Highly Stable Hybrid Capacitive Deionization with a MnO2 Anode and

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Highly Stable Hybrid Capacitive Deionization with a MnO Anode and a Positively Charged Cathode 2

Tingting Wu, Gang Wang, Shiyong Wang, Fei Zhan, Yu Fu, Huiying Qiao, and Jieshan Qiu Environ. Sci. Technol. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.estlett.7b00540 • Publication Date (Web): 22 Jan 2018 Downloaded from http://pubs.acs.org on January 24, 2018

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Environmental Science & Technology Letters

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Highly Stable Hybrid Capacitive Deionization with a MnO2

2

Anode and a Positively Charged Cathode

3

Tingting Wu†, Gang Wang†, *, Shiyong Wang†, Fei Zhan†, Yu Fu†, Huiying Qiao†, Jieshan

4

Qiu†, ‡, *

5

†

State Key Lab of Fine Chemicals, School of Chemical Engineering, Liaoning Key Lab for

6

Energy Materials and Chemical Engineering, Dalian University of Technology, Dalian

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116024, Liaoning, China.

8 9

‡

School of Chemical Engineering and Technology, Xi’an Jiaotong University, Xi’an 710049, China.

10 11

Corresponding Author:

12

Gang Wang, *E-mail: [email protected];

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Jieshan Qiu, *E-mail: [email protected], *Tel/Fax: +86-411-84986024

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1 ACS Paragon Plus Environment


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ABSTRACT

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Performance degradation caused by the oxidation of carbon anodes during the operation

17

of capacitive deionization (CDI) remains a major problem that may greatly restrict its

18

practical application. To improve the cyclic stability of CDI, carbon based anode materials

19

were replaced by pseudocapacitive MnO2 in this work. The cation-selective MnO2 anode was

20

assembled with an anion-selective quaternized poly (4-vinyl pyridine) coated activated

21

carbon (AC-QPVP) cathode into a hybrid CDI cell. The cell exhibited inverted CDI

22

performance with a wide operating voltage window of 1.4 V and salt adsorption capacity

23

(SAC) of 14.9 mg/g in 500 mg/L NaCl. The SAC retention ratio of the cell can be as high as

24

95.4% after 350 adsorption-desorption cycles at 1.0/0 V while that of the CDI cell consisting

25

of activated carbon electrodes was only 15.7% after 285 cycles. The enhanced cyclic stability

26

of the hybrid CDI cell is attributed to the employment of the MnO2 anode, which avoided the

27

use and oxidation of carbon anodes.

28


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INTRODUCTION

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Capacitive deionization (CDI) is a promising desalination technology with the

31

advantages of low cell voltage operation, high energy efficiency and environmental

32

friendliness. CDI usually employs two porous carbon electrodes to adsorb ions from water

33

with an external electric field as the driving force.1, 2 Recently, the exploration of new CDI

34

systems3-7 and electrode materials8-10, the optimization of operational processes,11-13 the

35

advances in related theories14,

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Nevertheless, the gradual performance degradation during the operation of CDI is still an

37

unsolved problem.16, 17

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etc. have greatly promoted the development of CDI.

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Although the cell voltage of CDI is usually lower than the theoretical value of water

39

electrolysis (1.23 V vs. NHE), parasitic reactions occur during the charging process and the

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carbon anode of CDI is continually oxidized.18,

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introduces negatively charged functional groups into the carbon surface and thus aggravates

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co-ion expulsion.16 Moreover, it may reduce the specific capacitance of the carbon anodes. As

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a result, salt adsorption capacity (SAC) of CDI gradually decreases during cycling. Therefore,

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the key to inhibit the performance degradation and improve the cyclic stability of CDI is to

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suppress the oxidation of the carbon anode. Up to now, low-cell voltage operation,20 polarity

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reversal,17 introducing ion exchange membranes or polymers,21, 22 etc. have been attempted to

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improve the cyclic stability of CDI. However, these methods usually have limited

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effectiveness and may lower the SAC or increase the operational cost of CDI.

