Polyvinylidene Fluoride Membranes Probed by ...

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AUTHOR SUBMITTED MANUSCRIPT - draft

Polyvinylidene Fluoride Membranes Probed by Electrochemical Impedance Spectroscopy Qi-Zhao Luo1, Qing Huang1, Zhe Chen*1, Lei Yao2, Ping Fu1, Zhi-Dong Lin1 1 Hebei Key Laboratory of Plasma Chemical and Advanced Materials & School of Materials Science and Engineering, Wuhan Institute of Technology, Wuhan, 430205, China 2 School of Electrical and Information Engineering, Wuhan Institute of Technology, Wuhan, 430205, China *Corresponding Author Zhe Chen

Email: [email protected]

ABSTRACT Electrochemical impedance spectroscopy (EIS) has been applied to characterize the structure of polyvinylidene fluoride (PVDF) membranes. The characteristic frequency, which directly obtained from the original EIS data, was used to clarify the difference of the membranes’ structures. The experimental data indicated the equivalence between the characteristic frequency and the membrane resistance fitted from the equivalent circuit. The results evidenced that the characteristic frequency obtained directly from original EIS data without any fitting calculation can be used for in situ characterizing a membrane instead of the membrane resistance.

KEYWORDS polyvinylidene fluoride membrane; electrochemical impedance spectroscopy; in situ characterization 1.Introduction: Electrochemical impedance spectroscopy (EIS) is an electrochemical method for measuring a disturbing signal due to a small amplitude sinusoidal potential or current, which has been applied as a powerful characterization technique providing detailed information about the electrode-electrolyte interface of an electrochemical system [1,2]. It has been widely applied to study the membrane structures [3,4] and membrane fouling [5.6]. Since the electrical properties are related to the membrane states, EIS could be potentially used as an in-situ real-time method for probing the membrane state during the separation process [7,8]. For example, Park’s work indicated that the organic fouling led to an increase in the resistance of ion exchange

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membranes [9]. Watkins found lignin sulfonate, an organic foulant, increased the capacitance of an electrodialysis membrane system [10]. Cen’s results showed that the pre-fouled membranes could be distinguished from un-fouled membranes [11] and the capacitance around 1 Hz was more sensitive than the resistance or flux in the early fouling stage [12]. In fact, EIS is based on the electrochemical relaxations related to the intrinsic properties of the membrane system, which is illustrated by impedance spectra. The interpretation is a little complicated for the parameters, such as the resistance and capacitance, which can only be obtained by fitting the experimental data with an equivalent circuit. Since the equivalent circuit modeling varies due to different operators, deviation may occur. In order to obtain a more reliable result, it is necessary to find a characteristic parameter from the original EIS data for evaluating the membrane structures. In this paper, we were motivated to develop the EIS characterization of the membrane structure by a simpler parameter. The objective of the study was to justify the effectiveness and practicability. A series of polyvinylidene fluoride (PVDF) membranes with different pore structures were prepared. The effects of the membrane structures and electrolytes concentrations on the EIS data were discussed. Particularly, a characteristic value in EIS data related to membrane structure was evaluated from the experimental data and theoretical calculation.

2.Experimental section 1. Fabrication of membranes Firstly, PVDF powder, N,N-Dimethylformamide (DMF), and polyethylene glycol (PEG, molecular weight 1000) were mixed and kept stirring at 60 ℃ for 6 hours to obtain a homogenous solution. Then the solution was cooled down to room temperature and casted on a glass by a casting knife with the thickness of 400 μm. The glass with casted film was immersed into deionized water for phase conversion. Finally, the membrane was formed as a sample. 2. EIS measurement

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Fig.1. The schematic of the EIS measurement setup, including an electrolytic cell inside the Faraday Cage, an Electrochemical Workstation, and a computer.

