Graphene supported bimetallic G–Co–Pt nanohybrid catalyst for enhanced and cost effective hydrogen generation

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Graphene supported bimetallic GeCoePt nanohybrid catalyst for enhanced and cost effective hydrogen generation Shubhanwita Saha a, Vaswar Basak a, Anish Dasgupta a, Saibal Ganguly b, Dipali Banerjee c, Kajari Kargupta a,* a

Chemical Engineering Department, Jadavpur University, Kolkata 700032, India Chemical Engineering Department, Universiti Teknologi Petronas, Malaysia c Department of Physics, Bengal Engineering and Science University, Shibpur, India b

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abstract

Article history:

A highly active and stable bimetallic nano-hybrid catalyst GrapheneeCobaltePlatinum

Received 5 April 2014

(GeCoePt) is proposed for the enhanced and cost effective generation of hydrogen from

Received in revised form

Sodium Borohydride. Three different nano-hybrid catalysts namely GrapheneeCobalt (G

18 May 2014

eCo), GrapheneePlatinum (GePt) and GrapheneeCobaltePlatinum (GeCoePt) are syn-

Accepted 19 May 2014

thesized, characterized using XRD, FTIR, SEM, HRTEM, EDAX and Cyclic voltammetry

Available online 20 June 2014

(CV) analysis and tested for hydrogen generation. The activity and stability of the catalysts are analyzed by estimating the turnover frequency (TOF), the electrochemically

Keywords:

active surface area (ECSA), the percentage decay of current density over ten cycles of CV

Graphene supported bimetallic

and the decay in the rate of hydrogen generation with the age of catalyst. Among the

catalyst

three catalysts GeCoePt exhibits the highest catalytic activity (TOF ¼ 107 min1,

Grapheneecobalteplatinum cata-

ECSA ¼ 75.32 m2/gm) and stability. The evaluated value of activation energy of the

lyst

catalytic hydrolysis using GeCoePt is 16 ± 2 kJ mol1.

Nanohybrid

Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Hydrogen generation NaBH4

Introduction Due to the increasing global demand of affordable, environmentally sustainable energy, the production of clean and renewable Hydrogen has received much attention. Hydrogen is considered to be one of the most promising alternative to traditional fuels (oil, coal, and natural gas) since it can be used directly in proton exchange fuel cells (PEMFC) and in internal combustion engines. The use of Hydrogen fuel cells in vehicles

or in portable electronic devices requires “on-board” Hydrogen generation or lightweight H2 storage. In particular, for portable Hydrogen generation systems, the most important issues are safety, ease to control, high release rate of Hydrogen and high Hydrogen storage capacity. There have been reports on Hydrogen generation using ethanol steam reforming [1], water electrolysis [2], hydrolysis of Hydrazine Borane [3]. During the recent past, the hydrolysis of chemical hydrides has gained renewed interest as a solution for onboard Hydrogen storage and generation [4,5]. Among

* Corresponding author. Tel.: þ91 9748829349; fax: þ91 33 2414 6378. E-mail addresses: [email protected], [email protected] (K. Kargupta). http://dx.doi.org/10.1016/j.ijhydene.2014.05.131 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

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hydrides, Sodium Borohydride (NaBH4) has gained much attention due to its environmentally benevolent nature, high Hydrogen content (10.8 wt%), easy availability and acceptable Hydrogen generation rate even at low temperature using low cost catalyst [6]. Schlesinger et al. [7] established that Hydrolysis of 1 atom of NaBH4 produces 4 atoms of Hydrogen gas and watersoluble Sodium meta borate (NaBO2), in the presence of appropriate catalyst according to the following stoichiometric equation: NaBH4 þ 2H2 O/4H2 þ NaBO2 þ Heat ð300 KJÞ

(1)

