Microbial Fuel Cells Keith Scott CONTENT Fuel Cells
Microbial Fuel Cells Keith Scott CONTENT § Fuel Cells and Biological Fuel Cells § Mechanisms § Research Challenges § MFC performance § MFC prospects
FUEL CELL Grove Genesis 1839 - 1842 O 2 H 2 lyte + 4 e- H+ Electro 4 H+ Cathode (Pt catalyst) 2 H 2 Pt Anode (Pt catalyst) e- 4 H+ The simplest realisation – the H 2/O 2 fuel cell + 4 e- + O 2 Pt 2 H 2 O Positively charged ions to pass through the electrolyte. The negatively charged electron must travel H 2 O 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 along an external circuit to the cathode, creating an electrical current.
Bioelectrochemical energy generation Biological fuel cells Enzyme and whole cell catalysis Enzymatic fuel cells Use isolated and purified enzymes to act as specific catalysts e Microbial fuel cells Use whole living cells to continously supply the biocatalysts R e - - better defined system - poisoning - reaction pathways more difficult Glucose e- Acetate e- PQQ ANODE e. Cytc Gluconic acid - more robust - oxidizing substrate completely - mixed substrates O 2 e. FAD R - COx H 2 O CATHODE e- O 2 e- Microorganism ANODE CO 2 H 2 O CATHODE
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 Electrochemical model Substrate e- MOX Bacteria r. E S Cathode • Reactions occur in the bulk liquid, the biofilm and the electrode surface Bulk liquid Membrane • Multiple biological, chemical and electrochemical reactions Biofilm A Anode • Several metabolic types Electrical load mod Oxygen r. B MRED Biofilm Boundary cells layer P anaerobic Aerobic
Microbial fuel cells: Biology/Chemistry/Physics Performance limitations R e- catalysts for the ORR: - Pt/C, high cost detrimental - few non-platinised catalysts - Mn. Ox/C 4 products P e. H+ 2 O 2 + 4 H+ + 4 e- → 2 H 2 O e 5 6 7 H+ 3 10 9 1 S 8 ANODE CATHODE O 2 substrate 1 2 3 substrate and mediator transport (bulk, boundary layer, biofilm) anodic reaction 4 5 6 7 electrical resistance 8 9 oxygen transport (bulk, b. l. ) 10 H+ transport cathodic (bulk, b. l. , membrane) reaction
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, Mn. Ox, biological……) § Separators (Tyvek, Scimat, Entek, ptfe. . ) § Electrode structure (gas diffusion, ptfe bonded…) § Parameters (Temp, p. H, COD, HRT, conductivity) § Cell design (anode structure, scale-up, flow through) § Modelling
Microbial Fuel Cells: Mechanisms of electron transfer 1. Product electron transfer Pox e- Iox Pred Ired Sox Sred cytochromes Iox e- Ired Sox 2. Direct electron transfer Sred
Microbial Fuel Cells: Mechanisms of electron transfer 3. Newer hypotheses for direct transfer e. Iox Sox "nano-wires" Ired e. Iox Sred Sox metal oxides Ired Sred
Microbial Fuel Cells: Mechanisms of electron transfer 4. Mediated electron transfer mediator M e- Iox Ired Sox non-diffusive Sred Mox e- Iox Mred mediator Sox diffusive Ired Sred
Microbial Fuel Cells COD REMOVAL Faster at temperatures above 30ºC METHANOGENESIS CO 2 + H 2 ACETOGENESIS VFAs + METHANOGENESIS Acetic ANODOPHILIC OXIDATION ACIDOGENESIS Carbohydrates HYDROLYSIS Aminoacids Fatty acids Lípids Glucose Proteins Extracellular enzymes Metano e- + CO 2 + H+ ANODOPHILIC OXIDATION Faster at temperatures below 10ºC
Biofilms on anodes What is the biofilm area? Biofilm on graphite cloth Biofilm on graphite paper
Biofilms on anodes What is the anode area? Biofilm on reticulated vitrous carbon Biofilm surface on graphite
Microbial Fuel Cells Cathode Material § Linear sweep voltammetry of O 2 reduction: Iron phthalocyanine supported on KJB (Fe. Pc-KJB) carbon demonstrated higher activity towards oxygen reduction than Pt in neutral media. Pt and metal phthalocyanine on KJB; . (Passive electrode without air sparging, catalyst loading 1 mg/cm 2, 50 m. M phosphate buffer with nutrients, p. H=7. 0, T= 30 o. C, scan rate 1 m. V/s
Microbial Fuel cell Power Cathode Material § MFC polarisation and power density- With Fe. Pc-KJB as the MFC cathode catalyst, a power density of 634 m. W 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 m. W m-2. CARBON FELT Various cathode catalysts (50 m. M phosphate buffer, T= 30 o. C). CARBON BRUSH Fe. Pc catalyst cathode and a graphite brush (30 o. C, 200 m. M PBM, p. H 7. 0, 1 g L-1 acetate).
