Farklı asitlerle katkılanmış polianilinin sentezi, karakterizasyonu ve süperkapasitör uygulaması
Synthesis, characterization and supercapacitor application of polyaniline doped with different acids
Yazar: MERVE METİN ÇELENK
Danışman: PROF. DR. FATMA SENİHA GÜNER ; DR. YURDANUR TÜRKER
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Kimya Mühendisliği Ana Bilim Dalı / Kimya Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2022
Konu: Enerji = Energy ; Kimya Mühendisliği = Chemical Engineering ; Polimer Bilim ve Teknolojisi = Polymer Science and Technology
Dizin: Fosil yakıtlar = Fossil fuels ; Kondansatör = Capacitor ; Polianilin = Polyanilines

Özet:
Fosil yakıt kaynaklarının hızla tükenmesi ile birlikte günümüzde sürdürülebilir, çevre dostu enerji sistemlerine ihtiyaç duyulmaktadır. Bu kaynakların etkili bir biçimde kullanılabilmesi için üretilen enerjinin minimum kayıpla ve maksimum nitelikle depolanması gerekmektedir. Bu durum ise enerji depolama sistemlerine olan ihtiyacı zaruri kılmaktadır. Tipik bir enerji depolama sürecinde, bir tür enerji depolanır ve gerektiğinde kullanılmak üzere başka bir enerji çeşidine dönüştürülür. Bu nedenle, farklı enerji kaynaklarının doğru kullanımına yönelik çeşitli enerji depolama sistemleri geliştirilmektedir. Enerji depolama sistemlerinden biri olan süperkapasitörler, mükemmel çevrim ömrü, yüksek güç yoğunluğu ve hızlı şarj-deşarj süreci gibi umut verici özelliklerinden dolayı yaygın olarak kullanılmaktadır. Süperkapasitörlerin performansı; malzeme tasarım stratejilerinin, sentez tekniklerinin, karakterizasyon metodolojilerinin ve farklı elektrot/elektrolit malzemelerinin geliştirilmesiyle hızla ilerlemektedir.
Süperkapasitörler temel olarak elektrikli çift katmanlı kapasitörler (EDLC'ler) ve psödokapasitörler olarak sınıflandırılabilir. Çift katmanlı süperkapasitörler elektriği elektrostatik olarak depolar, başka bir deyişle, süreç temel olarak elektrolit ve elektrotlar arasındaki arayüzlerde iyon adsorpsiyonundan kaynaklanmaktadır. Bununla birlikte, psödokapasitans, hızlı geri dönüşümlü faradayik elektrokimyasal reaksiyonlara dayanır. Çift katmanlı süperkapasitör mekanizmasında karbon nanotüpler (CNT'ler), grafen ve aktif karbon gibi çeşitli karbon bazlı malzemeler elektrot malzemeleri olarak yaygın olarak kullanılmaktadır. Karbonlu malzemeler süperkapasitör uygulamalarında yaygın olarak kullanılmasına rağmen, kapasitans değerleri psödokapasitans sergileyen iletken polimerlere kıyasla nispeten düşüktür. Polianilin (PANI), polipirol (PPY) ve poli (3,4-etilendioksitiyofen) (PEDOT) gibi psödokapasitif malzemeler, zahmetsiz işlenmesi, kolay sentezlenmesi, hafifliği ve düşük maliyeti sayesinde son yıllarda büyük ilgi görmektedir. Bu polimerler arasında PANI çevre dostu olması, yüksek iletkenliği, kimyasal kararlılığı ve çok çeşitli uygulamalarıyla büyük ilgi görmüştür. PANI bazlı elektrot malzemesinin yükleme/boşaltma işlemi, polimer ve elektrolit arasındaki iyon değişiminin eşlik ettiği tipik bir doping/de-doping işlemidir. Sentez esnasında farklı kimyasal yapılara sahip katkı maddelerinin kullanılmasının yüzey morfolojisi, çözünürlük, kristalinite derecesi, moleküler ağırlık ve iletkenlik gibi çeşitli özellikleri değiştirdiği bilinmektedir.
