Mg-C Interaction Induced Hydrogen Uptake and Enhanced Hydrogen

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Mg-C Interaction Induced Hydrogen Uptake and Enhanced Hydrogen Release Kinetics in MgH-rGO Nanocomposites 2

Sweta Shriniwasan, Tathagata Kar, Manoj Neergat, and Dr. Sankara Sarma V Tatiparti J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b05483 • Publication Date (Web): 12 Sep 2018 Downloaded from http://pubs.acs.org on September 14, 2018

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The Journal of Physical Chemistry

Mg-C Interaction Induced Hydrogen Uptake and Enhanced Hydrogen Release Kinetics in MgH2-rGO Nanocomposites

Sweta Shriniwasan, Tathagata Kar, Manoj Neergat, Sankara Sarma V Tatiparti* Department of Energy Science & Engineering, Indian Institute of Technology Bombay, Mumbai-400076, India

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Corresponding author email: [email protected]

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Abstract Hydrogen uptake at 250 °C, PH2 > 15 bar and release at 320, 350 °C by MgH2 mixed with 10 wt.% rGO, alleviates incubation period (slow kinetics) encountered during hydrogen release by pure MgH2. Ball milling establishes Mg-C interactions (~283 eV) in these nanocomposites through electron-transfer from Mg to π* of C and weakens the C-C π bond. These Mg-C interactions persist in the nanocomposites upon subsequent hydrogen uptake and release. These interactions change the hybridization of C from sp2 to sp3, aiding hydrogen uptake by C (C-H). On hydrogen release, H releases from C-H and electrons are donated back from C to Mg. This electron back-donation weakens the Mg-H bond and enhances hydrogen release from MgH2. The persistent Mg-C interactions are crucial for alleviating the incubation period. For the present study, X-ray diffraction, Raman, X-ray photoelectron spectroscopy (C-1s core level, valence band) and Fourier transform infrared spectroscopy are used.

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Introduction MgH2 is a promising material for hydrogen storage with a high gravimetric capacity (7.6 wt.% H2).1 However, the key issues hindering its applicability are high enthalpy of formation (~75 kJ mol-1 H2) and slow hydrogen uptake and release kinetics.2 Moreover, cyclability and oxidation of the hydrides also need to be addressed.2 The slow hydrogen release from pure MgH2 at ~300-400 °C appears as ‘incubation period’,3-4 and is due to the partly ionic and covalent nature of MgH2.3 Several attempts were reported in the literature to understand the reasons for this incubation period.3-5 Recently, we showed that, during incubation period H-release from MgH2 initiates by formation of H-interstitials and subsequently H-vacancies. Further, we reported the mechanism of hydrogen release from MgH2 during incubation period.6 Hence, for successful application of MgH2, alleviation of incubation period is inevitable. Strategies such as catalyst addition7 and nanosizing8 were reported in the literature to minimize the incubation period in MgH2. These strategies can be implemented by ball milling.9 Improved ball milling techniques assisted by dielectric-barrier discharge plasma can also be used, which not only improves the kinetics of hydrogen release from MgH2, but also its thermodynamics.10-12 Mg based nanocomposites (Mg-Ti-H) showed improved hydrogen release kinetics at ~150 °C and PH2≈ 1 mbar, where 1.8 wt. % hydrogen from the composite was released in ~17 min.13 Also, improved hydrogen release from Mg based composites14-16 and alloys such as Mg2Ni,10 Mg3La and Mg3LaNi0.117; Mg3Pr and Mg3PrNi0.118 was reported. Interestingly, Mg85Ag5Al10 alloy did not show any incubation period during hydrogen release at 320 °C.19 Zlotea et. al20 showed that embedding Mg nanoparticles in microporous ordered carbon enhanced hydrogen uptake and release kinetics from MgH2. The enhanced kinetics was attributed to smaller hydrogen diffusion lengths leading to improved hydrogen mobility in the 3



