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Electrodes consisting of PEDOT modified by Prussian Blue analogues deposited onto titania nanotubes – their highly improved capacitance
SOSI-14164; No of Pages 5 Solid State Ionics xxx (2016) xxx–xxx
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Electrodes consisting of PEDOT modified by Prussian Blue analogues deposited onto titania nanotubes – Their highly improved capacitance M. Szkoda a,⁎, K. Trzciński a, J. Rysz c, M. Gazda d, K. Siuzdak b, A. Lisowska-Oleksiak a a
Faculty of Chemistry, Gdansk University of Technology, Narutowicza 11/12, 80-233 Gdansk, Poland Center for Plasma and Laser Engineering, The Szewalski Institute of Fluid Flow Machinery, Fiszera 14, 80-231 Gdansk, Poland M. Smoluchowski Institute of Physic, Jagiellonian University, Lojasiewicza 11, 30-348 Krakow, Poland d Department of Solid State Physics, Faculty of Applied Physics and Mathematics, Gdansk University of Technology, 80-233 Gdansk, Poland b c
a r t i c l e
i n f o
Article history: Received 30 July 2016 Received in revised form 21 December 2016 Accepted 23 December 2016 Available online xxxx Keywords: Titania nanotubes Capacitors Inorganic-organic heterojunction Specific capacitances
a b s t r a c t In this work we present the outstanding energy storage of prepared inorganic-organic heterojunction where hydrogenated ordered titania nanotubes (H-TiO2NT) were modified by the hybrid made of poly(3,4ethylenedioxythiophene) (pEDOT) and iron hexacyanoferrate centres (Fehcf, Prussian Blue). The material TiO2NT/pEDOT:Fechf was obtained electrochemically by means of: anodization, hydrogenation and finally, ions. Inorganic-organic hybrids were characterelectropolymerization of EDOT in the presence of Fe(CN)3−/4− 6 ized using Raman spectroscopy and secondary ion mass spectrometry (SIMS). The morphology of obtained materials was inspected using scanning electron microscopy (SEM). Electrodes were tested using cyclic voltammetry and galvanostatic charge/discharge cycles in an aqueous electrolyte. The characterization of capacitance was studied by means of multiple (up to 10,000) charge/discharge cycles with the current density of 0.45 mA cm−2. Electrode materials consisting of H-TiO2, pEDOT and Prussian Blue exhibited the highest capacitance of 26 mF cm−2 even after 10,000 cycles. Thus, the capacitance of TiO2NT/pEDOT:Fehcf was c.a. 15 and 8 times higher than the capacitance registered for pure and hydrogenated TiO2, respectively. © 2016 Published by Elsevier B.V.
1. Introduction Electrochemical supercapacitors, as energy storage devices, have attracted growing interest in recent years [1]. Capacitors in general make use of three main classes of materials: porous carbon, transition metal oxides and conducting polymers deposited onto various electrode collectors [2–4]. Porous carbon materials exhibit a relatively low electrochemical double-layer capacitance. Ruthenium oxide is characterized by a very high electrochemical pseudo-faradic capacitance, however its high cost hampers possible commercial application. Conductive polymers are characterized by a high electrical capacitance and cyclability, but unfortunately they exhibit a narrow potential range of electrochemical activity and stability. The extension of potential window and the improvement of specific capacitance can be achieved by fabrication of organic-inorganic heterojunction with TiO2 [5,6]. Additionally, increase in charge capacity can be achieved by introduction of redox inorganic network, i.e. Prussian Blue analogues into the polymer matrix [7,8]. ⁎ Corresponding author. E-mail address: [email protected] (M. Szkoda).
Highly ordered TiO2 nanotubes are promising as supercapacitor electrodes, because of the large specific surface area and direct pathway for charge transport in nanotubes as well as facile fabrication directly onto the charge collector substrate. Thus, utilization of TiO2 nanotubes obtained onto the Ti support as an electrode material might offer an opportunity to improve the capacitance of supercapacitors. However, it should be mentioned that the semiconducting nature of TiO2 limits the electrical conductivity and hinders specific capacitance in the charge–discharge process and therefore titania requires further modification for further application in energy storage devices [9]. The electrodeposition of polymer films with modified TiO2 proceeds more efficiently in comparison to pure TiO2 [6]. Here, studies on the capacitance properties of organic-inorganic heterojunction containing highly ordered titania nanotubes and Prussian Blue embedded into the pEDOT matrix is presented. All investigated electrodes were prepared via electrochemical methods that could be easily scaled up. The presence of each part of heterojunction was confirmed using Raman spectroscopy whereas SIMS technique allows the composition tracking from the material surface down to the base of titania. The obtained heterojunction containing TiO2, Prussian Blue and pEDOT was characterized using prolonged reversible polarization
http://dx.doi.org/10.1016/j.ssi.2016.12.025 0167-2738/© 2016 Published by Elsevier B.V.
