Boosting the hydrogen storage performance of magnesium hydride with metal organi

Yan Zhang, Jiaguang Zheng,*, Zhiyu Lu, Mengchen Song, Jiahuan He, Fuying Wu, Liuting Zhang,*

1 School of Energy and Power, Jiangsu University of Science and Technology, Zhenjiang 212003, China

2 Analysis and Testing Center, Jiangsu University of Science and Technology, Zhenjiang 212003, China

3 State Key Laboratory of Silicon Materials, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China

Keywords:Hydrogen Absorption Catalyst MgH2 Bimetallic materials Synergistic effect

ABSTRACT In this study, a MOF-derived bimetallic Co@NiO catalyst was synthesized and doped into MgH2 to improve the hydrogen desorption and resorption kinetics.The Co@NiO catalyst decreased the onset dehydrogenation temperature of MgH2 by 160°C,compared with un-doped MgH2.The MgH2+9%(mass)Co@NiO composite released 6.6%(mass)hydrogen in 350 s at 315°C and uptook 5.4%(mass)hydrogen in 500 s at 165°C,showing greatly accelerated de/rehydrogenation rates.Besides,the desorption activation energy of MgH2 + 9% (mass) Co@NiO was decreased to (93.8 ± 8.4) kJ∙mol-1.Noteworthy, symbiotic Mg2NiH4/Mg2CoH5 clusters were in-situ formed from bimetallic precursors and inlaid on MgH2 surface,which are considered as ‘‘multi-step hydrogen pumps”, and provides surface pathways for hydrogen absorption.Meanwhile, the introduced Mg2NiH4/Mg2CoH5 interfaces could provide numerous low energy barrier H diffusion channels, therefore accelerating the hydrogen release and uptake.This research proposes new insights to design high-efficiency bimetallic catalyst for MgH2 hydrogen storage.

1.Introduction

Hydrogen energy is considered as a promising strategy to replace fossil fuel and develop low-carbon energy systems, due to its high gravimetric energy density and environmental friendliness [1,2].However, how to store and transport hydrogen in an efficient and safe way is currently the bottleneck for the utilization of hydrogen [3,4].Considerable attentions have been focused on solid-state hydrogen storage materials with high hydrogen storage capacities [5,6].Among these materials, MgH2occupies high hydrogen capacity of 7.6% (mass) together with good reversibility and low cost, which is considered to be an ideal hydrogen carrier[7,8].Meanwhile,some obstacles such as high operating temperature(>300°C)and slow dehydrogenation kinetics seriously hinder its practical application [9,10].

In order to tackle these drawbacks, numerous attempts have been dedicated to help improving the reaction kinetics of MgH2,including nano-sizing[11–13],alloying[14–16]and catalysts doping [17–19].Over the past decade, transition metal catalysts (Ni[20], V [21], Nb [22], Fe [23], Co [24], Mn [25],etc.) have been widely studied as highly effective catalysts to expedite the hydrogen storage in Mg.Particularly, Co and Ni have received extensive concern due to their high catalytic activity for hydrogen dissociation.For instance, Xieetal.[26] found the presence of Ni accelerated the combination of hydrogen atoms at MgH′2s surface,which led to the promoted desorption kinetics.Jiaetal.[27]pointed out that ultrafine Ni NPs doped MgH2exhibited dramatically enhanced dehydrogenation kinetics due to the uniform dispersion of Ni nanoparticles.Also, Zhangetal.[28] revealed that MgH2-Co FCC started to dehydrogenate at 195 °C and can reach a dehydrogenation capacity of 6.5% (mass).The performance of monometallic Ni or Co catalyst in MgH2showed great potential to stimulate its fast hydrogen absorption and desorption.However,the possible multi-elemental catalysis in transition metal based catalysts,which may lead to improved catalyzing effect,still needs to be further investigated.

