INTRODUCTION TO WATER ELECTROLYSIS
What is an Electrolyser?
We know that electrical energy can drive water
electrolysis. Now let us look at the physical
device in which the electrochemical process occurs.
🔗 From the reaction to the device
In the previous block, we described water
electrolysis using the overall reaction:
It takes place inside an electrolyser.
H₂O
+
Electrical Energy
→
H₂
+
½O₂
But where does this electrochemical conversion
actually take place?
It takes place inside an electrolyser.
DEFINITION
An
electrolyser
is an electrochemical device that uses
electrical energy
to drive a chemical reaction.
In water electrolysis, the electrolyser enables water to be converted electrochemically into hydrogen and oxygen.
In water electrolysis, the electrolyser enables water to be converted electrochemically into hydrogen and oxygen.
Let us build a simple electrolyser cell
We will start with a simple electrochemical cell
containing an electrolyte and two
electrodes.
An electrolyte is an ionically conducting medium that contains mobile ions and allows electrical charge to be transported through the electrochemical cell by the movement of those ions.
An electrode is an electronically conducting part of an electrochemical cell through which electrons enter or leave the electrochemical reaction region.
Electrochemical reactions occur at the electrode–electrolyte interface, where the electronically conducting electrode is in contact with the ionically conducting electrolyte.
In simple terms:
The two electrodes have different names according to the reaction occurring at them: anode and cathode.
An electrolyte is an ionically conducting medium that contains mobile ions and allows electrical charge to be transported through the electrochemical cell by the movement of those ions.
An electrode is an electronically conducting part of an electrochemical cell through which electrons enter or leave the electrochemical reaction region.
Electrochemical reactions occur at the electrode–electrolyte interface, where the electronically conducting electrode is in contact with the ionically conducting electrolyte.
In simple terms:
- Electrodes conduct electrons.
- The electrolyte conducts ions.
- The electrode–electrolyte interface is where electrochemical reactions occur.
The two electrodes have different names according to the reaction occurring at them: anode and cathode.
What are the important parts of an electrolyser cell?
Before looking at the reactions in more detail,
let us define the main parts that make
electrolysis possible.
⚙️ ELECTRODE
An electrode is an
electronically conducting part of an
electrochemical cell through which
electrons enter or leave the
electrochemical reaction region.
Its surface provides an interface at which electrochemical charge transfer can occur.
An electrolyser cell contains two electrodes: anode and cathode .
Its surface provides an interface at which electrochemical charge transfer can occur.
An electrolyser cell contains two electrodes: anode and cathode .
💧 ELECTROLYTE
An electrolyte is the
ionically conducting medium of an
electrochemical cell.
It allows ions to transport electrical charge between reaction regions inside the cell.
The nature of the electrolyte and the charge-carrying ion depend on the electrolyser technology.
It allows ions to transport electrical charge between reaction regions inside the cell.
The nature of the electrolyte and the charge-carrying ion depend on the electrolyser technology.
⚡ DC POWER SUPPLY
The DC power supply provides
the electrical energy required to
drive water electrolysis.
It establishes the electrical conditions required to drive oxidation at the anode and reduction at the cathode.
It establishes the electrical conditions required to drive oxidation at the anode and reduction at the cathode.
🔌 EXTERNAL CIRCUIT
The external circuit consists
of electronically conducting
connections between the electrodes
and the power supply.
Electrons travel through this electronic pathway.
They do not travel through the electrolyte as the ionic charge carrier.
Electrons travel through this electronic pathway.
They do not travel through the electrolyte as the ionic charge carrier.
⇄ IONS
Ions are electrically charged
species.
Their movement through the electrolyte provides the ionic conduction pathway required for charge transport through the cell.
Which ion carries charge, and its direction of transport, depends on the electrolyser technology.
Their movement through the electrolyte provides the ionic conduction pathway required for charge transport through the cell.
Which ion carries charge, and its direction of transport, depends on the electrolyser technology.
↔ ELECTRODE–ELECTROLYTE INTERFACE
The
electrode–electrolyte interface
is the region where an electronically
conducting electrode meets the
ionically conducting medium.
It is important because electrochemical charge transfer reactions occur at or near these electrode surfaces.
It is important because electrochemical charge transfer reactions occur at or near these electrode surfaces.
The key distinction:
⚙️ Electrodes and wires conduct electrons.
💧 The electrolyte conducts ions.
Electrolysis requires these electronic and ionic conduction pathways to work together.
⚙️ Electrodes and wires conduct electrons.
💧 The electrolyte conducts ions.
Electrolysis requires these electronic and ionic conduction pathways to work together.
Why are the electrodes in contact with
the electrolyte?
Electrochemical reactions occur at
electrode–electrolyte interfaces.
The electrodes provide electronically conducting pathways, while the electrolyte provides an ionically conducting pathway.
At the interface, electronic charge in the electrode and ionic species in the electrolyte can participate in electrochemical charge transfer reactions.
In this simple liquid-electrolyte representation, the active portions of both the anode and cathode are therefore immersed in the electrolyte.
The electrodes provide electronically conducting pathways, while the electrolyte provides an ionically conducting pathway.
At the interface, electronic charge in the electrode and ionic species in the electrolyte can participate in electrochemical charge transfer reactions.
In this simple liquid-electrolyte representation, the active portions of both the anode and cathode are therefore immersed in the electrolyte.
