WO2004025769A2

WO2004025769A2 - Membrane electrochemical generator

Info

Publication number: WO2004025769A2
Application number: PCT/EP2003/010170
Authority: WO
Inventors: Katia Ciapponi; Paolo Marzupio
Original assignee: Nuvera Fuel Cells Europe S.R.L.
Priority date: 2002-09-12
Filing date: 2003-09-12
Publication date: 2004-03-25
Also published as: WO2004025769A3; ITMI20021939A1; AU2003260507A8; AU2003260507A1

Abstract

The membrane electrochemical generator (100) is fed with gaseous reactants and comprises a plurality of modules (101) connected in series or in parallel to each other. Each module (101) represents a mechanically autonomous generating unit and is delimited by a couple of conductive terminal plates (102) which enclose a plurality of reaction cells (103). The reaction cells (103) are connected to each other in series and assembled according to a filter-press configuration.

Description

MEMBRANE ELECTROCHEMICAL GENERATOR

DESCRIPTION OF THE fNVENTION

The present invention related to a membrane electrochemical generator with improved assembling and maintenance.

Processes of conversion of chemical energy into electrical energy based on membrane electrochemical generators are known in the art.

An example of membrane electrochemical generator is schematically shown in figure 1. The electrochemical generator is formed by a plurality of reaction cells 2 which are connected to each other in series and assembled according to a filter-press configuration.

Each reaction cell 2 converts the free energy of reaction of a first gaseous reagent (fuel) with a second gaseous reagent (comburent) without completely degrading it to the state of thermal energy, and therefore without being subject to the limitations of Carnot's cycle. The fuel is fed to the anodic chamber of the reaction cell 2 and consists for example of a mixture containing hydrogen or light alcohols, such as methaπol or ethanol, whereas the comburent is fed to the cathodic chamber of the same ceil and consists for example of air or oxygen. The fuel is oxidised within the anodic chamber simultaneously releasing H⁺ ions, whereas the comburent is reduced within the cathodic chamber consuming H⁺ ions. An ion-exchange membrane separating anodic chamber and cathodic chamber allows the continuous flow of H* ions from the anodic chamber to the cathodic chamber simultaneously preventing the passage of electrons. In this way, the difference of electric potential which is established at the poles of the reaction cell 2 is extremely high. the passage of electrons. In this way, the difference of electric potential which is established at the poles of the reaction cell 2 is extremely high.

More in detail, each reaction cell 2 is delimited by a couple of conductive bipolar plates 3 which enclose, going from the inside towards the outside, the ion-exchange membrane 4; a couple of porous electrodes 5; a couple of catalytic layers 6 deposited at the interface between the membrane 4 and each of the porous electrodes 5; a couple of current collectors/distributors 7, which electrically connect the conductive bipolar plates 3 to the porous electrodes 5 and simultaneously distribute the gaseous reactants; a couple of gaskets 8 designed to seal the periphery of the reaction cell 2 in order to prevent the leakage of gaseous reactants.

The conductive bipolar plates 3 and the gaskets 8 of each reaction cell 2 are provided with feed and discharge openings openings, not shown in figure 1, which are connected to the anodic chamber and the cathodic chamber of the cell itself by means of distribution channels, also not shown in figure 1. The distribution channels are obtained in the thickness of the gaskets 8 and have a comb-shaped structure. They distribute and collect in a uniform way inside each reaction cell 2 the gaseous reactants and the reaction products, the latter mixed with possible residual reactants.

The conductive bipolar plates 3 and the gaskets 8 are also provided with openings for feeding and discharging a cooling fluid (typically deionised water).

In filter-press configuration, the coupling between the afore mentioned feed openings and discharge openings determines the formation of relevant longitudinal ducts. In particular, the longitudinal ducts 9, one only shown in figure 1, define channels for feeding the gaseous reactants and the longitudinal ducts 10, only one shown in figure 1, define the channels for discharging the reaction products (water) mixed with possible residual reactants (inert gases and non converted fraction of reactants).

Outside of the assembly of the reaction cells 2, two conductive terminal plates 1 are present delimiting the electrochemical generator 1. One of the two conductive terminal plates 11 is provided with nozzles, not shown in figure 1, for fluid connection of the longitudinal ducts to the external circuits. Moreover, both terminal plates 11 are provided with appropriate holes (not shown in figure 1 as well) for the accommodation of tie-rods by means of which the tightening of the electrochemical generator 1 is obtained.

