The recent discovery of layered transition metal carbides (MXenes) is one of the most important developments in two-dimensional (2D) materials. Preliminary theoretical and experimental studies suggest a wide range of potential applications for MXenes. The MXenes are prepared by chemically etching ‘A’-layer element from layered ternary metal carbides, nitrides and carbonitrides (MAX phases) through aqueous acid treatment, which results in various surface terminations such as hydroxyl, oxygen or fluorine. It has been found that surface terminations play a critical role in defining MXene properties and affects MXene performance in different applications such as electrochemical energy storage, electromagnetic interference shielding, water purification, sensors and catalysis. Also, the electronic, thermoelectric, structural, plasmonic and optical properties of MXenes largely depend upon surface terminations. Thus, controlling the surface chemistry if MXenes can be an efficient way to improve their properties. This research mainly aims to perform surface modifications of two commonly studied MXenes; Ti2C and Ti3C2, via chemical, thermal or physical processes to enhance electrochemical energy storage properties. The as-prepared and surface modified MXenes have been studied as electrode materials in Li-ion batteries (LIBs) and supercapacitors (SCs). In pursuit of desirable MXene surface, we have developed an in-situ room temperature oxidation process, which resulted in TiO2/MXene nanocomposite and enhanced Li-ion storage. The idea of making metal oxide and MXene nanocomposites was taken to the next level by combining a high capacity anode materials – SnO2 – and MXene. By taking advantage of already existing surface functional groups (–OH), we have developed a composite of SnO2/MXene by atomic layer deposition (ALD) which showed enhanced capacity and excellent cyclic stability. Thermal annealing of MXene at elevated temperature under different atmospheres was carried out and detailed surface chemistry was studied to analyze the change in surface functional groups and its effect on electrochemical performance. Also, we could replace surface functional groups with desirable heteroatoms (e.g., nitrogen) by plasma processing and studied their effect on energy storage properties. This work provides an experimental baseline for surface modification of MXene and helps to understand the role of various surface functional groups in MXene electrode electrochemical performance.

Master’s thesis deals with the design concept of DC/DC convertor usable for a backup source with supercapacitors. The paper describes the theoretical knowledge of supercapacitors technology, principle of basic DC/DC convertors and their use in electrical energy storage systems. The thesis contains a description of the designed backup power system and its properties, which has been measured.

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<p>Electrochemical supercapacitors (ES) have become an attractive research interest in advanced power systems and found many applications as an energy storage device in number of areas. The fabrication of advanced electrodes with novel materials and new techniques plays a key part in determining the properties of ES. Conducting polymer polypyrrole (PPY) has been found to be a promising electrode material for ES due to its high pseudo-capacitance and good electrical conductivity.</p> <p>Polypyrrole (PPY) films were successfully obtained on stainless steel substrates by anodic electropolymerization. Anionic dopants such as 2,6-naphthalenedisulfonic acid disodium salt (NSA), chromotropic acid disodium salt (CHR) and gallic acid were used for the synthesis of PPY. The roles of additives in the electrodeposition process have been discussed. The deposition was performed galvanostatically or potentiodynamically and the electrochemical properties of PPY have been investigated and compared by using different characterization techniques.</p> <p>The comparison of the experimental data for NSA, CHR and gallic acid showed the influence of aromatic ring and OH groups on the capacitive behaviour of PPY films. Adherent PPY films were obtained from pyrrole solutions containing CHR as dopant. The specific capacitance (SC) increased with increasing pyrrole and dopant concentration in the solutions used for deposition. The PPY films prepared on stainless steel substrates by electropolymerization are promising electrode materials for ES.</p> / Master of Applied Science (MASc)

