Solid-State NaBH 4 / Co Composite as Hydrogen Storage Material : Effect of the Pressing Pressure on Hydrogen Generation Rate

A solid-state NaBH4/Co composite has been employed as a hydrogen-generating material, as an alternative to sodium borohydride solutions, in the long storage of hydrogen. Hydrogen generation begins in the presence of cobalt-based catalysts, immediately after water is added to a NaBH4/Co composite, as a result of sodium borohydride hydrolysis. The hydrogen generation rate has been investigated as a function of the pressure used to press hydrogen-generating composites from a mechanical mixture of the hydride and cobalt chloride hexahydrate. The hydrogen generation rate was observed to increase with the increase of this pressure. Pre-reduction of the cobalt chloride, using a sodium borohydride solution, leveled this dependence with a two-fold decrease in the gas generation rate. According to TEM and XPS data, oxidation of the particles of the pre-reduced cobalt catalyst took place during preparation of the composites, and it is this oxidation that appears to be the main reason for its low activity in sodium borohydride hydrolysis.


Introduction
Exhaustion of the traditional sources of energy, as well as progress made in CO 2 -free fuel technologies, has stimulated a search for alternative energy carriers.To date, the main focus in the transition to CO 2 -free energy carriers has been on the accumulation of hydrogen to increase hydrogen capacity (important stage of the hydrogen cycle) [1].Compact hydrogen storage is a major challenge in hydrogen economy, and it is especially critical when developing hydrogen sources for low-temperature proton-exchange membrane fuel cells (LT-PEMFC) for unmanned aircraft [2][3][4][5] and for mobile applications [6,7].For such applications, the hydrogen-accumulating material needs to have a high hydrogen capacity and be capable of producing hydrogen, without an additional supply of heat.The high-sensitivity of electrocatalysts to catalytic poisons requires that the hydrogen gas used for a LT-PEMFC contains no CO, NH 3, or other impurities.In addition, for a stable LT-PEMFC operation, humidifying of the hydrogen is necessary [8].Catalytic sodium borohydride hydrolysis (hydrogen content-10.5 wt%), which produces a highly pure and humidified hydrogen at ambient temperatures, without a supply of energy, meets all of these requirements [9,10]: Due to their high activity and low cost, cobalt-containing catalysts are the most studied catalysts for sodium borohydride hydrolysis [11][12][13][14], compared with platinum group metals [15].Some researchers [16][17][18] have proposed the addition of cobalt compounds to the solid-state composites based on NaBH 4, which are considered as substitutes for sodium borohydride solutions, the traditional hydrogen storage and generation media [19,20].In storage, sodium borohydride solutions lose hydrogen because of spontaneous interactions between the hydride and water, even in the presence of the stabilizing agent, sodium hydroxide.This results in a three-fold decrease in hydrogen content after one year of storage [21,22].Using the solid-state NaBH 4 /Co composite as a source of hydrogen, gas generation starts immediately after the addition of water, with the evolution rate varying depending on the physicochemical properties of the catalyst: the particle size of the active component [23] and the crystal structure of the cobalt compounds [24].To date, the effect of the preparation conditions of such solid-state NaBH 4 /Co composites on the kinetics of hydrogen generation has not been considered in the literature.
In this work, the kinetics of hydrogen generation from solid-state NaBH 4 /Co composites prepared from a mechanical mixture of sodium borohydride and cobalt chloride hexahydrate, by pressing at different pressures after the addition of water, has been studied.The obtained results have been compared with the results for solid-state NaBH 4 /Co composites from sodium borohydride and a cobalt catalyst, which were prepared by reducing cobalt chloride in an aqueous sodium borohydride solution.

