Jun Chen,
Cong Wang,
Yong Wu,
He Fu,
Jie Zheng and
Xingguo Li*
Beijing National Laboratory for Molecular Sciences (BNLMS), College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China. E-mail: xgli@pku.edu.cn; Fax: +86-10-62765930; Tel: +86-10-62765930
First published on 31st March 2016
The reaction of water with hydrides (hydrolysis reaction) is very attractive for onsite hydrogen generation. Instead of using liquid water like in most cases, we demonstrate that hydrogen generation from hydrolysis reactions can also occur in the solid state. By simply heating mixtures of hydrated solids and hydrides, hydrogen generation is readily achieved through the recombination from the protonic hydrogen in the hydrated solids and the hydridic hydrogen in the hydride. The composites 5CaH2 + Na4P2O7·10H2O and NaBH4 + H2C2O4·2H2O give attainable gravimetric hydrogen storage capacity of 2.76% at 40 °C and 2.79% at 70 °C with rapid response, respectively. The dehydrogenation temperature can be controlled by the dehydration temperature of the hydrated solids. This innovative hydrogen generation approach provides a temperature activated, easy to control solution for onsite hydrogen generation.
The hydrolysis reaction of hydrides or reactive metals is able to produce hydrogen in mild conditions with highly competitive gravimetric hydrogen storage capacity (GHSC), which is very attractive for onsite hydrogen generation.4–8 However, in practical applications, hydrogen generation from hydrolysis is more complicated than just mixing the metals/hydrides with water. Almost all the hydrolysis reactions of reactive metals/hydrides are thermodynamically highly favourable, accompanied by substantial exothermic effects. The reaction heat will further accelerate the reaction. Thus, once the reactive metal/hydride and water is thoroughly mixed, the reaction can be easily out of control. In a practical hydrogen generation system, the hydride/metal and water has to be stored separately and allowed to contact in a controlled way. This will require additional storage space and controlling units, which will cause significant loss of the hydrogen storage capacity on a system basis.
Currently, most study on hydrolysis hydrogen generation is focused on optimizing the materials to improve the hydrogen yield and hydrogen generation kinetics. Much less attention is paid to improve the reaction process to facilitate compact system integration and more convenient operation, which is equally important for practical application. It is illustrative to recall the hydrogen generation from thermal decomposition of metal hydrides, in which dehydrogenation will only occur above a certain temperature threshold, i.e. is temperature activated. As a result, hydrogen generation is very easy to control. Moreover, all the materials involved in this case are solids, which is much more convenient to handle.
For hydrogen generation from hydrides, an interesting question is whether it is possible to combine the merits of high GHSC from the hydrolysis approach and the easy operation from the thermal decomposition approach? In this work, we demonstrate that hydrogen generation from the hydrolysis reactions of hydrides can also be carried out in the solid state and become temperature activated. This can be achieved by replacing the liquid water by hydrated solids. Upon heating, the water released from the hydrated solids can react with the hydride to generate hydrogen. The mechanism is just like the hydrolysis reaction, while no liquid state water is involved. Two all solid state hydrogen generation systems provide competitive GHSC above 2.7%, which are highly attractive for onsite hydrogen generation.
In the temperature programmed hydrogen generation measurement, the mixture is loaded in a bubbler like glass reactor. The glass reactor is heated by an oil bath at a constant heating rate of 2 K min−1 in a constant Ar flow of 50 standard cubic centimetres per minute (sccm). The released gas is detected by a residual gas analyser (Omnistar, Pfeiffer Vacuum).
