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International audience; Electrochemical noise (EN) generated by a 3500 W PEM fuel cell stack (50 cells) has been measured during micro-cogeneration durability tests. The experiments have been provided during long term campaign of measurements (more than 1200 h and 100 h have been analyzed) in well controlled operational conditions following The U.S. Department of Energy (DOE) recommendations. The micro-cogeneration operation mode involves two different regimes, one at 5 A (0.02 A.cm-²) and the other 99 A (0.45 A.cm-²). Thus, recorded signals are non-stationary, under dynamic load and specific data treatment procedure has been developed to overcome this difficulty. For both currents robust and stable statistical noise descriptors have been obtained based on Power Spectral Density (PSD) in the frequency range 0.01 Hz < f < 103 Hz. The noise signature of these currents involves two fractional noises (1 / f α) with different slopes α and pronounced peaks, at the characteristic frequency f = 1.6 Hz. The obtained noise signature is similar to the one obtained previously with the same type of the stack under stationary operation mode. It was demonstrated that noise signature varies slightly during stack operation. It seems that the observed evolution of the noise signature is related with reversible phenomena because after sudden interruption of the stack operation the PSD spectra return back to its initial shape. The proposed methodology of data treatment demonstrates that EN can be used as a tool for diagnostics of possible faults in long-term operation of electrochemical sources of energy under non-stationary conditions.

Efficiency of fuel cell is dependent on reactant distribution, products evacuation, pressure losses and many of these factors is dependent on the design of flow field plate. With an effective design, reactant distribution, pressure drop, and water and heat management can be further improved. In this work, two new designs, as multi-serpentine set-up with additional slots and hybrid geometry, on stainless steel bipolar plates, are presented. Electrical performance, and pressure head losses are analyzed by electrochemical methods such as polarization curve and use of electrochemical noise as a diagnostic tool to further understand the impact of water management on performance. On the one hand, multi-serpentine design shows the best electrical performance with an increase of 0.2 V (66%) at 0.9 A/cm2 in comparison of traditional serpentine design. On the other hand, hybrid design reveals the lowest pressure head losses, with a decrease of 2 mbar (about 50%) in comparison of traditional serpentine design, and a higher stability with time that can be useful to downsize compressor and provide lower impact on fuel cell stack durability.

International audience; An estimation of low frequency parameters at the interface of a ferri-ferrocyanide solution on a Pt rotating disk electrode is presented. The determination of the total resistance and effective (low frequency) capacity is based on a Foster's model extrapolation using electrochemical impedance spectroscopy measurements. The feasibility of the proposed approach has been demonstrated at equilibrium conditions and under two polarization points. Results show a good agreement with the identified and experimental values particularly in the extra-low frequency domain. Results also demonstrate that using factorized expression (poles and zeros) of the impedance allow the stability of the identified parameters and the calculation of the first time-constant of the system. Consequently, Foster's models as equivalent to Voight-type equivalent circuit can be used for the zero frequency extrapolation and predict the response of the interface in time domain that it is certainly useful for practical electrochemical systems.

International audience; Stainless steels are broadly used thanks to their specific physical properties such as their high corrosion resistance in poorly aggressive solutions. However, only few studies have been reported in the literature concerning their electrochemical behavior in low concentration electrolytes medium. Accordingly, the present work aims to study the immersion time influence on the solid-liquid interface properties of the austenitic stainless steel AISI 304L, immersed in a low-concentrated (0.01 M) sodium chloride (NaCl) solution. The electrochemical behavior of the interface was evaluated by electrochemical impedance spectroscopy (EIS) and open circuit potential (OCP) monitoring. The morphological features and the modification of the surface composition were evaluated by optic microscopy, scanning electron microscopy, energy dispersive X-ray spectrometry, atomic force microscopy, white light interferometry and X-ray photoelectron spectroscopy. It was determined by OCP measurement that the characteristic time of the interface stabilization is very long (several months). After an immersion of 2 months in NaCl solution, a second time constant on impedance phase diagram appears. Surface characterizations reveal a significant modification of the morphology and chemistry of the AISI 304L surface that can be linked to OCP/ EIS observations. It can be noticed that the repeatability deviation of the EIS method was about 1 % while its reproducibility deviation was estimated to 35 %.

