Modeling of a membraneless single-chamber microbial fuel cell with molasses as an energy source

Bunpot Sirinutsomboon 0 0 B. Sirinutsomboon (&) Department of Chemical Engineering, Faculty of Engineering, Thammasat University , 99 Paholyotin Road, Klong-Luang, Pathumthani 12120, Thailand Microbial fuel cell (MFC) is a novel bio-electrochemical system that can use various organic substances as energy source. Computational models of MFC are needed for prediction and optimization of the MFC performance. A comprehensive computational modeling of a membraneless single-chamber MFC, in which bacteria consumed molasses as a substrate, is reported here. The simulated cathode had a layer of polytetrafluoroethylene, which allowed oxygen molecules to diffuse through to take part in the reduction reaction. The substrate molecules diffused through the biofilm, which deposited on the anode surface, and were oxidized by the bacteria localized within the film. The simulation program accepted inputs such as the initial amount of molasses, thickness of the biofilm layer, and dimensions of the MFC chamber. Some outputs of the program include concentration profiles of molasses and oxygen as functions of time and location, and the opencircuit voltage of the MFC as a function of time. As the cathode thickness decreased or the biofilm increased, the voltage increased. To obtain a higher voltage, increasing the biofilm thickness was more effective than decreasing the cathode thickness when the initial COD levels were [5,000 mg/L. - An electrochemical cell that employs bacteria as biocatalyst is called microbial fuel cell (MFC) [1]. An MFC can treat wastewater containing organic matters, which bacteria can consume, while simultaneously generating electricity. Molasses, as a by-product of cane sugar production process, has extensively been used to produce renewable energy such as ethanol and biogas. Wastewater from molasses-based distilleries can potentially be a substrate for MFCs [2]. Microbial fuel cells typically appear in two configurations: two chambers separated by a proton exchange membrane (PEM) or a single chamber. The focus here is in a membraneless single-chamber MFC [3]. The MFC has a cathode electrode with one side in contact with the liquid, while the other side is directly exposed to air. Oxygen in the air can passively diffuse through the cathode and involve in the reaction of oxygen reduction. This process requires no liquid aeration, which is energy intensive, thus saving cost and energy. A study [3] showed that the air cathode MFC produced higher power output in the absence of PEM, which can obstruct the flow of protons. PEMs are also generally quite expensive. As a result, a membraneless single-chamber MFC is simple and inexpensive to build. Figure 1 shows a conceptual diagram of the MFC having molasses, of which primary sugar is sucrose (C12H22O11) [4], as a main source of energy. Computational simulation of an MFC requires a mathematical model that can predict the output and performance of the MFC. Knowledge in electrochemistry, reaction kinetics, and mass transport is required to derive the model. Various attempts to develop simulation models of biofilm on anode and two-chambered MFCs have been discussed [58]. The biofilm model only explained a portion of the Fig. 1 A conceptual diagram of a membraneless single-chamber MFC MFC process, while this paper presents a model describing the entire process of a single-chambered MFC. This type of modeling has never been reported in any literature. Here the results from a computational simulation based on the model are reported, in contribution to an ongoing study on the kinetic nature of the MFC process. For the MFC simulation of interest, the cathodic reaction was the reduction of oxygen [1] as follows: Sucrose was assumed to be the main substrate for the MFC, and made up about 50 % by weight of molasses [9]. With a similar stoichiometric approach as the oxidation of acetate [1], the equation for the oxidation of sucrose was derived. The anodic reaction is then given as follows: C12H22O11 25H2O ! 12HCO3 60H 48e The simulation was developed using Visual Basic program. The simulation program was mainly divided into two sequential segments; cathode and biofilm/anode. Each segment has a number of equations to be solved. The calculated outputs of the former segment were used by the latter. The cathode was assumed to have a layer of polytetrafluoroethylene (PTFE), which is permeable to oxygen but not water. The concentration profile of oxygen in the layer, O2 cat=liq O2 cat=liq H cat=liq which was a function of both time and location, was calculated using Ficks second law of diffusion [10] as follows: concentration of oxygen in the PTFE layer, mmol dm-3 diffusivity of oxygen in PTFE, lm2 min-1 location in the PTFE layer, lm Presumably, only oxygen molecules that completely diffused through the PTFE layer and became dissolved in water could be reduced to yield water molecules. The reduction reaction thus occurred only at the cathode/liquid interface. The rate of oxygen reduction was speculated to follow the Monod equation and the ButlerVolmer equation [7, 11]. The two equations were thus combined into the following equation. rate of oxygen reduction per area, mmol dm-2 min-1 rate constant of oxygen reduction per area, mmol dm-2 min-1 concentration of oxygen at cathode/liquid interface, mmol dm-3 half velocity constant for oxygen, mmol dm-3 electron-transfer coefficient of cathode electron equivalence of oxygen, mmol-electron mmol-oxygen-1 Faradays constant, C mol-1 gas constant, J mol-1 K-1 temperature, K cathode voltage, V standard cathode voltage, V ce =O2 F O2 cat=liqH c4at=liq concentration of oxygen at cathode/liquid interface, mol dm-3 concentration of hydrogen ion at cathode/ liquid interface, mol dm-3 The cathode voltage was calculated using the Nernst equation as follows [1]: jbio z s t ce =Suc ce =b biofilm conductivity, mS lm-1 location in biofilm, lm time conversion = 60 s min-1 volume conversion = 1015 lm3 dm-3 electron equivalence of sucrose, mmol-electron mmol-sucrose-1 electron equivalence of active biomass, mmolelectron mg-VS-1 (assuming C5H7O2N for VS [5]) The local voltage in the biofilm was related to the local potential and described by the following equation [5]. The local voltage at the biofilm/anode interface would be the anode voltage Eano. Sucrose was assumed to be the main substrate consumed by the bacteria, which were localized and dispersed throughout the biofilm. The biofilm was assumed to conduct electrons and considered to be part of the anode [5, 8, 11]. The rate of sucrose consumption was the same as the rate of exogenous respiration by the bacteria. The rate was described by the NernstMonod equation [5, 8] shown below. The active biomass referred to the live bacteria. cCOD=Suc rate of exogenous respiration in biofilm, mmol dm-3 min-1 density of active biomass, mg-VS dm-3 (VS, volatile solids, a measure of biomass) specific growth rate of active biomass, min-1 volume fraction of activ (...truncated)