Synthesis of Carbon Quantum Dots (Cqds) from Coal and Electrochemical

Synthesis of Carbon Quantum Dots (CQDs) from Coal and Electrochemical

Characterization

A thesis presented to

the faculty of

the Russ College of Engineering and Technology of Ohio University

In partial fulfillment

of the requirements for the degree

Master of Science

Mohammadreza Rostami

August 2019

This thesis titled

Synthesis of Carbon Quantum Dots (CQDs) from Coal and Electrochemical

Characterization

MOHAMMADREZA ROSTAMI

has been approved for

the Department of Chemical and Biomolecular Engineering

and the Russ College of Engineering and Technology by

John A. Staser

Associate Professor of Chemical and Biomolecular Engineering

Mei Wei

Dean, Russ College of Engineering and Technology 3

ABSTRACT

ROSTAMI, MOHAMMADREZA, M.S., August 2019, Chemical Engineering

Synthesis of Carbon Quantum Dots (CQDs) from Coal and Electrochemical

Characterization (104 p.)

Director of Thesis: John A. Staser

Carbon quantum dots (CQDs) were synthesized using two top-down approaches from bituminous coal. More specifically, perchloric acid and hydrogen peroxide were used as oxidizer agents for CQDs synthesis. An acid recovery system was suggested for method of synthesis by perchloric acid to make this synthesis route more efficient. 30%

CQDs production yield was achieved by adding a liquid charging pipette to the method of synthesis by hydrogen peroxide. By adding MoS2 to CQDs that were synthesized by perchloric acid, the specific capacitance increased up to 130 F/g from 12 F/g. Size of

CQDs synthesized by hydrogen peroxide were 3.26 nm on average and showed a specific capacitance as high as 200 F/g with more than 15000 charge-discharge cycle stability with 90% capacitance retention.

CQDs physical and electrochemical properties were investigated by different characterization methods such as transmission electron microscopy (TEM), Fourier- transform infrared spectroscopy (FTIR), ultraviolet–visible spectroscopy (UV-Vis), scanning Electron Microscopy (SEM), cyclic voltammetry (CV), and galvanic charge discharge (GCD). 4

DEDICATION

To my mother Parvaneh Azodifar, my father Alireza Rostami, my brother Ali Rostami,

and my dear friends for their unconditional love, support, and belief in me.

I also dedicate this thesis to my teachers and advisors who taught me great lessons

throughout life. 5

ACKNOWLEDGMENTS

I want to thank my kind graduate advisor Dr. John Staser for his patience, academic and financial support; and guidance throughout my research, study, and preparation of my thesis. I am grateful for the opportunity that was given to me by him to study my masters under his supervision. I also would like to thank my co-advisor Dr.

Toufiq Reza who advised me throughout my research. I would like to thank my graduate committee members Dr. Jason Trembly and Dr. Martin Kordesch for accepting to serve on my graduate committee.

I would like to thank my colleague Akbar Saba for helping me to set up the reactor and teaching me how to operate the reactor. I also would like to thank Ari Blumer from physics department for providing MoS2 and assisting me to run electrochemical tests. I would like to show my gratitude to my colleague in our center Fazel Bateni for his valuable suggestions throughout preparation of my proposal and thesis.

I would like to thank the Center for Electrochemical Engineering Center (CEER) staff. This research would not be possible without CEER facilities and complete laboratory equipment. I also would like to thank John Goettge, lab coordinator at CEER, for his help throughout my research.

Finally, I like to thank Ohio University for this opportunity. The outcome of my graduate study at OU is not just a master’s degree; the lessons that I learnt in life and friends that I found are priceless. This journey would not be possible without endless support and love from my great friends Emmett Mascha and Aly Griffin. 6

Page

Abstract ...... 3 Dedication ...... 4 Acknowledgments...... 5 List of Tables ...... 8 List of Figures ...... 9 Chapter 1: Introduction ...... 12 1.1. Project Overview ...... 12 1.2. Statement of Objectives ...... 13 1.3. Significance of the Research ...... 15 Chapter 2: Literature Review ...... 17 2.1. Carbon Quantum Dots (CQDs) ...... 17 2.1.1. Background ...... 17 2.1.2. Applications ...... 19 2.1.3. Synthesis ...... 21 2.1.4. Surface Passivation ...... 23 2.2. Carbon Based Electrochemical Supercapacitors ...... 24 2.2.1. Background ...... 24 2.2.2. Carbon Quantum Dot as Supercapacitor Electrode Material ...... 26 Chapter 3: Experimental Setup and Methodology ...... 28 3.1. Materials ...... 28 3.2. Synthesis of CQDs by Perchloric Acid ...... 28

3.3. Capacitance Enhancement by Adding MoS2 to CQDs ...... 30 3.4. Recovery of Perchloric Acid ...... 31

3.5. Synthesis of CQDs by H2O2 in a Semi Batch Reactor...... 32 3.6. Characterization of CQDs ...... 37 Chapter 4: Results and Discussion ...... 41 4.1. Synthesized CQDs by Perchloric Acid ...... 41 4.1.1. Physical Characterization of CQDs ...... 41

4.1.2. Electrochemical Characterization of CQDs and MoS2 ...... 47 4.2. Synthesized CQDs by Recovered Perchloric Acid ...... 62 4.2.1. Physical Characterization of CQDs ...... 62 7

4.1.2. Electrochemical Characterization of CQDs ...... 64 4.3. Synthesized CQDs by Hydrogen Peroxide ...... 66 4.3.1. Physical Characterization...... 66 4.3.2. Electrochemical Characterization ...... 74 4.3.3. Conversion and Yield ...... 86 Chapter 5: Conlusions and Recommendations for Future Work ...... 90 5.1. Conclusions ...... 90 5.2. Recommendations for Future Work...... 91 References ...... 93 Appendix: Perchloric Acid Safety data sheet ...... 98 8

LIST OF TABLES

Page

Table 1: Reaction conditions and mass of used coal...... 35

Table 2: Specific capacitance of CQDs synthesized by perchloric acid, MoS2, and their mixtures in two electrode system...... 56

Table 3: Specific capacitance of CQDs synthesized by H2O2 at 180 °C in a two electrode system...... 78 Table 4: Coal conversion and CQDs synthesized by hydrogen peroxide final yield...... 87 Table 5: Effect of liquid charging pipette on the conversion...... 88 Table 6: Coal conversion and CQDs final yield in series of 1- and 3- hour reactions at 180 °C (20% hydrogen peroxide)...... 89

LIST OF FIGURES

Page

Figure 1: Schematic structure of CQDs [21]...... 19 Figure 2: CQDs chemiluminescence mechanism illustration [16]...... 20 Figure 3: Different approaches to fabricate CQDs both via bottom up and top down approach [11]...... 21 Figure 4: Schematic illustration of the selective oxidation reaction of CDs synthesis by hydrogen peroxide [31]...... 23 Figure 5: Ragone plot of different energy conversion and storage devices [45]...... 26 Figure 6: A schematic of CQDs production setup by perchloric acid...... 30 Figure 7: Acid recovery setup used to synthesize CQDs...... 32 Figure 8: Reactor setup under fume hood. Consist of (1) Liquid charging pipette (2) blowdown tank (3) reactor vessel (4) Heater (5) controller...... 33 Figure 9: FTIR spectra of synthesized CQDs by perchloric acid...... 41 Figure 10: FTIR spectra of CQDs, N-CQDs and Coal...... 42 Figure 11: TEM images and size distribution diagram of CQDs synthesized by perchloric acid (a) TEM image of CQDs in final solution (b) TEM image of single CQD particle in final solution (c) Frequency of CQDs sizes in final solution...... 43 Figure 12 UV-vis absorption spectra of CQDs synthesized by perchloric acid...... 44 Figure 13: SEM images of (a1& a2) blank carbon paper. (b1& b2) CQDs on carbon paper. (c1& c2) Mixture of 50% CQDs and 50%MoS2 on carbon paper...... 46 Figure 14: Cyclic voltammetry curve of blank carbon paper and carbon paper loaded with CQDs made with perchloric acid at 20mV/s scan rate in 0.05 M H2SO4 (three-electrode system)...... 48 Figure 15: Cyclic voltammetry curve of CQDs and N-CQDs made with perchloric acid at 20mV/s scan rate in 0.05 M H2SO4 (three-electrode system)...... 49 Figure 16: Cyclic voltammetry curve of CQDs made with perchloric acid at 20, 50, and 100 mV/s scan rates in 0.05 M H2SO4 (three-electrode system)...... 50 Figure 17 Continued: Cyclic voltammetry curve of (c) 75% CQDs made with perchloric acid and 25% MoS2 (b) MoS2 at 20, 50, and 100 mV/s scan rates in 0.05 M H2SO4 (three- electrode system)...... 52 Figure 18: Cyclic voltammetry curve of blank carbon paper at 20, 50 mV/s scan rates (two-electrode system)...... 53 Figure 19: Cyclic voltammetry curve of (a) CQDs made with perchloric acid (b) Mixture of 75% CQDs and 25% MoS2 at 20 and 50 scan rates (two-electrode system)...... 54 10

Figure 20: Cyclic voltammetry curve of (a) Mixture of 50% CQDs and 50% MoS2 (b) Mixture of 25% CQDs and 75% MoS2 at 20 and 50 scan rates (two-electrode system). . 55

Figure 21: Cyclic voltammetry curve of MoS2 at 20 and 50 scan rates (two-electrode system)...... 56

