Recent advances in nanomaterial-based solid-state hydrogen storage

Materials Today Advances 6 (2020) 100022

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Recent advances in nanomaterial-based solid-state hydrogen storage Emmanuel Boateng, Aicheng Chen* Electrochemical Technology Centre, Department of Chemistry, University of Guelph, 50 Stone Road East, Guelph, Ontario, N1G 2W1, Canada

a r t i c l e i n f o

a b s t r a c t

Article history: Received 11 July 2019 Received in revised form 18 September 2019 Accepted 25 September 2019 Available online xxx

The hydrogen economy is a system that is proposed as a long-term solution for a secure energy future. Hydrogen production, storage, distribution, and utilization make up the fundamental elements of an envisaged hydrogen economy system. These elements have been the subject of intense research for decades; however, the development of a viable safe and efficient strategy for the storage of hydrogen remains the most challenging. Solid-state hydrogen storage research has expanded significantly, with the potential to fulfill the targets of the United States Department of Energy. This review highlights recent advances in the nanomaterial-based solid-state hydrogen storage. In addition, characterization techniques, including gravimetric and volumetric techniques, as well as electrochemical methods are discussed. Moreover, several promising approaches and an outlook for the enhancement of hydrogen storage are addressed. © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords: Hydrogen economy Nanomaterials Spillover effect Kubas interaction

1. Introduction The United States Energy Information Administration projected a 28% increase in global energy demands between 2015 and 2040, compared with 2008 [1]. A hydrogen economy is a proposed system to provide solutions to the world's increasing energy demands and climate change. The use of hydrogen, which is the most abundant element on earth, with an energy mass density of 142 MJ/ kg [2,3], as a primary energy source is considered a promising alternative energy technology to conventional fossil fuels. The rationale behind a hydrogen economy is that hydrogen fuel is clean and environmentally compatible that generates low or negligible levels of greenhouse gases [4]. Despite the promising future envisioned through a hydrogen economy, several significant technical and scientific challenges associated with the technology must be resolved. The lack of a practical, safe, easy, and cost-effective strategy for hydrogen storage remains one of the critical bottlenecks toward the proposed economy. In addition, onboard hydrogen storage continues to be challenging because gaseous hydrogen should be contained within a small volume without adding significant weight to a vehicle. For practical onboard applications, much hydrogen storage research is devoted to technologies with the potential to meet the hydrogen storage targets set by the United States Department of

* Corresponding author. E-mail address: [email protected] (A. Chen).

Energy (US DOE) [5]. The most stringent US DOE criteria is that by the year 2020, a system with a hydrogen gravimetric (4.5 wt.%) and volumetric capacity (0.030 kg H2/L) should be developed for a target driving range of 300 miles. Besides the requirement of high gravimetric and volumetric capacities, a desired system should exhibit rapid sorption kinetics at near-ambient temperatures, high reversibility (operational cycle life), high stability, and costeffectiveness [6,7]. The overview shown in Fig. 1 reveals the current status of hydrogen storage technologies with their respective estimated costs (based on 500,000 units) [8,9]. It also highlights the 2020, 2025, and “Ultimate Full Fleet” aimed gravimetric and volumetric capacity, as well as the cost targets for onboard applications. As shown in Fig. 1, the gravimetric and volumetric capacity measured for the current different hydrogen storages, including compressed and cryocompressed hydrogen, liquefied hydrogen, chemical hydrides, and complex hydrides, were between 1.5 and 6 wt.%, and 20 and 40 g/L, respectively. Although hydrogen gas and/ or liquid storages are the most mature technologies that are close to meeting the capacity targets, their safety and energy efficiency concerns present a bottleneck for their implementation as a storage medium. For example, liquid hydrogen storage systems show good volumetric storage efficiencies; however, special handling requirements, long-term storage losses from liquid boil-off, and cryogenic liquefaction energy requirements are penalties against their applicability [7,10e13]. Recent interests have thus been shifted toward solid-state hydrogen storage systems. This review presents the recent development in nanomaterialbased solid-state hydrogen storages that show great promise in

https://doi.org/10.1016/j.mtadv.2019.100022 2590-0498/© 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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E. Boateng, A. Chen / Materials Today Advances 6 (2020) 100022

Fig. 1. Current status of hydrogen storage systemsdvolumetric and gravimetric hydrogen density of existing developed hydrogen storage systems with respect to US DOE targets [8,9].

this exciting and rapidly expanding field of research in the sustainable energy community. The focus of this review, as highlighted in Fig. 2, is on metal hydrides, complex hydrides, metal-organic frameworks (MOFs), and carbonaceous nanomaterials. Various strategies that have been explored for the enhancement of hydrogen storage capacities in porous nanomaterials, such as MOFs and carbonaceous nanomaterials, are also discussed.

change in its sample mass. A microbalance housed within a pressure and vacuum-rated vessel is used to measure the mass change. With smaller quantities of sample, the gravimetric storage capacities can also be determined using thermogravimetric analysis (TGA). A disadvantage of TGA is the complexity of the measurement procedure [17].

