1. Microbial Fuel Cell Methodology & Technology Logan et al., 2006 EST;
Changwon Kim

2. MFC Structure Anode Cathode Bacterium Membrane e- Load, Resistor
CO₂
Glucose H+ e-
MEDnd
MEDDX O2
H2O
Current
Chemical mediator (neutral red) or Mediator-less
Parameters; Temp. pH, e- acceptor, substrate, electrode – material, surface area, reactor size, mediator, bacteria, CEM
Oxidzer : O2, ferricyanide,
Mn(IV), NO3
Reference electrode ◘
Graphite granules (Rabaey & Verstraete, 2005)
Graphite granules, wire mesh
(CEM, PEM; Nafion, Ultrex)

3. Fundamentals of voltage generation in MFC
Reaction evaluation by Gibb’s free energy
ΔGr = ΔGro + RT ln (Π)
Overall cell electromotive force (Eemf)
= potential difference between cathode & anode
= maximum attainable cell voltage
W(J) = Eemf Q = - ΔGr Q = nF RT
Eemf = - ΔGr / nF = Eemfo ln (Π) nF
Π = [Activity of product] / [Activity of reactant]
Q = No of electrons exchanged in the reaction
n = No of electrons per reaction mol, Coulomb (C)
F = Faraday’s const.

4. Standard electrode potential, at 298 oK, 1 bar, 1 M
= reported relative to normal hydrogen electrode (NHE)
Maximum attainable cell voltage can be calculated by,
Eemf = Ecat – Ean
Ex) acetate oxidized at anode & oxygen used as e-acceptor at cathode
2 HCO3- + 9H+ + 8e-  CH3COO- + 4 H2O
O2 +4H+ +4e-  2H2O
standard potential = 0 at standard conditions.
Ean = Ean0 – RT/8F ln ([CH3COO-]/[HCO3-]2[H+]9)
Ecat = Ecat0 – RT/4F ln (1/pO2[H+]4)
Eemf = Ecat - Ean

5. Electric current (I, [ampere (A)]) is the flow of electric charge, (Q, [coulomb] and equal to a flow of one coulomb of charge per second.
I = Q/t
Ohm's law predicts the current in an (ideal) resistor to be applied voltage divided by resistance (R, [ohms (Ω])
I = V/R
V is the potential difference [volts]
Current density [amperes/m2] is defined as a vector whose magnitude is the electric current per cross-sectional area.
Electric (electrostatic) potential [volts] is the potential energy per unit of charge associated with a static (time-invariant) electric field.

6. Identifying factors that decreasing cell voltage
Open Circuit Voltage (OCV) = measured after some time in absence of current, lower than Eemf due to overpotential.
Measured Cell Voltage (Ecell )
Ecell = Eemf – (Σηa + / Σηc/ + IRΩ)
= OCV – IRint
Σηa + / Σηc/ = overpotential of (anode + cathode)
= activation loss + bacterial metabolic loss + conc. loss
IRΩ = Ohmic loss = (current) (Ohmic resistance)
IRint = internal loss, max. MFC output when IRint = IRext

7. MFC performance should be evaluated based on Overpotential & Ohmic losses (polarization) or OCV & Internal losses.
Ohmic losses : resistance to flow of (e- thru electrode & interconnection + ion thru CEM & electrolytes)
- Reduced by minimizing electrode spacing, using low resistivity membrane, checking all contacts, and increasing solution conductivity.
Overpotential = losses in (activation + bacterial + conc.)

8. Activation losses : occur during transfer of e- from or to mediator and e-acceptor reacting at electrode surface.
- Strong increase at low currents, steadily increase when current density increase.
- Reduced by increasing electrode surface area, improving electrode catalysts, increasing temp, enrichment biofim.
Bacterial metabolic losses :
- To maximize MFC voltage, keep anode potential low. But if it’s too low, e- transport is inhibited.
Concentration (mass transport) losses :
- Conc. losses occur when species mass transport rate to or from electrode limits current production.