Binary transition metal-based catalysts for MgH2hydrogen storage have been recently studied by a large number of researchers due to the enhancement of desorption and absorption performances and the so-called ‘‘synergistic catalyzing effect” between different metal elements [29–31].In 2019, Liuetal.[32] first proposed and explained in detail the synergistic catalytic mechanism of Co/Pd bimetallic catalyst.MgH2-Co/Pd@B-CNTs could uptake 1.91% (mass) H2in 100 s at a low temperature of 50 °C.It was found that Pd and Co species act together as effective bidirectional catalysts accelerating the hydrogen de/absorption of MgH2.Chenetal.[33] discovered that MgH2catalyzed by Ni/TiO2released 6.5% (mass) H2within 7 min at 265 °C, showing the favorable collaborative catalytic effect from Ni and TiO2.It is believed that the understanding of bimetallic catalysts can develop a potential method for enhancing the catalytic activity in MgH2.The multielemental nature inside bimetallic composites could bring great potential to build heterogenous structures and interfaces.Building novel structures in bimetallic catalysts will strongly benefit their catalyzing activities.

Metal organic frameworks (MOF), the material self-assembled with metal cations/clusters and organic linker, have received considerable attention in adsorption, photocatalysis and catalysis fields, due to their controllable pore size, modified pore surface and high surface area [34–36].MOF-derived materials, usually metal oxides, could gain special structures inherited from the MOF precursor[37].Huangetal.[38]found that Ni@C-MXene catalysts, where the Ni-MOF was used as precursor, was able to reduce the initial dehydrogenation temperature of MgH2to 230°C.Shaoetal.[39]pointed out that MgH2-Ni-BTC300 composite exhibited excellent cycling stability due to the stable MOF-like structure.For maintaining a favorable hydrogen storage performance,MOFs and their derivatives can be used as buffer materials to prevent agglomeration during the ball milling and de/hydriding process[40,41].

Enlightened by the above results,we successfully synthesized a MOF-derived bimetallic Co@NiO catalystviaa facile hydrothermal method and an annealing treatment, and the catalyst was milled with MgH2to improve its hydrogen storage performance.The dehydrogenation and absorption properties of MgH2-Co@NiO composites were systematically investigated.It was observed that the Co@NiO catalyst could greatly reduce the onset dehydrogenation temperature of MgH2to 190 °C, while accelerating its de/rehydrogenation kinetics.The structural and morphological measurements identified the evolution of Co@NiO bimetallic catalyst during cycling,and the catalytic mechanism have been discussed in detail.

2.Experimental

2.1.Synthesis of Co@NiO catalyst

The Co@NiO catalyst was synthesizedviaa two-stages method including a facile hydrothermal process and an annealing treatment [42].First, 1 mmol Ni(NO3)2∙6H2O, 0.429 mmol Co(NO3)2-∙6H2O, 0.253 mmol 2,5-dihydroxyterephthalic acid, and 0.026 mmol PVP were dissolved in 45 ml mixture solution [DMF:ethanol: deionized water=1:1:1, v/v/v] and stirred for 20 min.The mixture was heated at 160°C in a sealed steel autoclave.After 12 h,the sediment was collected by washing with alcohol and centrifuging and then dried in vacuum.Finally, the dried sample was calcined at 400 °C for 3 h in H2atmosphere to obtain Co@NiO.For the sake of contrast, the Co and NiO was also prepared by the same method, only without adding Ni(NO3)2∙6H2O or Co(NO3)2-∙6H2O, respectively.

2.2.Preparation of MgH2 + Co@NiO composites

The MgH2was preparedviarepeated ball-milling and hydrogenation.Firstly, the Mg powder was annealed at 380 °C under a 6.5 MPa H2atmosphere for 3 h.Then the hydrogenated sample was transferred to the stainless steel jar and milled for 5 h at 450 r∙min-1.Finally, the ball-milling sample was further hydrogenated to obtain the high output MgH2.The Co@NiO was combined with MgH2by mechanical milling with different mass ratios of Co@NiO (5%, 7%, 9%, 11% (mass)).The mixture was ball milled for 6 h at 400 r∙min-1and the mass ratio of the ball to powder was 40:1.All operations were carried out under argon gas.

