Carbon Dioxide-Facilitated Low-Temperature Hydrogen Desorption

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Carbon Dioxide-Facilitated Low-Temperature Hydrogen Desorption from Polyaminoborane Ran Xiong,† Junshe Zhang,‡ and Jae W. Lee†,‡,* †

Department of Chemical Engineering, The City College of New York, 140th St and Convent Avenue, New York, New York 10031, United States ‡ Department of Chemical and Biomolecular Engineering, KAIST, 291 Daehak-ro, Yuseong-gu, Daejeon, 305-701, South Korea ABSTRACT: Gaseous CO2 environments dramatically enhance the thermolytic dehydrogenation of polyaminoborane (PAB) at 85 - 90 °C. By exposing PAB to 2.07 MPa CO2, 5.48 wt % hydrogen based on the mass of ammonia borane (AB) desorbs in less than 1 h, while 4 h is required to generate 5.04 wt % hydrogen by maintaining PAB at 120 °C without using CO2. The total hydrogen desorbed from the initial AB is 11.38 wt % under the CO2 atmosphere at 85 °C, including the hydrogen release during the transition from AB to PAB. The main contributing factors to the facile hydrogen release are the promoting effect of newly formed formic acid and the exothermic effect of the reaction between PAB and CO2. Spectroscopic and elemental analyses of the solid spent PAB confirm the degree of dehydrogenation and indicate that the decomposed residue has an empirical formula of BN0.86H2.34(CO2)0.17.

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INTRODUCTION Storing hydrogen (H2) safely and affordably is a major challenge for the development of H2-powered vehicles. Ammonia borane (AB, NH3BH3) is one of the promising solid state on-board H2 storage media, because it has a gravimetric H2 density of 19.6% and is a nonflammable, nonexplosive solid at ambient conditions. AB decomposes in three sequential steps in a temperature range of 80 to 1500 °C, with about 6.5 wt % H2 liberated in each step.1−4 Although the first step of dehydrogenation commences at temperatures below 100 °C, the H2 release is relatively low in this temperature range. Numerous efforts have been focused on enhancing H2 release around 85 °C, which is the typical operating temperature of polymer electrolyte membrane (PEM) fuel cells. The proposed approaches include confining AB within porous materials,5−8 dispersing AB in liquids,9,10 and exposing AB to gaseous media.11,12 The first two options have the following drawbacks: (1) material-based H2 storage capacity is low due to the weight of media; (2) the recovery of spent AB and the recycle of media are of practical concern; and (3) the cost of media makes the process less economically viable. The use of gaseous media such as CO2, however, can circumvent these issues associated with condensed media. We recently found that about 10.1 wt % H2, based on the mass of initial AB, desorbs from CO2-treated AB in 1 h at 85 °C.11 However, due to the CO2 treatment of AB, the mass increase in AB was 13 wt % and the final H2 desorption decreased to 9 wt % based on the total mass of CO2-treated AB. Thus, the current work aims at further increasing H2 release from AB above 10 wt % in terms of the total mass of the © 2013 American Chemical Society

storage material. To this end, this work investigates the dehydrogenation of polyaminoborane (PAB), spent AB with around 6 wt % H2 desorbed in a low temperature range of typical fuel cell operation (85−90 °C). H2 release from PAB generally commences at temperatures of 120 to 200 °C.13,14 However, this study demonstrates that a CO2 atmosphere of 1.38 to 2.07 MPa at 85−90 °C is sufficient to desorb around 5.14 to 5.48 wt % H2 from PAB. Considering the H2 release during the preparation of PAB, it is determined that more than 11 wt % H2 desorbs from AB at 85−90 °C. Upon combining the results of elemental analyses and spectroscopic investigations, the mechanism behind the CO2 enhanced thermal dehydrogenation of PAB is proposed.

