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Graphene oxide supported Pd-Fe nanohybrid as an efficient electrocatalyst for proton exchange membrane fuel cells
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Graphene oxide supported Pd-Fe nanohybrid as an efficient electrocatalyst for proton exchange membrane fuel cells Sunil Dhali a, Manoj Karakoti a, Sandeep Pandey a, Boddepalli SanthiBhushan b, Ravindra Kumar Verma c, Anurag Srivastava b, Rajaram Bal d, S.P.S. Mehta a, Nanda Gopal Sahoo a,* a
Nanoscience and Nanotechnology Centre, Department of Chemistry, D.S.B. Campus Kumaun University, Nainital, 263002, India b Atal Bihari Vajpayee - Indian Institute of Information Technology and Management, Gwalior, 474015, Madhya Pradesh, India c National Mission of Himalayan Studies, G.B. Pant National Institute of Himalayan Environment and Sustainable Development, Kosi Katarmal, Almora, India d Conversion & Catalysis Division, CSIR-Indian Institute of Petroleum, Dehradun, India
highlights
graphical abstract
GO supported PdeFe nanohybrid has been successfully synthesized for PEMFCs. Size of Pd and Fe nanoparticles were in the range of 2.5e3.0 nm in the GO sheets. Computational analysis has been done for the validation of the GOPd-Fe nanohybrid. ECSA of GO-Pd-Fe (58.08 m2/g) was higher
than
Pt/C
catalyst
(37.87 m2/g).
article info
abstract
Article history:
The experimental realization and computational validation for graphene oxide (GO) sup-
Received 28 March 2019
ported palladium (Pd)-iron (Fe) nanohybrids as a new generation electrocatalyst for proton-
Received in revised form
exchange membrane fuel cells (PEMFCs) has been reported. The experimental apprehen-
9 August 2019
sion of the present catalyst system has been initiated with the graphene oxide, followed by
Accepted 13 September 2019
the doping of Pd and Fe via thermal inter calation of palladium chloride and iron chloride
Available online xxx
with the in-situ downstream reduction to get nanohybrids of the GO-Pd-Fe. These nanohybrids are subsequently characterized by RAMAN, FT-IR, UVeVis, XRD, SEM, EDS, TEM
Keywords:
and HRTEM analysis. Furthermore, the first principle calculations based on Density
Catalyst
Functional Theory (DFT) with semi-empirical Grimme DFT-D2 correction has been
* Corresponding author. E-mail address: [email protected] (N.G. Sahoo). https://doi.org/10.1016/j.ijhydene.2019.09.131 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Dhali S et al., Graphene oxide supported Pd-Fe nanohybrid as an efficient electrocatalyst for proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.131
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Cyclic voltammetry
performed to support the experimental findings. Computational results revealed the
DFT
alteration of graphene electronic nature from zero-band gaped to metallic/semi-metallic
Fuel cell
on adsorption of transition metal clusters. Moreover, the defect sites of the graphene
Graphene oxide etc
surface are more favorable than the pristine sites for transition metal adsorption owing to the strong binding energies of the former. Electrochemical studies show that GO-Pd-Fe nanohybrids catalyst (Pd: Fe ¼ 2:1) demonstrates excellent catalytic activity as well as the higher electrochemical surface area of (58.08 m2/g PdeFe)1 which is higher than the commercially available Pt/C catalyst with electrochemical surface area 37.87 m2/(g Pt)1. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Proton exchange membrane fuel cells (PEMFCs) are regarded as imperative electrochemical energy devices to overcome the problems associated with combustion of fossil fuels and to control the emission of green house gases. In recent decades, Pt based materials have emerged as efficient catalysts for the development of next generation fuel cells [1e4]. These catalysts have been extensively used as anode and cathode electro-catalyst of fuel cells for oxygen reduction reaction to generate energy [5]. However, the Pt based catalyst has limited practical application of PEMFCs due to their significantly high cost, poor utilization coefficient and slow kinetics. Besides, the pure Pt based electro-catalyst normally endures several disadvantages such as poisoning during the oxidation reduction reaction due to the formation of CO-like species [6,7]. In recent years, graphene supported novel metal catalyst systems are replacing the Pt-based electro-catalysts. Graphene, a single layer thick of carbon atoms arranged in a 2dimensional macromolecule, has been actively pursued as an enthralling material with unusual properties. This hexagonal ring matrix has attracted much attention of the researchers as a new generation material for fuel cell application owing to its large surface area and high electrical conductivity [8e12]. These properties of graphene and graphene oxide exhibit their use as a suitable electro catalyst support and nano-catalyst material for PEMFCs. Several reports have been studied about graphene as an efficient catalyst support for various metal nanoparticles and demonstrated impressive catalytic efficacy and durability. Recently, palladium based nano hybrids catalysts like PalladiumeIron, PalladiumeCobalt, Palladium-Nikel, PalladiumeCopper, PalladiumeSilver and PalladiumeGold have been investigated as promising electro-catalysts for the oxygen reduction reaction in basic medium [13e24]. Recently, Gahlot et al. showed the graphene oxide based nanocomposite membrane for the practical application of DMFC and electrodialysis with superior properties. However, dispersability of the graphene based nanocomposite plays a significant role for the enhancement in the efficiency of different kinds of fuels cells. In this regard Kansara et al. reported that graphene nanosheets exhibits excellent high loading efficiency for paclitaxel when produced a stable dispersion up to 0.1 wt% [25,26]. While according to Baronia et al. the catalytical activity of Pt based binary anodic catalyst works as an admirable electrocatalyst for electro chemical oxidation reactions of fuel cells.
They have reported that rGO supported PteCo based electrocatalyst exhibits high performance for ethylene glycol electro oxidation process which follows the reductive pathway [27]. Similarly Liu et al. synthesized PdeAg/rGO supported with 1:1 mass ratio catalyst, where they have found that PdeAg/rGO has much better catalytic performance for electro-oxidation of ethanol [28]. Esabattina et al. have developed eletrocatalyst based on PtePd/rGO system, which shows the prominent electrocatalytic activity in the direction of methanol oxidation compared to bare Pt-Pd [29]. Further, Li et al. showed that CoePd/ rGO, and Ni/rGO, have the maximum electrocatalytic activity, electrochemical stability and better tolerance to CO poisoning, whereas their results are also applicable for formic acid oxidation and H2O2 reduction, respectively [30]. Recently, Feng et al. have prepared surfactant free PdeFe nanoparticles supported on rGO, which demonstrated impressive catalytic activity for formic acid oxidation. Their findings showed that 1:5 ratio of PdeFe in PdeFe/rGO hybrid exhibit maximum performance with a specific activity of 2.72 mA/cm2 at the mass activity of 1.0 A/mg, which is comparatively higher than Pd/rGO and commercially available Pd/C catalysts [31e36]. However, still the appropriate ratio PdeFe metal loading in graphene nanosheets is one of the challenging issues for the development of GO-Pd-Fe nanohybrids. Therefore in the present study, we are reporting, a placid and eco-friendly ethylene glycol based reducing predecessor to synthesize graphene oxide supported PdeFe (2:1 ratio) nanohybrid electro-catalyst, through uniform distribution of Pd and Fe nanoparticales overthe surface of graphene oxide. Further the electrocatalytic performance along with theoretical calculations has been investigated to verify the efficacy of thus prepared GOPd-Fe nanohybrid electrocatalyst for oxidation reduction reaction of PEMFCs. Our nanohybrid electrocatalyst showed the high electrochemical surface area and soaring abundance of the Pd/ Fe metals with tolerable catalytic activity of nanohybrids in comparison to the Pt-based catalyst systems. Moreover, the present nanohybrids offers enhanced catalytic activity and with the higher surface area i.e. 58.08 m2/g in comparison to the commercially available Pt/C 37.87 m2/g catalysts.
Materials and methods Materials Expanded graphite powder of 100 mm (approx.) was purchased from Sigma Aldrich. Potassium permanganate (KMnO4),
Please cite this article as: Dhali S et al., Graphene oxide supported Pd-Fe nanohybrid as an efficient electrocatalyst for proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.131
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sodium nitrate (NaNO3), sulphuric acid (H2SO4), hydrochloric acid (HCl), sodium hydroxide (NaOH), hydrogen peroxide (H2O2 30% aq), iron (II) chloride tetra hydrate 99.99% trace metal basis, Palladium (II) chloride (PdCl2) and ethylene glycol (C2H6O2) were purchased from SRL, and sillicone oil purchased from Merk. Commercial available Pt/C catalyst purchased from Sigma Aldrich and used as received.
