Hydrogen storage and spillover kinetics in carbon nanotube-Mg composites

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Short Communication

Hydrogen storage and spillover kinetics in carbon nanotube-Mg composites Efrat Ruse a,b,*, Svetlana Pevzner a, Ilan Pri Bar b, Roey Nadiv b, Vladimir M. Skripnyuk c, Eugen Rabkin c, Oren Regev b,d,** a

Department of Chemistry, Nuclear Research Center Negev, P.O.B.9001, 84190 Beer Sheva, Israel Department of Chemical Engineering, Ben-Gurion University of the Negev, 84105 Beer Sheva, Israel c Department of Materials Science & Engineering, Technion e Israel Institute of Technology, 32000 Haifa, Israel d Ilse Katz Institute for Nanoscale Science and Technology, Beer-Sheva 84105, Israel b

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abstract

Article history:

We explored the hydrogen storage kinetics of Pd-Mg composites upon addition of different

Received 3 December 2015

carbonaceous spillover agents (activated carbon and a wide spectrum of carbon nanotube

Accepted 5 December 2015

types). We found that the hydrogen (loading or release) kinetics is strongly dependent on

Available online xxx

the nanocarbon morphology and configuration (e.g., length, diameter and Pd distribution). We therefore define a figure of merit quantifying the de/hydriding performance of previ-

Keywords:

ously reported systems and the system investigated in the present study. It demonstrates

Hydrogen

that the fastest kinetics is obtained for our Pd-decorated carbon nanotubes having the

Spillover

largest diameter. We found a clear structure-function relation between the spillover agent

Storage

properties and the Mg de/hydriding rates, which could be applied in replacing the heavy

Magnesium

and expensive transition metal catalyst by lightweight nanocarbon additive.

Carbon nanotube

Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Hydrogen storage is essential for applications related to hydrogen energy economy. Although metal hydrides have high hydrogen storage efficiency [1e4], no metal hydridebased system has met the USA Department of Energy (DOE) requirements for a vehicular on-board hydrogen storage system [2]. This is mostly related to the slow hydriding and dehydriding kinetics of metallic hydrogen acceptors, which dictate high operation temperatures.

Magnesium is one of the most investigated materials for hydrogen storage applications [3,5e9]; it has a very high hydrogen capacity (7.7 wt% and 101 gH2/L) while being relatively cheap. However, this metallic hydrogen acceptor has a poor de/hydriding kinetics and requires temperatures above 300
C to obtain a hydrogen plateau pressure of around 0.1 MPa [4,5,8,10e14], which does not meet the DOE requirements [2]. Attempts to enhance Mg de/hydriding kinetics have involved Mg particle-size reduction [5e7,15e19], and loading of heavy transition metals [14,20e23] or metal oxides [24]. For example, Kimura et al. [25] reported that Mg catalyzed by Nb2O5 absorbs

* Corresponding author. Department of Chemistry, Nuclear Research Center Negev, P.O.B.9001, 84190 Beer Sheva, Israel. ** Corresponding author. Department of Chemical Engineering, Ben-Gurion University of the Negev, 84105 Beer Sheva, Israel. E-mail addresses: [email protected] (E. Ruse), [email protected] (O. Regev). http://dx.doi.org/10.1016/j.ijhydene.2015.12.017 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Ruse E, et al., Hydrogen storage and spillover kinetics in carbon nanotube-Mg composites, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.12.017

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Table 1 e Summary of CNT properties. CT>50a

Fig. 1 e Illustration of hydrogen atoms path during Mg de/ hydriding in composites containing two types of additives: Pd-supported activated carbon (Pd/AC) and Pd-decorated CNT (Pd/CNT). First, H2 dissociates on the Pd surface (1). Then, H atoms spill onto the carbonaceous allotrope (2) towards the Mg bulk (3). The obtained H atoms flux, penetrating the Mg bulk, is controlled by the H atoms spillover rate. This path is reversible and may describe the dehydriding reaction, as well. H2 even at 50
C. Moreover, loading the Mg by light carbonaceous additives increases the concentration of hydrogen adsorption sites, and promotes the nucleation of a hydride phase. It also reduces the composite material weight, compared to Mg loaded with heavy-metal catalysts [6,7,9,26e29]. Carbonaceous materials, such as carbon nanotubes (CNT) may assist in hydrogen atom diffusion in solid mixtures through spillover [11,15,30e33]; this phenomenon describes the transport of H atoms, adsorbed on one surface (e.g., metal catalyst) onto another surface (Fig. 1). The carbonaceous allotropes are considered as promising spillover agents for improving hydrogen transport rates [11,31,32], and therefore may tune the Mg de/hydriding kinetics. We have previously demonstrated hydrogen spillover behavior in an irreversible hydrogenation system (1,2Diphenyl Acetylene) [33], where we analyzed the hydrogenation kinetics in the presence of spillover agents such as carbon

