Boron-enriched advanced energy materials

Inorganica Chimica Acta 471 (2018) 577–586

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Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica

Review article

Boron-enriched advanced energy materials Yinghuai Zhu a,⇑, Shanmin Gao b, Narayan S. Hosmane c a

School of Pharmacy, Macau University of Science and Technology, Avenida Wai Long, Taipa 999078, Macau School of Chemistry and Materials Science, Ludong University, Yantai 264025, China c Department of Chemistry and Biochemistry, Northern Illinois University, DeKalb, IL 60115, USA b

a r t i c l e

i n f o

Article history: Received 27 October 2017 Accepted 21 November 2017 Available online 24 November 2017 Keywords: Borohydride Sustainable energy Hydrogen storage Supercapacitor

a b s t r a c t Fossil fuels have been used as one of the essential energy sources for centuries. Considering the rapid depletion of fossil fuels and increase in environmental pollution, caused by vast fossil-fuel consumption, it is essential to minimize the use of energy wisely and efficiently and, therefore, rapid exploration of green and sustainable energy sources is warranted. In this context, the element boron plays an important role in the area of sustainable energy due to its unique physical and chemical properties. Thus, the boron-enriched composites have proved to be unique materials for a number of applications. This mini-review summarizes the most advanced progress in the area of boron-based nanostructured and macromolecular compounds for energy applications, particularly for its conversions, and as carriers and storage materials. Ó 2017 Elsevier B.V. All rights reserved.

Contents 1. 2. 3. 4. 5. 6. 7.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Boraneous materials for supercapacitors . . . . . . . . Boraneous materials for lithium (Li)-ion batteries . Boraneous materials for nuclear reactors . . . . . . . . Boraneous materials for hydrogen storage . . . . . . . Borohydrides as hydrogen carriers . . . . . . . . . . . . . Conclusions and future perspectives. . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . Appendix A. Supplementary data . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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1. Introduction To date, fossil fuels make main contribution to the ever-increasing global energy demand. With the worldwide rapid development of the economy and population explosion, the consumption of fossil fuels accelerates significantly, and thus the energy sources deplete rapidly. In addition, fossil fuels produce carbon dioxide upon combustion, which is called a green-house gas and

⇑ Corresponding author at: School of Pharmacy, Macau University of Science and Technology, Avenida Wai Long, Taipa 999078, Macau. E-mail address: [email protected] (Y. Zhu). https://doi.org/10.1016/j.ica.2017.11.037 0020-1693/Ó 2017 Elsevier B.V. All rights reserved.

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contributes to global warming, as well recognized. Therefore, intensive effort has been devoted to the development of green, renewable energy, highly efficient methods of energy conversion and energy storage technologies. Concerning these goals, advanced devices with high efficiency, such as supercapacitors and fuel cells, are especially important in energy storage and conversion. Highperformance materials play key roles in the area of sustainable energy [1–3]. Among the materials exploited, carbon nanostructures are vital for the advanced devices as main electrode composites and for hydrogen storage. Boronated carbon nanostructures have been well investigated to show promising properties as functional materials for these devices due to electron-deficient nature

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of the atomic boron. This mini-review surveys advanced progress in boraneous materials for use in energy and hydrogen storage, and thus it should provide the fundamental insight offering important guidelines for the future design of nanostructured materials in energy applications.

2. Boraneous materials for supercapacitors Supercapacitor is potentially promising electrochemical energy storage and power output device due to its high power and energy density as well as its reasonable reusability [4]. They are of great interest as portable electronics, electric vehicles, and renewable energy systems operated on intermittent sources such as solar and wind mills. In general, there are two types of supercapacitors, namely pseudo-capacitor and electric double layer capacitor. Supercapacitors work on different mechanism from commonly used battery. They store and release energy through the interaction of the electrode interface and an electrolyte, while the batteries store and release energy via a chemical redox reaction [5,6]. Nanostructured carbons have been widely investigated as the excellent electrode composites due to their diversity of structures that generally provide extremely high surface areas and high electrical conductivity, along with their low-cost in production. Therefore, it is the material of great interest in the development of supercapacitors. Energy storage in carbon-based supercapacitors depends on charge uptake in the carbon/electrolyte interfacial surface region. It has been repeatedly demonstrated that the boron doping may improve the specific capacitance per surface area for the nanostructured carbons [7]. Boron is an electron deficient element that has been studied for decades as substitution in nanostructural carbons to promote the performances of Li-ion insertion, oxidation resistance and so on [8]. Boron enters the carbon lattice by substituting for carbon at the trigonal sites [9] and acts as electron acceptor because of its three valence electrons, causing a shift in the Fermi level to the conducting band and hence modifying the electronic structure of boron-doped carbon [10,11]. The change in electronic structure of carbon electrode materials can affect the electrical double layer capacitance. Most importantly, low-level boron doping shows catalytic effect on oxygen chemisorption on carbon surface, rendering the introduction of redox reactions related to oxygen functional groups on carbon surface [12,13]. Therefore, boron doping is able to modify the electrochemical capacitance of carbon materials, involving electrical double layer capacitance and pseudocapacitance [14]. Chen et al. prepared the mesoporous carbon with homogeneous boron dopant by co-impregnation and carbonization of sucrose and boric acid confined in mesopores of SBA-15 silica template [14]. Low-level boron doping shows catalytic effect on oxygen chemisorption at edge planes and alters the electronic structure of space charge layer of doped mesoporous carbon. These characteristics are responsible for substantial improvement of interfacial capacitance by 1.5–1.6 times higher in boron-doped carbon than that in boron-free carbon with alkaline electrolyte (6 M KOH) and/or acid electrolyte (1 M H2SO4). Such boron-doped mesoporous carbon can be expected to show maximum capacitance if further optimization of the local boron doping environment is done. This finding should be very useful for developing new doped carbon electrode materials for supercapacitors. Guo et al. prepared the boron and nitrogen co-doped porous carbons through a facile procedure using citric acid, boric acid and nitrogen as C, B and N precursors, respectively [15]. The resulting boron and nitrogen enriched carbon materials showed prominent capacitances. The nitrogen, boron and oxygen incorporated into the carbon matrix enhanced the wettability between the electrolyte and electrode