9. Load, Resistor
Anode
Cathode
Bacterium
Membrane
CO₂
Glucose H+ e-
MEDnd
MEDDX O2
H2O
Ohmnic polarization Activation polarization Bacterial metabolic loss Concentration polarization ◘

10. Instrumentation for measurement
Voltagemeter
Multimeter
Data acquisition system
Potentiostat : potential or current control
voltametry test
+ Frequency response analyzer : electrochemical impedance spectroscopy (EIS) measurement -> Ohmic & internal resistance measurment.

11. Calculations and Procedures for Reporting Data
Electrode potential ([voltage, V])
Reference electrode; NHE (0 Vt), Ag/AgCl (0.197 V)
Standard Calomel (0.242 V)
dependant on electrode used, pH, conc. of electron accepter
@pH=7 typical anode potential = 0.4~ V as Ag/AgCl
cathode potential = 0.10~0.0 V as Ag/AgCl
Power (P, [watt, W])
Overall performance of MFC based on power output & coulomb efficiency.
P = I ·Ecell = Ecell 2/Rext
Ecell = measured cell V across a fixed external resistance Rext
I = current calculated from Ohm’s law = Ecell / Rext
Maximum power is calculated from polarization curve.

12. Power density [W/m2]
Normalization of power output to projected electrode surface area Pan = Ecell 2/Aan · Rext
Reactor volume based.
Ohmic resistance (RΩ) using current interrupt technique
Ohmic resistance is determined by operating MFC at a current at which no concentration losses occur. Electrical circuit open and steep initial potential rise (ER, Ohmic losses) and then followed by a slow potential increase to OCA (EA, electrode overpotentials).
Ohmic losses (I RΩ) is a function of produced current and Ohmic resistance.

13. Polarization curve ; periodical decrease of load & measure V
with Potentiostat & variable resistor box
A O C A : Activation loss
O : Ohmic’s loss
C : Conc. loss
V Internal resistance (Rint) by increased RΩ
Power curve ; calculated from polarization curve
maximum power point (MPP) : O major mW
drops due to increasing A & O
short circuit condition mA

14. Treatment efficiencies
BOD, COD, TOC, soluble & particulate, nutrient
COD converted into:
- electrical current via Coulomb efficiency
- biomass via growth yield
- reactions with e- acceptors, O2, NO3, SO3
Coulombic efficiency (εc)
For batch : εcb = [M ƒ I dt] / [F b Van ΔCOD]
For continuous εcc = [M I] / [F b q ΔCOD]
Growth yield (Y)
Net (observed) yield = x /COD
MFC net yield = 0.07~0.22 g biomass COD/g substrate COD
Sludge combustion cost in Europe = 600 € /ton.

15. COD balance
Ζ = 1- εc - Y
Loading rate
Volumetric loading rate, MFC = 3 kg COD/m3-d
High rate anaerobic digestion = 8~20
Activated sludge = 0.5~2
MFC loading to total anode surface area = 25~35 g COD/m2-d
RBC = 10~20
Energy efficiency
εc = [ƒ Ecell I dt] / ΔH Madded ]
= 2~50% MFC, 40% Methane
ΔH = heat of combustion (J/mole)
Madded = amount (mol) of substrate added

16. Distinguishing methods of electron transfer
Presence of mediators
Activation losses due to
- direct membrane shuttle
- mobile suspended shuttle
- nanowire
distinguish by cyclic voltammetry; potentiostat
Extent of redox mediation and midpoint potentials
Presence of nanoweirs
Electrically conductive bacterial appendage; Pili.

17. Outlook Critical issues ; above issues + scale up; Stacked cells?
Success application on wastewater depends on;
- conc. & biodegradability of organic, temp., toxic.
Material cost : anode –graphite, catalyst for cathode.
Removal of non-carbon based substrate; N, S, P. particulate.
Applications
Food processing wastewater, digester effluent.
Sludge production decreased.
Ex) 7500 kg COD/d ~ 950 kW /d power
if 1 kW/m3 , then 350 m3 reactor volume => 2.6 M€
if energy production value = 0.3 M€/year (0.1 €/kWh)
Then 10 years pay-back period.