Comparison of Nonprecious Metal Cathode Materials
for Methane Production by Electromethanogenesis
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Abstract
In methanogenic microbial electrolysis
cells (MMCs), CO<sub>2</sub> is reduced to methane using a methanogenic
biofilm on the cathode
by either direct electron transfer or evolved hydrogen. To optimize
methane generation, we examined several cathode materials: plain graphite
blocks, graphite blocks coated with carbon black or carbon black containing
metals (platinum, stainless steel or nickel) or insoluble minerals
(ferrihydrite, magnetite, iron sulfide, or molybdenum disulfide),
and carbon fiber brushes. Assuming a stoichiometric ratio of hydrogen
(abiotic):methane (biotic) of 4:1, methane production with platinum
could be explained solely by hydrogen production. For most other materials,
however, abiotic hydrogen production rates were insufficient to explain
methane production. At −600 mV, platinum on carbon black had
the highest abiotic hydrogen gas formation rate (1600 ± 200 nmol
cm<sup>–3</sup> d<sup>–1</sup>) and the highest biotic
methane production rate (250 ± 90 nmol cm<sup>–3</sup> d<sup>–1</sup>). At −550 mV, plain graphite (76 nmol
cm<sup>–3</sup> d<sup>–1</sup>) performed similarly
to platinum (73 nmol cm<sup>–3</sup> d<sup>–1</sup>).
Coulombic recoveries, based on the measured current and evolved gas,
were initially greater than 100% for all materials except platinum,
suggesting that cathodic corrosion also contributed to electromethanogenic
gas production