Outer diameter [nm]b Length [mm]b Carbon purity [wt%]b Specific surface area [m2/g]b Impurities [wt%]b Defect density (Raman ID/IG)c a b c

50e80 10e20 95 60

CT20 e30a 20e30 10e30 95 110

NC7000a 9.5 1.5 90 250e300

2.4 (Fe, Ni, S) 1.2 (Ni, Fe) 10 (metal oxides) 0.5 0.89 1.2

For CNTs nomenclature see materials section. Supplier data. Measured by Raman spectroscopy.

allotropes (nanotubes, activated carbon and graphene) and a carbon supported metal catalyst. We found that the hydrogenation kinetics is directly affected by the concentration of the carbon allotropes, and could therefore be discussed in terms of hydrogen spillover [33]. In the present study, we extend the model to a reversible, Mg-based system (Fig. 1), and compare the efficiency of various types of spillover agents at the same metal catalyst concentration. Here, the carbonaceous materials and the metallic catalysts are integrated into the Mg bulk, forming a composite.

Materials and methods Materials Palladium on activated carbon (AC) e 5 wt% Pd (Pd/AC), ethylene glycol (107-21-1) (EG), N-methyl pyrolydinon 99% (872-50-4) (NMP), potassium tetrachloropalladate (10025-98-6)

Fig. 2 e Mg-based composites hydriding and dehydriding kinetics: (a) t50 is the time required to reach theoretical 50% of MgH2 full hydrogen storage capacity (t50 hydriding), as demonstrated for a composite containing 95 wt% Mg and 5 wt% Pd/CNT ([Pd] ¼ 0.25 wt%; CNT type: CT>50). In this figure, t50 ¼ 0.55 min; Inset: illustration of the obtained kinetic vector for Pd/CT>50; (b) Time required for 50% MgH2 dehydriding (t50) versus t50 of Mg hydriding for several composites containing Pd/CNT- and Pd/AC-based additives. T ¼ 300
C. [Pd] ¼ 0.25 wt%. For CNT specifications see Table 1. Please cite this article in press as: Ruse E, et al., Hydrogen storage and spillover kinetics in carbon nanotube-Mg composites, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.12.017

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Fig. 3 e Pd/CNTs morphologies: (a) Pd particles deposited onto CT>50, TEM (b) Pd/NC7000 agglomerate, TEM. Both images (a) and (b) were taken prior to milling and de/hydriding procedures; (c) Pd/CT20-30, post milling and de/hydriding, and after dissolving the Mg bulk, TEM; (d) Mg matrix post milling and de/hydriding incorporated with a few Pd/CT>50, SEM; (e) Pd/ CT>50 network (arrows), SEM and (f) discrete Pd/NC7000 agglomerate on Mg matrix, SEM. For CNT specifications see Table 1.

(Sigma Aldrich). Multiwall CNTs: NC7000 (Nanocyl), CT2030 nm and CT>50 nm (Cheaptubes) (Table 1). Magnesium: Dead Sea Magnesium Ltd, ASTM-9980A; 99.8 wt% purity. All the materials are used as received.

Methods Pd-decorated CNTs (Pd/CNT) are synthesized by dissolution of Potassium tetrachloropalladate (20 mg) in DI water (2 ml) and EG (5 ml), followed by 5 min stirring at ambient temperature. This solution is added to a pre-mixed mixture of pristine CNT (35 mg) and NMP (18 ml), and stirred for an additional 15 min. Then, the mixture is microwaved (90 W) intermittently (10 s pulses separated by 60 s intervals) until a temperature of 140
C is reached. The mixture is centrifuged (20 min, 4000 rpm),

