Why Is Graphite Used As An Electrode In Electrolysis?

Graphite, a naturally occurring amorphous form of carbon, is gifted with many unique properties that make it suitable for use in various electrochemical processes. Due to high electrical conductivity, thermal stability, chemical inertness, longevity and purity, graphite finds many uses as an electrode on a commercial level.

Electrolysis, a process of extracting pure elements using direct electric current, is yet another electrochemical process that can be efficiently conducted using graphite.

Electrolysis in scientific terms

During electrolysis, two electrodes are placed in an electrolytic solution (a solution that aids the passage of current). In the presence of an external current source, electrolysis is performed. This setup works like a battery.

Graphite – a common electrode material

Generally, a conductive material like metals namely gold, silver, platinum, copper and brass are used to make electrodes. However, as these metals are expensive, using them on a commercial level is not viable.

Graphite, on the other hand, possesses all the preferred qualities of a good electrode and is readily available making it suitable for electrolysis.

Chemical composition and occurrence of Graphite

Graphite is a form of carbon that occurs naturally and has a crystalline structure. Under standard conditions, graphite remains stable and unreactive. In industries like foundries, lithium-ion batteries industries, etc., graphite is used on a large scale. Its low cost, availability and thermal and chemical inertness make it useful in many industrial applications.

Physical properties

Graphite has an opaque and shiny appearance. It is very soft and slippery to the touch.  Additionally, it is not soluble in water or organic solvents. It is non-inflammable and breaks perfectly in the form of flexible sheets.

Why Graphite is suitable for electrolysis

Here are the chemical and physical properties of graphite that make it suitable for electrolysis:

• Electrical conductivity– Graphite’s unique internal structure makes it a good electrical conductor. Thus, electrolysis can be conducted efficiently.
• Chemical inertness– Being chemically inert indicates that graphite does not react with most organic and inorganic media so the odds of contamination are negligible.
• High melting point– The melting point of 3600 degrees Celsius makes Graphite applicable in various high-temperature applications.
• Low cost and availability– Graphite is commonly found in deposits at various sites in the form of metamorphic rocks like marble, schist, and gneiss. Its plentiful availability makes it cheap and highly in demand.

Graphite as an Electrode in Electrolysis

Role in conducting electricity

Due to the hexagonal structure of graphite, it allows the free flow of electricity. Moreover, free electrons in the structure move through carrying charge.

Interaction with electrolytes and ions

Graphite’s performance as an electrode varies depending on the type of electrolyte used and the ions involved. Commonly, lithium ions , sodium ions and potassium ions perform well with graphite as an electrode. Thus, using it with the right electrolyte can give preferable results.

Advantages of Graphite used as an Electrode

Durability and longevity

Graphite’s robust structure allows it to maintain performance over extended periods thus reducing replacement frequency, minimizing downtime and lowering overall operational costs.

Versatility

Graphite electrodes are extensively used across multiple industries from the manufacturing of phosphorus, calcium carbide and silicon metals to steel production industries.

Resistance to corrosion

Graphite’s natural resistance to corrosion makes it suitable for use in environments where electrodes are exposed to aggressive chemicals or electrolytes. Thus, it can outperform in highly reactive settings like chemical industries.

Ability to withstand high temperatures

Graphite can withstand temperatures up to 3000°C (5432°F) without melting and has good resistance to thermal shock. Thus, graphite electrodes can handle high and rapidly changing temperatures without cracking or losing structural integrity in industrial applications.

Cost-effectiveness

While the initial cost may be higher, the overall cost-effectiveness of graphite electrodes combined with their efficiency makes them an economically viable choice for commercial use.

Specific Applications of Graphite Electrodes in Electrolysis

Graphite electrodes play a crucial role in various electrolysis processes due to their unique properties. Some of its specific applications are:

1. Chlor-alkali Production

In the chlor-alkali industry, graphite electrodes are used as an anode for the electrolysis of sodium chloride solution to produce chlorine, sodium hydroxide and hydrogen.

2. Aluminum Production

Graphite forms both the cathode and the anode in the Hall-Héroult process used for aluminum production.

3. Zinc Electrowinning

In the zinc industry, graphite electrodes are used in the electro-winning process to extract zinc from zinc sulfate solutions.

4. Copper Refining

Graphite electrodes are used to produce high-purity copper from less pure copper anodes in the electrolytic refining process.

5. Water Treatment

In electrochemical water treatment processes, graphite electrodes are used for the generation of oxidants for disinfection.

Limitations and Considerations of Using Graphite as an Electrode

With numerous advantages, there are certain limitations and considerations while using graphite as an electrode. For instance, graphite can oxidize at high temperatures, typically above 400°C (752°F) in air. This can alter the chemical process. Thus, temperature control is needed. Furthermore, under high mechanical stress or abrasion, graphite electrodes may wear faster than metal alternatives.

Also, the porous structure of graphite can lead to gas permeability causing electrolyte penetration. This can affect the overall lifespan. In applications requiring ultra-high purity, graphite can introduce carbon particles. Graphite has a relatively high coefficient of thermal expansion, which means it can expand with temperature rise. This can cause issues in applications with large temperature fluctuations. Such systems must be designed by taking thermal expansion into account.

By considering these limitations, an optimum electrolysis system can be designed with either graphite or more suitable alternatives.

Alternatives to Graphite Electrodes

While graphite electrodes are widely used, certain alternative materials can be considered for use depending on their specific requirements. Metal electrodes like platinum have excellent chemical stability while gold is highly conductive. Metal oxide electrodes due to their high stability, long lifespan and catalytic properties are yet another option for applications like chlorine production and water treatment.

Conclusion

Graphite makes an excellent electrode in electrolysis due to its chemical and physical properties. Being readily available and cheap, it is considered as a great option for commercial applications involving electrolysis. However, due to certain limitations, it is important to consider its alternatives for use in certain applications.