Hierarchical Porous Carbon by Ultrasonic Spray Pyrolysis Yields

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Hierarchical Porous Carbon by Ultrasonic Spray Pyrolysis Yields Stable Cycling in Lithium−Sulfur Battery Dae Soo Jung,† Tae Hoon Hwang,† Ji Hoon Lee,† Hye Young Koo,‡ Rana A. Shakoor,§ Ramazan Kahraman,§ Yong Nam Jo,∥ Min-Sik Park,∥ and Jang Wook Choi*,† †

Graduate School of Energy, Environment, Water, and Sustainability (EEWS) and Center for Nature-inspired Technology (CNiT), KAIST Institute NanoCentury, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehakro, Yuseong-gu, Daejeon 305-701, Republic of Korea ‡ Powder Technology Department, Korea Institute of Materials Science (KIMS), 797 Changwondaero, Seongsan-gu, Changwon, Gyeongnam 641-831, Republic of Korea § Department of Chemical Engineering, Qatar University, P.O. Box 2713, Doha, Qatar ∥ Advanced Batteries Research Center, Korea Electonics Technology Institute (KETI), 25, Saenari-ro, Bundang-gu, Seongnam-si, Gyeonggi-do 463-816, Republic of Korea S Supporting Information *

ABSTRACT: Utilizing the unparalleled theoretical capacity of sulfur reaching 1675 mAh/g, lithium−sulfur (Li−S) batteries have been counted as promising enablers of future lithium ion battery (LIB) applications requiring high energy densities. Nevertheless, most sulfur electrodes suffer from insufficient cycle lives originating from dissolution of lithium polysulfides. As a fundamental solution to this chronic shortcoming, herein, we introduce a hierarchical porous carbon structure in which meso- and macropores are surrounded by outer micropores. Sulfur was infiltrated mainly into the inner meso- and macropores, while the outer micropores remained empty, thus serving as a “barricade” against outward dissolution of longchain lithium polysulfides. On the basis of this systematic design, the sulfur electrode delivered 1412 mAh/gsulfur with excellent capacity retention of 77% after 500 cycles. Also, a control study suggests that even when sulfur is loaded into the outer micropores, the robust cycling performance is preserved by engaging small sulfur crystal structures (S2−4). Furthermore, the hierarchical porous carbon was produced in ultrahigh speed by scalable spray pyrolysis. Each porous carbon particle was synthesized through 5 s of carrier gas flow in a reaction tube. KEYWORDS: Bimodal porous carbon, hierarchical porous carbon, lithium−sulfur battery, small sulfur, spray pyrolysis

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lithium polysulfides return to the sulfur electrode, completing the “shuttling”. Throughout repeated shuttling processes, active material from the sulfur electrode is continuously lost and the interfaces on both electrodes also become destabilized. The inherently low conductivity of sulfur (5 × 10−30 S/cm at 25 °C) is another significant hurdle in design of sulfur electrodes.5 One of the most effective approaches to address these issues is encapsulation of sulfur into porous conductive media made of amorphous carbon,1,6−10 carbon nanotube,11−13 porous carbon fibers decorated with porous structure,14 graphene,15−17 conducting polymers,18,19 pyrolyzed polyacrylonitrile,20,21 and metal oxides.22,23 While these research outcomes represent substantial progresses in the sulfur electrode design, some studies paid main attention to the pore size of the conductive media24 and observed superior electrochemical properties for

he timely advent of future lithium ion battery (LIB) applications, such as advanced mobile electronics and hybrid electric vehicles, demands higher energy densities than those covered by conventional LIBs relying on intercalation electrodes. To meet ever-increasing such demand, the battery community has invested considerable efforts focusing on new chemistry for the electrode reaction with Li ions, and lithium− sulfur (Li−S) and lithium−air batteries are most representative along this direction. In particular, Li−S systems have received exceptional attention due to the unparalleled theoretical capacity of sulfur reaching 1675 mAh/g alongside other additional advantages related to sulfur including abundance of raw material, low cost, and eco-friendly characteristic.1,2 Despite these promising features, Li−S batteries have suffered from insufficient cycle lives mainly due to the socalled shuttling process:3 long-chain lithium polysulfides (Li2Sn, 4 ≤ n ≤ 8) formed at the initial lithiation stage of each cycle are soluble into most electrolytes and diffuse toward the Li metal electrode.4 Upon further lithiation at the Li electrode, the © XXXX American Chemical Society

