Pitaya pulp structural cobalt–carbon composite for efficient hydrogen generation from borohydride hydrolysis

Journal of Alloys and Compounds 808 (2019) 151774

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom

Pitaya pulp structural cobaltecarbon composite for efficient hydrogen generation from borohydride hydrolysis Li Wang a, Yanyan Liu b, c, *, Saima Ashraf a, Jianchun Jiang b, c, **, Guosheng Han a, Jie Gao d, Xianli Wu a, ***, Baojun Li a a

College of Chemistry, Zhengzhou University, 100 Science Road, Zhengzhou, 450001, PR China Institute of Chemical Industry of Forest Products, CAF; National Engineering Lab. for Biomass Chemical Utilization, Key Lab. of Chemical Engineering of Forest Products, National Forestry and Grassland Administration, Key Lab. of Biomass Energy and Material, Jiangsu Province, PR China c Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, Nanjing, 210042, PR China d Integrated Analytical Laboratories, 273 Franklin Road, Randolph, NJ, 07869, United States b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 20 March 2019 Received in revised form 4 July 2019 Accepted 8 August 2019 Available online 8 August 2019

The design and synthesis of high efficiency catalysts for hydrogen generation and storage are the key factors for large-scale hydrogen use. Herein, a Pitaya pulp structural cobalt-based carbon nanocatalyst is synthesized. The 5e10 nm Co/CoOx particles are uniformly distributed in the 500-nm spherical carbon matrix and enable efficient catalytic performance during hydrogen generation from the alkaline NaBH4
1
1 and NH3BH3 solutions at 3998 mL min
1$g
1 Co for NaBH4 and 4677 mL min $gCo for NH3BH3. The catalyst nanoparticles were prepared by a facile one-pot solution reaction, pyrolysis at high temperature, and oxidation in air. The coexistence of various valence states of Co in the composite material is observed by utilizing various characterization methods. Co nanoparticles fixed in the carbon matrix effectively prevent particles from aggregation and loss. This unique structure's similarity with Pitaya pulp, its synergistic effects, and its self-stirring mode enable its effective catalytic performance and good stability. The results provide a reference direction for the design and synthesis of unique structure catalysts for efficient hydrogen production. © 2019 Elsevier B.V. All rights reserved.

Keywords: Borohydride hydrolysis Cobalt-carbon composite Hydrogen generation Pitaya pulp structure Pyrolysis

1. Introduction Hydrogen is now widely regarded as the promising alternative energy carrier to replace fossil fuels because of its high energy density, sustainability, and its socioeconomic and environmental advantages [1]. There remains a considerable challenge to create highly efficient technology for hydrogen generation and storage for large-scale applications [2e5]. Currently, chemical hydrides, such as

* Corresponding author. Institute of Chemical Industry of Forest Products, CAF; National Engineering Lab. for Biomass Chemical Utilization; Key Lab. of Chemical Engineering of Forest Products, National Forestry and Grassland Administration; Key Lab. of Biomass Energy and Material, Jiangsu Province, PR China. ** Corresponding author. Institute of Chemical Industry of Forest Products, CAF; National Engineering Lab. for Biomass Chemical Utilization; Key Lab. of Chemical Engineering of Forest Products, National Forestry and Grassland Administration; Key Lab. of Biomass Energy and Material, Jiangsu Province, PR China. *** Corresponding author. E-mail addresses: [email protected] (Y. Liu), [email protected] (J. Jiang), [email protected] (X. Wu). https://doi.org/10.1016/j.jallcom.2019.151774 0925-8388/© 2019 Elsevier B.V. All rights reserved.

NaBH4 and NH3BH3 (AB) with high densities of available hydrogen, are considered as convenient hydrogen sources through hydrolysis or pyrolysis [6e10]. Effectively controlling the release of hydrogen is the key step to use the hydrogen contained in the chemical hydrides. Therefore, the design and synthesis of effective catalysts for releasing hydrogen is the decisive factor [11]. Currently, catalysts with high efficiencies for hydrogen production via hydrolysis usually contain noble metals, such as Pt/C [12], PtRu/metal oxide [13], Pd/C [14], Ru/anion-exchange resin [15], AgeNi core-shell nanoparticles (NPs) [16], AueCo bi-metals [17], and CoePt core-shell nanoparticles [18]. However, the use of noble metals is severely restricted due to their high cost and scarcity on our planet. It is imperative to explore low-cost catalysts for large-scale industrial production. Cobalt-based catalysts with superior catalytic activity are one kind of promising candidate to replace precious metal catalysts [19]. Although there have been many positive advances, there still is a serious problem involving the stability of cobalt-based catalysts used in liquid phase reactions because of the relatively low atomic weight of Co and its high mobility.