nanoparticles.20 In addition to the composites and alloys mentioned above, use of graphene based MgH2 composites can also exhibit enhanced hydrogen release.8, 21 Graphene-based composites are promising for they not only improve hydrogen release kinetics of the active phase but also minimize its oxidation and agglomeration with minimal loss of gravimetric capacity.22 For e.g., Cho et. al22 showed that reduced graphene oxide (rGO) nanolaminate-encapsulated Mg exhibited oxidation resistance. These composites released ~5.8 wt.% hydrogen in ~78 min at 300 °C without any incubation period.22 In another work, ball milled MgH2-rGO nanocomposites showed ~30 °C decrease during non-isothermal hydrogen release.21 Moreover, the isothermal hydrogen release from these nanocomposites was attributed to 1D/2D Mg phase growth in MgH2.21 Another study by the same group reported ~87% of hydrogen release from MgH2 within 1 min at 300 °C.8 This enhanced hydrogen release was mainly attributed to the increased phase boundaries generated by the contact between C-edges with MgH2, acting as channels for H-diffusion.8 Such enhancements in hydrogen release are feasible due to Mg-C interactions. Hence, these interactions need to be understood. The expected Mg-C interactions could arise from a possible electron-transfer between Mg and C.23-25 In fact, density functional theory based analysis showed that such transfer can be expected between metal based catalysts (Eg. Cu, Nb, Zr, Ni and Nb2O5) and Mg leading to weakening of Mg-H bond, which enhances hydrogen release.7, 26-28 Further experimental investigations are necessary to understand these interactions and their consequences, particularly in MgH2-rGO nanocomposites, during the enhanced hydrogen release from them. In the present study, the hydrogen uptake and release by ball milled MgH2-10 wt.% rGO nanocomposites is investigated. The emphasis of this study is on the possible Mg-C interactions 4


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and their consequences, in terms of electron-transfer, changes in the bonding of carbon (sp2 to sp3)24, subsequent H-uptake by C and weakening of Mg-H bonds. X-ray diffraction, Raman spectroscopy, Transmission electron microscopy, X-ray photoelectron spectroscopy (C-1s core level and valence band) and Fourier transform infrared spectroscopy are used to support our analysis. Moreover, the cycling behavior of these nanocomposites up to ten cycles was also confirmed.

Experimental Methods Graphene oxide (GO) was prepared from graphite flakes (7-10 µm, 99% pure, Alfa Aesar®) using modified Hummers’ method as described in a previous report.29 Reduced graphene oxide (rGO) was obtained by chemical reduction of GO using hydrazine and ammonia solution.29 MgH2 (particle size: ~106 µm, 97 % pure, Alfa Aesar®) was ball milled with 10 wt.% rGO in a Fritsch Pulverisette 7, Premium Line planetary mill for 20 h in an inert atmosphere using tungsten carbide balls to produce MgH2-rGO nanocomposites. The ball-to-powder ratio was 40:1 and the rotation speed employed was 450 rpm.6 Hydrogen uptake and release from the nanocomposites were performed in the reaction chamber of an indigenous Sievert’s type apparatus. For hydrogen uptake experiments, the nanocomposites were heated to 250 °C with pressurized hydrogen in the apparatus (PH2 > 15 bar) for 300 min. Hydrogen release experiments were performed under vacuum at 320, 350 and 380 °C (Scheme S1, Supporting Information) for 300 min. Intermittent hydrogen release experiments were performed for 10, 35 min at 320 °C and for 2, 5 min at 350 °C as described in our earlier 5