2. Characterization techniques

While the accuracy and reproducibility of gas sorption techniques (e.g. gravimetric and manometric [Sievert] methods) could be uncertain, hydrogen storage through galvanostatic

The accurate analysis and characterization of the properties of novel nanomaterials for the hydrogen storage are of paramount importance toward the realization of a hydrogen economy. Sorption capacity, kinetics, as well as thermodynamic and cycling stability are among the essential hydrogen storage properties that are evaluated for a candidate material, using conventional techniques. Current characterization techniques that are typically used for the measurements of hydrogen storage properties in ideal solid-state materials may be broadly classified into (i) gas sorption-based approaches, for example, Sievert-type volumetric and gravimetric technique; and (ii) electrochemical methods.

2.3. Electrochemical methods

2.1. Volumetric technique Sieverts apparatus, as illustrated in Fig. 3a, is the most versatile, economical, and simple approach to measure the hydrogen storage properties of a candidate material. Volumetric measurements are used to determine the hydrogen gas sorption and desorption from the pressure change at a fixed, known volume. A sorption capacity (absorption and adsorption) is evident with a decrease in the observed pressure, whereas the reverse is true for desorption [14,15]. One major problem associated with this technique is that an error from continuous calculation of the gas content is accumulated over time [16]. 2.2. Gravimetric technique Gravimetric measurements (Fig. 3b) are used to determine the hydrogen sorption and desorption of a material by measuring the

Fig. 2. Schematic representation of the main topics discussed in this review, including various nanomaterials as potential candidates for solid-state hydrogen storage, and some methods to enhance their storage capacity.

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electrochemical decomposition may provide accurate and reproducible measurements [18]. During electrochemical measurements, the adsorption of hydrogen atoms occurs at the electrode/ electrolyte interface. The electrode becomes the material of interest for hydrogen storage, where it is either deposited on it, or the material itself is the electrode. In conventional hydrogen storage techniques, hydrogen molecules are physically stored over a large area of solid materials. Electrochemical hydrogen storage, on the other hand, stores hydrogen ions and atoms at the working electrode while the counter electrode completes the current flow. In general, electrochemical hydrogen storage consists of adsorption of hydrogen ions on the film surface of the electrode and insertion within the bulk materials [15,19,20]. The oxygen evolution reaction and the oxygen reduction reaction may occur at the counter electrode during the hydrogen charging and discharging process. It is noteworthy that the phenomena of the electrochemical hydrogen storage are similar to other electrochemical energy storage technologies such as batteries and supercapacitors [19e21]. For electrochemical hydrogen storage measurements, the specific capacity depends on various parameters, including physicochemical properties, acidic or basic electrolyte, concentration, and potential cutoffs. The most common electrolyte solution used for electrochemical hydrogen storage experiment consists of KOH and H2SO4 [18e20]. The former uses the reduction of water to produce H-adatoms, while the latter provides H3Oþ as an available proton source in the initial charge transfer. There are several electrochemical techniques that may be used to qualitatively and quantitatively measure reversible hydrogen that is stored on potential candidate materials. These include cyclic voltammetry (CV) [19,22], linear sweep voltammetry (LSV), amperometry [19], galvanostatic charge/discharge measurements [19,20,23], and the open-circuit voltage method [24,25]. Voltammetry techniques are well-established electrochemical methods that are utilized to acquire qualitative and quantitative information on the kinetics of reversible hydrogen storage. In a study conducted by our group [22,26], a combination of several electrochemical techniques, such as CV, LSV, and chronoamperometry were used for measuring the hydrogen

