- Email: [email protected]
Nano-engineered Mg–MgH2 system for solar thermal energy storage
Solar Energy 150 (2017) 532–537
Contents lists available at ScienceDirect
Solar Energy journal homepage: www.elsevier.com/locate/solener
Nano-engineered Mg–MgH2 system for solar thermal energy storage Sanjay Kumar a,b,⇑, Yoshitsugu Kojima a, Vivekanand Kain b a b
Natural Science Centre for Basic Research and Development, Hiroshima University, 739-8530, Japan Materials Processing & Corrosion Engineering Division, Bhabha Atomic Research Centre, Mumbai 400 085, India
a r t i c l e
i n f o
Article history: Received 12 November 2016 Received in revised form 11 April 2017 Accepted 1 May 2017
Keywords: Solar thermal energy Ultrafast Kinetics Activation energy Enthalpy Doping
a b s t r a c t The solar thermal energy could be stored and reused at a desired locations and conditions. The prerequisite is to develop a suitable media which could able to store the solar thermal energy reversibly. The metal-metal hydride system could be one of the option to store the thermal energy in the form of metal and hydrogen which on recombination will form metal- hydride and release the stored thermal energy with high efficiency. Besides the high hydrogen storage capacity, the ultrafast hydrogenationdehydration kinetics is desirable for the viable commercial applications. In connection to this, magnesium – magnesium hydride system has been considered as a potentials candidate. However, the sluggish hydrogenation-dehydrogenation kinetics is an issue. In the present study nano-engineered Mg-V composite has been developed using MgH2 and V2O5 as a precursor for magnesium and vanadium, respectively. The composite has shown an ultrafast hydrogenation-dehydrogenation kinetics at remarkable low temperature. The hydrogenation of composite has efficiently released the thermal energy. The hydrogenated composite could be dehydrogenated using compact solar power (CSP) even below 200 °C. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Hydrogenation-dehydrogenation of metals and alloys are associated with the massive thermal energy transformation. In general, thermal energy will be absorbed during dehydrogenation process whereas it will be released during hydrogenation (Gerard et al., 1992. Law et al., 2016; Taxak et al., 2013; Kumar et al., 2013a,b). Thus the metal - metal hydride system could be a suitable candidate to store the solar thermal energy. The prerequisite is that the hydrogenation-dehydrogenation should be reversible at the desirable temperature and pressure conditions (Bogdanovic et al., 2002. Kumar et al., 2015). The concept is based on the energy conversion from thermal energy to chemical energy (metal and hydrogen) during dehydrogenation (Kumar et al., 2013b). The solar thermal energy could be condensed using concentrated solar panel (CSP) cells (Arto et al., 2016) to achieve the desired temperature for dehydrogenation. The thermal energy stored in the form of metal and hydrogen will be released during hydrogenation. The solar thermal energy storage using metal-metal hydride is commonly known as a thermochemical process. A schematic concept of the CSP coupled with metal-metal hydride system has been illustrated in Fig. 1 (Michhael et al., 2009). The thermochemical method is
⇑ Corresponding author at: Materials Processing & Corrosion Engineering Division, Bhabha Atomic Research Centre, Mumbai 400 085, India. E-mail address: [email protected] (S. Kumar). http://dx.doi.org/10.1016/j.solener.2017.05.001 0038-092X/Ó 2017 Elsevier Ltd. All rights reserved.
considered superior as compared to other heat storage method because, high amount of thermal energy could be stored per unit mass of storage material (Michhael et al., 2009). In this process, hydrogen will be confined in a closed system, and it will be preserved during thermal energy storage cycle as illustrated in Fig. 1 (Michhael et al., 2009). The stored thermal energy will be liberated during hydrogenation, and it will serve as a useful heat source (Eberle et al., 2006. Ewa et al., 2010). Besides the high hydrogen storage capacity, the ultrafast hydrogenation-dehydrogenation kinetics is desirable for the viable commercial applications. In connection to this, magnesium – magnesium hydride has been considering as a potentials candidate (Beckmann and Gilli, 1984; Bogdanovic´ et al., 1995; Varin et al., 2004). However; the large activation barriers for hydrogenation - dehydrogenation of pure MgH2 (Eabs > 70, Edes > 150 kJ/mol) (Kumar et al., 2017a,b,c,d,e) requires undesirable high working temperature (Tabs > 250 °C, Tdes > 350 °C). Many catalysts have been explored in this regards, and some of them found suitable to overcome the kinetics barrier of dehydrogenation (Webb, 2015; Kumar and Tiwari, 2017, Jun et al., 2010, Reiser et al., 2000). However; hydrogenation is poorly explored, particularly for the thermal energy released during hydrogenation process. In the present study nano-engineered Mg-V composite has been developed and its hydrogenationdehydrogenation cycles have been tested up to 100 cycles at room temperature (20 °C) and 200 °C. The composite has shown ultrafast hydrogenation-dehydrogenation with high efficiency of thermal
S. Kumar et al. / Solar Energy 150 (2017) 532–537
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contamination during measurement. The argon gas flow during dehydrogenation was kept at the rate of 18 L/h to maintain the negligible hydrogens back pressure. The apparent activation energy of dehydrogenation was evaluated by a non-isothermal process using H2 – MS (m/z: 2) peak temperatures of thermal desorption mass spectroscopy conducted in the TG-DTA-MS unit. The Kissinger method was employed to calculate the kinetic parameters in the form of Eq. (1) as given below:
ln
Fig. 1. A schematic diagram of the concentrated solar panel coupled (CSP) the MgMgH2 system to store the solar thermal energy and released on demands.
