BIO301 Industrial Bioprocessing and Bioremediation
Chemostat report (Draft)
Microbial Fuel Cell
Good Report already: (C-D range because of
Good understanding.
Good work in general
Sufficient detail
Improvoments could be :
Less grammar mistakes
Consistent use of tenses (past tense)
Better and more complete figures
More focused writing of results :
Need of experiment (could point out literature)
Aim and significance of the aim for each figure followed by
how it was done and
what was found and
concluding wether the aim was reached, then perhaps
referring to literature
Group 1 & 2
Asiah, Jonathan, Melissa, Ben, Florence, Beatrice[RC1]
Optimization of Graphite-mediated, Acetate-fed Microbial Fuel Cell.
- INTRODUCTION
Energy is the single greatest challenge facing humanity[RC2]. Microbial fuel cells (MFC) can be used to convert biochemical energy to electrical energy (Allen and Bonetto, 1995).The MFC utilize bacteria that capture and process energy by converting chemical energy from organic matter to electrical energy via both abiotic and biotic catalysis (Bullen et al. , 2006).
Bacteria grow naturally by catalysing chemical reactions and storing energy in the form of ATP. In some bacteria, reduced substrates are oxidized and electrons are transferred to respiratory enzymes by NADH. These electrons flow down the respiratory chain thus producing translocating protons and consequently a proton gradient. These electrons are finally released transferred to soluble terminal electronegative electron acceptors such as sulphate, nitrate or oxygen[RC3].
The MFC are composed of an anode, a cathode and the electrolyte. In the MFC, bacteria catalyse the oxidation of reduced substrates thus releasing some of the electrons that are produced from cell respiration to the anode where they flow through an external circuit to the cathode and eventually current is created (Heller 2004). To maximize the deposition of electrons on the anode and to close the electrical cycle, a proton exchange membrane is can be installed to separate the anodic and the cathode compartments. The charge balance is maintained by transferring the protons i.e. H+ through the proton exchange membrane to the cathode chamber where it forms water (Mohan et al., 2007).
Not all the substrate is utilized by the bacteria and therefore not all the high energy electrons will be transferred to the cathode chamber. The coulombic efficiency i.e. the fraction of electrons recovered as current versus the maximum possible recovery is used as a measure of how much usable energy is available during discharging compared with the energy used to charge the cell. The amount of electrons transferred to the cathode chamber is expressed in terms of the coulombic efficiency (%) which calculates the number of electrons transferred to produce electricity over the total electrons oxidized to the cathode chamber. This parameter is a useful measure of the overall efficiency of the MFC (Liu et al., 2004)[RC4].
A Polarization curve is used to establish the relationship between the anodic potential and the microbial activity in the MFC so that a steady potential of the anode is established. As a result, this measures the power output of the MFC. The use of a larger resistor causes an increase in the anodic potential. Substrate oxidation rate increases with the anodic potential and therefore electron flow from the anode to the cathode is proportional to the rate of substrate oxidation by the bacteria (Serway and Faughn, 2003).
The reactions occurring in the MFC can be analyzed in terms of the half cell reactions, or the separate reactions occurring at the anode and the cathode. The anode potential determines, apart from the metabolic pathway used, the theoretical energy gain for the biocatalyst (Schroder 2007). Thus, the lower the anode potential, the less energy per electron transferred there is available for growth and cell maintenance. A higher anode potential may enhance the growth rate of bacteria, resulting in a higher biocatalyst density, faster starting up of the electricity generation and overall, higher current generation. However, in order to maximize the electrical energy output of a MFC, for a set current the anode potential should be as low and the cathode potential as high as possible (Logan et al.. 2006). This leads to a trade-off between the anode potential wanted for the end user and the biocatalyst, suggesting the existence of an optimal anode potential range satisfying both[RC5][RC6].
In this study, we aimed to obtain a efficient microbial fuel cells using acetate as the substrate at optimum conditions by
- investigating the optimum resistance using a Polarization [RC7]curve
- investigating the difference in anode potential
- investigating the substrate limiting conditions within a microbial fuel cellquantifying the response of a MFC to a substrate spike by determining the coulombic efficiency.
