Electromethanogenesis is a form of electrobiofuel production through a microbial electrolysis cell (MEC) where methane (CH4) is directly produced from an electrical current and carbon dioxide (CO2) using a cathode. With the aim of maximizing methanogenesis in an MEC, this study utilized granular activated carbon (GAC) and a transition metal catalyst to fabricate nickel (Ni) nanoparticle (NP)-loaded GAC (Ni-NP/GAC) composites and incorporated these into MECs. In this set-up, GAC acted as the main electrical conduit for direct interspecies electron transfer (DIET) between exoelectrogens and methanogenic electrotrophs, and the Ni-NPs served as a catalyst to further improve microbe-to-GAC electron transfer. The Ni-NP/GAC-composites were prepared using two different methods (microwave irradiation and solution plasma ionization). The Ni NPs were determined to be well doped on the GAC surface according to a field emission scanning electron microscope (FE-SEM) and energy-dispersive X-ray (EDX) spectroscopy analysis. Adding GAC into MECs improved CH4 production. The Ni-NP/GAC composites prepared by solution plasma ionization showed the highest CH4 production (20.7 ml), followed by the Ni-NP/GAC composite prepared by microwave irradiation (19.6 ml), bare GAC (15.6 ml), and GAC-free control (9.6 ml). In the methanogenic MECs, 40.6% of CH4 was produced from an electrode reaction (i.e., reduction of CO2 to CH4), and the remaining 59.4% was generated by nonelectrode reactions. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.