两电极, 三电极和四电极实验介绍

Introduction

Electrochemistry studies and develops applications by controlling a single type of chemical reaction and measuring the many physical phenomena it produces. For its part, a large variety of experiments have been conducted over the years to benefit such research. Experiments range from simple potentiostats (timing currents) to cyclic voltammetry (dynamic potentials) to complex AC techniques such as impedance spectroscopy. Not only that, but each independent technology has many possible experimental settings, all of which have one of the best options. This technical report discusses a portion of the experimental setup: the number of electrodes used.

Four-electrode potentiostat

Gamry potentiostats are all 4-electrode systems. This means that four related electrodes need to be placed in a given experiment. The working electrode (green) and the auxiliary electrode (red) are used to load the current, and the working sensing electrode (blue) and the reference electrode (white) measure the potential (potential).

Color coding of Gamry electrodesFigure 1 Color coding of Gamry electrodes

The four-electrode instrument can be easily changed to achieve 2-electrode, 3-electrode and 4-electrode testing. Understand why, how to use different modes is confusing.

electrode

The discussion of several electrode experiments needs to figure out what the electrodes are. The electrode is a conductor or semiconductor and contacts the solution to form an interface. The usual design is the working electrode, the reference electrode and the counter electrode (or auxiliary electrode).

The working electrode is connected to the electrode under investigation. It may be an electrode material in the battery test, and in the corrosion test, it is likely to be a corrosive metal material. In physical analysis experiments, often inert materials - usually gold, platinum or graphite - can transfer current to other molecules without being affected by themselves.

The counter electrode or auxiliary electrode is the current path in the battery. All electrochemical experiments have a pair of working and counter electrodes. In most experiments, the counter electrode is a simple conductor, and a relatively inert material such as graphite or platinum is the ideal electrode, although it is not necessary. In some experiments, the counter electrode is also part of the study, and its material composition and design will be different.

参比电极,顾名思义,是用来当作实验参考点。它们是电势的参照物。因此,在试验中参比电极必须保持一个恒定的电势,在一个绝对标度。这样可以有两种途径获得,其一没有电流经过时,其平衡性很好,即使电流通过电位也不会改变。许多电极能够很好的保持电位稳定,常用的有:Ag/AgCl电极,饱和甘汞电极,汞/氧化汞电极,汞/硫酸汞电极,铜/硫酸铜电极等等。也有其他如今不常用的参比电极,如标准氢电极。

两电极试验

两电极试验的设置最简单,但常有更为复杂的结果和相对应的分析。两电极试验设置中负载电流的电极也用来敏感度测试。

两电极电池的物理设置是将测电流端和电位端连在一起:W和WS连接在工作电极上,参比电极和对电极连接在第二个电极上,如图2所示。

Two-electrode battery connection

图2 两电极电池的连接

两电极试验测的是这个电池,也就是说,电位端测电流流过整个电池时的电位降:工作电极,溶液和对电极。如果整个电池的电势图如图3所示,则两电极体系是将WS端连在A点,参比电极端连在E点,所以测得的是整个电池的电位降。

The potential map of the entire battery.  The working electrode end is connected to point A, and the counter electrode end is connected to point E.

图3 整个电池的电势图。工作电极端连在A点,对电极端连在E点

两电极体系可用与下列情况。一种情况是想得到整个电池的电压降,例如电化学能源装置(电池,燃料电池,太阳能电池)。另一种情况就是,在试验整个过程中对电极的电位不漂移。通常出现在低电流或者相对较短的时间范围,对电极的电势要非常稳定,如微小的工作电极和相对较大的银电极。

三电极试验

三电极模式下,参比电极与对电极分开,连接在第三个电极上。这一电极通常放置在于工作电极很近的位置,工作电极连接W和WS。三电极体系的设置如图4所示。

Three-electrode system setting

图4 三电极体系的设置

图3中电位端在A和粗略的B点。3电极体系较2电极体系有很大优点:3电极体系只测量电池的一半,也就是说测量工作电极电势的改变,使其不受由对电极引起的电势影响。

This way of separating the reference electrode and the counter electrode is a more accurate study of the electrochemical reaction. Therefore, the three-electrode system is the most commonly used method in electrochemical tests.

Four-electrode test

The four-electrode system separates WS from W and is similar to the reference electrode. The four-electrode system is shown in Figure 5.

The four-electrode system measures the potential drop between B and D in Figure 3, and that point C has the potential to affect. This system is relatively less used in electrochemical tests, but still useful. The potential drop caused by the electrochemical reaction occurring at the working electrode or the counter electrode surface in the four-electrode system is not detected. Only the current drawn through the solution or the potential drop caused by the obstacle in the solution is measured.

Four-electrode system diagram

Figure 5 four electrode system diagram

Figure 5 Four-electrode system diagram The four-electrode system is typically used to measure the impedance of the solution phase interface. Such as membrane or liquid-liquid phase boundaries. Can also be used to accurately measure solution resistance, or metal surface resistance (solid state battery)

Special case settings: ZRA mode

A zero resistance galvanometer test is a special case. Briefly mention it. In ZRA mode, the working electrode and the counter electrode are short-circuited, such as the entire battery has no potential drop. In the Gamry instrument, this mode is similar to the 3-electrode system, with an orange CS end connected to the counter electrode. The reference electrode is not important in this test, but the potential can be applied to the working and counter electrode.

Figure 6 shows the potential of the battery in ZRA mode.  W/WS serializes point A, and C/CS connects to point E.  Note that this potential map was incorrect in Helmholtz.  B and D represent the most recent measurable value

Figure 6 shows the potential of the battery in ZRA mode. W/WS serializes point A, and C/CS connects to point E. Note that this potential map was incorrect in Helmholtz. B and D represent the most recent measurable value

The ZRA mode is Figure 3 re-displayed in accordance with Figure 6. The potential at point A is equal to point E. The reference electrode can be connected at point B, C or D. The reference electrode in the solution measures the potential drop caused by the position, current and solution resistance.

The ZRA mode is used for galvanic corrosion, electrochemical noise and a few special tests.