© Serials Publications IJCE : 7(1), 2014: 1-9

Bioelectricity Generation in a Microbial Fuel Cell: Study of Cement /Ash Electrode Akuma O., Ijoma C., Opara C. C. and Oduola M. K. Department of Chemical Engineering, University of Port Harcourt Port Harcourt, Nigeria, E-mail: [email protected]

ABSTRACT: Dual chamber MFC with saline catholyte was used in this laboratory scale study on the effects of composites of cement and charcoal as electrode on energy output. Charcoal-cement electrodes were prepared of varying compositions from 20% to 50% cement in the composite. All the electrodes were manufactured to the same dimensions with area of 2.42×102 m2. Produce water was used as the substrate, with its microorganism as the biocatalyst. The cells were operated at room temperature and pH of 6.6. All the cells were operated for 28days. The set-up of 50/50 ratio of charcoal to cement produced the maximum voltage of 85mV on the 17th day of the while the MFC units with a 40/50 ratio of charcoal to cement composition produce maximum voltage of 57mV after 21days. The electrode with a ratio of 70/50 was observed to have the lowest voltage; this can be attributed to the toxicity of the cement to the microorganisms at this ratio. Keywords: Microbial fuel cell, proton exchange membrane, charcoal, cement.

INTRODUCTION A microbial fuel cell (MFC) is a hybrid of biological and electrochemical reactors. The system is made up of an Anode chamber where anaerobic breakdown of organic and non-organic substrates takes place by anode-respiring bacteria (Torres et al. 2007, Picioreanu et al. 2007). This bacteria also shuttle the produce electron in this chamber through the anode-biofilm to the external circuit. The dualistic nature of the unit results in the generation of electric current and biodegradation of the substrate tact (mostly wastewater – agro, industrial and domestic) (Park and Zeikus 2003). MFC technology offers great promise for generating renewable energy and treatment of wastewater. The perceived advantage of MFC technology created a drive for research on how to harvest electricity from the system and a quest to elucidate challenges posed by low power generation from MFCs. (Nevin et al. 2008). The typical MFC (see Figure 1) unit consists of the anode and cathode chambers separated by a proton exchange membrane (PEM), the anode and cathode electrodes and external circuit or load. The anode chamber holds the substrate

(fuel) while the cathode chamber contains a solution of an oxidizing agent where anaerobic degradation of organic matter occurs in the anode chamber, a process which leads to production of proton (H +) and electron (e–) the electron thus produced are transferred to the anode electrode while the H+ passes through the Proton exchange membrane to the cathode chamber where the electron combine with the proton and oxygen to form water.

Figure 1: A General Layout of a MFC (Pant et al. 2009)

Anodic Reaction C6 H12 + 6H2 O � 6CO2 + 24H+ + 23e– Cathodic Reaction O2 + 2H2O + 4e– � 4OH– Bacteria; exogenous mediators (those external to the cell) such as methylene blue, thionine, neutral red, etc.; were initial used for direct transfer of electrons from electrochemically active bacteria cells (cytochromes) which shuttle the electron

