Biofuels: Hot Topics. Microbial Fuel Cells:

Transcription

1 Washington University in St. Louis Science Outreach July 19th, 2007 Biofuels: Hot Topics A Workshop for High School Teachers Miriam Rosenbaum (The Angenent Lab): Microbial Fuel Cells: Making Waste into Power 1

2 Introduction and Motivation The continuously growing demand for electrical power and the constantly increasing amounts of anthropogenic wastes are two main consequences of the global population growth and cumulative industrialization processes. To guarantee the worldwide energy demand and the ecologically and environmentally justified disposal of all the anthropogenic wastes are tremendous challenges for economy, science and politics. As fossil fuels will be exhausted in the near future, huge centralized power plants probably will loose importance. Instead, smaller, decentralized, tailored solutions may be designed for individual energy demands. Wastes could play a distinguished role as a resource for decentralized provision of energy as they are not useless end products as the name may imply, but valuable energy resources. The organic load of municipal wastewater is exceptionally energy-rich. Special industrial wastewaters, e.g., from food industry (dairy, brewery or sugar industry) are even richer in content. Different techniques have already been installed and approved in wastewater treatment plants to regain parts of this stored energy and to help cover the enormous power costs the treatment process itself requires. Heat pumps, biogas formation and conversion in combined heat and power plants (CHP) or biogas purification, reformation and conversion in fuel cells currently represent important elements on the way for the realization of an energy selfsufficient treatment plant. A novel concept for direct energy production from wastewater are microbial fuel cells. In the early 1970s, the first tangible ideas and studies for direct conversion of chemical into electrical energy by the exploitation of microbial processes arose. But mainly because of the low prize for fossil oil and gas it only attracted temporary attention. It was not until the mid 1990s, as interest in sustainable and renewable power sources grew, that the idea was revived and research was intensified. At present, especially two fundamental conceptions are promising the application of microbial biofilms in direct electronic interaction with an energy collecting electrode and the utilisation (oxidation) of energy rich microbial metabolites at catalytically active electrodes while early approaches, e.g., the application of synthetic mediators to facilitate the electron transfer from the microbes to an electrode, have been ruled out for ecological and economical reasons. 2

3 How does a microbial fuel cell work? (physico-chemical principles) Principle of Fuel Cells A fuel cell represents an electrochemical device for the direct conversion of chemical into electrical energy. Thereby, electrons are withdrawn from an electron-rich fuel, conducted over an external load and afterwards transferred to the oxidant. The circle is closed by an ion- or proton-exchange connection between the fuel and the oxidant chamber. In contrast to a battery with a definite fuel stock, the fuel is constantly feed into a fuel cell device. e - H + HO 2 e - anode cathode Figure 1 Schematic drawing of a fuel cell The effectiveness of the electron transfer at the electrode surface or more precisely the kinetics of fuel oxidation at the anode and the reduction of the oxidant at the cathode surface are decisively determined by the action of electrocatalysts. The differences in the standard electrode potentials the fuel, and E θ A, for the reversible anodic oxidation of E θ C, for the reversible cathodic reduction of the electron acceptor, determine the content of stored chemical energy ( Δ G, change in Gibbs energy with the complete oxidation of the fuel) and define the theoretical electromotive force, e.m.f. th., the theoretical reversible cell potential, E conditions: th. FC., or the theoretical open circuit potential, OCP th., of the cell under open circuit 3

4 th. th. th. ΔG θ θ FC C A em.. f. = E = OCP = = E E Equation 1 zf When current flows in the external circuit, activation overpotentials or polarization, η act, occur at both electrodes, which diminish the reversible potential in the following way 1 : E = E + η η Equation 2 ' th. FC FC C A The larger the difference between the individual electrode potentials and therefore the cell potential and the lower the overpotential terms, the more electrical energy can be generated. The energetic performance of a fuel cell may be characterized by the electrical power representing the gained electrical energy per time. The electrical power, P, can be calculated as product of cell potential and related current: P = E i Equation 3 ' FC Figure 2 illustrates a typical fuel cell performance plot with a schematic drawing of this mathematical relationship. The cell potential (left axis) can be measured at specific current densities by changing defined external resistance loads cell potential in mv power density in mw cm current density in ma cm -2 Figure 2 Schematic drawing of the cell potential (left axis) and the power density (right axis) of a typical fuel cell experiment. The dotted line represents ideal potential or theoretical e.m.f.. 1 Williams, K. R. (1966). An Introduction to Fuel Cells. Amsterdam: Elsevier 4

