Jochen Kerres

Development of ionomer membranes for fuel cells

In this contribution an overview is given about the state-of-the-art at the membrane development for proton-conductive polymer (composite) membranes for the application membrane fuel cells, focusing on the membrane developments in this field performed at ICVT. For preparation of the polymers, processes have been developed for sulfonated arylene main-chain polymers as well as for arylene main-chain polymers containing basic N-containing groups, including a lithiation step. Covalently cross-linked polymer membranes have been prepared by alkylation of the sulfinate groups of sulfinate group-containing polymers with α,ω-dihalogenoalkanes. The advantage of the covalently cross-linked ionomer membranes was their dimensional stability even at temperatures of 80–90°C, their main disadvantage their brittleness when drying out, caused by the inflexible covalent network. Sulfonated and basic N-containing polymers (commercial polymers as well as self-developed ones) have been combined to acid–base blends containing ionic cross-links. The main advantage of these membrane type was its flexibility even when dried-out, its good to excellent thermal stability, and the numerous possibilities to combine acidic and basic polymers to blend membranes having fine-tuned properties. The main disadvantage of this membrane type was the insufficient dimension stability at T>70–90°C, caused by breakage of the ionic cross-links, where the ionic cross-links broke as easier as lower the basicity of the polymeric base was. Some of the acid–base blend membranes were applied to H2 membrane fuel cells and to direct methanol fuel cells up to 100°C, yielding the result that these membranes show very good perspectives in the membrane fuel cell application.

Synthesis and characterization of novel acid–base polymer blends for application in membrane fuel cells

Abstract In this contribution novel acid–base polymer blend membranes are introduced. The membranes are composed of sulfonated poly(etheretherketone) sPEEK Victrex or poly(ethersulfone) sPSU Udel® as the acidic compounds, and of PSU Udel® diaminated at the ortho position to the sulfone bridge, or poly(4-vinylpyridine), poly(benzimidazole) PBI CELAZOLE®, or poly(ethyleneimine) PEI (Aldrich) as the basic compounds. The membranes showed good proton conductivities at ion-exchange capacities IEC of 1 (IEC=meq SO 3 H/g dry membrane), and they showed excellent thermal stabilities (decomposition temperatures >270°C). Two of the membranes were tested in a H 2 membrane fuel cell and showed good performance. The specific interaction of the SO 3 H groups and of the basic N groups was investigated via FTIR for the sulfonated PSU/diaminated PSU and for the sulfonated PSU/poly(4-vinylpyridine) (Pyr) blend. It could be proved that in the dry membranes polysalt groups exist formed by the following acid–base reaction: PSU–SO 3 H+H 2 N–PSU→[PSU–SO 3 ] − + [H 3 N–PSU], and PSU–SO 3 H+P→[PSU–SO 3 ] − + [H–Pyr].

New membranes for direct methanol fuel cells

The performance of direct methanol fuel cells (DMFC) is limited by the cross-over of methanol through the electrolyte. Electrolyte membranes prepared by blending of sulfonated arylene main-chain polymers like sulfonated PEEK Victrex (sPEEK) or sulfonated PSU Udel (sPSU) with basic polymers like poly(4-vinylpyridine) (P4VP) or polybenzimidazole (PBI) show excellent chemical and thermal stability, good proton-conductivity, and good performance in H2 PEM fuel cells. Furthermore, these materials have potentially lower methanol cross-over when compared to standard Nafion-type membranes. In this work, membrane electrode assemblies (MEAs) have been prepared from such membranes according to the thin-film method. The catalyst layer was spray-coated directly on the heated membrane using an ink consisting of an aqueous suspension of catalyst powder and Nafion solution. Unsupported catalysts were used for anode and cathode. A rather high catalyst loading was chosen in order to minimize the effects of limited catalyst utilization due to flooding conditions at both electrodes.