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-1 0. 2 $ m-2 Mn. Ox/C 23 $ m-2 Pt/C
Packed Bed of Graphite Granules anode. Variation of current with time for electrochemicallyactive 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 9 th batch was performed with AW containing 1000 ppm as COD and no inoculum. Anode cross sectional area: 12. 5 cm 2. External resistance: 500 Ω.
MFC Performance Continuous Operation
MFC generate electricity from fullstrength brewery wastewater (2, 239 mg-COD/L, 50 m. M PBS added) with the maximum power density of 483 m. W/m 2 (12 W/m 3) at 30 C and 435 m. W/m 2 (11 W/m 3) at 20 C, respectively- Y Feng et al WST 2008
Trickle Flow Tower Reactor
Effect of the loading rate on the SCMFC performance. Rext 100 Ω. Complex System. Use models to better understand behaviour
Model - biofilm+suspended cells and mediator diff. react. adv. Biofilm model (solutes) diff. react. Liquid Biofilm Electrode Solute concentration diff. react. adv. Substrate Electrochemical reactant Electrochemical product Product Here we measure! Biofilm thickness distance
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 (ADM 1), IWA Task Group for Mathematical Modelling of Anaerobic Digestion Processes. London: IWA Publishing.
Integrating modeling and experimentation Examining effect of external load on MFC properties 40 30 35 25 Current density 30 20 25 20 15 15 Charge 10 10 5 5 0 0 0 5 10 Time (days) 15 Total charge (C) Current density, j (m. A/m 2) 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
Integrating modeling and experimentation • 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
Effect of external resistance on COD removal COD (g/m 3) 600 • Experimental 100 Ohms 500 COD (exp), 0. 1 K (1) 400 COD (exp), 0. 1 K (2) COD (sim), 0. 1 K 300 COD 200 100 0 0 2 4 Time (days) 6
Effect of external resistance on COD removal 100 80 70 60 50 40 30 20 10 control OCV 50 kohms 25 kohms 10 kohms 0 1 kohms Systems run as MFC have improved COD removal compared to controls 90 0. 1 kohms • Higher external load shows reduction in COD removal efficiency detected experimentally COD removal (%) •
Effect of external resistance on the anode community • Denaturing gradient gel electrophoresis of anode communities M I 1000 M 10000 25000 M 50000 • Anode bacterial communities developed at 100 to 50, 000 ohms characterized • Anode biomass harvested at end of experimental run • DNA extracted and 16 S r. RNA gene fragments amplified by PCR • 16 S r. RNA gene fragments analyzed by DGGE to provide a community fingerprint OCV M Cont.
Prospects for wastewater MFC and biological treatment • 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 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 § Waste. Water polishing
Reactor Cost § If we take a potential power capability of 1. 0 k. W per m 3 of reactor containing 100 cell equivalents giving power of 10 W/m 2 of crosssectional area. § The energy produced would be 8000 k. Wh/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 To. K 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 § Ph. Ds- Amor Larrosa Guerrora, Jamie Hinks, Sharon Velasquez Orta, Beate Christgen
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