Water Electrolysis – VLE Simulations

What is Electrolysis of Water – Electrolysis of water is the process of decomposing water into hydrogen and oxygen gas by passing electricity through its aqueous solution (water).

The electrolysis of water is not simple and easy for many reasons.

1. The electrolysis of pure water will be a very slow process due to a) slow dissociation of water into hydrogen and hydroxide ions, and b) electrolysis involving the charge carriers, for the current to flow. So, water with few ions is a bad conductor of electricity. 
2. In water electrolysis, electrodes are inert solids like platinum/palladium, whereas electrolyte is a solute in a solution and the product is a gas.
3. As we know, pure water does not conduct electricity and requires excess energy to overcome the activation barrier. Therefore, the electrolysis is carried out in the presence of an acid to improve the electrical conductivity by increasing the hydrogen ion (H+) concentration. Examples of such electrolytes are sulfuric acid (H2SO4) and sodium nitrate salt (NaNO3).

Factors Affecting Efficiency of Electrolysis

The efficiency of electrolysis or the electron transfer depends on many factors such as:

i) The number of cations and anions present in the solution.

ii) Mobility rate of the ions to reach the electrode.

iii) Activation energy is needed for the electron transfer from the electrode to the electrolyte ions.

Principle of Electrolysis

To understand the process of electrolysis of water, first, let us prepare the electrolytic solution and then using the electrolytic solution construct an electrolytic cell. 

1st Step: Preparation of electrolytic solution

Prepare an aqueous solution of Na₂SO₄ by dissolving salt in distilled water which is used as an electrolytic solution. Then check the neutrality of the electrolytic solution by adding a few drops of universal indicator. The green colour appears, which indicates it is a neutral solution.

2nd Step: How to construct electrolytic cell

1. Two electrodes or plates that are made from an inert metal such as platinum or iridium are placed in the water. A Direct Current (DC) electrical power source is connected to these plates. The electrode connects to the positive end of the battery also known as the anode.

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2. The other electrode’s negative end connects to the plug key, also known as the cathode.

3. Finally connect conducting wire to the battery and plug key.

4. Now we need to click on the plug key to pass electric current through the electrolytic solution.

At the cathode part (where electrons enter the water), hydrogen gas will appear. On the anode side, oxygen gas will appear. If we consider the ideal faradaic efficiency, hydrogen will produce twice the amount of oxygen. On the other hand, both will be proportional to the total electrical charge conducted by the solution. However, in some cells side reactions can occur and different products are formed with less than ideal faradaic efficiency.

Electrolysis of Water – Reaction

The chemical reaction for the electrolysis of water can be split into two half-reactions occurring at the cathode and anode.

A reduction reaction occurs at the cathode when water acquires two electrons, and is converted into hydrogen gas. The half-reaction is as follows:

2H₂O (l) + 2e^– → H₂ (g) 

An oxidation reaction occurs when water molecules give electrons to the anode and liberate oxygen gas at the anode. The half-reaction is shown below:

2 H₂O (l) → O₂ (g) + 4 H⁺ (aq.) + 4 e^–

The overall chemical reaction can be written as follows:

2 H₂O (l) + electrical energy → 2 H₂ (g) + O₂ (g)

Thus, the electrolysis of water reaction shows the separation of hydrogen gas and oxygen gas from water. For two moles of water, two moles of hydrogen gas and one mole of oxygen gas are liberated. The number of moles of hydrogen generated is twice the oxygen. Also, charges are transferred between the electrodes and the electrolyte. For every mole of hydrogen, 2 electrons are transferred from the cathode to the electrolyte. For every mole of oxygen, 4 electrons are transferred from the electrolyte to the anode.

Water Electrolysis in Presence of Acids

The chemical reaction for the electrolysis of water in the presence of acids can be split into two half-reactions occurring at the cathode and anode.

A reduction reaction occurs at the cathode when hydrogen ions acquire electrons, and are converted into hydrogen gas. The half-reaction in an acidic medium is as follows:

At cathode: 2H⁺ + 2e^– → H₂ 

An oxidation reaction occurs when water molecules give electrons to the anode and liberate oxygen gas at the anode. The half-reaction in an acidic medium is shown below.

