Study of Low Frequency Electrolytic Behavior with an Accurate Impedance Method
Authors: Ho Kung-Chu , Su Vin-Cent , Ye Jia-Hao , Kuan Chieh-Hsiung 
Published: February 2020

INTRODUCTION

The behavior of electrolytic solutions operated in low frequency regime are investigated through an accurate impedance method realized with a specific microfluidic device and high resolution instruments. The results reveal better repeatability and accuracy of the proposed method, especially in lower end of the investigated frequency regime. The dielectric constant decreases with increase in frequency and finally approaches almost a constant at higher frequencies. All electrolytic solutions appear well-known relaxation frequency at each peak value of dielectric loss.

EXPERIMENTAL

Fig
ure
1(
a) shows the microfluidic device and the measurement system including a function generator (AFG-3252) and a lock-in amplifier (SR-830). Channel 1 of AFG-3252 was connected to the device, the device output was then connected to the current input of SR-830. The reference-input of SR-830 was connected to the channel 2 of AFG-3252. F
ig
.
1(
b) shows the electrode's double layer and the corresponding equivalent circuit. Rb and Csol(ω) are the bulk resistance and the capacitance of electrolytes, respectively[
1
]. Fig. 1(c) is the device photo. We commissioned rapid prototyping of device, which contained a precision cut square reservoir on the resin surface (which can hold 13.5-μl-fixed amount solutions). Then we evaporated Au/Ge/Ni alloy and Auelectrodes on both sidewalls of the device. The system was enclosed in a shielding box which prevents the interference and ensures the system grounded properly.

DISCUSSION

Figure 2 shows the standard deviation (SD) versus frequency of NaCl solutions with different concentrations biased at 100 mVp-p, computed from ten measurements. With a higher concentration, the SD of NaCl solutions becomes smaller. The results reveal better repeatability and accuracy of the proposed impedance method[2]. Fig. 3(a) shows the frequency dispersion of AC conductivity (σ(ω)) for NaCl solutions with various concentrations (biased at 100 mVp-p). For a given concentration, the σ(ω) of NaCl solutions decreases with decrease in frequency[3] because ions cannot pass through the electrodes region at lower frequencies and hence face highest resistivity. At higher frequencies, the periodic reversal of electric field represents so fast that ions less diffuse in the electric field direction. This lead to mobility improvement, and is responsible for conduction mechanism. With a given frequency, σ(ω) of all electrolytes are also increased with increase in NaCl concentrations, which results in relatively more effective charges. These ions move in the solutions, and hence σ(ω) increases. In concentrated cases, the frequency- independent region in σ(ω) clearly occurs at higher frequency

Figure 3(b) shows the frequency-independent region of conductivity (σ) and bulk resistance (Rb) versus NaCl concentration, where Rb is calculated through the intercept on the real axis at low frequencies of Nyquist plots[4]. In the concentrated electrolytes, the carrier number increases and hence Rb decreases. Fig. 4 shows the frequency dispersion of dielectric constant (ε’) for different NaCl concentrations. The log-scale diagram is shown in the inlet of figure. The initial ε’ is large, then decreases with increase in frequency, and finally approaches almost a constant at higher frequencies with given concentration. The reason is that ions not only accumulate at electrode-electrolyte interface but align themselves along the electric field direction. Therefore they fully contribute the total polarization. As frequency is increased, ions cannot follow rapid electric field inversion and hence diminish their contribution to the total polarization. At lower frequencies, higher ε’ occurring at larger concentration implies more ions are present since ε’ measures stored charges.

Figure 5 shows the frequency dispersion curve of dielectric loss (ε"). The ε" increases up to a specific frequency after which it decreases. All NaCl solutions appear relaxation frequency at each peak value of dielectric loss revealing the relaxing total polarization. The relaxation frequency with larger concentrations occurs at higher frequency owing to stronger total polarization coming from conductivity increasing as well as lower resistance in the solutions. So, in the low frequency before relaxation, indicates the capacitive property of the electrode-electrolyte interface and low conductivity, whereas in high frequency, above the relaxation frequency, can be characterized as a parallel combination of Rb and Csol(ω)[
5
].

SUMMARY

In this paper, the better repeatability and accuracy of the proposed impedance method have been reported. The dispersion relation occurs at low frequencies because of the total polarization from ions accumulating at interface and aligning themselves along the electric field. At higher frequencies, the fast periodic reversal of electric field makes ions less diffuse in the electric field direction, leading to conduction improvement. All solutions appear relaxation frequency at each peak value of dielectric loss. In more concentrated electrolyte, relaxation frequency becomes higher due to stronger total polarization coming from higher conductivity as well as lower resistance.

Figure 1