APPARATUS AND PROCESS FOR MAKING ACID-DOPED PROTON EXCHANGE MEMBRANES

Abstract

A continuous automated process and production line for preparing an acid doped polybenzimidazole, PBI, polymer membrane film for use in a fuel cell, the process having a washing stage, a drying procedure, and a doping stage.

Claims

1.-15. (canceled)

16. A continuous automated process for preparing an acid doped polybenzimidazole, PBI, polymer film membrane sheet for use in a fuel cell, the automated process comprising the following sequences: providing a PBI film membrane sheet from a roll as an endless strip and continuously moving the endless strip over rollers through the various stages for processing; in a washing stage, exposing the membrane sheet to water for removing solvents, for example N,N-dimethylacetamide DMAc, from the membrane sheet by the water; in a drying procedure, drying the membrane sheet at temperatures elevated above ambient temperature; and in a doping stage, after the drying procedure, exposing the PBI membrane sheet for a duration in the range of 10 seconds to 5 minutes to orthophosphoric acid at a concentration higher than 85 wt. % at a temperature in the range of 90-100° C. for doping the membrane sheet with the acid.

17. A process according to any preceding claim 16, wherein the drying procedure comprises two drying phases, wherein the first drying phase is made at a temperature in the range of 1-10 degrees below the boiling point of water, for example in the range of 90-99° C. if at atmospheric pressure, in order to evaporate water without bubble formation, and wherein the second drying phase is made at a temperature in the range of 1-10 degrees below the boiling point of DMAc or 1-10 degrees below the boiling point of an azeotropic mixture of acetic acid with DMAc for evaporating DMAc without bubble formation.

18. A process according to claim 16, wherein the method comprises, in a chemical-reaction stage, between the washing stage and the drying stage, exposing the membrane sheet to water-diluted orthophosphoric acid having a concentration in the range of 0.01 wt. % to 1 wt. % for removing further DMAc from the PBI membrane sheet by chemical reaction of the DMAc with the diluted orthophosphoric acid to form acetic acid.

19. A process according to claim 16, wherein the method comprises, in a pre-doping stage, between the drying stage and the doping stage, exposing the membrane sheet to orthophosphoric acid at a concentration higher than 65 wt. % but lower than the concentration at the doping stage and dissolving low molecular weight molecules of the PBI polymer of the membrane by the orthophosphoric acid.

20. A process according to claim 16, wherein the method comprises providing the various stages as liquid baths in corresponding containers and submerging the membrane sheet in the liquid.

21. An automated production line for a continuous automated process according to any preceding claim, the production line comprising, a sheet receiver for receiving a polybenzimidazole, PBI, film membrane sheet; a washing stage for exposing the membrane sheet to water and removing solvent, for example N,N-dimethyl acetamide DMAc, from the membrane sheet by the water; a drying apparatus for drying the membrane sheet at temperatures elevated above ambient temperature; a doping stage after the drying apparatus for exposing the PBI membrane sheet to orthophosphoric acid at a concentration higher than 85 wt. % for doping the membrane sheet with the acid at a temperature in the range of 90-100° C. and for performing the doping in the doping stage in the range of 10 seconds to 5 minutes or 10 seconds to less than 5 minutes; wherein the sheet receiver is configured for receiving the sheet from a roll as an endless strip over rollers for continuously moving the endless strip through various stages wherein the automated production line in the continuous automated process is configured for providing the orthophosphoric acid in the doping stage at a temperature in the range of 90-100° C. and for performing the doping in the doping stage in the range of 10 seconds to 5 minutes or 10 seconds to less than 5 minutes.

22. The production line of claim 21, comprising a first roll for receiving a quasi-endless strip of PBI membrane sheet and a plurality of correspondingly arranged further rollers for unrolling the strip from the first roll while guiding the strip over the further rollers through the various preparation stages in a sequence as described in the following: one or more water containers with water as part of the washing stage for washing the strip by guiding the strip through the water in the water 5 containers; a chemical-reaction stage comprising a diluted-acid container with water-diluted orthophosphoric acid having a concentration in the range of 0.01 wt. % to 1 wt. % for removing further DMAc from the PBI membrane sheet after the washing stage and by chemical reaction of the DMAc with 10 the diluted orthophosphoric acid to form acetic acid by guiding the strip through the diluted acid in the diluted-acid container.