19

The oxidation of the carbon anode

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In 2015, Gao et al. proposed inverted CDI (i-CDI) consisting of a carbon anode with net

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negative surface charges and a carbon cathode with net positive surface charges.5 When the 3 ACS Paragon Plus Environment


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cell is charged, the electrodes are polarized towards their potential of zero charge (Epzc) and

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desorb ions. When it is discharged, ions are adsorbed to neutralize the surface charges in the

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electrodes. I-CDI effectively avoided the negative effects of co-ion expulsion on CDI

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performance and exhibited excellent cyclic stability during 600 adsorption-desorption cycles.

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However, it suffers from relatively narrow operating voltage window and SAC (0.8/0 V, ~1.8

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mg/g). In addition, oxidation of carbon anodes still occur in i-CDI, which may affect the

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properties of carbon anodes and thus greatly reduce the SAC of i-CDI.23

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Recently, various pseudocapacitive or battery materials have been explored as electrode

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materials of different deionization systems.24-26 MnO2 is a low-cost and environmentally

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friendly pseudocapacitive metal oxide and MnO2/carbon composites with different structure

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have been used as CDI electrode materials.27-30 However, these works focused on the

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preparation of electrode materials and few of them investigated the impact of MnO2 anode on

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the cyclic stability of CDI. In this work, a hybrid CDI cell (MnO2//AC-QPVP) consisting of a

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MnO2 anode and a quaternized poly (4-vinyl pyridine) coated activated carbon (AC-QPVP)

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cathode was assembled. Due to the ion selectivity of the electrodes, MnO2//AC-QPVP

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exhibited inverted CDI performance with a wide operating voltage window and a high SAC.

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Moreover, as the cell avoided the use of carbon anode materials, it exhibited good cyclic

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stability with a SAC retention ratio of 95.4% during 350 adsorption-desorption cycles at 1.0/0

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V.

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MATERIALS AND METHODS

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Preparation of MnO2.

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MnO2 was prepared by the reaction of KMnO4 with HCl.31 Typically, 3.6 g KMnO4 was 4 ACS Paragon Plus Environment

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first dissolved in 40 mL deionized water at 85 ºC. Then 20 mL 1:1 (v/v) HCl was added into

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the KMnO4 solution dropwise under vigorous stirring. After the addition of HCl was

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completed, the resulting product was collected, washed with deionized water and finally dried

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in a vacuum oven at 110 ºC for 6 h. Quaternized poly (4-vinylpyridine) coated activated

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carbon (AC-QPVP) was prepared through polymerization, crosslinking and quaternization

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processes, as reported in our previous work.23

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Characterization.

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The morphology and elemental composition of MnO2 were examined using

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field-emission scanning electron microscopy (FESEM, FEI NOVA NanoSEM 450) equipped

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with an energy-dispersive spectrometer (EDS), and transmission electron microscopy (FEI

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Tecnai G20). The crystalline structure of MnO2 was characterized by X-ray diffractometer

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(Rigaku D/Max 2400) using Cu Kα radiation (λ=1.5406 Å). To characterize the pore structure

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of MnO2, the sample was first degassed at 150 ºC for 12 h under vacuum and then nitrogen

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adsorption-desorption isotherms were measured using a Micromeritics 3-Flex surface

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characterization analyzer at 77 K. Inductively coupled plasma atomic emission spectroscopy

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(ICP-AES) was used to analyze the composition of the prepared MnO2 and determine the

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concentration of Mn in the tank after the cyclic test of MnO2//AC-QPVP.

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CDI tests.