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Fig. 1 (a) Schematic illustration of the solid state hydrolysis reaction for hydrogen generation. (b–d) Elemental mapping in SEM of the 5CaH2 + Na4P2O7·10H2O composites. |
We first demonstrate this strategy using CaH2, a highly reactive hydride which reacts vigorously with water. When CaH2 is mixed with Na4P2O7·10H2O, a hydrated inorganic salt with high water percentage (40.4%), the X-ray diffraction pattern (Fig. 2a) suggests that both CaH2 (ICSD 98-026-0873) and Na4P2O7·10H2O (ICSD 98-001-5389) remain unchanged after mixing. Elemental mapping (Fig. 1b–d) suggests that the two components are macroscopically homogeneously mixed in the composite. The CaH2 + Na4P2O7·10H2O composites are stable at room temperature as shown in Fig. 3c (the lower panel). During the temperature programmed hydrogen generation measurement process, hydrogen evolution is not observed by MS detecter before the temperature is up to 40 °C. When the composite is heated to 40 °C, clear hydrogen evolution is observed. More than 70% theoretical hydrogen is released in the first 10 min and the final yield is 92%, corresponding to GHSC of 2.76% (Fig. 2b). After hydrogen release, the products are composed of Ca(OH)2 (ICSD 98-020-2220) and Na4P2O7 (ICSD 98-001-0370), as suggested by XRD (Fig. 2a). This suggests that the hydrogen generation is from the reaction between CaH2 and the water in Na4P2O7·10H2O, which can be described as:
5CaH2 + Na4P2O7·10H2O → 5Ca(OH)2 + Na4P2O7 + 10H2 | (1) |
The above mechanism is in accordance with the dehydration behaviour of Na4P2O7·10H2O. Thermal gravimetric analysis (TGA, Fig. 3b) suggests that Na4P2O7·10H2O shows one single step weight loss of 39.5% in the temperature range between 40 and 75 °C, which matches well with theoretical mass percentage of H2O in Na4P2O7·10H2O (40.4%). Thus, the hydrogen generation is still due to the hydrolysis reaction of CaH2, while there is no liquid state water involved. We denote this new hydrogen generation scheme as the solid state hydrolysis (SSH) reaction. Mass spectroscopy (MS) measurement (Fig. 3a) suggests that the released gas is composed of H2 with only small amount of water vapour without other impurities, which can be used for hydrogen fuel cells directly or with simple dehydration.
Clearly, other hydrated inorganic salts can also be used as the water carriers. Several hydrated salts with high water weight percentage are also tested for the SSH reaction with CaH2. The results are summarized in Fig. 4 and Table 1. Although the hydrogen generation kinetics and the final yields vary significantly, there is an interesting correlation between the dehydrogenation temperature of the composites and the dehydration temperature of the hydrated salts. The peak dehydration temperature (Tw) is obtained from the derivative of the TGA curves (Fig. 3c, the upper panel). The dehydrogenation peak temperature (TH) is obtained from the hydrogen release profile during the temperature programed heating process detected by MS (Fig. 3c, the lower panel). There is a clear positive correlation between TH and Tw, as shown in Fig. 3d. This correlation is more convincingly demonstrated by MgCl2·6H2O, which exhibits two well separated dehydration steps before 150 °C. Correspondingly, there are also two dehydrogenation peaks. The above results are fully in agreement with the dehydrogenation mechanism proposed for the SSH reaction, as shown in eqn (1). This also provides the flexibility in tuning the dehydrogenation temperature in the SSH reaction by rational choice of hydrated solids with proper dehydration temperature.
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Fig. 4 Isothermic dehydrogenation kinetics curves of Na4P2O7·10H2O at 40 °C, KAl(SO4)2·12H2O at 50 °C, MgCl2·6H2O at 60 °C, ZnSO4·7H2O at 60 °C and NiSO4·6H2O at 80 °C. |
Composition | Theoretical GHSC (%) | Tw (°C) | ΔwH (kJ mol−1 H2O) | TH (°C) | Attained GHSC (%) | Enthalpy (kJ mol−1 H2) |
---|---|---|---|---|---|---|
a GHSC: gravimetric hydrogen storage capacity. Tw: the dehydration peak temperature of the hydrated solid. TH: the dehydrogenation peak temperature of the SSH composite. ΔwH: the enthalpy to generate 1 mol H2O from the hydrated solid. | ||||||
5CaH2 + Na4P2O7·10H2O | 3.0 | 54 | 37.9 | 49 | 2.76 | −78.2 |
6CaH2 + KAl(SO4)2·12H2O | 3.3 | 70 | 19.5 | 54 | 1.91 | −96.6 |
3CaH2 + MgCl2·6H2O | 3.6 | 87 | 23.6 (ref. 9) | 65 | 1.40 | −92.5 |
3CaH2 + MgCl2·6H2O | 3.6 | 120 | 100 | 1.62 | ||
3CaH2 + NiSO4·6H2O | 3.1 | 90 | 36.8 | 80 | 1.24 | −79.3 |
3.5CaH2 + ZnSO4·7H2O | 3.2 | 62 | 13.9 | 63 | 1.02 | −102.6 |
Direct hydrolysis of NaBH4 is rather slow and incomplete.10,11 In fact, NaBH4 can form rather stable aqueous solution, particularly in alkaline solution. The H in the BH4− group shows much less hydridic characteristics compared to that in alkaline or alkaline earth metal hydrides, due to the very similar electronegativity of boron and hydrogen. Consequently, hydrogen with stronger acidity is required to facilitate the hydrogen generation. Demirci et al. demonstrated that hydrolysis of NaBH4 can be rather complete in aqueous acid solution such as hydrochloride acid and acetic acid.12 However, this approach has not been widely adopted for onsite hydrogen generation due to the inconvenience in storing and carrying the acid solution.