2.3.Sample characterization

A pressure-composition-temperature (PCT) instrument (Sievert’s type) was employed to perform the de/rehydrogenation experiments.For temperature programmed desorption (TPD),about 120 mg of each sample was heated from 25 °C to 450 °C at 5 °C∙min-1.The hydrogen pressure was set at 3 MPa and the sample was heated and maintained at desired temperature in the isothermal experiments.X-ray diffractometer (XRD, PANalytical,the Netherlands) was utilized to investigate the material phase compositions with Cu Kα radiation and a step of 2 (°)∙min-1.The structure and morphology of the sample were characterized through Scanning electron microscopy (SEM) and transmission electron microscopy(TEM,Tecnai G2 F30 operated at 300 kV)technology.The elemental distributions were researched by an energy dispersive spectrometer (EDS).

3.Results and Discussion

3.1.Characterization

The synthesis procedure of Co@NiO was diagrammed in Fig.1.After mixing Ni(NO3)2∙H2O, Co(NO3)2∙H2O and PVP and a hydrothermal treatment at 160 °C for 12 h, nanoflower-shaped NiCo MOF could be synthesized.Subsequently, the sample was annealed to a MOF-derived catalyst in hydrogen atmosphere at 400 °C for 3 h.

Fig.1. The illustration on the synthesis of MOF-derived Co@NiO catalyst.

The phase compositions of as-prepared MOF-derived composite was studied by XRD analysis.The XRD patterns in Fig.S1 (in Supplementary Material)proved the successful synthesis of NiCo MOF,which were in consistence with the previous results of Zhangetal.[42].The XRD patterns of the hydrogen-annealed MOF are provided in Fig.2.Three prominent diffraction peaks of NiO could be seen at around 37.2°, 43.3° and 62.8°, matching well with the PDF card of NiO (PDF#44–1159).Besides, the peaks located at 44.2° and 51.5° could also be fitted with Co (PDF#15–0806).The consistency of results strongly manifests the successful fabrication of Co and NiO in the annealed MOF-derived material.In addition,the broadening in diffraction peaks of NiO and Co indicate the poor crystallization status, which may be due to the size reduction in both NiO and Co particles [43].

Fig.2. XRD pattern of as-prepared Co@NiO.

The morphologies and microstructures of the NiCo MOF precursor and MOF-derived material were studied by TEM and EDS mapping analysis.Fig.3(a) showed that the TEM image of NiCo MOF presented as nanoflowers, which was composed of numerous nanosheets.After calcination, the nanosheets maintained their original morphology (Fig.3(b)).A novel structure could be clearly identified from Fig.3(c) that nanoparticles with sizes of approximately 2–3 nm were scattered on nanosheets which were about 5 nm thickness.From the selected area electron diffraction(SAED)pattern(Fig.3(d)),diffraction rings of NiO(012),(101)and Co(111)could be observed.The HRTEM image confirmed the interplanar spacing of 0.241 nm for NiO (101) and 0.205 nm for Co (111), further proving that ultrafine Co nanoparticles were anchored on the NiO nanosheets.EDS mapping results could manifest that Ni,Co,O,C elements share a homogeneous dispersion in Co@NiO(Fig.3(e)).Based on N2absorption–desorption curve, the mesoporous structure dominates NiCo MOF and Co@NiO material [44,45].After the calcination of NiCo MOF, the BET specific surface area was increased from 35.5 to 97.2 m2∙g-1and the total pore volumes was rose from 0.099 to 0.207 cm3∙g-1(Fig.S2).Combining the above results, it could be concluded that MOF-derived Co@NiO with a novel heterogenous structure has been successfully fabricated.

