Partager l'article
In a typical electrochemical experimental setup, the working electrode is not directly connected to the ground. This allows the use of a grounded potentiostat/galvanostat (PGSTAT) for the electrochemical research. However, there are times when electrochemists need to experiment with working electrodes that are intrinsically grounded (e.g., for water pipes, rebar in concrete, etc.). There are also situations where the electrochemist needs to ground the counter electrode or ground the cell body of their electrochemical setup. This blog post highlights these different grounding configurations and discusses their relevant applications.
Definitions: grounded and floating modes
Depending on the ground state of the analog electronics of an electrochemical instrument (in this case the PGSTAT), it can operate in either a «grounded» or «floating» mode. These options allow researchers to create a variety of conditions for experimentation. A grounded instrument has the analog (or signal) electronics connected to the EARTH ground. A floating instrument has the analog electronics disconnected from the EARTH ground (i.e., floating).
Ground is a place with a stable potential value, regardless of the amount of charge exchanged. It is also called the «ground point». The potential of the ground is defined to be 0 V. Ground can be the planet Earth itself, as is the case for the electrical wiring grid in buildings. A direct electrical connection with the Earth is made via one or more metal poles inserted into the ground while also connected to the electrical wiring grid of the building. This ensures that the entire building is grounded.
When the electronics are considered to be floating, they have no direct electrical connection to the Earth.
Non-floating vs. floating instrument
In order to define how grounded and floating modes are implemented in a PGSTAT instrument, it is necessary to present a short overview of how the electronics of a PGSTAT work.
The electronics of any PGSTAT are divided in two parts, each with a different role: the power electronics and the analog (or signal) electronics. They are connected to each other through the transformer as shown in Figure 1.
The power electronics (Figure 1, red) manage the power from the electrical grid while the analog electronics (Figure 1, blue) manage the signals coming from the electrochemical cell or from the device under test (DUT).
The power electronics are connected directly to the mains socket with three terminations. Two of these carry the current and the voltage. The value of the voltage (230 V, 50 Hz or 110 V, 60 Hz) is determined by the potential difference between the live wire and the neutral wire. The third termination is the ground wire which connects the metal chassis of the instrument to the ground point of the electrical grid via the Earth connection. For safety reasons, this direct connection to the ground is always present and cannot be removed.
A transformer is positioned between the power and the analog electronics, and it transforms the current and voltage into values used by the analog electronics.
The analog electronics are connected to the electrochemical cell. The analog electronics have multiple connections available for the different potential and current values needed. Such connections come in pairs to define the potential difference between the two wires included in each pair. The analog electronics have a ground point called analog ground (AGND), to which all the potentials are referred. In addition, the AGND switch connects the analog electronics ground to the chassis of the instrument. This connection can be removed, leaving the potential of the analog electronics floating through values that differ from the 0 V value of the Earth ground.
Therefore, when the PGSTAT operates in non-floating mode, the analog electronics ground (analog ground, AGND) is connected to the Earth (EARTH) through the chassis of the instrument.
When the instrument operates in floating mode, the AGND of the analog electronics is not connected to the chassis of the instrument, but to the Earth (Figure 2).
Flexible ground: working electrode, counter electrode, or cell body
Potentiostatic circuits that employ operational amplifiers are routinely used to apply a polarization potential to an electrode (usually referred to as a «working electrode») with respect to a reference electrode. This helps to avoid the flow of any current through the reference electrode in order to keep it strictly nonpolarized. Ultimately, a third electrode (i.e., the «counter electrode») is added to deliver the required current.
Grounded potentiostatic circuits can exist in three different basic configurations, namely with: grounded work (GW) electrode, grounded counter (GC) electrode or, grounded cell body, depending on which electrode is held at ground potential [1].
In principle, the current can be measured at the counter electrode or at the working electrode. From an electrical point of view, the GW and GC configurations are nearly identical.
However, choosing the electrode from which to measure the current has consequences on the accuracy of the cell, since the impedance of the current-measuring circuitry is added to the impedance of the electrode. Taking this into account, the GW configuration has been, by far, the most frequently used configuration.
VIONIC powered by INTELLO has the option to select the grounded state of the experiment. The different floating modes are selected in the INTELLO software as shown in Figure 3. No additional hardware is needed to convert the VIONIC from Non-Floating (standard) to Floating (grounded electrodes).
Application examples for grounded setups
WE grounded:
- Corrosion on bridges, buildings, or pipelines connected to Earth
- Hydrogen permeation in a Devanathan-Stachurski cell with two independent potentiostats
- In-situ electronic microscopy to obtain adequate images and reliable electrochemical data
CE grounded:
- Bioreactors in which two independent PGSTATs were first employed
- Multiple working electrode systems
- Shielded working electrodes
Cell body grounded:
- Autoclave – for safety reasons or the entire autoclave is used as counter electrode for the electrochemical cell [2]
- Tribocorrosion setup – friction is applied to a metal sample which is attached to the cell body
Conclusion
The ability to ground different electrodes (e.g., the counter electrode) may offer users some advantages over the typical working electrode setup. For example, the grounded counter electrode can be used in non-conventional systems where the solutions or the working electrodes are not isolated electrically. As reported by Busoni et al., «the grounded counter configuration, though scarcely known and used in electrochemical work, should be preferred over the grounded working configuration whenever a capacity measurement is involved» [3].
The selectable floating feature of VIONIC gives users the flexibility to choose the ground state of the cell setup, offering even greater experimental possibilities. The electrochemical experiment can therefore be configured to the exact specifications needed and is not limited by the electronics of the PGSTAT.
References
[1] Yarnitzky, C. N. Part I. Design and Construction of a Potentiostat for a Chemical Metal-Walled Reactor. Journal of Electroanalytical Chemistry 2000, 491 (1), 160–165. DOI:10.1016/S0022-0728(00)00150-9
[2] Holm, T.; Dahlstrøm, P. K.; Burheim, O. S.; et al. Method for Studying High Temperature Aqueous Electrochemical Systems: Methanol and Glycerol Oxidation. Electrochimica Acta 2016, 222, 1792–1799. DOI:10.1016/j.electacta.2016.11.130
[3] Busoni, L.; Carlà, M.; Lanzi, L. A Comparison between Potentiostatic Circuits with Grounded Work or Auxiliary Electrode. Review of Scientific Instruments 2002, 73 (4), 1921–1923. DOI:10.1063/1.1463715
Your knowledge take-aways
Compact potentiostats/galvanostats (PGSTATs) from Metrohm Autolab
(Un)Grounded: Grounded and floating measurements and their application in electrochemical research
In this White Paper, details of the electronic (PGSTAT) and electrochemical cell grounding are presented together with the necessity of using a floating PGSTAT for different application and experimental examples. Due to the wide variation of experimental requirements and kinds of electrochemical cells, the use of an electrochemical instrument with a selectable floating feature (such as VIONIC) which brings additional versatility to the user is recommended.
