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Papers by Rostand Ngameni
Electrochimica Acta, 2011
Analysis of phase transformation processes observed in most hydrogen-absorbing materials (pure me... more Analysis of phase transformation processes observed in most hydrogen-absorbing materials (pure metals, alloys or compounds) is still a matter of active research, in view of electrochemical or chemical hydrogen storage applications. Most metal hydride systems present a large and irreducible hysteresis, indicating that non-linear phenomena are taking place during hydrogen sorption/desorption reactions. Investigation of hydriding kinetics therefore requires non-harmonic spectroscopies
International Journal of Hydrogen Energy, 2011
Proton exchange membrane (PEM) technology is used in water electrolysers, H2/O2 fuel cells and un... more Proton exchange membrane (PEM) technology is used in water electrolysers, H2/O2 fuel cells and unitized regenerative fuel cells (URFCs). Such electrochemical devices are of particular interest in view of the so-called hydrogen economy. Several prototypes (kW power range) have been developed and tested in the course of the GenHyPEM project (2005–2008), a STREP research program financially supported by the Commission of the European Communities in the frame of the sixth Framework Programme. The purpose of this communication is to report on the current status of each device in terms of performances and technological development. Current limitations and future perspectives are discussed.
International Journal of Hydrogen Energy, 2011
Permeation across metallic membranes is a process used in the industry for purifying hydrogen. In... more Permeation across metallic membranes is a process used in the industry for purifying hydrogen. In conventional technology, a few tens of micrometers thick metallic membranes made of palladium alloys are used in the 400e600 C temperature range, using a driving force of several bars for enhanced kinetics. In stationary conditions of flow, the diffusion-controlled transport of atomic hydrogen across the membrane is usually rate-determining. When thin (sub-micron thick) membranes are used, surface rate contributions become more significant. To optimize permeation performances, there is therefore a need for separately measuring surface and bulk rate contributions. In this communication, we report on the kinetics of hydrogen permeation across Pd 77 Ag 23 metallic membranes using pneumato-chemical impedance spectroscopy. The role of different operating parameters (temperature, surface state, membrane microstructure) on the kinetics of permeation is analyzed and discussed. (P. Millet).
International Journal of Hydrogen Energy, 2011
Isotherms Impedance a b s t r a c t
International Journal of Hydrogen Energy, 2010
Hydrogen permeation is a process used in the industry for purification purposes. Palladium alloys... more Hydrogen permeation is a process used in the industry for purification purposes. Palladium alloys (PdAg and PdCu) are commonly used as membrane material. In this communication, we report on the kinetics of hydrogen permeation across Pd0.47Cu0.53 metallic membranes which can be used in catalytic crackers of biofuels. The permeation mechanism is a multi-step process including surface chemisorption of molecular hydrogen (upstream side of the membrane), hydrogen diffusion across bulk regions, hydrogen recombination (downstream side of the membrane) and evolution. The role of different operating parameters (temperature, surface state, sample microstructure) is analyzed and discussed using both time and frequency domain experiments. Experimental pneumato-chemical impedance diagrams show that there is no significant rate-limitation at surfaces, except at low temperatures close to room temperature. Diffusion-controlled transport of hydrogen across the membrane is rate-determining. However, the value of the hydrogen diffusion coefficient does not rise exponentially with operating temperature in the 40–400 °C temperature range under investigation, as expected for a thermally activated diffusion process. At temperatures as low as 300 °C, new rate-limitations appear. They can be attributed to recrystallization and/or phase transformation processes induced by temperature and the presence of hydrogen.
International Journal of Hydrogen Energy, 2009
Hydrogen permeation across palladium-alloys membranes is an industrial process used for purificat... more Hydrogen permeation across palladium-alloys membranes is an industrial process used for purification purposes. In state of the art systems, several tens of microns thick metallic membranes can be used and rate limitations generally come from atomic H diffusion. Cost considerations (for example for application in the automotive industry) require a reduction of the membrane thickness and operation at lower temperature. In the micron-thick range, surface contributions become increasingly rate-determining. To optimize permeation membranes, there is therefore a need to separately measure surface and bulk rate contributions to the overall permeation process. In this paper, pneumato-chemical impedance spectroscopy (PIS) is used to analyze the dynamics of hydrogen permeation across Pd77Ag23 membranes. Experimental pneumato-chemical transfer functions of the membrane are measured at different temperatures. Model impedances are calculated and fitted to the experimental ones, yielding microscopic rate parameter values such as surface resistance and hydrogen diffusion coefficient.
