Ni Nano-particle Encapsulated in Hollow Carbon Sphere Electrocatalyst in Polymer Electrolyte Membrane Water Electrolyzer

Electrochimica Acta 167 (2015) 429–438

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Ni Nano-particle Encapsulated in Hollow Carbon Sphere Electrocatalyst in Polymer Electrolyte Membrane Water Electrolyzer Jayeeta Chattopadhyay a, * , Tara Sankar Pathak b , R. Srivastava a,c , A.C. Singh a a b c

Department of Applied Chemistry, Birla Institute of Technology, Mesra, Deoghar Extension Campus, Deoghar 814142, Jharkhand, India Department of Chemistry, Surendra Institute of Engineering and Management, Dhukuria, Siliguri 734009, Darjeeling, West Bengal, India Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India

A R T I C L E I N F O

A B S T R A C T

Article history: Received 1 January 2015 Received in revised form 17 March 2015 Accepted 17 March 2015 Available online 19 March 2015

The present study evaluates the synthesis by solvo-thermal method and electrocatalytic activity of nickel nano-particles encapsulated in hollow carbon sphere, in hydrogen and oxygen evolution reaction in PEM water electrolyzer. The XRD patterns have ascertained the formation of nickel metal with different planes in face centered cubic (fcc) and hexagonal closed pack (hcp) form. SEM and TEM images have confirmed the nickel nano-particles with diameter of 10–50 nm inside the 0.2 mm sized hollow carbon spheres. The BET surface area values gradually decreased with greater encapsulation of nickel; although the electrochemical active surface area (ECSA) values have been calculated as quite higher. It confirms the well dispersion of nickel in the materials and induces their electrocatalytic performance through the active surface sites. The cyclic voltammetric studies have evaluated hydrogen desorption peaks as five times more intense in nickel encapsulated materials, in comparison to the pure hollow carbon spheres. The anodic peak current density value has reached the highest level of 1.9 A cm
2 for HCSNi10, which gradually decreases with lesser amount of nickel in the electrocatalysts. These electrocatalysts have been proved electrochemically stable during their usage for 48 h long duration under potentiostatic condition. ã 2015 Elsevier Ltd. All rights reserved.

Keywords: Hollow carbon spheres Nickel encapsulation Polymer Electrolyte Membrane Cyclic voltammetry Water Electrolyzer

1. Introduction: Remove fossil fuels from the human equation and modern industrial civilization would cease to exist. The transformation to a fossil-fuel civilization occurred more rapidly than any other change in energy regimes in world history. Governments across the world are making continuous effort in exploration of clean, renewable alternatives and stimulating the utilization of renewable energies and resources such as, solar, wind, water and biomass [1,2]. Hydrogen holds the promise as a dream fuel of the future with many social, economic and environmental benefits to its credit [3]. It has the long-term potential to reduce the dependence on foreign oil and lower the carbon and criteria emissions from the transportation sector. These days, polymer electrode membrane (PEM) water electrolyzer system is getting enormous attraction as one of the most important hydrogen production methods. These

* Corresponding author at: Department of Applied Chemistry, Birla Institute of Technology, Mesra, Deoghar Extension Campus, Deoghar 814 142, Jharkhand, India. Tel.: +91 85 4400 8411. E-mail address: [email protected] (J. Chattopadhyay). http://dx.doi.org/10.1016/j.electacta.2015.03.122 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.

PEM systems show many advantages over the typical alkali-based water electrolyzer [4]. Usually in the PEM water electrolysis system, oxygen evolution electrode attains the largest over potential values at typical operating densities. Several studies have been reported on electrocatalytic properties noble metal oxides [5–7] despite their high cost and limited availability make the researchers bound to search for the economically favorable alternative materials. Submicrometer – sized hollow spheres have received enormous attention due to their physical properties and potential applications e.g. photonic crystals, controlled release capsules of various substances, fillers, storage media, adsorbents, electrode materials and catalysts [8–12], but it's very difficult to prepare by standard chemical methods. There are various methods, such as templating method [13], sono-chemical method [14], micro-wave treatment [15], hydrothermal method [16], spray drying method [17] and so on, which have been reported as the preparation procedures for the inorganic and organic materials with hollow spherical structures. Hollow carbon spheres become extremely popular particularly because of their excellent reactivity, thermal insulation, low density, high compressive strength and large cavity space [18–20]. Carbon support technologies usually provide important