- METHODS AND MATERIALS
2.1.Microbial Fuel Cells(MFC) Start up
A reactor with one compartment of microbial fuel cell is used in this experiment. The fuel cell is divided into two chambers where with the anode lined at the bottom of the cells while cathode is at the top. These two electrodes are separated by a cloth membrane[RC8].
Anode consists of 2/3 the total reactor volume and dimension. Both anode and cathode were filled with conductive granular graphite (provided by Cheng Ka Yu ; Chenget al.. 2008) which then reduced the total liquid volume of reactor to 160mL. Describe the Material in terms of size Temperature is kept at 30 °C using two aquarium heaters in water bath.
The MFC was operated as a batch mode with sodium acetate as the only electron donor. Yeast extract was added to anode (0.1 g/L final concentration) every three to five days to enhance growth. The anode chamber is managed in an anaerobic condition where limited exposure to air as much as possible.
The activated sludge used as the inoculum in this experiment (provided by Cheng Ka Yu ; Chenget al.. 2008) has thehad a[RC9] biomass concentration about 2.0 g/L. Combination of ten percent of this activated sludge (v/v), synthetic wastewater, 1 mL/L of trace element and 50 mM phosphate buffer was mixed and inoculated into the reactor. The composition of the synthetic wastewater and the trace element is shown in Table 1 and Table 2 respectively (Chenget al.. 2008). Acetate is injected to anode chamber using a sterile syringe to replenish the electron donor in the rector while it was running in a batch mode.
Table 1 Composition of different components in synthetic wastewater.
Components / Composition (mg/L)NH4Cl / 125
NaHCO3 / 125
MgSO4 ·7H2O / 51
CaCl2 ·2H2O / 300
FeSO4 ·7H2O / 6.25
Table 2 Composition of different components in trace element.
Components / Composition (g/L)ethylenediamine tetraacetic acid (EDTA) / 15
ZnSO4 ·7H2O / 0.43
CoCl2 ·6H2O / 0.24
MnCl2 ·4H2O / 0.99
CuSO4 ·5H2O / 0.25
NaMoO4 ·2H2O / 0.22
NiCl2 ·6H2O / 0.19
NaSeO4 ·10H2O / 0.21
H3BO4 / 0.014
NaWO4 ·2H2O / 0.050
good
2.2.Calculation and Analysis
2.2.1.Determination of Voltage, Current and Power generation
Graphite rods were used to connect the external circuit with anode and cathode in the reactor. A variable resistor is placed between these rods which can be adjusted to determine a known resistant, R in the circuit. Potential difference between the anode and cathode (voltage, V) is measured using National Instruments LabVIEW 7.1 software connected via LabJack as shown in Figure 1 below. This software also calculated the current, I by using the Ohm’s Law (I = V/R).
2.2.2.Polarization Curve generation
Polarizations Curvesis were generated to determine the optimum resistancet[RC10] that gives rise to maximum power that the MFC can generate. Power is calculated according to P = V x I. While the reactor is was running as in batch mode, the reactoriis was leaved left[RC11] at open circuit for an hour. The resistancetis was[RC12] changed by switching the variable resistor to smaller resistant in a systematic order. For every resistant[RC13], a period of 15 minutes is allowed for the voltage reading to stabilised before changing the next resistant. Reverse polarization curve is generated by using the same known resistorsant but in reinverse order.
2.2.3.Measuring the Anodic Potential
By measuring the potential against the silver/silver chloride reference electron, the anodic potential can be determined. This reference electron is placed within the anode chamber. The reference electrode is also connected to LabJack and monitored by the software as the anode and cathode.
2.3.Chemostat Start up
To start up the chemostat, the batch culture is was flushed in and out with Medium (with no electrons donor of acetate). Using the peristaltic pump, medium (containings 1 mM of sodium acetate) with a flow rate of 5mL of medium per hour were adjusted to[RC14] feed into anode as a fed-batch mode. The medium is sterilised, stirred and placed in ice to avoid contamination.
Current is converted to amount of electron generated by using the following formulaes, 1 coulomb (C) = 1 amp x 1 s and 1 C = 6.24x 1018electron[RC15]. Coulombic efficiency is calculated by the integration of current against time plot[RC16][RC17].
- RESULTS AND DISCUSSION
3.1.Polarization Curve.
