EP2304829A1

EP2304829A1 - Device and method for improved electrochemical cell

Info

Publication number: EP2304829A1
Application number: EP09800608A
Authority: EP
Inventors: Patrick Roodenburg; Machiel Saakes; Christiaan Haldir Goeting; Hubertus Victor Marie Hamelers
Original assignee: Magneto Special Anodes BV
Current assignee: W&F TECHNOLOGIES B.V.
Priority date: 2008-07-21
Filing date: 2009-07-14
Publication date: 2011-04-06
Also published as: NL1035728C2; WO2010011135A1; CN102099949B; CN102099949A

Abstract

The present invention relates to a device and a method for performing electrochemical/biological reactions, like biological oxidation of a biological compound. The device comprises: - an anode compartment provided with an anode, placed in an anode fluid comprising reagents for an oxidation reaction; and - a cathode compartment that is separated from the anode compartment, the cathode compartment provided with a cathode placed in a cathode fluid, wherein at least one of the anode and cathode compartments, and at least one of the anode and cathode, are arranged such that at least one of the anode fluid and cathode fluid flows at least partially through at least one of the anode and cathode.

Description

DEVICE AND METHOD FOR IMPROVED ELECTROCHEMICAL CELL

The present invention relates to a device comprising an anode compartment provided with an anode, placed in an anode fluid comprising reagents for an oxidation reaction, and a cathode compartment that is separated from the anode compartment, the cathode compartment provided with a cathode placed in a cathode fluid. Such a device can, for example, be a fuel cell with which electric energy may be generated. Also, such a device can be a microbial fuel cell (MFC) , enzymatic fuel cell or redox flow fuel cell.

Fuel cells with which electric energy can be generated are known in the art. In such a fuel cell electric energy is generated, for example, by electrochemical combustion of hydrogen (H₂) and oxygen (O₂) . The reactions that may occur herein are the following:

(1) oxidation at the anode: H₂ -> 2H⁺ + 2e^~

(2) reduction at the cathode: O₂ + 4H⁺ + 4e^~ -> 2H₂O.

The reduction and oxidation reaction occur in two separate compartments. Because the anode and cathode are electrically connected, due to these reactions, electron transport occurs between the anode and cathode. This creates an electric current. The charge balance is maintained because transport of cations is possible via a cation- conducting material, with which the anode compartment and the cathode compartment are separated.

In a biological fuel cell an anaerobic biological oxidation reaction may also occur at the anode. Such a reaction is catalysed by a biocatalyst, which uses the anode either directly or via a redox mediator as terminal electron acceptor. Examples of such biocatalysts are anodophylic microorganisms and redox enzymes. If the anaerobic oxidation reaction at the anode is performed by a microorganism, reference is also made to a microbial fuel cell.

If O₂ is used as terminal electron acceptor, the reactions that occur in a microbial fuel cell may be presented as follows:

Anode : C_aH_bO_c + (2a - c)H₂O → CiCO₂ +(4a + b- Ic)H⁺ +(4a + b- 2c)e^~

(oxidation)

Cathode : [ ^{4a + b ~ 2c} )ø₂ + (4a + b - Ic)H⁺ + (Aa + b - 2c)e^~ → (Aa - 2c)H₂O

( reduction )

This reaction can be rewritten: For example for acetic acid applies a=2, b=4 and c=2.

A microbial fuel cell allows combining water purification with electricity generation as the microorganisms may convert different substrates, which are present in waste water. The reaction at the anode may be performed by microorganisms, which can use the anode as electron acceptor. Examples of such microorganisms are Geobacter sulferreducens, Shewanella putrefaciens, Geobacter metallireducens and Rhodoferax ferrireducens or a consortium of these organisms. WO 2007/094658 discloses a cathode system providing improvements relative to platinum cathodes. Use is made of the Fe ( II ) /Fe ( III ) redox couple to transfer electrons from the cathode to the terminal electron acceptor. During this process Fe(III) is reduced to Fe(II) at the cathode .

WO 2007/011206 discloses a biological fuel cell with an improved configuration of the anode compartment and the cathode compartment by arranging the anode and cathode compartment so that the compartment of the one kind encloses a number of compartments of the other kind.

Important in the design of devices like biological fuel cells are the resistances that limit the performance of these devices. The present invention has for its object to provide a device aimed at a reduction of the internal resistances and/or internal losses thereby improving the overall efficiency.