Bu çalışmada, süperkapasitörlerde elektrot malzemesi olarak kullanılmak üzere PANI bazlı iletken polimerler, farklı asit katkı maddeleri (HCl, HNO3, H2SO4, H3PO4, HClO4) kullanılarak kimyasal oksidatif polimerizasyon yoluyla sentezlenmiş ve asit katkısının polimerin elektrokimyasal özelliklerine katkısı incelenmiştir. İletken polimerlerin kimyasal yapısı Fourier dönüşümlü kızılötesi (FTIR) spektroskopisi, X-ışını kırınımı (XRD), Taramalı elektron mikroskobu (SEM) ve Termogravimetrik analiz (TGA) ile doğrulanmıştır. Elektrokimyasal özellikler Döngüsel voltametri (CV), Galvanostatik şarj/deşarj testi (GCD) ve Elektrokimyasal empedans spektroskopisi (EIS) ile incelenmiştir. Sülfürik asit kullanılarak sentezlenen PANI/H2SO4 elektrodu, 0.1 A.g-1'lık akım yoğunluğunda 1200 F.g-1'lik bir spesifik kapasitans sağlayarak en yüksek değeri vermiştir.

Abstract:
Climate change and the aim of reducing dependency on the fossil fuels require scientists to move towards renewable energy sources. Therefore, different types of energy storage systems are gaining more and more importance day by day. They are needed to enable efficient, versatile, and environmentally friendly uses of energy including electricity. In a typical energy storage process, one type of energy is converted into another form of energy that can be stored and converted for use when needed. Therefore, various energy storage systems are being developed aimed at proper utilization of different energy sources. 
The main energy storage techniques can be classified into six categories, these are:  magnetic systems, electrochemical systems, hydro systems, pneumatic systems, mechanical systems and thermal systems.
Basically electrochemical energy storage/release is achieved by electron and ion charge/discharge. Supercapacitors and batteries are two kinds of typical electrochemical energy storage devices, both of which store electricity in electrochemical processes. An electrochemical energy storage device is generally composed of an anode, a cathode, a separator, and an electrolyte.
Supercapacitors, as one of the most promising candidates, have attracted increasing attention owing to long cycle life time, fast charge-discharge, high power density etc. The energy stored in the supercapacitors is clearly proportional to their capacity and the square of the voltage between the terminals of the electrochemical cell, while the capacity is proportional to the electrode-surface area and inversely proportional to the distance between the electrodes. For this reason, the main difference between capacitors and supercapacitors is the use of porous electrodes with high surface-areas by the latter ones, providing higher energy densities to the system. The performance of supercapacitors have been progressing quickly with the development of materials design strategies, synthesis techniques, characterization methodologies and electrode/electrolyte materials. 
Supercapacitors can be basically classified into two kinds, electrical double-layer capacitors (EDLCs) and pseudocapacitors. EDLC stores electricity electrostatically, in other words, the process basically origin from ion adsorption on interfaces between electrolyte and electrodes. However, pseudocapacitance is based on fast reversible faradic electrochemical reactions on the electrodes producing high specific capacitances that are 10 to 100 times larger than those delivered by EDLCs. For the EDLCs, various carbon based materials, such as carbon nanotubes (CNTs), graphene and activated carbon, have been widely employed as electrode materials. Although carbonaceous materials are widely used in supercapacitor applications, their capacitance values are relativelty low compared to transition metal oxides and conducting polymers that exhibit pseudocapacitance. By virtue of their effortless processing, facile synthesis, lightweight and low cost, pseudocapacitive materials such as polyaniline (PANI), polypyrrole (PPY) and poly (3,4-ethylenedioxythiophene) (PEDOT) have been receiving considerable attention in recent years.
