A high energy density fuel cell apparatus and system with a hydride-based hydrogen generator comprising the following: a fuel cell and its balance of plant components; a water storage vessel comprising liquid water and its balance of plant components a reactor vessel and its balance of plant components comprising a reaction chamber adapted to contain magnesium hydride fuel and all of the essential other reactor components, a heater which is provided in the reaction chamber; a tubing adapted to supply the liquid water from the water storage vessel to reactor vessel; a water pump disposed between the water storage vessel and the reactor vessel; a check valve disposed in a discharge line connecting the water pump to the reactor vessel; where the fuel cell is based on one of the following fuel cell technologies: proton exchange membrane fuel cell, alkaline fuel cell, solid oxide fuel cell, or phosphoric acid fuel cells, where an array of fuel cells are arranged either in parallel or series in order to produce an appropriate power output level that is needed for the intended application, where the reactor vessel has a single stage hydrolysis reaction zone and has the capability of hydrolyzing magnesium hydride due to its high stability and high hydrogen storage capacity, the single stage hydrolysis reaction zone has a steam dispensing microporous medium and a single steam injection point being attached to the steam dispensing microporous medium, wherein the liquid water located in water storage vessel is first transferred to the tubing via water pump in desired quantities via a check valve so as to enter the reactor vessel, and wherein the heater 36 is directly connected to the microporous medium at a single location for steam to be injected into hollow internals of this microporous medium, where the hydrolysis reaction mechanism is based on the exothermic MgH2 hydrolysis reaction pathway in order to produce the maximum amount of exothermic heat for the self-sustainment of the hydrolysis reaction, wherein the hydrolysis of the MgH 2 is controlled by controlling the amount of water fed through the water pump according to a demand of an electric load connected to an electric output port, where an array of the hydride-based hydrogen generators (single or multiples) are arranged either in parallel or series in order to generate an appropriate hydrogen flow rate for the intended application, where the high energy density fuel cell apparatus and system, water storage vessel and reactor vessel are in fluid communication, and where the discharge line comprises a tubing with a predetermined rupture pressure range. a fuel cell and its balance of plant components; a water storage vessel comprising liquid water and its balance of plant components a reactor vessel and its balance of plant components comprising a reaction chamber adapted to contain magnesium hydride fuel and all of the essential other reactor components, a heater which is provided in the reaction chamber; a tubing adapted to supply the liquid water from the water storage vessel to reactor vessel; a water pump disposed between the water storage vessel and the reactor vessel; a check valve disposed in a discharge line connecting the water pump to the reactor vessel; where the fuel cell is based on one of the following fuel cell technologies: proton exchange membrane fuel cell, alkaline fuel cell, solid oxide fuel cell, or phosphoric acid fuel cells, where an array of fuel cells are arranged either in parallel or series in order to produce an appropriate power output level that is needed for the intended application, where the reactor vessel has a single stage hydrolysis reaction zone and has the capability of hydrolyzing magnesium hydride due to its high stability and high hydrogen storage capacity, the single stage hydrolysis reaction zone has a steam dispensing microporous medium and a single steam injection point being attached to the steam dispensing microporous medium, wherein the liquid water located in water storage vessel is first transferred to the tubing via water pump in desired quantities via a check valve so as to enter the reactor vessel, and wherein the heater 36 is directly connected to the microporous medium at a single location for steam to be injected into hollow internals of this microporous medium, where the hydrolysis reaction mechanism is based on the exothermic MgH2 hydrolysis reaction pathway in order to produce the maximum amount of exothermic heat for the self-sustainment of the hydrolysis reaction, wherein the hydrolysis of the MgH 2 is controlled by controlling the amount of water fed through the water pump according to a demand of an electric load connected to an electric output port, where an array of the hydride-based hydrogen generators (single or multiples) are arranged either in parallel or series in order to generate an appropriate hydrogen flow rate for the intended application, where the high energy density fuel cell apparatus and system, water storage vessel and reactor vessel are in fluid communication, and where the discharge line comprises a tubing with a predetermined rupture pressure range.