Abstract This paper develops a novel passive fractional-order sliding-mode control (PFOSMC) of a supercapacitor energy storage (SCES) system in microgrid with distributed generators. Firstly, a storage function is constructed and thoroughly analysed to investigate the inherent physical characteristics of SCES systems. Then, the beneficial terms are carefully retained for the sake of transient responses improvement, while the other detrimental terms are fully removed to achieve a globally control consistency. In order to further enhance the robustness of the closed-loop system, a fractional-order sliding-mode control (FOSMC) framework is synthesized as an additional input, which employs the fractional-order P D α sliding surface as well as an energy reshaping mechanism to realize a more flexible control performance. Four case studies, including (a) Active power and reactive power supply, (b) System restoration under power grid fault, (c) Power support under stochastic solar energy and wind energy integration, and (d) Robustness with system parameter uncertainties, are carried out to study the control performance of PFOSMC compared to that of PID control, interconnection and damping assignment passivity-based control (IDA-PBC), and FOSMC, Finally, a hardware-in-the-loop (HIL) experiment using dSpace platform is undertaken to validate its implementation feasibility.