filtered (Fluoropore membrane, Merck Millipore type GVWP, 0.22 mm), rinsed (75 ml DI) and dried overnight at 70
C. The Pd nanoparticles yield is 90 wt% of the initial Pd weight (thermogravimetric analysis (TGA) e Mettler Toledo Star, STDA85). The Pd nanoparticles on the CNT and the composites are analyzed by transmission electron microscope (TEM e T1230 JEOL) and scanning electron microscope (SEM e JSM-7400F, JEOL). Defect density of the CNT is compared by measuring the D-to-G intensity ratio (1360 and 1560 cm1, respectively) in Raman spectrum [35] (Jobin-Yvon HR LabrRam micro-Raman operated at 514 nm). Composites are prepared by milling Mg powder (18 h at 800 rpm) in a planetary ball micro-mill (Fritsch Pulverisette 7, hexane environment) using a stainless-steel bowl and balls (balls to powder weight ratio is 20:1). The Pd/AC or Pd/CNT

Please cite this article in press as: Ruse E, et al., Hydrogen storage and spillover kinetics in carbon nanotube-Mg composites, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.12.017

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Fig. 4 e The figure of merit (Eq. (1)) of the kinetics of catalyzed de/hydriding for various carbonaceous additives loaded into the Mg powder at T ¼ 300
C. Two types of carbonaceous additives were studied: CNTs (CT>50, CT2030 and NC7000 e black) and AC-containing composites (gray). The Pd concentration is in the range of 0.25e0.8 wt%. The catalyst concentrations in Refs. [38,39] are 4 wt% and 5 wt%, respectively (white).

additives are then loaded into the Mg powder for an additional 30 min of milling.

Results and discussion The reaction rates of the Mg de/hydriding (Sieverts-type at 300
C) are normalized to a single, easily comparable kinetic variable, t50, which represents the time required to reach or deplete 50% of the theoretical full Mg hydrogen storage capacity (Fig. 2). We compared the kinetic behavior of Mg-based composites loaded by two types of carbonaceous additives, namely, Pd/AC and Pd/CNT, where three types of CNTs were used as a support for Pd nanoparticles or as an additive to Pd/AC. The added Pd/CNT reduces the values of the obtained t50 for Mg hydriding and dehydriding by more than two and one orders of magnitude, respectively (Fig. 2). The CNT morphology significantly affects the kinetics: The CT20-30 and CT>50 CNTs yield the fastest de/hydriding kinetics despite their relatively low specific surface area (Table 1). In the followings, we study structural parameters of the CNT, the Pd nanoparticles, and their interaction. Our synthesized Pd/CNT are decorated by spherical Pd nanoparticles (<40 nm) deposited over the surfaces of all the CNT types, (Fig. 3; Fig. S1 in the supplementary data). The CT>50 and CT20-30 CNTs are longer, thicker and have lower defect densities compared to the NC7000 (Table 1). The latter also forms entangled agglomerates (Fig. 3b). The Pd/CT were not dramatically damaged by the milling and the de/hydriding procedures (Fig. 3c), and may be used in further de/hydriding cycles. The CNT function both as a catalyst support (Fig. 3d) and as a hydrogen spillover agent at the Mg matrix. A uniform Pd/CT>50 network is formed in the Mg after the milling and

the de/hydriding procedures (Fig. 3e), unlike the dense agglomeration of the Pd/NC7000 in the Mg matrix (Fig. 3f). The morphology and moreover, the dispersion state of the CNT (i.e., spillover agent) determine their efficiency: The Mg dehydriding rate is increased by over an order of magnitude (Fig. 2b) compared to pure Mg by loading a uniform CNT network (CT>50; Fig. 3e and CT20-30; not shown). However, with an entangled CNT state (NC7000; Fig. 3f), the effect on the rate is minor (a factor of 2; Fig. 2b), clearly pointing at hydrogen spillover mechanism. The penetration of Pd/CT>50 additive into the Mg is important and significantly affects its kinetic performance by promoting hydrogen spillover from the Pd catalyst to remote Mg particles (ranging in size from ~30 nm [36] to agglomerates of a few mm (Fig. 3f)). High outer diameter and low defect density (Table 1) of the CT>50 CNT enable a superior Mg de/hydriding kinetics compared to all nanocarbon additives studied. As expected, by increasing the Pd concentration in the Pd/ CNT additive, one could further enhance the Mg de/hydriding kinetics, reflected by the decrease in t50 (Fig. S2 in the supplementary data). Our results (Fig. 2) indicate that the loading of a nanometric catalyst support, Pd/CT>50, demonstrates accelerated de/ hydriding kinetics (a factor of 3.5) over the micron-sized Pd/ AC, in spite of the smaller Pd nanoparticles of the latter (2e5 nm [34]). The integration of pristine CNTs into Pd/AC additive (AC surface area: 780 m2/g) does not dramatically affect the reaction kinetics, suggesting that a spillover between different carbonaceous allotropes is a rate-limiting step in the Mg system, similar to the case of the irreversible hydrogenation reaction [33]. In order to quantify the kinetic improvement in both the hydriding and the dehydriding kinetics of each Mg-based composite, we employ a figure of merit (FOM, Eq. (1); Fig. 4) e the ratio between the de/hydriding rates and the catalyst concentration in the composite. The reactions rates are calculated as the inverse length of the {t50(hydriding), normalized t50(dehydriding) [37]} kinetic vector (Fig. 2a inset). sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi   ffi!1 ðt50 ðhydridingÞÞ þ t50 ðdehydridingÞ 100

