Nanosized MnO2 powders were grown using a green synthesis based on citrates via redox reaction between KMnO4 and natural reducing agents, i.e. lemon juice and peel extracts, without generating hazardous waste. X-ray diffraction (XRD) and Raman studies show that the samples grow as cryptomelane polymorph, α-MnO2, crystallized with the tetragonal cryptomelane structure. Both J-MnO2 prepared from lemon juice and P-MnO2 prepared from citrus peel consist of nanosized particles as observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Thermogravimetric analysis (TGA) confirms the typical thermal decomposition of MnO2 at temperatures of up to 500 °C. Improved electrochemical properties are obtained and discussed for P-MnO2, with a reversible capacity that reaches 160 mAh g−1 (initial capacity of 212 mAh g−1) at a current density of 10 mA g−1 and a good rate capability.