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electrosorption properties of PdeCd nanomaterials with compositions of Cd ranging from 0 to 20 at% (Fig. 4). Fig. 4a displays typical SEM images of the formed PdeCd nanostructures, showing the drastic surface morphology changes as the Cd percentage was increased. The effect of the applied electrode potential on the kinetics and capacity of the hydrogen sorption was studied using LSV and chronoamperometry. Fig. 4b and c presents a series of LSV curves of the Pd nanoparticles and the PdeCd (10 at%) nanodendrites, respectively. By integrating the area under the hydrogen desorption/oxidation peak as shown in Fig. 4b and c, the overall hydrogen oxidation/discharge and the ratio of H/(Pd þ Cd) of the different nanomaterials were calculated (Fig. 4d). The results revealed that the hydrogen sorption resulted in two different phases: the a-phase (solid solution) and b-phase (metal hydrides). The galvanostatic charge/discharge method is another technique that is used to measure the quantity of hydrogen that is reversibly stored on potential materials. A key feature of electrochemical method for hydrogen storage application is the easy control of hydrogen release, in contrast to physical adsorption methods. The electrochemical hydrogen sorption isotherm (i.e. electrochemical pressure-composition [EPC] isotherm) obtained from the open-circuit voltage method can be utilized to determine the thermodynamic properties of a potential hydrogen storage material. The EPC isotherm is analogous to the pressure-composition-temperature (PCT) isotherm of
ski et al. the gasesolid phase measurements [25]. Skowron measured the EPC and PCT isotherm of the prepared AB2.4 alloys; based on the Nernst equation, agreeable correlation between the equilibrium pressure measured in gas phase and the potential value of the metal alloys in the electrolyte solution was established [24]. 2.4. Complementary techniques In general, both volumetric and gravimetric techniques provide the same information. However, owing to their different operational principles, discrepancies in experimented data may often be observed [27,28]. This was evidently illustrated by Zlotea et al. where several laboratories across Europe were sent identical

Fig. 3. Schematic diagram of (a) Sievert-type apparatus. Open/close valves allow for the flow of hydrogen gas inlet and outlet to the sample and vacuum, respectively. This allows for the control of hydrogen pressure in the reference volume. Pressure of hydrogen gas in the reference volume to the sample volume is measured using the manometer. (b) Gravimetric sorption apparatus. Open/close valves allow the flow of hydrogen gas inlet and outlet to the sample and vacuum respectively. The mass of the sample before and after the introduction of hydrogen gas is determined by a microbalance, with the help of a predetermined counterweight mass.

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Fig. 4. (a) SEM images nanostructured PdeCd alloys with varying Cd compositions up to 15 at.% (reproduced with permission from Ref. [22]. © 2009 American Chemical Society); (b)Linear sweep voltammogram of the desorption of hydrogen from Pd (c) Linear sweep voltammogram of the desorption of hydrogen from PdeCd (10 at.%) (reproduced with permission from Ref. [26]. © 2010 American Chemical Society); (d) Overall hydrogen oxidation charges, ratio of H/(Pd þ Cd), of nanostructured PdeCd alloys with varying Cd compositions from 0 at.% to 15 at.% (reproduced with permission from Ref. [26]. © 2010 American Chemical Society).

carbon samples. Although the same instructions for sample preparation, data collection, and analysis were provided, considerable disparities were observed among the obtained results [29e31]. The accuracy and reproducibility of hydrogen storage capacity measurements remain a significant challenge in the research community. To avoid contradictory experimental results obtained from the conventional gas sorption measurement methods, complementary characterization approach is necessary. The gas sorptionebased method may be complemented with a range of other techniques, including neutron scattering, secondary-ion mass spectrometry (SIMS), thermal desorption spectroscopy (TDS), etc. In the case of metal hydrides, neutron scattering may be used to observe the presence and position of hydrogen within the crystal structure [30,32].

hydrogen in solid-state form via chemisorption (as hydrides form with binding energies from 50 to 100 kJ/mol) or physisorption (adsorption on porous materials with binding energy >10 kJ/mol) under various conditions [35,36]. These processes have several advantages over the conventional hydrogen storage in terms of safety and cost-effectiveness. However, they also possess their own disadvantages: high energy barriers, slow kinetics, and poor reversibility accompanies chemisorption, whereas low (cryogenic) temperatures are required for a reasonable physisorption uptake [37]. Physisorption plays a vital role in the carbon-based nanomaterials and MOFs for hydrogen storage, whereas chemisorption is mainly involved with metal hydrides and complex hydrides. 3.1. Carbonaceous materials

3. Solid-state hydrogen storage Currently, most mature hydrogen storage technologies are either via compressed gas or by liquefaction (stored in a liquid state), whereas the storage of hydrogen in a solid-state form has gained considerable interests recently. The identification and development of lightweight, low-cost solid-state hydrogen storage systems with a high capacity and fast kinetics can circumvent the difficulties of onboard applications. In ongoing solid-state storage research, nanoporous materials such as carbon-based nanomaterials, metal-doped carbon-based nanomaterials, MOFs, covalent-organic frameworks, complex chemical hydrides, clathrates, amides, zeolites, and metallic or intermetallic hydrides are considered as promising materials for future hydrogen storage [33,34]. These novel nanomaterials may facilitate the storage of