3.2.Hydrogen desorption properties

TPD analyses were conducted to confirm the catalyzing activity of Co@NiO to the hydrogen desorption of MgH2(Fig.4(a)).For comparison, NiO-doped MgH2, Co-doped MgH2and un-doped MgH2were also tested.The pristine MgH2could hardly release hydrogen below 350°C and only after 420°C could the hydrogen be released totally.When Co or NiO was doped, the onset dehydrogenation temperatures of MgH2were reduced to 183°C and 215°C,respectively,indicating modification effect to the hydrogen desorption of MgH2.The onset dehydrogenation of MgH2catalyzed by Co@NiO occurred at 190 °C, which was 25 °C lower than that of MgH2catalyzed by NiO,and was close to that of MgH2+9%(mass)Co composite.But the major hydrogen release of MgH2+ 9% (mass)Co@NiO could be fully accomplished at about 300 °C, significantly lower than that of MgH2+ 9% (mass) Co (350 °C), showing much faster desorption rates.These results manifest that bimetallic Co@NiO exhibits excellent catalytic performance in MgH2hydrogen storage system,compared with both monometallic Co and NiO.

To explain the influence of the amount of Co@NiO on the dehydrogenation property of MgH2,TPD experiments of MgH2+Co@NiO composites with different doping amounts were carried out(Fig.4(b)).5% (mass) Co@NiO significantly reduced the initial dehydrogenation temperature of MgH2to 225°C.For the MgH2+7%(mass)and 9% (mass) Co@NiO samples, the onset temperatures further decreased to 200 °C and 190 °C, respectively.As for desorption capacities, MgH2+ 5%, 7%, 9% (mass) Co@NiO released about 7.2%(mass), 6.9% (mass), 6.8% (mass) hydrogen, respectively.When the amount of Co@NiO was increased to 11% (mass), the composites started to release hydrogen at 185 °C.Neverthless, the hydrogen desorption content of MgH2+11%(mass)Co@NiO composites were merely 6.5%(mass),which was reduced greatly due to excess catalysts.In view of all factors including dehydrogenation temperatures and amounts, MgH2+ 9% (mass) Co@NiO was regarded as the composite which had the optimal hydrogen storage properties,and it was chosen for the following kinetic tests.

To further manifest the catalyzing performance of bimetallic MOF-derived Co@NiO on MgH2, isothermal dehydrogenation experiments at various temperatures were performed.(Fig.4(c)).The initial hydrogen pressure was set at 0.003 MPa.The asprepared MgH2exhibited a quite sluggish dehydriding rate, only releasing 0.6%(mass)hydrogen in 500 s even at a high temperature of 335 °C.The MgH2+ 9% (mass) Co@NiO released 3.5% (mass)hydrogen in 500 s at 270 °C.When temperatures was set at 285 °C, the MgH2catalyzed by Co@NiO system released 6.0%(mass) hydrogen in 500 s.At 300 °C, the composites released the highest hydrogen capacity of 6.4% (mass) hydrogen in 400 s.As the temperature rose to 315 °C, the composites reached the faster dehydrogenation rate,which released 6.6%(mass)in 350 s.Therefore, the presence of Co@NiO significantly enhanced the dehydrogenation kinetics of MgH2.

To further investigate the effect of bimetallic MOF-derived Co@NiO on the dehydriding reaction of MgH2, the Johnson–Meh l–Avrami–Kolmogorov (JMAK) and Arrhenius equations were employed to calculate the activation energy (Ea) [46].The JMAK model can be expressed as follows:

Fig.3. TEM image of the NiCo MOF (a), TEM images of Co@NiO ((b) and (c)), HRTEM and SAED images of Co@NiO (d), EDS mapping images of Co@NiO (e).

where α means the extent of reaction,tis the reaction duration,nis the Avrami exponent,kis the rate constant.Subsequently, theEawas ensured using the Arrhenius equation:

whereTrepresents the isothermal hydrogen absorption temperature,Rdenotes the gas constant andArepresents the preexponential factor.The isothermal dehydrogenation curves of undoped MgH2were also shown in Fig.S3 and the JMAK curves were shown in Fig.S4.The hydrogen desorption activation energy values of the as-prepared MgH2and the MgH2+ 9% (mass) Co@NiO composites were estimated to be (102.6 ± 13.3) kJ∙mol-1and(158.8 ± 11.8) kJ∙mol-1, respectively (Fig.4(d)).