International audience; Electrochemical noise (EN) measurements have been provided with PEM fuel cell stack of 600 W nominal power. These measurements demonstrate that EN can be used as a tool for diagnostics of possible faults in long-term running of electrochemical sources of energy. The noise measurements were performed in well controlled operational conditions during 100 h. Robust and stable statistical noise descriptors of the stack have been obtained both in frequency and time domains. The spectral noise signature of the stack based has been obtained in the frequency range 0.1 <

International audience; Electrochemical noise (EN) generated by a PEM fuel cell stack (600 W, 8 cells with surface area 220 cm2) has been measured in well-controlled operational conditions following DOE recommendations for 100 h. For the first time, robust and stable statistical noise descriptors of a PEM fuel cell stack have been obtained based on PSD (power spectral density) spectra in the frequency range of 0.1 Hz

Electrochemical Impedance Spectroscopy (EIS) is widely used in different types of applications to model accurately the internal battery behavior. EIS requires very long time to obtain experimental data, especially for diffusion phenomenon (low frequency). Therefore, fast chronopotentiometry experiment represents an interesting approach to solve this problem. This paper deals with the identification of diffusion electrochemical impedance parameters in time domain using fractional systems where EIS diffusion spectrum is obtained through the identified parameters from chronopotentiometry data. An Equivalent Electric Circuit (EEC) modeling using fractional systems is used to identify diffusion parameters. The method is validated using an experimental electrochemical cell and it shows accurate estimation for the diffusion parameters. Consequently, the fractional system is used as an identification tool to study the impact of external parameters on the diffusion aspects such as polarization point and working electrode rotation speed.

Electrochemical noise analysis (ENA) technique has been performed for the diagnosis of proton exchange membrane fuel cell (PEMFC) under various operating conditions. The interest of electrochemical noise (EN) measurements relates with its non-invasive character and the possibility of online diagnosis of commercial fuel cells without interruption of the system. A new approach for the interpretation of electrochemical noise (EN) measurements has been proposed. This approach is based on internal intermittence of the recorded fluctuating signal (cell voltage). Namely, statistical descriptors in time domain (standard deviation, skewness, flatness), calculated for small time windows (short-time analysis), are rather unstable. This phenomenon can be called the internal intermittence of EN. Our experiments show that the level of internal intermittence is very sensitive to water management and increases drastically when a fuel cell meets either flooding or drying conditions. This conclusion has been confirmed using three statistical moments (standard deviation, skewness, and flatness), four different relative humidities, and three operation points (OCV, 2.5 A, 8 A). The level of internal intermittence can be detected easily by different ways and can be used for the characterization of possible faults of water management in practical applications as commercial fuel cell stacks. From a practical point of view, the measurement of the level of the internal intermittence is rather simple and avoids a time drift. In our knowledge, the internal intermittence of EN has not been used previously for studies of fuel cells. It will be interesting to apply this approach for other electrochemical systems and processes.

The paper presents an overview of the statistical theory of turbulent mass transfer in electrochemical systems and some new results which generalize the previously obtained relations for the flows of complex geometry. The developed theory does not use traditional semi-empirical hypotheses and analogies, but directly addresses to the solving of the critical for turbulent transfer the closure problem. The mathematical procedure for solving of the closure problem makes use of new equations for the correlations between concentration and velocity fluctuations in different space points and at different time moments; the dumping of turbulent pulsations in the viscous sublayer allows to neglect high order moments and obtain a closed equation for the turbulent mass flux. In general, the relation between the turbulent mass flux and the mean concentration gradient is non-local. Using available experimental information, the non-local equation for the turbulent mass flux is reduced to the traditional local one and the functional form of the turbulent diffusion coefficient is obtained. It is demonstrated that Reynolds analogy cannot been used for the prediction of the turbulent diffusivity. Applications of the developed theory to chemical engineering and to electrochemical flow diagnostics (prediction of flow characteristics using limiting diffusion current measurements) are discussed.

This work considers the possibility of applying electrochemical noise analysis to fuel cell diagnostics. Theoretical hypothesis and experimental result have shown that spectral characteristics of electrical fluctuations depend on water management processes inside the PEMFC. It has been established that the spectrum of electrical fluctuations in low frequency range has the nature of flicker noise. The frequency ranges of 0.1-1 Hz convenient for single cell as well as for stack diagnostics are revealed. The results show that the proposed approach can be considered as an e ffective tool to diagnose of fuel cells, namely allowing for prediction of drying and flooding.

Electrochemical noise analysis (ENA) has been performed for the diagnosis of proton-exchange membrane fuel cell (PEMFC) under various operating conditions. Its interest is related with the possibility of a non-invasive on-line diagnosis of a commercial fuel cell. A methodology of spectral analysis has been developed and an evaluation of the stationarity of the signal has been proposed. It has been revealed that the spectral signature of fuel cell, is a linear slope with a fractional power dependence 1/fα where α = 2 for different relative humidities and current densities. Experimental results reveal that the electrochemical noise is sensitive to the water management, especially under dry conditions. At RHH2 = 20% and RHair = 20%, spectral analysis shows a three linear slopes signature on the spectrum at low frequency range (f < 100 Hz). This results indicates that power spectral density, calculated thanks to FFT, can be used for the detection of an incorrect fuel cell water balance.

