ISTANBUL TECHNICAL UNIVERSITY GRADUATE SCHOOL OF SCIENCE ENGINEERING AND TECHNOLOGY
Ph.D. THESIS
Department of Polymer Science and Technology Polymer Science and Technology Programme
INVESTIGATION OF CAPACITIVE BEHAVIOUR OF EMULSION POLYMERIZED PEDOT AND ITS NANOCOMPOSITES
Deniz GÜLERCAN
ISTANBUL TECHNICAL UNIVERSITY GRADUATE SCHOOL OF SCIENCE ENGINEERING AND TECHNOLOGY
Deniz GÜLERCAN (515102006)
Department of Polymer Science and Technology Polymer Science and Technology Programme
Thesis Advisor: Prof. Dr. A. Sezai SARAC
JUNE 2019
INVESTIGATION OF CAPACITIVE BEHAVIOUR OF EMULSION POLYMERIZED PEDOT AND ITS NANOCOMPOSITES
İSTANBUL TEKNİK ÜNİVERSİTESİ FEN BİLİMLERİ ENSTİTÜSÜ
EMÜLSİYON POLİMERİZASYONU İLE SENTEZLENEN PEDOT VE NANOKOMPOZİTLERİNİN KAPASİTİF ÖZELLİKLERİNİN İNCELENMESİ
Polimer Bilim ve Teknolojisi Anabilim Dalı Polimer Bilim ve Teknolojisi Programı
Deniz GÜLERCAN (515102006) DOKTORA TEZİ
Tez Danışmanı: Prof. Dr. A. Sezai SARAÇ
İSTANBUL TEKNİK ÜNİVERSİTESİ FEN BİLİMLERİ ENSTİTÜSÜ
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Prof. Dr. Hayal Bülbül Sönmez ... Gebze Technical University
Dr. Serkan ÜNAL ... Sabancı University
Thesis Advisor: Prof. Dr. A. Sezai SARAÇ ... Istanbul Technical University
Jury Members: Prof. Dr. İ. Ersin SERHATLI ... Istanbul Technical University
Prof. Dr. Cemal ÖZEROĞLU ... Istanbul University
Date of Submission : 31.05.2019 Date of Defence : 24.06.2019
Deniz GULERCAN, a Ph.D. student of ITU Graduate School of Science Engineering and Technology student ID 515102006, successfully defended the thesis/dissertation entitled “INVESTIGATION OF CAPACITIVE BEHAVIOUR OF EMULSION POLYMERIZED PEDOT AND ITS NANOCOMPOSITES”, which she prepared after fulfilling the requirements specified in the associated legislations, before the jury whose signatures are below.
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“Once you stop learning, you start dying.” Albert Einstein
ix FOREWORD
I would like to thank my advisor, Prof. Dr. A. Sezai SARAC, for his guidance, continuous encouragement throughout this work, and discussions during my Ph.D. studies.
I am grateful to Prof. Dr. İ. Ersin SERHATLI, Prof. Dr. Cemal ÖZEROGLU and Dr. Qiao CHEN for their invaluable help, good suggestions and supports.
I would like to thank to Selin GÜMRÜKÇÜ, Ezgi İŞMAR SIR, Aslı GENÇTÜRK, İlknur GERGİN, and Daniel COMMANDEUR for their collaboration and their friendship.
I would like to thank my brother, Hasan Onur TOPRAK for all his help. I would like to thank my husband Mustafa GÜLERCAN for his special helps and encouragment during preparation of my thesis. If I ever lost interest, my husband kept me motivated. I would like to give my special thanks to my son Çağan GÜLERCAN, his incredible smiles always relieve my tiredness.
Finally, I would like to offer the most gratitude to my mum for her patience, understanding, moral support and encouragement during all stages in the preparation of this PhD study, her wise counsel and kind words have served me well as always.
June 2019 Deniz GÜLERCAN (Chemist MSc)
xi TABLE OF CONTENTS Page FOREWORD……… ix TABLE OF CONTENTS……… xi ABBREVIATIONS……… xiii LIST OF SYMBOLS……… xv
LIST OF TABLES……… xvii
LIST OF FIGURES……… xix
SUMMARY……… xxiii
ÖZET………...xxvii
1. INTRODUCTION……… 1
1.1 Purpose of Thesis ... …….8
2.1 Experimental ... 13
2.1.2.1 Synthesis of P(AN-co-St) nanoparticles……….14
2.1.2.2. Synthesis of P(AN-co-St)/PEDOT nanoparticles………..14
2.2 Results and Discussion ... 15
2.2.2 UV-Visible spectrophotometric analysis………..17
2.2.3. Morphological characterization of nanoparticles ... 21
2.2.3.1. AFM studies ... 21
2.2.3.2. SEM studies………...21
2.2.4. Electrochemical impedance spectroscopy and equivalent circuit modelling (EIS) ... 24
2.3 Conclusions ... 29
3. PREPARATION AND ELECTROCHEMICAL PERFORMANCES OF GRAPHENE OXIDE /PEDOT AND REDUCED GRAPHENE OXIDE /PEDOT NANOFIBERS AND NANOCOMPOSITES …...31
3.1 Experimental ... 33
3.1.1 Materials ... 33
3.1.2 Preparation of graphene oxide and reduced graphene oxide ... 34
3.1.3 Preparation of a GO-PEDOT and rGO-PEDOT nanocomposites ... 34
3.1.4 Preparation of electrospinning solutions for nanofibers ... 34
3.2 Results and Discussion ... 36
3.2.1 Spectral characterization ... 36
3.2.3 Electrochemical impedance spectroscopy ... 40
3.3 Conclusions ... 46
4. A TERNARY PEDOT-TiO2-REDUCED GRAPHENE OXIDE NANOCOMPOSITE FOR SUPERCAPACITOR APPLICATIONS………..47
4.1 Experimental ... 48
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4.1.2 Synthesis of PEDOT and PEDOT-TiO2 nanoparticles ... 49
4.1.3 Preparation of PEDOT- TiO2-rGO composites ... 49
4.1.4 Structural characterization... 50
4.2 Results and Discussion ... 51
4.2.1 UV-Vis spectrophotometric studies ... 51
4.2.2 ATR-FTIR studies ... 51
4.2.3 XRD studies ... 52
4.2.4 Morphology and surface area characterization ... 53
4.2.5 Electrochemical characterization ... 56
4.2.5.1 Cyclic voltammetry studies ... 57
4.2.5.2 Galvanostatic charge–discharge behaviour ... 60
4.2.5.3 Electrochemical impedance spectroscopy ... 62
4.3 Conclusions ... 67
5.CONCLUSIONS AND RECOMMENDATIONS………...69
REFERENCES……… 73
APPENDICES………..87
APPENDIX A: ... 88
xiii ABBREVIATIONS
AC : Alternating Current AFM : Atomic Force Microscope APS : Ammonium persulfate
ATR-FTIR : Attenuated Total Reflectance Fourier Transform Infrared BET : Brunauer-Emmett-Teller
CP : Conducting Polymer CPE : Constant Phase Element CV : Cyclic Voltammetry DMF : Dimethyl formamide
EIS : Electrochemical Impedance Spectroscopy EDOT : Ethylenedioxythiophene
FTO : Flourinated Tin Oxide GO : Graphene Oxide ITO : Indium Tin Oxide PAN : Polyacrylonitrile
PEDOT : Poly (3,4-ethylenedioxythiophene) RE : Reference Electrode
rGO : Reduced Graphene Oxide PTh : Polythiophene
SCE : Saturated Calomel Electrode SDS : Sodium Dodecyl Sulfate
SEM : Field Emmision Scannin Electron Microscopy St : Styrene
UV-Vis : Ultraviolet visible XRD : X-ray Diffraction
xv LIST OF SYMBOLS
C : Capacitance
Csp : Specific capacitance Cdl : Double Layer Capacitance D : Diffusion constant
F : Faraday Constant R : Resistance
Rs : Solution resistance
Rct : Charge Transfer Resistance W : Wardburg resistance Q : Constant Phase Element v : Scan rate
xvii LIST OF TABLES
Page Table 3.1 : Fitting values for the equivalent circuit elements by simulation of the
impedance spectra of nanofibers composites ………... ………..51 Table 4.1 : Surface area and adsorption pore range percentage of PEDOT, TiO2,
PEDOT-TiO2-10, PEDOT-TiO2-15 and PEDOT-TiO2-20. ... 56
Table 4.2 : Fitting values for the equivalent circuit elements by simulation of the impedance spectra of PEDOT and PEDOT nanocomposites. ... 63 Table 4.3 : Summary of specific capacitance values calculated from three different methods of measurement, CV (20 mV s-1), GCD (0.1 mA cm-2) and EIS. 66
xix LIST OF FIGURES
Page
Figure 1.1 : The distribution of renewable energy sources with respect to 2015. ... 2
Figure 1.2 : Polymerization mechanism of 3,4-ethylenedioxythiophene. ... 3