work.6 These intermittent durations were chosen on the basis of the observed incubation period during hydrogen release. The results of the hydrogen release experiments on nanocomposites were compared with those from pure MgH2. All the results of the experiments at 320 °C are described here. For brevity, only the main results of the experiments performed at 350 °C are presented here and the details are furnished in the Supporting Information. Preliminary cyclability tests were performed on the ball milled MgH2-rGO nanocomposites and pure MgH2. For these tests, hydrogen uptake was performed at 320 °C, PH2 > 15 bar for 300 min. It was followed by subsequent hydrogen release for 300 min in vacuum at 320 °C. Ten cycles were performed on MgH2-rGO nanocomposites and three cycles were performed on pure MgH2. The hydrogen release curves after each cycle are represented in Supporting Information, Scheme S2. The samples at all the processing conditions were characterized using the techniques described below. Characterization techniques X-ray diffraction (XRD) X-ray diffraction patterns were obtained from PANalytical EMPYREAN goniometer (30 mA, 40 kV, CuKα radiation of wave length (λ) = 1.5406 Å). The obtained results were corrected for baseline and the phases present were indexed using the appropriate ICSD reference codes (MgH2: 155807, Mg: 642651, and MgO: 170905). The phase fractions were estimated through Rietveld refinement using PANalytical X’pert Highscore Plus software (Version 3.0) as described in our earlier study,6 and the results are summarized in the Supporting Information (Scheme S3). 6


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Raman Spectroscopy Phonon excitation modes for bonding in the samples were studied using Raman spectroscopy (Thermo Electron Corporation Nicolet Almega XR dispersive Raman). This spectroscopic analysis was performed at room temperature using a green Ar laser (531 nm) with an exposure of 60 s in backscatter mode. Transmission Electron Microscopy (TEM) Bright field images and the corresponding diffraction patterns of all the samples were obtained using TEM (FEI, Tecnai G2 F30). The samples were prepared by ultrasonicating the powders in toluene and drop-casting them onto lacey carbon grids. These grids were dried in infrared light and taken to TEM. The results are shown in Supporting Information, Scheme S5. .X-ray Photoelectron Spectroscopy X-ray photoemission measurements were recorded using Axis Supra photoelectron spectrometer (Kratos Analytical) using Al Kα source (X-ray Power = 75 W, hν = 1486.6 eV) with a pass energy of 20 eV. All the samples were ultrasonicated in toluene and were drop-cast onto an aluminum foil to obtain a thin film to be used for analysis. Fourier Transform Infrared Spectroscopy (FTIR) FTIR spectroscopy was performed in a Vertex 800 analyzer. For analysis, the samples were mixed with KBr and were pelletized. The functional groups present were analyzed for all the processing conditions.

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Results and Discussion

Figure 1. Hydrogen release curves of MgH2-rGO nanocomposites and pure MgH2 at (a) 320 and (b) 350 °C. Insets show the incubation period. Figures 1(a) and (b) represent the hydrogen release curves (wt.%) of MgH2-rGO nanocomposites and pure MgH2 at 320 and 350 °C, respectively. The curves of pure MgH2 exhibit slow hydrogen release during the initial stages (60 min in 320 °C, 11 min in 350 °C), corresponding to an incubation period.6 This is represented as ‘Incubation’ and is shown in the insets in Figure 1. Interestingly, the nanocomposites show more than an order of enhancement in the hydrogen release rates, eliminating incubation. This is most likely due to any possible interactions between Mg and C in the nanocomposites. As expected, the rate of hydrogen release 8


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shows an increase with temperature for all the samples. A careful observation of the inset of Figure 1(a) reveals that further enhancement occurs during hydrogen release from the nanocomposites, beyond 2 min at 320 °C. This suggests that either the mechanism or the source of hydrogen release is changing around 2 min in the nanocomposites. Similar trend could not be observed at 350 °C (Figure 1(b), inset), possibly because of the higher kinetics at this temperature. Figure 2 shows the XRD patterns of rGO; physical mixture of MgH2 and 10 wt.% rGO (PM); nanocomposites after ball milling (BM), hydrogen uptake (HY) and release at 320 °C at 10 and 35 min. Peaks corresponding to rGO appear at ~24° (002) and ~43° (100) in Figure 2.30 However, they are feeble in all the other patterns because of the weak crystallinity of rGO compared to the other phases present viz. MgH2, Mg and MgO. The phase percentages were estimated using Rietveld refinement of the XRD patterns (Supporting Information, Scheme S3(B)). The appearance of Mg peaks at 10 and 35 min of hydrogen release at 320 °C corroborates with the enhanced kinetics observed in Figure 1. The estimated grain sizes, using Williamson-Hall analysis,31 are ~9-13 nm for Mg and ~4-7 nm for MgH2 (Supporting Information, Scheme S3(C)). From Figure 2, MgO is present in all the samples. A comparison of the XRD patterns of pure MgH2 and nanocomposite after ball milling clearly shows that the MgO fraction is higher in the latter (Supporting Information, Scheme S3(D)). This indicates that the source of oxygen for oxidation of Mg is rGO in the nanocomposites.