The device according to the present invention comprises:

- an anode compartment provided with an anode, placed in an anode fluid comprising reagents for an oxidation reaction; and

- a cathode compartment that is separated from the anode compartment, the cathode compartment provided with a cathode placed in a cathode fluid, wherein at least one of the anode and cathode compartments, and at least one of the anode and cathode, are arranged such that at least one of the anode fluid and cathode fluid flows at least partially through at least one of the anode and cathode . An important aspect relating to the resistance or losses present in the device is a gradient in the concentration of reactants or products, and/or pH, and/or potential. In existing configurations with an anode and cathode a flow is forced along the surface of the electrodes. This means that only the surface area of the electrodes takes part in the process. This is not only inefficient as a substantially part of the electrode material does not take part in the process; it also leads to gradients in the fluid and to increased gradients in concentrations and/or pH . By forcing a flow at least partially through at least one of the anode and cathode, and preferably through both the anode and cathode, gradients and resistances are minimized. This results in a decreased over- potential.

By improving mass transport the current density is increased. Also the control of the pH is improved. To enable forced flow through an electrode this electrode must be porous. The present invention can be applied to MFC's with restrictions caused by gradients as explained above. In such MFC a bio-anode and/or bio-cathode can be used, however, also other anodes and/or cathode are possible.

To minimize the gradient in the fluid, preferably the fluid, at least one of the anode fluid (or anolyte) and cathode fluid (or catholyte) , flows substantially perpendicular to the surface of the at least one of the anode and cathode and there through. By providing a flow through the electrode in a direction perpendicular to its main surface the pressure drop over the electrode is minimal. Depending on the configuration of the process the flow can be in either of two directions perpendicular to the surface of the electrode. Also, the pressure drop is independent of the surface dimensions of the electrode. Additionally, the thickness of the 3-dimensional electrode can be adapted to the specific requirements of the process relatively easy.

For example, an over-potential in a known configuration, as described in WO 2007/094658 for generating energy from an anaerobic biological oxidation reaction, was found to be 240 mV with pH of 2.5 and Fe(III) concentration of 17 mM. In a device according to the present invention with similar conditions and pH of about 2 the over-potential is reduced to 1 mV. This increases the power density to 1.85 W/m². The configuration of the device according to the present invention was such that a fluid flowed through the cathode in longitudinal direction. The cathode comprised several layers of felt with a (compressed) thickness of about 12 mm and a length of about 100 mm. An alternative configuration of the device according to the present invention wherein a fluid flows through the electrode in a transversal/perpendicular direction further improves the power increase as in this configuration the pressure drop is reduced.

The anode and/or cathode are preferably porous to enable the fluid to at least partially flow through the anode and/or cathode. Also preferably the anode and/or cathode are made of Felt (for example Carbon or Graphite and preferably Graphite) , carbon or graphite granules or fibers, (coated) sintered Titanium, or combinations thereof. As alternative to sintered Titanium other metals like Nickel, Zicronia, Iron, Tungsten, Molybdenum, Steel, Stainless Steel and alloys may be used. Besides shaped as felt-type electrodes, the material can be shaped as foam type metals, woven/non-woven, sintered, stretched metal and/or coated material. The anode and/or cathode may be coated with a layer comprising of for instance mixed metal oxides, RuO₂, Irθ2, TiC>2, Zrθ2, Ta₂O₃, Pt, Pd, Ir, Ru, ironphosphates (e.g. FePo₄), M0S2, MoS₄ or a combination thereof. For example, the use of sintered Titanium, that may be coated, using a current feeder lowers the internal losses as compared to for example pressure contact using Felt. In case of using sintered titanium as anode it is preferably provided with a coating of for example mixed metal oxides, RuO₂, IrO₂, TiO₂, ZrO₂, Ta₂O₃, Pt, Pd, Ir, Ru, or combinations thereof. The porosity lies in the range of 25-85 % for example for sintered Titanium to 80-95 % for Felt. To enable a sufficient flow through at least a part of an electrode the porosity preferably is in the range 45-95 %, and more preferably in the range of 65-95 %. In existing configurations the Graphite Felt electrodes, that have a specific surface of 200-300 cm^"1, are only subjected to fluids on the surfaces of the electrodes. Therefore, these electrodes are in existing devices operated as 2-dimensional electrodes. By forcing fluids at least partially through the electrode this electrode is operated as a 3-dimensional electrode. In this respect it is noted that the current density of Graphite Felt in known devices, at low alkalinity in the order of 10 mM, is about 4 A/m², which is similar as that for a flat surface electrode. This means that indeed in known devices the electrode acts as a 2-dimensional electrode. In a device according to the invention the entire 3-dimensional specific surface area is used. This results in improved overall process efficiency. The process can be performed using a device according to the invention that uses a membrane or uses another means for separating the anode and cathode compartments. As an example of an embodiment without membrane a porous dividing wall may be used, for instance a standard polymer filter, in combination with means to minimize the diffusion of oxygen towards the anode compartment .

