Performance Analysis of a Microbial Fuel Cell Using Different Substrate Materials for Different loads

This research paper analyses the current density and power density ofaduelchambermicrobial fuel cell using different substrate materials for different resistive loads. Sucrose, glucose and starch were treated as substrate and potassium ferricyanide was used as electron acceptor. The separation of the cathode and anode cell is provided by proton exchange membrane (PEM) in most of the microbial fuel cell (MFC). The most popular proton exchange membranes are nafion, hyflon, zirfon, ultrex CMI-7000 etc. But all of them are not available to use in this part of the world. As an alternate, salt bridge was used in this study as a PEM which is receptive to other ions and chemicals. Different organic materials like sucrose, glucose, starch can be used as substrate as those were available in organic wastage. As the container of bacteria, the sludge of drain of DhakaTreatment Plant was utilized. The voltage and current were measured across 9.81 kΩand 5.91 kΩ resistors. 716.32 mV was measured as thehighest voltage across 9.81 kΩ resistor while 4.65 mA/m2 and 3.09mW/m2 were recorded as maximum current density and power density respectively across 5.91 kΩ resistor for sucrose as substrate. The anode chamber was maintained in anaerobic condition. The temperature during these experiments was 22±2º C.


INTRODUCTION 1
As the energy sources are not sustainable due to the majority share of fossil fuel in energy mix, Bangladesh is an energy deficit country [1,2]. On the contrary per capita waste generation rate is computed as 0.56 kg/capita/day based on the total estimated urban population of the year 2005. In Dhaka city of Bangladesh 4,634.52 tons wastes aredaily generated and 76% is from residential sector [3][4][5]. Generation of electricity and treatment of organic or inorganic wastes can be achieved by the microbial fuel cell.A new form of renewable energy by generating electricity from what would otherwise be considered waste was represented by Microbial fuel cell (MFC) technology. MFCs are electrochemical devices that are similar to conventional fuel cells in which energy from a chemical reaction is converted into useful electricity. This technology can use bacteria already present in wastewater as biocatalysts to generate electricity while simultaneously wastewater istreated [6,7]. Anode and cathode chambers are the compulsory parts of an MFC connected by an external circuit and detached by proton exchange membrane (PEM). Decomposition of organic substrates by microbes produces electrons and protons in the anode chamber. Those electrons and protons are transported through external circuit and proton exchange membrane respectively to the cathode [8]. Electrochemically active microorganisms (EAM) are employed to generate electricity the MFC device which is capable of providing the energy demands for small devices [9,10]. While accomplishing biodegradation of organic matters and wastes in the MFC, as microbe is the most preferred to generate electricity [11,12]. Hydrogen can be produced in Microbial Fuel Cell and it happens smoothly from the fermentation of sucrose in presence of Clostridium butyricum, as overall biochemical reaction is given as follows [13]:

Microbial fuel cell operation
In MFCs, a substrate as carbon source is consumed by microorganisms in anaerobic conditions, produce carbon dioxide, electrons and protons in anode chamber [14]. The simplified FMC concept is shown in Figure 1. Electrons are transferred to the negative terminal (anode) and reach to the positive terminal (cathode) through a load which are produced from the substrates by microorganisms. Several chemicals known as mediators like neutral red, methylene blue, thionine are used to accelerate the generation of current in MFC. Protons are also produced in oxidation reaction and passed to the cathode by proton exchange membrane. Direct link between the electrode surface and membrane bound proteins or by conduction through nanowires by microorganism that contact cells to the electrode surface is responsible for the transfer of electrons [15].

Standard electrode potentials
The half-cell reactions can analyzereactions occurring in the microbial fuel cellstated as follows: For sucrose oxidation, we therefore have, For the cathode potential Ecat if we consider the case where Potassium Ferricyanide is used as the electron acceptor for the reaction, we can written as follows: The Cell EMF is computed as follows [8]: Eemf =Ecat -EAn (6)

Experimental setup and data analysis
Based on the proposed model three setups were developed and observed. All the cells were kept in anaerobic condition and readings were measured across no load, 5.91 kΩ and 9.84 kΩ. Equal amount of surface area was provided for each of the experiments. Plastic bottles were use to construct anode and cathode chamber. The anode surface area was 0.0242 m 2 . The temperature during these experiments was 22±2ºC. The measurements were taken using a multimeter (Model no. KOOCU-DT9205A).

Preparation of Salt bridge
The materials mentioned in the proposed model were used to construct the salt bridges. Sodium chloride was used as the sample of salt to mix with agar powder. Each salt bridge was 5-inch-long and 0.5 inch thick. It consists of 5 g agar, 25 g sodium chloride as the salt sample dissolved in 250 mL of water for construction of the salt bridge. At first all the components were taken in a pot then the materials were mixed during the heating process. The mixture was heated until the temperature reached 85°C. The hot solution was poured into PVC pipes having length of 5 inch and thickness of 0.5 inch. The temperature was measured by a digital thermometer. The dimensions of different parts of constructed MFC for setup 1, 2 and 3 are provided in Table 1.

Setup 1
For the first setup 400 mL of Potassium Ferricyanide was used in cathode chamber. Zinc and copper plates were used as electrode material in anode and cathode compartments, respectively. A salt bridge was placed between anode and cathode to provide a chanel for the protons to pass from anode to cathode. The amount of starch used was 125 g. As the container of bacteria thick muddy layer of Dhaka North City Corporation's drain was used. At first 1 kg of muddy layer was mixed with 0.25 litre of drain water. Then 0.5 litre liquid was used from that mixture in the anode chamber. The measured voltage, current and power obtained from setup 1 were represented in Figures 3, 4

for different loads
Comparisons among practical setups Two resistors (9.81 kΩ and 5.91 kΩ) were treated as the load to measure the current generated by the MFC. The values of the current were obtained in micro range (10-6) which leads to inadequate power production from the experiments. The maximum current density recorded for setup 1, setup 2 and set up 3 were 3.05 mA/m2, 4.65 mA/m2, 3.46 mA/m2, respectively. The maximum power density recorded for setup 1, setup 2 and set up 3 were 3.05 mW/m2, 3.09 mW/m2, 2,09mW/m2, respectively. All the cells were kept in below 250C. From the result recorded it is seen that sucrose provided more stable outputs than other substrates. As all the substrates used here are available in organic wastes (like rotten rice, vegetables) so these experiments will show the way to utilize the wastes in a desired manor through MFCs. The current density and power density obtained against 9.81 kΩ and 5.91 kΩ from three setups are illustrated in Figures 12, 13, 14 and 15, respectively.

CONCLUSIONS
Alternative energy production technologies are gaining importance for battling future energy crisis. MFCs could be an effective means to accelerate bioelectricity production. This paper was unable to examine the entire field of MFC research in detail but hopes to highlight some important points. Low current density is a barrier for implementation of MFC in practical level. A parallel combination of such type of MFC could increase the current value in a modest level might be a formal solution. Further researches about bacteria and anode, cathode material could lead this experiment in a fruitful situation.