operation of flower-like ZnO nanostructured particles as anode in Li-ion batteries, but also to demonstrate the versatile and multifunctional use of the same material for two different applications, i.e. both dye-sensitized solar and lithium-ion cells.

2.2. Characterization techniques Powder XRD (X-ray diffraction) analysis was performed on a X’Pert diffractometer with Cu Ka X-ray radiation source (l z 1.54  A). Morphologies of the as-obtained products were observed on a FESEM (Field Emission Scanning Electron Microscope, Dual Beam Auriga from Carl Zeiss, operating at 5 keV). The BET (BrunauereEmmetteTeller) specific surface area was evaluated by using nitrogen sorption isotherms measured at 77 K on a Quadrasorb instrument from Quantachrome by multipoint method within the relative pressure range of 0.1e0.3 p/p0. Pore size distribution was derived from the desorption branch using DFT (Density Functional Theory) model. 2.3. Dye-sensitized solar cell assembly and characterization A ZnO paste, obtained dispersing as-prepared flower-like particles in acetic acid-based solution [48], was preliminarily deposited onto cleaned FTO (Fluorine-doped Tin Oxide)-covered glasses, used as conductive substrates (7 U/sq, Solaronix), by doctor-blade technique. The coated films were initially baked at 90  C for 30 min and subsequently thermally treated at 450  C for 10 min in air, thus obtaining a w14 mm-thick ZnO layer. Photoelectrodes were then heated at 70  C for 5 min, soaked for 6.5 h into a 0.4 mM N719 (Ruthenizer 535bis-TBA, Solaronix) ethanol-based sensitizing solution having a pH of 10.7 at ambient temperature. Finally the photoelectrodes were rinsed in pure ethanol to remove the nonadsorbed dye molecules. A commercial liquid electrolyte solution (Iodolyte AN 50, Solaronix) with the addition of 0.5 M N-methylbenzimidazole was employed. The counter electrode fabrication, the cell assembly procedure and the electrolyte filling process were already detailed in a previously published work [49]. The active area of the cells was 0.78 cm2 and the electrical characterization was performed with a 0.22 cm2 black rigid mask. Photocurrente voltage (IeV) measurements were carried out using a source measure unit (model 2440, Keithley) and a solar simulator (provided by a Newport 91195A class A solar simulator) under AM1.5G irradiation, with power output (100 mW cm2) calibrated by a Si reference solar cell. In addition, IeV characteristics were also acquired at different light intensities in the range between 0.2 and 1 sun, by using neutral density filters purchased from Newport. EIS (Electrochemical impedance spectroscopy) measurements were performed in dark conditions through an electrochemical workstation (760D, CH Instruments) in the frequency range 100 mHze 10 kHz at various applied bias voltages, with an AC signal amplitude of 10 mV. The experimental data were fitted through an equivalent circuit [22] in order to obtain information about the transport properties. From the fitting parameters RT and RCT, the diffusion length values Ln were calculated through the following Equation (E1):

2. Experimental

Ln ¼ d$ðRCT =RTÞ1=2

(E1)

2.1. Synthesis procedure where d is the photoanode thickness. To prepare the flower-like ZnO particles, 2.8 g potassium hydroxide (KOH, 1 M, Merck) and 7.4 g zinc nitrate hexahydrate (ZnNO3$6H2O, 0.5 M, Sigma) were dissolved separately in 50 mL bidistilled water (from Direct-Q System, Millipore). The zinc nitrate solution was dropped into KOH under vigorous stirring, thus the total volume was 100 mL. The obtained white gel was transferred in a closed Teflon bottle at 70  C for 4 h. At the end of this time, ZnO particles were separated from the solution by filtration, washed repetitively with deionized water until the pH neutralization, and dried at 60  C overnight in air.

