vacancy ordering on desalination performance of NaxCoO2

Desalination 478 (2020) 114301

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An insight into the promotion effect of Na+/vacancy ordering on desalination performance of NaxCoO2

T

Ruijuan Zhou, Xiaoxu Guo, Xiaoman Li, Yongshuai Kang, Min Luo

⁎

State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering, School of Chemistry and Chemical Engineering, Ningxia University, Yinchuan 750021, Ningxia, China

GRAPHICAL ABSTRACT

ARTICLE INFO

ABSTRACT

Keywords: NaxCoO2 Na+/vacancy ordering pattern Intermediate phases domain Faradaic electrode Desalination

In this work, we used NaxCoO2 as a model system for the first time to explore the effect of Na+/vacancy ordering on desalination performance. An asymmetrical FDI device was developed where NaxCoO2 as the Faradic electrode material and AC act as the Cl-storage electrode material. For x = 0.50, this configuration yields an ultrahigh desalination capacity (130.2 mg g−1), an excellent charge efficiency (100%), and a superior desalination rate (2.61 mg g−1 min−1) in 1000 mg L−1 NaCl solution at 1.4 V. The experimental results reveal that the desalination capacity depends on the range of reversible adjacent intermediate phases region and the desalination rate is affected by the sodium ions mobility in the interlayer determined by the Na+/vacancy ordering pattern between the CoO2 slabs. The excellent desalination capacity and rate are encouraging.

1. Introduction Desalination provides effective solutions to the global shortage of fresh water resources [1,2]. Capacitive deionization (CDI) as a promising water desalination technique has been developed exponentially during the past five years for the unique advantages of low energy consumption, reliable regeneration, and environmental friendliness [3–5]. Generally, high desalination capacity and rate are urgently ⁎

required for desalinating water on a large scale [6]. In contrast with traditional porous carbon materials of CDI which store ions in the electrical double layer (EDL) by electrosorption [7], Faradaic (battery) electrodes of Faradaic deionization (FDI) capture ions through Faradaic redox reactions in the surface and body phases of electrode materials [8]. FDI is enjoying renewed interest as a feasible alternative to the CDI for the large-scale application in a variety of industries. With the emergence of hybrid CDI and dual-ion CDI

Corresponding author at: School of Chemistry and Chemical Engineering, Ningxia University, Yinchuan 750021, China. E-mail address: [email protected] (M. Luo).

https://doi.org/10.1016/j.desal.2019.114301 Received 16 August 2019; Received in revised form 18 December 2019; Accepted 20 December 2019 0011-9164/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 1. The schematic of CDI unit.

configuration, various Faradaic positive electrode materials (Bi [9], BiOCl [10], Ag [11], AgCl [12], etc.) and negative electrode materials (Prussian blue [13], metal oxides [14], sodium layered oxides [15], MXene [16,17], etc.) have been intensively explored to expand its application range and enhance performance [9–17]. A high desalination capacity (up to 100 mg g−1) and charge efficiency (≥85%) were obtained [18]. Recently, sodium layered oxides (NaxMO2, M refers to transition metals) have gained immense interest as the CDI electrode material [19,20]. NaxCoO2 (NCO) has good electronic conductivity and high theoretical capacity. Therefore, in this context, NCO was used as a model system to explore the effect of Na+/vacancy ordering on desalination performance. Among various phases, P2-type NCO which is a layer structure consisting of alternate CoO2 slabs and sodium ions layers with sodium ions occupying two sodium environments noted as the octahedral site (Na1) and the prismatic site (Na2), exhibits the most stable structure during the sodium ions extraction process owing to a direct sodium ions diffusion and an open framework [21–26]. P2-type NCO usually exhibits various single-phase domains with different Na+/ vacancy patterns depending on the sodium content [27]. Moreover, the frameworks of P2-type NCO can be stabilized at different configurations of sodium ions and vacancies, which corresponds to the ordered intermediate metastable phases during the sodium electrochemical extraction/insertion process [28]. The content of sodium affects electronic conductivity and sodium vacancy ordering in the structure [29], which in turn affects the CDI performance. Therefore, it is very significant to study the relationship between the sodium content and desalination performance. Herein, layered NCO (0.45 ≤ x ≤ 0.71) with different Na concentrations were synthesized via a sol-gel and chemical deintercalation method to present a comprehensive study on the influence of Na+/ vacancy ordering on their desalination properties.