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advantages including determination of particle size, well distribution of supported catalyst nanoparticles and have significant effects on catalytic performance and stability of the supported catalysts [21,22]. Earlier we had worked on tin-doped hollow carbon spheres materials as electrocatalysts with the huge formation of open nano-channels and nano-pores on all over the hollow carbon spheres [23]. The large surface area and pore formation have induced their electrocatalytic activity in hydrogen and oxygen production from water electrolysis. Previously, we have also reported on the electrocatalytic nature of Ni-doped TiO2 hollow spheres in aqueous and PEM water electrolyzers [24,25]. The selection of nickel as loading metal over titania spheres was on the basis of its inexpensive nature. Few groups have already utilized nickel as electrocatalysts in methanol and ethanol water electrolyzers and solid oxide fuel cell [26,27]. Now-a-days, metal encapsulated hollow carbon spheres are getting popular as anode materials for Li ion batteries [28,29]. One of the most promising strategies behind their application is to disperse nano-sized metal oxides into a carbon matrix, where carbon acts as both structural buffer and electrochemically active material during the lithium insertion/extraction [30,31]. The present work emphasizes on the synthesis of nickel nano-particles encapsulated in hollow carbon spheres, and further has been utilized as the electrocatalysts for hydrogen and oxygen evolution reaction (HER and OER) in the PEM water electrolyzer. The void space created inside the carbon hollow sphere with presence of encapsulated Ni nano-particles can be effective in electrocatalytic performance during water electrolysis.

2. Experiments 2.1. Preparation of Nickel Nano-particles Encapsulated in Hollow Carbon Spheres Materials In the preparation of nickel nano-particles encapsulated in hollow carbon spheres (HCSNi) were prepared by using glucose (Aldrich, Germany) and nickel (II) sulphate (Merck, India) as the precursor materials for the carbon shell and nickel particles, respectively. The anionic surfactant, sodium dodecyl sulphate (SDS) (CH3(CH2)11OSO3Na) (Sigma–Aldrich, Germany) was taken into account as the starting material in the preparation of hollow carbon spheres. The HCSNi materials were synthesized using the following method: Requisite amount of SDS was dissolved in deionized (DI) water, followed by the addition of 10 gm of glucose to get the aqueous solution. 0.1 M aqueous solution of Ni(SO4)2 was then added drop wise with the amount of 5, 8 and 10 ml to these solutions under vigorous stirring for three different materials. After stirring for 30 mins, the whole solution was transferred into a sealed Teflon-based stainless steel autoclave under hydrothermal condition at 250  C for 5 h. The obtained particles were then cooled down to ambient temperature, centrifuged and washed with ethanol and water repetitively. This material was finally dried at 100  C in air and further calcined at 350  C in N2 atmosphere consecutively for 5 h. The hollow spheres were initially heated in air for physical drying of the samples, and then calcined in N2 atmosphere so a great part of surfactant and water molecules can be removed through endothermic decomposition, instead of

Fig. 1. Schematic Diagram of Hollow Carbon Spheres Encapsulated Ni Nano-particles.