energy exchange which was measured by the temperature increase during hydrogenation. Thus; the composite could be a potential candidate to store the solar thermal energy for their useful applications.
b T 2p
¼
Ea RK o þ ln RT p Ea
ð1Þ
where Tp is the onset peak temperature of the events, b is the heating rate, Ea is the apparent activation energy, Ko is the collision frequency, and R is the universal gas constant. The phase analysis was done by powder-X-ray diffractometer (pXRD; RINT-2100, Rigaku, Cu Ka radiation). The powder morphology and the elemental mapping were done using scanning electron microscope (SEM; JSM-6380A, JEOL) attached with energy dispersive spectroscopy (EDS). The XPS analysis (XPS; ESCALAB 250Xi, Thermo-Fisher Scientific, Al Ka = 1486.6 eV) has been carried out on the nano-composite to evaluate the chemical states of the vanadium and magnesium. It has been ensured that the sample should not be exposed to an open atmosphere before and during all the operations.
2. Experimental 3. Results and discussion The Mg-V composite was prepared using MgH2 and V2O5 as a precursor of magnesium and vanadium, respectively. The MgH2 powder of purity 98.8 mass% and divanadium pentoxide (V2O5, particle size 32 nm) of purity 99.9 mass% both obtained from Sigma–Aldrich was used. The sample was prepared by ball milling of 1 g of as received MgH2 with 5 wt.% V2O5 additive. A hardened stainless steel pot containing 20 pieces of hardened stainless steel balls of diameter 7 mm was used for the ball milling. The sample was ball milled for 10 h at the milling frequency of 400 rpm with one-hour milling and half an hour pause time pattern. Argon atmosphere was maintained throughout the ball milling. The Mg-V nanocomposite was prepared by dehydrogenating the ball milled sample (MgH2-V2O5) at 175 °C for 12 h under the turbomolecular pump. The hydrogenation was studied in a Sievert’s apparatus (Suzuki Shokan Co. Ltd.-Japan) using constant volume and variable pressure principle (Kumar et al., 2014, 2017a,b,c,d,e). In the process, a known hydrogen pressure (4 MPa) was kept into the gas reservoir and then connected to the blank reaction chamber. The pressure drops down to 1.9 MPa in the system (reservoir – blank reaction chamber) which remains unchanged for a long duration. Next; about 1000 mg sample was kept inside the reaction chamber, and the same initial hydrogen pressure (4 MPa) was introduced into the hydrogen gas reservoir. The hydrogen gas reservoir was connected to the reaction chamber. The pressure drop due to hydrogen absorption by the sample was recorded with respect to blank reaction chamber. The amount of hydrogen absorbed by the sample in wt.% was calculated using pressure drop data. It is worth to mention here that the change in the volume of the reaction chamber due to sample volume has also been considered during the calculation. The thermal energy released during hydrogenation was evaluated by measuring the temperature outside the sample chamber by a K-type thermocouple. The hydrogenation kinetics was studied in a high-pressure differential scanning calorimetric unit (HP-DSC; Q10PDSC, TA Instruments) placed inside the argon gas glove box. The dehydrogenation experiments were conducted in thermoanalyser (TG-DTA; TG-8120, Rigaku) unit attached with a mass spectrometer (MS; M-QA200TS, Anelva). The system was in the inert atmosphere (argon gas) glove box to avoid oxidation or
3.1. Hydrogenation Hydrogenation of Mg-V composite has been studied in the Sievert’s apparatus at room temperature (20 °C) and 0 °C. The outer cell of the reactor was kept inside the tap-water to maintained 20 °C, and ice-water bath to maintain 0 °C. The sample was exposed to 1.9 MPa (initial pressure after connection to hydrogen gas reservoir) at constant volume, and the instantaneous pressure drop (due to hydrogen absorption by the composite) was recorded. The instantaneous mass gain by the composite has been evaluated using pressure drop data, and the results are presented in Fig. 2(a). The results have shown an ultra-fast hydrogenation kinetics by the composite at room temperature (20 °C). The sample has absorbed about 6.9 wt.% hydrogen in less than a minute (the 40 s) at room temperature (20 °C). The absorbed hydrogen was found to be >97 wt.% of theoretical hydrogen storage capacity. The experiment has also been conducted without controlling the temperature of the reaction chamber during hydrogenation. The intensity of thermal energy released during the hydrogenation was measured by the rate of temperature rise during hydrogenation, and the result is presented in Fig. 2(b). Remarkably, the temperature during hydrogenation has increased to 192.5 °C within the 40 s at the rate of 4.8 °C/s. It is noteworthy to mention here that the temperature was measured outside of the reaction chamber. The reaction chamber was made of stainless steel having moderate thermal conductivity. Therefore, it could be assumed that the temperature close to the sample may be >200 °C. The results were interesting and relevant to the reversible solar thermal energy storage application using the developed Mg-V composite. The hydrogenation has also been studied at 0 °C isothermal temperature under the similar experimental conditions. The hydrogenation result at 0 °C is presented in Fig. 2(a). The composite has shown good hydrogen absorption kinetics even at 0 °C. The composite has absorbed >1.5 wt.% hydrogen even at 0 °C within the 40 s (the time in which the composite has absorbed 6.9 wt.% at 20 °C). The kinetics and the hydrogen storage capacity at 0 °C was found to be better than many bcc alloys (Taxak et al., 2013; Kumar et al., 2017a,b,c,d,e). The hydrogenation was continued for an hour at 0 °C and found
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S. Kumar et al. / Solar Energy 150 (2017) 532–537
Fig. 2. The hydrogenation rate of nano- engineered Mg at room temperature (20 °C), and 0 °C, the temperatures were controlled by dipping the reactor into the water and icewater mixture to maintained 20 °C and oC temperature respectively. (b) The temperature rises during uncontrolled hydrogenation at 20 °C. The inset of (b) is showing the schematic of nano-engineered Mg/MgH2 hydrogenation process.