Figure 22: Cyclic voltammetry curve of CQDs in 1 M Na2SO4 and 0.05 M H2SO4 at 20 mV/s scan rate (three-electrode system)...... 57 Figure 23: Galvanic charge discharge curve of (a) Blank carbon paper (b) CQDs made by perchloric acid in 1 M Na2SO4 electrolyte (three electrode system)...... 59 Figure 24: Galvanic charge discharge curve of carbon papers loaded with CQDs and MoS2 mixtures in 1 M Na2SO4 electrolyte (three electrode system)...... 61 Figure 25: FTIR spectra of CQDs synthesized by recovered perchloric acid...... 62 Figure 26: TEM images and size distribution diagram of synthesized CQDs by recovered perchloric acid (a) TEM image of single CQD particle in final solution (b) TEM image of CQDs in final solution (c) Frequency of CQDs sizes in final solution...... 64 Figure 27: Cyclic voltammetry curve of carbon paper with CQDs made by recovered perchloric acid and by perchloric acid before recovery at 20mV/s scan rate in 0.05 M H2SO4 (three-electrode system)...... 65 Figure 28: FTIR spectra of CQDs synthesized by hydrogen peroxide (20%) at 180 and 120 °C in 1- and 3-hours reaction...... 67 Figure 29: TEM images and size distribution diagram of CQDs synthesized by hydrogen peroxide (20%) in a 1-hour reaction show (a) TEM image of CQDs in final solution (b) TEM image of single CQD particle in final solution (c) Frequency of CQDs sizes in final solution...... 69 Figure 30: TEM images and size distribution diagram of CQDs synthesized by hydrogen peroxide (20%) in a 3-hour reaction shows (a) TEM image of CQDs in final solution, (b) TEM image of single CQD particle in final solution, (c) Frequency of CQDs sizes in final solution...... 71 Figure 31: UV-vis absorption spectra of CQDs synthesized by hydrogen peroxide (20%) at 180 ͦC in a (a) 1-hour reaction and (b) 3-hour reaction...... 73 Figure 32: Cyclic voltammetry curve of carbon paper loaded with CQDs made with hydrogen peroxide (20%) at 120 °C in 1-hour reaction and perchloric acid at 20mV/s scan rate in 0.05 M H2SO4 (three-electrode system)...... 75 Figure 33: Cyclic voltammetry curve of an RDE tip loaded with CQDs synthesized by 20% hydrogen peroxide in 1- and 3-hour reaction at 180 °C at 20mV/s scan rate in 0.05 M H2SO4 (three-electrode system)...... 76 Figure 34: Cyclic voltammetry curve of an RDE tip loaded with CQDs synthesized by 10% and 20% hydrogen peroxide at 180 °C in a (a) 3-hour and (b) 1-hour reaction at 180 °C at 20mV/s scan rate in 1 M Na2SO4 (three-electrode system)...... 77 11

Figure 35: Cyclic voltammetry curve of an RDE tip loaded with CQDs and Vulcan XC- 72 at 20mV/s scan rate in 1 M Na2SO4 (three-electrode system)...... 79

Figure 36: (a) Galvanic charge discharge curve of CQDs synthesized by 20% H2O2 at 120 th th th th °C in 3-hour reaction at 10 , 500 , 2000 , and 4000 cycle in 1 M Na2SO4 electrolyte (three electrode system) (b) Capacitance retention in first 8000 cycles...... 81

Figure 37: (a) Galvanic charge discharge curve of CQDs synthesized by 20% H2O2 at 180 th th th th °C in 1-hour reaction at 10 , 100 , 500 , and 2000 cycle in 1 M Na2SO4 electrolyte (three electrode system) (b) Capacitance retention in first 8000 cycles...... 83

Figure 38: (a) Galvanic charge discharge curve of CQDs synthesized by 20% H2O2 at 180 th th th th th °C in 1-hour reaction at 10 , 300 , 2000 , 8000 , and 12000 cycle in 1 M Na2SO4 electrolyte (three electrode system) (b) Capacitance retention in first 20000 cycles...... 85

CHAPTER 1: INTRODUCTION

1.1. Project Overview

Coal is an abundant source of energy. It has been the primary fuel for electricity generation in the United States (U.S.) for many decades. However, the availability of cleaner and less expensive energy sources has led to a steep decline in coal production.

Environmental concerns were another factor: coal consumption has been falling since

2007. U.S. coal usage in 2018 reached its lowest level since 1979, and it has been predicted that coal consumption drops by 8% in 2019. With the decline of the coal industry, alternative uses for coal have become of interest [1].

One possible use is to convert coal into nanostructured materials such as carbon quantum dots (CQDs). CQDs are nanoparticles as small as 5-10nm that possess attractive optoelectronic properties. The surface chemistry of this particle, which bear carboxyl moieties, renders it water-soluble. There are several applications for CQDs: including in light-harvesting devices, sensors, biomedical imaging, and drug delivery and as electrode materials [2]. This thesis focuses on studying the electrochemical properties of CQD as supercapacitor electrode material.

In order to achieve large-scale production of CQDs, it is necessary to find a feasible, economical and suitable synthesis method. After that method has been developed, it is necessary to analyze CQD physical properties and electrochemical performance.To accomplish these objectives two different method of synthesis were compared, and their final product properties was studied, which are: synthesis by hydrogen peroxide and synthesis by perchloric acid. The more desirable method for large 13 scale production was optimized to increase the conversion of coal to CQDs. In both methods, the oxidizer breaks down and reforms coal bonds and adds oxygen to its surface. A number of characterization tests were performed in this research.

From an economic perspective, using hydrogen peroxide as the oxidizer can reduce overall cost of oxidizer agent by at least 50% compared to the most common used oxidizers for CQDs synthesis from coal which are sulfuric acid and nitric acid [3].

1.2. Statement of Objectives

The increase in greenhouse gas emissions and related environmental issues has made fossil fuels and energy sources such as coal less desirable. The future of the coal market is uncertain, especially for power generation. Finding alternative uses for coal could help bolster the industry and prevent further job losses.

Although there are several methods currently used to synthesize CQDs, there are limited number of reports of synthesis of CQDs from coal. Most of these methods have used a strong acid such as sulfuric acid or nitric acid as the oxidizer. Most of the reports focused on analyzing CQDs for applications like bio-imaging and sensors, but there are only a few reports that have focused on their application in energy storage devices such as supercapacitors. The goal of this thesis is to convert coal to CQDs that are suitable to be used in electrochemical capacitor electrodes or supercapacitors. The two main objectives of this project are to:

1- Derive a suitable process to convert coal to CQDs

2- Evaluate CQDs as electrochemical capacitor electrodes for energy storage 14

To satisfy the first goal, two oxidation methods was executed, and their results were compared. The main difference between the two is their oxidation agents: one is a strong acid (70% Perchloric acid) and the other is a common oxidizer (30% Hydrogen peroxide). Using acid as the oxidizer is the most common method to synthesize CQDs in the literature although solutions containing acid, especially perchloric acid, are not desirable for industrial processes [2], [4], [5].

For the second objective a number of tests are necessary to determine the quality of the product and characterize the CQDs particles: including TEM (transmission electron microscopy, FTIR (Fourier-transform infrared spectroscopy), UV-vis, cyclic voltammetry, and galvanic charge discharge. FTIR and UV-vis were used to study CQDs surface functionalization groups and the chemical bonds. TEM images were used to study size distribution. Cyclic voltammetry was used to determine specific capacitance of

CQDs. Galvanic charge discharge was used to study their electrochemical behavior and cycle stability.

Overall, the main goal of this project is to obtain CQDs from coal with an ideal procedure which is green, safe and fast and test them as the electrode material for supercapacitors application. Characterizing them and studying their electrochemical behavior at the Center for Electrochemical Engineering Research (CEER) is necessary to achieve this goal.

1.3. Significance of the Research

Coal has been used as a source of energy for centuries now, but the future of this valuable carbon source is uncertain as the US Environmental Protection Agency (EPA)

Clean Power Plan (CPP) shows that the reduction of greenhouse gas emission should be regulated by limiting coal fired power plants emissions. Therefore, the proposed rule for

2023 may force many of the coal power plants to retire. In turn, coal’s value is expected to decrease substantially as renewable energy becomes more dominant. The U.S is not the only nation who is trying to retire coal power plants; Germany also proposed to phase out coal by 2038 [1].

High surface area carbon electrodes are ideal for electrochemical capacitors (also known as supercapacitors or ultracapacitors). Supercapacitors or ultracapacitors can store and release energy when it is needed. The market for these electrochemical capacitors is expected to grow 30% in the next 5 years and reach 4.6 billion dollars. Increase in popularity of hybrid vehicles had a huge impact on the market [6]. The significance of this study is to facilitate an environmentally friendly synthesis process of converting coal to CQDs and optimizing this process for possible large-scale production.

There has been a lot of research on this nanomaterial in these past few years that has focused on either the synthesis process or investigating its properties, but not many researchers have focused on both. Highest specific capacitance that was reported for

CQDs alone is 84.4 F/g [7]. One of the goals of this thesis is to synthesis CQDs with specific capacitance of 200 F/g and more than 15000 cycles charge-discharge stability.

This is why it is important to study the synthesis process, electrochemical properties, and 16 structure of the CQDs in parallel to study the effects of synthesis method and synthesis conditions like temperature and reaction time on electrochemical properties of these nanoparticles.

Most of the methods that have been proposed have used acid as oxidizer and involve heating up to 12 hours; in addition using strong acids to synthesize CQDs from coal can liberate toxic gasses which can be dangerous and harmful to the environment

[8]–[10]. However, one of the methods that has been proposed in this thesis is faster (1 to

3 hours), safer, and environmentally friendly since we use hydrogen peroxide which is a common oxidizer as the oxidizer to break down the bonds in coal and oxidize it to form new bonds. 17

CHAPTER 2: LITERATURE REVIEW

2.1. Carbon Quantum Dots (CQDs)

2.1.1. Background

Quantum dots (QDs) are semiconducting nanoparticles as small as 2 to 10 nanometers that were discovered in 1980. Their properties depend on the shape and materials of which they are made [11]. Their optical and electronic properties such as broad excitation profiles and photostability make them a good candidate for potential applications in photodetector devices, light emitting diodes, and in vivo imaging [12]–

[14]. QDs can have a uniform internal composition as the core and a shell around the core. They are known as core-shell quantum dots (CSQDs). CdSe (core)/ ZnS (shell) is an example of CSQDs [15]. QDs can be chalcogenides (sulfides, tellurides or selenides) of metals like zinc, lead or cadmium such as CdTe or PbS [16].

Research into carbon has been ongoing for years [17]. CQDs were discovered more than two decades after the introduction of quantum dots, in 2004 [18].They were found by coincidence as a byproduct when Xu et al. were doing experiments on carbon nanotubes [19]. Although quantum dots in general were a point of interest in semiconductor research, recently CQDs have gained a lot of attention due to their less toxic nature compared to conventional semiconductor materials based on heavy metals

[20]. Moreover, good biocompatibility, low cost fabrication methods, and high fluorescent properties alongside low toxicity compared to other QDs have played a significant role in drawing scientists’ attention to CQDs [21]. CQDs can be an electron 18 mediator, or a photosensitizer. They can also act as a sole photocatalyst or a spectral converter [2].