materials, and the introduction of heteroatoms may result in the pseudocapacitive effect. Two samples of the boron and nitrogen enriched carbons (BNC), BNC-9 and BNC-15 were prepared with high specific surface areas of 894 and 726 m2/g and showed the large specific capacitance up to 268 and 173 F/g, respectively, with the current of 0.1 A/g. When the current was set as 1 A/g, the energy densities were 3.8 and 3.0 Wh/kg and the power densities were 165 and 201 W/kg for BNC-9 and BNC-15, respectively. Thus, BNC-15 is more suitable to apply in high-power-demanded occasion, while BNC-9 tends to store more energy. Wu et al. have demonstrated a simplified prototype device of high-performance all-solid-state supercapacitors, based on threedimensional nitrogen and boron co-doped monolithic graphene aerogels [16], and this device possesses an electrode-separatorelectrolyte integrated structure, in which the graphenes acted as additive/binder-free electrodes and a polyvinyl alcohol (PVA)/ H2SO4 gel as a solid-state electrolyte and thinner separator. The graphene composites show 3D interconnected frameworks with a macroporous architecture, which are favorable for ion diffusion and electron transport in bulk electrode. In addition, the monolithic boron and/or nitrogen-doped graphenes can be easily processed into thin electrode plates with a desirable size upon physical pressing. Consequently, the resulting composites exhibited not only minimized device thickness, but also showed high specific capacitance (
62 F/g) and enhanced energy density (
8.65 Wh/kg) or power density (
1600 W/kg) with respect to undoped graphenes, or a layer-structured graphene paper [17]. Ling et al. prepared B/N co-doped carbon nanosheets by assembling the gelatin molecule in long-range order on 2D crystals of boric acid, followed by annealing [18]. The resulting B/N co-doped carbon nanosheets are very thin with ultrahigh aspect ratio, and excellent flexibility. The doped carbon nanosheets demonstrated excellent performance as supercapacitor electrodes, with an excellent high-rate capability of up to 100 A/g, and long lifetime of over 10,000–15,000 cycles with 105%–113% capacitance retention [18]. In addition, the synthetic approaches are feasible, economical and scalable. Synthesis and applications of an ultrafine amorphous nickel–boron alloy have been reported very recently [19]. The alloy has a high specific capacitance of 2230 F/g at 1 A/g in a three-electrode system, and the capacitance remained at 986 F/g when the current density is increased up to 20 A/g. Interestingly, the alloy was used to make an asymmetric supercapacitor with activated carbon, in which the amorphous Ni–B alloy was assembled as the cathode and activated carbon as the anode. The resulting device exhibited a high specific capacitance of 135.5 F/g at 1 A/g. It was claimed to deliver a maximum energy density of 59.3 Wh/ kg at a power density of 1004 W/kg [19]. This device could be reused more than 5000 times with 88.2% specific capacitance retention.

3. Boraneous materials for lithium (Li)-ion batteries Lithium-ion (Li-ion) batteries (LIBs), due to their reduced weight, high energy storage capability, and low self-discharge, have been widely used in many portable devices such as smart phones, laptops, digital cameras, electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs) and so on [20,21]. The secondary lithium-ion batteries are currently the best portable energy storage device for the consumer electronics market. A high energy density in batteries can be achieved by increasing the discharge capacity of the electrodes or by increasing the working potential of the cathode materials. Among the three key components (cathode, anode and electrolyte) of LIB, cathode material is usually the most expensive one with highest weight in the battery, which justifies the intense

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research focus on this electrode. In fact, cathode materials play an important role in the determination of energy density, safety and life cycle of Li-ion batteries. Therefore, research and development of cathode materials is one of the popular topics at main international meetings on Li-ion batteries. Borates of the type LiMBO3 have received much attention because of its lightest polyanion group, BO3, which ensures higher theoretical energy density than in other polyanion cathode materials. Legagneur et al. first reported the electrochemical properties of LiMBO3 (M = Mn, Fe, Co), which can only (de)intercalate 0.04 Li per formula, i.e. 9 mAh g1, at a rate of C/250 (the theoretical capacity is 220 mAh/ g) [22]. The structure of LiFeBO3 is shown in Fig. 1. The threedimensional FeBO3 framework is built from FeO5 bipyramids and BO3 trigonal planar. The FeO5 bipyramids share edges along [0 0 1] direction forming single chains, and the BO3 are corner shared with three chains. Within this three-dimensional framework, Li occupies two tetrahedral sites sharing an edge, which forms chains running along the [0 0 1] direction. It was later found by Dong et al. [23] that they can obtain 91.8 mAh/g for the initial discharge. They also applied carbon as a coating material as well as for making the LiFeBO3/C composite, and higher discharge capacity values (158.3 mAh/g at 5 mA/g and 122.9 mAh/g at 50 mA/g) were obtained [24]. The full potential of this material was not optimized until 2010, by Yamada et al. [25], approaching a capacity of 200 mAh/g that was supported by both experimental and computational results. The inherent activity of LixFeBO3, supported by ab initio calculations, indicated its potential as an electrode material with no thermodynamic limitation for approaching a theoretical capacity of 220 mAh/g. According to their opinions, surface poisoning by moisture in air is the main source of contamination happening in previous studies. With proper handling of the samples and electrodes, the theoretical capacity was almost achieved at C/20 rate, and more than 75% of the theoretical capacity was achieved at 2C rate. Besides LiFeBO3, the manganese (Mn)-based borates have also been studied over the past two years [22,26,27]. The LiMnBO3 has two polymorphs, hexagonal [22,26] and monoclinic [27]. The hexagonal phase was claimed to have an initial discharge capacity of 75.5 mAh/g with a wide voltage window range, 1.0–4.8 V [26], while for the monoclinic phase of LiMnBO3, electrochemical data was not shown until 2011 [27], despite the fact that a second discharge of 100 mAh/g with good retention over multiple cycles on its carbon coated material was observed. Borates being one of the newest generations of the Li-intercalation materials, its performance is relatively poor comparing to