Synthesis of GO GO was synthesized by using modified Hummer's method [37]. Briefly, 1 g of graphite powder was placed in a beaker and 50 ml of concentrated H2SO4 with 1 g of NaNO3 was added to subsequently and stirred for 4 h, followed by addition of 6 g of KMnO4 in ice bath with at a temperature of 35 C for 2 h. Thereafter, 90 ml of de-ionized water was added slowly to the mixture and maintained the temperature at 95 C for 2 h with continuous stirring. Subsequently, the heat supply was turned off and 200 ml de-ionized (DI) water was added to the mixture while stirring for 1 h. The mixture was cooled and 20 ml of 30% H2O2 was added, and again stirred the solution for 1 h at room temperature. Finally, the obtained GO was washed with 5% HCl and subsequently washing of GO with distilled water was done by using centrifugation techniques at 7000 rpm until the pH level reached to its neutral state.
Preparation of GO-Pd-Fe nanohybrids The amalgamation of PdeFe metal nanoparticles with the synthesized GO was done via chemical route (Scheme 1). Briefly, 80 mg of GO in 20 ml of ethylene glycol solution was sonicated for 1 h at room temperature. On the other hand, 22.22 mg PdCl2 (13.34 mg Pd) and 23.47 mg FeCl2$6H2O (6.66 mg Fe) were dispersed into another 20 ml ethylene glycol and sonicated it for 1 h. After that, both the solutions were mixed and sonicated again for 1hr maintaining the pH of solution at 12 with drop by drop addition of 15 ml 1M NaOH solution. Next, the mixture was heated at 130 C for 1 h at stirring condition for obtaining homogeneous dispersion of the metals nanoparticles into the GO suspension. Thus prepared GO-PdFe systems was cooled to room temperature while maintaining the pH at 4 through the addition of nearly 20 ml of 1 M HCl and washing with DI water through centrifugation to get the final GO-Pd-Fe nanohybrids.
Characterization The characterization of this synthesized graphene oxide (GO) and graphene oxide-palladium-iron (GO-Pd-Fe) nanohybrids was performed using various spectroscopic and microscopic techniques. In this order, Raman spectroscopy was performed by using Raman spectrophotometer (Research India RIRMLP1519-532 nm) for the confirmation of GO and GO-Pd-Fe nanohybrids. FT-IR analysis was performed by using KBr pellet method (Model IR Perkin Elmer, Spectrum 2000) within the scan range of 400 and 4000 cm1 for the detection of functional groups, XRD analysis was carried out by using Rigaku Japan (Ultima IV X-ray diffractometer) for the analysis of degree of graphitization and crystallization in the GO and
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GO-Pd-Fe nanohybrids, UVeVis spectrophotometer was performed out by using Agilent Technologies (Cary 60-UV-Vis spectrophotometer), Scanning electron microscopy (SEM) analysis was performed by using JOEL (Model-JSM-7610F) for the analysis of surface morphology, Transmission electron microscopy (TEM), High resolution Transmission electron microscopy (HRTEM) and Energy, dispersive X-ray spectroscopy (EDS) has been performed by using (JEOL JEM 2100 microscope) and Cyclic voltammograms (CV), were carried out by a Galvanostat-Potentiostat (PGSTAT 300N, Autolab, the Netherlands) for the estimation of oxygen-reduction reaction (ORR) and Electro chemical surface area (ECSA) activity.
Results and discussion Raman spectroscopy is a widely used spectroscopic tool for the analysis of graphene oxide. Fig. 1(a) shows two characteristic peaks at 1317 cm1 and 1550 cm1 corresponding to D and G bands, respectively. The D band at 1317 cm 1 associated with structural defects and disordered structure of the sp2 carbon atom of graphene sheet. The D band also shows the conversion of sp2 hybridized carbon atoms into sp3 hybridized carbon atoms, which is due to the oxidation of graphite during the oxidation process. The second band is G band at 1550 cm1, which arises due to the aromatic sp2 carbon atom used to elucidate the degree of graphitization. On the other hand, Raman spectra of GO-Pd-Fe nanohybrid, the D band has found with intensity rise sharply in compare to the D band of the graphene oxide, due the generation of defects on the surface of the GO nanosheets and presence of excessive incorporation of Pd and Fe nanoparticles in the edge of the graphene oxide sheets Fig. 1(b). Due to the incorporation of Pd and Fe nanoparticles over the sheet of graphene oxide, a significant blue shift also observed in the Raman spectrum of the GO-Pd-Fe. Further, the D band slightly shifts form 1317 cm1 to 1302 cm1 due to the more edge defects in GO sheet during the intercalation of Pd and Fe nanoparticles. Similarly, G bands shifted from 1550 cm1 to 1535 cm1 due to more intercalation and more defected GO sheets. However, this shifting of D and G band is expected due to the more scattered array of metal nanoparticles over the graphene oxide sheets. Further, the peak shifting in the Raman spectra of GO-Pd-Fe also depicted that dispersion of Pd and Fe nanoparticles over the graphene oxide sheets established the relation between the organic and inorganic form regarding the inelastic phase of the catalyst system. One of the another cause regarding the blue shift in the Raman spectrum of GO-Pd-Fe catalyst is the transformation of nanohybrid system to metallic phase, which is also confirmed by the computational analysis in the present study. The computational study also suggested the view that these PdeFe metal nanoparticles more abundantly occupies the defected sides of graphene oxide sheets, therefore we may seen such kinds of the shift in the Raman spectra of the GO-Pd-Fe catalysis system. Moreover, the intense peak of D band in the Raman spectrum of GO-Pd-Fe nanohybrid is due to the dispersion of metal nanoparticles over the graphene oxide sheets, which developed the formation of electrocatalytic active sides for the ORR reactions. Additionally, the higher intensity of the D band in the nanohybrids
Please cite this article as: Dhali S et al., Graphene oxide supported Pd-Fe nanohybrid as an efficient electrocatalyst for proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.131
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Scheme 1 e Flow chart regarding the synthesis of the GO-Pd-Fe nanohybrid electrocatalyst.
indicates the higher distribution rate of these nanoparticles over the sheets at the localized sp2 hybridized carbon atoms, where the d-orbital's of the metal nanoparticles interact with the localized sp2 hybridized carbon atoms to get the stability by forming local bonds between the metal d-orbital's and the p-bonds of the sp2 hybridized carbon atoms, thereby stabilizing the dispersion of the metal nanoparticles. Beside this, the tendency of making local bonds between the Fe nanoparticles and sp2 hybridized carbon atoms of the reduced graphene oxide sheets reduces the tendency of the formation of iron oxide of the iron nanoparticles, thereby enhances the performance of the electrocatalyst for the ORR reactions [38]. The X-Ray diffraction (XRD) analysis of GO shows peaks at 2q ¼ 12.0 and (002) at 2q ¼ 27.3 with an interlayer space of 3.379A, Fig. 1(c) indicating amorphous and graphitic nature of synthesized GO by the modified Hummers' method. The XRD peaks in the 12 and 13.64 are assigned for the exfoliated graphene nanosheets due to the chemical exfoliation by using oxidizing agents for the synthesis procedure and the reduction of graphene oxide due to the presence of reductive solvent used for the synthesis procedure of GO-Pd-Fe nanohybrids. Further, it also showed the interlayer spacing of 7.49 A because of intercalation of Pd and Fe metal nanoparticles over the graphene oxide sheets. The high intensity sharp peak at 2q ¼ 39.96 in XRD spectrum of GO-Pd-Fe is analogous to (111) interlayer 2D spacing due to the crystallographic property of Pd metal, while the lower intensity peak of
Fe at 46.41 corresponds to (111) diffraction plane with the inter layer spacing along c-axis. Hence, the results indicated the successful reduction of graphene oxide and metal precursors for the synthesis of GO-Pd-Fe nanohybrid system. Further, FT-IR spectra of GO Fig. 2(a) indicates the presence of various oxygenating groups and showed various peaks, such as peak at 3420 cm1 corresponds to OeH stretching vibration, peak at 1725 cm1 and 1640 cm1 corresponds to C]O stretching frequency due to presence of carboxylic and carbonyl functional groups, respectively. Also the peaks at 1225 cm1 and 1050 cm1 are due to the presence of CeO stretching vibrations. UVeVis spectrum of GO and GO-Pd-Fe catalyst are shown in Fig. 2 (b), where GO showed the characteristic peak at 203 nm due to the p / p* transition of the C]C double bond. The dispersion of PdeFe over the GO was shifted the peak towards the longer wavelength of 263 nm, thus showing an enhanced red shift because of the dispersion of metal nanoparticles of Pd and Fe. Moreover, this dispersion of Pd and Fe nanoparticles was clearly seen in TEM images, where the average size of the Pd and Fe nanoparticles was found to in the range of 2.5e3 nm over the GO sheets, which exhibits an average sheet thickness of 2e3 nm (Fig. 3c and d). HRTEM analysis was also conducted (Fig. 4) to confirm the presence of fringes of palladium and iron in the synthesized nanohybrid system of GO-Pd-Fe. Therefore, the Fig. 4c and d indicated that there was no formation of fringes or sometime the fringes of
Please cite this article as: Dhali S et al., Graphene oxide supported Pd-Fe nanohybrid as an efficient electrocatalyst for proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.131
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Fig. 1 e (a) Raman spectra of GO (b) Raman spectra of GO-Pd-Fe (c) XRD spectra of GO (d) XRD spectra of GO-Pd-Fe.
