KR20070018790A - Electrochemical generator - Google Patents

Abstract

The present invention describes a new design of an ionomer membrane fuel cell comprising a circumferentially sealed gasket, a plurality of stainless steel bipolar plates, an electrode and a membrane contained between compression plates. The novel design relates to preventing the accumulation of metal ions in the membrane and the resulting voltage degradation, for which migration of metal ions contained in the cooling fluid into the ionomer membrane is prevented.

Electrochemistry, generators, fuel cells, membranes, ionomers, stacks, bipolar plates.

Description

Electrochemical Generator {ELECTROCHEMICAL GENERATOR}

TECHNICAL FIELD The present invention relates to electrochemical generators of electrical energy, and more particularly, to a polymer membrane fuel cell stack.

Fuel cells have long been known as devices for the direct conversion of chemical energy from a combination of fuels such as hydrogen and oxidants such as air into electrical energy. The fuel cell is not limited by the known Carnot's cycle and is thus characterized by a particularly high efficiency compared to the efficiency of conventional electrical energy generating devices in which there is an intermediate thermal stage.

Among various known types, ionomer ion exchange membrane fuel cells (hereinafter using PEMFC, abbreviation of ionomer ion-exchange membrane fuel cell) have their function for response to rapid power demand and Due to the simplicity of the associated auxiliary system, particular attention has been paid to automotive applications and to the generation of small fixed power for small groups or household appliances.

PEMFCs typically consist of electrochemical units suitable for stacking with other equivalents according to a filter-press type modular arrangement, commonly referred to as a stack. PEMFCs generally consist of a perfluorinated form, as known in the art, commercially available from DuPont, USA under the tradename Nafion ^(R) , or polystyrene or Consisting of a carbon hydroxide derived from a polymer such as polyetherether ketone), the surface of which is applied two electrodes in the form of a porous membrane containing a suitable catalyst, namely a negative anode and a positive cathode. do. The outer surfaces of these electrodes, in turn, are in contact with a common planar porous structure, which is suitable for optimal electrical conduction and homogeneous distribution of reactants, eg hydrogen and oxygen, and due to this dual function Otherwise known as a collector or distributor. The entire assembly resulting from the electrochemical unit associated with the collector (hereinafter defined as MEA, acronym for the membrane electrode assembly) is finally made up of two sheets that are impermeable to the reactant, electrically conductive, and suitably shaped. It is housed between a pair of bipolar plates.

Fuel and oxidant are supplied through appropriate openings made in the bipolar plate and distributed through the collector to the anode and cathode respectively. Fuel, for example hydrogen, is oxidized to produce protons and electrons. Both move across the ionomer membrane and participate in the oxygen reduction reaction of the anode forming water. Electrons necessary for the reduction reaction are introduced from the cathode through an external circuit. Although the conversion efficiency of chemical reaction energy into electrical energy is significantly higher than that of conventional generators, it drops significantly at 100%, and the portion of the chemical energy that is not converted to electricity is dissipated as thermal energy, which usually results in an internal cell temperature of 60 In order to maintain around -100 ° C, it must be drawn out with a suitable cooling device. The cooling device is preferably air cooled for small power systems and has demineralized water circulation for higher power systems. Cooling is performed, for example, by flowing water along one of the bipolar plates, so that the bipolar plates consist of double-walled hollow cells. Additional measures necessary to ensure proper operation of the PEMFC are given by preliminary humidification of the reactants. The purpose of the preliminary humidification is to supply the PEMFC with a certain amount of water useful to keep the membrane hydration at its maximum level, which is at the highest proton migration capacity and thus the lowest ohmic resistance and highest operating voltage. Corresponds.

According to the prior art, bipolar plates defining cells can be made of graphite, more generally graphite polymer binder composites. In a very good alternative, they can also consist of metal, possibly chromium / nickel / molybdenum series stainless steel. The stray current caused by the high voltage concentrated in the stack internal manifold across the water causes corrosion in this case, in particular stainless steel concentrated on the plate surface which comes into contact with the cooling water. In general, this corrosion, which is not strong enough to cause structural damage to the bipolar plates, in some way results in progressive enrichment of metal ions, mainly Ni ⁺⁺ , Cr ⁺⁺ and Fe ⁺⁺⁺ ions, in the circulating water. Cause. This thickening can be limited by introducing a cation exchange resin containing filter into the cooling water circuit, but the operation of such a filter introduces some operational complexity into the overall system. As an alternative, the use of high alloy stainless steel with enhanced corrosion resistance can greatly reduce metal ion release. High alloyed steels, characterized by high chromium, nickel and molybdenum contents, are also expensive and significantly increase the investment required for stack fabrication. Metal ions are also released by auxiliary components of the overall system, such as resin filter vessels, collection tanks for circulating water, required heat exchangers, various piping and control sensors. However, these components are not subjected to the high voltage of the stack to accelerate the dissolution of the metal, so the contribution to ion enrichment in the circulating water is generally negligible. With the extension of the area where liquid water separates, it is necessary to remind the mechanism that the accelerated release and metal ion enrichment mechanisms are partially effective even within the stack manifold where the reactant and exhaust gases intersect with changes in strength. There is.

