Elsevier

Desalination

Volume 496, 15 December 2020, 114750
Desalination

High speed capacitive deionization system with flow-through electrodes

https://doi.org/10.1016/j.desal.2020.114750Get rights and content

Highlights

  • TiO2@CNF designed with electrical conductivities and high specific surface areas for CDI applications.

  • 3D nanostructured TiO2@CNF integrates the features of both the TiO2@CNF and the FTE CDI architecture.

  • A high salt removal rate of 1.26 mg g−1 min−1 in the capactive desalination process.

  • A removal capacity of 14.5 mg g−1 was obtained under external power ~ 1.4 V.

Abstract

Capacitive deionization (CDI) is considered as a promising approach to sustain fresh water supply with environmental friendliness and convenient electrode regeneration. As a novel CDI system, flow-through electrode (FTE) CDI is drawing researchers' attention due to its structural simplicity, highly compact cells, cost effectiveness, and fast salt adsorption kinetics that are applicable for large-scale desalination of saline water. However, the FTE CDI architecture requires electrodes with robust structures and preferable permeability, considering the direct flow through mechanism, which limits their choices of electrode materials. Herein, we propose a facial electrospinning method to fabricate three-dimensional TiO2 encapsulated carbon nanofiber (TiO2@CNF), which possesses good mechanical stability and highly permeable macroporous-mesoporous structure to endure the reasonable feed pressure upon high-speed influent flushing. Moreover, the TiO2@CNF electrode shows evident pseudo-capacitive performance as well as high electrical conductivity. By integrating the features of both the TiO2@CNF and the FTE CDI architecture, the as-fabricated system displays a salt removal capacity of 15.50 mg g−1 and a desalination rate of 1.26 mg g−1 min−1 at 1.4 V. The TiO2@CNF provides a promising alternative for FTE CDI towards the future desalination technologies.

Introduction

Water crisis is one of the most inevitable challenges faced worldwide [[1], [2], [3]]. And desalination is considered as a promising strategy to address the accessibility of clean and reliable freshwater [4,5]. However, the current desalination technologies such as reverse osmosis, membrane separation and thermal methods suffer from high energy consumption and the necessity of indispensable expensive infrastructures [[6], [7], [8]]. Therefore, capacitive deionization (CDI) is emerging as a suitable alternative due to its energy effectiveness and easy operation especially for brackish water desalination [[9], [10], [11], [12]]. In a typical CDI cell, the salt ions in concentrated feed solution can be stored in the electrical double layers of the two porous carbon electrodes with an applied electrical voltage. When the adsorption process reaches equilibrium, the electrodes are able to be regenerated by a reverse voltage or short-circuiting, releasing salt ions back to the concentrated stream [9,13,14]. The most common CDI architecture is a flow-by (or flow-between) system where the feed solution flows between the two working electrodes [15]. Enormous electrode materials are explored in the traditional flow-by CDI system to improve the desalination efficiency and lower the expenditure [[16], [17], [18], [19]]. Among which, electrospinning is considered a promising method to prepare upscalable and cost-effective electrode materials. This flow-by architecture delivers an environmental-friendly, energy efficient and simply operable desalination performance, while there are still some limitations of flow-by CDI systems for practical desalination applications, such as slow salt adsorption kinetics and inefficient desalination performance in saline water [20]. Besides, in flow-by electrodes, the separator thickness requires careful optimization to obtain influent flow through and low electrical resistance simultaneously, which involves extra complexity for the cell design [20]. To further improve the desalination kinetics and simplify the cell configuration, a novel alternative of CDI architectures, called flow-through electrode (FTE) CDI has been developed [20]. FTE CDI allows feed solution to flow directly through the mesopores and macropores of the electrode materials and facilitates the accessibility of influent into the electrode material. The desalination rates can be improved, and the cells configuration is more compact by reducing the flow resistance and minimizing spacer thickness [21]. For the advancement of FTE CDI system, exploring appropriate electrode materials for FTE CDI is necessary. Various carbon-based materials such as carbon aerogel [22,23], activated carbon [[24], [25], [26]], carbon cloth [15,21] and multi-walled carbon nanotubes (MWCNTs) [27] have been investigated as electrode materials for FTE CDI. However, the significant electrode degradation in FTE CDI has been noticed as a considerable pressure is necessary to pump water directly through the electrode materials [15,20]. Hence, the electrode materials for FTE CDI require not only the favourable properties of high adsorption performance (e.g., high surface area and electrical conductivity, excellent wettability), but also outstanding mechanical stability [15], which limits the choices of FTE CDI electrode candidates. Moreover, other factors such as pseudocapacitive properties are also viable to enhance the salt adsorption capacity yet seldom investigated. Meanwhile, titanium dioxide (TiO2), as a key material that has been extensively explored in lithium-ion batteries [[28], [29], [30], [31], [32], [33]], sodium-ion batteries [[34], [35], [36]], and water splitting [37], has demonstrated excellent rate performance due to its high pseudocapacitive contribution [38]. Especially, the high psedocapacitance of TiO2 has been proven to enhance the salt adsorption rates of traditional CDI [[39], [40], [41]]. In addition to the high psedocapacitance, TiO2 also has been demonstrated as a versatile electrode material with cost-effectiveness, environmental friendliness, high chemical stability, and diverse composites with various carbon structures [35]. However, the application of TiO2 in FTE CDI hasn't yet been investigated, especially for their pseudocapacitive performance. Herein, we propose a pseudocapacitive TiO2/carbon nanofiber composite (TiO2@CNF) as the flow-through electrode materials with good mechanical stability via a facile electrospinning method. This electrospinning strategy introduces three-dimensional (3D) carbon nanofiber network with numerous effective electron/ion transport pathways and enables high mass production for industrial applications. In addition, the 3D carbon networks of TiO2@CNF provide a mesoporous and macroporous structure as well as a high surface area of 178 m2 g−1 with a total pore volume of 0.35 cc g−1. The high electrical conductivity and high surface area of 3D carbon networks and the large specific capacitance of TiO2 nanoparticles of TiO2@CNF endow a prompted desalination performance in a FTE CDI system. As a result, the FTE CDI system with TiO2@CNF electrodes achieves an enhanced salt removal capacity of 15.50 mg g−1 and desalination rate of 1.26 mg g−1 min−1 at 1.4 V, respectively. This work provides a facile solution without utilisation of membranes and sheds light on the material design for the practical application of CDI systems.