Differential Scanning Calorimeter (DSC) analysis further affirmed the catalysis of Co@NiO on desorption kinetics of MgH2.Fig.5(a) and 5(b) showed the DSC curves of MgH2+ 9% (mass)Co@NiO and MgH2at various heating rates(5°C∙min-1,8°C∙min-1,10°C∙min-1and 12°C∙min-1).The thermal event arrow signs represent the endothermic peak direction.With the introduction of Co@NiO,the hydrogen decomposition peak shifts to lower temperature at the same heating rates.Furthermore, the desorption activation energyEawas measured according to the Kissinger method[47]:

Fig.4. TPD curves of the MgH2, MgH2 + 9% (mass) Co@NiO, MgH2 + 9% (mass) NiO and MgH2 + 9% (mass) Co (a), TPD curves of the MgH2 + x% Co@NiO (x=5, 7, 9, 11) (b),Isothermal dehydrogenation curves of MgH2 and MgH2 + 9% (mass) Co@NiO composites (c), fitted Arrhenius curves of MgH2 and MgH2 + 9% (mass) Co@NiO (d).

Fig.5. DSC curves of MgH2 (a) and MgH2 + 9% (mass) Co@NiO (b), and the Kissinger plots of MgH2 and MgH2 + 9% (mass) Co@NiO(c).

where β is the heating rates,Tpis the peak temperature,Ris the gas constant,Ais the pre-exponential factor.Hence, theEawas determined through the slope of the fitting line (Fig.5(c)).TheEaof MgH2+ 9% (mass) Co@NiO was reduced to (93.8 ± 8.4) kJ∙mol-1,comparing to un-doped MgH2((133.8±9.1)kJ∙mol-1).These activition energies were slightly lower than those calculated from JMAK methods (Fig.4(d)) due to the difference in measuring and calculating methods.The result expounds why the Co@NiO catalyst could enable MgH2to release hydrogen at a lower temperature.The catalyzing effect of bimetallic MOF-derived Co@NiO can be attributed to the diminution of dehydrogenation activation energy of MgH2.

Fig.6(a) and 6(b) displayed pressure-content-temperature(PCT) measurements of MgH2+ 9% (mass) Co@NiO and MgH2to evaluate the thermodynamic hydrogen storage performance.The PCT rehydrogenation curves of MgH2and MgH2+ 9% (mass)Co@NiO was added to form hysteresis cycle at 325 °C and displayed in Fig.S5.The desorption plateau pressures of MgH2+ 9%(mass) Co@NiO were measured to be 0.14, 0.31, 0.58 and 1.07 MPa at 300, 325, 350 and 375 °C, while those of pure MgH2were surveyed to be 0.23, 0.42, 0.87 and 1.66 MPa at 325, 350,375 and 400°C,respectively.Combined with the plateau pressures and temperatures of PCT curve, the decomposition enthalpy change ΔHof MgH2+ 9% (mass) Co@NiO and MgH2were calculated by using van’t Hoff Equation [48]:

WherePis pressure,ΔSis the decomposition entropy,ΔHis the decomposition enthalpy,Ris universal gas constant,Tmeans temperature.The computed van’t Hoff plot was shown in Fig.6(c)and 6(d).The ΔHwas estimated to be (83.0 ± 1.8) kJ∙mol-1and(87.3 ± 2.9) kJ∙mol-1for MgH2+ 9% (mass) Co@NiO and MgH2.Due to the facts of the above calculation, we could speculate that the boosting effect of bimetallic Co@NiO acts on the hydrogen storage kinetic performance rather than the thermodynamic properties.Also, the slight reduction of dehydrogenation plateau in MgH2+9%(mass)Co@NiO composites may indicate some changes in the catalyzing agents which may possibly form small amount Mg-Ni or Mg-Co alloys with different plateaus.