the maximum hydrogen absorption as 4 wt.% in one hour. The slow hydrogenation at 0 °C could be probably due to high activation energy barrier for (a) H2 dissociation into H atom at the composite surface, and (b) slow diffusion of hydrogen at such a low temperature (Kumar et al., 2017a,b,c,d,e). 3.2. Dehydrogenation The dehydrogenation of the hydrogenated composite has been studied to understand the cyclic performance. The dehydrogenation experiments were conducted in the TG-DTA-MS unit at a various heating rate (1, 2, 5 and 10 °C/min) under the flow of Ar gas at 18 L/h. The thermogravimetric analysis result is presented in Fig. 3. The simultaneous MS analysis of hydrogen evolution of the same experiments is presented as an inset of Fig. 3. It is evident from the figure that the composite starts dehydrogenation even at 150 °C. The minimum dehydrogenation temperature of MgH2 reported earlier was above 200 °C as per the author’s
knowledge. However; in present case dehydrogenation started at 150 °C as shown in Fig. 3. The kinetic improvement of dehydrogenation reaction of composite has been quantified by measuring the apparent activation energy. The Kissinger equation (1) and peak temperatures (Tp) of H2-MS analysis (m/z: 2) (Fig. 3) at a different heating rate, were used for the calculation. The linear plot of Kissinger equation is presented in Fig. 4. The apparent activation energy of dehydrogenation was calculated using Kissinger plot and found to be 68 ± 2 kJ/mol. The apparent activation energy is substantially less than the pristine MgH2 which was reported as 143 ± 2 kJ/mol (Kumar et al., 2014, 2016, 2017a,b,c, d,e). The remarkably reduced activation energy of dehydrogenation seems to be the possible reason of decreased dehydrogenation temperature. The cyclic hydrogenation-dehydrogenation of the composite have been tested in the Sieverts apparatus between 20 °C and 200 °C. The composite has shown cyclic stability over 100 hydrogenation-dehydrogenation without losing its hydrogen storage capacity.
Fig. 3. Thermogravimetric analysis in an argon flow at the heating rate of 1, 2, 5 and 10 °C/min to understand the heating rate effects of dehydrogenation. The inset of the Figure is. The mass spectroscopy analysis at the heating rate of 1, 2, 5 and 10 °C/min of the thermogravimetric. The peak temperature of MS analysis has been used for the activation energy calculation using Kissinger equation.
S. Kumar et al. / Solar Energy 150 (2017) 532–537
Fig. 4. The linear plot of the Kissinger equation to evaluate the apparent activation energy of dehydrogenation.
3.3. The XPS study The nanocomposite was analyzed by XPS to evaluate the exact chemical states of Mg and vanadium at the interface. The XPS of pristine MgO and V2O5 have also been measured separately to compare the results. The XPS results of Mg2s1/2 electrons are presented as an inset of Fig. 5. The binding energy peaks of Mg2s1/2 electron in the composite has appeared at 88.4 eV which was at the same position of the Mg2s1/2 electron of MgO as it can be seen in the inset of Fig. 5. Prima facie the XPS results indicate the presence of MgO at the interface of the composite. The XPS analysis of V2p electron of the composite and pristine V2O5 have also been carried out, and the results are presented in Fig. 5. The binding energy peaks of V2p electrons of pristine V2O5 were found to be at 517.2 and 524.5 eV respectively, which indicate the V5+ chemical state with marginal peaks shift. The appearance of double peaks is because of spin-orbit (S-L) coupling due to which, V2p energy level split into high spin
535
2p3/2 and low spin 2p1/2 energy levels. The peaks shift could be due to chemical shift (http://srdata, 2015). In the case of the composite, multiple binding energy peaks of V2p electrons were observed as shown in Fig. 5. The broader peak at 511.7 eV seems to be an overlap of two peaks which may originate at 513.0 (shoulder is clearly visible in Fig. 5) and 511.7 eV. The peaks at 513.0 and 511.7 eV could be the second peaks (2p3/2 electrons) of V0 and V3+, respectively with marginal peak shift (http:// srdata, 2015). The first peaks of V and V2O3 appear at 519.7 and 517.2 eV respectively. The peak shift of V0 as compared to NIST data could be due to chemical interaction and doping of nascent vanadium over the Mg surface. The doping leads to diffused V2p electrons which marginal shift the peaks to 517.2 eV as compared to NIST peak position of 517 eV for V0. To confirm the XPS results; the thermodynamic calculations has been performed for the reduction of V2O5 by MgH2. It has been found that V2O5 could be reduced to metallic vanadium by MgH2 even at room temperature as per the reaction: 5MgH2 + V2O5 = 5MgO + 2V + 5H2, DG°: 1246 kJ/mole V2O5. The XPS analysis already confirms the presence of MgO and V. The SEM-EDS analysis of the dehydrogenated catalyzed MgH2 (MgV composite) surface have been performed, and the results are presented in Fig. 6a–d. The circled portion of the EDS analysis of oxygen and vanadium have shown an uneven distribution of oxygen (circle A) and vanadium (circle B). The SEM-EDS results could be due to the reduced product of V2O5 as metallic vanadium and low oxidation state vanadium oxide which could have been formed during the ball milling and subsequent dehydrogenation. The density functional theory (DFT) calculations have shown that the vanadium - doped Mg surface favors the dissociation and diffusion of hydrogen and as a consequence, the kinetics will improve (Ling et al., 2013; Kecik et al., 2009; Pozzo et al., 2009). The hydrogenation - dehydrogenation results obtained in the current study are in accordance with the theory. As a conclusion, during ball milling and dehydrogenation of MgH2-V2O5 composite, V2O5 seems to be reduced to metallic vanadium and homogeneously doped over the Mg surface. The V-doped Mg surface has shown ultrafast hydrogenation kinetics and remarkably improved dehydrogenation kinetics. 3.4. PCT and thermodynamics The thermodynamics of hydrogenation - dehydrogenation of the Mg-V composite have been studied by pressurecomposition isotherm (PCT). The PCT curves were obtained at 275 °C, 300 °C, and 325 °C respectively and presented as an inset of Fig. 7. The enthalpies of the hydrogenation-dehydrogenation reaction of the composite was calculated using equilibrium hydrogen pressure of PCT curves using van’t-Hoff equation (4) (Sanjay et al., 2014)
ln
Fig. 5. The XPS analysis of vanadium in the composite to confirm the chemical state. The inset is the XPS analysis of Mg 2s electron to ratify the chemical of Mg at the surface.