In a preferred embodiment according to the present invention the anode and cathode are separated by a membrane or a non-conductive porous layer, wherein at least one of the anode and cathode are placed at a distance from the membrane .

In known configurations for biological fuel cells the electrodes are placed directly against the membrane to minimize resistances in the device. By placing the electrode at a distance from the membrane the fluid that is forced through the electrode can be removed. Preferably, the distance is about 0.1-10 mm, and more preferably about 1 mm. In experiments a distance of about 1 mm showed a good efficiency of the overall process. In case of relatively high salt concentrations a larger distance like about 5 mm is possible with the additional benefit that flow resistances are minimized. Also preferably, a spacer is arranged between the membrane and the electrode to maintain the distance. The spacer ensures a distance between the electrode and the membrane. Also, the spacer distributes forces acting upon the electrode over a large area of the membrane thereby decreasing the effect of such forces. In addition, a spacer may enhance mixing and, therefore, improve mass transport. In one of the possible embodiments according to the invention this spacer has the shape of a gauze. Alternatively, in stead of a spacer a so-called structured electrode may be used that comprises integral distance-holders (protrusions and/or channels), preferably of the electrode material, that maintain the desired distance between the main body of the electrode and the membrane. Also, as a further alternative, in stead of a structured electrode also a structured membrane can be used. Preferably such structured membrane is provided with a structure, for example protrusions and/or channels, on both sides of the membrane. An advantage of the alternative configuration is the spacerless design of the device thereby minimizing the number of parts and also minimizing risks like leakage.

In a preferred embodiment of the present invention at least one of the anode and cathode is provided with a thickness of about 0.1 - 10 cm and preferably about 0.1-1.0 cm.

By providing an electrode in the range of 0.1 - 10 cm in combination with a forced flow through the electrode a 3-dimensional use of the electrode is realized. Experiments have shown an efficient operation with a thickness of about 0.5-1.0 cm.

In a further preferred embodiment according to the invention the device comprises guide means for directing at least one the anode fluid and cathode fluid to the at least one of the anode and cathode. To ensure a forced flow through the electrode guide means are arranged that direct the flow through the electrodes. In addition, the guide means can be configured such that the fluid is distributed evenly over the entire surface of the electrodes so that also the fluid that is forced through these electrodes is distributed evenly. Also, the guide means may act as current collector.

In a further preferred embodiment according to the present invention the flow through the at least one of the anode and cathode has a substantial flow component in the longitudinal direction of the at least one of the anode and cathode .

An advantage of forcing a fluid in a longitudinal direction of the electrode through this electrode would be that the pressure upon the membrane by the flow through the electrode is reduced as compared to the configuration with a perpendicular flow through the electrodes. This may outweigh the fact that a longitudinal forced flow through the electrodes may result in a higher pressure drop, internal resistance and potential difference over the 3-dimensional electrodes as compared to a forced flow substantially perpendicular to the surface of this electrode and there through. Also, it is possible to use a combination of a forced flow with both a transversal and a longitudinal component to achieve the benefits, at least partially, of both configurations.

In a further preferred embodiment according to the present invention the anode compartments and cathode compartments are arranged in such a way that the compartment of the one kind substantially encloses at least one of the compartments of the other kind.

