EIS im6

Electrochemical Impedance Spectroscopy

? Zahner 02/2010

1. Introduction_______________________________5

2. EIS Main Page_____________________________6 2.1 File Operations (7)

2.1.1 Open (7)

2.1.2 Save (7)

2.2 Display Spectrum (8)

2.3 Impedance Spectra Analysis (8)

2.4 Signal Acquisition (8)

3. Cell Connections___________________________9 3.1 Connected Probes (12)

3.1.1 U-Buffer (12)

3.1.2 HiZ-Probe (Option) (14)

3.1.3 CVB120 (Option) (16)

3.1.4 FRA-Probe (Option) (17)

3.2 Electrode Schemes (18)

3.2.1 Two Electrodes (18)

3.2.2 Three Electrodes (19)

3.2.3 Four Electrodes (20)

3.3 LoZ Cable Set (Option) (21)

4. Control Potentiostat_______________________22 4.1 Potentiostatic Mode (23)

4.2 Galvanostatic Mode (26)

4.3 Pseudo-Galvanostatic Mode (27)

4.4 Rest Potential Mode (28)

4.5 DC/AC Settings (29)

4.6 Instrument Displays (31)

4.7 Graphic Realtime AC Displays (33)

5. EIS Setup________________________________34 5.1 Global Current Limit (34)

5.2 Default on-line display (35)

5.3 DC-Mode Parameters (35)

5.4 AC-Mode Parameters (38)

6. Calibration_______________________________41

7. EIS Measurement_________________________42 7.1 Sweep Mode (43)

7.2 Steps per Decade (43)

7.3 Measure Periods (44)

7.4 Runtime (44)

7.5 Start Recording (44)

7.6 Quick Guide (45)

7.7 Recording Page (46)

7.8 Display Spectrum (48)

7.8.1 Crosshair Mode (49)

7.8.2 Save Measurement (49)

7.8.3 Export ASCII List (50)

7.8.4 Export Drawing (50)

7.8.5 Hardcopy (51)

7.8.6 Import ASCII List (51)

7.8.7 Select Diagram (52)

8. Series Measurement_______________________53 8.1 Scan Mode (54)

8.1.1 Linear Scan (54)

8.1.2 Linear scan over time (54)

8.1.3 Linear scan over potential/current (54)

8.1.4 Linear scans controlled by analogue I/O-channels (55)

8.1.5 Setpoint List (55)

8.1.6 Manually (55)

8.2 Scan Variable or Device (56)

8.2.1 Internal signals (56)

8.2.2 External signals (56)

8.2.3 Channels (57)

8.2.4 File name (57)

8.2.5 Start series measurement (58)

9. Appendix________________________________59

1. Introduction

The measurement program EIS(E lectrochemical I mpedance S pectroscopy) offers all tools for the investigation of impedance in surface engineering, electrochemical power generation, corrosion research, catalysis or even in basic electrochemical research. At the same time, EIS provides some global setting pages useful also for other measuring methods.

As you may have already noticed, the Zennium hardware has got only a few knobs (mainly for emergency purpose). The entire system is controlled by software. This makes working with the IM system very easy and comfortable. No confusing knobs, no manual switch settings, etc. In addition, software controlled functions (in contrast to hardware knobs) can be automated. E.g. ranging, calibration and measurements will be carried out automatically. All this contributes to the high accuracy, the reliability and the easy handling of the IM systems.

The Series Measurement feature of EIS lets you record impedance spectra in dependence of one additional quantity measured by the system through extra analogue or digital inputs (via optional hardware plug-ins or via network virtual instruments). Also inherent and internal quantities like time, potential or controlled current can be taken to control recording.

Due to its graphics-based user-interface EIS is easy to handle. All settings are guided by dialogues, control routines will check inputs for range violations. During measurement, EIS provides you with an online-display with a selectable graph (Bode, Nyquist, etc.), time domain displays for U and I as well as FFT spectrum displays for U and I. With these you will have a complete visual control of the measurement and you are able to detect disturbances directly.

