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Hydrogen for Vehicle Applications from Hydrothermolysis of Ammonia Borane: Hydrogen Yield, Thermal Characteristics, and Ammonia Formation Hyun Tae Hwang, Ahmad Al-Kukhun, and Arvind Varma* School of Chemical Engineering, Purdue UniVersity, 480 Stadium Mall DriVe, West Lafayette, Indiana 47907
Among chemical hydrides, ammonia borane (AB) is of interest as a hydrogen storage material due to its high hydrogen capacity (19.6 wt %). In this paper, our recently developed hydrothermolysis process was investigated over a wide range of AB weight percentages (wt %) in water, pressures, and heating rates. It was found that hydrogen yield and thermal characteristics were influenced by these operating conditions. Ammonia generation was also observed during AB hydrothermolysis, where 14-53% of AB was converted to NH3 depending on the reaction conditions. It is remarkable that some AB (2-4%) was converted to NH3, which must be removed for use in proton exchange membrane (PEM) fuel cells, even by neat thermolysis. It was also found that during the hydrothermolysis reaction at Treactor of 85 °C, the Tsample can exceed 500 °C, where AB can be completely decomposed to boron nitride (BN). The 11B NMR characterization of hydrothermolysis products showed compounds with B-O and B-N bonds. This paper suggests directions for future research to identify optimal conditions, where the hydrothermolysis process provides the best balance between H2 yield and undesirable products, for PEM fuel cell vehicle applications. 1. Introduction The concerns over diminishing resources and the environmental impact of burning fossil fuels have increased attention on the use of hydrogen as an alternative energy carrier. Hydrogen is a clean and environmentally friendly fuel because, with oxygen in proton exchange membrane fuel cells (PEM FC) to generate electricity, its only product is water. One of the major obstacles for the development of hydrogen powered vehicles is the lack of safe and efficient means for on-board hydrogen storage. Current approaches for this purpose include compressed hydrogen gas, cryogenic and liquid hydrogen, adsorbents, metal hydrides, and chemical hydrides.1-5 Among these alternatives, chemical hydrides offer the advantages of high hydrogen gravimetric capacity, along with ease of hydrogen release.6,7 Among the chemical hydrides, ammonia borane (H3NBH3, AB) has attracted considerable interest because of its high hydrogen content (19.6 wt %). Hydrogen can be released from AB by either hydrolysis or thermolysis. Due to limited AB solubility in water, hydrolysis (eq 1) provides low theoretical H2 yield (∼5.6 wt %) and it also requires catalysts.7-10 Thermolysis (eqs 2-4), on the other hand, requires relatively high temperature to release two moles of hydrogen per mol of AB and above 500 °C for complete dehydrogenation, forming boron nitride (BN).11,12 From a spent fuel regeneration viewpoint, however, BN is not preferred due to its high chemical and thermal stability.13 Hydrolysis: NH3BH3 + 3H2O f B(OH)3 + NH3 + 3H2 (1) 1 Thermolysis: NH3BH3 f (NH2BH2)x + H2 (90 - 117oC) x
(2)
* To whom correspondence should be addressed. Tel.: +1-765-4944075. Fax: +1-765-494-0805.E-mail:
[email protected].
