(233c) Microbial Electro-Methanogenesis Coupled with in-Situ Hydrogen Generation for Biogas Upgrading

Methanogenic microbes from the Archaea domain are the natural catalysts for converting carbon
dioxide and hydrogen into methane (Eq1). Microbial catalytic activity for CO₂ conversion can be
coupled with electrochemical pathways of hydrogen generation (Eq2) from water splitting. This
combined technology is a promising power-to-gas technology for lowering the carbon intensity of
carbon-containing fuels while storing excess renewable energy into useful chemical energy.

CO₂ + 4H₂ ↔ CH₄ + 2H₂O Eq1
H⁺ + e^- → ½ H₂ E^o’= -410 mV vs. NHE (at pH 7) Eq2

Bio-electrochemical technologies are promising for upgrading raw biogas streams from sources
such as wastewater treatment plants. Biogas is predominantly composed of methane and carbon
dioxide. In order to upgrade it for use as pipeline-quality renewable natural gas, the CO₂ must be
removed, either by scrubbing it out or converting it into additional methane. Microbial electromethanogenesis
processes are advantageous compared to direct electrochemical CO₂ reduction
into methane mainly due to higher product selectivity and lower energy intensity of the hydrogen
evolution reaction.

In the present work, we studied electro-methanogenesis catalyzed by methanogenic microbes
using in-situ produced hydrogen. Electrochemically generated hydrogen is the both electron and
proton donor for methane production in microbial electrolysis of carbon dioxide. Hydrogen
evolution at neutral pH was achieved using electrochemically deposited Ni and Mo catalytic layers
on high-surface area and porous electrodes fabricated using advanced material manufacturing
technologies. Different reactor designs with ion selective membranes were devised to overcome
mass transport limitations and improve conversion efficiency in the anaerobic methanogenesis
process. More than 90% of conversion efficiency could be achieved in continuous operation of
CO₂ conversion in a tubular, flow-through reactor design.