INTRODUCTION TO WATER ELECTROLYSIS
What is Water Electrolysis?
We now apply the idea of electrolysis specifically
to water.
🔗 From electrolysis to water electrolysis
In the previous block, a generic substance was
represented as AB, and electrolysis was
introduced as the electrically driven splitting
of AB into A and B.
Now we consider a specific substance: water, H₂O.
When electrical energy is used to drive the electrochemical splitting of water, the process is called water electrolysis.
Now we consider a specific substance: water, H₂O.
When electrical energy is used to drive the electrochemical splitting of water, the process is called water electrolysis.
💡 Definition
Water electrolysis is the use of
electrical energy to drive the
electrochemical splitting of
water into
hydrogen
and
oxygen.
H₂O
+
Electrical Energy ⚡
→
H₂
+
O₂
Electrical energy is the
energy input that drives the process.
It is not a chemical reactant and does not become
part of the hydrogen or oxygen molecules.
The overall water splitting reaction
2H₂O
→
2H₂
+
O₂
Two water molecules contain a total of
four hydrogen atoms and
two oxygen atoms.
During electrolysis, these atoms are rearranged to form two hydrogen molecules and one oxygen molecule.
The atoms are not created or destroyed. They are rearranged into new molecules.
During electrolysis, these atoms are rearranged to form two hydrogen molecules and one oxygen molecule.
The atoms are not created or destroyed. They are rearranged into new molecules.
▶ Watch Water Molecules Split
Press Start Water Electrolysis.
Follow the water into the reaction region and
then watch bonded H—H and O—O molecules leave
through separate product pathways.
Ready — press Start.
What is actually happening?
Water contains hydrogen atoms and oxygen atoms.
During water electrolysis, electrical energy drives electrochemical reactions that rearrange those atoms.
The hydrogen atoms ultimately form H₂ molecules , represented visually as H—H.
The oxygen atoms ultimately form O₂ molecules , represented visually as O—O.
The two gaseous products then leave through separate product pathways.
This separation is important because hydrogen and oxygen are distinct products and should be collected separately.
During water electrolysis, electrical energy drives electrochemical reactions that rearrange those atoms.
The hydrogen atoms ultimately form H₂ molecules , represented visually as H—H.
The oxygen atoms ultimately form O₂ molecules , represented visually as O—O.
The two gaseous products then leave through separate product pathways.
This separation is important because hydrogen and oxygen are distinct products and should be collected separately.
Two products — two separate pathways
Water electrolysis produces
two different gases.
In the animation, the pathways are deliberately
separated so that you can follow each product
independently.
🔵 H₂ pathway
Hydrogen atoms originating from water
combine to form
H—H (H₂).
The hydrogen then leaves through the blue product pathway .
The hydrogen then leaves through the blue product pathway .
🟠 O₂ pathway
Oxygen atoms originating from water
combine to form
O—O (O₂).
The oxygen then leaves through the orange product pathway .
The oxygen then leaves through the orange product pathway .
For now:
focus simply on the idea that water is converted
into two separate gaseous products.
Later blocks will explain exactly
where each gas forms inside an
electrolyser.
🧩 Follow the atoms
Consider two water molecules:
START
2H₂O
4 hydrogen atoms
2 oxygen atoms
2 oxygen atoms
ELECTRICAL ENERGY
⚡
drives rearrangement
PRODUCTS
2H₂
+
O₂
4 hydrogen atoms
2 oxygen atoms
2 oxygen atoms
Nothing disappears.
The same hydrogen and oxygen atoms that were
initially present in water appear in the final
H₂ and O₂ molecules.
Why do we write H₂ and O₂?
The gaseous products of water electrolysis are
molecular hydrogen and molecular oxygen.
Hydrogen gas is written as H₂ because each hydrogen molecule contains two bonded hydrogen atoms.
Oxygen gas is written as O₂ because each oxygen molecule contains two bonded oxygen atoms.
Hydrogen gas is written as H₂ because each hydrogen molecule contains two bonded hydrogen atoms.
Oxygen gas is written as O₂ because each oxygen molecule contains two bonded oxygen atoms.
H — H
H₂
O — O
O₂
The reaction also gives us a ratio
From the balanced equation:
2H₂O
→
2H₂
+
O₂
Two moles of water produce
two moles of hydrogen
and
one mole of oxygen.
Therefore:
Therefore:
H₂
:
O₂
=
2
:
1
At the same temperature and pressure, this
molar relationship also corresponds to a
2:1 gas-volume relationship.
Why do we sometimes write ½O₂?
The balanced reaction can also be written on a
one-mole-of-water basis:
H₂O
→
H₂
+
½O₂
This does not mean that half an oxygen
molecule physically exists as the product.
