LiBH4 Electronic Destabilization with Nickel(II) Phthalocyanine

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LiBH4 Electronic Destabilization with Nickel(II) Phthalocyanine - Leading to a Reversible Hydrogen Storage System Qiwen Lai, Md Zakaria Quadir, and Kondo-Francois Aguey-Zinsou ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01087 • Publication Date (Web): 14 Nov 2018 Downloaded from http://pubs.acs.org on December 4, 2018

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ACS Applied Energy Materials

LiBH4 Electronic Destabilization with Nickel(II) Phthalocyanine - Leading to a Reversible Hydrogen Storage System Qiwen Laia, Md Zakaria Quadirb and Kondo-Francois Aguey-Zinsoua,* aMERLin,

School of Chemical Engineering, The University of New South Wales, Sydney NSW 2052, Australia, E-mail: [email protected]

bMicroscopy

& Microanalysis Facility, John de Laeter Centre, Curtin University, WA 6102, Australia

ABSTRACT Precipitation of a LiBH4 solution into an anti-solvent led to formation of nanoparticles in the size range of 2 to 18 nm. By direct deposition of these nanoparticles onto a Nickel(II) phthalocyanine substrate, LiBH4 was destabilized and the hydrogen release temperature was dramatically reduced to 350 °C through a single step decomposition. Remarkably, upon hydrogen release and uptakes the morphology of the material evolved to single crystal “plates” like particles and a reversible hydrogen storage capacity of 3.2 mass% at 350 °C under 6 MPa H2 pressure was observed. As evident by XPS analysis, such an enhancement is believed to result from the effective electron transfer interplay between LiBH4, LiH, B and the nickel(II) phthalocyanine, enabling a destabilization of LiBH4 and the facile rehydrogenation of LiH and B into LiBH4. This study thus reveals a novel approach to destabilize LiBH4 by the use of an “electron active” substrate.

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KEYWORDS:

Hydrogen

storage;

Borohydride;

Nanosizing,

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Nickel,

Electronic

destabilization.


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1. INTRODUCTION Hydrogen is believed to be a future energy carrier enabling clean energy systems. However, a safe and efficient way to store hydrogen with high density is needed to enable the large-scale deployment of hydrogen-based technologies across many sectors including on-board vehicles. LiBH4 is a light-weight metal borohydride with the capability to potentially deliver a solution for the storage of hydrogen with high density owing to its high hydrogen content (18.4 mass%). However, the high decomposition temperature, slow hydrogen desorption kinetics and extreme conditions of pressure and temperature for reversibility remain challenges for practical use. LiBH4 starts to release hydrogen at 380 °C, and full decomposition occurs at a temperature above 727 °C, with a further decomposition of LiH into Li.1 The decomposition mechanism of LiBH4 is relatively complex due to the formation of stable intermediates like Li2B12H12 and several potential intermediates releasing hydrogen following the proposed mechanism:2, 3 12 LiBH4 → Li2B12H12 + 10 LiH + 13H2

(1a)

-Li2B12H12 + 10 LiH → Li2B12H12-x +10LiH +x/2 H2

(1b)

Li2B12H12-x + 10LiH → (12)-BLi(y+2)/12 + (10-y)LiH + (12-x+y)/2 H2

(1c)

The enthalpy of reaction of LiBH4 from LiH and B was claimed to be of -75 kJ mol-1 H2, based on theoretical calculations.4 Experimental measurement using PCT also showed a similar value of 74 kJ mol-1 H2.5 Therefore, rehydrogenation of LiBH4 from LiH and B can only be accomplished at temperatures >600 °C and hydrogen pressures >35 MPa. This might be possible to implement on an industrial site, but it is not suitable for on-board use.6



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Methods to improve the thermodynamic properties of LiBH4 include the stabilization of the decomposition products via the addition of metal hydrides such as MgH2,7-9 LiNH210 or their combination.11-13 For example, a reduction in enthalpy of 25 kJ mol-1 H2 was observed for the LiBH4 + ½MgH2 system.9 Addition of catalysts based on transition metals,10, halides,14-18 or oxides,14,

