Cathode (–)
H₂
Hydrogen bubbles off here
Membrane
Anode (+)
O₂
Oxygen bubbles off here
Chapter 1
How electrolysis works
Electrolysis uses electricity to break water into its two elements. Give it clean power and you get clean hydrogen fuel — plus pure oxygen as a by-product.
The core reaction
Pass a direct electric current through water and the water molecules split apart. Two water molecules yield two molecules of hydrogen gas and one of oxygen gas:
2 H₂O + electricity → 2 H₂ + O₂
Water + energy → hydrogen gas + oxygen gas
The only inputs are water and electricity; the only outputs are hydrogen and oxygen. When the electricity comes from solar, wind, or hydro, no carbon dioxide is released in the process.
Inside an electrolyzer
An electrolyzer is a stack of cells. Each cell has two electrodes sitting in (or against) an electrolyte, separated by a membrane or diaphragm that lets charge through but keeps the two gases apart.
A power supply pushes electrons onto the cathode (negative side) and pulls them from the anode (positive side).
At the cathode, water molecules gain electrons and release hydrogen gas (H₂).
At the anode, water gives up electrons and releases oxygen gas (O₂).
The membrane keeps the two gases separated for safety and purity while letting ions carry the charge across.
The specific electrolyte and membrane are what separate the main electrolyzer technologies — AEM, PEM, and SOEC.
How much energy per kilogram?
The energy stored in hydrogen sets a hard floor on how much electricity you must put in. In theory, splitting water needs roughly 39 kWh of energy per kilogram of hydrogen. No real machine reaches that ideal — some energy always turns into heat.
In practice, commercial systems typically use somewhere around 50–55 kWh of electricity per kilogram of hydrogen, giving system efficiencies broadly in the 70–80% range. Because the electricity bill dominates the cost, even a few percentage points of efficiency matter a great deal — a theme we return to in the economics chapter.
A useful anchor: one kilogram of hydrogen holds roughly the usable energy of a gallon of gasoline, and making it takes on the order of 50 kWh of electricity — about what a typical U.S. home uses in a day and a half.
Don't forget the water
Splitting water obviously consumes water — about 9 kilograms of water per kilogram of hydrogen by chemistry alone, and more once you account for purification, since electrolyzers need clean, deionized feedwater. In the arid western United States, water sourcing is a real planning constraint, not an afterthought.