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Figure 2. XRD patterns of rGO and samples processed at various conditions. Figure 3 shows the Raman spectra of the samples at different processing conditions. Carbon from graphene results in G (~1580 cm-1), D (~1360 cm-1) and 2D (~2700 cm-1) bands in the Raman spectra.32 G band indicates the in-plane vibrations of the E2g phonons at the Brillouin zone corresponding to the in-plane stretching of the C-C sp2 bonds.33-34 D band corresponds to 10


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the breathing mode of the A1g vibration and appears when the hexagonal symmetry is disrupted (i.e. presence of defects).34 The ratio of intensities of the D and G bands (defect ratio: ID/IG), identifies the structural features in rGO such as edges, Stone-Wales and vacancy defects (i.e., absence of hexagonal symmetry); and also the changes from sp2 to sp3 hybridization.35 The 2D band appears due to in-plane E2g two phonon intervalley double resonance scattering.36 The presence of Mg-H bonds in MgH2 is expected to show vibration modes B1g, Eg and A1g at 300, 950 and 1270 cm-1,37 respectively. However, these modes do not appear as the intensity of vibration of the D and G bands in rGO are significantly higher. The bands in the Raman spectra (Figure 3) were deconvoluted using the Lorentzian function38 and are tabulated in the Supporting Information, Scheme S4. In PM, D and G bands are observed at 1334 and 1577 cm-1, respectively (Figure 3(a)). Here, ID/IG is 1.059 (Figure 3(b)) and a sharp and wide 2D band appears at 2476-2787 cm-1 (Figure 3(a)).36 On ball milling, ID/IG decreases to 0.477 (Figure 3(b)), indicating a relative increase in the G band intensity (Figure 3(a)). This suggests that O detaches from rGO and causes increase in the C-C sp2 hybridization.39 The released O most likely forms MgO during ball milling causing significant MgO peaks in the XRD pattern of the BM sample (Figure 2). On ball milling, a blue shift of the 2D band is also observed (Figure 3(a)). This could suggest (i) possible electron-transfer from C to Mg40 and/or (ii) compressive strain induced in rGO on ball milling.40-41 The reason for this 2D blue shift will be explored through XPS results, here.

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Figure 3. (a) Raman spectra D, G and 2D bands and (b) Defect ratio (ID/IG) from Raman spectroscopy of the samples processed at various conditions.

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On hydrogen uptake (HY), ID/IG increases to 0.804 (Figure 3(b)), pointing to a relative increase in D band intensity (Figure 3(a)), suggesting C-H bond formation by sp3 hybridization.42 Further blue shift in the 2D band upon hydrogen uptake reinforces the possibility of C-H bond formation.36 During hydrogen release, ID/IG decreases initially (10 min at 320 °C and 2 min at 350 °C) and increases eventually (35 min at 320 °C and 5 min at 350 °C) (Figure 3(b)). The initial ID/IG decrease can be due to the reversal of the trend observed in HY, thus, indicating a change in sp3 (C-H) to sp2 (C-C) hybridization.33 Such hybridization change, as a consequence of H removal from C-H, is also suggested by the red shift of the 2D band (Figure 3(a)). The eventual increase in ID/IG (Figure 3(b)) can imply (i) C-H formation in rGO and/or (ii) any other changes in rGO. C-H formation is impossible as hydrogen release was under vacuum. This is consistent with the absence of blue shift in 2D (Figure 3(a)). Hence, ID/IG increase corresponds most likely to any other changes in rGO during hydrogen release. To further examine any possible structural changes to rGO during the process, TEM results are considered (Supporting Information, Scheme S5). The elongated spots and the presence of diffused rings corresponding to rGO, upon ball milling (Figures S5(A)(i) and (iii), Supporting Information) indicate that the structure of rGO is changed upon ball milling. This is in conjunction with the information from Raman (ID/IG ratio, BM, Figure 3(b)). Diffused rings of rGO appear upon hydrogen uptake and release, suggesting that the crystallinity of rGO is decreased. However, the graphene based composites produced by novel techniques such as dielectric-barrier discharge plasma, can retain their structural integrity even after ball milling.12, 15, 43

