In a hydrogen-powered vehicle, the hydrogen fuel cell replaces the typical internal combustion engine.  So, instead of igniting a fuel-air mixture to generate power, hydrogen gas and air are directed to a fuel cell stack to produce an electric current.  The electricity produced is stored in an ultra-capacitor bank and used by an electric motor to drive the vehicle.  A typical fuel cell consists of two plates, two electrodes (anode and cathode), and two thin layers of a platinum base catalyst that are separated by a Polymer Electrolyte Membrane (PEM), also called Proton Exchange Membrane.  Through electrochemical reactions, hydrogen gas from an onboard storage tank and oxygen from external air are fed through channels in the stack plates with hydrogen on one side of the cell block and air on the other.  The catalyst splits hydrogen molecules into protons and electrons.  The protons are allowed to pass through the membrane while the electrons are blocked and must seek an alternative path through an external circuit which creates the electric current.  On the air side of the membrane, protons and electrons react with oxygen, at the second catalyst layer, to form water and generate heat as a by-product of this reaction.

To drive the rear wheel, a hub motor will be used rather than a crank motor.  In an ordinary crank motor, there is a ring-shaped permanent magnet (the stator) that remains static, and an inner metallic core (the rotor) that rotates inside it with a spinning axle running through the middle.  The rotating axle drives the crank which is connected to the wheel through a combination of gears and/or chain links.  In a hub motor, the axle remains fixed to a static frame.  As the electromagnetic field is applied to the stationary motor windings, the outer hub (with permanent magnets) tries to follow the field around which in turn rotates the wheel.  The advantage to using a hub motor is that they are easy to install, quiet, efficient on flat surfaces and fewer moving parts.  The disadvantage to using a hub motor is that they are expensive and perform poorly on inclines.