Received: April 15, 2014 Revised: July 1, 2014

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Figure 1. Schematic illustration of the electrode structures and their electrochemical processes. (A) Hierarchical porous carbon particles have micropores in the outer shell, surrounding the inner meso- and macropores. The dissolution of the soluble long-chain lithium polysulfides in the inner macro- and mesopores is suppressed by the outer micropores serving as a barricade. (B) Conventional activated carbon (AC1600) containing micropores and mesopores in a random geometry. The long-chain lithium polysulfides are liable to dissolution through the open pore ends.

the pore sizes below 2 nm.25−27 Such micropores have indeed turned out to be efficacious in mitigating soluble lithium polysulfides via various mechanisms: (1) strong adsorption of lithium polysulfides,26 (2) construction of solvent-free environment by desolvation of carrier ions,27 and (3) formation of insoluble small sulfur allotropes.25,28 In a distinct cell configuration, Manthiram et al. implemented carbon interlayers with micropores between the sulfur electrode and separator, and observed suppressed migration of dissolved lithium polysulfides.29 Impressed by the significantly improved electrochemical performance based on these micropore approaches, the current study has combined sulfur encapsulation and micropores into a single electrode component by developing a hierarchical porous carbon (HPC) structure. In this HPC structure, the majority of sulfur is encapsulated in the core meso- and macropores, while these pores in the core are surrounded by micropores in the shell. Hence, stable cycling was achieved by the outer micropores that shut down dissolution of lithium polysulfides while most of active sulfur remained loaded in the larger inner pores. Furthermore, such HPC was produced by ultrahigh speed spray pyrolysis (∼5 s flow time), and its synthesis is therefore compatible with highthroughput manufacturing processes.30 Figure 1 comparatively illustrates the effects of pore geometry on the electrochemical stability for the two electrode templates: HPC and conventional activated carbon (AC). In the sulfur-loaded HPC electrode, denoted as HPC-S (Figure 1A), the micropores existing in the outer shell and in the walls between the core meso- and macropores prevent migration of the soluble long-chain lithium polysulfides into most electrolytes due to the aforementioned mechanisms associated with micropores. By contrast, the classical ACs whose pores are created by simple activation processes in alkali media tend to have irregular pore structures (Figure 1B) even with larger surface areas. Hence, the soluble long-chain lithium polysulfides

could be randomly formed anywhere close to the surface and initiate the fatal shuttling process. Such beneficial HPC structure was produced by one-pot synthesis of high-throughput spray pyrolysis without involving any sacrificial templates. The precursor solution contained sucrose and sodium carbonate (Na2CO3) as a carbon source and a base catalyst for efficient decomposition of sucrose, respectively.31 Remarkably, the present spray pyrolysis is based on an ultrafast flow condition, as the passing duration of the flow from the bubble formation to the end of the reaction tube (length = 60 cm) was only 5 s (see the setup in Supporting Information Figure S1). Even during such fast flow condition, the pores with the two distinct dimensions were produced by engaging different pore generation mechanisms simultaneously; micropores were generated by formation of various gases including CO, CO2, and H2O during the thermal decomposition of sucrose31 or removal of the nanosized Na salt formed from the chelated compound between sucrose and Na2CO3, C12H21O11−Na+.9 On the other hand, the formation of the meso- or macropores was mediated by water-soluble salt byproducts whose dimensions correspond to those of the final meso- or macropores (i.e., sodium bicarbonate (NaHCO3), sodium oxide (Na2O), and precipitated Na2CO3). The presence of these salts was verified by the X-ray diffraction (XRD) spectrum of the spray pyrolysis product (Supporting Information Figure S2A). The formation of the final meso- or macropores was completed by washing away these salts with distilled water (Supporting Information Figure S2B,C). Apparently, one of the most noticeable features in the synthesis of the current HPC is that the byproduct salts are spontaneously encapsulated in the core region of the carbon matrix even during the ultrafast flow condition. This unique phenomenon can be explained by sequential processes during the spray pyrolysis; as the solvent evaporates from the outer surfaces of the liquid droplets, the solvent in the core B