Results and Discussion
Hydrogen generation occurred after water was added to a solid-state NaBH 4 /Co composite prepared from a mechanical mixture of sodium borohydride and cobalt chloride hexahydrate, by pressing it at 13.5-45 kgf•cm −2 .After pressing, the color of the NaBH 4 /Co composite was changed from white to grey as shown in Figure 1.This visual observation may be due to the reduction of cobalt, as a result of the solid-state interaction of cobalt chloride hexahydrate with the crystalline hydride under pressure.Some researchers [16][17][18] have proposed the addition of cobalt compounds to the solid-state composites based on NaBH4, which are considered as substitutes for sodium borohydride solutions, the traditional hydrogen storage and generation media [19,20].In storage, sodium borohydride solutions lose hydrogen because of spontaneous interactions between the hydride and water, even in the presence of the stabilizing agent, sodium hydroxide.This results in a three-fold decrease in hydrogen content after one year of storage [21,22].Using the solid-state NaBH4/Co composite as a source of hydrogen, gas generation starts immediately after the addition of water, with the evolution rate varying depending on the physicochemical properties of the catalyst: the particle size of the active component [23] and the crystal structure of the cobalt compounds [24].To date, the effect of the preparation conditions of such solid-state NaBH4/Co composites on the kinetics of hydrogen generation has not been considered in the literature.
In this work, the kinetics of hydrogen generation from solid-state NaBH4/Co composites prepared from a mechanical mixture of sodium borohydride and cobalt chloride hexahydrate, by pressing at different pressures after the addition of water, has been studied.The obtained results have been compared with the results for solid-state NaBH4/Co composites from sodium borohydride and a cobalt catalyst, which were prepared by reducing cobalt chloride in an aqueous sodium borohydride solution.

Results and Discussion
Hydrogen generation occurred after water was added to a solid-state NaBH4/Co composite prepared from a mechanical mixture of sodium borohydride and cobalt chloride hexahydrate, by pressing it at 13.5-45 kgf•cm −2 .After pressing, the color of the NaBH4/Co composite was changed from white to grey as shown in Figure 1.This visual observation may be due to the reduction of cobalt, as a result of the solid-state interaction of cobalt chloride hexahydrate with the crystalline hydride under pressure.However, according to XRD (Figure 1c) the diffraction peaks of cobalt chloride did not disappear after the pressing.Moreover, there is no halo at 40°, characteristic of amorphous cobalt borides [25].It appears that the observed change in color was caused by a reduction of an insignificant amount of cobalt.
The hydrogen generation rate from the solid-state NaBH4/Co composite was found to increase with an increase in the pressing pressure (Figure 2a).The changes were especially noticeable for pressures ranging from 13.5 to 22.5 kgf•cm -2 , while further increases in pressure had a smaller effect.It can be suggested that the higher the pressure that is used to press the NaBH4/Co composite into a tablet, the greater is the extent of the solid-state interaction between the hydride and cobalt salt to However, according to XRD (Figure 1c) the diffraction peaks of cobalt chloride did not disappear after the pressing.Moreover, there is no halo at 40 • , characteristic of amorphous cobalt borides [25].It appears that the observed change in color was caused by a reduction of an insignificant amount of cobalt.
The hydrogen generation rate from the solid-state NaBH 4 /Co composite was found to increase with an increase in the pressing pressure (Figure 2a).The changes were especially noticeable for pressures ranging from 13.5 to 22.5 kgf•cm −2 , while further increases in pressure had a smaller effect.It can be suggested that the higher the pressure that is used to press the NaBH 4 /Co composite into a tablet, the greater is the extent of the solid-state interaction between the hydride and cobalt salt to form the low-temperature cobalt boride (Co x B-Cl) as an active-phase catalyzing borohydride hydrolysis [26,27].
Energies 2019, 12, x FOR PEER REVIEW 3 of 7 form the low-temperature cobalt boride (CoxB-Cl) as an active-phase catalyzing borohydride hydrolysis [26,27].Hence, pre-reduction of the cobalt chloride by sodium borohydride will allow a leveling of the dependence of the generation rate on the pressing pressure used in preparing tablets of the NaBH4/Co composite.To verify this assumption, the kinetics of gas evolution from NaBH4/Co composites containing CoxB-Cl catalyst, prepared by the reduction of cobalt chloride in an aqueous sodium borohydride solution, was investigated.Indeed, with the pre-reduced cobalt catalyst, the hydrolysis rate remained constant for pressing pressures ranging from 13.5 to 36 kgf•cm −2 (Figure 2b) and it was only at 45 kgf•cm -2 that a decrease in the gas generation rate was noted, resulting from a slow dissolution of the tablet.Thus, the solid-state interaction of the cobalt salt with the hydride, at the pressing stage, plays an important role in the formation of the active phase of the cobalt catalyst.
