Potassium-birnessite (K-OL-1) samples of different morphologies (nanowire networks, nanoparticles and mesoporous hierarchical nanowire networks) and textural properties were synthesized from KMnO₄ by using oxalate-reduction method under ambient conditions. Cryptomelane (K-OMS-2) samples of similar morphologies were synthesized by the calcination of birnessite samples. The synthesized MnO₂ samples were characterized by XRD, TGA, DTA, SEM, TEM, nitrogen adsorption–desorption and ICP-OES analyses. One each of the birnessite and the cryptomelane samples showed mesoporous nature (pore size ∼50 Å and 66 Å respectively). The pH of the reaction medium has been found to have a key role in the properties of the resulting MnO₂ samples. Electrochemical studies of MnO₂–carbon black composites were performed in 0.1 M aqueous Na₂SO₄ solution using cyclic voltammetry and galvanostatic charge–discharge cycling. One of the birnessite samples with nanowire network morphology and a BET surface area of 202 m² g⁻¹ exhibited a high capacitance value of 234 F g⁻¹. Electrochemical studies revealed that the specific capacitance values of the studied MnO₂ materials have a direct relationship with their BET surface area values. Birnessite samples have exhibited better capacitance behavior and values than their cryptomelane derivatives. The effect of the morphology has also been reflected in the values of the specific capacitance.