Fast hydrogen generation from NaBH4 hydrolysis catalyzed by carbon aerogels supported cobalt nanoparticles

Carbon aerogels (CAs) with oxygen-rich functional groups and high surface area are synthesized by hydrothermal treatment of glucose in the presence of boric acid, and are used as the support for loading cobalt catalysts (CAs/Co). Cobalt nanoparticles distribute uniformly on the surface of ACs, creating highly dispersed catalytic active sites for hydrolysis of alkaline sodium borohydride solution. A rapid hydrogen generation rate of 11.22 L min⁻¹ g_(cobalt)⁻¹ is achieved at 25 °C by hydrolysis of 1 wt% NaBH₄ solution containing 10 wt% NaOH and 20 mg the CAs/Co catalyst with a cobalt loading of 18.71 wt%. Furthermore, various influences are systematically investigated to reveal the hydrolysis kinetics characteristics. The activation energy is found to be 38.4 kJ mol⁻¹. Furthermore, the CAs/Co catalyst can be reusable and its activity almost remains unchanged after recycling, indicating its promising applications in fuel cell.     

A multifactor study of catalyzed hydrolysis of solid NaBH4 on cobalt nanoparticles: Thermodynamics and kinetics

In the present work, hydrogen generation through hydrolysis of a NaBH_4(s)/catalyst_(s) solid mixture was realized for the first time as a solid/liquid compact hydrogen storage system using Co nanoparticles as a model catalyst. The performance of the system was analysed from both the thermodynamic and kinetic points of view and compared with the classical catalyzed hydrolysis of a NaBH₄ solution. The kinetic analysis of the NaBH_4(s)/catalyst_(s)/H₂O_(l) system shows that the reaction is first order with respect to the catalyst concentration, and the activation energy equal to 35 kJ mol_NaBH4⁻¹. Additionally, calorimetric measurements of the heat evolved during the hydrolysis of NaBH₄ solutions evidence the global process energy (−217 kJ mol_NaBH4⁻¹). Characterization of the cobalt nanoparticles before and after the hydrolysis associated with the calorimetric measurements suggests the “in situ” formation of a catalytically active Co_xB phase through “reduction” of an outer protective oxide layer that is regenerated at the end of reaction.     

Highly efficient acid-treated cobalt catalyst for hydrogen generation from NaBH4 hydrolysis

The effect of cobalt-based catalysts, i.e. CoCl₂(20 wt% Co)/Al₂O₃ treated by different acids, on NaBH₄ hydrolysis was investigated. Five acids were used: oxalic acid, citric acid, acetic acid, sulfuric acid and hydrochloric acid. Two ways of acid treatment were considered: (i) ex-situ addition of acid to CoCl₂(20 wt% Co)/Al₂O₃ at room temperature and (ii) in-situ addition by mixing CoCl₂, Al₂O₃ and acid (one-step process). Both ways showed that adding an acid to the catalyst contributed to an important increase of the catalytic activity towards the NaBH₄ hydrolysis. The best performances were obtained with the catalysts treated with either HCl or CH₃COOH as the global activity of CoCl₂(20 wt% Co)/Al₂O₃ was increased up to 50%.     

A highly active Co-B catalyst for hydrogen generation from sodium borohydride hydrolysis

Co-B catalysts were prepared by the chemical reduction of CoCl₂ with NaBH₄ for hydrogen generation from borohydride hydrolysis. The catalytic properties of the Co-B catalysts were found to be sensitive to the preparation conditions including pH of the NaBH₄ solution and mixing manner of the precursors. A Co-B catalyst with a very high catalytic activity was obtained through the formation of a colloidal Co(OH)₂ intermediate. The ultra-fine particle size of 10 nm accounted for its super activity for hydrogen generation with a maximum rate of 26 L min⁻¹ g⁻¹ at 30 °C. The catalyst also changed the hydrolysis kinetics from zero-order to first-order.     