The use of catalyst plays a key role in controlling Hydrogen generation from the NaBH4 solution. The hydrolysis reaction rate can be controlled by supported catalysts of some precious noble metals such as Pt [8], Ru [9] and PtRu alloy [10]. In earlier € study, S. Ozkar et al. [11] established Ruthenium(0) nanocluster to be a highly active catalyst for the hydrolysis of Sodium Borohydride. However, high cost of the precious catalysts confines their broad industrial application. Some low-cost metals and alloys, including Mn, Cr, Fe, Cu, Al [12], Nickel and Cobalt [13], Nickel Boride [14] and Cobalt Boride [15] have also been reported to accelerate the hydrolysis of NaBH4 solution to generate pure Hydrogen. However, the activities of these low cost catalysts are low compared to the precious catalysts due to their metallic properties. Consequently, to improve its activity and to reduce the cost, it is needed to synthesize high-quality nano-hybrids which contain both high and low cost metals in rational proportion dispersed on to a support. In a recent review [16], it is reported that polymetallic heterogeneous catalysts, mainly consisting of bimetallic metal nanoparticles supported on inorganic oxides, allow, in principle, to obtain high performances (increased activity, and selectivity to desired products, extended lifetime, high resistance to poisoning/coke deposition/sintering/etc.) due to the positive (surface and structure) effects arising from the interactions between the (two) metals. The particle size and degree of dispersion are the crucial factors for enhancement of catalytic activity. Small particle size and improved dispersion allow satisfactory contact of reactant with the catalysts, which is essential for enhancement of the reaction rate and hence allow for lower catalyst loading. In this context, catalyst support materials having high surface to volume ratio and high electrical conductivity could be introduced to disperse the metallic nanohybrids. Modified Carbon materials such as activated carbon, carbon nanofibers, and carbon nanotubes have potential for synthesis of carbon based catalysts. Dongyan Xu et al. [17]. used platinum-active carbon based catalysts, (carbon black, and alumina supports) for enhanced Hydrogen generation from catalytic hydrolysis of aqueous NaOH-stabilized Sodium Borohydride solutions. Graphene, which is a 2D single layer of carbon atoms with the hexagonal packed structure, has emerged as a promising option. In Graphene structure, carbon bonds are sp2 hybridized, where the in-plane s CeC bond is one of the strongest bonds in materials and the out-of-plane p bond that contributes to a delocalized network of electrons, is responsible for the electron conduction of Graphene and provides the weak interaction among Graphene layers or

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between Graphene and substrate [18e25]. Graphene has a large theoretical specific surface area (2630 m2 g1), high intrinsic mobility (200,000 cm2 v1 s1), high Young's modulus (~1.0 TPa) [25], high thermal conductivity (~5000 Wm1 K1) [26,27] and good electrical conductivity [28,29]. Owing to above properties Graphene has been considered as an ideal catalyst support material for dispersion of inorganic nanoparticles, thus forming novel nanohybrid catalysts. Recently, Liu et al. [30] reported the Graphene supported Platinum nanoparticles as anode electro catalyst for the application of direct Borohydride fuel cell. Fei Zhang et al. [31]. developed Graphene sheets/Cobalt (GRs/Co) nanocomposites for enhancement of the production of clean and renewable Hydrogen through the hydrolysis of Sodium Borohydride. In recent times, Mondal et al. [32]. synthesized graphene based copper nanoparticles for catalytic enhancement of electrode. Very newly, some group of researcher explored bi-metallic catalyst for hydrogen generation. Wenqi Feng et al. [33] prepared graphene supported Co/Ni bi-metallic catalyst for dehydrogenation of ammonium borane. Lan Yang et al. [34] synthesized trimetallic Ag-based coreeshell nanoparticles toward hydrolytic dehydrogenation of amine boranes. It was found that the highly conductive surface and increased surface area of the Graphene support enhanced the rate of hydrogen generation. In this paper a novel nanohybrid, namely Graphene supported CobaltePlatinum (GeCoePt) bimetallic catalyst is proposed for enhanced dehydrogenation of NaBH4. Noble bimetallic nanohybrid GeCoePt and two other noble monometallic nanohybrids, GrapheneeCobalt (GeCo) and GrapheneePlatinum (GePt) were synthesized using chemical routes and characterized. The sizes and surface to volume ratios of Pt and Co grains were estimated using HRTEM and XRD analysis. Kinetic studies of Hydrogen generation from hydrolysis of NaBH4, using different nanohybrid catalysts were performed. Catalyst activities of different nanohybrids were investigated based on electrochemically active surface area (ECSA)value and turnover frequency (TOF) value. The deactivation of catalysts over time was studied experimentally.

Experimental Materials Graphite, Chloroplatinic Acid (H2PtCl6.6H2O) and Cobalt Nitrate hexahydrate (Co(NO3)2,6H2O) were obtained from Sigma Aldrich, India. Ethylene Glycol (C2H6O2), KMnO4, NaBH4 H2SO4, H3PO4, HCl, and 30% H2O2 were obtained from E MERCK, Mumbai, India.

Methods Synthesis of graphite oxide (GO) Graphite Oxide was prepared by Improved method [35]. In this method, a 9:1 mixture of concentrated H2SO4/H3PO4 (360:40 mL) was added to a mixture of graphite flakes (3.0 g, 1 wt equiv) and KMnO4 (18.0 g, 6 wt equiv), producing a slight exotherm to 35e40  C. The reaction setup was kept in an ice bath and stirred for 6 h. Subsequently, the suspension was diluted to approximately 4 L with warm water and was treated

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with 30% Hydrogen peroxide (15 mL H2O2) in order to reduce the residual permanganate and manganese dioxide to colorless soluble manganese sulfate. The remaining material was then washed in succession with 400 mL of water, 200 mL of 30% HCl. Upon treatment with the peroxide the suspension turned brownish yellow. The mixture was then neutralized with ethanol and was subjected to several cycles of centrifugation and washing with deionized water. After centrifugation the yellow colored supernatant and brown colored suspension were obtained. The dried powder was stored and dispersed in solvents as needed.