23. The production line according to claim 21, comprising a first zone and a second zone of the drying apparatus with two corresponding drying phases for drying the strip while guided through the first and then the second zone, wherein the first zone is programmed to provide a drying temperature in the range of 1-10 degrees below the boiling point of water, for example in the range of 90-99° C. if at atmospheric pressure, in order to evaporate water without bubble formation in the first drying phase, and wherein the second zone is programmed to provide a temperature in the range of 1-10 degrees below the boiling point of DMAc or 1-10 degrees below the boiling point of an azeotropic mixture of acetic acid with DMAc for evaporating DMAc without bubble formation in the second drying phase.

24. The production line according to anyone of the claim 21, comprising a pre-doping container as part of the pre-doping stage with orthophosphoric acid at a concentration higher than 65 wt. % for dissolving low molecular weight molecules of the PBI polymer of the membrane by the orthophosphoric acid by guiding the strip through the acid in the pre-doping container (10); a doping container as part of the doping stage with orthophosphoric acid at a concentration higher than 85 wt. % for doping the membrane sheet with the acid by guiding the strip through the acid in the doping container after the pre-doping stage.

25. The production line of claim 21, comprising a first roll for receiving a quasi-endless strip of PBI membrane sheet and a plurality of correspondingly arranged further rollers for unrolling the strip from the first roll while guiding the strip over the further rollers through the various preparation stages in a sequence as described in the following: one or more water containers with water as part of the washing stage for washing the strip by guiding the strip through the water in the water containers; a chemical-reaction stage comprising a diluted-acid container with water-diluted orthophosphoric acid having a concentration in the range of 0.01 wt. % to 1 wt. % for removing further DMAc from the PBI membrane sheet by chemical reaction of the DMAc with the diluted orthophosphoric acid to form acetic acid by guiding the strip through the diluted acid in the diluted-acid container; a first zone and a second zone of the drying apparatus with two corresponding drying phases for drying the strip while guided through the first and then the second zone, wherein the first zone is programmed to provide a drying temperature in the range of 1-10 degrees below the boiling point of water, for example in the range of 90-99° C. if at atmospheric pressure, in order to evaporate water without bubble formation in the first drying phase, and wherein the second zone is programmed to provide a temperature in the range of 1-10 degrees below the boiling point of DMAc or 1-10 degrees below the boiling point of an azeotropic mixture of acetic acid with DMAc for evaporating DMAc without bubble formation in the second drying phase; a pre-doping container as part of the pre-doping stage with orthophosphoric acid at a concentration higher than 65 wt. % for dissolving low molecular weight molecules of the PBI polymer of the membrane by the orthophosphoric acid by guiding the strip through the acid in the pre-doping container; a doping container as part of the doping stage with orthophosphoric acid at a concentration higher than 85 wt. % for doping the membrane sheet with the acid by guiding the strip through the acid in the doping container; a collection roller for collecting the endless strip after doping.

Description

SHORT DESCRIPTION OF THE DRAWING

[0042] The invention will be described in the following with reference to the drawing, in which

[0043] FIG. 1 is a sketch of a continuous process for doping membranes;

[0044] FIG. 2 is a graph foradsorption isotherms for PBI membranes in 85 wt. % H.sub.3PO.sub.4 at different temperatures;

[0045] FIG. 3 is a graph for content of H.sub.3PO.sub.4 absorbed by PBI membranes depending on its concentration at 100° C. for 1 min;

[0046] FIG. 4 is a graph for adsorption isotherms for PBI membranes in 90 wt. % H.sub.3PO.sub.4 at 100° C.;

[0047] FIG. 5 illustrates tensile strength of PBI membranes before and after doping with different process parameters.