Figure 1.3 : Chemical structure of graphene. ... 5
Figure 1.4 : Electrochemical cell of a) two and b) three electrode configuration. ... 5
Figure 2.1 : Schematic diagram for the PEDOT on the P(AN-co-St) nanoparticles...15
Figure 2.2 : In situ ATR-FTIR spectra of P(AN-co-St)/PEDOT nanoparticles at various time intervals of polymerization. ... 16
Figure 2.3 : Absorbance-time curve for asymmetric stretching of C=C and inter-ring stretching of C-C at 1368 cm-1 for P(AN-co-St)/PEDOT nanoparticles, inset graph shows asymmetric stretching of C=C and inter ring stretching of C-C at 1368 cm-1 at various time intervals. ... 17
Figure 2.4 : Absorbance-time curve for stretching and bending at 1488 cm-1 at various time intervals, inset graph shows stretching and bending of sp3 of C-H in PSt. ... 18
Figure 2.5 : ATR-FTIR spectra of P(AN-co-St) and P(AN-co-St)/PEDOT from their solids form, inset graph shows the 800-1700 cm-1 region. ... 19
Figure 2.6 : Comparasion of absorbance results of co-St) and P(AN-co-St)/PEDOT nanoparticles. ... 19
Figure 2.7 : UV-Visible spectra, obtained at different times indicated, corresponding to aqueous micelleous P(AN-co-St)/PEDOT solution. ... 20
Figure 2.8 : Change of absorbance values at 253, 580 and 770 nm with time for P(AN-co-St)/PEDOT aqueous miscellous samples. ... 20
Figure 2.9 : AFM image of P(AN-co-St)/PEDOT nanoparticles 1.03x1.03 µm area on mica glass surface, at 360 min. ... 21
Figure 2.10 : AFM image of P(AN-co-St)/PEDOT nanoparticles 1.03x1.03 µm area on mica glass surface, at 1440 min. ... 22
Figure 2.11 : AFM image of P(AN-co-St)/PEDOT nanoparticles 1.03x1.03 µm area on mica glass surface, at 1694 min. ... 22
Figure 2.12 : Size of selected nanoparticles from AFM image at 1440 min. of polymerization of P(AN-co-St)/PEDOT measured by image programme. ... 23
Figure 2.13 : (a) SEM image of P(AN-co-St) matrix nanoparticles with 1 µm, at 100,000 x magnification (b) SEM image of P(AN-co-St) nanoparticles with 500 nm scale bar at 200,000 x magnification. ... 23 Figure 2.14 : (a) SEM image of P(AN-co-St)/PEDOT nanoparticles with 1 µm at 100,000 x magnification. (b) SEM image of P(AN-co-St)/PEDOT nanoparticles with 500 nm scale bar at 200,000 x magnification. 24
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Figure 2.15 : Schematic diagram of electrode fabrication process and equivalent
electrical circuit model for the simulation of the EIS spectra of PEDOT covered P(AN-co-St) nanoparticles. ... 25
Figure 2.16 : (a) The Nyquist plot, (b) The Bode Magnitude plot for different time intervals for the P(AN-co-St)/PEDOT. ... 27 Figure 2.17 (continued): (c) The Bode Phase plot for different time intervals for the P(AN-co-St)/PEDOT. ... 28 Figure 2.18 : Correlation between absolute impedance IZI and Roughness as a function of time for P(AN-co-St)/PEDOT core shell nanoparticles formation. ... 28 Figure 3.1 : Scheme of preparation of GO-PEDOT and rGO-PEDOT nanocomposites.
... 35 Figure 3.2 : ATR-FTIR spectra of (a) GO, (b) rGO-PEDOT, (c) GO-PEDOT,
(d) PEDOT, and (e) rGO. ... 36 Figure 3.3 : UV-vis spectra of (a) GO sheets, PEDOT and GO-PEDOT nanoparticles (b) rGO sheets, PEDOT and rGO-PEDOT nanoparticles. ... 37 Figure 3.4 : Interaction mechanism of GO during the in-situ polymerization of EDOT monomer. ... 38 Figure 3.6 : AFM image of (a) GO sheets in 2.17 µm (b) PEDOT nanoparticles in 1.00 µm scale (c) GO-PEDOT nanocomposite in 1.16 µm magnification. ... 39 Figure 3.7 : SEM micrographs and the diameter distribution of nanofibers (a)
PEDOT/P(AN-co-St), (b) GO-PEDOT/P(AN-co-St), and (c) rGO-PEDOT/P(AN-co-St). ... 41
Figure 3.8 : Scheme of preparation of nanofibers via electrospinning method. ... 42 Figure 3.9 : Nyquist Plots of nanofibers indicated with measured and calculated data from the model with 0.01Hz to 100 kHz. (inset: Nyquist Plots of nanofibers indicating with measured and calculated data from the model with high frequencies). ... 43 Figure 3.10 : Bode phase plots of nanofibers composites indicated with measured and calculated data from the model (inset shows the electrochemical circuit equivalent model). ... 45 Figure 3.11: Bode magnitude plots of nanofibers composites indicating with measured and calculated data from the model. ... 45 Figure 4.1 : Schematic representation of preparation of PEDOT-TiO2 particles and PEDOT- TiO2-rGO electrodes. ... 50 Figure 4.2 : UV-vis spectra of (a) TiO2-15 (b) TiO2-10 (c)
PEDOT-TiO2-5 and (d) PEDOT composites. ... 52 Figure 4.3 : ATR-FTIR spectra for (a) PEDOT-TiO2-5 (b) PEDOT-TiO2-10 (c) PEDOT-TiO2-15 and (d) PEDOT samples. ... 53 Figure 4.4 : X Ray diffraction pattern for PEDOT and PEDOT–TiO2 composites. ... 54 Figure 4.5 : SEM images of (a) PEDOT, (b) TiO2, (c) rGO, (d) PEDOT- TiO2
-10-rGO, (e) PEDOT- TiO2-15-rGO and (f) PEDOT-TiO2-20-rGO nanocomposites. ... 55 Figure 4.6 : (a) and (c) Cyclic voltammetry (CV) curves of PEDOT-TiO2 and PEDOT-TiO2-rGO composites with various weight content of TiO2 as the
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electrode materials, obtained at scan rate of 60 mV s−1. (b) and (d) Cyclic voltammetry curves for PEDOT-TiO2-15 and PEDOT-TiO2-15-rGO at various scan rates. ... 59 Figure 4.7 : Curves of current density versus the square root of scan rate for
PEDOT, PEDOT-TiO2 and PEDOT-TiO2-15-rGO at various scan rates. 60 Figure 4.8 : Specific mass capacitances of PEDOT-rGO and PEDOT-TiO2-rGO composites at various scan rates from CV measurements. ... 61 Figure 4.9 : a) Galvanic charge–discharge curves performed by maintaining the current density at 0.1 mA g-1 b) Galvanostatic charge/discharge curves of PEDOT-TiO2-15-rGO at 0.1, 1 and 10 mA g-1 in 1M Na2SO4. ... 62 Figure 4.10 : Schematic presentation of double layer at the electrode–electrolyte interface for prepared system. ... 63 Figure 4.11 : Nyquist plots of the PEDOT- TiO2 and PEDOT- TiO2-rGO composites indicated with measured and calculated data from the model with 0.1 Hz to 10 kHz. a) PEDOT, PEDOT- TiO2, PEDOT- TiO2-5, PEDOT-TiO2-10, PEDOT- TiO215; b) PEDOT-rGO, PEDOT- TiO2-5 -rGO, PEDOT-TiO2-10 -rGO and PEDOT-TiO2-15 -rGO; c) The equivalent circuit model for PEDOT-TiO2-rGO composites. ... 65 Figure 4.12 : Specific capacitance by area and Rct relation with amount (wt%) of TiO2 in PEDOT-TiO2-rGO composites. ... 66
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INVESTIGATION OF CAPACITIVE BEHAVIOUR OF EMULSION POLYMERIZED PEDOT AND ITS NANOCOMPOSITES
SUMMARY
Since the end of the last century, although renewable energy resources depleted, demand to energy supplies for electronic and hybrid units increases. Therefore, materials that have good conductivity and mechanical properties have attracted researchers’ attention to store energy. For energy storage, batteries are not highly efficient comparing with supercapacitors as they have two energy storage mechanisms, one is electrical double-layer (EDL) capacitance and the other is pseudo capacitance.
