Andreas Ullrich

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].

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.

Synthesis and characterization of polyaryl blend membranes having different composition, different covalent and/or ionical cross-linking density, and their application to DMFC☆

Abstract In this contribution, differentionomer blend membrane types which show high proton conductivity, thermal stability, and good direct methanol fuel cell (DMFC) performance, are presented: (1) Covalently cross-linked blend membranes from polyaryl sulfinates and polyaryl sulfonates where the sulfinate groups were crosslinked by alkylation with 1,4-diiodobutane; (2) ionically cross-linked blend membranes from polyaryl sulfonates and poly(het)aryl N bases; (3) covalent-ionically cross-linked blend membranes from polyaryl sulfinates, polyaryl sulfonates, and poly(het)aryl N bases; and (4) blend membranes which additionally contain an inorganic compound. The inorganic compound was mixed into the membrane. As aryl polymers, different poly(ethersulfone)s and different poly(etherketone)s have been used, as hetaryl N base, polybenzimidazole PBI Celazole® has been applied. The membrane characterization yielded the following results: (1) high proton conductivities of the membranes could be realized; (2) the TEM micrographs showed that phase-separated or homogeneous morphologies could be realized in the membranes; (3) the DMFC application of the membranes showed that the developed nonfluorinated ionomer membranes have a DMFC performance comparable to perfluorinated ionomer membranes, reaching peak power densities of around 0.25 W/cm 2 at 110°C. It was also found that the addition of SiO 2 powder dramatically reduced the MeOH permeability, but also led to a worse DMFC performance, probably caused by a worse contact membrane-electrode because of a rougher membrane surface caused by the inorganic compound.