Fig. 1. The structure of LiFeBO3 (green: transition metal ions; orange: B ions; red: Li ions) [22]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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other cathode materials. The kinetic polarization and the moisture sensitivity should be the main limiting factors and much work is still needed to explore the optimized synthesis and operational conditions [28]. The recent development of the secondary lithium ion batteries has been achieved by using selective innovated structural materials as an anode. Various elements have been utilized in innovative structures to enable these anodes, which can potentially increase the energy density, while decreasing the cost of preparing Li-ion batteries [29]. The performance of these secondary batteries, in terms of charge/discharge capacity, voltage profile and cyclic stability, depends strongly on the microstructure of the anode materials made of carbon and graphite. Due to the specific contribution of the carbon materials used in the anode, the capacity of the typical Li ion battery has been improved 1.7 times. However, investigations are currently in progress to identify the key parameters of carbons that provide the improved anodic properties. Since carbon and graphite materials have large varieties in their microstructure, texture, crystallinity and morphology, depending on their synthetic approaches and precursor materials in various forms, including as powder, fibers and spherule [30]. As the structural factors affecting the anode characteristics of graphite, particularly the capacity less than the theoretical value of 372 mAh/g, the effects of the a–b axis crystallite size [31], stacking fidelity [32] and defect of the basal planes have been proposed. To improve the capacity of these graphite systems, it is essential to dope the hetero atoms, such as B, P and N, into the graphite materials [33]. Heteroatom-doped carbon, such as BCx, BCxN and CxN, has been suggested for potential applications as anode materials in Li ion batteries, because of their layered structures [34–36]. Boron-doped carbon materials have been experimentally and theoretically investigated not only for the fundamental scientific aspect of electronic properties, but also for their potential applications toward high temperature oxidation protector for carbon/carbon (C/C) composite, along with its capability as an anode material for Li ion batteries. Since the boron-doping is inductive for the creation of electron acceptor level of Li-ion batteries [34–36], the enhanced capacity is expected. Many researchers [34–36] have suggested several preparative methods for boron-doped carbons, including co-deposition by CVD pyrolysis of organic molecules containing boron atoms and the substitutional boron-doping process into carbon structures. It is important to note that the voltage profiles of boron-doped samples are higher than those of the undoped samples by about 40 mV, and this could be useful for practical cell applications. For discharge cycle of the boron-doped samples, shoulder plateau is characteristically observed at about 1.3 V by inducing an electron acceptor level so that the lithium insertion yields a higher voltage compared to undoped samples [35]. The irreversible capacity is calculated as the average ratio of the capacities for the discharge and charge processes. It is interesting to note that the irreversible capacity loss for boron-doped samples is lower than that of the corresponding undoped samples. These results may be related to the redistribution of the Fermi level of the boron-doped samples, which is lowered after boron-doping by introducing an electron acceptor in the lattice [35]. The degradation of the lithium insertion capacity has been observed in some boron-doped graphite that could be due to the presence of borons arising from boron nitride and boron carbide precursors. Also, the unexpected opposite effects of boron-doping could be related to the heterogeneous growth of the crystallites dimension, L, due to borons acting as catalyst for graphitization. To maximize the positive effects of boron-doping, depending on the carbon materials obtained from a different precursor with a wide variety of shapes and microstructures, the doping conditions, such as the pressure, concentration and kinetics of heating rates

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should be carefully adopted. This could provide one of the new types of carbon or graphite electrodes with materials of ideal design. Besides as cathode and anode materials in lithium-ion batteries, the boron compounds have also been used as electrolyte materials. Lithium-ion batteries are composed of highly energetic electrode materials with specifically designed electrolyte. To stabilize carbonate-based electrolytes on 5 V-class cathode surfaces at room temperature, many additives have been investigated. Among various additives, lithium bis(oxalato) borate (LiBOB) has been recognized as an effective additive that forms a stable solid electrolyte interphase (SEI) not only on Si electrodes, but also on graphite electrodes [37]. Being environmental friendly, the LiBOB shows several advantages over traditional lithium salt, including good thermal stability, low cost, etc. While the graphite structure can be effectively stabilized by LiBOB even in pure propylene carbonate (PC) solvent [38], using only LiBOB as an additive led to the formation of an SEI film, and thereby graphite anode materials are protected from exfoliation by the PC-rich electrolyte. However, the effect is insignificant enough to have a negative impact on its bulk properties, including ion transport [39]. Very recently, the effect of LiBOB at room temperature as an additive on the cycling performance of high-voltage cathodes, LiNi0.5Mn1.5O4 and LiCoPO4, has been investigated [40,41]. The study by Ha et al. has indicated that the electrolytes with an LiBOB additive can improve the discharge capacity retention of LiNi0.5Mn1.5O4 at 60 °C, since the LiBOB additive can form a stable cathode SEI by inhibiting electrolyte decomposition. This SEI layer could inhibit direct contact of the electrolyte components with the high-voltage cathode, and thus considerably mitigates the oxidative decomposition of the electrolyte solution on prolonged cycles. A comparison of the performance cycles of LiNi0.5Mn1.5O4 with electrolytes between the reference and LiBOB-containing one is presented in Fig. 2. Although the fading of capacity still exists in the cells with the LiBOB additive, the discharge capacity retention of LiNi0.5Mn1.5O4 was significantly improved from 66.9 to 78.7% after 80 cycles at 60 °C. As expected, Li/LiNi0.5Mn1.5O4 half cells without a LiBOB additive showed a low coulombic efficiency of 95%, indicating the loss of 5% of the Li on each cycle largely due to the electrolyte decomposition during the performance at 60 °C. The XPS of the separators, retrieved from cells charged up to 5.0 V, clearly indicated that the electrolytes decomposed significantly at the cathode to form the light brown colored by-products. The ATR-FTIR and XPS results for the delithiated cathodes, along with the measurement of the leakage current at a constant voltage of 5.0 V, confirmed that the

Fig. 2. Discharge capacity retention (filled symbol) and coulombic efficiency (blank symbol) of LiNi0.5Mn1.5O4/Li cells at 60 C [42].

oxidative decomposition of electrolyte components is effectively alleviated by the LiBOB-derived SEI [42]. The impact of LiBOB electrolyte addition on the performance of full lithium-ion cells, pairing the high-voltage spinel cathode with the graphite anode, was systematically investigated by Pieczonka et al. [43]. Adding 1 wt% of LiBOB to the electrolyte significantly improved the cycle life and Coulombic efficiency of the full-cells at 30 and 45 °C. As the LiBOB was preferentially oxidized and reduced, when compared to LiBOB-free electrolyte during cycles, their relative contributions to the improved capacity retention in full-cells was gauged by pairing fresh and LiBOB-treated electrodes with various combinations. The results indicated that a solid-electrolyte interphase (SEI) film on graphite, produced by the reduction of the LiBOB additive, is more robust and stable against Mn dissolution problem during cycling process at 45 °C than with the SEI formed by the reduction of the base (LiBOB-free) electrolyte. In addition, a 3 wt% LiBOB-added electrolyte showed reduced Mn dissolution when compared with the base electrolyte after one month of storing the fully charged Li1xNi0.42Fe0.08Mn1.5O4 (LNFMO) electrodes at 60 °C. It is believed that LiBOB stabilizes the electrolyte by trapping the PF5, i.e., sequestering the radical which tends to oxidize EC and DEC electrolyte solvents. Thus, the oxidation is suppressed on the particles of carbon black in the positive electrode. Consequently, HF generation is also suppressed, which in turn results in less Mn dissolution from the spinel cathode. Although LiBOB has many advantages, it is a low-soluble electrolyte additive with inferior conductivity in many solvents, including the linear alkyl carbonate solvents, as examples. The SEI layer formed on the surface of carbonaceous anode material, when used in a cell, has high impedance of harming the cryogenic property and discharge capacity of the cell, severely [44]. To overcome these shortcomings, other boron compounds, such as lithium difluoro(oxalato)borate (LiODFB) and tris(trimethylsilyl)borate (TMSB), were employed as an alternative salt for lithium-ion batteries. As a prospective salt for lithium-ion battery, LiODFB has many advantages [44]: (1) it is more soluble than LiBOB in linear alkyl carbonate solvents which is essential not only to lower the viscosity and increase wettability of the electrolyte, but also to improve the rate capability and low-temperature cycle performance of lithium-ion battery, (2) as with LiBOB, the LiODFB has also been known to facilitate strongly the formation of SEI film on the surface of carbonaceous anode material even in pure PC, but results in less irreversible capacity of above 1.5 V in the first cycle of lithium-ion battery due to lower concentration of oxalate group in its molecule, and (3) for LiODFB-based electrolyte, the SEI layer formed on the surface of carbonaceous anode material has lower electrical impedance than the one with LiBOB-based electrolyte, which is conducive to improving the power capability and low-temperature cycle performance of lithium-ion battery. Therefore, to raise the level of competitiveness, the exploration of a simple synthetic technique with appropriate solvents to construct lithium-ion batteries is of special interest. Li et al. have synthesized the LiODFB (99.5%, by weight) with the co-product of LiBF4 (99.5%, by weight) by an improvised method, that is cheaper, simpler, more productive and more environmentally friendly than any of the previous methods. The electrochemical behaviors of sulfolane (SL) with LiODFB were studied by employing dimethyl sulfite (DMS) as a mixed solvent. It showed that 0.9 mol/L LiODFB-SL/DMS (1:1, by volume) electrolyte has a considerable stability against oxidative decomposition (>5.5 V) and exhibited satisfactory conductivity. When used in Li/MCMB cells, 0.9 M LiODFB-SL/DMS electrolyte not only exhibited excellent film-forming characteristics, but also proved its stable cycle performance. But, when used in LiFePO4/Li cells, 0.9 M LiODFBSL/DMS electrolyte exhibited several advantages compared to the