A high surface area to volume ratio in carbon-based nanomaterials, especially in

CQDs, is one particularly desirable property. This property results in high capacitance since capacitance is the ratio of stored charge to potential difference in a capacitor and higher surface area can store more charge [22]. In addition, heteroatoms in the CQDs structure can drive faradaic reactions that can increase psudeocapacitance effects and result in high total capacitance [23]. In addition, the diamond-like sp3 hybridized carbon insertions carboxyl moieties cover the entire oxidized CQDs surface and, due to this fact, they can dissolve in polar solvents including water (Fig. 1). CQDs owe their functionalization to reactive amino groups [24].The source material used to synthesize

CQDs plays an important role on the final structure and results in different physicochemical and optical properties in particular.

Figure 1: Schematic structure of CQDs [21].

2.1.2. Applications

Among numerous applications that CQDs have, the most important are as follows: chemical sensing, bio sensing, bioimaging, drug delivery, photodynamic therapy, photocatalysis, electrocatalysis, and in supercapacitors [21]. CQDs show high stability and sensitivity in both aqueous solutions and live cells. Aside from using the fluorescence of CQDs as an analytical signal, CQDs show amazing chemiluminescence and electrochemiluminescence [25].

A large and growing body of literature has investigated the application of CQDs in photo-reduction and oxidation, as sensors (Fig. 2), photo- electro- catalysts for organic photovoltaic devices, and detectors, for solar cell devices, light emitting diodes, energy transfer, and ink-free luminescent patterns. CQDs are biocompatible in nature, and therefore they can be used in bioimaging, for theranostic photodynamic therapy, cancer 20 therapy, gene nanocarriers, antimicrobial agents, and as drug delivery systems [17], [21],

[26]–[33].

Figure 2: CQDs chemiluminescence mechanism illustration [16].

Although a limited number of publications report applications of CQDs as supercapacitor material, CQDs have great properties that make them suitable for supercapacitors like high surface area, long lifespan, high coulomb efficiency, and high stability. CQDs can be combined with other materials like carbon and metal oxides to enhance conductivity and facilitate charge transfer [34], [35]. Xie et al. claimed that

CQDs almost doubled polypyrrole (PPy)/TiO2 specific capacitance. Specific capacitance at 0.5 A/g increased from 482 to 849 F/g [36]. Polypyrrole is a conductive polymer that has attracted much attention in electronic devices and for chemical sensors due to its particular electronic properties [37]. 21

To date, most literature has focuses on effects of CQDs in combination with other

materials and not many reports study CQDs alone. In this thesis CQDs will be studied in

combination with molybdenum disulfide (MoS2) and alone [34].

2.1.3. Synthesis

Synthesis of CQDs can be divided into two main groups; top down approaches and

bottom-up approaches (Fig. 3).

Figure 3: Different approaches to fabricate CQDs both via bottom up and top down approach [11].

Top down approaches are: using discharge, laser ablation and electrochemical

oxidation methods to convert larger carbon particles like coal, nanodiamonds, graphite,

carbon nanotubes, carbon soot, activated carbon and graphite oxide to CQDs. One of the

cheapest and most recent approaches to synthesis CQDs is oxidizing carbon with one step

thermal method. 22

Bottom up approaches are: combustion/thermal treatments, supported synthetic and microwave synthetic routes that can be used to synthesize CQDs from molecular precursors such as citrate, carbohydrates and polymer–silica nanocomposites [21].

Among bottom-up routes, the hydrothermal treatment has gained a lot of attention because it is an economical, efficient, environmentally friendly and nontoxic synthesis method. This method has the potential to be used in production of CQDs at large scale. The raw material can be a cheap source of carbon like waste peels (like watermelon peel), juice, or coal, etc [38].

Currently, the most widely used method of CQDs synthesis from coal is combining sulfuric acid with nitric acid to form the oxidizer agent. Sulfuric acid of 98.0 wt.% and nitric acid of 65.0 wt.% can be added to a carbon source such as coal in a flask that has a stirrer and reflux system and be heated up to 120 °C for 12h [18]. Major limiting issue with this method for large scale production is the difficult process needed for removing the inorganic salts and excess oxidizing agents produced during the neutralization phase. Additionally, this method is dangerous since using strong acids to oxidize coal can liberates toxic gases [4].

Hu et al. suggested a safe method by using H2O2 instead of acid which resulted in high-yield production [4]. 30% H2O2, ground up coal, DI water and a heater that can heat up to 80 °C are all they needed to start the process. Carbon dots were produced with a oxidation reaction which is selective. This oxidation reaction can utilize hydroxyl radicals

(•OH) from hydrogen peroxide (H2O2) to separate organic amorphous carbon in coal since crystalline carbon and organic carbon have different stability. This method is green, 23 fast, safe and has a high yield without liberation of toxic gas emissions. This method also does not require a costly and tedious purification step [4]. Chen et al. also reported a hydrothermal carbonization method using H2O2 by using lignin as the carbon source which resulted in production of 2-10 nm CQDs [39]. It is worth mentioning that this method is rapid and only takes 1-4 hours depending on what carbon source has been used and how big particles are before the reaction. Figure 4 shows the reaction process.

Figure 4: Schematic illustration of the selective oxidation reaction of CDs synthesis by hydrogen peroxide [31].

2.1.4. Surface Passivation

Surface passivation of CQDs was suggested for the first time by Sun et al. Surface passivation is needed for CQDs in order to eliminate environment effects such as contamination of CQDs as much as it is possible. CQDs are sensitive and their properties might change with contamination. By adding a layer of polymeric material on CQDs surface it is possible to passivate them. One of the most important factors to determine which polymer is suitable for this, is that this material must not contain visible or near- 24

UV chromophores which would make them nonemissive at visible wavelengths [40]. In other words, polymeric materials that do not contain visible or near-UV chromophores leave passivated CQDs’ colorful luminescence emissions intact [17].

Surface passivation can weaken electrochemiluminescence activities but increase quantum yield and results in stronger fluorescence [21]. Quantum yield is the ratio of the number of photons emitted to the number of photons absorbed. Quantum yield shows how efficient a fluorescent chemical compound (fluorophore) is in converting light into fluorescence [41]. CQDs were first passivated by Sun et al. using PEG1500N; quantum yield reached 55-60% at the last fraction in this method. Imposing different defects on the

CQD surface by adding functional groups such as amine and carboxyl groups can result in large fluorescence emissions variation. One of the reasons that nitric acid was introduced as an oxidation agent to produce CQDs is that this strong acid can result in functionalization of CQDs [21].

Consequently, in order for the successful largescale production of CQDs to come to fruition, it is essential that green and facile CQD production strategies are developed.

In this thesis a safe and fast method for production of CQDs from coal is proposed. To study the effects of functionalization on capacitance, N-CQDs were prepared and were tested alongside CQDs in this thesis.

2.2. Carbon Based Electrochemical Supercapacitors

2.2.1. Background

Supercapacitors (SCs) in general can store charges on their surface electrostatically which means they store electrical energy. Electrostatic forces store this 25 energy in the electrical double layer without phase transformation. With respect to the energy density formula (E= ½ cV2), it can be said that increasing specific capacitance (c) is a way to increase energy density (E). Because of that, much research has been conducted to find higher capacitance materials to be used in SCs in the past decade [23].

Supercapacitors (electrochemical capacitors) have a promising future in the energy storage industry since they have high power density. Depending on their energy storage mechanism, SCs can be electrochemical double layer capacitors (EDLCs) or pseudocapacitors [34]. Faradic reactions lead to pseudocapacitance [42]. EDLCs, on the other hand store energy at the electrode/electrolyte interface. Hybrid SC is a bridge between these two types which can store charge both electrostatically and electrochemically. Hybrid SCs not only have higher energy density, they have higher power density compared to EDLCs and pseudocapacitors which makes them a great candidate to complement battery system in the near future. Present rechargeable batteries have limited charging rate and power density because intercalation and de-intercalation of cations is depended on diffusion [43]. Supercapacitors can charge/discharge faster than batteries, so they are candidates for everything ranging from transportation applications to grid load leveling. Figure 5 compares energy density versus. power density in different energy storage devices.

The main challenge limiting supercapacitor applications is developing and synthesizing electrode materials that have high specific capacitance, power density, cycle life, cell voltage and stability altogether. In recent years, there has been an increasing 26 interest in developing nano-sized electrode materials that can reduce diffusion length

[44].

Figure 5: Ragone plot of different energy conversion and storage devices [45].

2.2.2. Carbon Quantum Dot as Supercapacitor Electrode Material

Carbon based electrode materials are good candidates for supercapacitor electrodes because of their high surface area, conductivity, stability, and pore size distribution. There is a large volume of published studies describing the role of activated carbon as an electrode material for supercapacitors [44], [46]–[48]. However, a major problem with activated carbon is that micropores are usually not accessible for electrolyte ions and they cannot support an electrical double layer [49]. 27

Novel high surface area carbon nanomaterials such as CQDs have been used and tested to improve energy density of SCs. High packing density (higher than activated carbon which is typically 0.4 ~ 0.8 g cm−3) is an essential property to be considered while the material is being selected [7]. There are some reports in the past couple of years that have investigated CQDs potential to be used in SCs, but none of them used CQDs synthesized from coal as their electrode material, which is one of objectives in this thesis.

CQD composites have a bright future in SCs; they can improve surface roughness, electrical conductivity, and chemical stability of hybrid composites. A number of studies have examined CQD composites like CQDs–PPy/TiO2, CQDs–PPy,

CQDs/PPy-NW, CQDs-PANI/CFs, and graphene oxide/carbon dots/polypyrrole as supercapacitor electrodes. All of these studies suggest CQDs can enhance cycle stability

(up to 5000 cycles) and increase specific capacitance (up to 849 F/g) [50]–[53].

To achieve even higher electrochemical performance of CQD based supercapacitors, the high surface area carbon could be combined with other materials that impart pseudocapacitive behavior [19]. MoS2 has a large van der Waals gap making it a suitable pseudocapacitive nanomaterial because a large gap lowers the guest-host interaction. Most reports have focused on MnO2 and other metal oxide materials, but recently scientists have become attracted to sulfate oxide materials [53], [54]. As a part of this thesis, mixtures of CQDs/MoS2 specific capacitance have been measured alongside

CQDs. 28

CHAPTER 3: EXPERIMENTAL SETUP AND METHODOLOGY

The specific objectives of this thesis are: 1. Derive a suitable process to convert coal to CQDs using an acid and a common oxidizer 2. Evaluate CQDs as electrochemical capacitor electrodes for energy storage and to compare effects of different synthesis techniques on CQDs electrochemical properties.