2

L. Wang et al. / Journal of Alloys and Compounds 808 (2019) 151774

Core-shell structured nanocomposites (CSNCs) have become a widely researched topic among studies of composite materials because of their unique physical and chemical properties and wide application prospects [20e25]. CSNCs generally are composed of the centre cores and outer coating shells [24,25]. As cores, the NPs with high specific surface area and surface energy provide highly catalytic properties. The outer shells protect core NPs from agglomerating and enhance the structural stability of composite materials [25e27]. These features ensure that CSNCs can become ideal high performance catalysts in a wide variety of applications [27e35]. Co NPs can be protected as cores by being wrapped in shells composed of porous polymers [25e29], zeolites [30e32], carbon [33e36], and other materials. The core-shell structure is beneficial to the stability of the metal NPs in composite materials [37e39]. For Co-based catalysts, CoOx NPs provide higher catalytic activity than pure metallic Co [40,41]. Co NPs can endow composite catalysts with super-paramagnetism and ensure complete and efficient magnetic momentum transfer by acting as nanoactuators [42e44]. Therefore, to enhance catalytic performance, Co/CoOx heterojunctions should be constructed effectively via the partial oxidation of Co NPs. To achieve the above purposes, a simple and facile protocol is urgently required to replace those complex procedures for effective catalyst preparation. In this article, a series of cobalt-based carbon composites similar to Pitaya pulp is synthesized by one-pot solution reaction, pyrolysis at different temperatures, and partial oxidation. Co NPs of 5e10 nm are evenly dispersed in a spherical carbon matrix of 500 nm. The spherical nanometer composite material, similar in looks to the spherical fruit pulp of Pitaya, demonstrates an efficient catalytic performance for hydrogen generation from alkaline NaBH4 and NH3BH3 solutions, with good stability of magnetic momentum transfer, providing a promising direction for the design and synthesis of uniquely structured catalysts for efficient hydrogen production.

Carl Zeiss NTS GmbH) and transmission electron microscopy (TEM, JEOL, JEM-2010F electron microscope, operating at 200 kV). Raman spectra were recorded on a Renishaw RM-1000 with excitation from the 514-nm line of an Ar-ion laser with a power of approximately 5 mW. The phase structure of the prepared product was characterized via X-ray powder diffraction (XRD, Bruker D8 Advance with Cu Ka, l ¼ 1.5418 Å). N2 sorption isotherms were measured on an ASAP 2420 Surface Area and Porosity Analyzer at 77 K. Prior to the measurements, the samples were degassed under high vacuum for 4 h at 523 K. The specific surface areas (SBET) were measured by applying the Brunauer-Emmett-Teller (BET) model to the portions of the adsorption branches of isotherms, where P/P0 was between 0.05 and 0.35. The pore size distributions were evaluated using the Barrett-Joyner-Halenda (BJH) model from the desorption branch. Thermogravimetric analyses (TGA, NETZSCH STA-499-F3) were performed from room temperature to 80  C at a heating rate of 10  C$min
1 under an Ar atmosphere. X-ray photoelectron spectroscopy (XPS) was conducted using a PHI Quantera SXM spectrometer with Al Ka ¼ 1486.6 eV excitation source, where binding energies were calibrated by referencing the C1s peak (284.8 eV) to reduce the sample charge effect. Hydrogen generation in the batch reactor. The hydrolysis of NaBH4 and NH3BH3 was performed through a water displacement method [45]. The structure of the device is shown in Scheme S1. First, catalyst (20 mg) was preloaded into a round-bottom flask (100 mL). Then, NaOH solution (1 M, 20 mL) containing NaBH4 or NH3BH3 (80 mg) was rapidly injected into the flask using a gastight syringe. A gas burette (200 mL) filled with water connected to the other side of reaction flask was employed to collect the gas. The experiment was conducted by a constant-temperature magnetic stirring apparatus, and the stirring speed was kept at 400 rpm. The specific rate of hydrogen generation was calculated using the information from the stabilizing stages (140 mL of hydrogen generated) according to the following formula:

2. Experimental section

rB ¼

Preparation of materials. KOH (1.1296 g) was dissolved in H2O (160 mL). Co(NO3)2$6H2O (2.9172 g) was dispersed in the above solution. Then, citric acid (1.9207 g) and triethylamine (1.4 mL) were added, and the mixture was stirred until complete dissolution. Resorcinol (2.5559 g), formaldehyde (3.84 mL), and ethylenediamine (2.56 mL) were then added into the reaction system. The mixture was sonicated for 30 min, transferred into a Teflonlined stainless-steel autoclave (500 mL) and then heated and maintained at 180  C for 24 h. The collection was dried in air at 80  C for 24 h after the hydrothermal process. The product was obtained and labelled as PF (9.0374 g). The abovementioned PF powder (1.000 g) was heated at 500  C for 2 h at a heating ramp of 3  C$min
1 in N2 flow and then passivated with ethanol before being removed from the tube furnace. The collection was dried in air at 30  C for 12 h. The final product was obtained and named as PF5 (0.5662 g) after being ground. PF7 and PF9 were prepared in the same way, except for different pyrolysis temperatures of 700  C for PF7 and 900  C for PF9. They are denoted as the PFX series (X is related to the calcination temperature). The weights of PF7 and PF9 were 0.4438 g and 0.2626 g, respectively. PF5eI, PF7eI, and PF9eI (PFX-I) were obtained by exposing PFX to air at 200  C for 22 h. Characterization. High-resolution transmission electron microscopy (HRTEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of the prepared products were acquired on an FEI Tecnai G2F20 S-TWIN electron microscope at 200 kV. The morphology of the prepared products was studied using scanning electron microscopy (SEM,