30 min to form a homogenous slurry. Then, the slurry uniformly adhered to the graphite paper. The electrodes was dried at 80 °C for 12 h. The mass ratio of the two electrodes was dominated at around 1:1, and the total mass of active materials, including NCO and AC electrode, were 26.6 mg, 26.3 mg, 24.1 mg, and 28.5 mg for x = 0.71, 0.61, 0.50, and 0.45, respectively. 2.3. Electrochemical test The electrochemical performance was evaluated by the three-electrode method on a CHI660D workstation (CH Instruments, TX, USA), which includes the working electrode (NCO), the counter electrode (Pt), and the reference electrode (KCl saturated Ag/AgCl). Cyclic voltammetry (CV) and Electrochemical Impedance Spectrum (EIS) were measured in 1 M NaCl solution with the cell potential from −0.8 to 0 V. 2.4. Device assembling The batch-mode experiment of the two-electrode system was carried out. The experimental system composed of a CDI unit, a peristaltic pump, a DC power, a conductivity, temperature, and pH monitor, and a current collector. The setup of the CDI unit showed in Fig. 1 [31]. In the process of desalination, a peristaltic pump was used to continuously circulated NaCl solution at a flow rate of 39 mL min−1 from a 40 mL NaCl reservoir. When the CDI unit was applied a constant voltage provided by DC power, the conductivity, temperature, and the pH value of the feed were continuously monitored at the same time. Also, a current probe was used to collect current transient data. 2.5. Performance measurement and evaluation Noting that each experiment was conducted until the electrode saturated with sodium ions. The desalination capacity (Γ), desalination rate (SAR), and charge efficiency (Λ) were calculated by the Eqs. (1)–(5):

2. Experimental 2.1. Materials synthesis

=

Parent P2-type Na0.71CoO2 was synthesized as reported previously [30]. Low-sodium-content samples of NaxCoO2 (x = 0.61, 0.50, 0.45) were obtained from Na0.71CoO2 through oxidative deintercalation of sodium ions utilizing iodine in acetonitrile with various stoichiometries, namely 0.5 X, 10 X and 50 X (1 X is the amount that is theoretically needed of iodine to replace all sodium).

n × v 1.89m

(1) (2)

n

=

m

w

=

NA × e 2 (10 pH DH 3O+ + 10 pH kB × T

SAR =

2.2. Electrode fabrication

=

The CDI electrode was fabricated following our previous work [31]. In details, the NCO or AC、Super P and polytetrafluoroethylene (PTFE) with the mass ratio of 5:1:4 were mixed in a mortar and ground for

w

OH

)

(3) (4)

t

58.44

14D

×F (5)

where m is the total mass of active materials (g), v is the volume of Na2q```r1qrCl solution (40 mL), σn, σm, σw and are the corrected 2

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conductivity (μS cm−1), the measured conductivity (μS cm−1), and the water conductivity (μS cm−1), respectively, NA (6.02 × 1023 mol−1), F (96,485C mol−1), and kB (1.38 × 10−23 m2 kg s−2 K−1) are constant, DH3O+ (9.3 × 10−9 m2 s−1) and DOH− (5.3 × 10−9 m2 s−1) are diffusion coefficient, t is the desalination time (min), and Σ is the electric charge (C g−1) [32].