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cell with 1.0 M LiPF6-EC/DMC or 0.9 M LiODFB/DMS electrolyte. The advantages of this electrolyte are the more stable electrochemical performances even at elevated and also at relatively low temperatures. It has been suggested that the LiODFB-SL/DMS is an ideal electrolyte material for lithium-ion batteries [45]. The TMSB has been successfully applied to improve the cyclability of various cathodes including Li[Li0.2Ni0.13Mn0.54Co0.13]O2 [46], LiNi0.5Mn1.5O4 [47] and LiMn2O4 [48], which is based on the SEI film formed by TMSB. It has been established that TMSB is more easily oxidized electrochemically than the electrolyte in use, and SEI film is formed from its decomposition products of oxidation, along with silicon and boron-containing compounds. Unlike the electrolyte decomposition, no gaseous products are formed from TMSB decomposition and the resulting SEI film can provide a protection for cathodes [49]. The study by Zuo et al. has demonstrated that TMSB additive in the electrolyte can dramatically improve the performance cycles of LiNi0.5Co0.2Mn0.3O2/graphite cell at the higher voltage operation. In the voltage range of 3.0–4.4 V, LiNi0.5Co0.2Mn0.3O2/graphite cell with TMSB in the electrolyte retains about 92.3% of its initial capacity compared to the cell without the additive in the electrolyte that retains only 28.5% of its initial capacity after 150 cycles, showing the promising future for TMSB at higher voltage. The enhanced performance cycles are attributed to the thinner film originated from TMSB on the LiNi0.5Co0.2Mn0.3 O2 and the combination of TMSB with PF 6 and F in the electrolyte, which not only protected the undesirable decomposition of EC solvents but also resulted in lower interfacial electrical impedance [50]. A three-dimensional boron/nitrogen-doped hybrid structures of carbon nanotubes (CNTs)/carbon nanosheets by a one-pot reaction of glucose, boric acid, urea, NaCl and h-SWCNTs (single-walled carbon nanotubes) has been reported [51]. In this synthesis, the reaction mixture was treated with a high rate of freezing step, followed by freeze-drying. It was claimed that the process enables the CNTs and carbon-heteroatoms sources confined in the limited space of the self-assembled NaCl salts, which are then heat-treated to obtain the B/N-doped hybrid [51]. The nanostructural hybrid integrated advantages of (1) promoting electrochemical properties of carbon matrix by the doped heteroatoms; (2) the high specific surface area of the warp-proof nanosheets; and (3) enhanced electron conductivity and reinforced skeletons of the architecture by the extracted and embedded CNTs. This material was used as the lithium-ion battery anode, and exhibited a high reversible storage capacity of 1165 mAh/g at 0.1 A/g with a stable long cycle performance at high rate of 1000 cycles at 2 A/g [51]. Furthermore, it could also be used as the electrode of supercapacitors by providing high specific capacity at different current densities of 389 and 129 F/g, at 1 and 20 A/g, respectively [51]. 4. Boraneous materials for nuclear reactors At present, electricity is mainly generated from fossil fuels and around 33% of the carbon entering the atmosphere, annually [52]. Fossil fuels are non-renewable and non-green energy resources. But, renewable energy resources of wind and solar power can only provide a small fraction of energy required for our daily life. Therefore, the replacement of non-renewable carbon-based fuels by nuclear power is becoming more attractive. Nuclear reactor has been incorporated in many nuke plants and in aircraft carriers and submarines. In the operation of a nuclear reactor, control of the generated neutrons, that are produced by fission reactions, is one of the basic requirements. The 10B isotope of boron possesses a very large neutron absorption cross section, about 3840 barns for thermal neutrons [53]. The reaction of thermal neutrons with boron is:

Fig. 3. Boron-based control rod assembly for PWR nuclear reactor. 10

3þ

B þ n ! 4 He2þ þ 7 Li

þ að2:31MeVÞ þ cð0:48 MeVÞ½53:

The resulting products (helium and lithium) are stable isotopes. Boron has been widely used as the main composition in control rods (see Fig. 3) in nuclear reactors to absorb neutrons and thus control the rate of fission by quenching the chain reactions that generate them. Boron-based control-rod materials are particularly useful for the predominantly pressurized water reactors (PWRs) and boiling water reactors (BWRs). In addition, boron is also used as a neutron shield due to its wide adsorption spectrum. However, the elementary form of boron is unsuitable for nuclear reactor due to its weak mechanical properties, instead alloys or compounds of boron are more suitable as control rods. In fact, boron carbide and refractory metal borides are commonly used as control-rod materials in nuclear reactors. They have attractive properties of high melting point, hardness, low density, chemical inertness and excellent thermal and electrical characteristics [54–56]. Boron carbide can be prepared by a carbothermic reduction route at a temperature of greater than 1500 °C from commercially available boric acid as per the reaction [56]: 4H3BO3 + 7C ? B4C + 6H2O + 6CO. However, this route gives a relatively poor yield of boron carbide with 60–65% in terms of boron content. A new method has been developed to prepare boron carbide by solidphase reaction between elementary boron and carbon as indicated by the equation [57]: 4B + C ? B4C. In summary, boron-based neutron absorber materials are crucial in nuclear reactors. Processes for the synthesis of high quality boron carbides and related materials are in the developing stage. Application of advanced nanotechnologies, including three-dimensional printing, should improve the process significantly with the desired properties. 5. Boraneous materials for hydrogen storage Hydrogen storage focuses on storing the lightweight hydrogen molecule for subsequent applications as a compact energy carrier. It is well recognized that hydrogen storage will be playing important roles in grid energy storage, especially for the renewable energy sources such as wind power. Hydrogen is also used directly as the fuel for transportation of airplanes, ships and space shuttles