80ðmLÞ ½t140
t60ðminÞ  wcðgÞ

(1)

where rB is the hydrogen generation specific rate, t140 represents the time for 140 mL of hydrogen generation, t60 for 60 mL, and Wc is the Co weight in the catalyst. In all of the hydrogen production experiments, the hydrogen production from NaBH4 or NH3BH3 was carried out in self-stirring mode (self-stirring mode means that the reaction system was stirred by an external magnetic field through the ferromagnetic catalysts in the absence of magneton). The recycling test was conducted at 298 K. 3. Results and discussion The precursor, PF, was obtained by a one-pot homogeneous polymerization reaction. During the reaction, the phenolic resin forms through the polymerization of resorcinol, formaldehyde, and ethylenediamine and wraps around the complex generated from Co(NO3)2$6H2O, citric acid, and triethylamine. By pyrolysis at 500  C, 700  C, and 900  C, the phenolic resin gradually decomposes into a carbon texture framework, while the metal complex turns into Co NPs as a core. In this process, the organic components in the system act as reducing agents to convert Co2þ into a simple Co substance. Then, those Co NPs in the carbon coating layer are partially oxidized in air to transform into Co/CoOx NPs (PFX-I, Scheme 1). To investigate the morphology and microstructure of the PFX-I composite, typical TEM and HRTEM images are obtained. The

L. Wang et al. / Journal of Alloys and Compounds 808 (2019) 151774

3

Scheme 1. Structural evolution route of PFX from organic and metallic precursors.

shape of PF is a black sphere, shown in Fig. 1a. Fig. 1b and c present the TEM images of PF9eI and PF7eI. Both of them are composed of hollow Co NPs and carbon texture. The distributions of Co NPs are not even, and the sizes of Co NPs are much larger than those of PF5eI in Fig. 1d. The sizes of those Co NPs in PF7eI range from 30 to 40 nm, while the sizes of Co NPs in PF5eI are between 5 and 10 nm. The Co NPs of PF9eI range in size from 60 to 70 nm. PF9eI has the largest size of the three samples. High temperatures trigger the aggregation of Co NPs. Fig. 1deg reveal that PF5eI is composed of Co NPs and carbon matrix, and the Co NPs are dispersed evenly throughout the carbon texture, similar to pitaya seeds evenly dispersed in the fruit flesh. The SEM images (Fig. 1i) and low magnification TEM images (Fig. 1e) show that the diameter of the carbon ball is approximately 500 nm. In general, the structure of PF5eI is similar to that of spherical Pitaya pulp, and it is significantly different from PF5 (Fig. 1h). The HRTEM image (Fig. 1d) shows that cobalt NPs comprise C, Co, Co3O4, and CoO, which was further confirmed by XRD patterns. The lattice fringes, with spacings of 0.206, 0.243, and 0.155 nm, are identified as (111) plane of Co, the (111) plane of CoO, and the (511) plane of Co3O4,

respectively. The spacing of 0.357 nm corresponds to the (002) plane of C materials. EDX spectra of PF5eI show the uniform distributions of C, Co, and O (Fig. 1kem). PF5eI, PF7eI, and PF9eI appear as black powders, while PF is a brown powder (Fig. S1). XRD technology was employed to analyse the composition and crystal structures of the samples. The peak present at approximately 28 is attributed as belonging to graphitized carbon. Comparing the XRD patterns of PF and PF5 in Fig. 2a, one can find that the crystallinity decreases and the positions of different peaks shift, illustrating that the chemical species change. New peaks appear at 44.2 and 51.5 in the XRD pattern of PF5, being ascribed to the (111) and (200) lattice facets of cubic b-Co (JCPDS card no. 89e4307) [46,47]. PF7 and PF9 also have the same peak (Fig. S2). Contrasting with the XRD pattern of PFX, there are five new peaks emerging at 36.5 , 36.8 , 59.3 , 61.5 , and 65.2 in the XRD patterns of PFX-I. The new peaks at 36.5 and 61.5 are assigned to the (111) and (220) lattice facets of CoO (JCPDS card No. 48e1719), and the peaks at 36.8 , 59.3 , and 65.2 are in line with the (311), (511), and (440) lattice facets of the pure cubic spinel phase of Co3O4 (JCPDS card no. 42e1467) [48], respectively. These

Fig. 1. TEM images of (a) PF, (b) PF9eI, (c) PF7eI, (deg) PF5eI, (h) PF5, (i) SEM of PF5eI, (j) HAADF-STEM image, and (kem) EDX-STEM element mapping images (O, C, Co) of PF5eI.

4

L. Wang et al. / Journal of Alloys and Compounds 808 (2019) 151774

Fig. 2. (a) XRD patterns of various catalysts, (b) Raman spectra of PF, PF5, PF5eI, PF7eI, and PF9eI, and (c, d) XPS spectra of Co 2p and O 1s of PF5eI and PF5.

