Lorenz Gubler

Radiation Grafted Membranes

The development of proton-exchange membranes for fuel cells has generated global interest in order to have a potential source of power for stationary and portable applications. The membrane is the heart of a fuel cell and the performance of a fuel cell depends largely on the physico-chemical nature of the membrane and its stability in the hostile environment of hydrogen and oxygen at elevated temperatures. Efforts are being made to develop membranes that are similar to commercial Nafion

Celtec-V A Polybenzimidazole-Based Membrane for the Direct Methanol Fuel Cell

Celtec-V is a proton exchange membrane based on polybenzimidazole (PBI) comprising an interpenetrating network of polyvinylphosphonic acid designed for application in the direct methanol fuel cell. The properties and fuel cell performance of Celtec-V are investigated and compared against a Nafion 117 standard. It is shown that with the PBI-based membrane, fuel cell performance can be sustained to higher methanol feed concentration at around half the methanol crossover rate. Above 1.0 M methanol, Celtec-V outperforms Nafion 117. Furthermore, lower water permeation is observed, with Celtec-V having an electro-osmotic drag coefficient of around 1 compared to a value of 4-5 for Nafion 117. Room for improvement is identified in the ohmic resistance of the membrane and the cathode-membrane interface, where higher losses are observed at increasing current density.

Cross-Linker Effect in ETFE-Based Radiation-Grafted Proton-Conducting Membranes I. Properties and Fuel Cell Performance Characteristics

Cross-linking of styrene grafted and sulfonated fuel cell membranes is essential to obtain well-performing and durable membranes. In this study, the properties and fuel cell performance of radiation grafted membranes based on poly(ethylene-alt-hexafluoropropylene) (ETFE) (25 μm) with different extent of cross-linking, using divinylbenzene (DVB) as a comonomer to styrene, were investigated. The water uptake and proton conductivity decrease as a function of increasing the extent of cross-linking. Also, the membranes become more brittle. Fuel cell performance is limited by the high ohmic resistance of the membrane at high degrees of cross-linking, whereas at low degrees of cross-linking the membrane-electrode interface is of poor quality. Optimum performance is obtained at an extent of cross-linking corresponding to a styrene:DVB monomer ratio (v/v) of 95:5 in the grafting solution.

Performance-Enhancing Asymmetric Separator for Lithium–Sulfur Batteries

Asymmetric separators with polysulfide barrier properties consisting of porous polypropylene grafted with styrenesulfonate (PP-g-PLiSS) were characterized in lithium-sulfur cells to assess their practical applicability. Galvanostatic cycling at different C-rates with and without an electrolyte additive and cyclic voltammetry were used to probe the electrochemical performance of the cells with the PP-g-PLiSS separators and to compare it with the performance of the cells utilizing state-of-the-art separator, Celgard 2400. Overall, it was found that regardless of the applied cycling rate, the use of the grafted separators greatly enhances the Coulombic efficiency of the cell. An appropriate Li-exchange-site (-SO3(-)) concentration at and near the surface of the separator was found to be essential to effectively suppress the polysulfide shuttle without sacrificing the Li-ion mobility through the separator and to improve the practical specific charge of the cell.

Materials for polymer electrolyte fuel cells

The commercial success of the polymer electrolyte fuel cell (PEFC) will to a large extent be determined by the nature, properties, functionality, and cost of the electrochemical sub-components used in the membrane electrode assembly (MEA). Materials research activities in Switzerland for the PEFC are being pursued at the Paul Scherrer Institut (Villigen AG) and the Swiss Federal Institute of Technology in Lausanne with different objectives. The radiation grafted proton exchange membrane developed at the Paul Scherrer Institut (PSI) has been brought to a near-product-like quality level with encouraging performance close to state-of-the-art materials and a life-time of several thousand hours. Furthermore, the membrane shows low methanol crossover in the direct methanol fuel cell. In addition, polyarylene block copolymer membranes have been investigated as an option for fluorine-free membranes. The electrocatalysis of Pt in acidic solution and in contact with a solid electrolyte, the development of new methanol oxidation and oxygen reduction catalysts, and co-sputtering of Pt and carbon as an alternative method for catalyst preparation are areas of fundamental research. More applied research is performed in the characterization of commercial electrodes in single cells, using standard as well as advanced diagnostic tools developed in-house. This article gives an overview over the research and development projects in Switzerland related to materials and components for the PEFC.