Peq DH DS : ¼ RT R Po
ð2Þ
where Peq: plateau pressure, Po: standard pressure, DH°: molar enthalpy, DS°: molar entropy, R: universal gas constant and T is the temperature in Kelvin. It has been found that the enthalpy corresponding to hydrogenation- dehydrogenation has been increased marginally as compared to pristine MgH2 which was found to be in the range of 75–76 kJ/mol (http://www.factsage, 2016). The results confirmed that vanadium had not formed the homogeneous solid solution with Mg, it only acts as a catalyst. The vanadium doped Mg surface remarkable enhanced the hydrogenationdehydrogenation kinetics; however; it has marginally affected the thermodynamics of the process.
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S. Kumar et al. / Solar Energy 150 (2017) 532–537
Fig. 6. The SEM-EDS analysis of nano-engineered Mg: (a) microstructure of the portion of the sample of which EDS mapping has been done (b) magnesium mapping, (c) oxygen mapping, and (d) vanadium mapping. The analysis indicated the concentration inhomogeneity of vanadium (circled as (A)) and oxygen (circle as (B)).
Fig. 7. The Enthalpy calculation hydrogen-dehydrogenation reaction. Inset figure is the PCT of the composite at 275 °C, 300 °C, and 350 °C. The enthalpies of hydrogenationdehydrogenation have shown marginal change as compared to pristine MgH2.
4. Conclusions The thermal energy released during hydrogenation will be helpful to design a system based on the developed nanocomposite for the storage of solar thermal energy. The kinetics of dehydrogenation of the composite also improved substantially, and as a consequence, the temperature lower than 200 °C is required to dehydrogenate the composite. The temperature could be easily
achieved using CSP. The XPS analysis has shown that the ball milling of MgH2-V2O5 followed by dehydrogenation leads to Mg-V composite. The vanadium is uniformly distributed over the Mg surface. The V-doped Mg surface remarkably reduced the activation energy of hydrogen dissociation process leading to ultrafast hydrogenation kinetics. The remarkable results of the present study are the rise in temperature up to 192.5 °C within the 40 s.
S. Kumar et al. / Solar Energy 150 (2017) 532–537
Acknowledgments This research work is supported by JSPS-Japan under the ‘‘Postdoctoral Fellowship Program for Overseas Researchers” granted to Sanjay Kumar (JSPS ID: P15078).
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Kumar, S., Tiwari, G.P., Krishnamurthy, N., 2015. Tailoring the hydrogen desorption thermodynamics of V2H by alloying additives. J. Alloys Compounds 645 (2015), S252–S256. Kumar, S., Singh, K., Jain, U., Krishnamurthy, N., 2014. Solid solubility of hydrogen in Pb83Li17 alloy: a preliminary investigation. Fusion Eng. Des. 89, 1351–1355. Kumar, S., Jain, U., Jain, A., Miyaoka, H., Ichikawa, T., Kojima, Y., Dey, G.K., 2017a. Development of Mg-Li-B based advanced material for on-board hydrogen storage solution. Int. J. Hydrogen Energy 42 (7), 3963–3970. Kumar, S., Miyaoka, H., Ichikawa, T., Dey, G.K., Kojima, Y., 2017b. Micro-alloyed Mg2Ni for better performance as a negative electrode of Ni-MH battery and hydrogen storage. Int. J. Hydrogen Energy 42 (7), 5220–5226. Kumar, S., Singh, A., Tiwari, Gyanendra Prasad, Kojima, Yoshitsugu, Kain, V., 2017c. Thermodynamics and kinetics of nano-engineered Mg-MgH2 system for reversible hydrogen storage application. Thermochemica Acta 652, 103–108. Kumar, S., Jain, A., Yamaguchi, S., Miyaoka, H., Ichikawa, T., Mukherjee, A., Dey, G.K., Kojima, Y., 2017d. Surface modification of MgH2 by ZrCl4 to tailor the reversible hydrogen storage performance. Int. J. Hydrogen Energy 42 (9), 6152–6159. Kumar, S., Tiwari, G.P., 2017. Thermodynamics and kinetics of MgH2-nfTa2O5 composite for reversible hydrogen storage application. J. Mater. Sci. 52 (12), 6962–6968. Kumar, S., Jain, A., Ichikawa, T., Kojima, Y., Dey, G.K., 2017e. Development of vanadium based hydrogen storage material: a review. Renew. Sustain. Energy Rev. 72, 791–800. Ling, G., Yanfei, Y., 2013. Theoretical investigation of molecular hydrogen absorption and dissociation on AlnV(n = 1–13) clusters. Int. J. Hydrogen Energy 38, 3640–3649. Law, E., Kay, W., Merlinde, T., Robert, A., 2016. Calculating the financial value of a concentrated solar thermal plant operated using direct normal irradiance forecasts. Sol. Energy 125, 267–281. Michael, F., Borislav, B., 2009. High-temperature metal hydrides as heat storage materials for solar and related applications. Int. J. Mol. Sci. 10, 325–344. Pozzo, M., Alfe, D., 2009. Hydrogen dissociation and diffusion on transition metal (Ti, Zr, V, Fe, Ru Co, Rh, Ni, Pd, Cu, Ag)-doped Mg(00001) surfaces. Int. J. Hydrogen Energy 34, 1922–1930. Reiser, A., Bogdanovic, B., Schilichte, K., 2000. The application of Mg-based metalhydrides as heat energy storage systems. Int. J. Hydrogen Energy 25, 425–430. Taxak, M., Kumar, S., Krishnamurthy, N., 2013. Thermodynamic parameters for the Ta-Cr-H Solid Solution from Equilibrium P-C-T Data. J. Chem. Thermodyn. 