The oxygen evolution reaction (OER) is recognized as a key half-reaction in the electrolysis of water to produce clean hydrogen. Nanocages have exhibited potential as electrocatalysts for water splitting because their large specific surface areas can provide more exposure to catalytically active sites and abundant pathways for ion (or electrolyte) diffusion. In this report, we present a cation exchange strategy, between Ni2+ and Mn3O4, to facilely synthesize spinel NiMn2O4 nanocages in combination with rGO nanosheets (NMO/rGO). When the synthesized NMO/rGO nanocages were tested as electrocatalysts for OER in water splitting, they exhibited a low overpotential of 284 mV at a stable current density of 10 mA cm(-2) with a Tafel slope of 82.2 mV dec(-1). This performance was superior to Mn3O4/rGO nanoparticles. The successful synthesis of these nanocages offer pathways for easy accessibility of highly efficient OER electrocatalyst. (c) 2018 Elsevier Ltd. All rights reserved.