NOVELTY - An auxiliary power unit for generating electrical power comprises a fuel cell system and a hydrogen storage and retrieval system (10) containing a hydrogen storage member (50) and a control system (20). The hydrogen storage member comprises porous silicon having interior and exterior surfaces, where the interior surfaces have dangling bond sites at which reversible chemisorption of hydrogen atoms occurs. The control system is operable to liberate chemisorbed hydrogen atoms (52) from the dangling bond sites and release the liberated hydrogen atoms (56) as hydrogen gas.    USE - As an auxiliary power unit, for generating electrical power installed in a vehicle particularly in e.g. floor, fender, quarter panel, rocker panel, door, column, post, trunk or roof of the vehicle (claimed).    ADVANTAGE - The auxiliary power unit includes hydrogen storage and retrieval system that provides low-cost, efficient, and safe storage of hydrogen. The adsorption and desorption of elemental hydrogen may be tailored to suit the system to particular applications. The elemental hydrogen remains safely adsorbed within the storage material in the event of catastrophic failure of the system. The size, weight, and volume of a housing within which the elemental hydrogen is stored can also be adapted to application-specific requirements.    DETAILED DESCRIPTION - An auxiliary power unit for generating electrical power comprises a fuel cell system operable to combine hydrogen and oxygen to generate electrical power; and a system (10) for storing and retrieving elemental hydrogen that is then supplied as hydrogen gas to the fuel cell system. The hydrogen storage and retrieval system comprises a hydrogen storage member (50) and a control system (20). The hydrogen storage member comprises porous silicon having interior and exterior surfaces, where the interior surfaces have dangling bond sites at which reversible chemisorption of hydrogen atoms occurs. The control system is operable to liberate the chemisorbed hydrogen atoms from the dangling bond sites and release the liberated hydrogen atoms (56) as hydrogen gas for use by the fuel cell system. The porous silicon is a porous silicon mass, or a porous mesh of silicon columns. The fuel cell system is selected from solid oxide fuel cell system and proton exchange membrane system.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of an elemental hydrogen storage and retrieval system.    Hydrogen storage and retrieval system (10)    Light Source (14)    Current source (16)    Voltage source (18)    Control system (20)    Hydrogen storage member (50)    Chemisorbed hydrogen atoms (52)    Liberated hydrogen atoms (56)