EIS measurements were carried out with a CS310H impedance analyzer operating in a frequency range from 1MHz to 0.1Hz. EIS experiments were conducted with an electrochemical workstation with a three-electrode system. The electrolytic cell was placed in a Faraday cage to reduce the interference from the outside. Experiments were performed at atmospheric pressure, and the temperature of the electrolyte solution was maintained room temperature (approximately 25℃ ). PVDF membrane was first soaked in the electrolyte solution for 24 hours before attaching to the ITO glass. The exposed sample area was kept as 1.76 cm2 for the membrane. The electrolytic cells should be rinsed by deionized water, and then rinsed by the electrolyte solution. Before the experiment was performed, the electrolytic cells were filled with an electrolyte solution and the system was left for about 1 h for equilibration. The EIS equipment and control software are provided by the Corr Test Inc. The conductivity and resistance parameters of the equivalent circuits were fitted by Zview program.

3.Result and discussion


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Fig.2. The Nyquist plots for PVDF membranes with different PEG contents (1%, 2%, 3%, 4% and 5%) in 0.1m/L KCl solution.

The Nyquist plots of the PVDF membranes are shown in Fig.2, It shows that each plot consists of several semicircles, which implies that the system composes of several different time constants in series. According to our previous studies [3,4], the semicircle appearing at higher frequency in Nyquist plot can be contributed to the membrane characteristic property. And the semicircle appearing at lower frequency may be due to the charge diffusion around the electrode. According to the plots shown in Fig. 2, a higher PEG content leads to a smaller semicircle, which is in agreement with our previous studies [3,4]. The membrane resistance for each membrane fitted from the Nyquist plot is listed as the following:

Table 1 The value of membrane resistance with different PEG contents calculated by ZView with the fitted equivalent circuit. Membrane

Rm [Ω]

Deviation

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1%

3772

4.9305%

2%

2901

6.7769%

3%

1623

1.9631%

4%

1349

3.1961%

5%

1001

1.7237%

The membrane resistance decreases with increasing PEG content, indicating that it is easier for charge transferring through the membrane with higher PEG. If the foulants were filled in the pores of the membrane, implying less porous structures for the membrane, the membrane resistance would increase. Thus EIS data can be potentially applied to monitor the membrane state. However, from the view of the fitting error for this system, the accuracy of the membrane resistance is not perfect enough. Different operator may obtain different fitting results from the same EIS data due to the selection of different fitting range and initial value. Therefore, it is necessary to find a parameter related to the membrane structure, which could be easily obtained and understood for technicians. The Bode plots of the membranes are also presented in Fig. 3. All the plots exhibit two peaks corresponding to different time constants. As mentioned above, the time constant appearing at higher frequency is related to the membrane structure. Fig. 3 shows that the characteristic frequency shifts to higher value with increasing PEG content. It suggests that a membrane with more porous structure would exhibit a higher characteristic frequency in Bode plot.


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Fig.3.The Bode plots of the PVDF membranes with different PEG contents in 0.1mol/L KCl aqueous solution.

According to the theory, for the present PVDF membrane immersing in electrolyte, the real part, imaginary part and phase angle can be expressed as 𝑅𝑅

𝑍𝑍𝑟𝑟 = 𝑅𝑅𝑠𝑠 + 1+(𝜔𝜔 𝑅𝑅𝑚𝑚 𝐶𝐶 𝜔𝜔 𝐶𝐶 𝑅𝑅 2

2 𝑚𝑚 𝑚𝑚 )

𝜔𝜔 𝐶𝐶 𝑅𝑅𝑚𝑚 2

𝑍𝑍𝑗𝑗 = 1+(𝜔𝜔 𝑖𝑖𝑅𝑅 𝑖𝑖𝐶𝐶 )2 + 1+(𝜔𝜔𝑚𝑚𝑅𝑅 𝑖𝑖 𝑖𝑖

𝑍𝑍

∅ = tan−1 �𝑍𝑍𝑗𝑗 �

𝑅𝑅

+ 1+(𝜔𝜔 𝑅𝑅𝑖𝑖 𝐶𝐶 )2 𝑖𝑖 𝑖𝑖

2 𝑚𝑚 𝐶𝐶𝑚𝑚 )

