Development of Ceramic Microbial Fuel Cells to Generate Electrical Energy from Urea

Abstract

Microbial fuel cells (MFC) are an ingenious technology that allows the integration of electricity generation on conventional biotechnology processes [1]. Wastewater treatment for instance is expensive and highly energy demanding and MFC can be an opportunity to address some of its issues. We have been developing a ceramic microbial fuel cell (CMFC) that can treat and clean water whilst generating electricity. The CMFC consists of a carbon cloth coated with biofilm of anaerobic sludge as bioanode and carbon cloth coated with chemically deposited Polytetrafluoroethylene as a gas diffusion cathode. Both electrodes were tied with a nickel-chromium wire to allow for the electrical measurements. The separator consisted of a cup shaped ceramic containing bioanode inside and cathode outside (Figure 1a). The ceramic cup allows for the electrolyte to flow through the pores of the ceramic from the anodic chamber to the cathode to maintain the ionic conduction as well as to filter the wastewater by holding the organic matter inside the chamber and in contact with the bioanode. Electrochemical experiments showed the capacity for this CMFC to degrade urea and acetate as nitrogen and carbon sources, respectively, with the generation of electricity. Although the CMFC showed great potential for the urea bioelectrochemical oxidation, it was observed that the carbon source is largely a limiting factor for the biofilm growth as well as the bioelectrochemical performance, but changes on pH or substrate concentration can be other limiting factors [2]. For a continuous two-week experiment, polarization curves were performed on the CMFC daily to measure the electrochemical capacity to generate electricity as well as to evaluate the microbial fuel cell performance. For this experiment, it was observed that the open circuit potential (OCP) increased from around 400 to 460 mV while electrical current increased from around 480 µA to around 2400 µA. The polarization curves shape appeared to have a mass transfer limitation performance on the first week, but this changed drastically to a non-mass transfer limitation performance on the second week (Figure 1b). In fact, the biofilm that covered only the very top of the carbon surface on the first days appeared to be more uniformly distributed around the carbon fibers after two weeks which appears to be an adaptation to the electrochemical system. The water collected from the cathodic side appeared to be clean in contrast to the darkish wastewater. We believe that this is an opportunity to be used on a wastewater treatment plant, since the ceramic can be easily formed with different shapes, and it may be possible to develop pipes containing electrodes to transport wastewater while treating it and generating electricity.[1] Liu, H.; Ramnarayanan, R.; Logan, B.E. Production of Electricity during Wastewater Treatment Using a Single Chamber Microbial Fuel Cell. Environmental Science and Technology, v. 38, no. 7, 2004.[2] Xu, W.; Wu, Z.; Tao, S. Urea-Based Fuel Cells and Electrocatalysts for Urea Oxidation. Energy Technology, v. 4, i. 11, 2016.The authors are greateful to FAPESP-Brazil (process 2020/01627-0) for financial support. Figure 1