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combustion. The schematic diagram in Fig. 1 is represented the detailed synthetic procedure of nickel encapsulated in hollow carbon spheres materials. The materials with three various amount of nickel encapsulation inside the hollow carbon spheres are denoted as HCSNi5, HCSNi8 and HCSNi10. We have also tried to synthesize nickel encapsulated in hollow carbon sphere materials with greater amount of nickel encapsulization (with the addition of 12, 15 and 20 ml of Ni(SO4)2), but the materials could not sustain their hollow spherical structure during the synthesis. For the obvious reason, Ni particles could not be encapsulated for these materials. 2.2. Physical Characterizations of Materials The morphology of nickel nano-particles encapsulated in hollow carbon spheres materials were evaluated by Scanning electron microscope (SEM) and Transmission electron microscope (TEM) using JSM-100 JEM-2010, respectively. The surface studies of HCSNi10 sample were performed to make a 3-dimensional view of the surface structure using ImageJ to ensure that, nickel particles are really encapsulated inside the spheres. ImageJ is a Java based image processing program, which has clearly exhibited encapsulation of nickel particles. The energy dispersive X-ray analysis (EDAX) was also performed for inner and outer surface of HCSNi materials. X-ray diffraction (XRD) analysis was done with D/MAX-3C equipment of Rigaku Denki Co. Ltd. with the scan rate of 2 (2u)/min. Brunauer–Emmett–Teller (BET) surface area with BJH adsorption isotherms of HCSNi materials were analyzed using Micromeritics ASAP equipment model 2010 at 77.35 K. Thermogravimetric analysis (TGA) is performed for the encapsulated material in Shimadzu TGA-50H apparatus by heating up to 650  C with a heating rate of 10  C min
1 under air atmosphere (30 ml min
1). The chemical composition of nickel in encapsulated materials was analyzed by inductively coupled plasma (ICP) emission spectroscopy on a Thermo Jarrell-Ash ICP-9000 (N + M) spectrometer. 2.3. Electrochemical Characterizations of Materials in PEM Water Electrolyzer The electrocatalysts ink was loaded over the Nafion1 membrane using very simple spraying technique. The detailed method of PEM cell preparation has already been reported in our previous publication [25]. The catalyst ink was prepared by mixing the catalyst powder with Nafion solution and i-propyl alcohol under sonicating bath for 20 mins. The catalyst ink was cast onto Nafion 115 using spray gun to load approximately 2 mg cm
2 of catalyst. Before loading of catalyst, membrane was pretreated according to the technique mentioned by Srinivasan et al. [32]. Here, Pt on Vulcan XC-72 R (E-TEK) was used as the counter electrode, which was cast in similar manner onto the opposite side of the membrane with loading amount of 0.5 mg cm
2. Thus, the MEAs contain working electrode of the studied electrocatalysts (Ni nano-particle encapsulated in hollow carbon spheres) and counter electrode of the Pt on Vulcan XC-72 R (E-TEK), both have been casted on opposite side of the Nafion 115 membrane. The nickel encapsulated materials were loaded on anode side, whereas Pt on Vulcan XC-72 R (E-TEK) on cathode side. After spraying both the materials, the membrane ware hot pressed 110  C and 15 kg cm
2 for 5 min, followed by boiling in dil. HCl and deionized water repeatedly. The porous titanium sinters were pressed against these catalytic layers to provide the electrical contacts. The schematic diagram of the PEM water electrolysis cell is presented in Fig. 2. The whole electrochemical process was carried out in 0.1 N H2SO4 solution with Ag/AgCl in 1 N KCl solution as reference electrode. All electrochemical measurements e.g. cyclic voltammetry,

Fig. 2. Schematic Diagram of PEM Water Electrolysis Cell.

polarization curve, Tafel plot etc. were performed in Potentiostat/Galvanostat (Princeton Applied Research, Parastat1 4000). The cyclic voltammetric data were recorded in the potential range of
2.0 V to 2.1 V vs. Ag/AgCl electrode with the scan rate of 100 mV sec
1. The polarization curves and Tafel plots were recorded with the scan rate of 0.5 mV sec
1. 3. Results and Discussion 3.1. Physical Characterizations The inner morphology of the synthesized materials was analyzed by Scanning Electron Microscopy and Transmission Electron Microscopy. The SEM and TEM images presented in Fig. 3(A)–(C) have confirmed that the Nickel nano-particles were formed inside the hollow cavity of carbon sphere in the obtained materials. It is confirmed from the TEM images that, the average size of the nickel particles was 10–50 nm, encapsulated inside the hollow carbon spheres with 0.2 mm average diameter. TEM image of pure carbon spheres is shown in Fig. 3(D). The TEM pictures are clearly depicting the transformation in the morphology of the encapsulated materials from that of pure carbon spheres. These images have also confirmed that, porosity of the materials have been enhanced with nickel encapsulation inside the carbon spheres, which is also supporting the porosity results for all the materials. Energy Dispersive X-ray Analysis (EDX) of HCSNi materials were performed to ensure the encapsulation of nickel particles inside the spherical structure. Fig. 4(A) and (B) are representing elemental compositions through the EDX spectra of inner and outer side of HCSNi10 sample. The results of outer shell ensure the peak assigning for Carbon, with the weak ambiance of peaks for O, S and Na, derived from surfactant (Fig. 4(B)); whereas the encapsulation of nickel particles inside the carbon shell are confirmed from Fig. 4(A) [33]. Table 2 has described the EDX results with comparative bulk composition of nickel in materials. The surface studies of HCSNi10 sample were performed to make a 3-dimensional view of the surface structure using ImageJ to ensure that, nickel particles are really encapsulated inside the spheres; they are not just attached to the surface (Fig. 5(A)–(C)). At the same time, it has also exhibited the double – layer structure on the outer wall of the shell (Fig. 5(D)).