3.1. Materials

Grinded clarion type #4A coal is the feedstock in this thesis (collected between a 40 and 60 mesh). This bituminous coal with 10% moisture originates from Southeast Ohio and was obtained from Sands Hill Mining LLC. [55]. Coal particles were between 0.25 and 0.42 mm. Perchloric acid (70%) and sulfuric acid (95 to 98%) were purchased from

Fisher Scientific. Hydrogen peroxide (30%) and ammonium hydroxide (28-30%) were purchased from Fisher Chemical. Sodium sulfate (99.0% min) was purchased from Alfa

Aesar.

The Ag/AgCl reference electrode was purchased from Fisher Scientific Accumet. 5 to

10 μm filter paper (Q5 7cm) was purchased from Fisherbrand. 3.5K MWCO dialysis cassette (30 ml) were purchased from Thermo Fisher. Carbon papers (Spectracarb

2050A-0550) were purchased from the Fuel Cell Store.

3.2. Synthesis of CQDs by Perchloric Acid

Since using nitric acid is not environmentally friendly, reaction time is quite long, and its use consumes an excess amount of energy, it is necessary to find an acid that can break down the bonds in coal and oxidize it to form CQDs under 10nm more efficiently. Among all known acids that have oxidizing properties, perchloric acid is considered one of the 29 most powerful. This feature of HClO4 results in faster conversion and lower energy consumption compared to using nitric acid.

Oxidation of coal (Clarion type #4A) was carried out in a perchloric acid solution (70%,

Fisher Scientific) to break down some of its bonds and form CQDs with size distribution under 10 nm.

Following this procedure, 2 grams of coal were added to 100 milliliters of HClO4 in a round bottom flask and stirred at 60 rpm. Then, the flask was clamped, and the acidic mixture was successively refluxed at 120 °C for 3 hours and then at 150 °C for 12 hours to obtain a light-yellow solution containing CQDs (Figure 6). To neutralize the solution, 8 ml of the solution was mixed with 80 ml deionized (DI) water and the pH was adjusted to 9.1 by adding 8 ml ammonium hydroxide (28-30%, Fisher Chemical). Finally, to remove the salts and large particles, the solution was filtered in a vacuum filtration system using 5 to

10 μm filter paper (Q5 7cm, Fisherbrand) and then dialyzed for 24 hours in a 3.5K MWCO dialysis cassette (30 ml, Thermo Fisher) in DI water at room temperature.

The experimental procedure to produce nitrogen-doped N-CQDs began with mixing

100 mL of the prepared CQD solution with 30 mL of ammonium hydroxide in a glass reaction vessel. Nitrogen doping is expected to render the CQDs suitable for further functionalization with, for example, ionic liquids or conducting polymers. 30

Figure 6: A schematic of CQDs production setup by perchloric acid.

3.3. Capacitance Enhancement by Adding MoS2 to CQDs

To achieve higher electrochemical performance of CQDs, different concentrations and configurations of CQDs and MoS2 were studied in this work. MoS2 can increase psuedocapacitance and as a result total capacitance. MoS2 was previously prepared from sodium cholate using the method described by Backes et al. [56]. To study the overall effect of mixing MoS2 and CQDs, a number of electrochemical tests were conducted in a conventional three-electrode and a two-electrode system.

Cyclic voltammetry measurements were carried out to evaluate the effect of addition of MoS2 to CQDs and calculate the overall capacitance of each mixture. The mixture was sonicated for 15 min in an ultrasonic bath (Branson 2800, 40 kHz) followed by stirring at 300 rpm for 1 h. With this procedure, different concentrations (25%, 50%, 31

75%, 100%) of CQDs and MoS2 were studied. MoS2 sheets with 50 nm in length are likely supporting carbon dots. It is possible that CQDs attach to these sheets by van der Waals force [57, p. 2].The capacitance for each solution was calculated using the equation (5):

푑푉 (1) 퐼 = 푚 × 퐶 × 푑푡

푑푉 (2) 푘 = 푑푡

퐼 × 푑푉 = 푚 × 퐶 × 푘 × 푑푉 (3)

푉2 푉2 (4) ∫ 퐼 × 푑푉 = ∫ 푚 × 퐶 × 푘 × 푑푉 푉1 푉1

푆1 = (푉2 − 푉1) × 푚 × 푘 × 퐶 (5)

I is the current (Amps), k is the scan rate (mV/s), S1 is area under the curve relative to the horizontal axis, m is weight of used material on carbon paper (g), V2 and V1 are potential windows (V). C is the specific capacitance (F/g) [58].

3.4. Recovery of Perchloric Acid

To make the process environmentally friendly and more economical, the perchloric acid in the experiment was recovered during the synthesis process by adding a condenser to the system. A mixture of water and ice cubes were used to reduce condenser temperature.

Figure 7 shows a schematic of the system that was used to recover acid.

To recover the acid during synthesis, 2g of coal and 100 ml of 70% perchloric acid were added into a round bottom flask. A magnetic stirrer was added to the flask.

Temperature was fixed at 200 °C. 32

To collect CQDs at the bottom of the flask after this process, DI water was added immediately, and the temperature was set to 80 °C for ten minutes while the solution was stirred. Recovered acid was reused to obtain more CQDs from more coal and recovered again. To compare recovered acid with the original acid that was used in the process, the pH was compared in both solutions.

Figure 7: Acid recovery setup used to synthesize CQDs.

3.5. Synthesis of CQDs by H2O2 in a Semi Batch Reactor

Making CQDs with acid was challenging, time consuming, and a dangerous process which cannot be used in mass production. Therefore, a facile, fast and green top down approach needs to be developed. Hu et al. proposed a method that used 30% H2O2 instead of acids to make CQDs. They used anthracite as carbon source [4].

In this thesis, a reactor has been used instead of a flask since mixing hydrogen peroxide with water and coal can produce a large amount of oxygen and a little amount of toxic gas (H2S). High pressure during the reaction can be dangerous without a reactor, 33 especially at high temperatures. Figure 8 shows the diagram of the reactor setup that was used in this thesis.

The specific components of the reactor were: a Parr stainless steel 300 ml pressure reactor model 452HC T316 070191 4783 (2000 psi 350C 1991), a Parr 4841 controller used to control the temperature and the stirrer speed, and a Parr liquid charging pipette

2113-150 ml used to add hydrogen peroxide.

Water input

Water output

Figure 8: Reactor setup under fume hood. Consist of (1) Liquid charging pipette (2) blowdown tank (3) reactor vessel (4) Heater (5) controller.

According to previous reports, only 20% CQDs production yield (from coal) was achieved in 24 h synthesis process by an strong acid [59]. A higher yield can be achieved by adding oxidizer gradually during the process. This modified reactor has a liquid charging pipette to add H2O2 during the process. This pipette not only makes the process 34 safer but will lower energy consumption since the reaction between H2O2, H2O and coal is exothermic.

To run this process, first 5 g or the desired amount of coal (Clarion type #4A) was weighed and loaded in the reactor glass liner (300 ml, Parr Instrument Company), then 40 ml or the desired amount of DI water was added to the glass liner. Then the glass liner was inserted into the reactor vessel (Parr Stainless Steel 300 ml Pressure Reactor Model

452HC). The reactor head, which has a stirrer, thermometer, rupture disk pipe, and liquid charger was sealed on top of the vessel. Before starting the reaction, the vessel was moved inside the heater and all the connections and pipes were connected to it. Connections that are required: thermocouple, stirrer and heater. The thermocouple and stirrer are connected to the head of the reactor as it is shown in Figure 8. Water input and output pipes were connected to the head of the reactor vessel to cool down the magnetic drive (stirrer) during the reaction to prevent overheating. The heater is controlled with a controller (Parr 4841,

Parr Instrument company) which reads the temperature inside the vessel using a thermocouple. A pipe line is connected between the reactor and the blow down tank.

Finally, the liquid charging pipette was connected to the vessel. Pressure can be controlled by the needle valve shown in the diagram in case of the need for pressure release to avoid high pressure in the reactor vessel. The pressure was controlled to be under 800 psi.

40 ml or half of the total desired amount of hydrogen peroxide (30%, Fisher chemical) was added with the liquid charging pipette (150 ml, Series A2113HC, Parr

Instrument Company) using Ar gas. The liquid charging pipette works based on pressure difference between the vessel and an Ar tank. The desired temperature (120 °C or 180 °C) 35 and stirrer speed were set with a controller (Parr 4841, Parr Instrument company). 40 ml or the other half of the hydrogen peroxide was added to the reactor during the process, gradually. To have 20% hydrogen peroxide in the vessel 40 ml of DI water and 80 ml of

30% hydrogen peroxide are needed in total. To have 10% hydrogen peroxide in the vessel

80 ml of DI water and 40 ml of 30% hydrogen peroxide is needed in total.

To find the highest yield possible, different temperatures, reaction times, hydrogen peroxide concentrations, and coal mass were tested. Table 1 shows the reaction conditions that were used to synthesize CQDs using hydrogen peroxide in this thesis.

Table 1: Reaction conditions and mass of used coal. Temperature (°C) Coal mass (g) H2O2 Concentration (%) Reaction

Time (hours)

120 3 10 1

120 5 10 1

120 5 20 1

120 5 10 3

120 5 20 3

180 3 10 1

180 5 10 1

180 5 20 1

180 5 20 3

Since water expands during the heating process and hydrogen peroxide is converted into water over time, it is necessary to calculate this expansion to avoid overloading the reactor using the following equations:

1 (6) 푣푓 = 휌푓

1 (7) 푣푔 = 휌푔

푥 = (푣 − 푣푓)/ (푣푔 − 푣푓) (8)

푚퐻표푡 푙𝑖푞푢𝑖푑 = (1 − 푥) ∗ 푚푤푎푡푒푟 (9)

푣퐻표푡 푙𝑖푞푢𝑖푑 = 푚 ∗ 푣푓 (10)

vf and vg are specific volume of the saturated liquid and vapor. ρf and ρg are liquid and vapor densities. Since densities are known, specific volumes can be calculated to get x. x can be used to get the mass of hot hydrogen peroxide and final volume of hot hydrogen peroxide from equation (10).

A bucket (5 Gallons, Lowe’s) was used as a blowdown tank (in case reaction pressure and temperature go over the reactor limit which results in bursting the rupture disc). In this case the blow down line (Figure 8) can transfer the vapor to the blowdown tank to reduce the pressure and temperature in the reactor vessel, protecting the reactor from damage.