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Figure 2. Characterization of HPC prepared by spray pyrolysis. SEM images of (A) an HPC sphere and (B) its fractured piece showing the inner meso- and macropores. (C) A TEM image of a HPC sphere exhibiting the inner meso- and macropores. (D) An HRTEM image of the outer shell of an HPC sphere. The gray scale contrast indicates the presence of micropores. (E) Nitrogen adsorption−desorption isotherms. The surface areas and pore volumes are denoted inside; the total surface area (SBET), total pore volume (VT), and micropore volume (Vmicro). (F) The pore size distribution.

continuously migrates outward through capillary force. During this process, the chelated C12H21O11−Na+ dissolved in the solvent migrates together toward the shell of the droplets. Upon complete evaporation of the solvent and the subsequent carbonization of the chelated compound, the outer microporous carbon shell is formed.32 Furthermore, it has been known that the sizes of the meso- or macropores can be controlled by changing the sucrose/Na2CO3 ratio in the precursor solution.31,33 The formation mechanism is commensurate with that of the salt-assisted spray pyrolysis by Okuyama group that was used for synthesis of metal oxide nanoparticles.34 Detailed experimental procedure is described in the Supporting Information Experimental Procedures. As displayed in Figure 2A, the scanning electron microscope (SEM) image of the final HPC indicates that the HPC particles hold smooth outer surfaces and their diameters are uniform around 2 μm. The cross-sectional SEM image of a fractured HPC piece confirmed the existence of the inner meso- or

macropores (Figure 2B). Moreover, the transmission electron microscope (TEM) image (Figure 2C) of the HPC particles not only reconfirmed the meso- or macropores confined in the core region but also suggested interconnected nature of those pores. The high-resolution TEM (HRTEM) image taken for the outer surface region supports the view on the HPC structure by exhibiting the outer micropore formation (Figure 2D). The porosity of the HPC was assessed quantitatively by nitrogen adsorption−desorption measurements (Figure 2E). The HPC particles were found to have a Brunauer−Emmett− Teller (BET) surface area of 807 m2/g, and the micropores turned out to account for ∼39% of the total pore volume (0.93 cm3/g). While the hysteresis between the adsorption and desorption curves in the isotherm is the first indication of the mesopore formation, the pore size distribution (PSD) obtained from the desorption branch using the density functional theory (DFT) method provided a detailed statistics on the pore C

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Figure 3. Characterization of the HPC-S composite. (A) TEM and (B) STEM images of a HPC-S sphere showing that the inner meso- and macropores are completely filled with sulfur. (C) The corresponding line-scans of sulfur and carbon across the composite sphere (red line in B). (D) The sulfur and carbon elemental mapping showing that the infiltrated sulfur exists mainly in the inner meso- and macropores and only a minor amount of sulfur is left in the outer microporous carbon shell layer. (E) TGA curve of the HPC-S composite under N2 at a heating rate of 5 °C/min. The curve indicates that the sulfur content is 46 wt %.

volume of the inner meso- and macropores (0.57 cm3/g) as well as the diminished sulfur concentration in the outer shell (Figure 3C,D), it is anticipated that most of molten sulfur was sucked thoroughly into the inner pores via capillary force during the infiltration step. CR2032 coin-type cells were fabricated to examine the electrochemical performance of HPC-S, and all of the specific capacities were calculated based on the mass of sulfur only. Figure 4A displays the first discharge−charge voltage profiles of HPC-S and bare elemental sulfur. One of the most noticeable features in these profiles is that HPC-S exhibited two distinct discharging (= lithiation) plateaus at 2.3 and 2.1 V, which is in contrast with the result of the reported microporous carbonbased electrode that showed only one lower plateau at 1.85 V.25−27 At a quick glance, the outer micropores in HPC-S are also expected to constitute a reaction-front in the lithiation and thus give rise to a similar single plateau behavior. However, as evidenced by the elemental mapping in Figure 3C,D, the sulfur did not seem to be located in most of the outer microporous region in our HPC-S. Hence, the sulfur in the inner meso- and macropores accounts for most of the active sulfur and, as a result, showed the observed two discharging plateau behavior. In the overall electrode structure viewpoint, the outer micropores serve as a barricade against the long-chain lithium polysulfides dissolution, as the microporous carbon interlayers did in the Manthiram group’s works.29,35 It is also notable that poly(acrylic acid) (PAA) binder, instead of commonly used poly(vinylene difluoride) (PVDF) binder, was chosen in the current study based on better kinetics of PAA reported in the recent literature,36 which is indeed confirmed in our characterization (Supporting Information Figure S5).

