Electrolysis uses electricity to split water into hydrogen and oxygen. Two electrodes sit in water with an electrolyte. Current flows between them. Hydrogen bubbles form at the cathode. Oxygen bubbles form at the anode. The gases are collected separately.

The process is clean if the electricity is clean. A wind-powered electrolyzer produces hydrogen with almost no emissions. A coal-powered one produces hydrogen with a large footprint. That is why the term "green hydrogen" matters. It refers to hydrogen made with renewable electricity.

Electrolysis has been used industrially for over a century, mainly for chlorine and aluminum. Hydrogen production is now the fastest-growing use. Costs are falling as electrolyzer manufacturing scales up. Efficiency ranges from 60 to 80 percent, depending on the technology. Alkaline and PEM electrolyzers dominate today. Solid oxide designs promise higher efficiency at higher temperatures.

Key types

• Alkaline electrolysis
• PEM electrolysis
• Solid oxide electrolysis
• Anion exchange membrane
• Green vs. gray hydrogen