Comparison of Nonprecious Metal Cathode Materials for Methane Production by Electromethanogenesis

In methanogenic microbial electrolysis cells (MMCs), CO₂ 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^-3 d^-1) and the highest biotic methane production rate (250 ± 90 nmol cm^-3 d^-1). At -550 mV, plain graphite (76 nmol cm^-3 d^-1) performed similarly to platinum (73 nmol cm^-3 d^-1). 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.