Microbial Fuel Cells Keith Scott CONTENT  Fuel Cells and Biological Fuel Cells  Mechanisms  Research Challenges  MFC performance  MFC prospects

H2H2 H+H+ Cathode (Pt catalyst) Anode (Pt catalyst) O2O2 H2OH2O e-e- 2H 2 4H + + 4e - 4H + + 4e - + O 2 2H 2 O The simplest realisation – the H 2 /O 2 fuel cell Pt Electro lyte 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 CELL Grove Genesis

Biological fuel cells Enzyme and whole cell catalysis Enzymatic fuel cells Use isolated and purified enzymes to act as specific catalysts Microbial fuel cells Use whole living cells to continously supply the biocatalysts R ANODE CATHODE Gluconic acid e-e- H2OH2O e-e- Glucose FAD PQQ e-e- O2O2 CO x Cyt c e-e- e-e- Bioelectrochemical energy generation R ANODE CATHODE CO 2 e-e- H2OH2O Acetate e-e- O2O2 e-e- Micro- organism - better defined system - poisoning - reaction pathways more difficult - more robust - oxidizing substrate completely - mixed substrates

MFC Applications  Enhanced Water and Waste treatment  Energy Production  Hydrogen Generation  Alternative Reductions- e.g. production of peroxide  Alternative oxidations- using electron accepting (cathodic) bacteria

MFC- 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 Anode Oxygen Membrane Cathode A Biofilm cells e - r E P S M RED M OX r B Electrical mod Boundary layer Substrate Bacteria Anode Oxygen Membrane Cathode A Biofilm cells Electrochemical model anaerobic Bulk liquid Biofilm e - r E e - r E P S M RED M OX r B P S M RED M OX r B Electrical Electrical load Boundary layer Aerobic

Performance limitations R ANODECATHODE substrate products O2O2 e-e- P S 1 2 e-e- H+H+ 3 4 H+H e-e substrate and mediator transport (bulk, boundary layer, biofilm) 3 anodic reaction 4 electrical resistance 5 H + transport (bulk, b.l., membrane) 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 - MnO x /C O 2 + 4H + + 4e - → 2H 2 O

Research Themes Research Themes  Anode materials – carbons (WC,…)  Biofilm mechanisms and anodophiles- Geobacteraceae, Desulfuromonaceae, Alteromonadaceae, Enterobacteriaceae, Pasteurellaceae, Clostridiaceae, Aeromonadaceae, and Comamonadaceae are able to transfer electrons to electrodes.  Cathodes (activated carbons, porphyrins, MnOx, biological……)  Separators (Tyvek, Scimat, Entek, ptfe.. )  Electrode structure (gas diffusion, ptfe bonded…)  Parameters (Temp, pH, COD, HRT, conductivity)  Cell design (anode structure, scale-up, flow through)  Modelling

1. Product electron transfer P ox e- I ox I red S red S ox P red 2. Direct electron transfer e- I ox I red S red S ox Microbial Fuel Cells: Mechanisms of electron transfer cytochromes

3. Newer hypotheses for direct transfer e- S red "nano-wires" I ox I red S ox metal oxides e- S red I ox I red S ox Microbial Fuel Cells: Mechanisms of electron transfer

4. Mediated electron transfer M red M ox e- S red diffusive I ox I red S ox Microbial Fuel Cells: Mechanisms of electron transfer M e- non-diffusive I ox I red S red S ox mediator

Aminoacids Fatty acids Glucose VFAs Acetic e-e- Metano CO 2 + H 2 Carbohydrates Lípids Proteins + ANODOPHILIC OXIDATION METHANOGENESIS ACIDOGENESIS ACETOGENESIS Extracellular enzymes ANODOPHILIC OXIDATION + CO 2 + H + HYDROLYSIS Faster at temperatures above 30ºC Faster at temperatures below 10ºC COD REMOVAL Microbial 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 vitrous carbon Biofilms on anodes Biofilm surface on graphite

Microbial Fuel Cells 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 Iron phthalocyanine supported on KJB (FePc-KJB) carbon demonstrated higher activity towards oxygen reduction than Pt in neutral media.  Linear sweep voltammetry of O 2 reduction: Iron phthalocyanine supported on KJB (FePc-KJB) carbon demonstrated higher activity towards oxygen reduction than Pt in neutral media.

Microbial Fuel cell Power Cathode Material  With FePc-KJB as the MFC cathode 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 anode the power density was increased to 2011 mW m -2.  MFC polarisation and power density- With FePc-KJB as the MFC cathode 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 anode the power density was increased to 2011 mW m -2. 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). CARBON FELTCARBON BRUSH

Microbial Fuel cell Power from Wastewater Cathode Material COST ( 0.1 mg/cm 2 Pt. 1.0 mg/cm 2 Mn) Mn: 0.02 $ g -1 Pt: 23 $ g $ m -2 MnO x /C 23 $ m -2 Pt/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 Performance Continuous Operation MFC Performance Continuous Operation

MFC generate electricity from full- strength 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 Reactor Trickle Flow Tower Reactor

Effect of the loading rate on the SCMFC performance. Rext 100 Ω. Complex System- Use models to better understand behaviour

Liquid Biofilm Electrode Solute concentration distance diff. adv. react. diff. adv. react. diff. Biofilm thickness Here we measure! Substrate Product Electrochemical product Electrochemical reactant Biofilm model (solutes) Model - biofilm+suspended cells and mediator

Microbes meet with resistance The biochemical model is based on the IWA anaerobic digestion model with electrogenic acetate oxidation and an electron-transfer mediator 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 experimentation Integrating modeling and experimentation Examining effect of external load on MFC properties Time (days) Current density, j (mA/m2) Total charge (C) Current density Charge Model outputs Time-dependent production of Current, Voltage Time-dependent bulk substrate, intermediate and product concentrations Power, Coulombic yields Current-voltage, current-power curves Spatial distributions of chemical species Spatial 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 experimentation

Effect of external resistance on COD removal Time (days) COD (g/m 3 ) 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 removal kohms 1 kohms 10 kohms 25 kohms50 kohms OCV control COD removal (%) 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 community M I M M 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 biological treatment

PROSPECTS The key issues for a microbial (bio-electrochemical) fuel cell reactor for the recovery of energy or production of valuable chemicals relate to:  Reactor Cost (in relation to product value including wastewater treatment)  MFC Reactor Design  Reactor scale-up to suitable plant production size  Reactor Durability  ANODE DESIGN and CONFIGURATION CRUCIAL  WasteWater polishing

Reactor Cost  If we take a potential power capability of 1.0 kW per m 3 of reactor containing 100 cell equivalents giving power of 10 W/m 2 of cross- sectional area.  The energy produced would be 8000 kWh/year with a value of approximately £800/year.  Working on a simple payback over 5 years this would be equivalent to a cost from generating electricity of £4000.  Thus the cost of an individual cell would be of the order of £40 (/m 2 ).  These are quite challenging costs and rule out the use of precious metal (including silver) for catalysts and ion-exchange membrane materials that are frequently used in microbial fuel cells.

Acknowledgments  EU Marie Curie ToK  EPSRC  Northumbrian Water  Research Group- Profs I M Head, T Curtis Dr E Yu  Drs K Katuri, M Di Lorenzo, I Roche, M Ghangreghar, B Erable, N Duteanu, Y Feng  PhDs- Amor Larrosa Guerrora, Jamie Hinks, Sharon Velasquez Orta, Beate Christgen