International audience; High frequency concomitant voltage and pressure measurements on a single proton exchange membrane fuel cell are presented in this paper. Voltage, anodic and cathodic pressure measurements were shown to be highly sensitive to gas relative humidity and temperature of the cell. Fast voltage dips corresponding to pressure spikes at the anode were observed and indicate a flooding occurrence due to the blockage of anode inlet by liquid water. In a generalize flooding of the cell, pressure drop fluctuations are easily recognizable and result in voltage fluctuations and drift. A methodology based on short‐time analysis on both, voltage and pressure signals, is used to quantify the effects of flooding and drying within proton exchange membrane fuel cell (PEMFC) systems. Intermittent behavior is highlighted on the standard deviation of the voltage for flooding and drying cases. Simultaneous analysis of the standard deviation short‐time for voltage and pressure signals allows for the differentiation between flooding and drying cases. Concomitant pressure and voltage fluctuations can be easily detected and monitored for a reliable diagnostic of proton exchange membrane fuel cell systems.

Electrochemical noise diagnostics (END) has been applied to commercial Li-ion batteries. This diagnostics is based on recording small voltage fluctuations of the battery during discharge. In order to avoid apparatus noise the direct measurements of voltage have been used without filtering and statistical descriptors of process have been calculated, namely standard deviation (STD), skewness (SK) and kurtosis (KU). It was demonstrated, as a function of State of Charge (SOC), that the noise increases drastically at the beginning (SOC > 97%) and the end (SOC < 4%) of the discharge. The noise is quasi-uniform and non-Gaussian with SK = 0 and KU =–1.2. In the boundaries between the beginning and the middle and between the middle and the end, an important variations of SK and KU have been detected.

A promising direction for improving proton exchange membrane fuel cells (PEMFCs) is the increase of their reliability and operating time without failure. Therefore, the problem of a reliable diagnostic tool is important. It has been theoretically deduced that a non uniform current distribution causes the increase of power spectral density of the flicker-noise component of electrical fluctuations, which was experimentally confirmed for the case of excess humidification. Theoretical investigations allow to conclude that other reasons of current non uniformity, for example incorrect stoichiometry, can lead to similar results as well. Experimental findings show that the behavior and parameters of electrical fluctuations depend on the fuel cell operation mode and technical state of its components. This paper shows that electrical fluctuations reflect physical processes inside the fuel cell. Thus, the electrical fluctuations can be considered as a reliable diagnostics tool for PEMFC.

A novel method for in situ temperature measurement inside a PEMFC cell is explored. This technique is based on the tantalum micro resistive thermal sensors technologies. The temperature sensors used in this work are robust, good chemical resistant, ductile and easily to introduce in homemade or commercial MEAs. Real time thermal and electrical behaviors of the cell are investigated. Measurements show that as function of the fuel cell working regime (constant voltage or constant current), more or less important overheat are possible inside the cell and that the active layer temperature can be really different (several degrees higher) than the temperature of bipolar plate. This, lead to a better understanding of the fuel cell operation and can help the improvement of fuel cell test protocols and metrology.

One of the main disadvantages of the modern fuel cells (FC) is their relatively short non-failure operating time, and prediction of its technical condition is one of the most promising ways to solve this problem. Therefore, the development of effective methods applied to fuel cell diagnostics becomes extremely important. In this paper new approach for FC diagnostics based on the Prony's spectral analysis of the quasi-periodic (QP) processes is proposed. The results of FC voltage (current) signals analysis are presented. It is shown that the proposed approach is able to evaluate the FC technical state. The proposed method allows to increase the accuracy of the fluctuating processes description and can be considered as a reliable diagnostic tool for PEMFC.

International audience; Electrochemical Impedance Spectroscopic (EIS) represents an important technique to characterize the impedance of electrochemical systems. The internal impedance phenomena can be modeled using Electrical Equivalent Circuit. Fractional operator is used to characterize the charge transfer and the diffusion phenomena. This may be attractive due to the ability of fractional systems to be simulated either in frequency or time domain. The impedance model using fractional systems is represented by a transfer function that is identified using Levenberg-Marquardt algorithm. The fractional model identification is validated on an experimental test-bench using two platinum electrodes immersed into Ferri-Ferro solution. Results indicate the ability of fractional system to identify properly the different impedance parameters. In order to decrease the long experiment time of EIS, minimization of the low frequency measurement points is performed and results show the ability of fractional systems to identify EIS data.