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that use liquid or slush hydrogen. Theoretically, hydrogen storage technologies can be classified as physical and chemical storages. In physical storage, the hydrogen molecules are stored by its compression and liquefaction, whereas chemical storage use hydrides which reversibly release hydrogen. As of today, various approaches of using highly pressurized, liquid or slush hydrogen, along with porous material hydrogen absorption and using chemical compounds of hydrides, have been explored for hydrogen storage. However, liquid hydrogen is not an economical source for hydrogen storage because both liquefaction and storage of liquid hydrogen require extremely low temperature (252.9 °C), thus a large energy is needed to cool hydrogen down to the required low temperature and maintain it. In addition, liquid hydrogen possesses less energy density by volume than gasoline. Compared to hydrocarbons, compressed hydrogen (
350 bar) has relatively higher energy density by weight rather than by volume. To prepare and store compressed hydrogen, large tanks are required. Further, production of compressed hydrogen also costs more energy. Hydrogen carriers, based on nanostructured carbons, have also been reported. Thus, this section discusses current progress in hydrogen storage using boronated carbonaceous materials. Interactions of hydrogen with materials are defined as physisorption, in which hydrogen is absorbed by means of van der Waals forces, and the chemisorption, through formation of hydrides, requires dissociation of the absorbing hydrogens. The physisorption is generally considered as a storage model in porous materials, and lower temperatures and higher pressures will benefit the absorption. Under moderate storage conditions (pressure less than 100 bar and temperature of around 300 K), carbonaceous materials were found to store hydrogen without excess of 2 wt% and that was far below the US Department of Energy’s (DOE) target of 5.5 wt% [58]. The low capacity is due to low binding energies of the molecular hydrogen and easy desorption from the carbonaceous materials. Extensive efforts are being made to improve the hydrogen storage capacity, significantly. Small amounts of boron, nitrogen and transition metals have been incorporated with the nanostructural carbons either by physically mixing or chemically doping. Boron and other heteroatoms have been investigated as substituents in carbonaceous materials to enhance their stability and electrochemical behaviors. Boron is usually doped into the carbon frameworks in the trigonal coordination form and it behaves as an electron acceptor due to its three valence electrons. Therefore, boron-doping may cause a shift in Fermi level to the conduction band, thus affecting its electrochemical capacity and influencing the oxygen chemisorption process on the carbon surface. In addition, it can change both physical and chemical properties of the materials, including their polarizability and solubility. Ariharan et al. prepared boron substituted carbon materials by pyrolysis of a mixture of resorcinol and triethylborate in an inert atmosphere [59]. The resulting boron-incorporated carbonaceous materials showed a sheet-like morphology with a boron atomic percent of about 10.5%. Evidently, the boron content, surface area and its hydrogen storage capacity could be controlled by the temperature used in pyrolysis. The material showed 5.9 wt% hydrogen storage capacity at 298 K and 100 bar H2 pressure [59]. Boron-incorporated carbon materials were also obtained by template and bulk synthesis from sucrose using boric acid as a boron source [59]. Accordingly, the ordered mesoporous silica (SBA-15) was used as a template to prepare boron-doped carbon materials in the presence of boric acid. The boron content in the ordered mesoporous carbons is around 1.5 wt% determined by acid-base titration using mannitol. The material resulting from the template carbonization of sucrose in SBA-15 in the presence of boric acid has hexagonal morphology and surface area of 870 m2/g with a mesoporous diameter of ca. 4 nm and volume of ca. 1.0 cm3/g [60]. It showed

a hydrogen storage capacity of
1.2 wt% and 37.5 mg/cm3 in micropores with the hydrogen pressure of 760 torr at 77 K, and a gravimetric capacity near 270 F/g. The B-doped materials exhibited higher values of gravimetric and interfacial capacity, when compared to those materials without boron. Lin et al. theoretically studied the hydrogen adsorption capacity of boron-substituted carbon nanostructures, decorated with alkaline earth metals, through ab initio calculations, systematically [61]. It was expected that B-substituted carbon nanostructures with alkaline earth metals, such as Mg2+, can enhance the hydrogen storage capacity. The B-doped nano-carbon-Be2+ can serve as a material of high hydrogen storage capacity reaching up to 13.38 wt%, and that is consistent with the experimental results [61]. Li et al. demonstrated that the hydrogen storage capacity of a B-doped graphene can reach to 15.1 wt% [62]. Nitrogen-containing carbonaceous materials such as N-doped activated carbons are also potential candidates for hydrogen storage and, as such, various carbon species have been explored as storage materials for hydrogen energy applications. Despite several advantages of high surface area and low density, carbonaceous materials also have inherent drawback that may limit their applications. One main drawback is their low adsorption heat, which is 15.1 kJ/mol at 298 K [63]. Hydrogen molecules are adsorbed on the surface of carbonaceous materials by weak van der Waals forces. Therefore, ultra-microporous carbons my offer high hydrogen storage capacity due to their narrower pores which strengthen the interaction between hydrogen and surface [64]. Sethia et al. reported the high nitrogen microporous activated carbons with high nitrogen loading and surface area of 526–2386 m2/g and pore volumes of 0.26–1.16 cm3/g [65]. This type of carbonaceous materials is prepared by heating a nitrogen-rich carbon precursor with KOH at varying temperature in the range of 550–700 °C. Accordingly, these carbons showed high hydrogen storage capacity at 77 K and 1 bar hydrogen pressure because of their increasing porosity. One optimizing carbonaceous material with a nitrogen content of 22.3 wt% and a porosity of 0.59 nm in size, exhibited a hydrogen storage capacity of 2.94 wt%. It has been found that the capacity is linearly related to the porosity (0.5–0.7 nm) of the materials rather than their total surface area or the total pore volume [65]. The ultra-micropore volume mainly controls hydrogen adsorption at 77 K, whereas the large pores (>1nm) are involved in hydrogen adsorption at higher pressure. In summary, nanostructured carbons provide considerable opportunities to overcome the drawbacks of traditional bulk hydrogen storage technologies. The nanostructured hybrids of boron and/or nitrogen-doped carbons may benefit in terms of both thermodynamics of hydrogen adsorption and the kinetics of the hydrogen uptake and release processes.

6. Borohydrides as hydrogen carriers The sodium borohydride (NaBH4), one of the most common hydrides of boron, has undergone extensive research efforts globally, as part of sponsored projects of governmental agencies, due to their outstanding ability for hydrogen storage. Borohydrides possess excellent hydrogen densities, release rates and safety characteristics, and they can hold substantial hydrogen by weight and volume [66,67]. Also, they have an excellent potential for high energy density storage at room temperature and at atmospheric pressure [68], and thus they have been extensively studied to evaluate its potential for portable, automobile and stationary applications. It has the advantages of a high potential hydrogen density (maximum 10.9 wt%) together with a safe and ready hydrogen release through a hydrolysis reaction that can be controlled catalytically [69,70].

