Application of the modified Dubinin-Astakhov equation for a better understanding of high-pressure hydrogen adsorption on activated carbons

International audienc

Application of the modified Dubinin-Astakhov equation for a better understanding of high-pressure hydrogen adsorption on activated carbons

International audienc

Effect of spark plasma sintering and high-pressure torsion on the microstructural and mechanical properties of a Cu–SiC composite

This investigation examines the problem of homogenization in metal matrix composites (MMCs) and the methods of increasing their strength using severe plastic deformation (SPD). In this research MMCs of pure copper and silicon carbide were synthesized by spark plasma sintering (SPS) and then further processed via highpressure torsion (HPT). The microstructures in the sintered and in the deformed materials were investigated using Scanning Electron Microscopy (SEM) and Scanning Transmission Electron Microscopy (STEM). The mechanical properties were evaluated in microhardness tests and in tensile testing. The thermal conductivity of the composites was measured with the use of a laser pulse technique. Microstructural analysis revealed that HPT processing leads to an improved densification of the SPS-produced composites with significant grain refinement in the copper matrix and with fragmentation of the SiC particles and their homogeneous distribution in the copper matrix. The HPT processing of Cu and the Cu-SiC samples enhanced their mechanical properties at the expense of limiting their plasticity. Processing by HPT also had a major influence on the thermal conductivity of materials. It is demonstrated that the deformed samples exhibit higher thermal conductivity than the initial coarse-grained samples

Development of a hybrid hydrogen compressor : electrochemical at low pressure/ adsorption at high pressure

La preuve de concept d’un compresseur non-mécanique d’hydrogène a été réalisée dans le cadre de ce travail de thèse. Le système étudié est hybride puisqu’il est constitué de (i) une première étape de compression électrochimique, qui comprime l’hydrogène de 1 bar jusqu’à 40-80 bar et ; (ii) une deuxième étape de compression par adsorption-désorption qui complète la compression jusqu’à 700 bar. Des modèles numériques ont été développés pour vérifier la faisabilité d’un tel système, et leur validité a été prouvée par les données expérimentales obtenues avec les prototypes réalisés pour chacune des deux étapes de compression. Concernant l’étape de compression électrochimique, un profil de densité de courant le long du compresseur électrochimique a été observé à l’aide d’une cellule segmentée, et le modèle pseudo-2D développé a permis de prouver que la stabilité de la densité de courant dépend fortement de la teneur locale en eau de la membrane. En effet, il a été observé une diminution de la densité de courant de 0.75 à 0.65 A/cm2 entre l’entrée et la sortie du compartiment basse pression. Cette variation correspond à une diminution du taux d’humidité dans le flux d’hydrogène, de 90 à 55%, le long des canaux de distribution des réactifs côté anodique (à 0.66 A/cm2 x 0.36 V et à 333 K). Concernant l’étape de compression par adsorption-désorption, le modèle modifié de Dubinin-Astakhov (MDA) a été mis en œuvre pour décrire l’adsorption d’hydrogène sur des charbons actifs en fonction des conditions de température et de pression. Cette loi, associée aux bilans de masse et d’énergie ont permis d’étudier la faisabilité d’un tel compresseur. Les résultats de modélisation ont été validés par comparaison avec des données expérimentales obtenues grâce à un prototype de 0.5 L, conçu et construit pour ce travail de thèse, et contenant 0.135 kg de charbon actif MSC-30 (Kansai, Japon). Lorsque le réservoir est rempli d’hydrogène à 80 bar et 77K, son réchauffement jusqu’ à 315 K permet d’obtenir des débits de 30 NL/h à 700 bar. Le compresseur hybride proposé pourrait être une alternative valable aux compresseurs mécaniques placés dans des installations décentralisées telles que les stations-service d’hydrogène de faible ou moyenne capacité.The proof of concept of a non-mechanical hydrogen compressor has been carried out in the present study. It is a hybrid compressor since it consists of: (i) a first electrochemical compression step, which compresses hydrogen from 1 bar up to 40-80 bar and; (ii) a second compression step based on the thermally-driven cyclic adsorption-desorption which allows compressing hydrogen up to 700 bar. Numerical models have been developed to verify the feasibility of such a system, and their validity has been proved by the experimental data obtained with the prototypes built for each of the two compression stages. Concerning the electrochemical compressor, a current density distribution along the electrochemical was observed using a segmented cell, and the developed pseudo-2D model proved that the stability of the current density strictly depends on the local water content of the membrane. Indeed, the current density was found to decrease from 0.75 A/cm2 to 0.65 A/cm2 between the first and the last segment of the compressor, which corresponds to a decrease of the relative humidity in the inlet hydrogen flow from 90% to 55% along the gas channels at the anode side (at 0.66 A/cm2 x 0.36 V and 333 K). Concerning the adsorption-desorption compressor, the Modified Dubinin-Astakhov model (MDA) was implemented to describe hydrogen adsorption on activated carbons as a function of the temperature and the pressure. It was used along with the mass and the energy balance equations to study the feasibility of such a compressor. The results from the numerical simulation were validated with the experimental data, which were obtained using a prototype of 0.5 L, designed and built for the present study, and containing 0.135 kg of the activated carbon MSC-30 (Kansai, Japan). 30 NL/h of high-pressure hydrogen at 700 bar were obtained when introducing hydrogen at 80 bar into the compressor, previously cooled to 77 K, and when heating it up to 315 K. The proposed hybrid hydrogen compressor could be a valid alternative to traditional mechanical compressors, and it could be used in small and decentralized facilities using hydrogen as a fuel, e.g. a hydrogen refuelling station

