Neutral pH Gel Electrolytes for V2O5Â·0.5H2O-Based Energy Storage

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Neutral pH Gel Electrolytes for V2O5·0.5H2O-based Energy Storage Devices Aniu Qian, Kai Zhuo, Padmanathan Karthick Kannan, and Chan-Hwa Chung ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12672 • Publication Date (Web): 28 Nov 2016 Downloaded from http://pubs.acs.org on December 5, 2016

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ACS Applied Materials & Interfaces

Neutral pH Gel Electrolytes for V2O5·0.5H2O-based Energy Storage Devices Aniu Qian, Kai Zhuo, Padmanathan Karthick Kannan, and Chan-Hwa Chung* School of Chemical Engineering, Sungkyunkwan University, Suwon 16419, Republic of Korea KEYWORDS: Gel electrolyte, Vanadium oxide hydrates, Electrochemistry, Cyclic stability, Hybrid super-capacitors

ABSTRACT: Gel electrolytes are considered to be promising candidates for the use in supercapacitors. It is worthy to systematically evaluate the internal electrochemical mechanisms with a variety of cations (PVA-based Li+, Na+, and K+) toward redox-type electrode. Herein, we describe a quasi-solid-state polyvinyl alcohol (PVA)-KCl gel electrolyte for V2O5·0.5H2O-based redox-type capacitors, effectively avoiding electrochemical oxidation and structural breakdown of layered V2O5·0.5H2O during 10000 charge-discharge cycles (98% capacitance retention at 400 mV s-1). With the gel electrolyte, symmetric V2O5·0.5H2O-reduced graphene oxide (V2O5·0.5H2O-rGO) devices exhibited a volumetric capacitance of 136 mF cm-3, which was much higher than that of 68 mF cm-3 for PVA-NaCl and 45 mF cm-3 for PVA-LiCl. Additionally, hybrid full cells of activated carbon cloth (ACC)//V2O5·0.5H2O-rGO delivered an energy density of 102 µWh cm-3 and a power density of 73.38 mW cm-3 over a wide potential window of 2 V. The present study provides direct experimental evidence for the contribution of PVA-KCl gel

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electrolytes toward quick redox reactions for redox-type capacitors, which is also helpful for the development of neutral pH gel electrolytes for energy storage devices.

1. INTRODUCTION Supercapacitors (SCs) as renewable energy sources are energy storage devices that can be used to harvest energy and offer particularly high power densities.1 Considering a mechanism of charge accumulation in electrochemical double layer capacitors (EDLC),2 a key approach to increase charge storage capacitance is to utilize pseudo-capacitors that arise from reversible redox-reactions.3 Long et al. mentioned that pseudo-capacitive oxide materials such as MnO2, RuO2, and Nb2O5, exhibit rectangular-shaped curves upon cyclic voltammetry measurements, while other oxide materials such as bulk NiO, Ni(OH)2, Co3O4, and V2O5, display redox-type electrode behaviors.4 Among them, nano-crystalline V2O5 is an attractive redox-type electrode alternative, due to its multiple oxidation states (II-V),5 layered structures,6 and low cost (of $12 per kg) as compared to RuO2 and NiO.7, 8 However, structural deterioration and irreversible electrochemical oxidation in aqueous electrolytes are still major challenges, although graphenedecorated or conducting-polymer modified V2O5 nanostructures exhibit favorable performance with regard to redox-type capacitors. Exploring appropriate electrolytes can yield promising solutions to the aforementioned problems. In particular, organic electrolytes and/or gel electrolytes have been considered as a proper candidate electrolyte to stabilize V2O5 nanostructures. Unfortunately, the former still suffer from capacitance degradations.9, 10 Alternatively, gel electrolytes are emerging as an attractive candidate.11 Their semi-solid characteristics function as elastic coatings to prevent the structural degradation of electrode materials;12 meanwhile, high ionic conductivity (10-4 ~ 10-1 S cm-1) in conjunction with flexibility enables excellent solid-state supercapacitor performance.13 Despite these advantages,