S1) [34]. Besides, we found that the percentage of O2 increased from 63.4% to 77.3%, indicating oxygen deficiencies were simultaneously created as the variation in the valence of cobalt. XRD patterns were shown in Fig. 2c, revealing that all samples can be classified to representative P2 layered structure with P63/mmc space group. No impurity phases were noticed. Obvious, the position of (002) shifted to a lower 2θ and the (100) reflection shown an opposite trend as the decreasing of x (Fig. 2d–e). Accordingly, the c-axis length expanded and the a-axis length shrank due to the decreased Na+−O2– attraction and Na+−Na+ repulsion [35]. The Raman spectrum was plotted in Fig. 2f. For x = 0.71, two distinct Raman modes (A 1g: 570.5 cm−1, E 2g: 448 cm−1) of oxygen were observed, while oxidized samples exhibited a Raman-active of A 1g + 3 E 2g mode. The extra E 2g is the xy displacements of sodium ions caused by the rearranged distribution and lower mobility of sodium ions between CoO2 slabs [36,37]. Besides, the A 1g mode is known to be strongly sensitive to caxis. The A 1g mode shifted to the low frequency, indicating the c-axis length expanded [38]. This result was strongly consistent with XRD analysis. The morphology of NCO particles before and after deintercalation of Na+ were examined by SEM and TEM. The as-oxidized samples exhibited a typical accordion-like multilayer structure (Fig. S2), which was consistent with the report [39]. The HRTEM was introduced to further confirm the change of interlayer spacing. The lattice fringes of 0.545, 0.552, 0.555 and 0.557 nm corresponding to the x = 0.71, 0.61, 0.5 and 0.45 demonstrated that the c-axis length expanded as the decreasing of x (Fig. 3a–h), and the corresponding electron diffraction image (Fig. S3) displayed that it was characteristic of a single crystalline materials. The EDS mapping images of Na0.50CoO2 exhibited that Na, Co and O elements were evenly distributed throughout the whole sample, affirming our effective synthesis method (Fig. 3i–l),

2.6. Characterization Compositions were confirmed using inductively coupled plasma atomic emission spectroscopy (ICP-AES, ICP6000, Thermo Fisher). XRD data were obtained in a 2θ from 5° to 85° with Cu Kα radiation (10° min−1). All samples were examined with scanning electron microscopy (SEM, Zeiss Supra 40), transmission electron microscope (TEM, Hitachi HT7700), and X-ray photoelectron spectroscopy (XPS, thermal science Escalab 250 xi). 3. Results and discussion 3.1. Chemical compositions and structure The crystal structure of NaXCoO2 is depicted in Fig. 2a–b, it is composed of alternating stack of Na and CoO2 layers, and the Na ions are highly mobile among two different lattice sites: Na1 is placed above the Co site of CoO2 layers, while Na2 occupies the lattice point above the center of triangular lattice of CoO2 layers. Chemical analysis revealed that the chemical composition of the parent compound of sodium cobalt oxide was Na0.71(1)CoO2, which was lower than the nominal value because of the evaporation of sodium (Table S1). The precise Na/Co ratio of the as-oxidized samples were confirmed by ICP-AES, and the samples were denoted as Na0.61(1)CoO2, Na0.50(1)CoO2, and Na0.44(9)CoO2. Considering that the charge deviation resulted from the deintercalation of sodium ions would be compensated by the increase of Co4+, a cerimetric titration method was employed to accurately detect the content of Co3+ and Co4+ (Table S2) [33]. With the decreasing of x, the percentage of Co4+ gradually increased from 18.0% to 47.0% to balance the valence state. This result was verified by XPS analysis (Fig.

3.2. Electrochemical performances To assess the electrochemical performance, CV was measured. In Fig. 4a, a pair of main oxidation/reduction peaks of NCO were observed on the CV curve at the scan rate of 5 mV s−1, implying that highly

Fig. 2. (a) The structure of NCO, (b) view parallel to the c-axis to show the site positions of Na1 and Na2, (c–e) XRD of NCO samples, (f) the Raman spectrum. 3

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Fig. 3. The TEM images of (a) Na0.71CoO2, (b) Na0.61CoO2, (c) Na0.50CoO2, (d) Na0.45CoO2 and (e-h) the corresponding HRTEM images, (i–l) EDS maps of Na0.50CoO2 sample.

reversible Faradaic reaction occupied in the process of charging and discharging. Sodium ions can be deintercalated/intercalated from/into interlayer, which achieves the transformation between Co3+ and Co4+ on oxygen network to compensate the electrons moving out from the NCO. Linear of the ΔJ (the difference between the oxidation current and the reduction current) at 0.1 V versus the scan rate was depicted in Fig. 4b. It is well known that the linear slope represents the corresponding electroactive surface area (ECSA). Therefore, ECSAs of Na0.71, Na0.61, Na0.50, and Na0.45 electrodes were 0.389, 0.644, 1.49, and 0.964 mF cm−2, respectively. Apparent, the largest ECSA can be obtained when x = 0.50 in NaxCoO2 system. Fig. 4c displayed the Nyquist plots in the frequency range from 10−2 Hz to 106 Hz. The value of the semicircle was associated with the charge-transfer resistance [40]. Normally, the smaller the diameter of the semicircle, the better the electron transfer rate. Obvious, the electron transfer rate increased as the decreasing of x.

