*2.1. Materials*

Polymeric precursor (polyurethane) was obtained from Aekyung Petrochemical Co., Ltd. (Wanju, Korea) [27]. Portions of the polymeric precursor (5 g) were heated to 900 ◦C at 10 ◦C/min in a self-made cylindrical steel tube furnace (SiC heater: length 1000 mm, diameter 100 mm) under N2 gas (99.999%) and kept at a target temperature for 1 h to obtain 2 g of carbonized polymer-based HC.

#### *2.2. Steam Activation Optimizaiton*

Activation of the HC was performed using steam as the physical reagent. Steam activation was performed in the same steel tube used for carbonization. The HC was heated (10 ◦C/min) to the required activation temperature (900 or 1000 ◦C) under N2 (300 mL/min), before the N2 was replaced with H2O (0.5 mL/min, liquid) with various activation times. The sample was then allowed to cool under N2 (300 mL/min). The samples were named as activated polymer-based hard carbons (APHS) and named APHS-activation temperature-activation time sequence according to activation conditions. The prepared AC was pulverized to 10 μm size using a ball mill (Pulverisette 23, Idar-Oberstein, Germany).