To remove the salts and large particles, the solution was filtered in a vacuum filtration system using 5 to 10 μm filter paper (Q5 7cm, Fisherbrand) and then dialyzed for 37

24 hours in a 3.5K MWCO dialysis cassette (30 ml, Thermo Fisher) in a bucket of water at room temperature.

The conversion was calculated based on remaining coal on the filter paper. The filter paper was dried in an oven (Model 30GC Lab Oven, Quincy Lab., Chicago, IL, USA) at 80 °C before weighing.

푐표푛푣푒푟푠푖표푛 (11)

퐶표푎푙 푢푠푒푑 푎푡 푠푡푎푟푡 표푓 푟푒푎푐푡푖표푛 − 푅푒푚푎푖푛푖푛푔 푐표푎푙 표푛 푓푖푙푡푒푟 푝푎푝푒푟 (푔) = 퐶표푎푙 푢푠푒푑 푎푡 푠푡푎푟푡 표푓 푟푒푎푐푡푖표푛 (푔)

Final yield was calculated based on the mass of CQDs produced. The final solution after filtration was dried in oven at 80 °C until CQDs remained in solid form.

3.6. Characterization of CQDs

High-resolution transmission electron microscopy (HRTEM) on a JEM-2100 (JEOL,

Japan) operated at 200 kV was used to capture images of CQDs that were synthesized by hydrogen peroxide and perchloric acid. These images were used to compare size distributions and investigate morphological characteristics of CQDs.

Fourier-transform infrared (FTIR) spectroscopy was performed by means of a

VERTEX 70v FTIR Spectrometer, in the range of 500 and 4000 cm-1 with a resolution of

4 cm-1 on CQDs that were synthesized by hydrogen peroxide and perchloric acid. Resulting spectra were used to determine the CQDs chemical bonds.

Ultraviolet–visible spectroscopy (uv-vis) absorption specta of CQDs synthesized by hydrogen peroxide and perchloric acid were collected using a Hewlett Packard 8452A 38

Diode-Array Spectrophotometer. The resulting spectra were used to determine surface functional groups [60].

The electrochemical oxidation tests were carried out in conventional three and two electrode configurations using a Solartron Analytical CellTest 1470E (AMETEK, Inc.). In the three-electrode configuration, a Pt ring was used as the counter electrode, and an

Ag/AgCl or Hg/HgO electrode was used as reference electrode. H2SO4 (0.5 and 0.05 molar,

Fisher Scientific) and Na2SO4 (1 molar, Alpha Aesar) were used as electrolytes. The working electrode is either RDE (with Pt tip) or a carbon paper. The working electrode was prepared by drop casting solutions that contains CQDs on commercial gas diffusion layer

(GDL) carbon papers (Fuel Cell Store, Spectracarb 2050A-0550) and RDE (Pt tip, rotating disk electrode). Loaded carbon papers and RDE tip which is made from Pt were dried in laboratory oven at 80 °C. RDE was fixed and it does not rotate during the tests.

In the two-electrode configuration, both working electrode and counter electrodes are carbon papers loaded with CQDs. CQDs were loaded on carbon papers using the method mentioned above. A membrane (Nafion 115) was used as solid-state electrolyte to separates these carbon papers.

Cyclic voltammetry (CV) test was performed to investigate capacitance behavior of

CQDs and to calculate specific capacitance of CQDs. Cview (Version 3.5f, Scribner

Associates Inc) software was used to calculate area under the curve (charge). The potential window in the CV tests was set to -0.2 to 1 V, 0 to 1 V, 0 to 0.8 V, and -0.4 to 0.7 V vs. the reference electrode. CV tests were performed under 20mV/s, 50mV/s, and 100mV/s scan rates. 39

Galvanic charge discharge (GCD) tests were performed in two-electrode system to investigate stability of CQDs as the electrode material in supercapacitors and study their electrochemical behavior. Capacitance retention was calculated at the nth cycle using equation (12). Since capacitors are not stable at first cycles, specific capacitance at nth cycle was compared to specific capacitance at the 10th cycle. Based on equation (13), if mass, potential window, and discharge currents are equal in both capacitors (which are equal since we are comparing the same electrode during the test), capacitance retention can be calculated using equation (14). In other words, capacitance retention is the ratio of discharge time at the nth cycle to discharge time at the 10th cycle [61]. The potential range was set to 0 to 1 V and 0 to 0.8 V. Applied currents for GCD tests were ±10-5, ±10-6,

±5*10-6, ±10-7 Amps. CQDs show stable electrochemical behavior in these potential windows in selected electrolytes. If voltage excess in a positive or negative direction happens then dissolution may occur and it may affect stability of the electrode and also degrade the electrolyte [62], [63]. The reaction mechanism was not studied in this thesis.

푆푝푒푐푖푓푖푐 푐푎푝푎푐푖푡푎푛푐푒 푎푡 푛푡ℎ 푐푦푐푙푒 (퐹) (12) 퐶푎푝푎푐푖푡푎푛푐푒 푟푒푡푒푡푎푡푖표푛 = 푆푝푒푐푖푓푖푐 푐푎푝푎푐푖푡푎푛푐푒 푎푡 10푡ℎ 푐푦푐푙푒 (퐹)

퐼∆푡 (13) 퐶 = 푚∆푉

∆푡 푎푡 푛푡ℎ 푐푦푐푙푒 (14) 퐶푎푝푎푐푖푡푎푛푐푒 푟푒푡푒푡푎푡푖표푛 = ∆푡 푎푡 10푡ℎ 푐푦푐푙푒

Where C (F g−1) is the specific capacitance; ∆t (s) is the discharge time; I (A) is the discharge current; ∆V (V) is the potential window; and m (mg) is the mass of materials loaded in working electrode.

CHAPTER 4: RESULTS AND DISCUSSION

4.1. Synthesized CQDs by Perchloric Acid

4.1.1. Physical Characterization of CQDs

A common way to synthesize CQDs in the past decade has been using acid to oxidize carbon-based materials such as coal. In this work, perchloric acid was used to synthesize CQDs.

The CQDs were analyzed with FTIR spectroscopy (Figure 9). Peaks around 1100 cm-1 correspond to C-O bond, 1400 cm-1 correspond to C-N/C-O bonds, and 1550-1600 cm-1 correspond to C=C bonds. The peak around 3200 cm-1 correspond to O-H bonds

[64]. Peaks were similar to CQDs made with a common procedure which involves mixing nitric acid and sulfuric acid [18]. The comparison shows successful conversion of coal to CQDs. Figure 10 compares the FTIR spectra of coal to CQDs and N-CQDs. The coal FTIR spectra shows no functional groups such as C-O or C-OH. FTIR spectra of N-

CQDs is similar to CQDs with the same peaks at the same wavenumbers.

Figure 9: FTIR spectra of synthesized CQDs by perchloric acid. 42

Figure 10: FTIR spectra of CQDs, N-CQDs and Coal.

The size distribution of CQDs was investigated by high-resolution transmission electron microscopy (HRTEM). CQDs made with perchloric acid are 10nm or smaller. It can be seen in Figure 11(a) that these nanoparticles are well distributed in solution.

Figure 11(c) shows the CQDs size distribution based on 20 randomly selected particles from the TEM image. The average size is 8.15 nm. Figure 11(b) shows a crystalline structure of single particle and lattice spacing of 0.16 nm that is close to the diffraction facets (100) of graphite carbon [4]. 43

Figure 11: TEM images and size distribution diagram of CQDs synthesized by perchloric acid (a) TEM image of CQDs in final solution (b) TEM image of single CQD particle in final solution (c) Frequency of CQDs sizes in final solution.

UV-vis was performed (Figure 12) to investigate the transition of aromatic domains in the carbonized core. The peak at 256 nm is ascribed to the π–π* transition of aromatic domains in the carbonized core of CQDs [5]. According to Hemment et al., C6 ring aromatic domains (sp2 bonding) is the origin of the optical properties [65].

Figure 12 UV-vis absorption spectra of CQDs synthesized by perchloric acid.

To investigate the effect of CQDs and mixture of CQDs/MoS2 on the surface of carbon papers that was used in electrochemical characterization tests, scanning electron microscopy (SEM, JEOL-JSM-6930LV) images were taken. Figure 13 shows images of blank carbon paper and carbon paper loaded with CQDs and 50% CQDs/ 50% MoS2 mixture. These images show that the surface area of the electrode increased significantly.

Comparing Fig 13(c) and (b) can prove that mixtures of these nanoparticles can increase the surface area even more than CQDs alone. We can see cluster of CQDs and the mixture on both images. 46

(a1) (a2)

(b1) (b2)

(c1) (c2)

Figure 13: SEM images of (a1& a2) blank carbon paper. (b1& b2) CQDs on carbon paper. (c1& c2) Mixture of 50% CQDs and 50%MoS2 on carbon paper.

4.1.2. Electrochemical Characterization of CQDs and MoS2

To study electrochemical behavior of CQDs and mixture of CQDs/MoS2, their performance under cyclic voltammetry (CV) was investigated. By analyzing CV curves, the specific capacitance of these materials can be calculated and compared.

Figure 14 compares the results obtained from preliminary analysis of CV tests performed on a blank carbon paper and a carbon paper loaded with CQDs (less than 10-4 g). This test was performed in a three-electrode system with Pt as the counter electrode,

0.05 M H2SO4 as the electrolyte, and a Hg/HgO electrode as the reference electrode (at 20 mV/s scan rate). Based on area under the curve, total charge is 2.0541*10-4 Coulomb for blank carbon paper and 4.2909*10-4 Coulomb for carbon paper loaded with CQDs. These results indicated that CQDs possess good capacitance and may suitable as capacitor electrodes. 48

Figure 14: Cyclic voltammetry curve of blank carbon paper and carbon paper loaded with CQDs made with perchloric acid at 20mV/s scan rate in 0.05 M H2SO4 (three-electrode system).

Figure 15 compares CQDs and N-CQDs CV curves. The test was performed in the same three electrode system at 20 mV/s scan rate. What is interesting in this figure is that the behavior of the two different electrode materials is similar and based on equation

(5) since mass of loaded materials are equal, specific capacitance is almost equal. In other words, no significant differences (less than 1%) were found between specific capacitance of CQDs and N-CQDs that were functionalized by ammonium hydroxide.

1.50E-05

1.00E-05

5.00E-06

0.00E+00

-5.00E-06

N-CQDs I I (Amps) -1.00E-05 CQDs

-1.50E-05

-2.00E-05

-2.50E-05 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 1.2 E (Volts)

Figure 15: Cyclic voltammetry curve of CQDs and N-CQDs made with perchloric acid at 20mV/s scan rate in 0.05 M H2SO4 (three-electrode system).

