This paper describes the energy management controller design of a mid-sized vehicle driven by a fuel cell/battery plug-in hybrid powertrain, where an experimentally validated high temperature polymer electrolyte membrane fuel cell model is used. The power management strategy results from the application of the Pontryagin's Minimum Principle, where the optimal control parameter is derived in order to minimize fuel consumption under certain constraints. In particular, the vehicle is also equipped by an autothermal reformer and, in order to minimize the hydrogen buffer size, the control algorithm is subject to constraints on the maximum hydrogen buffer level. The effectiveness of the system is analyzed when feeding the autothermal reformer with different hydrocarbon fuels and over different driving conditions. The obtained solutions are compared in terms of hydrogen consumption, fossil fuel consumption, system efficiency, money saving and equivalent CO2 emissions.