Fact sheet n°2 : Producing hydrogen using an electrolyser
As of today, hydrogen is mostly produced from natural gas which leads to tremendous CO2 emissions. Another way of producing hydrogen is coming into being, that is electrolysis. A process through which hydrogen can be produced from decarbonised or renewable electricity. But…
WHAT IS
ELECTROLYSIS?
The principle of electrolysis is quite simple. A direct current is injected into an electrolytic solution, to separate the water molecules into hydrogen and oxygen.
LET’S SEE HOW AN ALKALINE ELECTROLYSER WORKS
An alkaline electrolyser cell consists of two electrodes immersed in a bath of electrolytic solution. Strong direct currents are injected, and that’s it!
Hydrogen will appear at the cathode and oxygen will appear at the anode.
The electrolyser is made up of hundreds of cells stacked one on top of the other, and the whole thing is commonly known as a stack.
HOWEVER ALKALINE ELECTROLYSIS IS NOT
THE ONLY TECHNOLOGY USED.
THE MAJOR FAMILIES ARE:
PEM
Proton Exchange Membrane electrolysers
Alkaline
Alkaline electrolysers
AEM
Anion Exchange Membrane electrolysers
SOEC
Solid Oxide Electrolyser Cell electrolysers
Operating temperature - efficiency - maturity - deployment:
The general principle is the same: breaking up water molecules with electricity using different techniques:
PEM
The electrolyte solution is replaced by a solid polymer membrane.
The protons (H+) pass through it and form H2 at the cathode.
Alkaline
These are the electrolysers described above.
The electrolyte is an aqueous liquid solution. The hydroxide ions (OH-) flow through the membrane to form O2 at the anode.
AEM
Unlike PEM electrolysers, here it is the anions (H-) that pass through and form the H2 at the anode.
SOEC
Here the electrolyte is in the form of a solid ceramic.
At the cathode, the hydrogen is separated from the oxygen ions (O2–), which pass through the ceramic membrane and form O2 at the anode.
THE MAIN DIFFERENCES:
Each technique has its advantages and disadvantages.
PEM:
- Advantages: compact size, high current density, rapid production time
- Disadvantages: Use of Rare materials, high production cost, sensitive to bivalent ions (calcium and iron)
Alkaline:
- Advantages: reliability, low production costs, not sensitive to bivalent ions
- Disadvantages: low efficiency, operation at low current density, bulky, long start-up time
AEM:
- Advantages: compact size, high current density, rapid production time, reliability, low production costs, not sensitive to bivalent ions
- Disadvantages: less mature than other conventional techniques
SOEC:
- Advantages: high efficiency
- Disadvantages: requires high temperatures, low maturity level