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Metal borohydrides are formally called metal tetrahydroborate, an extensive class having the general composition as M(BH4)n, where M may be Li, Na, K, Mg, Ca, Sc, Ti, V, Cr, Mn, Zn, Zr, Al, U, and so on, and n is normally between 2 and 4. Metal borohydrides can contain substantial amounts of hydrolytically or thermally accessible hydrogen on both the weight and volume basis, depending on the atomic number and valence requirements of the metal. They can appear in either solid or liquid form, and can be heated directly, passed through a catalyst-containing reactor, or combined with water (i.e. hydrolysis) or other reactants to produce hydrogen. It was first noted by Schlesinger et al. that it is possible to form a highly stable aqueous solution of NaBH4 by dissolving it in basic solution. The hydrolysis reaction (NaBH4 + 2H2O ? NaBO2 + 4H2; DH = 75 kJ mol1) can then be initiated on demand by bringing the solution into contact with a heterogeneous catalyst, making the release of hydrogen very easy and controlled [69,70]. Based on the reaction stoichiometry, 1 g of fully hydrolyzed NaBH4 will produce 2.37L of hydrogen at standard temperature and pressure. On a reactants-only basis, this gives a gravimetric hydrogen storage capacity (GHSC) of 10.8 wt%, which is more than required storage density target set by DOE. Aqueous borohydride is stable under ordinary conditions and liberates hydrogen in a safe and controllable manner and that makes it a potentially promising approach in hydrogen storage, particularly for portable applications. This system is also much safer and more controllable than the hydrolysis of solid hydrides, because the solution effectively acts as a thermal buffer by absorbing the exothermic heats of reaction and preventing thermal loss. The release rate of hydrogen is easily regulated by controlling the amount of solution in contact with the catalyst (or vice versa), allowing the system to meet the dynamic power demands of a fuel cell vehicle. A paper published in 2000 by Millennium Cell, Inc. was the first to demonstrate a portable hydrogen storage system based on aqueous NaBH4 solutions, and thus stimulated further research in this field [70,71]. However, it is arguable about the ability of such systems to meet the DOE targets. The GHSC of real storage systems will invariably be lower than the theoretical 10.8 wt%, due to excess water, required to dissolve the NaBH4 and its by-product, NaBO2, as well as due to added mass of the reaction and storage vessels. These shortcomings were identified in the DOE’s 2007 publication of a review paper on NaBH4 as a hydrogen storage material [72]. Unlike the reversible complex hydrides, borohydrides are considered as ‘‘one-way” single-use fuels as the waste materials or byproducts must be removed from the vehicle for off-board regeneration [73]. During the course of catalyst development for the borohydride hydrolysis reaction to release hydrogen, low cost and highly effective transition metal catalysts such as Ru, Pd, Pt, Pd-Pd, Ru-Pd alloys [70,74–78], and Ni-Co-B powders [79] exhibited outstanding activity in the essential hydrogen generation system for practical onboard applications. Amendola et al. [70,74] reported the application of Ru-catalyzed hydrolysis of aqueous BH4 solution as hydrogen generator for proton exchange membrane fuel cells (PEMFC). While the noble metal catalysts show excellent catalytic activity, their use in practical applications is restricted by their high material cost. A highly stable and active nickel boride catalyst (NixB) was prepared and tested for the catalytic hydrolysis of alkaline NaBH4 solution [80]. It was found that after heat treatment at 150 °C in vacuum, the NixB catalyst showed greatly enhanced catalytic activity and operational stability. Under suitable experimental conditions, the hydrolysis reaction can produce 6:75 wt% hydrogen at 45 °C and >4.0 wt% hydrogen even at room temperatures, exhibiting much higher hydrogen storage capacity than currently used alloys for hydrogen storage. Since the NixB catalyst is inexpensive and easy to prepare, it is feasible to use this catalyst

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in the construction of practical hydrogen generators for portable and in situ applications. Among numerous boron compounds, ammonia borane is indeed a promising chemical hydrogen storage material [81–83]. The appeal for some chemical hydrides originates from their high gravimetric and volumetric capacities and near-ambient operating conditions which are often less than 80 °C (at 0.1 MPa hydrogen pressure). For example, ammonia borane (NH3BH3) contains over 19 wt% H2 and 150 g H2/L of hydrogen by weight and volume, respectively (materials basis) and practically releases over one equivalent of H2 rapidly at 70 °C using a transition metal catalyst [84]. Likewise, materials related to ammonia borane, for example ammonia triborane (NH3B3H7) [85], are being actively pursued as potential hydrogen storage materials. The development of ammonia borane in the field of solid-state chemical hydrogen storage is however hindered by issues relating to thermolytic decomposition. Upon heating (e.g., over the temperature range 80–200 °C), ammonia borane decomposes more than its dehydrogenation and high amounts of undesired gaseous by-products (e.g., borazine, diborane and ammonia) are released along with two equivalents of hydrogen [86]. Concomitantly, the formation of a solid residue of complex nature, with the empirical formula [BNHx]n where x < 2, is possible (as suggested elsewhere). Therefore, it is most likely to obtain a mixture of polyaminoborane [H2N-BH2]n, polyiminoborane [HN = BH]n, o-polyborazylene [B3N3H4]n/3 and graphitic crosslinked polymer [B3N3Hy]n/3 with y < 4 in such decomposition processes [87]. Evidently, the decomposition scheme of ammonia borane is not acceptable from an application point of view. The temperatures are too high and the hydrogen is not pure. The solid residue cannot be totally and properly dehydrogenated by chemical recycling [88]. In this context, strategies of destabilization of ammonia borane have been investigated with the purposes of decreasing the dehydrogenation temperature below 100 °C, releasing pure hydrogen (while avoiding any by-products), and forming a solid residue of simple composition, ideally that of polyborazylene. To date, five different destabilization strategies have been reported: (1) solubilization in organic solvent or in ionic liquid; (2) solubilization and addition of homogeneous catalyst; (3) doping with solid-state oxidant or acid; (4) nanoconfinement into the porosity of a host material; and (5) chemical modification towards the formation of derivatives of alkali amidoboranes (e.g., LiNH2BH3). By solubilizing ammonia borane in a suitable solvent, the intermolecular dihydrogen N-H  H-B network is disrupted [89]. This results in destabilization of ammonia borane by dehydrocoupling to release hydrogen at temperatures lower than 100 °C. An example of this is the triglyme (b.p. 216 °C) solution of 6 M ammonia borane, which can generate one equivalent of hydrogen in less than 1 h at 70 °C, and with no induction period [90]. Several metal acetylacetonates, Fe(O2C5H7)3, Co(O2C5H7)2, Ni (O2C5H7)2, Pd(O2C5H7)2, Pt(O2C5H7)2 and Ru(O2C5H7)3, are considered for assisting dehydrocoupling of ammonia borane in diglyme (0.135 M) at 50 °C [91,92]. The dehydrogenation kinetics can be improved by using metal-based catalysts. For example, ammonia borane (0.4 M) in toluene (b.p. 110.6 °C) can release one equivalent of hydrogen in less than 1 h when catalyzed by homogeneous ruthenium acetylacetonate-based catalyst at 60 °C [93]. From a mechanistic point of view, cyclic intermediates with singly dehydrogenated boron atoms are supposed to form first, that subsequently polymerizes to yield polyborazylene. Formation of cyclic intermediates, such as B-(cyclodiborazanyl)amine-borane, B-(cyclo-triborazanyl)amine-borane and cyclotriborazane, were indeed reported [94]. Among those metal acetylacetonates catalysts, palladium acetylacetonate was found to be very reactive towards ammonia borane, even in the glove box when both solids were