results indicate the coexistence of Co, CoO, and Co3O4 in PFX-I. The results also indicate that the extent of crystallinity will rise with the increase of calcination temperature, especially at 900  C, revealing that Co particles more easily aggregate to form large crystal particles at higher temperatures. The aggregation is unfavourable for engaging in catalytic activity due to reduced exposure of active sites. Raman spectra of PF5 and PFX were obtained to acquire more structural information of the materials, as shown in Fig. 2b (Raman spectra of PF7 and PF9 are in Fig. S3). The D bands at approximately 1351 cm
1 and G bands at approximately 1584 cm
1 are clearly observed in the spectra of PFX-I and PFX, demonstrating the presence of carbonaceous content. The ID/IG values of PF5, PF5eI, PF7eI, and PF9eI are 0.97, 0.99, 0.88, and 0.81, respectively, further illustrating that PF9eI exhibits the highest crystallinity and PF5eI the lowest crystallinity. The 2D bands at approximately 2691 cm
1 suggest that there are several carbon layers in the samples. The weak and wide band at 790 cm
1 in PFX-I is assigned to the CoOx surface species. The bands at 192, 472, 519, 608, and 680 cm
1 are also detected in the spectra of PFX-I, being assigned to the five Raman-active modes (F2g, Eg, F2g, F2g, and A1g) caused by Co3O4 [49]. To investigate the thermal stability of the samples, TG curves were obtained, shown as Figs. S4 and S5. The total weight loss rates of PF5, PF7,PF9, PF5eI, PF7eI, and PF9eI were 40.96%, 44.1%, 40.6%, 49.5%, 55.8%, and 48.9%, respectively, when the temperature changed from 25  C to 800  C, revealing that PFX-I exhibits similar thermal stability, while PF5 is more stable. The contents of cobalt in the series of catalysts are obtained from the TG analysis. The mass of cobalt contained in every 20 mg of PF, PF5, PF7, PF9, PF5eI, PF7eI, and PF9eI was 6.0, 6.0, 6.5, 6.0, 7.3, 8.2, and 7.2 mg, respectively. To characterize the chemical and electronic states of elements on the surfaces of PFX and PFX-I, XPS refined spectra were acquired, as illustrated in Fig. 2c and d, S6, and S7. From Fig. 2c, it can be found that a Co3þ peak at 781.7 eV emerges in the XPS spectrum of PF5eI,

in addition to Co0 at 778.5 and 792.7 eV and Co2þ at 780.2 and 794.6 eV, which are also in PF5 [50e52]. The fine XPS spectrum of O1s of PF5eI in Fig. 2d shows that there is a new peak at 529.32 eV. The peak is attributed to O¼Co, while only three peaks at 530.14 (OeCo), 531.67 (-OH), and 533.08 eV (water) are found in the spectrum of PF5. Elemental Co partially transforms into cobalt oxide via oxidation in air. Because of the pyrolysis treatment, there is a certain amount of Co2þ in the PFX series. After activation, PFX-I shows more O¼Co bonds via oxidation than PFX. Similar results are obtained for PF7, PF9, PF7eI, and PF9eI in Figs. S6 and S7. Combining the given XPS results, the areas of each peak are calculated by integrating the characteristic peaks corresponding to Co, CoO, and Co3O4, thereby obtaining the molar ratio of Co, CoO, and Co3O4. Due to the features of XPS testing, this method can only be used to calculate the proportion of each component on the surface of the material. Therefore, a detailed molar ratio was not obtained from the XPS. Nitrogen adsorption-desorption isotherms measured at 77 K were used to evaluate the specific surface areas and porosities of the PFX-I. The samples exhibit type-IV isotherms with hysteresis loops, indicating the presence of mesoporous structures (Fig. 3a) [53,54]. The SBET values of PF5eI, PF7eI, and PF9eI were 8.5, 18.3, and 36.8 m2 g
1, respectively. As the temperature of pyrolysis increases, the specific surface area increases. The temperature promotes the decomposition of the surface carbon shell and the aggregation of Co NPs. These porosities were confirmed by pore distribution analysis (Fig. 3b). PFX-I possesses mesopores with a predominant diameter of 3e4 nm, while pore size distribution became very narrow when the calcination temperature of 900  C was utilized. The sizes of pores cover 2e20 nm in PF5eI and PF7eI. The pore size features are well suitable for the mass transfer of reactants and products during catalytic reactions. The catalytic properties of PF, PFX, and PFX-I were evaluated by

L. Wang et al. / Journal of Alloys and Compounds 808 (2019) 151774

5

Fig. 3. (a) N2 adsorption
desorption isotherms, and (b) pore-size distributions of PF5eI, PF7eI, and PF9eI.

the studies of the hydrolysis of alkaline NaBH4 and NH3BH3. The hydrogen generation process of alkaline NaBH4 catalysed by various samples was recorded at 298 K, as shown in Fig. 4a. The catalytic performances of PFX-I were the best, followed by PFX and then PF under the same conditions. The rB values of PF, PF5, PF7, PF9, PF5eI, PF7eI, and PF9eI in Fig. 1b, calculated from Fig. 4a, were 136, 1242, 901, 594, 3998, 1733, and 1552 mL min
1$g
1 Co , respectively. PF5eI represents the best catalytic activity among all of the samples. The higher catalytic performances of PFX-I versus corresponding PFX are attributed to the coexistence of Co and CoOx NPs inside the carbon shells. The reasons for adding alkali into the reaction system are as follows: 1) the alkali promotes the stability of NaBH4, and 2) alkali can increase the rate of hydrogen production by the decomposition of NH3BH3 and act as a cocatalyst. The hydrogen generation activity without NaOH is clearly low, and the details are shown in Figs. S8 and S9. Fig. 4c shows that the rB of PF5eI clearly increases with reaction temperature because the mass transfer rates of reactants and products become faster with the increase of reaction temperature. When the temperature reaches 328 K, the rB of PF5eI hydrogen production reaches 12512 mL min
1$g
1 Co . As expected, the Arrhenius plot of lnk vs. the reciprocal of the absolute temperature (1/T) is a straight line (as shown in the inset of Fig. 4c). The Arrhenius energy (Ea) of the catalytic reaction can be calculated using the following Arrhenius equation:

  Ea Ko ¼ A exp
RT

(2)

where k0 denotes the rate constant, A is the pre-exponential factor, R represents the ideal gas constant, and T is the reaction temperature. The slope of the Arrhenius plot provides the activation energy (Ea ¼ 30.84 kJ mol
1). The impact of the initial NaBH4 concentration on hydrogen production has also been investigated. For low concentrations from 0.05 to 0.1 M, the impact of the concentration is obvious, while its influence becomes trivial for high

concentrations from 0.1 to 0.2 M, as seen in Fig. 4d. This is because the mass transfer rate tends to be stable when the concentration of reactants reaches a certain value, making the reaction kinetics more strongly enhanced for high-concentration substrates than for low concentrations [55]. The hydrogen production rates at different concentrations are compared here. The hydrogen production rates are calculated using data between 60 and 140 mL, so only the hydrogen production data for 0e200 mL is given in the high concentration case. Complete data (0e400 mL) are exhibited in Fig. S10. Different stirring modes also influence the reaction performance, especially in the second half of the reaction, as shown in Fig. 4d. The self-stirring method is more conducive to hydrogen generation than are the magneton stirring modes. The catalytic performances of samples for the hydrogen generation of alkaline NH3BH3 solutions at room temperature have been characterized, as exhibited in Fig. 4e and f. The rB values of PF, PF5, PF7, PF9, PF5eI, PF7eI, and PF9eI were 647, 1916, 1604, 969, 4677, 4230, and 3213 mL min
1$g
1 Co , respectively. The influences of the concentration of reactants, reaction temperature, and stirring modes on rB have been investigated, with results similar to those of NaBH4, as shown in Fig. S11. The value of Ea of 38.16 kJ mol
1 is somewhat higher than that of NaBH4. The higher Ea value indicates a serious energy barrier, which is necessary to overcome as the reaction proceeds. Compared with NaBH4, the NH3BH3 is more sluggish in accelerating the hydrolysis reaction. The stability of PF5eI has been investigated by the recycling of catalysts, as shown in Fig. S12. The results reveal that the catalyst exhibits good stability during the hydrolysis of both NaBH4 and NH3BH3. Although the rate of hydrogen production decreases after several cycles, PF5eI still exhibits some catalytic ability after five cycles. The ferro magnetism of the samples is confirmed by the attraction to a magnet, as shown in Fig. S13. The results show that the strong magnetization of PF5 and PFX-I is fundamental to realize the self-stirring mode. The high catalytic activities of PF5-1 originate from the following factors: 1) high dispersion of Co NPs assures exposure of

6

L. Wang et al. / Journal of Alloys and Compounds 808 (2019) 151774

Fig. 4. (a) Hydrogen generation curves of different materials during the hydrolysis of alkaline NaBH4 solutions at room temperature, (b) the corresponding rates of (a), (c) hydrogen generation curves of PF5eI at different temperatures, where the inset is the corresponding Arrhenius plot, (d) hydrogen generation curves of PF5eI at various NaBH4 concentrations and under different stirring modes, (e) hydrogen generation curves of different materials during the hydrolysis of alkaline NH3BH3 solutions at room temperature, and (f) the corresponding rates of (e).

more active sites, 2) synergistic effects of different valence states of Co endow more active sites and stronger catalytic activities, 3) the unique structure, similar to Pitaya pulp, guarantees the high dispersion of active sites and prevents them from aggregation and loss, and 4) the self-stirring mode promotes further catalyst activity and saves energy.

synergistic effects of different valence states of Co, the unique Pitaya pulp-like structure, and an efficient self-stirring mode. This research is expected to be developed into a novel route for preparation of highly active catalysts that have unique structure for easy and economical H2 generation from the hydrolysis of borohydrides. Author information

4. Conclusions Notes In conclusion, a new type of cobalt-based carbon composite similar to Pitaya pulp was easily obtained by one-pot homogeneous reaction, pyrolysis, and partial oxidation in air. In the catalyst, 5e10 nm Co NPs are uniformly distributed in the 500-nm spherical carbon matrix, with various coexisting valence states of Co. The catalyst reveals very good catalytic activities for hydrogen production from alkaline NaBH4 and NH3BH3 solutions, maximum specific H2 generation rates of 3998 and 4677 mL min
1$g
1 Co in 0.1 M NaBH4 and 0.13 M NH3BH3 solutions at room temperature, and a high stability. The high catalytic activities of this cobalt-based carbon composite are ascribed to the good dispersion of Co NPs, the

The authors declare no competing financial interest. Acknowledgment Financial supports from the National Natural Science Foundation of China (no. 31530010), the Jiangsu Province Key Laboratory of Biomass Energy and Materials (no. JSBEM-S-201906), and the Special Project of Guangdong Province to Introduce Innovation and Entrepreneurship Team (no. 2016ZT06N467) are acknowledged. All the authors thank the Communist Party of China.

