Microbial Fuel Cell Methodology & Technology Logan et al., 2006 EST; 2006. 10. 27 Changwon Kim

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)

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.

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

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.

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

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.)

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.

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

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.

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~ -0.48 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.

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.

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

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.

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

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.

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.