Article pubs.acs.org/JPCC
Characterization of the Dehydrogenation Process of LiBH4 Confined in Nanoporous Carbon Stephen D. House,† Xiangfeng Liu,‡ Angus A. Rockett,† Eric H. Majzoub,§ and Ian M. Robertson*,∥ †
Department of Materials Science, University of Illinois at Urbana−Champaign, 1304 West Green Street, Urbana, Illinois 61801, United States ‡ College of Materials Science and Optoelectronic Technology, University of Chinese Academy of Sciences, Beijing, 10049, P. R. China § Center for Nanoscience and Department of Physics and Astronomy, University of MissouriSt. Louis, One University Boulevard, St. Louis, Missouri 63121, United States ∥ College of Engineering, University of WisconsinMadison, 1415 Engineering Drive, Madison, Wisconsin 53706, United States S Supporting Information *
ABSTRACT: Annealing in the transmission electron microscope allows direct comparison of the same nanoporous carbon scaffold particles following dehydrogenation of the confined LiBH4. At a nominal temperature of 200 °C, a granular crust of cubic and cuboidal LiH nanocrystals ∼8−15 nm wide grew on the outer surface of the scaffold. Furthermore, these crystals migrated on the carbon support film away from the scaffold. Ejection of material from the scaffold also occurred upon dehydrogenation in a differential scanning calorimeter. The form of the ejected material was different, being bristles instead of cubes or cuboids. This ejection of LiH and thus preferential segregation of lithium from boron is proposed to explain the observed continual decrease in hydrogen storage capacity with the number of cycles.
1. INTRODUCTION Developing a suitable lightweight onboard regenerative hydrogen storage system remains a challenge in the quest for a practical H2-powered fuel cell car.1 The size and weight restrictions imposed by automotive transportation applications make complex metal hydrides candidate materials for such a fuel storage system.2−5 This class of materialscomposed of light metal cations (e.g., Li, Na, Mg) and anion hydrides (e.g., AlH4−, NH2−, BH4−)has high gravimetric and volumetric storage capacities compared to other H2-storage options, but unfavorable thermodynamics and kinetics of hydrogen uptake and release hinder their practical use. The addition of various catalysts, typically metals or nanostructured carbon, has been shown to reduce the temperatures required for hydrogen absorption and desorption, in some cases quite dramatically.6,7 Mixing stable hydrides with certain other complex metal hydrides can exhibit a destabilizing effect, in which the new alloys or compounds that form during dehydrogenation possess a lower enthalpy of reaction, yielding more favorable thermodynamics.8−10 A combination of first-principles calculations and experimental efforts have predicted and shown significant gains in kinetics in certain hydride materials when reduced to nanoscale particle sizes. This has been attributed to the increase in surface area, increase in interfacial contact, and decrease in diffusion distances.11−15 Sintering or agglomeration of the nanoparticles © 2014 American Chemical Society
during hydrogen cycling, however, could result in an effective coarsening of particle size and a concomitant loss of the benefits attained at the nanoscale. Infiltration of the storage medium into a nanoporous host scaffold offers a way both to restrict the particle size as well as hinder agglomeration and/or segregation of the constituents.16 Lithium borohydride (LiBH4) possesses one of the highest theoretical gravimetric hydrogen storage capacities, 18.5 wt %, but requires temperatures around 460 °C to dehydrogenate and only rehydrogenates at 600 °C under 350 bar of H2.17 Additionally, decomposition can result in the formation of stable closoborane species18 or diborane (B2H6).19,20 Numerous approaches have been investigated, with mixed success, to improve on these properties. Doping with transition metal, metal oxide, and metal halide catalysts as well as alloying with other hydrides has in many cases improved the reversibility of the LiBH4 system.20−23 The benefits thus far have been proven to be limited, enhancing one performance aspect or resulting in undesirable stable compounds. Nanoconfinement of LiBH4 has met with somewhat better success.23−30 Gross et al. infiltrated LiBH4 into carbon aerogels with 13 and 25 nm average pore sizes and activated carbon with 2 nm average pore size and Received: October 2, 2013 Revised: January 24, 2014 Published: April 3, 2014 8843