Among the vast range of potential materials for hydrogen storage, carbon-based systems have garnered immense research interest due to their low mass densities, high surface areas, and chemical stabilities [5,38]. The physical adsorption of hydrogen stored in light nanoporous carbon-based materials is dominated by weak van der Waals forces with a binding strength less than 10 kJ/ mol [35]. Activated carbons, graphite nanofibers, graphene, fullerenes, and carbon nanotubes comprise the various carbon materials under consideration for future hydrogen storage by the large scientific research community. This is due to their high surface areas, relatively low costs, fast kinetics, and other beneficial adsorption properties [39]. Several studies have revealed a linear correlation between gravimetric hydrogen storage capacity and the surface area of

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carbon-based porous nanomaterials and MOFs. In Fig. 5a, Talyzin et al. [27] demonstrated an improvement in the gravimetric capacity of various graphene samples with different surface areas at 77 K. Fig. 5b shows the gravimetric wt.% vs BET surface area trends at 77 K and 296 K for various reduced graphene oxide (rGO) samples [14]. The linear correlation from Fig. 5b was developed following a general trend known as Chahine's rule. This correlation occurs as higher surface areas provide a platform for additional available/accessible sites per sample mass, where hydrogen can be physisorbed [15,40]. Many experimental studies of hydrogen storage in carbon-based materials have produced promising results with high hydrogen storage capacities that may exceed the DOE targets [41]. 3.2. Metal-organic frameworks Owing to their high surface areas of up to 6000 m2/g, permanent porosity, tunable pore sizes, rigidity, structural flexibility, and thermal stability, MOFs have gained considerable interest for hydrogen storage and other applications [42e44]. MOFs constitute a large family of crystalline porous materials and are a unique class of inorganic-organic hybrid materials that are formed from metal ions or clusters coordinated by oxygen or nitrogen atoms. They are bound together by “linkers” (organic molecules) such as malonic acid, oxalic acid, citric acid, pyrrodiazole, etc. The significant number of new MOF structures being synthesized every year suggests that the possibility of varying a few parameters during the early stage of their synthesis might make them suitable for specific applications. For example, tailored pore structures and threedimensional frameworks may be achieved via the modification of organic linkers, reaction temperature, and metal/ligand ratios [43,45]. Furthermore, the physisorption of hydrogen molecules onto the surfaces of MOF nanopores may be enhanced through the removal of trapped guest molecules (via solvents) that occupy the nanopores. However, moisture instability, low heat of adsorption, and difficulties in large-scale process are some drawbacks which should be overcome for MOFs to be competitive media for hydrogen storage [42,45,46]. 3.3. Metal hydrides The use of metal hydrides to store hydrogen has received extensive research interest due to their exceptional attributes. These include high storage capacities at a low pressure and strong binding energies of ~40e100 kJ/mol, while maintaining volumetric

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densities comparable with the conventional methods for hydrogen storage [35,36,47]. Metal hydrides are a class of hydrogen compounds that are created when metal alloys reversibly react with hydrogen. A requirement of metal hydrides for a practical hydrogen storage application is to maintain their reactivity and capacity over thousands of cycles [48]. Although various types of metal hydrides can be formed, only a few are suitable for hydrogen storage applications. This special group may include materials from AB5, AB2, and AB types (A and B are strong and weak hydriding elements, respectively) with their desired intermediate thermodynamic affinities for hydrogen. Examples of such metal hydrides are LaNi5-xAlx, TiV2-xMnx, and FeTi1xMnx, with x as a variable for adjusting the equilibrium pressure and tailoring their stability [48,49]. The high volumetric hydrogen density storage enabled by metal hydrides is one of the main advantages they have over the conventional mature compressed gas and liquid hydrogen storage methods. However, the low gravimetric hydrogen storage capacity of metal hydrides remains a hurdle for their use as efficient hydrogen storage materials [34,50]. Another disadvantage of a metal hydride (e.g. MgH2) as a storage medium is the high temperature required for hydrogen desorption, slow desorption kinetics, and high reactivity toward air and oxygen. Of all the reversible hydrides that are appropriate for hydrogen storage, magnesium-based hydrides have the highest hydrogen storage capacity of 7.6 wt.% (110 g/L H2). Their high thermodynamic stability and robust binding energy can lead to dehydrogenation temperatures as high as 350e400  C under an ambient pressure [34,51,52]. As a consequence, many efforts have been invested to overcome this drawback, which has prevented the use of MgH2-based materials for hydrogen storage applications. For example, Ren et al. [53] reported a significant improvement in both the hydrogenation and dehydrogenation kinetics of magnesium-hydride via the use of V-based additives. The dehydrogenation properties of MgH2 Vbased additives at several heating rates were characterized by TGA. Fig. 6a shows a comparison between the TGA curves of the different MgH2 samples with various V-based additives at a 5  C/min heating rate. The figure reveals that the dehydrogenation temperature of various prepared hydrides was within the range of 217e233  C, which was almost 100  C lower than that of the pure MgH2. Fig. 6b depicts the isothermal dehydrogenation curves of the MgH2 samples with various V-based additives at the temperature of 240  C and a pressure of 0.01 bar, showing that all the samples with the Vbased additives underwent full dehydrogenation within 6e10 min,