dimensions (Figure 2F), not to mention the hierarchical pore generation. The HPC-S particles were completed by infiltration of elemental sulfur into the HPC particles via capillary reaction of molten sulfur at 155 °C. The disappeared brightness in the TEM image that was assigned to the meso- and macro-pores was the first evidence of the sulfur infiltration (Figure 3A). The HPC-S particles preserved the original spherical morphologies without leaving any bulk elemental sulfur (Supporting Information Figure S3A). An energy-dispersive X-ray (EDX) analysis on the HPC-S composite suggested a sulfur content of 47.3 wt % (Supporting Information Figure S3B). The elemental line and 2D mappings exhibited coherent results. The line scan along the HPC-S particle whose scanning transmission electron microscopy (STEM) image is shown in Figure 3B exhibited a pronounced sulfur peak throughout the entire core region, whereas the relatively strong carbon peaks were observed around the outermost regions (Figure 3C), which is attributed to the dominant micropore formation in these regions. The 2D mapping of carbon and sulfur showed consistent results (Figure 3D), such as the complete infiltration of sulfur and the concentrated carbon around the outermost region. In addition, a separate thermal gravimetric analysis (TGA) indicates that the sulfur content in the HPC-S is 46 wt %. Although the maximum available sulfur content from the given pore volume is 66 wt % based on the total pore volume of HPC (0.93 cm3/g) and the density of elemental sulfur (2.07 g/cm), we introduced the sulfur amount corresponding to 50 wt % to ensure the void micropores in the shell layer as well as to give extra void space with the purpose of accommodating the volume expansion of sulfur. From the final sulfur content that is a little smaller than the value (54 wt %) achievable based on the D

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Figure 4. (A) Discharge/charge voltage profiles of the HPC-S and bare S electrodes at a 0.06C rate (1C = 1675 mAh/g). (B) Comparative cycling performance of HPC-S and AC1600-S when measured at 0.3C. Selected discharge/charge voltage profiles of (C) HPC-S and (D) AC1600-S. (E) Discharge/charge capacities of HPC-S and (F) the corresponding discharge/charge voltage profiles, when the electrode loading increased by 2.5 times to 2.5 mg/cm2. Both discharge/charge were measured at a rate of 2 A/g. (G) Long-term cycling performance and Coulombic efficiencies of HPC-S at 2.4C. (Inset) The SEM image and corresponding elemental mapping of a HPC-S particle after the 500 cycles whose data is shown in this figure.

To compare the electrochemical stability of HPC-S with the conventional activated carbon (AC1600, specific surface area = 1600 m2/g) containing micropores and mesopores in a random geometry (Supporting Information Figure S6), AC1600-sulfur composite (denoted as AC1600-S) was prepared by the same procedure. Also, the sulfur content (∼48 wt %) in AC1600-S was controlled to be almost the same as that (∼46 wt %) of HPC-S for direct comparison. The detailed analyses of AC1600-S using SEM, TEM, and XRD are presented in Supporting Information Figure S7. This series of data indicate that the sulfur was infiltrated in the pores of AC1600 under the amorphous phase. When measured at 0.3C (1C = 1675 mAh/ g), HPC-S retained 90% (= 899 mAh/g) of the initial capacity after 100 cycles. The average CE during this cycling period was

The Coulombic efficiency (CE) defined by charging capacity/discharging capacity was also distinct between both samples in the first cycles. HPC-S and bare S exhibited the first CEs of 98% and 111%, respectively. While the CE of HPC-S close to 100% indicates highly reversible character of HPC-S by assistance of the outer micropores that suppress the dissolution of lithium polysulfides, the CE of bare S surpassing 100% is reflective of the shuttling process that is accompanied by unwanted side reactions including those at the Li metal anode.37 In the case of bare S, the much shorter lowdischarging plateau (∼2.1 V) as compared to that of the HPC-S is a direct indication of the serious dissolution of the long-chain lithium polysulfides even in the early period of the first discharge. E