Synthesis of GeCo and GePt, GeCoePt nanocatalyst Nanocatalysts were synthesized using Polyol method [36]. Graphite oxide was mixed with ethylene glycol using ultrasonication for 1hr. Subsequently, Cobalt nitrate (in 3:1 ratio with Graphite oxide) was added to the solution. The pH of the mixture was adjusted to 8 by using 1 M NaOH solution. The mixture was stirred constantly for 4 h at 120  C and the color of solution transformed from light yellow to orange. Subsequently, 20 mL of NaBH4 was dissolved in EG and added to the solution, kept at 120  C for 1 h. After 1 h the material was cooled to room temperature and the solution was added dropwise in a beaker containing double distilled water. Cobalt nitrate and graphite oxides were reduced and blackish Graphene cobalt nanohybrid was precipitated out from the solution. Resulting material was filtered and washed with double distilled water 3 times and dried in oven at 80  C. The dried catalyst was stored inside vacuum desiccators not more than 48 h and used as catalyst for Hydrogen generation. Similarly, GePt hybrids were prepared, using Chloroplatinic Acid (H2PtCl6.6H2O), (100 mg) as the metal precursor. In case of GeCoePt catalyst, Cobalt Nitrate hexahydrate (Co(NO3)2,6H2O) and Chloroplatinic Acid (H2PtCl6.6H2O) were used as metal precursor (in 3:1:1 ratio with Graphite oxide).

Characterization A JEOL JEM 2100 High Resolution Transmission Electron Microscope (HRTEM with SAED system, EDAX) at an accelerating voltage of 200 kV was used for analysis of the morphology and elemental identification of mono and bi-metallic catalyst. The samples were prepared by placing several drops of dilute particle dispersion (small amount of sample and ethanol) on carbon coated copper TEM grids. XRD analyses were performed in 0e90 2q range using Ultima III, (Rigaku, Japan) applying 40 kV voltage, 30 mA current, 1.2 kW power. Fourier transform infrared (FTIR) spectra were recorded on a Perkin Elmer FTIR Spectrometer and the samples were pressed into a pellet with KBr. JEOL JSM-6360 Scanning Electron Microscopy (SEM) and INCAx-sight (Oxford Instrument) (SEM with EDAX) was used for surface morphology analysis and elemental identification, Varian AA140 (Netherlands) Atomic Mass Absorption Spectrophotometer was used for quantitative determination of metals present in catalyst product.

traditional three-electrode electrochemical cell was used for electrochemical measurements. Prepared catalyst coated electrode, platinum mesh, Ag/AgCl and 0.5 M H2SO4 solution were used as the working electrode, counter electrode, reference electrode and electrolyte, resepctively. Carbon toray paper coated electrodes (0.785 cm2) were prepared using dropcast method. Tetrahydrofuran and Nafion cotaining catalyst suspension was used as a coating materials. The tests were conducted using 0.5 M H2SO4 acid aqueous solution which was purged with nitrogen for about 30 min. After the purge, a continuous stream of nitrogen was introduced into the cell above the liquid surface to maintain an inert atmosphere over the testing solution. The cyclic voltammetry (CV) curves were recorded over a voltage span 0.5 V and 0.5 V, with respect to the reference electrode with a scan rate of 20 mV/s. To analyze the electrocatalytic stability of the catalysts measurement, 10 cycles of cyclic voltammetric curves were recorded for 1000 s using the same experimental conditions stated above. Percentage decay of peak current density over time was estimated. The integrated area under the desorption peak in the CV curves between 0.5 and 0.0 V, represents the total charge during hydrogen adsorption, and was used to determine the electrochemically active surface area (ECSA) by employing the Eq. (2).  
ECSA cm2 Pt g Pt ¼ charge ½QH mC=cm2  210 ½mC=cm2   electrode loading ½g Pt=cm2 

(2)

Hydrogen generation Hydrogen generation by hydrolysis of NaBH4 The hydrolysis reaction was carried out in a 250 mL threenecked flask, where the precisely weighed Graphene supported catalyst was preloaded. An appropriate amount of water and NaBH4 were fed into contact with the catalyst in the reactor using a control valve. Typically, the dropping rate of the aqueous solution was controlled at around 3 g min1. The generated Hydrogen gas was passed through a heat exchanger to cool to room temperature and collected in a graduated gas jar by water displacement followed by contacting with silica drier to remove water vapor. The reaction temperature was monitored using a thermometer embedded in the solid fuel bed and recorded carefully. Each experiment was repeated two times. The determined relative error was not more than ±3%. Hydrolysis of NaBH4 was carried out with Graphene and three different catalysts namely GePt, GeCo and GeCoePt, using a specified amount (0.01 gm) of catalysts and at a constant operating temperature. To study the effect of operating temperature on the reaction kinetics dehydrogenation of Sodium Borohydride was performed at different temperatures (e.g. 20  C, 30  C, 40  C, 50  C) and the reaction kinetics were analyzed.