67, 48–54. Varin, R.A., Calka, Li.S.A., Wexler, D., 2004. Formation and environmental stability of nanocrystalline and amorphous hydrides in the 2Mg-Fe mixture processed by controlled reactive mechanical alloying (CRMA). J. Alloys Compd. 373, 270–286. Webb, C.J., 2015. A review of catalyst-enhanced magnesium hydride as a hydrogen storage materials. J. Phys. Chem. Solids 84, 96–106.
Contents lists available at ScienceDirect
Solar Energy journal homepage: www.elsevier.com/locate/solener
Nano-engineered Mg–MgH2 system for solar thermal energy storage Sanjay Kumar a,b,⇑, Yoshitsugu Kojima a, Vivekanand Kain b a b
Natural Science Centre for Basic Research and Development, Hiroshima University, 739-8530, Japan Materials Processing & Corrosion Engineering Division, Bhabha Atomic Research Centre, Mumbai 400 085, India
a r t i c l e
i n f o
Article history: Received 12 November 2016 Received in revised form 11 April 2017 Accepted 1 May 2017
Keywords: Solar thermal energy Ultrafast Kinetics Activation energy Enthalpy Doping
a b s t r a c t The solar thermal energy could be stored and reused at a desired locations and conditions. The prerequisite is to develop a suitable media which could able to store the solar thermal energy reversibly. The metal-metal hydride system could be one of the option to store the thermal energy in the form of metal and hydrogen which on recombination will form metal- hydride and release the stored thermal energy with high efficiency. Besides the high hydrogen storage capacity, the ultrafast hydrogenationdehydration kinetics is desirable for the viable commercial applications. In connection to this, magnesium – magnesium hydride system has been considered as a potentials candidate. However, the sluggish hydrogenation-dehydrogenation kinetics is an issue. In the present study nano-engineered Mg-V composite has been developed using MgH2 and V2O5 as a precursor for magnesium and vanadium, respectively. The composite has shown an ultrafast hydrogenation-dehydrogenation kinetics at remarkable low temperature. The hydrogenation of composite has efficiently released the thermal energy. The hydrogenated composite could be dehydrogenated using compact solar power (CSP) even below 200 °C. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Hydrogenation-dehydrogenation of metals and alloys are associated with the massive thermal energy transformation. In general, thermal energy will be absorbed during dehydrogenation process whereas it will be released during hydrogenation (Gerard et al., 1992. Law et al., 2016; Taxak et al., 2013; Kumar et al., 2013a,b). Thus the metal - metal hydride system could be a suitable candidate to store the solar thermal energy. The prerequisite is that the hydrogenation-dehydrogenation should be reversible at the desirable temperature and pressure conditions (Bogdanovic et al., 2002. Kumar et al., 2015). The concept is based on the energy conversion from thermal energy to chemical energy (metal and hydrogen) during dehydrogenation (Kumar et al., 2013b). The solar thermal energy could be condensed using concentrated solar panel (CSP) cells (Arto et al., 2016) to achieve the desired temperature for dehydrogenation. The thermal energy stored in the form of metal and hydrogen will be released during hydrogenation. The solar thermal energy storage using metal-metal hydride is commonly known as a thermochemical process. A schematic concept of the CSP coupled with metal-metal hydride system has been illustrated in Fig. 1 (Michhael et al., 2009). The thermochemical method is
⇑ Corresponding author at: Materials Processing & Corrosion Engineering Division, Bhabha Atomic Research Centre, Mumbai 400 085, India. E-mail address: [email protected] (S. Kumar). http://dx.doi.org/10.1016/j.solener.2017.05.001 0038-092X/Ó 2017 Elsevier Ltd. All rights reserved.