Fig. 2 e (a) FT-IR spectra GO (b) UV spectra of GO and GO-Pd-Fe.
Please cite this article as: Dhali S et al., Graphene oxide supported Pd-Fe nanohybrid as an efficient electrocatalyst for proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.131
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Fig. 3 e (a) SEM images of GO (b) SEM image of GO-Pd-Fe, the distribution of 1catalyst over graphene oxide sheet (c) TEM image of GO (d) TEM image of GO-Pd-Fe.
Fig. 4 e (a) HRTEM images of GO-Pd-Fe nanohybrid shows the dispersion of Pd-Fe particle over GO nanosheet, (b) Pd with GO lattice Fringes (c) and (d) GO-Pd-Fe nanohybrid without firings. Please cite this article as: Dhali S et al., Graphene oxide supported Pd-Fe nanohybrid as an efficient electrocatalyst for proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.131
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Fig. 5 e (a) Map sum spectra of GO-Pd-Fe (b) EDX layered images of GO-Pd-Fe, shows the distribution of catalyst on graphene oxide sheet.
Table 1 e The distribution concentration of Pd and Fe nanoparticles over the GO sheet by EDS. Element C N O Fe Pd Total:
Line Type K K K K K
series series series series series
k Factor
k Factor type
2.781 3.530 2.028 1.136 8.900
Pd disordered structures due to the intercalation of Fe particle in to the fcc lattice of Pd. Thus, the HRTEM images also demonstrated that Fe particle was hooked on the Pd crystal lattice and gained a distinct phase fcc muddled structure, which showed a excellent conformity with the result of XRD [39]. Moreover, it was also reported that graphene oxide sheets behaved as corrosion protector for metal catalyst, thereby, the iron metal hooked inside the Pd fcc lattice structure was protected from the corrosion effect [40]. The distribution concentration of Pd and Fe nanoparticles over the GO sheet were evaluated by EDS analysis (Fig. 5a and b). EDS analysis showed that GO-Pd-Fe nanohybrid displayed the occurrence of mainly carbon, palladium and iron. The present elements and their concentrations in the GO-Pd-Fe nanohybrid are tabulated in Table 1. Further, to confirm the effective adsorption of Pd and Fe nanoparticles over the graphene oxide sheets, scanning electron microscopy was performed for GO and GO-Pd-Fe catalysis system. SEM images of graphene oxide (Fig. 3a and b) showed few layered stacks of graphene oxide sheets with clearly depicted edge defects. These few layer stack of graphene oxide sheets when came into the contact of PdeFe nanoparticles, behaves as a matrix for the effective adsorption of these metal nanoparticles. To better understand Pd/Fe nanoparticle adsorption on the GO surface, Density Functional Theory (DFT) calculations have been performed for Pd/Fe nanocluster adsorption on pristine and defected graphene surface. Though GO was used for experimental
Absorption Correction
Wt%
Wt% Sigma
1.00 1.00 1.00 1.00 1.00
56.95 4.26 21.40 0.79 16.61 100.00
8.86 2.91 4.49 0.36 4.30
synthesis of the nanohybrid, the simulations are performed on graphene sheet to avoid unnecessary computational cost. The DFT calculations are performed using Quantum ATK code [41]. The Perdew-Burke-Ernzerhof (PBE) functional within Generalized Gradient Approximation (GGA) is used to describe the exchange-correlation interaction energy of electrons. The Van der Waals interaction between the transition metal cluster and graphene sheet is included with the help of semi-
Table 2 e The binding strength of Pd/Fe nanoparticles at the defect sites of GO is estimated by the Perdew-BurkeErnzerhof (PBE) functional within Generalized Gradient Approximation (GGA). Configuration Pristine Graphene Graphene þ Fe Graphene þ Pd Graphene þ Pd_Fe (Pd:Fe ¼ 1:1) Graphene þ Pd_Fe (Pd:Fe ¼ 2:1) Defected Graphene Defected Graphene þ Fe Defected Graphene þ Pd Defected Graphene þ Pd_Fe (Pd:Fe ¼ 1:1) Defected Graphene þ Pd_Fe (Pd:Fe ¼ 2:1)
Binding Energy (eV)
Electronic Nature
e 5.36 4.88 3.35 3.33 e 9.45 8.95 8.64
Zero Band Gap Semi-Metallic Metallic Semi-Metallic Metallic Metallic Metallic Metallic Metallic
7.07
Semi-Metallic
Please cite this article as: Dhali S et al., Graphene oxide supported Pd-Fe nanohybrid as an efficient electrocatalyst for proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.131
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All structures were relaxed with 12*12*1 Monkhorst-Pack of kpoints, while the DOS plots are obtained at 27*27*1 k-points for accuracy. The influence of transition metals (Pd, Fe) on graphene-Pd-Fe nanohybrid was assessed through the calculation of structural and electronic properties of graphene in the presence of metal clusters. Fe cluster of 8 atoms, Pd cluster of 8 atoms, PdeFe cluster of 8 atoms with Pd:Fe ratio 1:1, and PdeFe cluster of 9 atoms with Pd:Fe ratio 2:1 were adsorbed separately on periodic hexagonal 4*4 graphene sheet of 32 atoms, so as to maintain the transition metal concentration at ~20% in the nanohybrid. Fig. 7(aec) depicts various metal clusters prepared and the adsorption of Fe cluster on pristine graphene sheet for representative purpose. The adsorption strength of these metal clusters on graphene surface is assessed through calculation of binding energies (EB) using the expression below. Fig. 6 e CV analysis of GO-Pd-Fe, commercial Pt/C catalyst system at 0.1 M H2So4 the scan rate of 10 mV/s.
empirical Grimme DFT-D2 correction [42]. The valance electrons are described by localized pseudoatomic orbitals with a double zeta polarized basis set, and a large density mesh cutoff of 130 Hartree considered for accuracy of calculations.
