Abstract:

Capacitive deionization (CDI) has been considered one of the most promising desalination technologies in the past decade. However, it faces challenges related to low salt removal efficiency in high salinity water. To address this issue, ion intercalation materials have been developed as anodes for CDI due to their abundant electroactive sites capable of accommodating large salty ions. V₂O₃, a typical intercalation host, has garnered significant attention in the field of metal-ion batteries and appears to be a suitable candidate for CDI. Nevertheless, structural instability and slow ion diffusion, resulting from large volume expansion and low intrinsic electron/ion conductivity, present obstacles to its commercial application. Given their high specific surface area, abundant ion diffusion channels, and excellent conductivity, derivatives of metal-organic frameworks (MOFs) have become highly attractive in the electrochemical research community. In this study, 2D V₂O₃@porous carbon (V₂O₃@PC) nanosheets were prepared using homologous metal V₂CF_x MXene as a precursor for CDI anodes, aiming to enhance salt removal capacity. The structure, crystallinity, wettability, graphitization degree, and electrochemical behavior of V₂O₃@PC were investigated by adjusting carbonization temperatures. The findings reveal that V₂O₃@PC exhibits a typical 2D nanosheet structure, with highly crystalline V₂O₃ nanoparticles securely enveloped by graphitized PC. The electronic coupling between PC and V₂O₃ ensures high electron conductivity. This unique structure demonstrates excellent interfacial wettability and high conductivity, facilitating electrolyte penetration, accelerating interfacial charge transfer, and enhancing salt ion diffusion. Additionally, the PC effectively restricts the volume expansion of V₂O₃. Moreover, reversible electrochemical conversion between V³⁺/V⁴⁺ contributes to Na⁺ storage, aiding the desalination/regeneration process. Notably, X-ray diffraction (XRD) analysis revealed the preferential growth of V₂O₃ crystal planes at different carbonization temperatures. Consequently, the optimized V₂O₃@PC-850 electrode exhibits remarkable desalination performance, including a desalination capacity of 2.20 mmol·g⁻¹, desalination rate of 0.13 mmol·g⁻¹·min⁻¹, water recovery rate of 62%, and energy consumption of 24.0 Wh·m⁻³ at 1.2 V in 1000 μS·cm⁻¹ NaCl solutions. Compared to V₂O₃@PC-750 and V₂O₃@PC-950, the superior performance of V₂O₃@PC-850 can be attributed to its enhanced interfacial wettability, which promotes charge transfer and improves salt ion diffusion kinetics. Additionally, the preferential growth of the (110) crystal plane in V₂O₃@PC-850 enhances ion storage capacity, contributing to its superior desalination performance. This study offers new insights into the synergistic enhancement of electrochemical ion removal characteristics through the utilization of metal oxide and carbon nanomaterials.

Key words: Capacitive deionization, V₂O₃, Metal organic frameworks, MXene, Electrochemistry