L. Wang et al. / Journal of Alloys and Compounds 808 (2019) 151774

Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.151774. References [1] A.M. Abdalla, S. Hossain, O.B. Nisfindy, A.T. Azad, M. Dawood, A.K. Azad, Hydrogen production, storage, transportation and key challenges with applications: a review, Energy Convers. Manag. 165 (2018) 602e627. [2] P. Chen, M. Zhu, Recent progress in hydrogen storage. Mater, Today 11 (2008) 36e43. [3] I.P. Jain, P. Jain, A. Jain, Novel hydrogen storage materials: a review of lightweight complex hydrides, J. Alloy. Comp. 503 (2010) 303e339. [4] J. Yang, A. Sudik, C. Wolverton, D.J. Siegel, High capacity hydrogen storage materials: attributes for automotive applications and techniques for materials discovery, Chem. Soc. Rev. 39 (2010) 656e675. [5] C. Liang, Y.F. Liu, H.L. Fu, Y.F. Ding, M.X. Gao, H.G. Pan, LieMgeNeH-based combination systems for hydrogen storage, J. Alloy. Comp. 509 (2011) 7844e7853. [6] K. Guo, H.L. Li, Z.X. Yu, Sizeedependent catalytic activity of monodispersed nickel nanoparticles for the hydrolytic dehydrogenation of ammonia borane, ACS Appl. Mater. Interfaces 10 (2018) 517e525. [7] W. Wang, Z.H. Lu, Y. Luo, A.H. Zou, Q.L. Yao, X.S. Chen, Mesoporous carbon nitride supported Pd and Pd
Ni nanoparticles as highly efficient catalyst for catalytic hydrolysis of NH3BH3, ChemCatChem 10 (2018) 1620e1626. [8] X.L. Wu, X.Y. Zhang, G.S. Han, Y.Y. Liu, B.Z. Liu, J. Gao, Y.P. Fan, B.J. Li, Reaction of Co3O4 nanocrystals on graphene sheets to fabricate excellent catalysts for hydrogen generation, ACS Sustain. Chem. Eng. 7 (2018) 8427e8436. [9] Q.L. Yao, Z.H. Lu, Y.Q. Wang, X.S. Chen, G. Feng, Synergetic catalysis of nonnoble bimetallic Cu
Co nanoparticles embedded in SiO2 nanospheres in hydrolytic dehydrogenation of ammonia borane, J. Phys. Chem. C 119 (2015) 14167e14174. [10] Q.L. Yao, K. Yang, X.L. Hong, X.S. Chen, Z.H. Lu, Base-promoted hydrolytic dehydrogenation of ammonia borane catalyzed by noble-metal-free nanoparticles, Catal. Sci. Technol. 8 (2018) 870e877. [11] L. Cui, Y.H. Xu, L. Niu, W.R. Yang, J.Q. Liu, Monolithically integrated CoP nanowire array: an on/off switch for effective on-demand hydrogen generation via hydrolysis of NaBH4 and NH3BH3, Nano Res. 10 (2017) 595e604. [12] U.B. Demirci, About the technological readiness of the H2 generation by hydrolysis of B(-N)-H compounds, Energy Technol. 6 (2018) 470e486. [13] P. Krishnan, T.H. Yang, W.Y. Lee, C.S. Kim, PtRu-LiCoO2-an efficient catalyst for hydrogen generation from sodium borohydride solutions, J. Power Sources 143 (2005) 17e23. [14] G. Guella, C. Zanchetta, B. Patton, A. Miotello, New insights on the mechanism of palladium-catalyzed hydrolysis of sodium borohydride from 11B NMR measurements, J. Phys. Chem. B 110 (2006) 17024e17033. [15] S.C. Amendola, S.L. Sharp-Goldman, M.S. Janjua, N.C. Spencer, M.T. Kelly, P.J. Petillo, M. Binder, A safe, portable, hydrogen gas generator using aqueous borohydride solution and Ru catalyst, Int. J. Hydrogen Energy 25 (2000) 969e975. [16] H. Guo, Y. Chen, X. Chen, R. Wen, G.H. Yue, D.L. Peng, Facile synthesis of nearmonodisperse Ag@Ni coreeshell nanoparticles and their application for catalytic generation of hydrogen, Nat. Nanotechnol. 22 (2011) 195604. [17] J.M. Yan, X.B. Zhang, T. Akita, M. Haruta, Q. Xu, One-step seeding growth of magnetically recyclable Au@Co core-shell nanoparticles: highly efficient catalyst for hydrolytic dehydrogenation of ammonia borane, J. Am. Chem. Soc. 132 (2010) 5326e5327. [18] X. Yang, F. Chen, Z. Tao, J. Chen, Hydrolytic dehydrogenation of ammonia borane catalyzed by carbon supported Co coreePt shell nanoparticles, J. Power Sources 196 (2011) 2785e2789. [19] V.I. Simagina, O.V. Komova, A.M. Ozerova, O.V. Netskina, G.V. Odegovaa, D.G. Kellerman, O.A. Bulavchenko, A.V. Ishchenko, Cobalt oxide catalyst for hydrolysis of sodium borohydride and ammonia borane, Appl. Catal. Gen. 394 (2011) 86e92. [20] X. Wang, Y.C. Zhao, X.L. Peng, C. Jing, W.J. Hu, S.S. Tian, J.N. Tian, In situ synthesis of cobalt-based tri-metallic nanosheets as highly efficient catalysts for sodium borohydride hydrolysis, Int. J. Hydrogen Energy 41 (2016) 219e226. [21] M. Muthu, D. Ramachandran, N. Hasan, M. Jeevanandam, J. Gopal, S. Chun, Unprecedented nitrate adsorption efficiency of carbon-silicon nano composites prepared from bamboo leaves, Mater. Chem. Phys. 189 (2017) 12e21. [22] Q.Y. Yu, J. Hu, C. Qian, Y.Z. Gao, W. Wang, G.P. Yin, CoS/N-doped carbon core/ shell nanocrystals as an anode material for potassium-ion storage, J. Solid State Electrochem. 23 (2019) 27e32. [23] J.T. Zhang, Q.M. Di, J. Liu, B. Bai, J. Liu, M. Xu, J.J. Liu, Heterovalent doping in colloidal semiconductor nanocrystals: cation-exchange-enabled new accesses to tuning dopant luminescence and electronic impurities, J. Phys. Chem. Lett. 8 (2017) 4943e4953. [24] D. Miyajima, F. Araoka, H. Takezoe, J. Kim, K. Kato, M. Takata, T. Aida, Ferroelectric columnar liquid crystal featuring confined polar groups within core
shell architecture, Science 336 (2012) 209e213. [25] J.T. Zhang, Y. Tang, K. Lee, M. Ouyang, Nonepitaxial growth of hybrid