EB ¼ EGraphene sheet þ EMetal Cluster EHybrid Composite
(1)
Where, E represents energy. The calculated binding energies (EB) for each metal cluster adsorption case are tabulated in Table 2. In general, higher binding energy implies strong adsorption. All the binding energies listed in Table 1 are positive indicating stable adsorption; please note that the binding energies in this work are calculated by subtracting the energy of product from that
Fig. 7 e (a) Fe, Pd clusters and Pd-Fe clusters in 1:1 and 2:1 ratio. (b) Pristine Graphene sheet. (c) Top view and side view of Fe cluster adsorption on pristine graphene for representative purpose (d) Defected Graphene sheet. (e) Top view and side view of Fe cluster adsorption on defected graphene for representative purpose. Please cite this article as: Dhali S et al., Graphene oxide supported Pd-Fe nanohybrid as an efficient electrocatalyst for proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.131
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Fig. 8 e (aee), (a) Density of states (DOS) profile of pristine graphene sheet. (bee) DOS profiles of transition metal graphene hybrid composites (black colored upper plot) and the contribution of transition metal clusters to DOS (blue colored lower plot). (Fermi level EF is located at energy zero). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
of reactants. Large binding energies are observed owing to the chemisorptions of metal clusters on graphene surface. The binding energy data reveals relatively stronger adsorption of Fe metal than Pd on the graphene surface. The binding energy of Fe cluster on graphene is 5.36 eV, while it is 4.88 eV for the case of Pd cluster. Similarly, the binding energy reduced from 3.35 eV to 3.33 eV when the Pd share is doubled in the PdeFe cluster. Moreover, the binding strength of Pd/Fe nanoparticles at the defect sites of GO is estimated by adsorbing the PdeFe clusters on the defected graphene. Fig. 6(dee) shows the defected graphene sheet having defect concentration of 3.12% and the adsorption of transition metal cluster on defected graphene. From Table 2, the binding strength of metal clusters at defect site of graphene has increased dramatically in comparison to the pristine case, indicating strong chemisorptions. The chemical bonds between metal cluster and defected graphene were almost doubled in comparison to the pristine case. This is due to the availability of dangling bonds on both the defect site of graphene and metal cluster surface that prompted the strong adsorption. Similar
to the pristine case, the Fe cluster demonstrated relatively strong adsorption on the defected graphene surface than the Pd cluster. The binding energy of Fe cluster at defect site of graphene is 9.45 eV, while it is 8.95 eV for the case of Pd cluster. Correspondingly, the binding energy reduced from 8.64 eV to 7.07 eV when the Pd share is doubled in the PdeFe complex of defected graphene based hybrid composite. To conclude the adsorption analysis, the Pd/Fe metal clusters are more active towards the defect sites of graphene surface in comparison to the pristine sites, owing to the strong binding energies. This infers that the Pd/Fe nanoparticles of synthesized GO-Pd-Fe nanohybrid are most likely occupied the defect sites of GO. Furthermore, the influence of Pd/Fe nanoclusters on the electronic nature of the pristine and defected graphene has been estimated through computation of density of states (DOS) profiles, as shown in Fig. 8 and Fig. 9. From Fig. 8, pristine graphene exhibits a perfect zero band gap at the dirac point, whereas all other graphene-Pd/Fe nanohybrids exhibit either semi-metallic or metallic nature as described in Table 1. Graphene þ Fe and graphene þ Pd/Fe (Pd:Fe ¼ 1:1)
Please cite this article as: Dhali S et al., Graphene oxide supported Pd-Fe nanohybrid as an efficient electrocatalyst for proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.131
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Fig. 9 e (a) Density of states (DOS) profile of Defected Graphene (DG) sheet. (bee) DOS profiles of transition metal defected graphene hybrid composites (black colored upper plot) and the contribution of transition metal clusters to DOS (blue colored lower plot). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
nanohybrid exhibited semi-metallic nature, while graphene þ Pd and graphene þ Pd/Fe (Pd:Fe ¼ 2:1) hybrid composites exhibited metallic nature. From Fig. 5(a), creation of a vacancy defect in graphene has altered the electronic nature from zero-band gaped to metallic, which is a result of Fermi level movement into the valence band in comparison to the pristine graphene due to the vacancy induced electron deficiency. The nanohybrids formed by adsorption of transition metal clusters on defected graphene have also retained the metallic nature, except the Pd/Fe (Pd:Fe ¼ 2:1) cluster adsorption case that resulted in semi-metallic behavior to the defected graphene. The common observations made from both Fig. 8(aee) and Fig. 9(aee) is that, the DOS profiles of nanohybrids are completely dominated by the peaks from transition metal clusters. Moreover, Fe induced DOS peaks are distributed in both valence and conduction bands, while the Pd induced peaks are limited mainly to the valance band region of DOS profile. This might be due to the presence of
completely filled d e sub shell in the valence of Pd (electron configuration of Pd: [Kr] 4d10), unlike Fe that has partially filled d e sub shell in its valence (electron configuration of Fe: [Ar] 3d64s2). To conclude the DOS analysis, the adsorption of Pd/Fe clusters on graphene surface has resulted in enhanced band nature of graphene from zero-band gaped to metallic/semimetallic. The enhanced band nature of nanohybrid may prove beneficial during its application as anode and cathode catalyst as the metallic natured materials transport electrons more efficiently than the semiconducting and insulating materials. As the performance of the GO-Pd-Fe nanohybrid catalyst as anode and cathode catalyst highly dependent upon the electrochemical surface area (ECSA), which is a measure of the electrochemically active sites per gram of the catalyst, Therefore cyclic voltametry (CV) analysis was performed to analyze the catalytic properties of the GO-Pd-Fe nanohybrid (Fig. 6). showed the CV analysis of commercial Pt/C and GOPd-Fe nanohybrid catalytic system at a scan rate 10 mVs1.
Please cite this article as: Dhali S et al., Graphene oxide supported Pd-Fe nanohybrid as an efficient electrocatalyst for proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.131
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The evaluations of ECSA were obtained by integrating area under the curve in the region of hydrogen adsorption and desorption after double layer correction. CV analysis regarding the ECSA evaluation showed that GO-Pd-Fe nano hybrids catalyst showed higher value of mass specific of ECSA (58.08 m2/g), than the commercial Pt/C, (37.87 m2/g). The ECSA value obtained from the developed GO-Pd-Fe nanohybrid is considered to be very high, which is comparable or even higher than most of the previous reports of Pd-based catalysts. For example, Seo et al. [43] estimated the ESCA value for the Pd doped graphene nanosheets to be 54.9 m2/g, whereas Zhang et al. [44] found the ESCA value of GO/Pd1/Pt3 as 49.8 m2/ g. In a very recent report, Yang et al. [45] reported that Pt77Pd23 ACSNDs/rGO could show the ECSA value of 55.92 m2/g. In this regard, our study showed that our nanohybrid electrocatalyst showed better performance than commercially available Pt/C catalyst and previously reported systems. Therefore, the present GO-Pd-Fe nanohybrid substantiates the implication of enhancing catalytic activity than the commercial Pt/C catalyst. This is due to the amalgamation of Pd and Fe nanoparticles, when dispersed in the ratio of 2:1 over the GO sheet. Also, computational studies suggest that the 2:1 ratio triggers metallic nature in the nanohybrids, therefore facilitates electron transportation via the matrix of GO nanosheets. Further, the strong binding energy of the Fe nanoparticles for the GO sheets improves chemical reactivity and electrocatalytic performance of graphene nanosheets. Therefore, the present study of the GO-Pd-Fe nanohybrids depicted the similarities between the experimental and computational analysis.
Conclusion In conclusion, we have successfully demonstrated the utility of GO-Pd-Fe nanohybrids catalyst for the oxidation-reduction reaction in PEMFCs. The present work explores noticeable properties of the GO-Pd-Fe nanohybrids as catalyst, experimentally as well as computationally. The matrix of graphene oxide used as frame of support for the growth of palladium (Pd) and iron (Fe) nanopaticles over graphene oxide sheet showed that these nanohybrids could be used in PEMFCs. Further, the present work also clarified via computational analysis with the help of Density Functional Theory (DFT) with semi-empirical Grimme DFT-D2, which demonstrates good agreements with the experimental study. Experimental result showed that the nanohybrid of GO-Pd-Fe nanohybrids electro catalyst showed an enhanced catalytic activity with electrochemical surface area of (58.08 m2/g PdeFe)1, which is higher than the commercial available Pt/C catalyst with electrochemical surface area 37.87 m2 (g Pt)1, which supports the computational results obtained from the computational analysis. Thus overall study showed that GO-Pd-Fe nanohybrids with the metallic ratio of 2:1 of Pd and Fe not only showed an enhanced performance for the ORR reaction for the PEMFCs, but it also showed an effective way to synthesize cost effective metal catalyst to replace the costly platinum based catalyst.