International audience; In this paper, identification of the diffusion impedance parameters of an electrochemical cell in time domain is presented. As Electrochemical Impedance Spectroscopy (EIS) requires long time to obtain experimental data for low frequencies, a method using chronopotentiometric experiments is studied in order to reach more quickly diffusion parameters. The electrochemical cell is modeled using an equivalent electrical circuit describing separately the charge transfer and the diffusion phenomena. The electrochemical model provides then a transfer function whose parameters can be identified only in the frequency domain. Thus, thanks to a fractional operator approximation, a half order integrator is able to perform simulations in the time domain. Therefore, a fractional model is proposed in order to identify the slow dynamics due to diffusion thanks to the Levenberg-Marquardt algorithm. Experimental results obtained from an electrochemical test-bench including Ferri-Ferro solution demonstrate the interest of fractional modeling. Diffusion parameters are identified in the time domain and cross-validation in the frequency domain using EIS measurements demonstrates the relevance of the proposed method.

Nanostructured electrocatalysts for hydrogen electrochemical systems with solid polymer electrolyte were synthesized by the platinum magnetron–ion-beam sputtering onto nanostructured carbonaceous support Vulcan XC-72 premetallized by the impregnation–reduction method. To be able to perform comparative analysis, a number of mono- and bimetallic catalyst samples was synthesized using impregnation–reduction, a traditional method. Thus prepared catalysts were examined by thermogravimetric method, transmission electron microscopy, X-ray diffraction analysis, X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and cyclic voltammetry. The electrodes prepared with the synthesized catalysts were tested in fuel cell and water electrolyzer with solid polymer electrolyte. This study confirmed the possibility of preparation of multicomponent catalysts with complicated structures (such as the core–shell structure) over nanostructured carbon support by magnetron sputtering and demonstrated the effectiveness of their performance as parts of electrochemical systems with solid polymer electrolyte. The effect of additional ionbeam processing of the Pt/C electrocatalysts on their efficiency is also studied.

Conducting an experimental investigation, we show that the electrical conductivity σ (Sm −1 ) of the gas diffusion layer (GDL) and bipolar plates in stainless steel foam (SSF), as the heart of a Proton Exchange Membrane Fuel Cell (PEMFC), depends in fact strongly on mechanical compression arising in an operating system. Thus, this mechanical compression significantly affects the current density and σ; while it is not so far introduced in modelling and performance analysis. By mean of a developed metrology for this purpose and using Van der Pauw method, we studied, upon applied mechanical loads by varying the compression p , the electrical properties of different layers of a carbon cloth for GDL, of SSF, and, as in operating fuel cell, of the combination GDL + SSF in a sandwich form. A strong dependency is observed for each of these materials and their combination up to saturation for high enough p which is seen then to reduce the electrical resistance; while the number of layers has no influence. The obtained results are analysed in term of rate of increase of σ with an interesting conclusion for their application.

Electrochemical noise analysis (ENA) is performed on ICR 18650 commercial lithium-ion batteries. The interest of ENA relates with the possibility of in-situ diagnostics during charging or discharging of the battery. Thus, the extraction of small voltage fluctuations should take into account the time evolution of the mean signal. The non-stationary character of the phenomenon (charging and discharging battery) limits the use of traditional methods of signal filtering and attenuation, so a special methodology has been developed to calculate the noise standard deviation (STD). A good reproducibility of the results has been demonstrated, and V-shape form curves have been obtained with a minimum STD value at about 55 % of state of charge (SOC). It can be noted also that fast discharge provided with 3.3 Ω load is noisier than the slow one with 5 Ω load. Some promising results have been obtained regarding the possibility of battery state of health determination.

Carbon-supported platinum electrocatalysts for electrochemical systems with a proton-exchange membrane were prepared by ion plasma sputtering in a DC magnetron sputtering unit. Vulcan XC-72, carbon nanotubes, and nanofibers were used as substrates. The starting substrates were occasionally subjected to preliminary chemical treatment or metallization. The synthesized electrocatalyst samples were analyzed by thermogravimetric, X-ray diffraction, and X-ray fluorescent analysis, scanning and transmission microscopy, and cyclic voltammetry. The electrochemical activity of the synthesized Pt/C and Pt-Pd/C materials was studied in a fuel cell, water electrolyzer, and bifunctional fuel cell with a solid polymer electrolyte. Several samples were found to have high specific surface areas (up to 44 m2/g), chemical purity, and electrochemical activity. The qualitative dependences of the characteristics of the catalysts on the parameters of metal deposition and the properties of the starting substrate were found.