As the membrane is situated between the anode and cathode compartments the membrane is also at least partially enclosed by the enclosing compartment. In turn the membrane will at least partially enclose the enclosed compartment. Thus, there is a relatively large contact area between the fluid in the anode and cathode compartments and this membrane. This results in an improved mass transfer between these compartments in comparison to commonly used fuel cells. As in this configuration the surface area of the membrane, relative to the volume of the compartment which it encloses, is relatively large the electrical output of the biological fuel cell will be increased. It is possible that a number of anode compartments enclose a number of cathode compartments. It is also possible that a number of cathode compartments enclose a number of anode compartments. This choice depends on the type of reactions taking place. In case the reaction at the anode is the limiting reaction it is preferred to have an anode area that is relatively large relative to the cathode area. It is also possible to have several rings of compartments enclosing each other and/or having several compartments of one type enclosed by one compartment of the other kind.

In a further preferred embodiment according to the present invention at least one of the anode and cathode is provided with an additional layer to stretch the material of the at least one of the anode and cathode.

By stretching the material of the electrode by providing an additional layer, preferably comprising Titanium and Platinum, the electrode maintains its shape during the operation. As alternative for Titanium and Platinum for the additional layer also other (precious metal based) electro catalysts can be used. With the additional layer forces acting on the electrode are distributed more evenly by the additional layer. Also the additional layer may act as current collector. In case of using carbon or graphite felt as material of the electrode the electrode is compressed.

In a further preferred embodiment according the present invention the device comprises tensioning means for pre-stretching the anode and cathode. By pre-stretching the anode and/or cathode leakage of fluid in the device can be prevented or minimized. Preferably the electrodes are being used in combination with a spacer provided with an 0-ring to prevent crushing of the electrodes. Also preferably, the tensioning means compress the electrode material, for example graphite felt, with about 20%. This compression showed good conductivity.

The invention further relates to a method for performing electrochemical/biological reactions, like biological oxidation of a biological compound, comprising the steps of:

- providing a device according to any of the claims 1- 14; - electrically connecting the anode and the cathode;

- performing a biological oxidation reaction in the anode compartment; and

- arranging at least one of the anode and cathode compartment, and at least one of the anode and cathode, such that at least one of the anode fluid and cathode fluid flows at least partially through at least one of the anode and cathode.

Such a method provides the same effect and advantages as those stated with reference to the device. In case of anaerobic oxidation the method also involves forming and/or maintaining anaerobic conditions in the at least one anode compartment .

Further advantages, features and details of the invention are elucidated on the basis of preferred embodiments thereof, wherein reference is made to the accompanying drawings, in which:

- Figure 1 shows a schematically view of a microbial fuel cell;

- Figure 2 shows a cross section through a biological fuel cell with enclosing compartment;

- Figure 3 shows an alternative embodiment of the fuel cell of figure 2;

- Figure 4 shows a lengthways section through another alternative embodiment of a biological fuel cell; - Figure 5 shows a biological fuel cell according to the present invention;

- Figure 6 shows a section through the fuel cell of figure 5; - Figure 7 shows a schematically view of an electrode provided with guide means;

- Figure 8 shows an alternative configuration for an electrode provided with guide means; - Figure 9 shows a further alternative embodiment of an electrode provided with guide means;

- Figure 10 shows an alternative embodiment with enclosing compartments; and

- Figure 11 shows another alternative embodiment of an electrode provided with guide means.

Figure 1 schematically shows the operation of a microbial fuel cell and how at the anode 4 (A) of a microbial fuel cell 2 organic matter OM together with water is oxidised anaerobically to CO₂ and protons. The electrons, elaborated herein, are transferred to the anode 4 and flow to the cathode 6 (C) via an electric system 8. At the cathode 6 the electrons together with the protons are used for the reduction of oxygen to water. The charge balance in the system is maintained because protons may flow through the membrane 10 from the anode compartment to the cathode compartment. Due to the electron flow from the anode 4 to the cathode 6, electric work may be performed in the electric system 8. A biological fuel cell 12 (figure 2) comprises an anode compartment 14, enclosing a cathode compartment. Both the anode compartment and the cathode compartment have a circular cross section. The anode compartment and the cathode compartment are separated by a partition wall 16, formed by Nafion 117, which at the side of the cathode compartment is coated with a graphite layer 18, acting as cathode, and which is provided with a electron transfer catalyst, in this case a platinum material. The anode compartment 14 is filled with a porous graphite felt, acting as anode, and whereon a consortium of Geobacter sulferreducens, Shewanella putrefaciens, Geobacter metallireducens and Rhodoferax ferrireducens, which use the anode 2 as an electron acceptor, grows. The anode compartment 14 is fed with a waste water stream, comprising biologically oxidizable organic molecules. Through the open space 20 of the cathode compartment a gas stream is fed, comprising oxygen as electron acceptor. The anode 14 and cathode 18 are electrically connected (not shown) to allow electron transport from the anode to the cathode 18.

