Plasma-assisted preparation of carbon cloth-supported NiCoAl-LDHs with large interlayer spacing for electrochemical deionization

Abstract

Capacitive deionization (CDI) has been regarded as an emerging desalination technology in recent years, especially for brackish water, due to its economic and energy-saving advantages. However, research on chloride removal electrodes is limited, and slow desalination kinetics also restrict their development. In this work, NiCoAl-LDHs nanosheet arrays were grown in situ on flexible carbon cloth (ACC) after surface acid treatment and then subjected to Ar plasma treatment, producing Ar-NiCoAl-LDHs@ACC with enlarged interlayer spacing. The carbon cloth substrate inhibited the agglomeration of NiCoAl-LDHs nanosheets and improved electrical conductivity, while Ar plasma treatment further expanded the interlayer spacing and enhanced hydrophilicity, providing fast chloride ion diffusion channels and releasing more interlayer active sites, achieving high desalination kinetics. Ar-NiCoAl-LDHs@ACC was used as the chloride removal electrode and assembled with activated carbon into a hybrid capacitive deionization (HCDI) device. In 1000 mg L−1 NaCl solution at 1.2 V, the salt removal capacity reached 93.26 mg g−1, the salt removal rate reached 0.27 mg g−1 s−1, and the charge efficiency was as high as 0.97. In 300 mg L−1 NaCl solution at 0.8 V, the capacity retention remained above 85% after 100 cycles. This work provides a new strategy for the controllable preparation of two-dimensional metal hydroxide materials with large interlayer spacing and the design of high-performance electrochemical chloride removal electrodes.

1. Introduction

The global increase in water consumption has led to high water stress both domestically and internationally, making it urgent to address the shortage of fresh water resources. Desalination is one of the effective methods to solve this problem [1,2]. Traditional thermal, membrane, and ion exchange methods suffer from high equipment costs, high energy consumption, and secondary pollution [3,4], which are not conducive to sustainable development. Capacitive deionization (CDI) has attracted attention due to its energy efficiency, economy, and environmental friendliness, emerging as a promising desalination technology [5,6]. CDI works by applying an electric field across electrodes, causing ions in the solution to migrate toward oppositely charged electrodes, thereby reducing ion concentration and achieving desalination.

Conventional CDI typically uses carbon materials such as activated carbon [7], carbon aerogels [8], carbon nanotubes [9], and graphene [10] as electrodes, which store charge via electrical double layers, offering fast desalination rates [11]. However, carbon electrodes suffer from co-ion repulsion effects, leading to low charge efficiency [12]. In contrast, Faradaic electrodes operate through ion intercalation/deintercalation, providing higher ion selectivity, charge efficiency, and salt removal capacity [12,13]. Most research has focused on sodium-removal materials (e.g., manganese oxides, titanium oxides, Prussian blue analogs), while studies on chloride-removal electrodes are relatively scarce. Current chloride removal electrodes mainly include Ag/AgCl and Bi/BiOCl [14], but these materials exhibit slow chloride removal rates and are prone to hydrogen/oxygen evolution reactions [15]. Layered double hydroxides (LDHs) have high electrochemical activity and can undergo rapid reversible redox reactions, leading to faster desalination rates. LDHs are two-dimensional layered materials with the general formula [M2+1−xM3+x(OH)2]2+[An−x/n·mH2O], where M2+ and M3+ are divalent and trivalent metal cations, and An− is an interlayer anion [16]. LDHs can store anions through interlayer intercalation, making them suitable for anion adsorption.