5 The schematic curve depicts the characteristic three phases of a fuel cell polarization curve. The actual potential is decreased from its ideal equilibrium potential (dotted line) by irreversible losses. At high resistance and low current flow (for the illustrated case ma cm -2 ) a short, sharp drop of the OCP (1000 mv to 800 mv) indicates the activation polarization, η act, of the electrodes which is directly related to the rates of electrochemical kinetics of the electrode reaction. Between 0.25 and 2 ma cm -2 an ohmic polarization (following Ohm s law) caused by the internal resistance is determining as follows: η ohm = i R Equation 4 in The slope of this linear part delivers the sum value of the fuel cell systems internal resistance, including, e.g., solution and membrane resistances and electrode reaction resistances. This parameter is an important character for the estimation of the overall fuel cell performance and should be as low as possible. At current densities above 2 ma cm -2 the steep potential drop is called concentration polarization, η conc, and indicates mass transfer limitation, which means that transport processes of the reactants to the electrode surface are now rate determining 2. The power density (right axis in Figure I.2) was calculated using Equation I.3. For this case, the maximum power point (MPP) is reached just before diffusion limitation comes into play. Ideally, mass transfer should limit the performance only at (very) high current densities and thus, the drop of the power density curve should be observed most asymmetrically at high current densities. Since, the losses by internal resistance are currently still the main constraints of microbial fuel cell performance, most of the real performance curves are more symmetrically with a MPP at lower current densities. Principles of Biofuel Cells In a biofuel cell, electrons are made accessible from a non-electroactive fuel by the use of biocatalysts. On one hand enzymes or enzyme complexes can be used as biocatalyst in enzymatic fuel cells. On the other hand whole living organisms are used in microbial fuel cells (MFCs). Enzymatic fuel cells usually employ immobilized enzymes (commonly redox enzymes) as catalysts to accelerate highly specific reactions. Purely enzymatic fuel cells have the great advantage of being very small scaled. Due to the small size of the enzymes and the 2 Fuel Cell Handbook, EG&G Technical Service, Inc.; US Dept. of Energy, November

6 high specificity of the anode and cathode reactions, high turnover rates and thus high power densities can be realized (and a membrane separation of the electrodes is often unnecessary). Therefore, they give the chance to construct low energy power supply units for in vivo applications, e.g., medical implantations. But currently, the immobilization and long-term stability of the enzymes still remain great challenges. An excellent review on the state of the art of enzymatic fuel cells is given by Dr. Shelley Minteer from St. Louis University 3. Figure 3 Schematic drawing of a biofuel cell Microbial fuel cells exploiting whole living cells aim to gain energy in a much larger scale. In the early times of MFC research, chemical redox-mediators (often dyes as methylene blue or neutral red) have been used to shuttle metabolic energy (reducing equivalents in the form of electrons) from the cytoplasma of bacteria to an anode electrode. Today, these dyes play no longer play a role for the development of practical MFCs, mainly because in continuous systems these mediators would have to be added and recycled permanently. But mediators still are a very good academic device to study electron transfer processes of microorganisms. As already mentioned in the introduction, the recent research may be classified into two basic concepts which will be introduced subsequently: I) the evolution of microbial biofilms in direct electronic interaction with electrodes and II) the in situ oxidation of microbial metabolites at highly active electrocatalytic anodes. 3 Minteer, S. D., Liaw, B. Y. & Cooney, M. J. (2007). Curr. Opin. Biotechnol. 18,

