MXene as a Cation-Selective Cathode Material for Asymmetric Capacitive Deionization

Abstract

Capacitive deionization (CDI) has become a promising method to solve the shortage of freshwater resources recently. However, the co-ion expulsion effect obviously hinders electrosorption capacity and charge efficiency of CDI. In this work, an asymmetric CDI cell is assembled in which Na+-intercalated Ti3C2T x (NaOH-Ti3C2T x ) serves as a cation-selective cathode, while the activated carbon (AC) serves as the anode. The NaOH-Ti3C2T x with negatively charged surface groups (−OH, −O, and −F) is adopted to weaken the co-ion expulsion effect. Benefited from the synergistic effect of the reduced co-ion expulsion effect and expanded interlayer space, the asymmetric CDI cell achieves a higher electrosorption capacity of 12.19 mg g–1 and a higher charge efficiency of 0.826 compared with the symmetric one composed of AC (4.55 mg g–1 and 0.306) in 100 mg L–1 NaCl solution. High cyclic stability of the as-prepared asymmetric CDI cell is also observed. The improved desalination performance indicates that NaOH-Ti3C2T x is a promising alternative as cation-selective cathode material for asymmetric CDI cells. The desalination mechanism is discussed in detail to lay the foundation for further improvement of the CDI performance of other 2D materials like MXene.

Host–Guest Intercalation Chemistry in MXenes and Its Implications for Practical Applications

“…Relating to the discussions in EES devices, the process happened in MXenes is more accurately termed pseudocapacitive deionization (PCDI), and Srimuk et al provided a proof-of-concept study on the PCDI performance of multilayer Ti 3 C 2 T x . Possible ways to improve the salt adsorption capacity include the tuning of surface moieties, optimization of the intercalation process, and construction of porous electrodes with high SSA, − which have been already discussed in the EES section.…”

In Situ Formation of Prussian Blue Analogue Nanoparticles Decorated with Three-Dimensional Carbon Nanosheet Networks for Superior Hybrid Capacitive Deionization Performance

“…With the growing human population and the fast-developing economy, the scarcity of freshwater has become one of the most challenging and significant global issues. , Recently, researchers have been focusing on seeking a water purification technology with low cost and high efficiency because desalination has been identified as an ideal way to solve this issue . Capacitive deionization (CDI), as a newly innovative water purification technology, has drawn great attention owing to its low driven energy, low operating cost, reliable regeneration, and environmentally friendly property. − Salt ions can be moved toward and absorbed on the oppositely charged electrodes when a certain voltage is imposed between the two electrodes, and thereby the fresh water is obtained. − An electrode material is the core part of CDI and one of the key factors that determine the performance of CDI. − Carbonaceous materials, such as activated carbon (AC), carbon aerogel (CA), and graphene, , were used as CDI electrodes frequently because of their high conductivity and high specific surface area. However, the desalination capacity of these carbonaceous CDI electrodes remain unsatisfactory and shows results far from the commercial standards in practical application, which is due to the low effective specific surface area, complex pore structures, and electrical double layer (EDL) overlapping effect .…”

The structural evolution of 3D-RGO with reduction temperature and its effect on capacitive deionization performance

“…This means that the number of ions desorbed from the electrode is more than the ones adsorbed. [47][48][49][50] The deeper reason may be the presence of a certain amount of oxygen-containing functional groups in 3D-RGO-650 °C due to a relatively low reduction temperature. These electronegative groups electrostatically adsorb some cations.…”