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EXPERIMENTAL SECTION Materials. Ammonia borane (NH3BH3, AB) with a purity of 97% was purchased from Sigma-Aldrich. Carbon dioxide (CO2) with a purity >99.8% was obtained from T.W Smith. Ethane (C2H6) with a purity >99% was supplied by Praxair. All chemical were used as received. Preparation and Identification of PAB. Approximately 100 mg of AB was loaded into a high-pressure cell with a volume of 11.65 mL, followed by charging ethane to a pressure of 1.38 MPa, which is inert in terms of AB thermolysis.11,12 The reactor was then kept at 85 °C for 4 h in an oil bath. The mole fraction of H2 in the gas phase was subsequently analyzed by Received: November 15, 2012 Revised: January 28, 2013 Published: January 31, 2013 3799

dx.doi.org/10.1021/jp311315t | J. Phys. Chem. C 2013, 117, 3799−3803

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Characterization of Solid Residues. ATR-FTIR (attenuated total reflection Fourier-transformation infrared) spectra were recorded on a Bruker Alpha-P spectrometer (Bruker Optik Gmbh) at a resolution of 2 cm−1 from 500 to 4000 cm−1. Raman spectra were obtained by using an HR800 Horiba Jobin Yvon Dispersive-Raman system equipped with a CCD detector and an Olympus BX41 microscope with a 50× objective lens. Samples were placed on a cover glass and excited with 514.5 nm Ar ion laser radiation. The spectrum was obtained by multiple-spectral bandpasses between 100 and 4000 cm−1. Elemental analyses were performed on an EA1110 CHNS-O (Thermo Finnigan) elemental analyzer. 13C solid state MAS NMR (magic angle spinning-nuclear magnetic resonance) spectra were recorded at 100.6 MHz on a Bruker AVANCE 400WB with a 4 mm probe, operating in a 9.4 T magnet. Samples were spun at 15 kHz for 13C with the sample temperature maintained at 25 °C. The 13C solid state crosspolarization (CP)-MAS NMR spectra were referenced to DSS (4,4-dimethyl-4-silapentane-1-sulfonic acid) at 0 ppm. All data were processed with 50 Hz line broadening.

gas chromatography (GC) and the solid product was referred to as PAB. The elemental analyses (Table 1) demonstrate that Table 1. Elemental Analyses of PAB and Solid Decomposition Residuesa sample a sample b sample c

N (wt %)

H (wt %)

C (wt %)

48.06 46.72 37.19

14.48 9.43 7.21

6.10

Sample a: PAB obtained from AB decomposition in 4 h at 85 °C; sample b: PAB solid residue in 4 h at 120 °C under 2.07 MPa C2H6; and sample c: PAB solid residue in 1 h at 90 °C under 1.38 MPa CO2. a

the mass fraction of nitrogen (N) and hydrogen (H) elements for PAB is 48.06 and 14.48 wt %, respectively. The mass fraction of boron (B) is found to be 37.46 wt %, because the amount of other elements except for H, B, and N in PAB is negligible. Also, the mole ratio of B to N is 1.01, suggesting that no ammonia forms during AB thermolysis at 85 °C. The empirical formula for PAB can be written as (NBH4.2)n. Assuming that the H2 is the only volatile compound during the AB thermolysis, the H2 yield is calculated to be 5.84 wt %, which is very close to the measured value (5.9 wt %) by GC. These results exhibit that the thermolysis products are PAB and pure H2. In addition, the infrared spectrum of PAB reveals that the PAB produced here has the same structure as that of earlier reports.11−13,15 Thermolysis of PAB on a Differential Scanning Calorimeter (DSC). After loading about 5 mg of PAB into the sample cell of the Micro-DSC VII (SETARAM), it was pressurized with carbon dioxide or ethane to 1.38 MPa. The sample and reference cells were kept at 25 °C for 5 min, and then they were heated to 115 °C at 1 °C/min. Dehydrogenation of PAB in a High Pressure Cell. After PAB was prepared from 100 mg of AB in the high-pressure cell, the reactor was purged by ethane twice, followed by charging ethane to 0.689 MPa at room temperature. Carbon dioxide was then charged to a desired pressure. Once the temperature of the oil bath reached a set point, the reactor was quickly immersed into the oil bath and maintained at this temperature for 1 h. Upon the completion of dehydrogenation, the gas phases and solid residues were collected for further analyses. For PAB dehydrogenation in the ethane atmosphere, the reactor was pressurized with only ethane after several purges. Hydrogen Yield Measurements. After the dehydrogenation of PAB, the mole ratio of ethane to H2 for the gas phase was determined by using a HP 5890 SII GC equipped with a MolSieve 5A Plot capillary column (30 m × 0.53 mm, SigmaAldrich). On the basis of the initial mole number of ethane charged to the reactor, the H2 mass yield is calculated by the following equation: Y(H 2) = MW(H 2) × ρ × V ×