1 1 (NH2BH2)x f (NHBH)x + H2 (150 - 170oC) x x
(3)
1 1 (NHBH)x f (NB)x + H2 (>500oC) x x
(4)
New methods have been proposed to generate hydrogen from AB at near PEM FC operating temperatures. Bluhm and Sneddon reported that dehydrogenation of AB in ionic liquids such as 1-butyl-3-methylimidazolium chloride (bmimCl) releases 0.5, 0.8, and 1.1 equiv of H2 in 1 h at 85, 90, and 95 °C, respectively.14 Recently, Himmelberger et al. found that AB reactions in bmimCl containing 5.3 mol % (28 wt %) bis(dimethylamino) naphthalene (Proton Sponge) released 2 equiv of H2 in 171 min at 85 °C.15 Autrey et al. found that a nanocomposite of mesoporous silica and AB releases hydrogen at 50 °C with a half-reaction time of 85 min, as compared to a half-reaction time of 290 min at 80 °C for neat AB.16 Heldebrant et al. found that addition of trace quantities of diamoniate of diborane ([NH3BH2NH3][BH4], DADB), a product of AB isomerization, to neat AB significantly reduces the induction time and onset temperature at which hydrogen is released.17 Nanophase boron nitride (nano-BN) additives to AB play a similar role as DADB and also serve as a scaffold, both decreasing the onset temperature of H2 release.18 These methods, however, require either relatively high temperature or involve additives for lower temperature operation and shorter induction period, which increase the overall system weight. It is noteworthy that both molar equivalent (equiv) and wt % yield are commonly used to express H2 generation efficiency. The H2 molar equivalent is the number of H2 moles generated per mole of AB, while H2 yield is the ratio of H2 weight generated to the total mixture weight (i.e., AB and all other materials, such as water, ionic liquid, scaffold, proton sponge, etc.). From the application viewpoint, H2 yield is the critical number. Therefore, the weight of additives or solvent, which
10.1021/ie100520r 2010 American Chemical Society Published on Web 09/10/2010
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Figure 1. Schematic diagram of the experimental setup.
are used along with AB for thermolysis or hydrolysis, must be taken into account to calculate material-based H2 yield. For example, 2 equiv of H2 from AB/bmimCl/Proton Sponge mixture,15 described above, corresponds to only 3.5-6.5 material-wt % H2 yield, depending on the AB wt % in the mixture. Similarly, >2.5 equiv of H2 (or 18 wt % H2 based on AB alone) obtained at 60 °C by Keaton et al.19 becomes only 4.5 wt % material-based H2 yield when the weight of solvent is included. Finally, although 11.4 material-wt % H2 was obtained from an AB-ionic liquid mixture, it was operated at temperature >110 °C,20 which is too high to utilize waste heat from a PEM FC (operated at ∼85 °C). We have recently proposed and demonstrated a new method involving noncatalytic hydrothermolysis of AB-water mixtures.21,22 Using this approach, the maximum hydrogen storage capacity, obtained at 77 wt % AB and Treactor of ∼85 °C along with rapid kinetics, was 11.6 and 14.3 wt % at pressure 14.7 and 200 psia, respectively.22 To our knowledge, on a material basis, the AB hydrothermolysis process is the first one to provide such high H2 yield values at near PEM FC operating temperature. The 11 B NMR spectra reported in our previous study showed that BNH compounds and boric acid were the main solid products of hydrothermolysis. It is known that, in the products, B-O bonds from AB hydrolysis are thermodynamically more stable than BNH bonds from thermolysis. For this reason, B-O bonds are more expensive to break; however, methods for regenerating AB from both hydrolysis and thermolysis have recently been published.13,23,24 As shown in eq 1, AB hydrolysis produces ammonia. For use in PEM FCs, ammonia must be removed from the H2 stream. In addition to potentially poisoning the anode catalyst site, ammonia can also cause ion exchange with protons in the polymer electrolyte.25 It has been reported that as low as 10 ppm NH3 can decrease the FC performance and that the degradation is irreversible for long-term exposure to 10-20 ppm