Please cite this article as: M. Szkoda, et al., Electrodes consisting of PEDOT modified by Prussian Blue analogues deposited onto titania nanotubes – Their highly improved capa..., Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.12.025
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M. Szkoda et al. / Solid State Ionics xxx (2016) xxx–xxx
cycles, which allowed to demonstrate enhancement of the capacitance as compared to the pure and hydrogenated TiO2 nanotubes, as it was reported previously [9–11]. 2. Experimental 2.1. Preparation of H-TiO2 nanotubes Highly ordered titania nanotubes were prepared via a two-step electrochemical anodization of a Ti plate (Strem, 99.7%) in a fluoride-containing solution. The procedure that leads to the formation of TiO2 nanotubes was given previously [12]. Anodization process was realized in a two-electrode configuration with a cathode: (Pt grid) and anode (Ti plate) placed 2 cm from each other. Both steps of anodization were performed at temperature (23 °C), electrolyte composition (0.27 M NH4F in 1%/99% v/v water/ethylene glycol solution) and anodization parameters (40 V, 2 h). Before the second anodization step, the as-formed nanotubes during the first stage were removed by overnight etching in an oxalic acid solution (0.5% wt). To remove surface debris, the titanium plates covered with nanotubes were immersed in 0.05% wt HF for 180 s. The as prepared amorphous TiO2 nanotubes were transformed to anatase phase by thermal annealing performed at 450 °C for 2 h (heating rate: 2 °C min−1). The electrochemical hydrogenation process, similar to that proposed by Xu et al. [13], was performed in a three-electrode configuration at room temperature, using the crystallized TiO2 nanotubes as working electrode, Ag/AgCl as reference and platinum mesh as counter electrode. The hydrogenation was performed in 0.5 M K2SO4 aqueous solution under the cathodic potential of −1.5 V vs. Ag/AgCl (0.1 M KCl) applied for 20 s, charge consumed equals to 60 mC cm−2. Afterwards, the TiO2 nanotubes were taken out, washed with deionized water and dried in the air. The hydrogenation process activates the titania surface by lowering of the resistivity and facilitates monomer adsorption and nucleation, resulting in the efficient electropolymerization process [14]. 2.2. Fabrication of H-TiO2/pEDOT:Fehcf Electrochemical polymerization was carried out to obtain the inorganic-organic heterojunction H-TiO2NTs/pEDOT:Fehcf. The electrochemical deposition was performed according to the two stage procedure proposed in Ref. [15]. The first stage covers potentiostatic electropolymerization from an aqueous solution containing monomer EDOT in the presence of Fe(CN)3−/4− ions. As a working electrode flat 6 titanium plate or hydrogenated titania NTs layer were used. The
reference electrode was Ag/AgCl(0.1 M KCl) whereas a platinum mesh was used as a counter electrode. The potentiostatic electropolymerization was realized under the potential of 1.5 V vs. Ag/ AgCl(0.1 M KCl) and consuming the charge of 50 mC cm−2. For comparative studies, the H-TiO2/pEDOT:Cl, H-TiO2/Fehcf were also produced. Electrochemical depositions were performed in an aqueous solution containing 0.1 M NaCl and 0.001 M EDOT (in order to obtain H-TiO2/ pEDOT:Cl) or Fe(CN)36 −/4− [16] (in order to obtain H-TiO2/Fehcf), by potentiostatic polymerization of 1.5 V vs. Ag/AgCl(0.1 M KCl) consuming charge of 50 mC cm−2. 2.3. Apparatus The surface morphology and cross-section view were examined using the Schottky field emission scanning electron microscopy (FEI Quanta FEG 250) with an ET secondary electron detector. The Raman spectra were recorded by a confocal micro-Raman spectrometer (InVia, Renishaw) with sample excitation, by means of an argon ion laser emitting at 514 nm operating at 5% of its total power (50 mW). The crystal structure of TiO2 and H:TiO2 samples was also determined from X-Ray diffraction patterns (XRD), using X-ray diffractometer (Xpert PRO-MPD, Philips) with copper Kα radiation (λ = 1.5404 Å). The chemical composition of modified titania nanotubes was examined using Time of Flight Secondary Ion Mass Spectrometer (TOF SIMS). The TOF SIMS 5 (ION-TOF GmbH) working in dual beam mode was used to obtain composition versus depth profiles. Bi+ 30 keV ions and Cs+ 2 keV ions, both incident at 45° to the surface normal, were used as the analysis and sputter. The Cs+ sputter beam was rastered over 350 μm·350 μm while the 100 μm·100 μm central region of the sputter crater was analyzed with the Bi+ ions. Negatively charged secondary ions induced by Bi primary ions were collected using time of flight mass spectrometer. Intensities of characteristics signals versus sputtering time were analyzed using SurfaceLab 6 software (ION-TOF GmbH). To determine variations in vertical composition of the samples the following signals were chosen: C– (m/z = 12.00), 18O− (m/z = 17.99), CN− (m/z = 26.00), 34 − S (m/z = 33.97), Ti− (m/z = 47.95), Fe− (m/z = 55.94) and 46 TiO− 2 (m/z = 77.94). Electrochemical experiments (cyclic voltammetry – CV, chronopotentiometry – CP, electrochemical impedance spectroscopy – EIS) were performed using the AutoLab PGStat 302N potentiostatgalvanostat system (Methrom, Autolab) in a standard three-electrode assembly, where different working electrodes were tested: TiO2, HTiO2, H-TiO2/pEDOT:Cl, H-TiO2/Fehcf and H-TiO2/pEDOT:Fehcf with the geometric surface area of 0.3 cm2 but characterized the real surface
Fig. 1. Surface SEM image of TiO2, H-TiO2 and composite materials: H-TiO2/Fehcf, H-TiO2/pEDOT:Cl and H-TiO2/pEDOT:Fehcf and cross-section image of H-TiO2.
