BACKGROUND Microbial electrosynthesis (MES) is a biocathode‐driven process, producing high‐value chemicals, from CO2. However, the low efficiency of the biocathode hinders the MES process efficiency significantly. RESULTS A novel 3D graphene–nickel foam (G‐NF) cathode has been fabricated, by hydrothermal approach for the improvement of microbially‐catalyzed reduction at the MES cathode. An increase of 1.8 times in the volumetric acetate production rate was obtained, compared with the untreated nickel foam. In MES with G‐NF, a volumetric acetate production rate of 3.11 mmol L‐1 day‐1 has been achieved; 70% of the electrons consumed were recovered and the final acetate concentration reached 5.46 g L‐1 within 28 days. CONCLUSION The hierarchical porous G‐NF cathode improved bacterial colonization and the efficiency of mass, nutrients and protons transfer due to its 3D composition; the graphene coating considerably increased the effective surface area for microbial adhesion, as well as the electron transfer rate of biofilm in the MES. This study attempted to improve the efficiency of the biocathode, and provides a promising large electrode for large‐scale MES devices. © 2017 Society of Chemical Industry

Background: The conversion of CO 2 into high value-added products has a very important environmental and economic significance. Microbial electrosynthesis (MES) is a promising technology, which adopts a bioelectrochemical system to transform CO 2 into organic chemicals. Results: In this study, Clostridium scatologenes ATCC 25775 T , an anaerobic acetogenic bacterium, demonstrated its utility as a biocatalyst in a MES system, for the first time. With the cathodic potential of the MES system decreased from − 0.6 to − 1.2 V (vs. Ag/AgCl), the current density of the MES, and the production of organic chemicals, increased. Combining the genetic analysis and the results of the wet lab experiments, we believe C. scatologenes may accept electrons directly from the cathode to reduce CO 2 into organic compounds at a potential of − 0.6 V. The acetic and butyric acid reached a maximum value of 0.03 and 0.01 g/L, respectively, and the maximum value of total coulombic efficiency was about 84%, at the potential of − 0.6 V. With the decrease in cathodic potentials, both direct electron transfer and exogenous electron shuttle, H 2 might be adopted for the C. scatologenes MES system. At a potential of − 1.2 V, acetic acid, butyric acid and ethanol were detected in the cathodic chamber, with their maximum values increasing to 0.44, 0.085 and 0.015 g/L, respectively. However, due to the low H 2 utilization rate by the C. scatologenes planktonic cell, the total coulombic efficiency of the MES system dropped to 37.8%. Conclusion: Clostridium scatologenes is an acetogenic bacterium which may fix CO 2 through the Wood-Ljungdahl pathway. Under H 2 fermentation, C. scatologenes may reduce CO 2 to acetic acid, butyric acid and ethanol. It can also be used as the biocatalyst in MES systems.

For Cr(VI)-removal microbial fuel cell (MFC), a more efficient biocathode in MFCs is required to improve the Cr(VI) removal and electricity generation. RVC-CNT electrode was prepared through the electrophoretic deposition of carbon nanotube (CNT) on reticulated vitreous carbon (RVC). The power density of MFC with an RVC-CNT electrode increased to 132.1 ± 2.8 mW m−2, and 80.9% removal of Cr(VI) was achieved within 48 h; compared to only 44.5% removal of Cr(VI) in unmodified RVC. Cyclic voltammetry, energy-dispersive spectrometry and X-ray photoelectron spectrometry showed that the RVC-CNT electrode enhanced the electrical conductivity and the electron transfer rate; and provided more reaction sites for Cr(VI) reduction. This approach provides process simplicity and a thickness control method for fabricating three-dimensional biocathodes to improve the performance of MFCs for Cr(VI) removal.