Cathode Improvements in Microbial Fuel Cell (MFC): from the Platinum-based Cathode to the Bio-cathode

Microbial fuel cell is a promising technology to clean wastewater and generate environmentally-friend electricity. Microbial fuel cells (MFCs) had several problems that hindered the technology to achieve the real world applications. Low power generation, low organic compounds removal and especially high electrode costs relegate the MFC technology still in lab-scale applications. A reduction of costs has been achieved with the membrane removal in a single chamber membraneless air-cathode microbial fuel cell (SCMFC) system. Even if the platinum is the best-known catalyst for the oxygen reduction reaction, facing the anodic solution due to the membrane removal, the catalyst is easily poisoned by the organic substances naturally present in the wastewater. The research aims to a better understanding of cathode processes in a long terms operation. Power curves measurements were necessary in order to compare the MFC systems and electrochemical LSV were used to compare different electrodes (structure and compositions) separately. Microbiological analysis was also conducted in order to confirm the results obtained by the electrochemical tests. Chemical Oxygen Demand (COD) removal measurements were used to study the organic compounds degradation in the MFC system over time. Particularly, the proposed research aims to improve the cathode structure, reduce the platinum loading (and consequently the costs), use the biofilm as bio-catalyst and at last use enzyme as catalyst in order to increase the cathode reaction, power generation and organic compounds removal. The results showed that: firstly, the addition of the micro porous layer (MPL) between the catalyst layer and the carbon cloth and the removal of the external PTFE layers increased significantly SCMFC output and organics removal due to a better oxygen penetration to the cathode catalyst. Secondly, the reduction in platinum loadings at the cathode of 2 orders of magnitude (from 0.5 to 0.005 mgPt/cm2) decreased the performances of just 10-30% despite the significant reduction in costs. Thirdly, platinum-based and platinum-free cathode SCMFC have been investigated in 20 weeks experiments and in long terms operation, the biofilm formation acted as biocatalyst enhancing the oxygen reduction at the cathode and similar performances with the Pt cathode were achieved. Fourth, for the first time, enzymatic based cathode have been compared to the platinum cathode. Better cathode polarization curves due to a much higher catalytic activity of the bilirubine oxidase used as cathode was reached. Moreover, double power generation compared with the Pt cathode was obtained. The research has been also enlarged with the study of water transport through the cathode materials and detection of oxygen into the anodic chamber and the utilization of human urine as fuel for SCMFC. Water transport decreased over time due to the biofilm formation and mainly to the precipitation of salts through the fibers. At last, power was generated using human urine as fuel in SCMFC and struvite has been recovered due to the high pH in the anodic chamber that enhanced the salts precipitation in the system.