Electrochemical measurements

Stability of the graphene based catalyst in the hydrolytic dehydrogenation of NaBH4

The electrochemical measurements were performed with an Autolab AUT84999 electrochemical station (Metrohm). A

In order to check the decay in the activity of the catalyst, it was synthesized and the total amount of catalysts was divided

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into different parts which were stored in different vacuum desiccators separately for different time periods. Time period of storage i.e. the age of the catalysts varied from 3 days to 60 days. Dehydrogenation of Sodium Borohydride was carried out using the constant amount of catalyst stored for a definite time period.

Results and discussion Characterization of nanocatalysts Three different nanohybrid catalysts (GePt, GeCo and GeCoePt) were synthesized from Graphite Oxide (GO) by employing Polyol method. GO and the catalysts products are

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characterized using SEM, EDAX, X-ray Diffraction, FTIR, HRTEM, EDAX and Atomic mass absorption spectroscopy.

Characterization: scanning electron microscopy (SEM) Fig. 1A shows SEM image of Graphene supported bi-metallic catalyst GeCoePt. The corresponding images 1C, 1D and 1E depict the elemental identification and proper dispersion of Co and Pt, respectively on graphene sheet. The EDAX spectrum shown in Fig. 1B, revealed the presence of Co and Pt in the single GeCoePt nanohybrid. Fig. 1F and G shows the surface morphology of Graphene supported mono-metallic catalysts GeCo and GePt, respectively, using Scanning Electron Microscopy (SEM). Corresponding plane dispersion images and the EDAX spectrums were attached in S1 (Supplementary material).

Fig. 1 e SEM analysis of GeCoePt (1A.-1E.),GeCo(1F.) and GePt (1G.) nano-hybrid; 1A. SEM image of GeCoePt nanoensembles; 1B. EDAX spectrum of deposited Co and Pt metals in GeCoePt nano-ensembles; 1C., 1D., and 1E. corresponding metals Co and Pt dispersion on graphene; 1F. SEM image of GeCo nanohybrid; 1G. SEM image of GePt nanohybrid.

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Characterization: atomic mass absorption spectrophotometer Percentage of the metal content (Co, Pt, and both) in catalysts (GeCo, GePt, GeCoePt) were measured by using Atomic Mass Absorption Spectrophotometer. Spectroscopic analysis revealed that the weight percent of Pt and Co present in GeCoePt catalyst are 7.96% and 16%, respectively. In case of monometallic catalysts GeCo and GePt, the metal content of Co and Pt are 28.5% Co and 6.45%, respectively. These values of metal content of catalysts are of significance for determination of TOF values of the catalysts which is discussed in Effect of catalyst activity and its loading on Rate of Hydrogen Generation Section.

3.1.3.

Characterization: X-ray diffraction

Fig. 2 shows the X-ray Diffraction (XRD) patterns of GO and GePt, GeCo, GeCoePt hybrids using the same scale for intensity. As shown in the Fig. 2, the sharp, small peak at around 2q ¼ 10.8 (pattern of GO) corresponds to the (001) reflection of GO, indicating that GO forms a well-ordered layered structure [31]. In other three cases (GeCo, GeCoePt, GePt) a common broad peak that is observed at 24.3 , can be attributed to the graphite structure (002) plane of the Graphene nanosheet. For the sample of GeCo, two another diffraction peaks at 2q ¼ 51.5 , and 81.3 are observed, which can be indexed as the (200), (311) planes of cobalt, respectively. The figure shows another four different diffraction peak at 2q ¼ 39.75, 46.23, 67.51, 81.24 for the GePt sample, which can be indexed as (111), (200), (220) and (311) planes respectively, corresponding to the face-centered cubic (FCC) structure of Platinum

nanoparticle. In case of GrapheneeCoePt nanohybrid, the figure shows two diffraction peaks at 2q ¼ 39.75, 81.24, which can be assigned to (111), and (220) planes those correspond to Pt nanoparticle; while the other two peaks match well with the (200), and (311) planes of cobalt. To briefly summarize, during the exfoliation of GO sheets, metal ions intercalated into the layered GO sheets and absorbed on to them; subsequently, the GeCo, GeCoePt and GePt hybrids were formed by the reduction of GO to Graphene and crystal growth of metals on it. It is interesting to note that incorporation of Co with Pt (in GeCoePt) results in the enhancement of the intensity of Pt (111) peak, which indicates the reduction of the corresponding crystallite grain size. For three different hybrids, the average crystallite grain sizes of the Pt and Co nanoparticles/nanostructures are calculated from broadening of the diffraction peak using Scherrer equation [30] d¼

0:9l b1=2 cos q

(3)

where d is the average particle size (nm), l is the wavelength of the X-ray used (1.54056 Å), q is the angle at the maximum of the peak (rad), and b1=2 is the width of the peak at half height in radians. The estimated particle sizes of Pt and Co nanoclusters for three different hybrid catalysts are tabulated in Table 1. It may be inferred that the crystalline grain sizes of both the Pt and Co are reduced in case of GeCoePt compared to GePt and GeCo, respectively.

