A novel carbon foam with microporous structure (CFMS), with the advantages of a simple fabrication process, low energy consumption, large specific surface area and high conductivity, has been prepared by a facile one-step carbonization. In addition, the carbon foam possesses suitable interlayer spacing in short range which is flexible to accommodate the deformation of carbon layer caused by the ion insertion and deinsertion at the charge and discharge state. Furthermore, a low cost carbon-based symmetric potassium dual-ion capacitor (PDIC), which integrates the virtues of potassium ion capacitors and dual-ion batteries, is successfully established with CFMS as both the battery-type cathode and the capacitor-type anode. PDIC displays a superior rate performance, an ultra-long cycle life (90% retention after 10000 cycles), and a high power density of 7800 W kg−1 at an energy density of 39 Wh kg−1. The PDIC also exhibits excellent ultrafast charge and slow discharge properties, with a full charge in just 60 s and a discharge time of more than 3000 s.