3.3.Hydrogen absorption properties

Isothermal absorption experiments were conducted at various temperatures in Fig.7(a) and compared with the pure MgH2.The initial hydrogen pressure was set at 3 MPa.Dehydrogenated MgH2+ 9% (mass) Co@NiO absorbed 5.4% (mass) hydrogen in 500 s at 165 °C, while MgH2only re-absorbed 0.8% (mass) hydrogen at 200 °C under the same time.At the temperature of 150 °C,135 °C and 120 °C, MgH2+ 9% (mass) Co@NiO absorbed 4.3%(mass), 3.2% (mass) and 2.2% (mass) hydrogen within 500 s,respectively.As a result, the introduction of Co@NiO significantly improved the hydrogenation performance of MgH2.In order to demonstrate the boosting hydrogen absorption in bimetallic Co@NiO doped MgH2, some comparisons had been made between recently studied bimetallic and monometallic catalysts, saw in Table 1.For instance, 6.15% (mass) H2was absorbed at 250 °C for MgH2-Co@CNTs composites [49].For MgH2-Ni@C composites,4.78% (mass) H2was obtained when the temperature increases to 300 °C [50].MgH2-FeCo composites absorbed 3.5% (mass) H2at 150 °C [51].At 125 °C, MgH2-FeNi/rGO composites reached a hydrogen content of 4.3% (mass) [52].For MgH2-NiCu/rGO composites,5.0%(mass)H2was uptook at 200°C[53].From the overall comparison it could be found that bimetallic catalysts,which hold more than one metal components, exhibit lower hydrogen uptaking temperatures in MgH2composite.Also, these results again demonstrate that Co@NiO exhibited superior catalytic performance on the hydrogen absorption performance of Mg among the state-of-the-art.

Table 1Comparative study on the hydrogen storage performances for some currently studied bimetallic or monometallic catalysts-doped MgH2

Fig.6. PCT curves of MgH2 (a) and MgH2 + 9% (mass) Co@NiO (b) at various temperature, the van’t Hoff plot of MgH2 (c) and MgH2 + 9% (mass) Co@NiO (d).

Fig.7. Isothermal rehydrogenation curves (a) and fitted Arrhenius curves (b) of MgH2 and MgH2 + 9% (mass) Co@NiO.

Furthermore,the absorption activation energyEawas measured to further evidence the brilliant hydrogen absorption in Co@NiO catalyzed MgH2(Fig.7(b)).TheEawas calculated from Johnson–Mehl-Avrami-Kolmogorov (JMAK) model and Arrhenius equation.The isothermal rehydrogenation curves of MgH2were shown in Fig.S6 and the JMAK curves were shown in Fig.S8.For accuracy,the isothermal rehydrogenation curves of MgH2+ 9% (mass)Co@NiO at 140 °C was also employed to calculate theEa(Fig.S7).TheEaof MgH2+ 9% (mass) Co@NiO was estimated to be(55.4 ± 5.7) kJ∙mol-1, which was lower than that of MgH2((87.0 ±7.5) kJ∙mol-1), confirming the superior hydrogen absorption property of MgH2+ 9% (mass) Co@NiO.

Fig.8. Cycling hydrogen desorption and absorption curves of MgH2 + 9% (mass)Co@NiO for 20 times at 300 °C.