Characterization: fourier transformed infrared spectroscopy (FT-IR) Fig. 3 shows the FTIR spectra of GO and the reduction products obtained by chemical routes. In spectrum of GO, a strong and broad absorption is observed at 3405 cm1 due to OeH stretching vibration. The C]O stretching of COOH group situated at edges of GO sheets is observed at 1729 cm1. Other oxygen-containing functional groups of graphite oxide are indicated by the bands at 1382, 1240 and 1062 cm1. These correspond to CeOeH deformation peak, CeOH stretching peak and CeO stretching peak, respectively [31] and indicate that the full oxidation of graphite powders can be achieved within a short period of 6 h. The IR spectrum of Hybrids (GeCo, GeCoePt, GePt) in comparison to GO indicates almost complete removal of functional groups from GO under reaction conditions employed for synthesis.

Characterization: high resolution transmission electron microscopy (HR-TEM)

Fig. 2 e The XRD patterns of Graphite oxide (GO, Red), GrapheneeCobalt (GeCo, Green), grapheneeCobaltePlatinum (GeCoePt, blue) and GrapheneePlatinum (GePt, orange) hybrids, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 4A, B and C show the HRTEM images of the GePt nanohybrid, GeCo nanohybrid, GeCoePt nanohybrid, respectively. Here the particles are distributed randomly on the surface and edges of the Graphene with almost no free metal nanoparticle observable outside the Graphene sheet. Fig. 4D and E shows the elemental identification of the attached nancomposites (NCs), performed using energy-dispersive X-ray spectroscopy (EDAX). The spectrum of Fig. 4D reveals the presence of Co in the hybrid; Fig. 4E verifies the presence of Co and Pt in the hybrid. Also, it can be found from Fig. 4A that the Pt nanoclusters (Pt NC) are almost uniformly distributed on Graphene

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Table 1 e Comparative analysis according to grain size and surface area to volume ratio (S.A:V) of Co and Pt for different nanocatalysts. Name of nanocatalysts GeCo GeCoePt GePt

Grain size from X-ray diffraction

Grain size from HRTEM

Surface area to volume ratio (S.A:V)

Pt

Co

Pt

Co

Pt

Co

e ~5 nm ~18 nm

~21 nm ~10.2 nm e

e ~3 nm ~15 nm

~22 ~8 e

e 1.95 0.19

0.2823 1.430 e

sheets. Fig. 4B shows the rod like nanostructures of Co dispersed in to Graphene. Fig. 4C shows the bimetallic PteCo nano-ensembles [16] dispersed on surface and edges of Graphene. The average grain sizes of Pt and Co in GeCoePt and in GePt and GeCo respectively, obtained from HRTEM analysis, are tabulated in Table 1. More than two fold reduction of grain size of Co (from 22 nm to 8 nm) is observed for the case of GeCoePt compared to GeCo. Almost 5 fold reduction (from 15 nm to 3 nm) of Pt grain size is obtained for GeCoePt hybrid compared to GePt. The HRTEM results and approximate results on grain size estimated from XRD analysis match closely. The surface to volume ratio of the Pt and Co grains are calculated from the HRTEM data given and tabulated (Table 1). Almost 10 times enhancement of surface to volume ratio of Pt

grains is obtained for GeCoePt compared to GePt. Almost 5 times enhancement of surface to volume ratio of Co is obtained for GeCoePt compared to GeCo. In summary, compared to monometallic GePt and GeCo catalysts, the average grain size of both Pt and Co nanoparticles/structures are reduced in bimetallic GeCoePt catalyst where Pt and Co nano-ensembles are formed. Two principal effects caused by the interaction between two metals in a bimetallic catalyst are surface effect and structural phenomena; surface effects are of two category: geometric and electronic effect [16]. Geometric effect causes the size reduction of surface ensembles which has dramatic effect on catalytic performance since many reactions are catalyzed by a threshold number of surface atoms organized in an ensemble [16]. Reduced size and higher dispersion have strong effect on activity of bimetallic catalyst. This will be discussed below in the following section.

Results of hydrogen generation using graphene based catalysts The catalytic activities of the synthesized GePt, GeCo and GeCoePt nano-hybrids are investigated for the hydrolysis of NaBH4 in aqueous solution.