Over the last decades, conducting polymers and their composites have attracted many interests due to their applications in various disciplines. Among the conducting polymers, poly(3,4-ethylenedioxythiophene) (PEDOT) has more advantageous properties than other conducting polymers because of their low oxidation potential and band gap width and good stability in their oxidized states. After the invention of Graphene in 2004, it has received many researchers’ attraction because of its good conductivity, high surface area to volume ratio and good mechanical properties. Therefore, graphene-based composites have become good candidates for supercapacitor applications.
To produce materials with high efficiency and lower cost, TiO2 has been used
together with conducting polymers in many researches. TiO2 has many applications
including solar cells, dye sensitive solar cells (DSSC), capacitors and photovoltaic cells.
Stability of supercapacitor materials during charging/discharging process is a critical point that needs to be developed. Although carbon based materials such as activated carbon and carbon nanotubes have enough stability, their capacitance values are limited.
Therefore, composites including carbon based materials or metal oxides with conducting polymers have been improved to obtain more stable and capacitive supercapacitors. For this purpose, different types of composites were fabricated including PEDOT as a conducting polymer.
In this thesis, there are three main parts, the excluding introduction part, in order to follow step by step how to produce composite electrodes. In the first part, PEDOT polymer was synthesized via micro emulsion polymerization method in the aqueous media of P(AN-co-St) and polymerization monitored by time and samples were taken from polymerization media in certain time intervals. Then, these samples were coated on glass electrode, during polymerization. Nanoparticles characterized by UV-vis and FTIR spectrophotometry, SEM and AFM methods Afterwards, electrochemical properties of fabricated electrodes were characterized by
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electrochemical impedance spectroscopy (EIS). The electrochemical impedance spectroscopy data were well fitted with Electrical Circuit Model of R(C(R(Q(R))))(CR).
The results indicated that in situ emulsion polymerized PEDOT/P(AN-co-St) core shell structures have spherical, 25-65 nm sized nanoparticles and this method improved morphological features of the PEDOT nanocomposites.
In the second part of the study, synthesized graphene oxide and reduced graphene oxide were added into in situ emulsion polymerization system of PEDOT to improve capacitance of PEDOT. Nanofibers have been fabricated using P(AN-co-St) as a carrier latex by using electrospinning technique to prepare light, flexible and high surface area/volume ratio of as-synthesized PEDOT-GO and PEDOT-rGO composites. Nanofibers have been characterized by EIS and SEM. SEM images showed that beadless and with diameter range less than 300 nm nanofibers were formed. Capacitance features were analysed by EIS and the results exhibited that integration of both GO and rGO to PEDOT has improved PEDOT capacitance value. However, comparing with PEDOT-rGO, PEDOT-GO showed more capacitive behaviour. It can be explained with solubility characteristics of GO and rGO, GO has more polar groups (hydroxy, carbonyl, epoxy etc.) attached to carbon atoms whereas rGO just has a few polar groups on the single layer of carbon atoms. Therefore, GO was dispersed in aqueous emulsion media better than rGO. Moreover, PEDOT polymer has polar groups in the ring that can interact with GO and it has been proved that GO acts as a dopant during polymerization in the literature. As a consequence of this, PEDOT and GO exhibited good synergy comparing with rGO and PEDOT-GO composite was obtained more uniform and nanofibers were more capacitive.
In the third part of the study, to prepare low cost, more capacitive and reproducible electrodes in a one-pot step polymerization, TiO2 was integrated to in situ
polymerization system of EDOT, using same conditions, surfactant and initiator. This time, in order to overcome the low solubility problem of rGO and increase capacitance of system, rGO and GO (in DMF solutions) were blended with PEDOT-TiO2 composites when the polymerization completed. PEDOT-TiO2 nanocomposites
were prepared in various compositions while the PEDOT amount kept constant. For the composites, four different concentrations of TiO2 (5, 10, 15, 20 w%) were added
to surfactant solution to synthesize composites, based on previous literature studies. When the PEDOT-TiO2 nanocomposites were synthesized, they were pyhsically
blended with GO and rGO separately in 1:1 w% ratio. After that, PEDOT-TiO2-GO
and PEDOT-TiO2-rGO ternary composites were obtained for various amount of
TiO2. To investigate electrochemical features of composites, they were dispersed in
DMF and ultrasonicated during 1 h and then the electrodes were fabricated via drop casting method onto FTO. Afterwards, they were kept in the oven 90°C- for 30 min to evaporate their solvent. For electrochemical studies CV (Cyclic voltammetry), GCD (Galvanostatic Charging Discharging) and Electrochemical Impedance Spectroscopic (EIS) measurements were applied to electrodes in three electrodes system. From the impedance results, PEDOT and its composites have semi arc at high frequency region and distorted warburg line at intermediate frequency were obtained as has been mentioned in several studies. These features have been arised from the interaction between electrolyte and electrode surface, non-uniformity of the surface of the coated material and charge diffusion along with redox reactions. Moreover, thickness of the as-prepared films played an important role for the results.
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It has been deduced that TiO2 has a synergestic effect which can facilitate electron
transportation from PEDOT to rGO/GO in the ternary composite system, and thus reduce both the internal resistance (Rs) and charge transfer resistance (Rct) of the
electrode materials.
GCD results exhibit that PEDOT-TiO2-15-rGO has represented most specific
capacitance (Csp value which was 18.9 F.g-1 at 0.1 mA cm-2 current density).
PEDOT-TiO2-15-rGO, presenting a 92% enhancement over PEDOT-rGO at a 0.1
mA cm-2 current density. CV results well agreed with both GCD and EIS results and Csp value has been found as 31.7 F.g-1 at 0.1 mA cm-2 current density for
PEDOT-TiO2-15-rGO.
It can also have been correlated with BET (Brunauer–Emmett–Teller) results, because pore size of materials which coated on FTO electrodes have influence on charge transport from solution to electrode and PEDOT-TiO2-15 composites which
have pore size 25.7 nm whereas PEDOT-TiO2-20 composites have 4.93 nm and
PEDOT has 11.9 nm.
It can be concluded that, the contribution of TiO2 with rGO promote the capacitance
of PEDOT polymer and thus PEDOT-TiO2-15-rGO could be used as a
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EMÜLSİYON POLİMERİZASYONU İLE SENTEZLENEN PEDOT VE NANOKOMPOZİTLERİNİN KAPASİTİF ÖZELLİKLERİNİN
İNCELENMESİ ÖZET
Son yıllarda iletken polimerlerin sentezlenmesi ve bunların uygulamalarına yönelik çalışmalar oldukça ilgi görmektedir. Bu malzemelerin hem iletkenlik göstermesi hem de organik yapıya sahip olmalarından dolayı, enerji depolama, biyolojik sensör, güneş pilleri gibi birçok uygulamada tercih edilmesini beraberinde getirmiştir. Polianilin, politiyofen, polipirol ve poli (3,4-etilendioksitiyofen) (PEDOT) gibi iletken polimerler bu alandaki uygulamalarda daha çok kullanılanlar arasındadır. Bunun sebebi bu polimerler genelikle p-katkılandırılmış ve bundan dolayı n-katkılandırılmış polimerlere göre daha kararlı bir yapıya sahip olmalıdır.
İletken polimerler arasında PEDOT, iletkenlik bandının düşük olması, yükseltgenmiş halinin kararlılığı, iyi iletkenlik gibi özellikleri ile diğer polimerlere göre daha fazla uygulama alanına sahiptir.
İletken polimerler genel olarak kimyasal oksidasyon veya elektrokimyasal sentez metodu ile sentezlenebilmektedirler. Kimyasal oksidasyon yönteminde, uygun çözücü içerisinde dağılan monomere başlatıcı eklenerek polimerizasyon yapılırken, elektrokimyasal yöntemde elektrolit çözelti içerisindeki monomerin, anodik polimerizasyon ile çalışma elektrotu üzerine elektropolimerizasyon yöntemi ile polimerleşmesi gerçekleştirilmektedir.