Calibration of electrodiffusion (ED) probes with respect to the wall velocity gradient measurements has been experimentally studied in a fully developed turbulent channel flow over the Reynolds number range 14000–23000. In steady state conditions, direct calibration concerning the mean wall velocity gradient can be provided using simultaneous transient and steady state diffusion limiting current measurements. Indirect calibration of the ED probes regarding the turbulent fluctuation has been undertaken using a spectral analysis performed with probes of different size and geometry. This indirect calibration method has been supported using the measurements of thermo-dependence of molecular diffusivity. In this case, only one ED probe can be used for calibration. Dynamic calibration provides necessary information about critical longitudinal dimension of ED probes. When the longitudinal dimension of the probes exceeds the critical length, the probes becomes sensitive to the normal fluctuation as well. This fact has been confirmed using different orientation of the rectangular ED probe with respect to the flow direction.

The quasi-distributed sensor allowing measuring of steady-state temperature fields has been proposed. The sensor comprises of thin sensing resistive elements arranged in grid structure. The sensor can be used in the system wherein only external boundary is available for measurement equipment connection such as PEMFC membrane electrode assembly. The proposed sensor can be adapted to measure other physical fields such as mechanical tension, deformations, etc.

A microscopic model of electrification phenomena in steady state and transient regimes has been developed. The model has been applied for analysis of experimental data failing to be explained in the frame of classical macroscopic approach. Various boundary conditions have been studied regarding the possibility of experiential data fitting. The hybrid boundary condition has been suggested for explaining steady state and transient experiments. The proposed boundary condition can be related to the coexistence of two reactions on the active surface. The first reaction runs under the diffusion control (fast reaction) and the second one is limited by the kinetics (slow reaction).

In this study we present, analyze and compare the power spectral density of the wall shear stress in a turbulent plane channel flow obtained with different techniques. Experimentally the instantaneous wall shear stress was measured with the electrochemical technique using different probes, which give approximately the same results after applying the transfer function for correction of the probe’s inertia. Numerically, the time evolution of the wall shear stress has been determined using direct numerical simulations (DNS) and large eddy simulations (LES). The results of DNS are in a good agreement with the electrochemical flow measurements. However the power spectra of the wall shear stress obtained with LES shows deviations with respect to DNS at high frequencies because of the spatial filtering inherent to the LES technique.

Power spectral density (PSD) of wall velocity gradient fluctuations has been measured using electrochemical (electrodiffusion) method in two rectangular channels with different hydraulic diameters and aspect ratio. The problem of the probes inertia at high frequency range has been smoothed by means of the transfer function; the reliability of the obtained results is justified by using of two circular probes with different diameter. For studied range of Reynolds numbers (6 × 103 < Reh < 4 × 104) all spectra functions tend to a unique curve when a suitable dimensionless coordinates are used. These coordinates depend on the mean value of the velocity gradient S, the intensity of the velocity gradient fluctuations \(\overline {s'^2 }\) and the liquid viscosity ν, only. Thus, the obtained results reveal the similitude of the turbulence in the viscous sublayer with respect to Re number and flow geometry. Sure, this hypothesis should be tested with more representative base of experimental data. The revealed similitude can be important for chemical engineering applications. Indeed, for these applications the Reynolds numbers are rather small and correspond to the studied range and the intensity of the velocity gradient fluctuations are well related with the mean value of the velocity gradient value. So, the knowing of the mean velocity gradient S allows the prediction of the PSD of the velocity gradient fluctuations, if the viscosity of the fluid ν is known.

International audience; The present study focused on the improvement of some aspects of the membrane electrode assembly (MEA) fabrication and on some working conditions of the SAMFC (Solid Anionic Membrane Fuel Cells) fed with glycerol. The fuel solution composition has a great importance. Higher performances were achieved with 1 M glycerol +6 M NaOH solution composition. Further increase of the glycerol concentration led to transport limitations due to the increasing of the mixture viscosity. The electrical performance of a SAMFC reached the maximum value with a glycerol flow of 10 mL min(-1); the oxygen flow rate displayed no significant influence on the electrical performance. The temperature has a great effect on the fuel cell performance. The anion exchange membrane available in the present study led to higher performance in the temperature range between 60 and 70 degrees C. The fuel cell measurements performed with monometallic Pt/C. Pd/C materials, and bimetallic PtPd/C, PtBi/C and PdBi/C compounds as anode catalysts showed encouraging results with respect to the decrease of platinum loading and elaboration of Pt-free catalysts. Low platinum loaded Pt5Pd5/C and non-platinum based Pd9Bi1/C catalysts allowed reaching fuel cell performances very close or even higher than those obtained with a Pt/C catalyst at 25 degrees C and 60 degrees C.