Please cite this article as: M. Szkoda, et al., Electrodes consisting of PEDOT modified by Prussian Blue analogues deposited onto titania nanotubes – Their highly improved capa..., Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.12.025
M. Szkoda et al. / Solid State Ionics xxx (2016) xxx–xxx
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Fig. 2. (a) Raman spectra of TiO2, H-TiO2 and composite materials, (b) The X-ray diffraction patterns of pristine and hydrogenated TiO2 and (c) SIMS composition versus depth profiles of hydrogenated titania nanotubes modified with pEDOT:Fehcf hybrid. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
area 120 times higher than the geometric one [17]. In order to estimate gravimetric capacitance, the mass of the electrode before and after pEDOT:Fehcf deposition was measured using automated-microbalance Metler-Tolledo XP6 characterized with readability of 1 μg. 3. Results and discussion 3.1. Morphology and structure studies In Fig. 1 the top-view and cross-section images of TiO2, H-TiO2 and different composite materials are presented. The pure and hydrogenated titania layers are composed of regular nanotubes with the internal radius of 50 nm, the wall thickness of 25 nm and the length of 2.5 μm. Thus, no difference was observed in the nano-architecture between the hydrogenated and pure titania proving any impact of the hydrogenation process onto the titania nanostructure. Other images show the layer of titania nanotubes modified by Fehcf, pEDOT:Cl and pEDOT:Fehcf, respectively. Comparing SEM images registered for hydrogenated titania and composite samples, one may observe the growth of polymer and Prussian Blue films mainly on the wall-edges of the tubes. The Raman spectroscopy measurements of electrode materials were carried out to confirm the crystalline phase of TiO2 and successful deposition of polymer films onto the titania layer. A number of bands characteristic for the pure anatase crystalline form of TiO2 were identified for pristine as well as for hydrogenated titania and composite material (see Fig. 2a) The maxima located at 144, 198, 395, 516 and 637 cm−1 are attributed to Eg(1), Eg(2), B1g, A1g, and Eg(3) active anatase modes, respectively [18,19]. Fig. 2b illustrates the XRD patterns of pristine and hydrogenated titania nanotubes. The strong XRD diffraction peaks indicate that titania nanotubes are present in a form of anatase for both, pristine
and hydrogenated samples and any typical peaks for rutile and brookite was detected. Peaks marked as “Ti” is a signal from the metallic substrate. No significant changes between patterns before and after modification were noticed. Similar behaviour was observed for hydrogenated titania in a form of powder [20]. Thus, changes in bulk material structure in both cases are negligible for XRD measurements. All Raman bands characteristic for pEDOT are observed in Fig. 2a (blue line). The main band at 1433 cm−1 can be attributed to symmetric (C_C)\\O bond vibrations in the thiophene ring. Two signals at 1495 cm−1 and 1561 cm−1 could be interpreted as asymmetric C_C modes in polymer chains [21]. Raman signals registered at 1267 cm−1 and 1366 cm−1 are attributed to single bonded carbons in both thiophene ring and polymer chain [22]. Furthermore, the oxyethylene ring deformations can be identified at 988, 571, and 442 cm−1 [23]. Nevertheless, no clear signal attributed to the C`N arrangement was observed. Therefore, SIMS depth profiles were recorded to indicate Prussian Blue presence and to determine whenever the organic component penetrate uniformly into the titania nanotube matrix. Fig. 2c shows the variation of different signals characteristic for: i) titania nanotubes 18 − (46TiO− O ); ii) pEDOT (34S− and also C−); iii) Prussian 2 and also − Blue species (Fe and CN−) with sputtering time (reflecting increasing distance from original film surface), iv) titania NT and Ti substrate (Ti−). After some variation close to the surface (c.a. first 1000 s of sputtering time) that is related with the interaction of ions with the outer layer of the sample, both the Fe− and CN− as well as C−, and 34S− signals become almost constant. Since 4500 s, the sharp falling edge of the 46TiO− 2 and 18O− signals at the Ti substrate is a sign that the height distribution of the nanotubes is very narrow. As it could be observed, when the decrease of signals attributed to titania appears, simultaneous fall of the signals assigned to the pEDOT:Fehcf matrix are also registered. Thus, obtained composition versus depth profiles confirm that pEDOT with
Fig. 3. (a) The cyclic voltammetry curves of pure and hydrogenated titania, and composite materials (electrolyte: 0.5 M K2SO4, v = 50 mV s−1), (b) Specific capacitance performance of materials with respect to the cycle number. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Please cite this article as: M. Szkoda, et al., Electrodes consisting of PEDOT modified by Prussian Blue analogues deposited onto titania nanotubes – Their highly improved capa..., Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.12.025
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Table 1 The comparison of specific areal capacitances (C) for each sample estimated after 5000 charge/discharge cycles. Sample
C/mF cm−2
Sample
C/m cm−2
H-TiO2NTs/pEDOT:Fehcf H-TiO2NTs/pEDOT:Cl H-TiO2NTs/Fehcf
26.1 7.9 5.1
H-TiO2 NTs TiO2 NTs Ti/pEDOT:Fehcf
3.3 1.8 0.7
imbedded Prussian Blue species are available at the same level at surface of the TiO2 nanostructures as well as along the tubes down to the base of titania NT. Therefore, proposed electrochemical synthesis method allows for the uniform modification of hydrogenated TiO2 by polymeric material containing redox centres. 3.2. Electrochemical properties The cyclic voltammetry measurements of TiO2, H-TiO2 and composite electrodes have been carried out to investigate their electrochemical properties (Fig. 3a). Detailed discussion on TiO2 and H-TiO2 electrochemical properties was given previously [14]. Titania nanotubes electrodes modified with i) Fehcf, ii) pEDOT:Cl and iii) pEDOT:Fehcf are characterized with higher charging current comparing to pure and hydrogenated TiO2NT samples. Furthermore, when polymer matrix with embedded Fehcf network was deposited onto titania tubular surface, the reversible redox peaks are observed at the formal potential of +0.17 V vs. Ag/AgCl (0.1 M) KCl. The observed redox activity is attributed to the Prussian White to Prussian Blue transformation, based on high spin iron centre activity (Fe coordinated via nitrogen atoms), as shown for pEDOT:Fehcf hybrid deposited onto the Pt disc electrode [24]. Further anodic polarization leads to the current increase (see orange line) due to low spin iron centre Fe(II)/Fe(III) faradaic reaction. This one is related to Prussian Blue to Berlin Green transition. The performance of specific capacitance with respect to the charge/ discharge cycle numbers is depicted in Fig. 3b. The values of the capacitance were calculated from galvanostatic charge/discharge measurements (ja = jc = 0.45 mA cm2, ΔE = 1.6 V). The specific capacitance of H-TiO2/pEDOT:Fehcf declines gradually from 40 down to 28 mF cm−2 during the first 2500 cycles, presenting 70% retention of initial capacity. After 10,000 charge/discharge cycles the composite was still electroactive with capacitance retained at 26 mF cm−2. Noteworthy, the composite electrode material exhibits satisfying stability
