Stress and plastic deformation of MEA in running fuel cell

Numerical modelling of mechanical stresses in running fuel cell is provided. The evolution of stresses and plastic deformations in the membrane has been obtained during the turn-on phase. The operating conditions have been taken into account by imposing the heating sources and the humidity field. The results have been presented on two scales: the global scale reflects the stress evolution in the entire fuel cell and the local one corresponds to the tooth/channel structure. It has been shown that the stresses are strongly heterogeneous on the both scales and time dependent. From the mechanical point of view, the most sensible zone is under the GDL/seal joint interface. In the running fuel cell the heterogeneity decreases while it rises in the fuel cell just after the assembly. The stresses reach the maximum values during the humidification step, the magnitude of these stresses is sufficient for initiation of the plastic deformations in the Nafion membrane.     

Temperature recovery and contrast computations in NDE thermographic imaging systems

Thermographic imaging systems find application in many nondestructive evaluation problems. However, because of the inherent nature of the image-formation process, several degradations arise which preclude a more generalized use of thermographic NDE. In this paper, we analyze radiometric and noise degradations and propose a calibration process for the imaging systems which allows recovery of object surface temperature evolution. Moreover, a constrast computation technique which combines both the time and spatial reference techniques is shown to increase the defect visibility and make possible a more quantitative (sizing) defect evaluation.     

PWR steam generator R&D in France: thermal-hydraulic and vibratory aspects

Concomitant with the launching of the French pressurized water reactor (PWR) nuclear power program, a large research and development (R&D) effort was initiated, devoted to the steam generators (SGs). This program, managed cooperatively by Framatome, the SG designer and manufacturer; Electricité de France (EDF), the French electrical utility; and the Commissariat à l'Energie Atomique (CEA), the French Atomic Energy Commission, primarily responsible for nuclear research; was focused on four main objectives:

1. (1) To obtain a better understanding of the physical phenomena existing in these steam generators and leading to SG performance alterations or operating life reductions.

2. (2) To test and validate improved design solutions for the model 51 Framatome steam generator, which was the first one designed under Westinghouse license.

3. (3) To test and validate new Framatome SG designs.

4. (4) To test and validate new, high-performance design tools.

This vast R&D program covers the following theses:

• - SG thermal-hydraulics,

• - SG tube vibration and wear,

• - SG materials (production, corrosion, etc.),

• - Primary and secondary fluid chemistry,

• - SG technology (manufacturing processes, NDT, etc.),

• - SG in-service inspection, and

• - SG maintenance.

These themes are too numerous to be dealt with in a single article. Consequently, the present article will focus on only the first two themes.     

Proton-conducting ionic liquid-based Proton Exchange Membrane Fuel Cell membranes: The key role of ionomer-ionic liquid interaction

The paper deals with the synthesis and characterisation of proton-conducting ionic liquids (PCILs) and their polymer electrolytes obtained by blending modified Nafion membranes with different concentrations of PCILs. The PCILs are obtained by the neutralization of triethylamine with different organic acids. The first part of the paper studies the influence of acidity and acid structure on PCIL thermal and electrochemical performance, while the second part examines membrane conductivity and reveals it to depend more on PCIL structure than on its intrinsic conductivity. At 130 °C, conductivities exceeding 10 mS cm⁻¹ were obtained in fully anhydrous conditions.     

Development and operation of alternative oxygen electrode materials for hydrogen production by high temperature steam electrolysis

High temperature steam electrolysis (HTSE) is one of the most promising ways for hydrogen mass production. To make this technology suitable from an economical point of view, each component of the system has to be optimized, from the balance of plant to the single solid oxide electrolysis cell. At this level, the optimization of the oxygen electrode is of particular interest since it contributes to a large extent to the cell polarization resistance. The present paper is focused on alternative oxygen electrode materials with improved performances compared to the usual ones mainly based on perovskite structure. Two nickelates, with compositions La₂NiO_4+δ and Nd₂NiO_4+δ are investigated and evaluated in HTSE operation at the button cell level. The performances of the Ln₂NiO_4+δ - containing cells (Ln = La, Nd) is improved compared to a cell containing the classical Sr-doped LaMnO₃ (LSM) perovskite oxygen electrode showing that nickelates are promising candidates for HTSE oxygen electrodes, especially for operation below 800 °C. Indeed, current densities determined at 1.3 V are 1.1 times larger for the La₂NiO_4+δ - containing cell and 1.6 times larger for the Nd₂NiO_4+δ one compared to the LSM - containing cell at 850 °C, whereas at 750 °C they are 1.8 and 4.4 times larger, respectively. Thanks to the use of a reference electrode, by coupling impedance spectroscopy and polarization measurements, the overpotential of each working electrode is deconvoluted from the complete cell voltage under HTSE operating conditions.     