Effect of catalyst activity and its loading on rate of hydrogen generation

Fig. 3 e FTIR Image of Graphite oxide (GO, Red), GrapheneeCobalt (GeCo, Green), GrapheneeCobaltePlatinum (GeCoePt, blue) and GrapheneePlatinum (GePt, orange) hybrids, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 5A shows the volume of Hydrogen generated versus time during the catalytic hydrolysis of (0.015 mol) NaBH4 solution in the presence of Graphene and three different nanohybrid catalysts i.e. GeCo, GePt and GeCoePt of the same amount (10 mg) at 30 ± 0.5  C. It may be inferred from Fig. 5A that the rate of hydrogen generation is remarkably increased using the bimetallic GeCoePt catalyst. Use of GeCoePt enhances the rate of Hydrogen generation (averaged over a period of 600 s) by ~6.5, 4.5 and 4 times compared to that using Graphene and monometallic GeCo and GePt catalysts, respectively. Also, compared to the rate of hydrogen production in absence of any catalyst, almost 2.5, 3.5, 4 and 17 times enhancement is achieved using graphene and graphene based GeCo, GePt and GeCoePt catalysts, respectively (refer to Fig. 5A and inset). Fig. 5B shows the plot of the volume of hydrogen generated versus time during the catalytic hydrolysis of 0.015 mol Sodium Borohydride in presence of different amount of GeCoePt catalysts (10 mg  50 mg), which corresponds to 0.0312 mmole0.156 mmol metal catalysts at 30 ± 0.5  C. Five times increase in catalyst loading (from 10 mg to 50 mg) enhances the average rate of hydrogen generation by almost 12 times.

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Fig. 4 e A.,B.,C. HR-TEM images of GeCo, GePt, GeCoePt nanocomposite (D) and (E)corresponding EDX spectrum of the deposited GeCo and GeCoePt nanocatalyst.

A comparative study on activity of different catalysts for generation of hydrogen from hydrolysis of sodium borohydride is performed. GeCoePt catalysts were found to be highly active. For instance, the catalytic hydrolysis of 0.015 mol Sodium Borohydride was completed to give stoichiometric amount of hydrogen gas (0.06 mol) within 1080 s in presence of 10 mg GeCoePt (0.0312 mmol metal catalyst), which corresponds to TOF of 107 min1. Table 2 shows the activities in terms of TOF values (mol H2 (mol catalyst)1 min1) of the noble catalysts tested for hydrogen generation from the hydrolysis of Sodium Borohydride at room temperature. Among all the catalysts reported in literature and in this paper, GeCoePt exhibits the highest TOF value. The TOF of 107 min1 of GeCoePt is even greater than the best TOF value of reduced graphene oxide based Pd nanocatalyst recently reported for hydrolysis of ammonia borane [38].

The remarkable enhancement of Hydrogen generation and higher catalyst activity of GeCoePt may be attributed to the smaller grain size of Pt and Co nano-ensembles due to geometric effect and resulting higher surface to volume ratio of the same as discussed in Characterization: High Resolution Transmission electron microscopy (HR-TEM) Section. It is mentioned in literature [39] that higher surface to volume ratio of nanostructures has strong influence on sensing property of nano-material. The present findings of better catalytic activity of GePteCo having higher surface to volume ratio, commensurate with the earlier reported inference [36].

Stability of graphene based catalysts Fig. 6A, B and C demonstrate the decay in catalyst activity over a time period of 60 days for the three catalysts GeCo, GePt and GeCoePt, respectively. The decay in volumetric hydrogen

Fig. 5 e The volume of hydrogen generated versus time plots at constant sodium borohydride concentration A. using three different nanocatalysts, B. using several amount of GeCoePt nanocatalyst.

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Table 2 e Activities in terms of TOF values (mol H2 (mol catalyst)¡1 min¡1) of the noble metal catalysts or their alloys for hydrogen generation from sodium borohydrate (The TOF values were estimated from the data given in respective references.). Catalyst PVP-stabilized nickel(0) nanoparticles Ru(0) nanoclusters Co G/Co G/Pt G/Co G/Co/Pt

Amount of metal catalyst (gm mol)

Maximum eqv. H2 generated

Completion time (min)