Fig. 5. Linear relationship of hydrogen storage uptake as a function of the BET specific surface area (SSA) illustrated through (a) H2 adsorption isotherms for graphene samples with various surface areas at 77 K (reproduced with permission from Ref. [27]. ©2015 Elsevier); (b) H2 uptake as a function of BET surface area (reproduced with permission from Ref. [14]. ©2015 Royal Society of Chemistry).

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Fig. 6. Hydrogen isotherm properties of magnesium-based metal hydride: (a) TGA curves of as-milled MgH2 and MgH2 with V-based additives; (b) PCT dehydrogenation kinetics of MgH2 with V-based additives; (c) PCT hydrogenation kinetics of MgH2 with V-based additives under RT and 1 hydrogen pressure; (d) PCI hydriding measurements of as-milled MgH2 and MgH2- 5% V75Ti5Cr20 at 300  C. (Reproduced with permission from Ref. [53]. ©2014 American Chemical Society).

whereas the pure MgH2 failed to dehydrogenate at the applied temperature. The authors subsequently performed a series of hydrogenation experiments using PCT curves (Fig. 6c), demonstrating significant MgH2 hydrogen absorption with the various V-based additives under 1 bar pressure at room temperature. Fig. 6d depicts pressure-composition-isothermal (PCI) curves from thermodynamic behavior studies of pure MgH2 and MgH2 with V75Ti5Cr20 samples at 300  C. From the PCI measurements, there was negligible difference in the equilibrium pressure between the two samples (pure MgH2 with and without the V-additives). The Vadditives imparted a potent effect on the kinetics of MgH2 (de) hydrogenation, but no influence on its thermodynamic stability. This suggested that the additives functioned similarly only as catalysts but did not participate in the MgeH reactions. 3.4. Complex hydrides Recently, complex hydrides have garnered considerable interests as potential solid-state hydrogen storage materials. Complex hydrides are referred to as materials of Group I and II salts of alanates [AlH4]-, amides [NH2]-, and borohydrides [BH4]- [33,34,54]. Unlike metal hydrides, complex hydrides are saline materials, whereby hydrogen molecules are covalently bonded to the central atoms of complex anions. Ball milling is the most mature technique for the formation of complex nanostructured hydrides. During the ball milling process, powdered nanoparticles or nanocrystalline materials can be synthesized [55,56]. In fact, the mechanical ball milling technique has been shown to greatly enhance the performance of hydrogen storage (e.g. (de)hydrogenation kinetics, desorption temperatures, etc.) of complex hydride systems. Like metallic hydride systems, alloying and additive functionalization are also common methods for overcoming the dehydrogenation and hydrogenation issues of complex hydride systems. In 2007, the US DOE reached a “no-go” decision on the hydrolysis of sodium

borohydride (NaBH4) for hydrogen storage application because of its poor energy efficiency and high regeneration cost (i.e. irreversibility) [57]. To address these issues, Zhong et al. recently demonstrated a single-step method for the regeneration of NaBH4 using Mg/MgH2 or magnesium silicide as additives [58,59]. Different milling parameters such as the type of mill, milling time, the number and size of balls, and milling atmosphere can positively affect the hydrogen sorption performance of metal and complex hydride systems. Complex nanostructured hydrides are attractive as hydrogen storage media due to their extreme theoretical gravimetric hydrogen densities (between 7 wt.% and 18 wt.%) as a result of their binding energy strength in the range of 40e100 kJ/mol [36,46,53]. However, their application as viable hydrogen storage media has been impractical as the result of their poor thermodynamics, kinetics, and limited reversibility. Complex hydride hydrogen systems, in contrast to metallic and ionic hydrides, are not fully understood, because of the limited knowledge as relates to dehydrogenation pathways and hydrogen absorption mechanisms.

