I. Introduction
A fuel cell (FC)–ultracapacitor (UC) hybrid combines the high energy density of hydrogen FC with the high power density of UC, resulting in a system with improved performance and reduced size [1], [2]. Due to the limited response rate of its reactant supply, a proton exchange membrane FC is limited in following fast transients in power demand [3], [4]. Supplementing the FC with an energy storage device, which can provide the needed power during quick power transients, results in an improved load-following capability. Moreover, the lifetime of the FC stack is improved by reducing the large transients that the stack must provide [5]–[7]. In such a hybrid system, the FC can be sized to meet the expected power demand at steady state [8], [9], and the energy storage device is sized to buffer the power transients.