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Fig. 3. (A): SEM image of HCSNi10 Material, (B)–(C): TEM images of Ni Nano-particles encapsulated in Hollow carbon spheres, (D): TEM images of Pure Carbon Spheres.

for face centered cubic (fcc) nickel have also confirmed their formation from the corresponding diffraction peaks [34]. The diffraction pattern of the hollow carbon spheres without nickel encapsulization is presented in Fig. 6(B). The diffraction peaks formed are attributed to the formation of (0 0 2) and (1 0 1) planes of graphitic carbon [35]. The broad peak formed around 25 is

Fig. 6(A) and (B) are the XRD patterns of as-prepared hollow carbon spheres with and without encapsulating nickel nanoparticles, respectively. Fig. 6(A) is clearly showing various diffraction peaks situated, which are assigned to the formation of (0 1 0), (0 0 2), (0 11), (2 0 0), (0 1 2) and (11 0) planes of hexagonal closed pack (hcp) nickel. Two planes (111) and (2 2 0)

(B)

(A) 25

30

C 25

C 20

20

Ni 15

15

Ni 10

10

5

o

5

Na

S

o

0

Na S

0 0

2

4

6

8

10 keV

0

2

4

6

Fig. 4. Electron Dispersive X-ray Analysis of HCSNi10 Sample (A) Inner Side and (B) Outer Shell.

8

10 keV

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433

Table 1 Surface Area Studies, Pore Volume, Pore Size and Skeletal density Results of Hollow Carbon Spheres with and without Encapsulated Ni Nanoparticles. Sample

Stotal(m2/g)

Smicro (m2/g)

Vtotal (cm3/g)

Vmicro (cm3/g)

DBET (nm)

rs (g/cm3)

Percent porosity (%)

Hollow Carbon Sphere HCSNi5 HCSNi8 HCSNi10

200.5 121.3 118.8 115.4

86.4 48.1 52.4 55.7

0.88 1.51 1.55 1.67

0.091 0.19 0.22 0.25

4.93 12.4 13.04 14.4

1.95 2.42 2.77 3.01

63.3 78.6 81.1 83.5

Stotal = Total surface area calculated by BET method; Smicro = Micropore area calculated by the BET method; Vtotal = Single-point total pore volume at relative pressure (P/Po) of 0.99; Vmicro = Micropore volume of pores less than 3.5 nm; DBET = Average pore size calculated by Vtotal/Stotal; rs = skeletal density measured by helium pychnometry; Percent porosity = Vtotal/[Vtotal + 1/rs]  100, where 1/rs = skeletal volume per mass.

Table 2 Calculated and Bulk Composition of nickel, EDAX and ECSA Results of Hollow Sphere Materials. Sample

Hollow Carbon Sphere HCSNi5 HCSNi8 HCSNi10

Calculated Ni mass% during synthesis

Ni content mass % (ICP Analysis)

ECSA value (m2 g
1)

EDAX results C

O

Na

S

0

0

91.1

4.9

2.1

1.9

12.5 18.7 25.1

10.5 15.4 20.1

77.0 72.5 66.5

5.2 4.4 4.6

3.9 3.5 3.1

2.1 2.2 1.9

Ni 0.00 11.8 17.4 23.9

50.1 75.6 70.5 68.9

Fig. 5. 3D Surface Studies of Hollow Carbon Spheres Encapsulated Ni Nano-particles (HCSNi) Materials.

Fig. 6. (A): XRD Pattern of HCSNi Materials with Various Composition of Nickel, (B): XRD Pattern of Hollow Carbon Sphere without Encapsulation of Nickel.