Sodium borohydride hydrolysis kinetics comparison for nickel,cobalt, and ruthenium boride catalysts

Hydrolysis tests have been performed at a constant temperature of 60 °C over a range of sodium borohydride (2.5–30 wt%) and sodium hydroxide (2.5–30 wt%) concentrations. Catalysts used to initiate the hydrolysis reaction were developed through the metal salt reduction method with sodium borohydride. These catalysts were identified as nickel boride, cobalt boride, and ruthenium with each catalyst having similar morphology. Catalysts were tested in loose, powder form free of binders or substrates. Hydrolysis rate comparisons show that reaction rates decrease linearly with increasing NaBH₄ concentrations due to mass transfer limitations. Increasing NaOH concentration has been shown to drive a non-catalyzed intermediate reaction with the rate of the overall reaction hindered by the catalysts’ ability to bind hydrogen to active sites. Maximum hydrogen production rates for the Ni₃B, Co₃B, and Ru catalysts were found to be 1.3, 6.0, and 18.6 L min⁻¹ g_cat⁻¹, respectively.     

Synthesis of solid-state NaBH4/Co-based catalyst composite for hydrogen storage through a high-energy ball-milling process

Solid-state composites of NaBH₄ and Co-based catalyst have been fabricated for hydrogen generation via a novel mechanochemical technique, i.e. the high-energy ball milling, in which the gravimetric storage capacity of hydrogen has reached 6.7 wt%, meeting the 2010 target of at least 0.06 kg H₂/kg set by the U.S. Department of Energy (DOE). The active catalysts used in the hydrolysis reaction of sodium borohydride for hydrogen generation are mainly cobalt oxides. Controlled addition of water, namely water used as a limiting agent, to the solid composites of NaBH₄ and Co-based catalyst greatly improves the H₂ storage capacity and resolved the issues of low gravimetric H₂ storage in conventional aqueous system of sodium borohydride. Factors influencing the performance of hydrogen production such as amounts of H₂O added, catalyst loadings and durations of ball-milling processes are investigated. Moreover the hydrolyzed products of NaBH₄ and spent catalysts are analyzed as well.     

Microstructural control of catalyst-loaded PVDF microcapsule membrane for hydrogen generation by NaBH4 hydrolysis

Polymer microcapsules were prepared and used as catalyst support for hydrogen generation from sodium borohydride. Polyvinylidene fluoride (PVDF) porous microcapsule membranes immobilized with metal salt (cobalt (II) chloride hexahydrate) catalyst and cobalt–boron catalyst were prepared, denoting them as MS and MP method respectively. Non-solvent coagulation bath consisting of a mixture of water and isopropanol (IPA) were used to prepare the microcapsules. The compositions of the non-solvents were changed with a ratio of 10:90 (v/v%)–50:50 (v/v%) with 1 wt% NaOH and 0.5 wt% NaBH₄. The effects of a number of parameters such as the kinds of additives, the size and morphology of the resulting microcapsules were studied on hydrogen generation. The structures and physical–chemical properties of the metal catalyst-loaded microcapsule membranes were characterized using SEM and EDX. The MS method used in preparing the microcapsule showed good performance in hydrogen generation from sodium borohydride. There was also improved performance in hydrogen generation with increasing IPA composition used in the metal salts loaded microcapsule preparation. The control of three regions inside the microcapsules (hollow region, crust region and skin layer) as well as the specific loading of metal catalysts gave a good hydrogen generation performance. The catalyst-loaded microcapsule also maintained an appreciable performance and stability after many runs of hydrolysis reaction for the hydrogen generation.     

A novel design of solid-state NaBH4 composite as a hydrogen source for 2 W PEMFC applications

Hydrolysis of sodium borohydride (NaBH₄) is a promising method for on-board hydrogen supply to polymer electrolyte membrane fuel cells (PEMFC). This article presents an attempt to design a novel solid-state NaBH₄ composite, which is made up of NaBH₄ powder, Co²⁺/IR-120 catalyst and silicone rubber, for hydrogen generator. The silicone rubber can act as a stabilizer in the solid-state NaBH₄ composite because of its surface hydrophobicity leading to reduced diffusion rate of water into the composite. The solid-state NaBH₄ composite can produce hydrogen stably near 25 mL min⁻¹ for at least 2 h without employment of any mechanical control system. Using the hydrogen generated from the solid-state NaBH₄ composite, a 2 W PEMFC stack is successfully operated to power a cellular phone.     