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CDI electrodes were prepared by coating the homogeneous slurry of the active material,

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acetylene black, polyvinyl butyral (PVB) and polyvinylpyrrolidone (PVP) in a weight ratio of

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82.5:10:6:1.5 onto graphite paper (57 cm2) followed by drying at 80 ºC overnight. The CDI

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cell consisting of a MnO2 anode and an AC-QPVP cathode was denoted as MnO2//AC-QPVP 5 ACS Paragon Plus Environment


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while that consisting of two AC electrodes was denoted as AC//AC; the CDI cell with a

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MnO2 anode and an AC cathode was denoted as MnO2//AC and that with an AC anode and

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an AC-QPVP cathode was denoted as AC//AC-QPVP. A silicon gasket with the thickness of

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~1.3 mm and two pieces of nonwoven fabric with the thickness of ~120 m were used as the

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spacer and the separator of the CDI cells, respectively. All CDI tests were conducted under

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constant voltage operation. In a typical cycle, the CDI cells were first charged at a certain cell

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voltage for 5 min and then short-circuited for 5 min. Single-pass tests were conducted during

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which 2.5 L NaCl solution in a tank was pumped continuously through the CDI cell and then

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back to the tank. An electrochemical workstation (CHI 660D) was used to supply the voltage

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needed. Conductivity and pH changes in the NaCl solution at the outlet of CDI cells were

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recorded by the conductivity and pH monitor system. The salt adsorption/desorption capacity

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(, mg/g), charge (Σ, C/g) passed during the charging and discharging processes, and charge

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efficiency () were calculated according to eqs 1, 2 and 3, respectively:

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 

    c0  ct 

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

110



m

 idt

(1)

(2)

m

 F  

(3)

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where Ф refers to the flow rate of NaCl solution and is 9.2 mL/min in this work, c0 and ct

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(mg/L) refer to the influent and effluent NaCl concentration, respectively, m is the total mass

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of the two electrodes in a CDI cell (0.23-0.27 g in this work), i (A) is the charge current, F

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refers to Faraday constant (96485 C/mol) and M is the molar mass of NaCl (58.5 g/mol).

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RESULTS AND DISCUSSION 6 ACS Paragon Plus Environment

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Properties of MnO2 and AC-QPVP.

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The morphology and surface properties of AC-QPVP have been reported in our previous

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work.23 There are positively charged quaternary ammonium groups on the surface of

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AC-QPVP. As a result, the AC-QPVP electrode is anion selective and its Epzc can be as

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negative as -0.745 V vs. Ag/AgCl. The structure and morphology of the prepared MnO2 are

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summarized in Figure S1. The obtained product is birnessite-type MnO2 with submicron

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flower-like structure composed of highly interconnected carbon nanosheets. Its Brunauer-

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Emmett- Teller (BET) specific surface area is 193 m2/g and the pore size distribution is

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centered at about 8.6 nm. MnO2 can reversibly intercalate/deintercalate Na+ in the positive

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potential range following the eqs 4.32, 33

126 127

MnO2 + Na + + e-

MnOONa

(4)

CDI performance at different cell voltages.

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Figure 1 shows the CDI performance of AC//AC and MnO2//AC-QPVP at different cell

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voltages in 500 mg/L NaCl. As shown in Figure 1a, AC//AC exhibits conventional CDI

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performance, i.e. ions are adsorbed when the cell is charged and desorbed when the cell is

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discharged. By contrast, MnO2//AC-QPVP exhibits inverted performance at different cell

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voltages. When the cell is charged, ions are released from the electrodes into the electrolyte.

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During this process, the MnO2 electrode is oxidized and cations in the MnO2 structure are

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deintercalated to maintain charge balance. Simultaneously, the AC-QPVP electrode is

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polarized towards its Epzc and desorbs Cl-. When the cell is discharged, ions are stored into

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the electrodes during which the MnO2 electrode is reduced and intercalate Na+ while the

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AC-QPVP electrode stays far away from its Epzc and adsorbs Cl-. ICP results indicate that K+ 7 ACS Paragon Plus Environment


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exists in the prepared MnO2 and the amount is 19.9 mg/g. In addition, it has been reported

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that a certain amount of H3O+ exists in MnO2.34 During the CDI tests, K+ and H3O+ in the

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MnO2 structure were gradually replaced by Na+,35 which can be demonstrated by the EDS

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results shown in Figure S2.