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180 160 140 120 100 80 60 40 20 0 0.0

0.2

0.4

0.6

0.8

1.0

Fig. 7. N2 Adsorption-Desorption Isotherms: I – Hollow Carbon Spheres; II – HCSNi8; and III –HCSNi10 Materials.

exhibiting the hollow carbon sphere materials with low crystalinity and low graphitization [28]. The N2 adsorption–desorption isotherms and pore size distribution with surface area values for hollow carbon spheres materials with and without nickel encapsulation, are presented in Fig. 7 and Table 1. The BET surface area studies have revealed that, pure hollow carbon spheres possess greater surface area value (200.5 m2 g
1); whereas it get reduced considerably with the encapsulation of nickel nanoparticles. At the same time, the surface area of the materials tends to possess the declining nature with an increase in the amount of nickel during the synthesis, i.e. 121.3, 118.8 and 115.4 m2 g
1 of BET surface area values for HCSNi5, HCSNi8 and HCSNi10 samples, respectively. In the Fig. 7, the adsorption isotherms for pure hollow carbon spheres, HCSNi5, HCSNi8 and HCSNi10 samples were observed with typical type IV characteristics content H1 type hysteresis loops (P/P0 > 0.5). The pure hollow carbon sample (without encapsulation) possesses pore size distribution in 0.4 to 25 nm range, with the maximum pore size at 2 nm. The pore size distribution results of hollow carbon spheres (Fig. 8(A)) shows that, pores inside the sample consist of both micro and mesopores. Fig. 8(B) presents the pore size distribution results of HCSNi10 sample. It shows the bimodal distribution with the first peak around 1.5 to 5 nm (micropores) and second peak at between 10 to 25 nm (small mesopores). It is easy to understand by correlating the pore size distribution with the TEM results that the small micropores are related to the (A)

partially filled hollow sphere, and mesopores are formed for the outer shell of carbon spheres. The single-point total pore volume at relative pressure (P/Po) of 0.99 has been revealed as 1.6 cm3 g
1 for nickel encapsulated materials, which is almost 1.8 times greater than that of pure carbon spheres. The hollow carbon spheres possess wide range of pore size distribution, thus their pore volume value has not revealed as the only controlling factor of their surface area. On the other hand, larger pore volume of encapsulated materials has complex bimodal pore size distribution. The skeletal density of all the materials have also been evaluated by helium pychnometry, and presented in the Table 1. The percent porosity was calculated, using the total pore volume obtained from the BET analysis and the skeletal volume evaluated by helium pychnometry. The percent porosity calculation reveals that all the nickel encapsulated materials have nearly identical porosity of 80%, which is significantly higher than the pure carbon spheres. This may be attributed to the combined effect of higher fractions micropores and mesopores in nickel encapsulated materials. The larger pore volume of nickel encapsulated materials may possess the greater electrocatalytic activity of the corresponding materials through better adsorption of hydrogen and oxygen gases during the electrolysis reaction. 3.2. Electrochemical Performance in PEM water electrolyzer Cyclic voltammetric studies were initially conducted to evaluate the electrocatalytic activity of the nickel encapsulated in hollow carbon sphere materials in the PEM water electrolyzer, which was examined at the scan rate of 100 mV s
1 over the potential range of
2.0 V to 2.0 V. Fig. 9 represents the cyclic voltammetric curves for all the encapsulated materials with that of a pure hollow carbon sphere at the inset. It is clearly evident from the results that, electrocatalytic activity for hydrogen and oxygen evolution reactions during the electrolysis has been greatly induced by the nickel encapsulation. At the same time, the peak position and intensity have also been influenced in great extent for the nickel encapsulated materials. The hydrogen desorption peak formed at
0.2 V during the anodic sweep, is almost five times intense for the HCSNi materials to that of pure carbon hollow spheres. The highest anodic peak current density value has been evaluated as 1.9 A cm
2 for HCSNi10 sample; this peak is actually assigned to the oxygen evolution reaction. It can be seen from the voltammograms that, this peak becomes less intense with lowering of nickel amount during the encapsulation process. For HCSNi5 and HCSNi8 samples, the current density is only 1.4 and 1.5 A cm
2, respectively. During the hydrogen desorption (anodic sweep) and evolution process (cathodic sweep), the similar trend (B)

1.0

0.8

0.6

dV/dD (cm3/nm/g)

dV/dD (cm3/nm/g)

0.8

1.0

0.4

0.2

0.0

0.6

0.4

0.2

0.0 0

5

10

15

Pore Diameter (nm)

20

25

0

5

10

15

20

Pore size (nm) Fig. 8. (A): Pore size distribution curve for Hollow Carbon Sphere, (B): Pore size distribution curve for HCSNi10 Material.

