Conductive Polymers' Applicatipns for Electrochemistry (İletken Polimerlerin Elektrokimyadaki Uygulamaları, İng)

The science that studies the exchanges between chemical and electrical energy is called electrochemistry. Electrochemical processes are either spontaneous chemical reactions that produce electricity or processes in which an electric current causes a chemical reaction. Reactions that occur during electrochemical processes are redox reactions in which reduction and oxidation occur. Corrosion, which is an undesirable and destructive reaction, can be given as an example to understand the mentioned terms. Corrosion, which has different effects on metal, polymer and ceramic materials, is represented by dissolution or film formation in metals, degradation in polymers, while ceramic materials are resistant to these changes [1]. Indeed, corrosion is an electrochemical process and it can be explained by the movement of electrons in a reaction. This science can also be called the science that studies the phenomena that occur in oxidation-reduction reactions, in which the exchange of electrons is studied, monitored and used for practical purposes.

When we look at the oxidation reaction, an M metal with a valence of n is oxidized at the end of the reaction, becoming an n⁺ ion.

In the reduction reaction, the M metal changes from the ionic state to the neutral metallic state.

Electrolysis, another term that should be known in electrochemistry, is the decomposition of a chemical reaction that does not occur spontaneously into its components by electrical energy sent from outside, and electricity is converted into chemical energy. This change is caused by the substance giving or receiving electrons. The electrolysis process is applied in an electrolysis container or tank (Figure 1). Two electrodes are immersed in a solution of a compound divided into positive and negative charged ions in the electrolysis tank in such a way that they do not touch each other. When the electrodes are connected to a current source, the voltage causes the ions to move towards the oppositely charged electrode. So, oxidation occurs at the anode and reduction occurs at the cathode. Also, atoms or molecules whose charges balance at the opposite pole initiate new reactions with molecules in the electrolyte.

Figure 1 Simple schematic of an electrochemical cell.

When looking Figure 1, it can be said that a copper electrode is immersed in a copper sulfate solution, while a zinc electrode is immersed in a zinc sulfate solution. A porous barrier or salt bridge separates two components and the electrons are transferred from zinc atoms to copper ions.

The cells are shown by schematization from oxidation to reduction or from anode to cathode. According to the indicated notation,

anode | anode electrolyte | | cathode electrolyte | cathode

The anode is on the left, the cathode is on the right, the electrode solution boundary with one perpendicular line, the salt bridge with two perpendicular lines, the semi-permeable membranes with a dashed perpendicular line, and the ion concentrations are shown in parentheses.

If we consider Figure 1, the cell diagram will be as follows:

Zn(s) | ZnSO₄ (aq) | | CuSO₄ (aq) | Cu(s) 

Table 1 Articles of conducting polymers in the use of electrochemical applications.

Year	Article	Reference
2020	polyaniline nanocomposites as electrode material for microbial fuel cells	[2]
2019	PANI-HClO4 sensor to different acids	[3]
2018	drug delivery system based on PEDOT	[4]
2020	polypyrrole composites for supercapacitors	[5]
2020	ionic conductivity of polyaniline thin films	[6]
2021	PPy/Ni2P/GO composites for high-performance supercapacitor electrodes	[7]
2017	biosensor based on poly(pyrrole-co-3,4-ethylenedioxythiophene) for lactose determination	[8]
2022	conductive polymer structures for the proton exchange membrane water electrolysis	[9]
2019	polypyrrole-coated fiber-scaffolds	[10]
2019	copper oxide particles/polypyrrole: application in direct ethanol fuel cell	[11]

Polymers

Polymer materials are materials with long chains in their structures and have high molecular weights. Their structure consists mostly of hydrogen and carbon, but they also contain other elements, so the properties they acquire change. Proteins, enzymes and rubber can be given as an example of natural polymers; polystyrene (PS) and polyethylene (PE) can be given as an example of synthetic polymers.

Polymers can be classified in different ways.

Figure 2 Classification of polymers.

Polymers are obtained by the chemical reaction of monomers. In nature, this process results in the formation of natural polymers, while synthetic polymers are produced by humans. Both natural and synthetic polymers play a remarkable role in facilitating human life and are used in fields such as medicine, nutrition, communications, transportation, clothing, buildings, highways. Without synthetic and natural polymers, human life would be difficult.

When we look at the general properties of polymers, high strength or weight modulus, toughness, flexibility, resistance to corrosion, lack of conductivity (heat and electricity), color, transparency, processing and low cost come to the fore.

With increasing technology, the properties of polymers, which are considered disadvantages, can be improved, in particular, the conductivity values are increased. Hideki Shirakawa, Alan Heeger, and Alan MacDiarmid, who received the Nobel Prize in 2000 for their work on this subject, discovered conductive polymers in 1974.