Experimental evidence of local heterogeneities in a PEM Electrochemical Hydrogen Compressor

International audienceElectrochemical hydrogen compressor (EHC) has proven to be a valid solution for the development of Fuel Cell Vehicles and hydrogen refueling stations. Hydrogen pressures up to 1000 bar can be reached by using an EHC. However, low compression efficiencies are obtained at very high pressures, mainly because of difficulties in sealing and hydrogen back diffusion through the polymer electrolyte membrane (PEM). Moreover, the performances of an EHC are affected by local dehydration of the PEM. A segmented cell of active area 30 x 1 cm 2 and divided into 20 segments of 1.5 cm 2 was used to investigate the local behavior of an EHC. Such a system allowed measuring the current density along the channel direction and the resistances of each single segment by impedance spectroscopy, making possible the evaluation of the local water transport through the PEM. In fact, heterogeneities in the distribution of the electric resistances were observed, reflecting the unbalanced contribution of the two water transport mechanisms along the PEM, i.e. electro-osmosis and water back diffusion. A pseudo 2D model, which simulates the mass and energy transfer occurring in the EHC, was developed along with the experimental investigation. It was observed that these heterogeneities can significantly alter the efficiency of an EHC. Hence, several parameters affecting the performance of an EHC, such as the humidity of the inlet flow, temperature, PEM thickness, discharge pressure and stoichiometric ratio, were investigated in order to optimize the system and enhance its overall efficiency

Operating heterogeneities in a PEM Electrochemical Hydrogen Compressor

International audienceIn this study, the local behavior of an electrochemical hydrogen compressor (EHC) was investigated. A local dehydration of the polymer electrolyte membrane (PEM) was experimentally observed, due to the unbalanced contribution of the electro-osmosis flow and the back diffusion of water across the membrane. Such operating heterogeneities can significantly affect the overall efficiency of an EHC. A pseudo 2D model was developed along with experimental studies in order to estimate the physical parameters enhancing the overall efficiency of the system. 1. Introduction Electrochemical hydrogen compressor (EHC) has proven to be a valid solution to compress hydrogen. Even though pressures up to 1000 bar can be reached using an EHC [1], very high pressure gradients can cause issues related to sealing and the diffusion of dissolved hydrogen molecules across the membrane from the high pressure cathode to the low pressure anode, decreasing the overall efficiency. Rohland et al. [2] showed that hydrogen permeation across the membrane is a function of both the pressure gradient and the temperature. In particular, they showed that the higher the EHC temperature, the higher the hydrogen permeation. Grigoriev et al. [3] showed that it is possible to compress hydrogen from atmospheric pressure to 48 bar in a single step with an energy consumption of 0.3 kWh/Nm 3 and an efficiency around 50%. This value is in average higher than those obtained with mechanical compressors [4]. Water management is an important issue in an EHC. As in proton electrolyte membrane fuel cells (PEMFCs), the polymer electrolyte membrane (PEM) has to be hydrated in order to enhance its proton conductivity. Nevertheless, contrarily to PEMFCs, water is not a reaction product in an EHC, thus it needs to be fed along with hydrogen in order to preserve the optimal hydration degree of the membrane. Onda et al. [5] found that both the hydrogen concentration in the gas distribution channels and the current density distribution decrease along the channel direction during operation. This behavior was found to be directly related to the water transport across the membrane. Heterogeneities in the distribution of the electric resistances are the main consequence of the unstable water flow across the membrane: specifically, the local dehydration of the membrane can lead to an increase of the electric resistance of the system, which in turn makes the current density decreasing. In this study, the local behavior of an EHC is investigated and the effect of the humidity of feed gas, temperature and membrane thickness was evaluated and discussed. A pseudo 2D model taking into account the overall mass and energy balance occurring in the EHC, as well as the heterogeneities introduced above, was developed along with the experimental investigation