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Acknowledgements The work is supported by the National Mission of Himalayan Studies, Kosi Katarmal, Almora, India (Ref No. NMHS/MG2016/002/8503-7) and DST INSPIRE division (IF150750), New Delhi, India.
references
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Graphene oxide supported Pd-Fe nanohybrid as an efficient electrocatalyst for proton exchange membrane fuel cells Sunil Dhali a, Manoj Karakoti a, Sandeep Pandey a, Boddepalli SanthiBhushan b, Ravindra Kumar Verma c, Anurag Srivastava b, Rajaram Bal d, S.P.S. Mehta a, Nanda Gopal Sahoo a,* a
Nanoscience and Nanotechnology Centre, Department of Chemistry, D.S.B. Campus Kumaun University, Nainital, 263002, India b Atal Bihari Vajpayee - Indian Institute of Information Technology and Management, Gwalior, 474015, Madhya Pradesh, India c National Mission of Himalayan Studies, G.B. Pant National Institute of Himalayan Environment and Sustainable Development, Kosi Katarmal, Almora, India d Conversion & Catalysis Division, CSIR-Indian Institute of Petroleum, Dehradun, India
highlights
graphical abstract
GO supported PdeFe nanohybrid has been successfully synthesized for PEMFCs. Size of Pd and Fe nanoparticles were in the range of 2.5e3.0 nm in the GO sheets. Computational analysis has been done for the validation of the GOPd-Fe nanohybrid. ECSA of GO-Pd-Fe (58.08 m2/g) was higher
than
Pt/C
catalyst
(37.87 m2/g).
article info
abstract
Article history:
The experimental realization and computational validation for graphene oxide (GO) sup-
Received 28 March 2019
ported palladium (Pd)-iron (Fe) nanohybrids as a new generation electrocatalyst for proton-
Received in revised form
exchange membrane fuel cells (PEMFCs) has been reported. The experimental apprehen-
9 August 2019
sion of the present catalyst system has been initiated with the graphene oxide, followed by
Accepted 13 September 2019
the doping of Pd and Fe via thermal inter calation of palladium chloride and iron chloride
Available online xxx
with the in-situ downstream reduction to get nanohybrids of the GO-Pd-Fe. These nanohybrids are subsequently characterized by RAMAN, FT-IR, UVeVis, XRD, SEM, EDS, TEM
Keywords:
and HRTEM analysis. Furthermore, the first principle calculations based on Density
Catalyst
Functional Theory (DFT) with semi-empirical Grimme DFT-D2 correction has been
* Corresponding author. E-mail address: [email protected] (N.G. Sahoo). https://doi.org/10.1016/j.ijhydene.2019.09.131 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Dhali S et al., Graphene oxide supported Pd-Fe nanohybrid as an efficient electrocatalyst for proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.131
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Cyclic voltammetry
performed to support the experimental findings. Computational results revealed the
DFT
alteration of graphene electronic nature from zero-band gaped to metallic/semi-metallic
Fuel cell
on adsorption of transition metal clusters. Moreover, the defect sites of the graphene
Graphene oxide etc
surface are more favorable than the pristine sites for transition metal adsorption owing to the strong binding energies of the former. Electrochemical studies show that GO-Pd-Fe nanohybrids catalyst (Pd: Fe ¼ 2:1) demonstrates excellent catalytic activity as well as the higher electrochemical surface area of (58.08 m2/g PdeFe)1 which is higher than the commercially available Pt/C catalyst with electrochemical surface area 37.87 m2/(g Pt)1. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Proton exchange membrane fuel cells (PEMFCs) are regarded as imperative electrochemical energy devices to overcome the problems associated with combustion of fossil fuels and to control the emission of green house gases. In recent decades, Pt based materials have emerged as efficient catalysts for the development of next generation fuel cells [1e4]. These catalysts have been extensively used as anode and cathode electro-catalyst of fuel cells for oxygen reduction reaction to generate energy [5]. However, the Pt based catalyst has limited practical application of PEMFCs due to their significantly high cost, poor utilization coefficient and slow kinetics. Besides, the pure Pt based electro-catalyst normally endures several disadvantages such as poisoning during the oxidation reduction reaction due to the formation of CO-like species [6,7]. In recent years, graphene supported novel metal catalyst systems are replacing the Pt-based electro-catalysts. Graphene, a single layer thick of carbon atoms arranged in a 2dimensional macromolecule, has been actively pursued as an enthralling material with unusual properties. This hexagonal ring matrix has attracted much attention of the researchers as a new generation material for fuel cell application owing to its large surface area and high electrical conductivity [8e12]. These properties of graphene and graphene oxide exhibit their use as a suitable electro catalyst support and nano-catalyst material for PEMFCs. Several reports have been studied about graphene as an efficient catalyst support for various metal nanoparticles and demonstrated impressive catalytic efficacy and durability. Recently, palladium based nano hybrids catalysts like PalladiumeIron, PalladiumeCobalt, Palladium-Nikel, PalladiumeCopper, PalladiumeSilver and PalladiumeGold have been investigated as promising electro-catalysts for the oxygen reduction reaction in basic medium [13e24]. Recently, Gahlot et al. showed the graphene oxide based nanocomposite membrane for the practical application of DMFC and electrodialysis with superior properties. However, dispersability of the graphene based nanocomposite plays a significant role for the enhancement in the efficiency of different kinds of fuels cells. In this regard Kansara et al. reported that graphene nanosheets exhibits excellent high loading efficiency for paclitaxel when produced a stable dispersion up to 0.1 wt% [25,26]. While according to Baronia et al. the catalytical activity of Pt based binary anodic catalyst works as an admirable electrocatalyst for electro chemical oxidation reactions of fuel cells.
They have reported that rGO supported PteCo based electrocatalyst exhibits high performance for ethylene glycol electro oxidation process which follows the reductive pathway [27]. Similarly Liu et al. synthesized PdeAg/rGO supported with 1:1 mass ratio catalyst, where they have found that PdeAg/rGO has much better catalytic performance for electro-oxidation of ethanol [28]. Esabattina et al. have developed eletrocatalyst based on PtePd/rGO system, which shows the prominent electrocatalytic activity in the direction of methanol oxidation compared to bare Pt-Pd [29]. Further, Li et al. showed that CoePd/ rGO, and Ni/rGO, have the maximum electrocatalytic activity, electrochemical stability and better tolerance to CO poisoning, whereas their results are also applicable for formic acid oxidation and H2O2 reduction, respectively [30]. Recently, Feng et al. have prepared surfactant free PdeFe nanoparticles supported on rGO, which demonstrated impressive catalytic activity for formic acid oxidation. Their findings showed that 1:5 ratio of PdeFe in PdeFe/rGO hybrid exhibit maximum performance with a specific activity of 2.72 mA/cm2 at the mass activity of 1.0 A/mg, which is comparatively higher than Pd/rGO and commercially available Pd/C catalysts [31e36]. However, still the appropriate ratio PdeFe metal loading in graphene nanosheets is one of the challenging issues for the development of GO-Pd-Fe nanohybrids. Therefore in the present study, we are reporting, a placid and eco-friendly ethylene glycol based reducing predecessor to synthesize graphene oxide supported PdeFe (2:1 ratio) nanohybrid electro-catalyst, through uniform distribution of Pd and Fe nanoparticales overthe surface of graphene oxide. Further the electrocatalytic performance along with theoretical calculations has been investigated to verify the efficacy of thus prepared GOPd-Fe nanohybrid electrocatalyst for oxidation reduction reaction of PEMFCs. Our nanohybrid electrocatalyst showed the high electrochemical surface area and soaring abundance of the Pd/ Fe metals with tolerable catalytic activity of nanohybrids in comparison to the Pt-based catalyst systems. Moreover, the present nanohybrids offers enhanced catalytic activity and with the higher surface area i.e. 58.08 m2/g in comparison to the commercially available Pt/C 37.87 m2/g catalysts.
Materials and methods Materials Expanded graphite powder of 100 mm (approx.) was purchased from Sigma Aldrich. Potassium permanganate (KMnO4),
Please cite this article as: Dhali S et al., Graphene oxide supported Pd-Fe nanohybrid as an efficient electrocatalyst for proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.131
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sodium nitrate (NaNO3), sulphuric acid (H2SO4), hydrochloric acid (HCl), sodium hydroxide (NaOH), hydrogen peroxide (H2O2 30% aq), iron (II) chloride tetra hydrate 99.99% trace metal basis, Palladium (II) chloride (PdCl2) and ethylene glycol (C2H6O2) were purchased from SRL, and sillicone oil purchased from Merk. Commercial available Pt/C catalyst purchased from Sigma Aldrich and used as received.