25

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2.0 1.6 1.2 0.8 0.4 0.0 -0.4 -0.8 -1.2 -1.6 -2.0

-1.6

-1.2

-0.8

-0.4

0.0

0.4

0.8

1.2

1.6

2.0

Fig. 9. Cyclic Voltammetric Results of Hollow Carbon Sphere Materials with and without encapsulated with Nickel nano-Particle.

has been noticed. HCSNi10 sample has been proved as the material with greatest electrocatalytic activity, which has been followed by HCSNi8 and HCSNi5 respectively. Thus, it can be concluded that, nickel capsulation has influenced both in hydrogen and oxygen evolution reaction; although pure carbon spheres have also shown good electrocatalytic activity. Earlier it is reported by many researchers that, hollow carbon spheres exhibit similar behavior like graphite and fullerene; graphite and fullerene are well-known for their oxygen adsorption characteristics [36]. Initially the graphitic nature of the carbon spheres with their void space has initiated the good desorption–adsorption process of hydrogen and oxygen gases, thereafter the encapsulated well-dispersed nickel nano-particles has influenced in the adsorption of gases, resulting in the greater electrocatalytic activity. It is already mentioned in the literature that, nickel is a very good candidate in hydrogen and oxygen gas adsorption process. Especially, the Ni metallic plane like (111) and (11 0) present in the encapsulated materials, are highly active in the adsorption process [37]. The correlation between the morphological and electrochemical studies of all the materials has revealed that, the electrocatalytic activity has been greatly induced by the nickel encapsulation and also with the transformation in the morphological structures. This remarkable enhancement of the electrocatalytic activity may be attributed to the dissociation-spillover effect of the encapsulated Ni nano-particles. Usually, hydrogen is stored in carbonaceous materials, through physical adsorption, whereas nickel can dissociate hydrogen molecule into atom, resulting in the greater electrocatalytic activity by chemical adsorption with higher hydrogen uptake [38]. Actually, the improvement of electrocatalytic activity for Ni encapsulated in hollow carbon spheres is a result of several factors: 1) enhancement of ECSA for the great dispersion of nickel particles in the carbon spheres; 2) these materials create inter and trans-particle porosity better; 3) their geometric condition facilitates the escape of the hydrogen molecules from the catalyst surface; 4) extra conductive properties results in the easier electron exchange with the hydrogen protons, smoother way to form adsorbed hydrogen atom, and further hydrogen molecule. 5) carbonaceous materials are good in hydrogen storage through physical adsorption; whereas nickel nano-particles can dissociate hydrogen molecule into atom through chemical adsorption process. The materials with greater nickel encapsulation possess pores with larger volume; as we all know that larger pore volume is always effective in the better electrocatalytic process. But the BET surface area values have been reduced with nickel encapsulation in hollow carbon spheres. Although in electrochemical processes, the

435

surface area which actually influences on the electrocatalytic activity of a particular material is electrochemically active surface area (ECSA) under working conditions, which is highly dependent on the number of reactive surface sites. The materials with low roughness factor always possess the ECSA value very close to the geometric surface area. But the nickel encapsulated hollow carbon sphere materials are highly porous in nature with high roughness factors. In this case, an activity expressed per geometrical surface area cannot be used in the electrocatalytic evaluation process; as when comparing between two different catalysts, higher current per geometrical surface area can merely mean higher surface area and not higher intrinsic catalyst activity [39]. Moreover, all parts of surfaces are not electrochemically accessible, which means that electrochemically active surface is different from nitrogen adsorption surface. Nickel encapsulated materials possess both the effect of graphitic carbon and active sites of nickel in oxygen and hydrogen process, and induce the ECSA value for encapsulated material in comparison to the pure carbon material. The ECSA of catalysts can be obtained by calculating the charges accumulated during the H desorption. ECSA = 100.Q/m.c Where, Q is the charge for H desorption, c is the electrical charges associated with monolayer adsorption of H (0.21 mC cm
2) and m is the corresponding catalyst loaded. In the present study, the calculated ECSA value has been revealed as 75.6, 70.5 and 68.9 m2 g
1 for HCSNi10, HCSNi8 and HCSNi5, respectively; whereas this value has been calculated as 50.1 m2 g
1 for pure hollow carbon spheres. In this case, ECSA has been expressed per total mass, included total mass of nickel and carbon, instead of per nickel content as active phase, as their electrocatalytic activity has been compared with carbon spheres. Apparently, the ECSA values are quite high for nickel encapsulated materials, due to the good dispersion of nickel nano-particles in the carbon spheres. Nickel encapsulated materials possess both the effect of graphitic carbon and active sites of nickel in oxygen and hydrogen process, and induce the ECSA value for encapsulated material in comparison to the pure carbon material. Fig. 10 shows the galvanostatic polarization curves for all the hollow carbon sphere nickel encapsulated materials measured at the scan rate of 0.5 mV s
1 at 25  C. At the low current density range (around 1 mA cm
2), all the electrocatalysts are showing almost similar results, whereas HCSNi10 sample has depicted the highest electrocatalytic activity by producing comparatively lower over potential value, at higher current density range. This over potential