19-21

14

their

was also investigated in an attempt to lower the activation

energy and improve the kinetics for hydrogen release. However, the kinetics and reversible conditions (> 350 °C, 10 MPa) are still very poor. An alternative approach is to utilize the idea of H+/H- coupling, by adding an amine to the metal borohydride and thus thermodynamically destabilize the complex hydride. With the help of additional catalysts, such as metal chlorides and oxides,22-24 ammine metal borohydride systems were found to release a higher amount of hydrogen without significant impurities in the form of ammonia (NH3). For instance, the LiBH4NH3/AlCl3 system released 9.1 mass% H2 accompanied by only 2 mass% of NH3.25 Similarly, LiMg(BH4)3(NH3)/LiBH4 was found to release 8 mass% H2 up to 200 °C, with no NH3 release.24 However, the exothermic nature of dehydrogenation reaction of the LiBH4NH3 based systems remains a significant challenge toward direct hydrogen reversibility and to date, the only option is the off-board regeneration of the spent fuel.26 Nanoconfinement is a potential approach to overcome both kinetic and thermodynamic issues of complex hydrides through the restriction of their particle size within the nanometer range.27-30 To date, most approaches toward the nanoconfinement of complex hydrides are through the use of porous inorganic hosts and in particular porous carbons.31,

32

Porous

carbons may possibly contribute to the improvements in hydrogen properties observed owing to (i) a destabilization of the confined complex hydride through an “active” hydride/carbon wall interface, (ii) a stabilisation of dehydrogenated products and/or (iii) an increased ionic mobility at the nanoscale and in particular that of hydrogen.33-35 Notably, NaAlH4 4 ACS Paragon Plus Environment

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nanoconfined within C60 or carbon nanotubes showed destabilized properties assigned to the electronegative nature of the hosting porous substrates and a partial charge transfer that weakened the Al-H bond.36, 37 With respect to LiBH4 nanoconfined in porous carbons, similar observations have been made,38 and hydrogen reversibility has been shown to occur to some extent under less drastic conditions of pressure and temperature once the complex hydride is nanoconfined.39-43 Despite these improvements some challenges associated with the use of porous carbons as host materials still remain in the form of: i) the overall hydrogen capacity owing to the “dead” mass and volume of the carbon scaffold, ii) the significant amount of oxygen groups at the surface of the carbon scaffolds44 – these may contribute to facilitate the release of hydrogen through oxidation of the borohydride but also significantly impede the reversible hydrogen capacity,45 and iii) the formation of stable intermediates, i.e. Li2B12H12, that contribute to the suppression of the release of B2H6 under nanoconfined conditions but also leads to decreased hydrogen cycling capacities.46,

47

Hence, it is important to find

alternative substrates that can support the nanoparticles of borohydrides while providing additional destabilization effects. Inspired by previous work,48 this study investigates a novel approach using nickel(II) phthalocyanine as a support for LiBH4 nanoparticles (Scheme 1). The chemically inert porphyrin film49 should allow nanosized LiBH4 to be cycled without degradation, since porphyrins are stable at high temperatures (> 400 C).50, 51 Furthermore, the unique electronic structure of porphyrins could provide a path for additional destabilization of the B-H bond owing to their ability to support extensive electronic delocalisation.26, 52 In addition, Ni has been found to improve the hydrogen kinetics of LiBH4.10, 53, 54



Scheme 1. Molecular structure of nickel(II) phthalocyanine (NiPc)

2. MATERIALS AND METHOD 2.1 Materials Oleic acid (≥99%), LiBH4 solution (2M in THF), LiBH4 as a solid powder (95%), pentane and Nickel(II) phthalocyanine (NiPc, C32H16N8Ni, 85%) were purchased from Sigma-Aldrich. Cyclohexane was purchased as HPLC grade from Sigma Aldrich and dried using a LC Technology SP-1 Solvent Purification System. Except for LiBH4,55 all chemicals were used without further purification. All operations were carried out under inert atmosphere in an argon-filled LC-Technology glove box (< 1 ppm O2 and H2O). 2.2 Synthesis of the LiBH4 nanoparticles In 0.2 mL oleic acid, 50 mL of pentane was added and this was transferred to a jacketed flask. The mixture was cooled down to 5 °C under constant magnetic stirring. Separately, 10 mL of a 2 M LiBH4 in THF solution was placed in a small bottle and heated to 45 °C under constant magnetic stirring. To this solution, an additional amount of purified LiBH4 solid (45 mg) was added and the solution was further stirred for 30 min. The hot LiBH4 solution was then dropped into the cold pentane with a dropping funnel to allow the precipitation of LiBH4 nanoparticles. The resulting suspension was aged for 2 h and centrifuged at 20,000 rpm at 6 ACS Paragon Plus Environment