Amongst these polymers polyaniline has attracted a lot of attention due to its environmental friendliness, high conductivity, chemical stability and large variety of applications. Despite the mentioned advantages, PANI suffers from limited capacity property and poor cycling stability due to its compactness leading to the low accessible surface areas and volume changes during the repeating charge/discharge processes. The charging/discharging process of PANI based electrode material is a typical doping/dedoping process accompanied by ions exchange between the polymer and the electrolyte. The dopants used during the polymerization process would have a significant effect on the electrochemical properties of PANI-rGO hybrid electrodes. It is known that utilizing dopants with different chemical structures could change PANI properties such as surface morphology, solubility, crystallinity, molecular weight and conductivity. 
In this study, polyaniline (PANI) doped with different acids (H2SO4, HClO4, HCl, HNO3, H3PO4) have been successfully polymerized with ammonium persulfate (NH4)2S2O8 as an oxidant by using chemical oxidative polymerization method.
The chemical structure of conducting polymers has been confirmed by Fourier-transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), Scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET), Gel permeation chromatography (GPC), Contact angle measurements and Thermogravimetric analysis (TGA). 
The electrochemical properties have been investigated by Cyclic voltammetry (CV), Galvanostatic charge/discharge test and Electrochemical impedance spectroscopy (EIS). To determine the performance of the electrodes, symmetrical supercapacitor cells were prepared. Polyaniline is used as the main active material for the pseudocapacitive effect and reduced graphene oxide is expected to enhance electrodes stability and provide double layer contribution with its porous structure of the resulting electrodes. Firstly, rGO was prepared by thermal reduction of graphene oxide, which was oxidized by hummers method. The synthesis of polyaniline doped with various acids was carried out by chemical oxidative polymerization. Subsequently, polyaniline and rGO were coated on the current collector surface. 12 mg of reduced graphene oxide, 9 mg of polyaniline, 3 mg of carbon black, and 3 mg of polyvinylidene fluoride were dispersed in N-methyl-2 pyrrolidone solvent. The slurry-like dispersion was dropped onto the carbon foam surface with a diameter of 1.1 cm dried at 60  °C  for 24 h. After the separator was immersed in the liquid crystal gel, it was sandwiched between two identical electrodes and taken into the split cell. Electrolyte-electrode assembly was placed in the split cell and analyzes were made with two electrode configuration, in order to simulate real supercapacitor performance. 
Each cyclic voltammetry measurement was made with 3 cycles and voltammograms of the 2nd cycles are presented. The voltammograms of the full cell with PANI which doped with H2SO4 have the largest enclosed area compared to the full cells prepared with different polyanilines. This indicates the highest gravimetric specific capacitance. The lowest area was obtained in the supercapacitor prepared with polyaniline doped with HNO3.
EIS analyzes were performed in the range of 100.000 Hz to 0.01 Hz. After Kramers–Kronig analysis, it was determined that the range of 100.000 Hz-0.1 Hz was suitable for fitting and calculations were made for this range.
All galvanostatic charge-discharge tests were performed for 10 cycles. Specific capacitance calculations were performed over the 4th, 5th and 6th cycles. GCD profiles maintain a nearly triangular shape without a high IR drop, indicating both good capacitive behavior and have good symmetry. In addition, a low IR drop indicates that the contact resistance between the current collector and the stainless steel of the split cell is low. On the other hand, the charge and discharge times obtained in PANI-H2SO4 supercapacitor are very close to each other. This is noticeable from the triangular shape charge-discharge curve. This indicates that the coulombic efficiency is also high for H2SO4-PANI symetrical supercapacitor. Similar to results shown for the cyclic voltammetry, electrodes with H2SO4 doped PANI active material exhibit the longest discharge time compared to other working electrodes.
The PANI/H2SO4 electrode, obtained by chemical oxidative polymerization using sulfuric acid, provided a specific capacitance of 1200 F.g-1 at a current density of 0.1 A.g-1. According to the chemical characterization results examined, it is concluded that the specific capacitance depends on many parameters such as surface area, conductivity, stability, molecular weight, contact angle or crystallinity. Despite the high power density of supercapacitors, their low energy density is a major disadvantage, this handicap is improved by using the PANI-H2SO4 electrode. With this electrode, a supercapacitor cell with high specific capacitance and high energy density was obtained and one of the main objectives in supercapacitor studies was achieved.