Figure 16 compares CQDs CV curves at different scan rates (20, 50, and 100 mV/s). This test was performed in a three-electrode system which has Pt as counter electrode, 0.05 M H2SO4 as electrolyte, and Hg/HgO electrode as reference electrode.

The working electrode was an RDE with CQDs loaded on Pt tip. These results show that at all scan rates peaks centering at the same voltages which means that the electron transfer is reversible.

Figure 16: Cyclic voltammetry curve of CQDs made with perchloric acid at 20, 50, and 100 mV/s scan rates in 0.05 M H2SO4 (three-electrode system).

Figure 17 compares CQD and MoS2 mixtures (at different mass ratios) CV curves at different scan rates (20, 50, and 100 mV/s). This test was performed in a three- electrode system with Pt as the counter electrode, 0.05 M H2SO4 as the electrolyte, and a

Hg/HgO electrode as the reference electrode. The working electrode was an RDE with

CQDs and MoS2 mixture loaded on Pt tip. These results show that at all three scan rates peaks centering at the same voltages which means that the electron transfer is reversible. 51

Figure 17: Cyclic voltammetry curve of (a) 25% CQDs made with perchloric acid and 75% MoS2 (b) 50% CQDs made with perchloric acid and 50% MoS2 at 20, 50, and 100 mV/s scan rates in 0.05 M H2SO4 (three-electrode system).

Figure 17 Continued: Cyclic voltammetry curve of (c) 75% CQDs made with perchloric acid and 25% MoS2 (b) MoS2 at 20, 50, and 100 mV/s scan rates in 0.05 M H2SO4 (three- electrode system).

To calculate and compare specific capacitance of CQDs and mixtures of

CQDs/MoS2, a CV test was performed in a two-electrode system. The working electrode

and counter electrode are both carbon papers loaded with CQDs and mixtures of

CQDs/MoS2. A Nafion membrane was used between these carbon papers as the solid

electrolyte. Figure 18 shows CV curves of blank carbon paper before loading the

materials on them. Figures 19, 20, and 21 show CV curves at 20 and 50 mV/s scan rates.

Table 2 shows the specific capacitance from equation (1), mass of the loaded material on

both carbon papers, and the scan rate. The highest specific capacitance (130 F/g) belongs

to a mixture of 75% CQDs and 25% MoS2. A significant increase in specific capacitance

can be seen by comparing specific capacitance of CQDs and MoS2 to the mixture of these

materials. Carbon based supercapacitor materials has specific capacitance in range of 40-

300 F/g and according to Dubey et al. specific capacitance more than 100 F/g is

considered large value for these materials [66].

Figure 18: Cyclic voltammetry curve of blank carbon paper at 20, 50 mV/s scan rates (two-electrode system). 54

(a)

(b)

Figure 19: Cyclic voltammetry curve of (a) CQDs made with perchloric acid (b) Mixture of 75% CQDs and 25% MoS2 at 20 and 50 scan rates (two-electrode system). 55

(a)

(b)

Figure 20: Cyclic voltammetry curve of (a) Mixture of 50% CQDs and 50% MoS2 (b) Mixture of 25% CQDs and 75% MoS2 at 20 and 50 scan rates (two-electrode system). 56

Figure 21: Cyclic voltammetry curve of MoS2 at 20 and 50 scan rates (two-electrode system).

Table 2: Specific capacitance of CQDs synthesized by perchloric acid, MoS2, and their mixtures in two electrode system. Material on carbon Charge Specific Material Scan rate paper (Coul) capacitance (F/g) mass (g) (mV/s)

MoS2 0.0014 8.67 0.0024 50

MoS2 0.0032 48.36 0.0024 20

25% CQD -75% MoS2 0.0020 12.71 0.0023 50

25% CQD -75% MoS2 0.0031 49.36 0.0023 20

50% CQD -50% MoS2 0.0080 79.81 0.0036 20

50% CQD -50% MoS2 0.0054 21.53 0.0036 50

75% CQD -25% MoS2 0.0134 130.01 0.0037 20

75% CQD -25% MoS2 0.0086 33.51 0.0037 50 CQDs 0.000663 12.45 0.0019 20 CQDs 0.000692 5.20 0.0019 50 Blank carbon paper 1.84*10-05 0.0309 0.0213 20 Blank carbon paper 1.16*10-05 0.0077 0.0213 50 57

To compare electrochemical behavior of CQDs in a different electrolyte, a CV test was conducted in a three-electrode system. In both systems the counter electrode was a Pt wire and the working electrode was an RDE loaded with CQDs by the drop cast method. In the first system the electrolyte was 1 M Na2SO4 and the reference electrode was Ag/AgCl. In the second system the electrolyte was 0.05 M H2SO4 and the reference electrode was Hg/HgO. It can be seen in Figure 22 that CV curve in the first system has greater area under it. The first system charge was 8.3211*10-4 which means specific capacitance doubled compared to the second system. The slope up in the CV (as opposed to being symmetric around the axis) suggests resistive behavior in addition to pseudocapacitance behavior due to the presence of peaks in the CV curve.

Figure 22: Cyclic voltammetry curve of CQDs in 1 M Na2SO4 and 0.05 M H2SO4 at 20 mV/s scan rate (three-electrode system).

Galvanic charge-discharge tests were performed in a three-electrode system. The counter electrode was a Pt wire, the working electrode was a carbon paper loaded with

CQDs by the drop cast method, the reference electrode was Ag/AgCl, and the electrolyte was 1 M Na2SO4.

Figure 23 (a)-(b) presents the charge-discharge responses of a blank carbon paper and a carbon paper loaded with CQDs corresponding to a constant charge and discharge current of 10-6 Amps. Discharge time of the carbon paper loaded with CQDs is 13 seconds in average. Discharge time of the blank carbon paper is less than 2 seconds in average. According to equation 13 specific capacitance of blank carbon paper is negligible compared to CQDs. A reversible faradaic reaction can be seen from the CQDs

GCD curve. Reuleaux triangular shape of CQDs GCD curve shows a typical supercapacitor material GCD curve. The logarithmic discharge curve during charge and discharge shows CQDs pseudocapacitance electrochemical behavior [67].

(a)

(b)

Figure 23 : Galvanic charge discharge curve of (a) Blank carbon paper (b) CQDs made by perchloric acid in 1 M Na2SO4 electrolyte (three electrode system). 60

Similar charge-discharge experiments were carried out on the CQD-MoS2 mixtures. The counter electrode was a Pt wire, the working electrode was a carbon paper loaded with CQDs by the drop cast method, the reference electrode was Ag/AgCl, and the electrolyte was 1 M Na2SO4.

Figure 24 presents the GCD responses of blank carbon papers loaded with CQDs

-6 and MoS2 corresponding to a constant charge and discharge current of 10 Amps.

Discharge time of the carbon paper loaded with 75% CQDs/ 25% MoS2 is 265 seconds on average. Discharge time of the carbon paper loaded with 50% CQDs/ 50% MoS2 is

113 seconds on average. Discharge time of the carbon paper loaded with 25% CQDs/

75% MoS2 is 3 seconds in average. These results confirm that highest specific capacitance belongs to 75% CQDs/ MoS2 mixture. These results also confirm that adding

MoS2 to CQDs can increase specific capacitance. The logarithmic shape of discharge curve from the mixtures GCD curves suggest that a reversible faradaic reaction is occurring. This reaction might be the redox reactions of hydroxide groups on CQDs surface [68].

Figure 24: Galvanic charge discharge curve of carbon papers loaded with CQDs and MoS2 mixtures in 1 M Na2SO4 electrolyte (three electrode system).

4.2. Synthesized CQDs by Recovered Perchloric Acid

As mentioned earlier, perchloric acid was recovered by adding a condenser to the system. 100 ml perchloric acid was used in the process and 80 ml was recovered. The pH changed from 0.1 to 0.4. CQDs were synthesized by recovered acid, using 2 g of coal.

Perchloric acid was recovered again during the synthesis process. The pH changed from

0.4 to 0.6 after the experiment. 70 ml perchloric acid was recovered in the second synthesis process.

4.2.1. Physical Characterization of CQDs

The CQDs were analyzed with FTIR spectroscopy (Figure 25). Peaks around

1100 cm-1 correspond to C-O bonds, 1400 cm-1 corresponds to C-N/C-O, and 1550-1600 cm-1 corresponds C=C bonds. The peak around 3200 cm-1 corresponds to O-H bonds. The peak around 2550 cm-1 corresponds to C-H bonds [64].

Figure 25: FTIR spectra of CQDs synthesized by recovered perchloric acid. 63

The size distribution of these CQDs was investigated by high-resolution transmission electron microscopy (HRTEM). CQDs made with recovered perchloric acid are 10nm or smaller. It can be seen in Figure 26 (b) that these nanoparticles are not well distributed in solution. Figure 26 (c) shows CQDs size distribution based on 20 randomly selected particles from the TEM image. The average size is 7.18 nm. Figure 26 (a) shows a crystalline structure of single particle and lattice spacing of 0.13 nm that is close to the diffraction facets (100) of graphite carbon [4]. These CQDs are 12% smaller compared to

CQDs that were synthesized by original perchloric acid. It’s possible that the size is different due to the temperature effect. 64

(a) (b)

(c)

Figure 26: TEM images and size distribution diagram of synthesized CQDs by recovered perchloric acid (a) TEM image of single CQD particle in final solution (b) TEM image of CQDs in final solution (c) Frequency of CQDs sizes in final solution.

4.1.2. Electrochemical Characterization of CQDs

Figure 27 compares the results obtained from preliminary analysis of CV tests performed on a carbon paper loaded with CQDs made by recovered acid and a carbon paper loaded with CQDs made by acid before recovery. This test was performed in a three-electrode system with Pt as the counter electrode, 0.05 M H2SO4 as the electrolyte, and a Hg/HgO electrode as the reference electrode (at 20 mV/s scan rate). Based on area 65 under the curve, total charge is 4.4945*10-4 Coulomb for carbon paper loaded with CQDs made by recovered acid and 4.2909*10-4 Coulomb for carbon paper loaded with CQDs made by acid before recovery. The difference is less than 5% and it is negligible. Smaller

CQDs results in higher total surface area and therefore they can store more charge, that can be the reason why CV curve of CQDs made by recovered acid have larger area under the curve.