A thermo-electrical model of fuel cell proton exchange membrane is developed and the stability of the system is studied. The marginal curves have been obtained as a function of governed dimensionless parameters. The determined dimensionless parameters allow analyzing different scenarios of non-controlled membrane heating. It has been shown that a positive thermo-electrical feedback can cause instability development and membrane damage in real systems. Recommendations for the improvement of heat management in fuel cells have been proposed using the provided stability study. In particular, it has been demonstrated that decreasing of gas diffusion layer thickness has positive effect and stabilizes the system.

An advanced optical technique based on digital version of μ-PIV and speckle technologies has been applied for flow microstructure analysis in a semi-transparent model of a PEM fuel cell. The line-of-sight speckle photography has been used for quantitative measurements of the instantaneous temperature fields in 3D unsteady flows. Using μ-PIV, the sequence of flow images has been visualised by micro-particles. The images obtained have been evaluated then by cross-correlation and autocorrelation techniques and instantaneous temperature, velocity and vorticity maps have been obtained with sub-millimeter spatial resolution. Up to 250,000 quantitative velocity vectors are reconstructed simultaneously using one digitally recorded specklogram.

Accurate information on the temperature field and associated heat transfer rates is particularly important for proton exchange membrane fuel cells (PEMFC) and PEM electrolyzers. An important parameter in fuel cell and electrolyzer performance analysis is the effective thermal conductivity of the gas diffusion layer (GDL) which is a solid porous medium. Usually, this parameter is introduced in modeling and performance analysis without taking into account the dependence of the GDL thermal conductivity λ (in W · m−1 · K−1) on mechanical compression. Nevertheless, mechanical stresses arising in an operating system can change significantly the thermal conductivity and heat exchange. Metrology allowing the characterization of the GDL thermal conductivity as a function of the applied mechanical compression has been developed in this study using the transient hot-wire technique (THW). This method is the best for obtaining standard reference data in fluids, but it is rarely used for thermal-conductivity measurements in solids. The experiments provided with Quintech carbon cloth indicate a strong dependence (up to 300%) of the thermal conductivity λ on the applied mechanical load. The experiments have been provided in the pressure range 0 < p < 8 MPa which corresponds to stresses arising in fuel cells. All obtained experimental results have been fitted by the equation λ = 0.9log(12p + 17)(1 − 0.4e−50p ) with 9% uncertainty. The obtained experimental dependence can be used for correct modeling of coupled thermo/electro-mechanical phenomena in fuel cells and electrolyzers. Special attention has been devoted to justification of the main hypotheses of the THW method and for estimation of the possible influence of the contact resistances. For this purpose, measurements with a different number of carbon cloth layers have been provided. The conducted experiments indicate the independence of the measured thermal conductivity on the number of GDL layers and, thus, justify the robustness of the developed method and apparatus for this type of application.

A linear elastic–plastic 2D model of fuel cell with hardening is developed for analysis of mechanical stresses in MEA arising in cell assembly procedure. The model includes the main components of real fuel cell (membrane, gas diffusion layers, graphite plates, and seal joints) and clamping elements (steel plates, bolts, nuts). The stress and plastic deformation in MEA are simulated with ABAQUS code taking into account the realistic clamping conditions. The stress distributions are obtained on the local and the global scales. The first one corresponds to the single tooth/channel structure. The global scale deals with features of the entire cell (the seal joint and the bolts). Experimental measurements of the residual membrane deformations have been provided at different bolts torques. The experimental data are in a good agreement with numerical predictions concerning the beginning of the plastic deformation.

The transient hot-wire technique is widely used for measurements of the thermal conductivity of most fluids. However, for some particular liquids such as concentrated nitric acid solutions or similar nitric mixtures, for which the thermal properties are important for industrial or security applications, this technique can be difficult to use, which is essentially due to incompatibility between measurement probe materials and highly electrically conducting and corrosive liquids. Moreover, the possible highly energetic (explosive) character of these liquids requires minimum volume liquid samples, and safety measurement devices and processes. It is the purpose of this paper to report on a new patented instrument, based on tantalum short-hot-wire probe technology, which responds to the above requirements and allows safe automated thermal-conductivity measurements of concentrated acid nitric solutions and similar nitric mixtures for liquid samples less than 2 cm3, with uncertainties better than 5%.