2.4. Lithium cell assembly and electrochemical characterization The electrochemical properties of the as-obtained ZnO flowerlike materials were investigated using a three-electrode T-cell configuration at ambient temperature with liquid electrolyte. The working electrodes were fabricated by drop casting a solution of ZnO flower-like particle on stainless steel discs without the addition of any electronic conductivity enhancer and/or binder. In a typical preparation, a 0.5 mg mL1 suspension of the synthesized

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powders was dispersed in isopropanol and sonicated for 10 min. The suspension was then added drop wise directly on a SS (stainless steel-316) disc acting as current collector and left drying at ambient temperature overnight. The obtained electrodes were successively dried under high vacuum at 130  C for 5 h to ensure the complete removal of traces of water/moisture, as they may create problems in long-term properties of rechargeable Li-based batteries. After their transfer in the inert atmosphere of a dry glove box (MBraun Labstar, O2 and H2O content <0.1 ppm) filled with extra pure Ar 6.0, they were weighed before their electrochemical evaluation in the cells and, by subtraction of the average weight of the SS-316 discs, the weight of the coating mixture was calculated. The T-cells were assembled as follows: ZnO on SS-316 disk (area 0.785 cm2) as the working electrode, 1.0 M lithium perchlorate (LiClO4, Aldrich) in a 1:1 (w/w) mixture of EC (ethylene carbonate, Fluka) and DEC (diethyl carbonate, Aldrich) electrolyte solution soaked on a WhatmanÒ GF/A separator and a lithium metal foil (high purity lithium foils, Chemetall Foote Corporation) as the counter electrode. For cyclic voltammetry, a second lithium foil was added at the third opening, in direct contact with the electrolyte, acting as the reference electrode. The evaluation of the electrochemical behavior was carried out by galvanostatic discharge/charge cycling (cut off potentials: 0.02e 2.8 V vs. Liþ/Li) and cyclic voltammetry (between 0.02 and 3.0 V vs. Liþ/Li, scan rate of 0.100 mV s1) at ambient temperature, using an Arbin Instrument Testing System model BT-2000 as the controlling instrument. Discharge reaction corresponds to lithium insertion into the electrode material structure while charge reaction corresponds to the lithium extraction. The discharge/charge cycles were set at the same rate at 0.1 C, 0.2 C, 0.5 C and 1 C, respectively. Note that 1 C corresponds to about 100 mA g1 with respect to a ZnO active material mass of about 0.1 mg, based on a calculated theoretical specific capacity of ZnO of 987.8 mAh g1 and about 200 mA cm2 (for an electrode area of 0.785 cm2). Clean electrodes and fresh samples were used for each of the above reported tests. To confirm the obtained results, the tests were performed at least three times on different fresh samples. 3. Results and discussion 3.1. ZnO material characterization The shape-controlled synthesis of ZnO flower-like microparticles with nanostructured petals was carried out by simple hydrothermal process under mild reaction temperature (70  C) from the aqueous solutions of KOH and Zn(NO3)2, with some modification from the previous literature [50,51]. Fig. 1 shows the representative Field Emission Scanning Electron Microscopy (FESEM) images of the ZnO particles. ZnO microparticles have approximately uniform morphologies and dimensions, consisting in several flower-like aggregates with nanosized multipetals and diameters of about 2 mm. The higher magnification image (Fig. 1b) shows that each flower is composed of many thin nanosheets with a thickness of about 50 nm, which are assembled spokewise, projected from a common central zone to form the flower-like architecture. Furthermore, each nano-petal has almost the same thickness, indicating that the sheet growth is strictly oriented and limited to the 2D plane during the whole growing process. X-ray diffraction pattern (Fig. 2a) shows diffraction peaks which could be all assigned to single phase wurtzite ZnO (JCPDS 80-0074, a ¼ 0.3253 nm, c ¼ 0.5215 nm, hexagonal, space group P63mc). In addition, the diffraction peaks are sharp, indicating that the product has a high degree of crystallinity and high purity. The main dimension of the crystalline domains is equal to 60 nm, as

641

Fig. 1. Field Emission Scanning Electron Microscopy characterization. FESEM images of the flower-like nanostructured ZnO particles at different magnifications.

evaluated through the DebyeeScherrer equation, thus demonstrating that each petal is almost mono-crystalline and grows along the [001] direction. Therefore a growth mechanism can be speculated, also from the literature evidence [47,50]. During the hydrothermal reaction, the hydroxyl groups of KOH bound to zinc cations Zn2þ through coordination or electrostatic interactions, forming the growth units ZnðOHÞ2 4 (see Equations R1 and R2). Therefore, ZnO nucleated from the solution of Zn(OH)2 4 forming multinuclei aggregates (Equation (R3)).

