1 Bioelectrochemical Fuel Cells

Transcription

1 VCH Herr Schmidt REED/REHM, Vol Bioelectrochemical Fuel Cells DIETER SELL Frankfurt/Main, Germany 1 Introduction 6 2 History of Bioelectrochemical Fuel Cells 6 3 Global Dimensions of Bioenergetics 7 4 A Brief Introduction to Fuel Cells The Hydrogen Oxygen Fuel Cell 9 5 Principle of Bioelectrochemical Fuel Cells Classification of Bioelectrochemical Fuel Cells Indirect Bioelectrochemical Fuel Cells Direct Bioelectrochemical Fuel Cells Application of Redox Mediators Cathode Systems in Bioelectrochemical Fuel Cells Examples for Different Approaches to Bioelectrochemical Fuel Cells 15 6 The Role for Bioelectrochemical Fuel Cell Research in the Development of Biosensors and Bioelectronics 17 7 Conclusions 20 8 References 21

2 6 1 Bioelectrochemical Fuel Cells 1 Introduction Today only the direct incineration or gasification of biomass is of practical importance for the use of biological resources for industrial energy generation. In a broader sense the incineration of non-regenerative, fossil sources of energy coal, petroleum, and natural gas also represents an energetic use of biological metabolism (OSTEROTH, 1989). A further possibility will be illustrated in this chapter: the application of fuel cell devices which are powered by biological redox reactions, performed by intact microorganisms or isolated enzymes. Typically, a fuel cell works by the oxidation of a reduced fuel at an anode with concomitant transfer of electrons, via a circuit, to a suitable electron acceptor molecule, e.g., oxygen, at the cathode. The power output of the fuel cell depends on the number and rate of transfer of electrons and on the potential difference between anode and cathode. In theory, fuel cells are the most effective devices for the conversion of chemical energy to electrical energy because they avoid the limitations of the Carnot cycle (BOCKRIS and REDDY, 1970). Microorganisms are able to utilize a vast range of organic and inorganic substances, which can be regarded as potential fuels for combustion in a fuel cell. The direct combustion of these substances cannot be performed due to lack of suitable electrocatalysis. Microorganisms dispose of a large set of enzymes for the highly efficient and controlled oxidation of different substrates with concomitant electron transfer by way of a sequence of reactions to a terminal electron acceptor (oxygen for aerobic catabolism). The key enzymes for these processes are the oxidoreductases, which catalyze the electron transfer for the redox reactions involved, a property that is highly desirable for good fuel cell performance. 2 History of Bioelectrochemical Fuel Cells The concept of bioelectrochemical fuel cells was extremely popular in the early 1960s when it was taken up by NASA (National Aeronautics and Space Administration, USA). NASA s interest in studying the production of electricity by means of biochemical reactions stemmed from the association with the problems of waste management in the closed system of a space shuttle during longer space flights: human waste should be reprocessed in order to attain a closed or nearly closed ecology in the spacecraft (LEWIS, 1966). Therefore, algae and bacteria were among the first organisms used in this phase of experiments with bioelectrochemical fuel cells. Even in current publications this concept of electricity generation from waste materials is proposed for outer space applications (PARK and ZEI- KUS, 2000). In 1963 bioelectrochemical fuel cells were already commercially available and they were promoted as power sources for radios, powered radio beacons, signal lights and other apparatus at sea (Anonymous, Financial Times, 1963a, b). However, these fuel cells were not a commercial success and they soon disappeared from the market. With the successful development of technical alternatives, e.g., photovoltaic technologies for the energy supply on space flights and later on for many different fields of application, interest in bioelectrochemical systems declined for a while until there was a revival of interest in the late 1970s/early 1980s, stimulated by the bottleneck in world oil supplies and the growing importance of biotechnology as a field with broad industrial applications. The generation of electric energy was, of course, just one aspect of the research activities engaged in by groups all over the world (VIDELA and ARIVA, 1975; HIGGINS and HILL, 1979; TANAKA et al., 1983). However, it was soon realized that bioelectrochemical fuel cells were also promising for a different application in the area of measurement and control due to the fact that the voltage or current ge-