Section snippets

Experimental section

Sodium chloride (NaCl), polyvinyl pyrrolidone (PVP), Titanium butoxide (Ti(OBu)4), ethanol (99%) and acetic acid used in this work were purchased from Sigma-Aldrich.

Illustration of the FTE CDI system

Fig. 1(a) presents the schematic diagram of the material synthesis process. The TiO2@CNF is fabricated via an electrospinning method which enables high mechanical stability and large-scale production. The TiO2 nanoparticles are encapsulated inside the carbon nanofibers, facilitating its chemical stability during salt adsorption process. The schematics of FTE CDI cell are depicted in Fig. 1(b, c), where the feed solution flows directly through the TiO2@CNF electrodes during

Conclusion

A rationally designed TiO2@CNF material is fabricated via electrospinning for FTE CDI system. The strong mechanical stability accommodates the feasibility of the greatly simplified FTE CDI system, enabling a membrane-less and energy efficient desalination technique. Besides, the uniform distribution of mesopores and macropores approves feed solution to flow through the cell with a relative low pressure which lowers the energy consumption of the whole system. The high specific surface area of

CRediT authorship contribution statement

L. Guo and H. Y. Yang: Conceptualization, Methodology, Analysis of data.

L. Guo, M. Ding, D. Yan: Data curation, Writing- Original draft preparation, Visualization, Investigation.

M. E. Pam, S. Vafakhah, C. Gu, W. Zhang, P. V. y Alvarado, Y. Shi: experimental data collections and Validation.

L. Guo, H. Y. Yang: Writing- Reviewing and Editing,

Declaration of competing interest

Please check the following as appropriate:

  • All authors have participated in (a) conception and design, or analysis and interpretation of the data; (b) drafting the article or revising it critically for important intellectual content; and (c) approval of the final version.

  • This manuscript has not been submitted to, nor is under review at, another journal or other publishing venue.

  • The authors have no affiliation with any organization with a direct or indirect financial interest in the subject

Acknowledgement

This research is funded by Singapore Ministry of Education research grant Tier 2 (MOE2018-T2-2-178).

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