Working Electrodes

We will describe some tips in using working electrode in following series of letter. It is not appropriate to distinguish which one is working electrode or counter electrode whenever we employ a system of two electrodes. In that case, it is just possible to discriminate them as anode or cathode. However, it is still unable to say exact values of potential of both anode and cathode, so that it remains uncertain whether purposed reaction is occurring or not. It is important to examine redox potential corresponding to intended reaction, for which the potential of working electrode should be measured.

Distinct difference between working and counter electrode is applicable under using potentiostat. We can define the potential of a working electrode versus a reference electrode under so-called three electrode configuration (as shown in Figure below) by controlling with potentiostat. The understanding about potentiostat is desirable, even if not saying indispensable, which will be given later time.

Although only connecting a platinum wire, which is immersed in a test solution, to the reference terminal of potentiostat makes electric contact with the test solution, so called pseudo-reference electrode, but that could not guarantee the stability or reproducibility of potential measurement. It might be worthy to note that reproducible measurement is possible only by connecting a reliable reference electrode.

We receive often the question that how the potential of counter electrode is going. It is quite reasonable question. The answer to this question is as follows. Amount of current flow in working electrode is exactly the same in counter electrode. The difference is just oxidative or reductive. If the current in working electrode is reductive (oxidative), the one in counter electrode should be oxidative (reductive). Hence, if depolarizing material (whatever it is) on counter electrode side without respect with either oxidative or reductive is scarce compared to the reaction of working electrode side, the overpotential in counter electrode side may glow exceedingly resulting in violation of compliance voltage. This difficulty can be mitigated by increasing the surface area of counter electrode in order to decrease current density. This is the reason why counter electrode with large surface area compared with working electrode is recommended, especially for large current application like bulk electrolysis.

Electrodes can be divided into two types: polarizable and non-polarizable electrodes.
The characteristic of an ideal polarizable electrode is that no faraday current flow when the electrode potential is varied. This type of electrodes usually can be used as Working or Counter electrodes.
The feature of a non-polarizable electrode is once the electrode potential be changed, the Faraday current flows out. In generally, this type of electrodes can be used as Reference electrodes.
An ideal polarizable electrode can be represented by a capacitor (condenser) in equivalent circuit as shown in Fig. 1-1. However, the extremely weak current flows at an actual polarizable electrode indeed. Therefore a high value resistance (Rhi) is required to parallel with the capacitor to represent the actual polarizable electrode in equivalent circuit.

If a redox species coexists with the polarizable electrode, the redox reaction of species is occurred on the electrode surface and the Faraday current flows under the certain potential. In this case, a potential depending variable resistance (RF) should be added to the equivalent circuit in parallel. Furthermore, the effect of the species diffusion should be included, and a Warburg Impedance element is connected to Faraday resistance (RF) in equivalent circuit.
Figure 1 is a schematic view of above description. In the absence of a redox system, even electrode potential has been changed, the capacitance and high resistance parallel circuit could still be maintained, this variable potential range is so called as potential window.
The wide potential window of the polarized electrodes is a very important condition in practical application. Platinum, Gold and Carbon (e.g., glassy carbon etc.) electrodes can meet this condition in general.

 Gold, Platinum and other metal electrodes

Incidentally, figure 2 in above was borrowed from the old literature, negative potential located at the potential axis right side which was known as the classic graphic display method. This reflected the epoch in which the polarography was popularly used in the metallic ions reduction. Currently, the opposite direction graphic display of IUPAC is mainly used (positive right).

Platinum is the most common polarizable electrode material, due to its high stability in physical and chemical properties. Hydrogen generation in reduction zone should be noted when the Pt used in aqueous solution. In addition, a proton adsorption process accompanied with the pre-step before hydrogen reduction should be cared. In oxidation zone, oxidation occurs at electrode surface and generates a corresponding reduction current.
This redox paired phenomenon can be founded in most of the solid metal electrode.
When the concentration of the target component is very low, such background current will interfere the measurement and the potential window becomes narrow. Although the electrode potential window is theoretically regulated by the electrode capacitance potential range, there should be no influence for the application if the background current does not interfere to the measurement.
Incidentally, figure 2 in above was borrowed from the old literature, negative potential located at the potential axis right side which was known as the classic graphic display method. This reflected the epoch in which the polarography was popularly used in the metallic ions reduction. Currently, the opposite direction graphic display of IUPAC is mainly used (positive right).