Although well subjected to chemical erosion by the vagus current, graphite produces carbon dioxide as a corrosion product, and therefore does not raise metal thickening, but raises it to a microscopic level corresponding to its impurity content. Nevertheless, this undoubted advantage is the reduced thickness of the bipolar plate obtainable with stainless steel, for example more balanced by 0.1-0.5 mm, so that more compact stacks with high elasticity which are usually metal It is possible to produce and generally guarantees particularly satisfactory mechanical resistance and thermal shock resistance.

Stacks made of stainless steel bipolar plates have been observed to exhibit performance decay of terminal cells after variable operating times of around 200-300 hours. This degradation contributes to the higher thermal dissipation that characterizes the terminal cell, and the temperature may thus be lower than the average of the cells in the stack. Lower temperatures excessively condense the water vapor contained in the reactant gas, resulting in flooding of the catalytic film porosity. The presence of liquid water in the pores of the catalyst membrane prevents the diffusion of the reactant gas towards the reaction site located at the interface with the ionomer membrane. Therefore, it is proposed to insert the heating element in contact with the outer surface of each terminal cell, and the appropriate control device can be used to increase the terminal cell temperature to match the average temperature of the remaining cells in the stack.

An alternative variant consists in disposing a suitable thermal device consisting of crossed hollow cells by circulating water in contact with the outer surface of each terminal cell, depending on the configuration equivalent to that applied for cooling of a single cell. By appropriately adjusting the water temperature, it is possible to maintain the terminal cell temperature at a level sufficient to prevent harmful condensation of the water vapor.

A further proposal of the performance degradation mechanism is based on the heterogeneous current distribution in the terminal cell, and as a solution, it has been proposed to extend the size of the contacts to the external electrical circuit. However, the inventors have found that prior art refinements can delay the occurrence of stack performance degradation only up to 500-1000 operating times. In particular, the inventors have observed that after this improved operating period, a new type of performance degradation problem arises with a limited number of cells arranged at the stack extremity on the negative terminal side. Surprisingly, the performance of the cells disposed at the anode terminal side ends and the remaining cells of the stack remained unchanged at a satisfactory level as a whole. Performance degradation has been shown to decrease gradually in cell voltages where the terminal phase undergoes extremely harmful polarity inversion. To prevent this serious situation, it is necessary to monitor the cells and short-circuit them in a timely fashion to those whose voltage results are reduced below a predetermined threshold. It has also been recognized that short circuits tend to exacerbate degradation of adjacent cells.

The fact that these problems also occur for longer periods of time in stacks with heating devices for terminal cells, the deterioration worsens in adjacent cells at the time of short-circuit, and the deterioration only occurs on the negative terminal side stack is a breakdown analysis of the prior art. It clearly shows that this cannot be reliably applied to the kind of degradation observed by the present inventors. Therefore, in conclusion, the prior art cannot explain and solve the long term deterioration of the performance of the cell near the negative terminal side of a stack made of stainless steel bipolar plates.

It is an object of the present invention to provide a design of a stack of cells made of stainless steel plates that overcomes the disadvantages of the prior art and prevents deterioration of the cells disposed in the vicinity of the stack negative terminal.

In one aspect, the present invention consists in the design of a stack made of metal bipolar plates that prevents transverse migration of metal ions contained in the cooling fluid into the ionomer membrane.