As the world population grows, a persistent pressure on natural resources remains. Resource requirements have extensively expanded due to industrialization. Several technological advancements continually aim to alleviate these resource shortages by targeting existing shortcomings in effective and efficient material design. Practical, high-performing, and economical materials are needed in several key application areas, including energy storage, energy harvesting, electronics, catalysis, and water purification. Further development into high-performing and economical materials remain imperative. Innovators must seek to develop technologies that overcome fundamental limitations by designing materials and devices which address resource challenges. Carbon serves as a versatile material for a wide range of applications including purification, separation, and energy storage owing to excellent electrical, physical, and mechanical properties. One-dimensional (1D) carbon fiber in particular is renowned for excellent strength with high surface-to-volume ratio and is widely commercially available. Although an exceptional candidate to address current energy and environmental needs, carbon fibers require further investigation to be used to their full potential. Emerging strategies for carbon fiber design rely on developing facile synthetic routes for controlled carbon structures. The scientific community has shown extensive interest in porous carbon fabrication owing to the excellent performance enhancement in separation, filtration, energy storage, energy conversion, and several other applications. This dissertation both reviews and contributes to the recent works of porous carbon and their applications in energy and environmental sciences. The background section shows recent development in porous carbon and the processing methods under investigation and current synthetic methods for designing porous carbon fibers (PCF). Later sections focus on original research. A controlled radical polymerization method, reversible addition-fragmentation chain transfer (RAFT), enabled a synthetic design for a block copolymer precursor, poly(methyl methacrylate) (PMMA) and polyacrylonitrile (PAN). The block copolymer (PMMA-b-PAN) possesses a unique microphase separation when electrospun and develop narrowly disperse mesopores upon carbonization. The PMMA and PAN domains self-assemble in a kinetically trapped disordered network whereby PMMA decomposes and PAN cross-links into PCF. The initial investigation highlights the block copolymer molecular weight and compositional design control for tuning the physical and electrochemical properties of PCF. Based on this study, mesopore (2 – 50 nm) size can be tuned between 10 – 25 nm while maintaining large surface areas, and the PAN-derived micropores (< 2 nm). The mesopores and micropores both contribute to the development of the unique hierarchical porous carbon structure which brings unprecedented architectural control. The pore control greatly contributes to the carbon field as the nano-scale architecture significantly influences performance and functionality. The next section uses PCF to clean water sources that are often tainted with undesirable ions such as salts and pollutants. Deionization or electrosorption is an electrochemical method for water purification via ion removal. I employed the PCFs as an electrode for deionization because of their high surface area and tunable pore size. Important for deionization, the adsorption isotherms and kinetics highlight the capacity and speed for purification of water. I studied PCF capacitive filtration on charged organic salts. Because PCF have both micropores and mesopores, they were able to adsorb ions at masses exceeding their own weight. The PFC adsorption efficiency was attributed to the diffusion kinetics within the hierarchical porous system and the double layer capacitance development on the PCF surface. In addition, based on the mechanism of adsorption, the PCFs showed high stability and reusability for future adsorption/desorption applications. The PCF performance as an electrosorption material highlights the rational design for efficient electrodes by hierarchical interconnected porosity. Another application of PFCs is updating evaporative desalination methods for water purification. Currently distillation is not widely used as a source of potable water owing to the high cost and energy requirement. Solar desalination could serve as a low-cost method for desalination; however, the evaporation enthalpy of water severely limits practical implementation. Here I apply the pore design of PCF as a method for water nano-confinement. Confinement effects reduce water density and lowers evaporation enthalpy. Desalination in PCF were studied in pores < 2 nm to 22 nm. The PCF pore size of ~ 10 nm was found to be the peak efficiency and resulted in a ~ 46% reduction in enthalpy. Interestingly, the PCF nano-confinement also contributed to the understanding in competing desorption energy for evaporation in micropores. The pore design in PCF also shows confinement effects that can be implemented in other environmental applications. Lastly, the block copolymer microphase morphology was explored in a vapor induced phase separation system. The resulting PCF properties showed a direct influence from the phase separation caused by nonsolvent. At low nonsolvent vapor, a disordered microphase separation occurred, however upon application of nonsolvent vapor, the polymer chains reorganized. The reorganization initially improved mechanical properties by developing more long-range ordered