Synthesis of GO GO was synthesized by using modified Hummer's method [37]. Briefly, 1 g of graphite powder was placed in a beaker and 50 ml of concentrated H2SO4 with 1 g of NaNO3 was added to subsequently and stirred for 4 h, followed by addition of 6 g of KMnO4 in ice bath with at a temperature of 35 C for 2 h. Thereafter, 90 ml of de-ionized water was added slowly to the mixture and maintained the temperature at 95 C for 2 h with continuous stirring. Subsequently, the heat supply was turned off and 200 ml de-ionized (DI) water was added to the mixture while stirring for 1 h. The mixture was cooled and 20 ml of 30% H2O2 was added, and again stirred the solution for 1 h at room temperature. Finally, the obtained GO was washed with 5% HCl and subsequently washing of GO with distilled water was done by using centrifugation techniques at 7000 rpm until the pH level reached to its neutral state.
Preparation of GO-Pd-Fe nanohybrids The amalgamation of PdeFe metal nanoparticles with the synthesized GO was done via chemical route (Scheme 1). Briefly, 80 mg of GO in 20 ml of ethylene glycol solution was sonicated for 1 h at room temperature. On the other hand, 22.22 mg PdCl2 (13.34 mg Pd) and 23.47 mg FeCl2$6H2O (6.66 mg Fe) were dispersed into another 20 ml ethylene glycol and sonicated it for 1 h. After that, both the solutions were mixed and sonicated again for 1hr maintaining the pH of solution at 12 with drop by drop addition of 15 ml 1M NaOH solution. Next, the mixture was heated at 130 C for 1 h at stirring condition for obtaining homogeneous dispersion of the metals nanoparticles into the GO suspension. Thus prepared GO-PdFe systems was cooled to room temperature while maintaining the pH at 4 through the addition of nearly 20 ml of 1 M HCl and washing with DI water through centrifugation to get the final GO-Pd-Fe nanohybrids.
Characterization The characterization of this synthesized graphene oxide (GO) and graphene oxide-palladium-iron (GO-Pd-Fe) nanohybrids was performed using various spectroscopic and microscopic techniques. In this order, Raman spectroscopy was performed by using Raman spectrophotometer (Research India RIRMLP1519-532 nm) for the confirmation of GO and GO-Pd-Fe nanohybrids. FT-IR analysis was performed by using KBr pellet method (Model IR Perkin Elmer, Spectrum 2000) within the scan range of 400 and 4000 cm1 for the detection of functional groups, XRD analysis was carried out by using Rigaku Japan (Ultima IV X-ray diffractometer) for the analysis of degree of graphitization and crystallization in the GO and
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GO-Pd-Fe nanohybrids, UVeVis spectrophotometer was performed out by using Agilent Technologies (Cary 60-UV-Vis spectrophotometer), Scanning electron microscopy (SEM) analysis was performed by using JOEL (Model-JSM-7610F) for the analysis of surface morphology, Transmission electron microscopy (TEM), High resolution Transmission electron microscopy (HRTEM) and Energy, dispersive X-ray spectroscopy (EDS) has been performed by using (JEOL JEM 2100 microscope) and Cyclic voltammograms (CV), were carried out by a Galvanostat-Potentiostat (PGSTAT 300N, Autolab, the Netherlands) for the estimation of oxygen-reduction reaction (ORR) and Electro chemical surface area (ECSA) activity.
Results and discussion Raman spectroscopy is a widely used spectroscopic tool for the analysis of graphene oxide. Fig. 1(a) shows two characteristic peaks at 1317 cm1 and 1550 cm1 corresponding to D and G bands, respectively. The D band at 1317 cm 1 associated with structural defects and disordered structure of the sp2 carbon atom of graphene sheet. The D band also shows the conversion of sp2 hybridized carbon atoms into sp3 hybridized carbon atoms, which is due to the oxidation of graphite during the oxidation process. The second band is G band at 1550 cm1, which arises due to the aromatic sp2 carbon atom used to elucidate the degree of graphitization. On the other hand, Raman spectra of GO-Pd-Fe nanohybrid, the D band has found with intensity rise sharply in compare to the D band of the graphene oxide, due the generation of defects on the surface of the GO nanosheets and presence of excessive incorporation of Pd and Fe nanoparticles in the edge of the graphene oxide sheets Fig. 1(b). Due to the incorporation of Pd and Fe nanoparticles over the sheet of graphene oxide, a significant blue shift also observed in the Raman spectrum of the GO-Pd-Fe. Further, the D band slightly shifts form 1317 cm1 to 1302 cm1 due to the more edge defects in GO sheet during the intercalation of Pd and Fe nanoparticles. Similarly, G bands shifted from 1550 cm1 to 1535 cm1 due to more intercalation and more defected GO sheets. However, this shifting of D and G band is expected due to the more scattered array of metal nanoparticles over the graphene oxide sheets. Further, the peak shifting in the Raman spectra of GO-Pd-Fe also depicted that dispersion of Pd and Fe nanoparticles over the graphene oxide sheets established the relation between the organic and inorganic form regarding the inelastic phase of the catalyst system. One of the another cause regarding the blue shift in the Raman spectrum of GO-Pd-Fe catalyst is the transformation of nanohybrid system to metallic phase, which is also confirmed by the computational analysis in the present study. The computational study also suggested the view that these PdeFe metal nanoparticles more abundantly occupies the defected sides of graphene oxide sheets, therefore we may seen such kinds of the shift in the Raman spectra of the GO-Pd-Fe catalysis system. Moreover, the intense peak of D band in the Raman spectrum of GO-Pd-Fe nanohybrid is due to the dispersion of metal nanoparticles over the graphene oxide sheets, which developed the formation of electrocatalytic active sides for the ORR reactions. Additionally, the higher intensity of the D band in the nanohybrids
Please cite this article as: Dhali S et al., Graphene oxide supported Pd-Fe nanohybrid as an efficient electrocatalyst for proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.131
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Scheme 1 e Flow chart regarding the synthesis of the GO-Pd-Fe nanohybrid electrocatalyst.
indicates the higher distribution rate of these nanoparticles over the sheets at the localized sp2 hybridized carbon atoms, where the d-orbital's of the metal nanoparticles interact with the localized sp2 hybridized carbon atoms to get the stability by forming local bonds between the metal d-orbital's and the p-bonds of the sp2 hybridized carbon atoms, thereby stabilizing the dispersion of the metal nanoparticles. Beside this, the tendency of making local bonds between the Fe nanoparticles and sp2 hybridized carbon atoms of the reduced graphene oxide sheets reduces the tendency of the formation of iron oxide of the iron nanoparticles, thereby enhances the performance of the electrocatalyst for the ORR reactions [38]. The X-Ray diffraction (XRD) analysis of GO shows peaks at 2q ¼ 12.0 and (002) at 2q ¼ 27.3 with an interlayer space of 3.379A, Fig. 1(c) indicating amorphous and graphitic nature of synthesized GO by the modified Hummers' method. The XRD peaks in the 12 and 13.64 are assigned for the exfoliated graphene nanosheets due to the chemical exfoliation by using oxidizing agents for the synthesis procedure and the reduction of graphene oxide due to the presence of reductive solvent used for the synthesis procedure of GO-Pd-Fe nanohybrids. Further, it also showed the interlayer spacing of 7.49 A because of intercalation of Pd and Fe metal nanoparticles over the graphene oxide sheets. The high intensity sharp peak at 2q ¼ 39.96 in XRD spectrum of GO-Pd-Fe is analogous to (111) interlayer 2D spacing due to the crystallographic property of Pd metal, while the lower intensity peak of
Fe at 46.41 corresponds to (111) diffraction plane with the inter layer spacing along c-axis. Hence, the results indicated the successful reduction of graphene oxide and metal precursors for the synthesis of GO-Pd-Fe nanohybrid system. Further, FT-IR spectra of GO Fig. 2(a) indicates the presence of various oxygenating groups and showed various peaks, such as peak at 3420 cm1 corresponds to OeH stretching vibration, peak at 1725 cm1 and 1640 cm1 corresponds to C]O stretching frequency due to presence of carboxylic and carbonyl functional groups, respectively. Also the peaks at 1225 cm1 and 1050 cm1 are due to the presence of CeO stretching vibrations. UVeVis spectrum of GO and GO-Pd-Fe catalyst are shown in Fig. 2 (b), where GO showed the characteristic peak at 203 nm due to the p / p* transition of the C]C double bond. The dispersion of PdeFe over the GO was shifted the peak towards the longer wavelength of 263 nm, thus showing an enhanced red shift because of the dispersion of metal nanoparticles of Pd and Fe. Moreover, this dispersion of Pd and Fe nanoparticles was clearly seen in TEM images, where the average size of the Pd and Fe nanoparticles was found to in the range of 2.5e3 nm over the GO sheets, which exhibits an average sheet thickness of 2e3 nm (Fig. 3c and d). HRTEM analysis was also conducted (Fig. 4) to confirm the presence of fringes of palladium and iron in the synthesized nanohybrid system of GO-Pd-Fe. Therefore, the Fig. 4c and d indicated that there was no formation of fringes or sometime the fringes of
Please cite this article as: Dhali S et al., Graphene oxide supported Pd-Fe nanohybrid as an efficient electrocatalyst for proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.131
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Fig. 1 e (a) Raman spectra of GO (b) Raman spectra of GO-Pd-Fe (c) XRD spectra of GO (d) XRD spectra of GO-Pd-Fe.
