Open Access

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.

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.

Lithium-ion batteries and supercapacitors have become indispensable energy storage devices for the steadily growing electrification. Both technologies possess unique advantages and disadvantages due to the inherently different energy storage principles involved. Hybrid supercapacitors (HSC) combine the advantages of the individual devices and have thus attracted considerable attention in recent years. In this thesis, electrode hybridization was investigated based on an optimized and synergetic combination of commercial lithium-ion battery and supercapacitor materials. This cost-effective approach is highly versatile since the electrode recipe can be precisely adjusted to a certain application via simple variation of the active material ratio or active material combination. A special focus of this thesis was set on the reference electrode design for HSC characterization, the influence of electrode microstructure on the electrochemical performance and the improvement of the rate performance of hybrid supercapacitors via enhancing the electrical conductivity of the active material. The latter was achieved via adjustment of the oxygen defect concentration and the associated titanium valence state of lithium titanate. This enabled the reduction of the carbon concentration of lithium titanate electrodes to 5 mass%, while yielding a high electrode capacity of about 70 mAh/g (82 mAh/g normalized to the active mass) at ultra-high C-rates of 100 C. When combined with an activated carbon / lithium manganese oxide composite cathode, an excellent energy and power performance of 70 Wh/kg and 47 kW/kg, respectively, was obtained (82 Wh/kg and 55 kW/kg normalized to the active mass), while maintaining 83 % of its energy ratings after 5,000 cycles at 10 C (78 % after 15,000 cycles at 100 C).

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.

Energy storage technologies are at the focal point of current research activities, propelled by the increasing demand on mobile storage for portable electronics and the rapidly growing implementation of renewable energy sources. To fulfill the requirements of an advanced energy storage device - high power and high energy - a hybridization of redox-active materials and electrical double-layer materials can be used. In this PhD thesis, several strategies and material architectures will be employed: Wet-chemical carbon coating procedures with transition metal oxides, carbon functionalization with redox-active surface groups, and the synthesis of core-shell particles. A special focus was on the optimization of the carbon substrate properties, such as porosity, surface area, and structure. It was shown that carbon nanomaterials, like carbon onions, with exclusively outer surface area and high conductivity are ideal substrates for battery and pseudocapacitor hybrids. This is exemplified for pseudocapacitive manganese oxide, a quinone-based surface functionalization, and lithium-sulfur batteries. Coating nanoporous carbons led to substantial pore blocking and thus a reduction of the specific surface area and the resulting capacity. To take advantage from the high double-layer capacitance, nanoporous carbons can be effectively used in core-shell particles, with the battery material in the core and the nanoporous carbon in the shell, which avoids the pore blocking issues.