Kimyasal oksidasyon yöntemlerinin uygulanması ve emülsiyon polimerizasyonu ile nano boyutta polimerlerin daha düşük maliyette ve büyük ölçeklerde üretilmesi sağlanabilmektedir. Emülsiyon polimerizasyonu uygulamasında kimyasal oksidasyon yöntemine ek olarak sentez ortamına yüzey aktifleştirici madde eklenmektedir. Yüzey aktifleştirici maddeler sulu çözelti ortamında monomerlerin etrafını kaplıyarak polimerizasyonun miseller içeresinde gerçekleşmesini ve böylece nano ölçekte polimerler elde edilmesini sağlamaktadır.
Nanofiber yapıların küçük çapta ve hafif olması, yüksek yüzey/hacim oranı ve gözenek boyutunun kontrol edilebiliyor olmasından dolayı enerji depolama, doku mühendisliği, medikal alanlarda kullanılması yaygınlaşmaktadır. Kompozitlerin nanofiber olarak elde edilmesinde genellikle elektrospin yöntemi daha avantajlıdır. Elektrospin tekniğinde temel olarak 3 öğe bulunmaktadır. 1. Voltaj ünitesi 2. Besleme ünitesi (şırınga vb.) 3. Toplayıcı. Genellikle nanofiber istenen iletken polimer kompoziti, taşıyıcı ve iletken olmayan bir matris ile elektriksel alan kuvvetinden faydalanarak bir toplayıcı üzerinde toplanır.
Sentezlenen yapıların difüzyon empedansı, çift yüzey kapasitansı, çözelti direnci, sistemdeki yük taşıma ve iletim hızı gibi elektrokimyasal özelliklerinin incelenmesi için bilinen en iyi yöntem elektrokimyasal empedans spektroskopisi (EIS) dir.
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Bu yöntemde seçilen akım veya potansiyelde uygun elektrolit çözeltisi içerisinde ikili veya üçlü elektrot sistemleri kullanılarak yapının elektrokimyasal karakterizasyonu gerçekleştirilir.
Yakın zamanda yapılan çalışmalar göstermiştir ki, organik yapıdaki iletken polimerler, inorganik maddeler ile birleştirildiğinde oluşan yeni kompozitin iletken polimere oranla iletkenlik, kapasitans gibi bazı özelliklerinde önemli oranda gelişme gözlenmiştir. Son yıllarda polimerik kompozitlerle birlikte kullanılan katkı maddeleri arasında başta karbon ve türevleri gelmektedir, grafen ve grafen oksit karbon türevleri arasında en çok kullanılanlar arasındadır. Grafen, karbon atomunun iki boyutlu, yüksek yüzey alanına sahip ve çok iyi iletkenlik özelliği gösteren bir allotropudur. Grafen, grafen oksitin indirgenmesi ile elde edilebilir. Grafenin yüzeyinde grafen oksite oranla daha az bulunan fonksiyonel grupların (oksi, peroksi, hidroksi v.b) bulunması moleküller arası etkileşimin daha az etkin olmasına sebep olduğundan, grafen oksit yerine indirgenmiş grafenin kullanılması durumu iyi sonuçlar vermemektedir. Bundan dolayı grafen ve türevleri genellikle polimer veya diğer yapılar ile kompoziti halinde kullanılmaktadır.
Polimerik yapılarla kompozit yapıda dolgu malzemesi olarak kullanılan bir diğer grup ise metal oksitlerdir, bunlar arasında en yaygın kullanılanlardan biri ise titanyum dioksittir (TiO2). TiO2, düşük maliyeti, geniş yüzey alanı, kimyasal
kararlılığı ve çevre dostu olmasından dolayı son yıllarda polimerik kompozitlerde oldukça fazla kullanılmaktadır.
Bu çalışmada emülsiyon polimerizasyon yöntemi ile edilen PEDOT polimerinin farklı malzemelerle oluşturulan nanokompozitleri elde edilmiş, elektrokimyasal özellikleri ve spektroskopik özellikleri incelenmiştir. İlk çalışmada emulsiyon polimerizasyon yöntemi ile akrilonitril (AN) ve stiren (St) monomerlerinden P(AN-ko-St) kopolimeri elde edildi. Elde edilen bu kopolimerin sulu ortamında EDOT monomerinin polimerizasyonu, yüzey aktifleştirici sodyum dodesil sülfat (SDS) varlığında, amonyum persülfat (APS) başlatıcısı eklenerek gerçekleştirildi. Bunun sonucunda P(AN-ko-St) nanopartiküllerinin PEDOT ile kaplanması sağlandı. Polimerizasyon süresince reaksiyon ortamından alınan örneklerle polimerleşme zamana göre takip edilerek polimer nanotaneciklerinin büyümesi spektroskopik ve elektrokimyasal olarak takip edildi. Ayrıca elde edilen PEDOT/P(AN-ko-St) nanokompozitinin özellikleri saf PEDOT ile karşılaştırıldı. SEM ve AFM sonuçları göstermiştir ki saf P(AN-ko-St) nanotaneciklerinin boyutu 40-80 nm arasında iken ile PEDOT kaplı P(AN-ko-St) nanotaneciklerin ortalama tanecik büyüklüğü 25-65 nm dir. Bunun yanısıra polimerizasyonun 120. dakikasına kadar EDOT monomerinin hızlıca polimerleştiği fakat uzun polimerizasyon sürelerinde polimer zincirlerinin kümelenmesinden dolayı yüzey pürüzlülüğünde artma gözlenmiştir. EIS ölçümleri için AC sinyali 10 mV ve frekans aralığı 0.01 Hz–100 kHz olarak tanımlanmıştır. PEDOT polimerinin P(AN-ko-St) nanotaneciklerinin üzerinde oluşumunu takip etmek ve tanecik büyüklüğünün elektrokimyasal özelliğe etkisini görebilmek için, polimerizasyon süresince seçilen zaman aralıklarında reaksiyon ortamından numuneler alınmış, damlatma yöntemi ile cam üzerinde filmleri oluşturulmuş ve elektrokimyasal karakterizasyonu yapılmıştır. Bunun sonucunda elde eldilen verilerle nanofilmlerin en iyi R(C(R(Q(R))))(CR) eşdeğer devre modellemesine uyduğu sonucuna varılmıştır. Ayrıca Bode magnitüt diyagramı göstermektedir ki, 1440. dakikaya kadar PEDOT, P(AN-ko-St) üzerine daha ince bir tabaka olarak kaplandığından IZI değeri azalırken, pürüzlülük değeri artmaktadır.
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İleri polimerizasyon sürelerinde ise bunun tam aksi gözlenmektedir. Buna göre uzun polimerizasyon sürecinde PEDOT nanotaneciklerinin P(AN-ko-St) üzerinde tutunamadığı ve çözeltide asılı kaldığı söylenebilir. Dolayısıyla kompozitte istenen özelliğe göre optimum polimerizasyon süresi seçilmelidir.
Çalışmanın bir sonraki aşamasında PEDOT polimerinin elektrokimyasal özelliklerinin geliştirilebilmesi için grafen oksit (GO) ve indirgenmiş grafen oksit (rGO) ile nanokompozitleri sentezlendi. Bunun için grafitten başlayarak Hummers metodu ile önce grafen oksit ardından NaOH kullanılarak indirgenmiş GO elde edildi. EDOT monomeri daha önceki aşamada kullanılan yüzey aktifleştirici SDS ve başlatıcı APS ile ayrı ayrı GO ve rGO bulunan sulu çözelti ortamlarında emülsiyon polimerizasyonu ile PEDOT-GO, PEDOT-rGO ve saf PEDOT nano yapıları elde edildi. Elde edilen nanokompozitlerin yapısal karakterizasyonu için UV-Vis, ATR-FTIR spektrofotometrik yöntemleri ile AFM ve SEM morfolojik yöntemleri kullanıldı. Elde edilen UV-Vis ve FTIR sonuçlarının literatürde önerilen pik değerleri ile önemli ölçüde örtüştüğü görüldü. Aynı şekilde SEM görüntüleri literatürde gösterilen görüntülerle benzerlik göstermiştir. Görüntü olarak saf PEDOT polimeri karnabahar şeklinde iken GO ve rGO pullu tabaka şeklindedir. Dolayısıyla PEDOT-GO and PEDOT-rGO SEM görüntülerinde PEDOT ın küresel partikülleri arasında tabakalar halinde GO ve rGO yapıları gözlenmektedir. Elde edilen nanokompozitlerin literatür verileri ile uyumluluğu görüldükten sonra, mukavemeti, yüksek hacim/alan oranı ve geniş kullanım alanı oluşturmak için nanofiberleri üretildi. Nanofiberler elde edilirken daha önceki çalışmada kullanılan P(AN-ko-St) taşıyıcı matris olarak seçildi. Öncelikle toz halinde olan P(AN-ko-St) DMF çözücüsü içerisinde 5% (kütlece) oranında dispers edildi. Daha sonrasında PEDOT, PEDOT-GO and PEDOT-rPEDOT-GO nanotanecikleri bu dispersiyon içerisinde kütlece 0.3% derişimde olacak şekilde hazırlanıp 25°C de 1 saat karıştırıldıktan sonra, 10 kV DC voltajda, 5.5μl/sa dan 400 ml/sa kadar olan besleme hızında nanofiberler 1cm2 lik
ITO-PET üzerinde toplandı. Toplama ünitesi ile iğne ucu arasındaki uzaklık 16 cm olarak seçilmiş ve toplam işlem süresi 25 dak. sürmüştür.