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RESULTS AND DISCUSSION The thermal decomposition of PAB under CO2 and C2H6 atmospheres was investigated on a high-pressure DSC. The detailed description of this apparatus was given in a prior study.16 As Figure 1 shows, no significant exothermic or

Figure 1. Thermograms for PAB exposed to 1.38 MPa CO2 and 1.38 MPa C2H6.

endothermic events are detected at temperatures of 25 to 115 °C under 1.38 MPa of C2H6. However, a sharp exothermic peak is observed in the DSC thermogram under 1.38 MPa of CO2 and it commences at around 80 °C and maximizes at 105 °C. This exothermic peak is tentatively attributed to the reaction between CO2 and PAB. Hereafter, the pressure of CO2 or C2H6 is that at room temperature and is denoted as Po. To measure hydrogen yield and explore the mechanism, we performed thermal decomposition of PAB in a high-pressure cell. As shown in Figure 2, at the initial stage (around 5 min), a pressure rise of 0.82 MPa is observed under a CO2 partial pressure of 1.38 MPa. Concurrently, the temperature rapidly rises above 120 °C and then quickly decreases to 90 °C. Under 2.07 MPa C2H6, neither pressure nor temperature spikes are observed as the temperature approaches each set value. The decomposition behavior in the C2H6 atmosphere mainly depends on the temperature. At 120 °C, a pressure increase of about 1.2 MPa is achieved in 4 h. At 90 °C, however, the pressure increase is lower than 0.2 MPa in 4 h, because the

M(H 2) ÷ W × 100% M(C2H6) (1)

V: volume of reactor, 11.65 mL W: initial AB mass, g ρ: density of ethane at 0.689 MPa and room temperature, mol/mL MW(H2): molecular weight of H2, 2 g/mol M(H2)/M(C2H6): mole ratio of ethane to H2. 3800

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and 2.07 MPa CO2 at 85 and 90 °C. The H2 yields at the end of decomposition are given in Figure 4. For 0.689 MPa CO2, the

Figure 2. Pressure (upper) and temperature (lower) profiles for PAB dehydrogenation in C2H6 and CO2 atmospheres. Black: 2.07 MPa of C2H6 at 90 °C; Blue: 2.07 MPa of C2H6 at 120 °C; Red: mixture with 1.38 MPa CO2 and 0.689 MPa C2H6 at 90 °C.

Figure 4. Hydrogen yield for PAB decomposition in 1 h at different temperatures and under different CO2 pressures.

decomposition of PAB usually occurs at temperatures around 120 °C.13,14 The H2 yield is 5.32 wt % within 1 h at 90 °C under 1.38 MPa of CO2. In contrast, 4 h is needed to obtain 5.04 wt % H2 at 120 °C under 2.07 MPa C2H6. Therefore, we conclude that CO2 promotes PAB dehydrogenation at lower temperatures. Moreover, a comparison of the temperature and pressure profiles among the above cases reveals that some active species related to the decomposition of PAB form in the CO2 atmosphere, because the thermal effect alone cannot give rise to the fast decomposition rate, as observed in the ethane environment at 120 °C. Subsequently, we investigated the effect of CO2 pressures (0.689, 1.38, and 2.07 MPa) and temperatures (80, 85, 90, and 95 °C) on the PAB decomposition behavior. As Figure 3

H2 yield increases from 0.775 to 3.49 wt % as the temperature increases from 85 to 95 °C. For 1.38 and 2.07 MPa CO2, the H2 yield is between 5.14 and 5.49 wt % at 85 and 90 °C. Specifically, the H2 yield is 5.48 wt % for 2.07 MPa CO2 at 90 (or 85) °C. After considering the H2 desorption during the preparation of PAB (5.9 wt.%), the total H2 released from AB is 11.38 wt.% at 85 and 90 °C. To obtain more insight into the decomposition behavior of PAB, we acquired the IR and Raman spectra of PAB and its solid decomposition residues. For both residues prepared under 1.38 MPa CO2 at 90 °C in 1 h and 2.07 MPa C2H6 at 120 °C in 4 h, a new band at 3430 cm−1 appears in the spectra (Figure 5).