0.001

4

67

0.0002 1.27E-03 1.27E-03 6.98E-06 4.838E-05 3.124E-05

4 4 4 4 4 4

5 41 40 120 150 18

generation shown in the figures designates the decay in activity of catalyst over time. The possible reason of decay in activity may be due to unstable/reduced dispersion over time. To quantify the deactivation of catalyst, the percent decay in rate of hydrogen generation (averaged over a time period), mol of hydrogen per minute per mol of catalyst is evaluated. For instance for fresh GeCoePt catalyst the initial rate of hydrogen generation (averaged over 720 s), mol hydrogen per min per mol of catalyst is 96.3 min1. After 60 days (from the day of synthesis) this value is reduced to 85.9 min1. Thus over a period of 60 days (age of catalyst) percent decay is 10.8%. The comparative analysis of stability of different catalysts is tabulated in Table 3. Among the three catalysts the bimetallic GeCoePt exhibits the highest stability. Fig. 7A and B demonstrate the detailed analysis of decay in the activity of GeCoePt catalyst over time. The decay in activity in terms of percent decay in mol hydrogen generation per minute per mol of catalyst values is plotted as a function of age (day) of the catalyst in Fig. 7B. Another variable, the mean percentage deviation of volumetric hydrogen generation (over the total reaction time) is also plotted in Fig. 7B to quantify the decay in P ðviHj v Hj Þ f

activity of catalyst. It is defined as

vi Hj

j¼1;n

n

 100; where vHj is

the volume of hydrogen generated at any jth time and the superscripts i and f denote the initial (0) and final age of catalyst, respectively. The maximum value of mean % deviation evaluated is 16% over a time period of 60 days. It may also be inferred from Fig. 7B catalyst aging and deactivation is almost negligible up to its age of 20 days. Beyond 20 days the decay is steep.

TOF (mol H2 mol catalyst1 min1) 0.392417 18.11157 0.114 0.218 82.6 12.5 107

Reference [37] [9] [31] [31] This study This study This study

In case of a bimetallic catalyst, apart from the above mentioned structural/morphological effects other important positive effects arise from the attraction of one metal to another. In this regards, reports are there in literature on improvement of coke resistance [40e45], poisoning resistance [46,47] and on improved thermal stability [48] of bimetallic inorganic oxide based catalysts compared to monometallic one. Increased dispersion and enhanced interaction with the support material offer better stability of the catalysts.

Effect of temperature on the kinetics: evaluation of activation energy for the hydrolysis of NaBH4 catalyzed by GeCoePt catalyst The GeCoePt catalyzed hydrolysis of Sodium Borohydride is carried out at various temperatures (20e50  C). Initial amount of NaBH4 is 0.568 gm and catalyst loading is 10 mg. Fig. 8A shows the volume of Hydrogen generated versus time for different operating temperatures. The corresponding rates of Hydrogen generation are evaluated as a function of Borohydride concentration. The Borohydride concentrations at different times are obtained from the values of hydrogen produced using stoichiometry. The values of rate constant k at different temperatures are calculated from the intercept of logelog plots of rate versus concentration (Sodium Borohydride). The apparent activation energy is calculated from Arrhenius plot shown in Fig. 8B. The calculated value of activation energy of GeCoePt catalyst appa ¼ 16.443 kJ/mol K. It is comparable with the other Pd Ea metal-based catalyst systems reported for the hydrolysis of Ammonium Borane [38].

Fig. 6 e Effect of catalyst deactivation on volumetric hydrogen generation for A. GeCo, B.GePt, C.GeCoePt catalysts.

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Table 3 e Comparative analysis of stability of different nanocatalysts. Stability testing Amount of catalyst (mg) Amount of metal in catalyst (gm mol) Rate (mol hydrogen min1 mol catalyst) for fresh catalyst Rate (mol hydrogen min1 mol catalyst) for catalyst's age 60 days Decay of activity (%)

GeCo

GePt

GeCoePt

10 4.838E-05

10 6.98E-06

10 3.124E-05

12.5

82.6

96.3

7.9

67.2

85.9

36.6

18.6

10.8

Results of electrochemical stability study using graphene based catalysts Fig. 9A shows the cyclic voltammetry (CV) curve for three different catalysts. The calculated values of ECSA estimated from Fig. 9A using Eq. (2) show the descending trend as: GeCoePt (75.386 m2/g) > GePt (50.80 m2/g) > GeCo (23.221 m2/ g). Graphene supported CoePt catalysts have much larger surface area than GePt and GeCo catalysts. GeCoePt catalyst not only affords more active sites for electrochemical reaction, but it also facilitates more than one effective electron transfer pathway through the electrode surface, therefore resulting in outstanding electro-catalytic activity. To investigate the electrochemical stability and potential of catalysts, the 10 cycles of CV curves were measured and analyzed. Fig. 9B, C and D show the CV pattern variation over 10th cycles and the decay of current density with respect to time. Among the three catalysts, GeCoePt shows the highest current density (0.27 A/cm2) and the lowest % (13.6) decay of current density after 10 cycles.