A comparison of the experimental results, in Figure 2, reveals that the gas generation rate was two times faster in the presence of cobalt chloride, than it was when a CoxB-Cl catalyst was added to the NaBH4/Co composite, even with a two-fold increase in the metal content.To explain this outcome, the CoxB-Cl catalyst was studied before and after its contact with the crystalline hydride within a solid-state NaBH4/Co composite.Using TEM, it was found that the starting CoxB-Cl catalyst consisted of particles joined together, under the action of their own magnetic fields (Figure 3), to form three-dimensional structures (Figure 4a).Hence, pre-reduction of the cobalt chloride by sodium borohydride will allow a leveling of the dependence of the generation rate on the pressing pressure used in preparing tablets of the NaBH 4 /Co composite.To verify this assumption, the kinetics of gas evolution from NaBH 4 /Co composites containing Co x B-Cl catalyst, prepared by the reduction of cobalt chloride in an aqueous sodium borohydride solution, was investigated.Indeed, with the pre-reduced cobalt catalyst, the hydrolysis rate remained constant for pressing pressures ranging from 13.5 to 36 kgf•cm −2 (Figure 2b) and it was only at 45 kgf•cm −2 that a decrease in the gas generation rate was noted, resulting from a slow dissolution of the tablet.Thus, the solid-state interaction of the cobalt salt with the hydride, at the pressing stage, plays an important role in the formation of the active phase of the cobalt catalyst.
A comparison of the experimental results, in Figure 2, reveals that the gas generation rate was two times faster in the presence of cobalt chloride, than it was when a Co x B-Cl catalyst was added to the NaBH 4 /Co composite, even with a two-fold increase in the metal content.To explain this outcome, the Co x B-Cl catalyst was studied before and after its contact with the crystalline hydride within a solid-state NaBH 4 /Co composite.Using TEM, it was found that the starting Co x B-Cl catalyst consisted of particles joined together, under the action of their own magnetic fields (Figure 3), to form three-dimensional structures (Figure 4a).Hence, pre-reduction of the cobalt chloride by sodium borohydride will allow a leveling of the dependence of the generation rate on the pressing pressure used in preparing tablets of the NaBH4/Co composite.To verify this assumption, the kinetics of gas evolution from NaBH4/Co composites containing CoxB-Cl catalyst, prepared by the reduction of cobalt chloride in an aqueous sodium borohydride solution, was investigated.Indeed, with the pre-reduced cobalt catalyst, the hydrolysis rate remained constant for pressing pressures ranging from 13.5 to 36 kgf•cm −2 (Figure 2b) and it was only at 45 kgf•cm -2 that a decrease in the gas generation rate was noted, resulting from a slow dissolution of the tablet.Thus, the solid-state interaction of the cobalt salt with the hydride, at the pressing stage, plays an important role in the formation of the active phase of the cobalt catalyst.
A comparison of the experimental results, in Figure 2, reveals that the gas generation rate was two times faster in the presence of cobalt chloride, than it was when a CoxB-Cl catalyst was added to the NaBH4/Co composite, even with a two-fold increase in the metal content.To explain this outcome, the CoxB-Cl catalyst was studied before and after its contact with the crystalline hydride within a solid-state NaBH4/Co composite.Using TEM, it was found that the starting CoxB-Cl catalyst consisted of particles joined together, under the action of their own magnetic fields (Figure 3), to form three-dimensional structures (Figure 4a).The Co x B-Cl catalyst that was extracted from the solid-state hydride-containing composite, immediately after pressing, showed the presence of thin films (Figure 4b) with an amorphous structure (Figure 4c) that were blocking the particles of the active component.According to energy-dispersive X-ray microanalysis (EDX), these films contain considerable amounts of oxygen (Figure 3d), indicating an oxidized state of the cobalt.