Zn2þ þ 2OH /ZnðOHÞ2

(R1)

ZnðOHÞ2 þ 2OH /ZnðOHÞ2 4

(R2)

 ZnðOHÞ2 4 /ZnO þ H2 O þ 2OH

(R3)

At the low temperature used here (70  C) both the nucleation and growth mechanism of ZnO structure proceeded slowly, leading to thermodynamically stable products. This is because the crystallizing structures tend to aggregate and to follow the lowest-energy path. Being the (001) face the highest-energy surface of the wurtzite ZnO crystal, the c-axis resulted the fastest growth direction [44]. In our case, with the reaction proceeding, the multinuclei aggregates can serve as the initial central sites for the growth of bidimensional ZnO nanostructures along the [001] direction, radiating from the center, to form flower-like ZnO nanostructures with randomly distributed mono-crystalline petals under low thermal conditions.

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travel through the petals following a direct and fast pathway, with a low recombination rate, until their collection at the electrode. We fabricated DSCs exploiting a microfluidic architecture [52] using conventional N719 sensitizer and iodine-based electrolyte (for further details see Section 2.3). In order to limit the ZnO dissolution and the Zn2þ/dye aggregate formation, which can lower the photoconversion efficiency [26], the sensitization procedure was carefully controlled with a multivariate optimization by means of a chemometric approach. Details about the fine tuning procedure will be comprehensively discussed in a forthcoming publication. The results of the chemometric optimization gave the optimal parameters for the sensitization step, namely 0.4 mM as dye concentration, 10.7 as pH of the solution and 391 min as incubation time. Moreover, to further enhance the cell performances, 0.5 M of NMBI (N-methylbenzimidazole) was added to the electrolytic solution. The results of the photovoltaic characterization performed under AM1.5G illumination are reported in Fig. 3a. The cell exhibits a photoconversion efficiency of 3.6% (short circuit current density

Fig. 2. Micro-structural and surface properties characterization. (a) X-ray diffraction pattern and (b) nitrogen sorption isotherm with DFT pore size distribution of the flower-like ZnO particles.

Nitrogen sorption isotherm (Fig. 2b) shows the presence of hysteresis above p/p0 ¼ 0.8, indicating the presence of inter-particle porosities. Density Functional Theory (DFT) model was applied to estimate the pore volume and the pore size distribution, as reported in the inset of Fig. 2b. The presence of small porosities, with a maximum distribution at 3.8 nm in diameter, is assessed, showing a broad pore size distribution. Pore volume is 0.042 cm3 g1. The BET surface area was measured by multipoint method within the relative pressure range of 0.1e0.3 p/p0 leading to a value of 21.8 m2 g1. The relatively high values of both pore volume and surface area are attributed not only to the detected nanopores, but also to the presence of several nanostructured petals, radiating from the center of each flower-like particle, as previously shown in Fig. 1. 3.2. ZnO as photoanode in dye-sensitized solar cells (DSCs) In principle, the micrometer-scale interconnected platelets, with a nanometer-scale porosity, present a particularly favorable morphological feature for the application as anode material in DSCs. In fact, the wide exposed surface area allows a superior dye loading, leading to a huge amount of photogenerated electrons. Such carriers find a self-interconnected structure and can thus

Fig. 3. Photoelectrochemical performance and diffusion length dependence of the ZnO flower-like particles. (a) Current density vs. voltage curve for DSCs fabricated with flower-like ZnO photoanode, N719 sensitizer and iodine-based electrolyte containing 0.5 M NMBI. In the inset, the short-circuit current density dependence on the light intensity is reported; (b) Dependence of the electron diffusion length on the applied voltage for DSCs assembled with ZnO flower-like photoanode.

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643

Table 1 Literature comparison between the performances of some ZnO-based photoanodes in Dye-sensitized Solar Cells. For simplicity, here are considered some examples of ZnO nano- and micro-structures with relatively high surface area, excluding nanowires, nanotubes and thin films. Morphology

Sensitizer

Light intensity [mW cm2]

Photoconversion efficiency h [%]

Short circuit current density Jsc [mA cm2]

Open circuit voltage, Voc [mV]

Fill factor, FF

Ref.

