Electrolysis

Consider two electrodes in water with a voltage difference between them. There is an electric field in the water between them. **Hydrogen ions** get pulled one way, **OH- ions** get pulled the other. The electrode where the **H+ ions** gather has a **negative voltage** (cathode) and too many **electrons**. **Electrons** can leave the electrode and join up with a pair of **H+ ions** to make the **hydrogen molecule**.



The reaction over where the **OH- ions** gather is caused by donating electrons to the **positive electrode** which is the **anode**. Electrons can form **water molecules** and **O₂ molecules**. This process doesn't use up the ions, because more **H₂O** water molecules keep falling apart to make up more **H+** and **OH- ions**.



When there's no voltage, the process is also happening but it's balanced by **ions** recombining to make water molecules.



The **electrolysis** of one mole of **water** produces a mole of **hydrogen gas** and a half-mole of **oxygen gas** in their normal diatomic forms. 

Water electrolysis is used to **generate oxygen** for example for the **International Space Station**. Hydrogen may later be used in a fuel cell as a storage of energy and water.