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Electrospun carbon hybrid fibers as binder-free electrodes for electrochemical energy storage
(2018)
There is a great need for the development and improvement of electrochemical energy storage devices for applications ranging from energy and power management to portable electronic devices. My work explores electrode materials for devices with higher energy storage capacity and rate handling, namely electrical double-layer capacitors, lithium-ion batteries, and sodium-ion batteries. To this end, I report the synthesis and properties of electrospun fiber mats composed of nanoporous carbon, transition metal oxide/carbon hybrid material, or silicon oxycarbide. Based on a comprehensive array of structural and chemical analysis and electrochemical benchmarking, this work evaluates the potential and drawbacks of electrospun materials as electrodes. Key findings demonstrate that electrospinning of molecular precursor is an attractive approach for the synthesis of carbon and hybrid fiber mats as free-standing electrodes. By following a one-pot synthesis approach, material properties such as phase composition, crystal structure, and phase distribution are well tuned to achieve the desired electrochemical properties. Compared to polymer-bound free-standing electrodes, the continuous fiber network yields a superior gravimetric electrochemical performance, related to the absence of additives and the continuous path for electron transport. However, the large interfiber space and low electrode density limit the usefulness of adopting electrospun fiber mats to size-sensitive applications
As industrial and agricultural activities expand along with a growing global population, numerous regions are experiencing shortages of water and essential metals. To address these challenges, electrochemical separation methods utilizing electroactive materials and interfaces offer an efficient and straightforward approach to water purification and targeted ion extraction. Although carbon-based materials have been extensively studied and have the advantages of stability and low cost, they suffer from low desalination capacity, particularly for high-salinity water, and low selectivity. This dissertation investigates the potential of Faradaic materials and processes for electrochemical ion/water separation, as well as ion/ion separation, with a focus on alkali and alkaline earth metal ions, which are vital for industrial development but challenging to separate. The study includes synthesizing several Faradaic materials to achieve high ion removal capacity in seawater desalination. This work also develops a strategy to exploit the nuanced differences of ion intercalation kinetics in 2D material to achieve specific ion separation. The study also examines the selectivity and stability of LiFePO4 and presents new ways to optimize its performance. Finally, the study establishes a novel electrochemical process based on redox flow batteries, which promises a more efficient and continuous extraction of lithium ions from seawater.
Porous carbon materials are widely used in electrochemical applications for intermediate energy storage or water desalination. This work aimed to synthesize nanoporous carbons with well-controlled properties (e.g., specific surface area, average pore size, chemical composition) to correlate them to the performance in electrochemical applications (e.g., supercapacitors, LiS batteries). Especially the surface chemistry of highly porous carbons with different oxygen and nitrogen groups influences the electrochemical behavior. The carbon materials were obtained from polymeric precursors, including phenolic resins and polysilsesquioxanes. A physical activation with CO 2 or NH 3 that additionally introduced nitrogen groups was applied to adjust the porosity of the phenolic resin-derived carbons. Thereby, it was possible to obtain materials with different properties from the same precursor. The polysilsesquioxanes were first pyrolyzed and then thermally treated with chlorine gas to produce carbide-derived carbons. The porosity was tuned by the composition of the precursor and the synthesis temperature. The intermediate product (silicon oxycarbide) is also an attractive electrode material for Li-ion batteries. It was shown that optimization of the carbon content resulted in extended cycling stability.
The growing use of portable devices and a global transition to electric vehicles has tremendously increased the demand for energy storage devices such as lithium-ion batteries and supercapacitors. Especially the interest is established for better devices exceeding the energy and power performance of current technology. The hybrid supercapacitor (HSC) concept addresses the limits of each device and utilizes the distinct electrochemical features of lithium-ion batteries and supercapacitors. The focus of this Ph.D. thesis is the nano-design of hybrid materials of metal oxides and carbon for better electrochemical performance in lithium- and sodium-ion hybrid energy storage devices. The hybridization of metal oxide and carbon substrate can be achieved by tailored sol-gel synthesis, yielding a homogeneous distribution of nanosized metal oxide domain in the hybrid material. The performance of the hybrids was superior to the composite concept electrodes, but this is not a statement that can be generalized for all sorts of (nano)composites. In addition to the electrode material, also the electrolyte choice has a strong impact on the device operation and safety. The use of alternative solvents and Li- or Na-containing ionic liquids allows to increase the upper temperature and cell voltage at which Li- and Na-based systems can be safely operated at.
Electrochemical water desalination is an emerging technology known for its high efficiency and low energy consumption in removing ions from aqueous media. The present thesis begins by explaining the fundamentals of a typical electrochemical water desalination system and presenting relevant performance metrics. The significance and limitations of the latter metrics are then discussed based on the generations of the electrodes developed during the past few decades. This report seeks to expand the scope by investigating MXene (titanium carbide) as a purely pseudocapacitive material characterized by a capacitor-like electric response achieved through ion intercalation. Afterward, the merit of MXene when utilized as an electrode in electrochemical desalination is investigated for both single-salt and multi-salt aqueous solutions, ultimately establishing qualitative insights into the relationship between MXene properties and its electrochemical desalination behavior. Finally, the thesis goes beyond MXene and explores its sibling materials, such as MBene (transition metal boride), for lithium-ion battery electrodes. As another application of 2D nanolamellar materials at the water-energy nexus, we have explored MXene conversion into transition metal dichalcogenides by sulfidation heat treatment and its merit as electrodes for hydrogen electrocatalysis. These findings can contribute to developing more efficient and sustainable energy storage, conversion, and desalination technologies.
