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Microbial Fuel Cells Keith Scott
CONTENTCONTENT Fuel Cells and Biological Fuel CellsFuel Cells and Biological Fuel Cells Mechanisms Mechanisms Research ChallengesResearch Challenges MFC performanceMFC performance MFC prospectsMFC prospects
H2
H+C
athod
e (Pt catalyst)A
nod
e (P
t ca
taly
st)
O2
H2O
e-
2H2 4H+ + 4e- 4H+ + 4e- + O2 2H2O
The simplest realisation – the H2/O2 fuel cell
Pt Pt
Ele
ctro
l
yte
The electrolyte can be liquid, solid or polymeric and essentially:Separates fuel and oxidant•Facilitates ion transport between anolyte and catholyte•Prevents electrical short circuit between anode and cathode
Positively charged ions to pass through the electrolyte. The negatively charged electron must travel along an external circuit to the cathode, creating an electrical
current.
FUEL CELLGrove Genesis 1839 - 1842
Biological fuel cellsEnzyme and whole cell catalysis
Enzymatic fuel cellsUse isolated and purified enzymes to act as specific catalysts
Microbial fuel cellsUse whole living cells to continously supply the biocatalysts
R
ANODE CATHODEGluconicacid
e-
H2O
e-
Glucose
FAD
PQQ
e-
O2
COx
Cytc
e-e-
Bioelectrochemical energy generation
R
ANODE CATHODECO2
e-
H2O
Acetate
e-
O2
e-
Micro-organism
- better defined system- poisoning- reaction pathways more difficult
- more robust- oxidizing substrate completely- mixed substrates
MFC ApplicationsMFC Applications
Enhanced Water and Waste Enhanced Water and Waste treatmenttreatment
Energy ProductionEnergy Production Hydrogen GenerationHydrogen Generation Alternative Reductions- e.g. Alternative Reductions- e.g.
production of peroxideproduction of peroxide Alternative oxidations- using Alternative oxidations- using
electron accepting electron accepting (cathodic) bacteria(cathodic) bacteria
MFC- A Complex SystemMFC- A Complex System• Anode-attached
and suspended biomass
• Several metabolic types
• Multiple biological, chemical and electrochemical reactions
• Reactions occur in the bulk liquid, the biofilm and the electrode surface
Substrate
Bacteria
An
od
e
Oxygen
Me
mb
ran
e
Ca
tho
de
A
Biofilmcells
e-
rE
P
S
MRED
MOX
rB
Electrical mod
Boundarylayer
Substrate
Bacteria
An
od
e
Oxygen
Me
mb
ran
e
Ca
tho
de
A
Biofilmcells
Ele
ctr
och
em
ical
mod
el
anaerobic
Bulk liquid Biofilm
e-
rE
e-
rE
P
S
MRED
MOX
rB
P
S
MRED
MOX
rB
Electrical Electrical load
Boundarylayer
Aerobic
Performance limitationsR
ANODE CATHODE
substrate
products
O2
e-
P
S
1
2
e-
H+
3
4
H+65 7
9
8
e-
10
1 2 substrate and mediator transport (bulk, boundary layer, biofilm)
3 anodic reaction
4 electrical resistance
5 H+ transport(bulk, b.l., membrane)
67
8 9 oxygen transport (bulk, b.l.)
10 cathodic reaction
Microbial fuel cells: Biology/Chemistry/Physics
catalysts for the ORR:- Pt/C, high cost detrimental- few non-platinised catalysts- MnOx/C
O2 + 4H+ + 4e- → 2H2O
Research ThemesResearch Themes Anode materials – carbons (WC,…) Anode materials – carbons (WC,…) Biofilm mechanisms and anodophiles- Biofilm mechanisms and anodophiles- Geobacteraceae, Geobacteraceae,
Desulfuromonaceae, Alteromonadaceae, Desulfuromonaceae, Alteromonadaceae, Enterobacteriaceae, Pasteurellaceae, Clostridiaceae, Enterobacteriaceae, Pasteurellaceae, Clostridiaceae, AeromonadaceaeAeromonadaceae, and , and Comamonadaceae Comamonadaceae are able to are able to transfer electrons to electrodes. transfer electrons to electrodes.