Engineered Water Highways in Fuel Cells: Radiation Grafting of Gas Diffusion Layers

A novel method to produce gas diffusion layers with patterned wettability for fuel cells is presented. The local irradiation and subsequent grafting permits full design flexibility and wettability tuning, while modifying throughout the whole material thickness. These water highways have improved operando performance due to an optimized water management inside the cells.

Taming the polysulphide shuttle in Li–S batteries by plasma-induced asymmetric functionalisation of the separator

A novel microporous separator for lithium–sulphur batteries, in the form of an asymmetric membrane with cation-exchange functional groups, was synthesised by a one-step plasma-induced graft co-polymerisation. This separator consists of commercial porous polypropylene modified with styrene sulfonate at or near the surface of the material. Both successful grafting and membrane asymmetry were confirmed by attenuated total reflectance Fourier transform infrared spectroscopy. Morphological changes as a function of graft level were analysed using scanning electron microscopy. Additionally, the separators were tested for their ability to inhibit polysulphide diffusion, which showed a strong dependence on the amount of negatively charged –SO3− groups introduced onto the hydrophobic substrate. Cycling tests in model Li–S cells with the modified separators showed improved coulombic efficiency.

Polymer-bound antioxidants in grafted membranes for fuel cells

Proton conducting membranes for fuel cells containing antioxidants were synthesized by radiation grafting. Styrene and a “linker” monomer were pre-irradiation cografted onto an ETFE base film of 25 µm thickness. Tyramine, a phenol type antioxidant, was subsequently covalently attached to the “linker” units, and the styrene units were sulfonated. FTIR and SEM-EDX analyses of the synthesized films and membranes were performed to confirm uniform grafting, functionalization and sulfonation steps. The obtained membranes were characterized in terms of ion exchange capacity (IEC), water uptake and through-plane conductivity. The cografted membranes were assembled into a fuel cell and tested under accelerated aging conditions to assess their chemical stability. The membranes containing the tyramine moieties degraded substantially less compared to membranes lacking these phenolic groups. Post-test FTIR and IEC analyses confirmed the results of the fuel cell test, which supports the notion that the polymer-bound antioxidants effectively mitigate oxidative aging of the polymer electrolyte in a fuel cell.

Radiation-Grafted Polymer Electrolyte Membranes for Water Electrolysis Cells: Evaluation of Key Membrane Properties

Radiation-grafted membranes can be considered an alternative to perfluorosulfonic acid (PFSA) membranes, such as Nafion, in a solid polymer electrolyte electrolyzer. Styrene, acrylonitrile, and 1,3-diisopropenylbenzene monomers are cografted into preirradiated 50 μm ethylene tetrafluoroethylene (ETFE) base film, followed by sulfonation to introduce proton exchange sites to the obtained grafted films. The incorporation of grafts throughout the thickness is demonstrated by scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDX) analysis of the membrane cross-sections. The membranes are analyzed in terms of grafting kinetics, ion-exchange capacity (IEC), and water uptake. The key properties of radiation-grafted membranes and Nafion, such as gas crossover, area resistance, and mechanical properties, are evaluated and compared. The plot of hydrogen crossover versus area resistance of the membranes results in a property map that indicates the target areas for membrane development for electrolyzer applications. Tensile tests are performed to assess the mechanical properties of the membranes. Finally, these three properties are combined to establish a figure of merit, which indicates that radiation-grafted membranes obtained in the present study are promising candidates with properties superior to those of Nafion membranes. A water electrolysis cell test is performed as proof of principle, including a comparison to a commercial membrane electrode assembly (MEA).

Effects of cell and electrode design on the CO tolerance of polymer electrolyte fuel cells

Injection of low levels of oxygen or air, in the order of 1–5%, into the fuel stream is a popular method to obtain full CO tolerance to 100 ppm or more, in polymer electrolyte fuel cells (PEFC) operating on reformed, i.e. CO contaminated fuel. The susceptibility of the fuel cell towards this “O2 bleeding” technique was investigated with a focus on cell design and electrode structure effects. Two different cell constructions were explored, a one-dimensional (1-D) cell with homogeneous gas distribution over the active area, and a two-dimensional (2-D) cell with a gas channel flow field. It was found that if Pt–Ru is used as anode electrocatalyst instead of Pt, less oxygen in the fuel stream is required to achieve full tolerance to 100 ppm CO in H2. In the 2-D cell hardware, oxygen bleeding was less effective, probably due to shorter contact time. By using a bilayer anode, comprising a gas phase catalyst layer, it was possible to promote in situ catalytic CO oxidation, reducing thereby CO poisoning of the electrocatalyst and increasing the susceptibility towards O2 bleeding in the 2-D cell.