[26]

[27]

[28]

[29]

[30]

[31]

[32]

[33]

[34]

[35]

[36]

[37]

[38]

[39]

[40]

[41]

[42]

[43]

[44]

[45] [46]

[47]

[48]

[49]

7

core
shell nanostructures with large lattice mismatches, Science 327 (2010) 1634e1638. G.M. Jiang, H.Y. Zhu, X. Zhang, B. Shen, L.H. Wu, S. Zhang, G. Lu, Z.B. Wu, S.H. Sun, Core/shell face
centered tetragonal FePd/Pd Nanoparticles as an efficient non
Pt catalyst for the oxygen reduction reaction, ACS Nano 9 (2015) 11014e11022. Y.J. Zhong, H.B. Dai, Y.Y. Jiang, D.M. Chen, M. Zhu, L.X. Sun, P. Wang, Highly efficient Ni@Ni
Pt/La2O3 catalyst for hydrogen generation from hydrous hydrazine decomposition: effect of Ni
Pt surface alloying, J. Power Sources 300 (2015) 294e300. F. Wang, X. Wang, D.P. Liu, J.M. Zhen, J.Q. Li, Y.H. Wang, H.J. Zhang, High
performance ZnCo2O4@CeO2 core shell microspheres for catalytic CO oxidation, ACS Appl. Mater. Interfaces 6 (2014) 22216e22223. Y. Wan, Y.L. Min, S.H. Yu, Synthesis of silica/carbon
encapsulated core
shell spheres: templates for other unique core
shell structures and applications in situ loading of noble
metal nanoparticles, Langmuir 24 (2008) 5024e5028. F. Tao, M.E. Grass, Y.W. Zhang, D.R. Butcher, J.R. Renzas, Z. Liu, J.Y. Chung, B.S. Mun, M. Salmeron, G.A. Somorjai, Reaction
driven restructuring of Rh
Pd and Pt
Pd core
shell nanoparticles, Science 322 (2008) 932e934. M. Gu, S. Ko, S. Yoo, E. Lee, S.H. Min, S. Park, B.S. Kim, Double locked silver
coated silicon nanoparticle/graphene core/shell fiber for high
performance lithium
ion battery anodes, J. Power Sources 300 (2015) 351e357. V.M. Dhavale, S. Kurungot, Tuning the performance of low
Pt polymer electrolyte membrane fuel cell electrodes derived from Fe2O3@Pt/C core
shell catalyst prepared by an in situ anchoring strategy, J. Phys. Chem. C 116 (2012) 7318e7326. J.Y. Sun, Z.K. Wang, H.S. Lim, S.C. Ng, M.H. Kuok, T.T. Tran, X.M. Lu, Hypersonic vibrations of Ag@SiO2 (cubic core)
shell nanospheres, ACS Nano 4 (2010) 7692e7698. Z. Chen, Y.J. Xu, Ultrathin TiO2 layer coated
CdS spheres core
shell nanocomposite with enhanced visible
light photoactivity, ACS Appl. Mater. Interfaces 5 (2013) 13353e13363. Y.P. Wang, A.Q. Pan, Q.Y. Zhu, Z.W. Nie, Y.F. Zhang, Y. Tang, S.Q. Liang, G.Z. Cao, Facile synthesis of nanorod
assembled multi
shelled Co3O4 hollow microspheres for high
performance supercapacitors, J. Power Sources 272 (2014) 107e112. B.L. Chen, M. Eddaoudi, S.T. Hyde, M.O. Keeffe, O.M. Yaghi, Interwoven metal
organic framework on a periodic minimal surface with extra
large pores, Science 291 (2001) 1021e1023. Y. Chen, V. Lykourinou, C. Vetromile, T. Hoang, L.J. Ming, R.W. Larsen, S.Q. Ma, How can proteins enter the interior of a MOF? Investigation of cytochrome translocation into a MOF consisting of mesoporous cages with microporous windows, J. Am. Chem. Soc. 134 (2012) 13188e13191. P. Cubillas, M.W. Anderson, M.P. Attfield, Crystal growth mechanisms and morphological control of the prototypical metal
organic framework MOF
5 revealed by atomic force microscopy, Chem. Eur J. 18 (2012) 15406e15415. D.Q. Zhu, F.C. Zheng, S.H. Xu, Y.G. Zhang, Q.W. Chen, MOF
derived self
assembled ZnO/Co3O4 nanocomposite clusters as high
performance anodes for lithium-ion batteries, Dalton Trans. 44 (2015) 16946e16952. H. Zhang, T. Ling, X.W. Du, Gas-phase cation exchange toward porous singlecrystal CoO nanorods for catalytic hydrogen production, Chem. Mater. 27 (2015) 352e357. J. Mahmood, S.M. Jung, S.J. Kim, J. Park, J.W. Yoo, J.B. Baek, Cobalt oxide encapsulated in C2N-h2D network polymer as a catalyst for hydrogen evolution, Chem. Mater. 27 (2015) 4860e4864. Y.Y. Liu, J. Zhang, X.J. Zhang, B.J. Li, X.Y. Wang, H.Q. Cao, D. Wei, Z.F. Zhou, A.K. Cheetham, Magnetic catalysts as nanoactuators to achieve simultaneous momentum transfer and continuous-flow hydrogen production, J. Mater. Chem. 4 (2016) 4280e4287. C.C. Xing, Y.Y. Liu, Y.H. Su, Y.H. Chen, S. Hao, X.L. Wu, X.Y. Wang, H.Q. Cao, B.J. Li, Structural evolution of Co-based metal organic frameworks in pyrolysis for synthesis of coreeshells on nanosheets: Co@CoOx@carbon-rGO composites for enhanced hydrogen generation activity, ACS Appl. Mater. Interfaces 8 (2016) 15430e15438. S.S. Duan, G.S. Han, Y.H. Su, X.Y. Zhang, Y.Y. Liu, X.L. Wu, B.J. Li, Magnetic Co@ g-C3N4 coreeshells on rGO sheets for momentum transfer with catalytic activity toward continuous-flow hydrogen generation, Langmuir 32 (2016) 6272e6281. C.B. Rong, N. Poudyal, G.S. Chaubey, V. Nandwana, Y. Liu, Y.Q. Wu, M.J. Kramer, M.E. Kozlov, R.H. Baughman, J.P. Liu, Appl. Phys. Lett. 103 (2008), 07E131. G.L. Yu, J. Sun, F. Muhammad, P.Y. Wang, G.S. Zhu, Cobalt-based metal organic framework as precursor to achieve superior catalytic activity for aerobic epoxidation of styrene, RSC Adv. 4 (2014) 38804e38811. Y.X. Zhou, Y.Z. Chen, L.N. Cao, J.L. Lu, H.L. Jiang, Conversion of a metaleorganic framework to N-doped porous carbon incorporating Co and CoO nanoparticles: direct oxidation of alcohols to esters, Chem. Commun. 51 (2015) 8292e8295. K.Y.A. Lin, F.K. Hsu, W.D. Lee, Magnetic cobaltgraphene nanocomposite derived from self-assembly of MOFs with graphene oxide as an activator for peroxymonosulfate, J. Mater. Chem. 3 (2015) 9480e9490. Y.W. Liu, C. Xiao, M.J. Lyu, Y. Lin, W.Z. Cai, P.C. Huang, W. Tong, Y.M. Zou, Y. Xie, Ultrathin Co3S4 nanosheets that synergistically engineer spin states and exposed polyhedra that promote water oxidation under neutral conditions, Angew. Chem. Int. Ed. 54 (2015) 11231e11235.