Flower-like Flower-like Nanoparticles Nanoparticles Branched nanowires Branched nanowires Branched nanowires Nanosheets Nanosheets Jacs-like Nanocristallytes/ hierarchical aggregates Commercial nanopowders Hierarchical NWs

N719 N719 N719 N719 N719 N719 N719 N719 N719 N719 N719

100 100

100

3.6 1.9 2.1 1.2 0.5 1.1 1.1 3.3 3.9 1.8 7.5

9.1 5.5 0.7 5.4 1.6 3 4.7 13.8 11.2 5.5 19.8

605 650 480 510 740 e 510 560 580 590 640

0.65 0.53 e 0.43 0.38 e 0.48 0.44 e 0.54 0.59

Present paper [53] [54] [55] [56] [25] [57] [58] [59] [60] [61]

100 100

6.6 2.6

18.1 8.8

621 680

0.58 0.53

[48] [62]

N719 N719

100 100 100 100 100

Jsc ¼ 9.1 mA cm2, open circuit voltage Voc ¼ 605 mV, fill factor FF¼ 0.65), thus accounting for the effective use as photoanode material for DSC application of the prepared flower-like ZnO structures. In the literature are reported several studies about the performances of ZnO-based photoanodes in DSCs. Table 1 shows some examples of the photovoltaic parameters reported for ZnO microand nano-structures. It can be concluded that the photovoltaic performances reported in the present work (here shown in the first row of Table 1) are in line with the literature values. In addition, in order to check the possible presence of mass transfer limitations in the electrolyte for high light intensity, the Ie V characteristic was measured under different illumination condition (using neutral filters). As can be clearly observed from the inset of Fig. 3a, the Jsc linearly depends on the light intensity, demonstrating that the charge diffusion in the electrolyte does not limit the overall photocurrent density of the cells. In order to evaluate the transport properties of the flower-like ZnO nanostructures, electrochemical impedance spectroscopy measurements were performed on DSCs at different applied bias voltages. From the fitting of the experimental data, the diffusion length was calculated (see Section 2.2 for details about the fitting procedure) and the results are reported in Fig. 3b. The diffusion length values lie in the range 150e200 mm, which represents a noticeable result among ZnO nanostructures, usually ranging from some mm to some tenth of mm [22]. The valuable transport properties of these

Fig. 4. Cyclic voltammetry (CV) characterization. Typical ambient temperature cyclic voltammograms of the flower-like ZnO; scan rate: 0.100 mV s1 and potential range: 0.02 and 3.0 V vs. Liþ/Li.

nanostructures can be attributed to the high crystalline quality of the material, with reduced presence of defects that would increase the charge recombination probability. Therefore, the ability to tune the morphology of ZnO at the nanoscale can open interesting opportunities in the control of charge transfer dynamics in the photoelectrode. We have indeed shown that it is possible to combine the fast direct transport of the ZnO nanostructures with the wide exposed area for dye-sensitization typical of our multi-petal particles. 3.3. ZnO flower-like particles as anode in lithium-ion batteries In order to demonstrate the promising prospects of the novel flower-like ZnO material as Li-ion battery anode, the electrodes

Fig. 5. Electrochemical performances of flower-like ZnO particles as electrode in lithium-ion cells. Ambient temperature cycling behavior in lithium cell of the asprepared flower-like ZnO electrode: (a) typical potential vs. time profiles at different cycles and (b) specific discharge (lithiation) and charge (de-lithiation) capacities plotted as a function of the cycle number and at different current regimes.

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(prepared by simple drop casting on stainless-steel discs) were assembled in lithium cells and the electrochemical behavior tested at ambient temperature in terms of CV (cyclic voltammetry) and prolonged galvanostatic discharge/charge (i.e. lithiation/de-lithiation) cycling at different current rates. The cyclic voltammetric (CV) analysis of the as-prepared flowerlike ZnO film drop casted onto the stainless steel substrate is shown in Fig. 4. The CV profiles reveal the typical electrochemical redox behavior of ZnO, as expected on the basis of previous reports [13,63,64], showing several cathodic and anodic peaks. In particular, the following equations are expected during the first cathodic scan towards 0.02 V vs. Li:

ZnO þ 2Liþ þ 2e /Li2 O þ Zn

(R4)

Zn þ Liþ þ e /LiZn

(R5)