graphic chains in the PAN-derived carbon. However, at higher nonsolvent vapor concentrations, the fibers experienced polymer precipitation which resulted in bead and clump formation in the fiber mats. The beads and clumps lowered both mechanical properties and electrochemical performance. The vapor induced phase separation showed a method for enhancing mechanical properties without compromising electrochemical performance in flexible carbon fibers. / Doctor of Philosophy / Nanomaterials possess mechanical, physical, and electrical properties to address important growing demands for precious resources such as clean water and energy. Many advancements in nanomaterials focus on improving fine-tune architectures which facilitate efficiency in composites, filtration systems, catalytic systems, energy storage devices, and electronics. Carbon material has remained a valuable candidate in these fields owing to its abundancy economical cost, and excellent properties. Several carbon forms provide unique characteristics including 0D dots, 1D fibers, 2D sheets, and 3D monoliths. Of these, 1D fibers possess excellent strength, resiliency, and conductivity and have been commercially employed in modern automotive, airplanes, membranes, and conductors. However, traditional carbon fiber fabrication does not match the growing needs in performance. Therefore, in this dissertation I explore the design and processing of carbon fibers for controlled architectures. These designs were then systematically studied in filtration systems, solar desalination, and flexible electronics. Block copolymers provide a new way to combine polymers for drastically new materials and effects. Firstly, I conducted a comprehensive study on the synthesis and composition of this block copolymer which laid the foundation for future carbon fiber design. The polymer consists of two chains – one chain to develop carbon structures upon heating; the second which decomposes into pores upon heating. Therefore, with these two chains, a highly porous carbon fiber can be created. The reaction I studied could mostly be controlled with time to change the length of each chain. Ultimately, the pore size and surface area depend on the relative lengths of each chain. Future studies, including ones in this work, could therefore tune pore size and surface area for many applications. Carbon fibers with graphitic structure are inherently conductive and thereby attract charged molecules in a solution. Diffusion and capacity serve as major factors in these types of systems. With the aforementioned control of the carbon fibers a diffusion study was conducted with charged pollution ions. Owing to the conductive nature, a voltage supply was attached to the fibers, which would adsorb ions electrostatically, termed "electrosorption". The electrosorption performance within the carbon fibers elucidated the interconnected porous structure and how ions orientate themselves along the surface of the fibers. In addition, with the development of ion orientation along the surface of the fibers, a greater than 1:1 ratio of carbon weight to ion weight adsorbed developed owing to the diffusion and ion stacking capabilities. Additionally, the study provides deeper investigation into movement of ions within confined nano-porous material. The ever-growing need for renewable resources such as fresh water has pressured development into more efficient material. Solar desalination has attractive qualities which makes it a focus for micro-scale studies. One of the major limitations lies in the high energy input change liquid water into vapor. At 100 °C for boiling, desalination lacks sufficient efficiency for large-scale applications in evaporation. However, by utilizing nano-scale material, the fundamental properties of water can be altered. The carbon fibers were then created with various nano-pore sizes which revealed nano-confinement effects when subject to solar heating. With the shrinking of pore sizes, the density of water also decreased. A lower density means less energy was required to convert water from a liquid to a vapor state. The carbon fibers helped reveal real applications into confinement effects on water based on pore size. Apart from just desalination, this means future environmental application can utilize this knowledge for more effective and smart designs. The carbon fibers outstanding electrical and mechanical properties have spurred research and development since the mid-1900s. Since then, carbon fiber technologies have grown from facile and efficient productions means, to high end, high performance smart design. The work presented here furthers two major components: first, the high-performance design of porous carbon fiber; second, the fundamental principles in nano-material properties and their applications. By first constructing a design of polymer synthesis and then subsequent studies, development of nano-porous carbon energy progresses knowledge on smart and efficient designs. These materials provide a platform for future energy and environmental sciences.