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Figure 5. (A) Schematic illustration showing the formation of HPC-S400 and its electrochemical process. HPC-S400 was prepared by an additional heat-treatment process of HPC-S at 400 °C to infiltrate small sulfur (S2−4) into the outer micropores. (B) The TEM image and (C) the sulfur and carbon elemental mapping of HPC-S400 showing that the sulfur existing in inner meso- and macropores migrated to the outer micropores. (D) The first discharge/charge voltage profiles of HPC-S400 at 0.05C (1C = 1675 mAh/g) and (E) its cycling performance at 0.2C.

from the abrupt current rate increase from the precycle to the subsequent one (0.1 A/g → 4 A/g). Such capacity fading upon an abrupt current density increase was generally observed in Li−S cells39 perhaps reflecting structural and interfacial stabilization of the sulfur electrode or a small amount of residual sulfur on the carbon surface40,41 but requires further investigation. The average CE in the cycling range of 2−500 was 99.1%. The SEM and EDX elemental analyses of HPC-S after the same 500 cycles also support the robust cycling performance (Figure 4G inset). It was observed that HPC-S preserved the consistent spherical morphology after the cycling, and sulfur was detected only in the confined region inside the particles, reflecting the suppressed dissolution of soluble lithium polysulfides. It would be instructive to see the electrochemical performance when sulfur is loaded only in the outer micropores of HPC because it is not fully guaranteed that the outer micropores in our HPC-S remain void throughout the cycling (Figure 5A). For this purpose, a control sample was prepared by heat-treating HPC-S at 400 °C for 5 h in a sealed glass container. Hereafter, we denote this control sample as HPCS400. During the heat-treatment, the S8 in the inner macro- and mesopores migrates to the outer micropores while undergoing phase change to smaller sulfur (S2−4). The detailed sulfur infiltration process of HPC-S400 is described in Supporting Information Figure S9. From TEM and element mapping analyses (Figure 5B,C), it was found that the brightness for the inner macro- and mesopores was darkened compared with that of HPC-S (Figure 3A,B), indicative of emptying of those pores

99.5%. By contrast, the capacity retention of AC1600-S was only 64% after the same number of cycles (Figure 4B), implying that AC1600-S even with a good portion of micropores still allows for dissolution of the long-chain lithium polysulfides. The voltage profiles of HPC-S were persistent throughout the entire cycling period (Figure 4C) in contrast against those of AC1600-S (Figure 4D). When the discharge capacities from the upper plateaus over cycling were compared (Supporting Information Figure S8) because these plateaus are directly related to the formation of the soluble long-chain lithium polysulfides,38 those for HPC-S were maintained better than those of AC1600-S. Although the excellent capacity retention of HPC-S could be attributed partially to the moderate mass loading of the electrode (∼1 mg/cm2), the superior performance of HPC-S compared to that of AC1600-S under the same condition verifies the critical role of the HPC structure in the cycling performance. Even when the electrode loading increased by 2.5 times (1 → 2.5 mg/cm2), HPC-S retained 91% of the original capacity after 100 cycles at 2 A/g (Figure 4E,F), reconfirming the superior structural advantage of HPC. HPC-S preserved 539 mAh/g even after the prolonged cycling period of 500 cycles measured at 2.4C (Figure 4G), which corresponds to 77% capacity retention with respect to its capacity in the fifth cycle, validating the structural stability of the present hierarchical porous structure in the commercial level of lifetime testing. The capacity at the fifth cycle was set to the standard value for the capacity retention evaluation to compensate the capacity drop in the first five cycles originating F

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carbonate (EC)/diethyl carbonate (DEC), v/v = 1:1) was used (Supporting Information Figure S13). This commercial electrolyte did not contain any additive such as lithium nitrate (LiNO3). This result confirms again that the small sulfur confined in the micropores is resistant against dissolution into the liquid electrolyte by utilizing the shortened sulfur chain lengths. Thus, it can be concluded that whether a certain portion of the outer micropores in the HPC-S become filled with sulfur or not during the cycling, the unique electrode structure is able to facilitate good electrochemical performance. In conclusion, we have achieved highly stable cycling of Li−S batteries by employing the HPC structure prepared by ultrahigh speed spray pyrolysis. The micropores in the outer shell have played a critical role in achieving the good electrochemical performance, as they inhibit the fatal dissolution of the lithium polysulfides into the electrolyte. A control experiment also indicates that regardless of the presence of sulfur in the outer micropores, the micropores consistently serve as a barricade against the lithium polysulfide dissolution and consequently facilitate robust cycling. The current investigation offers a new design principle in development of high capacity sulfur electrodes that can be especially fabricated based on scalable synthetic procedures.