DETAILED DESCRIPTION

[0048] In the following, a high-speed roll-to-roll process is described in which production parameters are precisely controlled in order to produce high quality membranes for fuel cells based on polybenzimidazole, PBI, sheet material and doped with orthophosphoric acid H.sub.3PO.sub.4. A production line for preparation of H.sub.3PO.sub.4-doped PBI membranes with useful properties is presented in the following.

[0049] FIG. 1 illustrates a general production line, including washing and acid doping of PBI membranes by a roll-to-roll process.

[0050] A roll 1 is provided of an undoped quasi-endless strip of PBI membrane sheet material. Optionally, the PBI membrane is provided by casting or coating PBI material onto a polymer film, for example polyester film. The quasi-endless polymer membrane sheet 18 is unwound from the roller 1, and by guiding rollers 2 brought into a container 3 with deionized water. Normally, after coating onto a polymer film, the PBI membrane contains up to 20 wt. % (weight percentage) of solvent, for example including N,N-dimethylacetamide (DMAc), and in the water container 3, the main part of this solvent is removed.

[0051] Although, the containers in FIG. 1 are shown as having equal size, this is typically not the case. As the membrane strip is guided over rollers so that all parts of the strip are moving with the same speed, the length of the path through the baths in the various containers 3, 6-11 can be varied by varying the size of the individual containers and, correspondingly, the length of the time it takes for a portion of the membrane strip to pass through a container.

[0052] After the water container 3, the PBI membrane only comprises a small content of solvent inside, typically less than 2 wt. %. The membrane is then easily detached from the substrate, for example polyester substrate, which is collected on film collection roller 4.

[0053] The quasi-endless membrane strip is guided via tension-controlling roller 5 to a second water container 6 that also contains deionized water. The water in the second container 6 is steadily or periodically replenished in order to keep the concentration of the solvent in the second water container 6 low. The concentration of the removed solvent in the first water container 3 is substantially higher than in the second water container 6 why the water with the low concentration of solvent from the second water container 6 is advantageously used for replenishing the water for the first water container 3, from which the water is then discarded into water receptacle 16, which is optionally a drain or which is used for recycling of the water if combined with corresponding cleaning options. The use of water from the second water container 6 for use in the first water container reduces the overall necessary consumption of water in the process. For example, the replenishing step is continuous during the production process.

[0054] Despite the two-step washing of the PBI membrane, it can still contain some residuals of solvent, for example DMAc due to the strong interaction between the polar groups in PBI and the DMAc molecules, see also reference [2].

[0055] In order to further remove residuals of solvent, in particular DMAc, a chemical-reaction step is performed in low-acidity container 7, which contains diluted orthophosphoric acid, having a concentration of less than 1 wt. % in water.

[0056] An exact concentration of the orthophosphoric acid in container 7 is defined by the volume of container 7 used and the volume of membrane rolled through the container per specified time. In this container, a chemical reaction takes place between the solvent, in particular DMAc, and the orthophosphoric acid. For DMAc as the solvent, the process forms acetic acid, which is described by the equation below, where DMAc is marked here as CH.sub.3CON(CH.sub.3).sub.2).

3CH.sub.3CON(CH.sub.3).sub.2+3H20+H.sub.3PO.sub.4.fwdarw.3CH.sub.3COOH+[(CH.sub.3).sub.2NH.sub.2.sup.+].sub.3PO.sub.4.sup.3−

[0057] This washing in container 7 removes the solvent, in particular DMAc. This is important as this process prevents the products of the acid hydrolysis of DMAc from getting into the fuel cell stack.

[0058] Before the final doping, the PBI membrane should be dried from all liquids besides orthophosphoric acid, especially the water, possibly remaining trace amounts of DMAc and acetic acid, which have boiling points 100, 166 and 118° C. at standard pressure, respectively, see also reference [5]. Also, other reaction products should be removed. It is pointed out in EP1551522, equivalent to US2004/000470, that acetic acid forms an azeotropic mixture with DMAc, which boils at 171° C.