Comparative performance analysis of three graphene based catalysts Fig. 10 summarizes the comparative analysis of chemical and electrochemical activity, and stability of three graphene supported catalysts GePt, GeCo and GeCoePt, respectively. It is observed that bimetallic GeCoePt catalyst enhances the TOF value by 29.5% and ECSA by 50% compared to GePt catalyst. Among the three catalysts, GeCoePt exhibits the minimum values of decay of activity and decay of current density over 10

cycles of CV, indicating the best chemical and electrochemical stability. The enhanced activity of GeCoePt may be attributed to (i) the increase in the number of catalytic surface site due to a higher surface to volume ratio of Pt and Co grains (refer to Table 1) and (ii) increased number of possibilities or ways of electron transfer due to presence of bi metal. The different possibilities of transfer of electron with one Hydrogen atom to adjacent metal/carbon atom are via (i) Pt nanoparticle to adjacent Pt or Co atom (ii) Co nanostructure to adjacent Pt/Co atom (iii) Pt/Co nanostructure to adjacent Pt/Co nanostructure passing through Graphene sheet (iv) GeCoePt nano ensemble to adjacent Carbon atom (vi) GeCoePt structure to adjacent Pt/ Co/carbon atom directly or via Graphene sheet. The number of different ways of electron transfer depicted in a bimetallic GeCoePt catalyst is considerably higher than only three ways of electron transfer for monometallic GrapheneeCobalt catalyst, reported by Zhang et al. [31].

Conclusion A highly active, stable noble bimetallic nano-hybrid catalyst named as GeCoePt, is proposed for enhanced Hydrogen generation from hydrolysis of Sodium Borohydride under ambient condition. The study highlights on the comparative analysis of Hydrogen generation from NaBH4 using three different graphene based nano-hybrid catalysts, GeCo, GePt and GeCoePt. The catalysts were synthesized employing chemical routes and characterized by Scanning Electron Microscopy (SEM), High Resolution transmission electron microscope (HRTEM), Energy-dispersive X-ray spectroscopy (EDAX), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectra and Atomic mass absorption spectroscopy. Both the HRTEM and XRD analysis revealed that presence of bimetals Pt and Co on to Graphene essentially reduce the grain size of Pt, and Co compared to a monometallic nano-hybrid catalyst GePt and GeCo, respectively. Resulting enhanced surface to volume ratio and number of catalyst sites per unit volume significantly increases the catalyst activity. Furthermore, in case of a bimetallic Graphene supported catalyst the number of possibilities of electron transfer of ion to adjacent metal atom increases, which may enhance its activity. The

Fig. 7 e A. Effect of catalyst deactivation on volumetric hydrogen generation for GeCoePt catalyst, B. Mean % Variation and % of decay with respect to time.

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Fig. 8 e A. Volume of hydrogen generated versus time for catalytic hydrolysis of NaBH4 at different temperatures using constant amount of catalyst GeCoePt, B. The Arrhenius plot (ln k versus the reciprocal absolute temperature 1/T (K¡1)).

experimental results of hydrolysis of Sodium Borohydride reveals that the use of Graphene, GeCo, GePt and GeCoePt catalysts enhances the rate of Hydrogen generation by ~2.5 times, 3.5 times, 4 times and 17 times, respectively compared to the rate of Hydrogen generation in the absence of any catalyst. Use of bimetallic GeCoePt enhances the rate of Hydrogen generation by ~6.5,4.5 and 4 times more compared to that using Graphene and Graphene based monometallic GeCo and GePt catalysts, respectively. The bimetallic

GeCoePt catalysts provide turnover frequency of 107 min1, e the best among all other nanocatalysts tested in the hydrolysis of Sodium Borohydride. The calculated value of activa¼ 16.443 kJ/mol K. The tion energy of GeCoePt catalyst Eappa a decay of the activity of catalysts with its age is studied experimentally and estimated. In terms of electrochemical activity, GeCoePt exhibits the highest electrochemically active surface area (ECSA ¼ 75.38 m2/g). Cyclic voltammetry over ten cycles and corresponding decay in mean current

Fig. 9 e A. Cyclic voltammetric (CV) curves of G/Pt (black line), GeCo (blue line), and GeCoePt (red line) catalysts in 0.5 M H2SO4 at a sweep rate of 20 mV s¡1. B., C. and D. 10 cycles of CV and corresponding current density changes with respect to time (sec) using GeCoePt, GePt, GeCo catalyst respectively. (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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Fig. 10 e Comparative analysis of performance indices of three graphene supported catalysts GePt, GeCo and GePteCo.

density predict the highest electrochemical stability of GeCoePt among the three catalysts.

Acknowledgment Financial assistance received from Department of Science and Technology, Ministry of Science and Technology, India (DST File No. SR/S3/CE/084/2010) is gratefully acknowledged. Financial assistance for PhD research scholar received from Council of Scientific and Industrial Research (CSIR File No. 09/ 096(780)/2013-EMR-I) is also acknowledged. School of Materials Science & Nanotechnology, Jadavpur University, Kolkata, India is gratefully acknowledged for HRTEM analysis.

Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2014.05.131.

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