NH3.26 Ammonia can be removed from gaseous streams by various methods such as membrane separation, selective catalytic oxidation, absorption, and adsorption.27-30 In this context, it is important to find optimal conditions, where the hydrothermolysis process provides the best balance between H2 yield and undesirable products from hydrolysis, i.e., NH3 and B(OH)3. In the present work, to determine these conditions, the hydrothermolysis process was investigated over a wide range of AB wt %, pressures, and heating rates (β). 2. Experimental Section The hydrothermolysis experiments were conducted in a stainless steel reactor (Parr Instrument Company, Model 4592) with external heating (Figure 1). The reactor volume, including added fittings and tubing, was determined to be 70 mL. The samples (∼0.5 g) were prepared by mixing AB (97% pure, Sigma Aldrich) with H2O in varying weight ratios. The mixture was placed in a small glass vial (3 mL) inside the reactor, under an argon (99.99% pure) environment (15-400 psia). Starting at room temperature, with a programmed heating rate (0.2-4 °C/min), the reaction vessel was maintained for a 30 min hold at the set point value (TSP). The reactor pressure was monitored using a transducer (Omega Engineering PX35D1). Apart from the reactor temperature (Treactor), the sample temperature (Tsample) was also recorded by inserting another fast-response thermocouple (0.25 s response time) inside the sample. Note that to better understand the thermal characteristics, the reactor used in this study was of smaller volume (70 mL) than in our previous work (360 mL).22 In our previous work, it was found that, for g43 wt % AB, the pressure and temperature increased sharply at Treactor in the range of 75-85 °C (depending on AB wt %) and ∼85 °C was sufficient to release the same amount of hydrogen as at 135 °C.22 For this reason, all experiments in this study were
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Figure 2. Effect of AB wt % on H2 yield and NH3 formation (Pi ) 200 psia, TSP ) 85 °C, β ) 1 °C/min).
conducted at TSP of 85 °C. Additionally, 200 psia initial pressure was selected as the base case to avoid water evaporation, except when the effect of pressure was investigated. The product gas composition was analyzed by massspectrometry (Hiden Analytical HPR-20) after cooling the reactor to room temperature at the end of the experiment. The hydrogen generation was calculated using the gas composition analysis, along with pressure increase during the experiment. Each data point is an average of 2-4 experiments, and the standard deviation is indicated by the error bars. In addition to mass spectrometry, NH3 was also measured by titration technique (Figure 1). The gases from the Parr reactor were passed through methanol to capture NH3, which boils at -33.5 °C and becomes liquefied in methanol maintained at -78 °C using dry ice. The gases further pass through a 5 wt % CuSO4 solution to verify complete NH3 capture. Any escaped NH3 from methanol would produce a pale blue precipitate with CuSO4. The NH3 in methanol was titrated with 0.1 N HCl solution using methyl orange as indicator. The solid products were characterized by solid-state 11B NMR, where the spectra were recorded using a Chemagnetics CMX400 spectrometer and were referenced to NaBH4 (-42.2 ppm). The samples were run with magic angle spinning at 9 kHz. 3. Results and Discussion 3.1. Effect of AB Weight Percentage. Figure 2 shows the effect of AB wt % on H2 yield and NH3 generation. For 200 psia and a heating rate of 1 °C/min, increasing AB loading from 40 to 60 wt %, hydrogen yield increased from 6.9 to 11.6 wt % and 12.4 wt % H2 yield was obtained at 80 wt % AB. The H2 yield for pure AB thermolysis was only 6.8 wt % (∼1.05 H2 molar equivalent). This is in good agreement with our previous study,22 which showed that, with increasing AB loading up to 77 wt %, the H2 yield increased and a further increase in AB wt % decreased the H2 yield. Figure 2 also shows the mole ratio of AB converted to NH3 (NH3/AB) in the gaseous product, indicating a decrease of NH3/AB ratio with increasing AB wt %. From 50 to 80 wt % AB, the NH3/AB ratio decreased from 0.53 to 0.15. Since NH3 is a product of hydrolysis, it was expected that the NH3/AB ratio would increase with water content in the AB-water mixture. Particularly for 40 wt % AB, however, some ammonia dissolved in water was observed in the reactor vessel after the reaction was completed. Therefore, the actual amount of ammonia produced from 40 wt % AB was higher than the value (29%) presented in Figure 2. For comparison with hydrothermolysis, neat AB thermolysis was also conducted. In this case, care was taken to ensure the
Figure 3. Effect of AB wt % on thermal characteristics (Pi ) 200 psia, TSP ) 85 °C, β ) 1 °C/min). (a) Profiles of reactor and sample temperature with time. (The detailed temperature-time profiles for the rectangular region are shown in the inset.) (b) Sharp evolution temperature and maximum sample temperature.