and durability in a cyclic charge/discharge process (after first 2500 cycles). The H-TiO2/pEDOT:Fehcf electrode exhibits 15 times higher specific capacitance than pure and nearly 8 times higher than hydrogenated titania. Moreover, from a practical point of view the thickness change of the material due to hybrid pEDOT:Fehcf deposition is negligible as it is equal to c.a. 3 nm [25]. The values of specific capacitances after 5000 cycles are listed in Table 1. It is worth noting that HTiO2/pEDOT:Fehcf layer exhibits higher specific capacitance than those reported by other authors for pristine, hydrogenated TiO2 nanotubes as well as for modified titania nanotubes of the similar geometry [3,9, 26–28]. If only the polymer matrix is regarded as an active material, as it is assumed by Xie et al. [29] or Dziewoński et al. [30], the gravimetric capacitance od H-TiO2/pEDOT:Fehcf equals 183 F/g. Finally, the H-TiO2/pEDOT:Fehcf electrode was tested using electrochemical impedance spectroscopy performed at the rest potential before the charge/discharge experiment. Proposed model consists of two Warburg impedance elements, thus diffusion impedance is characterized by at least two charge fluxes with different time constants. The same phenomenon was detected for pEDOT:Fehcf films electrodeposited on the Pt substrate [31] One may expect three diffusion elements due to the presence of i) TiO2 [32] ii) PEDOT matrix and iii) Prussian blue, however our simplified circuit provided satisfactory fitting results (χ2 = 8 × 10−5). The Warburg open element (Wo) present in the equivalent circuit is assigned to the impedance of finite-length diffusion with reflective boundary as presented by formula (Eq. (1)) [32]: h pffiffiffiffiffiffii W or Z Wo ðωÞ ¼ pffiffiffiffi ð1−jÞ coth W oc jω ω
ð1Þ
Impedance of constant phase element (CPE) is represented by: Z ðωÞ ¼ P −1 ðjωÞ−n
ð2Þ
Experimental and resulting from fitting procedure data are presented in Fig. 4. 4. Conclusions In summary, we have demonstrated that electrochemical modification of hydrogenated titania by organic matrix improves significantly the electrochemical performance of TiO2NT as promising electrode materials for supercapacitors. Among all fabricated materials, only the H-
Fig. 4. The registered (exp) and fitted (fit) impedance spectra for H-TiO2NTs/pEDOT:Fehcf recorded in contact with 0.5 M K2SO4 at the rest potential (E = +0.24 V vs. Ag/AgCl (0.1 M KCl)), geometric surface area equals to 0.3 cm2.
Please cite this article as: M. Szkoda, et al., Electrodes consisting of PEDOT modified by Prussian Blue analogues deposited onto titania nanotubes – Their highly improved capa..., Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.12.025
M. Szkoda et al. / Solid State Ionics xxx (2016) xxx–xxx
TiO2/pEDOT:Fehcf exhibits reversible redox activity Fe(II)/Fe(III) and is characterized by the highest specific capacitance whilst excellent rate capability and good cycle stability is preserved. The specific capacitance of TiO2/pEDOT:Fehcf material equals to 26 mF cm− 2 in 0.5 M K2SO4 electrolyte solution. This result is 15 times higher in comparison with capacitance of pure TiO2 nanotubes and c.a. 8 times higher than HTiO2 nanotubes. Proposed inorganic/organic nanostructure with a very high surface area and nanotubular morphology provides facilitated path for charge percolation between the electrode/electrolyte interface and electrode substrate, leading to the promoted reversible reaction in a charge/discharge processes. Because whole material is directly fabricated onto the current collector (Ti plate) and any additional mechanical deposition of electroactive layer is not required, proposed synthesis approach could be considered form the commercial point of view. These findings could open up the new opportunities for application of titania nanotubes on Ti support in high-performance supercapacitors as well as other energy storage devices. Acknowledgement This work received financial support from the Polish National Science Centre: Grant No. 2012/07/D/ST5/02269. Authors would like to acknowledge PhD Jakub Karczewski for SEM images. References [1] Y. Zhang, H. Feng, X. Wu, L. Wang, A. Zhang, T. Xia, H. Dong, X. Li, L. Zhang, Progress of electrochemical capacitor electrode materials: a review, Int. J. Hydrog. Energy 34 (2009) 4889–4899. [2] A. Lisowska-Oleksiak, M. Wilamowska, V. Jasulaitiene, Organic-inorganic composites consisted of poly(3,4-ethylenedioxythiophene) and Prussian Blue analogues, Electrochim. Acta 56 (2011) 3626–3632. [3] Y. Xie, H. Du, Electrochemical capacitance performance of polypyrrole-titania nanotube hybrid, J. Solid State Electrochem. 16 (2012) 2683–2689. [4] L.Z. Fan, J. Maier, High-performance polypyrrole electrode materials for redox supercapacitors, Electrochem. Commun. 8 (2006) 937–940. [5] X. Yang, L. Chi, C. Chen, X. Cui, Q. Wang, The nearly 100% filling of PEDOT in TiO2 nanotube array by a simple electropolymerization method, Phys. E Low-Dimens. Syst. Nanostruct. 66 (2015) 120–124. [6] K. Trzcinski, A. Lisowska-Oleksiak, Electrochemical characterization of a composite comprising PEDOT/PSS and N doped TiO2 performed in aqueous and non-aqueous electrolytes, Synth. Met. 209 (2015) 399–404. [7] A. Lisowska-Oleksiak, A.P. Nowak, Metal hexacyanoferrate network synthesized inside polymer matrix for electrochemical capacitors, J. Power Sources 173 (2007) 829–836. [8] A.P. Nowak, M. Wilamowska, A. Lisowska-Oleksiak, Spectroelectrochemical characteristics of poly(3,4-ethylenedioxythiophene)/iron hexacyanoferrate film-modified electrodes, J. Solid State Electrochem. 14 (2010) 263–270. [9] X.H. Lu, G.M. Wang, T. Zhai, M.H. Yu, J.Y. Gan, Y.X. Tong, Y. Li, Hydrogenated TiO2 nanotube arrays for supercapacitors, Nano Lett. 12 (2012). [10] M. Salari, S.H. Aboutalebi, K. Konstantinov, H.K. Liu, A highly ordered titania nanotube array as a supercapacitor electrode, Phys. Chem. Chem. Phys. 13 (2011) 5038–5041. [11] M. Salari, K. Konstantinov, H.K. Liu, Enhancement of the capacitance in TiO2 nanotubes through controlled introduction of oxygen vacancies, J. Mater. Chem. 21 (2011) 5128–5133.