In a preferred embodiment, the invention is preferably made of a stainless steel bipolar plate consisting of AISI 316 L type (DIN X 2 CrNiMo 1712 or 1713, 16-18% chromium / 10-14% nickel / 2-3% molybdenum) The design of the stacked stack thus prevents migration of metal ions towards the membrane, thereby enabling the use of ionomer membranes with reduced outer dimensions. In particular, a perimeter should be placed in the middle region of the perimetrical sealing gasket contained between the circumference of the cooling water supply and discharge holes and the edge of the active area. As an alternative, the use of molybdenum-free stainless steel with higher nickel and chromium content, such as the content of series CrNi 2520 (19-22% Ni, 24-26% Cr) according to DIN standards, is preferred for the constituent material.

In another preferred embodiment, the transverse migration of metal ions towards the membrane active region is characterized by the fact that the circumference of the supply and discharge holes of water, optionally reactants and exhaust gases, is planar gasket, O-ring or curable. It is suppressed by using an ionomer membrane having a non-conductive material, which preferably has elastic properties, applied in the form of a liquid film and having the same dimensions as that of the bipolar plate.

In another preferred embodiment, the present invention is directed to a metal directed toward the membrane active region by homogenizing the electric field in the coolant inlet and outlet manifolds on the stack cathode terminal side when installing a bipolar plate without supply holes as the outer bipolar plate of the final cell. Solve the ion migration problem.

The invention will now be described with reference to the accompanying drawings, which are merely illustrative and serve as non-limiting.

1 is a schematic cross-sectional view of a PEMFC stack according to the prior art.

2 shows another component of the first cell of the stack of FIG. 1 separately;

Fig. 3 shows another component of the first cell of the stack according to the first embodiment of the present invention separately.

Fig. 4 is a diagram showing separately the other components of the first cell of the stack according to the second embodiment of the present invention.

5 is a diagram illustrating the vagus current distribution of the stack of FIGS. 3 and 4.

FIG. 6 is a view showing separately the other components of the first cell of the stack according to the third embodiment of the present invention. FIG.

FIG. 7 shows the vagus current distribution of the stack of FIG. 6. FIG.

For example, referring to the prior art shown in US Pat. No. 6,482,792, FIG. 1 shows a longitudinal section of a PEMFC stack, where ionomer membrane 2, cathode 3, anode 4 and collector 5 are shown. Each comprising a MEA assembly 1, number 6 represents a bipolar plate, number 7 a single cell, each cell between two bipolar plates 6 with a circumferential sealing gasket 8. Containing a housed MEA unit 1, number 9 being a cooling device consisting of a desalted water supply cell defined by two adjacent bipolar plates 6, which has a peripheral sealing gasket 10 and A conductive spacer 11 adapted to maintain longitudinal electrical continuity between two adjacent bipolar plates, number 12 being two pieces of electrically conductive material for connecting the stack to an external electrical circuit. Represents a sheet, each of which is a gas By 14, in contact with the terminal bipolar plate through the conductive element 13 is isolated from the external environment. In addition, the number 15 denotes two plates of low flexibility which allow to hold the cooling device 9 and the plurality of cells 7 under compression, which is a tie-rod 16 with a non-conductive coating. Under the action of, the tie rod is optionally provided with a spring (not shown in the figure) to compensate for thermal expansion / contraction of the stack component, plate 15, bipolar plate 6, sheet (12) and the gaskets 8, 10, 14 are distributed in an appropriate number along the periphery. Number 17 represents a sheet of non-conductive material for electrical insulation of the stack plate 15, and number 18 is a fuel and oxidant gas, for example, disposed on the plate 15 on the anode terminal side of the stack. Connections to external circuits for the supply of hydrogen and air, for the injection and discharge of cooling water, and for the extraction of residual gas and product water. Fuel and oxidant, which proceed from the external circuit through the connection 18, are each obtained, for example, at the thickness of the gasket 8, into a longitudinal manifold formed in the stack by juxtaposition of the appropriate holes in the various components. It is supplied to the cathode and the anode by means of a distribution channel that is connected. In an equivalent form, the coolant is injected into the device 9, the residual gas mixed with the product water is extracted and the coolant is discharged.