4. Strategies for enhanced hydrogen storage capacity Many research publications suggest that pristine carbon nanomaterials are promising candidates for the hydrogen storage because of their high surface areas and other attributes. However, the reported hydrogen storage capacities for these nanomaterials are ~1 wt.% under ambient conditions [60]. This is rather impractical for a hydrogen storage system as it fails to meet the US DOE targets. To significantly enhance the hydrogen storage capacity of pristine carbon materials, numerous research efforts have focused on chemical modifications through the substitution of heteroatom groups and metal nanoparticles, defect formation to increase the surface area, etc. Several strategies that may be used to

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optimize the hydrogen storage capacity of carbon materials are discussed as follows. 4.1. “Kubas” interaction One of the goals of the hydrogen storage system set by the US DOE is for a material to possess a hydrogen adsorption enthalpy of between 20 and 30 kJ/mol under ambient conditions [61]. Metal hydrides or chemical hydrides suffer from slow chargeedischarge kinetics and poor reversibility because of strong binding interactions between hydrogen molecules and metals. Metal hydrides have a binding enthalpy of >60 kJ/mol [61] (see Fig. 7a). On the other hand, carbon materials and other porous materials that undergo physisorption suffer a lower storage capacity because of the weak van der Waals interactions between the adsorbent and hydrogen molecules. They possess a significantly low hydrogen binding enthalpy (<10 kJ/mol) [62,63]; hence, they are too weak to bind the hydrogen to the adsorbents, which leads to a low storage capacity. None of these fulfill the goals being set; thus, to overcome this drawback, the Kubas interaction has been proposed and investigated. A metal catalyst is incorporated into an adsorbent storage material. However, there is no breakage of the H2 bonds; rather, the H2 bonds lengthen. It is estimated that after the Kubas interaction, the final bond distance between H2 molecules is ~20% longer than the distance between the atoms in free H2 [61,63,64]. Compared with simple physisorption, this essential factor provides a stronger interaction between the H2 molecules and the metal catalyst. Morris et al. [65] developed a porous manganese hydride (KMH-

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1 ¼ Kubas manganese hydride-1) that was easily synthesized from inexpensive precursors. The group demonstrated a reversible excess hydrogen adsorption performance of 10.5 wt% at 120 bar under ambient temperature. Using data obtained from computational studies and inelastic neutron scattering, the group attributed the high gravimetric hydrogen uptake to Kubas binding as the principal mechanism. During the Kubas interaction of H2 binding to a metal, an electron is donated from a H2 s-bonding orbital to an empty d-orbital of the transition metal. Simultaneously, p-backdonation from a filled transition metal d-orbital into vacant s* antibonding orbital occurs [66]. Fig. 7b illustrates the Kubas binding mechanism using two Kohn-Sham molecular orbital interactions between MnH2 and H2. Research into the Kubas interaction as a strategy for enhancing hydrogen storage capacities remains in its nascent stages and is yet to be fully understood. As a result, most associated literature has been limited to computational studies. 4.2. Spillover effect In recent years, the “spillover” phenomenon for hydrogen storage has received tremendous interest, particularly in the field of heterogeneous catalysis. The spillover mechanism consists of the transport of active species that are sorbed or formed on an initial surface, which then migrate onto a final substrate that does not sorb or form active species under the same conditions. The hydrogen spillover mechanism involves three primary steps including (i) chemisorptive activation or dissociation of gaseous hydrogen molecules on a transition metal catalyst; (ii) migration of hydrogen atoms from the catalyst to the substrate; and (iii)

Fig. 7. (a) Binding energy strength of physisorption and chemisorption materials, including hydrogen spillover and “Kubas” interaction enhancement mechanism [35,36]; (b) KohnSham molecular model of “Kubas” orbital interaction between MnH2 and H2, shown at left is s donation from the filled H2 s bonding MO into a Mn d orbital and (right) p backdonation from a filled Mn d orbital into the H2 s* orbital (reproduced with permission from Ref. [65]. © 2019 The Royal Society of Chemistry); (c) Illustration of hydrogen spillover mechanism [69].