International Journal of Hydrogen Energy, 2010
Proton Exchange Membrane (PEM) water electrolysis can be used to produce hydrogen from renewable ... more Proton Exchange Membrane (PEM) water electrolysis can be used to produce hydrogen from renewable energy sources and can contribute to reduce CO2 emissions. The purpose of this paper is to report on recent advances made in PEM water electrolysis technology. Results obtained in electrocatalysis (recent progresses made in low-cost electrocatalysis offer new perspectives for decentralized and domestic applications), on low-cost membrane electrode assemblies (MEAs), cell efficiency, operation at high current density, electrochemical performances and gas purity issues during high-pressure operation, safety considerations, stack design and optimization (for electrolyzers which can produce up to 5 Nm3 H2/h) and performance degradations are presented. These results were obtained in the course of the GenHyPEM project, a 39 months long (2005–2008) research program supported by the European Commission. PEM technology has reached a level of maturity and performances which offers new perspectives in view of the so-called hydrogen economy.
Review of Scientific Instruments, 2007
Hydrogen storage remains a bottleneck process on the way to the hydrogen economy. For practical a... more Hydrogen storage remains a bottleneck process on the way to the hydrogen economy. For practical applications, metal hydride systems offer interesting features, in particular, the possibility of reversibly storing large amounts of hydrogen at low or moderate pressure. However, they still suffer from unfavorable specific energy, with mass-fraction values ranging from 0 up to 5 wt % whereas transport applications require 6 wt % and more. Besides this, higher sorption/desorption kinetics and better chemical stability over long-term cycling are also needed. This is why many studies are carried out in the research community on hydride-forming systems, to develop new materials meeting these requirements. Development and optimization of metal hydride reactors require coupled thermodynamic and kinetic characterization of metal-hydrogen systems. In particular, it is necessary to analyze the kinetics in terms of reaction mechanism, in order to identify the different steps of commonly observed multistep reaction paths, and to measure their individual rate parameters. By analyzing hydriding kinetics in the frequency ͑Fourier͒ domain, pneumatochemical impedance spectroscopy ͑PIS͒ now offers the possibility of measuring experimental impedances and identifying reaction steps. However, measurement of such impedances is indirect and nontrivial. The purpose of this paper is to detail the experimental requirements needed for correctly measuring gas-phase impedance diagrams. In particular, practical conditions of data sampling and data treatment are described. Experimental results obtained with the model LaNi 5 -H 2 ͑g͒ system are presented to illustrate the potentialities of PIS analysis.
International Journal of Hydrogen Energy, 2009
Pneumato-chemical impedance spectroscopy (PIS) is a tool derived from electrochemical impedance s... more Pneumato-chemical impedance spectroscopy (PIS) is a tool derived from electrochemical impedance spectroscopy (EIS). PIS can be used to analyze the kinetics of various solid-gas reactions, such as the hydriding kinetics of metals in the presence of hysteresis.
Electrochimica Acta, 2011
Analysis of phase transformation processes observed in most hydrogen-absorbing materials (pure me... more Analysis of phase transformation processes observed in most hydrogen-absorbing materials (pure metals, alloys or compounds) is still a matter of active research, in view of electrochemical or chemical hydrogen storage applications. Most metal hydride systems present a large and irreducible hysteresis, indicating that non-linear phenomena are taking place during hydrogen sorption/desorption reactions. Investigation of hydriding kinetics therefore requires non-harmonic spectroscopies
International Journal of Hydrogen Energy, 2011
Proton exchange membrane (PEM) technology is used in water electrolysers, H2/O2 fuel cells and un... more Proton exchange membrane (PEM) technology is used in water electrolysers, H2/O2 fuel cells and unitized regenerative fuel cells (URFCs). Such electrochemical devices are of particular interest in view of the so-called hydrogen economy. Several prototypes (kW power range) have been developed and tested in the course of the GenHyPEM project (2005–2008), a STREP research program financially supported by the Commission of the European Communities in the frame of the sixth Framework Programme. The purpose of this communication is to report on the current status of each device in terms of performances and technological development. Current limitations and future perspectives are discussed.
International Journal of Hydrogen Energy, 2011
Permeation across metallic membranes is a process used in the industry for purifying hydrogen. In... more Permeation across metallic membranes is a process used in the industry for purifying hydrogen. In conventional technology, a few tens of micrometers thick metallic membranes made of palladium alloys are used in the 400e600 C temperature range, using a driving force of several bars for enhanced kinetics. In stationary conditions of flow, the diffusion-controlled transport of atomic hydrogen across the membrane is usually rate-determining. When thin (sub-micron thick) membranes are used, surface rate contributions become more significant. To optimize permeation performances, there is therefore a need for separately measuring surface and bulk rate contributions. In this communication, we report on the kinetics of hydrogen permeation across Pd 77 Ag 23 metallic membranes using pneumato-chemical impedance spectroscopy. The role of different operating parameters (temperature, surface state, membrane microstructure) on the kinetics of permeation is analyzed and discussed. (P. Millet).