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put into contact without solvent [95]. Ruthenium acetylacetonate showed to be an efficient precursor for the in-situ formation of homogeneous catalyst for ammonia borane dehydrocoupling and the homogeneous character was unequivocally demonstrated with the help of the mercury poisoning test [93]. Perez et al. studied the dehydrocoupling of liquid-state ammonia borane NH3BH3 by the metal acetylacetonate-aided catalytic process [93]. The catalytic effect of the metal acetylacetonates was compared by monitoring the hydrogen evolution with time (over a maximum of 2 h). The main objective was to define the time at which the dehydrocoupling reaction could be stopped after the evolution of less than 0.3 mol of hydrogen per mole of ammonia borane (conversion rates lower than 30%) and to analyze the intermediates at the early stages of the reaction. The hydrogen evolution curves are shown in Fig. 4. The most efficient metal acetylacetonate is Ru(O2C5H7)3 with the release of one equivalent of hydrogen in less than two hours. This is in good agreement with the results reported by Duman and Özkar [93] where one equivalent of hydrogen was liberated in about 70 min at 60 °C (mol ratio NH3BH3/Ru of 160). The mechanisms of ammonia borane dehydrocoupling are roughly independent on the metal nature of the acetylacetonate salts. Metal acetylacetonates catalyze ammonia borane’s dehydrocoupling by accelerating the reaction; they mainly have effect on the kinetics. Except for the borohydrides and the ammonia borane compounds, boron-based nanostructures can also be used as the hydrogen storage materials [96,97]. Nanostructured form of boron materials has been the focus in recent research on hydrogen storage [68,98]. Materials at nanoscales can have advantages over their bulk counterparts with respect to molecular adsorptions on large specific surface area with potentially high binding energy. For storage materials operating at near room temperature, the binding energy of hydrogen should be in the range of
0.2–0.5 eV, which was addressed by the van’t Hoff equation. It demonstrated the possibility of designing nanometer materials with appropriate hydrogen binding properties. Recently, the tubular form of boron nitride has been shown to store hydrogen at elevated temperatures [99,100]. It has been reported that boron nitride (BN) nanotubes can uptake 1.8–2.6 wt% hydrogen under
10 MPa at room temperature [101] and collapsed BN nanotubes exhibit an even higher hydrogen adsorption capacity (4.2 wt%) than any multiwalled carbon nanotubes [102]. Jhi et al. have shown through computational simulations that BN can be a good hydrogen storage medium. Their

study shows that deviations from sp2 bonding tends to increase the binding energy of hydrogen in BN. It is possible that layered materials of ionic character, more ionic than boron nitrides, with a moderate substitutional doping would have a substantially large binding energy, enough for storing hydrogen at room temperatures [99]. Chen et al. synthesized BN nanotubes through chemical vapor deposition over a wafer made by a LaNi5/B mixture and nickel powder at 1473 K. The results verified that the BN nanotubes could store hydrogens by means of an electrochemical process. It was tentatively concluded that the improvement of the electro-catalytic activity by the surface modification with metal or alloy would enhance the electrochemical hydrogen storage capacity of BN nanotubes [103]. Jhi et al. theoretically studied the activated forms of BN nanotubes for potential applications to hydrogen storage with the use of pseudopotential density functional method [104]. The binding and diffusion energies of adsorbed hydrogen were particularly calculated. The calculated binding energy of hydrogen on activated boron nitride nanotubes was found to be in the right range for room-temperature storage. It was further shown that the diffusion through the active sites enables hydrogen to access the inner surface of the nanotubes, which leads to increase in the storage capacity [104]. This study provides a tangible solution to increase the operating temperature and capacity of hydrogen storage based on heteropolar nanomaterials such as boron nitride nanotubes. Li et al. investigated hydrogen adsorption and storage in Ca-coated boron fullerenes and nanotubes by means of density functional computations [105]. Their study shows that Ca can bind strongly to the surface of boron-80 (B80) fullerene and boron nanotubes, thus avoiding the notorious clustering problem. Fullerene B80 coated with 12 Ca atoms can store up to 60 H2 molecules with an average binding energy of 0.12–0.40 eV, corresponding to a gravimetric density of hydrogen storage of 8.2 wt%. The hydrogen storage capacity of a Ca-covered boron nanotube is 7.6 wt% with a binding energy of 0.10–0.30 eV [105]. The strong interaction between Ca and boron fullerenes and nanotubes is attributed to the charge transfer. The optimal molecular hydrogen adsorption energies make reversible hydrogen adsorption and desorption feasible at ambient conditions. Nonetheless, the hydrogen storage capacity of such boron structures will significantly decrease in their corresponding macroscopic boron materials. It remained a big challenge to assemble the suitable macroscopic materials for practical hydrogen storage. Porous structures with Ca-coated boron nanostructures, as building blocks, might be useful for high gravimetric and volumetric hydrogen storage capacity. In summary, for the hydrogen storage, new materials with improved performance, or new approaches to the synthesis and/or processing of existing materials, are highly desirable. Given the limitations identified in the conventional systems, a novel approach to design new materials is required to achieve the targets set by DOE.

7. Conclusions and future perspectives

Fig. 4. Time evolution of hydrogen release by dehydrocoupling of ammonia borane (0.135 M) solubilized in diglyme at 50 °C and in the presence of metal acetylacetonate (mol ratio NH3BH3/M of 100; with M as Fe, Co, Ni, Pt or Ru for the metal of the acetylacetonate salts) [92].