For a better understanding, the components of the first cell arranged on the side of the negative electrode terminal of the associated cooling device are shown in FIG. 2 in a continuous form from (a) to (p) in FIG. 2 corresponding to the stacking order. In particular, FIG. 2A schematically shows a front view of a compression plate 15 having an associated hole 19 for the passage of the tie rod 16, and FIG. 2B shows a plate 15 of the stack. Shows a sheet of non-conductive material, such as EPDM rubber, preferably having elastic properties, and having an aperture 19 for passing the tie rods 16 oriented to ensure electrical insulation of (C) optionally comprises, for example, a coating suitable for preventing the increase of contact electrical resistance over time, for example, aluminum plating, and also with holes 19; The conductive sheet 12 of a highly conductive material such as copper is shown, and FIG. 2D shows a complete contact between the sheet 12 and the first bipolar plate 6 in the presence of small surface irregularities or small deflections. Conductive yoke, having the same size as the cell active area, having residual strain and elasticity to achieve FIG. 2 (e) shows a gasket 14 oriented to insulate the space containing the element 13 from the external environment to prevent possible degradation due to moisture and oxygen contained in the air. To show. 2 (f) shows a front view of the first bipolar plate 6, where numbers 20a and 20b denote holes for supplying fuel and oxidant gas, and numbers 20c and ( 20d) denotes a fuel and oxidant exhaust extraction hole, numbers 20e and 20f denote cooling water injection and discharge holes, and finally number 20e denotes a hole for passing the tie rod 16 through. As will be explained in more detail below, attention should be paid to how concentrated the corrosion phenomenon is, which corresponds to the annular surfaces of the holes 20e and 20f in contact with the coolant, resulting in deterioration of the cell disposed close to the stack cathode terminal.

The holes 19 and holes 20a to 20f also appear in the first outer circumferential sealing gasket 8, shown in FIG. 2g, where the central hollow part 21 is again illustrated in FIG. 2 h as a front view. The site of the first collector 5 is formed. The surface of the gasket of FIG. 2 (g) facing the viewer is in contact with the first bipolar plate, and in addition to the holes indicated, there are provided distribution channels 22, 23 obtained at a thickness, indicated by dashed lines. Distribution channels 22 and 23 communicate holes 20a and 20d, respectively, with the central portion 21 of the gasket, where the first collector 5 is received and in turn, the first of the porous catalyst membranes applied to the membrane surface. Contact with the first one. In this way, fuel gas, for example hydrogen, is supplied to the collector and thus distributed over the catalytic porous membrane. The exhaust gas is discharged from the hole 20d, optionally with water. In (i) of FIG. 2, the membrane is shown as a catalytic porous membrane, which is the hollow central portion 21 and the collector of the outer sealing gasket 8 applied on the central portion (active area, only one visible) on both surfaces. It has the same dimensions as (5). In FIG. 2 (m), a second outer circumferential sealing gasket 8 is shown, which has holes 19 and holes 20a-20f as already shown in the case of the first gasket.

The surface of the second outer circumferential gasket 8 of FIG. 2 l facing the viewer is arranged to be in direct contact with the second bipolar plate 6 and is generally equivalent to those shown for the first gasket, but is biased therefrom. And a distribution channel 22 and 23, which are represented by the continuous line: in fact the channel is in contact with the second catalytic porous membrane applied to the membrane, containing the second collector 5 (shown in FIG. 2L). The holes 20b and 20c are connected to the central portion 21 of the second outer circumferential gasket. In this way, an oxidant gas, such as air, is supplied to the second collector through the channel 22 and thus distributed over the second catalytic porous membrane. The residual oxidant gas mixed with the product water is discharged through the channel 23 in the extraction hole 20c.

In FIG. 2 (n), a front view of the second bipolar plate 6 is shown, which is generally equivalent to the first bipolar plate of FIG. 2 f. 2 (o) and 2 (p) finally show the conductive spacer 11 and the outer sealing gasket 10 of the cooling device. In particular, the gasket 10 has cooling water inlet and outlet channels 24 and 25.

The stacking of the elements of FIGS. 2 (f) to 2 (p) results in an iterative module constituting a single cell with an associated cooling device. The stack of FIG. 1 consists of a number of repetitive modules housed between two compression plates 15 and two connecting sheets 12: overlaying various holes from hole 20a to hole 20f. ) Determines the formation of the longitudinal manifold, each of which has a connection 18 for connection to an external circuit, each in communication with a cathode or an anode or a cooling device, depending on its function.

The above description makes it clear that, under normal operating conditions, the longitudinal manifold formed by the overlapping of the holes 20e and 20f is completely filled with circulating coolant, the circulating coolant being dependent on the level of conductivity and the various cells. Under the influence of the high electrical voltage resulting from the sum of the voltages of < RTI ID = 0.0 >, < / RTI > This parasitic parasitic loss of electrical efficiency is minimized by desalting cooling water. It is not possible to reduce the vagus current to zero because the circulating water always maintains a certain conductivity due to the incomplete efficiency of the enrichment and desalination of ions of various sources.

