MICROBIAL FUEL CELLS KENAN DALKILI Content A brief

MICROBIAL FUEL CELLS KENAN DALKILIÇ

Content A brief presentation of microbial fuel cells (MFCs) Mechanism of electron transfer Mediators and mediatorless MFCs Microorganisms in MFCs Anode and cathode materials (grafite, cloth coating, metals) Membranes in MFCs (bipolar, anion, cation and micro, ultra m. ) and membraneless MFCs Operational conditions (p. H, temp. , OLR, HRT) MFC designs (single, two chmaber, stack, tubular, upflow) MFC applications, advantages and disadvantages (cost, treatment, remote sensors)

History of MFCs A microbial fuel cell (MFC) is a bioreactor, that converts chemical energy in the chemical bonds in org. comp. to electrical energy through catalytic reactions of microorg. under anaerobic conditions. First demonstration of current and voltage generation via microorg. was shown by a botany prof. Michael Cresse Potter, in 1911. Dependency on energy derived from finite fossil fuel and environmental concerns led researches on MFC accelerate for the last two decades. 5000 4500 4000 3500 3000 2500 2000 1500 1000 500 0 1991 # MFC studies 1996 2001 2006 Year 2011 2016 2021 (search as: microbial fuel cell in elsevier)

Microbial Fuel Cell

Mechanism of electron transfer Basic respiration in the cell, involves the transfer of electrons from a low redox potential electron donor to a electron acceptor at a high redox potential. Nicotinamide adedine dinucleotide NAD+ NADH Hydrogen atoms removed from carriers (NADH) simultaneously are seperated from electrons. While electrons transferred to next electron acceptor (carrier), protons are pumped outside the cell. This is occured by tricarboxylic acid (TCA) cycle in the cell.

Electron transfer pathways Possible microbial interactions with the anode. Electrons can be transferred through immobilized structures such as membrane-bound proteins (A) electrically conductive pilus (B) through mobile components (mediators) that are alternatively oxidized and reduced at the electrode (C).

Mediators in MFC Except the anodophiles, the microbes are incapable of transferring electrons directly to the anode. The outer layers of the majority of microbial species are composed of non-conductive lipid membrane that hinder the electron transfer. Mediators increase the electron transfer from bacteria to anode by a oxidation-reduction cycle between bacteria and anode. A good meadiator should be: (1) able to cross the cell membrane easily (2) able to grab electrons from the electron carries of the electron transport chains (3) possessing a high electrode reaction rate (4) having a good solubility in the anolyte (5) Nonbiodegradable and non-toxic to microbes (6) low costed Dyes and metallorganics such as neutral red (NR), methylene blue (MB), Fe(III)EDTA… Disadvantages: toxicity and instability of synthetic mediators, additional cost.

Mediatorless MFCs Anodophiles can directly transfer electrons to the anode. 1. Have electrochemically redox enzymes (cytochrome) on the outer membrane. 2. They may produce soluble redox mediators (pyocyanin) to transfer the electrons to the anode. 3. They can built membrane proteins (nanowires-pili) to transfer e-. Certain mic. org. have two of these abilities. (1) Mediator electron transfer (a) Non-diffusive mediator electron transfer and (b) Diffusive mediator electron transfer (2) Product electron transfer (3) Direct electron transfer. (4) Nano-wires.

Microbes used in MFCs Du, Z. et al. , 2007, Biotechnology Advances

Microbes used in MFCs • EET (extracellular electron transfer) capacity is widespread in nature and it is found among most of bacteria. • The existence of these microorganisms in nature is also confirmed by the diversity of inocula that can be used to start up lab-scale MFCs. • Raw sewage, activated sludge, anaerobic and methanogenic sludge, river sediment, and seawater • Acetate, propionate, butyrate, ethanol, lactate, glucose, domestic wastewater and beer-processing wastewater.

Electrodes (Anodes and Cathodes) Because different materials have different activation polarization losses, anode and cathode material type is important. Platinum, platinum coated carbon, graphite felt, carbon-cloth, carbon paper and some metals Platinum and carbon based materials Graphite or carbon based materials are cheaper and have enormous surface area. 1. Surface area and the roughness of anode material 2. Rough surfaces decrease the current density 3. Electrode modification is very useful for increasing MFC performances (Mn graphite anode, mediator )

Electrodes (Anodes and Cathodes) An electron acceptor is needed to maintain the process at the cathode chamber. Oxygen is widely used as an electron acceptor. Ferricyanide, hydrogen peroxide. Oxygen has slow reduction kinetic and limited dissolution in water, a catalyst (platinum, cobalt oxide, binded mediators on graphite electrode) is needed. Air-cathode electrodes and biocatalysts Ferric sulfate, cobalt and manganese based materials, molybdenum are employed on graphite electrodes. In biocathode chamber, oxygen, nitrate, sulfate and iron can be used as electron acceptor by microorganisms.

Membranes in MFC The ideal function of an MFC membrane is to seperate the anode and cathode rxn while permitting selective transport of protons from anode to cathode and preventing transport of oxygen into the anode chamber. o good ionic conductor o an insulator o ion selective o durable o chemically stable o biocompatible o unsusceptible to fouling and clogging o inexpensive Ion EM, cation EM, anion EX, proton EM (Nafion, Ultrex, Hyflon) Power output is proportional with the membrane`s surface area. Ø Membrane-less MFCs : cloth cathode assembly (CCA), cloth electrode assembly (CEA) Ø Cloth assemblies(seperators) reduces the anode-cathode spacing and internal resistance, enhancing power generation.