Improved hydrogen generation from alkaline NaBH4 solution using cobalt catalysts supported on modified activated carbon

Hydrogen production from alkaline sodium borohydride (NaBH₄) solution via hydrolysis process over activated carbon supported cobalt catalysts is studied. Activated carbons are used in their original form and after liquid phase oxidation with HNO₃. The changes in surface functional groups of the activated carbon are detected by FTIR spectroscopy. The effects of HNO₃ oxidation on the properties of the activated carbon and the resulting catalyst performance are investigated. FTIR analysis reveals that the oxidative treatment leads to the formation of various functional groups on the surface of the activated carbon. Cobalt catalysts supported on the modified activated carbon are found to exhibit higher activity and stability.     

Effects of crystallinity and morphology of solution combustion synthesized Co3O4 as a catalyst precursor in hydrolysis of sodium borohydride

Solution combustion synthesized (SCS) cobalt oxide (Co₃O₄) powder has been studied as a catalyst precursor for the hydrolysis of sodium borohydride (NaBH₄). Synthesis is completed in less than two minutes and results indicate SCS is capable of reproducibly synthesizing 98.5–99.5% pure Co₃O₄ nano-foam materials. SCS materials demonstrate an as-synthesized specific surface area of 24 m² g⁻¹, a crystallite size of 15.5 nm, and fine surface structures on the order of 4 nm. Despite having similar initial surface areas and sample purities, SCS-Co₃O₄ outperforms commercially available Co₃O₄ and elemental cobalt (Co) nano powders when used as a catalyst precursor for NaBH₄ hydrolysis. Hydrogen generation rates (HGR) using 0.6 wt% NaBH₄ in aqueous solution at 20 °C were observed to be 1.24 ± 0.2 L min⁻¹ g_cat⁻¹ for SCS nano-foam Co₃O₄ compared to 0.90 ± 0.09 and 0.43 ± 0.04 L min⁻¹ g_cat⁻¹ for commercially available Co₃O₄ and Co, respectively. The high catalytic activity of SCS-Co₃O₄ is attributed to its nano-foam morphology and crystallinity. During the hydrolysis of NaBH₄, the SCS-Co₃O₄ converts in-situ to an amorphous active catalyst with a specific surface area of 92 m² g⁻¹ and exhibits a honeycomb type morphology.     

Catalytic hydrolysis of sodium borohydride by a novel nickel–cobalt–boride catalyst

With the aim of designing an efficient hydrogen generator for portable fuel cell applications nickel–cobalt–boride (Ni–Co–B) catalysts were prepared by a chemical reduction method and their catalytic hydrolysis reaction with alkaline NaBH₄ solution was studied. The performance of the catalysts prepared from NaBH₄ solution with NaOH, and without NaOH show different hydrogen generation kinetics. The rate of hydrogen generation was measured using Ni–Co–B catalyst as a function of the concentrations of NaOH and NaBH₄, as well as the reaction temperature, in the hydrolysis of alkaline NaBH₄ solution. The hydrogen generation rate increases for lower NaOH concentrations in the alkaline NaBH₄ solution and decreases after reaching a maximum at 15 wt.% of NaOH. The hydrogen generation rate is found to be constant with respect to the concentration of NaBH₄ in the alkaline NaBH₄ solution. The activation energy for hydrogen generation is found to be 62 kJ mol⁻¹, which is comparable with that of hydrogen generation by a ruthenium catalyst.     

Hydrogen generation by sodium borohydride hydrolysis on nanosized CoB catalysts supported on TiO2, Al2O3 and CeO2

A series of nanosized CoB catalysts supported on TiO₂, Al₂O₃, and CeO₂ were prepared. The catalysts were prepared by incipient-wetness impregnation. The sample was dried at 100 °C and then dispersed in water and reduced by an aqueous solution of sodium borohydrate at room temperature. An unsupported CoB cluster was used for comparison. The activities of the supported CoB catalysts were higher than that of unsupported one. The reaction rates of these supported CoB catalysts decreased in the order: CoB/TiO₂ > CoB/Al₂O₃ > CoB/CeO₂ > unsupported CoB. The reaction kinetics on various catalysts was also investigated.     