Based on these two reactions, and considering three lithium ions per formula unit, the calculated theoretical specific capacity of ZnO is considered to be 987.8 mAh g1. Briefly, the destruction of the crystal oxide structure occurs followed by the formation of Zn metal and Li2O (Eq. (R4)) which is then followed by the formation of LiZn alloy (Eq. (R5)) upon deep discharge, along with various minor irreversible processes. This behavior is reflected in the first cathodic profile (black line), where at voltages below 0.7 V vs. Li the reduction of ZnO into Zn occurs along with the growth of the SEI (solid electrolyte interphase) layer. The SEI is the gel-type layer which deposits at the surface of the micro-particles upon the decomposition of some of the electrolyte components [13]. The SEI layer formation leads to an extra lithium consumption which largely contributes to the huge drop in specific capacity during the first galvanostatic cycles, shown in Fig. 5. The potentials at which these reactions take place are very close, thus only one broad intense peak is observed, centered at w0.4 V, along with a shoulder at w0.3 V (see Fig. 4). Then, the LiZn alloy formation occurs below 0.2 V. In the subsequent anodic scan towards 3.0 V vs. Li (black curve, upper side), different oxidation peaks are observable, located in the 0e0.75 V vs. Li range, easily ascribable to the average potentials of the multi-step process of LiZn dealloying [64], finally leading to metallic zinc. The broad peak ranging from 1.0 to slightly above 2.0 V may be ascribed to the formation of ZnO by the conversion reaction between Li2O and Zn, even if controversial literature reports can be found [13,63,64]. Finally, a steep irreversible peak centered at about 2.6 V vs. Li can be found, which is related to the partial conversion of Zn to ZnO [63]. In the second cathodic

scan, the peaks related to the LiZn alloying process shift to about 0.6 and 0.4 V vs. Li. After the initial cycles, where the intensities of all the peaks decrease during cycling because of capacity fading, the curves tend to stabilize in shape and intensity of the redox peaks, accounting for more reversible electrode reactions as observable by the quasi-overlapping of the cycle 5 (cyan line) and cycle 10 (magenta line) profiles. The galvanostatic cycling results show the typical potential vs. time profiles at different cycles (Fig. 5a) and the specific capacity of the ZnO electrode as a function of the cycle number and at different current regimes (Fig. 5b). The ZnO/Li cell delivers a specific capacity of about 1330 mAh g1 during the first lithiation (discharge) step at low 0.1 C current, largely exceeding the theoretical value, and about 685 mAh g1 in the first de-lithiation/charge step, which drastically decrease at about 640 and 515 mAh g1 during the second lithiation and de-lithiation, respectively. The noticeable irreversible capacity loss during the first cycle is mainly ascribable to the expected side reaction with the electrolyte components, which were reduced at the surface of active nanomaterial particles. As mentioned before, they formed the insoluble quasi-solid passivating film at their surface (SEI) [63,64]. After the initial cycles, where structural rearrangements and irreversible processes took place, the cell demonstrates a stable behavior at each of the tested currents. It can deliver a specific capacity exceeding 175 mAh g1 after 200 cycles at reasonably high 1 C rate (or 200 mA cm2, see also Fig. S-1 in the Supporting Information), which is about 42% of the initial reversible capacity at 0.2 C (cycle 20) and about 72% of the initial reversible capacity at the same 1 C rate. Thus, the capacity retention while increasing the current regime is overall satisfying and the cell behaves correctly for prolonged cycling, which is noticeably good considering the characteristics of ZnO electrodes. In Fig. 5a, typical discharge and charge profiles, obtained at different cycles, are shown. They are well in accordance with the characteristic behavior upon lithiation/de-lithiation of ZnO electrodes [64] and with the CV results. In addition, after the first cycles, the Coulombic efficiency rapidly increases to above 97% at 0.5 C, then remaining highly stable throughout the cycles (even approaching 100% at the higher rate). This indicates good mechanical stability of the anode during the Liþ ions intercalation/deintercalation process and excellent reversible cycling after the surface reactions are completed. These highly valuable characteristics may be ascribed to the nanometricsized 1-D plates and the highly porous structural features of the flower-like ZnO aggregates. Both these features assure very high transport kinetics of Li-ions and effectively accommodate the large mechanical stresses caused by volumetric expansion/shrinkage

Table 2 Literature comparison between the performances of ZnO-based anodes for Li-ion batteries. Sample

Active material structure

Carbon coating

Additives (even during electrode preparation)

Reversible capacity (mAh g1) @ cycle number

Current regime

Potential scan range (V vs. Liþ/Li)

Total number of cycles

Ref.

