Low-temperature water-splitting by sodium redox reaction

Low-temperature water-splitting by sodium redox reaction

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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 7 7 0 9 e1 7 7 1 4

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Low-temperature water-splitting by sodium redox reaction Hiroki Miyaoka a,*, Takayuki Ichikawa b,c, Naoya Nakamura c, Yoshitsugu Kojima b,c a

Institute for Sustainable Sciences and Development, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8530, Japan b Institute for Advanced Materials Research, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8530, Japan c Graduate School of Advanced Sciences of Matter, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8530, Japan

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abstract

Article history:

Thermochemical water-splitting by sodium redox reactions was investigated from mate-

Received 21 July 2012

rial science point of view as a future hydrogen production method. The reaction system

Received in revised form

consists of three separate reactions, which are hydrogen generation by NaOH-Na reaction,

31 August 2012

metal separation by thermolysis of Na2O, and oxygen generation by hydrolysis of Na2O2.

Accepted 12 September 2012

Although the current techniques of thermochemical water-splitting required a tempera-

Available online 12 October 2012

ture higher than 800  C for whole reaction cycle, the sodium system was able to be operated below only 400  C by using nonequilibrium techniques to control the entropy of the

Keywords:

chemical reactions. Therefore, this system should be recognized as a potential water-

Hydrogen

splitting technique that can widely utilize any heat sources in contrast to the conven-

Hydrogen production

tional methods.

Water-splitting

Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

Nonequilibrium process

reserved.

Sodium

1.

Introduction

Hydrogen is an attractive energy storage material for establishing a sustainable energy cycle based on primary natural resources such as solar, hydro, and wind energy. Once fossil fuels are exhausted, only natural energy is available. To utilize such fluctuating energy, conversion techniques to hydrogen as secondary energy are necessary for the storage and distribution of energy. As a technique of producing hydrogen by natural energy, water-splitting techniques via thermochemical reactions [1e3] are attractive. So far, the possible thermochemical water-splitting reactions for hydrogen production have been sorted by using a thermodynamic database of materials and investigated from the engineering

point of view. The thermochemical reactions considered, namely, 2-step water-splitting [4e13], the IeS process [14e16], and the UT-3 process [17e19], require a temperature higher than 800  C to generate hydrogen, suggesting that the heat source that can run the systems is limited to large-scale solar heat plants (tower-type) [20e22]. Thus, the construction of thermochemical hydrogen production plants is limited by the location, cost, and safety issues at present. If breeding technology will be established, high-temperature gas-cooled reactor, a kind of nuclear reactor, will be able to be used for water-splitting reactions [23e25] after the depletion of fossil fuels. However, we must consider the benefit and risk of utilizing nuclear reactors. The limits of heat sources, i.e. operating temperature, will be a critical problem after fossil

* Corresponding author. Tel./fax: þ81 82 424 4604. E-mail address: [email protected] (H. Miyaoka). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.09.085

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fuel depletion. If the operating temperature of water-splitting can be lowered, various types of heat sources, which are smaller-scale solar heat systems and unused energy resources such as exhaust heat from factories, can be utilized as a wide practical application. From thermodynamic points of view, the important point in reducing the reaction temperature is entropy control, which can be realized by using a nonequilibrium process. As a suitable system for satisfying the above requirements, we focus on thermochemical hydrogen production by watersplitting via sodium (Na) redox reactions as follows,

2NaOH(s) þ 2Na(l) / 2Na2O(s) þ H2(g),

(1)

2Na2O(s) / Na2O2(s) þ 2Na(g),

(2)

Na2O2(s) þ H2O(l) / 2NaOH(s) þ 1/2O2(g).

(3)

This system is composed of three kinds of reactions, which are (1) H2 generation by a solideliquid reaction (DH ¼ 11 kJ, DS0 ¼ 36 J/mol$K, Teq ¼ 32  C), (2) metal separation by thermolysis (DH ¼ 540 kJ, DS0 ¼ 250 J/mol$K, Teq ¼ 1870  C), and (3) O2 generation by hydrolysis (DH ¼ 55 kJ, DS0 ¼ 66 J/mol$K), where DH is the enthalpy change, DS0 is the standard entropy change, and Teq is theoretical reaction temperature at equilibrium condition without contribution of gaseous pressure. DH and DS0 of each reaction were estimated by using databases [26,27]. Generally, although it is thought that it is difficult to utilize the corrosive materials such as compounds including alkali metals for safety issues, some attractive systems using sodium hydroxide such as 3-step cycles were recently reported [28e30]. The Na system is mentioned in the patent assigned by Gaz de France [31] as a series of the K system, which substitutes K for Na in the equations (1)e(3) [1,2], to utilize the high-temperature heat energy (more than 1000  C) obtained from nuclear reactor. To reduce the reaction temperature of the endothermic reactions in the Na system, the control of entropy by using nonequilibrium techniques is a key-point. The thermodynamics of a chemical reaction is generally expressed by the following Gibbs free energy DG, DG ¼ DH  TDS;

