Solid-state hydrogen storage and generation : from sodium borohydride NaBH4 to hydrazine borane N2H4BH3 : catalysis, kinetic and mechanisms

Parmi les procédés de stockage d’hydrogène étudiés actuellement, le stockage solide de l’hydrogène dans les hydrures chimiques, associée à sa génération par une réaction de solvolyse, est une technologie prometteuse. La première partie de cette thèse s’articule donc autour de l’étude de la solvolyse de deux composés étudiés ces dernières années, le borohydrure de sodium NaBH4 et l’ammoniaborane NH3BH3. Le dégagement contrôlé d’hydrogène peut alors se faire par des réactions d’hydrolyse. La comparaison de la cinétique de la réaction d’hydrolyse du NaBH4 avec celle de la méthanolyse du NaBH4 nous a permis de décrire ces réactions avec le modèle de Langmuir-Hinshelwood. Concernant la réaction d’hydrolyse du NH3BH3, nos recherches se sont focalisées sur la préparation in situ de catalyseurs présentant de fortes activitéspar l’étude des hydrolyses spontanées et catalysées de mélanges NH3BH3-NaBH4. La seconde partie de la thèse est consacrée au développement d’un nouveau système N2H4BH3-eau pour la génération d’hydrogène. Les premiers essais, réalisés avec des métaux de transition comme catalyseur, nous ont permis de mettre en évidence que cette réaction se faisait en deux étapes catalytiques, l’hydrolyse de BH3, puis la décomposition de N2H4.Une faible sélectivité pour la décomposition complète de N2H4 étant atteinte dans ces conditions, la suite de l’étude a porté sur la préparation de catalyseurs sélectifs. La stratégie adoptée a été l’utilisation de matériaux bimétalliques Ni-Pt. La sélectivité de la réaction est alors dépendante de la teneur en Pt et une sélectivité maximum de 93 % pour la seconde étape de la réaction a été obtenue avec Ni0,89Pt0,11Hydrogen use as a potential alternative solution to fossil fuels is hindered by engineering problems, its storage being one of the most prominent. Various storage methods are under investigation but solid-state storage in chemical hydrides appears to be convenient with regards to their storage capacities, safety and cost. The first part of this thesis deals with the solvolysis reaction of two well known compounds, sodium borohydride NaBH4 and ammonia borane NH3BH3. The hydrogen can be easily released by hydrolysis at ambient temperature. We focused on understanding the kinetics and reaction mechanisms of NaBH4 hydrolysis. Thus, we compared this reaction with NaBH4 methanolysis, and found that the Langmuir-Hinshelwood model well captures the kinetics of the reaction. Concerning the NH3BH3 hydrolysis reaction, we concentrated our efforts on the in situ preparation of highly-active catalysts. This was achieved by studying the spontaneous and catalyzed hydrolysis of NaBH4-NH3BH3 mixtures. The second part of the thesis is dedicated to the development of the N2H4BH3-water system for hydrogen generation. Initial tests using transition metals as catalysts allowed us to determine that the reaction takes place in two steps, the hydrolysis of BH3 and the N2H4 decomposition. Since Rh as catalyst exhibits only a 29 % selectivity for the complete decomposition of N2H4, the strategy was set up to use Ni-Pt bimetallic nanoparticles. It has been found that the selectivity for the reaction is dependent on the Pt content in the Ni-Pt alloy and a selectivity of 93 % was reached in the presence of Ni0,89Pt0,11 nanoparticle

Effect of ZIF-8 Crystal Size on the O2 Electro-Reduction Performance of Pyrolyzed Fe–N–C Catalysts

International audienceThe effect of ZIF-8 crystal size on the morphology and performance of Fe–N–C catalysts synthesized via the pyrolysis of a ferrous salt, phenanthroline and the metal-organic framework ZIF-8 is investigated in detail. Various ZIF-8 samples with average crystal size ranging from 100 to 1600 nm were prepared. The process parameters allowing a templating effect after argon pyrolysis were investigated. It is shown that the milling speed, used to prepare catalyst precursors, and the heating mode, used for pyrolysis, are critical factors for templating nano-ZIFs into nano-sized Fe–N–C particles with open porosity. Templating could be achieved when combining a reduced milling speed with a ramped heating mode. For templated Fe–N–C materials, the performance and activity improved with decreased ZIF-8 crystal size. With the Fe–N–C catalyst templated from the smallest ZIF-8 crystals, the current densities in H2/O2 polymer electrolyte fuel cell at 0.5 V reached ca. 900 mA cm −2 , compared to only ca. 450 mA cm −2 with our previous approach. This templating process opens the path to a morphological control of Fe–N–C catalysts derived from metal-organic frameworks which, when combined with the versatility of the coordination chemistry of such materials, offers a platform for the rational design of optimized Metal–N–C catalysts

The Quest for Polysulfides in Lithium–Sulfur Battery Electrolytes: An Operando Confocal Raman Spectroscopy Study

International audienceConfocal Raman spectra of a lithium–sulfur battery electrolyte are recorded operando in a depth-of-discharge resolved manner for an electrochemical cell with a realistic electrolyte/sulfur loading ratio. The evolution of various possible polysulfides is unambiguously identified by combining Raman spectroscopy data with DFT simulations

Cobalt, a reactive metal in releasing hydrogen from sodium borohydride by hydrolysis: A short review and a research perspective

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Ex-situ characterization of N2H4-, NaBH4- and NH3BH3-reduced cobalt catalysts used in NaBH4 hydrolysis

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High-extent dehydrogenation of hydrazine borane N2H4BH3 by hydrolysis of BH3 and decomposition of N2H4

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Hydrazine borane: synthesis, characterization, and application prospects in chemical hydrogen storage

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