A hydrogen reformer is a device that extracts the hydrogen contained in other fuels. A reformer allows existing fuel supply and distribution systems to supply fuel cell-powered vehicles.

Reformers are based on a catalytic reaction that separates the hydrogen from the carbon in the fuel, then mixes the carbon to form carbon dioxide which is released into the atmosphere. This is a fairly simple process, although it does drive up the overall need for catalyst.

Most reformers work best at a temperature of several hundred degrees. This presents problems for the sort of "start up and go" performance people expect from their cars today. Considerable effort has been put into researching this problem, and a number of reformer systems have addressed it to one degree or another.

A much more difficult problem is that the fuel fed into a reformer is typically not very "clean". In the case of commercial gasoline and diesel fuels, this is compounded by the addition of a variety of other chemicals intended to help clean the engine and keep the fuel flowing. Reformers have a much harder time cleaning these substances out of the fuel, and many of them have a direct negative impact on the reformer itself. This is why many reformer projects have focussed on "new" fuels like methanol; they are not widely used today, so it would not be too difficult to have the market supply it in a form tailored to use in a reformer.

The biggest problem for reformer based systems remains in the fuel cell itself. The catalyst used in the common polymer-electrolyte-membrane fuel cell, the device most likely to be used in transportation roles, is very sensitive to any leftover carbon in the fuel, which reformers never completely remove. The membrane is "clogged up" with the carbon and its performance degrades.

Even with these problems, the reformer–fuel-cell system is considered almost certain to be the system that will power cars and trucks in the future. One running on existing fuels would be the best solution to quickly and cheaply replace existing engines, but in the long run a fuel like methanol may be preferable. The overall cost of making, transporting and storing the fuel is the key issue, and liquid fuels that can be grown or dug up will always be financially advantageous over pure hydrogen, which is expensive to separate, bulky, and difficult to store.