by the sulfur migration. Instead, the colors for the outer shell and interconnected walls were enhanced due to the sulfur infiltration into these regions. The elemental line scan also shows a consistent sulfur distribution (Supporting Information Figure S10A) such that the sulfur signal over the microporous regions, such as the outer shell and the walls between the inner meso- and macropores, was markedly enhanced. This structural change was indeed reflected in XRD and TGA results. After the heat-treatment, the XRD peaks assigned to the orthorhombic phase of sulfur present in the inner large pores disappeared, and only the amorphous phase was observed (Supporting Information Figure S10B). This phase transformation can be explained by the crystal structure change during the annealing process. At such high temperature, the S8 ring tends to break into shorter sulfur chains (S2−4), but recovers the original S8 ring structure after typical cooling to room temperature because the S8 ring structure is thermodynamically more stable.42 However, the smaller S2−4 could be stabilized in the micropores due to the limited pore dimensions for accommodation of the large S8 ring structure, as in the previous literature.25 The small domain sizes originating from the pore sizes could also contribute to the amorphous XRD spectrum. As the sulfur was stuck in the micropores, its onset temperature for evaporation was affected. According to the TGA profiles (Supporting Information Figure S10C), HPC-S and HPC-S400 exhibited the onset temperatures at 180 and 260 °C, respectively, suggesting that the sulfur confined in the micropores requires more energy for evaporation, perhaps due to enhanced binding with the pore walls through C−S bonds28,41,43,44 that could be formed during the heat treatment. The formation of the C−S bonds was suggested by X-ray photoelectron spectroscopy (XPS) data (Supporting Information Figure S11). Additionally, the surface area and pore volume originating from the micropores completely disappeared after the heat-treatment (Supporting Information Figure S12), reconfirming the sulfur infiltration into the micropores. The sulfur content in HPC-S400 turned out to be 42 wt %, which is consistent with the hypothetical value when the given micropores are completely filled with sulfur. For this calculation, the micropore volume of 0.36 cm3/g was used. When galvanostatically tested, HPC-S400 showed a single plateau in each charge and discharge (Figure 5D). In particular, during the discharge HPC-S400 did not show the high-voltage plateau near 2.3 V. This behavior is attributed to the small sulfur that is prevented from forming the long-chain lithium polysulfides and is indeed consistent with the previous observation obtained from the similar microporous samples.25,26 As can be expected from the absence of the longchain lithium polysulfides, HPC-S400 exhibited excellent cycling performance (Figure 5E). It retained 83% of the initial capacity after 200 cycles. Also, HPC-S400 exhibited smaller gravimetric capacities than those of HPC-S and other typical sulfur electrodes. While further in-depth investigation is required for compelling explanation, it is speculated that when sulfur exists in short chain forms, the stoichiometric Li reaction with sulfur (Li/S = 2:1) to reach the theoretical specific capacity is not available perhaps due to the existing bonding to neighboring carbon, and only a smaller amount (1 < Li/S < 1.5) of Li is allowed to form the bonds with sulfur in the given chain in a reversible manner. Interestingly, HPC-S400 maintained excellent electrochemical performance as well as the single-plateau voltage profiles even when a commercial carbonate-based electrolyte (lithium hexafluorophosphate (LiPF6) in electrolyte

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ASSOCIATED CONTENT

S Supporting Information *

Detailed spray pyrolysis set up, further characterization of HPC, HPC-S, HPC-S400, and AC1600-S. More battery results of the electrode materials. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +82-42-350-1719. Fax: +82-42-350-2248. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.W.C. acknowledges the financial support by the National Research Foundation of Korea (NRF) Grant funded by the Korea government (MEST) (NRF-2010-C1AAA001-0029031, NRF-2012-R1A2A1A01011970, and NRF2014R1A4A1003712) and the Qatar National Research Fund (NPRP Grant 5-569-2−232).

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