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20 °C for 30 min. The resulting white precipitate was then washed with pentane 3 times and finally dried under vacuum at room temperature for 12 h. 2.3 Synthesis of the LiBH4/nickel(II) phthalocyanine composite material 100 mg of LiBH4 nanoparticles were suspended in 40 mL of cyclohexane in a double jacketed flask. The mixture was cooled down to 10 °C. 30 mg of Ni porphyrin dissolved in 10 mL cyclohexane was then added dropwise to the LiBH4 solution. The mixture was aged at 10 °C overnight and centrifuged at 13,500 rpm at 20 C for 30 min. The wet solid was washed with cyclohexane 3 times and dried under vacuum for at least 12 h. 2.4 Characterization Transmission Electron Microscopy (TEM), Scanning Transmission Electron Microscopy (STEM) and Energy Dispersive X-ray Spectroscopy (EDS) were performed with a Philips CM200 field emission gun (FEG) TEM operated at 200 kV. The materials were dispersed in cyclohexane, sonicated and dropped onto a carbon coated copper grid and dried in an argon filled glovebox before transfer to the microscope in a quick manner as to minimize air exposure. X-ray Diffraction (XRD) was performed by using a PANalytical X'pert Multipurpose XRD system operated at 40 mA and 45 kV with a monochromated Cu Kα radiation (λ = 1.541 Å), step size = 0.01, 0.02 or 0.05, time per step = 10 or 20 s/step. The materials were protected against oxidation from air by a Kapton foil. Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC) in conjunction with Mass Spectrometry (MS) were conducted at 10 °C min-1 under an argon flow of 20 mL min-1 using a Mettler Toledo TGA/DSC 1 coupled with an Omnistar MS.



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Masses between m/z = 2 and 100 were followed and 70 L alumina crucibles were used. The MS enabled the determination of the hydrogen desorption profiles of the materials. Infrared analysis was carried out on a Bruker Vertex 70V equipped with a Harrick Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) Praying Mantis accessory. The materials were loaded in an air-tight chamber in an argon filled glovebox and the chamber was fitted on the Praying Mantis. Spectra were acquired with a 1 cm-1 resolution with a MCT detector. The chemical properties of the surface of the nanoparticles were characterized by X-Ray Photoelectron Spectroscopy (XPS) using a Thermo Scientific ESCALAB250Xi, UK spectrometer (base pressure below 2.10-6 Pa). Pellets of the materials were prepared inside an argon filled glove box and quickly transferred to the spectrometer to minimize exposure to air. The XPS spectra were collected using a mono-chromatic Al Kα (1486.7 keV) X-ray source at 150 W power. Survey scans were collected at 100 eV pass energy with an energy step of 0.5 eV, while detailed scans were acquired at 20 eV pass energy and 0.1 eV energy step. The data were analysed using the Advantage software. Hydrogen cycling was characterized using a high pressure magnetic balance of 1 μg resolution equipped with capability for simultaneous density measurements (Rubotherm). The materials were first desorbed at 350 °C. Hydrogen cycling was then performed at 350 °C. 40 mg of material was used and a hydrogen pressure of 6 MPa for absorption and 0.01 MPa for desorption. With the high pressure balance, hydrogen uptake and release were determined from the weight changes. For an accurate determination of the amount of hydrogen stored, a blank measurement with the empty sample holder was performed at the cycling temperature to determine the mass and volume of the sample holder. Further measurements were performed at the cycling temperature under a He atmosphere with the material fully desorbed 8 ACS Paragon Plus Environment

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to determine the density of the materials and corresponding parameters for buoyancy corrections.

3. RESULTS 3.1 LiBH4 nanoparticles and properties LiBH4 nanoparticles were synthesised via anti-precipitation method.56, 57 The supersaturated and hot LiBH4 solution was injected in an cold anti-solvent, where rapid nucleation and growth immediately took place. With the aid of oleic acid as a surfactant, the growth of the LiBH4 nanoparticles was limited and precipitation of nanosized LiBH4 particles (nano-LiBH4) occurred. The morphology and structure of nano-LiBH4 was characterized by TEM and compared to its pristine material, i.e. bulk LiBH4. As shown on Figure 1, nano-LiBH4 mainly consisted of isolated particles between 2 to 18 nm, while some larger particles up to 120 nm were also observed. In comparison, bulk LiBH4 is mainly composed of micron sized particles (Supporting Information Figure S1). XRD analysis further confirmed that nano-LiBH4 retained the same orthorhombic crystalline structure as bulk LiBH4, (Figure S2), suggesting that nanosizing did not change the structure of the borohydride. However, the crystallite size of the nano-LiBH4, as determined from the Scherrer equation, was found to decrease from 67 to 30 ± 2 nm in comparison to bulk LiBH4. FTIR analysis also revealed that the B-H stretching and bending modes of nano-LiBH4 remained in similar wavelengths regions as its pristine material (Figure S3). However, some additional splitting of B-H stretching peak between 2406 and 2151 cm-1 was interpreted as the interaction between LiBH4 and oleic acid, leading to a change in site symmetry of the BH4- ions and crystal field effects at the LiBH4 nanoparticles’ surface.58, 59 The hydrogen desorption profile of bulk LiBH4 and nano-LiBH4 were compared under the same heating conditions (Figure S4). Bulk LiBH4 showed four endothermic events during its decomposition in agreement with the literature (Figure S4a).60 9 ACS Paragon Plus Environment