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Please cite this article as: M. Szkoda, et al., Electrodes consisting of PEDOT modified by Prussian Blue analogues deposited onto titania nanotubes – Their highly improved capa..., Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.12.025
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Electrodes consisting of PEDOT modified by Prussian Blue analogues deposited onto titania nanotubes – Their highly improved capacitance M. Szkoda a,⁎, K. Trzciński a, J. Rysz c, M. Gazda d, K. Siuzdak b, A. Lisowska-Oleksiak a a
Faculty of Chemistry, Gdansk University of Technology, Narutowicza 11/12, 80-233 Gdansk, Poland Center for Plasma and Laser Engineering, The Szewalski Institute of Fluid Flow Machinery, Fiszera 14, 80-231 Gdansk, Poland M. Smoluchowski Institute of Physic, Jagiellonian University, Lojasiewicza 11, 30-348 Krakow, Poland d Department of Solid State Physics, Faculty of Applied Physics and Mathematics, Gdansk University of Technology, 80-233 Gdansk, Poland b c
a r t i c l e
i n f o
Article history: Received 30 July 2016 Received in revised form 21 December 2016 Accepted 23 December 2016 Available online xxxx Keywords: Titania nanotubes Capacitors Inorganic-organic heterojunction Specific capacitances
a b s t r a c t In this work we present the outstanding energy storage of prepared inorganic-organic heterojunction where hydrogenated ordered titania nanotubes (H-TiO2NT) were modified by the hybrid made of poly(3,4ethylenedioxythiophene) (pEDOT) and iron hexacyanoferrate centres (Fehcf, Prussian Blue). The material TiO2NT/pEDOT:Fechf was obtained electrochemically by means of: anodization, hydrogenation and finally, ions. Inorganic-organic hybrids were characterelectropolymerization of EDOT in the presence of Fe(CN)3−/4− 6 ized using Raman spectroscopy and secondary ion mass spectrometry (SIMS). The morphology of obtained materials was inspected using scanning electron microscopy (SEM). Electrodes were tested using cyclic voltammetry and galvanostatic charge/discharge cycles in an aqueous electrolyte. The characterization of capacitance was studied by means of multiple (up to 10,000) charge/discharge cycles with the current density of 0.45 mA cm−2. Electrode materials consisting of H-TiO2, pEDOT and Prussian Blue exhibited the highest capacitance of 26 mF cm−2 even after 10,000 cycles. Thus, the capacitance of TiO2NT/pEDOT:Fehcf was c.a. 15 and 8 times higher than the capacitance registered for pure and hydrogenated TiO2, respectively. © 2016 Published by Elsevier B.V.
1. Introduction Electrochemical supercapacitors, as energy storage devices, have attracted growing interest in recent years [1]. Capacitors in general make use of three main classes of materials: porous carbon, transition metal oxides and conducting polymers deposited onto various electrode collectors [2–4]. Porous carbon materials exhibit a relatively low electrochemical double-layer capacitance. Ruthenium oxide is characterized by a very high electrochemical pseudo-faradic capacitance, however its high cost hampers possible commercial application. Conductive polymers are characterized by a high electrical capacitance and cyclability, but unfortunately they exhibit a narrow potential range of electrochemical activity and stability. The extension of potential window and the improvement of specific capacitance can be achieved by fabrication of organic-inorganic heterojunction with TiO2 [5,6]. Additionally, increase in charge capacity can be achieved by introduction of redox inorganic network, i.e. Prussian Blue analogues into the polymer matrix [7,8]. ⁎ Corresponding author. E-mail address: [email protected] (M. Szkoda).
Highly ordered TiO2 nanotubes are promising as supercapacitor electrodes, because of the large specific surface area and direct pathway for charge transport in nanotubes as well as facile fabrication directly onto the charge collector substrate. Thus, utilization of TiO2 nanotubes obtained onto the Ti support as an electrode material might offer an opportunity to improve the capacitance of supercapacitors. However, it should be mentioned that the semiconducting nature of TiO2 limits the electrical conductivity and hinders specific capacitance in the charge–discharge process and therefore titania requires further modification for further application in energy storage devices [9]. The electrodeposition of polymer films with modified TiO2 proceeds more efficiently in comparison to pure TiO2 [6]. Here, studies on the capacitance properties of organic-inorganic heterojunction containing highly ordered titania nanotubes and Prussian Blue embedded into the pEDOT matrix is presented. All investigated electrodes were prepared via electrochemical methods that could be easily scaled up. The presence of each part of heterojunction was confirmed using Raman spectroscopy whereas SIMS technique allows the composition tracking from the material surface down to the base of titania. The obtained heterojunction containing TiO2, Prussian Blue and pEDOT was characterized using prolonged reversible polarization
http://dx.doi.org/10.1016/j.ssi.2016.12.025 0167-2738/© 2016 Published by Elsevier B.V.