3 3 Global Dimensions of Bioenergetics 7 nerated by such a fuel cell is directly dependent on the activity of the biological component, be it an isolated enzyme or an intact microorganism. Work on bioelectrochemical fuel cells, therefore, represents the first systematic approach to investigating the interaction of biological systems with electrodes and for this reason can be regarded as the forerunner of the development of electrochemical biosensors, bioelectrochemical syntheses, and modern bioelectronics. The first experiments in the field of bioelectrochemical fuel cells were already performed in It was POTTER who first postulated a relationship between electrode potentials and microbial activity (POTTER, 1911). POTTER measured the potential difference between two electrodes, one of which was inserted into a bacterial culture and the other into a sterile culture medium. He actually made what can be considered to be the first biochemical fuel cell battery by assembling six cells, each of which consisted of a yeast glucose half-cell and a glucose half-cell without microorganisms. Similarly, in 1931 COHEN studied the potential differences arising between various cultures and sterile media; he also built a bacterial battery which produced a small current for a short period of time. Such an experimental set-up is shown in Fig. 1. COHEN found that the potential of a vigorously growing bacterial culture amounted to V over the control medium. The greatest deficiency of the microbial half-cell was that its current output was generally very low (10 P5 10 P6 A). 3 Global Dimensions of Bioenergetics From an energetic standpoint, the existence of the whole terrestrial biosphere, including man, depends solely on extraterrestrial solar radiation. The decisive factor is that it is not solar energy itself that is the driving force for all life processes, but the conversion of electromagnetic radiation into electrochemical, and ultimately chemical, energy that takes place in living organisms (RENGER, 1983). Fig. 1. Early bioelectrochemical fuel cell developed by COHEN (1931). The half-cell on the left side contained a glucose medium with a growing bacterial culture; the half cell on the right side just contained a sterile glucose medium. Both half-cells were connected via an agar-kcl bridge (slightly modified according to COHEN, 1931).

4 8 1 Bioelectrochemical Fuel Cells Photoautotrophic organisms are capable of this conversion. They can use sunlight in the visible range ( nm) to synthesize biomass, a process referred to as photosynthesis (ZIEGLER, 1983). The crucial phase of this fundamental bioenergetic reaction took place during evolution in the cyanophytic stage. Cyanobacteria developed a complex molecular system enabling water to degrade into molecular oxygen by means of sunlight on the one hand, and into hydrogen bonded to suitable carriers by chemical means on the other hand. One consequence was that the hitherto anaerobic atmosphere of the earth became aerobic. This was advantageous for the group of chemoorganotrophic organisms which cannot use sunlight directly as a source of energy; instead they metabolize exogenic nutrients from which they derive energy to sustain their vital functions. Oxygen provided these organisms with a reactant which facilitated a substantially more effective exploitation of substrates. Thus, for instance, under anaerobic conditions glucose can only be degraded to ethanol and carbon dioxide by yeasts, producing 197 kj mol P1. Under aerobic conditions it can be completely degraded into carbon dioxide and water, supplying the organism with 2,874 kj mol P1 (LEHNINGER, 1977). Only the availability of oxygen, permitting an increased energy yield, enabled heterotrophic organisms to develop highly organized mobile forms of life with a high energy turnover. This bioenergetic principle was strongly favored throughout the further evolution processes: With all forms of life above the evolution stage of cyanobacteria and with the overwhelming majority of bacteria, the central system for the realization of fundamental bioenergetic reactions that has prevailed is the oxidation of carrier-bonded hydrogen using oxygen as the oxidant with the concomitant formation of water. Sunlight causes water to be broken down by photosynthesis by which carrier-bonded hydrogen and oxygen are produced. This process is reversed by heterotrophic processes consuming oxygen, and water is produced again (Fig. 2). It will be shown that the universality of this metabolic concept is very advantageous for Fig. 2. The system of carrier-bonded hydrogen as fundamental bioenergetic principle: Photosynthesis as a source and heterotrophic processes as sink for this universal energy carrier which can be used directly for intracellular energy production in the catabolic metabolism or can be used for anabolic processes.

