Quantitative study of carbonates deposition in biocathodes by 3-D X-ray microcomputed tomography

Irreversible processes on the electrodes severely affect performances and full-scale application of microbial fuel cells (MFCs). Single Chamber MFCs are mainly limited by the cathodic oxygen reduction reaction, which is kinetically hindered, and further obstructed by fouling and deposition phenomena on the cathode. pH increases through the cathode, facilitating salts and inorganic forms precipitation. The study of biofouling and precipitation processes is therefore crucial for a deeper understanding and optimization over long term operation of MFCs. In this work, a carbon-based cathode, consisting of carbon cloth covered with a carbon microporous layer, was investigated. The electrochemical behaviour of the cathode has been analysed over time during the MFC life. X-ray microcomputed tomographies (microCTs) have been carried out at progressive stages of cathode inactivation. The technique provides cross-sectional images and 3D reconstruction of volumes. Lower grayscale values in the image indicate lower X-ray attenuation, (i.e., lower atomic density) thus allowing to distinguish biofilm from inorganic fouling on the basis of the value of the linear attenuation coefficient calculated voxel (3D pixel). MicroCT was combined with SEM and EDX techniques in order to recognise chemical species in each different layer of the cathode’s section. Results correlated the calcium and sodium carbonate deposits, in the inner and outer part of the cathode respectively, with the produced electric current over time. A specific microCT-related software quantified the time-dependent salts deposition, identifying a correlation between the decreasing performances and the increasing quantity of calcium carbonate deposits.