2

2

FOM ¼

½Metal Catalyst

(1)

The FOM may be used as a simple and facile comparative tool for evaluating the kinetic dependence on various nanocarbon additives and catalyst concentrations in the hydrogen storage systems. The FOM values are strongly dependent on the processing route and the measurement method, and therefore a comparison with other studies is not straightforward. Nevertheless, similar experimental conditions (e.g., milling duration or de/hydriding temperature) make it possible to compare the kinetics of Mg de/hydriding with previously reported studies (Fig. 4). Higher values of FOM indicate faster de/hydriding rates and/or lower metal catalyst concentrations (e.g., Pd, La etc.). Pd/CNT additive with a [Pd] ¼ 0.25 wt% provides the best kinetic effect (FOM ¼ 5.26). For the sake of comparison, we calculated the best FOMs, based on the previously reported Mg de/hydriding rates under similar de/hydriding conditions: 0.55

Please cite this article in press as: Ruse E, et al., Hydrogen storage and spillover kinetics in carbon nanotube-Mg composites, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.12.017

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for Pd/CNT additive (4 wt% Pd) [38] and 0.06 for 5 wt% LaNi5 loading [39] (Fig. 4). The correlation between CNT morphology and Mg de/ hydriding kinetics is applicable to the replacement of heavy and expensive transition metal catalyst metals by lightweight nanocarbons additives. The FOM analysis indicates that by manipulating the CNT type (i.e., NC7000, CT20-30 or CT>50) at a constant Pd concentration, the de/hydriding rates may be substantially accelerated (a factor of 5). Though some acceleration may be achieved when increasing the Pd concentration in a Pd/CNT additive, this is definitely not as efficient as manipulating the CNT type. For example, increasing [Pd] from 0.25 wt% to 0.8 wt% in the CT20-30 CNT composite maintains the same FOM, while changing the CNT type to CT>50 (lowest defect density and higher outer diameter) at [Pd] ¼ 0.25 wt%, doubles the FOM. In other words, we find that the type of the spillover agent is more important than the concentration of the metal catalyst.

Conclusions We have shown that nanometric Pd/CNT additives are superior to micron-sized Pd/AC ones in accelerating Mg de/ hydriding kinetics. The CNT properties and the Pd concentrations may be used for tuning the kinetics when nanocarbons are integrated into Mg-based composites. We found that acceleration of the Mg de/hydriding kinetics is optimized by addition of well dispersed carbonaceous spillover agents in the Mg matrix (i.e., longer and thicker CNT with lower defect density values). Our developed FOM is an effective mean for comparing different nanocarbon-based composites, which may be used to evaluate the net effect of the spillover agent type on de/ hydriding rates in various hydrogen acceptors (Mg-based and others). The FOM emphasizes the significant effects of our synthesized nanostructures (Pd/CNT). These findings are applicable as an alternative to expensive metal catalysts in light-metal hydrogen storage devices and fuel cells.

Acknowledgments This research was carried in the framework of the Energy Initiative Program and supported by the Adelis Foundation for renewable energy research and was partially performed in the framework of Grand Technion Energy Program (GTEP).

Appendix A. Supplementary data Two figures are included, describing the Pd particle-size distribution of different types of Pd/CNTs, and Mg hydriding and dehydriding rates as a function of Pd concentration of various carbonaceous additives to Mg composite. This material is available online via the internet at http://www.sciencedirect.com. Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2015.12.017.

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