Fig. 2 e (a) FT-IR spectra GO (b) UV spectra of GO and GO-Pd-Fe.
Please cite this article as: Dhali S et al., Graphene oxide supported Pd-Fe nanohybrid as an efficient electrocatalyst for proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.131
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Fig. 3 e (a) SEM images of GO (b) SEM image of GO-Pd-Fe, the distribution of 1catalyst over graphene oxide sheet (c) TEM image of GO (d) TEM image of GO-Pd-Fe.
Fig. 4 e (a) HRTEM images of GO-Pd-Fe nanohybrid shows the dispersion of Pd-Fe particle over GO nanosheet, (b) Pd with GO lattice Fringes (c) and (d) GO-Pd-Fe nanohybrid without firings. Please cite this article as: Dhali S et al., Graphene oxide supported Pd-Fe nanohybrid as an efficient electrocatalyst for proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.131
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Fig. 5 e (a) Map sum spectra of GO-Pd-Fe (b) EDX layered images of GO-Pd-Fe, shows the distribution of catalyst on graphene oxide sheet.
Table 1 e The distribution concentration of Pd and Fe nanoparticles over the GO sheet by EDS. Element C N O Fe Pd Total:
Line Type K K K K K
series series series series series
k Factor
k Factor type
2.781 3.530 2.028 1.136 8.900
Pd disordered structures due to the intercalation of Fe particle in to the fcc lattice of Pd. Thus, the HRTEM images also demonstrated that Fe particle was hooked on the Pd crystal lattice and gained a distinct phase fcc muddled structure, which showed a excellent conformity with the result of XRD [39]. Moreover, it was also reported that graphene oxide sheets behaved as corrosion protector for metal catalyst, thereby, the iron metal hooked inside the Pd fcc lattice structure was protected from the corrosion effect [40]. The distribution concentration of Pd and Fe nanoparticles over the GO sheet were evaluated by EDS analysis (Fig. 5a and b). EDS analysis showed that GO-Pd-Fe nanohybrid displayed the occurrence of mainly carbon, palladium and iron. The present elements and their concentrations in the GO-Pd-Fe nanohybrid are tabulated in Table 1. Further, to confirm the effective adsorption of Pd and Fe nanoparticles over the graphene oxide sheets, scanning electron microscopy was performed for GO and GO-Pd-Fe catalysis system. SEM images of graphene oxide (Fig. 3a and b) showed few layered stacks of graphene oxide sheets with clearly depicted edge defects. These few layer stack of graphene oxide sheets when came into the contact of PdeFe nanoparticles, behaves as a matrix for the effective adsorption of these metal nanoparticles. To better understand Pd/Fe nanoparticle adsorption on the GO surface, Density Functional Theory (DFT) calculations have been performed for Pd/Fe nanocluster adsorption on pristine and defected graphene surface. Though GO was used for experimental
Absorption Correction
Wt%
Wt% Sigma
1.00 1.00 1.00 1.00 1.00
56.95 4.26 21.40 0.79 16.61 100.00
8.86 2.91 4.49 0.36 4.30
synthesis of the nanohybrid, the simulations are performed on graphene sheet to avoid unnecessary computational cost. The DFT calculations are performed using Quantum ATK code [41]. The Perdew-Burke-Ernzerhof (PBE) functional within Generalized Gradient Approximation (GGA) is used to describe the exchange-correlation interaction energy of electrons. The Van der Waals interaction between the transition metal cluster and graphene sheet is included with the help of semi-
Table 2 e The binding strength of Pd/Fe nanoparticles at the defect sites of GO is estimated by the Perdew-BurkeErnzerhof (PBE) functional within Generalized Gradient Approximation (GGA). Configuration Pristine Graphene Graphene þ Fe Graphene þ Pd Graphene þ Pd_Fe (Pd:Fe ¼ 1:1) Graphene þ Pd_Fe (Pd:Fe ¼ 2:1) Defected Graphene Defected Graphene þ Fe Defected Graphene þ Pd Defected Graphene þ Pd_Fe (Pd:Fe ¼ 1:1) Defected Graphene þ Pd_Fe (Pd:Fe ¼ 2:1)
Binding Energy (eV)
Electronic Nature
e 5.36 4.88 3.35 3.33 e 9.45 8.95 8.64
Zero Band Gap Semi-Metallic Metallic Semi-Metallic Metallic Metallic Metallic Metallic Metallic
7.07
Semi-Metallic
Please cite this article as: Dhali S et al., Graphene oxide supported Pd-Fe nanohybrid as an efficient electrocatalyst for proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.131
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All structures were relaxed with 12*12*1 Monkhorst-Pack of kpoints, while the DOS plots are obtained at 27*27*1 k-points for accuracy. The influence of transition metals (Pd, Fe) on graphene-Pd-Fe nanohybrid was assessed through the calculation of structural and electronic properties of graphene in the presence of metal clusters. Fe cluster of 8 atoms, Pd cluster of 8 atoms, PdeFe cluster of 8 atoms with Pd:Fe ratio 1:1, and PdeFe cluster of 9 atoms with Pd:Fe ratio 2:1 were adsorbed separately on periodic hexagonal 4*4 graphene sheet of 32 atoms, so as to maintain the transition metal concentration at ~20% in the nanohybrid. Fig. 7(aec) depicts various metal clusters prepared and the adsorption of Fe cluster on pristine graphene sheet for representative purpose. The adsorption strength of these metal clusters on graphene surface is assessed through calculation of binding energies (EB) using the expression below. Fig. 6 e CV analysis of GO-Pd-Fe, commercial Pt/C catalyst system at 0.1 M H2So4 the scan rate of 10 mV/s.
empirical Grimme DFT-D2 correction [42]. The valance electrons are described by localized pseudoatomic orbitals with a double zeta polarized basis set, and a large density mesh cutoff of 130 Hartree considered for accuracy of calculations.