Nanofiberlerin çap kalınlığı SEM ile belirlendi ve nanofiberlerin düzgün pürüzsüz olduğu ayrıca çap kalınlığının saf PEDOT polimerine göre GO ve rGO katılımıyla sırasıyla 190±170 den 80±172 ve 150±100 nm ye düştüğü gözlemlenmiştir.
Nanofiberlerin elektrokimyasal karakterizasyonu için elektrokimyasal empedans spektroskopisi (EIS) yöntemi kullanılmıştır. Elektrolit olarak literatürdeki çalışmalara benzer olarak 0.01 M LiClO4 seçilmiş ve üçlü elektrot sistemi
kullanılmıştır. EIS ölçümlerinde frekans aralığı 10 mHz ile 100 kHz olarak seçilmiş ve Nyquist, Bode faz ve Bode magnitüt grafikleri elde edilmiştir. Bu grafiklerdeki veriler ZSimpwin programı kullanılarak eş değer devre modellemesi yapılmıştır. Nanofiber sistemi icin, devre modellemesi ZSimpwin programı ile gerçekleştirilmiş ve deneysel sonuçlarla en uyumlu eşdeğer devre modeli Rs(Qdl(RctW))(QelRel) olarak
saptanmıştır. Nyquist grafiğinde yüksek frekans aralığı bölgesinde yarım çemberler gözlenirken, düşük frekans bölgesinde y eksenine paralel doğrusal bir grafik görülmüştür. Literatür çalışmaları göstermektedir ki, Nyquist grafiğinde düşük frekans bölgesinde oluşan doğrusal grafiğin açısı 45°ye yakınlaştıkça kapasitif özellik artmaktadır. Elde edilen değerler, PEDOT-GO ve PEDOT-rGO nanokompozitlerinin P(AN-ko-St) matrisiyle oluşturdukları nanofiberlerin yük iletme direnç (Rct) değerlerinin tek başına PEDOT’ın P(AN-ko-St) kopolimeriyle
oluşturduğu nanofibere oranla daha düşük olduğu göstermiş, ayrıca çözelti direncinin (Rs) ise daha yüksek olduğu gözlenmiştir.
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Rct değeri elektrot yüzeyindeki elektronların cözeltiye iletilmesi sonucu olusan
direnci ifade ettiğinden dolayı GO ve rGO nun PEDOT yapısına katılması ile Rct
değerlerinde azalma olması PEDOT-GO ve PEDOT-rGO nanokompozitlerinin kapasitif özellige katkıda bulunduğunu kanıtlamaktadır. GO ve rGO ile elde edilen kompozitler ayrı ayrı karşılatırıldığında ise GO in kapasitif özelliğe katkısı, iletken olan rGO e göre daha fazla olduğu gözlenmiştir. Bu durum, rGO in GO a kıyasla polimer ile daha zayıf etkileşimi ve rGO in sulu ortamda GO ya göre daha az dispers olmasından kaynaklanmaktadır, literatürdeki bulgular da bu bulguyu desteklemektedir. Bundan dolayı PEDOT-rGO ve PEDOT-GO nanokompozitleri sentezlenirken EDOT monomeri ile rGO nun yeterince etkileşememiş olması ve rGO nun bir kısmının dibe çökmesi, rGO nun polimer zincirleri arasında homojen bir şekilde dağılamamasına sebep olmuştur. Buna karşın, GO in yapısında bulunan oksi, peroksi ve hidroksi vb. fonksiyonel gruplardan dolayı sulu emülsiyon polimerizasyon ortamında çok iyi bir dağılım göstermesi ve EDOT monomerindeki oksijen ve kükürt atomları içeren kısımlarla kolay etkileşim göstermesinden dolayı PEDOT-GO nanokompozitinin P(AN-ko-St) matrisi üzerinde elde edilen nanofiberlerinin kapasitif özelliği daha iyidir.
Çalışmanın bir sonraki aşamasında, emülsiyon polimerizasyonu ile sentezlenen PEDOT nanokompozitlerinin süperkapasitör olarak uygulamasına yönelik kapasif özelliklerinin iyileştirilmesi hedeflendiğinden dolayı, taşıyıcı matris olarak kullanılan yalıtkan özellik gösteren, organik P(AN-ko-St) kopolimeri yerine metal oksit olan ve iletken polimerlerle iyi bir uyumluluk gösterdiği daha önceki çalışmalarla ispatlanan TiO2 kullanılmasına karar verilmiştir.
PEDOT-TiO2 nanokompozitleri elde edilirken aynı yüzey aktifleştirici (SDS) ve
başlatıcı (APS) seçilmiştir ve bu sentez sırasında PEDOT miktarı sabit tutulup kütlece değişken TiO2 değerleri alınmıştır. TiO2 nun PEDOT’a kütlece sırasıyla 5,
10, 15 ve 20% olarak eklenmiş ve elde edilen nanokompozitlerin spektroskopik ve elektrokimyasal karakterizasyonları gerçekleştirilmiştir. UV-vis, FTIR spektrofotometrik ve XRD ölçümleri sonucu elde edilen verilerin literatürle örtüştüğü gözlendikten sonra, bu oluşum SEM görüntüleriyle de desteklenmiş ve ayrıca oluşan taneciklerin gözenek büyüklüğü ve yüzey alanı BET yöntemi ile ölçülmüştür. Bir sonraki aşamada elekrokimyasal karakterizasyon için kompozitlerin GO ve rGO ile fiziksel karışımı elde edilmiştir. Bu işlem için farklı bileşim oranlarındaki PEDOT- TiO2 örnekleri, daha öncesinde ayrı ayrı ultrasonik cihazda
bir saat DMF te dağıtılan GO ve rGO homojen dispersiyonlarına eklenmiş (1:1% kütlece) ve tekrar 20 dak. kadar birlikte ultrasonik cihazda homojen olarak karışması için bırakılmıştır. Elde edilen örnekler PEDOT-TiO2-x-GO ve PEDOT-TiO2-x-rGO
olarak etiketlenmiştir. (-x TiO2 miktarını göstermek üzere)
Bu örneklerin elektrokimyasal özelliklerinin incelenmesi için FTO üzerine damlatma yöntemiyle filmleri oluşturulmuştur. Hazırlanan filmlerin kalınlığını kontrol etmek amacıyla aynı sıcaklıkta her örnekten 1mL kullanarak ve eşit ölçülerde FTO üzerine damlatma gerçekleştirilmiştir. Oluşturulan filmler çalışma elektrodu olarak, EIS yöntemiyle üçlü elektrot sisteminde ve 1M Na2SO4 elektroliti içerisinde, empedans,
döngülü voltametri ve galvanostatik şarj–deşarj ölçümleri yapılmıştır.