Oxalic acid is a solid organic acid with high percentage of protonic hydrogen. The solid state composite NaBH4 + H2C2O4·2H2O exhibits theoretical GHSC of 4.9%. XRD study shows that the physical mixture of NaBH4 and H2C2O4·2H2O (molar ratio 1:
1) is stable up to 60 °C (Fig. 5a). During the temperature programmed heating process, the composite shows a sharp hydrogen release peak at 65 °C. Rapid dehydrogenation is achieved in isothermic dehydrogenation at 70–80 °C (Fig. 5b and c). The above features are very similar to the CaH2 + hydrated salt systems. About 55% of theoretical hydrogen is released in about 2 min, corresponding to GHSC of 2.70%. MS analysis shows that the released gas is composed of hydrogen and trace amount of water vapour, without impurities like CO or CO2 from decomposition of oxalic acid (Fig. 5d). After dehydrogenation, the solid is mainly composed of NaB(C2O4)2 (ICSD 98-028-1622) and the unreacted NaBH4, as suggested by XRD (Fig. 5a). There is no unreacted H2C2O4·2H2O detected. The main dehydrogenation reaction, therefore, is given by:
NaBH4 + 2H2C2O4·2H2O → NaB(C2O4)2 + 4H2 + 4H2O | (2) |
The above reaction gives 50% conversion of NaBH4 in the NaBH4 + H2C2O4·2H2O (molar ratio 1:
1) composite, which is in good agreement with the experimental results (55% conversion of NaBH4). The additional 5% may be resulted from the water vapour induced dehydrogenation:
NaBH4 + 4H2O → NaBO2 + 4H2 | (3) |
The molar ratio of NaBH4 and H2C2O4·2H2O in the composite is adjusted to optimize the GHSC. As shown in Fig. 6, increasing the fraction of H2C2O4·2H2O can increase the conversion percentage of NaBH4. In the NaBH4 + 3H2C2O4·2H2O composite, more than 95% NaBH4 is converted. However, the GHSC of the composite decreases with the H2C2O4·2H2O fraction. The NaBH4 + H2C2O4·2H2O (molar ratio 1:
1) gives the highest GHSC of 2.78%.
Despite of the inevitable loss in GHSC due to using hydrated solids instead of water, the SSH reactions remain provide very competitive GHSC. As shown in Table 1, the theoretical GHSC of several SSH composites well exceeds 3%. The attained GHSC for 5CaH2 + Na4P2O7·10H2O and NaBH4 + H2C2O4·2H2O reaches 2.76% at 40 °C and 2.78 wt% at 70 °C, respectively. For comparison, the metal hydrides with moderate dehydrogenation temperature typically show theoretical GHSC lower than 2% (LaNi5H6: 1.4%,13 TiFeH1.95: 1.8%,14 TiMn1.5H2.4: 1.8%15).