As known, the number of reaction cells 2 which constitute the electrochemical generator 1 varies according to the total voltage the generator is required to supply. Said voltage in fact corresponds to the sum of the voltages generated by each individual reaction cell 2.

Similarly, the area of each reaction cell 2 varies according to the total current that the generator is required to supply. In particular, the total current corresponds to the product of the current density developed by each reaction cell 2 by its active area (i.e. the cell area where the reaction properly takes place).

As a consequence of what written above, once the total voltage and current to be supplied by the electrochemical generator 1 are determined, both the number of reaction cells 2 composing it and the dimensions of their active area are univocally determined as well.

At present the electrochemical generators 1 are manufactured on customers' specific request, adapting the number of reaction cells 2 and the active area thereof to specific requirements (voltages and currents).

However, this kind of approach implies that the assembling of the electrochemical generator 1 cannot start until the customer has identified the required specifications in a relevant purchase order. In these conditions the assembling cannot be planned with due advance and continuity in the space of the year.

Also the purchase of the materials necessary to assemble the electrochemical generator 1 are subject to the receipt of an order from the customer, in order to avoid unsustainable stocks. This means that the delivery time of the electrochemical generator 1 to the customer is rather high being bound both by the delivery time of materials and by the assembling of the generator itself.

Furthermore, when the electrochemical generator 1 has to be repaired, for instance due to a defective reaction cell 2 (defective reaction cell 2 means a cell wherein even only one of the porous electrodes 5 or the catalytic layers 6 or the current distributors/collectors 7 does not correctly work, or finally the membrane 4 is pierced), it must be completely disassembled to allow the cell itself to be replaced or repaired.

This operation, besides requiring long service times, also involves the replacement of all the current collectors/distributors 7 with rather high costs. In fact, the current collectors/distributors 7, once pressed while tightening the electrochemical generator 1, do no longer ensure a good electrical contact with the conductive bipolar plates 3 and with the membrane-electrode assembly 4-5 when the electrochemical generator 1 is secured again after the necessary replacements have been performed. Moreover, the electrochemical generator 1 cannot normally be repaired at the customer's site, since the reassembling of the generator usually requires the presence of a rather complex assembling equipment.

It is one object of the present invention to obtain a membrane electrochemical generator free of the above described inconveniences.

According to the present invention a membrane electrochemical generator is obtained as defined in claim 1.

For a better understanding of the invention, some embodiments are now described as mere non limiting examples and with reference to the appended drawings, wherein:

- figure 1 shows an exploded side view of a membrane electrochemical generator according to the prior art;

- figure 2 shows a first embodiment of the membrane electrochemical generator according to the invention;

- figure 3 shows a second embodiment of the membrane electrochemical generator according to the invention;

- figure 4 shows a front view of a component of the membrane electrochemical generator of figure 2;

- figure 5 shows a third embodiment of the membrane electrochemical generator according to the invention;

- figure 6 shows a fourth embodiment of the membrane electrochemical generator according to the invention.

Figure 2 shows an electrochemical generator 100 obtained according to the invention. The electrochemical generator 100 is formed by a plurality of modules 101, connected to each other in series or in parallel (the latter solution not shown in figure 2) and having the same structure as the electrochemical generator 1 of figure 1. In this way each module 1Q1 represents a mechanically autonomous generating unit

In particular, each module 101 is delimited by a couple of conductive terminal plates 102 enclosing in-between a plurality of reaction cells 103, just the same as the reaction cells 2 of figure 1. The reaction celts 103 are connected to each other in series and assembled according to a filter-press configuration.

Advantageously, the number of reaction cells 103, which constitute each module 101, must be such as to generate not a very high total power at preset current and voltage.

For instance, if one would like to have the electrochemical generator 00 to supply a voltage of 60 V and a current of 83 A, corresponding to a power of 5 k , 10 modules 101 can be connected m series to each other, where each one, which has a power of 0.5 kW, is constituted by 10 reaction ceils 103 having for example active area of 225 cm², and is capable of delivering a current of 83 A with a vαttage of 6 V. The tie-rods 106, crossing the channels formed by the alignment of holes provided in the different components (bipolar plates, gaskets, optionally electrodes and membranes not shown in figures), allow for each module the reaction cells 103 to be pressed between the relevant conductive terminal plates 1Q2.

The latter have a thickness calculated in a way to minimise flexure resulting from the peripheral localisation of the tie-rods 106. An excessive bending of the terminal plates would actually cause an inhomogeneous distribution of pressure on the active surface of the reaction cells 103, with negative impact on the performances.