Cathodes (activated carbons, porphyrins, MnOx, Cathodes (activated carbons, porphyrins, MnOx, biological……)biological……)
Separators (Tyvek, Scimat, Entek, ptfe.. )Separators (Tyvek, Scimat, Entek, ptfe.. ) Electrode structure (gas diffusion, ptfe bonded…)Electrode structure (gas diffusion, ptfe bonded…) Parameters (Temp, pH, COD, HRT, conductivity)Parameters (Temp, pH, COD, HRT, conductivity) Cell design (anode structure, scale-up, flow through)Cell design (anode structure, scale-up, flow through) ModellingModelling
1. Product electron transfer
Poxe-Iox
IredSred
Sox
Pred
2. Direct electron transfer
e-
Iox
IredSred
Sox
Microbial Fuel Cells: Mechanisms of electron transfer
cytochromes
3. Newer hypotheses for direct transfer
e-
Sred
"nano-wires"
Iox
Ired
Sox
metal oxides
e-
Sred
Iox
Ired
Sox
Microbial Fuel Cells: Mechanisms of electron transfer
4. Mediated electron transfer
Mred
Mox
e-
Sred
diffusiveIox
Ired
Sox
Microbial Fuel Cells: Mechanisms of electron transfer
M
e-
non-diffusiveIox
IredSred
Sox
mediator
mediator
AminoacidsFatty acidsGlucose
VFAs Acetic
e-
MetanoCO2 + H2
CarbohydratesLípidsProteins
+
ANODOPHILIC OXIDATION
METHANOGENESIS
METHANOGENESIS
ACIDOGENESIS
ACETOGENESIS
Extracellularenzymes
ANODOPHILIC OXIDATION
+ CO2 + H+
HYDROLYSIS
Faster at temperatures above 30ºC
Faster at temperaturesbelow 10ºC
COD REMOVALMicrobial Fuel CellsMicrobial Fuel Cells
Biofilm on graphite cloth
Biofilm on graphite paper
What is the biofilm area?
Biofilms on anodes
What is the anode area?
Biofilm on reticulated vitrouscarbon
Biofilms on anodes
Biofilm surface on graphite
Microbial Fuel CellsMicrobial Fuel Cells Cathode Material Cathode Material
Pt and metal phthalocyanine on KJB;. (Passive electrode without air sparging, catalyst loading 1 mg/cm2, 50 mM phosphate buffer with nutrients, pH=7.0, T= 30 oC, scan rate 1mV/s
Linear sweep voltammetry of O2 reduction: Iron phthalocyanine supported on KJB (FePc-KJB) carbon Iron phthalocyanine supported on KJB (FePc-KJB) carbon demonstrated higher activity towards oxygen reduction than Pt in neutral media. demonstrated higher activity towards oxygen reduction than Pt in neutral media.
Microbial Fuel cell Power Microbial Fuel cell Power Cathode Material Cathode Material
MFC polarisation and power density- With FePc-KJB as the MFC cathode With FePc-KJB as the MFC cathode catalyst, a power density of 634 mW m-2 which was higher than that obtained catalyst, a power density of 634 mW m-2 which was higher than that obtained using the precious-metal Pt cathode. Using a high surface area carbon brush using the precious-metal Pt cathode. Using a high surface area carbon brush anode the power density was increased to 2011 mW manode the power density was increased to 2011 mW m-2-2..
Current density J/mA cm-2
0.0 0.1 0.2 0.3 0.4
Cel
l vol
tage
V/V
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Pow
er d
ensi
ty P
/mW
m-2
0
200
400
600
800
1000FePc-KJBPt (in house)CoTMPP-KJBPt (commercial)
Various cathode catalysts (50 mM phosphate buffer, T= 30 oC).
FePc catalyst cathode and a graphite brush (30 oC,
200 mM PBM, pH 7.0, 1 g L-1 acetate).
