How to measure AC impedance?

Jacktang

How to measure AC impedance? [Copy link]

Delahay systematically discussed the problem of using the alternating current method to study the kinetic model of the photoelectrocatalytic process from a theoretical perspective. The EIS (Electrochemical Impedance Spectroscopy, referred to as EIS) study on the hydrogen evolution reaction published by H. Gerischer and W. Mehl in 1955 may be the most important early EIS study on irreversible electrode processes. In this study, they found that the measured impedance spectrum had inductive reactance and phase-frequency characteristics of both inductive components. H. Gerischer used the network analysis method of linear thermal elements to do important work on the impedance spectrum of the electrode system measured by the AC bridge, in which the equivalent circuit method was adopted. It was also found that the Faraday impedance of the electrode process can have different equivalent circuit forms. In the early 1960s, the French physicist J. H. Sluyters insisted on the application of the alternating current impedance spectroscopy method in the study of the photoelectrocatalytic process in experiments. In addition, Smith et al. studied the same problem from a different perspective, that is, adding a small amplitude AC current signal on the basis of DC steady state and observing the valley value of the current without response. This method is called AC voltammetry or AC polarography. The results obtained by the two methods are the same.
　　According to the definition of impedance itself, the input excitation signal of the measured system should be current, and the non-response signal in photoelectrocatalytic measurement is the electrode potential. For the electrode system of reversible electrode reaction, it is very convenient to use current as the oscillation signal for impedance measurement, because the potential difference of the reversible electrode reaction is at the equilibrium potential difference. For irreversible electrode reaction, it is very complicated. The Faraday current flowing through the electrode is much higher than the exchange current of the electrode reaction. To maintain a certain level of irreversibility, it is necessary to maintain a certain Faraday current flowing through the electrode or maintain the electrode system at a certain non-equilibrium potential difference. It is very difficult to keep the electrode system in a certain potential difference range for a long time by manipulating the current.
　　The AC impedance method is to apply a small amplitude sinusoidal oscillation signal of different frequencies to the electrode system, and obtain the electrode impedance from the relationship between the non-response of the electrode system and the oscillation signal, and infer the equivalent circuit of the electrode, so as to analyze the dynamic process and principle of the electrode system, and estimate the dynamic parameters of the electrode system from the parameter values of related components in the equivalent circuit, such as the electrode double layer capacitor, the reaction resistor of the charge transfer process, and the parameters of the external diffusion convection heat transfer process, etc.
　　Measurement method of AC impedance
　　The AC impedance method is a very important method in photoelectrocatalytic testing technology, and is an important method for studying the dynamic model and phenomenon of electrode processes. Especially in recent years, the detection accuracy of AC impedance has become higher and higher, and the ultra-high frequency signal impedance spectrum also has good reproducibility. With the development of electronic information technology, the automation technology level of impedance spectrum analysis is getting higher and higher, which enables us to better understand the double layer structure of the electrode surface, the transformation of active coatings, the initiation, development, cessation of pitting corrosion, and the adsorption and desorption process of specific substances.

(1) Communication impedance: Communication impedance is impedance. In electromagnetism, it refers to the composite characteristics of resistance and reactance of electronic components to communication excitation signals; in photocatalysis, it refers to the composite characteristics of resistance and reactance of electrodes to communication excitation signals released by the system. The impedance modulus is ohms, and the impedance angle (phase angle) is angle or degree.

(2) AC impedance spectrum: In the process of measuring impedance, if the frequency of the AC excitation signal is continuously changed, a series of impedance statistics that change with frequency can be measured. This combination of impedance statistics that change with frequency is called impedance frequency spectrum or impedance spectrum. The impedance spectrum is a complex function of frequency, which can be expressed as a combination of amplitude-frequency characteristics and phase-frequency characteristics; it can also be expressed in the complex plane with frequency as a parameter to show the real and imaginary parts of the impedance. The wider the measurement frequency range, the more detailed the impedance spectrum information that can be obtained. The frequency range of the RST5200 electrochemical workstation is: 0.00001Hz~1MHz, which can perform impedance spectrum measurements very well.

(3) Electrochemical impedance spectroscopy: Electrochemical impedance spectroscopy is a photoelectrocatalytic test method, and the technology used is a small signal communication steady-state measurement method. For parameters such as aqueous resistance, double-layer capacitance and Faraday resistance in the photoelectrocatalytic electrode system, the electrochemical impedance spectroscopy method can be measured very accurately; while the precision is somewhat lower when measured by transient methods such as current step and potential difference step. In addition, the characteristics that require long-term measurement, such as the external diffusion convection heat transfer process, cannot be achieved by transient methods, which is indeed the strength of electrochemical impedance spectroscopy.