absence of moisture in the reactor. It is remarkable that some AB (2-4%) was converted to NH3 even by neat thermolysis. Note that there are many published articles on neat thermolysis and hydrolysis of AB but, to our knowledge, only recently Neiner et al. reported the formation of ammonia for neat thermolysis.18 They reported that ca. 200 ppm of NH3 concentration was measured from neat AB thermolysis, which releases 2.6 equivalents of hydrogen at 150 °C after 900 min.18 The amount of gas along with the NH3 was not reported in this work, so it is not possible to quantify the fractional conversion of AB to NH3. While heating the reactor to reach set point, TSP, the Tsample, and Treactor increased at the same rate until 60-65 °C, and subsequently, Tsample increased at a higher rate. The typical temperature profiles obtained during AB hydrothermolysis are shown in Figure 3a. Sharp heat evolution, indicating rapid reaction, was observed for all hydrothermolysis experiments in this study. It was found that sharp evolution reactor temperature (Tshev) decreases with increasing AB wt %. This rapid evolution started at ∼85, 76, 74, and 72 °C for 40, 50, 60, and 80 wt % AB, respectively (Figure 3b). This indicates that the mixture with higher water content requires higher Treactor to initiate sharp H2 evolution, which results in simultaneous increase of Treactor. Figure 3b also shows that the maximum sample temperature (Tsample,max) increases with increasing AB loading up to 60 wt % and decreases with a further increase in AB wt %. It is known that the release of hydrogen from AB via both thermolysis (first and second release of H2) and hydrolysis is exothermic, where the latter is more exothermic than the former.6,12,18,31 For lower AB wt %, Tsample,max is lower because of the higher total heat
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Figure 4. Effect of pressure on H2 yield and NH3 formation (60 wt % AB, TSP ) 85 °C, β ) 1 °C/min).
capacity of the mixture. For higher AB loading (i.e., 80 wt % AB), however, the Tsample,max decreased. The reason for this decrease is related to lower heat release from hydrolysis due to smaller water content. It was observed that Tsample,max reached values ∼340, 480, 510, and 350 °C for 40, 50, 60, and 80 wt % AB, respectively (Figure 3b). For neat AB thermolysis (Figure 3a), the sharp evolution was not observed and Tsample,max was found to be only ∼103 °C, indicating that water (hydrolysis) plays a critical role to drive sharp heat evolution resulting in high H2 yield. It is noted that, by the hydrothermolysis reaction, for AB loading in the range 50-60 wt %, Tsample reached above 500 °C, where AB can be decomposed to boron nitride (BN). Due to high stability of BN, however, this is not preferred from the spent fuel regeneration viewpoint. 3.2. Effect of Pressure. Baitalow et al. found that the release of hydrogen by AB thermolysis is not significantly affected by pressure up to 600 bar.32 On the other hand, Nylen et al. reported that the mechanism of hydrogen generation by thermolysis of poly aminoborane (obtained after 1 mol H2 release from AB) changes with pressure.33 In our previous study,22 no pressure effect was observed for 60 wt % AB on the hydrogen yield when 7 °C/min heating rate was used. In the present work, we investigated the effect of initial pressure for 60 wt % AB at a 1 °C/min heating rate, and the results are shown in Figures 4 and 5. It was found that the H2 yield increases with increasing the pressure up to 200 psia (Figure 4); thus, 9.6 wt % H2 yield was obtained at 14.7 psia, and it increased to 11.6 wt % at 200 psia. Further increase in pressure did not increase H2 yield. Similarly, NH3/AB ratio increased with increasing pressure. It is likely that less water was consumed by hydrolysis at lower pressure (