3.4.Cycling performance

Due to the significantly improvement of hydrogen ab/desorption of MgH2+9%(mass)Co@NiO composites,we also attach great importance to the cycling stability,which plays a pivotal role in the practical application.For the MgH2+ 9% (mass) Co@NiO composites,20 cycles of isothermal hydrogen absorption–desorption were conducted at 300 °C (Fig.8).The composites released 6.4% (mass)hydrogen within 5 min in the first cycle and 6.2%(mass)hydrogen was still desorbed during the fifth cycle.After 20 cycles, a little hydrogen storage capacity degradation was observed, and the composites maintained 95%of the original hydrogen capacity,indicating the outstanding cycling stability of MgH2+ 9% (mass)Co@NiO composites.Table 1 also tabulates the cycling performances of MgH2doped with several transition metal based monometallic or bimetallic catalysts.It could be found that MgH2+ Co@NiO composites (this work) present superior cycling properties among these current studies.

Fig.9. XRD patterns of MgH2 + 9% (mass) Co@NiO: after ball-milling, after dehydrogenation and after rehydrogenation.

3.5.Catalytic mechanism

To explore the catalytic mechanism of Co@NiO catalyst,the XRD measurements of MgH2+ 9% (mass) Co@NiO after ball-milling,dehydrogenation and rehydrogenation were performed (Fig.9).It was clear that the phases of MgH2,MgO and Ni/Co had been exhibited in the as-milled composition.Because the peak positions of Ni and Co are similar, Co and Ni were not distinguished apart at around 40.0°.The traces of MgO was observed at around 42.9°,owing to the fractional oxidation reaction of MgH2.After dehydrogenation, except Mg and MgO, Mg2Ni appeared in the composites,along with the disappearance of Ni phase.These results confirmed that Ni reacted with Mg to form Mg2Ni during dehydrogenation,which could be written out by the following equations:

Notably the peak of Mg2Co and Mg2CoH5was also detected in the patterns of the rehydrogenated sample, indicating another alloying reaction:

Fig.10. TEM images of as-cycled MgH2+ 9%(mass) Co@NiO(a and b),HRTEM images of as-cycled MgH2 +9 %(mass) Co@NiO(c and d),EDS mapping images of as-cycled MgH2 + 9 % (mass) Co@NiO (e).

Hence,it could be found that both Ni and Co could react actively with MgH2/Mg to form Mg2NiH4and Mg2CoH5alloy hydrides,which could further serve as hydrogen pumps to propel the hydrogen ab/desorption of MgH2, boosting the hydrogen storage of MgH2+ 9% (mass) Co@NiO.Furthermore, XPS analysis of Ni element further confirmed the formation of Mg2Ni alloy phase during dehydrogenation, and the results are shown in Fig.S9.

Further TEM, HRTEM and EDS characterizations were carried out on the as-cycled MgH2+9%(mass)Co@NiO to make a further inquiry into the morphology of the composites during cycling.Fig.10(a) and (b) showed that the particle size of the irregularshaped MgH2+ 9% (mass) Co@NiO composites were detected to be about 500 nm, and catalyst particles were still inlaid on MgH2surface.MgH2(200) plane with interplanar distances of 0.226 nm could been confirmed in Fig.10(c), and the interface between MgH2and doped catalysts had also been found and magnified in Fig.10(d), indicating the existence of Mg2Co (511), Mg2Ni (203),Mg2CoH5(112) and Mg2NiH4(220) planes.These different phases stay closely to each other, providing numerous boundaries and interfaces.EDS mapping images showed that Ni element and Co element distributed in the same areas.It could be found from the HRTEM and EDS images that bimetallic Co@NiO actually served as precursor toin-situform symbiotic Mg2NiH4/Mg2CoH5clusters which were inlaid on MgH2surface.Furthermore, it should be noted that the uniform distribution of bimetallic Co@NiO on MgH2surface was usually related to the unique framework structure of MOF derivatives.This particular structure is not restricted to reduce particle size of catalysts to the nanoscale and facilitate the homogeneous dispersal on the surface of MgH2to improve de/rehydrogenation kinetics, but can also act as a grain inhibitor in preventing the growth of MgH2particles to improve the cycling stability [54,55].