Please cite this article as: M. Szkoda, et al., Electrodes consisting of PEDOT modified by Prussian Blue analogues deposited onto titania nanotubes – Their highly improved capa..., Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.12.025
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M. Szkoda et al. / Solid State Ionics xxx (2016) xxx–xxx
cycles, which allowed to demonstrate enhancement of the capacitance as compared to the pure and hydrogenated TiO2 nanotubes, as it was reported previously [9–11]. 2. Experimental 2.1. Preparation of H-TiO2 nanotubes Highly ordered titania nanotubes were prepared via a two-step electrochemical anodization of a Ti plate (Strem, 99.7%) in a fluoride-containing solution. The procedure that leads to the formation of TiO2 nanotubes was given previously [12]. Anodization process was realized in a two-electrode configuration with a cathode: (Pt grid) and anode (Ti plate) placed 2 cm from each other. Both steps of anodization were performed at temperature (23 °C), electrolyte composition (0.27 M NH4F in 1%/99% v/v water/ethylene glycol solution) and anodization parameters (40 V, 2 h). Before the second anodization step, the as-formed nanotubes during the first stage were removed by overnight etching in an oxalic acid solution (0.5% wt). To remove surface debris, the titanium plates covered with nanotubes were immersed in 0.05% wt HF for 180 s. The as prepared amorphous TiO2 nanotubes were transformed to anatase phase by thermal annealing performed at 450 °C for 2 h (heating rate: 2 °C min−1). The electrochemical hydrogenation process, similar to that proposed by Xu et al. [13], was performed in a three-electrode configuration at room temperature, using the crystallized TiO2 nanotubes as working electrode, Ag/AgCl as reference and platinum mesh as counter electrode. The hydrogenation was performed in 0.5 M K2SO4 aqueous solution under the cathodic potential of −1.5 V vs. Ag/AgCl (0.1 M KCl) applied for 20 s, charge consumed equals to 60 mC cm−2. Afterwards, the TiO2 nanotubes were taken out, washed with deionized water and dried in the air. The hydrogenation process activates the titania surface by lowering of the resistivity and facilitates monomer adsorption and nucleation, resulting in the efficient electropolymerization process [14]. 2.2. Fabrication of H-TiO2/pEDOT:Fehcf Electrochemical polymerization was carried out to obtain the inorganic-organic heterojunction H-TiO2NTs/pEDOT:Fehcf. The electrochemical deposition was performed according to the two stage procedure proposed in Ref. [15]. The first stage covers potentiostatic electropolymerization from an aqueous solution containing monomer EDOT in the presence of Fe(CN)3−/4− ions. As a working electrode flat 6 titanium plate or hydrogenated titania NTs layer were used. The
reference electrode was Ag/AgCl(0.1 M KCl) whereas a platinum mesh was used as a counter electrode. The potentiostatic electropolymerization was realized under the potential of 1.5 V vs. Ag/ AgCl(0.1 M KCl) and consuming the charge of 50 mC cm−2. For comparative studies, the H-TiO2/pEDOT:Cl, H-TiO2/Fehcf were also produced. Electrochemical depositions were performed in an aqueous solution containing 0.1 M NaCl and 0.001 M EDOT (in order to obtain H-TiO2/ pEDOT:Cl) or Fe(CN)36 −/4− [16] (in order to obtain H-TiO2/Fehcf), by potentiostatic polymerization of 1.5 V vs. Ag/AgCl(0.1 M KCl) consuming charge of 50 mC cm−2. 2.3. Apparatus The surface morphology and cross-section view were examined using the Schottky field emission scanning electron microscopy (FEI Quanta FEG 250) with an ET secondary electron detector. The Raman spectra were recorded by a confocal micro-Raman spectrometer (InVia, Renishaw) with sample excitation, by means of an argon ion laser emitting at 514 nm operating at 5% of its total power (50 mW). The crystal structure of TiO2 and H:TiO2 samples was also determined from X-Ray diffraction patterns (XRD), using X-ray diffractometer (Xpert PRO-MPD, Philips) with copper Kα radiation (λ = 1.5404 Å). The chemical composition of modified titania nanotubes was examined using Time of Flight Secondary Ion Mass Spectrometer (TOF SIMS). The TOF SIMS 5 (ION-TOF GmbH) working in dual beam mode was used to obtain composition versus depth profiles. Bi+ 30 keV ions and Cs+ 2 keV ions, both incident at 45° to the surface normal, were used as the analysis and sputter. The Cs+ sputter beam was rastered over 350 μm·350 μm while the 100 μm·100 μm central region of the sputter crater was analyzed with the Bi+ ions. Negatively charged secondary ions induced by Bi primary ions were collected using time of flight mass spectrometer. Intensities of characteristics signals versus sputtering time were analyzed using SurfaceLab 6 software (ION-TOF GmbH). To determine variations in vertical composition of the samples the following signals were chosen: C– (m/z = 12.00), 18O− (m/z = 17.99), CN− (m/z = 26.00), 34 − S (m/z = 33.97), Ti− (m/z = 47.95), Fe− (m/z = 55.94) and 46 TiO− 2 (m/z = 77.94). Electrochemical experiments (cyclic voltammetry – CV, chronopotentiometry – CP, electrochemical impedance spectroscopy – EIS) were performed using the AutoLab PGStat 302N potentiostatgalvanostat system (Methrom, Autolab) in a standard three-electrode assembly, where different working electrodes were tested: TiO2, HTiO2, H-TiO2/pEDOT:Cl, H-TiO2/Fehcf and H-TiO2/pEDOT:Fehcf with the geometric surface area of 0.3 cm2 but characterized the real surface
Fig. 1. Surface SEM image of TiO2, H-TiO2 and composite materials: H-TiO2/Fehcf, H-TiO2/pEDOT:Cl and H-TiO2/pEDOT:Fehcf and cross-section image of H-TiO2.