Döngülü voltametride 20-200 mV tarama hızları aralığında çalışılmış ve 20 mV tarama hızı tüm örnekleri karşılaştırmak için seçilmiştir. Döngülü voltametride elde edilen dikdörtgene yakın grafikler ve artan tarama hızına karşı alınan akım değerinin artması nanopartiküllerin pseudokapasitif özellik taşıdığını göstermektedir. Ayrıca
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kaplanan materyalin artan tarama hızında elektrolitle daha yüzeysel ve hızlı etkileştiği gözlenmiştir. Sadece PEDOT olan örnek ile rGO ve PEDOT-TiO2-15-rGO nun döngülü voltametri grafiklerinin altında kalan alandan hesaplanan
spesifik kapasitans değerleri karşılaştırıldığında yaklaşık olarak sırasıyla 35.4% ve 89.6% artış olduğu saptanılmıştır. Galvanostatik şarj-deşarj ölçümleri sonrasında elde edilen tam ve simetrik üçgen grafikleri de döngülü voltametreyi desteklemektedir. Her iki karakterizasyonda da elde edilen grafiklerde spesifik kapasitans değerleri hesaplandığında numuneler arasında, PEDOT-TiO2-15-rGO
örneğinin en yüksek (0.1 mA.cm-2 akım yoğunluğunda 18.9 F.g-1) spesifik
kapasitansı gösterdigi gözlenmistir. GCD grafiklerinin eğiminden hesaplanan spesifik kapasitans değerleri de CV sonuçlarını desteklemektedir. 0.1 mA.cm-2 akım
yoğunluğu kullanılarak alınan ölçümlerde sırasıyla kütlece spesifik kapasitans PEDOT-rGO, PEDOT-TiO2-5-rGO, PEDOT-TiO2-10-rGO, PEDOT-TiO2-15-rGO
ve PEDOT-TiO2-20-rGO için 0.54 F. g-1, 4.42 F.g-1, 5.75 F.g-1, 9.49 F.g-1 ve 1.32
F.g-1 olarak hesaplanmış ve ayrıca spesifik alan kapasitans değerleri 1.47 F.cm-2, 9.74 F.cm-2, 11.5 F.cm-2,18.9 F.cm-2 and 2.64 F.cm-2 olarak bulunmuştur.
Sonuçlar göstermiştir ki PEDOT-TiO2 nanokompozitlerinde %20 kütlece TiO2
miktarına kadar artan TiO2 miktarı ile kapasitif özellik artmakta fakat %20 kütle
oranı ve sonrasında kapasitansta azalma olmaktadır. Nanokompozitlerin Empedans değerlerinin ZSimpwin modelleme programı kullanılarak elde edilen eş değer devre modeli Rs(Qdl(RctW)) olarak bulunmuştur. Yine bu grafiklerden elde edilen en düşük
Rct değeri 7.41 ohm olup PEDOT-TiO2-15-rGO nanokompozitine aittir.
Emülsiyon polimerizasyon tekniği ile sentezlenen PEDOT ve PEDOT nanokompozitlerinin elektrokimyasal verilerine dayanarak bu çalışmada en kapasif olan örnek PEDOT-TiO2-15-rGO olup ilerideki çalışmalarda süperkapasitör
1 1. INTRODUCTION
Over the decades, due to developing technology and pollution, it has been becoming harder each day to accumulate and store enough energy to meet increasing demands of mankind. This problem forces researchers to find new approaches to accumulate and store energy. Energy resources can be divided into two groups; renewable energy sources (solar and wind) and power plants (nuclear power stations, fossil fuels, biomass and geothermal). Statistical studies and researches show that existing renewable energy resources could not be enough for near future generations. Among the other types of energy sources, the share of renewable energy in 2015 is shown in Figure 1.1 (Libich et al., 2018). In order to store and accumulate energy from various renewable energy resources, several methods have been invented so that it can be used whenever it is needed. Conventional batteries and capacitors have been most the common methods for energy storage up to recent years however, these methods are not enough to store more energy for future generations. Therefore, researchers have started to look for new materials and methods. First experimental studies for supercapacitors have been started in the early 1950s by General Electric to design a bridge between traditional capacitors and batteries. One of the most important advantage of supercapacitors is that they have higher energy density than that of conventional dielectric capacitors (Wang, Y. et al., 2009b). Supercapacitors have been developed for fast charging process of any devices and therefore two capacitors have been connected in series with conducting liquid linking between them (Snook et al., 2011), this kind of supercapacitors are called electrochemical double-layer capacitors (EDLC). In the EDLC type, charges are accumulated on the electrode surface and ions are build-up on the surface of a double layer capacitor, therefore, no electrons are transferred between chemical species and charge can only be stored physically. Hence, the key factor to form highly capacitive EDLC is the large electrode surface area and Helmholtz layer thickness. On the other hand, pseudo capacitors store energy chemically via fast reversible redox reactions on the electrode surface so that they act as batteries rather than capacitors.
2
Among the other materials, conductive polymers are commonly known pseudo capacitive electrodes due to their ability to give fast faradaic reversible multi-step redox reactions, relatively high theoretical capacity, lightweight and mechanical flexibility (Yue, B. et al., 2013a). One of the disadvantageous of pseudo capacitors is low stability and degradation of electrode during charging–discharging cycles.
Figure 1.1 : The distribution of renewable energy sources with respect to 2015. New generation electrochemical capacitors have been manufactured based on carbon derivatives, metal oxides and conductive polymers in recent years. These materials have been selected because of their conductivity and surface size and porosity. Conductive polymers can be synthesized by electrochemical, chemical oxidative and chemical vapour deposition methods. (Groenendaal et al., 2003) One of the derivatives of thiophene polymers is diethoxysubstituted thiophene poly(3,4-ethylenedioxythiophene) PEDOT has attracted attention because of its low band gap and high chemical stability during redox reactions. EDOT monomer has five-membered thiophene ring with the dioxy-ethyl substituent group at the 3 and 4 positions and two reactive hydrogens at the 2 and 5 positions and can be polymerized to PEDOT as shown in Figure 1.2.
In order to achieve high capacitive performance supercapacitors, conducting polymers have been assembled with carbon materials due to their high mechanical properties and conductivities. Among the other carbon materials, graphene has the most promising carbon allotropes, which has a two-dimensional hexagonal lattice
3
structure obtained from graphite (Figure 1.3). It has almost zero band gap between valence and the conduction bands therefore, it exhibits unusual physical, chemical and mechanical properties, and in recent times, has been used in supercapacitors with and without conducting polymers as electrode materials (Ho et al., 2015).
S O O e -S O O + 2 2 2 S O O S O O + + - 2 H+ S O O S O O S O O S O O S O O S O O S O O S O O n Figure 1.2 : Polymerization mechanism of 3,4-ethylenedioxythiophene. Inorganic fillers have been incorporated to supercapacitor electrodes to enhance capacitive properties of materials, which include conducting polymers, and carbon materials. Transition metals and their oxides are commonly used for inorganic fillers. Recently, TiO2 has been chosen in researches for several applications due to its low
cost and high chemical stability, and eco-friendly properties (Reddy et al., 2013; Vu et al., 2005). Electrochemical characteristics of the supercapacitors can be investigated via electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), chronopotentiometry and chronoamperometry methods. Electrochemical cells can be designed using two electrode or three electrodes configurations depending on requirement. Figure 1.3 shows schematic view of two electrodes and three electrodes cell configurations.
Essentially, in the three electrode cell configuration, the current flows through the counter electrode (CE) and working electrode (WE), and the corresponding voltage is measured or controlled between the reference electrode (RE) and the WE. For two cell configuration, there is no reference electrode (RE). The RE used for this purpose should have an ideal non-polarizable characteristics i.e. its voltage is constant over a
4
large range of current densities. Thus, three electrode configuration is more reliable as working voltage is measured accurately.
Electrochemical impedance spectroscopy (EIS) analysis can be used to understand the response of the system as a function of the frequency and displays internal dynamics and the kinetic feature of the ion diffusion, which is responsible for charge storage property of the electrode. Impedance data enlightens the interface, material’s structure and reactions taking place at the interface of electrode/electrolyte. Moreover, EIS can give correlation between the real system and ideal equivalent circuit, which consists of individual electrical components (R, C and L) in their series and parallel combination. The Nyquist plot provides charge transfer resistance (Rct)
value that related with system kinetic. For instance, if the diameter of semicircle in high frequency region (Rct) is small then the system is kinetically facile (Saranya et
al., 2015).
Cyclic voltammetry (CV) is mostly used to characterize performance of various electrical energy storage devices. In this method, because of oxidation and reduction process, electrons are transferred between electrolyte and electrode and this process monitors reversibility of electrons in the form of current-potential diagram (Levi et al., 1997). In other words, electric potential is applied to the electrodes during time and current is recorded (Evans et al., 1983).
Another method to evaluate electrochemical capacitance of materials is galvanostatic charge discharge method (GCD) and it is called as chronopotentiometry. This method is mostly common in supercapacitor industry. In this method, current is applied to the working electrode and the resulting potential is measured against a reference electrode as a function of time. Various parameters can be obtained such as; Capacitance, resistance and charge-discharge cycle numbers.
In this thesis, PEDOT and its nanocomposites have been prepared via microemulsion polymerisation.
However, the electrochemical polymerisation of conducting polymers mostly used to produce thin films, the need of conducting substrate for polymerisation can cause difficulty for many applications.
Chemical oxidative method is one of the most effective method to have mass production of films (Dai et al., 2008).
