Electrode material properties for designing effective microbial electrosynthesis systems
The electrode material is one of the key components of a bioelectrochemical system (BES) as the biocatalyst or the electroactive biofilm will develop on this base material and hence plays a pivotal role in regulating the type and rate of electron transfer processes and bioelectrochemical conversions. Microbial electrosynthesis (MES) requires biocompatible electrode materials with a high surface area that can support the effective development of microbial biomass at high current densities to provide high product titers. Carbon-based materials are the best available practice today; yet, resolving which of their properties are truly relevant to achieve these goals remains elusive. The present study shows that biofilm coverage and the relative abundance of cell-bound polymeric filaments are directly proportional to the total charge consumed over time. The combination of high biofilm coverages and high relative abundance of the cell-bound polymeric filaments resulted in low charge transfer resistances, as determined by electrochemical impedance spectroscopy. Although a wide variety of the physicochemical parameters of the supporting carbon electrode materials (electric conductivity, specific surface area, porosity, roughness, thermogravimetric mass spectrometry, etc.), were characterized in this study, the only one that showed a consistent correlation with the total charge consumed by the electroactive biofilms was the contact angle. This suggests that the hydrophilic moieties and surface tension are two fundamental parameters to consider for an effective design of microbial electrosynthesis biocathodes. Among the carbon electrode materials used in the present study, the activated carbon-based VITO CORE™ electrode was considered the most suitable electrode material to provide such desired features while other electrodes exhibited roughnesses much higher than this biologically-amenable range. Thus, the superior wetting characteristics of the VITO CORE™ electrode seem highly reliant on the roughness provided by the manufacturing method (i.e., cold-rolling).