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diffusion of hydrogen atoms onto substrate (receptor) surfaces [15,26]. Fig. 7c provides an illustration of the hydrogen spillover mechanism. The substrate is considered as inert toward inactivated hydrogen, where only the activated atomic hydrogen that migrates from the catalyst is accepted [67]. To improve the hydrogen storage capacity under ambient conditions, Divya et al. [63] modified hydrogen-exfoliated graphene sheets (HEG) with platinum nanoparticles (Pt NPs). The composite, Pt-HEG, exhibited 1.4 wt% at 25  C and 3 MPa of hydrogen uptake, which was higher than that of bare HEG (0.5 wt.%). The incorporation of the Pt NPs enhanced the hydrogen storage capacity through the hydrogen spillover process. Using electrochemical hydrogen storage studies, our group successfully demonstrated the hydrogen spillover effect on decorating rGO with a palladium (Pd) metal catalyst (Fig. 8) [68]. In the case of the rGO sample (Fig. 8a inset), there were no observable adsorption/desorption peaks, while pure Pd and Pd-rGO revealed adsorption/desorption peaks (Fig. 8a). To investigate the hydrogen uptake kinetics for the Pd-rGO thin film, chronoamperometry and LSV were used, revealing that the maximum hydrogen uptake was reached within a 3-min holding time. Fig. 8b shows the overall hydrogen oxidation charges of the Pd-composites at various electrode potentials. In the case of the Pd-rGO composites, the hydrogen spillover mechanism began with the adsorption of H atoms to the Pd nanoparticles, after which the H atoms diffused onto the rGO surface that resulted in an enhanced hydrogen storage capacity [68,69]. In general, the decoration of transition metals on carbon materials was found to enhance the H2 storage capacity by 30% without decreasing their desorption kinetic properties. Because adsorption in the spillover process is reversible, it is being thoroughly investigated as a technique for the enhancement of onboard hydrogen storage capacities. However, structural stability in transition metal dispersions remains a large challenge, as metal atoms tend to aggregate because of strong metal cohesion forces [63,70]. 4.3. Functionalization It is well-known that carbon-based nanostructures, such as carbon nanotubes, fullerenes, graphene, etc. are promising candidates for hydrogen storage due to their high surface areas, low mass density, and chemical stability. However, unsubstituted or pristine carbon-based materials possess a low storage capacity (<1 wt.%). As discussed previously, the incorporation of metal catalysts into a

porous adsorbent enhances their storage capacity through spillover effects, or the Kubas interaction. One major problem with the aforementioned phenomena is the structural stability of transition metal dispersions due to their tendency of aggregation, resulting in a low storage capacity [71]. To effectively utilize the “spillover” mechanism for enhancing hydrogen storage, a metal catalyst must be uniformly distributed on the adsorbent surface. Yang et al. [72] synthesized an N-doped microporous carbon, which demonstrated an 18% increase of hydrogen storage capacity, in contrast to pristine microporous carbon. They further decorated Pt NPs onto the N-doped microporous carbon and obtained a hydrogen storage capacity of 1.26 wt.% at 298 K and 10 MPa, an enhancement factor of 2.4 compared with the pristine material. Computational studies performed by Deng et al. [73] showed a significantly enhanced hydrogen adsorption strength when they introduced boron atoms into platinum-decorated graphene composites. Metal clustering may be suppressed by modifying sorbent materials with heteroatom dopants such as boron, nitrogen, phosphorous, and so on. Substitutional heteroatoms in sorbent materials not only enlarge the surface area, but also introduce defects and active sites for the metal to bind in a uniformly distributed manner, thus greatly improving the hydrogen storage capacity [71]. 5. Summary and outlook With the impending concerns associated with the climate change and the depletion of fossil fuels, an envisaged hydrogen economy remains a viable alternative to the subsequent energy issues. To achieve a hydrogen economy, hydrogen storage remains the most significant challenge, as conventional storage systems (compressed hydrogen and liquid hydrogen) may not be efficient and safe for onboard applications. Several nanomaterials and nanocomposites have been explored recently as potential candidate for the solid-state hydrogen storage, which are discussed in this review and summarized in Table 1. Their performance for hydrogen storage strongly depends on the nature of the nanomaterials (e.g. composition and surface area) as well as the testing conditions such as temperature and pressure. Chemical storage systems, including metal hydrides and complex hydrides, are promising; however, their poor reversibility and cost-effectiveness should be addressed for a practical application. Physical adsorption on carbonaceous materials and other porous materials with large surface areas has also gained much research attention because of its facile reversibility and rapid kinetics. However, a very low

Fig. 8. Electrochemical hydrogen storage studies of graphene composites demonstrating the hydrogen spillover effect with Pd-decoration on graphene nanocomposite: (a) CV curves for rGO, pure Pd NP and Pd-decorated rGO with the Pd load of 0.108 mg/cm2; (b) Overall hydrogen desorption charge vs holding electrode potentials. (Reproduced with permission from Ref. [68]. © 2015 Elsevier).