to the electrode. The toxicity of these mediators has made the mediatorless MFC more adaptive in practice. Two mechanisms for transfer of electrons from the bacteria to the anode are recognized in literature: electron shuttles (Rabaey and Verstraete, 2005) and conduction (Reguera et al., 2006). In the electron-shuttle mechanism, the electrons are transferred from bacterial cells to a soluble Anode electrode biofilm (i.e., the shuttle, also known as a mediator). The reduced shuttle electron then diffuses to the anode to discharges its electrons, and then diffuses back to the bacterial cells to repeat the process. This mechanism laid claim to the need for optimisation of electricity generation in MFC as electrode dependent, both in the generation of biofilm and site for the anaerobic metabolic reaction. Researchers highlighted the link between electrical potential and the bulk substrate in the anode and the diffusion of electron through the anode electrode (Wang 2000, Lee et al. 2009 , Liu and Logan, 2004, Liu et al., 2005, Lovley, 2008; Logan and Regan, 2006; Marsili et al., 2008). MFC has remained a promising source of energy necessitating a lot of research works on different substrates and other reactor configurations, PEM, oxygen sink in the cathode chamber etc. This research work is aim at checking the effect of composition of electrode made of Charcoal-Cement mixture on the power generation of a Microbial Fuel Cell using a dual chamber MFC, water catholyte and Produce water as substrate on a laboratory scale. This can produce a cheap material for MFC unit with additional reduction of the unit cost. MATERIALS AND METHOD The Charcoal used for the cement-charcoal electrode produced from Sawdust collect from a saw mill at Rumuosi, a community near University of Port Harcourt, Port Harcourt, Nigeria. The cathode and anode chambers were plastic container of 5 liters each, while the proton exchange membrane (PEM) casing was made from a PVC plastic pipe. The MFC form the experiment was tagged based on the ratio of Charcoal-cement mixture a shown in table 1. Preparation of the Electrodes The Sawdust was checked to ensure that it is free of foreign objects. It was then placed in an oven tray and heated at 200 °C to ash. The charcoal was ground to a fine powder. The charcoal was mixed with cement at a certain ratio (See table 1) with small quantity of water 100 ± 5ml. A PVC trunk was used as mold to the desired length (24.2cm) the mixture was poured into the mold with a flexible wire inserted at the middle. The charcoal-cement mixture was allowed to solidify. Preparation of the PEM, Salt Bridge Agar-agar (at concentration of 40g/L) was dissolved into distilled water and salt added (7.5g of salt in 40g of agar); the mixture was autoclaved at about 121°C for

Table 1 MFC Setup Tag Number S/N

Ratio of Charcoal to Cement

1 2 3 4 5

30g/50g 40g/50g 50g/50g 60g/50g 70g/50g

MFC Unit Tag MFC30 MFC40 MFC50 MFC60 MFC70

15mins. The agar/salt mixture was poured into PVC plastic pipe while still warm and then allowed to cool and solidify (about 30mins). The PEM pipe was sterilized with ethanol to free the internal surface of the pipe from microorganisms; the open ends were sealed with masking tape to prevent microorganisms from acting on the internal surface after sterilization. Preparation of Cathode and Anode Chambers The Anode chamber contains substrate, produce water (collected from an onshore Oil and Gas facility in Niger delta, Nigeria). The cathode chamber contains water mixed with salt to improve conductivity (salt-water – 250g of salt is dissolved in 30litres of water). The high strength Produce water was poured into the 5.0L anode chamber of each set-up. MFC Set Up The prepared produce water was poured into each of the anode chambers and the saline water in the cathode chamber for the five (5) set-ups of the MFC. The anode was covered and sealed to prevent oxygen from entering. The anode chamber must be anaerobic. The cathode chamber (the oxidant) was filled with the catholyte. Power Generation and Parameter Monitoring The MFC cells were monitored simultaneously for a period of 28days during which the power generated was measured each day. The current and voltage values were measured using a digital multimeter (DT-830). The derived readings of current and power densities were determined by applying the experimental data on the applicable equations (1 and 2). Power density was expressed in mW/m 2 normalized to the projected surface area of the graphite anode (m2) (Rabaey and Verstraete, 2005).

P

Current(mA) Volts( v ) Surface area of projected anode(m2 )

And current density C, expressed as:

(1)

C

Current produced mA Surface area of projected anode m 2

(2)

The data were also analyzed graphically by plots voltage and current versus time to obtain the cell performance and polarization curves (Rabaey and Verstraete 2005). Measurement of Chemical Oxygen Demand (COD) and Biological Oxygen Demand (BOD) The BOD, COD, temperature and pH were measured in the anodic chambers of Cells during the operation according to standard methods (APHA, 1998). Thus, the performance of MFC was evaluated by estimating removal efficiency. The initial and final substrate BOD and COD was measured. The pH, BOD and COD were determined at the commencement of operation of the cell using standard methods. The data of the MFC set up is presented in table 2. Table 2 Table of MFC Unit Data and Substrate Parameter Electrode Size