A similar situation can also arise in the reactant and exhaust passages formed by the overlapping of the holes 20a, 20b, 20c and 20d: In this case, the conditions for the passage of small vagus currents separate the product water from the residual gas. And by the presence of liquid water formed by the condensation of humidified steam generated by stack heat dissipation to the external environment.

3 shows a first embodiment of the present invention, wherein parts common to those in the case of FIG. 2 are denoted by the same reference numerals. Numeral 2 denotes an ionomer membrane whose periphery is arranged in the region of the outer circumferential sealing gasket surface comprised between the circumference of the feed, injection, extraction and discharge holes and the edge of the active region. This region can be a planar on the surface in contact with the membrane and thus on the surface opposite the surface comprising the distribution channels 22, 23, 24 and 25: alternatively, it is a gas, a production One or more ridges or rings may be provided to prevent both water and cooling water leakage into the external environment and contact of the cooling water with the membrane edges in a safe manner.

Figure 4 shows the components of a single cell of a second embodiment of the invention, wherein the unchanged parts have the same reference numerals with respect to that of Figure 2. Numeral 2 denotes an ionomer membrane having a periphery coinciding with the periphery of the bipolar plate, with feed, inlet, extract and outlet holes of sections larger than equivalent holes in the gasket. The outer circumference of the hole of the membrane has a planar ring 26 made of EPDM, for example. EPM rubber and low hardness polymeric materials, which generally have elastic properties, can likewise be used. The purpose of the ring 26 is to seal the interface of each pair of sealing gaskets corresponding to the holes to prevent the possibility of both leakage of gas, product water and cooling water to the outside and contact between the cooling water and the membrane. As an alternative, the planar ring can be replaced with an O-ring. As a further alternative, in the region of the outer circumferential sealing gasket defined by the circumference of the hole in the membrane and the circumference of the hole in the outer gasket, a film of liquid material which is cured and polymerized by UV irradiation or heat treatment, or by a catalyst is applied. . Suitable materials may be given by liquid silicone resins, which maintain good elasticity and low hardness even after completion of the curing process. As an alternative to planar rings, O-rings or cured polymerizable membranes, suitable lips or steps may be provided, as known in the art.

FIG. 5 shows the longitudinal, negative, side of the stack of one of the manifolds formed by the sequential overlap of the appropriate holes 20e and 20f present on the bipolar plate, the gasket and the membrane, with respect to the stack of FIGS. The system of distribution of both the longitudinal and radial directions of the vagus currents emitted by the annular surface of each bipolar plate hole along the directional section is shown, number 27 denotes the annular surface of the bipolar plate hole in contact with the coolant ( 28 represents the Americas emitted by 28), number 29 represents the generated equipotential surface, and the remainder are denoted by the same reference numerals as in the previous figures.

FIG. 6 shows the negative terminal of the third embodiment of the invention, characterized in that it has a first bipolar plate on the negative side, completely free of supply, injection, extraction and discharge holes for cooling water and for gaseous reactants and exhaust; The component of the 1st cell of the side is shown.

FIG. 7 shows the longitudinal and radial distribution scheme of the vagus currents for the stack of FIG. 6.

[First Embodiment]

In an attempt to solve the problem of performance deterioration of a cell located on the negative terminal side of a stack made of stainless steel bipolar plates, the present inventors have shown in FIG. 1 composed of a plurality of 50 cells, interposed with the same number of demineralized water supply cooling devices. A series of tests was performed using the stack of the illustrated type. The stainless steels used were the illustrated type AISI (DIN X2 CrNiMol7 13 2, 16-18% Chromium / 10-14% Nickel / 2-3% Molybdenum, Carbon <0.03%, a small percentage of Silicon and Manganese, the rest being steel ) As ionomer membrane, Nafion 112 type provided by DuPont, USA was used. Catalytic porous membranes comprise 1 mg / cm ² of platinum and are multicatalytic porous membranes provided by the E-TEK division of De Nora North America, USA.

A metallic reticulated material coated with a chemical resistant chromium layer was used as the collector as disclosed in US Pat. No. 5,480,792.

The stack used for the test differs in its internal design as shown below:

Stack A (based on prior art): component as indicated in FIG. 2. In particular, the membrane dimensions are consistent with those of the bipolar plate and have supply, injection, extraction and discharge holes that can be accurately overlaid on equivalent sealing holes present on the outer sealing gasket and the bipolar plate.

