Electrolysis of water is a popular method used for different applications in various industries, mainly in the food industry, metallurgy, power plants amongst others. Besides, the components of water which include hydrogen and oxygen have many applications. For instance, hydrogen obtained through electrolysis is a clean, renewable and efficient fuel source.
The electrolysis of water is mainly carried out to yield pure hydrogen and oxygen gases. It involves passing an electric current through the water which results in the decomposition of water into hydrogen and oxygen.
However, the electrolysis of water is not simple and easy for many reasons.
- Water is very weakly dissociated into hydrogen and hydroxide ions. The concentration of the ions in neutral water are equal (= moles per litre). Electrolysis involves the charge carriers, for the current to flow. So, water with a very small amount of ions is a bad conductor of electricity. So, electrolysis of pure water will be a very slow process.
- The hydrogen ion is associated with other water molecule and exists as hydronium ion. So, any hydroxide ion, moving towards the anode, will be neutralized by the hydronium ion, even before it reaches the anode to form oxygen gas. Similarly, any hydrogen present will be neutralized by the hydroxyl ion present near the cathode and will not be, reduced to hydrogen. So, electrolysis of water to hydrogen and oxygen will be very small. The electrolysis also involves the transfer of electrons from the anion to the anode and cathode to cations.
- In the electrolysis of water, electrodes are inert solids like platinum/palladium whereas electrolyte is a solute in a solution and the product is a gas.
Factors Affecting the Efficiency of Electrolysis
The efficiency of electrolysis or the electron transfer depends on many factors such as;
i) The number of available cations and anions in the solution.
ii) Mobility rate of the ions to reach the electrode.
iii) Activation energy needed for the electron transfer from the electrode to the electrolyte ions.
iv) The effect of the gas bubble surrounding the electrode on the further electrotransfer etc.
Crossing over of several interfaces (solute- liquid, solute-solid, solid-gas) results in the increase of energy requirements for the electrolysis (overvoltage) than predicted by the thermos-dynamical Gibbs energy.
Electrolysis of Water – Cell Potential and Thermodynamic Feasibility
Half reactions in the electrolysis of pure water at pH=7, and at 25°Care-
At cathode: 2H2O(l) + 2e– → H2(g) + 2OH– E° = -0.42 V
At anode: 2H2O → O2(g) + 4H+ + 4e– E° = +0.82 V
The net reaction of electrolysis of water is given as;
2H2O(l) → 2H2(g) + O2(g) E° = -1.24 V
The cell potential of electrolysis of pure water is negative and hence is thermodynamically unfavourable. Because of the low concentration of ions and the interfaces to be crossed electrons an extra voltage (Overvoltage) at each electrode is needed to about 0.6V.
In practice, continuous electrolysis of pure water is possible only at an external voltage of 2.4V. Since the electrolysis of pure water is thermodynamically non-feasible, methods to make it kinetically feasible are being investigated.
One of the methods is to increase the conductivity by increasing the number of ions available by adding acid, base, or non-reacting salts.
Water Electrolysis in the Presence of Acids (pH lower than 7)
Additional hydrogen ions from acid will be reduced at the cathode while water will be oxidized at the anode. Half reactions in an acid medium are;
At cathode: 2H+ + e– → H2 E° = +0.0 V
At anode: 2H2O → O2(g) + 4H+ + 4e– E° = +1.23 V
Net reaction is written as 2H2O → O2(g) + 2H2 E° = -1.23 V
The electrolysis takes place at a much lower potential than pure water (2.4V).
Water Electrolysis in the Presence of a Base (pH higher than 7)
Additional hydroxyl ions, release their electrons to anode, while electrons at cathode oxidize water molecules near it. Half reactions of electrolysis in the presence of a base are-
At cathode: 2H2O(l) + 2e– → H2(g) + 2OH– E° = -0.83 V
At anode: 4OH– → O2 + 2H2O + 4e– E° = +0.4 V
Net reaction is 2H2O → O2(g) + 2H2 E° = -1.23 V
Like electrolysis in acid medium, electrolysis in the basic medium also needs much lower potential.
Pourbaix diagram gives the equilibrium regions of water, hydrogen and oxygen at various electrode potentials.
Water Electrolysis in the Presence of Salts
Salts are 100%dissociate into cations and anions in water and hence increase the ionic concentration for increasing 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 the electrolysis of water.
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 and allow hydrogen ions from water to hydrogen.
Non-reactive anions like nitrate, sulphate ions have 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), has been found to help electrolysis of water in 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. High surface area, larger activation centres are the ability of the catalyst in increasing the reactivity.
The activity of the inert electrode like platinum can be, enhanced by modification of the surface by
i) Increasing the surface area with nanoparticles or alloying with catalytic d-block elements and changing the electronic state coated with other catalytic substance to enhance the electrolysis.
ii) Coating the electrode surface with catalytically active substances, like enzymes.
The electrolytic cell used for the electrolysis of water is the electrolyzer. Depending on the transporter of the electrolyte, electrolyzer can be divided into three types;
Polymer Electrolyte Membrane (PEM) Electrolyzer
A polymer such as Nafion separates the electrodes and allows hydrogen ions formed by the oxidation of water at the anode to pass through it to the cathode compartment for discharge and form hydrogen gas.
Dilute aqueous sodium (or potassium) hydroxide used in the electrolysis provides and movement of hydroxide ions to the anode to form oxygen.
Solid Oxide Electrolyzer
Ceramic oxide separates the electrodes. At the cathode, water is reduced to hydrogen and oxide ions. The oxide ions pass through the ceramic oxide to the anode to become oxygen gas. This is used at high temperatures of 700 to 800°C to reduce the external voltage needed for electrolysis.