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Project Title:

Development of membranes for electrolysis and membrane fuel cells

Ref.No.: 139

Project Type and Category:

Basic research

Project Duration:

1.01.1998 through 31.12.98; See project ref. No.: 73

Project Participants:

Jochen Kerres, Wei Cui, Sabine Reichle, Gerhart Eigenberger ,Jochen Kerres, Wei Cui, Sabine Reichle, Gerhart Eigenberger ,Institut für Chemische Verfahrenstechnik, Universität Stuttgart

Sponsor:

Deutsche Forschungsgemeinschaft (DFG)

Project Budget and
Funding:

Phase A7: DM 71,800

Project Description and Objectives:

Objective of the research work in this partial project is the development and optimization of new chemically stable ion-exchange membranes (ionomer membranes) for the application in PEM electrolysis and PEM fuel cells (PEFC), which can substitute the perfluorinated ionomer Nafion® and other ionomers of the perfluorinated ionomer type. Up to now mainly Nafion® has been used as membrane polymer in these processes because of its outstanding chemical stability [1]. Driving force for our research has been the high price of Nafion® which to date hinders an increased application in PEM electrolysis and fuel cells.
Besides Nafion and other perfluorinated ionomers only few other polymers have been tested for suitability as membrane polymers in PEFCs and electrolysis up to now. Here the research of Scherer et al. [2], [3], [4] can be mentioned: Scherer grafted g-irradiated perfluoropolymer foils with styrene. The perfluoropolymer/styrene graft foils were then sulfonated by immersion in chlorosulfonic acid/dichloroethane mixtures. The ionomer foils showed a fairly good performance in PEFCs. However, the oxidation stability of these graft foils was limited because of the limited oxidation stability of the tertiary H of the styrene units. Another research group developed ionomers produced by soaking of polybenzimidazole foils with phosphoric acid, where the phosphoric acid acted as the proton-conducting species [5]. These blend membranes had the disadvantage that the phosphoric acid gradually diffused out of the membrane together with the water formed within action of the fuel cell so that the membrane lost its initial ionic conductivity. Ledjeff [6] proposed the application of cross-linked sulfonated poly(ethersulfone) ionomers, produced via electrophilic sulfonation as protonic conductors in PEFCs. However, no PEFC application data were provided in this article. In a patent paper, sulfonated PEEK (polyether ether ketone) membranes were proposed as proton conductor in a PEFC [7]. In this patent a good chemical stability of the sulfonated PEEK membranes was Reported. A new publication of Ledjeff et al. Suggested the application of organic solvent-soluble, thermally and chemically stable polyphenylenes as protonic conductors in PEFC [8]. However, no PEFC application data of these membranes have been reported to date. Recently, the BALLARD company announced the development of a non-fluorinated proton-conducting membrane based on an arylic main-chain polymer which showed a good performance in a PEFC [9]. However, the chemical nature of the base polymer was not disclosed, as was to be expected.

Technical Goals:

The most promising candidates for substitution of other ionomers for Nafion are polymers with an arylic main chain bearing the following components: Components of arylic main-chain polymers show very good chemical and thermal stabilities. One of these polymers is poly(ethersulfone) PSU Udel®, which we selected as basis polymer for the modification with sulfonic acid groups because it shows a very good chemical stability. We developed a new process for the sulfonation of PSU [10], [11], [12], [13].
The advantages of the presented sulfonation procedure are: (a) in principle all polymers which can be lithiated can be subjected to this sulfonation process, (b) the sulfonic acid group is inserted into the more hydrolysis-stable part of the molecule, (c) no chlorinated hydrocarbons are required in the synthesis process. The overall yield of the sulfonated PSU polymer, based on relation n-buLi/PSU reaches up to 90%. The most useful oxidation reagents for the sulfinated polymer are NaOCl, H2O2 and KMnO4 in aqueous phase. The sulfonated PSU was Characterized, formed into membranes and applied in a PEFC.