The establishment of a polarization curve enables us to determine fuel cell function based onthe steady state conditions at various resistanceresistances. Two[RC18] polarization curves (Forward and backward) were set up in this study
As shown in Figure[RC19] 2, the resistor of 100 ohms produced maximum power as for both forward(87.52 mW) and backward polarization (93.55 mW) (Figure 2). In the figure[RC20], the optimum resistance for the MFC is found to be 100 ohms. However, there is a significant difference in the level of power output in both curves. This can be attributed to the difference in level of activity microbes to generate electrons.
In the forward polarization curve, the resistor was changed from a higher resistance ( 1 M ohms ) to lower resistance (5 Ohms). On the other hand, backward polarization curve was carried out by altering resistance from lower resistance (5 Ohms) to higher resistance ( 1 M ohms[RC21]).
Considering the amount of activity the activated sludge have to put in to overcome the resistance, the difference in power output is significant in figure 2. For forward polarisation curve, the MFC is first exposed to a level of high resistance, forcing the cell to increase its activity (substrate degradation). As resistance is lowered at a time interval of 15 minutes, the pressure for the MFC to generate electricity is reduced. Therefore, it is easier for the cell to generate a high level of electricity, giving a higher power output.
On the other hand, if the resistance is increasing (as seen in a backward polarisation curve), the cell would be exposed to a higher resistance every 15mins. Therefore, the MFC have to adapt to the increasing pressure to overcome the resistance, thus giving a lower power output[RC22].
FIGURE[RC23]2: Polarization Curve of the microbial fuel cell obtained by plotting power MW against Current mV. 1mV produced the highest power of 87mW operating at 100ohms resistor for the forward curve while 0.68mV produced the highest power of 46.46mW for the backward curve operating at 100ohms resistor[RC24].
3.2.Anode Potential[RC25]
Microbial Fuel Cells (MFCs) are devices involving activated bacterial cells as catalysts in order to oxidise either organic or inorganic matter to generate current. It is established in Logan et al. (2006) that electricity can be generated from any biodegradable material, ranging from pure compounds such as acetate, to complex mixtures of organic matter such as domestic wastewater.
In a microbial fuel cell, electrons produced by the bacteria from the breakdown of substrate (acetate in this case) are drained off and transferred to the anode (negative terminal), through a conductiveon material containing a resistor, and flowing into the cathode (positive terminal), at which the electrons combine with protons and electron acceptor (oxygen) to form water (Logan et al., 2006[RC26]).
The microbial fuel cell converts energy available in a bio-convertible substrate (acetate) directly into electricity by bacteria switching from their natural electron acceptor (such as oxygen) to an insoluble acceptor (MFC anode) (Rabaey et al., 2005). Since the difference in potential generated by the electron flow produces electricity in the fuel cell, electron liberation at the anode and subsequent electron consumption at the cathode are the defining characteristics of an MFC[RC27].
It is also important to note that electrons within a MFC can be transferred from the bacteria to the anode by either electron mediators or shuttles (direct membrane associated electron transfer) or nanowires produced by bacteria (Logan et al., 2006[RC28])
Discussion of Anode Potential
Figure 3: Anode potential, Cathode potential and voltage over time. Addition of sodium acetate as substrate at 2.30pm[RC29].
3.2.1.Positive anode potential before acetate addition
From the figure above, it is observed that the anode potential is positive before acetate is pumped in at 2.30pm. It is attributed to the presence of oxygen found within the anaerobic anode section of the MFC. Having just established and starting up a chemostat, oxygen found within the fresh medium, deionised water and tygon tubings may have contributed to the positive anode potential by acting as a strong electron acceptor, taking in any moving electrons found within the anode.
3.2.2.Decreasing anode potential
As the bacterial cells found within the anode breaks down acetate as an electron donor, electrons are transferred to the anode, causing a gradual decrease in anode potential. Since electrons are considered to be negatively charged, the anode potential is observed to be more negative (decreasing anode potential) because the graphite found in the anode is accepting the electrons, before transferring them through the resistor, to the cathode for oxidation.
3.2.3. Decreasing cathode potential
A decreasing cathode potential is also identified in this graph along with the decreasing anode potential. This observation could be attributed to the presence of a stronger anode as compared to the weaker cathode. In this case, the rate at which the anode is transferring electrons over to the cathode is much faster than the rate at which oxygen accepts the electrons on the cathode. Hence, electrons are said to be accumulated at the cathode, causing a gradual decrease in cathode potential[RC30].