2.42×10-2 m 2

pH

6.63

Temperature

29 °C

BOD

280mg/L

COD

2100mg/L

Anode Substrate

Produce water

Catholyte

Salt Water

Reactor Volume

5 Liter

Reactor Material

PVC

RESULTS AND DISCUSSION The data from the use of five (5) MFC units with different electrode compositions are presented at this point highlighting the volume of power generated by the various units. MFC30 The values of voltage produced (see figure 2) from this unit having 30g of charcoal and 50g of cement showed a maximum voltage of 50mV on the 11th day of operation. Total voltage for the 28days was 17% of the total. The volume of the binder was more than the reaction (carbon) surface and the anode biofilm build up and reaction support being commensurate to the reaction surface (Lovley, 2010). The reduction in performance must not be unconnected to the active reaction surface and a

corresponding negative effect on the rate of electron transport across the biofilm. The total cumulative value of voltage generated is 1051mV for the 28 days recording the lowest cumulative voltage of five (5) set ups. The maximum power density (equation 1) of 14.42mW/m2 was recorded on the 11th day. MFC40 The MFC40 having 40g of charcoal generated maximum voltage of 57mV on the 21st day. Total cumulative voltage records 1141 mV difference from the MFC 30. The effect of the quantity of binder and toxicity of the cement has a role that is not very prevalent in this unit. The maximum power density of 14.48mW/m 2 was recorded on the 21st day. The current generation was relatively stable without much plateau as seen in figures 2 and 3. MFC50 The MFC50 produced the highest cumulative voltage of 1743mV for the 28days. This unit produced the lowest voltage of (21±1 mV) between 3rd and 7th day and recorded a continuous increase after the 7th day to the maximum voltage of 85mV on the 17th day and the highest Power density of 18.13mW/m 2 on the 21st day. This charcoal-cement unit recorded the best mixture of the five (5) units. This unit also recorded a relatively stable voltage after the 14th day and further recorded low effect of internal resistance in the system.

Figure 2: Voltage against Time

Time (Days) Figure 3: Current against Time

Figure 4: Power Density against Time

MFC60 As observed in Figure 2, the effect of reduction in the binding effect of the cement and its toxicity played a role on voltage generation as it recorded lower value than the MFC50 unit. Although the unit was relatively stable, the maximum voltage recorded was 52mV on the 20th day of operation and Power density of 14.51mW/ m2. The total cumulative voltage was 1135mV for the 28 days (see figures 3 and 4). MFC70 The MFC70 unit had increased effect on binder reduction and cement toxicity. This can be attributed to the role of pore spaces for electrode catalytic reaction leading to the lowest cumulative voltage of 968mV for the 28 days (see figures 3 and 4). The unit records a maximum voltage of 50mV on the 12th day and Power density of 17.60mW/m2. In general, the maximum power density was produced by the MFC with a ratio of 50g Charcoal to 50g cement. This unit also produced the maximum cumulative voltage 1743mV and Cumulative power density of 283.17mW/m 2. The high power output from the MFC50 was attributed to low toxicity of the mixture and high pore space for reaction that produces more electrons. CONCLUSION This experimental study presented a preliminary study on the performance of the designed MFC. Electricity was generated using produce water, at a power density that depends greatly on the composition of electrode of the MFC. The findings from this study imply that wood charcoal could be used as the electrodes of the MFC and the charcoal/cement composition of the electrode to a good extent has effect on the power generated. The readily available low cost of wood charcoal in this country can be a solution for the issues in economically scale-up of MFC. Further studies are required to determine the limiting factors of the toxicity of the cement using a microscope scan analysis of the electrode. REFERENCES [1] APHA, (1998), Standard Methods for Examination of Water and Wastewater. 20th Edition, American Public Health Association/American Water Works Association/Water Environment Federation, Washington, DC. USA. [2] Lee, H. S., Torres, C. I., Rittmann, B. E., (2009), Effects of Substrate Diffusion and Anode Potential on Kinetic Parameters for Anode-Respiring Bacteria. Environ. Scienc. Technol. 43, 7571–7577. [3] Liu H., Cheng S. A., Logan B. E., (2005), Production of Electricity from Acetate or Butyrate using a Single-chamber Microbial Fuel Cell. Environ Sci. Technol 39: 658–662. [4] Liu H., Logan B. E., (2004), Electricity Generation using an Air-cathode Single Chamber Microbial Fuel Cell in the Presence and Absence of a Proton Exchange Membrane. Environ Sci. Technol 38: 4040–4046.

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