The coefficient ½ is a stoichiometric amount. It means that for every one mole of water converted, the corresponding amount of oxygen produced is half a mole of O₂.
Multiplying the entire equation by two removes the fractional coefficient:
The coefficient ½ is a stoichiometric amount. It means that for every one mole of water converted, the corresponding amount of oxygen produced is half a mole of O₂.
Multiplying the entire equation by two removes the fractional coefficient:
2H₂O → 2H₂ + O₂
⚡ Where does electricity fit into the equation?
It is useful to distinguish between
matter and energy.
Water is the chemical feed. Hydrogen and oxygen are chemical products. Electrical energy is the energy input that drives the conversion.
Therefore, electricity is not treated as though it were another chemical molecule in the balanced material equation:
Water is the chemical feed. Hydrogen and oxygen are chemical products. Electrical energy is the energy input that drives the conversion.
Therefore, electricity is not treated as though it were another chemical molecule in the balanced material equation:
2H₂O → 2H₂ + O₂
Conceptually, however, we should always remember:
Water
+
Electrical Energy ⚡
→
Hydrogen
+
Oxygen
⚖️ What does this mean in terms of mass?
The reaction stoichiometry can also be converted
into a mass relationship.
Using approximate molar masses:
Using approximate molar masses:
WATER
H₂O
≈ 18 g mol⁻¹
HYDROGEN
H₂
≈ 2 g mol⁻¹
OXYGEN
O₂
≈ 32 g mol⁻¹
For the reaction:
2H₂O → 2H₂ + O₂
approximately:
36 g H₂O
→
4 g H₂
+
32 g O₂
Scale that relationship up
Dividing the previous mass relationship by four
gives:
9 kg H₂O
→
1 kg H₂
+
8 kg O₂
So, from ideal reaction stoichiometry,
producing 1 kg of hydrogen requires
approximately 9 kg of water to be chemically
converted and simultaneously produces
approximately 8 kg of oxygen.
This is a useful theoretical relationship. Real electrolyser systems can require additional water for purposes such as purification, circulation, cooling, or other balance of plant needs, depending on the system design.
This is a useful theoretical relationship. Real electrolyser systems can require additional water for purposes such as purification, circulation, cooling, or other balance of plant needs, depending on the system design.
Visualise the mass balance
💧
9 kg
H₂O
→
1 kg H₂
Hydrogen product pathway
8 kg O₂
Oxygen product pathway
Mass is conserved:
9 kg in
=
1 kg + 8 kg out
Important: reaction water is not always total plant water
The value of approximately
9 kg H₂O per kg H₂
comes directly from the chemical reaction.
It tells us the amount of water that must be chemically converted.
A complete hydrogen production facility may use more water than this because water can also be required elsewhere in the system.
Later in ElectrolyserPy, it will therefore be important to distinguish between:
It tells us the amount of water that must be chemically converted.
A complete hydrogen production facility may use more water than this because water can also be required elsewhere in the system.
Later in ElectrolyserPy, it will therefore be important to distinguish between:
- stoichiometric water consumption
- total system water demand
🎯 Quick Check
Check whether the key ideas of water electrolysis
are clear before moving on.
✓ Block 2 Takeaway
Water electrolysis uses
electrical energy to drive the
electrochemical splitting of water into
hydrogen and oxygen.
The overall reaction is:
Hydrogen is produced as bonded H—H molecules (H₂), while oxygen is produced as bonded O—O molecules (O₂).
After formation, the two gaseous products can follow separate pathways:
🔵 H₂ follows the hydrogen product pathway.
🟠 O₂ follows the oxygen product pathway.
The balanced reaction also gives:
The overall reaction is:
2H₂O
→
2H₂
+
O₂
The hydrogen and oxygen atoms in the products
originate from the water.
Electrical energy provides the energy required
to drive the conversion; it is
not a chemical reactant.
Hydrogen is produced as bonded H—H molecules (H₂), while oxygen is produced as bonded O—O molecules (O₂).
After formation, the two gaseous products can follow separate pathways:
🔵 H₂ follows the hydrogen product pathway.
🟠 O₂ follows the oxygen product pathway.
The balanced reaction also gives:
MOLAR / GAS RATIO
H₂
:
O₂
= 2 : 1
IDEAL MASS BASIS
9 kg H₂O
→
1 kg H₂
+
8 kg O₂
Next question:
We now know what water electrolysis does: water is converted into H₂ and O₂, and the two gaseous products can leave through separate pathways.
But what physical device actually carries out this electrochemical conversion?
That brings us to: What is an electrolyser?
We now know what water electrolysis does: water is converted into H₂ and O₂, and the two gaseous products can leave through separate pathways.
But what physical device actually carries out this electrochemical conversion?
That brings us to: What is an electrolyser?