In this article, we have studied the possibility of using carbon-based screen-printed probes for electrochemical flow and mass flux measurements. Such probes have, up to now, been mainly used as biosensors to study in vivo reactions. Our study shows that screen-printed sensors allow the measurement of mass flux and mean wallp shear stress with good accuracy and high reproducibility. The existence of a micro-porous layer covering the surface of screen-printed electrodes has been revealed by means of the impedance measurements and has been confirmed by Scanning Electron Microscopy. This layer influences the statistical characteristics of the turbulent limiting diffusion current and should be taken into account as an additional transfer function between the current and the velocity fluctuations. The use of screen-printed sensors opens new possibilities in the field of electrochemical flow diagnostics. The advantage of this new manufacturing technology lies in the possibility of serial mass production at very competitive prices (disposable sensors) combined with the possibility of the manufacture of segmented electrodes or matrix of electrodes. These new possibilities can be used for various industrial applications as well as for scientific studies on near-wall turbulent mass transfer. Furthermore, it is possible to envisage the development of a hybrid sensor (electrodiffusional/biochemical) allowing the study of in situ biochemical reactions in a flow.

We propose theoretical model of the heat transfer in the axisymmetric swirl pipe flows. The model is used to study influences of the type of vortex symmetry and of the vorticity distribution in the vortex core on the heat transfer enhancement. The study shows that traditional empirical correlations for swirl flows are, in general, insufficient. Indeed, two types of vortex structures can exist in the swirl flows with the same integral characteristics: vortices with left-handed and right-handed helical symmetry. Left-handed helical vortexes generate wake-like swirl flows and increase heat transfer in comparison with the axial flow. Right-handed vortex structures generate jet-like swirl flows and can diminish heat transfer. Two major factors of the heat transfer enhancement have been identified: (i) formation of the swirl flow with left-handed helical vortex; (ii) modification of the near wall velocity profile of the inviscid flow due to the different vorticity distribution in the vortex core.

parameters. However, in contrast to axial non-swirling flows, the comparison [4] of experimental data [1–4] for various set-ups and flow regimes did not reveal the generalized character of Eq. (1) for swirling flows. To verify the applicability of Eq. (1) to mass transfer in swirling flows, we consider the problem of mass transfer from a swirling flow to a microelectrode flash mounted with the wall of a cylindrical tube of radius R (corresponding measurements were reported, e.g., in [4]). In order to simplify the mathematical formulation of the problem, we examine only axisymmetrically developed flow regimes (turbulent pulsations and threedimensional effects are ignored). Let the distance z0 from the entrance of the cylindrical section to the microelectrode be so small that the boundary layer developed in this length is thin (its thickness δ ! R) and Sh cReSc, Re Ud ν -------. = =

Electrochemical method is applied to study coherent structures in a complex three-dimensional flow generated by a rotating magnetic rod. In order to detect these structures, the power spectral density (PSD) analysis of the limiting diffusion current fluctuations is provided. This analysis reveals the existence of resonance frequencies when the probe is located in the very vicinity of the stirrer. The main resonance is related to the mechanical excitation frequency. The detailed analysis of the PSD curves points out some mechanisms of development of turbulence as sub-harmonics and non-linear interactions. For different locations of the probe and different angular rates of the stirrer Ω, a similarity of the energy repartition is obtained when the PSD is plotted versus the adimensional frequency f/Ω. Flow visualization by laser tomography is also provided and the local secondary flow past the electrode probe is sketched. The laser Doppler velocimetry (LDV) at a given location in a horizontal z-plane indicates a tornado wise mean flow structure.

The measurements of the limiting diffusion current and its fluctuations are used to study mass transfer near the stagnation region in a tornado wise flow generated by a magnetic rod stirrer. The paper aims to investigate what kind of information could be provided by the electrochemical method when the probe is submerged in the flow. The power spectral density (PSD) of the current pulsations was measured regard to three angular rotation rates of the stirrer and four different locations on a radius. The analysis of the PSD revealed that the resonance frequencies obeyed to a sub-harmonic interaction mechanism. The existence of the sub-harmonics with their odd harmonics testify the role of some entertained eddies. As for the order of magnitude of the time average value of mass transfer parameters, the diffusion layer thickness right to the electrode and the mass transfer coefficient were calculated respectively to be about 10 μm and 0.10 mm s −1 . The turbulence intensity relative to the fluctuations of the limiting current, varies between 11% and 20%, in comparison with the value of 10% commonly retained in planar flows. These values seem account for the three-dimensional complexity of the flow.