In the illustrated biological fuel cell 22 (figure 3), a number of cathode compartments is enclosed by a single anode compartment. Each cathode compartment is herein separated from the anode compartment by means of the partition wall 24, constituted from Nafion 117, which is coated on the side of the cathode compartment with a graphite material acting as cathode 26, and which is furthermore provided with an electron transfer catalyst, made from a platinum material. Because of the scattering of the plurality of cathode compartments within the anode compartment, the distance for mass transport from the anode compartment to the cathode compartment 26 is small. The cathodes 26 in the cathode compartment are in mutual electrical contact (not shown) with the anode 28. Electrical current can be generated by flowing a waste water stream, comprising a biologically oxidizable organic compound, through the anode compartment and by flowing a fluid, comprising oxygen, through the cavity 30 of the cathode compartment.

Figure 4 shows a lengthways section of a biological fuel cell 32 according to the present invention in a modular housing. Also in this embodiment a cathode compartment is enclosed by an anode compartment. The cathode- and anode compartments are again separated by a partition wall 34 constituted from Nafion 117, which at the side of the cathode compartment is coated with a cathode material 36, made from graphite powder, and which furthermore comprises finely divided platinum particles as electron transfer catalyst. The graphite felt 38 in the anode compartment is in contact with a conducting material 40. The same is true for the cathode 36. Via the conducting material 40 the anode 38 and cathode 36 are electrically connected with an electrical apparatus 42, in this case a lamp, which can convert the generated electrical energy. The whole is packed together in a modular housing 44.

Apart from connecting the anode and the cathode with an electrical apparatus, the biological fuel cell according to the invention may also be connected in series with a number similar biological fuel cells, by connecting the anode with the cathode of a different biological fuel cell, and by the connecting the cathode with the anode of a different biological fuel cell. This increases the delivered voltage of the generated electrical current.

A fuel cell 46 (figure 5) is mounted in a frame 48 with tensioning means 50 that enables the provision of a pressure on fuel cell 46. The tensioning means 50 especially press the electrodes toward each other. Tension rods 52 are used to mount the parts of fuel cell 46 together. Between tensioning means 50 is provided a cathode 54 (figure 6) with a conductive layer 58. Between cathode 54 and membrane 60 is provided a spacer 62 for maintaining a distance between the cathode 54 and membrane 60. In the illustrated embodiment of fuel cell 46 this distance is about 2-3 mm. This distance ensures that a flow of catholyte supplied from input 64 passing transversely through cathode 54 entering the volume maintained by spacer 62 and finally leaves the cell 46 at output 66. On the other side of membrane 60 there is provided an anode 68 with a conductive layer or current feeder 70 and a spacer 72. In a possible embodiment according to the invention the material of the anode, like coated porous titanium, can be spot-welded to layer 70.

Anolyte is supplied at input 74 passing through anode 68 entering the volume maintained by spacer 72 and leaving fuel cell 46 at output 76. 0-rings 78 are provided to prevent leakage of the catholyte and anolyte from cell 46. The conductive layer is about 2 mm thick and comprises Titanium and Platinum as metals, or other conductive and stable metals. The titanium is, in case of the anode, provided with a coating comprising for example platinum. The diameter of the inputs 64, 74 and outputs 66, 76 is about 10 mm. The diameter of the cathode compartment 78 and anode compartment 80 is about 200 mm and the height of compartments 78, 80 is adjustable between 0 - 40 mm. Membrane 60 is constituted from Nafion 117. The anode 68 and cathode 54 are in the illustrated embodiment made of graphite felt. Locally the potential within the 3- dimensional electrode can be measured with sensor 82 using a Haber-Luggin capillary.