Figure 5. IR Spectroscopy of PAB (a), solid residue of PAB in 4 h at 120 °C under 2.07 MPa C2H6 (b), and solid residue of PAB in 1 h at 90 °C under 1.38 MPa CO2 (c).

Figure 3. Pressure profiles for PAB decomposition under three CO2 pressures at four temperatures.

shows, the pressure increase is negligible in 1 h under 0.689 MPa CO2 at 85 °C and under 2.07 MPa CO2 at 80 °C. It should be noted that the initial pressure increase is due to the reactor temperature increase to the set point. Under 0.689 MPa CO2, the pressure continuously but slowly increases until the reactor temperature reaches 90 °C. However, a small pressure spike is observed in the first 5 min, followed by a relatively slow pressure increase in the next 1 h, at 95 °C under the same CO2 pressure. Similar pressure profiles are also observed for 1.38

This band is assigned to the N−H stretching mode involving πbonded nitrogen,15 e.g., terminal NH2 groups or segments of polyiminoborane (PIB) that structurally approximate the solid decomposition product resulting from AB after desorbing 2 equiv. of H2. This band also can be observed at 3326 cm−1 in the Raman spectra of two deydrogenation residues (Figure 6). In addition, both N−H and B−H stretching modes are observed at higher wavenumbers for the decomposition solid 3801

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Figure 6. Raman spectrum of PAB (a), solid residue from PAB in 4 h at 120 °C under 2.07 MPa C2H6 (b), and solid residue from PAB in 1 h at 90 °C under 1.38 MPa CO2 (c).

Figure 7. 13C MAS NMR spectra for the solid residue of PAB decomposition under 1.38 MPa of CO2 at 90 °C.

residues than those for PAB. These observations exhibit that, for these two conditions, the decomposition residue is PIB-like. The empirical formula of solid residue can be inferred from the elemental analyses (Table 1). With 1.38 MPa CO2 at 90 °C, the empirical formula is BN0.86H2.34(CO2)0.17 if we assume that the atom ratio of O to C is 2 from gaseous CO2 and, under 2.07 MPa C2H6 at 120 °C, the formula is BN0.82H2.32. The results indicate that N-containing volatile species such as ammonia form during the decomposition, as the B:N ratio deviates from 1. It is thus expected that CO2 cannot inhibit the occurrence of these species. The fixation of oxygen and carbon from gaseous CO2 to solid dehydrogenation residue was also tracked in this work. As shown in Figure 5, the peaks for C−O, CO, and B−O bond are observed at 1242, 1686, and 1338 cm−1, respectively, in the IR spectra of the solid residue under CO2. The Raman spectra can provide more details about carbon and oxygen groups in the solid residue, as shown in Figure 6. The strong peak at 1346 cm−1 may be attributed to an asymmetrical B−O stretching mode of the BO3 group. B−O and CO stretching can be observed at 1389 and 1692 cm−1, respectively. The observation of C−O and B−O suggest that CO2 is chemically incorporated rather than physically absorbed into the decomposition residue. Furthermore, an aliphatic group and hydroxyl bonds were detected by Raman spectroscopy. The peaks at 2838 and 2965 cm−1 are assigned to C−H stretching. A hydroxyl bond can be observed at 3074 cm−1. However, IR and Raman spectroscopic investigations can only provide the basic information of chemical bonds (B−O, C−O, CO, C−H, and O−H). To obtain insight into the structure of carbon-containing groups, we further acquired 13C NMR spectra of the dehydrogenation residue under CO2. As shown in Figure 7, a formate (HCOO) group was located at 169 ppm, which was also reported in the solid product of the CO2 reaction with LiBH417 and AB.12,18 On the basis of computational studies, Zimmerman et al.19 proposed that formic acid (HCOOH) was the initial product from the reaction between AB and CO2 and two hydrogens were transferred from AB to CO2. In this study, a formate group was also observed in the dehydrogenation residue, thus experimentally confirming the mechanism proposed on the basis of the simulation.19 The transition from CO2 to formic acid can also be the first step for carbon fixation by PAB, since PAB partially contains the features of AB.