Stack B (first embodiment of the invention): Component of a single cell as shown in FIG.

Stack C (second embodiment of the invention): Component of a single cell as shown in FIG.

Of the two sizes employed for the membrane, one with respect to stack B has the advantage of reducing the amount of material used, which is particularly expensive. On the other hand, the membrane size used in Stack C may be more compatible with automated stack assembly procedures, since the outer periphery that matches that of the outer seal gasket and bipolar plate allows for fast and precise centering. to be.

The three stacks have elements equivalent to a cooling device inserted between each of the two outer cells on the negative and positive terminal sides and the electrical contacts to maintain their temperature at a value close to the remaining cells.

The three stacks operate at a temperature of 70 ° C, measured at the residual gas outlet, with pure hydrogen (above 20% stoichiometric, prehumidification at 70 ° C, 1.3 bar absolute) and air (50% stoichiometric, 60 ° C). Pre-humidification at 1.2 bar absolute). The current generated on the resistive substrate corresponds to a density of 0.5 A / cm ² called the active region. During operation, the cell voltages of all stacks are monitored and cells with voltages less than 0.2 Volt are shorted (using the abbreviation sc).

In the following, for each stack, the voltages of the first eight cells on the negative terminal side, the voltage of the last three cells on the positive terminal side and the average voltage of the remaining cells were reported.

# Stack A

100 hours [0.70, 0.72, 0.70, 0.69, 0.70, 0.70, 0.71, 0.71] [0.70, 0.72, 0.71] [0. 71]

250 hours [0.69, 0.72, 0.70, 0.70, 0.69, 0.71, 0.71, 0.71] [0.70, 0.70, 0.71] [0.71]

500 hours [0.45, 0.55, 0.60, 0.65, 0.69, 0.71, 0.70, 0.71] [0.70, 0.71, 0.70] [0.71]

750 hours [0.20, 0.30, 0.45, 0.55, 0.60, 0.65, 0.70, 0.71] [0.70, 0.71, 0.70] [0.71]

1000 hours [s.c., 0.25, 0.30, 0.40, 0.52, 0.60, 0.65, 0.71] [0.70, 0.71, 0.70] [0. 71]

1250 hours [s.c., s.c., s.c., 0.30, 0.40, 0.55, 0.60, 0.67] [0.70, 0.71, 0.70] [0.71]

1500 hours [s.c., s.c., s.c., s.c., 0.30, 0.35, 0.45, 0.55] [0.70, 0.70, 0.71] [0.71]

#Stack B

100 hours [0.70, 0.70, 0.71, 0.69, 0.70, 0.71, 0.70, 0.69] [0.69, 0.71, 0.70] [0.70]

250 times no significant change, voltage oscillation ≤0.1Volts

500 times same as above

750 times same as above

1000 times same as above

1250 hours as above

1500 hours same as above

#Stack C

100 hours [0.71, 0.69, 0.70, 0.72, 0.70, 0.70, 0.71, 0.71] [0.70, 0.70, 0.71] [0.71]

250 times no significant change, voltage oscillation ≤0.1Volts

500 times same as above

750 times same as above

1000 times same as above

1250 hours as above

1500 hours same as above

Analysis of these data clearly indicates that stacks B and C are substantially unaffected by performance degradation for at least the selected test time. Stack A, on the contrary, exhibits severe instability of the first cell on the negative terminal side immediately after 500 hours. The voltage density increased and caused the operator to perform a short (s.c.) when the single cell voltage dropped below the fixed threshold of 0.2 Volt as described above. In addition, it is important to note that shorting cells with severely dangerous voltages accelerates degradation of nearby cells and clearly advances uninterrupted toward the center of the stack. The voltage degradation characteristic makes it possible to exclude that the cause of the problem is the heat dissipation into the environment or the current distribution due to incorrectly dimensioned sheet connections to the external electrical circuit, as suggested in the prior art analysis. If this is the case, in fact, the problem should only exist in the first cell and in no case exist at both ends, the cathode and the anode of the stack. To clarify the mechanism of degradation that occurs on stack A, the inventors performed a series of checks on components of both the cell and the shorted cell that still feature correct functionality. The single most relevant indicator, indicating a mild variation in metal ion content, was provided by the analysis of the membrane. Table 1 shows the detection of the outer and central regions of the active area of the first shorted cell on the negative terminal side (PEMFC 1) of the unshorted cell close to the center cell (PEMFC 20) and the shorted cell (PEMFC 8). Report the average nickel and calcium content:

TABLE 1

PEMFC 1 Outer zone Central area Ni (mg / cm ² ) 12.9 12.7 Ca (mg / cm ² ) 0.4 0.5 PEMFC 8 Ni (mg / cm ² ) 11.2 0.6 Ca (mg / cm ² ) 0.5 0.1 PEMFC 20 Ni (mg / cm ² ) 0.3 0.1 Ca (mg / cm ² ) 0.1 0.1

These data clearly show that the voltage degradation observed on the cell is directly linked to the thickening of the membrane of metal ions running from the cooling water, as indicated by the presence of calcium. In fact, the cooling water is characterized by the presence of nickel (7.5 ppm) and calcium (2 ppm) in addition to other contaminants such as ions and chromium.

Regarding the surprising localization of the thickening in the membrane of a cell located at the stack end on the cathode terminal side, it does not limit the invention to any particular theory, but rather to the potential profile present inside the coolant inlet and outlet manifolds. It can be assumed that there is a cause.

This profile is derived from the radial and longitudinal distributions of the vagus currents emitted from the annular face of each bipolar plate hole, as shown in FIG. 5, for the region adjacent to the stack negative terminal. The current line and potential profile distributions are comparable in the same manifold area on the positive terminal side of the stack. The current emission peak coincides with the first bipolar plate disposed corresponding to the stack cathode and anode terminals (longer arrows in FIG. 5) and gradually decreases as it progresses toward the center of the stack (shorter arrows in FIG. 5).

The resulting potential distribution is characterized by strong asymmetry in the terminal region and substantial uniformity in the manifold region corresponding to the center cell. The asymmetric equivalence from the geometric standpoint in both manifold sections is maximum corresponding to the negative of the cell located at the stack end on the negative terminal side, and opposite to the positive of the cell located at the opposite end on the positive terminal side. This results in the formation of a potential gradient that becomes. The combination of the voltage of the single cell and the potential inside the manifold determines the transverse electrical gradient in the membrane, which promotes the migration of ions of cooling water toward the central region of the membrane, on the cathode terminal side, on the anode terminal side, It appears to work against. In the middle part of the manifold, where the electrical potential distribution is substantially uniform, it does not exhibit a particular acceleration or retardation action of the migration of ions, and therefore it penetrates inside the membrane, mainly by diffusion, which is a much slower process. In principle, similar mechanisms of transverse ion permeation inside the membrane are possible in the manifold for extracting and supplying residual gas containing product water and pre-humidized reactant gas. However, due to heat loss, the water condensed from the reactant gas is only present as droplets of limited size, in particular, to ensure that preliminary humidification is carried out in the correct form and that the produced water does not necessarily impede emissions of the exhaust. Only occupy a small portion of the extraction manifold section. In addition, the ionic content in the condensate and product water may be limited. Thus, the contribution to ion migration into the membrane is almost negligible.

Second Embodiment

In view of the premise of the membrane ion enrichment mechanism outlined above, the inventor has assembled a new stack, the following stack D, which is shown in FIG. 6 showing the components of the first cell on the cathode terminal side of stack D. Likewise, stack A is comparable except for the fact that it has a first bipolar plate on the side of the cathode that is completely free of supply, injection, extraction and discharge holes for the reactant gas, residual gas and cooling water. The remaining components (bipolar plates, gaskets, MEA units, electrical connection sheets, compression plates, tie rods, except the first one) are equivalent to those used for the construction of stack A as a whole. Stack D operates in a generally equivalent form to Stacks A, B and C. After 1500 hours, no significant degradation was found in the cell voltage, which was observed with a weakly remaining cell voltage around an average value of about 0.1 Volt. Analysis of the membrane of the first cell on the negative terminal side of the stack showed negligible ion enrichment, without any particular distinction between the outer and central regions. Without wishing to limit the present invention to any particular theory, the inventors have found that the presence of a holeless first bipolar plate alters the potential profile inside the manifold, leading to a first on the negative terminal side of the stack, as shown in FIG. It was assumed that the portion corresponding to the cell was already substantially uniform. Thus, with regard to metal ion enrichment, cells disposed on the negative terminal side of the stack are characterized by behavior consistent with one of the center cells, and the manifold potential distribution is physiologically uniform.