Development of a hydrogen electrochemical compression system for Joule-Thomson Cryocooler

Among the multiple complexities of extraterrestrial observation, there is the fact that a sensor of radio or infrared waves emits electromagnetic radiation. It is thus necessary to neutralize these waves by the extreme cold to improve the quality of the observation. The hydrogen electrochemical compressor developed in this study in collaboration with the European Space Agency, which has all the advantages of non-mechanical compressors1,2, would produce hydrogen at high pressure which would then be introduced into a Joule-Thomson expansion. This process would generate enough cold, up to -253°C. At this temperature, hydrogen is in a liquid state, which allows to cool the sensors and to neutralize the electromagnetic radiation they produce. The major challenge of such an electrochemical compression system remains water management3. Indeed, it is fundamental that the hydrogen is humified to improve the charge transfer through the polymer membrane. On the other hand, the high-pressure hydrogen stream that is produced must be rigorously dry before the Joule-Thomson expansion. The possibility of using a countercurrent membrane water exchanger in series with the electrochemical compressor is explored, which allows the produced hydrogen stream (which is compressed and wet) to be dried and the dry low-pressure hydrogen stream to be humidified at the same time. In this study, the design of an electrochemical hydrogen compressor operating between 1 and 100 bar, following a flow rate of 10 mg/s of hydrogen, is performed. The characterization of the base materials, in particular the polymer membrane and the gas diffusion layers, has been performed to identify the best solution to improve the energy efficiency of the system4,5. For this purpose, sintered titanium diffusion layers with different porosities were used, as well as different PFSA membranes. At the same time, a temperature gradient between the two compressor compartments was applied to avoid water condensation in the system. This solution improves the stability of the system. The performances obtained with the different materials and under different operating conditions will be presented in terms of efficiency, durability and compactness

Développement d’un compresseur électrochimique pour des applications aérospatiales

Parmi les mille complexités de l'observation extraterrestre, il y a le fait qu'un capteur d'ondes radio ou infrarouges émet des radiations électromagnétiques. Il est donc nécessaire de neutraliser ces ondes par le froid extrême pour améliorer la qualité de l'observation. Le compresseur électrochimique développé dans le cadre de cette étude en collaboration avec l'Agence Spatiale Européenne produirait de l'hydrogène à haute pression qui serait ensuite introduit dans une expansion Joule-Thomson. Ce processus générerait suffisamment de froid, jusqu'à - 253 °C. À cette température, l'hydrogène est à l'état liquide, ce qui permet de refroidir les capteurs et de neutraliser le rayonnement électromagnétique qu'ils produisent. Le défi majeur d'un tel système de compression électrochimique demeure la gestion de l'eau. En effet, il est fondamental que l'hydrogène soit humifié pour améliorer le transfert de charge à travers la membrane polymère. En revanche, le flux d'hydrogène à haute pression qui est produit doit être rigoureusement sec avant l'expansion Joule-Thomson. La possibilité d'utiliser un échangeur d'eau à membrane à contre-courant en série avec le compresseur électrochimique est explorée, ce qui permet de sécher le flux d'hydrogène produit (qui est comprimé et humide) et d'humidifier en même temps le flux d'hydrogène sec à basse pression. Dans cette étude, la compression de l’hydrogène de 1 à 100 bars a été réalisée en une seule étape. Un débit d'hydrogène de 6 NL/h à 100 bar et 20 °C a été obtenu en fournissant 3 W de puissance électrique dans une cellule élémentaire. Néanmoins, l'optimisation de l'efficacité énergétique de la compression de l'hydrogène nécessite également la sélection des matériaux les plus appropriés ainsi que des stratégies pour optimiser la gestion de l'eau. Des couches de diffusion en titane fritté avec différentes porosités ont été utilisées, ainsi que différentes membranes PFSA. Parallèlement, un gradient de température entre les deux compartiments du compresseur a été appliqué pour éviter la condensation de l'eau dans le système. Cette solution permet d'améliorer la stabilité du système. Les performances obtenues avec les différents matériaux et dans différentes conditions de fonctionnement seront présentées

Modélisation des mécanismes de réaction au sein d’électrodes à oxygène La2_2NiO4+δ_{4+δ}

International audienceLe matériau d'électrode sur-stoechiométrique en oxygène La$_2$NiO$_{4+δ}$ (LNO) est considéré aujourd'hui comme une solution alternative aux électrodes à oxygène classiques à base de pérovskite