INTRODUCTION TO WATER ELECTROLYSIS
What Happens at the Electrodes?
Water electrolysis is not one reaction happening
everywhere at once. Oxidation and reduction occur
separately at the two electrodes.
🔗 We now have the basic electrochemical cell
The previous block introduced three important
components:
🟠 ANODE
💧 ELECTROLYTE
🔵 CATHODE
We also connected the electrodes to a DC power supply.
Now we can look more closely at what happens at each electrode.
🟠 ANODE
💧 ELECTROLYTE
🔵 CATHODE
We also connected the electrodes to a DC power supply.
Now we can look more closely at what happens at each electrode.
ONE CELL — TWO ELECTROCHEMICAL PROCESSES
ANODE
OXIDATION
Electrons are released into the
external circuit.
CATHODE
REDUCTION
Electrons are supplied from the
external circuit.
What do oxidation and reduction mean?
Oxidation
Oxidation involves the
loss of electrons.
Species undergoing oxidation release electrons.
Species undergoing oxidation release electrons.
Reduction
Reduction involves the
gain of electrons.
Species undergoing reduction receive electrons.
Species undergoing reduction receive electrons.
OIL RIG:
Oxidation Is Loss —
Reduction Is Gain
Follow the electrons
Watch how oxidation and reduction are linked
through the external electrical circuit.
Ready — press Start.
⚠️ There is not just one set of half reactions
The overall water splitting reaction is the same,
but the species transported through the electrolyte
can differ between water electrolyser technologies.
For example, some electrolysers transport H⁺, while others transport OH⁻ or O²⁻.
Therefore, the individual anode and cathode half reactions depend on the type of electrolyser employed.
We will learn those technology-specific reactions when we introduce the four electrolyser types.
For example, some electrolysers transport H⁺, while others transport OH⁻ or O²⁻.
Therefore, the individual anode and cathode half reactions depend on the type of electrolyser employed.
We will learn those technology-specific reactions when we introduce the four electrolyser types.
For now, remember the universal picture
Regardless of the specific electrolyte chemistry,
oxidation occurs at the anode and reduction occurs
at the cathode.
AT THE ANODE
OXIDATION
• Electrons are released.
• Oxygen is produced on the oxidation side of water electrolysis.
• The anode is connected to the positive terminal of the DC power supply.
• Oxygen is produced on the oxidation side of water electrolysis.
• The anode is connected to the positive terminal of the DC power supply.
AT THE CATHODE
REDUCTION
• Electrons are supplied.
• Hydrogen is produced on the reduction side of water electrolysis.
• The cathode is connected to the negative terminal of the DC power supply.
• Hydrogen is produced on the reduction side of water electrolysis.
• The cathode is connected to the negative terminal of the DC power supply.
Put both electrode processes together
The oxidation and reduction half-reactions describe
what happens separately at the
anode and
cathode.
To obtain the overall water electrolysis reaction, we add the two half-reactions together. Species that appear on both sides of the combined equation cancel.
To obtain the overall water electrolysis reaction, we add the two half-reactions together. Species that appear on both sides of the combined equation cancel.
ANODE — OXIDATION
2H₂O
→
O₂
+
4H⁺
+
4e⁻
Water is oxidised at the anode, producing oxygen,
protons and electrons.
CATHODE — REDUCTION
4H⁺
+
4e⁻
→
2H₂
At the cathode, protons gain electrons and are
reduced to hydrogen gas.
ADD THE TWO HALF-REACTIONS
2H₂O
→
O₂
+
4H⁺
+
4e⁻
+ 4H⁺ + 4e⁻ → 2H₂
+ 4H⁺ + 4e⁻ → 2H₂
The
4H⁺
produced at the anode are consumed at the cathode.
The 4e⁻ produced at the anode are also consumed at the cathode.
Therefore, both H⁺ and e⁻ cancel when the two half-reactions are added.
The 4e⁻ produced at the anode are also consumed at the cathode.
Therefore, both H⁺ and e⁻ cancel when the two half-reactions are added.
4H⁺
cancels
4e⁻
↓
OVERALL WATER ELECTROLYSIS
2H₂O
→
2H₂
+
O₂
The intermediate
H⁺
and
electrons
do not appear in the overall reaction because
they are produced in one half-reaction and
consumed in the other.
Electrical energy is required to drive
the non-spontaneous reaction.