From 13C MAS NMR spectra, two more carbon groups: methanediol (OCH2OH) and methoxy (OCH3) groups were observed at 87 ppm and 51 ppm, respectively. Carbon atoms in these three groups have different valencies: +2 for the fomate group, 0 for the methanediol group, and −2 for the methoxy group, which may imply that the reduction of CO2 by PAB is a three-step process. This mechanism can be proposed as given below: CO2 → −OOCH → −OCH 2OH → −OCH3

(2)

Furthermore, the presence of the B−O bond may suggest all three groups are chemically connected with the solid residue rather than free molecules. Based on the results of the simulation study19 it was reported that the enthalpy (ΔH, 298 K) for the formation of HCOHOBH3 from −BH3 and HCOOH is −21.1 kcal/mol, and subsequently HCOHOBH3 decomposes into hydrogen and HCOOBH2 with ΔH = 4.3 kcal/mol. We can thus propose formic acid will connect with the solid residue by the B−O bond, which is very stable with a high bonding energy. Upon its formation, the B−O bond is very hard to break. The reduced groups (methanediol groups and methoxy groups) will also be fixed in the solid residue. In our previous works12,18 we reported that the reaction between CO2 and AB is strongly exothermic. This strong heat release may be due to the formation of the B−O bond, as mentioned before. The exothermic nature was also observed from the temperature peak up to 120 °C in the PAB decomposition under CO2 (Figure 2). This exothermic reaction will accelerate H2 release from PAB. However, hydrogen release is still slow when the temperature was increased to 120 °C under C2H6 (Figure 2). This means that the dramatic H2 release from PAB under CO2 is not only due to the exothermic effect, but also due to active species formed from the reaction between CO2 and PAB. The active species was posited to be formic acid according to the previous simulation work,19 which indicated that hydrogen is released from the reaction between formic acid and −BH3 or −BH2-group. The formation of this active species is the same true for the case of enhanced dehydrogenation from CO2 reaction with AB.11,12 In contrast, the dehydrogenation of AB without CO2 follows the formation of active species of DADB (diammoniate of diborane),7 linear or cyclic polymers,16,20 and vapor phase NH2BH2 in an autocatalytic route.20 3802

