3Fundamentals of voltage generation in MFC Reaction evaluation by Gibb’s free energyΔGr = ΔGro + RT ln (Π)Overall cell electromotive force (Eemf)= potential difference between cathode & anode= maximum attainable cell voltageW(J) = Eemf Q = - ΔGr Q = nFRTEemf = - ΔGr / nF = Eemfo ln (Π)nFΠ = [Activity of product] / [Activity of reactant]Q = No of electrons exchanged in the reactionn = No of electrons per reaction mol, Coulomb (C)F = Faraday’s const.
4Standard 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 – EanEx) acetate oxidized at anode & oxygen used as e-acceptor at cathode2 HCO3- + 9H+ + 8e- CH3COO- + 4 H2OO2 +4H+ +4e- 2H2Ostandard 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
5Electric 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/tOhm's law predicts the current in an (ideal) resistor to be applied voltage divided by resistance (R, [ohms (Ω])I = V/RV 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.
6Identifying 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 – IRintΣηa + / Σηc/ = overpotential of (anode + cathode)= activation loss + bacterial metabolic loss + conc. lossIRΩ = Ohmic loss = (current) (Ohmic resistance)IRint = internal loss, max. MFC output when IRint = IRext
7MFC 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.)
8Activation 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.
9Load, ResistorAnodeCathodeBacteriumMembraneCO₂GlucoseH+e-MEDndMEDDXO2H2OOhmnic polarization Activation polarization Bacterial metabolic loss Concentration polarization◘
10Instrumentation for measurement VoltagemeterMultimeterData acquisition systemPotentiostat : potential or current controlvoltametry test+ Frequency response analyzer : electrochemical impedance spectroscopy (EIS) measurement -> Ohmic & internal resistance measurment.
11Calculations 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/AgClcathode potential = 0.10~0.0 V as Ag/AgClPower (P, [watt, W])Overall performance of MFC based on power output & coulomb efficiency.P = I ·Ecell = Ecell 2/RextEcell = measured cell V across a fixed external resistance RextI = current calculated from Ohm’s law = Ecell / RextMaximum power is calculated from polarization curve.
12Power density [W/m2]Normalization of power output to projected electrode surface area Pan = Ecell 2/Aan · RextReactor volume based.Ohmic resistance (RΩ) using current interrupt techniqueOhmic 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.
13Polarization curve ; periodical decrease of load & measure V with Potentiostat & variable resistor boxA O C A : Activation lossO : Ohmic’s lossC : Conc. lossV Internal resistance (Rint) by increased RΩPower curve ; calculated from polarization curvemaximum power point (MPP) : O majormWdrops due to increasing A & Oshort circuit conditionmA
14Treatment efficiencies BOD, COD, TOC, soluble & particulate, nutrientCOD converted into:- electrical current via Coulomb efficiency- biomass via growth yield- reactions with e- acceptors, O2, NO3, SO3Coulombic 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 /CODMFC net yield = 0.07~0.22 g biomass COD/g substrate CODSludge combustion cost in Europe = 600 € /ton.
15COD balanceΖ = 1- εc - YLoading rateVolumetric loading rate, MFC = 3 kg COD/m3-dHigh rate anaerobic digestion = 8~20Activated sludge = 0.5~2MFC loading to total anode surface area = 25~35 g COD/m2-dRBC = 10~20Energy efficiencyεc = [ƒ Ecell I dt] / ΔH Madded ]= 2~50% MFC, 40% MethaneΔH = heat of combustion (J/mole)Madded = amount (mol) of substrate added
16Distinguishing methods of electron transfer Presence of mediatorsActivation losses due to- direct membrane shuttle- mobile suspended shuttle- nanowiredistinguish by cyclic voltammetry; potentiostatExtent of redox mediation and midpoint potentialsPresence of nanoweirsElectrically conductive bacterial appendage; Pili.
17Outlook 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.ApplicationsFood processing wastewater, digester effluent.Sludge production decreased.Ex) 7500 kg COD/d ~ 950 kW /d powerif 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.