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the application of gel electrolytes is still in preliminary stages, especially for blends of aqueous alkaline metal salts and polyvinyl alcohol (PVA) gel electrolytes. Several alternatives have been selected as ideal gel electrolytes for vanadium-based supercapacitors, such as PVA-LiCl,12, 14 PVA-H3PO4,15 and PVA-BMIMBF4 ionic liquid.16 All above alternatives play a significant role in relieving capacitance degradations and exhibiting excellent cyclic behavior in the ranges of 2000 cycles to 10000 cycles. Notably, neutral KCl electrolytes have been proved to exhibit higher specific capacitance under acid and neutral pH electrolytes in V2O5 supercapacitor applications.17 Nevertheless, few studies have been focused on neutral gel electrolytes of waterPVA-alkaline metal salts such as NaCl, KCl, and NaNO3. In EDLC-based porous carbon supercapacitors employing different alkali metal-based electrolytes with Li+, Na+, and K+, the Li+-containing electrolyte was found to exhibit the most promising capacitance performance, as Frackowiak et al. explained through clear experimental results and mechanisms.18 However, in redox-type V2O5-based capacitors, the internal electrochemical mechanisms are still unclear with regard to PVA-based Li+, Na+, and K+ electrolytes, although many studies reported neutral PVA-LiNO3 19 and PVA-LiCl 20, 21 gel electrolytes. Moreover, the effect of specific gel electrolytes on the capacitance of V2O5-based energy storage devices has not been compared. Herein, we put forward an optimized quasi solid-state PVA-KCl gel electrolyte to demonstrate the role of a polymer matrix affecting the elastic coating interface between V2O5 electrodes and electrolytes. Then we compared each capacitance performance of PVA-based Li+, Na+, and K+ electrolytes, respectively. We found that PVA-KCl gel electrolytes were essential for quick redox reactions due to their short diffusion paths and fast ion transfer at the redox-type electrode/electrolyte interface. Additionally, during 10000 charge-discharge cycles, the PVA-


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KCl gel network effectively prevented electrochemical oxidation and structural breakdown of V2O5·0.5H2O and maintained its chemical composition in the form of 98% capacitance retention at 400 mV s-1. Our study is helpful to utilize neutral PVA gel electrolytes in V2O5-based redoxtype capacitors. 2. EXPERIMENTAL SECTION 2.1. Chemical and Reagents. Carbon cloth (Plain, Fuel Cell Earth, USA), ammonium metavanadate (NH4VO3, 99%, Sigma-Aldrich), hydrochloric acid (HCl, 35%), PEG-6000 (Alfa Aesar), potassium chloride (KCl, 99%), lithium chloride (LiCl, 99%), sodium chloride (NaCl, 99%), and polyvinyl alcohol (PVA, Mw=146000-186000, 99% hydrolyzed) were used without further purification. 2.1.1. Preparation of the V2O5·0.5H2O-rGO Hybrid on Carbon Cloth. The V2O5·0.5H2Oreduced graphene oxide (rGO) (V2O5·0.5H2O-rGO) hybrid was grown on carbon cloth via hydrothermal methods with PEG-6000 surfactant served as a soft template. A piece of carbon cloth (CC, 3 cm × 3 cm, 14.8 mg cm-2) was sequentially cleaned with acetone, ethanol, and deionized (DI) water for 15 min each. Prior to reaction, the cleaned CC was immersed in ethanol to improve its hydrophilicity. 0.1 M NH4VO3 and 17.5 mM PEG-6000 were dissolved in 30 mL of DI water, of which the pH was adjusted to 2.0 by another 1.5 mL of 2 M HCl. After sonication for 1 h, a homogeneous dispersion of 16 mg of as-prepared GO in 8 mL of DI water was obtained and dropwise added into the above V2O5·0.5H2O precursor solution, followed by another sonication for 40 min. The solution mixture with immersed CC was transferred to a 50mL-capacity Teflon-lined autoclave and sealed at 160°C for 24 h. After the reaction, the CC was uniformly coated by a thick-layer dark-blue V2O5·0.5H2O-rGO hybrid. Then, the hybrid was


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carefully washed with DI water and freeze-dried with compressed air. Finally, the hybrid was annealed at 300°C in air for 2 h to obtain V2O5·0.5H2O-rGO product. 2.1.2. Preparation of the PVA-KCl Gel Electrolyte. 3 g of PVA powder was dissolved in 30 mL DI water at 95°C, until the PVA powder completely dissolved and became a clear solution. After the solution was cooled, 10 mL of 2 M KCl was dropwise added and stirred for 4 h to promote the cross-linking reaction. The mixture was subsequently evaporated to remove the excess 10 mL of DI water, yielding a transparent and sticky gel. The gel solution was stored for 24 h to remove the trace amounts of water. For PVA-LiCl and PVA-NaCl gel electrolytes, a similar process was conducted by adding 2 M LiCl and 2 M NaCl to the PVA mixture. 2.1.3. Device Assembly of the Symmetric Supercapacitor (SC). Scheme 1 illustrates the fabrication procedure of the symmetric device. Prior to assembly, two gold-coated V2O5·0.5H2OrGO electrodes were covered with PVA-KCl gel by a doctor blading and allowed the electrolyte to diffuse into the nanostructures. After the gel solidify for 6 h, a quasi-solid-state film was formed. Each gold-coated electrode without electrolyte covering was connected with a copper wire using silver paste. Subsequently, two gel-coated electrodes were soaked in another PVAKCl solution for 10 min and bonded together. Finally, the SC device was sealed in parafilm with aluminum foil to prevent short-circuits and water evaporation of the gel electrolyte. The device thickness was 0.44 mm with a 1 cm2 working area. For the hybrid supercapacitor, a similar process was employed by changing the activated carbon cloth (ACC). Detailed fabrication process for the ACC can be found in the Supporting Information (SI). 2.2. Electrochemical Measurements. All device electrochemical performance measurements were performed by a two-electrode system. Each electrode was grown on CC substrate as a current collector without the use of binders, which reduces an interfacial resistance and avoids