At anode: 2H₂O → O₂(g) + 4H⁺ + 4e^– 

The overall chemical reaction can be written as follows:

2H₂O → O₂(g) + 2H₂

In the presence of an acidic medium, the concentration of H⁺ ions increases in the solution. Additional hydrogen ions from acid will be reduced at the cathode, while water will be oxidized at the anode. The electrolysis takes place at a much lower potential than pure water.

Water Electrolysis in Presence of Bases

The chemical reaction for electrolysis of water in the presence of base can be split into two half-reactions occurring at the cathode and anode.

A reduction reaction occurs at the cathode when water acquires electrons and is converted into hydrogen gas. The half-reaction in the basic medium is as follows:

At cathode: 2 H₂O (l) + 2e^– → H₂ (g) + 2 OH^– (aq.)

An oxidation reaction occurs when hydroxide ions give electrons to the anode and liberate oxygen gas at the anode. The half-reaction in the basic medium is shown below.

At anode: 4 OH^– (aq.) → O₂ (g) + 2 H₂O (l) + 4 e^–

The overall chemical reaction can be written as

2H₂O → O₂(g) + 2H₂

In the presence of a basic medium, the concentration of OH^– ions increases in the solution. Additional hydroxyl ions from the base will be oxidized at the anode, while water will be reduced at the cathode. The electrolysis takes place at a much lower potential than pure water.

Water Electrolysis in Presence of Salts

Salts act as strong electrolytes. They dissociate entirely into cations and anions in water, augmenting the ionic concentration to increase conductivity. But the cations and anions from the salt also will be attracted towards the electrodes and hence become competitors to the decomposition of water to produce hydrogen and oxygen. So, the selection of salts with non-competing ions becomes necessary.

Salts containing lesser standard electrode potentials than hydrogen and hydroxide ions are suitable for water electrolysis.

Ions of first and second group elements (Li, Na, K, Mg, Ca, Ba, etc.) have lower standard potential than hydrogen ions and will not be reduced, allowing hydrogen ions from water to hydrogen.

Non-reactive anions, like nitrate and sulphate ions, have a lesser standard reduction potential than hydroxide ions. Sulphate oxidation to peroxy-sulphate has a reduction potential of +2.1V.

Non-soluble, solid polymeric ionic compounds (Nafion) have been found to help water electrolysis at less than 1.5V.

Electrolysis of Water using Electro Catalysts

Electro-catalysts are substances that accelerate electrochemical reactions without being consumed in the reaction like a catalyst in chemical reactions. Catalysts take the reaction through a different path of lower activation energy. The higher the surface area, the larger the activity is the catalyst’s ability to increase the reactivity.

1. The activity of the inert electrode like platinum can be enhanced by modification of the surface by:

a) Increasing the surface area with nanoparticles or alloying with catalytic d-block elements and changing the electronic state coated with other catalytic substances to enhance the electrolysis.; and

b) Coating the electrode surface with catalytically active substances, like enzymes.

1. Some of the best electrocatalysts are noble-metal oxides, which are rare and costly. Fortunately, nickel-based hydroxide (Ni(OH)2) compounds are a better alternative.

FAQs on Electrolysis of Water

Q.1. How would you show that the gas obtained at the cathode during electrolysis of water is hydrogen and that obtained at the anode is oxygen?
Ans. When we bring the burning splint near the electrode, and it makes a pop sound, then the gas liberated is hydrogen. When we bring the glowing splint near the electrode, and it reignites, then the gas is oxygen. From this, we can prove that the gas liberated at the cathode is hydrogen and that of the anode is oxygen.

Q.2. What happens during the electrolysis of water?
Ans. During the electrolysis of water, decomposition of water into oxygen gas and hydrogen gas occurs. This happens by passing an electric current through the solution. Hydrogen gas is produced at the cathode, whereas oxygen gas is produced at the anode. 

Q.3. Why is electrolysis of water carried out in an acidic medium but not in pure water?
Ans: Pure water cannot conduct electricity. It is almost non-electrolyte. When we add a small amount of acid to water, it dissociates into hydrogen and hydroxide ions and increases the concentration of H+ ions. Therefore, electrolysis is carried out in acidified water but not in pure water.

Q.4. Which type of chemical reaction is the electrolysis of water?
Ans: Electrolysis is an example of a decomposition reaction. It is because water splits into its constituent elements, hydrogen and oxygen, under the effect of an electric current.

Q.5. Which electrodes are used in the electrolysis of water?
Ans: Platinum electrodes are used in the electrolysis of water.

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