0.6

0.2

-0.2

-0.6

-1.0

-1.4

-1.8 0

0.4

0.8

1.2

1.6

2.0

2.4

2.8

Fig. 10. Galvanostatic Polarization Curves for Hollow Carbon Spheres Encapsulated Ni Nano-particles for HER in PEM Water Electrolyzer System.

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value is gradually inclined with rise in the amount of nickel in the material. It is well known that, the over potential value is highly influenced at the high current density range, by the ohmic resistivity and bubble formation during the reaction performed on the electrolysis cell. Therefore, the performance of an electrocatalyst at higher current density exhibits at the real operating conditions. Mostly, hollow carbon spheres and metal spheres have been reported as electrocatalysts in alcohol electrooxidation process [40,41]. B. FıcSıcılar et al. had worked on Pt-Pd bimetallic catalysts doped on hollow core mesoporous carbon spheres and utilized as electrocatalysts in PEM water electrolyzer [42,43]. Pt incorporated hollow carbon spheres had worked better than the commercial one, although their current density value during cyclic voltammetry shown much lesser value than these nickel incorporated materials. S.A. Grigoriev et al. had also reported similar results on carbon supported Pt and Pd nanoparticles electrocatalysts in PEM water electrolysis [44]. Xu et al. reported the electrocatalytic application of nickel hollow spheres in methanol and ethanol electrooxidation reaction with the peak current densities of 3.5 and 2.0 mA cm
2, respectively [45]. In our previous work, we determined the electrocatalytic performance of Ni-doped on titania hollow spherical materials in PEM water electrolyzer, where 30 wt% Ni-doped titania electrocatalyst had shown the best activity in hydrogen evolution reaction with 0.36 A cm
2 of peak current density value [25]. Our nickel encapsulated hollow carbon spheres have performed better than those bimetallic materials and inexpensive too. Tafel Eq. (1) relates the rate of an electrochemical reaction to the over potential, includes the uncompensated ohmic drops. E = a + blnI + IR

(1)

After differentiating Eq. (1) w.r.t. I, we will get: dE/dI = b/I + R

(2)

The average Tafel slope calculated for all these materials is 90 mV. This Tafel slope value is not the standard one [46]. Therefore the mechanism behind the reaction is composite. Fig. 11 represents a typical plot of log I vs. pH at E = 1.5 V for HCSNi10 and HCSNi5 samples. The average reaction order value has been calculated as
0.51 from the slope of the straight line. The chemically significant reaction order can be expressed by the following relation: EɤH+ = hɤ H+
ɣ

(3)

Where n is the reaction order at constant overpotential (ɤ) and potential (E), respectively, and ɣ is the observable transfer coefficient: ɣ = (RT/F) (dln E/dln j)

(4)

The oxygen evolution reaction involves water as the main reacting molecules: H2O + S ! S–OH* + H+ + e


(5)

Here, S is the surface active site on the electrocatalyst. This step is usually followed by a chemical step, which is considered as rate determining step at the low coverage of the surface intermediate. On the other hand, the rate determining step can be the step in which surface OH group rearranges to the more reactive intermediate, although the nature of rearrangement is not clear yet: S–OH* ! S–OH

(6)

