**4. Conclusions**

In this work, a polymeric precursor was modified using variations of steam activation methods to prepare APHSs with high specific surface area and a mesopore-rich structure. The specific surface area and specific capacitance of the physical APHS samples was up to 2240 m<sup>2</sup>/g and 136 <sup>F</sup>/g, respectively.

The activation condition (time and temperature) affected the pore structure of APHS. During the initial increase of the activation time, micropores with narrow PSD were formed by oxidation of amorphous areas or small crystallites. As the activation time increased, the oxidation of crystallites increased, leading to increased specific surface areas and mesopore volume. As the activation time increased further, the oxidation of crystallites increased, leading to increased micropore and mesopore volume. With additional extension of the activation time, the pores that initially developed started to deepen, enlarge, and perhaps merge, resulting in the observed increase in the average pore width. When the activation temperature increased from 900 to 1000 ◦C, the oxidation rate of APHS was found to increase about twice. As a result, at the activation temperature of 1000 ◦C, APHS exhibited broad PSD curves and wide pore diameter.

The specific capacitance was significantly dependent on the pore size distribution according to activation conditions. It was confirmed that the correlation between the specific capacitance in 1M TEABF4/PC and the pore characteristics of the APHS was determined by pores of diameter 1.5–2.5 nm. The specific capacitance of APHS-9-4 was higher than that of APHS-10-2, which had the highest specific surface area and mesopore volume. These results sugges<sup>t</sup> that the pore structure of APHS-9-4 is better optimized than is the pore structure of APHS-10-2 in 1M TEABF4/PC. In conclusion, APHS, created using the physical activation method, exhibited better specific capacitance than did MSP20 created using the chemical activation method.

**Author Contributions:** Conceptualization, S.-J.P. and B.-J.K.; methodology, S.-J.P. and B.-J.K.; validation, K.-H.A., S.-J.P. and B.-J.K.; formal analysis, H.-M.L. and B.-J.K.; investigation, K.-H.A.; resources, K.-H.A. and B.-J.K.; data curation, H.-M.L.; writing—original draft preparation, H.-M.L.; writing—review and editing, B.-J.K.; visualization, B.-J.K.; supervision, B.-J.K.; project administration, B.-J.K.; funding acquisition, B.-J.K.

**Funding:** This research was supported by the Nano·Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (Project no. 2018M3A7B9086636). This research was financially supported by the Ministry of SMEs and Startups (MSS), Korea, under the "Regional Specialized Industry Development Program (P0003187)" supervised by the Korea Institute for Advancement of Technology (KIAT).

**Conflicts of Interest:** The authors declare no conflict of interest.