Novel fabrication of solid-state NaBH4/Ru-based catalyst composites for hydrogen evolution using a high-energy ball-milling process

Solid-state NaBH₄/Ru-based catalyst composites have been fabricated for hydrogen generation through a high-energy ball-milling process, providing uniform dispersion of resin-supported Ru³⁺ catalysts among pulverized NaBH₄ (SBH) particles, so as to increase the contacts of SBH with active catalytic sites. Consequently, the gravimetric hydrogen storage capacity as high as 7.3 wt% could be achieved by utilizing water as a limiting reagent to overcome the issue of deactivated catalysts whose active sites are often blocked by precipitates caused by limited NaBO₂ solubility occurring in conventional aqueous SBH systems for hydrogen productions. Products of hydrolyzed SBH that greatly influence the gravimetric H₂ storage capacity are found to be most likely NaBO₂·2H₂O and NaBO₂·4H₂O from SBH/H₂O reacting systems with initial weight ratios, SBH/H₂O = 1/2 and 1/10, respectively, according to the TGA and XRD analyses.     

Hydrogen storage properties of various carbon supported NaBH4 prepared via metathesis

Sodium borohydride nanoparticles prepared via the metathesis reaction between LiBH₄ and NaCl were successfully deposited on various carbon supporting materials such as graphite, graphene oxide and carbon nanotubes. The X-ray diffraction analyses were conducted to identify the phase of NaBH₄ deposited on various carbon supporting materials. The transmittance electron micrograph analyses were also conducted to investigate the particle size and dispersion of NaBH₄ within carbon supporting materials. The particle size and size distribution of NaBH₄ on graphite were observed to be larger and broader than of other two supporting materials, graphene oxide and CNT due to the lower surface energy as compared to GO and CNT. The bonding state of NaBH₄ was confirmed by the Fourier-transformed infrared spectroscopy analysis. The TG and PCT results show that the hydrogen desorption of the NaBH₄ deposited on carbon supports takes place at temperature (130 °C～) significantly lower than that of pure NaBH₄ (above 500 °C) and the amount of desorption was in the order of graphene oxide (12.3 mass %) > CNT (9.8 mass %) > graphite (5.7 mass %). The reversibility of hydrogen adsorption after five cycles of adsorption-desorption showed that NaBH₄/GO and NaBH₄/CNT were much better than that of pure NaBH₄ due to excellent structural stability.     

A durable ruthenium catalyst for the NaBH4 hydrolysis

Ru-active carbon (Ru/C) catalysts are prepared by impregnation reduction method for hydrogen generation via hydrolysis of alkaline sodium borohydride (NaBH₄) solution. The corresponding activity and durability of the prepared catalysts are tested in an immobile bed reactor. The variation of hydrogen generation rate with the increasing of flux and concentration of NaBH₄ solution is measured. The durability of the catalysts prepared under various reductive pH values and reductants is tested by using different concentrations of NaBH₄ solution (10 & 15 wt%). It is found that the durability of catalyst in 15 wt% NaBH₄ solution is longer than that in 10 wt% NaBH₄ solution. The deactivation of Ru/C catalysts is considered as the comprehensive effect of three factors: the loss of Ru, the deposition of byproducts on the catalyst surface and the aggregation of Ru particles.     

Fully-integrated micro PEM fuel cell system with NaBH4 hydrogen generator

A fully-integrated micro PEM fuel cell system with a NaBH₄ hydrogen generator was developed. The micro fuel cell system contained a micro PEM fuel cell and a NaBH₄ hydrogen generator. The hydrogen generator comprised a NaBH₄ reacting chamber and a hydrogen separating chamber. Photosensitive glass wafers were used to fabricate a lightweight and corrosion-resistant micro fuel cell and hydrogen generator. All of the BOP such as a NaBH₄ cartridge, a micropump, and an auxiliary battery were fully integrated. In order to generate stable power output, a hybrid power management operating with a micro fuel cell and battery was designed. The integrated performance of the micro PEM fuel cell with NaBH₄ hydrogen generator was evaluated under various operating conditions. The hybrid power output was stably provided by the micro PEM fuel cell and auxiliary battery. The maximum power output and specific energy density of the micro PEM fuel cell system were 250 mW and 111.2 W h/kg, respectively.     