considered superior as compared to other heat storage method because, high amount of thermal energy could be stored per unit mass of storage material (Michhael et al., 2009). In this process, hydrogen will be confined in a closed system, and it will be preserved during thermal energy storage cycle as illustrated in Fig. 1 (Michhael et al., 2009). The stored thermal energy will be liberated during hydrogenation, and it will serve as a useful heat source (Eberle et al., 2006. Ewa et al., 2010). Besides the high hydrogen storage capacity, the ultrafast hydrogenation-dehydrogenation kinetics is desirable for the viable commercial applications. In connection to this, magnesium – magnesium hydride has been considering as a potentials candidate (Beckmann and Gilli, 1984; Bogdanovic´ et al., 1995; Varin et al., 2004). However; the large activation barriers for hydrogenation - dehydrogenation of pure MgH2 (Eabs > 70, Edes > 150 kJ/mol) (Kumar et al., 2017a,b,c,d,e) requires undesirable high working temperature (Tabs > 250 °C, Tdes > 350 °C). Many catalysts have been explored in this regards, and some of them found suitable to overcome the kinetics barrier of dehydrogenation (Webb, 2015; Kumar and Tiwari, 2017, Jun et al., 2010, Reiser et al., 2000). However; hydrogenation is poorly explored, particularly for the thermal energy released during hydrogenation process. In the present study nano-engineered Mg-V composite has been developed and its hydrogenationdehydrogenation cycles have been tested up to 100 cycles at room temperature (20 °C) and 200 °C. The composite has shown ultrafast hydrogenation-dehydrogenation with high efficiency of thermal
S. Kumar et al. / Solar Energy 150 (2017) 532–537
533
contamination during measurement. The argon gas flow during dehydrogenation was kept at the rate of 18 L/h to maintain the negligible hydrogens back pressure. The apparent activation energy of dehydrogenation was evaluated by a non-isothermal process using H2 – MS (m/z: 2) peak temperatures of thermal desorption mass spectroscopy conducted in the TG-DTA-MS unit. The Kissinger method was employed to calculate the kinetic parameters in the form of Eq. (1) as given below:
ln
Fig. 1. A schematic diagram of the concentrated solar panel coupled (CSP) the MgMgH2 system to store the solar thermal energy and released on demands.
energy exchange which was measured by the temperature increase during hydrogenation. Thus; the composite could be a potential candidate to store the solar thermal energy for their useful applications.
b T 2p
¼
Ea RK o þ ln RT p Ea
ð1Þ
where Tp is the onset peak temperature of the events, b is the heating rate, Ea is the apparent activation energy, Ko is the collision frequency, and R is the universal gas constant. The phase analysis was done by powder-X-ray diffractometer (pXRD; RINT-2100, Rigaku, Cu Ka radiation). The powder morphology and the elemental mapping were done using scanning electron microscope (SEM; JSM-6380A, JEOL) attached with energy dispersive spectroscopy (EDS). The XPS analysis (XPS; ESCALAB 250Xi, Thermo-Fisher Scientific, Al Ka = 1486.6 eV) has been carried out on the nano-composite to evaluate the chemical states of the vanadium and magnesium. It has been ensured that the sample should not be exposed to an open atmosphere before and during all the operations.
2. Experimental 3. Results and discussion The Mg-V composite was prepared using MgH2 and V2O5 as a precursor of magnesium and vanadium, respectively. The MgH2 powder of purity 98.8 mass% and divanadium pentoxide (V2O5, particle size 32 nm) of purity 99.9 mass% both obtained from Sigma–Aldrich was used. The sample was prepared by ball milling of 1 g of as received MgH2 with 5 wt.% V2O5 additive. A hardened stainless steel pot containing 20 pieces of hardened stainless steel balls of diameter 7 mm was used for the ball milling. The sample was ball milled for 10 h at the milling frequency of 400 rpm with one-hour milling and half an hour pause time pattern. Argon atmosphere was maintained throughout the ball milling. The Mg-V nanocomposite was prepared by dehydrogenating the ball milled sample (MgH2-V2O5) at 175 °C for 12 h under the turbomolecular pump. The hydrogenation was studied in a Sievert’s apparatus (Suzuki Shokan Co. Ltd.-Japan) using constant volume and variable pressure principle (Kumar et al., 2014, 2017a,b,c,d,e). In the process, a known hydrogen pressure (4 MPa) was kept into the gas reservoir and then connected to the blank reaction chamber. The pressure drops down to 1.9 MPa in the system (reservoir – blank reaction chamber) which remains unchanged for a long duration. Next; about 1000 mg sample was kept inside the reaction chamber, and the same initial hydrogen pressure (4 MPa) was introduced into the hydrogen gas reservoir. The hydrogen gas reservoir was connected to the reaction chamber. The pressure drop due to hydrogen absorption by the sample was recorded with respect to blank reaction chamber. The amount of hydrogen absorbed by the sample in wt.% was calculated using pressure drop data. It is worth to mention here that the change in the volume of the reaction chamber due to sample volume has also been considered during the calculation. The thermal energy released during hydrogenation was evaluated by measuring the temperature outside the sample chamber by a K-type thermocouple. The hydrogenation kinetics was studied in a high-pressure differential scanning calorimetric unit (HP-DSC; Q10PDSC, TA Instruments) placed inside the argon gas glove box. The dehydrogenation experiments were conducted in thermoanalyser (TG-DTA; TG-8120, Rigaku) unit attached with a mass spectrometer (MS; M-QA200TS, Anelva). The system was in the inert atmosphere (argon gas) glove box to avoid oxidation or