mode circulation device. When applied a positive cell voltage, the conductivity of saline decreased quickly, and the current was simultaneous decreased rapidly (Fig. 5a). The experiment was conducted until the conductivity does not decrease significantly with time, and then given a negative voltage between the circuit, the conductivity of saline recovered to the original value. The corresponding salt concentration changes displayed in Fig. S4, implying that the adsorbed ions could be released back to the solution as one cycle of desalination. The Co element was detected in the initial NaCl solution, desalted NaCl solution, and saline NaCl solution by ICP-AES, revealing that no Co element was leached during the CDI process (Table S3). XRD of the NCO electrodes after the desalination test were investigated, as shown in Fig. S5, indicating the crystal structure of all samples still remained after the desalination test. Fig. 5b illustrated the desalination capacity of NCO electrodes measured in 500 mg L−1 NaCl solution with a cell voltage between 0.8 V and 1.4 V. The conductivity and pH value transients for x = 0.71, 0.61, 0.50, and 0.45 were shown in Figs. S6–7. No matter which voltage was applied, the desalination capacity of Na0.50CoO2 was the highest among all samples. Both experimental and theoretical research

3.3. FDI performance The desalination performance of NCO was evaluated by a batch-

Fig. 4. (a) CV plots of NCO electrodes in 1 M NaCl solution at the scan rate of 5 mV s−1, (b) linear of the ΔJ at 0.1 V versus the scan rate, (c) the Nyquist curves of NCO. 4

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Fig. 5. (a) Conductivity and current transients of Na0.50CoO2 in 500 mg L−1 NaCl solution at 1.4 V, (b) the desalination capacity and (d) the range of reversible deintercalation of sodium ions, (c) the corresponding charge efficiency in terms of the potential for NCO.

84.7 mg g−1 where the sodium content was 0.45. The desalination capacity of Na0.50CoO2 in this work is better than that of most of HCDI systems and reported desalination batteries (Table S4). Besides, the desalination rate was 1.27 mg g−1 min−1, 2.61 mg g−1 min−1, and 1.80 mg g−1 min−1 for x = 0.61, 0.50, and 0.45, respectively, indicating the desalination rate of Na0.50CoO2 was superior. It can be ascribed to the formation of unique Na-ions ordering. It is reported that sodium ions in the Na2 site extract slightly faster than sodium ions in the Na1 site due to the higher in-plane Na+−Na+ electrostatic repulsion in the Na2 site when x is higher than 1/3 [42]. Therefore, the deintercalation rate of NCO depends on the hopping rate of sodium ions in the Na2 site during the electrochemical cycling. It is noticing that sodium ions prefer different in-plane orderings at different Na concentrations. Na0.71CoO2 forms an intriguing pattern marked LZZ, which Na1 is arranged in a zigzag pattern (Fig. S10a) [43,44]. When the sodium content reduces from 0.71 to 0.50, the ordering changes from LZZ to ROW in which the ordering is one row of Na1 and one row of Na2 arrange in the plane alternatively (Fig. 6c) [45]. The diffusion path of sodium ions is shorter and more spacious in ROW configuration than that in the other sodium-ion configurations (Fig. S10b) leading to the mobility of sodium ions in Na0.50CoO2 is faster than that of other oxidized samples. It is the first time that sodium ions ordering effects are reported and discussed in CDI application, though a lot of work has been done on NCO as a sodium-ion battery material [46]. The stability was further evaluated by prolonged cycling for 50 times. The desalination capacity of Na0.50CoO2 had no sign of obvious performance degradation in the first 26 cycles (Fig. 6d). As the number of cycles increased, a little decay of desalination capacity appeared and the final desalination capacity was 116.5 mg g−1. The retention rate of desalination capacity was 86.3%. Noting that the charge efficiency was 100% for 50 cycles.