Conductive (Conjugated) Polymers

With the important discoveries of Hideki Shirakawa, Alan Heeger, and Alan MacDiarmid, a new perspective was gained on polymers. This was because polyacetylene, the simplest conjugated polymer, could conduct electricity almost like a metal.

Figure 3 Microstructure of different conductive polymers [12]

Conducting polymers contain conjugated double bonds in their structure. In conjugation, the bonds between carbons line up as single and double. These are localized “sigma” (σ) and slightly localized “pi” (π) bonds. The electrons in the π bonds do not form a strong bond and are easily separated from their places, providing electrical conductivity in the polymer.

Figure 4 The electron band structure found in a) metals b) insulators c) semiconductors [1] .

Conjugated polymers have a narrow band gap and doping changes their band structure. This is done by either removing electrons from the valence band (p-doping) or adding electrons to the conduction band (n-doping).

The structure of dopants plays an important role in the stability of conductive polymers.

Table 2 Conductive polymers and some of their properties.

Indeed, conducting polymers have the potential to replace many materials used in fields such as electronics, aviation, communications, medicine, automobiles, and space technology.

Literature Review

Energy Storage

Devices used in energy storage can be classified as capacitors, supercapacitors, batteries and fuel cells. Batteries and fuel cells are obtained by converting chemical energy from the redox reactions taking place at the anode and cathode to electrical energy, while supercapacitors are in a separate place due to their high energy storage capacity and long life cycle capabilities [13].

Polypyrrole (PPy), which is one of the polymers with high conductivity, was used for supercapacitors in the studies of Shiyu Ji and Shuling Liu [5] [14].

In another study, Polyaniline (PANI) was produced as electrode material for use in Microbial Fuel Cells (MFC) [2]. According to the results, an increase was observed in the performance of the MFC.

A study of electrodeposition of copper particles on PPy film is another study for fuel cell applications [11]. In this study, the aim was to inhibit poisoning of electrode or, in other words, to help oxidization of ethanol. Indeed, with respect to the characterization, copper oxide (Cu₂O)/PPy/carbon paste electrode (CPE) has excellent electrocatalytic activity and a high tolerance to poisoning species.

Figure 5 Cyclic voltammograms of bare CPE, PPy/CPE and Cu2O/PPy/CPE [11].

Figure 5 indicates that because of PPy’s electronic properties, electronic transfer was improved.

Sensors - Biosensors

In the simplest way, sensors are devices that process physical information and convert it into an electronic signal. They convert physical information such as heat, light, pressure, humidity into electrical signals. These electrical signals can be analog or digital signals. These signals are also processed depending on the purpose and used for the system [15].

Analytical devices that show selective properties against biological samples in the environment and convert the structure and density information of the samples into measurable and processable electrical signals are called biosensors. Mostly used in medicine, agriculture, environment and defense industries, biosensors detect enzymes, viruses, drugs and other substances such as poisonous gases.

Conductive polymers are suitable materials for biosensor design with their immobilizable matrices for the purpose of quantifying very small amounts of biological materials [13].

PPy and poly(3,4-ethylenedioxythiophene) (PEDOT) was developed as lactose biosensor [8].

Figure 6 Preparation of enzyme-entrapped copolymer-modified Pt electrode [8].

According to the study, biosensors based on PPy-PEDOT has advantages such as wide linear range with a low detection limit, temperature stability, a short response time, a strong relationship between enzyme and substrate, and simplicity, which allows for rapid lactose detection [8]. When compared to conventional electrodes, the one that they produced has simple fabrication method with proper cost.  

Another study of conductive polymer in electrochemistry is films of PANI which are used as electrode materials to detect the selectivity to various acids [3]. Study shows that PANI film has different degrees of redox peaks for monobasic acid, dibasic acid, organic macromolecular acid, and inorganic acid as an electrode and sensor material and they have good repeatability and reproducibility.

CONCLUSION

When we look at the studies, in the products developed for different purposes in different fields, the possibilities of material selection have expanded with the technology. When factors such as material availability, ease of processing and price are considered, polymer materials come to the fore in the sector. However, polymers known to be insulators were not preferred until a while ago. Now, polymers that can be made conductive have started to take their place in products where metals are preferred.

Sensors, supercapacitors, membranes and many products that need conductivity are developed with conductive polymer materials. Polypyrrole, Polyaniline and PEDOT are just a few of them.

When we look at academic studies, conductive polymers, which have been used mainly in the energy sector, are now included in electrochemistry. Their structures continue to be discovered and their combinations are created with different materials.

REFERENCES

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[5]	S. Ji, J. Yang, J. Cao, X. Zhao, M. A. Mohammed, P. H, R. A. W. Dryfe and I. A. Kinloch, "A Universal Electrolyte Formulation for the Electrodeposition of Pristine Carbon and Polypyrrole Composites for Supercapacitors," ACS Applied Materials & Interfaces, vol. 12, no. 11, 2020.
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