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ments atmospheric thin window energy dispersive X-ray spectrometer equipped with a Si(Li) photon detector. The S/ TEM analysis was performed at an operating voltage of 200 keV. The SEM used was a JEOL 7000F SEM, equipped with a Schottky field-emission source. Specimens for microscopy were prepared by applying the powdered material to holey carbon coated 400-mesh Cu TEM grids (SPI supplies) inside a MBraun argon-filled glovebox maintained at nominal O2 and H2O concentrations below 0.1 ppm. To minimize exposure to the environment during the TEM and STEM experiments, a Gatan HHST 4004 environmental vacuum cell transfer stage was utilized. The grids were loaded into the stage inside the glovebox. Then the stage tip was retracted into the vacuum cell, where it was sealed and evacuated to a roughing vacuum, after which it was inserted into the microscope column. To minimize contamination, the stage was plasma cleaned for 5 min prior to loading into the glovebox. The samples for SEM examination, mounted on an aluminum stub, were briefly exposed to atmosphere during transfer from the glovebox to the microscope; exposure was typically less than 10 min. Samples in the discharged state were also examined for comparison with those discharged in the TEM. The material for these specimens had been outgassed in a differential scanning calorimeter (TQ2000) in an argon-filled glovebox, where they were heated to 550 °C at a ramp rate of 10 °C/min under a nitrogen flow rate of 50 mL/min.30 Specimen preparation was otherwise identical to that used for the charged samples. The STEM heating experiments were performed using the vacuum transfer stage. For each heating step, the stage tip was retracted into the vacuum cell, ramped to the desired temperature at a rate of 3−5 °C/min, held for 30 min, cooled at 8−10 °C/min, and then returned to the observation position. Nominal temperatures of 100, 200, and 250 °C were used; the actual temperature is not known because, other than the copper grid, the remainder of the material is not in direct contact with the heating element. Indeed, the actual temperature is expected to be higher than the nominal temperatures reported. An asymmetric center marking on the TEM sample grids allowed specific scaffold particles to be revisited after each heating step. Different S/TEM conditions for examination were explored to find the exposure conditions that caused minimum degradation to the scaffold and the enclosed material. In general, under normal operating conditions, STEM was found to be preferable to conventional TEM for minimizing material degradation due to electron beam effects. In order to acquire selected area electron diffraction patterns (SADP), for which conventional TEM is preferred, low current conditions were used to delay the onset of beam damage. It was noted that the infiltrated LiBH4 exhibited less sensitivity to the electron beam than did LiBH4 on the outer surface of the scaffolds. Additional discussion of the effects of the electron beam on the infiltrated scaffolds is included in the Supporting Information (Figure S1).
observed an increase in dehydrogenation rate with decreasing pore size, with rate enhancements up to 50 times faster than bulk LiBH4 at 300 °C. They were also able to rehydrogenate at 400 °C and 100 bar of H2. A greater retention of hydrogen storage capacity following cycling was also exhibited with decreasing pore size, compared to bulk LiBH4.26 Nielsen et al. showed significant improvement in desorption kinematics, and potentially thermodynamics, through nanoconfinement of LiBH4/MgH2 in carbon aerogels.28 By confining LiBH4 in porous carbon, Ngene et al. found a 100−150 °C decrease in desorption temperature along with improved reversibility, 5.8 wt % hydrogen release at 320 °C and 40 bar, an effect further enhanced by doping with Ni, to 9.2 wt %.23 The nanoconfinement of LiBH4 in highly ordered nanoporous carbon scaffolds, with 2 nm diameter pores, resulted in a significant decrease in desorption temperature, to 220 °C, as well as improved reversibility.29 Unlike previous scaffolds used, these highly ordered carbon structures possess a narrow distribution of pore sizes, which allowed study of the effect of confinement size on system performance. Reduction in diborane production with decreasing pore size was observed, down to complete elimination for the 2 and 4 nm pores, which also avoided the formation of closoboranes.30 They speculated that the loss of crystallinity in the LiBH4, resulting from the decreased particle size when nanoconfined, played a role in altering the decomposition reaction pathway. This is of particular interest, because the formation of diborane is known to reduce the storage capacity. In this paper, the results of direct characterization of LiBH4 confined in a carbon scaffold on dehydrogenation are presented. The results have implications regarding degradation of storage capacity during cycling.26,30