When volumetric hydrogen storage capacity (VHSC) is also taken into account as a major figure of merit, the SSH reactions become even more competitive compared to direct hydrolysis. For instance, the theoretical VHSC for directly hydrolysis of CaH2 is 66 g H2 L−1 (calculated from the total volume of the reactants CaH2 + 2H2O). In addition, the volume of the formed Ca(OH)2 is expanded by 33% compared to that of CaH2. Therefore, when generating hydrogen by gradually adding water into the CaH2 container (which is the most common way to realized controlled hydrogen generation), the CaH2 container should be at least 33% larger than the volume of CaH2 to accommodate the volume expansion, which gives theoretical VHSC of 58 g L−1 (based on the minimum internal volume of the CaH2 and water containers). The theoretical VHSC of the SSH mixture 5CaH2 + Na4P2O7·10H2O is 54 g H2 L−1 (density: 1.7 g cm−3 for CaH2 and 1.8 g cm−3 for Na4P2O7·10H2O). There is no need to leave additional space in the container, as the volume is contracted by 27% when it is converted to the spent fuel 5Ca(OH)2 + Na4P2O7 (density: 2.24 g cm−3 for Ca(OH)2 and 2.53 g cm−3 for Na4P2O7). Therefore, the theoretical VHSC of the SSH reaction for CaH2 is already comparable to that of direct hydrolysis.
For hydrogen generation from hydrolysis of NaBH4, the benefit of the SSH reactions is more than just excluding the liquid water. Although the theoretical GHSC of NaBH4 hydrolysis (NaBH4 + 2H2O) is as high as 10.8%, direct hydrolysis of NaBH4 is very slow and incomplete. In most practical applications, NaBH4 is stored as alkaline solution and the dehydrogenation needs to be catalysed by transition metal catalysts.16–19 To avoid precipitation of the hydrolysis product NaBO2 which will passivate the catalysts, the optimized concentration of NaBH4 solution is around 15% and the corresponding theoretical GHSC is significantly reduced to only 3.2%.20 The solid acid promoted SSH reaction described in this work achieves efficient hydrogen generation from NaBH4 without using transition metal catalysts, which is very promising for low cost, efficient onsite hydrogen generation from NaBH4. The attained GHSC 2.78% is very close to the theoretical value of 15% NaBH4 solution. Aqueous acid solution has been studied to promote hydrogen generation from NaBH4 solution,12,21 while the solid state hydrated oxalic acid is clearly much more convenient to handle compared to the acid solution.
An additional advantage of the SSH reactions compared to direct hydrolysis reactions is the lower reaction heat. Hydrolysis of most reactive hydrides or metals is highly exothermic. In the SSH reactions, however, this heavy thermal burden can be effectively alleviated by coupling the endothermic dehydration reaction. Here we illustrate this effect using CaH2. The dehydrogenation enthalpy ΔdHH of the SSH reactions of CaH2 is given by:
ΔdHH = ΔHyH(CaH2) + ΔwH = −116.05 + ΔwH | (4) |
The SSH reactions in this work are representative examples of how recombination of the hydridic and protonic hydrogen in the solid state can promote hydrogen generation. Many hydrogen storage/generation systems are based on this strategy, such as LiNH2–LiH,23 ammonia borane3,24 and its analogies,25 amides/borohydrides composites26,27 and ammoniates of borohydrides.28,29 A recent work on AlCl3·mNH3–nLiBH4 (ref. 30) (m = 3 to 6; n = 3 to 5) is a closer analogy to this work. Except very few examples, most of the above systems have very poor reversibility and practically can only serve as single use hydrogen carriers. Compared to the above systems, the SSH composites are composed of much cheaper materials and thus are more viable for onsite hydrogen generation.
Further improvement of the GHSC from the SSH reactions is possible. As summarized in Table 1, the GHSC of the SSH systems is generally much lower than the theoretical value. A major loss of the GHSC is due to the unreacted water vapour escaped from the system, as demonstrated by the MS analysis (Fig. 3a). By proper design of the reactor to elongate the residence time of the water vapour in the reactor, it is possible to reduce the unreacted water vapour and further enhance the GHSC from the SSH reactions. Moreover, better control of the hydrogen generation rate is also needed to meet the requirement of onsite hydrogen generation, which will be our next research focus.
Footnote |
† Electronic supplementary information (ESI) available: Details of sample preparation, additional structural and dehydrogenation characterization. See DOI: 10.1039/c6ra04410g |
This journal is © The Royal Society of Chemistry 2016 |