The society profits from a variety of electronic devices, which rely on electrons and holes as the charge carriers for information transmission and processing. In contrast, biological systems operate via ions of varying size to handle complicated tasks, including massive parallel information sensing, processing, storing, and behavior controlling in nature, which inspire the development of iontronics (such as ionic transistors, ionic diodes, and ionic resistive memristors) for further bioelectronic interface, in-memory computing, and artificial intelligence hardware.[1] In recent years, the electric double layer (EDL) formation has proven a powerful tool for the coupling of ions and electrons in iontronics. EDL electrically adsorbs/desorbs ions on the surface of electrodes to balance and store opposite charges in a controllable manner, which enables to operate ions and build iontronic devices. Nanoporous carbons with higher specific surface areas compared to widely-used metal electrodes in iontronics feature the higher volume and specific capacitances along with fine compatibility with biological systems, which are promising for ion manipulation in iontronics.[2,3] In recent years, a series of carbon-based capacitive iontronics were developed to realize the functions of conventional diodes and transistors.[4,5] Due to the demand of high performance (energy and power density) in above devices, toxic electrolytes were applied as the electrolytes,[4,5] which limits the implementation in biological applications. Various functionally bioactive ions are a requisite for complicated psychological, physiological, and behavioral processes, such as neurotransmitters (ranging from amino acids (e.g., glycine (Gly) and gamma-aminobutyric acid (GABA)), biogenic amines (e.g., dopamine and acetylcholine), to peptides (e.g., vasopressin and somatostatin)).[6] Moreover, some bioactive ions such as sodium ibuprofen (NaIbu) and sodium salicylate (NaSal) are common analgesic and inflammation drugs for the human health.[7,8] So far, there have been few reports about applying bioactive ions as the charge carriers in carbon-based EDL iontronics. A deep molecular-level mechanism of the adsorption of bioactive ions and the deliberate concentration control via nanoporous carbons with and without polarization remain unclear and unsolved, but are crucial for the design of neuromorphic devices, neurotransmitter sensors, and transmitter delivery. Given the varying sizes and structures of bioactive ions and the varying porosity structures of nanoporous carbons, there are some open questions for the interaction mechanism of bioactive ions and nanoporous carbons in the EDL devices as shown in the following: a) the influence of porosity structures of nanoporous carbons for the adsorption kinetics and thermodynamics of bioactive ions; b) the difference of the electrosorption and physisorption and their roles for manipulating ion behaviors; c) the influence of bioactive ion structure for the adsorption process; d) the adsorption mechanisms for electroneutral and charged neurotransmitters; e) the effects of the surface polarity and functional groups of nanoporous carbons for the bioactive ion adsorption process. This thesis focuses on revealing the interaction behaviors of bioactive ion electrolytes and nanoporous carbon electrodes from four main parts, with the aid of electrochemical methods and spectroscopic analyses. In Chapter 5.1, the adsorption kinetics of bioactive choline chloride (ChCl) in ACC with a narrow pore size distribution (PSD) and ROX with a broad PSD is explored. The comparison indicates a faster diffusion process of ChCl in ROX with a broad PSD. The evaluation of physisorption and electrosorption of ChCl in ROX with a broad PSD is conducted, which show that the amount of physically adsorbed ChCl in ROX is less than 6 μmol/g, while the amount of electrosorption-induced concentration changes in the polarized ROX electrode is up to 30 μmol/g. Electrosorption dominates the adsorption process for ChCl. Consequently, it can be concluded that the