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Figure 1.3 : Chemical structure of graphene.
Figure 1.4 : Electrochemical cell of a) two and b) three electrode configuration. Furthermore, it enables various deposition techniques on substrates which facilities to control composite combination and diversity. Micro-emulsion polymerisation is one of the most promising method among the other chemical oxidative polymerisation methods. Since EDOT, monomer is insoluble in water, its polymerisation in emulsifier free system gives low yield and poor conductivity. To overcome this problem, surfactants such as dodecyl benzene sulfonic acid (DBSA), sodium dodecyl benzenesulfate (SDBS) and sodium dodecyl sulphate (SDS) have been introduced to emulsion systems by researchers (Choi, J. W. et al., 2004b; DeArmittArmes, 1993; Kudoh et al., 1998; OriakhiLerner, 1995). The studies proved that adding the surfactants to oxidative chemical polymerisation has increased
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colloid stability, polymerisation yield and acted as a dopant, which increased conductivity of the particles. Considering these reasons, PEDOT has been synthesized via micro-emulsion polymerisation using SDS surfactant.
One of the important problem about synthesized conductive polymers is low processability because of their rigid structure of conjugated double bonds. In order to enhance processability of the conducting polymers, electrochemical stability in air as well as mechanical stability, some of the insulating polymers have been combined with conductive polymers in the last decades. There have been many tried methods, for example, by blending and combining or connecting more than two kinds of materials. Among the various methods to combine more than two polymers, core– shell formation has provided homogeneous conductive polymer/insulating polymer composites (Onoda et al., 1999). P(AN-co-St) copolymers composed of acrylonitrile and styrene structures and known SAN in thermoplastic industry. This copolymer has chemical resistance to hydrocarbons, grease and oils due to polar characteristic of acrylonitrile segment and have a higher softening point, a much better resistance to stress cracking and crazing and an enhanced impact strength comparing with polystyrene itself. Moreover, it is cheaper than the poly(methyl methacrylate) and cellulose acetate, which are the competitors of P(AN-co-St) copolymer. Hence, in the first part of study, when copolymer formation of AN and St monomers have been completed via the microemulsion polymerization, EDOT has been polymerized in this aqueous micelle system. This method helps to construct a network between PEDOT and P(AN-co-St) copolymer, which avoided heterogeneous two phase system. In order to elucidate core-shell formation, structural and morphological characterizations of prepared PEDOT/P(AN-co-St) composites have been examined via monitoring polymerization by time. To have better understanding of the electrical conductivity of the system, electrochemical characterization has been carried out using glass electrode and spin coating method. The results have been discussed in section 2.2.
Nanofibers have been attracting great interest as they have high surface area/volume ratio. Nanofibers of conductive polymers have been manufactured to integrate good electrical properties along with good environmental stability and processability. Electrospinning is the most simple and efficient method to obtain nanofibers. The straightforward method to electrospun conducting polymers is by blending with
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second latex polymer solution. Nanofibers of conducting polymers with insulating polymer not just only enhance processability and mechanical properties of conducting polymers but also influence on orientation of PEDOT molecules lengthwise direction of the fiber, thus potentially increasing its charge‐carrier mobility (Li, D. et al., 2004).
To improve PEDOT polymer functional performance, nanoscale reinforcement has been searched as well as polymerization techniques in this study. It has been seen that GO and rGO structures have been attracted rapidly increasing attention as they possesses high surface area which contribute to polymer chain confinement and thus lead to superior structural, mechanical and thermal properties compared to other common fillers (Hebbar et al., 2019). Since the micro emulsion polymerization technique has been successfully examined for PEDOT in our previous study (GulercanSarac, 2018), GO and rGO have been chosen to develop existing system. Besides, the advantage of nanofibers has been proved by our research group and obtained significant results (Gergin et al., 2017; Ismar et al., 2018). These are all taken into account to integrate micro-emulsion polymerization technique with most popular GO and rGO structures and studied about their nanofibers with P(AN-co-St) to enhance PEDOT functional performance and results has been discussed in section 3.2.
The latest researches in literature show that performance of conductive polymers have been improved by using metal oxides. Recently, ternary composites have been studied to obtain advanced materials. Ternary polypyrrole/graphene oxide/zinc oxide was synthesized for supercapacitor electrodes and gravimetric capacitance has been calculated in two electrodes system as 94.6 Fg−1 at 1 Ag−1 from charge/discharge (CD) curves (Chee et al., 2015). In another study, three electrode system has been used to measure gravimetric capacitance of ternary cobalt ferrite/graphene/polyaniline nanocomposites, which showed gravimetric capacitance of 1133.3 Fg−1 at scan rate of 1 mVs−1 (Xiong et al., 2014).
This significant capacitance values inspired us to design a three component system
including PEDOT, rGO and a metal oxide. TiO2 was chosen as metal oxide
component as it is cost effective and has good physical, optical and electrical
properties. In this part of the study, EDOT has been polymerized with TiO2 to obtain
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kept constant. rGO has been blended to binary system at the electrode preparation stage. The ternary composites have been characterized to have better understanding of each component effect on the capacitance and synergistic impact between each other. The results have been discussed in section 4.2.
1.1 Purpose of Thesis
The aim of this thesis is to enhance PEDOT functional performance via micro emulsion polymerization method and different components to obtain facile, cost effective composite materials. Supercapacitors are highly in demand particularly for automotive and power source industries. Comparing with conventional batteries, supercapacitors work based on EDLC, which is related with layer between the oppositely charged electrolyte/electrode surfaces and faradaic reversible reactions (pseudo capacitance). Therefore, unlike the batteries they are more flexible, lighter and are able to charge-discharge millions of times at high current densities (Purkait et al., 2018; Purkait et al., 2017). Graphene oxide and reduced graphene oxide are good candidates for supercapacitor devices due to their high surface area/volume ratio and inherent flexibility. However, graphene has large surface area/volume and good conductor, it still needs to be improved to use in supercapacitor applications. Therefore, in recent times, graphene has been used for supercapacitor researches including conductive polymers, transition metal oxides and materials containing oxygen or nitrogen groups on their surfaces (Basnayaka et al., 2013; Huang et al., 2012; Lehtimaki et al., 2015).
Several studies proved that metal oxides have improved physical, electrochemical and thermal properties of conductive polymer-graphene composites (Chee et al., 2015; Naderi et al., 2016; Ng et al., 2015; Park et al., 2002; Shayeh et al., 2015; Xie, Y. et al., 2014). Therefore, these studies lead our work to synthesize PEDOT-metal oxide nanocomposites.
In this thesis, we report the synthesis of novel composites based on PEDOT. Micro- emulsion polymerization of PEDOT, PEDOT/P(AN-co-St), GO, PEDOT-rGO and PEDOT-TiO2 has been conducted and their characterizations with EIS,
SEM, AFM, ATR-FTIR, and UV-Vis were studied. Nanofibers of PEDOT-GO, PEDOT-rGO on P(AN-co-St) were fabricated successfully and analysed for capacitive performances. In order to further increase capacitive properties of
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PEDOT-GO and PEDO-rGO, TiO2 metal oxide was incorporated, we have prepared
and investigated composites of PEDOT with graphene oxide and TiO2. All
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2. MORPHOLOGICAL AND ELECTROCHEMICAL IMPEDANCE SPECTROSCOPIC (EIS) STUDY OF
POLY(3,4-ETHYLENEDIOXYTHIOPHENE)-COATED POLY(ACRYLONITRILE-co-STYRENE) NANOPARTICLES1
Micro-emulsion polymerization has been improved to obtain nano-sized polymers. The particles are transferred into spherical aggregates toward the inside of the surfactant template with this process. Conducting polymer nanoparticles have been synthesized by some research groups, using micro emulsion polymerization method (Lei, Y. et al., 2005; Reung-U-Rai et al., 2008) (Jang et al., 2002; Ovando-Medina et al., 2011; YanXue, 1999). They reported particle sizes in the range of 50–200 nm. There would be two purposes of utilizing the surfactant in the polymerization. The first one is the micelle formation that creates a micro-reactor vessel, where monomer is confined in a localized environment created from encapsulation by the surfactant. The second reason is to improve processibility of polymers and also physical properties of polymers such as solubility, stability and conductivity in organic solvents (Stejskal et al., 2003).
A standard micro-emulsion polymerization usually consists of monomer mixture, water, surfactant, initiator and co-stabilizer. The micro-emulsion polymerizations are more stable than the ordinary emulsion polymerization by thermodynamically and do not form any phase separation (Yan et al., 1998).