7 Biological principles in MFCs Considered chemically, every biological degradation of organic matter is an oxidation process. However, if we keep the degradation anaerobic we get the chance to exploit this process for electron recovery (= power production). Anaerobic conversion of, e.g., sugars is realized either by bacterial fermentation leading to the formation of small, reduced energy-rich metabolic products as ethanol, acetate or hydrogen or by anaerobic respiration using another terminal electron acceptor instead of oxygen to take up the electrons coming from the sugar. Different MFC techniques allow us to utilize both of these anaerobic metabolic processes: One approach aims at the development of biocatalytic electrodes, which are based on direct physical and electronic interaction between the microorganisms and the anode surface. Figure 4 Showing bacteria fixed in a biofilm on a MFC anode. Electrons might be transferred directly, via conductive pili (protein filaments) or via redox active shuttling molecules (mediators) to the electrode. from: 4 For the development of power supply devices in marine sediments mainly iron, manganese and sulphate reducing microbes are exploited 5. For energy generation from wastewater, biofilm forming microbial consortia, enriched directly from the wastewater at the electrodes, represent a successful strategy 6. The initially low current densities have been overcome by establishing multilayer biofilms that are capable of using very different mechanisms to discharge their 4 Rosenbaum, M., Zhao, F., Schröder, U. & Scholz, F. (2006 ). Angew. Chem. int. Ed. 45, Lovley, D. R. (2006). Nat. Rev. Microbiol. 4, Logan, B. E. (2005). Water Sci. Technol. 52, Rabaey, K., Lissens, G., Siciliano, S. D. & Verstraete, W. (2003). Biotechnol. Lett 25, He, Z., Minteer, S. D. & Angenent, L. T. (2005). Environ. Sci. Technol. 39,

8 electrons to the electrode (as final metabolic electron acceptor). The different electron transfer pathways that are known up to now are illustrated in Figure 1. Undoubtedly, future will bring out a lot of further discoveries and new insights into inter-bacterial and bacteria-electrode interaction processes. from: 7 Figure 5 Sediment microbial fuel cell: Iron, manganese and sulphate reducing sediment bacteria are using an embedded anode as electron acceptor. The oxygen reduction cathode is placed in the overlaying aerobic water phase. Because of the natural separation between aerobic water and anaerobic sediment, no membrane is required. Figure 6 Tubular microbial fuel cell (a,b): Wastewater flows though outer anode chamber, which is separated from the inner cathode tube by an ionexchange membrane. from: 8 7 Lovley, D. R. (2006). Curr. Opin. Biotechnol. 17, Rabaey, K. & Verstraete, W. (2005). Trends Biotechnol. 23,

9 from: 9 Figure 7 Upflow microbial fuel cell: In accordance to the upflow anaerobic digestion technology, high loaded wastewater is entering the anode side of the reactor at the bottom. The cathode is placed above the anode chamber. This concept also realizes an active flow of ions towards the exchange membrane. The second highly promising approach is based on the preparation and investigation of electrocatalytic anodes, capable of an efficient and direct electro-oxidation of bacterial metabolites under the diverse and complex microbial growth conditions. Figure 8 Transformation of a primary substrate (wastewater) into a secondary fuel for the direct (in situ) electro-oxidation at catalytically active electrodes. from: 4 With this attempt not only electricity generation from wastewater is possible, but also individual devices for decentralized power supply, e.g. microbial solar cells or gastrobots 10, can be designed using suitable specific microbial populations. Up to that point, primarily platinum is used as catalyst for the utilisation of molecular hydrogen produced by 9 He, Z., Minteer, S. D. & Angenent, L. T. (2005). Environ. Sci. Technol. 39, Wilkinson, S. (2000). Autonomous Robots 9,

10 fermentative 11 or phototrophic 12 microorganisms. But very recently also other, non-noble metal catalysts which are cheaper and more robust than platinum have been shown to successfully convert such metabolites as hydrogen, formate and lactate into electricity 4. Figure 9 Chew chew the gastrobot: Autonomous robot just has to be fed with grapes, all other functions are energy selfsufficient. from: 10 Figure 10 A microbial solar cell: In left (anode) chamber a purple bacterium is using organic waste and sun light to produce molecular hydrogen which is directly converted into electricity. from: 13 Further information: For information on the worldwide status of MFC research watch out at he webpage of the MFC research community: Several books on the topic are currently in preparation. 11 Schröder, U., Nießen, J. & Scholz, F. (2003). Angew. Chem. int. Ed. 115, Rosenbaum, M., Schröder, U. & Scholz, F. (2005a). Appl. Microbiol. Biot. 68, Rosenbaum, M., Schröder, U. & Scholz, F. (2005b). Environ. Sci. Technol. 39,