Figure 27: Cyclic voltammetry curve of carbon paper with CQDs made by recovered perchloric acid and by perchloric acid before recovery at 20mV/s scan rate in 0.05 M H2SO4 (three-electrode system). 66

4.3. Synthesized CQDs by Hydrogen Peroxide

4.3.1. Physical Characterization

A facile and green method to synthesize CQDs by a one-step hydrothermal method that use hydrogen peroxide as an oxidizer was suggested in recent years. This method is faster and safer compared to the method that was mentioned earlier.

The CQDs that were synthesized by hydrogen peroxide (20%) at different temperatures and in different times were analyzed with FTIR spectroscopy (Figure 28).

Peaks around 1100 cm-1 correspond to C-O bond, 1400 cm-1 correspond to C-N/C-O bonds, and 1550-1600 cm-1 correspond to C=C bonds. The peak around 3200 cm-1 correspond to O-H bonds [64]. Peaks were similar to CQDs that were synthesized by perchloric acid. The comparison shows successful conversion of coal to CQDs. 67

Figure 28: FTIR spectra of CQDs synthesized by hydrogen peroxide (20%) at 180 and 120 °C in 1- and 3-hours reaction.

The size distribution of CQDs was investigated by high-resolution transmission electron microscopy (HRTEM). CQDs made with hydrogen peroxide (20%) are 10nm or smaller. Figure 29 shows TEM images of CQDs after 1-hour reaction in 20% hydrogen peroxide. It can be seen in Figure 29 (a) that these nanoparticles are well distributed in solution. Figure 29 (c) shows the CQDs size distribution based on 20 randomly selected particles from the TEM image. The average size is 3.65 nm. Figure 29 (b) shows a crystalline structure of single particle and lattice spacing of 0.21 nm that is close to the diffraction facets (100) of graphite carbon [4].

Figure 29: TEM images and size distribution diagram of CQDs synthesized by hydrogen peroxide (20%) in a 1-hour reaction show (a) TEM image of CQDs in final solution (b) TEM image of single CQD particle in final solution (c) Frequency of CQDs sizes in final solution.

Figure 30 shows TEM images of CQDs after 3-hour reaction in 20% hydrogen peroxide. It can be seen in Figure 30 (a) that these nanoparticles are well distributed in solution. Figure 30 (c) shows the CQDs size distribution based on 20 randomly selected particles from the TEM image. The average size is 3.26 nm which is less than the average size of CQDs that were synthesized in a 1-hour reaction. Figure 30 (b) shows a crystalline structure of single particle and lattice spacing of 0.11 nm. CQDs smaller interlayer spacing can be because of graphene sheets effective π- π stacking that have structural defects. Another explanation for the compact stacking of these layers is existence of hydrogen bonds among the functional groups that contain oxygen at the edge of graphene layers [18].

Figure 30: TEM images and size distribution diagram of CQDs synthesized by hydrogen peroxide (20%) in a 3-hour reaction shows (a) TEM image of CQDs in final solution, (b) TEM image of single CQD particle in final solution, (c) Frequency of CQDs sizes in final solution.

UV-vis was performed (Figure 31) to investigate the transition of aromatic domains in the carbonized core. The first peak around 260 nm is ascribed to the π–π* transition of aromatic domains in the carbonized core of CQDs [5]. According to

Hemment et al., C6 ring aromatic domains (sp2 bonding) is the origin of the optical properties [65]. The second peak is around 300-330 which is attributed to n–π* transition of –C=O, C–N or –C–OH bonds in the sp3 hybridized domains, which originated from carboxyl (–COOH) or amine (–NH2) groups that exist on the CQDs surface. 73

(a)

(b)

Figure 31: UV-vis absorption spectra of CQDs synthesized by hydrogen peroxide (20%) at 180 ͦC in a (a) 1-hour reaction and (b) 3-hour reaction.

4.3.2. Electrochemical Characterization

To study electrochemical behavior of CQDs, their performance under cyclic voltammetry (CV) was investigated. By analyzing CV curves, the specific capacitance of these materials can be calculated and compared.

Figure 32 compares the results obtained from preliminary analysis of CV tests performed on a carbon paper loaded with CQDs synthesized by hydrogen peroxide (less than 10-4 g) and a carbon paper loaded with CQDs synthesized by perchloric acid (less than 10-4 g). This test was performed in a three-electrode system with Pt as the counter electrode, 0.05 M H2SO4 as the electrolyte, and a Hg/HgO electrode as the reference electrode (at 20 mV/s scan rate). Based on area under the curve, total charge is

4.2909*10-4 Coulomb for carbon paper loaded with CQDs synthesized by perchloric acid

(20%) at 120 °C in a 1-hour reaction and 1.9605*10-3. These results indicated that CQDs synthesized by hydrogen peroxide possess good capacitance and may be suitable as capacitor electrodes.

Figure 32: Cyclic voltammetry curve of carbon paper loaded with CQDs made with hydrogen peroxide (20%) at 120 °C in 1-hour reaction and perchloric acid at 20mV/s scan rate in 0.05 M H2SO4 (three-electrode system).

Figure 33 compares the results obtained from analysis of CV tests performed on an RDE loaded with CQDs synthesized by hydrogen peroxide (20%) in a 1-hour and 3- hour reaction at 180 °C. This test was performed in a three-electrode system with Pt as the counter electrode, with 0.05 M H2SO4 as the electrolyte, and with a Hg/HgO electrode as the reference electrode (at 20 mV/s scan rate). Based on the shape of CV it can be concluded that CQDs that were synthesized in a 3-hour reaction show more reversibility and, therefore, they are more stable compared to CQDs synthesized in a 1-hour reaction.

Figure 33: Cyclic voltammetry curve of an RDE tip loaded with CQDs synthesized by 20% hydrogen peroxide in 1- and 3-hour reaction at 180 °C at 20mV/s scan rate in 0.05 M H2SO4 (three-electrode system).

Figure 34 compares the results obtained from analysis of CV tests performed on an RDE loaded with CQDs synthesized by 10% and 20% hydrogen peroxide in a 1-hour and 3-hour reaction at 180 °C. This test was performed in a three-electrode system with

Pt as the counter electrode, 1 M Na2SO4 as the electrolyte, and a Ag/AgCl electrode as the reference electrode (at 20 mV/s scan rate). Based on the area under the CV curve, it can be concluded that CQDs that were synthesized by 20% hydrogen peroxide possess better capacitance, and they are more suitable for supercapacitors electrode. 77

(a)

(b)

Figure 34: Cyclic voltammetry curve of an RDE tip loaded with CQDs synthesized by 10% and 20% hydrogen peroxide at 180 °C in a (a) 3-hour and (b) 1-hour reaction at 180 °C at 20mV/s scan rate in 1 M Na2SO4 (three-electrode system). 78

To calculate and compare specific capacitance of CQDs synthesized in different conditions at 180 °C, a CV test was performed in a two-electrode system. The working electrode and counter electrode are both carbon papers loaded with CQDs. A Nafion membrane was used between these carbon papers as the solid electrolyte. Table 3 shows the specific capacitance from equation (1), mass of the loaded material on both carbon papers, and the scan rate. The highest specific capacitance (203.5 F/g) belongs to CQDs synthesized by 20% hydrogen peroxide in a 3-hour reaction at 180 °C. The specific capacitance was almost doubled by increasing the reaction time from 1 hour to 3 hours.

Specific capacitance was also doubled by using more concentrated hydrogen peroxide to synthesize CQDs from coal. Highest specific capacitance of CQDs synthesized at 120 °C was 31.4 F/g which makes them undesirable for supercapacitor applications.

Table 3: Specific capacitance of CQDs synthesized by H2O2 at 180 °C in a two electrode system. CQDs on carbon Charge Specific Material Scan rate paper (Coul) capacitance (F/g) mass (g) (mV/s)

1 hour 10% H2O2 0.0163 65.630 0.0089 20

1 hour 10% H2O2 0.0062 10.045 0.0089 50

1 hour 20% H2O2 0.0624 118.560 0.0188 20

1 hour 20% H2O2 0.0279 21.272 0.0188 50

3 hour 10% H2O2 0.0138 88.182 0.0056 20

3 hour 10% H2O2 0.0085 21.785 0.0056 50

3 hour 20% H2O2 0.0079 203.505 0.0014 20

3 hour 20% H2O2 0.0032 33.083 0.0014 50 Blank carbon paper 1.84*10-05 0.0309 0.0213 20

To compare electrochemical behavior of CQDs with a commercial high surface area carbon, Vulcan XC-72 (FuelCellStore) performance under CV was investigated.

Figure 35 compares the results obtained from analysis of CV tests performed on an RDE tip loaded with XC-72 and a RDE tip load with CQDs synthesized by 20% hydrogen peroxide at 180 °C in a 3-hour reaction. This test was performed in a three-electrode system with Pt as the counter electrode, 1 M Na2SO4 as the electrolyte, and an Ag/AgCl electrode as the reference electrode (at 20 mV/s scan rate). Specific capacitance of XC-72 is less than 1 F/g in this system, and Tang et al. reported a value of 15.2 F/g for this material [69].

Figure 35: Cyclic voltammetry curve of an RDE tip loaded with CQDs and Vulcan XC- 72 at 20mV/s scan rate in 1 M Na2SO4 (three-electrode system).

Galvanic charge-discharge tests were performed in a three-electrode system to test

CQDs cyclic electrochemical stability. The counter electrode was a Pt wire, the working electrode was a carbon paper loaded with CQDs by the drop cast method, the reference electrode was Ag/AgCl, and the electrolyte was 1 M Na2SO4. The capacitance retention was calculated with equation 14.

Figure 36 (a) presents the charge-discharge response of an RDE tip loaded with

CQDs synthesized by 20% hydrogen peroxide in 3-hour reaction at 120 °C corresponding to a constant charge and discharge current of 5*10-5 Amps. As it can be seen from this figure CQDs show similar electrochemical behavior in first 4000th cycles but as cycles increase CQDs GCD curve get more triangular which means that the electrode performs more like an electric double-layer capacitor rather than a hybrid pseudocapacitor.

Figure 36 (b) presents the capacitance retention percentage (first 8000 cycles) which compares the capacitance of each cycle with 10th cycle. The electrode capacitance was maximum at 500th cycle with 118.3% capacitance compared to 10th cycle. This increase can be because of activation of the electrode after 500 cycles [70]. These CQDs show more than 80% capacitance retention in first 4000 cycles.