(4)

  DS ¼ DS0 þ R ln p0 =ppro ;

(5)

where R is the gas constant, p0 is the standard pressure (constant), and ppro is the partial pressure of the gaseous product. Since DH is determined by the difference between thermodynamic stability (heat of formation) of starting materials and products in chemical reaction, the reaction temperature (T ) becomes lower with larger DS to satisfy DG < 0. The value of the final entropy term in equation (5) is increased by reducing the partial pressure of the product ppro, indicating that the equilibrium condition of the reaction is changed. As a result, it is expected that the reaction proceeds at a low temperature. By continuously removing the gaseous product from the reaction field, the nonequilibrium condition

could be realized, and then the reaction could be completed at lower temperature. In the Na system, the metal separation is the most difficult reaction thermodynamically. Noteworthy, the entropy term is estimated to be large, assuming that gaseous Na is generated, where Na easily melts and generates vapor pressure (gaseous phase) below 100  C. Thus, thermodynamic control by using a nonequilibrium process can be adopted for all the endothermic reactions, which are the H2 desorption reaction (1) and the metal separation (2). In addition, it is noted that the hydrolysis reaction (3) generates O2 although H2 is released by a conventional hydrolysis reaction. This phenomenon indicates that O2 is easily produced by the exothermic reaction even though the O2 desorption from oxides by thermolysis generally requires a high temperature similarly to the 2-step reactions. In this work, we focused on water-splitting by the Na redox reactions as a thermochemical hydrogen production technique. The feasible conditions of each reaction in the Na system were separately investigated, where a nonequilibrium process was adopted for the endothermic reactions to control the entropy from the material science point of view. From the results, it is determined that low-temperature water-splitting can be realized.

2.

Experimental procedure

Commercial sodium (Na) (99.9%, Aldrich) and sodium hydroxide (NaOH) (99.998%, Aldrich) were used as the starting materials. Sodium oxide (Na2O) was synthesized by heat treatment at 300  C under a dynamic vacuum condition for 20 h of the sodium hydride (NaH) (95%, Aldrich) and NaOH mixture with 1:1 M ratio, which was prepared by ball-milling (P7, Fritsch) under 1 MPa of H2 for 2 h. Sodium peroxide (Na2O2) was synthesized by heating Na2O under O2 atmosphere to 100  C, where the molar ratio of Na2O to O2 was 2:1. The reaction (1) was performed with Na:NaOH of 1:1 M ratio at 300 and 350  C under Ar atmosphere for 20 h in a closed system by using home-made apparatus as shown in Fig. S1(a) (in supplementary data). Here, magnesium (Mg) catalyzed by 1 mol% of niobium oxide (Nb2O5), denoted as cMg, was used as H2 absorbent to remove the generated H2 from the reaction field. It was installed at separate part connected by the stainless still tube to the reaction field of Na and NaOH, suggesting that only generated H2 was able to reach there. c-Mg was reported by Hanada et al. and can absorb H2 even at room temperature [32]. The equilibrium pressure of cMg is about 1  102 Pa at room temperature, which is almost vacuum condition. c-Mg was synthesized by the same procedure as that reported in the literature. The molar ratio of c-Mg used as the H2 absorbent was chosen to be c-Mg/Na ¼ 1. Thus, half of c-Mg is hydrogenated when the reaction (1) proceeds completely, then its hydrogen capacity is 3.5 mass% (full capacity is about 7.0 mass%). For the metal separation reaction (2), the reactor has been designed and assembled to realize the nonequilibrium condition. It was equipped with the cooling part as shown in Fig. S1(b) to separate the metal vapor from the reaction field by condensation. The heat treatment of Na2O was carried out at 400 and 500  C for 20 h under a vacuum condition in a closed system. To investigate