After completion of a measurement, EIS offers a variety of ways to store, export, process and treat the data in a very flexible manner. Spectra may be stored together with their complete history as binary data, allowing a reliable way to reproduce measurements: simply opening a former measurement file will re-configure the instrument according to the prior measurement conditions. For an immediate analysis, the measured spectra may be transferred directly to the simulation & fitting program SIM, in parallel to the storage.

Data may be exported as plain ASCII text lists, as high-quality vector graphics (Windows?-EMF), or as screenshot-bitmap. The export path may be chosen via the Windows?-clipboard, via data file storage, re-directed for printer output or paste into the internal processing software ZEdit and CAD for further text- and/or graphical editing purposes.

2. EIS Main Page

Click on the icon on the Main Menu page to open the EIS main page.

display resident EIS

The EIS main page is the central page for measuring E lectrochemical I mpedance S pectra. Here you also set the recording parameters such as Sweep Mode, Frequency Range, Steps per Decade, Measure Periods and from here you have access to different sub-pages such as Series Measurement, Control Potentiostat or Display Spectrum.

When preparing an EIS measurement proceed in the following order:

- connect the cell

- check the cell connections

- check the object

- set up a measurement

- record and save an EIS spectrum

- check and document the results

2.1 File Operations

2.1.1 Open

This function allows you to load/save EIS data from/to the hard disk of your PC. Click on the icon and decide in the sub-menu, whether you

want to open previously stored EIS data or save actual EIS data.

A description box opens where you may input your measurement parameters and comments. Some lines will be filled automatically by the software (e.g. potential, current and measuring time) others are free for the user to fill and are not used for any calculation. The last three lines are reserved for series recording information. In a series EIS run they will be filled automatically. You may use these lines to define spectra for series analysis even if they were created as single measurements.

Accept the inputs by clicking on the button in the upper right corner or reject the inputs by clicking on the button. Click on the button to call the calculator.

In the following browser navigate to the desired path, Input a file name and Click on the button to save the data. Click on the button to cancel the saving.

2.2 Display Spectrum

The Signal Acquisition pages Setup and Acquisition let you configure and activate additional channels to be recorded or output along with the EIS spectrum. To do this, optional hardware (TEMP/U, DIO, DA-4, FE-42, etc.) or software drivers for virtual NET instruments are needed. The same pages are accessible from the Thales main menu. The Signal Acquisition pages are described in detail in a separate chapter of this manual.

3. Cell Connections

The first thing to do is connecting the electrochemical cell and/or the measuring object to the front terminals of the Zennium.

For standard impedance object (1 Ohm to 100 M?) use one of the BNC cable sets shipped with the system. Keep in mind: the shorter the cables the better the quality of the measurement.

For high impedance objects (100 M? and higher) we recommend to use the optional HiZ probe. Its use can make sense also in lower impedance ranges if the artefacts caused by non-perfect reference electrodes are too prominent. In addition, you should use a Faraday Cage for shielding your cell and the cell cables when measuring high impedances. The Faraday Cage has to be grounded at the Ground connector to the backside of the Zennium.

For low impedance objects (less than 1 ?) we recommend to use the optional LoZ-cable set. These are twisted-pair cables which reduce the mutual inductance which is prominent in low-impedance measurements noticeably.

The shields of the Zennium BNC-connectors and the Lemosa plugs are actively driven ! This minimizes the influence of the stray capacity of the cables.

!Do not connect the Zennium shield to ground or to the cell!

Do not short-cut the Zennium shields!

Before connecting your cell to the front terminals, go to the Control Potentiostat page (click on the button with the same name in the EIS Main page) and make sure that the potentiostat is off.

Then go to the page Check Cell Connections (click on the button with the same name in the Control Potentiostat page). Here you are able to select your connection scheme. You are guided step-by step through the connection sequence now.