Contrasting to aqueous electrolyte solution, in non-proton organic solvent， platinum electrode can be used in a wide potential range without any proton adsorption desorption and hydrogen evolution reaction occurring. It is necessary to be considered in advance, when high concentration of chloride ions are contained in aqueous solution, dissolution of platinum metal is occurred, due to the formation of chloroplatinic acid ion at high oxidation potential.
Same as platinum, gold is commonly used electrode material. The difference is that gold does not have any proton adsorption-desorption waves, and the over-potential for proton reduction to hydrogen formation is much higher than platinum, therefore the potential window of gold in aqueous solution is wider than platinum in reductive direction. Similar to platinum, in aqueous electrolyte solution containing high concentration of chloride ions may cause the gold metal dissolution due to the formation of gold chloride acid ion at high oxidation potential. Because the surface of gold can be easily chemical modified by the thiol compounds, it has been used for the purpose of many research fields.
Carbon which is usually used electrode material same as gold and platinum, has many types. Such as graphite, pyrolytic graphite, highly oriented pyrolytic graphite (HOPG), glassy carbon and boron-doped diamond electrodes etc. Glassy carbon is the most commonly used electrode material among them. Significant progress regarding the carbon electrode surface analysis and chemical modification is expected to explain in detail in the next future article.
Metallic mercury is liquid state at room temperature, accumulated mercury may drop down through the capillary by gravity, and it is used as repeatedly dropping microelectrode (dropping mercury electrode) in most cases. It is also frequently used as a stationary suspension electrode (hanging mercury electrode). This is the classic polarography.
Mercury is a pioneer electrode by which the electrochemical reduction analysis methods being developed up to today. Mercury electrode surface is smooth up to atomic level, and it is possible to prepare a high surface reproducibility electrode. Since the over-potential for the reduction of hydrogen ions is large, it is utilized for many heavy metal (Pb, Tl, In, Cd, Sn, Zn, Ni, Cu, Mn, Fe, Co, Sb, Mg, Ca, Sr, W, etc.）ions reductive detection. Mercury can not be used in oxidation area, due to the oxidative dissolution of mercury itself. Gold electrode surface coated with mercury is amalgam electrode, on which the heavy metals ions can be detected in high sensitivity using anodic stripping voltammetry. However, from the viewpoint of environmental pollution, mercury environmental standards are extremely strict and difficult for mercury use in Japan.
The typical working electrodes generally used in electrochemical measurements are described above and it is of course be able to use other types of electrodes for special purpose. In corrosion research field, iron electrode is used for polarization measurement of Tafel plot, a nickel electrode and a nickel titanium alloy electrode are used for selective detection of carbohydrate in alkaline solution etc. examples are not shortage in the enumeration. The point is any materials can be used as working electrode in proper applications according to the research purpose.

• Pyrolytic graphite (PG) is prepared by decomposing the hydrocarbon gas at the high temperature substrate. Pyrolytic graphite further dealt with high temperature and high pressure could be enhanced in the crystalline ordering, and highly oriented pyrolytic graphite (HOPG) can be obtained. The electrode performance is depended on the crystalline orderly ratio State of electrode surface can be detected by redox peak potential difference (ΔEp) in cyclic voltammetry (CV) diagram of ferrocyanide/ferricyanide. If the redox peak potential difference is higher than 700 mV (1M KCl supporting electrolyte solution, potential scan rate 0.2V/s), this surface is regarded as the basal plane of the highly oriented pyrolytic graphite (HOPG) (the peak potential difference of commonly used graphite electrode is around 60 mV). It is known that faster the electron transfer is, smaller the peak potential difference (ΔEp) will be, because the electron transfer rate on basal plane of the highly oriented pyrolytic graphite (HOPG) surface is very slow, that is why the large ΔEp value is appeared.
• Glassy carbon (GC) is the most commonly used electrode material, the structure of glassy carbon is shown in Fig. 4-1 (the left lower schematics). A lot of graphite structure shaped thin strip intertwine with each other in glassy carbon, although the microscopic structure has orderly morphology, but the macro structure can be regarded as amorphous (glassy) carbon. Therefore, as the electrode materials, allotropes of carbon are sp2 carbon in graphite structure (excluding diamond electrode). The surface of glassy carbon electrode (GC) is the mixture of basal plane and edge plane. Glassy carbon (GC) is dense and hard as glass, without gas or liquid permeability. In contrast, highly oriented pyrolytic graphite (HOPG) has a slippery flexible structure along the edge plane, peeling along the basal plane, a fresh surface can be obtained.
• Carbon paste is obtained by dispersed graphite powder in oil to paste state , can be used as an electrode (carbon paste electrode).
• Carbon fiber used for electrode preparation is called carbon fiber electrode. Usually, carbon fiber is used for microelectrodes preparation. Embedding the carbon fiber into the plastic or glass, after cutting, a cross-section is used as a microelectrode. Including glassy carbon, the basic structure is sp2 carbon bond.
• Positive holes in valence band are produced and conductivity is generated when boron element is mixed into sp3 bonded diamond (so called as boron-doped diamond electrode. If nitrogen element is incorporated, electronic conductivity is occurred.). A boron-doped electrode especially has very wide potential window. It has chemical stability like diamond, and can be used for special applications.