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For MnO2//AC and AC//AC-QPVP, negligible changes in the effluent NaCl

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concentration were observed (Figure S3a). However, they consumed comparable charge with

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AC//AC and MnO2//AC-QPVP (Figure S3b), indicating their low charge efficiency. It is

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supposed that the migration of a single kind of ion between the electrodes rather than the

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simultaneous removal/release of sodium and chloride ions dominates in MnO2//AC and

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AC//AC-QPVP, which results in their poor desalination performance.

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The NaCl concentration variation range of MnO2//AC-QPVP increases steadily as the

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cell voltage increases from 0.6 V to 1.4 V, which indicates that the operating voltage window

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of MnO2//AC-QPVP can reach 1.4 V. From Figure 1b, it can be seen that the current density

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of AC//AC and MnO2//AC-QPVP increases with the applied cell voltage, indicating a gradual

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increase in the charge passed.


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Figure 1. Effluent NaCl concentration profiles (a) and current density (b) of AC//AC and

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MnO2//AC-QPVP at different cell voltages in 500 mg/L NaCl.

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Figure 2 shows the SAC of AC//AC and MnO2//AC-QPVP according to eqs 1 and 3. As

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the cell voltage increases from 0.6 V to 0.8, 1.0, 1.2 and 1.4 V, the SAC of MnO2//AC-QPVP

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increases from 6.0 mg/g to 8.0, 10.0, 11.9 and 14.4 mg/g, respectively, while that of AC//AC

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is 3.8，6.3, 9.0, 9.7 and 9.7 mg/g, respectively (Figure 2a).



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Figure 2. SAC (a) and charge efficiency during charging (b) of AC//AC and

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MnO2//AC-QPVP at different cell voltages in 500 mg/L NaCl. The average values of

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triplicate experiments are shown and the error bars represent the standard deviations.

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From Figure 2b, it can be seen that the charge efficiency of both AC//AC and

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MnO2//AC-QPVP exhibits a downward trend as the cell voltage increases, which can be

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attributed to aggravated parasitic reactions at high cell voltages. Nevertheless, charge

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efficiency of MnO2//AC-QPVP stays above 0.67 and is higher than that of AC//AC at each

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cell voltage. This can be attributed to the ion selectivity of the MnO2 and AC-QPVP

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electrodes, which can greatly diminish co-ion expulsion. The SAC and charge efficiency


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results indicate that the CDI performance of MnO2//AC-QPVP at different cell voltages is

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much better than that of AC//AC.

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The salt desorption capacity, and charge efficiency during discharging of

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MnO2//AC-QPVP at different cell voltages are shown in Figure S4. The salt desorption

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capacity of MnO2//AC-QPVP is approximately equal to its SAC. However, the charge

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efficiency during discharging is above 0.81 and higher than that during charging, which is

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because that more charge is consumed by parasitic reactions during charging.

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Cyclic performance.

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The cyclic performance of AC//AC and MnO2//AC-QPVP was investigated in 500 mg/L

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NaCl at the cell voltage of 1.0/0 V. The obtained effluent NaCl concentration profiles, current

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density and effluent pH changes during the cyclic tests are shown in Figure S5, Figure S6 and

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Figure S7, respectively.

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For AC//AC, the variation range of effluent NaCl concentration at the 285th cycle is

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much smaller than that at the 5th cycle, indicating that a serious performance degradation

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occurred (Figure S8a). As shown in Figure 3a, the SAC of AC//AC at the 5th cycle is

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calculated to be 8.3 mg/g and falls rapidly to 1.3 mg/g at the 285th cycle. The SAC retention

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ratio is only 15.7%. Correspondingly, the charge efficiency decreases from 0.61 at the 5th

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cycle to 0.13 at the 285th cycle. After 288 adsorption-desorption cycles, the polarity of the

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applied cell voltage on AC//AC was reversed. The SAC recovers to 8.2 mg/g and the charge

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efficiency goes back to 0.49. However, significant performance degradation occurs again

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during the continued cyclic test. The SAC declines to 5.3 mg/g and the charge efficiency

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decreases to 0.29 at the 350th cycle. 11 ACS Paragon Plus Environment


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For MnO2//AC-QPVP, the adsorption process at the 350th cycle becomes slower than

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that at the 5th cycle, as shown in Figure S8b. Meanwhile, the current of MnO2//AC-QPVP

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gradually decreases during the cyclic test, as shown in Figure S6. The instantaneous current

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during charging at the 5th cycle is 0.1962 while that at the 350th cycle decreases to 0.1156 A.