Development and characterization of crosslinked ionomer membranes based upon sulfinated and sulfonated PSU : Crosslinked PSU blend membranes by disproportionation of sulfinic acid groups

Abstract Crosslinked sulfonated ion-exchange blend membranes have been produced via a new crosslinking process. The blends have been obtained from mixing PSU-SO3H and PSU-SO2H in different sulfinic acid/sulfonic acid relations in N-methyl pyrrolidone. The crosslinking process consists of the disproportionation between SO2H groups which occurs during membrane formation. These membranes have been characterized in terms of ion-exchange capacity, ion-resistance, swelling, ion-permeability, and ion-permselectivity. Some of the membranes have been applied to electro-membrane processes, as electrodialysis, and PEM fuel cells (PEM=polymer electrolyte membrane). The advantages of the sulfinate disproportionation crosslinking process are: (i) the crosslinking process is easy to do; (ii) the ion-exchange capacities of crosslinked membranes and thus their ionic resistance, swelling and permselectivity can be varied in a broad range. The crosslinked blend membranes show good thermal stabilities and are suitable for application in electrodialysis. Although the property profile of the blend membranes still has to be improved further, it is demonstrated that they are in principle suitable for application in PEM fuel cells.

New sulfonated engineering polymers via the metalation route. I. Sulfonated poly(ethersulfone) PSU Udel® via metalation‐sulfination‐oxidation

A new process has been developed for the sulfonation of arylene polymers which can be lithiated, like polysulfone Udel®. The sulfonation process consists of the following steps: (1) lithiation of the polymer at temperatures from −50 to −80°C under argon, (2) gassing of the lithiated polymer with SO2; (3) oxidation of the formed polymeric sulfinate with H2O2, NaOCl, or KMnO4; (4) ion-exchange of the lithium salt of the sulfonic acid in aqueous HCl. The advantages of the presented sulfonation procedure are: (1) in principle all polymers which can be lithiated can be subjected to this sulfonation process; (2) by this sulfonation procedure the sulfonic acid group is inserted into the more hydrolysis-stable part of the molecule; (3) this process is ecologically less harmful than many common sulfonation procedures. The sulfonated polymers were characterized by NMR, titration and elemental analysis, by IR spectroscopy, and by determination of ionic conductivity. Also the hydrolytic stability of the sulfonated ion-exchange polymers was investigated. Polymers with an ion-exchange capacity of 0.5 to 3.2 mequiv SO3H/g Polymer have been synthesized and characterized. The following results have been achieved: membranes made from the sulfonated polymers show good conductivity, good permselectivity (>90%), and good hydrolytic stability in 1N HCl and water at temperatures up to 80°C.

Development and characterization of ion-exchange polymer blend membranes

In the presented paper, the preparation and characterization of new ionomer blend membranes containing sulfonated poly(etheretherketone) PEEK Victrex® is described. The second blend components were Polysulfone Udel®-ortho-sulfone-diamine, polymide PA Trogamid P (producer: Huls) and poly(etherimide) PEI Ultem (producer: General Electric). In the blend membranes swelling was reduced by specific interaction, in the case of the blend components PA and PEI hydrogen bonds, and in the case of the blend component PSU–NH2 (partial) polysalt formation, leading to electrostatic interaction between the blend component macromolecules, and hydrogen bonds. The acid–base interactions also led to decrease of ionic conductivity by partial blocking of SO−3 groups for cation transport, compared with the ionic conductivity of the hydrogen bond blends. The acid–base blends showed better ion permselectivities than the hydrogen bond blends, even at high electrolyte concentrations, and thus better performance in electrodialysis. The thermal stability of the investigated blends was very good and in the case of the acid–base blends even better than the thermal stability of pure PEEK–SO3H. DSC traces of the blend membranes showed only one Tg. In addition, the membranes are transparent to visible light. But therefrom it cannot be concluded that the blend components are miscible to the molecular level: at the acid–base blend blends, the Tg of PEEK–SO3H is very similar to the Tg of PSU–NH2, and in the investigated hydrogen bond blends, the portion of PA or PEI, respectively, might be too low to be detected by DSC. The investigated blend membranes showed similar performance as the commercial cation-exchange membrane CMX in electrodialysis (ED) application. The performance of the acid–base blend membrane is better than the performance of the hydrogen bonded PEEK–PA blend, especially in the ED experiment applying the higher NaCl concentration. This is mainly due to the lower swelling and thus better ion permselectivity of the acid–base blend membrane, compared with the PEEK–PA blend. To get a deeper insight into the microphase structure of the investigated blends, dynamic mechanical analyses and TEM investigations of the prepared blend membranes are planned. In addition, due to their promising properties, the preparation of arylene main-chain acid–base blends with other polymeric acidic and basic components is planned. Furthermore, the acid–base blend membranes will be tested in H2 polymer electrolyte fuel cells and direct methanol fuel cells, because preliminary tests have shown that they have a good perspective in this application.

