Electrolysis: Understanding Key Concepts and Operating Modes
Electrolysis
Electrolysis applies to a reaction which causes a chemical change when an electric current passes through an electrolyte solution. Oxidation (loss of electrons) takes place at the anode, and reduction (gain of electrons) occurs at the cathode.
Electrode Potential
When a conductor (electrode) is placed in contact with an electrolytic solution, a spontaneous electrical potential difference occurs at the electrode-solution interface. This difference is the electrode potential and is measured in volts.
Voltage
Voltage is the potential difference in volts between the two electrode ends. It is the sum of several contributions:
- Equilibrium potential of the anode
- Cathode equilibrium potential
- Overvoltage
- Electrolyte ohmic drop
- Ohmic drop in the membranes
Intensity
Intensity is the flow of electrons flowing through the electrical circuit in the unit of time. It is measured in amps.
Current Density
Current density affects the speed of processing, selectivity, current efficiency, specific energy consumption, and the rate of corrosion of an electrode.
Electrical Resistance
Electrical resistance is the opposition that every conductor has to the process of passing electric current through it. It is measured in ohms.
Resistivity
Resistivity is the representation of the specific resistance of a material of 1 cm long and 1 cm2 cross-section. It is a function of temperature and the nature of the material.
Conductivity
Conductivity is the inverse of resistivity. It is measured in mhos cm-1.
Define Overpotential and its Three Types
Activation Overpotential
Represents the excess of potential to be applied with respect to equilibrium so that the electron energy exceeds the barrier property of each reaction.
Concentration Overpotential or Polarization
Arises from the reduction of electroactive species at the electrode-solution interface in the course of an electrochemical reaction.
Resistance Overpotential
Depends on the current density and the ohmic drop through the electrolyte, diaphragms, electrical connections, etc.
Operating Modes in Electrochemistry
How to Apply the Power
Constant Electrode Potential
This takes three electrodes: working electrode, counter electrode, and reference electrode. Voltmeter 1 measures the voltage of the cell, and voltmeter 2 measures the cathode potential. Both are incorporated into a potentiostat. The electrosynthesis speed and productivity are proportional to the current flowing through the cell (according to Faraday), and this, in turn, is proportional to the concentration of the electroactive species (operating below the limiting current density). In the process, the concentration of species declines over time at constant operating potential. If the current flowing through the cell declines, then so does the productivity.
Constant Intensity
As the reagent decreases, the electrode potential shifts to higher values in order to maintain the reaction rate imposed by the intensity. This can lead us to parallel electrochemical reactions: conversion of the reagent or product forms, reducing unwanted efficiency and selectivity.
The Way of Feeding the Reaction Medium to the Cell
Batch
Reagents, solvent, and electrolyte are added at the beginning of the process in a tank equipped with two electrodes. A potential difference is applied between them without adding reagents or withdrawing products, while the electrolyte is agitated. Once the operation is finalized, the electrolyte is removed, and the products and reagents are separated to restart the process.
Continuous
The reaction medium is continuously fed and removed at a given flow so that the composition inside the cell is constant once stationary phase conditions are achieved. The current density, potential, and concentration are maintained constant over time.