The electronic state of cobalt in the Co x B-Cl catalyst was characterized by XPS, both before and after its contact with the crystalline sodium borohydride.Figure 4e shows that cobalt is present, in at least two states, in the Co x B-Cl catalysts prepared in an aqueous sodium borohydride solution.Corresponding to the reduced metal (Co), the binding energy is 778.2 eV and the value for the oxidized state is 781.0 eV.It can definitely be said that the oxidized cobalt is Co 2+ , since the analyzed Co2p region has a satellite that shifted from the main Co2p 3/2 peak by about 4-6 eV towards higher binding energies [28].Taking into account this observation and the value of the binding energy of 781.0 eV, the oxidized state was attributed to cobalt hydroxide Co(OH) 2 [29][30][31].
Energies 2019, 12, x FOR PEER REVIEW 4 of 7 The CoxB-Cl catalyst that was extracted from the solid-state hydride-containing composite, immediately after pressing, showed the presence of thin films (Figure 4b) with an amorphous structure (Figure 4c) that were blocking the particles of the active component.According to energy-dispersive X-ray microanalysis (EDX), these films contain considerable amounts of oxygen (Figure 3d), indicating an oxidized state of the cobalt.
The electronic state of cobalt in the CoxB-Cl catalyst was characterized by XPS, both before and after its contact with the crystalline sodium borohydride.Figure 4e shows that cobalt is present, in at least two states, in the CoxB-Cl catalysts prepared in an aqueous sodium borohydride solution.Corresponding to the reduced metal (Co), the binding energy is 778.2 eV and the value for the oxidized state is 781.0 eV.It can definitely be said that the oxidized cobalt is Co 2+ , since the analyzed Co2p region has a satellite that shifted from the main Co2p3/2 peak by about 4-6 eV towards higher binding energies [28].Taking into account this observation and the value of the binding energy of 781.0 eV, the oxidized state was attributed to cobalt hydroxide Co(OH)2 [29][30][31].A comparison of the Co2p3/2 line intensities for metallic cobalt and its oxidized state has shown the Co/Co 2+ ratio to be equal to 0.71 for the initial CoxB-Cl catalyst, and to 0.17 for the catalyst extracted from the solid-state NaBH4/Co composite immediately after pressing.It appears that, in the course of tablet preparation, under air with a humidity of 54%, the metal oxidizes to form cobalt hydroxide.As has been found [23], this compound is considerably less active in sodium borohydride hydrolysis than in the cobalt chloride.Thus, the presence of cobalt hydroxide in the CoxB-Cl catalyst is the main reason for the slow hydrogen generation rate from the solid-state NaBH4/Co composite containing a pre-reduced catalyst.A comparison of the Co2p 3/2 line intensities for metallic cobalt and its oxidized state has shown the Co/Co 2+ ratio to be equal to 0.71 for the initial Co x B-Cl catalyst, and to 0.17 for the catalyst extracted from the solid-state NaBH 4 /Co composite immediately after pressing.It appears that, in the course of tablet preparation, under air with a humidity of 54%, the metal oxidizes to form cobalt hydroxide.As has been found [23], this compound is considerably less active in sodium borohydride hydrolysis than in the cobalt chloride.Thus, the presence of cobalt hydroxide in the Co x B-Cl catalyst is the main reason for the slow hydrogen generation rate from the solid-state NaBH 4 /Co composite containing a pre-reduced catalyst.

Materials and Methods
Solid-state NaBH 4 /Co composites were prepared from a mechanical mixture of sodium borohydride (CAS 16940-66-2; Sigma-Aldrich 452882) and a cobalt catalyst.The mass of sodium borohydride was 0.0465 g.For the CoCl 2 •6H 2 O and Co x B-Cl catalysts, the molar «NaBH 4 :Co» ratios were 60 and 30, respectively.Weighted amounts of the reagents were ground with a mortar grinder (PULVERISETTE 2, Germany) and compressed into tablets using a manual tablet press machine (TDP-0, China) under an air humidity of 54%.
The catalytic additives used were cobalt chloride hexahydrate-CoCl 2 •6H 2 O (CAS 7791-13-1; Sigma-Aldrich 255599) and a cobalt catalyst (Co x B-Cl) prepared by reducing cobalt chloride in a 0.12 M aqueous sodium borohydride solution at a molar «NaBH 4 :Co» ratio of 25:1.The physicochemical properties of this catalyst have been studied in detail [32].