Forcing a transversal flow through the electrodes 54, 68 the pH gradient is minimal as well as the substrate and product gradient inside the 3-dimensional electrode. Efficiency loss by pumping the fluid through the electrodes is minimized by transversally flowing. The potential gradient is minimized over the felt of electrodes 54, 68, by providing a current collector 58, 70 over substantially the entire felt surface. The specific surface of the 3- dimensional felt as electrode is about 200 cm². Electric resistances are minimized by providing electrical contacts over substantially the entire surface of electrodes 54, 68. The generation of energy is optimized by externally controlling the pressure on the current collectors 58, 70 by tensioning means 50. In the fuel cell 46 according to the present invention the thickness of the 3-dimensional electrodes and spacers, for example gauze, depends on the specific parameters of the entire operation. Additionally the stretching layer 58, 70 functions as distributing means of the electrolyte over the electrodes 54, 68. Compartments 78, 80 distribute the incoming fluid over the surface of the electrodes 54, 68. By having a cylindrical shape fuel cell 46 is optimized in relation to the distribution of electrolyte over the electrodes 54, 68 as there are fewer effects of side edges and corners. The 0-ring 78 between the electrodes 54, 68 and spacers 62, 72 prevents over pressing the felt of electrodes 54, 68 with the electrodes 54, 68 being 95% porous in the illustrated embodiment.

Temperature of fuel cell 46, and especially of the anolyte and catholyte, is maintained within a range by providing the anolyte and catholyte supplied at inputs 64, 74 at a specific temperature and/or by providing housing 84 of fuel cell 46 double-walled (not shown) . The housing 84 is made of polymethylmethacrylate (PMMA) or Perspex. Preferably the electrodes 62, 72 are larger than collectors 58, 70 to avoid passing of anode and/or cathode fluid across the borders. In an alternative embodiment (not shown) a flat shaped 0-ring is installed to prevent any passage of anode and/or cathode fluid across the borders of the current collectors 58, 70. This flat shaped 0-ring would preferably have an internal diameter of 180 mm and an external diameter of 206 mm for the illustrated embodiment.

Besides a circular configuration the fuel cell can have a rectangular configuration with rectangular electrodes 86 (figure 7) . Electrode 86 is made of a porous conductive material, for instance felt, woven/non-woven cloth, sintered metal and/or foam. Electrode 86 can be made of a metal, for instance Titanium, with or without a coating, or a carbon or graphite felt. In the illustrated embodiment the felt electrode 86 has fibres of graphite felt RVG-4000. This material has a thickness of the fibres of about 10 μm and an open space of about 100 μm. Bacteria with a length of about 1 μm and a diameter of about 0.5 μm can be positioned on a single fiber of the electrode material. The porosity is around 95% for a graphite felt material with a length of 2 cm and a width of 15 cm and a thickness of 0.5 cm. The cross-section is 7.5 cm². This leads to a resistance of 0.5 Ohm per cm in the longitudinal direction, in this case leading to a total resistance of 1.33 Ohm. In transversal direction the cross-section is 300 cm² leading to a resistance of 0.8 Ohm per cm and a total resistance of 0.0013 Ohm. Experiments have shown that a pressure leading to a compression of 20% of the felt gives good results. Between electrode 86 and membrane 88 there is provided a spacer 90 with a gauze structure. Guide means 92 direct the flow by channels 94 in the plate of graphite or coated metal. This plate acts as guiding means 92, directing the flow through electrode 86. In the illustrated embodiment of the guide means 92 the fluid enters electrode 86 in a transversal direction. In an alternative configuration guide means 96 (figure 8) has a flow through channels 98 substantially parallel to the surface of electrode 86 and membrane 88. Guide means 96 is a graphite plate, or a (coated) metal plate, with channels 98 which channels are dead-ended, thereby forcing a flow through electrode 86. Guide means 96 also acts as a current collector. In an alternative configuration of the electrode configuration (figure 9) the fluid is forced through porous electrode 102 by guiding means 104. The electrolyte enters the guiding means 104 through channel 106 with a diameter of about 3 mm in the illustrated embodiment. The fluid is forced transversely through electrode 102 and enters the spacer 108 that is provided between electrode 102 and membrane 110. A metal, for instance Titanium, current collector 112 is connected to guide means 104 and is in contact with electrode 102. The fluid leaves configuration

100 through output channel 114 with a diameter of about 3 mm in the illustrated embodiment. This diameter and that of channel 106 preferably lies in the range of 1-30 mm and more preferably in the range of 3-10 mm depending on the dimensions of the device. In a further alternative configuration the direction of the fluid is reversed so that the fluid enters at channel 114 and leaves at channel 106.

