Elsevier

Desalination

Volume 520, 15 December 2021, 115343
Desalination

Raspberry-structured silver-carbon hybrid nanoparticle clusters for high-performance capacitive deionization

https://doi.org/10.1016/j.desal.2021.115343Get rights and content

Highlights

  • Ag-carbon nanoparticle clusters as positive electrode via aerosol-based synthesis.

  • Direct gas-phase coupled characterization supports the aerosol-based synthesis.

  • Successful quantitative characterization of the Ag-C-NPC in aerosol state.

  • Remarkable high capacity (15.6 mg/g) and stability with a low cell voltage.

Abstract

An aerosol-based synthetic approach, in combination with a real-time characterization method using differential ion-mobility analyses coupled to temperature-programmed Fourier-transform infrared spectroscopy (DMA/TP-FTIR), was demonstrated for the development of raspberry-structured Ag‑carbon hybrid nanoparticle clusters (Ag-C-NPC). Physical size, number concentration, and compositions of the Ag-C-NPC were shown to be successfully characterized directly in the aerosol state on a quantitative basis. Remarkable high salt adsorption capacity (15.6 mg/g), good charging/discharging stability, and low requirement of cell voltage were attainable by using the Ag nanoparticle-decorated carbon nanoparticle clusters as the positive electrode material. This work provides a proof of concept of using the aerosol-based synthesis, with the support of in-situ characterization, for the development of Agsingle bondC nanocomposite clusters to achieve high CDI performance. The method also shows promise for the tuning of the material properties of Ag-C-NPC for the optimization of the CDI capacity and cyclic stability useful for a variety of the water desalination applications (e.g., brackish water).

Introduction

Capacitive deionization (CDI) is an attractive desalination technology and employed as an energy-efficient, economic alternative for a variety of applications in the field of water purification (e.g., brackish water desalination [1], [2], distilled water production [2], [3], wastewater treatment [4], [5], hardness control [6], [7], [8]). In principles, the charged species in the water are removed by electrostatic force on a pair of CDI electrodes under an electric field (i.e., the charging process). The ionic species stored in the electrical double layer (EDL) of the electrodes can be released from the electrodes for the subsequent regeneration process (i.e., the discharging process) [2], [5], [9], [10], [11], [12], [13]. Based on the mechanisms of deionization, six categories of the electrochemical desalination systems are reported recently [14]: (1) pseudocapacitive//EDL CDI, (2) pseudocapacitive//pseudocapacitive deionization (3) EDL//EDL CDI, (4) battery-type//EDL deionization, (5) desalination batteries, and (6) pseudocapacitive//battery-type deionization [15], [16], [17], [18], [19], [20], [21], [22]. Porous carbons are the common electrode materials employed in the conventional EDL//EDL CDI systems attributable to their advantageous large specific surface area and high electric conductivity [4], [5], [10], [15], [16], [23], [24], [25], [26], [27]. However, the classical carbon-based CDI system encounters the issue of low salt adsorption capacity, presumably due to the limited hydrophilic surface area available for the formation of the effective EDL (i.e., insufficient accessible area for accommodating ions) [11], [12], [13], [23], [24], [27]. Another critical challenge of the carbon-based CDI system is to maintain sufficiently high cyclic stability, where carbon oxidation at relatively highly positive potentials leads to the deterioration of carbon structure and the shift of point of zero charge, resulting in the decay of CDI performance [10], [11], [23], [24].

Recently, incorporation of the concepts of material hybridization shows promise to improve the capacity and cyclic stability of the CDI systems [11], [12], [15], [16], [24], [28]. In addition to being attracted by the electrostatic interaction within the EDL in the classical carbon-based system, ions in water are able to be captured by the formation of chemical bonds via faradaic reactions at the electrode/electrolyte interface [10], [13], [29], [30], [31]. Furthermore, the decorated species in the hybrid nanostructure with redox reversibility can inhibit carbon materials from degradation by preventing the so-called “memory effect” and prolonging the cyclic stability of desalination systems [14], [32]. In comparison to the conventional CDI, former studies have demonstrated that carbon-based hybrid electrode materials (e.g., polypyrrole-coated activated carbon [32], Ni- and MnO2- hybridized activated carbon [16], [26], hollow carbon@MnO2 [33], vanadium-pentoxide-decorated multiwalled carbon nanotubes [34] and molybdenum disulfide/reduced graphene oxide [35]) were employed to achieve superior deionization capacities via following a highly reversible redox reaction. Silver nanoparticle (Ag NP) is considered as one of the most useful materials to be hybridized with the carbon-based CDI electrode [13], [30], [36], [37], [38], [39]. Based on their high anion capturing ability (especially high selectivity toward the targeting Cl ions) and electrochemical stability in an aqueous system, both the capacity and cyclic stability of the carbon-based CDI were reported to be significantly enhanced through the incorporation of Ag NPs.

