36
C. Juhong et al.
electrode potential can be controlled by applying a driving potential difference Ed
between the reference electrode and the working electrode. In addition, by adding
the Luggin capillary and shortening the distance between the Luggin capillary and
the working electrode, most electrochemical tests do not require solution resistance
correction for iR drop. Therefore, this potential scanning technique is applicable to
most metal electrodes. Moreover, the scanning rate and the potential scanning interval
of the cyclic voltammetry curve are controlled, and the electrochemical process of the
metal or alloy surface is highly reproducible. Today, this potential scanning method
has been widely used in the research of various electrode processes.
Figure 3.2 shows a simplified potentiostat test circuit. The most important electronic device is the potential comparison circuit, which has the following two functions. The first is to make the control potential Ed (obtainable by the function generator) equal to the effective potential difference Em between the working electrode
and the reference electrode. Since the response times of the balanced potentials, Ed
and Em are in the order of microseconds, the input voltage allowed by the output
current can oscillate beyond 10 kHz. Another function is to convert the current signal
between the working electrode and the counter electrode into a voltage signal through
a resistor R. In order to ensure that the output current signal is only from the counter
electrode and the working electrode, the reference electrode needs to be connected
to a very high resistance R. The voltage drop of the resistor R and the potential Ed
are plotted as images by an x/y recorder or as digital signals.
The potentiostat has three interfaces that can be connected to the working electrode, the reference electrode and the counter electrode. Therefore, as long as an
external fixed resistor is used, the potentiostat can also be used for constant current
testing. The working electrode and the reference electrode are disconnected from
the potentiostat, and the input terminals of the potentiostat are connected to the
fixed resistor Rg. Then, the input terminal of the reference electrode is connected to
the working electrode, and the potential of the working electrode is referenced by
the reference electrode. If the output potential of the constant voltage meter is Ed,
the current flowing through the working electrode will be Ed/Rg. The potential on
the working electrode is changed by controlling this current. Both the galvanostatic
method and the potentiostatic method can be used to test the current potential curve
of a specified potential region. However, for an electrode reaction with a negative
resistance value (voltage increase current decreases), stabilizing one current value
may result in two voltage values and two oscillations at the same time.
In a cyclic voltammetric scan, the scan rate changes the scan direction at a set
reversal point. The generally selected reversal point can present the oxidation and
reduction reactions that may occur in the electrode process on the scan curves in
different directions, such as the oxygen process in the forward scan and the hydrogen
process in the negative scan. The linear scan curve is a one-way (forward or negative)
sweep volt-ampere curve over a specified potential interval. For the surface reaction
process of most metals or alloys, when the scanning rate is 50–500 mVs
−1 , the surface
state of the metal or alloy is basically unchanged after undergoing a complete cyclic
voltammetry scanning process, thus ensuring the test repeatability. Platinum, gold
and palladium, the test results are highly reproducible. Electrochemical testing of
C. Juhong et al.
electrode potential can be controlled by applying a driving potential difference Ed
between the reference electrode and the working electrode. In addition, by adding
the Luggin capillary and shortening the distance between the Luggin capillary and
the working electrode, most electrochemical tests do not require solution resistance
correction for iR drop. Therefore, this potential scanning technique is applicable to
most metal electrodes. Moreover, the scanning rate and the potential scanning interval
of the cyclic voltammetry curve are controlled, and the electrochemical process of the
metal or alloy surface is highly reproducible. Today, this potential scanning method
has been widely used in the research of various electrode processes.
Figure 3.2 shows a simplified potentiostat test circuit. The most important electronic device is the potential comparison circuit, which has the following two functions. The first is to make the control potential Ed (obtainable by the function generator) equal to the effective potential difference Em between the working electrode
and the reference electrode. Since the response times of the balanced potentials, Ed
and Em are in the order of microseconds, the input voltage allowed by the output
current can oscillate beyond 10 kHz. Another function is to convert the current signal
between the working electrode and the counter electrode into a voltage signal through
a resistor R. In order to ensure that the output current signal is only from the counter
electrode and the working electrode, the reference electrode needs to be connected
to a very high resistance R. The voltage drop of the resistor R and the potential Ed
are plotted as images by an x/y recorder or as digital signals.
The potentiostat has three interfaces that can be connected to the working electrode, the reference electrode and the counter electrode. Therefore, as long as an
external fixed resistor is used, the potentiostat can also be used for constant current
testing. The working electrode and the reference electrode are disconnected from
the potentiostat, and the input terminals of the potentiostat are connected to the
fixed resistor Rg. Then, the input terminal of the reference electrode is connected to
the working electrode, and the potential of the working electrode is referenced by
the reference electrode. If the output potential of the constant voltage meter is Ed,
the current flowing through the working electrode will be Ed/Rg. The potential on
the working electrode is changed by controlling this current. Both the galvanostatic
method and the potentiostatic method can be used to test the current potential curve
of a specified potential region. However, for an electrode reaction with a negative
resistance value (voltage increase current decreases), stabilizing one current value
may result in two voltage values and two oscillations at the same time.
In a cyclic voltammetric scan, the scan rate changes the scan direction at a set
reversal point. The generally selected reversal point can present the oxidation and
reduction reactions that may occur in the electrode process on the scan curves in
different directions, such as the oxygen process in the forward scan and the hydrogen
process in the negative scan. The linear scan curve is a one-way (forward or negative)
sweep volt-ampere curve over a specified potential interval. For the surface reaction
process of most metals or alloys, when the scanning rate is 50–500 mVs
−1 , the surface
state of the metal or alloy is basically unchanged after undergoing a complete cyclic
voltammetry scanning process, thus ensuring the test repeatability. Platinum, gold
and palladium, the test results are highly reproducible. Electrochemical testing of