Oxygen reduction reaction (ORR) electrocatalysts in constructed wetland-microbial fuel cells: Effect of different carbon-based catalyst biocathode during bioelectricity production

Abstract

The oxygen reduction reaction (ORR) catalytic activities that occur in constructed wetland-microbial fuel cells (CW-MFCs) during bioelectricity production through exudates released from macrophyte rhizosphere using different alternative carbon-based catalysts combined with biocathode were studied in this work. A catalytic layer of graphene/titanium dioxide (G/TiO2), graphene/nitrogen (G/N), and carbon/platinum (C/Pt) was spray-coated on the carbon cathode as electrocatalysts for the CW-MFC1, CW-MFC2, and CW-MFC3, respectively. Then, the development of the biofilm took place on the catalytic layer during the operation of CW-MFCs generating a biohybrid catalyst (carbon-based catalyst biocathodes). The electrokinetic parameters showed that the ORR of the different biohybrid catalysts and biocathode without carbon-based catalyst occurred through the direct reduction mechanism, which involves a transfer of four-electrons towards the formation of water. Tafel slopes of 110–184 mV/dec, charge transfer coefficients (α) ≤ 0.5, and exchange current density (io) of 2.0–4.9 × 10−2 mA/cm2 were obtained. These parameters demonstrated that the electrocatalysts increased the electrocatalytic activities of the ORR. The increase of the active sites on the biocathodes led to an increase in the bioelectricity production. The highest bioelectrochemical performance in the CW-MFCs was obtained with the G/TiO2, and C/Pt electrocatalysts with maximum voltages of 490 and 454 mV and power densities of 99.2 and 82.4 mW/m2, respectively. The biohybrid catalysts showed an increase of more than 100% in bioelectricity production compared with biocathode CW-MFCs without catalytic layer. Biolectricity production was directly related to the generation of sugars released from macrophyte rhizosphere.