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Figure 4. C-1s core level XPS spectra of the samples processed at various conditions, deconvoluted into Mg-C interaction, C-C sp2, C-H sp3 C-O-C, C-OH and π→ π* transition peaks.

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Figure 5. Valence band XPS spectra of the samples processed at various conditions, deconvoluted into C- π band, Mg-3s, O-2p. Dashed Purple, dark red, dark yellow and blue lines indicate literature values of Mg-3s peak in Mg,44 MgH2,45 MgO;46 and C-π,47 respectively. Insets show valence states near Fermi level (Ef). Figures 4 and 5 present XPS data showing various structural changes in rGO and interactions between Mg and rGO, H and rGO for different processing conditions. XPS of C-1s core level (Figure 4, black curve) is deconvoluted using Gaussian function48-49 considering C-C 15



sp2, C-H and C-C sp3, C-Mg, C-O-C and C-OH and π→π* transition peaks. The observed peak positions are close to their literature counterparts of ~284.5 eV50, ~285.1 eV 36, 50, ~283 eV22, ~286 eV51 and ~290 eV52, respectively. The trends in the peak positions with the processing conditions are given in Supporting Information, Scheme S6. Insets in Figure 4 show the spectra between ~282-285 eV, emphasizing the shoulder in C-1s arising from Mg-C bonds. The vertical red dashed line in Figure 4 acts as a guideline to observe any changes in the C-C sp2 peak with the processing conditions. The XPS of hydrogenated pure rGO is presented in Supporting Information, Scheme S7. Figure 5 shows the deconvoluted π band in C, Mg-3s, O-2p (~7-8.3 eV53-54) and Fermi level (Ef), which constitute the valence band spectra (Supporting Information, Scheme S8) of the samples. The π and σ band spectra of all the samples are presented in Supporting Information, Scheme S9. PM shows peaks corresponding to C-C sp2, π→π* transition and C-O-C and C-OH in Figure 4. The presence of C-C sp2 peak confirms the sp2 hybridization in rGO. On photoemission, an electron from the π band jumps to a lower energy level, leaving it half-filled. The electron in this π band, upon photoexcitation, can occupy π*. This process appears as a π→π* transition peak (Figure 4).25 The C-O-C and C-OH bands appear due to the functional groups present in rGO. In the valence band spectrum (PM, Figure 5), the π band in C appears at ~9.31 eV.47 On ball milling, interestingly, Mg-C peak appears suggesting their interactions. These interactions arise via electron-transfer from Mg to C, which occupies π* of C. With subsequent photoemission, this π* electron can occupy lower energy levels. The photoemitted electron eventually excites the electron in the filled π. This requires more energy than that for excitation of the electron in the half-filled π as in the case of PM.55 Thus, π→π* peak (BM, Figure 4) and 16