Structural studies on γ-MnO2 by transmission electron microscopy

Single-crystal electron diffraction patterns were obtained on five specimens of -MnO₂: one natural, two electrolytical (EMD) and two chemical (CMD) samples. EMDs are best described by the orthorhombic structure proposed by De Wolff which is derived from the ramsdellite structure. A CMD prepared from MnCO₃ fits the hexagonal cell of -MnO₂. Flaky grains from the natural sample and fibres from a CMD prepared from Mn(NO₃)₂ are hexagonal with a new cell:a 0.494,c 0.539 nm. No simple relation between chemical composition, morphology and structure could be found.     

As(V) retention and As(III) simultaneous oxidation and removal on a MnO2-loaded polystyrene resin

Based on KMnO4 oxidative capacity, a polystyrene matrix loaded with manganese dioxide was synthesized from an anionic commercial resin in chloride form. This medium, called R-MnO2, was tested for As(V) retention and for As(III) simultaneous oxidation and removal. Equilibrium was reached in 2 h and isotherms showed that R-MnO2 maximal capacities towards As(III) and As(V) are, respectively, 0.7 and 0.3 mmol/g. Various mechanisms were involved in As(III) retention: oxidation of H3AsO3(0) by MnO2(s) leading to the formation of HAsO4(2)- and Mn2+, fixation of As(V) formed on the resin beads and precipitation of Mn3(AsO4)2 with Mn2+ released. Successive arsenic desorption and retention steps were performed and showed that the quantity desorbed was low compared to the quantity removed during the first stage of the process. A second removal step, carried out under the same conditions as the first one, proved that the matrix second-removal capacity was weak. This solid sorbent, although not reusable, can be considered in field application as arsenic retention is really strong.     

Mössbauer resonance studies on the Ca2Fe2O5-LaFeO3 system

Phases between Ca₂Fe₂O₅ and LaFeO₃ with La_1?2yCa_2yFe³⁺O_3?y formulation have been prepared in order to study the oxygen defects in perovskite-related ferrites. X-ray-diffraction analysis and magnetic measurements have confirmed the previous results i.e. the existence of a critical concentration of vacancies above which long range ordering appears. A detailed Mössbauer resonance study shows a continuous evolution of the iron environment, tetrahedra being formed even for low values of y. This result has been discussed and compared with previous ones for homologous CaTi_1?2yFe_2yO_?y phases.     

Cohesiveness and hydrodynamic properties of young drinking water biofilms

Drinking water biofilms are complex microbial systems mainly composed of clusters of different size and age. Atomic force microscopy (AFM) measurements were performed on 4, 8 and 12 weeks old biofilms in order to quantify the mechanical detachment shear stress of the clusters, to estimate the biofilm entanglement rate ξ. This AFM approach showed that the removal of the clusters occurred generally for mechanical shear stress of about 100 kPa only for clusters volumes greater than 200 μm³. This value appears 1000 times higher than hydrodynamic shear stress technically available meaning that the cleaning of pipe surfaces by water flushing remains always incomplete. To predict hydrodynamic detachment of biofilm clusters, a theoretical model has been developed regarding the averaging of elastic and viscous stresses in the cluster and by including the entanglement rate ξ. The results highlighted a slight increase of the detachment shear stress with age and also the dependence between the posting of clusters and their volume. Indeed, the experimental values of ξ allow predicting biofilm hydrodynamic detachment with same order of magnitude than was what reported in the literature. The apparent discrepancy between the mechanical and the hydrodynamic detachment is mainly due to the fact that AFM mechanical experiments are related to the clusters local properties whereas hydrodynamic measurements reflected the global properties of the whole biofilm.