[0059] Advantageously, in order to obtain proper drying results, a two-zone oven 8 and 9 is used with a first zone, in which the temperature is kept around 100° C. for removing water, and a second zone at around 171° C. for removing the acetic acid-DMAc mixture.

[0060] In order to avoid bubbles from boiling of the liquids, which could produce voids in the membrane, the temperature in the first container is maintained at a temperature just below 100° C., for example in the range of 90-98° C., and in the second zone just below 171° C., for example in the range 160-170° C. By gradually increasing the temperature, for example in a multi zone drying tunnel, evaporation of the various liquids can be achieved without bubble formation.

[0061] After the drying process, the dried PBI membrane is moved through two subsequent containers 10,11 with concentrated orthophosphoric acid. The concentration in the first acid container 10 is above 65 wt. % but not necessarily as high as in second acid container, as the second container is used for the final doping of the membrane. The first acid container 10 is kept at elevated temperatures in the range of 40-80° C.

[0062] The role of the first acid container 10 is explained in the following. Normally, PBI has some spread in the molecular weight of the polymer so that both polymer with low molecular weight and with high molecular weight are present. The PBI polymer with low molecular weight will be at least partially dissolved in hot concentrated orthophosphoric acid. In order to optimize the doping process, two containers 10 and 11 are used, where the dissolution of PBI polymer with low molecular weight takes place dominantly or entirely in the first container 10, which during the dissolution process attains a lower concentration of acid than the acid concentration desired in the second container 11. As polymer with low molecular weight are dominantly removed in the first container 10, the membrane contains dominantly polymer with higher molecular weight when entering the second container 11. Accordingly, in the second container 11, the acid is maintained at a higher concentration due to a lower degree of contamination by dissolved polymer residuals. The higher concentration is advantageous for the doping, as will be explained in more detail further below.

[0063] All in all, the use of the cascade system of containers 10 and 11 assists in regenerating an optimized doping solution.

[0064] After doping of the PBI membrane with orthophosphoric acid in doping container 11, the H.sub.3PO.sub.4-PBI drops of acid on the membrane are removed by sponge-covered rollers 12, and the doped membrane is wound onto roller 13.

[0065] The concentration of DMAc and acetic acid in the water container 3 and the chemical-reaction container 7 should be controlled to avoid over-contamination of the working solution. If necessary, liquids are removed into waste containers 16, 17.

[0066] It is important to control the concentration of orthophosphoric acid in the chemical reaction container 7 for the cleaning of the membrane, in the pre-doping container 10 for the reduction of low-molecular weight polymer, and in doping container 11 for carrying-out of the overall complex doping process. Therefore, a first replenish container 14 with deionized water and a second container 15 with 99 wt. % H.sub.3PO.sub.4 are utilized to adjust the liquids to predetermined concentration levels in the various corresponding containers for the process.

[0067] Water contaminated with DMAc and orthophosphoric acid contaminated with products of hydrolysis of DMAc are collected in waste containers 16 and 17, respectively, for their further recycling.

[0068] Returning to the doping process and its mechanism, it should be mentioned here that doping of PBI with orthophosphoric acid occurs via bonding of one repeating unit of the polymer with two molecules of acid by means of coulombic forces. Further accumulation of acid within PBI membrane takes place due to the hydrogen bonds, see also reference [2]. Process parameters such as time, temperature, and concentration of orthophosphoric acid must be carefully considered in order to reach optimized and consistent doping levels and in order for the membrane not to lose its tensile strength. This is discussed in greater detail in the following with reference to experimental results.

[0069] FIG. 2 illustrates adsorption isotherms for PBI membranes in 85 wt. %

[0070] H.sub.3PO.sub.4 at temperatures of 50° C., 75° C., and 100° C. In order to reach a plateau region for the content of orthophosphoric acid, at least 2h of doping time are necessary at 50° C. when doped in a 85 wt. % orthophosphoric acid solution. A reduced doping time of 30 mins is necessary at 75° C., while less than 5 min can be used at 100° C. to reach a plateau region. For a fast production process, a short doping time is highly advantageous.