Current density J/mA cm-2
0.0 0.2 0.4 0.6 0.8 1.0
Cel
l Vol
tage
V/V
0.0
0.2
0.4
0.6
0.8
1.0
Pow
er d
ensi
ty P
/mW
m-2
0
500
1000
1500
2000
2500
CARBON FELT CARBON BRUSH
Microbial Fuel cell Power from Wastewater
Cathode Material
COST ( 0.1 mg/cm2 Pt. 1.0 mg/cm2 Mn)
Mn: 0.02 $ g-1
Pt: 23 $ g-1
0.2 $ m-2 MnOx/C23 $ m-2Pt/C
Packed Bed of Graphite Granules anode. Variation of current with time for electrochemically-
active bacterial enrichment of SCMFC
The first 8 batches were performed with anaerobic sludge as inoculum (0.5% by volume) and AW (1000 ppm COD). The 9th batch was performed with AW containing 1000 ppm as COD and no inoculum. Anode cross sectional area: 12.5 cm2. External resistance: 500 Ω.
MFC PerformanceMFC Performance Continuous OperationContinuous Operation
MFC generate electricity from full-MFC generate electricity from full-strength brewery wastewaterstrength brewery wastewater
(2,239 mg-COD/L, 50mM PBS added) with the maximum power density of 483mW/m2 (12W/m3) at 30C and 435mW/m2 (11W/m3) at 20C, respectively- Y Feng et al WST 2008
Trickle Flow Tower ReactorTrickle Flow Tower Reactor
Effect of the loading rate on the SCMFC performance. Rext 100 Ω.
Complex System- Use models to better understand behaviour
Liq
uid
Bio
film
Ele
ctro
de
Solu
te
con
cen
trati
on
distance
diff.adv.
react.diff.adv.
react.react.diff.
Biofilm thickness
Here we measure!
Substrate
ProductElectrochemical product
Electrochemical reactant
Biofilm model Biofilm model (solutes)(solutes)
Model - biofilm+suspended cells and mediator
Microbes meet with resistanceMicrobes meet with resistance• The biochemical model is
based on the IWA anaerobic digestion model with electrogenic acetate oxidation and an electron-transfer mediator
Glucose
Propionate
Val/Butyrate
Acetate H2
CH4
AcidogenesisXsug
AcetogenesisXpro Xbv
MethanogenesisXace Xh2
CO2
MOX
MRed
current
ElectroactiveXeab
anode
Glucose
Propionate
Val/Butyrate
Acetate H2
CH4
AcidogenesisXsug
AcetogenesisXpro Xbv
MethanogenesisXace Xh2
CO2
MOX
MRed
current
ElectroactiveXeab
anode
Batstone D.J., Keller J., Angelidaki I., Kalyuzhnyi S.V., Pavlostathis S.G., Rozzi A., Sanders W.T.M., Siegrist H., Vavilin V.A. (2002) Anaerobic Digestion Model No. 1 (ADM1), IWA Task Group for Mathematical Modelling of Anaerobic Digestion Processes. London: IWA Publishing.
Integrating modeling and experimentationIntegrating modeling and experimentationExamining effect of external load on MFC properties
0
50
100
150
200
250
300
350
400
450
500
0 5 10 15
Time (days)
Org
anic
s in
bul
k (g
CO
D/m
3 )
0
0.5
1
1.5
2
2.5
3
3.5
4
H2
in b
ulk
(mgC
OD
/m3 )
Glu
CH4
Ace
Pro
But
H2
0
5
10
15
20
25
30
35
40
0 5 10 15
Time (days)
Cu
rre
nt
de
nsi
ty,
j (m
A/m
2)
0
5
10
15
20
25
30
To
tal c
ha
rge
(C
)Currentdensity
Charge
Model outputsTime-dependent production of Current , VoltageTime-dependent bulk substrate, intermediate and product concentrationsPower, Coulombic yieldsCurrent-voltage, current-power curvesSpatial distributions of chemical speciesSpatial distributions of biomass species
• Qualitative predictions
• Increasing external resistance should reduce the rate of electron transfer from the substrate to the anode
• Electrogens become less competitive
• Methanogens become more competitive
• Reduced current/charge and Coulombic yield
• Community composition should alter with external resistance
• Biomass of electrogens should be reduced
Integrating modeling and experimentationIntegrating modeling and experimentation
Effect of external resistance on COD removalEffect of external resistance on COD removal
Time (days)
CO
D (
g/m
3)
0
100
200
300
400
500
600
0 2 4 6
COD (exp), 0.1 K (1)
COD (exp), 0.1 K (2)
COD (sim), 0.1 K
COD
• Experimental 100 Ohms
Effect of external resistance on COD removalEffect of external resistance on COD removal
0
10
20
30
40
50
60
70
80
90
100
0.1
kohm
s
1 ko
hms
10 k
ohm
s
25 k
ohm
s
50 k
ohm
s
OC
V
cont
rol
CO
D r
emov
al (
%)
• Higher external load shows reduction in COD removal efficiency detected experimentally
• Systems run as MFC have improved COD removal compared to controls
• Denaturing gradient gel electrophoresis of anode communities
Effect of external resistance on the anode communityEffect of external resistance on the anode community
M I 100 1000 M 10000 25000 M 50000 OCV M Cont.