In this study, an aerosol-based synthesis approach, with the ability of gas-phase in-situ characterization, is demonstrated for preparing the Ag nanoparticle-decorated carbon nanoparticle clusters (Ag-C-NPC), which can be used as a positive electrode of the hybrid CDI system. As depicted in Scheme 1a, stabilized colloids of acid-functionalized carbon nanoparticles (C-NP) are firstly nebulized altogether with soluble Ag precursor to become aerosols (Step 1). During the spray drying process, spherical clusters of C-NP (C-NPCs) are formed and are uniformly decorated by the dried Ag precursor crystallites via the principles of evaporation-induced self-assembly (EISA) (Step 2). Subsequently, the formation of raspberry-structured Ag‑carbon hybrid nanoparticle clusters (Ag-C-NPC) is achievable by selective decomposition of the dried Ag precursor crystallites to Ag NPs via an aerosol-phase thermal treatment (Step 3). The advantages of using the aerosol-based synthetic method are (1) continuous production and (2) creation of large amount of the Agsingle bondC interfaces to promote the reversible charge transfer in the redox reactions [40], [41], [42], [43], [44].

In addition, a new in-situ gas-phase characterization method, differential ion-mobility analyses (DMA) coupled to the temperature-programmed Fourier transform infrared spectroscopy (TP-FTIR), is established in this study. The method provides real-time analyses of the synthesized hybrid nanostructures directly in the aerosol state. As depicted in Scheme 1b, the aerosols of the synthesized Ag-C-NPCs were delivered downstream to the coupled analytical system, where the physical size and the number concentration of the aerosol particles can be quantified by the DMA mode (Route 1), and the mass concentrations of carbon in the aerosol particles can be obtained using the TP-FTIR mode (Route 2). The objectives of the study are in two folds: (1) development of a Ag-C-NPC hybrid nanostructure using the gas-phase evaporation-induced self-assembly process and (2) establishment of real-time quantitative characterization approach useful for providing understanding of interfacial phenomena of Ag-C-NPC correlated to the optimization of the CDI performance.

Section snippets

Materials

Nanocarbon (Vulcan XC72R, typical bulk density was 96.1 kg/m3, Cabot corporation, Boston, Massachusetts, USA) and silver nitrate (AgNO3, 99.8%, Honeywell, Charlotte, NC, USA) were chosen as the precursors of C-NPC and Ag NPs, respectively. Activated carbon powder (ACS25, China Steel Chemical Corporation, Taiwan, ROC) was used as the negative electrode material in the capacitive deionization (CDI) system. Poly(vinylidene fluoride) (PVDF. 99%; molecular mass: 530 kDa), n-methylpyrrolidone (NMP,

Preparation of carbon nanoparticle colloid

ATR-FTIR was employed for the study of the effectiveness of the acid-based surface functionalization of carbon nanoparticles. Fig. 1a shows the FTIR spectra of the nanocarbon over different periods of acid treatment (ta). In comparison to the sample without acid treatment (ta = 0 h), vibration bands were identified at 1200 cm−1, and 1660 cm−1 corresponding to the vibrational modes of Csingle bondO symmetric stretching and Cdouble bondO stretching, respectively. The results of ATR-FTIR indicate that the number of

Conclusions

The concept of gas-phase evaporation-induced self-assembly is demonstrated as a facile route for the fabrication of silver-decorated carbon hybrid nanoparticle clusters (Ag-C-NPC). With the support of the developed aerosol-based differential ion-mobility analyses coupled to the temperature-programmed Fourier-transform infrared spectroscopy, physical size, number concentration, and composition of the Ag-C-NPC are successfully characterized directly in the aerosol state. Remarkable high capacity

CRediT authorship contribution statement

Meng-Ting Chiang's contribution (1st author): Conceptualization; Data curation; Formal analysis; Investigation; Methodology; Validation; Writing – review, editing and revision.

Yi-Heng Tu's contribution (2nd author): Data curation; Formal analysis; Investigation; Methodology.

Hsin-Li Chiang's contribution (3rd author): Methodology; Validation; Formal analysis.

Chi-Chang Hu's contribution (co-corresponding author): Conceptualization; Formal analysis; Investigation; Methodology; Project

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors thank financial supports from Ministry of Science and Technology (MOST) of Taiwan, R.O.C. under the contract MOST 109-2223-E-007-002-MY3 and MOST 109-2221-E-007-038-MY3, and from National Tsing Hua University under the contract 110Q2708E1.

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