(4) Diversity of electrochemical impedance spectroscopy measurement: In terms of measurement principle, there is no difference between measuring the characteristic impedance spectrum of the electrode system in photoelectrocatalysis and measuring the characteristic impedance spectrum of electronic components in electromagnetism. Generally, we hope to obtain the electrochemical impedance spectrum of the electrode system when it is in a certain state. To maintain the state of the electrode system, the electrode potential must be kept constant. It is generally believed that a change of about 50mV in the electrode potential will destroy the current state. Therefore, in the electrochemical impedance spectroscopy measurement, it is necessary to pay attention to two key links, namely: the reference potential difference and the strength of the sinusoidal AC signal.

(5) The intensity of the sinusoidal AC signal: In order to avoid causing great damage to the photoelectrocatalytic electrode system and to ensure that it has a good linear response, the intensity of the sinusoidal AC signal can generally be set between 2 and 20 mV.

(6) Automatic debiasing: During the electrochemical impedance spectroscopy measurement process, because the reference potential difference may not be equal to the guide potential difference and a small amount of discrete system functions, there will be a DC component in the working electrode current. Removing this DC component (bias current) can expand the dynamic range of the AC signal and improve the signal-to-noise ratio. The RST5200 electrochemical workstation can dynamically adjust the debiasing current during the measurement process to make the obtained impedance spectrum statistics more accurate. In addition, the polarization current of the working electrode can be displayed in real time in the notification bar of the program interface for the operator to refer to. The left and right are related explanations of AC impedance. Below we briefly describe the terms encountered during the experimental setting process to help users better understand the AC impedance method. Measurement method of AC impedance
　　(1) Frequency band: In electrochemical impedance spectroscopy, describing the frequency change in most ways can make the impedance spectrum look compact without losing its characteristics. In most plane coordinates, we are more accustomed to using 10 as the base. Therefore, in the RST electrochemical workstation, the frequency range of 10 times the frequency change is called a frequency band. For example, the frequency range of 1Hz to 10Hz is called frequency band 6; the frequency range of 10Hz to 100Hz is called frequency band 7, and so on. In each frequency band, 1 to 24 frequency bands can be included, depending on the operator's settings. Generally, the frequency bands that need to be paid attention to can be set more, and the frequency bands that have a long operation time can be set less.

(2) Frequency band: The photoelectrocatalytic impedance is a function of frequency (for example, frequency is the independent variable in the amplitude-frequency characteristic and phase-frequency characteristic; frequency is the parameter in the impedance complex plane and the admittance complex plane). In order to more comprehensively describe the impedance characteristics of the photoelectrocatalytic system, we need to measure within a wider frequency range, usually more than a dozen frequencies. In the RST electrochemical workstation, this kind of separated measurement frequency is called a frequency band. After measurement, a set of measurement values will be obtained for each frequency point.

(3) Cycle: In the RST electrochemical workstation, the waveform generated by a sine wave that continues for a full cycle (position change = 2·60 degrees) is called a cycle. In the steady-state measurement of AC signals, the longer the measurement time, the higher the signal-to-noise ratio. Therefore, if the frequency of a certain frequency band is set a little higher, the measurement data of this frequency band will be more accurate, or the corresponding measurement time will be longer.

(4) Start and stop frequency, stop frequency: In the process of electrochemical impedance spectroscopy measurement, the first measurement frequency is called the start and stop frequency; the last measurement frequency is called the stop frequency. Tips: Because the frequency band with higher frequency requires shorter measurement time, if the start and stop frequency is set to high frequency and the stop frequency is set to high frequency, the overall picture of the characteristic impedance spectrum can be seen earlier in the measurement process. (5) Operation time: The operation time is closely related to the setting of the main parameters such as the start and stop frequency, stop frequency, number of frequency bands, and the frequency of each frequency band. In the mobile phone software of the RST electrochemical workstation, when the above parameters are changed, the operation time will be calculated immediately, which is convenient for the operator to measure.

(6) Reference potential difference: In the RST electrochemical workstation, the AC potential (relative to the reference electrode) applied to the working electrode in the electrolytic cell is called the reference potential difference. In electromagnetism, in order to facilitate signal analysis, the AC/DC mixed signal is often regarded as the sum of an AC signal and a DC signal. From the perspective of time waveform, the DC signal can cause the AC waveform to shift up or down, so it is called the reference signal. If it is described in the form of potential difference (working voltage), it is called the reference potential difference (working voltage). Most of the measurements of photoelectrocatalytic impedance are carried out under the guide potential difference standard. At this time, the external circuit current is zero, and there is no overpotential difference on the working electrode. When the AC signal applied to the working electrode is sufficient for a long time, such as 2mV~20mV, it is generally believed that this equilibrium state will not be destroyed. Warm reminder, the reference potential difference added to the working electrode at this time should be its guide potential difference. Because of the guiding potential difference of the photoelectrocatalytic system.