High-rate and energy-efficient microbial electrosynthesis of methane using 3D dual-layer cathodes

Microbial electromethanogenesis cells (MMCs) perform electromethanogenesis from captured CO₂ using microorganisms (methanogens) on a cathode with water splitting on the anode using renewable electricity. The key challenge for MMCs is increasing methane production rates while maintaining high energy efficiencies. A unique dual-layer cathode design was developed here to substantially improve methane production rates by placing a conductive 3D electrode (reticulated vitreous carbon (RVC) or multiple layers of carbon nanoparticle-coated stainless-steel mesh (CN-SSM)), on top of a thin carbon cloth with the catalyst (Pt/C). Using 3 mm thick 3D cathodes with flow directed through it generated 9−20 L/L/d of methane over multiple cycles, averaging 12 ± 3 L/L/d for RVC at 50 ± 5 A/m², and 16 ± 3 L/L/d for CN-SSM at 54 ± 5 A/m², with an applied voltage of 2.8 V. These rates were achieved with high energy conversion efficiencies (electricity to methane) of 20 ± 4% (RVC) and 23 ± 4% (CN-SSM). Thicker cathodes (6 mm) increased current but not methane production. Higher current densities of up to 148 A/m² were temporarily obtained by spiking the anode feed daily with water. The biocathode archaeal community was dominated by hydrogenotrophic methanogens of the genus Methanobacterium. Cathodic methane recovery was found to be the most important operational component based on our analysis using a machine learning model. These results show that both high energy efficiencies and methane gas flowrates can be achieved by using highly porous dual-layer cathodes in zero-gap MMCs.