Please cite this article as: M. Szkoda, et al., Electrodes consisting of PEDOT modified by Prussian Blue analogues deposited onto titania nanotubes – Their highly improved capa..., Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.12.025
M. Szkoda et al. / Solid State Ionics xxx (2016) xxx–xxx
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Fig. 2. (a) Raman spectra of TiO2, H-TiO2 and composite materials, (b) The X-ray diffraction patterns of pristine and hydrogenated TiO2 and (c) SIMS composition versus depth profiles of hydrogenated titania nanotubes modified with pEDOT:Fehcf hybrid. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
area 120 times higher than the geometric one [17]. In order to estimate gravimetric capacitance, the mass of the electrode before and after pEDOT:Fehcf deposition was measured using automated-microbalance Metler-Tolledo XP6 characterized with readability of 1 μg. 3. Results and discussion 3.1. Morphology and structure studies In Fig. 1 the top-view and cross-section images of TiO2, H-TiO2 and different composite materials are presented. The pure and hydrogenated titania layers are composed of regular nanotubes with the internal radius of 50 nm, the wall thickness of 25 nm and the length of 2.5 μm. Thus, no difference was observed in the nano-architecture between the hydrogenated and pure titania proving any impact of the hydrogenation process onto the titania nanostructure. Other images show the layer of titania nanotubes modified by Fehcf, pEDOT:Cl and pEDOT:Fehcf, respectively. Comparing SEM images registered for hydrogenated titania and composite samples, one may observe the growth of polymer and Prussian Blue films mainly on the wall-edges of the tubes. The Raman spectroscopy measurements of electrode materials were carried out to confirm the crystalline phase of TiO2 and successful deposition of polymer films onto the titania layer. A number of bands characteristic for the pure anatase crystalline form of TiO2 were identified for pristine as well as for hydrogenated titania and composite material (see Fig. 2a) The maxima located at 144, 198, 395, 516 and 637 cm−1 are attributed to Eg(1), Eg(2), B1g, A1g, and Eg(3) active anatase modes, respectively [18,19]. Fig. 2b illustrates the XRD patterns of pristine and hydrogenated titania nanotubes. The strong XRD diffraction peaks indicate that titania nanotubes are present in a form of anatase for both, pristine
and hydrogenated samples and any typical peaks for rutile and brookite was detected. Peaks marked as “Ti” is a signal from the metallic substrate. No significant changes between patterns before and after modification were noticed. Similar behaviour was observed for hydrogenated titania in a form of powder [20]. Thus, changes in bulk material structure in both cases are negligible for XRD measurements. All Raman bands characteristic for pEDOT are observed in Fig. 2a (blue line). The main band at 1433 cm−1 can be attributed to symmetric (C_C)\\O bond vibrations in the thiophene ring. Two signals at 1495 cm−1 and 1561 cm−1 could be interpreted as asymmetric C_C modes in polymer chains [21]. Raman signals registered at 1267 cm−1 and 1366 cm−1 are attributed to single bonded carbons in both thiophene ring and polymer chain [22]. Furthermore, the oxyethylene ring deformations can be identified at 988, 571, and 442 cm−1 [23]. Nevertheless, no clear signal attributed to the C`N arrangement was observed. Therefore, SIMS depth profiles were recorded to indicate Prussian Blue presence and to determine whenever the organic component penetrate uniformly into the titania nanotube matrix. Fig. 2c shows the variation of different signals characteristic for: i) titania nanotubes 18 − (46TiO− O ); ii) pEDOT (34S− and also C−); iii) Prussian 2 and also − Blue species (Fe and CN−) with sputtering time (reflecting increasing distance from original film surface), iv) titania NT and Ti substrate (Ti−). After some variation close to the surface (c.a. first 1000 s of sputtering time) that is related with the interaction of ions with the outer layer of the sample, both the Fe− and CN− as well as C−, and 34S− signals become almost constant. Since 4500 s, the sharp falling edge of the 46TiO− 2 and 18O− signals at the Ti substrate is a sign that the height distribution of the nanotubes is very narrow. As it could be observed, when the decrease of signals attributed to titania appears, simultaneous fall of the signals assigned to the pEDOT:Fehcf matrix are also registered. Thus, obtained composition versus depth profiles confirm that pEDOT with
Fig. 3. (a) The cyclic voltammetry curves of pure and hydrogenated titania, and composite materials (electrolyte: 0.5 M K2SO4, v = 50 mV s−1), (b) Specific capacitance performance of materials with respect to the cycle number. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Please cite this article as: M. Szkoda, et al., Electrodes consisting of PEDOT modified by Prussian Blue analogues deposited onto titania nanotubes – Their highly improved capa..., Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.12.025
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Table 1 The comparison of specific areal capacitances (C) for each sample estimated after 5000 charge/discharge cycles. Sample
C/mF cm−2
Sample
C/m cm−2
H-TiO2NTs/pEDOT:Fehcf H-TiO2NTs/pEDOT:Cl H-TiO2NTs/Fehcf
26.1 7.9 5.1
H-TiO2 NTs TiO2 NTs Ti/pEDOT:Fehcf
3.3 1.8 0.7
imbedded Prussian Blue species are available at the same level at surface of the TiO2 nanostructures as well as along the tubes down to the base of titania NT. Therefore, proposed electrochemical synthesis method allows for the uniform modification of hydrogenated TiO2 by polymeric material containing redox centres. 3.2. Electrochemical properties The cyclic voltammetry measurements of TiO2, H-TiO2 and composite electrodes have been carried out to investigate their electrochemical properties (Fig. 3a). Detailed discussion on TiO2 and H-TiO2 electrochemical properties was given previously [14]. Titania nanotubes electrodes modified with i) Fehcf, ii) pEDOT:Cl and iii) pEDOT:Fehcf are characterized with higher charging current comparing to pure and hydrogenated TiO2NT samples. Furthermore, when polymer matrix with embedded Fehcf network was deposited onto titania tubular surface, the reversible redox peaks are observed at the formal potential of +0.17 V vs. Ag/AgCl (0.1 M) KCl. The observed redox activity is attributed to the Prussian White to Prussian Blue transformation, based on high spin iron centre activity (Fe coordinated via nitrogen atoms), as shown for pEDOT:Fehcf hybrid deposited onto the Pt disc electrode [24]. Further anodic polarization leads to the current increase (see orange line) due to low spin iron centre Fe(II)/Fe(III) faradaic reaction. This one is related to Prussian Blue to Berlin Green transition. The performance of specific capacitance with respect to the charge/ discharge cycle numbers is depicted in Fig. 3b. The values of the capacitance were calculated from galvanostatic charge/discharge measurements (ja = jc = 0.45 mA cm2, ΔE = 1.6 V). The specific capacitance of H-TiO2/pEDOT:Fehcf declines gradually from 40 down to 28 mF cm−2 during the first 2500 cycles, presenting 70% retention of initial capacity. After 10,000 charge/discharge cycles the composite was still electroactive with capacitance retained at 26 mF cm−2. Noteworthy, the composite electrode material exhibits satisfying stability