TGA showed a total mass loss of 17.2 %, which is slightly lower than the theoretical hydrogen capacity (18.4 mass%) owing to the partial decomposition of LiH. It is believed that the decomposition of LiBH4 results in the formation of stable products i.e. LiH which requires a temperature above 700 °C to fully release the hydrogen.1 For nano LiBH4 (Figure S4b), the hydrogen release properties remained akin to that of bulk LiBH4. The polymorphic transformation temperature was still found to occur at 116 °C, while the melting temperature was slightly reduced to 284 instead of 286 C. However, the major hydrogen release occurred at 435 °C, instead of the 512 °C for bulk LiBH4. Even though the particle size of LiBH4 was below 20 nm, the improvements observed on the hydrogen desorption profile of such nanoparticles were limited. One explanation is that the interaction between the oleic acid and LiBH4 is not sufficient to restrict the phase transformation and melting behaviours of LiBH4. Additionally, the decomposition of oleic acid occurs between 150 and 300 °C (Figure S5). Therefore upon heating, the particle of nano-LiBH4 will have the tendency to agglomerate before hydrogen release. 3.2 Modification of LiBH4 with NiPc To better stabilize nano-LiBH4, we investigated the possibility of replacing the surfactant with a Ni porphyrin. To this aim, a nickel(II) phthalocyanine solution was added to a suspension of LiBH4 nanoparticles (molar ratio NiPc : LiBH4 = 1 : 87). After 12 h of reaction to enable ligand exchange, a composite of nano-LiBH4 and nickel(II) phthalocyanine (LiBH4NiPc) was obtained upon centrifugation. TEM analysis showed that the LiBH4-NiPc (Figure 2) had the morphology of the pristine porphyrin (Figure S6). However, STEM and elemental mapping revealed the overlapping of Ni, N and B signals suggesting that the LiBH4 nanoparticles were evenly intermixed with the nickel(II) phthalocyanine. According to XRD analysis, the main phases of LiBH4-NiPc remained that of orthorhombic LiBH4 and the monoclinic nickel(II) phthalocyanine (Figure 3b). Additionally, FTIR analysis of LiBH4-NiPc 10 ACS Paragon Plus Environment

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revealed that the B-H stretching modes exhibited three peaks at 2218, 2284 and 2378 cm-1, while the associated bending mode occurred as a sharp peak at 1120 cm-1. These vibrations are at similar wavelengths as compared to pristine LiBH4, but significantly shifted when compared to nano-LiBH4 as a result of the removal of most of the surfactant in favor of the interaction of LiBH4 with the nickel(II) phthalocyanine.59 Some vibrations of oleic acid (at 2926 and 2850 cm-1) were still observed by FTIR in addition to vibrations related to the C-H stretching and bending modes of the aromatic groups of NiPc (Figure 4a and 4b), and this suggested that NiPc replaced to some extent oleic acid at the LiBH4 surface (Figure 3b). The thermal stability and hydrogen desorption property of LiBH4-NiPc were further analysed by TGA/DSC/MS. As shown on Figure 5, a small hydrogen release was observed at 90 °C, with a 0.5 % mass loss before 200 °C. The main hydrogen evolution occurred at the reduced temperature of 350 °C, and was associated with a single decomposition step visible by TGA. This suggested that the decomposition path of LiBH4 was probably changed to a one step reaction. The mass loss of 14.5 % observed by TGA beyond 200 C was slightly higher than the theoretical hydrogen content of the composite LiBH4-NiPc material, i.e. 14.1 mass% after the addition of nickel(II) phthalocyanine and thus was attributed to the release of remaining oleic acid in the material (Figure 5b). It is noteworthy that the polymeric transformation and melting of LiBH4 still remained apparent in LiBH4-NiPc, although at a slightly reduced temperature of 114 °C and 266 °C, respectively (Figure 5a). XRD analysis of this material decomposed at 350 C overnight under vacuum (0.01 MPa) revealed LiH as the main phase after hydrogen release (Figure 3d). Hence, at such a low temperature of 350 C LiBH4 modified with NiPc was fully decomposed into LiH and B. In comparison, pristine LiBH4 requires a temperature of 570 C (Figure S4a). 3.3 Hydrogen storage properties of LiBH4-NiPc



