Because of avaricious global energy consumption, and serious environmental issues, the demand for clean energy technology has grown to a significant level, and the development of emerging effective energy devices and resources has become one of the most important topics in recent decades, and has attracted passionate research [1]. Among encouraging alternative sources of energy, supercapacitors are one of the promising electrochemical energy storage devices, with high power characteristics compared to batteries, high energy density compared to conventional capacitors, and long cycling times [2][3][4][5][6].Hence, supercapacitors have been practically used in various applications, and offer potential benefits in communications, consumer electronics, aviation, transportation and associated technologies [7][8][9]. It is well known that the performance of electrochemical double layer capacitors (EDLCs) typically depends on the high specific surface area and highly reversible redox reactions of the electrode materials [10]. Therefore, various carbonaceous materials, such as carbon nanotubes, activated carbons, carbon blacks, carbon aerogels and carbon fibers have been investigated as electrode materials for supercapacitors. Comparatively, carbon nanofibers (CNFs) have gained great importance in recent years due to their large axial ratio [11,12]. Presently, CNFs are extensively applied in many fields such as fuel cells, EDLCs, hydrogen energy and lithium-ion cells [13][14][15][16][17].On CeO 2 has received considerable attention due to its remarkable properties and wide applications in various fields such as oxygen permeation membrane systems [25,26], low-temperature water-gas shift reaction [27,28], oxygen sensors [29,30], glass-polishing materials [31,32], and fuel cells [33][34][35][36].In this paper, we demonstrate a methodology for the synthesis of Co/CeO 2 -doped CNFs, which are introduced as an electrode of electrochemical capacitors (EC). The proposed nanostructures were synthesized by the carbonization of an electrospun mat composed of cerium acetate, cobalt acetate, and poly(vinyl alcohol) (PVA) in an argon atmosphere at 700°C.Cobalt (II) acetate tetrahydrate (CoAc, 98%; Aldrich, St. Louis, MO, USA) and cerium (III) acetate hydrate (CeAc, 99.9%; Aldrich) were used as the precursors for Co and CeO 2 while PVA with a molecular weight 65,000 g/mol (Aldrich) was used as the precursor for the carbon fiber.A mixture containing 75 wt% CoAc and 25 wt% CeAc was mixed with 15 g PVA aqueous solution (10 wt%). Finally the mixture was stirred at room temperature for 5 h to obtain a transparent mixture. The obtained sol-gel was electrospun at a high voltage of 21 kV using a direct current (DC) power supply at room temperature. The produced mats were dried at room temperature for 10 h and then under vacuum overnight at 70°C and finally carbonized DOI: http://dx.doi.org/