Challenges to sustainable CO2-to-methane conversion in soil-based electromethanogenesis

Electromethanogenesis is a promising technology for renewable electricity storage, offering a sustainable pathway to convert CO2 emissions into “green” CH4. Its advantages include mild operating conditions, a renewable biocatalyst, and the potential for high-purity methane production. Moreover, the microorganisms involved show tolerance to impurities, enabling direct CO2 capture from natural sources such as geological soil emissions. Despite these advantages, several challenges limit the electromethanogenesis process to marginal efficiencies, which must be carefully considered in view of potential scale-up and the application in the field. This study investigated a simple, low-cost electromethanogenesis setup for CO2-to-CH4 conversion in soil environments, using terracotta pots and a graphite granule cathode, to critically address some of the main weaknesses. The tested system achieved a Coulombic Efficiency of 45 ± 13%, aligning with values reported in the literature. Anoxic conditions were established in the cell, confirming terracotta as a viable alternative to expensive components and membrane separators. Competing sulfate-reducing reactions and scaling phenomena were identified as the primary causes of the lowest methane productivity observed (8.5 ± 4.9 mL L−1 d−1), representing only 1.64 ± 0.99% of the total gas produced. In particular, scaling phenomena interfered with gas collection and measurement. The study highlights and discusses these key challenges as critical barriers to the application of electromethanogenesis in soil environments.