• Anode bacterial communities developed at 100 to 50,000 ohms characterized
• Anode biomass harvested at end of experimental run
• DNA extracted and 16S rRNA gene fragments amplified by PCR
• 16S rRNA gene fragments analyzed by DGGE to provide a community fingerprint
• External load has profound effects on the anode community and MFC performance
• Even if MFC never produce useful amounts of electricity, still potential benefits for wastewater treatment
• Can external load be used to “tune”
• Treatment performance?
• Sludge yield?
• If lower external load selects for electrogens, should MFC anode communities be conditioned under low external load to maximize electrogen colonization?
Prospects for wastewater MFC and Prospects for wastewater MFC and biological treatmentbiological treatment
PROSPECTSPROSPECTSThe key issues for a microbial (bio-electrochemical) fuel The key issues for a microbial (bio-electrochemical) fuel
cell reactor for the recovery of energy or production of cell reactor for the recovery of energy or production of valuable chemicals relate to:valuable chemicals relate to:
Reactor Cost (in relation to product value including Reactor Cost (in relation to product value including wastewater treatment) wastewater treatment)
MFC Reactor DesignMFC Reactor Design Reactor scale-up to suitable plant production sizeReactor scale-up to suitable plant production size Reactor DurabilityReactor Durability
ANODE DESIGN and CONFIGURATION CRUCIALANODE DESIGN and CONFIGURATION CRUCIAL WasteWater polishingWasteWater polishing
Biogas kWh
Wastewater treatment Anaerobic digestion MFC
Biogas kWh
Wastewater treatment Anaerobic digestion MFC
Reactor CostReactor Cost If we take a potential power capability ofIf we take a potential power capability of 1.0 kW per m1.0 kW per m33 of reactor of reactor
containing 100 cell equivalents giving power of 10 W/mcontaining 100 cell equivalents giving power of 10 W/m22 of cross- of cross-sectional area. sectional area.
The energy produced would be 8000 kWh/year with a value of The energy produced would be 8000 kWh/year with a value of approximately £800/year.approximately £800/year.
Working on a simple payback over 5 years this would be Working on a simple payback over 5 years this would be equivalent to a cost from generating electricity of £4000.equivalent to a cost from generating electricity of £4000.
Thus the cost of an individual cell would be of the order of £40 (/mThus the cost of an individual cell would be of the order of £40 (/m22).).
These are quite challenging costs and rule out the use of precious These are quite challenging costs and rule out the use of precious metal (including silver)metal (including silver) for catalysts and ion-exchange membrane for catalysts and ion-exchange membrane materials that are frequently used in microbial fuel cells. materials that are frequently used in microbial fuel cells.
AcknowledgmentsAcknowledgments
EU Marie Curie ToKEU Marie Curie ToK EPSRC EPSRC Northumbrian WaterNorthumbrian Water Research Group- Profs I M Head, T Curtis Dr E YuResearch Group- Profs I M Head, T Curtis Dr E Yu Drs K Katuri, M Di Lorenzo , I Roche, M Drs K Katuri, M Di Lorenzo , I Roche, M
Ghangreghar, B Erable, N Duteanu, Y FengGhangreghar, B Erable, N Duteanu, Y Feng PhDs- Amor Larrosa Guerrora, Jamie Hinks, PhDs- Amor Larrosa Guerrora, Jamie Hinks,
Sharon Velasquez Orta, Beate ChristgenSharon Velasquez Orta, Beate Christgen
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