capture and release of ChCl in aqueous solutions can be easily manipulated via electrochemical techniques. Chapter 5.2 builds on the investigation of the ChCl interaction behavior in the ROX carbon. The investigation is extended to a series of ammonium-based ionic liquid salts with different alkyl chain lengths paired with Cl- anions (CxAmOMCl, where x=2, 6, and 12). The increasing physisorption of these cations in the ROX carbon is observed with the alkyl chain length increasing. The role of alkyl chain is clarified in bioactive cations for the adsorption in nanoporous carbons. However, the bioactive anions with long alkyl chains showed a quite weaker adsorption in the ROX carbon, compared with bioactive cations with long alkyl chains. These results illustrate the synergistic effect of the hydrophobic interaction and electrostatic attraction for the bioactive ions strong adsorption in nanoporous carbons. In Chapter 5.3, the adsorption and charge balancing mechanism of electroneutral amino acids are further explored in ROX carbon electrodes. The weak physisorption of four amino acids (with linear structures) is observed, which results from the hydrophilic end groups and electroneutral properties. The charge balance mechanism of these electroneutral zwitterions (with amine and carboxylic acid groups) is clarified as the dissociation reaction of amino acid zwitterions, which produces anions to balance positive charges and cations to balance negative charges. In the buffered environment, the deliberate uptake and release of inhibitory neurotransmitters (Gly and GABA) are achieved by polarizing porous carbon electrodes, which implies the powerful abilities of electrosorption for controlling the concentration of neurotransmitters in aqueous and phosphate-buffered saline (PBS) solutions. In Chapter 5.4, we investigate the impurity effects for the carbon properties and bioactive ion adsorption processes. The impurity contents are very high in some commercial porous carbons. The washing process leads to the decrease of O and N contents, and reduces wettability of porous carbons. Moreover, some O, N, and other non-carbon contents, which are commonly considered as surface functional groups of carbons, are not bonded but adsorbed inorganic impurities on the carbon surface. In-situ UV-Vis experiments clarify that the adsorbed ionic impurities play a role in the charge balance process during the electric polarization, which partly explains the capacitances of porous carbons in pure water electrolytes. The questions addressed in this thesis provide a fundamental basis for the understanding of the interaction of various bioactive ions with nanoporous carbons, which benefit the development of EDL iontronics. Based on two different interaction modes (weak and strong adsorption), the interaction theory is further applied in the construction of iontronic devices. For weak adsorption, EDL transistors are deeply explored using bioactive ions (ChCl, NaIbu, Gly, and GABA). The capacitance switching behavior is confirmed in a 3D printed carbon-based ionic transistor. The concentration manipulation of bioactive ions in aqueous environments are promising for various potential applications, such as toxic ion removal, drug delivery, plant regulation, and bioelectronic devices. For strong adsorption, the confined cations with long alkyl chains (cations of C12AmOMCl) are irreversibly adsorbed and fixed on the porous carbon surface. The electric polarization cannot desorb confined cations, causing anion depletion and anion enrichment during electric polarization, which leads to the favorable memristive behavior for promising ionic memristors and in-memory computing applications in the future. References [1] C. Wan, K. Xiao, A. Angelin, M. Antonietti, X. Chen, Advanced Intelligent Systems 2019, 1, 1900073. [2] S. Z. Bisri, S. Shimizu, M. Nakano, Y. Iwasa, Advanced Materials 2017, 29, 1607054. [3] Y.-Z. Zhang, Y. Wang, T. Cheng, L.-Q. Yao, X. Li, W.-Y. Lai, W. Huang, Chemical Society Reviews 2019, 48, 3229. [4] S. Lochmann, Y. Bräuniger, V. Gottsmann, L. Galle, J. Grothe, S. Kaskel, Advanced Functional Materials 2020, 30, 1910439. [5] E. Zhang, N. Fulik, G.