Besides stack design of cells according to the present invention it is possible to have a cylindrical design 116 (figure 10) . Sidewall 118 of cylinder 116 is porous for supply of anolyte. In the centre 120 of cylinder 116 there is provided graphite particles acting as cathode and catholyte is supplied in the longitudinal direction of cylinder 116. Between centre 120 and side wall 118 there is provided graphite felt 122 acting as anode. Between centre 120 and anode 122 is membrane 124. The cylindrical design 116 may also have similar configurations as illustrated for biological fuel cells 12, 22.

An alternative design of the electrode configuration 126 (figure 11) there is provided a (gauze) spacer 128 on one side of the Felt electrode 130. The side of electrode 130 is closed by blocking means 132. Blocking means 132 force the fluid to flow from input 134 through electrode 130 to spacer 128. From spacer 128 the fluid is directed to output channel 136.

Experiments with the alternative design of the electrode configuration illustrated in figure 11 have shown that a current density of 15 A/m² is achieved with a 1.0 V over the cell using a graphite felt bio-anode. The pH at the anode is about 6 and at the cathode about 12. Without the blocking means 132 and without directing the flow through the electrode 130 a current density of 5.6 A/m² is achieved. In this experiment an increase with about factor 3 was achieved for the current density by forcing the fluid through the electrode.

The present invention is by no means limited to the above described preferred embodiments. The rights are defined by the following claims, within the scope of which many modifications can be envisaged. The device according to the present invention can be used for generation of energy. The device according to the invention can for example be used as a (biological) fuel cell and a microbial fuel cell wherein biological oxidation reactions involving bacteria and/or enzymes take place. Also, it is also possible to use the device according to the invention for other electrochemical/biological reactions, for example for production of hydrogen.

Claims

1. Device for performing electrochemical/biological reactions, like biological oxidation of a biological compound, comprising:

- an anode compartment provided with an anode, placed in an anode fluid comprising reagents for an oxidation reaction; and - a cathode compartment that is separated from the anode compartment, the cathode compartment provided with a cathode placed in a cathode fluid, wherein at least one of the anode and cathode compartments, and at least one of the anode and cathode, are arranged such that at least one of the anode fluid and cathode fluid flows at least partially through at least one of the anode and cathode .

2. Device according to claim 1, wherein at least one of the anode fluid and cathode fluid flows substantially perpendicular to the surface of the at least one of the anode and cathode and at least partially there through.

3. Device according to claim 1 or 2, wherein the at least one of the anode and cathode is porous.

4. Device according to claim 1, 2 or 3, wherein the anode and cathode are separated by a membrane or a non-conductive porous layer, and wherein at least one of the anode and cathode are placed at a distance from the membrane .

5. Device according to claim 4, wherein the distance is about 0.1 - 10 mm, and preferably about 1 mm.

6. Device according to claim 4 or 5, wherein a spacer is arranged between the membrane and the at least one of the anode and cathode to maintain the distance.

7. Device according to claim 4 or 5, wherein the membrane and/or at least one of the anode and cathode comprises flow channels to maintain the distance.

8. Device according to claim 6 or 7, wherein at least one of the anode and cathode is provided with a thickness of about 0.1 - 10 cm, and preferably about 0.5 - 1.0 cm.

9. Device according to any of claims 1-8, further comprising guide means for directing at least one of the anode fluid and cathode fluid to the at least one of anode and cathode.

10. Device according to any of claims 1-9, wherein the flow through the at least one of the anode and cathode has a substantial flow component in the longitudinal direction of the at least one of the anode and cathode.

11. Device according to any of claims 1-10, wherein the anode compartments and cathode compartments are arranged in such a way that a compartment of the one kind substantially encloses at least one of the compartments of the other kind.

12. Device according to any of claims 1-11, wherein at least one of the anode and cathode is provided with an additional layer to stretch the material of the at least one of the anode and cathode.

13. Device according to any of the claims 1-12, further comprising tensioning means for pre-stressing and/or compressing the anode and/or cathode.

14. Device according to claim 13, wherein the anode and/or cathode are compressed by the tensioning means up to about 20 %.

15. Method for performing electrochemical/biological reactions, like biological oxidation of a biological compound, comprising the steps of:

- providing a device according to any of the claims 1- 14;

- electrically connecting the anode and the cathode; - performing a biological oxidation reaction in the anode compartment; and