Electrolysis of water | General Awareness for SSC CGL PDF Download

Factors Influencing Electrolysis Efficiency

The effectiveness of electrolysis, or the transfer of electrons, is influenced by several key elements, including:

• The concentration of cations and anions present in the electrolyte solution. 
• The speed at which ions migrate toward the electrodes. 
• The activation energy required for electrons to move from the electrode to the ions in the electrolyte. 
• The interference caused by gas bubbles adhering to the electrode surface, which can hinder ongoing electron transfer, among other factors.

Navigating multiple phase boundaries (such as liquid-solute, solid-solute, and solid-gas interfaces) raises the energy demands for electrolysis beyond what standard thermodynamic calculations, like Gibbs free energy, would suggest—this additional demand is known as overvoltage.

Fundamentals of Water Electrolysis

In a basic setup, inert metal electrodes, such as those made from platinum or iridium, are submerged in water and connected to a direct current (DC) power supply. At the cathode—where electrons flow into the solution—hydrogen gas evolves. At the anode, oxygen gas is generated. Under perfect Faradaic efficiency, hydrogen production is twice that of oxygen, with yields directly tied to the total charge passed through the system. However, in certain setups, competing side reactions may produce unintended byproducts, reducing the Faradaic efficiency below ideal levels.

Water Electrolysis: Cell Voltage and Thermodynamic Viability

For pure water electrolysis at neutral pH (7) and 25°C, the relevant half-cell reactions are:

• Cathode: 2H₂O(l) + 2e^– → H₂(g) + 2OH^– (standard potential E° = -0.42 V)
• Anode: 2H₂O(l) → O₂(g) + 4H⁺ + 4e^– (standard potential E° = +0.82 V)

The overall process is: 2_H2O(l) → 2H₂(g) + O₂(g) (standard potential E° = -1.24 V)

The negative cell potential indicates that electrolysis of pure water is thermodynamically unfavorable. Due to sparse ion availability and the energy losses from crossing interfaces, an additional overvoltage of approximately 0.6 V per electrode is required. In real-world applications, sustained electrolysis of pure water demands an applied voltage of around 2.4 V. Given its thermodynamic challenges, research focuses on kinetic enhancements to make the process viable.

One common approach is boosting solution conductivity by introducing more ions via acids, bases, or inert salts.

Selecting Electrolytes for Water Electrolysis

Choosing an appropriate electrolyte is crucial for water electrolysis. Anions from the electrolyte can compete with hydroxide ions for oxidation at the anode; if an anion's standard electrode potential is lower than that of hydroxide, it will oxidize preferentially, preventing oxygen evolution. Similarly, for cations at the cathode, those with a higher standard reduction potential than hydrogen ions will reduce instead, blocking hydrogen gas formation.

Acidic Conditions in Water Electrolysis (pH < 7)

In acidic environments, excess protons from the acid reduce at the cathode, while water oxidizes at the anode. The half-reactions are:

• Cathode: 2H⁺ + 2e^– → H₂(g) (E° = 0.00 V)
• Anode: 2H₂O(l) → O₂(g) + 4H⁺ + 4e– (E° = +1.23 V)

Overall: 2H₂O(l) → 2H₂(g) + O₂(g) (E° = -1.23 V)

This setup operates at significantly lower voltages compared to pure water (well below 2.4 V).

Basic Conditions in Water Electrolysis (pH > 7)

In alkaline media, excess hydroxide ions oxidize at the anode, while water reduces at the cathode. The half-reactions include:

• Cathode: 2H₂O(l) + 2e^– → H₂(g) + 2OH^– (E° = -0.83 V)
• Anode: 4OH^– → O₂(g) + 2H₂O(l) + 4e– (E° = +0.40 V)

Overall: 2H₂O(l) → 2H₂(g) + O₂(g) (E° = -1.23 V)

Like acidic electrolysis, basic conditions also require much less applied voltage.

Pourbaix diagrams illustrate the stable domains for water, hydrogen, and oxygen across different electrode potentials and pH values.

Salt Addition in Water Electrolysis

• Salts fully dissociate in water into cations and anions, thereby elevating the ionic concentration and enhancing electrical conductivity. However, these ions can migrate toward the electrodes and interfere with the electrolysis process by competing against the breakdown of water into hydrogen and oxygen gases. To mitigate this, it's essential to choose salts featuring ions that do not compete in these reactions.
• Suitable salts are those whose cations exhibit standard electrode potentials lower than that of hydrogen ions, preventing their reduction and allowing hydrogen ions from water to preferentially form hydrogen gas. Ions from Group 1 and Group 2 elements—such as Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, and Ba²⁺—possess these lower potentials and thus remain unreduced during electrolysis.
• For anions, inert options like nitrate (NO₃⁻) and sulfate (SO₄²⁻) are ideal, as their standard reduction potentials are less positive than that of hydroxide ions (OH⁻), minimizing oxidation. For instance, the oxidation of sulfate to peroxydisulfate requires a high potential of +2.1 V.
• Additionally, insoluble solid polymeric ionic materials, such as Nafion, facilitate efficient water electrolysis at voltages below 1.5 V.

Enhancing Water Electrolysis with Electrocatalysts

Electrocatalysts speed up electrochemical processes without being depleted, much like traditional catalysts that lower activation energy via alternative pathways. Their efficacy stems from high surface areas and abundant active sites.

The performance of inert electrodes, such as platinum, can be improved by:

i) Boosting surface area through nanoparticles, alloys with transition metals, or electronic modifications via catalytic coatings to promote electron transfer. ii) Applying layers of active materials, including enzymes, to the electrode surface.

Electrolyzers

The electrolytic cell employed in water electrolysis is known as an electrolyzer. These devices are categorized into three main types based on the mechanism for transporting ions through the electrolyte:

Polymer Electrolyte Membrane (PEM) Electrolyzer

• A proton-conducting polymer membrane, such as Nafion, divides the anode and cathode compartments. At the anode, water oxidation generates hydrogen ions (H⁺), which migrate through the membrane to the cathode, where they are reduced to produce hydrogen gas.

Alkaline Electrolyzer

• This type utilizes a dilute aqueous solution of sodium hydroxide (NaOH) or potassium hydroxide (KOH) as the electrolyte. The solution facilitates the transport of hydroxide ions (OH⁻) toward the anode, where they participate in oxygen evolution.

Solid Oxide Electrolyzer

• A solid ceramic oxide electrolyte separates the electrodes. At the cathode, water is reduced to hydrogen gas and oxide ions (O²⁻), which then diffuse through the ceramic to the anode, recombining to form oxygen gas. These systems operate at elevated temperatures (700–800°C), which lowers the required external voltage for the electrolysis process.
  

Electrolysis of Pure Water

Pure water electrolysis demands substantial overpotential to surmount activation hurdles, as the reaction proceeds sluggishly—or not at all—without it. Water's minimal autoionization further limits conductivity (about 1/1,000,000th that of seawater). To enhance efficiency, incorporate suitable electrolytes (salts, acids, or bases) alongside electrocatalysts.