Ion removal performance and enhanced cyclic stability of SnO₂/CNT composite electrode in hybrid capacitive deionization

Abstract

Hybrid capacitive deionization (HCDI) is developed based on capacitive deionization (CDI) for water treatment, which combines CDI with a battery system. Developing the high-performing battery electrode materials is a significant point for HCDI. Herein, SnO₂/CNT was fabricated according to hydrothermal method, which was denoted as SnO₂/CNT-H, after calcination, the obtained product was denoted as SnO₂/CNT-HC. The as-prepared SnO₂/CNT-HC was used in HCDI system for the first time, which demonstrated a salt adsorption capacity of 9 mg g⁻¹ and ultrafast salt adsorption rate of 2.5 mg g⁻¹ min⁻¹ at 500 mg L⁻¹ initial NaCl concentration. The SnO₂/CNT-HC electrode also had a good cyclic stability of keeping 120 % of salt adsorption capacity after 100 adsorption-desorption cycles at 1.0 V and 500 mg L⁻¹ NaCl solution. Moreover, when this system was used to study the adsorption-desorption performance for different cations (Li⁺, K⁺ and Ca²⁺) and anions (F⁻, NO₃⁻ and SO₄²⁻), the phenomenon was revealed that the cation adsorption capacity of Li⁺ > K⁺ > Ca²⁺ captured by SnO₂/CNT-HC and the anion adsorption capacity of F⁻ > NO₃⁻ > SO₄²⁻ adsorbed by activated carbon. These results show that SnO₂/CNT-HC should be one of realistic electrode materials for HCDI.

Introduction

With a sharp increase in population and serious groundwater pollution, the shortage of clean water has become one of the major issues limiting social development and human healthy living in the next 50 years [1,2]. In response to these crisis, developing the desalination technology is imperative. Until now, several desalination technologies including reverse osmosis, multistage ash distillation, electrodialysis and multi effect distillation have been developed [[3], [4], [5]]. However, these desalination technologies have the disadvantages of high energy consumption and secondary pollution [6,7]. For these reasons, it is particularly important to design and develop a low-cost, high-efficiency and novel desalination technology.

CDI is one of the new desalination technologies which possess many advantages including high removal efficiency, simple, cheap and robust design, and has attracted more attention in recent years [8,9]. The typical CDI process includes two steps: first, salt solution flows through two electrodes and when the voltage is applied between the electrodes, the charged ions in salt solution are adsorbed into the opponent electrodes. Subsequently, after reaching adsorption equilibrium, the electrodes are regenerated using reversing the cell voltage [10]. The principle of CDI is in view of the electric double layer (EDL), suggesting that the charged ions are adsorbed in electrodes surface when voltage is applied [[11], [12], [13]]. The desalination performance of CDI is related to electrode materials. Carbon materials such as carbon aerogels [14,15], activated carbon [16,17], carbon nanotubes [18], mesoporous carbon [19,20], carbon nanofibers [21], and graphene [22] have been widely used in CDI due to their high suitable surface area and high electrical conductivity in recent decades. But, the traditional carbon materials also have inevitable defects of poor cycle stability [[23], [24], [25]].

The concept of HCDI was put forward at the first by Yoon at 2014 [26]. In their work, sodium manganese oxide (Na₄Mn₉O₁₈) prepared using a solid-state reaction between Mn₂O₃ and Na₂CO₃ was used in HCDI and exhibited more higher salt adsorption capacity (31.2 mg g⁻¹) than a typical CDI system (13.5 mg g⁻¹) and a fast salt adsorption rate and excellent stability in NaCl solution [26]. In recent years, varieties of battery materials were applied in HCDI for water desalination and made an explosive success. Cao’s group synthesized chemically exfoliated MoS₂ (ce-MoS₂) used in CDI [27] and the ce-MoS₂ material electrode demonstrated a high salt adsorption capacity of 8.81 mg g⁻¹, and excellent cycling stability. At 2018, Choi’s group prepared one kind of prussian blue with K_0.03Cu[Fe(CN)₆]_0.65·0.43H₂O by the ion coprecipitation [28]. An asymmetric CDI system consisting of activated carbon (AC) material electrode and prussian blue material electrode was applied in HCDI and demonstrated a high ion quality removal capacity of 20.4 mg g⁻¹, and keeping 99.51 % of ion quality removal capacity over 100 cycles. In recent years, more and more new battery materials with high salt capacity, fast salt removal rate and excellent cyclic stability were explored in HCDI.

Kinds of metal oxides such as MnO₂, V₂O₅, and TiO₂ have been widely used in sodium ion battery (NIB). Among these new metal oxides candidates, tin oxide (SnO₂) has drawn much attention because of it’s significant advantages of appropriate bandgap and high theoretical capacity (670 mA h g⁻¹) [[29], [30], [31]]. However, the disadvantages of poor conductivity and large volume expansion (>400 %) of SnO₂ isn’t suitable for HCDI. Constructing composite of nano-SnO₂ with carbon material such as carbon nanotubes (CNTs) seem to be a good way to solve these problems. CNTs have good electronic conductivity, suitable surface area, and excellent chemical stability, which can eliminate the defect of SnO₂ and enhance the adsorption performance of the electrode. SnO₂/CNT composite was prepared by hydrothermal method, which was denoted as SnO₂/CNT-H, after calcination, the obtained product was denoted as SnO₂/CNT-HC. CNTs were used as conductive carrier not only promote the conductivity of the composite but also interconnect the SnO₂ particles and inhibit the aggregation of SnO₂. Apart from these, the CNTs have many functional groups (COOH, OH), after hydrothermal and calcination, many chemical bonds of SnC will be formed connecting SnO₂ and CNTs. Because of these strong chemical bonds, the large volume change of SnO₂ will be inhibited and the Na⁺ transport will be accelerated [32].

Herein, SnO₂/CNT was investigated as HCDI electrode material for the first time which demonstrated a salt adsorption capacity of 9 mg g⁻¹ and excellent cyclic stability keeping 120 % of salt adsorption capacity over 100 adsorption-desorption cycles. These results indicate that SnO₂/CNT-HC should be one of practicable electrode materials for HCDI. The structure of HCDI was shown in Fig. S1. In this system, sodium ions are captured by redox reaction in the SnO₂/CNT-HC electrode, and chloride ions are adsorbed on the activated carbon electrode during the desalination process. Because the Na⁺ is stored into battery materials electrode based on Faraday reaction [33,34] (mainly include redox reaction, intercalation reaction, conversion reaction and alloy reaction), the structure of battery electrode materials is keeping stable and not limited by ions concentration in the adsorption/desorption process, so HCDI can survive in high-concentration saline.