Project Status

Finalized

Preliminary or Final Results:

Results of the Oxidation of the Polymeric Sulfinic Acids H2O2 was used as oxidant for the oxidation of sulfinated PSU with <1.5 meq SO2Li/g Polymer,alkalischen Elektrolyseuren durchgeführt. Besonderen Stellenwert hat dabei die Ermittlung der while for the oxidation of sulfinated PSU with >2 meq/g SO2Li/g Polymer KMnO4 and NaOCl were used as oxidants. The oxidation conditions are described in more detail in [11]. The oxidation proceeded completely, as could be proved by 1H-NMR and IR [11].
Specific Resistance of Membranes Made from the Sulfonated Polymers The sulfonated polymers show low resistances in dependence of the ion-exchange capacity [11,12,13]. The percolation threshold (opening of ion-conductive channels) of the membranes lies in the range of 0.6-0.8 meq/g. The preferred ion-exchange capacities lies between 1.3 and 1.8 meq/g because these membranes already show low resistances, connected with a not too high swelling.
For the ionomers, new cross-linking processes were developed in order to reduce swelling and to enhance the mechanical, thermal and chemical stability of the membranes which is particularly important for the PEFC application of the membranes.
Application of the Sulfonated PSU Membranes in a PEFC: One of the cross-linked membranes (PSU-M-41, IEC=1.4 meq/g) was tested in a H2/O2 PEFC (electrode area 25 cm2). The electrodes (GDE/E-tek, 0.4 mg Pt/cm2, 20% Pt onto Vulcan XC-72) were mechanically cold-pressed on the membrane. The temperature of the cell was 50°C.
The results of some I/U curves of the membrane in the PEFC, recorded in succession (run-1 to run-6) indicate good H+ transport properties. However, the formation of a three phase electrode/gas/membrane interlayer is not sufficient by simply cold-pressing together the components, as can be seen by a limited voltage and power density of the EME unit. The contact between membrane and electrodes might be improved by a modification of membrane casting process and/or coating the electrodes with a solution of the sulfonated polymer in a suitable solvent. The membrane was tested in the PEFC for 1 week. In this time range, no voltage decrease of the PEFC could be observed.

Related Reference Papers and Other Publications:

[1] Ledjeff, K.; Heinzel, A.; Mahlendorf, F.; Peinecke, V.; Dechema-Monographien Band 128, VCH Verlag, 103-118 (1993)
[2] Scherer, G. G.; Ber. Bunsenges. Phys. Chem. 94, 1008-1014 (1990)
[3] Scherer, G. G.; Büchi, F. N.; Gupta, B.; Rouilly, M.; Hauser, P. C; Chapiro, A.; Proceedings of the 27th Intersociety Energy Conversion Engineering Conference IECEC-92, San Diego, USA, Aug. 3-7, 3.419 - 3.424 (1992)
[4] Gupta, B.; Büchi, F. N; Scherer, G. G.; Solid State Ionics 61, 213-218 (1993)
[5] Wainright, J. S.; Wang, J.-T.; Savinell, R. F.; Litt, M.; Moaddel, H.; Rogers, C.:The Electochemical Society, Spring Meeting, San Francisco, May 22-27, Extended Abstracts, Vol. 94-1, 982-983 (1994)
[6] Nolte, R.; Ledjeff, K.; Bauer, M.; Mülhaupt, R.; J. Memb. Sci. 83, 211-220 (1993)
[7] Helmer-Metzmann, F.; Ledjeff, K.; Nolte, R., et al.; EP 0 574 791 A2 Offen (1993)
[8] Matejcek, L.; Nolte, R.; Heinzel, A.; Ledjeff, K.; Zerfass, T.; Mülhaupt, R.; Frey, H.; Jahrestagung 1995 der Fachgruppe Angewandte Elektrochemie der GdCh, Duisburg, Sept. 27-29, 1995, Abstract Poster Nr. 20 (1995)
[9] Steck, A. E.; First International Symposium on New Materials for Fuel Cell Systems, Montreal, Quebec, Kanada, July 9-13, 1995, Proeedings, 74-94 (1995)
[10] Kerres, J.; Schnurnberger, W.; Reichle, S.; Eigenberger, G.; German Patent Application, in processing
11] Kerres, J.; Cui, W.; Reichle, S.; J. Polym. Sci., in press (1996)
[12] Kerres, J.; Eigenberger, G.; Reichle, S.; Hetzel, K.; Schramm, V.; Lecture (Speaker J. Kerres), ""Jahreskolloquium 1994 des Sonderforschungsbereichs 270 - Energieträger Wasserstoff"", Jan. 20-21, 1994, VDI Verlag, S. 69-90 (1994)
[13] Kerres, J.; Cui, W.; Neubrand, W.; Springer, S.; Reichle, S.; Striegel, B.; Eigenberger, G.; Schnurnberger, W.; Bevers, D.; Wagner, N.; Bolwin, K.; Lecture paper (speaker J. Kerres), Euromembrane ´92 Congress, Bath (UK), Sept. 18-20, 1995; Proceedings Book 1.284 - 1.289 (1995)