The kinetics of sodium borohydride hydrolysis was studied using a temperature-controlled glass reactor at 40 • C, into which the solid-state NaBH 4 /Co composite was placed with a subsequent addition of 5 mL of water.The volume of the evolved hydrogen was measured using a 100 mL gas burette with a resolution of 0.2 mL The data obtained were corrected to Normal Temperature and Pressure-N.T.P.-(20 • C, 1 atm) based on three repeated experiments under the same conditions.The experimental uncertainty was less than 2%.
For the physicochemical studies, the catalysts were separated from the hydride-containing medium with a magnet, evacuated at room temperature for 24 h, and then stored in a desiccator under argon to prevent oxidation by air oxygen.
The XRD analysis was performed using an URD-63 (Seifert-FPM, Germany) diffractometer with a CuKα radiation.
The magnetic characterization was performed using a vibrating sample magnetometer (VSM-7407, Lake Shore) in an argon atmosphere at 20 • C. The hysteresis loop was recorded in an applied magnetic field between −5 kOe and +5 kOe.
High resolution transmission electron microscopy (HRTEM) images were obtained on a JEM-2010 microscope with an accelerating voltage of 200 kV and a resolution of 0.14 nm.The elemental analysis of the samples (with a locality of 50 nm) was carried out using EDX on an EDAX Phoenix spectrometer with a Si(Li) detector and an energy resolution of 130 eV or higher.The samples to be analyzed were supported onto a holey carbon film on a standard copper grid.
The XPS spectra were taken with a SPECS photoelectron spectrometer (Germany) using a PHOIBOS-150-MCD-9 hemispheric analyzer and a FOCUS-500 monochromator (AlKα, hν = 1486.74eV, 150 W).The binding energy scale of the spectrometer was calibrated beforehand using the Au4f 7/2 (84.0 eV) and Cu2p 3/2 (932.6 eV) core level peaks.The binding energies were determined with an accuracy of ±0.1 eV.The samples were applied onto conducting scotch tape and were studied without pretreatment.The sample charging was taken into account using C1s lines (284.8 eV).Analysis of the Co2p spectra allowed us to determine its electronic state [33].

Conclusions
Solid-state NaBH 4 /Co composites have been used as a hydrogen source.The hydrogen generation rate has been studied as a function of the pressure used for pressing a mechanical mixture of the sodium borohydride and cobalt chloride hexahydrate.It was shown that, with the increase of pressing pressure, there was an increase in the hydrogen generation rate, apparently because of the interaction rising between the hydride and the cobalt salt to form the low-temperature cobalt boride as a catalytically active phase.It was established that pre-reduction of the cobalt chloride by sodium borohydride in an aqueous solution leveled this dependence, but with a two-fold decrease in the hydrogen generation rate.The low activity of the pre-reduced Co x B-Cl catalyst was due to the oxidation of the active component to form cobalt hydroxide.Thus, for practical applications, we concluded that it is not expedient to use a pre-reduced cobalt catalyst in the solid-state NaBH 4 /Co composite, since it is not possible to avoid oxidation of the active component in the air during its preparation.

Figure 4 .
Figure 4. Morphology of (a) the initial CoxB-Cl catalyst and (b) the CoxB-Cl catalyst extracted from the solid-state NaBH4/Co composite with the molar ratio NaBH4:Co = 30:1; (c) the amorphous film without crystal lattice; (d) energy-dispersive X-ray microanalysis (EDX) from the chosen regions of the catalyst particles; (e) XPS spectra of the Co2p core level for these catalysts.

Figure 4 .
Figure 4. Morphology of (a) the initial Co x B-Cl catalyst and (b) the Co x B-Cl catalyst extracted from the solid-state NaBH 4 /Co composite with the molar ratio NaBH 4 :Co = 30:1; (c) the amorphous film without crystal lattice; (d) energy-dispersive X-ray microanalysis (EDX) from the chosen regions of the catalyst particles; (e) XPS spectra of the Co2p core level for these catalysts.