Materials Characterization Approaches for Optimization of Microbial Fuel Cell Electrodes
In this study, structure-to-property relationship of different carbonaceous materials suitable for anode and cathode electrodes development have been developed in details and related to the performance of these electrodes was explored in real MFCs. Commercially available carbon paper (Toray® paper) was studied as material for anodes preparation. Surface parameters, such as wettability, porosity and roughness have been determined and related to the bacterial attachment and biofilm formation, as well as MFCs start up time. On the other end “home-made” activated carbon was studied as a material for the design of cathode electrodes and optimized varying the applied pressure and the magnitude of the temperature treatment step. These two parameters showed significant influence on the cathodes surface chemistry and morphology and thus on the cathodes electrochemical behavior. In both cases surface-to-property relationship approach was applied to understand the similarities and differences in the studied electrodes and highlight the properties that are important for improved MFCs performance.
Carbon paper (Toray® H-090) with different PTFE content (0, 20, 40 and 60%wt PTFE) was used as anode material. The results showed that the increase in PTFE content led to an increase in roughness in both macro (100-300 μm) and micro-scale (5-10 μm) along with an increase of the porosity at macro-scale underling the presence of higher number of large pores . At the contrary, the higher PTFE content led to a lower number of small pores (5-10 μm) that are the one preferred by bacteria for bacterial attachment and biofilm formation . The contact angles measured varied between 135° and 155°, showing high hydrophobicity independent from the PTFE amount. After immersion in wastewater for 2 weeks, the contact angle dropped dramatically to slightly hydrophobic or completely hydrophilic. This phenomenon was mainly due to biofilm attachment on the surface that enhanced the wettability of the materials. Variation of anodes weight over time was also monitored to correlate the materials surface properties to bacteria attachment and further biofilm formation. The materials lost their hydrophobicity proportionally to the PTFE content and due to that the Toray® carbon paper with low PTFE content (0 and 20%wt PTFE) had higher increase in weight (wet and dry) compared to the other materials tested (40 and 60%wt PTFE) (Fig. 1) . The reason is the increased number of small pores, which enhanced biofilm formation. The start up trend followed the biofilm attachment with the faster start up achieved by the Toray® with no PTFE treatment (Fig. 1) . All of these points out the importance of the hydrophilic/hydrophobic properties and surface morphology on biofilm formation and subsequently the start up period of MFCs .
Even more detailed study was carried out for the developed gas-diffusion cathodes based on activated carbon. In this case except the surface morphology parameters, the surface chemistry and charge transfer resistance were included in the surface-to-property study and correlated with the electrochemical performance of the electrodes. Significant dataset was collected and processed through Principal Component Analysis (PCA), which is a statistical tool for data analysis (Fig. 2).
The change in surface chemistry (determined using x-ray photoelectron spectroscopy) due to PTFE variation was found to have significant influence on the power output. The highest current was observed for samples with largest amounts of carbon oxides and oxygenated tetrafluroethylene (cathodes treated at 200°C). At the contrary, the highest resistance as well as lowest performance was noticed for the sample having increased amount of fully fluorinated carbons. It was found out that the magnitude of the applied pressure also determines the resistance of the cathodes showing reverse proportionality. Based on this study we can conclude that a step forward in the MFCs development can be done only with detailed investigation of surface-to-property relationships. Further studies on materials towards deeper characterizations should be addressed and parameter considered until know as insignificant or completely ignored have to be examined.
 C. Santoro, Y. Lei, B. Li, P. Cristiani. Biochemical Engineering J. 2012;62:8–16.