3.2.4.Increasing voltage
Since voltage is defined as the difference in potential generated by the electron flow within a microbial fuel cell, a significant decrease in anode potential generated by electron transfer against a gradual decrease in cathode[RC31] potential generated by oxygen as an electron acceptor would identify a gradual increase in voltage observed within the cell.
Metabolism in microbial fuel cells
Figure 4: Current vs Anode Potential over time after addition of sodium acetate[RC32].
The determination of metabolic pathways governing microbial electron and proton flow is constantly used to assess bacterial electricity generation. Since current is defined as the flow/movement of electrons, the anode potential can be used to establish bacterial metabolism. A decrease in the potential of the anode after the addition of acetate shows that the bacteria is forced to deliver the electrons through more-reduced complexes, increasing MFC current flow. (Rabaey et al., 2005)
Bacterial cells in the MFC are able to gain energy simply by transferring electrons from a reduced substate at a low potential (acetate), to an electron acceptor with a high potential (oxygen)
Optimisation of Anode Potential[RC33]
Anode
Since electrons produced by the bacteria are transferred to an insoluble acceptor (graphite anode), anodic materials have to be conductive, biocompatible and chemically stable in the reactor solution (Logan et al., 2006). It is identified by Logan et al. (2006) that the most versatile electrode material is carbon graphite plates or rods as they are relatively inexpensive, easy to handle and have a defined surface area[RC34].
In order to improve the anodic performance of a MFC, different chemical and physical strategies could be implemented, including physical (larger surface area with graphite felt electrodes) and chemical strategies (Mn(IV) and Fe(III) to mediate electron transfer) to the anode (Logan et al., 2006).
An experiment by Chaudhuri et al. (2003) identified that increasing the surface area of graphite available for microbial colonization yielded increased power output. Similarly in a study led by Rabaey et al. (2005, 2003) have established faster increases in electricity conversion is observed when larger anode surfaces were available for bacterial growth. Likewise, soluble redox mediators have been added to MFCs to enble bacteria to have a sufficiently high turnover rate[RC35] in relation to the electrode.
Cathode
[RC36]Electrons transferred to the cathode are oxidised accepted by the atmospheric oxygen (electron acceptor for MFC) due to its high oxidation potential, availability, low[RC37] cost, sustainability and lack of chemical waste product (as the end product is water) Logan et al. (2006). Therefore, in order to yield a higher voltage output, the cathodic potential has been optimised in order to great a greater difference in potential energy for electricity conversion. Firstly, the surface area of the cathode has been increased by the addition of grooves on the graphite, maximising the surface contact between oxygen in the atmosphere and the electrons to be oxidised accumulated in the cathode.
Figure 5: The effect of increasing surface area of cathode on the voltage yield in the MFC.
In the graph above[RC38], it is clearly observed that the addition of grooves [RC39]on the cathode at around 2.28pm resulted in a net increase of voltage production by the cell through a higher cathode potential. This is because by maximising surface contact between oxygen and the electrons in the cathode, there is a larger potential difference created between the anode and cathode, thus driving the cell to generate more electricity in the form of electron flow.
However, it is also identified that oxygen is a poor electron acceptor on graphite due to its slow kinetics of oxygen reduction, resulting in a large overpotential at the cathode, and it restricts the use of noncatalysed material to systems Logan et al. (2006[RC40]).
3.3.Determination of Coulombic Efficiency & Results
Coulombic efficiency (CE) can be determined from a closed circuit microbial fuel cell through the introduction of a specified amount of sodium acetate into the anodic chamber of a starved culture. Data required for calculation of CE can be obtained in an excel spreadsheet from National Instruments LabVIEW 7.1 set to periodically record time and voltage.
Two parts of information are needed to calculate CE. These are the coulombs recovered from the addition of acetate which is based on current and time, as well as the theoretical coulombs expected from the oxidation of added acetate (Liu & Logan, 2004). The resistance used was kept constant at 100 ohms based on the results of the polarization curve and 0.5mM of Sodium Acetate was decided upon based on a prior CE trial.