In recent studies, conducting polymers have been widely studied due to their interesting chemical and physical properties. Among conductive polymers, PEDOT is very special since it is easily synthesized and has lower band gap. The PEDOT has ability to reach high level doping due to its reactivity, thus more electrically conductive materials can be produced.
1This chapter based on “D.Gülercan and A. S. Sarac (2018). Morphological and Electrochemical
Impedance Spectroscopy (EIS) Study of poly(3,4 ethylenedioxythiophene)-coated poly(acrylonitrile-co-styrene) nanoparticles, Int. J. Electrochem. Sci., 13, 433–451, doi: 10.20964/2018.01.30.”
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The in situ oxidative emulsion polymerization process is a facile preparation method for the PEDOT nanoparticles since it provides separated nanomicelles. The nanomicelles contain organic phase that are dispersed in the aqueous medium, and the chemical oxidative polymerization of EDOT only takes place in the organic droplets (Jin et al., 2011).
Emulsion polymerization of pyrrole and thiophene derivatives using monomeric surfactants have been studied by a few research groups. For the first time (DeArmittArmes, 1993) described the colloidal dispersions of surfactant-stabilized polypyrrole particles in aqueous media using an anionic surfactant, sodium dodecyl benzenesulfonate (SDBS). They also investigated average particle size of nanoparticles 200-500 nm and some polymerization conditions to obtain stable colloidal particles of polypyrrole. PPy and Polythiophene colloids have been synthesized by using a monomeric surfactant, SDBS (OriakhiLerner, 1995). They described the formation of a large number of polymer latexes containing nanoparticles via emulsion polymerization. Kudoh synthesized PPy and PEDOT in aqueous solution using Fe2(SO4)3 as oxidant and different monomeric surfactants.
Although they obtained insoluble surfactant–oxidant complex, the nanoparticles represented approximately 40 and 60 S/cm conductivity values, respectively (Kudoh, 1996; Kudoh et al., 1998). However, the nanoparticle sizes and morphologies have not been reported and particular clarification of the surfactant-oxidant complex effects on the high conductivity have not been explained in their study. Armes and co-workers reported the deposition of PEDOT onto near-monodisperse, micrometre-sized Polystyrene latexes. They also indicated that the optimum conducting polymer loading for well-defined PEDOT-coated PSt latexes with reasonable solid-state conductivities (10-2-10-3 S cm-1) (KhanArmes, 1999). Han et al. (2004) fabricated nanometre-sized PEDOT-silica core-shell particles and their corresponding hollow particles. The monomer, 3,4-ethylenedioxythiophene (EDOT) is relatively insoluble in water and iniators react with water and the surfactant-free emulsion polymerization of EDOT leads to poor conductivity and low yields. To overcome this problem, it has been proposed that adding monomeric surfactants to an aqueous solution of EDOT monomer improves yield of polymerization significantly (Choi, J. W. et al., 2004a). In this study, we successfully synthesized PEDOT nanoparticles on the prepared P(AN-co-St) matrix at appropriate monomeric surfactant concentration
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in aqueous solution. The micro emulsion polymerization was chosen among various emulsion polymerization methods as the method could help us to obtain nano-sized polymer particles.
Sodium dodecylsulfate (SDS) was selected as surfactant dopant since it is not too expensive and commercially available. ATR-FTIR and UV-Visible
spectrophotometers were also used to investigate the characteristics of nanoparticles. The progress of polymerization followed by spectroscopic and morphologic measurements with respect to time.
Moreover, there are a few studies about electrochemical impedance measurement and electric circuit model of PEDOT coated nanoparticles, these measurements were also used to evaluate the time-dependent capacitive behaviour of PEDOT-coated P(AN-co-St) nanoparticles.
2.1 Experimental 2.1.1 Materials
Acrylonitrile (AN), Styrene (St) was obtained from Sigma Aldrich. The initiator used for the polymerization was (NH4)2S2O8 (APS), Sodium dodecyl sulfate (SDS),
methanol and ethanol were purchased from Merck reagents. EDOT was used an Aldrich reagent.
The characteristic functional groups of the samples were analysed with Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) (PerkinElmer ATR-FTIR Spectrum One with a universal ATR attachment with a diamond and a ZnSe crystal, Shelton, USA). P(AN-co-St)/PEDOT and P(AN-co-St) polymers were characterized by UV-Visible (Perkin Elmer, Lambda 45) spectrophotometric analysis. P(AN-co-St)/PEDOT and P(AN-co-St) were dispersed 2% volume fraction in water for characterization by using UV-Visible spectrophotometer. Nanoparticles were characterized as morphological by Atomic Force Microscope (AFM) (Nanosurf EasyScan2 STM) and Scanning electron microscope (SEM).
Electrochemical impedance spectroscopy (EIS) measurement of nanoparticles was analysed in % 0.01M LiClO4 solution on glass with a platinum wire as a counter
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Electrochemical Analyser via electrochemical impedance spectroscopy measurement. The Nyquist, Bode Magnitude, Bode Phase and admittance data of the EIS was measured in the frequency range 0.01 Hz–100 kHz using Z SimpWin V3.10, AC-impedance data analysis software program.
2.1.2 Preparation of the P(AN-co-St)/PEDOT nanoparticles 2.1.2.1 Synthesis of P(AN-co-St) nanoparticles
Emulsion polymerization of P(AN-co-St) was conducted using Acrylonitrile and Styrene monomers (9:1 molar ratios) in the presence of monomeric surfactant (SDS) and initiator (APS) in 150 ml aqueous solution. For the first step, SDS was dissolved in 145 mL water and mixed with magnetic stirrer. Afterwards AN and St monomers dissolved in surfactant solution. The solution which contains monomers (AN and St) and surfactant was stirred and then was transferred to three neck flask and temperature was raised 70°C then APS solution was added. The temperature of synthesis was held at 70°C during 3h. The obtained P(AN-co-St) latex was highly stable and precipitation was not observed even after centrifugation at 6000 rpm for 30 min.
2.1.2.2. Synthesis of P(AN-co-St)/PEDOT nanoparticles
Aqueous micellar dispersion of P(AN-co-St) copolymer was prepared as mentioned section 2.1.2.1 and 10 mL of P(AN-co-St) micellar solution was taken, subsequently EDOT monomer was added and solubilized in the 10 mL of this micellar solution. The molar ratio of EDOT monomer was determined after several experiments and was added into for producing PEDOT coated P(AN-co-St) nanoparticles.
The 0.08g of (NH4)2S2O8 (APS) dissolved in 1 ml of distilled water and then was
added directly to the mixtures and they were stirred for 24 h at 25 °C (Fig.2.1). The molar ratio of EDOT monomer to APS is 2.5.
During this polymerization, same amount of samples were taken at several time intervals to verify the particle formation and growth mechanism.
The nanoparticles were filtrated and then they were washed with deionized water, methanol and ethanol respectively, afterwards they were dried at 60°C for 24 h under vacuum. To investigate colloidal stability and morphologies of the particles, small
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amount of the samples were taken from the washed products before drying process they were re-dispersed in methanol for 10 min with ultrasonication.
Figure 2.1 : Schematic diagram for the PEDOT on the P(AN-co-St) nanoparticles. 2.2 Results and Discussion
2.2.1. ATR-FTIR spectrophotometric characterization
For further confirmation, the growth of the PEDOT on P(AN-co-St) cores was examined by in situ ATR-FTIR analysis (Fig. 2.2). From the FTIR spectrum in Figure 2.2 the existence of a broad band between 2800 and 3700 cm−1 attributes the presence of -OH groups, which is caused by water solution medium during the synthesis. During the polymerization of EDOT in P(AN-co-St), C=C bonds disappear and are transformed into C–C single bonds. This transformation causes a new absorption bands at 1488 cm−1. This characteristic peak of PSt which were referred to bending and stretching of sp3 C–H bonds, absorbance value decreases with increasing time (Fig 2.4). Furthermore, the characteristic peaks at 705, 762 cm−1 of PSt disappears during polymerization. They can be attributed to the PEDOT formation on P(AN-co-St) core structure. The peak at 1635 cm−1 of APS overlaps with both 1565 cm−1 characteristic peak of PSt and the characteristic peak of PEDOT at 1520 cm−1. When APS is used as oxidant the existence of the peak at 1635 cm−1 (Fig 2.2.) demonstrates the presence of C=O group, which can be ascribed to the overoxidation. Since EDOT has no free carbonyl group in its structure, the peak at