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Project Title: Development of membranes for electrolysis and membrane fuel cells		Ref.No.: 139
Project Type and Category:	Basic research
Project Duration:	1.01.1998 through 31.12.98; See project ref. No.: 73
Project Participants:	Jochen Kerres, Wei Cui, Sabine Reichle, Gerhart Eigenberger ,Jochen Kerres, Wei Cui, Sabine Reichle, Gerhart Eigenberger ,Institut für Chemische Verfahrenstechnik, Universität Stuttgart
Sponsor:	Deutsche Forschungsgemeinschaft (DFG)
Project Budget and Funding:	Phase A7: DM 71,800
Project Description and Objectives:	Objective of the research work in this partial project is the development and optimization of new chemically stable ion-exchange membranes (ionomer membranes) for the application in PEM electrolysis and PEM fuel cells (PEFC), which can substitute the perfluorinated ionomer Nafion® and other ionomers of the perfluorinated ionomer type. Up to now mainly Nafion® has been used as membrane polymer in these processes because of its outstanding chemical stability [1]. Driving force for our research has been the high price of Nafion® which to date hinders an increased application in PEM electrolysis and fuel cells. Besides Nafion and other perfluorinated ionomers only few other polymers have been tested for suitability as membrane polymers in PEFCs and electrolysis up to now. Here the research of Scherer et al. [2], [3], [4] can be mentioned: Scherer grafted g-irradiated perfluoropolymer foils with styrene. The perfluoropolymer/styrene graft foils were then sulfonated by immersion in chlorosulfonic acid/dichloroethane mixtures. The ionomer foils showed a fairly good performance in PEFCs. However, the oxidation stability of these graft foils was limited because of the limited oxidation stability of the tertiary H of the styrene units. Another research group developed ionomers produced by soaking of polybenzimidazole foils with phosphoric acid, where the phosphoric acid acted as the proton-conducting species [5]. These blend membranes had the disadvantage that the phosphoric acid gradually diffused out of the membrane together with the water formed within action of the fuel cell so that the membrane lost its initial ionic conductivity. Ledjeff [6] proposed the application of cross-linked sulfonated poly(ethersulfone) ionomers, produced via electrophilic sulfonation as protonic conductors in PEFCs. However, no PEFC application data were provided in this article. In a patent paper, sulfonated PEEK (polyether ether ketone) membranes were proposed as proton conductor in a PEFC [7]. In this patent a good chemical stability of the sulfonated PEEK membranes was Reported. A new publication of Ledjeff et al. Suggested the application of organic solvent-soluble, thermally and chemically stable polyphenylenes as protonic conductors in PEFC [8]. However, no PEFC application data of these membranes have been reported to date. Recently, the BALLARD company announced the development of a non-fluorinated proton-conducting membrane based on an arylic main-chain polymer which showed a good performance in a PEFC [9]. However, the chemical nature of the base polymer was not disclosed, as was to be expected.
Technical Goals:	The most promising candidates for substitution of other ionomers for Nafion are polymers with an arylic main chain bearing the following components: Components of arylic main-chain polymers show very good chemical and thermal stabilities. One of these polymers is poly(ethersulfone) PSU Udel®, which we selected as basis polymer for the modification with sulfonic acid groups because it shows a very good chemical stability. We developed a new process for the sulfonation of PSU [10], [11], [12], [13]. The advantages of the presented sulfonation procedure are: (a) in principle all polymers which can be lithiated can be subjected to this sulfonation process, (b) the sulfonic acid group is inserted into the more hydrolysis-stable part of the molecule, (c) no chlorinated hydrocarbons are required in the synthesis process. The overall yield of the sulfonated PSU polymer, based on relation n-buLi/PSU reaches up to 90%. The most useful oxidation reagents for the sulfinated polymer are NaOCl, H2O2 and KMnO4 in aqueous phase. The sulfonated PSU was Characterized, formed into membranes and applied in a PEFC.