2. EXPERIMENTAL DETAILS The LiBH4-impregnated highly ordered nanoporous carbon (NPC) scaffolds were prepared by following the method detailed in refs 29−31. The scaffolds were produced using amphiphilic triblock copolymer templates and phenolic resins for the carbon precursors. The films were scraped off and ground to powder before calcination at 1200 °C under 2.5% O2 (balance N2). An ASAP2020 physisorption analyzer was used to measure nitrogen adsorption isotherms at 77 K for the carbon scaffolds. Prior to this analysis, the scaffolds were outgassed at 300 °C for 1 h under a vacuum. The specific surface area was determined using the Bruner−Emmett−Teller (BET) method, using adsorption data in a relative pressure range from 0.05 to 0.2 bar. Total pore volume and the pore size distribution were estimated using the Barrett−Joyner−Halenda (BJH) method. The scaffolds were impregnated with LiBH4 using melt-infiltration. The maximum practical fill fraction for the NPC was previously determined to be 35 wt % LiBH4.30 Two batches with different filling amounts (20 and 30 wt %) were examined in this study. The morphological and microstructural studies of these samples were performed using transmission electron microscopy (TEM) for imaging and electron diffraction, scanning transmission electron microscopy (STEM) for imaging and heating experiments, energy-dispersive X-ray spectrometry (EDS) for compositional analysis, and scanning electron microscopy (SEM) for surface imaging. The TEM used was a JEOL 2010 TEM equipped with a LaB6 thermionic emission source. The STEM used was a JEOL 2010F S/TEM equipped with a Schottky field-emission source and an Oxford Instru-
3. RESULTS 3.1. As-Synthesized Morphology. STEM examination of the unfilled carbon scaffolds shows their structure to be a highly ordered array of hexagonally packed columns, as shown in Figure 1, with a narrow distribution of pore diameters of around 3.6−4.3 nm and pore spacing around 9−9.8 nm. The scaffolds are subdivided into domains of columns, which have differing orientations with respect to each other. Analysis of the nitrogen adsorption measurement (Figure S2, Supporting 8844
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into the host scaffold pores is visible in S/TEM due to the reduction in pore/channel contrast that occurs in the filled scaffold (Figure 3a) relative to the unfilled one (Figure 1b).
Figure 1. High-angle annular dark field (HAADF) STEM micrographs of the unfilled highly ordered carbon scaffolds showing the hexagonally packed columnar pores (a) end-on and (b) in profile.
Information) determined the scaffolds to have a high specific surface area of 1133 m2/g and a pore volume of 0.75 cm3/g. The pore size distribution, centered around 3.8 nm, of the unfilled carbon scaffolds as determined by BJH (Figure 2) agrees well with the direct S/TEM observations. Although the light elements lithium and boron are not detectable with most current EDS systems, including the one used for the present study, the presence of material infiltrated
Figure 3. HAADF-STEM micrographs of the same LiBH4-infiltrated NPC scaffold particle after the (a) 100 °C and (b) 200 °C heating runs. The boundary between the two domains of columns is indicated by a black dotted curve in panel a along with straight lines showing the column orientation in each domain. The bright curved band reaching from the upper right corner of each image to the bottom is the edge of the underlying carbon support film, indicated by a white dotted line.
Previous X-ray diffraction (XRD) analysis of identically prepared specimens by Liu et al.29,30 found the infiltrated material to be LiBH4 in an amorphous state. The presence of LiBH4 and not contaminates is corroborated by EDS spectra of the infiltrated scaffolds, which show an absence of any contaminants above the detection limit (Figure S3, Supporting Information). Electron energy-loss spectroscopy (EELS) was not used, despite its sensitivity to light elements, since the spectrum acquisition time was found to be of sufficient duration for the electron beam to bore a hole through the particle. In both 20 and 30 wt % batches, residual noninfiltrated LiBH4 was sometimes observed attached to the outside of the scaffold; however, as expected, it was rarer in the 20 wt % filled
Figure 2. Pore size distribution of the nanoporous carbon scaffold, calculated using the BJH method from the nitrogen desorption measurement (Figure S2, Supporting Information). 8845
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material. The external LiBH4 took two main forms: irregular nodules or blisters. Both forms ranged in size from ∼50 nm up to over 1 μm across and were rounded and smooth, in contrast to the scaffold particles, the edges of which were typically angular or broken along layers of columns. Examples of these forms of noninfiltrated LiBH4 are shown in Figure 4; the arrows indicate examples of blisters. The morphological changes are discussed further later.
Figure 5. HAADF-STEM micrographs of the same LiBH4-infiltrated NPC scaffold particle after the (a) 100 °C and (b) 200 °C heating runs. The pore structure of the underlying scaffold is clearly visible and intact both before and after the nanocrystalline crust forms.