When ɤ = 1 in Eq. (4), the chemically significant reaction order is
1 with the Tafel slope of 60 mV, which is evident in the support of both electrochemical and chemical steps, presented in Eqs. (5) and (6). If the Tafel slope value increases (as in the present study) further, then the rate determining step will be shifted from Eq. (6) to (5). It is well known that, the reaction order at constant over potential is always zero, then EɤH+ =
ɣ, according to Eq. (3); therefore reaction order value will be decreased from –1 to –0.5. The electrocatalytic stability test was examined at potentiostatic condition for the HCSNi5 sample, by plotting the anodic peak current density value taken every after 2 h from cyclic voltammograms for 48 h of total duration. In the Fig.12, the electrocatalytic stability test is plotted against time. The result shows anodic peak current density around 1.6 A cm
2 for all along the experiment duration. 3.3. Mechanism behind the synthesis of metal encapsulated hollow carbon spheres The materials have synthesized using glucose and Ni(SO4)2 under hydrothermal condition using SDS as the sacrificial template. The surfactant molecules in aqueous solution form micelles when it achieves a certain level of concentration, called critical micelle concentration (CMC). The value of the CMC for SDS is about 8.27 mM at 30  C [47]. Thus, the concentrations of SDS we used in our experiments were mostly higher than the CMC value of this surfactant. During the synthesis, glucose, Ni(SO4)2 and SDS

0.14 0.12

1.875

Peak Current Demsity / ip / Acm

-2

0.10 0.08 0.06 0.04 0.02 0.00

HCSNi5

1.750

1.625

1.500

1.375

1.250

-0.02 0.6

0.8

1.0

1.2

1.4

1.6

1.8

1.125 0

Fig. 11. Determination of Reaction Order with respect to H+ for O2 evolution at E = 1.5 V for HCSNi5 and HCSNi10 Samples.

10

20

30

40

50

Time (h) Fig. 12. Potentiodynamic Electrocatalytic Stability Test for HCSNi5 Material.

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were dissolved in DI water to get a clear solution. The positively charged nickel ions present in the solution were reduced by hydrogen released from carbonization of glucose, resulting in the formation of Ni nano-particles, which further acted as the nucleation center. Therefore, already present negatively charged SDS micelles were forced to collapse by influence of this new nucleation center, and getting attached with the surface of the Ni nano-structures. In the next step, hydrophobic side of the SDS micelles tilted towards the surface of the Ni nano-particles leading to the formation of reverse micelles. This process resulted in the dehydration of glucose molecules present in water to form aromatic rings. In hydrothermal condition, glucose molecules underwent through dehydration, condensation, or polymerization and aromatization reactions, leading to the formation of carbonaceous product around the surface of the reverse micelles, leading to the formation of a carbonaceous shell. Here, SDS acted as sacrificial template which controls the size of the cavity within the hollow carbon spheres. 4. Conclusions The present study involves the electrocatalytic activity of nickel nano-particles encapsulated in hollow carbon spheres during water electrolysis in the PEM water electrolyzer. Nickel encapsulated materials are prepared under hydrothermal condition using Sodium dodecyl sulphate (SDS) as a surfactant. The Scanning Electron microscope (SEM) and Transformed Electron microscope (TEM) images have confirmed the encapsulation of nickel nanoparticles (Diameter 10–50 nm) inside the carbon hollow sphere (Diameter 0.2 mm). The presence of hexagonal closed pack (hcp) and face centered cubic (fcc) nickel has been ascertained in Ni encapsulated materials from the XRD studies. The pore size distribution results have shown that, the materials with greater nickel encapsulation have possessed pores with larger volume, which has actually induced their electrocatalytic activity. Although the BET surface area values get reduced in the samples with greater nickel encapsulation, but the electrocatalytic activity mainly depends upon of a particular material is an electrochemically active surface area (ECSA) under working conditions, dependant on reactive surface sites. In the present study, the ECSA values are quite high in nickel encapsulated materials, due to the good dispersion of nickel nano-particles in the carbon spheres. The cyclic voltammteric results have ensured that, nickel encapsulation in hollow carbon spheres has induced the electrocatalytic activity during hydrogen and oxygen evolution reaction. Hydrogen desorption peaks for nickel encapsulated samples are almost five times more intense than that of without encapsulated materials. HCSNi10 sample has produced 1.9 A cm
2 anodic peak current, which is assigned to oxygen evolution. According to the galvanostatic polarization experiment, HCSNi10 sample has depicted the highest electrocatalytic activity by producing comparatively lower over potential value, at higher current density range, which has been gradually inclined with rise in the amount of nickel in the material. Tafel slope calculated for all the nickel encapsulated materials is 90 mV with the reaction order of
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