Direct CO2 delivery with hollow stainless steel/graphene foam electrode for enhanced methane production in microbial electrosynthesis

Abstract

Microbial electrosynthesis system (MES) is a promising power-to-gas energy storage technology. Developing high-performance biocathodes is one of the most strenuous efforts in MES and the key factor limiting MES productivity. However, mass transfer limitation has sometimes been observed to occur in electrodes during operation, especially in three-dimensional (3D) electrodes with certain thicknesses. Herein, a porous hollow stainless steel/graphene foam (GF) electrode was developed to facilitate the direct delivery of CO2 to microorganisms colonized on graphene sheets. The 3D GF possesses a layered structure with interconnected pores, providing a high specific surface area for the colonization and proliferation of methanogenic microorganisms. The hollow stainless steel, coupled with the porous GF to form a hybrid electrode, allows efficient CO2 supply within the 3D biocathode and has a positive effect on maintaining a suitable pH for microbial metabolism inside it. This unique biocathode possesses a high biomass loading and thereby achieves a high CH4-production rate of 848.0 ± 124.5 mmol/m2/d at −1.0 V vs Ag/AgCl (3.6-fold higher compared with carbon cloth), while ensuring the Faradaic efficiency of CH4 up to 84.2 ± 7.7 %. The results indicate that developing 3D-network cathodes with CO2 diffusion assistance is an effective approach to enhance the performance of MES.