This Ph.D. thesis focuses on developing electrochemical energy storage devices that outperform existing lithium-ion batteries. The research investigates the design and modification of metal oxides and sulfides to enhance the electrochemical performance of commercial battery electrodes and presents the challenges met. By employing specific design strategies and derivatization methods, novel materials with unique properties are synthesized, distinct from those found in commercial batteries. For each material studied, the thesis examines the relationship between its electrochemical performance and various other material properties to address existing limitations. In the case of self-standing fibers, the influence of mechanical flexibility on electrochemical properties is analyzed. Similarly, for the conversion-type materials, the detrimental shuttling effect or electrode etching is mitigated by applying a stable coating to protect the active component from degradation. In parallel. this thesis aims to use pH-neutral syntheses and low-temperature derivatization to reduce the effect of harsh components and high-energy procedures and presents the challenges that arise from this. Additionally, this work explores complementary approaches to enhance the interface between the electrode and electrolyte after modifying the electrode material through materials engineering. These strategies are thoroughly investigated and presented as potential solutions to improve the overall performance of energy storage devices.
In recent decades, a new type of electric energy storage system has emerged with the principle that the electric charge can be stored not only at the interface between the electrode and the electrolyte, but also in the electrolyte by the redox activities of the bulk electrolyte itself. Such redox electrolytes are promising for non-flow energy storage (redox electrolyte aided hybrid energy storage systems, REHES) particularly when they are combined with electrodes made of nanoporous carbon. In this PhD work, I have established a fundamental understanding regarding ion diffusion, process kinetics, and adsorption of redox ions. For that, different REHES systems have been investigated including tetrapropylammonium iodide, zinc iodide, potassium iodide, potassium ferricyanide, vanadyl sulfate, tin sulfate, and tin fluoride. The basic understanding of REHES systems enabled the targeted improvement of the device performance throughout this PhD work. Compared to the energy storage capacity of a conventional (non-redox) electrical double layer capacitor of 4 Wh/kg (ca. 80 F/g), the use of the ZnI2 redox electrolyte yielded significantly higher performance of up to 226 Wh/kg. Furthermore, the specific power was also enhanced from 1.3 kW/kg to 20 kW/kg. As a key conclusion, this PhD work demonstrates the high attractiveness of REHES systems not only from a performance point of view, but also regarding low cost and simplicity of the system.
Owing to an expanding economy and growing population, there is increasing consumer demand for freshwater. However, with global climate change and water pollution issues, there is rising water stress in many countries worldwide. Electrochemical water desalination technologies such as capacitive deionization (CDI) utilize electrical energy to store ions in porous materials and provide energy-efficient water desalination. However, due to the cation and anion exchange process during the charging and discharging processes, CDI is considered suitable for low salinity water desalination (salinity of 1-10 g/L). This dissertation explores novel approaches to next-generation CDI for better desalination performances and water desalination at high ionic strength. In particular, the ability of sub-nanometer carbon pores (ultramicropores) to enable highly efficient CDI even at seawater salinity is demonstrated based on unexpected simulation predictions. This unique ability originates from the energy barrier of ion solvation for pores smaller than the solvation shell. Consequently, uncharged carbon ultramicropores behave ionophobic and overcome the limitation of CDI only to be suitable for remediation of brackish water. Ultramicropores also provide novel perspectives for ion separation via the interplay of intrinsic and kinetic ion selectivity. This work also establishes electrocatalytic fuel cell desalination, whereby conventional fuel cell technology can easily be adapted to generate electricity, heat, and desalinated water concurrently.
Key parts of an electrochemical energy storage device are the active material, the electrolyte, the binder, and the conductive additives. This dissertation investigates the role of such individual components on the device’s overall performance and how they interact with each other to influence the device’s ability to store energy and longevity. Three aspects of the performance of electric double-layer capacitors are investigated: (1) The role of the conductive additives on performance and longevity, where 5 wt% admixture shows the best capability. (2) The role of the active material and the electrolyte with an increased capacitance when the pore width matches the ion size. (3) The volumetric expansion of carbon electrodes during charging is depending on the size ratio of the ions and the pore width. Further, an asymmetry in charging mechanism is found for two-dimensional metal carbides, MXenes, in ionic liquids. The charging mechanism is based on cation (de-)intercalation. The role of binder properties on the performance of battery electrodes was investigated with intercalation-induced volumetric changes of the active material. Moreover, the multi-length scale approach using different in situ measurement techniques reveals a promising way to understand mechanisms in electrochemical energy storage devices. The combination of dilatometry with quartz-crystal microbalance, X-ray diffraction or small-angle X-ray scattering shed light on potential-induced structural changes in the systems.
An ever-growing global population leads to higher water consumption and demand for advanced remediation technologies. Thus, water stress intensifies in many countries around the world. Most global water remediation is accomplished by established techniques, such as reverse osmosis and thermal desalination. To lower the energy consumption per processed water volume, engineers and scientists investigate novel techniques like capacitive deionization technology (CDI). CDI based on carbon electrodes promises energy-efficient desalination by ion electrosorption but is limited to the remediation of brackish water, that is, very low salt concentration media. My doctoral thesis explores next-generation electrodes for electrochemical water desalination based on Faradaic materials. Unlike carbon, these materials accomplish ion removal by reversible electrochemical processes, such as ion insertion of crystalline structures or redox-reactions of dissolved ions. Faradaic materials not only provide a large potential for enhanced desalination capacity but also enable the remediation of seawater, that is, aqueous media with high molar strength. These features can be accomplished while maintaining a low level of energy consumption to enable energy- efficient water desalination for a more sustainable future.