EB ¼ EGraphene sheet þ EMetal Cluster EHybrid Composite
(1)
Where, E represents energy. The calculated binding energies (EB) for each metal cluster adsorption case are tabulated in Table 2. In general, higher binding energy implies strong adsorption. All the binding energies listed in Table 1 are positive indicating stable adsorption; please note that the binding energies in this work are calculated by subtracting the energy of product from that
Fig. 7 e (a) Fe, Pd clusters and Pd-Fe clusters in 1:1 and 2:1 ratio. (b) Pristine Graphene sheet. (c) Top view and side view of Fe cluster adsorption on pristine graphene for representative purpose (d) Defected Graphene sheet. (e) Top view and side view of Fe cluster adsorption on defected graphene for representative purpose. Please cite this article as: Dhali S et al., Graphene oxide supported Pd-Fe nanohybrid as an efficient electrocatalyst for proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.131
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Fig. 8 e (aee), (a) Density of states (DOS) profile of pristine graphene sheet. (bee) DOS profiles of transition metal graphene hybrid composites (black colored upper plot) and the contribution of transition metal clusters to DOS (blue colored lower plot). (Fermi level EF is located at energy zero). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
of reactants. Large binding energies are observed owing to the chemisorptions of metal clusters on graphene surface. The binding energy data reveals relatively stronger adsorption of Fe metal than Pd on the graphene surface. The binding energy of Fe cluster on graphene is 5.36 eV, while it is 4.88 eV for the case of Pd cluster. Similarly, the binding energy reduced from 3.35 eV to 3.33 eV when the Pd share is doubled in the PdeFe cluster. Moreover, the binding strength of Pd/Fe nanoparticles at the defect sites of GO is estimated by adsorbing the PdeFe clusters on the defected graphene. Fig. 6(dee) shows the defected graphene sheet having defect concentration of 3.12% and the adsorption of transition metal cluster on defected graphene. From Table 2, the binding strength of metal clusters at defect site of graphene has increased dramatically in comparison to the pristine case, indicating strong chemisorptions. The chemical bonds between metal cluster and defected graphene were almost doubled in comparison to the pristine case. This is due to the availability of dangling bonds on both the defect site of graphene and metal cluster surface that prompted the strong adsorption. Similar
to the pristine case, the Fe cluster demonstrated relatively strong adsorption on the defected graphene surface than the Pd cluster. The binding energy of Fe cluster at defect site of graphene is 9.45 eV, while it is 8.95 eV for the case of Pd cluster. Correspondingly, the binding energy reduced from 8.64 eV to 7.07 eV when the Pd share is doubled in the PdeFe complex of defected graphene based hybrid composite. To conclude the adsorption analysis, the Pd/Fe metal clusters are more active towards the defect sites of graphene surface in comparison to the pristine sites, owing to the strong binding energies. This infers that the Pd/Fe nanoparticles of synthesized GO-Pd-Fe nanohybrid are most likely occupied the defect sites of GO. Furthermore, the influence of Pd/Fe nanoclusters on the electronic nature of the pristine and defected graphene has been estimated through computation of density of states (DOS) profiles, as shown in Fig. 8 and Fig. 9. From Fig. 8, pristine graphene exhibits a perfect zero band gap at the dirac point, whereas all other graphene-Pd/Fe nanohybrids exhibit either semi-metallic or metallic nature as described in Table 1. Graphene þ Fe and graphene þ Pd/Fe (Pd:Fe ¼ 1:1)
Please cite this article as: Dhali S et al., Graphene oxide supported Pd-Fe nanohybrid as an efficient electrocatalyst for proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.131
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Fig. 9 e (a) Density of states (DOS) profile of Defected Graphene (DG) sheet. (bee) DOS profiles of transition metal defected graphene hybrid composites (black colored upper plot) and the contribution of transition metal clusters to DOS (blue colored lower plot). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
nanohybrid exhibited semi-metallic nature, while graphene þ Pd and graphene þ Pd/Fe (Pd:Fe ¼ 2:1) hybrid composites exhibited metallic nature. From Fig. 5(a), creation of a vacancy defect in graphene has altered the electronic nature from zero-band gaped to metallic, which is a result of Fermi level movement into the valence band in comparison to the pristine graphene due to the vacancy induced electron deficiency. The nanohybrids formed by adsorption of transition metal clusters on defected graphene have also retained the metallic nature, except the Pd/Fe (Pd:Fe ¼ 2:1) cluster adsorption case that resulted in semi-metallic behavior to the defected graphene. The common observations made from both Fig. 8(aee) and Fig. 9(aee) is that, the DOS profiles of nanohybrids are completely dominated by the peaks from transition metal clusters. Moreover, Fe induced DOS peaks are distributed in both valence and conduction bands, while the Pd induced peaks are limited mainly to the valance band region of DOS profile. This might be due to the presence of
completely filled d e sub shell in the valence of Pd (electron configuration of Pd: [Kr] 4d10), unlike Fe that has partially filled d e sub shell in its valence (electron configuration of Fe: [Ar] 3d64s2). To conclude the DOS analysis, the adsorption of Pd/Fe clusters on graphene surface has resulted in enhanced band nature of graphene from zero-band gaped to metallic/semimetallic. The enhanced band nature of nanohybrid may prove beneficial during its application as anode and cathode catalyst as the metallic natured materials transport electrons more efficiently than the semiconducting and insulating materials. As the performance of the GO-Pd-Fe nanohybrid catalyst as anode and cathode catalyst highly dependent upon the electrochemical surface area (ECSA), which is a measure of the electrochemically active sites per gram of the catalyst, Therefore cyclic voltametry (CV) analysis was performed to analyze the catalytic properties of the GO-Pd-Fe nanohybrid (Fig. 6). showed the CV analysis of commercial Pt/C and GOPd-Fe nanohybrid catalytic system at a scan rate 10 mVs1.
Please cite this article as: Dhali S et al., Graphene oxide supported Pd-Fe nanohybrid as an efficient electrocatalyst for proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.131
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The evaluations of ECSA were obtained by integrating area under the curve in the region of hydrogen adsorption and desorption after double layer correction. CV analysis regarding the ECSA evaluation showed that GO-Pd-Fe nano hybrids catalyst showed higher value of mass specific of ECSA (58.08 m2/g), than the commercial Pt/C, (37.87 m2/g). The ECSA value obtained from the developed GO-Pd-Fe nanohybrid is considered to be very high, which is comparable or even higher than most of the previous reports of Pd-based catalysts. For example, Seo et al. [43] estimated the ESCA value for the Pd doped graphene nanosheets to be 54.9 m2/g, whereas Zhang et al. [44] found the ESCA value of GO/Pd1/Pt3 as 49.8 m2/ g. In a very recent report, Yang et al. [45] reported that Pt77Pd23 ACSNDs/rGO could show the ECSA value of 55.92 m2/g. In this regard, our study showed that our nanohybrid electrocatalyst showed better performance than commercially available Pt/C catalyst and previously reported systems. Therefore, the present GO-Pd-Fe nanohybrid substantiates the implication of enhancing catalytic activity than the commercial Pt/C catalyst. This is due to the amalgamation of Pd and Fe nanoparticles, when dispersed in the ratio of 2:1 over the GO sheet. Also, computational studies suggest that the 2:1 ratio triggers metallic nature in the nanohybrids, therefore facilitates electron transportation via the matrix of GO nanosheets. Further, the strong binding energy of the Fe nanoparticles for the GO sheets improves chemical reactivity and electrocatalytic performance of graphene nanosheets. Therefore, the present study of the GO-Pd-Fe nanohybrids depicted the similarities between the experimental and computational analysis.
Conclusion In conclusion, we have successfully demonstrated the utility of GO-Pd-Fe nanohybrids catalyst for the oxidation-reduction reaction in PEMFCs. The present work explores noticeable properties of the GO-Pd-Fe nanohybrids as catalyst, experimentally as well as computationally. The matrix of graphene oxide used as frame of support for the growth of palladium (Pd) and iron (Fe) nanopaticles over graphene oxide sheet showed that these nanohybrids could be used in PEMFCs. Further, the present work also clarified via computational analysis with the help of Density Functional Theory (DFT) with semi-empirical Grimme DFT-D2, which demonstrates good agreements with the experimental study. Experimental result showed that the nanohybrid of GO-Pd-Fe nanohybrids electro catalyst showed an enhanced catalytic activity with electrochemical surface area of (58.08 m2/g PdeFe)1, which is higher than the commercial available Pt/C catalyst with electrochemical surface area 37.87 m2 (g Pt)1, which supports the computational results obtained from the computational analysis. Thus overall study showed that GO-Pd-Fe nanohybrids with the metallic ratio of 2:1 of Pd and Fe not only showed an enhanced performance for the ORR reaction for the PEMFCs, but it also showed an effective way to synthesize cost effective metal catalyst to replace the costly platinum based catalyst.
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Acknowledgements The work is supported by the National Mission of Himalayan Studies, Kosi Katarmal, Almora, India (Ref No. NMHS/MG2016/002/8503-7) and DST INSPIRE division (IF150750), New Delhi, India.
references
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Please cite this article as: Dhali S et al., Graphene oxide supported Pd-Fe nanohybrid as an efficient electrocatalyst for proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.131