3.1. Hydrogenation Hydrogenation of Mg-V composite has been studied in the Sievert’s apparatus at room temperature (20 °C) and 0 °C. The outer cell of the reactor was kept inside the tap-water to maintained 20 °C, and ice-water bath to maintain 0 °C. The sample was exposed to 1.9 MPa (initial pressure after connection to hydrogen gas reservoir) at constant volume, and the instantaneous pressure drop (due to hydrogen absorption by the composite) was recorded. The instantaneous mass gain by the composite has been evaluated using pressure drop data, and the results are presented in Fig. 2(a). The results have shown an ultra-fast hydrogenation kinetics by the composite at room temperature (20 °C). The sample has absorbed about 6.9 wt.% hydrogen in less than a minute (the 40 s) at room temperature (20 °C). The absorbed hydrogen was found to be >97 wt.% of theoretical hydrogen storage capacity. The experiment has also been conducted without controlling the temperature of the reaction chamber during hydrogenation. The intensity of thermal energy released during the hydrogenation was measured by the rate of temperature rise during hydrogenation, and the result is presented in Fig. 2(b). Remarkably, the temperature during hydrogenation has increased to 192.5 °C within the 40 s at the rate of 4.8 °C/s. It is noteworthy to mention here that the temperature was measured outside of the reaction chamber. The reaction chamber was made of stainless steel having moderate thermal conductivity. Therefore, it could be assumed that the temperature close to the sample may be >200 °C. The results were interesting and relevant to the reversible solar thermal energy storage application using the developed Mg-V composite. The hydrogenation has also been studied at 0 °C isothermal temperature under the similar experimental conditions. The hydrogenation result at 0 °C is presented in Fig. 2(a). The composite has shown good hydrogen absorption kinetics even at 0 °C. The composite has absorbed >1.5 wt.% hydrogen even at 0 °C within the 40 s (the time in which the composite has absorbed 6.9 wt.% at 20 °C). The kinetics and the hydrogen storage capacity at 0 °C was found to be better than many bcc alloys (Taxak et al., 2013; Kumar et al., 2017a,b,c,d,e). The hydrogenation was continued for an hour at 0 °C and found
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Fig. 2. The hydrogenation rate of nano- engineered Mg at room temperature (20 °C), and 0 °C, the temperatures were controlled by dipping the reactor into the water and icewater mixture to maintained 20 °C and oC temperature respectively. (b) The temperature rises during uncontrolled hydrogenation at 20 °C. The inset of (b) is showing the schematic of nano-engineered Mg/MgH2 hydrogenation process.
the maximum hydrogen absorption as 4 wt.% in one hour. The slow hydrogenation at 0 °C could be probably due to high activation energy barrier for (a) H2 dissociation into H atom at the composite surface, and (b) slow diffusion of hydrogen at such a low temperature (Kumar et al., 2017a,b,c,d,e). 3.2. Dehydrogenation The dehydrogenation of the hydrogenated composite has been studied to understand the cyclic performance. The dehydrogenation experiments were conducted in the TG-DTA-MS unit at a various heating rate (1, 2, 5 and 10 °C/min) under the flow of Ar gas at 18 L/h. The thermogravimetric analysis result is presented in Fig. 3. The simultaneous MS analysis of hydrogen evolution of the same experiments is presented as an inset of Fig. 3. It is evident from the figure that the composite starts dehydrogenation even at 150 °C. The minimum dehydrogenation temperature of MgH2 reported earlier was above 200 °C as per the author’s
knowledge. However; in present case dehydrogenation started at 150 °C as shown in Fig. 3. The kinetic improvement of dehydrogenation reaction of composite has been quantified by measuring the apparent activation energy. The Kissinger equation (1) and peak temperatures (Tp) of H2-MS analysis (m/z: 2) (Fig. 3) at a different heating rate, were used for the calculation. The linear plot of Kissinger equation is presented in Fig. 4. The apparent activation energy of dehydrogenation was calculated using Kissinger plot and found to be 68 ± 2 kJ/mol. The apparent activation energy is substantially less than the pristine MgH2 which was reported as 143 ± 2 kJ/mol (Kumar et al., 2014, 2016, 2017a,b,c, d,e). The remarkably reduced activation energy of dehydrogenation seems to be the possible reason of decreased dehydrogenation temperature. The cyclic hydrogenation-dehydrogenation of the composite have been tested in the Sieverts apparatus between 20 °C and 200 °C. The composite has shown cyclic stability over 100 hydrogenation-dehydrogenation without losing its hydrogen storage capacity.
Fig. 3. Thermogravimetric analysis in an argon flow at the heating rate of 1, 2, 5 and 10 °C/min to understand the heating rate effects of dehydrogenation. The inset of the Figure is. The mass spectroscopy analysis at the heating rate of 1, 2, 5 and 10 °C/min of the thermogravimetric. The peak temperature of MS analysis has been used for the activation energy calculation using Kissinger equation.
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Fig. 4. The linear plot of the Kissinger equation to evaluate the apparent activation energy of dehydrogenation.