Again referring to figure 2, each module 101 is enclosed inside a metal cage Q4 coated with an insulating material (for example resin or silicone). In this way any possible leakage of gaseous reactant from the module 101 is avoided and at the same time an appropriate electrical insulation towards the external environment is ensured.

As already said, the modules 101 are assembled in a convenient number so as to form the stack 100 capable of delivering the required electric power, as shown in the diagra of figure 2. This arrangement requires an electric continuity to be established between the different modules 101, in a way that the current may be drawn through connections present on the external terminal plates of the modules placed at the two ends of the stack. It is also necessary that continuity be established between the feeding ducts and the discharging ducts of the different modules to allow the connection with the external circuits feeding the reaction gases, discharging the products and exhausts, and the circuits feeding and discharging the cooling fluid.

A first embodiment of the two types of the above indicated continuity is shown in figure 2: the modules 101 are separated by a device 109 which consists, as shown in figure 4, of a rubber gasket 109a of low hardness containing in its central portion a conductive planar element 109b at least partially elastic and capable of establishing an effective contact of low electrical resistance between the faces of the two adjacent terminal plates. The planar element can consist of a metal mesh, or better of a multiplicity of superimposed metal meshes, of a metal sponge or of a metal wire mattress, or of similar structures in any case provided with electric conductivity and at least partially elastic. The constituent metal is preferably nickel, copper or silver.

The gasket 1Q9a, as clarified in figure 4, is provided with various types of openings, in particular the holes 117 for the passage of longitudinal tie-rods, as discussed further on, and the holes 1 8 for the feeding and the discharge of the gaseous reactants, the products and the cooling fluid respectively. With the stack assembled, the holes 116 and 118 match similar holes present in the various components (bipolar plates, gaskets, terminal plates, optionally membranes) of the individual modules and allow to create channels that cross the whole stack longitudinally.

During the assembling of the stack 100, the different modules 101, each representing a mechanically autonomous unit, are pressed against each other, with consequent compression of the gaskets 109a (which in this way ensure the connection between the feeding and discharge ducts of the different modules with a proper sealing preventing leaks towards the outside) and of the conductive elements 109b which establish the electric conductivity between the different conductive terminal plates 102. The modules are assembled by means_ of tie-rods which longitudinally cross the entire stack (not shown in figure 2) unlike the tie-rods 106 which only concern each individual module. The arrangement of the tie-rods is clarified in figure 4, which is a cross-sectional front view of the stack 100 of figure 2 taken in correspondence of the element 109; the holes 116 are intended for housing the tie-rods 106, whose tightening nuts are accommodated inside the thickness of the terminal plates 102 so as not to interfere with the element 109. The holes 116 are indicated by a dashed line, since they are provided in the components of the stack portion located behind the element 109, and they are not present on the element 109, since the tie-rods 106 do not extend beyond the terminal plates 102. The holes 117, aligned with similar ones present in the components of the various modules, allow the passage of the longitudinal tie-rods for the stack assembling: their tensioning by means of nuts and possibly springs, in this case located outside of the two external terminal plates, allows to compress the various modules against each other thus creating through the interposed elements 109 the necessary continuity for the passage of the gaseous reactants, the products and the cooling fluid and for the passage of the electric current.

If during operation a reaction cell 103 of one module undergoes a failure, the maintenance operation is extremely simplified, since it consists of removing the longitudinal tie-rods to free the different modules, in removing the module containing the defective reaction cell and in replacing it with a previously assembled new module, optionally in replacing the conductive elements 109b and in reassembling the stack by introducing again the longitudinal tie-rods and tensioning them by means of the appropriate nuts.

Alternatively, the continuity between the feeding ducts and the discharge ducts can be obtained by providing the conductive terminal plates 102 with male 107 or female 108 quick connections which fit to each other during the serial connection of the modules 101, as shown in figure 3.

Electric continuity can be ensured by the quick-fit connections themselves, if made of metal and in electrical connection with each terminal reaction cell or, alternatively, by a conductive element 109b inserted between each couple of adjacent terminal plates 102.

A further embodiment of the electrical continuity between the modules 101 envisages that a plurality of nuts 114, made of a conductive material (for instance copper), be positioned on the perimetrical portion of the conductive terminal plates 102, as shown in figure 5. In this case too, the nuts need to be in electric connection with the relevant terminal reaction cells.

