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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.
Abstract 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.
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 Mo2AlB2) as novel, high-performance electrode materials for LIBs and SIBs, is explored. It is discovered that Mo2AlB2 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 Mo2AlB2, 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, Mo2AlB2 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.
