2D nanolamellar materials toward water-energy nexus applications

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.Elektrochemische Wasserentsalzung ist eine vielversprechende Technologie, die für ihre hohe Effizienz und ihren geringen Energieverbrauch bei der Entfernung von Ionen aus wässrigen Medien bekannt ist. Die vorliegende Arbeit beginnt mit der Erläuterung der Grundlagen eines typischen elektrochemischen Wasserentsalzungssystems und der Darstellung relevanter Leistungskennzahlen. Die Bedeutung und Einschränkungen der letztgenannten Metriken werden dann anhand der in der Vergangenheit entwickelten Generationen von Elektroden diskutiert. Die vorliegende Arbeit untersucht MXene (Titankarbid) als rein pseudokapazitives Material, das durch eine kondensatorähnliche elektrische Reaktion gekennzeichnet ist, die durch Ioneninterkalation erreicht wird. Anschließend wird der Nutzen von MXene bei der Verwendung als Elektrode bei der elektrochemischen Entsalzung sowohl für wässrige Lösungen mit einem Salz als auch mit mehreren Salzen untersucht, um schließlich qualitative Erkenntnisse über die Beziehung zwischen den Eigenschaften von MXen und seinem elektrochemischen Entsalzungsverhalten zu gewinnen. Letztlich erforscht die Arbeit auch MBene (Übergangsmetallborid) sowie die Derivatisierung von MXenen hin zu Übergangsmetalldichalkogeniden durch Sulfidierung für den Einsatz in der Wasserstoffelektrokatalyse untersucht. Die Ergebnisse dieser Studien können zur Entwicklung effizienterer und nachhaltigerer Technologien zur Energiespeicherung, -umwandlung und -entsalzung beitragen

Layered Nano‐Mosaic of Niobium Disulfide Heterostructures by Direct Sulfidation of Niobium Carbide MXenes for Hydrogen Evolution

MXene-transition metal dichalcogenide (TMD) heterostructures are synthesized through a one-step heat treatment of Nb2C and Nb4C3. These MXenes are used without delamination or any pre-treatment. Heat treatments accomplish the sacrificial transformation of these MXenes into TMD (NbS2) at 700 and 900 °C under H2S. This work investigates, for the first time, the role of starting MXene phase in the derivative morphology. It is shown that while treatment of Nb2C at 700 °C leads to the formation of pillar-like structures on the parent MXene, Nb4C3 produces nano-mosaic layered NbS2. At 900 °C, both MXene phases, of the same transition metal, fully convert into nano-mosaic layered NbS2 preserving the parent MXene's layered morphology. When tested as electrodes for hydrogen evolution reaction, Nb4C3-derived hybrids show better performance than Nb2C derivatives. The Nb4C3-derived heterostructure exhibits a low overpotential of 198 mV at 10 mA cm−2 and a Tafel slope of 122 mV dec−1, with good cycling stability in an acidic electrolyte

Time‐Dependent Cation Selectivity of Titanium Carbide MXene in Aqueous Solution

Electrochemical ion separation is a promising technology to recover valuable ionic species from water. Pseudocapacitive materials, especially 2D materials, are up-and-coming electrodes for electrochemical ion separation. For implementation, it is essential to understand the interplay of the intrinsic preference of a specific ion (by charge/size), kinetic ion preference (by mobility), and crystal structure changes. Ti3C2Tz MXene is chosen here to investigate its selective behavior toward alkali and alkaline earth cations. Utilizing an online inductively coupled plasma system, it is found that Ti3C2Tz shows a time-dependent selectivity feature. In the early stage of charging (up to about 50 min), K+ is preferred, while ultimately Ca2+ and Mg2+ uptake dominate; this unique phenomenon is related to dehydration energy barriers and the ion exchange effect between divalent and monovalent cations. Given the wide variety of MXenes, this work opens the door to a new avenue where selective ion-separation with MXene can be further engineered and optimized

Toward MBenes Battery Electrode Materials: Layered Molybdenum Borides for Li‐Ion Batteries

Lithium-ion and sodium-ion batteries (LIBs and SIBs) are crucial in our shift toward sustainable technologies. In this work, the potential of layered boride materials (MoAlB and Mo2 AlB2 ) as novel, high-performance electrode materials for LIBs and SIBs, is explored. It is discovered that Mo2 AlB2 shows a higher specific capacity than MoAlB when used as an electrode material for LIBs, with a specific capacity of 593 mAh g-1 achieved after 500 cycles at 200 mA g-1 . It is also found that surface redox reactions are responsible for Li storage in Mo2 AlB2 , instead of intercalation or conversion. Moreover, the sodium hydroxide treatment of MoAlB leads to a porous morphology and higher specific capacities exceeding that of pristine MoAlB. When tested in SIBs, Mo2 AlB2 exhibits a specific capacity of 150 mAh g-1 at 20 mA g-1 . These findings suggest that layered borides have potential as electrode materials for both LIBs and SIBs, and highlight the importance of surface redox reactions in Li storage mechanisms