Conversion of CO2 to formate using activated carbon fiber-supported g-C3N4-NiCoWO4 photoanode in a microbial electrosynthesis system

Microbial electrosynthesis (MES) integrated with photocatalytic materials is a novel carbon dioxide (CO2) utilization technology with considerable potential to mitigate CO2 emission. The present study demonstrates for the first time an efficient formation of formate from CO2 in a photo-assisted MES system utilizing the activated carbon fiber (ACF)-supported g-C3N4-NiCoWO4 photoanode and g-C(3)N(4 )biocathode. The prepared NiCoWO4-g-C3N4/ACF formed a Z-scheme heterojunction resulting in the enhanced suppression of electron-hole pairs. Several characterization techniques confirm a successful incorporation of NiCoWO4 over g-C3N4/ACF. Atomic absorption spectroscopy revealed high chemical stability of photoanode without any metal leaching in the reaction medium. Cyclic voltammetry test showed an improved oxygen evolution reaction (OER) at the photo anode with a significantly low overpotential (-0.12 V). Transient photocurrent measurements showed-1.05 times higher photocurrent for NiCoWO4-g-C3N4/ACF relative to g-C3N4/ACF upon illumination of light, demonstrating the formation of the Z-scheme heterojunction between NiCoWO4 and g-C3N4/ACF. The band alignment, scavenger analysis, and electron spin resonance spectroscopy results further confirmed the Z-scheme charge transfer mechanism. Approximately 12.8 mM of formate was synthesized per day in the MES system under visible light irradiation, which is approximately twice that in dark. The solar to formate conversion efficiency was determined to be 1.48%. The proposed mechanism indicates the photoinduced holes-initiated OER at the photoanode, and the release of formate dehydrogenase by Escherichia coli, isolated from a wastewater, at the biocathode. This study provides an efficient supported photoanode for harvesting solar light, combined with a biocathode in the MES cell for achieving a sustainable high rate of biochemicals formation.

Automated summary

This study demonstrates for the first time the efficient formation of formate from CO2 in a photo-assisted microbial electrosynthesis system. The use of an activated carbon fiber-supported g-C3N4-NiCoWO4 photoanode and a g-C(3)N(4) biocathode leads to the enhanced suppression of electron-hole pairs and the formation of a Z-scheme heterojunction. The system achieves a high rate of formate synthesis under visible light irradiation.