8

L. Wang et al. / Journal of Alloys and Compounds 808 (2019) 151774

[50] Y. Zhang, J.W. Huang, Y. Ding, Porous Co3O4/CuO hollow polyhedral nanocages derived from metal
organic frameworks with heterojunctions as efficient photocatalytic water oxidation catalysts, Appl. Catal., B 198 (2016) 447e456. [51] Y.K. Zhang, Y.X. Lin, H.L. Jiang, C.Q. Wu, H.J. Liu, C.D. Wang, S.M. Chen, T. Duan, L. Song, Well
defined cobalt catalyst with N
doped carbon layers enwrapping: the correlation between surface atomic structure and electrocatalytic property, Small 14 (2017) 1702074. [52] Z.H. Wei, Y. Liu, Z.K. Peng, H.Q. Song, Z.Y. Liu, B.Z. Liu, B.J. Li, B. Yang, S.Y. Lu, Cobalt-ruthenium nanoalloys parceled in porous nitrogen-doped graphene as highly efficient difunctional catalysts for hydrogen evolution reaction and hydrolysis of ammonia borane, ACS Sustain. Chem. Eng. 7 (2019) 7014e7023.

[53] Y.Y. Liu, W.N. Zhang, S.Z. Li, C.L. Cui, J. Wu, H.Y. Chen, F.W. Huo, Designable yolkeshell nanoparticle@MOF petalous heterostructures, Chem. Mater. 26 (2014) 1119e1125. [54] Y.Y. Lü, W.W. Zhan, Y. He, Y.T. Wang, X.J. Kong, Q. Kuang, Z.X. Xie, L.S. Zheng, MOF-templated synthesis of porous Co3O4 concave nanocubes with high specific surface area and their gas sensing properties, ACS Appl. Mater. Interfaces 6 (2014) 4186e4195. € Metin, V. Mazumder, S. Ozkar, € [55] O. S.H. Sun, Monodisperse nickel nanoparticles and their catalysis in hydrolytic dehydrogenation of ammonia borane, J. Am. Chem. Soc. 132 (2010) 1468e1469.