Water to Hydrogen Generator installed in cars usually uses electrolysis as a method to split water into hydrogen and oxygen and/or in a form of hydroxy gas.
Here's an image of a simple concept of Electrolysis using pencils and metal plates.
The negatively charged pencil is where the Cathode is this is also where pure hydrogen is collected. The other pencil is the anode and that is where oxygen goes out. If the hydrogen and oxygen, although they’ve been split already, are still together and not a spark within the vicinity, the hydrogen-oxygen combination is called hydroxy - a.k.a Common Ducted Oxyhydrogen, HHO, Green Gas, Rhode's Gas, Di-Hydroxy, and Watergas.
So HHO is hydrogen and oxygen together but not bonded. If it is, then it is called WATER or H2O. To bond both molecules fire or spark is needed and that's how water is formed. The process of bonding hydrogen and oxygen produces energy in the form of electricity.
The use of HHO in cars is to mix it with the regular gasoline fuel and feed it into the combustion chamber whereby making the combustion process efficient and cleaner.
For now, majority of pure hydrogen produced is through Methane Steam Reforming - this is done mostly by the big oil companies, and then stock in very high pressured tanks. With this method of producing pure hydrogen a fuel cell is needed to convert hydrogen into electricity that will propel a motor that runs the car.
Other methods used to generate hydrogen from water is through catalytic substances, either metal base or biological, combined with sunlight. But most of these catalytic-solar combination technologies are still under research and development.
Others put up mechanisms to generate electricity from renewable sources such as Solar Panels, Wind Turbines, or Mini Hydro to electrolyze water into useful energy source. Enhanced photovoltaic panels are now being utilized for Hydrogen Highways - a network of hydrogen fueling stations for fuel cell cars. You can also make use of such Hydrogen Fuel System and install one in your home.
If you are planning and ready to increase your mileage or drive "green" car, there are miniaturized electrolyzers that fit on board a vehicle that skips the need for hydrogen storage tanks. These are installed together with the Solid Polymer Electrolyte (SPE)/Proton Exchange Membrane (PEM). Or, you can do- it-yourself with the Hydrogen on Demand generators.
These miniaturized hydrogen generator can also be applied to fuel cells installed into home appliances and equipments.
At the end of the day, Making-Hydrogen.com is about how to create hydrogen from solar power to bacteria! What is the coolest way you have discovered in making hydrogen?