In-situ spin trap electron paramagnetic resonance study of fuel cell processes

A novel method allows the monitoring of radical formation and membrane degradation in-situ in a working fuel cell which is placed in the microwave resonator of an electron paramagnetic resonance (EPR) spectrometer. By introduction of a spin trap molecule at the cathode the formation of immobilized organic radicals on the membrane surface is observed for F-free membranes, revealing the onset of oxidative degradation. For Nafion® there is much less evidence of degradation, and the hydroxyl radical is detected instead. At the anode, free radical intermediates of the fuel oxidation process are observed. No traces of membrane degradation are detected on this side of the fuel cell.

Proton-conducting polymers with reduced methanol permeation

The permeability of Nafion® 117 and some types of acid-base and covalently crosslinked blend membranes to methanol was investigated. The methanol crossover was measured as a function of time using a gas chromatograph with a flame ionization detector. In comparison to Nafion, the investigated acid-base and covalently crosslinked blend membranes show a significant lower permeation rate to methanol. Additionally, another method to reduce the methanol permeability is presented. In this concept a thin barrier layer is plasma polymerized on Nafion 117 membranes. It is shown that a plasma polymer layer with a thickness of 0.3 μm reduces the permeability to methanol by an order of magnitude.

Comparative investigations of ion-exchange membranes

Abstract The equilibrium and transport properties (conductivity, transport number, diffusion) of crosslinked ionomer membranes based on sulfinated and sulfonated PSU in aqueous solutions of HCl, NaCl and KCl have been investigated and compared with a Nafion 117 membrane. It has been found that these membranes are more compact and their conducting paths are of smaller dimension than that of the Nafion 117. The influence of length of crosslinking chain, changing from –(CH 2 ) 4 – to –(CH 2 ) 12 –, is particularly indicated by the diffusion coefficients; the conductivity and transport numbers of counterions are influenced only slightly. Practically no dependence of this effect on the transport number of H + has been found.

New sulfonated engineering polymers via the metalation route. II. Sulfinated/sulfonated poly(ether sulfone) PSU Udel and its crosslinking

New mixed sulfinated/sulfonated polysulfone PSU Udel has been produced by partial oxidation of sulfinated PSU with NaOCl. From the mixed sulfinated/sulfonated PSU, thin crosslinked polymer films have been produced by S-alkylation of the residual sulfinate groups with α,ω-diiodoalkanes having 4–10 (CH2) units. The advantages of the partial oxidation process using NaOCl are as follows: (1) The desired oxidation degree can be adjusted finely. (2) No side reactions take place during oxidation. (3) The partially oxidized polymers is stable at ambient temperature. By variation of the oxidation degree of the sulfinated/sulfonated prepolymer and by variation of the chain length of the diiodo crosslinker, crosslinked membranes with a large range of properties in terms of ionic conductivity, swelling, and permselectivity have been produced. The partially oxidized polymers have been characterized by redox titration, 1H-NMR, and FTIR. The crosslinked membranes have been characterized in terms of ionic conductivity (resistance), permselectivity, and swelling in dependence on ion-exchange capacity and oxidation degree of the prepolymers. In addition, the thermal stabilities of the membranes have been determined by TGA, and FTIR spectra have been recorded on the crosslinked films. Selected membranes show low ionic resistances, low swelling, and good temperature stability which makes them promising candidates for application in (electro)membrane processes.