Recent advances in cathode materials and configurations for upgrading methane in bioelectrochemical systems integrated with anaerobic digestion

Methane enrichment in anaerobic digestion (AD) is definitely required to enhance the calorific value of biogas to ensure its application as an off-grid energy source. Electromethanation in bioelectrochemical systems (BESs) integrated with the AD process is a promising technology because of its efficient conversion of CO2 to methane at the biocathode using microbes as catalysts. An additional advantage of this system is CO2 fixation, which is important in view of the increasing global-warming threat. BES performance is highly reliant on different operating and physical parameters, among which poor cathode materials and reactor design often limit the performance. The cathode serves as an electron reservoir for microorganisms to reduce CO2 to methane. Reactor design governs internal resistance, mass transport of reactants and products, and boundary-layer oxygen diffusion in the system, which determine the rate of ion flux and overall system performance. Detailed critical commentary on recent developments in cathode materials and their role in the bioconversion of methane and adopted efficient BES-AD designs is given in this paper. The current costs of catalysts and base-electrode materials for scaling up BES-AD have been also comprehensively discussed. Studies have suggested that an efficient cathode should possess biocompatibility, hydrophilicity, positive surface charge, and an extensive surface area. Graphitic-carbon materials, especially those with three-dimensional structure, have been demonstrated to be superior cathodes than metal electrodes. The modification of carbon-based electrodes with efficient catalysts could further enhance performance in methane production suggesting the beneficial action of catalyst on the microbes-electrode interaction and extracellular electron transfer. Further detailed investigations will help the development of cutting-edge catalysts and configurations for scale-up BES-AD systems.