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the π band in C (BM, Figure 5) would shift to higher energy. Since, XPS results strongly suggest electron-transfer from Mg to C, the blue shift of the Raman 2D band (BM, Figure 3(a)) on ball milling, that was discussed earlier, is most likely attributed to strain effects and not to electrontransfer from C to Mg.33 Also, interestingly, a very small C-H peak corresponding to sp3 hybridization of C appears in BM, Figure 4. Due to sp3 hybridization, C-C sp2 peak shifts to lower values. Eventual hydrogen uptake by these nanocomposites shows a stronger evidence of C-H sp3 peak. This peak is absent in hydrogenated pure rGO (Supporting Information, Scheme S7). This clearly shows that Mg-C interaction is a prerequisite for C-H formation under present conditions. The ratio of the intensities of the C-H sp3 bond and the C-C sp2 bond (i.e. I(sp3)/I(sp2)) is ~0.29. On hydrogen uptake, the π band (HY, Figure 5) becomes wider. The estimated Iπ /Iσ intensity ratios for PM, BM and HY are 0.26, 0.25 and 0.15, respectively (Supporting Information, Scheme S9). The lowest Iπ /Iσ ratio in HY suggests that the number of σ bands increases due to sp3 hybridization of C. This is mainly due to the electron occupation in π* through Mg-C interactions, which weakens the π band in C.25 This π band weakening changes hybridization of C to sp3. These additional sp3 orbitals can facilitate C-H formation leading to increased number of σ bands. The combined XPS results (HY, Figures 4 and 5); increase in ID/IG and blue shift of 2D band in Raman (HY, Figures 3(a) and (b)) confirm C-H formation. With sp3 hybridization in C, the probability of π*existence decreases due to the participation of the π band in hybridization. Hence, the electrons residing in π* would either return to Mg or migrate to π* of the neighboring sp2 C. Had the electrons returned to Mg, Mg-C interactions would have vanished in HY (Figure 4). However, the Mg-C peak in HY clearly

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indicates that these interactions prevail after hydrogen uptake. Hence, it is most likely that some of these electrons could migrate to the neighboring C. Hydrogen release for 10 min at 320 °C results in disappearance of the C-H sp3 peak (Figure 4). This indicates that hydrogen release in the initial stages has contributions from C-H bonds. Hence, the change in the rate of hydrogen release in Figure 1(a) could arise from initially C-H and eventually MgH2. Hydrogen release exhibits reversal in the trends of C-C sp2, π→π* transition peaks (Figure 4); and the C-π band (Figure 5). Moreover, the relative intensities of the C-C sp2 peak and the C-π band increase, suggesting the reversal of hybridization from sp3 to sp2. This is also seen from the increased Iπ /Iσ to ≥ 0.22 (Supporting Information, Scheme S9); the decrease of ID/IG and red shift of 2D band in Raman (320 °C, 10 min, Figures 3(a) and (b)). Further hydrogen release up to 35 min shows no significant changes beyond 10 min of hydrogen release. The absence of both C-H peaks (Figure 4) and blue shift of 2D band (Figure 3(a)) is a consequence of the impossibility of C-H interactions during hydrogen release under vacuum. The ID/IG increase observed in Raman (Figure 3(b)) on prolonged hydrogen release is mainly due to the increase in the oxygen functional groups.39 This is supported by (i) increase in binding energy of C-O-C & C-OH band (Figure 4, Supporting Information S6); and (ii) decrease in the relative intensity of G band (Figures 3(a) and (b)). The Mg-3s band is a convolution of contributions from Mg, MgH2 and MgO; and it experiences changes with the processing conditions. The corresponding literature values for these phases are indicated with vertical lines in Figure 5. The Mg-3s peak in PM corresponds to MgH2 (~5.36 eV45). On ball milling, a shoulder corresponding to Mg appears in the Mg-3s band at lower energy values (BM, Figure 5), supporting the in-situ Mg formation by hydrogen release. Part of this in-situ Mg formed undergoes oxidation, appearing as MgO peak in XRD (BM, 18