Figure 6. SEM micrograph of a LiBH4-infiltrated NPC scaffold particle (upper left half) supported on a holey carbon film (lower right half) after annealing at 250 °C. The granular crust of LiH nanocrystals can clearly be seen on the exterior of the scaffold particle. Nanocrystals are also evident on the carbon support film.
Figure 4. HAADF-STEM micrographs of overfilled scaffold particles showing examples of the form the noninfiltrated external LiBH4 takes. Arrows indicate some blisters of LiBH4: (a) side view and (b) top/ bottom view. Although these are larger than most examples, the shapes observed were the same, regardless of size. These were selected due to the increased contrast with size, for ease of viewing.
Electron diffraction was performed on the granular crusts to determine their crystal structure. By comparing the observed diffraction peaks against a library of known positions and intensities of crystal structures and materials, the crystal structure of the material comprising the crust was determined. The electron diffraction pattern (Figure 7a) is a polycrystalline ring pattern and does not show evidence of any orientation preference. The integrated radial intensity (Figure 7b) of the diffraction pattern was calculated using the Circular Hough Transform Diffraction Analysis plugin for Gatan’s Digital Micrograph.32 The pattern was compared with those of materials composed of Li, B, O, C, and/or H, with the closest match being LiH, for which the expected diffraction peak positions exhibited 95−99% matches with the measured patterns. Accompanying bright- and dark-field TEM micrographs (Figure 7c,d) of the particle confirm that the LiH rings originate from the nanocrystal crust. The size of the LiH nanocrystals in the dark-field image agrees with that assessed from the HAADF-STEM images (Figures 3b and 5b). Subsequent heating to 250 °C resulted in no significant additional changes, including crust growth.
3.2. Behavior During Outgassing. HAADF-STEM observations of both 20 and 30 wt % specimens heated to 100 °C using the TEM holder revealed no noticeable morphological differences compared to the unheated asprepared state. The micrograph presented in Figure 3a shows a thinner portion of a large scaffold particle that had been heated to 100 °C. Two domains of differing column orientations are visible, indicated by the black dotted line, though with reduced contrast due to infiltration. Upon subsequent heating to 200 °C, a significant change in morphology occurred: a granular crust composed of cubic and cuboidal nanocrystals formed on the exterior surface of the particles (Figure 3b). These nanocrystals were much larger than the NPC pore diameters, with typical side lengths of 8−15 nm, though sizes up to 20 nm and down to 3 nm were observed. The underlying scaffold structure remained intact beneath the crust (Figure 5), indicating that the nanocrystals resided on the exterior of the scaffold. This observation was corroborated in SEM images (Figure 6), in which nanocrystals are clearly visible on the exterior surface and carbon support film. 8846
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Figure 7. (a) Typical electron diffraction pattern from a LiBH4-infiltrated NPC particle following formation of granular crust via outgas. The contrast was adjusted in the left half in order to more clearly show the fainter outer rings. (b) Integrated radial intensity profile of panel a along with the expected diffraction peaks of LiH, indicated by dotted lines. (c) Bright-field and (d) dark-field TEM micrographs of the particle used for panel a. For panel d, the objective aperture was placed over a portion of the LiH 200 ring, indicated by the white circle in panel a, revealing some of the nanocrystals in the crust.
Migration of the LiH crystals away from the NPC scaffold and across the holey carbon support film was also observed. As can be seen in the HAADF-STEM micrograph presented in Figure 8a, the crystal size and density decrease with distance from the scaffold particle. This suggests that the migration occurs continuously during heating, with some of the crystals colliding during migration and agglomerating to form the larger crystals. Support for this agglomeration can be seen in the higher magnification images presented in Figure 8b, which show the presence of the smaller crystals. The size distribution of the migrating crystals (Figure 9) was determined using the particle analysis functionality of ImageJ.33 To use this function, a threshold level was set to distinguish the crystals from the carbon film. Spurious artifacts were manually removed, and a combination of watershed algorithm and manually segmenting was used to separate any particles that could not be automatically demarcated. The areal size of each particle was measured and then converted into a side length, assuming square particles, a reasonable selection given the observed crystal shapes. The resulting distribution agrees well with the sizes observed on the scaffold exteriors by STEM. The truncation of the distribution below 5 nm is a result of the lower areal bound placed on the analysis; visual inspection of the images suggests that the histogram should have a small tail down to around 3 nm. A lower bound was used because round