and durability in a cyclic charge/discharge process (after first 2500 cycles). The H-TiO2/pEDOT:Fehcf electrode exhibits 15 times higher specific capacitance than pure and nearly 8 times higher than hydrogenated titania. Moreover, from a practical point of view the thickness change of the material due to hybrid pEDOT:Fehcf deposition is negligible as it is equal to c.a. 3 nm [25]. The values of specific capacitances after 5000 cycles are listed in Table 1. It is worth noting that HTiO2/pEDOT:Fehcf layer exhibits higher specific capacitance than those reported by other authors for pristine, hydrogenated TiO2 nanotubes as well as for modified titania nanotubes of the similar geometry [3,9, 26–28]. If only the polymer matrix is regarded as an active material, as it is assumed by Xie et al. [29] or Dziewoński et al. [30], the gravimetric capacitance od H-TiO2/pEDOT:Fehcf equals 183 F/g. Finally, the H-TiO2/pEDOT:Fehcf electrode was tested using electrochemical impedance spectroscopy performed at the rest potential before the charge/discharge experiment. Proposed model consists of two Warburg impedance elements, thus diffusion impedance is characterized by at least two charge fluxes with different time constants. The same phenomenon was detected for pEDOT:Fehcf films electrodeposited on the Pt substrate [31] One may expect three diffusion elements due to the presence of i) TiO2 [32] ii) PEDOT matrix and iii) Prussian blue, however our simplified circuit provided satisfactory fitting results (χ2 = 8 × 10−5). The Warburg open element (Wo) present in the equivalent circuit is assigned to the impedance of finite-length diffusion with reflective boundary as presented by formula (Eq. (1)) [32]: h pffiffiffiffiffiffii W or Z Wo ðωÞ ¼ pffiffiffiffi ð1−jÞ coth W oc jω ω
ð1Þ
Impedance of constant phase element (CPE) is represented by: Z ðωÞ ¼ P −1 ðjωÞ−n
ð2Þ
Experimental and resulting from fitting procedure data are presented in Fig. 4. 4. Conclusions In summary, we have demonstrated that electrochemical modification of hydrogenated titania by organic matrix improves significantly the electrochemical performance of TiO2NT as promising electrode materials for supercapacitors. Among all fabricated materials, only the H-
Fig. 4. The registered (exp) and fitted (fit) impedance spectra for H-TiO2NTs/pEDOT:Fehcf recorded in contact with 0.5 M K2SO4 at the rest potential (E = +0.24 V vs. Ag/AgCl (0.1 M KCl)), geometric surface area equals to 0.3 cm2.
Please cite this article as: M. Szkoda, et al., Electrodes consisting of PEDOT modified by Prussian Blue analogues deposited onto titania nanotubes – Their highly improved capa..., Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.12.025
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TiO2/pEDOT:Fehcf exhibits reversible redox activity Fe(II)/Fe(III) and is characterized by the highest specific capacitance whilst excellent rate capability and good cycle stability is preserved. The specific capacitance of TiO2/pEDOT:Fehcf material equals to 26 mF cm− 2 in 0.5 M K2SO4 electrolyte solution. This result is 15 times higher in comparison with capacitance of pure TiO2 nanotubes and c.a. 8 times higher than HTiO2 nanotubes. Proposed inorganic/organic nanostructure with a very high surface area and nanotubular morphology provides facilitated path for charge percolation between the electrode/electrolyte interface and electrode substrate, leading to the promoted reversible reaction in a charge/discharge processes. Because whole material is directly fabricated onto the current collector (Ti plate) and any additional mechanical deposition of electroactive layer is not required, proposed synthesis approach could be considered form the commercial point of view. These findings could open up the new opportunities for application of titania nanotubes on Ti support in high-performance supercapacitors as well as other energy storage devices. Acknowledgement This work received financial support from the Polish National Science Centre: Grant No. 2012/07/D/ST5/02269. Authors would like to acknowledge PhD Jakub Karczewski for SEM images. References [1] Y. Zhang, H. Feng, X. Wu, L. Wang, A. Zhang, T. Xia, H. Dong, X. Li, L. Zhang, Progress of electrochemical capacitor electrode materials: a review, Int. J. Hydrog. Energy 34 (2009) 4889–4899. [2] A. Lisowska-Oleksiak, M. Wilamowska, V. Jasulaitiene, Organic-inorganic composites consisted of poly(3,4-ethylenedioxythiophene) and Prussian Blue analogues, Electrochim. Acta 56 (2011) 3626–3632. [3] Y. Xie, H. Du, Electrochemical capacitance performance of polypyrrole-titania nanotube hybrid, J. Solid State Electrochem. 16 (2012) 2683–2689. [4] L.Z. Fan, J. Maier, High-performance polypyrrole electrode materials for redox supercapacitors, Electrochem. Commun. 8 (2006) 937–940. [5] X. Yang, L. Chi, C. Chen, X. Cui, Q. Wang, The nearly 100% filling of PEDOT in TiO2 nanotube array by a simple electropolymerization method, Phys. E Low-Dimens. Syst. Nanostruct. 66 (2015) 120–124. [6] K. Trzcinski, A. Lisowska-Oleksiak, Electrochemical characterization of a composite comprising PEDOT/PSS and N doped TiO2 performed in aqueous and non-aqueous electrolytes, Synth. Met. 209 (2015) 399–404. [7] A. Lisowska-Oleksiak, A.P. Nowak, Metal hexacyanoferrate network synthesized inside polymer matrix for electrochemical capacitors, J. Power Sources 173 (2007) 829–836. [8] A.P. Nowak, M. Wilamowska, A. Lisowska-Oleksiak, Spectroelectrochemical characteristics of poly(3,4-ethylenedioxythiophene)/iron hexacyanoferrate film-modified electrodes, J. Solid State Electrochem. 14 (2010) 263–270. [9] X.H. Lu, G.M. Wang, T. Zhai, M.H. Yu, J.Y. Gan, Y.X. Tong, Y. Li, Hydrogenated TiO2 nanotube arrays for supercapacitors, Nano Lett. 12 (2012). [10] M. Salari, S.H. Aboutalebi, K. Konstantinov, H.K. Liu, A highly ordered titania nanotube array as a supercapacitor electrode, Phys. Chem. Chem. Phys. 13 (2011) 5038–5041. [11] M. Salari, K. Konstantinov, H.K. Liu, Enhancement of the capacitance in TiO2 nanotubes through controlled introduction of oxygen vacancies, J. Mater. Chem. 21 (2011) 5128–5133.
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Please cite this article as: M. Szkoda, et al., Electrodes consisting of PEDOT modified by Prussian Blue analogues deposited onto titania nanotubes – Their highly improved capa..., Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.12.025