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Figure 2). The remaining Mg corresponding to the shoulder in the Mg-3s band (BM, Figure 5) appears as a tiny peak in XRD (BM, Figure 2). The presence of MgH2 and Mg render the band shift to lower energies in XPS (BM, Figure 5). MgH2 formation by hydrogen uptake shifts the Mg-3s band to higher energy in HY (Figure 5). The eventual hydrogen release for 10 and 35 min at 320 °C, shifts the Mg-3s band towards the theoretical values of Mg and leads to appearance of intense Mg peaks in XRD (Figure 2). Mg-C interactions, which are crucial for enhancement of hydrogen release kinetics, are further investigated by Ef (Figure 5).56 Bands near Ef represent valence states in different phases (Mg, MgH2, MgO, C). Valence states in PM consist of contributions mainly from MgH2.45 Ef and its corresponding three peaks of π and σ valence states57-58 for pure rGO are shown in Supporting Information (Scheme S10). The electron-transfer from Mg to C upon ball milling (BM, Figure 4), shifts the valence states of Mg towards that of C (BM, Figure 5; Supporting Information, Scheme S10).56, 59 This shift is mainly due to the filling of π* in C. On hydrogen uptake, Ef shifts to higher values (HY, Figure 5) due to the decrease in sp2 C resulting from C-H formation.60 The valence states in HY are around the same energy values as that of C in rGO, reinforcing the persistence of Mg-C interactions. Subsequent hydrogen release shifts the valence states at Ef to higher values. This shift suggests electron back-donation from C to Mg leading to the weakening of Mg-H interactions and eventual H release from Mg. However, this electron back-donation is partial. This is supported by the Mg-C interactions in C-1s core level and the failure of the complete reversal of both π→π* (Figure 4) and C-π band (Figure 5) to the values of PM. The Mg-H weakening reveals the catalytic ability of C in rGO during hydrogen release. Similar catalytic effects for hydrogen release from MgH2 were reported in literature with other catalysts (Cu7 and NbO26). This observed catalytic ability of C during hydrogen release from MgH2 19



(Figure 5) is the main reason for the alleviation of incubation period during dehydrogenation (Figure 1).

Figure 6. FTIR spectra for C-H bands in hydrazine and samples processed at various conditions. Figure 6 presents the FTIR spectra showing the C-H bands and their variations with the processing conditions. C-H bands appear at ~2850-2960 cm-1 with distinct peaks mainly at ~2950, ~2930 and ~2850 cm-1 in rGO.61 Weak C-H bands appear in PM at ~2850 and ~2930 cm1

. A comparison with the bands from hydrazine suggests the presence of N-H group in the 20


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samples,62 due to its use in rGO synthesis. Well-defined C-H groups are mainly observed on hydrogen uptake (peaks at ~2955, ~2925 and ~2848 cm-1). This clearly supports the hydrogen uptake by C in rGO as revealed from the Raman (Figure 3(a)) and XPS (Figure 4). Hydrogen release attenuates these C-H bands (Figure 6). The combined analysis from XRD, Raman, XPS and FTIR clearly indicates (i) hydrogen uptake by C due to Mg-C interactions; and (ii) in-situ catalytic ability of C during hydrogen release from MgH2, thus eliminating the incubation period. The above-mentioned trends are also observed in the case of hydrogen release at 350 °C and are shown in Supporting Information (Schemes S3-S6, S8-S11). The apparent activation energies were estimated according to the method developed in our earlier work (Supporting Information, Scheme S12).6, 63-65 The estimated activation energies for nucleation and growth are 74±2 kJ mol-1 H and 116±5 kJ mol-1 H, respectively. In the absence of rGO, the activation energy required for growth was 209±8 kJ mol-1 H.6 The estimated value of activation energy for growth clearly shows that Mg growth is enhanced upon addition of rGO. The estimated overall activation energy is 132±5 kJ mol-1 H. The activation energies reported in literature for hydrogen release from Mg-based nanocomposites is 126±6 kJ mol-1 H, 127.7 kJ mol-1 H, 125.2 kJ mol-1 H, and 28.9 kJ mol-1 H, respectively. 10-12, 14, 16-18 This shows that addition of rGO to MgH2 is also effective in improving the kinetics of hydrogen release. A possible mechanism for hydrogen uptake and release from these nanocomposites is presented below. The physical mixture (PM) of MgH2 and rGO develops interactions between Mg and C upon ball milling (BM, Figures 3-5). These interactions are due to electron-transfer from Mg to C, which occupy π* in C (Figures 4 and 5). Such electron occupation in π* weakens the π band in C. This weakening of π bands leads to C-H formation (Figures 4-6), during hydrogen uptake (HY), through change in hybridization of C from sp2 to sp3 (Figures 3-5). The 21