Exit Check Cell Connections .

Shows the selected device. Change device on Control Potentiostat page.

Choose plugged probe set.

(FRA-Probe is only available with FRA software module)

Set controlled voltage, if a buffer or an external potentiostat is selected.

Choose electrode connection scheme.

!

Connect all the four terminals to the cell.

Connect each of the four cell cables to the terminal with the same color, only. Connect always in the sequences:

1. Test electrode power (black)

2. Test electrode sense (blue )

3. Reference electrode (green )

4. Counter electrode (red )

If you do not, the system and/or your object may get damaged.

The U-buffer shipped with the Zennium systems can be used to extend the voltage range of the system up to ±10 V. Another important function of the U-buffer is to de-couple the cable load of the connection lines from potentially high impedance reference electrodes. Use the U-buffer, if your reference electrode system impedance exceeds some K ?. The U-buffer is connected to the Lemosa terminal Probe E . Then, connect the Reference Electrode and the Test Electrode Sense to the U-buffer instead of to the BNC terminals.

Both, HiZ probe or the LoZ -cable set are to be plugged to the Lemosa terminals (Probe I and Probe E ) on the IM front panel. All the four electrodes are connected to the HiZ probe and the LoZ -cable set, respectively. !

When using the Lemosa terminals the BNC terminals must be left unconnected!

After you made your choice, click the middle mouse button or press the Esc key to return to the Control Potentiostat page.

If you did select an electrode setup with a reference electrode (three- or four-electrodes) you are now asked for the reference electrode potential. The most common electrode types are listed and the correct potential is automatically used when selecting one of these types.

If your type of reference electrode is not listed, or if you prefer to refer your data to the physical potential, you may use the entry User Defined Reference. In that case you have to enter the individual

reference electrode potential (in V) after clicking on . Enter zero, if you refer to the physical potential reference.

Clicking on will lead you back to the Control Potentiostat page.

!Always connect all the four terminals to the cell, even if you use a 2-electrode set-up.

The potentiostat will be switched off automatically as soon as you change any Check

Cell Connection setting.

!Before switching on the potentiostat make sure that cell and potentiostat are wired correctly. A missing sense connection - either reference electrode or working

electrode sense - may result in uncontrolled currents and floating potentials. Thus

the cell or the potentiostat may be damaged.

3.1 Connected Probes

3.1.1 U-Buffer

The U-Buffer is shipped with the Zennium systems by standard. It is used to decouple high source resistance reference electrodes from the cable load, or when an extended voltage range of up to +-10 V is needed. If you are dealing with high-impedance objects, do not use the U-Buffer. It is not calibrated to the IM-system individually. For those applications use the optional HiZ-Probe set instead. It is calibrated to each IM-system individually and is therefore able to provide optimal accuracy.

!RE and WE sense BNC connectors on the ZENNIUM front panel have to be left unconnected.

With U-Buffer the Zennium has the following input specifications:

Input resistance : > 100 G?

input capacitance : < 30 pF (compensated to 0 ± 5pF due to software calibration)

switch

gain

The U-Buffer amplifier may be used in the two-, three- and four-electrode arrangements if the voltage range of the potentiostat has to be increased to up to ± 10V. The U-Buffer has to be connected to the Probe E

connector on the front panel of the Zennium.

Don’t connect the U-Buffer to the Probe E

potentiostat (PP, XPOT, EL

buffer available by setting the buffer gain factor in the

Connections page.

!The setting of the gain switch of the U-Buffer cannot be detected automatically by the Thales software. Make sure that the correct gain factor (1 or 0.4) is set corresponding

to the controlled voltage menu of the Check Cell Connections page.

For details of the buffer function please refer to the appendix.

