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Carbon nanofiber (CNF)‐based supercapacitors have promising applications in the field of energy storage. It is desirable, but remains challenging, to develop CNF electrode materials with large specific surface area (SSA), high specific capacitance (SC), and high power density, as well as excellent cycling stability and high reliability. Herein, acrylonitrile–acrylic acid copolymer P(AN‐co‐AA) was synthesized for the preparation of nitrogen‐doped microporous CNFs. Thermal degradation of the AA segment leads to the formation of micropores that are distributed not only on the CNF surface, but also inside the material. The microporous structure and nitrogen content can be manipulated at the molecular level by adjusting the weight ratio between AN and AA, and the SSA and SC could reach as high as 1099 m2 g−1 and 156 F g−1, respectively. After KOH activation, the activated CNFs have an extremely high SSA of 2117 m2 g−1 and SC of 320 F g−1, which are among the highest values ever reported for electric double‐layer supercapacitors with an alkaline electrolyte. Furthermore, the capacitance retention, which can be maintained at 99 % even after 16 000 cyclic tests, reveals outstanding durability and repeatability. [ABSTRACT FROM AUTHOR]