results so far clearly show that presence of MgH2 is responsible for hydrogen uptake by C, which is not possible in the absence of MgH2 under same conditions (Supporting Information, Scheme S7). During hydrogen release from these nanocomposites incubation period was absent (Figure 1), unlike in the case of pure MgH2 in our earlier studies.6 This clearly shows that the presence of rGO aids hydrogen release in the initial stages (Figure 1). The initial hydrogen release has contributions from C-H and MgH2 (Figures 2-6). Since, C-H bonds are weaker than MgH2 bonds, H from C-H can release in the initial stages (Figure 1).25 H-release from MgH2 is triggered by electron back-donation from C to Mg, which weaken the Mg-H interactions (Figures 4 and 5). Thus, C catalyzes hydrogen release from MgH2. The results of the present study show that the persistent Mg-C interactions are crucial in alleviating the incubation period. The results discussed here are for the first cycle of hydrogen uptake and release. Further cycles (ten) exhibit enhanced hydrogen release from these nanocomposites, which is absent in pure MgH2 (Supporting Information, Scheme S2). This enhanced hydrogen release upon cycling in the MgH2-rGO nanocomposites is possibly due to the improved Mg-C interactions, which will be reported elsewhere.

Conclusions Hydrogen uptake (250 °C, PH2 > 15 bar) and release (320, 350 °C) of ball milled MgH2-10 wt.% rGO nanocomposites is investigated. Ball milling causes persistent Mg-C interactions through electron-transfer from Mg to C (π*). This is evidenced by Mg-C peak (~283 eV, C-1s core level); shift of π→π* transition peak, C-π band to higher values; and shift of Mg valence 22


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states near Fermi level (Ef) towards those of C in the valence band spectra. Electrons in π* weaken the C-π band, causing change in hybridization of C from sp2 to sp3, thus aiding its hydrogen uptake. The same is supported by C-H sp3 peak and decrease of Iπ/Iσ in the XPS; increased ID/IG, blue shift of 2D band in Raman; and C-H band appearance in FTIR. Hydrogen release from C-H and MgH2, is supported by the reversal of the above trends; disappearance of C-H in XPS and FTIR. This trend reversal suggests electron back-donation from C to Mg, weakening Mg-H bond and catalyzing H release from MgH2 in the early stages. Thus, persistent Mg-C interactions alleviate the incubation period in the nanocomposites.

Acknowledgements Financial support from Science and Engineering Research Board, Department of Science and Technology (Sanction Order number: EMR/2016/004969), India is appreciated. We acknowledge IIT Bombay and the facilities provided by its Department of Metallurgical Engineering and Materials Science for XRD, Department of Earth Sciences for Raman spectroscopy, Central Surface Analytical Facility (ESCA Lab) for XPS and Sophisticated Analytical Instrument Facility (SAIF) for FTIR spectroscopy and TEM.

Supporting Information Available : Supporting Information includes the results from hydrogen release curves of MgH2-rGO nanocomposites at 320, 350 and 380 °C; Cyclability tests of MgH2-rGO nanocomposites and pure MgH2 at 320 °C; XRD results: XRD patterns of nanocomposites at 350 °C; Percentages of constituted phases from Rietveld analysis; Estimation of crystallite sizes from Williamson-Hall analysis; Raman spectra with deconvoluted peaks and 23



their peak positions; TEM images and diffraction patterns of samples at various conditions; XPS spectra with deconvoluted peaks for nanocomposites after hydrogen release at 350 °C, various estimated binding energies, XPS of hydrogenated rGO, Valence band XPS for nanocomposites after hydrogen release at 350 °C, deconvoluted values of Mg-3s, C-2p, O-2p peaks, Valence band spectra for σ and π band in C, and valence states near Fermi level, ratios of intensities of π and σ bands; FTIR spectroscopy results; and Estimation of activation energies. This material is available free of charge via the Internet at http://pubs.acs.org.

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Mg-C Interaction Induced Hydrogen Uptake and Enhanced Hydrogen

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