3.1.2 HiZ-Probe (Option)

The HiZ Probe Set is an option which should be used when measuring on objects with an impedance >100 M? or if you encounter disturbances with high impedance objects. The HiZ Probe Set is calibrated together with the IM system it is used with. Therefore, it is not recommended to change HiZ probes, to mix the two amplifier boxes or to use a HiZ probe with a system it is not calibrated for. To reach the high accuracy the HiZ probe provides, you have to copy the HiZ calibration data (calfacj.bin) to the hard drive as described in the HiZ probe manual appendix.

gain switch

The HiZ Probe Set is to be connected to the Lemosa terminals Probe I and Probe E on the front panel of the Zennium

High Impedance Probe Current

Probe I High Impedance Probe Potential

For detailed information about installation and handling of the HiZ Probe Set please refer to the HiZ Probe manual appendix shipped with the HiZ Probe .

The HIZ -U-Buffer has to be connected to the Probe E Lemosa connector on the front panel of the Zennium and the HiZ I Probe has to be connected to the Probe I Lemosa connector. It is calibrated to each IM -system individually and therefore can provide optimal accuracy.

!

All BNC connectors on the ZENNIUM front panel have to be left unconnected.

With HiZ-Probe the Zennium has the following input specifications: Input resistance : > 1 T ? input capacitance : < 5 pF (compensated to 0 ± 1pF due to software calibration)

!

Before switching on the potentiostat make sure that cell and potentiostat are wired correctly. A missing sense connection - either reference electrode or test electrode sense - may result in uncontrolled currents and floating potentials. Thus, the cell or the potentiostat may be damaged.

For details please refer to the appendix.

3.1.3 CVB120 (Option)

The CVB120 compliance voltage booster is used to extend the controlled voltage of the Zennium up to 100V and the compliance voltage up to 120V. The CVB120 has to be connected to the Probe E and Probe I Lemosa connector on the front panel of the Zennium.

!All BNC connectors on the ZENNIUM front panel have to be left unconnected.

!Before switching on the potentiostat make sure that cell and potentiostat are wired correctly. A missing sense connection - either reference electrode or test electrode

sense - may result in uncontrolled currents and floating potentials. Thus, the cell or

the potentiostat may be damaged.

3.1.4 FRA-Probe (Option)

3rd party devices (electronic loads) can be interfaced from Thales with the FRA-Probe. The FRA-Probe is not available for the EIS software package. Therefore the radio button is deactivated.

To use the FRA-Probe start FRA software package from the Thales main menu. For further information please refer to the FRA-Probe manual.

!All BNC connectors on the ZENNIUM front panel have to be left unconnected.

3.2 Electrode Schemes

3.2.1 Two Electrodes

The two pole scheme may be understood as the standard Kelvin arrangement known from precision current measurements. The current lines as the potential sensing lines will be connected to the corresponding electrodes of the cell. In electrochemistry the two pole scheme allows the measurement of the full cell's impedance.

Connect the electrodes as follows:

2. working electrode sense U-Buffer - TEs

electrode

3. reference electrode U-Buffer - RE

electrode

4. counter electrode HIZ-I-buffer counter electrode

electrode

In the two-electrode arrangement the Zennium has the following input specifications: Input resistance : > 50 G ? input capacitance : < 50 pF (compensated to 0 ± 5pF due to software calibration)

!

Before switching on the potentiostat make sure that cell and potentiostat are wired correctly. A missing sense connection - either reference electrode or test electrode sense - may result in uncontrolled currents and floating potentials. Thus the cell or the potentiostat may be damaged.

The three electrode scheme will be used in the traditional potentiostatic arrangement of a half cell. In order to obtain a high-accuracy measurement of the absolute potential a reference electrode must be used. This electrode is to be connected to the reference electrode input of the potentiostat. The stray-capacitance of the cable has to be as small as possible. Use the IM cable set for best results. Try to avoid a twisting of the wires of the reference electrode and the counter electrode. If possible keep a large distance between these two cables.

