Features of the Degradation of the Proton-Conducting Polymer Nafion in Highly Porous Electrodes of PEM Fuel Cells.

The stability of new membrane-electrode assemblies of a proton-exchange membrane fuel cell with highly porous electrodes and low Pt loading, based on the proton-conducting polymer Nafion, was characterized in conditions of electrochemical aging. A comprehensive study of the effect of the microstructure on the evolution of the electrochemical characteristics of the new assemblies was obtained by voltammetry, electrochemical impedance spectroscopy, X-ray powder diffraction, and scanning electron microscopy. Because high (>70%) porosity provides intensive mass transfer inside an electrode, structural-modifying additives-long carbon nanotubes-were introduced into the new electrodes. PEM fuel cells with electrodes of a conventional composition without carbon nanotubes were used for comparison. The aging of the samples was carried out according to the standard accelerated method in accordance with the DOE (Department of Energy) protocols. The results show two fundamental differences between the degradation of highly porous electrodes and traditional ones: 1. in highly porous electrodes, the size of Pt nanoparticles increases to a lesser extent due to recrystallization; 2. a more intense "washout" of Nafion and an increase in ionic resistance occur in highly porous electrodes. Mechanisms of the evolution of the characteristics of structurally modified electrodes under electrochemical aging are proposed.

Control of perovskite film crystallization and growth direction to target homogeneous monolithic structures.

Getting performant organo-metal halide perovskite films for various application remains challenging. Here, we show the behavior of solvent and perovskite elements for four different perovskites families and nine different initial precursor solution systems in the case of the most popular preparation process which includes an anti-solvent dripping-assisted spin coating of a precursor solution and a subsequent thermal annealing. We show how the initial solution composition affects, first, the film formed by spin coating and anti-solvent dripping and, second, the processes occurring upon thermal annealing, including crystal domain evolution and the grain growth mechanism. We propose a universal typology which distinguishes three types for the growth direction of perovskite crystals: downward (Type I), upward (Type II) and lateral (Type III). The latter results in large, monolithic grains and we show that this mode must be targeted for the preparation of efficient perovskite light absorber thin films of solar cells.

What Can Glow Discharge Optical Emission Spectroscopy (GD-OES) Technique Tell Us about Perovskite Solar Cells?

The emerging broad range of applications of the glow discharge optical emission spectroscopy (GD-OES) technique in the field of perovskite solar cells (PSCs) research is reviewed. It can provide a large palette of information by easily and quickly tracking the depth distribution of light to heavy elements. After a discussion of the advantages and the limitations of the technique and a comparison with other analytical techniques, how GD-OES is employed to give structural information on perovskite solar cells is shown. GD-OES has allowed the full perovskite film formation process investigation, from the initial precursor layers containing soaking and complexed solvent to the final crystallized 3D perovskite layers. The A-site elemental cations distribution is followed-up during the film formation. In addition, this technique gives a deep insight into the action mechanism of additives and their effects on the film formation. It provides fruitful information on optimized light absorbing layers and on the selective contact layers which ensure the charge transport in PSCs. It allows to directly visualize halide ions migration and their blocking by ad-hoc chemical engineering and to study the films and PSCs ageing. GD-OES opens new perspectives to explain the final performances of the devices.

Corrosion Inhibition at Scribed Locations in Coated AA2024-T3 by Cerium- and DMTD-Loaded Natural Silica Microparticles under Continuous Immersion and Wet/Dry Cyclic Exposure.

Earlier studies on cerium-loaded naturally occurring silica microparticles (i.e., diatomaceous earth) demonstrated the potential to efficiently protect small scratches in epoxy-coated AA2024-T3 panels during relatively short immersion times. The current work investigates the potential of such inhibitor-loaded microparticles to protect wide and deep scribes (up to 1 mm wide) in long-time immersion testing and during cyclic (wet/dry) conditions. For this, cerium nitrate and 2,5-dimercaptothiadiazole (DMTD) were used as inorganic and organic corrosion inhibitors. The corrosion protection was evaluated using a hyphenated real-time optics/electrochemistry method and two individual local techniques measuring oxygen concentration and electrochemical impedance (LEIM) inside the scribe. SEM/EDS was used to analyze the samples after exposure. The results show significant levels of corrosion protection at damaged locations at low cerium concentrations (3.7 wt % Ce3+ relative to the total coating mass) during 30 days of immersion in salt solution. However, for a given scribe geometry, the protection was found to be dependent on the electrolyte volume with larger electrolyte/exposed metal ratios leading to short protection time. A partial replacement of the Ce3+ by DMTD in the microcarriers resulted in a higher degree of passivation than when DMTD was used alone. Wet/dry cyclic exposure tests showed that cyclic conditions can increase the buildup of stable inhibitor-containing layers in the case of cerium-loaded silica microparticles. This underlines the need for more research using wet/dry exposure conditions.

The effect of absorbed hydrogen on the dissolution of steel.

Atomic hydrogen (H) was introduced into steel (AISI 1018 mild steel) by controlled cathodic pre-charging. The resultant steel sample, comprising about 1 ppmw diffusible H, and a reference uncharged sample, were studied using atomic emission spectroelectrochemistry (AESEC). AESEC involved potentiodynamic polarisation in a flowing non-passivating electrolyte (0.6 M NaCl, pH 1.95) with real time reconciliation of metal dissolution using on-line inductively coupled plasma-atomic emission spectroscopy (ICP-OES). The presence of absorbed H was shown to significantly increase anodic Fe dissolution, as evidenced by the enhanced detection of Fe2+ ions by ICP-OES. We discuss this important finding in the context of previously proposed mechanisms for H-effects on the corrosion of steels.

Revisiting the electrochemical impedance spectroscopy of magnesium with online inductively coupled plasma atomic emission spectroscopy.

The electrochemical impedance of reactive metals such as magnesium is often complicated by an obvious inductive loop with decreasing frequency of the AC polarising signal. The characterisation and ensuing explanation of this phenomenon has been lacking in the literature to date, being either ignored or speculated. Herein, we couple electrochemical impedance spectroscopy (EIS) with online atomic emission spectroelectrochemistry (AESEC) to simultaneously measure Mg-ion concentration and electrochemical impedance spectra during Mg corrosion, in real time. It is revealed that Mg dissolution occurs via Mg(2+) , and that corrosion is activated, as measured by AC frequencies less than approximately 1 Hz approaching DC conditions. The result of this is a higher rate of Mg(2+) dissolution, as the voltage excitation becomes slow enough to enable all Mg(2+) -enabling processes to adjust in real time. The manifestation of this in EIS data is an inductive loop. The rationalisation of such EIS behaviour, as it relates to Mg, is revealed for the first time by using concurrent AESEC.