H2OME

Home / Uses / How electrolysis splits water into hydrogen

How electrolysis turns water and electricity into hydrogen

Electrolysis is the first half of a hydrogen storage system. It uses electricity to pull water molecules apart, storing energy in the hydrogen it produces.

The basic idea

Water is two hydrogen atoms bonded to one oxygen atom. Pass enough direct current through water between two electrodes and those bonds break. Oxygen bubbles off at one electrode, the anode, and hydrogen at the other, the cathode. The electrical energy you put in does not vanish. Much of it ends up stored chemically in the hydrogen, ready to be released again later in a fuel cell.

What it would keep running

  • Night-rate power into the electrolyser
  • Surplus wind turned into stored hydrogen
  • Overnight micro-hydro flow stored for later
  • De-ionised water as the feedstock
  • Hydrogen sent to the low-pressure buffer

Inside a PEM electrolyser

The system uses a PEM electrolyser rated at 25 kW DC. PEM stands for proton exchange membrane, a thin polymer sheet that lets hydrogen ions pass through but blocks gases and electrons. Water is fed to the anode side, where it splits into oxygen, hydrogen ions and electrons. The ions cross the membrane, the electrons travel round an external circuit, and they meet on the other side as hydrogen gas.

Because the membrane keeps the two gases apart, the hydrogen comes off clean and the oxygen is simply released to the air. Many of these cells are stacked together to reach the rated power, and the stack runs whenever there is cheap night-rate power, surplus wind or spare micro-hydro output to feed it.

How it works →
The H2OME loop: an electrolyser splits water into hydrogen, the hydrogen is stored in a tank for weeks, a fuel cell turns it into electricity for your home, and the water returns to the loop.

Where the energy goes

Electrolysis is not perfectly efficient. Some of the electricity becomes heat in the stack rather than chemical energy in the hydrogen. That is one of the losses behind the system's round-trip efficiency of 30% or less. The rest comes later, in the fuel cell and power electronics. It takes about 9 litres of water to make a kilogram of hydrogen, most of which comes back when the fuel cell runs.

Questions

Does electrolysis make dangerous oxygen levels?

The oxygen produced is vented outside, where it mixes into the open air. Air is already about one fifth oxygen, and the small flow from a home electrolyser disperses quickly. It is handled as part of the system's ventilation design.

Why does it need direct current?

Electrolysis needs current flowing one way so the gases form at fixed electrodes. Grid power is alternating current, so the system converts it to DC before it reaches the stack. That conversion has its own small loss.

Can you use tap water straight in?

No. Tap and rainwater contain dissolved minerals that would foul the membrane and electrodes. The water is filtered and de-ionised first, which is why water treatment is a core part of the system rather than an extra.

More in outages, weather and how it works

Elsewhere

Every use →

Join the pilot list

Pilot installs go to people on this list first. Rural homes, farms and remote sites especially. No spam, no payment.