The integrated formulation of the catalytic mechanism of Co@NiO on MgH2has been illuminated in Fig.11.From the above morphological observations, numerous nano-sized Mg2NiH4/Mg2-CoH5clusters were found generated on MgH2surface.Also, it has already been explained by Luetal.[56] that alloy hydrides facilitate the hydrogen absorption in MgH2due to a ‘‘hydrogen pump”effect.During rehydrogenation,the decomposition of H2molecules occur first on the surfaces of alloy particles, producing H atoms,and then the activated H atoms could be transferred from the alloy particles to Mg by hydrogen spillover and surface diffusion,resulting in the enhancement of hydrogen storage property [57].The nature of hydrogen spillover is a discrepancy of hydrogenation plateau pressure between MgH2and inlaid tiny alloy particles.Interestingly, in MgH2+ 9% (mass) Co@NiO composites, more than one hydrogen pumps could be evolved in symbiotic Mg2NiH4/Mg2CoH5conjunctions, and the plateaus of these pumps are seperated(0.3 MPa for Mg2CoH5at 360 °C [58], 0.45 MPa for Mg2NiH4at 300 °C [59] and 0.65 MPa for MgH2at 300 °C [60]).The seperated‘‘multi-step hydrogen pumps” may be beneficial for the hydrogen spillover in Mg2Ni/Mg2Co/Mg ternary composites, as illustrated in Fig.11.Simultaneously, in the MgH2+ 9% (mass) Co@NiO composite, plentiful interfaces would form, not only the interfaces between MgH2and catalysts,but also the Mg2Ni/Mg2Co interfaces.The more complicated interfaces provide numerous low energy barrier channels for hydrogen diffusion, therefore accelerating the hydrogen release and uptake [61].Similar phenomenon was also detected in the symbiotic CeH2.73/CeO2catalyst in MgH2by Linetal.[62].The interfacial effect of CeH2.73/CeO2greatly accelerates the hydrogen transportation,owing to reduced energy barrier of hydrogen release in the CeH2.73/CeO2boundary area.Overall, it can be concluded that the synergistic catalytic effect between the multi-step hydrogen pumps and active interfaces in MgH2+ 9%(mass) Co@NiO composite results in its enhanced hydrogen ab/desorption performances.This research proposes a referential solution to promote the hydrogen storage performance of MgH2by building novel structure in multielemental catalysts.

Fig.11. Schematic illustration of catalytic mechanism of the MgH2 + Co@NiO composite.

4.Conclusions

A bimetallic MOF-derived Co@NiO catalyst was fabricated by facile hydrothermal and annealing treatment.Micromorphology characterization revealed that the Co@NiO is mainly composed of ultrafine Co nanoparticles anchored on the NiO nanosheets.The MgH2+ 9% (mass) Co@NiO composites exhibited outstanding improved hydrogen sorption kinetics.At 315 °C and 300 °C, the MgH2+9%(mass)Co@NiO composites released 6.6%(mass)hydrogen in 350 s and 6.4%(mass)hydrogen in 400 s,respectively.In the term of rehydrogenation, the composites uptook 5.4% (mass)hydrogen in 500 s at 165 °C.Further mechanism study revealed the in-situ transformation from Co@NiO precursor into symbiotic Mg2NiH4/Mg2CoH5on MgH2surface.The nano-sized Mg2NiH4/Mg2CoH5clusters not only act as‘‘the multi-step hydrogen pump”to facilitate the hydrogen spillover and absorption, but also form plentiful interfaces to provide numerous channels for hydrogen diffusion.The strategy of utilizing multielemental catalysts could guide a promising method for regulating the bidirectional hydrogen storage properties of MgH2.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors appreciatively acknowledge the financial supports from the National Natural Science Foundation of China(51801078), the Natural Science Foundation of Jiangsu Province(BK20210884).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2022.06.026.