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the hydrolysis reaction (3), Na2O2 was heated to 100  C with an excess molar amount of liquid H2O in 0.1 MPa of Ar in a closed system. For the reactions (1) and (2), Ni-based alloy (Inconel) was used as material of the reactor to minimize the corrosion effect of Na compounds at high temperature. All the samples were handled in a glove box (MP-P60W, Miwa MFG) filled with purified Ar gas (>99.9999%) to avoid the effect of air. The solid samples obtained before and after each reaction were identified by X-ray diffraction (XRD) measurement (RINT-2100, Cu Ka radiation, Rigaku), where all the samples were covered with a polyimide sheet (Kapton, Du PontToray) in the glove box to avoid the influence of air during the XRD measurement. To quantitatively evaluate the H2 generated by reaction (1), c-Mg was analyzed by a thermal desorption mass spectroscopy (MS) (M-QA200TS, Anelva) connected to a thermogravimetry-differential thermal analysis (TG-DTA) device (TG8120, Rigaku). In this thermal analysis, high-purity helium (He) gas (>99.9999%) was flowed as a carrier gas and the heating rate was fixed at 5  C/min. The gaseous products were analyzed by gas chromatography (GC) (GC-14B, Shimadzu). For hydrolysis (3), the reactor was soaked into iced water during the GC measurement to freeze the remaining H2O in it. The temperature of the column was set to 100  C, and high-purity Ar gas was flowed as a carrier gas.

3.

Results and discussion

For the H2 generation reaction (1), since the enthalpy change of the reaction is not large, it is expected that 500  C of our target temperature is enough to release H2 for thermodynamic point of view. Fig. 1 shows the XRD pattern of the product formed by the reaction (1) at 350  C. The diffraction peaks corresponding to the starting materials of NaOH and Na were still observed, suggesting that the reaction was not completed. The other peaks generated by the reaction were assigned to Na2O and NaH. The formation of Na2O indicates the progress of the expected reaction. The NaH phase was an impurity, which was estimated to be formed by the reaction between the remaining Na and the released H2 during the cooling process after the heat treatment. The c-Mg used as the H2 absorbent was evaluated in order to qualitatively and quantitatively analyze the generated H2. The MS and TG-DTA results for c-Mg are shown in Fig. 2. The H2 desorption was clearly observed in the MS spectrum, indicating that H2 was generated by the reaction between NaOH and Na. The weight loss of c-Mg by the H2 desorption was about 2.8 mass%. From the result, the yield of reaction (1) at 350  C is estimated to be more than 80%, considering the stoichiometric H2 capacity of c-Mg is 3.5 mass% as described in experimental part. Furthermore, it was confirmed that the H2 generation reaction proceeded at 300  C, where the results for the XRD and TG-DTA-MS measurements revealed almost the same behavior as those at 350  C. Then, the amounts of Na2O formation and H2 generation were smaller, in which case the reaction yield was about 10%. Here, regarding the practical use, the reaction yield should be reached to 100%. If the reaction is not completed, the separation of Na2O in the remaining materials is quite difficult. Therefore, the reaction kinetics has to be improved by using catalysts and/or

Fig. 1 e XRD pattern of the product on H2 generation reaction (1). XRD patterns of Na2O, NaOH, Na, and NaH are referred from database.

scaffolds in further research, where the reaction should completely proceed under the nonequilibrium condition thermodynamically. The metal separation expressed as equation (2) is the most difficult reaction thermodynamically, where about 2000  C is required in thermodynamic calculation under equilibrium condition at ppro ¼ 0.1 MPa. The reaction temperature would be lowered to 500  C theoretically by reducing the Na vapor pressure down to 4  1019 Pa, which would be realized to condense Na as solid phase because vapor pressure of solid should be almost zero. For reaction (2), the water cooling system was attached to the upper part of the reactor as shown in Fig. S1(b) to condense the Na vapor, resulting in a nonequilibrium condition. The XRD patterns of the assynthesized Na2O, the product at the upper (cooling) part, and the product at the bottom part of the reactor after the reactions at 400 and 500  C are shown in Fig. 3. From the XRD pattern of the as-synthesized Na2O, it was confirmed that the main phase was Na2O although a small amount of NaOH was included as an impurity. After the reaction at 400  C, a metallic and ductile material was found at the cooling part, and it was identified as Na by XRD measurement as shown in the middle part of Fig. 3. These results indicated that the metal separation reaction proceeded even at 400  C. At 500  C, the amount of condensed Na was significantly larger and the diffraction peak intensity of the Na2O phase was lowered. In the XRD pattern of the product at the bottom part, the peaks with high intensity corresponded to Na2O and NaOH, although Na2O2 was the expected product of reaction (2). It was difficult to assign the

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Fig. 3 e XRD patterns of the asLsynthesized Na2O, the product condensed at the cooling part, and at bottom part of the reactor by the reaction (2) at 400 and 500  C. XRD patterns of Na2O2, NaOH, Na, and Na2O are referred from database.

