In this article, different sized CeO2 nanoparticles were synthesized using one step low-cost solvothermal method with various reaction time. Defect states were induced due to the reduction of Ce4+ into Ce3+ valence state. X-ray photoelectron spectroscopy results recommend that Ce3+ valence states and defects in the form of oxygen vacancies be present on the surface of CeO2 nanoparticles. Such availability of oxygen vacancies provided high specific capacitance 142.5 Fg(-1) at a current density of 0.25 Ag-1 in three electrode system using 1M Na2SO4 electrolyte. There is an increase in faradaic reactions taken place on the surface which is attributed to the high surface area, more oxygen vacancies, and increased diffusion rate. The highest energy density is obtained to be similar to 12.68 Wh/kg, and the stability result confirmed that the capacitance retention is similar to 75% after 1000 cycles of operation. These results indicate that CeO2@20h NPs is a potential candidate as electrode materials for supercapacitor applications due to their fast mutation between Ce4+ to Ce3+ oxidation state. (c) 2018 Elsevier Ltd. All rights reserved.