Kinetics of NaBH4 hydrolysis on carbon-supported ruthenium catalysts

In this work, different shapes (powder and spherical) of ruthenium-active carbon catalysts (Ru/C) were prepared by impregnation reduction method for hydrogen generation (HG) from the hydrolysis reaction of the alkaline NaBH₄ solution. The effects of temperature, amount of catalysts, and concentration of NaOH and NaBH₄ on the hydrolysis of NaBH₄ solution were investigated with different shapes of Ru/C catalysts. The results show that the HG kinetics of NaBH₄ solution with the powder Ru/C catalysts is completely different from that with the spherical Ru/C catalysts. The main reason is that both mass and heat transfer play important roles during the reaction with Ru/C catalysts. The HG overall kinetic rate equations for NaBH₄ hydrolysis using the powder Ru/C catalysts and the spherical catalysts are described as r = A exp (−50740/RT) [catalyst]^1.05 [NaOH]^−0.13 [NaBH₄]^−0.25 and r = A exp (−52,120/RT) [catalyst]^1.00 [NaOH]^−0.21 [NaBH₄]^0.27 respectively.     

Highly dispersed cobalt nanoparticles onto nitrogen-doped carbon nanosheets for efficient hydrogen generation via catalytic hydrolysis of sodium borohydride

Porous carbon nanostructures are promising supports for stabilizing the highly dispersed metal nanoparticles and facilitating the mass transfer during the reaction, which are critical to achieve the high efficiency of hydrogen generation from sodium borohydride dehydrogenation. Herein, the catalytically active porous architectures are simply prepared by using 2-methylimidazole and melamine as reactive sources. The structural and compositional characterizations reveal the coexistence of metallic cobalt and N-doped carbon in porous architectures. Electron microscopy observations indicate that the synthesized products are smartly constructed from the carbon nanosheets with densely dispersed Co nanoparticles. Due to the notable structural features, the prepared Co@NC-600 sample presents the highly efficient activity for catalytic hydrolysis of NaBH₄ with a hydrogen generation rate of 2574 mL min⁻¹ g_cat⁻¹ and an activation energy of 47.6 kJ mol⁻¹. The catalytically active metallic Co and suitable support-effect of N-doped carbon are responsible for catalytic dehydrogenation.     

CO2-promoted hydrolysis of KBH4 for efficient hydrogen co-generation

Hydrolysis of metal borohydrides in the presence of CO₂ has not been studied so far, although carbon dioxide contained in air is known to accelerate hydrogen generation. KBH₄ hydrolysis promoted by CO₂ gas put through an aqueous solution was studied by time-resolved ATR-FTIR spectroscopy, showing a transformation of BH₄⁻ into B₄O₅(OH)₄²⁻, and a drastically accelerated hydrogen production which can be completed within minutes. This process can be used to produce hydrogen on-board from exhaust gases (CO₂ and H₂O). We found a new intermediate, K₉[B₄O₅(OH)₄]₃(CO₃)(BH₄)·7H₂O, forming upon hydrolysis on air via a slow adsorption of the atmospheric CO₂. The same intermediate can be crystallized from partly hydrolyzed solutions of KBH₄ + CO₂, but not from the fully reacted sample saturated with CO₂. This phase was studied by single-crystal and powder X-ray diffraction, DSC, TGA, Raman, IR and elemental analysis, all data are fully consistent with the presence of the three different anions and of the crystallized water molecules. Its crystal structure is hexagonal, space group P-62c, with lattice parameters a = 11.2551(4), c = 17.1508(8) Å. Formation of the intermediate produces 16 mol of H₂ per mole of adsorbed CO₂ and thus is very efficient both gravimetrically and volumetrically. It allows also for an elimination of carbon dioxide from exhaust gases.