A membrane for water electrolysis is like a sponge soaked with water. Inside it, hydroxide ions, particles that carry electric charge, must move freely. But exactly how they move has long been a mystery: tracking each ion in an experiment is too difficult.

Scientists built a computer model that follows thousands of atoms over tens of nanoseconds. They found that when water is scarce, it gathers into isolated puddles, and ions get stuck near positively charged groups in the membrane, like pedestrians unable to cross scattered islands of dry land. But add more water, and the puddles merge into a continuous network, giving ions long pathways. Then they carry charge almost as fast as they do in a glass of water.

Now engineers can design membrane compositions in advance to better retain water and give ions more freedom. This will speed up the transition to clean hydrogen fuel.