Enhanced methane production by bimetallic metal–organic frameworks (MOFs) as cathode in an anaerobic digestion microbial electrolysis cell

Cathode materials play important roles in the optimization of methane upgrading in microbial electrolysis cell-coupled anaerobic digestion (MEC-AD) systems. In this study, metal organic framework (MOF)-derived carbon-based bimetallic hybrids (Ni/Co-NC) was synthesized as cathode material and its performances and microbial activities were evaluated in MEC-AD. A high methane production rate (0.57 m3 CH4/m3·d) and yield (0.34 m3 CH4/kgCOD) were achieved with a Ni/Co-NC cathode at a potential of 0.6 V. The methane content in the biogas was upgraded to ca. 90%. The enhanced methane production was likely related to the positive surface charge and the good conductive capacity and hydrogen evolution of the Ni/Co-NC cathode, as demonstrated by electrochemical assessment. Additionally, roughness and extensive surface areas provided additional room for cathodic biofilm adhesion via hydrogen bonding and electrostatic interactions. A significant enrichment of Methanobacterium (79.6%) in the MEC-AD-Ni/Co-NC system, especially in the biofilm of the cathode surface, indicated that the methane production pathway shifted from acetoclastic to hydrogenotrophic methanogenesis. Moreover, Methanosaeta harundinacea 6Ac species, which can directly accept electrons, were identified in the MEC-AD systems. These findings revealed that Ni/Co-NC cathode probably enhanced bioelectrochemical reduction of CO2 to CH4 by accelerating hydrogenotrophic methanogenesis and direct electron transfer.