Electrolysis of Hydrochloric Acid: Faradaic and Non-Faradaic Currents

Electrolysis of Hydrochloric Acid: Faradaic and Non-Faradaic Currents

Consider the electrolysis of hydrochloric acid using platinum electrodes. Initially when the applied potential is less than the cell Emf no electrolysis takes place and hence there will be no hydrogen or chlorine gas in contact with the respective electrodes. The electrodes will not be in equilibrium with the solution.

When an external Emf  is applied across the electrodes, hydrogen ions move towards the cathode and are discharged (reduced) to form hydrogen gas.

H⁺_(aq)+e ------------->1/2H_2(g)

Similarly chloride ions move towards the anode are discharged (oxidized) to form chlorine gas.

Cl^-_(aq) ----> 1/2Cl_2(g) + e

As soon as traces of these gases appear or are in contact with their respective electrodes the cell behaves as a galvanic (chemical) cell with an Enif which is in opposition to the applied Emf This

Emf known as the back Emf, will be given by equation (1) rewritten as;

E = E⁰_Cl-/Cl2 +                                                                           ( 5)

Eqn ( 5) shows that if the activities of H⁺ and Cl^- remain constant, the back Emf will increase as the partial pressures of the gases produced increase. When enough H₂ gas and Cl₂ gas that are produced during electrolysis have accumulated at the electrodes to produce a back Emf equal to the applied Emf, electrolysis should stop. Ideally, there should only be a transient current when a small Emf is applied to the cell. This transient current should consist partly of the current required to discharge the necessary amounts of products at the electrodes and partly of the current required to form the electrical double layer. The current responsible for electrolysis is known as faradaic current as it results in electrochemical action. The current that is required to form the double-layer is referred to as nonfaradaic current as no electrical change takes place in the cell. Non-faradaic current is usually a small current compared to faradaic currentl.