Resume Les sondes electrochimiques tri-segmentees sont couramment utilisees pour la mesure des composantes des contraintes parietales. La variation du courant de chaque segment en fonction de l'angle d'ecoulement θ, donne les caracteristiques directionnelles bien connues pour une sonde ideale de geometrie parfaite. L'utilisation de ces caracteristiques « ideales » entraine une erreur sur l'angle θ, de l'ordre de 15 ∘ . Cette note vise a ameliorer la precision de mesure sur θ, en faisant appel aux caracteristiques directionnelles calculees a partir de la photographie d'une sonde. Les courbes caracteristiques directionnelles obtenues permettent d'ameliorer la precision des mesures de l'angle d'ecoulement de 50 %. Pour citer cet article : F. Barbeu et al., C. R. Mecanique 330 (2002) 433–436.

A mathematical model of mass transfer in the boundary layer of swirl flow is developed. The equations obtained for Sherwood number and mass transfer coefficient generalize the equations obtained by Levich for the flat plate to the case of mass transfer in axisymmetrical swirl flow. For the first time it is shown that, in swirl flows, the mass transfer is not completely controlled by integral flow parameters (Reynolds number and the swirl number) but depends essentially on the type of vortex symmetry. The left-handed helical vortices generate wake-like swirl flows and increase mass transfer in comparison with axial flows. The right-handed helical vortices generate jet-like swirl flows, which can decrease mass transfer.

International audience; The present study deals with PEM fuel cells, namely with the optimization of the hot pressing process for membrane electrode assembly (MEA) fabrication. Designs of experiments (DoE) have been used for evaluating the effect of hot pressing parameters (pressure, temperature, and time) on the MEA electrical performances. Full factorial 2(3) DoE showed that the most important parameter is the pressing temperature. Surface response methodology indicated a non-monotonous behavior of the MEA electrical performances with respect to the pressing temperature. The MEA electrical performances increased with the pressing temperature in the temperature range from 100 to 115 A degrees C, and decreased significantly in the temperature range from 115 to 130 A degrees C. This behavior was attributed to drastic changes of the NafionA (R) 112 membrane properties and membrane/electrode interface over this temperature range. Observations of the MEA cross-section structure by scanning electron microscopy confirmed such hypotheses. Thermo-mechanical properties of NafionA (R) as determined by dynamic scanning calorimetry allowed estimating the glass transition temperature at ca. T (g) a parts per thousand aEuro parts per thousand 117 A degrees C in the conditions of the present study. The higher H-2/air fuel cell performance of ca. 0.8 W cm(-2) was obtained with the optimized pressing temperature for MEA fabrication of ca. 115 A degrees C close to the T (g) temperature of NafionA (R) 112, whereas for higher temperature the structure of the NafionA (R) membrane and of the membrane-electrode interface is damaged.

In situ continuous recording of glass alteration is a difficult experimental problem. In this paper, an electrochemical method is proposed for monitoring the concentrations of dissolved ions in the altering solution. This method was tested in a real case (alteration of a binary glass in water at room temperature). The possibility to measure local concentrations in the vicinity of the glass/solution interface is an interesting prospective of the method.

A model describing mass transfer from a swirling flow toward walls of a cylindrical reactor is developed for the first time. The model explains a possible decrease in the mass transfer caused by the flow swirling.

In the water-cooled blanket concept designed for fusion reactors, the liquid Pb-17Li alloy surrounding the water pipes is used to produce the tritium required for a self-sustaining fusion reaction. During operation, the steel box which acts as the liquid metal container can be corroded by the lithium–lead alloy flowing at low velocity. The corrosion products dissolved in the alloy are then transported with the flow. In some regions, characterised for example by low temperatures, these corrosion products can crystallise and form aggregates which can be deposited on the wall and can contribute to the plugging of the ducts. The mechanisms involved in corrosion, formation of aggregates and deposition depend on several factors such as hydrodynamics, solubility, kinetics of exchange at the solid/liquid interface, roughness of the wall, … In addition, they can also be affected by the high external magnetic field used to confine the plasma. The present paper gives an overview of the different types of possible magnetic field effects on the corrosion and deposition processes in the flowing liquid alloy.

In this paper, a coupled model of transfer phenomena within Proton Exchange Membrane Fuel Cell (PEMFC) developed from Stefan-Maxwell (in the diffusion and active layers), Butler-Volmer (in the active layer), and water mass transport (in the electrolyte membrane) equations is presented. This modeling allows interpreting experimental results, prediction of PEMFC electrical performances and guiding perspective investigations on optimization of PEMFC. The model helps the research of dominating sensitivity parameters, as well as the estimation of some badly known MEA (Membrane Electrode Assembly) parameters using fuel cell tests.