Obviously, it is not feasible to employ a holeless bipolar plate on the anode terminal side of the stack because the circulation of cooling water and gas reactants and exhaust is blocked. Thus, the potential distribution does not change and is asymmetric. Nevertheless, this situation has no critical meaning because, as illustrated by the test, the overlap of the cell voltage and the manifold potential on the positive terminal side of the stack prevents the migration of metal ions inside the membrane. to be.

The use of a single bipolar plate of a design different from the rest does not involve any particular complexity in the stack assembly. In addition, considering the need for automation of stack assembly, which is necessary for mass production, a solution based on the use of the first bipolar plate on the negative terminal side without holes may be preferable to the solutions described above for stacks B and C.

It is clear that any discomfort can be eliminated by using water that is completely free of metal ions, however, however, metal parts in contact with water (bipolar plates, various tubes, circulation pumps, control mechanisms that accelerate the effect of the viscous currents) In a closed cycle cooling system with a heat exchanger and a storage tank, unavoidable release of metal ions leads to a gradual accumulation in water, which can only be partially offset by insertion of an ion exchange resin mounting filter. Levels at least one order of magnitude lower, as the analysis of circulating water proves to be sufficient at levels of only a few ppm to trigger unacceptable voltage degradation at relatively reduced times. ) May be necessary to ensure sufficient functional stability. Similar levels of metal ion concentration in water are not easy to maintain with ion exchange resins unless relatively oversized filters are used in very frequent adsorption / regeneration cycles, and this frequent adsorption / regeneration cycle, in contrast, There are two measures that cannot coexist with cell system management, which should be as simple as possible.

It will be apparent to those skilled in the art that the present invention may be practiced based on other variations and modifications to the examples cited.

Therefore, it is to be understood that the above description is not intended to limit the invention, and that the scope may be utilized in accordance with other embodiments without departing from the scope thereof solely defined by the appended claims.

In the description and claims of this application, the words "comprise" and their modifications such as "comprising" and "comprising" do not exclude the presence of other elements or additional components.

Claims (14)

A stack of filter press modular structures comprising a plurality of single proton exchange membrane fuel cells and a cooling device, Each cell is formed by a pair of metal bipolar plates and includes an outer circumferential sealing gasket and an ion exchange membrane formed as a frame for accommodating a current collector in its hollow central portion, A bipolar plate and a gasket are provided in a stack having a through opening including holes for supplying a reactant gas, extracting residual gas together with the reaction product, and injecting and releasing thermostating fluid, And lateral migration of ions traveling from the thermostatic fluid inside the ion exchange membrane is prevented. The stack of claim 1 wherein the bipolar plate closest to the negative terminal has no through opening. 2. The stack of claim 1, wherein said lateral migration of ions is prevented by physical isolation of the ion exchange membrane from a thermostatic fluid. The stack according to claim 1, wherein the material of the metal bipolar plate is stainless steel comprising 15% -26% chromium, 10-22% nickel and optionally molybdenum. The stack according to claim 4, wherein the stainless steel is selected between steel of the CrNi 5220 series according to DIN and AISI 316L. The stack according to any one of claims 1 to 5, wherein the thermostatic fluid is demineralized water circulating in a closed circuit. The stack according to any one of claims 1 to 6, wherein the outer circumference of the ion exchange membrane is disposed in the middle region of the outer circumferential sealing gasket contained between the circumference of the passage opening and the edge of the central hollow portion. 8. The stack of claim 7, wherein the ion exchange membrane is isolated from the thermostatic fluid by a sealing element disposed in said intermediate region, optionally including a ridge or ring. The ion exchange membrane of claim 1, wherein the ion exchange membrane has a passage hole that matches the passage opening of the gasket and has a cross section larger than the opening, wherein the ion exchange membrane is in the form of a planar gasket or an O-ring. And a sealing element disposed between the passage opening of the gasket and the edge of the passage hole of the membrane, the ring including the ring or ridge of a non-conductive material of the insulating material. 10. The stack of claim 9, wherein the ring of non-conductive material consists of a rubber of low hardness, optionally EPM or EPDM. 10. The stack of claim 9, wherein the ring of nonconductive material consists of a liquid film applied at the moment of assembling the stack and is polymerized using a catalyst contained in the liquid film or by UV irradiation or heat treatment. . 12. The stack of claim 11 wherein the membrane has elasticity and reduced hardness after polymerization. 13. The stack of claim 11 or 12, wherein the liquid film is comprised of a silicone resin based polymerizable material. Substantially the same stack as described with reference to the accompanying drawings.

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