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Table 1 Hydrogen storage properties of different solid-state materials, characterization methods, and their enhancement techniques. Materiala

Surface area (m2/g)

T ( C)

P (MPa)

Measurement

H2 Capacity (wt.%)

Enhancement mechanism

Ref

MgH2 (TiCl3) Ti0.47V0.46Mn Ti0.50V0.51Mn FeTi (Zr7Ni10) a-[Mg3(O2CH)6] LaNi4.7Al0.3 MIL-100(Al) (Pd) MIL-100(Al) (Pd) MgH2 (Sic-Ni) rGO (Al2O3 þ Pd) 2MgH2eCo rGO (TiO2 þ N) Mg@C NPs 2LiBH4eMgH2@G MWCNT (Pd) MWCNT (Ni) AC (PdeNi) d-CNTs (PdeNi) AC (cellulose) GNs (MgH2eNi2P) PS/Si-GO (Pd) Mgx-Co100-x (Pd)

e

278

2

TPD

6.70

Enhanced kinetics alloying

[74]

e e e

29.9 29.9 25

12 12 3.5

VT VT VT

1.53 ± 0.05 1.56 ± 0.05 1.37

Alloying Alloying Alloying

[75] [75] [76]

496 e 380

196 20 24.9

2.7 e 4

VT VT VT

1.20 1.43 0.35

e Alloying Spillover

[77] [78] [79]

380

196

4

VT

1.30

Spillover

[79]

e

300

3

VT

5.10

Additives

[80]

62

24.9

1

GT

0.31

Kubas interaction

[81]

e 132.2

300 RT

3 0.8

TPD; VT GT

4.43 0.91

Synthesis method Functionalization

[82] [83]

e 794.7 29.93

400 350 196

4 e 6.5

VT GT; VT VT

6.30 8.9 0.37

Spillover Spillover; alloying Spillover

[84] [85] [86]

e

200

2

VT

0.298

Spillover

[87]

e

e

e

TPD

1.5

Spillover

[88]

e

e

e

TPD

6.6

Increase defects; spillover

[88]

3800

196

2

GT

8.1

High surface area; defects

[89]

e

325

e

TPD; VT

6.1

Alloying; spillover

[90]

e

25

e

EHS

2.1

Spillover

[91]

e

RT

e

EHS

1.04e1.85

Enhanced kinetics; cyclic stabilities

[92]

TPD: temperature programmed-desorption; VT: volumetric technique or Sievert-type apparatus; GT: gravimetric techniques; EHS: electrochemical hydrogen storage; MWCNT: multiwalled carbon nanotube; AC: activated carbon; GNs: graphite nanosheets. a Data in parenthesis are either catalysts or additives used to enhance hydrogen storage capacity.

temperature is required for a high-performance hydrogen sorption; most of the reported hydrogen storage systems exhibit less than 1 wt.% in capacity under ambient conditions. To enhance their hydrogen storage performance, metal catalysts may be incorporated into the surfaces of the adsorbents to induce the hydrogen spillover effect or Kubas interactions. To further enhance sorption performance, the adsorbent materials may be functionalized via heteroatom substitution. Traditional characterization techniques such as gravimetric, Sievert-type volumetric techniques, and electrochemical methods could be used to investigate the hydrogen sorption capacity, kinetics, thermodynamic stability, and cycling stability of potential storage materials. Both gravimetric and volumetric techniques provide similar information, despite their different mechanisms. The accuracy and reproducibility of hydrogen storage capacity measurements should be improved. To avoid contradictory experimental sorption results obtained from conventional gas sorption measurement methods, complementary characterization techniques such as SIMS, TDS, and neutron scattering may be helpful. The advantages of nanomaterial-based solid-state hydrogen storage have been well demonstrated; however, the associated challenges must be addressed to meet the onboard vehicular targets set by the US DOE. This short review highlights some recent advances in nanomaterial-based solid-state hydrogen storage. It is anticipated that new strategies and advanced functional nanomaterials will continue to emerge due to the active research and

development of the solid-state hydrogen storage, which would contribute significantly to a successful implementation of the hydrogen economy vision. Conflict of interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by a Discovery Grant from the Natural Sciences and Engineering Research Council of Canada (NSERC RGPIN-2015-06248). A. Chen acknowledges NSERC and the Canada Foundation of Innovation (CFI) for the Canada Research Chair Award in Electrochemistry and Nanoscience. References [1] L. Doman, EIA projects 28% increase in world energy use by 2040, U.S. Energy Information Administration (EIA), Today in Energy (2017). https://www.eia. gov/todayinenergy/detail.php?id¼32912. (Accessed 24 May 2019).
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