Project Status	Finalized
Preliminary or Final Results:	Results of the Oxidation of the Polymeric Sulfinic Acids H2O2 was used as oxidant for the oxidation of sulfinated PSU with <1.5 meq SO2Li/g Polymer,alkalischen Elektrolyseuren durchgeführt. Besonderen Stellenwert hat dabei die Ermittlung der while for the oxidation of sulfinated PSU with >2 meq/g SO2Li/g Polymer KMnO4 and NaOCl were used as oxidants. The oxidation conditions are described in more detail in [11]. The oxidation proceeded completely, as could be proved by 1H-NMR and IR [11]. Specific Resistance of Membranes Made from the Sulfonated Polymers The sulfonated polymers show low resistances in dependence of the ion-exchange capacity [11,12,13]. The percolation threshold (opening of ion-conductive channels) of the membranes lies in the range of 0.6-0.8 meq/g. The preferred ion-exchange capacities lies between 1.3 and 1.8 meq/g because these membranes already show low resistances, connected with a not too high swelling. For the ionomers, new cross-linking processes were developed in order to reduce swelling and to enhance the mechanical, thermal and chemical stability of the membranes which is particularly important for the PEFC application of the membranes. Application of the Sulfonated PSU Membranes in a PEFC: One of the cross-linked membranes (PSU-M-41, IEC=1.4 meq/g) was tested in a H2/O2 PEFC (electrode area 25 cm2). The electrodes (GDE/E-tek, 0.4 mg Pt/cm2, 20% Pt onto Vulcan XC-72) were mechanically cold-pressed on the membrane. The temperature of the cell was 50°C. The results of some I/U curves of the membrane in the PEFC, recorded in succession (run-1 to run-6) indicate good H+ transport properties. However, the formation of a three phase electrode/gas/membrane interlayer is not sufficient by simply cold-pressing together the components, as can be seen by a limited voltage and power density of the EME unit. The contact between membrane and electrodes might be improved by a modification of membrane casting process and/or coating the electrodes with a solution of the sulfonated polymer in a suitable solvent. The membrane was tested in the PEFC for 1 week. In this time range, no voltage decrease of the PEFC could be observed.
Related Reference Papers and Other Publications:	[1] Ledjeff, K.; Heinzel, A.; Mahlendorf, F.; Peinecke, V.; Dechema-Monographien Band 128, VCH Verlag, 103-118 (1993) [2] Scherer, G. G.; Ber. Bunsenges. Phys. Chem. 94, 1008-1014 (1990) [3] Scherer, G. G.; Büchi, F. N.; Gupta, B.; Rouilly, M.; Hauser, P. C; Chapiro, A.; Proceedings of the 27th Intersociety Energy Conversion Engineering Conference IECEC-92, San Diego, USA, Aug. 3-7, 3.419 - 3.424 (1992) [4] Gupta, B.; Büchi, F. N; Scherer, G. G.; Solid State Ionics 61, 213-218 (1993) [5] Wainright, J. S.; Wang, J.-T.; Savinell, R. F.; Litt, M.; Moaddel, H.; Rogers, C.:The Electochemical Society, Spring Meeting, San Francisco, May 22-27, Extended Abstracts, Vol. 94-1, 982-983 (1994) [6] Nolte, R.; Ledjeff, K.; Bauer, M.; Mülhaupt, R.; J. Memb. Sci. 83, 211-220 (1993) [7] Helmer-Metzmann, F.; Ledjeff, K.; Nolte, R., et al.; EP 0 574 791 A2 Offen (1993) [8] Matejcek, L.; Nolte, R.; Heinzel, A.; Ledjeff, K.; Zerfass, T.; Mülhaupt, R.; Frey, H.; Jahrestagung 1995 der Fachgruppe Angewandte Elektrochemie der GdCh, Duisburg, Sept. 27-29, 1995, Abstract Poster Nr. 20 (1995) [9] Steck, A. E.; First International Symposium on New Materials for Fuel Cell Systems, Montreal, Quebec, Kanada, July 9-13, 1995, Proeedings, 74-94 (1995) [10] Kerres, J.; Schnurnberger, W.; Reichle, S.; Eigenberger, G.; German Patent Application, in processing 11] Kerres, J.; Cui, W.; Reichle, S.; J. Polym. Sci., in press (1996) [12] Kerres, J.; Eigenberger, G.; Reichle, S.; Hetzel, K.; Schramm, V.; Lecture (Speaker J. Kerres), ""Jahreskolloquium 1994 des Sonderforschungsbereichs 270 - Energieträger Wasserstoff"", Jan. 20-21, 1994, VDI Verlag, S. 69-90 (1994) [13] Kerres, J.; Cui, W.; Neubrand, W.; Springer, S.; Reichle, S.; Striegel, B.; Eigenberger, G.; Schnurnberger, W.; Bevers, D.; Wagner, N.; Bolwin, K.; Lecture paper (speaker J. Kerres), Euromembrane ´92 Congress, Bath (UK), Sept. 18-20, 1995; Proceedings Book 1.284 - 1.289 (1995)