Microbial Fuel Cell Methodology & Technology

Logan et al., 2006 EST;

2006. 10. 27

Changwon Kim

Anode CathodeBacterium Membrane

CO₂

Glucose

H+

H+e-MEDnd

MEDDX

e- e-

H+

O2

H2Oe-

MFC StructureLoad, Resistor

Current

Parameters; Temp. pH, e- acceptor, substrate, electrode – material, surface area, reactor size, mediator, bacteria, CEM

(CEM, PEM; Nafion, Ultrex)

Reference electrode ◘

Chemical mediator (neutral red) or Mediator-less

Oxidzer : O2, ferricyanide,

Mn(IV), NO3

Graphite granules

(Rabaey & Verstraete, 2005)Graphite granules, wire mesh

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 = Eemf

o - ------ 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 appl

ied 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.

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.

Anode CathodeBacterium Membrane

CO₂

Glucose

H+

H+e-MEDnd

MEDDX

e- e-

H+

O2

H2Oe-

Ohmnic polarization Activation polarizationBacterial metabolic loss

Concentration polarization

Load, Resistor

◘

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/AgClPower (P, [watt, W])• Overall performance of MFC based on power output & coulo

mb 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 majormW 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 - YLoading 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 = 10~20Energy 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.