[0071] In order to reduce the doping time even more, the concentration of orthophosphoric acid is advantageously higher than 85 wt. %.

[0072] FIG. 3 shows the amount of orthophosphoric acid adsorbed by PBI membranes when doping at 100° C. for various concentrations of H.sub.3PO.sub.4 in the range of 65 to 90 wt. %. It should be noted that process time is fixed there on 1 min.

[0073] According to FIG. 3, the doping level is exponentially growing and exceeding the values of maximal adsorption in FIG. 1, approximately 11.5 mg/cm.sup.2. The strong growth is due to the change of the mechanism from monomolecular type to polymolecular adsorption. In the polymolecular adsorption regime, the PBI membrane film becomes gel-like. A pronounced gel-formation effect was observed at doping levels around 30 mg/cm.sup.2. At even higher levels, the membrane is dissolved in orthophosphoric acid.

[0074] Experimentally, it was shown that use of orthophosphoric acid with concentration levels above 85 wt. %, in particular at 90 wt. %, allowed a reduction of the doping time to 10 sec.

[0075] With reference to FIG. 4, showing adsorption isotherms for PBI membranes in 90 wt. % H.sub.3PO.sub.4 at 100° C., the transition from a monomolecular mechanism of adsorption to a polymolecular mechanism is clearly observed in the range of 10-15 mg/cm.sup.2. An illustrative transition level of 12 mg/cm.sup.2 is indicated in FIG. 4.

[0076] Thus, when implementing such accelerated doping procedure, care must be taken that the PBI membrane still has sufficient tensile strength. In order to verify this, the highly doped PBI membranes were compared to PBI membranes that were doped in mild conditions, i.e. at 2h at 50° C. in 85 wt. % H.sub.3PO.sub.4. Experimental results are illustrated in FIG. 5.

[0077] As seen from FIG. 5, the experimentally produced membranes doped at high temperature of 100° C. and high acid concentration of 90 wt. % have a tensile strength at least as high as membranes doped slowly at a low temperature of 50° C. and in more moderate acid concentration of 85 wt. %. This is the case, however, only as long as the doping time is not more than 15s. After 15 seconds, the tensile strength becomes lower, although only moderately lower until 45 seconds and still acceptable up to 1 minute.

[0078] This is in agreement with FIG. 4, where the transition from monomolecular to polymolecular adsorption regime occurs between 12 and 18 seconds.

[0079] As a conclusion, a process has been demonstrated in which through multi-step cleaning of the membrane material and careful adjustment of the parameters results in doping content and tensile strengths of the membrane similar to slow doping processes, however, where the process is much more suitable for large scale production due to its much higher speed. In particular, the proposed fast-doping process for PBI membrane with orthophosphoric acid can be automated and used as part of continuous production for fuel cells.

[0080] In order to summarize in comparison with some prior art, the following features are pointed out: [0081] 1) washing PBI membrane in a diluted solution of orthophosphoric acid in order to decompose the residuals of DMAc and make the drying process fast and efficient, which is different from the disclosures in references [7, 8] where casted PBI membranes are merely washed in non-solvents, such as water and/or alcohols, which is why drying in those cases requires long time to remove DMAc bonded to PBI, for example drying at 80° C. for 12 h, see reference [6]; [0082] 2) using a two-zone oven to safely remove the various liquid components with different boiling points from the PBI membrane, while also avoiding bubble formation therein; [0083] 3) keeping the adsorption in monomolecular state by strictly controlling the process parameters, for example at 100° C. in 90 wt. % H.sub.3PO.sub.4 for less than 1 min, for example in the range of 10-15 sec, for maintaining its mechanical properties after doping, which is different to the disclosures in references [10-14] where the PBI membranes are doped in more diluted solutions of orthophosphoric acid and at lower temperatures, which takes hours and therefore is not suited for fast large scale production.

REFERENCES

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