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The hydrogen desorption and absorption kinetics of LiBH4-NiPc were also analysed at 350 °C, where the major dehydrogenation occurred. As shown in Figure 6a, LiBH4-NiPc displayed fast absorption kinetics of hydrogen at 350 °C, with 90% of hydrogen uptake achieved within 50 min. The desorption kinetics were relatively slower, with 90 % of the hydrogen release occurring in 150 min. The slow kinetics may be due to the negative effect of amorphous boron on the dehydrogenation rate of borohydrides as previously reported.61 To confirm the reversibility and purity of the hydrogen released, TGA/DSC/MS of the absorbed materials was carried out (Figure 6b-c). The rehydrogenated material showed a hydrogen desorption profile very similar to the one observed for the as-synthesized material, with a 3.2 mass% loss and a single hydrogen peak at 334 °C. The polymorphic transformation and melting of LiBH4 remained visible by DSC at 115 and 265 °C, respectively. Further analysis by TEM showed a morphology evolution for the cycled material (Figure S7). Noteworthy, the shape of the particles became more defined as long and rectangular “plates” with a width of several m. Smaller “plates” were also observed. STEM and elemental mapping confirmed the overlap of the B, N, and Ni signals, suggesting that LiBH4 still remained intimately linked to the nickel(II) phthalocyanine. The alternation in morphology was most likely the result of the recrystallization of nano-LiBH4 and/or LiH phases upon successive hydrogen absorption/desorption cycles along certain planes. The nature of the reversibility was further investigated by XRD, FTIR and XPS. Upon heating LiBH4-NiPc at 260 °C, crystalline LiBH4 was still visible upon cooling as an orthorhombic phase by XRD (Figure 3c), and this further confirmed that the hydride did not fully decompose below 300 C in agreement with the TGA/DSC/MS analysis (Figure 5). However, diffraction peaks from nickel(II) phthalocyanine were not observed after heating LiBH4-NiPc at 260 C, i.e. close to the melting temperature of LiBH4, and this suggested an amorphous state or a “screening” effect owing to the melting and recrystallization of LiBH4 12 ACS Paragon Plus Environment

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on the nickel(II) phthalocyanine (Figure 3c). FTIR analysis further revealed that LiBH4 and nickel(II) phthalocyanine retained their structure upon heating at 260 C (Figure 4c). The B-H stretching and bending mode of LiBH4 remained the same as for the as-synthesised material. The peaks associated with nickel (II) phthalocyanine were still visible but the intensity was dramatically reduced, possibly due to the “screening” effect of molten nano-LiBH4 covering the nickel(II) phthalocyanine. After hydrogen release at 350 C, XRD analysis of LiBH4-NiPc only showed diffraction peaks assigned to LiH along smaller peaks corresponding to Li3BO3 and thus a partial oxidation of the material during XRD analysis (Figure 3d). FTIR analysis of the desorbed material also confirmed the disappearance of both B-H stretching and bending vibrations (Figure 4d). However, vibrations corresponding to Li2B12H12 were detected, and this confirmed that LiBH4 still partially decomposed through the formation of stable intermediates. Upon hydrogen absorption, the B-H stretching reappeared at 2380, 2289, and 2220 cm-1, while the bending peak came back 1123 cm-1, i.e. the same position as observed in assynthesized LiBH4-NiPc (Figure 4b and 4e). This proved the regeneration of LiBH4 in agreement with the hydrogen cycling and TGA/DSC/MS measurements (Figure 6). However, the corresponding crystalline phase was not observed by XRD (Figure 3e) and this suggested an amorphous state of the regenerated LiBH4. Based on these results, it can thus be assumed that the LiBH4-NiPc reversibly stored hydrogen following the reaction path: LiBH4-NiPc  LiH + NiPc + B + 3/2 H2

(2)

XPS was carried out to further characterize the electronic state of the porphyrin and gain insights in the destabilization effects of nickel(II) phthalocyanine on LiBH4 (Figure 7). In the absorbed state, LiBH4-NiPc revealed by XPS analysis a single Li1s peak at 56.3 eV assigned to the regeneration of LiBH462 and in agreement with the Li1s spectrum of as-synthesised 13 ACS Paragon Plus Environment


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LiBH4-NiPc (Figure S8) and FTIR results. The B1s narrow scan of the absorbed material also showed peaks at 188.9, 191.2 and 192.5 eV that were assigned to LiBH4,63, 64 oxidized boron species,62 i.e. BxOy (1.5

LiBH4 Electronic Destabilization with Nickel(II) Phthalocyanine

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