Effect of applied potential on the performance of an electroactive methanogenic biocathode used for bioelectrochemical CO2 reduction to CH4

BACKGROUND Biogas can be upgraded to biomethane, which can be used as vehicle fuel and natural gas substitute. Bioelectrochemical biogas upgrade is an innovative alternative to energy-consuming physicochemical processes and bio-upgrade methods which require H-2 supply. Bioelectrochemical biogas upgrade is conducted by methanogenic microorganisms that convert CO2 into CH4, in the biocathode of a bioelectrochemical system (BES), using electric current as energy source. The aim of the present work was to study the efficiency of an H-type BES in the conversion of CO2 into CH4, by applying different potentials at the electromethanogenic biocathode. RESULTS The H-type BES was operated in a three-electrode configuration (working: graphite rod; counter: Pt/Ti; reference: Ag/AgCl) with a potentiostat, which set the biocathode's potential initially at -0.7 V versus a standard hydrogen electrode (SHE) and monitored the current demand. Based on cyclic voltammetry runs, a highly electroactive methanogenic biocathode was developed in a short time. The methane production rate (MPR) at a cathode potential of -0.7 V versus SHE was 31.1 mmol m(-2) d(-1), with an electron capture efficiency of 77.6%. The efficiency of the BES was reduced by applying a potential of -0.5 V versus SHE at the biocathode, resulting in negligible CH4 production. The BES achieved its maximum performance at a potential of -0.9 V versus SHE with a MPR of 53.8 mmol m(-2) d(-1) and an electron capture efficiency of 86%. The CO2 consumption rate achieved was 0.8 mmol d(-1). CONCLUSIONS The H-type BES achieved an effective biolectrochemically driven methane production, while the biocathode electroactive behavior was evaluated during the whole operation of the system. (c) 2021 Society of Chemical Industry (SCI).

Automated summary

The study demonstrates that CO2 can be efficiently converted into CH4 in an H-type BES by applying different potentials, with the best performance achieved at a potential of -0.9 V. Additionally, the electroactive behavior of the biocathode plays a crucial role in the overall operation of the bioelectrochemical system.