A method for making an electrocatalyst containing manganese oxide nanoparticles present on carbon obtained from Albizia procera (MnOxNPs-C) for electrochemical water oxidation. The method includes a thermal decomposition and forms a product with specific morphological variations, including crystalline structure, elemental composition, and chemical compatibility. The manganese oxide nanoparticles are well dispersed over the carbon. The amount of manganese oxide nanoparticles increases by increasing the amount of precursor. Single-phase formation of the Mn3O4, and Mn3O4 along with MnO phase occurs at low and high amount of the precursor materials, respectively. The electrocatalyst can be used for the purpose electrolytic water splitting.