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Gas diffusion electrodes for electrochemical devices frequently consist of polytetrafluroethylene (PTFE) and carbon black-supported catalysts. The hydrophilic or hydrophobic character determines the wetting behaviour, which is important for media transport in the gas diffusion electrodes. The main components of the gas diffusion electrodes are an organic binder (typically PTFE) and a catalyst (for applications with acid electrolyte, a noble metal catalyst) that frequently is supported by carbon black. The chemical surface compositions of electrodes prepared in different ways from a mixture of carbon black and PTFE and mechanically and thermally modified were determined by x-ray photoelectron spectroscopy (XPS). The XPS measurements show differences between electrodes wet-prepared from a suspension and electrodes prepared with the dry preparation technique of DLR (Deutsches Zentrum für Luft-und Raumfahrt). During preparation of the electrodes by the dry technique, the carbon black is enriched on the surface. Mechanical modification reduces the enrichment of the carbon black and tempering increases the PTFE concentration on the surface. The results of the XPS measurements were related to the wetting behaviour: a concentration of ∼20 wt.% PTFE marks the transition between hydrophilic and hydrophobic surfaces.

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The Nb/α−Al2O3 system has been used as a model study for investigating the stability of different MBE grown epitaxial Nb films on α−Al2O3 substrates. The films were grown at 800 °C in ultrahigh vacuum. The growth process was monitored in situ by reflection high energy electron diffraction (RHEED). After deposition the structure of the film was investigated by x-ray diffraction (XRD) and conventional transmission electron microscopy (CTEM) which encompasses also selected area diffraction (SAD). Both techniques revealed the following orientation relationship between the Nb film and the α–Al2O3 substrate: (0001)α–Al2O3‖ (111)Nb; [2110]α–Al2O3‖ [110]Nb. The stability of the niobium films was investigated by annealing the Nb-film/α–Al2O3 system to temperatures up to 1500 °C for different periods of time. Surprisingly, the orientation relationship between the Nb film and the substrate changed to (0001)α–Al2O3‖ (110)Nb; [0110]α–Al2O3‖ [001]Nb. A model will be developed which shows that above a critical film thickness the growth orientation is metastable with respect to its crystallographic orientation. Furthermore, high resolution transmission electron microscopy (HREM) was performed to investigate the defect structure of the annealed Nb/α–Al2O3 interface.

Gas diffusion electrodes can be operated in a lot of electrochemical processes and devices, such as fuel cells, electrolysis and cleaning of waste water from electrochemical treatments of metals (electrochemical machining, electrochemical drilling and galvanic processes). Carbon black-supported platinum is mixed with polytetrafluoroethylene (PTFE) in a knife mill and rolled onto a metal web to form an electrode. The electrodes were characterized before and after electrochemical operation with x-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Induced by the rolling process of the PTFE-carbon black powder, the PTFE forms thin fibres that are spun to a web in the electrode.The surface composition of the electrodes for electrochemical metal treatment and cleaning of galvanic waste water changes during the operation time. Owing to the presence of metal ions in the liquid electrolyte in these applications, metal is deposited onto the electrode surface during the operation time.Depth profiling of the electrodes shows that metal is not only deposited on the surface but also in the electrodes.

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