Connect the electrodes in the following sequence:

2. working electrode sense U-Buffer - TEs

electrode

3. reference electrode U-Buffer - RE

electrode

4. counter electrode HIZ-I-buffer counter electrode

electrode

In the two-electrode arrangement the Zennium has the following input specifications: Input resistance : > 50 G ? input capacitance : < 30 pF (compensated to 0 ± 5pF due to software calibration)

!

Before switching on the potentiostat make sure that cell and potentiostat are wired correctly. A missing sense connection - either reference electrode or test electrode sense - may result in uncontrolled currents and floating potentials. Thus the cell or the potentiostat may be damaged.

The four electrode arrangement must be chosen when using a second reference electrode as test electrode sense . An application of the four electrode arrangement are measurements on symmetric cells. e.g. for arrangements with diaphragms. Both reference electrodes should be connected to the corresponding input of the potentiostat with cables of as little straw capacitance as possible. Use the IM cable set for best results. Try to avoid a twisting of the wires of the reference electrodes and the cables of the working/counter electrodes. If possible keep a large distance between these cables.

Connect the electrodes in the following sequence:

2. working electrode sense U-Buffer - TEs

electrode sense

3. reference electrode U-Buffer - RE

electrode

4. counter electrode HIZ-I-buffer counter electrode

electrode

In the four-electrode arrangement the Zennium has the following input specifications: Input resistance : > 100 G ? input capacitance : < 50 pF (compensated to 0 ± 5pF due to software calibration)

!

Before switching on the potentiostat make sure that cell and potentiostat are wired correctly. A missing sense connection - either reference electrode or test electrode sense - may result in uncontrolled currents and floating potentials. Thus, the cell or the potentiostat may be damaged.

!

Shielded cables will increase the parasitic capacitance but decrease the coupling capacitance. The best way to suppress both types of disturbances is to maximize the distances especially to the counter electrode and to shield the whole measuring arrangement, e.g. with a Faraday cage.

For details please refer to the appendix.

电化学阻抗谱的应用及其解析方法

电化学阻抗谱的应用及其解析方法 交流阻抗法是电化学测试技术中一类十分重要的方法，是研究电极过程动力学和表面现象的重要手段。特别是近年来，由于频率响应分析仪的快速发展,交流阻抗的测试精度越来越高，超低频信号阻抗谱也具有良好的重现性，再加上计算机技术的进步，对阻抗谱解析的自动化程度越来越高，这就使我们能更好的理解电极表面双电层结构，活化钝化膜转换，孔蚀的诱发、发展、终止以及活性物质的吸脱附过程。 阻抗谱中的基本元件 交流阻抗谱的解析一般是通过等效电路来进行的，其中基本的元件包括：纯电阻R ，纯电容C ，阻抗值为1/j ωC ，纯电感L ，其阻抗值为j ωL 。实际测量中，将某一频率为ω的微扰正弦波信号施加到电解池，这是可把双电层看成一个电容，把电极本身、溶液及电极反应所引起的阻力均视为电阻，则等效电路如图1所示。 图1. 用大面积惰性电极为辅助电极时电解池的等效电路 图中A 、B 分别表示电解池的研究电极和辅助电极两端，Ra 、Rb 分别表示电极材料本身的电阻，Cab 表示研究电极与辅助电极之间的电容，Cd 与Cd ’表示研究电极和辅助电极的双电层电容，Zf 与Zf ’表示研究电极与辅助电极的交流阻抗。通常称为电解阻抗或法拉第阻抗，其数值决定于电极动力学参数及测量信号的频率，Rl 表示辅助电极与工作电极之间的溶液 电阻。一般将双电层电容Cd 与法拉第阻抗的并联称为界面阻抗Z 。 实际测量中，电极本身的内阻很小，且辅助电极与工作电极之间的距离较大，故电容Cab 一般远远小于双电层电容Cd 。如果辅助电极上不发生电化学反映，即Zf ’特别大，又使辅助 电极的面积远大于研究电极的面积（例如用大的铂黑电极），则Cd ’很大，其容抗Xcd ’比串 联电路中的其他元件小得多，因此辅助电极的界面阻抗可忽略，于是图1可简化成图2，这也是比较常见的等效电路。 图2. 用大面积惰性电极为辅助电极时电解池的简化电路 Element Freedom Value Error Error %Rs Free(+)2000N/A N/A Cab Free(+)1E-7N/A N/A Cd Fixed(X)0N/A N/A Zf Fixed(X)0N/A N/A Rt Fixed(X)0N/A N/A Cd'Fixed(X)0N/A N/A Zf'Fixed(X)0N/A N/A Rb Free(+)10000N/A N/A Data File: Circuit Model File:C:\Sai_Demo\ZModels\12861 Dummy Cell.mdl Mode: Run Fitting / All Data Points (1 - 1) Element Freedom Value Error Error %Rs Fixed(X )1500N/A N/A Zf Fixed(X )5000N/A N/A Cd Fixed(X ) 1E-6 N/A N/A Data File: Circuit Model File:C:\Sai_Demo\ZModels\Tutor3 R-C.mdl Mode: Run Simulation / Freq. Range (0.01 - 10000Maximum Iterations: 100 B