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mass losses. Cyclic voltammetry (CV) measurements were performed with a Zahner Elektric IM6 electrochemical workstation. Electrochemical impedance spectroscopy (EIS) was carried out by applying an AC amplitude of 5 mV and a sinusoidal signal at 0 V within a frequency range from 10 mHZ to 100 kHZ. Specific capacitance (C) of two-electrode supercapacitor configuration was calculated from CV curves using the following equation:

where is the scan rate, ∆V is the device voltage, I is the current, and A is the surface area of electrode. Areal (Ca, F cm-2) or volumetric capacitance (CV, F cm-3) of the device were calculated though the following equations:

where V is the volume of the device configurations, E is the energy density (mWh cm-3), P is the power density (mW cm-3), and ∆t (h) is the discharge time. 2.3. Materials Characterization. Electrode morphology and composition was observed by a field emission scanning microscope (FE-SEM, JEOL JSM-7000F). Structural and chemical composition were obtained by X-ray diffraction (XRD, Bruker AXS), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR, JASCO FT/IR-4700 spectrometer), and Raman spectroscopy (Renishaw inVia Raman Microscope, 532 nm laser beam). 3. RESULTS AND DISCUSSION 3.1 Morphological Features and Chemical Composition of the PVA-LiCl, PVA-NaCl, PVA-KCl, V2O5·0.5H2O, and V2O5·0.5H2O-rGO Hybrid. To evaluate the morphology and chemical composition of each neutral pH gel electrolyte, we compared PVA-KCl with PVANaCl and PVA-LiCl, respectively, as shown in SEM images of Figure 1. The concentration of all


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salts (KCl, NaCl, and LiCl) is 2M concentration, which is well dispersed into polymeric PVA frameworks without any salt crystallization or agglomeration. Compared with PVA-LiCl gel is bundle-like in Figure 1a and 1d, PVA-KCl gel in Figure 1c and 1f apparently forms interconnected microporous networks which have been suggested that there are sufficient to provide high ion mobility for K+.22 Attenuated total reflectance technique of Fourier transform infrared (ATR-FTIR) spectroscopy in Figure 1g reveals that a peak intensity of H2O bending vibration decreases in PVA-NaCl and PVA-KCl as compared with that of in PVA-LiCl (ν= 1655 cm-1), since hydrophilic LiCl could form hydrogen bonds with water molecules.23 Notably, relative to the stretching vibration of O-H at ν= 3318 cm-1 in PVA-LiCl, a red-shift in the vibration bands of PVA-NaCl and PVA-KCl occurs at ν= 3269 cm-1 and 3247 cm-1. Besides, the peak intensity of O-H is PVA-KCl < PVA-NaCl < PVA-LiCl. These results imply an energy decrease in O-H vibration of PVA in PVA-KCl gel, which may be caused by hydrogen bond interactions between water molecules and O-H. These results demonstrate slow ion transport in PVA-LiCl gel electrolyte, since LiCl is accessible to adsorb water molecules and form large hydrated ions. In contrast, the decreased O-H stretching vibration and peak intensity of PVAKCl implies that water molecules can homogeneously disperse into PVA network. It not only helps to dissociate K+ into the PVA-KCl gel, but also contributes to enhance mobility of K+.

Scheme 1. Schematic illustration of the device assembly of symmetric V2O5-based supercapacitor.


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0.4

PVA-LiCl

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PVA-NaCl

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-OH


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PVA-KCl

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Wavenumber (cm ) Figure 1. Characterizations of neutral gels. SEM images of (a, d) PVA-LiCl, (b, e) PVA-NaCl, and (c, f) PVA-KCl. ATR-FTIR spectra (g) of PVA-LiCl, PVA-NaCl, and PVA-KCl.