3.3. The XPS study The nanocomposite was analyzed by XPS to evaluate the exact chemical states of Mg and vanadium at the interface. The XPS of pristine MgO and V2O5 have also been measured separately to compare the results. The XPS results of Mg2s1/2 electrons are presented as an inset of Fig. 5. The binding energy peaks of Mg2s1/2 electron in the composite has appeared at 88.4 eV which was at the same position of the Mg2s1/2 electron of MgO as it can be seen in the inset of Fig. 5. Prima facie the XPS results indicate the presence of MgO at the interface of the composite. The XPS analysis of V2p electron of the composite and pristine V2O5 have also been carried out, and the results are presented in Fig. 5. The binding energy peaks of V2p electrons of pristine V2O5 were found to be at 517.2 and 524.5 eV respectively, which indicate the V5+ chemical state with marginal peaks shift. The appearance of double peaks is because of spin-orbit (S-L) coupling due to which, V2p energy level split into high spin
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2p3/2 and low spin 2p1/2 energy levels. The peaks shift could be due to chemical shift (http://srdata, 2015). In the case of the composite, multiple binding energy peaks of V2p electrons were observed as shown in Fig. 5. The broader peak at 511.7 eV seems to be an overlap of two peaks which may originate at 513.0 (shoulder is clearly visible in Fig. 5) and 511.7 eV. The peaks at 513.0 and 511.7 eV could be the second peaks (2p3/2 electrons) of V0 and V3+, respectively with marginal peak shift (http:// srdata, 2015). The first peaks of V and V2O3 appear at 519.7 and 517.2 eV respectively. The peak shift of V0 as compared to NIST data could be due to chemical interaction and doping of nascent vanadium over the Mg surface. The doping leads to diffused V2p electrons which marginal shift the peaks to 517.2 eV as compared to NIST peak position of 517 eV for V0. To confirm the XPS results; the thermodynamic calculations has been performed for the reduction of V2O5 by MgH2. It has been found that V2O5 could be reduced to metallic vanadium by MgH2 even at room temperature as per the reaction: 5MgH2 + V2O5 = 5MgO + 2V + 5H2, DG°: 1246 kJ/mole V2O5. The XPS analysis already confirms the presence of MgO and V. The SEM-EDS analysis of the dehydrogenated catalyzed MgH2 (MgV composite) surface have been performed, and the results are presented in Fig. 6a–d. The circled portion of the EDS analysis of oxygen and vanadium have shown an uneven distribution of oxygen (circle A) and vanadium (circle B). The SEM-EDS results could be due to the reduced product of V2O5 as metallic vanadium and low oxidation state vanadium oxide which could have been formed during the ball milling and subsequent dehydrogenation. The density functional theory (DFT) calculations have shown that the vanadium - doped Mg surface favors the dissociation and diffusion of hydrogen and as a consequence, the kinetics will improve (Ling et al., 2013; Kecik et al., 2009; Pozzo et al., 2009). The hydrogenation - dehydrogenation results obtained in the current study are in accordance with the theory. As a conclusion, during ball milling and dehydrogenation of MgH2-V2O5 composite, V2O5 seems to be reduced to metallic vanadium and homogeneously doped over the Mg surface. The V-doped Mg surface has shown ultrafast hydrogenation kinetics and remarkably improved dehydrogenation kinetics. 3.4. PCT and thermodynamics The thermodynamics of hydrogenation - dehydrogenation of the Mg-V composite have been studied by pressurecomposition isotherm (PCT). The PCT curves were obtained at 275 °C, 300 °C, and 325 °C respectively and presented as an inset of Fig. 7. The enthalpies of the hydrogenation-dehydrogenation reaction of the composite was calculated using equilibrium hydrogen pressure of PCT curves using van’t-Hoff equation (4) (Sanjay et al., 2014)
ln
Fig. 5. The XPS analysis of vanadium in the composite to confirm the chemical state. The inset is the XPS analysis of Mg 2s electron to ratify the chemical of Mg at the surface.
Peq DH DS : ¼ RT R Po
ð2Þ
where Peq: plateau pressure, Po: standard pressure, DH°: molar enthalpy, DS°: molar entropy, R: universal gas constant and T is the temperature in Kelvin. It has been found that the enthalpy corresponding to hydrogenation- dehydrogenation has been increased marginally as compared to pristine MgH2 which was found to be in the range of 75–76 kJ/mol (http://www.factsage, 2016). The results confirmed that vanadium had not formed the homogeneous solid solution with Mg, it only acts as a catalyst. The vanadium doped Mg surface remarkable enhanced the hydrogenationdehydrogenation kinetics; however; it has marginally affected the thermodynamics of the process.
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Fig. 6. The SEM-EDS analysis of nano-engineered Mg: (a) microstructure of the portion of the sample of which EDS mapping has been done (b) magnesium mapping, (c) oxygen mapping, and (d) vanadium mapping. The analysis indicated the concentration inhomogeneity of vanadium (circled as (A)) and oxygen (circle as (B)).
Fig. 7. The Enthalpy calculation hydrogen-dehydrogenation reaction. Inset figure is the PCT of the composite at 275 °C, 300 °C, and 350 °C. The enthalpies of hydrogenationdehydrogenation have shown marginal change as compared to pristine MgH2.
4. Conclusions The thermal energy released during hydrogenation will be helpful to design a system based on the developed nanocomposite for the storage of solar thermal energy. The kinetics of dehydrogenation of the composite also improved substantially, and as a consequence, the temperature lower than 200 °C is required to dehydrogenate the composite. The temperature could be easily
achieved using CSP. The XPS analysis has shown that the ball milling of MgH2-V2O5 followed by dehydrogenation leads to Mg-V composite. The vanadium is uniformly distributed over the Mg surface. The V-doped Mg surface remarkably reduced the activation energy of hydrogen dissociation process leading to ultrafast hydrogenation kinetics. The remarkable results of the present study are the rise in temperature up to 192.5 °C within the 40 s.
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Acknowledgments This research work is supported by JSPS-Japan under the ‘‘Postdoctoral Fellowship Program for Overseas Researchers” granted to Sanjay Kumar (JSPS ID: P15078).
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