(完整版)电化学曲线极化曲线阻抗谱分析

电化学曲线极化曲线阻抗谱分析 一、极化曲线 1.绘制原理 铁在酸溶液中，将不断被溶解，同时产生H2，即：Fe + 2H+ = Fe2+ + H2 (a) 当电极不与外电路接通时，其净电流I总为零。在稳定状态下，铁溶解的阳极电流I(Fe)和H＋还原出H2的阴极电流I(H)，它们在数值上相等但符号相反，即：(1) I(Fe)的大小反映Fe在H+中的溶解速率，而维持I(Fe)，I(H)相等时的电势称为Fe／H+体系的自腐蚀电势εcor。 图1是Fe在H+中的阳极极化和阴极极化曲线图。图2 铜合金在海水中典型极化曲线 当对电极进行阳极极化(即加更大正电势)时，反应(c)被抑制，反应(b)加快。此时，电化学过程以Fe的溶解为主要倾向。通过测定对应的极化电势和极化电流，就可得到Fe／H+体系的阳极极化曲线rba。 当对电极进行阴极极化，即加更负的电势时，反应(b)被抑制，电化学过程以反应(c)为主要倾向。同理，可获得阴极极化曲线rdc。 2.图形分析 （1）斜率 斜率越小，反应阻力越小，腐蚀速率越大，越易腐蚀。斜率越大，反应阻力越大，腐蚀速率越小，越耐腐蚀。 （2）同一曲线上各各段形状变化 如图2，在section2中，电流随电位升高的升高反而减小。这是因为此次发生了钝化现象，产生了致密的氧化膜，阻碍了离子的扩散，导致腐蚀电流下降。 （3）曲线随时间的变动 以7天和0天两曲线为例，对于Y轴，七天后曲线下移（负移），自腐蚀电位降低，说明更容易腐蚀。对于X轴，七天后曲线正移，腐蚀电流增大，亦说明更容易腐蚀。 二、阻抗谱 1.测量原理 它是基于测量对体系施加小幅度微扰时的电化学响应，在每个测量的频率点的原始数据中，都包含了施加信号电压（或电流）对测得的信号电流（或电压）的相位移及阻抗的幅模值。从这些数据中可以计算出电化学响应的实部和虚部。阻抗中涉及的参数有阻抗幅模（| Z |）、阻抗实部（Z,）、阻抗虚部（Z,,）、相位移（θ）、频率（ω）等变量，同时还可以计算出导纳（Y）和电容（C）的实部和虚部，因而阻抗谱可以通过多种方式表示。