ZnO

Nanostructured flowerlike microaggregates

NO

NO

200

present work

Flower-like nanowall arrays

YES

NO

0.0e3.0

70

[65]

ZnOeC

Hierarchical flowerlike nanospheres Hierarchical nanorods Nanowire arrays Nanorods Nanorods Nanotubes Hexagonal nanoplates

YES

YES

0.2 C 0.5 C 1C 0.5 C 1C 2C 0.5 C

0.02e2.8

ZnOeC

410 @ 20 332 @ 40 179 @ 200 w320 @ 30 w220 @ 50 w160 @ 70 w400 @ 30

0.05e2.5

30

[66]

0.02e3.0 0.02e2.0 0.01e3.0 0.01e2.5 0.01e2.5 0.0e3.0

50 40 100 50 50 50

[69] [70] [67] [71]

ZnO ZnO ZnO ZnO ZnO

NO NO YES NO

NO YES NO NO

NO

YES

<200 @ 50 w200 @ 40 w200 @ 100 w120 @ 50 w400 @ 50 w500 @ 50

200 mA 120 mA 100 mA 0.5 C 0.5 C 100 mA

1

g g1 g1

g1

[68]

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during lithiation/de-lithiation processes without the need of foreign additives/buffers. Finally, it is important to note that the system behavior remains satisfying after prolonged cycling at higher regime, with no abnormal drift. In fact, reducing the rate (after 90 cycles) almost completely restores the specific capacity (only 1% decrease when comparing the specific capacity values of cycle 20 and 110 at the same 0.2 C rate in Fig. 5a). It is also worth mentioning that the positive electrochemical characteristics upon reversible reaction with lithium are even more remarkable when considering that our proposed anode material is pure ZnO, without any carbon-coating and/or additives/buffering agents. Furthermore, the electrodes were obtained by simple drop casting onto stainless-steel current collectors without addition of any electronic conductivity enhancer and/or binder, which means enhanced overall energy density output. This accounts for the highly valuable electrochemical characteristics of the flower-like ZnO electrodes elaborated in this study in terms of specific capacity and rate capability for prolonged cycling, i.e. 200 cycles, which is the highest number of cycles reported so far if compared to those obtained in other recent works by similar synthetic methods and/or having similar structural-morphological characteristics (see Table 2). In that cases eventually higher specific capacity values were shown, however assisted by carbon-coating and/or addition of buffers/foreign ingredients to enhance the performances [13,65e68]. 4. Conclusions Herein the shape-controlled synthesis of flower-like ZnO microparticles with nanostructured petals was carried out by simple hydrothermal process, being reliable, cost effective and easily scalable up to an industrial level, thanks to energy and time saving preparation. The promising prospects of the elaborated structures as multifunctional anodic material for energy storage and conversion devices were effectively demonstrated for the first time. A ZnO-based DSC was fabricated with a microfluidic architecture, exploiting conventional sensitizer and electrolyte, and a ZnObased lithium cell was assembled with a lithium metal counter electrode and a standard liquid electrolyte. A 3.6% value of sunlight conversion efficiency was obtained in DSC, as well as a stable longterm ambient temperature cycling behavior in Li cell even at increasingly higher currents. The abundance, low cost and eco-friendliness of Zn metal and the good electrochemical performance of the flower-like ZnObased electrodes enlighten the promising prospects of this material for the successful implementation as stable and long-term performing anode in the next generation of energy conversion and storage devices. Acknowledgment The help of Dr. Edvige Celasco for FESEM imaging is gratefully acknowledged. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.energy.2013.12.025. References [1] Zhang Q, Dandeneau CS, Zhou X, Cao G. ZnO nanostructures for dye-sensitized solar cells. Adv Mater 2009;21:4087e108. [2] Jose R, Thavasi V, Ramakrishna S. Metal oxides for dye-sensitized solar cells. J Am Ceram Soc. 2009;92:289e301.

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