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(7) Feaver, A.; Sepehri, S.; Shamberger, P.; Stowe, A.; Autrey, T.; Cao, G. Z. Coherent Carbon Cryogel-Ammonia Borane Nanocomposites for H2 Storage. J. Phys. Chem. B 2007, 111, 7469−7472. (8) Li, L.; Yao, X.; Sun, C.; Du, A.; Cheng, L.; Zhu, Z.; Yu, C.; Zou, J.; Smith, S. C.; Wang, P.; et al. Lithium-Catalyzed Dehydrogenation of Ammonia Borane within Mesoporous Cabon Framework for Chemical Hydrogen Storage. Adv. Funct. Mater. 2009, 19, 265−271. (9) Wright, W. R. H.; Berkeley, E. R.; Alden, L. R.; Baker, R. T.; Sneddon, L. G. Transition Metal Catalysed Ammonia-Borane Dehydrogenation in Ionic Liquids. Chem. Commun. 2011, 47, 3177− 3179. (10) Bluhm, M. E.; Bradley, M. G.; Butterick, R.; Kusari, U.; Sneddon, L. G. Amineborane-based Chemical Hydrogen Storage: Enhanced Ammonia Borane Dehydrogenation in Ionic Liquids. J. Am. Chem. Soc. 2006, 128, 7748−7749. (11) Xiong, R.; Zhang, J. S.; Zhao, Y.; Akins, D. L.; Lee, J. W. Rapid Release of 1.5 Equivalents of Hydrogen from CO2-Treated Ammonia Borane. Int. J. Hydrogen Energ. 2012, 37, 3344−3349. (12) Zhang, J. S.; Zhao, Y.; Akins, D. L.; Lee, J. W. CO2-Enhanced Thermolytic H2 Release from Ammonia Borane. J. Phys. Chem. C 2011, 115, 8386−8392. (13) Baumann, J.; Baitalow, E.; Wolf, G. Thermal Decomposition of Polymeric Aminoborane (N2BNH2)x under Hydrogen Release. Thermochim. Acta 2005, 430, 9−14. (14) Staubitz, A.; Robertson, A. P. M.; Manners, I. Ammonia-Borane and Related Compounds as Dihydrogen Source. Chem. Rev. 2010, 110, 4079−4124. (15) Frueh, S.; Kellett, R.; Mallery, C.; Molter, T.; Willis, W. S.; King’ondu, C.; Suib, S. L. Pyrolytic Decomposition of Ammonia Borane to Boron Nitride. Inorg. Chem. 2011, 50, 783−792. (16) Zhang, J. S.; Zhao, Y.; Akins, D. L.; Lee, J. W. Thermal Decomposition and Spectroscopic Studies of Preheated Ammonia Borane. J. Phys. Chem. C 2010, 114, 19529−19534. (17) Burr, J. G.; Brown, W. G.; Heller, H. E. The Reduction of Carbon Dioxide to Formic Acid. J. Am. Chem. Soc. 1950, 72, 2560− 2562. (18) Zhang, J. S.; Zhao, Y.; Guan, X. D.; Stark, R. E.; Akins, D. L.; Lee, J. W. Formation of Graphene Oxide Nanocomposites from Carbon Dioxide Using Ammonia Borane. J. Phys. Chem. C 2012, 116, 2639−2644. (19) Zimmerman, P. M.; Zhang, Z.; Musgrave, C. B. Simultaneous Two-Hydrogen Transfer as a Mechanism for Efficient CO2 Reduction. Inorg. Chem. 2010, 49, 8724−8728. (20) Zimmerman, P. M.; Paul, A.; Zhang, Z.; Musgrave, C. B. Oligomerization and Autocatalysis of NH2BH2 with Ammonia-Borane. Inorg. Chem. 2009, 48, 1069−1081. (21) Chandra, M.; Xu, Q. A High-Performance Hydrogen Generation System: Transition Metal-Catalyzed Dissociation and Hydrolysis of Ammonia-Borane. J. Power Sources 2006, 159, 855−860. (22) Stephens, F. H.; Baker, R. T.; Matus, M. H.; Grant, D. J.; Dixon, D. A. Acid Initiation of Ammonia-Borane Dehydrogenation for Hydrogen Storage. Angew. Chem., Int. Ed. 2007, 46, 746−749.

The enhanced AB dehydrogenation through acidic hydrolysis using CO2 was also observed and, in this case, the carbonic acid was first formed and catalyzed the AB hydrolysis.21 Other Lewis and Bronsted acids were also reported to accelerate the dehydrogenation of AB by first forming borenium cations.22 Thus, the combination of the exothermic effect and the promoting effect of formic acid can contribute to fast hydrogen desorption from PAB.

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CONCLUSIONS We have demonstrated that PAB decomposition at 85−90 °C is drastically accelerated by CO2. The fast decomposition of PAB at the low temperature is due to the promoting effect of formic acid, and the thermal effect of the exothermic reaction between CO2 and PAB. Formic acid is considered to be the initial product from the reduction of CO2 by PAB and incoporated to the solid as a formate group. It is further reduced to methanediol and then methoxy groups. Under a CO2 pressure of 2.07 MPa, the H2 yield for PAB decomposition is 5.48 wt % within 1 h. After taking account of the 5.9 wt % H2 release during the preparation of PAB from AB, 11.38 wt.% H2 desorbs based on the initial mass of AB. This result exhibits that CO2enhanced H2 desorption may be a feasible option for AB to meet the 2017 U.S. Department of Energy target (5.5 wt % based on the whole system including storage materials, tanks, valves, piping, regulators, and insulation) for on-board H2 storage.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +82-42-350-3940; fax: +82-42-350-3910; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Brain Korea 21 Program at KAIST. The authors are grateful to Dr. B.-K. Kim at the Common Facility of KAIST for helping with solid-state NMR measurements.

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REFERENCES

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