-P. Hao, H.-Y. Zhang, K. Kaneko, L. Borchardt, E. Brunner, S. Kaskel, Angewandte Chemie International Edition 2019, 58, 13060. [6] S. E. Hyman, Current Biology 2005, 15, R154. [7] S. A. Hawley, M. D. Fullerton, F. A. Ross, J. D. Schertzer, C. Chevtzoff, K. J. Walker, M. W. Peggie, D. Zibrova, K. A. Green, K. J. Mustard, B. E. Kemp, K. Sakamoto, G. R. Steinberg, D. G. Hardie, Science 2012, 336, 918. [8] N. Azum, A. Ahmed, M. A. Rub, A. M. Asiri, S. F. Alamery, Journal of Molecular Liquids 2019, 290, 111187.:Table of Contents I Abbreviations IV 1. Motivation 1 2. Background and Introduction 5 2.1. Biology and Ion-controlled Devices 5 2.2. Ion-related Biological Processes 5 2.2.1. Sensing and Signaling 5 2.2.2. Memory and Computing 7 2.2.3. Actuation Components 9 2.3. Bioinspired Iontronics 10 2.3.1. Ionic Diodes 11 2.3.2. Ionic Transistors 12 2.3.3. Ionic Resistive Memristors 14 2.4. Carbon-based Capacitive Iontronics 15 2.4.1. The Mechanism of Carbon-based Supercapacitors 15 2.4.2. Electrolytes for Supercapacitors 18 2.4.3. Nanoporous Carbons 22 2.4.4. Carbon-based Ionic Diodes 23 2.4.5. Carbon-based Ionic Transistors 24 2.4.6. The Interaction Mechanism of Bioactive Ions with Porous Carbons 26 3. Electrochemical Methods 28 3.1. Linear Sweep Voltammetry (LSV) 28 3.2. Cyclic Voltammetry (CV) 30 3.3. Electrochemical Impedance Spectroscopy (EIS) 31 4. Experimental Section 35 4.1. List of Used Chemicals 35 4.2. List of Used Materials 36 4.3. Preparation and Characterizations 37 4.3.1. Carbon Preparation 37 4.3.2. Electrode Preparation 38 4.3.3. 2-electrode Cells 38 4.3.4. 3-electrode Cells 38 4.3.5. 4-terminal Setups 39 4.3.6. Local pH Measurement 39 4.3.7. EIS Measurement 40 4.3.8. In-situ UV-Vis Measurement 40 4.3.9. Raman Spectroscopy 41 4.3.10. NMR and MS Measurement 41 4.3.11. Ninhydrin Reaction 42 4.3.12. Nitrogen Physisorption 42 4.3.13. Electrosorption Evaluation 42 5. Results and Discussion 43 5.1. Pore Structure and Ion Adsorption Kinetics 43 5.1.1. Introduction 43 5.1.2. Physiochemical Properties of Two Nanoporous Carbons 43 5.1.3. ChCl Physisorption Mechanism in Nanoporous Carbons 45 5.1.4. ChCl Electrochemical Stability and Performance 48 5.1.5. ChCl Electrosorption Mechanism in Nanoporous Carbons 52 5.1.6. Switchable Capacitive Transistor Analogues in Printed Structures 60 5.1.7. Summary 63 5.2. Ion Structures and Adsorption Kinetics 64 5.2.1. Introduction 64 5.2.2. Synthesis and Characterization of Ammonium-based ILs 65 5.2.3. Adsorption Behavior of Ammonium-based ILs in Nanoporous Carbons 67 5.2.4. Electrochemical Performance of Ammonium-based ILs 76 5.2.5. Adsorption Behavior of Organic Salts in Nanoporous Carbons 78 5.2.6. Strong Interaction and Ionic Memristor Behaviors 82 5.2.7. Weak Interaction and Ionic Transistor Applications 86 5.2.8. Summary 89 5.3. Electroneutral Neurotransmitter Adsorption Mechanism 90 5.3.1. Introduction 90 5.3.2. Amino Acid Physisorption Mechanism in Nanoporous Carbons 92 5.3.3. Amino Acid Electrochemical Behaviors in Electrically-polarized Nanoporous Carbons 96 5.3.4. Mechanism Investigation of Zwitterions in Electrically-polarized Nanoporous Carbons 99 5.3.5. Local pH Measurement of Amino Acid Electrolytes during Electric Polarization 105 5.3.6. Electrosorption-induced Capture and Release of Amino Acid Neurotransmitters 109 5.3.7 Neurotransmitter-based Bioinspired Iontronic Devices 113 5.3.8. Summary 115 5.4. Porous Carbon Impurities for Bioactive Ion Adsorption 116 5.4.1. Introduction 116 5.4.2. Qualitative Analysis of the Impurity Release from Porous Carbons 118 5.4.3. Electrochemical Evaluation for Ionic Impurities in Porous Carbons 121 5.4.4. The Effect of Adsorbed Ionic Impurities for Carbon Properties 126 5.4.5. The Charge Balance Mechanism of Ionic Impurities for Bioactive Ion Controlling 135 5.4.6. Summary 137 6. Conclusion and Outlook 139 7. References 142 A. Bibliography 152 B. List of Publications 154 C. Acknowledgements 155 D. Appendix 157 E. Versicherung und Erklärung 161