Comparison of studies 200 W/m 3 is enough power → to turn on 3 lamps (60 watt) at the same time, to pump 5 L water to a height of 1 m in one second with an appropriate pump. V. B. Oliveira et al. , 2013, Biochemical Engineering Journal

Operating Conditions Power generation of an MFC is affected by many factors: microbe type fuel biomass type and concentration ionic strength membrane type p. H temperature reactor configuration (space between electrodes)

Operating Conditions p. H is tended to due to the production of protons and slow transfer of protons through membrane. (Anode) p. H is tended to due to the consumption of protons by the oxygen reduction rxn and slow transfer of protons from anodic chamber. p. H conc. gradient on both side of membrane p. H changes affects microbial growth and power output A selectively membrane is needed To prevent the p. H limitations, p. H buffers and bipolar membranes can be used. Microbials cultures could be acclimatized to higher p. H values.

Operating Conditions Temperature System kinetics and mass transfer, microbial community Increase microbial activity and membrane permeability (COD removal and power output) Optimum 30 -45 o. C Organic Loading Rate An optimum OLR Too high OLR leads to domination of methanogens which compete with electrochemically active microorg.

Design of MFCs Two-compartment MFC systems Single-compartment MFC systems (SMFC) Upflow mode MFC systems Stacked MFCs Tubular MFC systems

Design of MFCs Stacked MFCs Several MFCs connected in series and in parallel. Enhanced current output and COD removal can be achieved especially in parallel stacked MFCs. Energy autonomous eco-bot 3

Applications of MFCs Ø Electricity generation Since 1999, power production in MFCs has improved 5– 6 orders of magnitude (2. 7– 6. 9 W/m 2 and COD removal as high as %95). On a volumetric basis, up to 2. 2 k. W/m 3 has been reported in the lab. 1 k. W/m 3 is needed to sustain pumping, heating, lighting, aeration, and other energy needs in a wastewater treatment facility. Although these results are promising, its should be noted that these reported power outputs were achieved at the bench scale (V < 1 L) Ø Wastewater treatment (50 -90% less sludge compared to activated sludge treatment) Ø Biohydrogen production: instead of electricity production in MFC, H 2 is produced with an external potential of 110 m. V combining electrons and protons in the cathode. Ø Biosensor : sensors for pollutant analysis, in situ monitoring and control, ex: BOD-coulumbic yield or maximum current.

Pilot scale applications of MFCs A stackable horizontal MFC (SHMFC) (aircathode) The system is the largest single MFC module by far. Domestic wastewater was fed into SHMFC in horizontal advection. During the stable operation period, a maximum current 0. 435 ± 0. 010 A in each module was observed under the external resistance of 1 ohm and the maximum power density was 116 m. W. The COD removal was 79 ± 7% and the TN removal rate was 71 ± 8%. (Feng et al, 2014)

Pilot scale applications of MFCs Brewery Wastewater Treatment Fosters, an Australian beer company, built the first pilot scale MFC (combined volume of 1 m 3) to clean its wastewater while generating electricity and clean water. Each long tube (3 m) is one large Microbial Fuel Cell. Downflow MFC (because of the ionic conductivity of wastewater, power: 8 W/m 3, must be improved) Partnered with the University of Queensland, Fosters’ plans to build a 3 m 3, 2 Kilowatt MFC that cleans all of the company’s wastewater. The power generated from cleaning the brewery wastewater is expected to pay for the initial cost of the Microbial Fuel Cell in ten years. Significant challenges have been observed when these systems are expanded to the pilot scale. Low power production may result from ohmic losses within the large systems and low conductivity of the wastewater. No need oxygen to achieve COD removal, and produce little biomass.

Limitations / further improving Energy losses in MFCs 1 -mass transport limitations: transfer of reactants and products to/from biofilm (concentration polarization) 2 -bacterial metabolic kinetics: microbial consortium and biomass density on the electrode (activation polarization) 3 -electron transfer to/from the electrode: limited by the finite rate of electron transfer between microorg. and solid phase of electrode (activation polarization) 4 -resistance of the electrolytes including membranes: conductivity, ionic strength, electrode spacing (Ohmic losses-internal resistance)

References Zhuwei Du, Haoran Li, Tingyue Gu, A state of the art review on microbial fuel cells: A promising technology for wastewater treatment and bioenergy, Biotechnology Advances 25 (2007) 464– 482 B Virdis, S Freguia, RA Rozendal, K Rabaey, Z Yuan, and J Keller, Microbial Fuel Cells, The University of Queensland, Brisbane, QLD, Australia. Betül Hande Gürsoy, Kimyasal Arıtma Uygulamalarının Organik Maddelerin Yapıları Üzerindeki Etkisinin Araştırılması: Karasu İle Örnek Çalışma, 2014, Doktora Tezi, İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü. V. B. Oliveira, M. Simões, L. F. Melo, A. M. F. R. Pinto, Overview on the developments of microbial fuel cells, Biochemical Engineering Journal 73 (2013) 53– 64. Ashley E. Franks and Kelly P. Nevin, Microbial Fuel Cells, A Current Review, Energies 2010, 3, 899 -919; doi: 10. 3390/en 3050899. Seokheun Choi, Microscale microbial fuel cells: Advances and challenges, http: //dx. doi. org/10. 1016/j. bios. 2015. 021 Minghua Zhou, Jie Yang, Hongyu Wang, Bioelectrochemistry of Microbial Fuel Cells and their Potential Applications in Bioenergy, http: //dx. doi. org/10. 1016/B 978 -0 -444 -595614. 00009 -7

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