5 4 A Brief Introduction to Fuel Cells 9 the development of bioelectrochemical fuel cells. Some forms of carrier-bonded hydrogen formed during metabolic processes are suitable forms of fuel for combustion in a bioelectrochemical fuel cell or can easily be transformed into a combustional form. Due to this fact, potentially a broad spectrum of microorganisms, heterotrophic as well as (photo-) autotrophic, can be used as biocatalysts for energy production by means of a fuel cell. With an estimated annual conversion of Gt (gigaton p 10 9 tons) carbon, photosynthesis and its heterotrophic conversion are the quantitatively most important chemical processes on the earth s surface (KRIEB, 1981). Considered on a global scale, when biomass builds up by photosynthesis the degree of effectiveness is not high. Approximately 0.12% of solar radiation energy on the whole earth is converted into chemically bonded energy in the form of biomass. Nevertheless approximately 170 Gt per year of biomass accumulate from the photosynthesis of all land and water plants. The energy content of this biomass amounts to about 3,000 EJ (Exa Joule) p J, corresponding to tenfold of mankind s annual energy needs (GRATHWOHL, 1983). The orders of magnitude of these figures explain why diverse investigations are being conducted to determine whether and how bioenergetic reactions can be relied on as a basis for industrial energy production. To understand the principle of bioelectrochemical fuel cells a look at the simplest and best known of all technical fuel cells will help: the hydrogen oxygen fuel cell, which today can already be found as a prototype in cars, buses, etc. 4.1 The Hydrogen Qxygen Fuel Cell The hydrogen oxygen fuel cell is the most highly developed fuel cell, since hydrogen is the most reactive fuel known. The principle of operation of such a fuel cell is illustrated in Fig. 3. This cell operates with hydrogen gas as the fuel and oxygen gas as the oxidant. The fuel cell consists of two electrodes, an anode at which oxidation occurs and a cathode at which reduction occurs. The electrodes are in contact with an electrolyte. Often an ion-exchange membrane separates the anode and cathode compartments. In operation, the fuel, hydrogen gas, passes over the surface of the anode and is electrochemically oxidized to hydrogen ions, which enter the electrolyte and migrate towards the cathode. Oxygen gas passes over the surface of the cathode and is reduced, combining with the hydrogen ions from the elec- 4 A Brief Introduction to Fuel Cells A fuel cell is a device for the direct energy conversion of chemical energy to electrical energy. It requires an anode, a cathode, a supporting electrolyte medium to connect the two electrodes, and an external circuit to utilize the energy. Reactants must be supplied to both electrodes as a source for the electron transfer reactions; catalysts must be present to provide a rapid rate of reaction at each electrode. Fig. 3. Principle of a hydrogen oxygen fuel cell.

6 10 1 Bioelectrochemical Fuel Cells trolyte to form water. The net result of the operation of the fuel cell is the combination of hydrogen and oxygen to form water with electrons flowing through the external circuit. This flow of electrons can be utilized to power a machine, a light bulb, or anything that runs on electricity. Other fuels for technical fuel cells are methane, natural gas, carbon monoxide, methanol, etc. More details about the state of the art in the field of fuel cell research are summarized by KORDESCH and SIMADER (1996). The electricity output of the hydrogen oxygen fuel cell is determined by the electrode reactions, the fuel oxidation at the anode and the oxidant reduction at the cathode. The reactions in a hydrogen oxygen fuel cell (with an acid electrolyte) are the following: Cathode: Anode: Total: O 2 c4h c c4e P ] 2H 2 O E 0 p1.23 V H 2 ] 2H c c2e P E 0 p0 V 2H 2 co 2 ] 2H 2 O E 0 p1.23 V (1) (2) (3) E 0 represents the standard half-cell potential, assuming reversible operation. The theoretical amount of useful work, W u, that can be obtained depends on the net release of Gibbs free energy, G, as the reaction proceeds from reactants to products. Both values are equal and are related to the net reversible potential difference E 0 for the anodic and cathodic half-cell reactions according to: P GpW u pn F E 0 (4) In this equation n is the number of electrons transferred per mole of fuel and F the Faraday constant (96,500 coulombs per mole). With regard to the first law of thermodynamics, the free energy is related to the change in enthalpy, H, and the change in entropy, S, at constant temperatures, T: Gp HPT S (5) From this, the efficiency of a fuel cell can be calculated as follows: Efficiencyp G p1p T S (6) The theoretical efficiency of fuel cell systems can be as high as 85 90%; this is considerably higher than the 40 60% theoretical efficiencies for the mechanical-thermal energy transfer in a combustion engine. The reason is that the fuel cell reactions are not coupled with an increase in temperature, so that the T S term amounts to only F10% of H (BOCKRIS and REDDY, 1970). 5 Principle of Bioelectrochemical Fuel Cells In a bioelectrochemical fuel cell the electrode reactions are similar to those in the hydrogen oxygen fuel cell, except for the fact that the fuel is not hydrogen gas but a form of carrier-bonded hydrogen, produced by physiological redox reactions. If we again look at Fig. 2 and recall the metabolic concept of carrierbonded hydrogen as fuel for the intracellular generation of energy, we can draw interesting parallels between the reaction in a hydrogen oxygen fuel cell, a bioelectrochemical fuel cell, and even the catabolic reactions in a living organism. The overall reaction of a hydrogen oxygen fuel cell and a bioelectrochemical fuel cell [with the coenzyme NAD (nicotine adenine dinucleotide) as the assumed hydrogen carrier] are: Hydrogen oxygen fuel cell: H 2 c 1 / 2 O 2 ph 2 O Bioelectrochemical fuel cell: E 0 p1.23 V Gp237 kj mol P1 (7) Carrier-H 2 c 1 / 2 O 2 ph 2 OcCarrier E 0 p1.14 V Gp220 kj mol P1 (8)