International Journal of Hydrogen Energy, 2011
Isotherms Impedance a b s t r a c t
International Journal of Hydrogen Energy, 2010
Hydrogen permeation is a process used in the industry for purification purposes. Palladium alloys... more Hydrogen permeation is a process used in the industry for purification purposes. Palladium alloys (PdAg and PdCu) are commonly used as membrane material. In this communication, we report on the kinetics of hydrogen permeation across Pd0.47Cu0.53 metallic membranes which can be used in catalytic crackers of biofuels. The permeation mechanism is a multi-step process including surface chemisorption of molecular hydrogen (upstream side of the membrane), hydrogen diffusion across bulk regions, hydrogen recombination (downstream side of the membrane) and evolution. The role of different operating parameters (temperature, surface state, sample microstructure) is analyzed and discussed using both time and frequency domain experiments. Experimental pneumato-chemical impedance diagrams show that there is no significant rate-limitation at surfaces, except at low temperatures close to room temperature. Diffusion-controlled transport of hydrogen across the membrane is rate-determining. However, the value of the hydrogen diffusion coefficient does not rise exponentially with operating temperature in the 40–400 °C temperature range under investigation, as expected for a thermally activated diffusion process. At temperatures as low as 300 °C, new rate-limitations appear. They can be attributed to recrystallization and/or phase transformation processes induced by temperature and the presence of hydrogen.
International Journal of Hydrogen Energy, 2009
Hydrogen permeation across palladium-alloys membranes is an industrial process used for purificat... more Hydrogen permeation across palladium-alloys membranes is an industrial process used for purification purposes. In state of the art systems, several tens of microns thick metallic membranes can be used and rate limitations generally come from atomic H diffusion. Cost considerations (for example for application in the automotive industry) require a reduction of the membrane thickness and operation at lower temperature. In the micron-thick range, surface contributions become increasingly rate-determining. To optimize permeation membranes, there is therefore a need to separately measure surface and bulk rate contributions to the overall permeation process. In this paper, pneumato-chemical impedance spectroscopy (PIS) is used to analyze the dynamics of hydrogen permeation across Pd77Ag23 membranes. Experimental pneumato-chemical transfer functions of the membrane are measured at different temperatures. Model impedances are calculated and fitted to the experimental ones, yielding microscopic rate parameter values such as surface resistance and hydrogen diffusion coefficient.
International Journal of Hydrogen Energy, 2010
Proton Exchange Membrane (PEM) water electrolysis can be used to produce hydrogen from renewable ... more Proton Exchange Membrane (PEM) water electrolysis can be used to produce hydrogen from renewable energy sources and can contribute to reduce CO2 emissions. The purpose of this paper is to report on recent advances made in PEM water electrolysis technology. Results obtained in electrocatalysis (recent progresses made in low-cost electrocatalysis offer new perspectives for decentralized and domestic applications), on low-cost membrane electrode assemblies (MEAs), cell efficiency, operation at high current density, electrochemical performances and gas purity issues during high-pressure operation, safety considerations, stack design and optimization (for electrolyzers which can produce up to 5 Nm3 H2/h) and performance degradations are presented. These results were obtained in the course of the GenHyPEM project, a 39 months long (2005–2008) research program supported by the European Commission. PEM technology has reached a level of maturity and performances which offers new perspectives in view of the so-called hydrogen economy.
Review of Scientific Instruments, 2007
Hydrogen storage remains a bottleneck process on the way to the hydrogen economy. For practical a... more Hydrogen storage remains a bottleneck process on the way to the hydrogen economy. For practical applications, metal hydride systems offer interesting features, in particular, the possibility of reversibly storing large amounts of hydrogen at low or moderate pressure. However, they still suffer from unfavorable specific energy, with mass-fraction values ranging from 0 up to 5 wt % whereas transport applications require 6 wt % and more. Besides this, higher sorption/desorption kinetics and better chemical stability over long-term cycling are also needed. This is why many studies are carried out in the research community on hydride-forming systems, to develop new materials meeting these requirements. Development and optimization of metal hydride reactors require coupled thermodynamic and kinetic characterization of metal-hydrogen systems. In particular, it is necessary to analyze the kinetics in terms of reaction mechanism, in order to identify the different steps of commonly observed multistep reaction paths, and to measure their individual rate parameters. By analyzing hydriding kinetics in the frequency ͑Fourier͒ domain, pneumatochemical impedance spectroscopy ͑PIS͒ now offers the possibility of measuring experimental impedances and identifying reaction steps. However, measurement of such impedances is indirect and nontrivial. The purpose of this paper is to detail the experimental requirements needed for correctly measuring gas-phase impedance diagrams. In particular, practical conditions of data sampling and data treatment are described. Experimental results obtained with the model LaNi 5 -H 2 ͑g͒ system are presented to illustrate the potentialities of PIS analysis.
International Journal of Hydrogen Energy, 2009
Pneumato-chemical impedance spectroscopy (PIS) is a tool derived from electrochemical impedance s... more Pneumato-chemical impedance spectroscopy (PIS) is a tool derived from electrochemical impedance spectroscopy (EIS). PIS can be used to analyze the kinetics of various solid-gas reactions, such as the hydriding kinetics of metals in the presence of hysteresis.