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Correspondingly, the ESR increases from 5.1  at the 5th cycle to 8.7  at the 350th cycle

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during the cycling. These phenomena may be related to the oxidation of the conductive agent

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(carbon black) in the anode. Despite this, most of the desalination performance of

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MnO2//AC-QPVP was retained after 350 adsorption-desorption cycles. The SAC of

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MnO2//AC-QPVP at the 5th cycle is calculated to be 8.7 mg/g. During the cyclic test, it

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increases a little first as a result of the activation of the MnO2 electrode and then decreases

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slightly to 8.3 mg/g. The SAC retention ratio is as high as 95.4%. To demonstrate the

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reproducibility of the results, the cyclic performance of another MnO2//AC-QPVP cell is

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provided in Figure S9. Its SAC at the 5th cycle and the 350th cycle is 8.4 mg/g and 7.6 mg/g,

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respectively, and the SAC retention ratio is 90.5%.


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Figure 3. SAC (a) and charge efficiency during charging (b) of AC//AC and

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MnO2//AC-QPVP during 350 adsorption-desorption cycles at the cell voltage of 1.0/0 V in

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500 mg/L NaCl.

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Charge efficiency of MnO2//AC-QPVP maintains above 0.8 during the cyclic test and is

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much higher than that of AC//AC. In a word, MnO2//AC-QPVP exhibited much better cyclic

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stability than AC//AC and the i-CDI cell with a negatively charged carbon anode reported in

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our previous work.23 The improved cyclic stability is attributed to the employment of the

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MnO2 electrode, which avoided the use of carbon anodes and the consequent irreversible

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carbon oxidation. 13 ACS Paragon Plus Environment


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Environmental implications.

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The concentration of Mn in the tank was tested by the ICP-AES method to evaluate the

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influences of MnO2 on the quality of produced water. After MnO2//AC-QPVP was tested for

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350 adsorption-desorption cycles, the concentration of Mn in the tank is 2.9 g/L, which is

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much lower than the usually acceptable level for drinking water (100 g/L). Therefore, the

221

impact of the MnO2 electrode on the quality of the produced water is negligible. In summary,

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this work demonstrated a novel strategy to enhance the cyclic stability of CDI by using

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non-carbon MnO2 as the anode material of CDI, which can be extended to other

224

pseudocapacitive electrode materials and may pave the way for the large-scale industrial

225

application of CDI.

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ASSOCIATED CONTENT

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Supporting Information. Structure and morphology of MnO2; EDS spectra of the MnO2

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electrodes; CDI performance of MnO2//AC and AC//AC-QPVP; salt desorption capacity, and

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charge efficiency during discharging of MnO2// AC-QPVP; effluent NaCl concentration

230

profiles, current density and effluent pH changes of AC//AC and MnO2//AC-QPVP during

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the cyclic tests; adsorption curves of AC//AC and MnO2//AC-QPVP at the start and end of

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the cyclic test; changes in SAC of another MnO2// AC-QPVP cell during cycling.

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NOTES

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The authors declare no competing financial interest.

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ACKNOWLEDGEMENTS

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This work was supported by the National Natural Science Foundation of China (NSFC,

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No. 21336001), and the Qaidam Salt Lake Chemical Joint Research Fund Project of NSFC 14 ACS Paragon Plus Environment

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and Qinghai Province State People's Government (No. U1507103).

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Highly Stable Hybrid Capacitive Deionization with a MnO2 Anode and

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