Electrocatalysts

Overall reaction: 2 H2O(l) → 2 H2(g) + O2(g)

An electrochemical reaction is when a chemical reaction occurs, but it either produces electrical power or requires electrical ability to function. For example, consider hydrogen gas and oxygen gas pumped in a balloon. Nothing will happen as it merely has a combination of gases. 

However, if one could get the two to react with each other in a regulated manner, it would create water. This is where an electrocatalyst can be used. Before knowing what an electrocatalyst is, we should understand what a catalyst is and learn how it is different from an electrocatalyst.

What is a Catalyst?

A catalyst is a substance that speeds up a chemical reaction without being consumed. It reduces the activation energy required for a chemical reaction from a thermodynamic standpoint. It also influences the activation energy in an electrochemical reaction. 

An electrocatalyst is a catalyst that influences the activation energy of an electrochemical reaction. In electrochemical reactions, the activation energy is proportional to the potential or voltage at which the reaction occurs. As a result, electrocatalysts often change the potential at which oxidation and reduction reactions occur. The primary purpose of electrocatalysts is to accelerate electrochemical processes that occur on electrode surfaces or at any solid-liquid interface.

Generally, electrocatalysts are divided into two types, homogeneous and heterogeneous electrocatalysts. A homogeneous electrocatalyst exists in the same phase as the reactants and helps in accelerating an electrochemical reaction in solution. Mostly, homogeneous electrocatalysts are soluble, aid in the transport of electrons between the electrode and reactants, and facilitate a chemical transformation characterised by an overall half-reaction. 

Certain enzymes have the ability to act as electrocatalysts. Nitrogenase, a MoFe cluster-containing enzyme, is used to fix atmospheric nitrogen, converting nitrogen gas into compounds like ammonia. Another example, the nickel-containing enzyme formate dehydrogenase, has sparked the development of synthetic compounds with comparable molecular architectures for CO2 reduction.

However, when it comes to heterogeneous electrocatalysts, it exists in a different phase than the reactants. Most heterogeneous electrocatalytic reactions require electron transfer between the solid catalyst, which can be metal, and the electrolyte, which can be a liquid solution and a polymer capable of ionic conduction. 

The reaction is influenced by both the catalyst and the electrolyte and the interface. It also distinguishes between solid, liquid, and gas components and between immiscible mixes such as oil and water. One of the best examples of heterogeneous electrocatalysts is nanoparticles. Though various nanoparticle materials have been proven to promote varied electrochemical processes, it depends on the size, form, and surface strain.

Electrochemical Reactions in Water Electrolysis

Water electrolysis is breaking down water into oxygen and hydrogen gas using electricity. This hydrogen gas may generate hydrogen fuel or oxyhydrogen gas, utilised in welding.

A reduction reaction occurs in pure water, which contains the negatively charged cathode. The electrons (e−) from the cathode are transferred to hydrogen cations, resulting in hydrogen gas. The half-reaction, balanced with acid, is:

Reduction at cathode: 2 H+(aq) + 2e− → H2(g)

When it comes to the positively charged anode, an oxidation reaction occurs, creating oxygen gas and giving out electrons to the anode. The following is:

Oxidation at anode: 2 H2O(l) → O2(g) + 4 H+(aq) + 4e−

Not all half-reactions must be balanced with acid or base under all circumstances. On the other hand, acid-balanced reactions are usually seen in acidic (low pH) solutions. However, it is dominated by essential (high pH) solutions when it comes to base-balanced reactions.

Cathode (reduction): 2 H2O(l) + 2e− → H2(g) + 2 OH−(aq)

Anode (oxidation): 2 OH−(aq) → 1/2 O2(g) + H2O(l) + 2 e−

When each half-reaction pair is combined, the total breakdown of water into oxygen and hydrogen is just the same.

Overall reaction: 2 H2O(l) → 2 H2(g) + O2(g)

In water electrolysis, the water reduces at the cathode to form H2, and water oxidises at the anode to produce O2. Here, both the reactions are kinetically slow, resulting in low energy efficiency. As a result, electrocatalysts are required to accelerate the reaction at the electrodes.

Water electrolysis is hailed for its ability to produce ultrapure hydrogen. However, the kinematic limits of the anodic oxygen evolution process restrict the energy efficiency of traditional water electrolysis systems. 

An innovative and much-improved version of the hybrid water electrolysis system has been developed and demonstrated throughout the last decade. The critical challenges in conventional water electrolysis are addressed by replacing the oxygen evolution reaction with more simplistic oxidation reactions and coupling with the cathodic hydrogen evolution reaction.

Advantages of Electrocatalysts

• Electrocatalysts are used in water electrolysis and help in the methanol oxidation reaction.

• They are used in metal electrodeposition and also used in hydrogen and oxygen reduction reactions.

• They are used in wastewater management treatment and employed in the synthesis of nanoparticles.

• It is helpful to study the various chemical reactions.

• They are used to avoid interferences in the electro-reduction process.

• They are used in physical, chemical and biological sciences.

• They are used to decrease the activation energy of charge transfer reduction.

Conclusion

Electrocatalysts play a vital role in everyday life, and it is essential in the development of many products. It is not only used in the field of chemistry but also in biological and physical sciences. It is also used in the development of fuels cells and is not the only use of catalysis in this field.