电化学阻抗谱的应用及其解析 2

电化学阻抗谱的应用及其解析方法 董泽华 华中科技大学 交流阻抗发式电化学测试技术中一类十分重要的方法,是研究电极过程动力学和表面现象的重要手段。特别是近年来,由于频率响应分析仪的快速发展,交流阻抗的测试精度越来越高,超低频信号阻抗谱也具有良好的重现性，再加上计算机技术的进步,对阻抗谱解析的自动化程度越来越高,这就使我们能更好的理解电极表面双电层结构，活化钝化膜转换，孔蚀的诱发、发展、终止以及活性物质的吸脱附过程。 1. 阻抗谱中的基本元件 交流阻抗谱的解析一般是通过等效电路来进行的，其中基本的元件包括：纯电阻R ，纯电容C ，阻抗值为1/j ωC ，纯电感L ，其阻抗值为j ωL 。实际测量中，将某一频率为ω的微扰正弦波信号施加到电解池，这是可把双电层看成一个电容，把电极本身、溶液及电极反应所引起的阻力均视为电阻，则等效电路如图1所示。 Element Freedom Value Error Error %Rs Free(+)2000N/A N/A Cab Free(+)1E-7N/A N/A Cd Fixed(X)0N/A N/A Zf Fixed(X)0N/A N/A Rt Fixed(X)0N/A N/A Cd'Fixed(X)0N/A N/A Zf'Fixed(X)0N/A N/A Rb Free(+)10000N/A N/A Data File: Circuit Model File:C:\Sai_Demo\ZModels\12861 Dummy Cell.mdl Mode: Type of Weighting: Data-Modulus 图1.用大面积惰性电极为辅助电极时电解池的等效电路 图中AB 分别表示电解池的研究电极和辅助电极两端，Ra,Rb 分别表示电极材料本身的电阻,Cab 表示研究电极与辅助电极之间的电容,Cd 与Cd ’表示研究电极和辅助电极的双电层电容,Zf 与Zf ’表示研究电极与辅助电极的交流阻抗。通常称为电解阻抗或法拉第阻抗，其数值决定于电极动力学参数及测量信号的频率，Rl 表示辅助电极与工作电极之间的溶液电阻。一般将双电层电容Cd 与法拉第阻抗的并联称为界 面阻抗Z 。 实际测量中，电极本身的内阻很小，且辅助电极与工作电极之间的距离较大，故电容Cab 一般远远小于双电层电容Cd 。如果辅助电极上不发生电化学反映，即Zf ’特别大，又使辅助电极的面积远大于研 究电极的面积（例如用大的铂黑电极），则Cd ’很大，其容抗Xcd ’比串联电路中的其他元件小得多,因此辅 助电极的界面阻抗可忽略,于是图1可简化成图2，这也是比较常见的等效电路。 Element Freedom Value Error Error % Rs Fixed(X )1500N/A N/A Zf Fixed(X )5000N/A N/A Cd Fixed(X )1E-6N/A N/A Data File:Circuit Model File:C:\Sai_Demo\ZModels\Tutor3 R-C.mdl Mode: Run Simulation / Freq. Range (0.01 - 100Maximum Iterations: 100Optimization Iterations: Type of Fitting: Complex 图2.用大面积惰性电极为辅助电极时电解池的简化电路 2. 阻抗谱中的特殊元件 以上所讲的等效电路仅仅为基本电路，实际上，由于电极表面的弥散效应的存在，所测得的双电层电容不是一个常数，而是随交流信号的频率和幅值而发生改变的，一般来讲，弥散效应主要与电极表面电流分布有关，在腐蚀电位附近，电极表面上阴、阳极电流并存，当介质中存在缓蚀剂时，电极表面就会为缓蚀剂层所覆盖，此时，铁离子只能在局部区域穿透缓蚀剂层形成阳极电流，这样就导致电流分布 极度不均匀，弥散效应系数较低。表现为容抗弧变“瘪”，如图3所示。另外电极表面的粗糙度也能影响弥散效应系数变化，一般电极表面越粗糙，弥散效应系数越低。 2.1 常相位角元件（Constant Phase Angle Element ，CPE ）