This paper designs a novel Passive Fractional-Order Sliding-Mode Control (PFOSMC) for a Supercapacitor Energy Storage (SCES) system. First, a storage function of SCES systems is developed in which inherent physical characteristics are thoroughly analyzed. Then, the beneficial terms are carefully retained to improve the dynamical responses of the SCES system, while the remaining detrimental terms are fully compensated for to achieve a global control consistency. Lastly, a Fractional-Order Sliding-Mode Control (FOSMC) framework is employed as an additional input to reshape the closed-loop system into an output of strictly passive form. At the same time a fractional-order sliding surface is designed to enhance system robustness. Simulation results verify that the PFOSMC can outperform the PID control, Interconnection and Damping Assignment Passivity-Based Control (IDA-PBC), and FOSMC under different operational conditions. This work is supported by National Natural Science Foundation of China (No. 61963020).