In recent years, energy materials are receiving tremendous attention and research interests due to the increasing concern on the sustainable development of energy, economy, and society, which is closely related to the high efficiency storage and consumption of energy. To fulfill the newly emerging applications, advanced energy materials with superior integrated performance that enables high energy and power density and environmentally benign, convenient, and flexible storage of energy are highly demanding. Nanostructural carbons materials are extremely important for energy storage, particularly for advanced energy devices, such as capacitors. Boron is an electron deficient element

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and able to enter the carbon lattice by substituting carbon atoms at the trigonal sites and acts as electron acceptor. The boron doping tunes the electronic structure of the carbonaceous material and thus affect the electric double layer capacitance. In addition, boron doping shows catalytic effects on oxygen chemisorption on carbon surface. Therefore, optimizing boron and/or other heteroatom doping is expected to modify the electrochemical capacitance of carbon materials. As discussed above, further investigation on improving overall performance and accelerated practical use of Li-ion battery is warranted. In addition, the compatibility among anode materials, cathode materials, electrolytes, and separators is also significantly important. Several general strategies for making advanced energy storage materials, such as nanostructuring, nano-/microcombination, hybridization, pore-structure control, configuration design, surface modification, composition optimization, and novel device design have been developed. These energy storage materials with high capacity, long life cycles, good safety, and good reliability will undoubtedly boost the performance of energy storage devices and facilitate their wider applications. For hydrogen storage, the nanostructural hybrids of borondoped carbons benefit both the thermodynamics and kinetics of hydrogen adsorption and release processes. It is expected to discover new materials with novel structures that strengthen both physisorption and chemisorption capability for hydrogen storage. For boron materials-based hydrogen adsorbents, it is essential to discover new methodologies to produce macroscopic boron materials with sustainable gravimetric and volumetric hydrogen storage high capacity, like that observed for their nanostructured counterparts. The Ca-coated boron nanostructures should prove to be the suitable building blocks to prepare porous boron materials with high capacity. Acknowledgements The authors thank financial support from the School of Pharmacy, Macau University of Science and Technology and Northern Illinois University. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.ica.2017.11.037. References [1] M.-M. Titirici, M. Antonietti, Chem. Soc. Rev. 39 (2010) 103–116. [2] X.Y. Lai, J.E. Halpert, D. Wang, Energy Environ. Sci. 5 (2012) 5604–5618. [3] J. Deng, M.M. Li, Y. Wang, Green Chem. (2016), https://doi.org/10.1039/ c6gc01172a. [4] J.R. Miller, A.F. Burke, Electrochemical capacitors: challenges and opportunities for real-world applications, in: The Electrochemical Society Interface, Spring, 2008, pp. 53–57. [5] M.D. Stoller, S. Park, Y. Zhu, J. An, R.S. Ruoff, Nano Lett. 8 (2008) 3498–3502. [6] J. Chmiola, G. Yushin, Y. Gogotsi, C. Portet, P. Simon, P.L. Taberna, Science 313 (2006) 1760–1763. [7] S. Shiraishi, M. Kibe, T. Yokoyama, et al., Appl. Phys. A: Mater. Sci. Process 82 (2006) 585–591. [8] X.X. Wu, L.R. Radovic, J. Phys. Chem. A 108 (2004) 9180–9187. [9] C.E. Lowell, J. Am. Ceram. Soc. 50 (1967) 142–144. [10] T. Morita, N. Takami, Electrochim. Acta 49 (2004) 2591–2599. [11] P.N. Vishwakarma, S.V. Subramanyam, J. Appl. Phys. 100 (2006) 113702. [12] L.R. Radovic, M. Karra, K. Skokova, P.A. Thrower, Carbon 36 (1998) 1841– 1854. [13] D.H. Zhong, H. Sano, Y. Uchiyama, K. Kobayashi, Carbon 38 (2000) 1199– 1206. [14] D.-W. Wang, F. Li, Z.-G. Chen, G.Q. Lu, H.-M. Cheng, Chem. Mater. 20 (2008) 7195–7200. [15] H.L. Guo, Q.M. Gao, J. Power Sources 186 (2009) 551–556. [16] Z.S. Wu, A. Winter, L. Chen, Y. Sun, A. Turchanin, et al., Adv. Mater. 24 (2012) 5130–5135. [17] D.W. Wang, F. Li, J.P. Zhao, et al., ACS Nano 3 (2009) 1745–1752.

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[100] R.Z. Ma, Y. Bando, T. Sato, D. Golberg, H.W. Zhu, et al., Appl. Phys. Lett. 81 (2002) 5225. [101] R.Z. Ma, Y. Bando, H.W. Zhu, T. Sato, C.L. Xu, et al., J. Am. Chem. Soc. 124 (2002) 7672–7673. [102] C.C. Tang, Y. Bando, X.X. Ding, S.R. Qi, D. Golberg, J. Am. Chem. Soc. 124 (2002) 14550–14551. [103] X. Chen, X.P. Gao, H. Zhang, Z. Zhou, W.K. Hu, et al., J. Phys. Chem. B 109 (2005) 11525–11529. [104] S.-H. Jhi, Phys. Rev. B 74 (2006) 155424. [105] M. Li, Y.F. Li, Z. Zhou, P.W. Shen, Z.F. Chen, Nano Lett. 9 (2009) 1944–1948. Dr. Yinghuai Zhu received his Ph.D. degree from Nankai University in 1997. He has been working at boron chemistry, nanomaterial and catalysis for more than 25 years and published around 80 papers in the peer-reviewed journals. His current research interest includes construction and application of nanocomposites in biological and catalytical areas. Shanmin Gao obtained his Ph.D. in Inorganic Chemistry at University of Science and Technology of China in 2003. Except for a one-year (2010–2011) academic visit at Northern Illinois University, he has worked at the School of Chemistry and Materials Science at Ludong University since 2004. Now he is a distinguished professor there. His main research interests are new functional nanometer materials for energy conversion and the electrochemical processes in these systems. His current research is fully devoted to the development of new materials for improving the photovoltaic conversion and power density of lithium ion batteries. Narayan S. Hosmane obtained his Ph.D. in inorganic at Edinburgh University in 1974. He is an Indian born cancer research scientist who made the featured article in NRI Achievers magazine and is currently a Distinguished Research Professor of Chemistry and Biochemistry and an Inaugural Board of Trustees Professor at Northern Illinois University. He received the coveted Humboldt Research Award for senior scientists twice. This award is presented annually, by Alexander von Humboldt Foundation, Bonn, Germany, to scientists worldwide as a tribute to their lifelong accomplishments. He was the founder of Boron in the Americas (formerly known as BUSA) and hosted the organization’s first meeting in Dallas in April 1988. He has published over 300 papers in leading scientific journals and was ranked by the Institute for Scientific Information (ISI) in the top 50% of the most cited chemists in the world from 1981 to 1997. In September 2007, at the launch of the NRI Institute’s Washington D.C. chapter, the NRI Institute presented him with its Pride of India Gold Award, in recognition of his outstanding accomplishments. That following January, he was honored by the same NRI Institute with its Lifetime Achievement Pravasi Award and Bharath Samman Medal during the Annual Conference held in New Delhi, India. A fellow of the Royal Society of Chemistry and the American Institute of Chemists, he has also been listed in Who’s Who in the World. His now research interests is synthetic and structural chemistry of polyhedral boron cage biomolecules and nanostructured materials for cancer therapy, catalysis and extraction of radionuclides.