ANODE
Electronic conductor
Oxidation occurs here
Electronic conductor
Oxidation occurs here
ELECTROLYTE
Ionic conductor
Transports ionic charge
Ionic conductor
Transports ionic charge
CATHODE
Electronic conductor
Reduction occurs here
Electronic conductor
Reduction occurs here
The two electrodes have names
Both the anode and cathode are
electrodes, meaning they are
electronically conducting parts of the
electrochemical cell.
They are named according to the electrochemical process occurring at them: oxidation at the anode and reduction at the cathode.
They are named according to the electrochemical process occurring at them: oxidation at the anode and reduction at the cathode.
A
ANODE
Oxidation electrode
The anode is the electrode at
which oxidation occurs.
Oxidation involves the loss of electrons.
In water electrolysis, oxygen is generated on the oxidation side of the cell.
In an electrolytic cell, the anode is connected to the positive terminal of the external power supply.
Oxidation involves the loss of electrons.
In water electrolysis, oxygen is generated on the oxidation side of the cell.
In an electrolytic cell, the anode is connected to the positive terminal of the external power supply.
C
CATHODE
Reduction electrode
The cathode is the electrode at
which reduction occurs.
Reduction involves the gain of electrons.
In water electrolysis, hydrogen is generated on the reduction side of the cell.
In an electrolytic cell, the cathode is connected to the negative terminal of the external power supply.
Reduction involves the gain of electrons.
In water electrolysis, hydrogen is generated on the reduction side of the cell.
In an electrolytic cell, the cathode is connected to the negative terminal of the external power supply.
🧠 Two names worth remembering
AN OX
ANode → OXidation
RED CAT
REDuction → CAThode
Two pathways carry charge
Electrochemical systems require both
electronic and ionic
conduction.
e⁻
Electronic conduction
Electrons move through
electronically conducting materials.
These include the electrodes, electrical contacts and external wires.
These include the electrodes, electrical contacts and external wires.
⊕ / ⊖
Ionic conduction
Ions carry charge through the
ionically conducting medium.
The ion that carries charge depends on the electrolyser technology.
Examples introduced later include H⁺, OH⁻ and O²⁻.
The ion that carries charge depends on the electrolyser technology.
Examples introduced later include H⁺, OH⁻ and O²⁻.
⚠️ This is a generic introductory cell
The diagram above is deliberately simplified.
Different electrolyser technologies use different electrolytes, separators or membranes, electrode structures, operating temperatures and charge-carrying ions.
Therefore, the liquid-electrolyte arrangement shown here should be understood as an introductory representation, not as the exact physical architecture of every electrolyser.
Those differences will become important when we examine the individual electrolyser technologies.
Different electrolyser technologies use different electrolytes, separators or membranes, electrode structures, operating temperatures and charge-carrying ions.
Therefore, the liquid-electrolyte arrangement shown here should be understood as an introductory representation, not as the exact physical architecture of every electrolyser.
Those differences will become important when we examine the individual electrolyser technologies.
🎯 Quick Check — Know the Cell
Choose the correct definition or function
for each component.
✓ Block 3 Takeaway
An electrolyser is an electrochemical
device that uses electrical energy to drive
a chemical reaction.
⚙️ ELECTRODE — an electronically conducting part of the cell through which electrons enter or leave the electrochemical reaction region.
🟠 ANODE — the electrode at which oxidation occurs.
🔵 CATHODE — the electrode at which reduction occurs.
💧 ELECTROLYTE — the ionically conducting medium through which ions carry charge.
↔ ELECTRODE–ELECTROLYTE INTERFACE — the region where an electronically conducting electrode meets the ionically conducting medium and electrochemical charge transfer occurs.
⚡ DC POWER SUPPLY — provides the electrical energy that drives the electrolysis process.
🔌 EXTERNAL CIRCUIT — connects the electrodes to the power supply and provides an electronic conduction pathway for electrons.
⇄ IONS — carry charge through the electrolyte, while electrons move through electronically conducting parts of the cell and external circuit.
The central picture is therefore:
The next step is to understand what actually happens at the anode and cathode during water electrolysis.
⚙️ ELECTRODE — an electronically conducting part of the cell through which electrons enter or leave the electrochemical reaction region.
🟠 ANODE — the electrode at which oxidation occurs.
🔵 CATHODE — the electrode at which reduction occurs.
💧 ELECTROLYTE — the ionically conducting medium through which ions carry charge.
↔ ELECTRODE–ELECTROLYTE INTERFACE — the region where an electronically conducting electrode meets the ionically conducting medium and electrochemical charge transfer occurs.
⚡ DC POWER SUPPLY — provides the electrical energy that drives the electrolysis process.
🔌 EXTERNAL CIRCUIT — connects the electrodes to the power supply and provides an electronic conduction pathway for electrons.
⇄ IONS — carry charge through the electrolyte, while electrons move through electronically conducting parts of the cell and external circuit.
The central picture is therefore:
⚡ Electrical Energy
↓
🟠 Anode + 💧 Electrolyte + 🔵 Cathode
↓
H₂ + O₂
We now understand the basic physical parts of
an electrolyser cell.
↓
🟠 Anode + 💧 Electrolyte + 🔵 Cathode
↓
H₂ + O₂
The next step is to understand what actually happens at the anode and cathode during water electrolysis.