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The photograph in Figure S1 shows thick-layer green hydrated V2O5 and dark-blue V2O5·0.5H2O-rGO hybrid that are uniformly grown on carbon cloth substrate. As shown in Figure 2a and 2c, morphological features of hydrated V2O5 reveal an intertwined and highly orientated structure of nanowires that are several tens of micrometers in length, promoting the formation of porous networks. Contrastively, hybrid V2O5·0.5H2O-rGO architectures proved by SEM images of Figure 2b and 2d show that rGO nanosheets that enwrapped ultrafine V2O5·0.5H2O nanowires shrink into extremely dense networks with interlaced voids, which may facilitate sufficient infiltration of electrolyte into the pores. Distinct to the V2O5 nanowires, increased pores and decreased lengths in the architectures are mainly caused by V2O5·0.5H2O nanowires cross-linking each other during the shrinking process of rGO nanosheets.24

Figure 2. SEM images of (a, c) ultra-long hydrated V2O5 nanowires, (b, d) three-dimensional V2O5·0.5H2O-rGO networks grown on CC substrate.

Phase compositions of electrode materials were confirmed by X-Ray diffraction (XRD) (Figure S2a). XRD patterns of hydrated V2O5 can be indexed to layered V2O5·0.5H2O (JCPDS No. 40-1297),25 while the XRD spectra peaks of powder hybrid V2O5·0.5H2O-rGO products are


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matched well with (00l) reflections of layered V2O5·0.5H2O and diffraction peaks of graphene at 2θ = 25.4°, which proves that V2O5·0.5H2O nanowires are successfully incorporated into the graphene layers.26, 27 To identify chemical composition and structure of the above products, ATR-FTIR technique was employed to collect the spectra in Figure S2b. For hydrated V2O5, diverse IR absorption bands at ν = 1617 cm-1, 3630 cm-1, 3256 cm-1, 1438 cm-1, 999 cm-1, and 717 cm-1 can be observed in ATR-FTIR spectroscopy, which demonstrated typical nanostructured V2O5.28 The ATR-FTIR spectrum of hybrid V2O5·0.5H2O-rGO nanostructure shows characteristic V2O5 vibration bands at ν = 999 cm-1 along with typical rGO vibration bands at ν = 1221 cm-1 (in phenolic) and ν = 1056 cm-1 (in alkoxy). An increased intensity ratio (ID/IG) and presence of characteristic V2O5 vibration bands in ranges of 100 cm-1 ~1000 cm-1 in Raman spectra of Figure S3 indicates highly disordered rGO and V2O5.28, 29 3.2 Electrochemical Performance of PVA-LiCl, PVA-NaCl, PVA-KCl. The above gel electrolytes were applied to symmetric V2O5·0.5H2O-rGO devices in cyclic voltammetry (CV) curves (Figure 3a), the device operating in PVA-KCl electrolyte delivers a specific capacitance of 136 mF cm-3 at a high scan rate of 400 mV s-1, which is much higher than those of 68 mF cm-3 in PVA-NaCl and 45 mF cm-3 in PVA-LiCl. To analyze the differentiated results, we evaluated electrochemical interface of the three devices by the electrochemical impedance spectroscopy (EIS) (Figure 3b). Specifically, the device operating in PVA-LiCl electrolyte reveals the highest equivalent series resistance (ESR) value of 11.5 Ω, which proves poor ion transfer of the electrolyte. The straight line under low frequency refers to slow ion diffusion, which is agreement with our above analysis by IR (Figure 1). For the device operating in PVA-NaCl electrolyte, an ESR value of 5.5 Ω indicates enhanced ion transfer of the electrolyte. Notably, the lowest ESR value of 0.8 Ω and the vertical curve under low frequency are observed for device


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operating in PVA-KCl electrolyte, which proves that such electrolyte facilitates fast ion transfer and diffusion. Furthermore, the smallest semicircle in PVA-KCl electrolyte under high frequency range reveals fast electron transfer that accelerates redox reactions at the electrolyte/electrode interface. Additionally, short diffusion pathways for ions through electrode micropores can be confirmed by the 45° slope of the curve in the middle frequency range (Warburg impedance) of the PVA-KCl electrolyte.

(a)

PVA-LiCl PVA-NaCl PVA-KCl

PVA-LiCl PVA-NaCl PVA-KCl

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Neutral pH Gel Electrolytes for V2O5Â·0.5H2O-Based Energy Storage

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