Fig. 2 e MS (upper) and TG-DTA (lower) profiles of c-Mg used as absorbent of H2 generated by reaction (1).

other weak peaks such as 34 and 36 , even though the position of some peaks might be close to those of Na2O2. The results suggested that unexpected reactions, e.g. a reaction between some sodium compounds and the reactor, might occur on a small scale to form unidentified phases. The corrosion at 400  C would be relatively weaker than that at 500  C because the unidentified peaks observed at 400  C were smaller. As one of possibility that Na2O2 was not observed in XRD measurement, it was considered that the generated Na2O2 was amorphous. To confirm the formation of Na2O2, the reaction between the products of reactions (2) and H2O was carried out, and then the generated gases were analyzed by GC. For the experiments, three types of hydrolysis reactions are expected. One is the hydrolysis of Na2O2 as expressed by equation (3). The other reactions are the hydrolysis of Na2O and Na as follows,

Na2O þ H2O / 2NaOH,

(6)

Na þ H2O / NaOHþ1/2H2.

(7)

Thus, when Na2O2 exists in the product, O2 should be generated by the reaction with H2O. The results of GC as shown in Fig. S2 (in supplementary data) revealed the presence of O2 and H2, suggesting that Na2O2 and Na existed in the

products generated by the metal separation reaction. However, the state of Na2O2 should be directly identified by other analyses such as solid NMR in future work to essentially understand the reaction properties. The reaction yield was roughly estimated by comparing the peak area of Na2O obtained before and after the reaction at 400 and 500  C, and the result was about 35 and 80%, respectively. From the results, the possibility that reaction (2) proceeded at 400  C by using nonequilibrium process was demonstrated. However, considering practical use, the reaction kinetics should be improved to complete the reaction like the reaction (1) and the corrosion effect should be minimized by further investigation such as development of reactor materials. The hydrolysis reaction (3) is exothermic. In the experiment, the reaction between Na2O2 and H2O of excess molar ratio was carried out at 100  C. Fig. 4 shows the XRD patterns of the as-synthesized Na2O2 and the products by the reaction with H2O. As shown in the upper XRD pattern, the assynthesized Na2O2 contained a small amount of NaOH as an impurity. After the hydrolysis reaction, it was confirmed that Na2O2 was completely changed to NaOH hydrate, indicating that the hydrolysis reaction proceeded. The NaOH hydrate was formed by a reaction between NaOH generated by reaction (3) and the remaining H2O because excess H2O was used in this experiment. To identify the gaseous product, GC was

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(3) easily proceeded by heating to 100  C. After the reaction, Na2O2 was completely changed to NaOH hydrate and O2 as the expected products. From the above results, the possibility of H2 generation below 400  C by the water-splitting reaction using Na redox reactions was demonstrated. Furthermore, the reaction temperature is expected to be further reduced by modifying the nonequilibrium processes. However, the yield of reactions (1) and (2) were lower than 100% under the conditions in this work. The problem was caused by kinetic properties of each reaction because the reaction should be completed under the nonequilibrium condition. For the development as practical application, the further research is necessary to improve the reaction kinetics. In addition, the reactor materials should be carefully chosen to withstand the corrosion for safety issue. Assuming that the complete reactions could be realized, the H2 generation cycle via this system can be controlled by the gases (H2 and O2) separation using membrane and the condensation/separation system of Na vapor. In this case, it is not necessary to transfer the solid phase, suggesting the reaction system can be simpler than the conventional water-splitting techniques including difficult material separations such as solidesolid and liquideliquid separations. Therefore, the Na system should be recognized as a potential thermochemical hydrogen production technique without the current limitations of heat sources.

Fig. 4 e XRD patterns of asLsynthesized Na2O2 and the product by the hydrolysis reaction (3). XRD patterns of NaOH hydrate, NaOH, and Na2O2 are referred from database.

performed for the gas inside the reactor after the reaction. The result is shown in Fig. S3. It was clearly clarified that the gaseous product was mainly O2 with small amount of H2 by comparison with the results for a H2 and O2 mixture as a reference, where the small amount of H2 would be caused by the impurity in the synthesized Na2O2. This result indicates that 100  C is sufficient for completing the hydrolysis reaction.

4.

Conclusions

In this work, the reaction conditions of water-splitting by the Na system were investigated as a possible future thermochemical hydrogen production system. In particular, a nonequilibrium process was adopted to control the entropy and lower the reaction temperature below 500  C. For reaction (1), it was clarified that H2 is generated from the NaOH and Na mixture at 350  C with the formation of Na2O. In the case of reaction (2), we succeeded in realizing nonequilibrium condition by using the reactor with cooling system. As a result, it was indicated that Na and Na2O2 were generated at 400  C, which is drastically lower than that under the equilibrium condition. Although the corrosion by the reaction between some sodium compounds and the reactor was suggested, it can be prevented by developing the reactor materials and/or reducing the operating temperature. The hydrolysis reaction

Acknowledgments The authors gratefully acknowledge Dr. Satoshi Hino for the good discussion and valuable help in this work. This work was partially supported by “The Japan Prize Foundation”.

Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2012.09.085.

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