Influence of thermal post-curing on the degradation of a cross-linked polybenzimidazole-based membrane for high temperature polymer electrolyte membrane fuel cells

The lifetime stability of membranes is one of the main requirements regarding reliability of high temperature polymer electrolyte membrane fuel cells. The present work has improved durability under cycled operation by thermal post-curing of cross-linked polybenzimidazole (PBI)-based membranes. The membranes were dried over 1, 2 and 3 h at 250 degrees C under air. Ex-situ experiments proved an increase in stability by post-curing. The liquid uptake and swelling in phosphoric acid increased with longer curing periods. The effect of thermal treatments on cycle stability, lifetime and begin-of-life performance of the membrane electrode assemblies (MEAs) was investigated. Longer post-curing periods of the membranes had no influence on the MEAs' begin-of-life performance and constant current behavior over 2300 h. However, the 3 h post-cured MEAs showed enhanced cycle stability. Post-mortem analysis was carried out to identify the occurring degradation mechanisms. While a significant loss of phosphoric acid and a reduction of electrochemical surface activity on the cathode were observed for both post-cured MEAs, the 3 h dried membrane sample had a significantly higher resistance against pinhole formation during the long term test. Altogether, this work presents thermal post-curing as a promising method for the reduction of degradation determining effects in fuel cell membranes. (C) 2014 Elsevier B.V. All rights reserved

Influence of the size and shape of silica nanoparticles on the properties and degradation of a PBI-based high temperature polymer electrolyte membrane

The life Lime stability of membrane material is one of the major parameters regarding reliability of high temperature polymer electrolyte membrane fuel cells. Present work has improved fuel cell durability and chemical stability by incorporating cross-linked silica particles in phosphoric acid doped poly(2,2`-m-phenylene-5,5'-bibenzimidazole) membranes. Three different silica particle contents were generated in membranes by in-situ sol-gel reaction from the precursor tetraethoxy silane and cross-linked to the polymer chains by using (3-glycidoxypropyl)-methyldiethoxysilane. The size, shape and distribution of the silica nanoparticles were examined by transmission electron microscopy. The amorphous characteristics and the chemical composition of the silica particles were investigated using X-ray diffraction, electron diffraction and energy dispersive X-ray spectroscopy. Detailed statistical analysis showed that by increasing the tetraethoxy silane content, the particle size was reduced while the amount of particles was increased. Ex-situ membrane characterization and in-situ membrane electrode assembly testing revealed a high influence of the silica content on the mechanical stability and start-stop-cycling behavior. The improved lifetime durability of the organic-inorganic composite membrane was proven in comparison to the pure polybenzimidazole membrane in membrane electrode assemblies over 1300 h under constant fuel cell operation in reformate. (c) 2013 Elsevier B.V. All rights reserved

Influence of membrane type and molecular weight distribution on the degradation of PBI-based HTPEM fuel cells

The degradation of high-temperature proton exchange membrane fuel cells with phosphoric acid-doped polybenzimidazole (PBI) based membranes is one of the key challenges for early commercialization. The aim of this work is to analyze the influence of different reinforcement strategies on the performance and degradation of PBI-based membranes. The effects on the membranes were analyzed by polarization curves, long term operation under constant load, electrochemical impedance spectroscopy, in-situ cyclic voltammetry and electron microscopy techniques. The results show that the molecular weight distribution of the PBI and the different types of reinforcement have a distinct influence on the poisoning of the cathode layer and the mass transport resistance of the cathode microporous layer. The reasons are discussed in detail and a new degradation mechanism is proposed. A significant reduction of the degradation was obtained by enhancing the interactions between the PBI polymer chains via cross-linking

Active stabilization for optically synchronized optical parametric chirped pulse amplification

The development of new high power laser sources tends toward optical parametric chirped pulse amplification (OPCPA) in recent years. One of the difficulties in OPCPA is the the temporal overlap between pump and seed pulses. In this work we characterize our timing jitter on a single-shot basis using spectrally resolved cross-correlation in combination with a position sensitive detector. A commercial beam stabilization is adapted to actively enhance temporal overlap. This delay-stabilizer reduces the RMS jitter from σ = 127fs down to σ = 24fs. The enhanced temporal overlap is demonstrated in our frontend and we propose the scheme to be applicable in many optically synchronized high-repetition-rate OPCPA systems

Carrier-envelope-phase-stable, 1.2 mJ, 1.5 cycle laser pulses at 2.1 mu m

We produce 1.5 cycle (10.5 fs), 1.2 mJ, 3 kHz carrier-envelope-phase-stable pulses at 2.1 μm carrier wavelength, from a three-stage optical parametric chirped-pulse amplifier system, pumped by an optically synchronized 1.6 ps Yb:YAG thin disk laser. A chirped periodically poled lithium niobate crystal is used to generate the ultrabroad spectrum needed for a 1.5 cycle pulse through difference frequency mixing of spectrally broadened pulse from a Ti:sapphire amplifier. It will be an ideal tool for producing isolated attosecond pulses with high photon energies