𝑟𝑟

𝜔𝜔 𝐶𝐶𝑖𝑖 𝑅𝑅𝑖𝑖 2 +𝜔𝜔𝐶𝐶𝑚𝑚 𝑅𝑅𝑚𝑚 2 +𝜔𝜔 3 𝑅𝑅𝑖𝑖 𝐶𝐶𝑖𝑖 𝑅𝑅𝑚𝑚 𝐶𝐶𝑚𝑚 (𝑅𝑅𝑚𝑚 𝐶𝐶𝑚𝑚 +𝑅𝑅𝑖𝑖 𝐶𝐶𝑖𝑖 ) � 𝑅𝑅𝑚𝑚 +𝑅𝑅𝑖𝑖 +𝑅𝑅𝑠𝑠 +𝜔𝜔 2 (𝑅𝑅𝑖𝑖 𝐶𝐶𝑖𝑖 )2 [𝑅𝑅𝑠𝑠 +𝑅𝑅𝑠𝑠 (𝜔𝜔𝑅𝑅𝑚𝑚 𝐶𝐶𝑚𝑚 )2 +𝑅𝑅𝑚𝑚 ]+𝜔𝜔 2 (𝑅𝑅𝑚𝑚 𝐶𝐶𝑚𝑚 )2 (2𝑅𝑅𝑠𝑠 +𝑅𝑅𝑖𝑖 )

= 𝑡𝑡𝑡𝑡𝑡𝑡−1 �

(1) (2)

(3)

Generally, for a given membrane, membrane fouling would result in the increase of membrane resistance. Equation (3) suggests that the higher membrane resistance leads to the phase angle shifts to lower value. In this case, the characteristic frequency can be applied to monitor the membrane state.

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In order to examine the effect of monitoring the membrane by the characteristic frequency, a PVDF membrane immersing in the electrolyte with different concentrations was probed by EIS, respectively. The Bode plots are shown in Fig. 4. It is found obviously that the characteristic frequency in the Bode plot shifts to larger value with increasing electrolyte concentration. Higher concentration means easier charge transferring through the membrane, which implies lower membrane resistance. And according to Equation (3), lower membrane resistance leads to higher frequency. Therefore, this confirmed that the frequency can be applied to monitor the state during the membrane separation process.

Fig.4.The Bode plots of PVDF membrane with the 5% PEG content immersing the KCl solution with different concentrations (0.001mol/L, 0.002mol/L, 0.01mol/L, 0.05mol/L, and 0.1mol/L).

4. Conclusion


A series of PVDF membranes with different pore structures were probed by EIS. The effect of the pore structure on the characteristic frequency directly obtained from the Bode plot was discussed. And the influence of the electrolyte concentration on the Bode plot was studied. It indicates that a membrane with more porous structure would exhibit a lower characteristic frequency, while a low electrolyte concentration would lead to a higher characteristic frequency. The results imply that the characteristic frequency obtained directly from the Bode plot could be potentially applied for monitoring the membrane state.

Acknowledgment This work was supported by National Science Foundation of China No.11205118 and Qinghai Salt Lake Industry Co., Ltd. Key Laboratory of Qinghai Salt Lake Resources Comprehensive Utilization (Q-SYS-201526-KF-04). References [1] Huang HL, Bu FR and Tian J, 2017, Influence of direct current electric field on corrosion behavior of Tin under a thin electrolyte layer, J. Electron Mater., 46, 6936-6946. [2] Zhang Q, Dai Z, Cheng G, Liu YL and Chen R, 2017, In-situ roo-temperature synthesis of amorphous/crystalline contact Bi2S3/Bi2WO6 heterostructures for improved photocatalytic ability, Ceram. Int., 43, 11296-11304. [3] Yin C, Wang S, Zhang YJ, Chen Z, Lin ZD, Fu P and Yao L, 2017, Correlation between the pore resistance and water flux of the cellulose acetate membrane, Environ. Sci.: Water Res. Technol., 3, 1037-1041. [4] Chen X, Zhou W, Chen Z, Yao L, 2017, Study of the photocatalytic property of polysulfone membrane incorporating TiO2 nanoparticles, Journal of Molecular and Engineering Materials, 5, 1750005. [5] Zhao ZJ, Shi SY, Cao HB and Li YP, 2017, Electrochemical impedance

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