identified that there are continuous structural transformations with various stable phases during the electrochemical de-intercalation process. For a structure with hexagonal symmetry, each loss of one out of six sodium ions from the lattice will lead to a new sodium-ion configuration for the stable structure, corresponding to the intermediate phase upon the electrochemical cycling process. It is observed in Ding's work [41] that the intermediate phases of NCO are when x = 1/3, 1/2, 5/8, 2/3, and 3/4, respectively. Therefore, it is speculated that sodium ions in NCO only reversibly exchange between two adjacent intermediate phases as mentioned above in the FDI process because the driving force required to beyond this range is much > 1.4 V. For example, Na0.71CoO2, the sodium ion deintercalation range fall in between 2/3 and 3/4, while Na0.61CoO2 fall in between 1/2 and 5/8, and Na0.45CoO2 fall in between 1/3 and 1/2 (Fig. 5c). Thus, the amount of de-intercalated sodium ions in NCO is in the range of the adjacent intermediate phase, and in turn affects desalination capacity. In the more specific case, Na0.50CoO2 is a stable phase in the structure, the presence of Na+/vacancy ordering will decrease the energy barrier for sodiumion exchange between adjacent prismatic positions and consequently the range of deintercalated sodium ion expand from two-phase to threephase. The highest desalination capacity of Na0.50CoO2 may arise from this widest three-phase region with a range of 1/3–5/8, which is wider than that of the other three samples. The corresponding charge efficiency of the oxidized samples at 1.4 V was 100%, while it was 91.5% for Na0.71CoO2 in the same case (Fig. 5d), the corresponding charge efficiency of AC electrodes (Fig. S8) was 56.7%, 59.1%, 69.7%, and 62.3% for x = 0.71, 0.61, 0.50, and 0.45, respectively, indicating that the increase of electron transfer rate caused by deintercalation of sodium ions effectively improved the charge efficiency. To verify the advantage of Na0.50CoO2, we investigated the desalination capacity in 1000 mg L−1 NaCl solution for comparison (the conductivity transients and pH value were shown in Fig. S9). The comparative results on desalination capacity and charge efficiency were depicted in Fig. 6a, and the effect of sodium content on the CDI Ragone Kim-Yoon-Plot was plotted in Fig. 6b. The desalination capacity increased from 39.2 mg g−1 to 130.2 mg g−1 when the sodium content decreased from 0.71 to 0.50, and then decreased drastically to

4. Conclusions In summary, layered NaxCoO2 (0.45 ≤ x ≤ 0.71) with tuning sodium content was successfully synthesized via a facile sol-gel and chemical deintercalation method. As the decreasing of sodium content, 5

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Fig. 6. (a) The desalination capacity and charge efficiency of NCO in 1000 mg g−1 NaCl solution at 1.4 V, and (b) the corresponding CDI Ragone Kim-Yoon-Plots, (c) in-plane Na-ions ordering of Na0.50CoO2 (yellow balls: Na-ions on octahedral site, pink balls: Na-ions on prismatic site), (d) the cyclability of Na0.50CoO2 for 50 times in 1000 mg L−1 NaCl solution at 1.4 V. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

both the spacing between the CoO2 slabs and the oxygen vacancies increased, resulting in the enhanced transport of sodium ions and electrons within material. Benefiting from this, the superior desalination performance was achieved. The desalination capacity is 39.2 mg g−1, 61.1 mg g−1, 130.2 mg g−1 and 84.7 mg g−1 in 1000 mg L−1 NaCl solution at 1.4 V for x = 0.71, 0.61, 0.50, and 0.45, respectively. Moreover, as the cell voltages and salt concentrations increased, desalination capacity increased as well. Our experiment has provided the first dataset for the effect of Na+/vacancy ordering on desalination performance of P2-type sodium layered oxide and proposes an innovative theory for the factors affecting the desalination capacity and rate.

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CRediT authorship contribution statement Ruijuan Zhou: InvestigationData curation, Validation, Writing original draft. Xiaoxu Guo: Investigation, Validation. Xiaoman Li: Validation, Supervision. Yongshuai Kang: Investigation, Validation. Min Luo: Conceptualization, Validation, Writing - original draft, Supervision. 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. Acknowledgments This work is sponsored by the National Natural Science Foundation of China (Grant Nos. 21965027, 21561026, and 21802078), the National First-rate Discipline Construction Project of Ningxia: Chemical Engineering and Technology (Grant No. NXY-LXK2017A04). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.desal.2019.114301. 6

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