Splitting water means breaking each water molecule into its two components: hydrogen gas and oxygen gas. The most accessible way to do this is electrolysis, which uses electricity to force the split. At minimum, you need 1.23 volts of electrical energy to start the reaction, though real-world setups require closer to 1.8 to 2.0 volts because of energy losses at the electrodes. You can do this at home with a 9-volt battery and a few hardware store supplies, or at industrial scale with specialized equipment designed to produce hydrogen fuel.
What Happens When Water Splits
A water molecule is two hydrogen atoms bonded to one oxygen atom. Those bonds are stable, so energy has to be added to break them apart. In electrolysis, that energy comes from electricity flowing through two electrodes submerged in water. At one electrode (the cathode), water molecules pick up electrons and release hydrogen gas. At the other electrode (the anode), water molecules give up electrons and release oxygen gas.
The result is always a 2:1 ratio: two volumes of hydrogen for every one volume of oxygen. This matches the formula of water itself, H₂O. If you collect the gases in separate tubes, you can see this ratio directly, with the hydrogen tube filling twice as fast.
How to Split Water at Home
A simple electrolysis setup requires just a few items: two stainless steel screws (at least 1.5 inches long), a 9-volt battery, a glass of water, and a small amount of Epsom salts. Pure water conducts electricity poorly, so the Epsom salts act as an electrolyte, allowing current to flow between the electrodes. Dissolve about a teaspoon into a cup of water, submerge the screws, and connect each one to a battery terminal.
Within seconds, you’ll see tiny bubbles forming on both screws. The screw connected to the negative terminal produces hydrogen, and the one on the positive terminal produces oxygen. The hydrogen side will produce noticeably more bubbles.
A critical safety note: do not substitute table salt for Epsom salts. Table salt (sodium chloride) can produce chlorine gas at the anode, which is toxic even in small amounts. Epsom salts (magnesium sulfate) and baking soda (sodium bicarbonate) are both safe alternatives that dissolve easily and improve conductivity.
Collecting the Gas
To actually capture and measure the hydrogen and oxygen separately, you can use an inverted water-filled test tube or jar over each electrode. As gas bubbles rise, they displace the water in the tube and collect at the top. A laboratory device called a Hoffmann voltameter does this precisely with graduated tubes, letting you read the exact volume of each gas and confirm the 2:1 ratio. For home experiments, any clear container that can be inverted over the electrode works. Use platinum or stainless steel electrodes if possible. Carbon electrodes can partially convert the oxygen to carbon dioxide, which dissolves back into the water and throws off your measurements.
Why It Takes More Energy Than Theory Predicts
The theoretical minimum voltage to split water is 1.23 volts at room temperature. In practice, you always need more. The oxygen-producing reaction at the anode is particularly sluggish, requiring an extra 0.5 volts or more beyond the theoretical minimum before the process runs at any useful speed. This extra energy, called overpotential, is essentially wasted as heat. It’s the main reason electrolysis systems focus so heavily on electrode materials and catalyst design: better catalysts lower the overpotential and make the process more efficient.
Industrial systems currently use platinum and iridium as catalysts because they minimize this energy penalty, but both metals are rare and expensive. Researchers are developing alternatives based on abundant materials like iron, cobalt, and nickel compounds. Selenium-doped iron oxyhydroxide, for instance, shows strong performance for the oxygen side of the reaction, and cobalt phosphosulfide works well for the hydrogen side. These substitutions could eventually bring down the cost of large-scale water splitting significantly.
Industrial Electrolysis Technologies
Two main types of electrolyzers dominate commercial hydrogen production. Alkaline electrolyzers are the older, more established technology. They use a liquid alkaline solution (typically potassium hydroxide) as the electrolyte and operate at low pressure. Their energy efficiency sits around 70%, meaning roughly 70% of the electrical energy input ends up stored in the hydrogen gas, with the rest lost as heat. They’re relatively cheap to build and have long track records in industry.
Proton exchange membrane (PEM) electrolyzers use a solid polymer membrane instead of a liquid electrolyte. They achieve high voltage efficiency and can ramp up and down quickly, which makes them well suited to pairing with intermittent renewable energy sources like wind and solar. The tradeoff is higher cost, partly because PEM systems rely more heavily on precious metal catalysts.
Green hydrogen, produced entirely from renewable electricity, currently costs between $3.50 and $6.00 per kilogram. That makes it the most expensive form of hydrogen production today, but costs are falling as renewable electricity gets cheaper and electrolyzer technology improves. Government incentives are accelerating this: the U.S. Inflation Reduction Act, for example, offers tax credits of up to $3.00 per kilogram of green hydrogen.
Splitting Water With Heat
Electrolysis isn’t the only option. Thermochemical water splitting uses extremely high temperatures to drive chemical cycles that break water apart without direct electricity. The most studied version is the sulfur-iodine cycle, which involves a series of chemical reactions using sulfuric acid and hydrogen iodide. The reactions ultimately consume only water and heat, recycling all the other chemicals in a closed loop. The catch is that the temperatures involved (above 800°C for the sulfuric acid decomposition step) typically require concentrated solar power or nuclear reactor heat.
Thermochemical cycles remain mostly experimental, but they’re attractive because they could potentially achieve higher overall efficiencies than electrolysis by using heat energy directly rather than converting it to electricity first.
Splitting Water With Sunlight
Photocatalytic water splitting skips the electricity step entirely. A semiconductor material absorbs sunlight and uses that energy to drive the water-splitting reaction on its surface. Think of it as a solar panel and an electrolyzer merged into one material. The most promising recent results come from an indium gallium nitride photocatalyst, which achieved a solar-to-hydrogen efficiency of 9.2% using pure water. That same system managed about 7% efficiency with ordinary tap water and seawater, and 6.2% in a larger-scale outdoor system running on natural sunlight with a capacity of 257 watts.
These numbers are still far below what electrolyzers achieve when paired with solar panels, but photocatalytic systems could eventually be simpler and cheaper because they eliminate the need for separate electrical equipment.
Safety With Hydrogen Gas
Hydrogen is flammable across an unusually wide range of concentrations in air, from 4% to 75% by volume. That means even a small accumulation in an enclosed space can become ignitable. For a home experiment producing tiny amounts of bubbles, the risk is minimal as long as you work in a ventilated area and don’t try to collect large volumes. Never use a flame to test for hydrogen near your electrolysis setup while it’s still running, since the oxygen being produced simultaneously creates an especially reactive mixture.
At industrial scale, facilities use inert gases like nitrogen and carbon dioxide to narrow the flammable range of hydrogen during storage and transport. Proper ventilation is the single most important safety measure at any scale, because hydrogen is colorless and odorless, making leaks impossible to detect without sensors.