The Electrolysis of water in Hydrogen Hybrid Cars happens when an electric current passes through the water causing the water (H2O) to break down into its two basic elements, oxygen (O2) and hydrogen gas (H2). It's interesting to note that Industrial applications don't use this method of hydrogen production much since they can do it more affordably by getting it out of fossil fuels.
The way this works in Hydrogen Hybrid Cars is by taking an electrical power source and connecting it to two electrodes, or plates, (or multiples of two) usually made out of some kind of stable metal like aluminum or stainless steel, then putting them into the water. If this cell is made correctly then Hydrogen will form on the cathode (metal plate that's negatively charged), and oxygen will form on the anode (metal plate that's positively charged). Since there's two hydrogen molecules to every one oxygen, molecule in water the amount of hydrogen produced is twice that of oxygen and both are in proportion to the electrical charge that runs through the water.
Seawater is about a million times more conductive to electricity than pure water because of all the electrolytes in it like salt. Many electrolytic cells also may lack these needed electro-catalysts. Electrolysis in Hydrogen Hybrid Cars is far more efficient with the addition of some of these electrolytes such as salt or an acid or a base.
The history of electrolysis dates all the way back to 1789 when Jan Rudolph Deiman and Adriaan Paets van Troostwijk used an electrostatic machine to make electricity and discharged it into gold electrodes placed in a Leyden jar full of water. Then, in 1800, Alessandro Volta invented the voltaic pile, and soon after William Nicholson and Anthony Carlisle used that for the first electrolysis of water. Eventually, in 1869, Zénobe Gramme invented the Gramme machine that became a cheap way to produce hydrogen through the electrolysis of water.
Next, in The Science Behind Hydrogen Hybrid Cars - The Equations of Electrolysis, we'll look at the scientific details.