Capacitance retention of CQDs synthesized by 10% hydrogen peroxide at 120 and

180 °C in 1-hour reaction dropped below 50% after 300 cycles. Capacitance retention of

CQDs synthesized by 10% hydrogen peroxide at 120 and 180 °C in 3-hour was below

50% after 500 cycles. Capacitance retention of CQDs synthesized by 20% hydrogen peroxide at 120 °C in 1-hour reaction dropped below 50% after 1000 cycles which makes them undesirable in supercapacitor application because of their low cycle stability. 81

(a)

(b)

Figure 37 (a) presents the charge-discharge response of an RDE tip loaded with

CQDs synthesized by 20% hydrogen peroxide in a 1-hour reaction at 180 °C corresponding to a constant charge and discharge current of 5*10-5 Amps. As it can be seen from this figure CQDs show similar electrochemical behavior in first 2000th cycles but as cycles increase CQDs GCD curve get more triangular which means that the electrode performs more like an electric double-layer capacitor rather than a hybrid pseudocapacitor.

Figure 37 (b) presents the capacitance retention percentage (first 8000 cycles) which compares the capacitance of each cycle with 10th cycle. These CQDs show high cycle stability during 8000 cycles with 80% capacitance retention. These CQDs show a better cycle stability compared to CQDs synthesized at 120 °C. 83

(a)

(b)

Figure 37: (a) Galvanic charge discharge curve of CQDs synthesized by 20% H2O2 at th th th th 180 °C in 1-hour reaction at 10 , 100 , 500 , and 2000 cycle in 1 M Na2SO4 electrolyte (three electrode system) (b) Capacitance retention in first 8000 cycles.

Figure 38 (a) presents the charge-discharge response of an RDE tip loaded with

CQDs synthesized by 20% hydrogen peroxide in 3-hour reaction at 180 °C corresponding to a constant charge and discharge current of 5*10-5 Amps. As it can be seen from this figure CQDs show similar electrochemical behavior in first 12000th cycles but as cycles increase CQDs GCD curve get more triangular which means that the electrode performs more like an electric double-layer capacitor rather than a hybrid pseudocapacitor.

Figure 38 (b) presents the capacitance retention percentage (first 20000 cycles) which compares the capacitance of each cycle with 10th cycle. These CQDs show high cycle stability during 12000 cycles with 100% capacitance retention. Capacitance retention start to drop below 80% after 18000 cycles. These CQDs show a remarkable cycle stability which makes them a good candidate for a supercapacitor material. CQDs synthesized in a 3-hour reaction show a better cycle stability compared to CQDs synthesized in a 1-hour reaction.

(a)

(b)

Figure 38: (a) Galvanic charge discharge curve of CQDs synthesized by 20% H2O2 at th th th th th 180 °C in 1-hour reaction at 10 , 300 , 2000 , 8000 , and 12000 cycle in 1 M Na2SO4 electrolyte (three electrode system) (b) Capacitance retention in first 20000 cycles.

4.3.3. Conversion and Yield

To investigate effect of temperature, reaction time and coal amount at the start on

CQDs production rate conversion and yield was calculated using equation (11). Table 4 presents conversion and final yield of synthesized CQDs by hydrogen peroxide in the reactor (with liquid charging pipette). Increasing temperature from 120 to 180 °C resulted in a 41% increase in the conversion in a 3-hour reaction (20% hydrogen peroxide). Increasing reaction time from 1-hour to 3-hour resulted in a 13.3% increase in the conversion at 180 °C (20% hydrogen peroxide). Increasing hydrogen peroxide concentration from 10% to 20% at 180 °C resulted in a 25.9% increase in the conversion

(3-hour reaction). To produce 1 g of CQDs, 3 g of coal need to be converted. This means that only 0.56 g CQDs were produced in a 3-hour reaction at 180 °C by using 20% hydrogen peroxide. Increasing amount of coal more than 5 g at the start of reaction or reaction time more than 3 hours did not increase the amount of produced CQDs.

Reactions at higher temperatures (more than 180 °C) reduce the CQDs production rate since CQDs decompose at higher temperatures [4]. 87

Table 4: Coal conversion and CQDs synthesized by hydrogen peroxide final yield. Temperature Coal mass (g) H2O2 Reaction Unreacted Coal conversion CQDs final yield

(°C) Concentration Time Coal (g) (%) (%)

(%) (hours)

120 3 10 1 2.4 20% 6.66%

120 5 10 1 4 20% 6.66%

120 5 20 1 3.8 24% 8%

120 5 10 3 3.9 22% 7.33%

120 5 20 3 3.7 26% 8.66%

180 3 10 1 2.34 22% 7.33%

180 5 10 1 3.85 23% 7.66%

180 5 10 3 3.65 27% 9%

180 5 20 1 3.5 30% 10%

180 5 20 3 3.3 34% 11.33%

180 7 20 3 5.2 25.7% 8.57% 88

To investigate effect of adding hydrogen peroxide gradually into the reactor, conversion of a reactor that utilized a liquid charging pipette and a reactor without a liquid charging pipette were compared in table 5. Up to 50% increase in conversion can be seen by adding liquid charging pipette to the reactor. Adding H2O2 gradually to this reaction not only increase CQDs production rate but lower the energy consumption since reaction between water and hydrogen peroxide is exothermic and heat up the reactor.

Table 5: Effect of liquid charging pipette on the conversion. Temperature Coal mass H2O2 Reaction Conversion of reactor Conversion of reactor with

(°C) (g) Concentration Time (hours) without liquid charging liquid charging pipette (%)

(%) pipette (%)

120 5 10 1 10% 20%

120 5 10 3 14% 22%

120 5 20 1 16% 24%

120 5 20 3 20% 26%

180 5 10 1 16% 23%

180 5 20 3 24% 34% 89

To achieve higher conversion, the solution was filtered and washed 2-3 times after each reactor experiment with DI water since the CQDs are adhesive and can adhere to coal. The remaining coal on the filter paper was dried in a laboratory oven at 80 °C for 12 hours. The remaining coal was used in the reactor two or three more times and the solution was filtered after each experiment. Series of 1- and 3- hour reactions (4 and 9 hours in total) was performed. Table 6 compares the CQDs final yield and coal conversion in these reactions at 180 °C (20% hydrogen peroxide). The maximum 30% conversion be achieved by this method of CQDs synthesis which is higher than reported rate of 20% by using a strong acid as the oxidizer in the same amount of time. Hu et al. reported 10% conversion using hydrogen peroxide as the oxidizer [4].

Table 6: Coal conversion and CQDs final yield in series of 1- and 3- hour reactions at 180 °C (20% hydrogen peroxide). Time for each Number of Total time Total Coal at Remaining Coal final CQDs final Produced experiment experiments (hours) 30% start (g) the end (g) conversion yield (%) CQDs (g) (hours) H2O2 (%) used (ml) 3 hours 3 9 hours 180 ml 5 g 0.5 g 90% 30% 1.5 g

2 hours 4 8 hours 240 ml 5 g 0.7 g 86% 28% 1.4 g

1 hour 4 4 hours 240 ml 5 g 2.5 g 50% 16.66% 0.83 g

CHAPTER 5: CONLUSIONS AND RECOMMENDATIONS FOR FUTURE

WORK

5.1. Conclusions

In this study, we have developed two synthesis methods to synthesize carbon quantum dots (CQDs) from bituminous coal. These CQDs were characterized physically and electrochemically to analyze their properties as supercapacitor electrode materials.

This study has shown that the method of synthesis by hydrogen peroxide is more suitable for CQDs large scale production. The method of synthesis by perchloric acid was time consuming and dangerous compared to the method of synthesis by hydrogen peroxide. Price range of 2000-4000$ per ton for perchloric acid (70%) compared to 180-

240$ per ton for hydrogen peroxide (30%) is one of the other limiting factors that makes

HClO4 an undesirable oxidizer for CQDs large scale production. The method of synthesis by H2O2 had two less steps since the final solution that contains CQDs did not need neutralization or dialysis. This is method of synthesis with hydrogen peroxide is also faster, safer and more economical compared to the common method of synthesis by nitric acid and sulfuric acid (400-500$ per ton) [3]. In addition, final CQDs yield (up to 30% yield) was three times more when perchloric acid or sulfuric acid was used as the oxidizer agent.

Size distribution analysis showed that CQDs synthesized by H2O2 had an average size of 3-4 nm which is half of the average size of CQDs that were synthesized by perchloric acid. Data from UV-vis analysis show that CQDs synthesized by H2O2 have 91 carboxyl (–COOH) or amine (–NH2) groups on their surface which makes them water soluble.

Electrochemical characterization of CQDs has shown that CQDs synthesized by

HClO4 have low specific capacitance of 12 F/g compared to CQDs synthesized by H2O2 with specific capacitance of 200 F/g. This study has shown that adding MoS2 to CQDs synthesized by HClO4 can increase their capacitance up to 130 F/g which is almost 10 times more than specific capacitance of these CQDs alone. CQDs synthesized by 20% hydrogen peroxide at 180 °C in a 3-hour reaction showed a high cycle stability of 18000 charge discharge cycles with more than 80% capacitance retention in addition to high specific capacitance of 200F/g which makes them a good candidate for supercapacitor electrode material.

Taken together, these findings suggest that synthesis of carbon quantum dots by

20% hydrogen peroxide at 180 °C in a 3-hour reaction from coal, not only has the highest yield of production among all applied methods in this thesis but also results in a stable electrode material with a high specific capacitance.

5.2. Recommendations for Future Work

Further research is needed to investigate reaction mechanism of CQDs and mixtures of CQDs and MoS2 in order to explain how MoS2 can enhance electrochemical behavior of CQDs. A future study investigating electron-transfer kinetics of CQDs can help in explaining what can cause the peaks in cyclic voltammetry curve and why CQDs act like a pseudocapacitor material. Further research on other combinations of CQDs with metal oxides can impact the future of supercapacitor materials. An equivalent circuit 92 model can help on studying the mechanism. Charge discharge curves can be used for this circuit model.

CQDs have more common applications such as in bio-imaging, sensors, and photovoltaics. As a future work, photoluminescence properties and quantum yield of these CQDs can be analyzed and compared for their other applications other than energy storage devices such as supercapacitors. Method of synthesis can impact properties of

CQDs significantly, and the new method of synthesis by perchloric in this thesis was never used before; therefore, the study on these CQDs in other application can be interesting.

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APPENDIX: PERCHLORIC ACID SAFETY DATA SHEET

Safety data sheet (SDS) of perchloric acid (70%) is presented below [71]. 99 100 101 102 103 104

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