As an alternative to the presence of the nuts 114, the electric continuity between the modules 101 can be obtained through an external electrical connection 115, as shown in figure 6.

As for the continuity of the feeding and discharge channels, it can be obtained by using the already considered gasket 109a. It should be noted that if the connecting devices between the different modules (109b, 107-108, 114, 115) are provided with contacts accessible from the outside (e.g. welded cables), the modules can be connected in parallel instead of in series as assumed so far.

The advantages achievable by the above described invention are the following.

First of all, the modular structure of the electrochemical generator 100 allows to plan the assembling with due advance and with continuity in the space of the year without waiting for orders from customers. In fact the modules 101 can be prepared independently of the receipt of an order from a customer, and they can be assembled together to obtain the electrochemical generator 100 only when required.

Also the purchase of the materials can be made in a regular way during the year, being all the modules 101 constituted by the same materials. In this way, expensive stocks are avoided.

Moreover, the delivery time of the electrochemical generator 100 is considerably reduced, since the time between a customer's order and the generator shipping is only the time necessary to connect the modules 101 to each other.

Another advantage is the reduction of the time needed to repair the electrochemical generator 100. In fact, in case of defective reaction ceil 103, the latter can be changed by simply replacing the module 101 containing it, with a considerable decrease of maintenance costs as well, while the other modules remain unchanged.

Moreover, considering its simplicity, the repairing of the electrochemical generator can be carried out at the customer's site, because there is no more need of a complex assembling equipment.

Claims

1. A membrane electrochemical generator (100) provided with channels for feeding gaseous reactants, channels for discharging the products and channels for feeding and discharging a cooling fluid, comprising a plurality of modules (101) connected to each other, each module (101) being delimited by a couple of conductive terminal plates (102) which enclose a plurality of reaction cells (103) connected to each other in series and assembled according to a filter-press configuration, each module (101) representing a mechanically autonomous generating unit.

2. A generator according to claim 1, wherein said modules (101) are connected to each other in series.

3. A generator according to claim 1, wherein said modules (101) are connected to each other m parallel.

4. A generator according to any of the preceding claims, wherein each module (101) is enclosed inside a metal cage (104).

5. A generator according to claim 4, wherein said metal cage (104) is coated with an insulating material.

6. A generator according to claim 1, wherein each module (101) of said plurality is coated with insulating material.

7. A generator according to claim 5, wherein each module (101) is provided with holes (116) for the accommodation of tie-rods (106) suitable for compressing said terminal plates (102), of holes ( 18) for the accommodation of longitudinal tie-rods to compress the plurality of modules (101) by means of the external terminal plates (102) of the two modules located at the ends of said generator, and with openings for the passage of said reactants, products and cooling fluid.

8. A generator according to any of the preceding claims, characterised in that it comprises a device (109) positioned between the two contiguous terminal plates (102) of a module (101) and the following one, said device (109) providing continuity in the feeding and discharge of said gaseous reactants, products and cooling fluid, and electrical continuity between said plurality of modules (101).

9. A generator according to claim 8, wherein said device (109) comprises a conductive central element (109b) and a gasket (109a) provided with holes (117) for the passage of longitudinal tie-rods and with openings (118) for the passage of said reactants, products and cooling fluid.

10. A generator according to claim 9, wherein said conductive element (109b) is provided with at least partial elasticity.

11. A generator according to claim 10, wherein said conductive element is constituted by metal meshes, metal wire mattresses and metal foams.

12. A generator according to claim 9, wherein said gaskets (109a) are made of low hardness rubber.

13. A generator according to any of the claims 1 to 7, wherein said conductive terminal plates (102) are provided on their external sides with male (107) and female (108) quick connections, said male (107) and female (108) quick connections being suitable to fit to each other to create a serial connection of said modules (101) with a continuity in the feeding and discharge of said gaseous reactants, products and cooling fluid.

14. A generator according to claim 13, characterized in that it comprises a conductive element (109b) inserted between the contiguous terminal plates (102) of one module (101) and the following one, said element (109b) providing an electric continuity between the modules (101) of the generator (100).

15. A generator according to any of the claims 1 to 7, wherein said conductive terminal plates (102) are provided with a plurality of conductive nuts (114) located on a relevant perimetrical portion, said plurality of conductive nuts (1 4) providing an electrical continuity between said modules (101).

16. A membrane electrochemical generator of improved maintenance substantially as described with reference to the annexed figures.