PCCP View Article Online

Published on 30 October 2013. Downloaded by University of Michigan Library on 31/10/2014 13:11:04.

PAPER

View Journal | View Issue

Cite this: Phys. Chem. Chem. Phys., 2013, 15, 20333

Graphene–cobaltite–Pd hybrid materials for use as efficient bifunctional electrocatalysts in alkaline direct methanol fuel cells† Chandra Shekhar Sharma,a Rahul Awasthi,a Ravindra Nath Singh*a and Akhoury Sudhir Kumar Sinhab Hybrid materials comprising of Pd, MCo2O4 (where M = Mn, Co or Ni) and graphene have been prepared for use as efficient bifunctional electrocatalysts in alkaline direct methanol fuel cells. Structural and electrochemical characterizations were carried out using X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, chronoamperometry and cyclic, CO stripping, and linear sweep voltammetries. The study revealed that all the three hybrid materials are active for both methanol

Received 14th September 2013, Accepted 2nd October 2013

oxidation (MOR) and oxygen reduction (ORR) reactions in 1 M KOH. However, the Pd–MnCo2O4/GNS

DOI: 10.1039/c3cp53880j

hybrid electrode exhibited the greatest MOR and ORR activities. This active hybrid electrode has also outstanding stability under both MOR and ORR conditions, while Pt- and other Pd-based catalysts

www.rsc.org/pccp

exhibited superior ORR activity and stability compared to even Pt in alkaline solutions.

undergo degradation under similar experimental conditions. The Pd–MnCo2O4/GNS hybrid catalyst

Introduction Concerted efforts have been made during recent years towards the development of alkaline direct methanol fuel cells (DMFCs),1–3 because methanol oxidation reaction (MOR) kinetics are significantly improved in an alkaline medium, compared to an acid one.1–3 Also, the selection range of metals other than Pt as catalysts gets widened in this medium. Among the non Pt metals investigated,2–5 Pd appears to be a promising candidate as a Pt substitute, because it has similar properties and relatively low cost and high availability.6 Further, Pd shows an inert behavior towards MOR in acid solutions, while it is highly active in alkaline solutions.6 To improve the catalytic activity and CO poisoning tolerance, Pd has been alloyed with other metals, such as Ni,7,8 Ag,9 Au,10 Sn,11 Ru,12 Ru–Sn,13 Co,14 etc.5 and produced in the form of dispersed nanoparticles (NPs) on high surface area carbon supports, e.g. multi-walled carbon nanotubes (MWCNTs),5,7,8,15 carbon microspheres (CMSs),16 nanowire arrays (NWAs),17 and graphene nanosheets (GNSs).11–13,18–20 Mixed valence oxides of transition metals, particularly Co-based with the spinel structure, are an important class of materials, a

Department of Chemistry, Centre of Advanced Study, Faculty of Science, Banaras Hindu University, Varanasi-221005, India. E-mail: rnsbhu@rediffmail.com, [email protected]; Fax: +91-542-2368127; Tel: +91-542-6701596 b Department of Chemical Engineering, Indian Institute of Technology, Banaras Hindu University, India † Electronic supplementary information (ESI) available: Results of deconvolution of the Co 2p X-ray photoelectron spectra, Table S1. See DOI: 10.1039/c3cp53880j

This journal is

c

the Owner Societies 2013

which display bifunctional catalysis towards the oxygen reaction (evolution/reduction) in alkaline solutions, and have been extensively studied.20,21 Investigations regarding the use of these oxides in electrocatalysis of MOR are scarce; only very recently nano-sized oxides with molecular formula, CuxCo3 xO4 (where x = 0, 0.3 and 1) have been studied as electrocatalysts for MOR in alkaline solutions.22 These mixed valence oxides were catalytically active and did not show any surface poisoning by methanol oxidation intermediates/products. It is known that Co-based mixed valence oxides are stable in alkaline medium and their surface are covered by adsorbed OH ions.23 Under anodic potential conditions these adsorbed OH ions produce adsorbed OH radicals through the charge transfer process.24 It is almost agreed that in the electrolysis of MOR, the surface adsorbed OH species promote dehydrogenation of adsorbed alcohol and also desorption of poisoning residue.9–14 In view of these, it is considered that introduction of a mixed valance oxide to the palladium–carbon composite material might be beneficial for electrocatalysis of methanol oxidation. We, therefore, have synthesized ternary composites of Pd, GNS and a mixed valence oxide (MnCo2O4, CoCo2O4 or NiCo2O4) and studied these as electrocatalysts for the MOR and oxygen reduction reaction (ORR) in 1 M KOH. In each composite, the composition of Pd has been maintained at a constant 40 wt%. Preliminary investigations carried out on 40 wt% Pd–x wt% MnCo2O4/GNS composites with x = 5, 10, 20 and 30 in 1 M KOH at 25 1C exhibited the best catalytic performance towards MOR when x = 10 and best ORR when x = 5. Considering these results,

Phys. Chem. Chem. Phys., 2013, 15, 20333--20344

20333

View Article Online

Paper

PCCP

composites of Pd, MCo2O4 (M = Mn, Co or Ni) and GNS containing 5 and 10 wt% of the oxide were prepared and investigated for electrocatalysis of MOR and ORR in 1 M KOH at 25 1C. Details of the results of this investigation are described in this paper.

Published on 30 October 2013. Downloaded by University of Michigan Library on 31/10/2014 13:11:04.

Experimental Hybrid catalyst preparation Graphene has been prepared through the reduction of graphite oxide (GO) by NaBH4.25,26 GO was prepared by the modified Hummers and Offenmans method.25 Metal cobaltites with nominal compositional formula MCo2O4 (where, M = Mn, Co and Ni) were prepared by a precipitation method using metal sulphates as the precursors, details of which are described elsewhere.27 Pd–cobaltite/GNS hybrid materials have been synthesized by a microwave-assisted polyol reduction.25 In a typical procedure, 10–11 mg of GNS and 1–2 mg of nano-sized cobaltite powders were dispersed in 40 mL ethylene glycol (EG, Merck) solvent by ultrasonication for 1 h and to this 6.6 mL of 0.01 M PdCl2 (Merck) was added under ultrasonication. The pH of this mixture was adjusted to B10 with 0.8 M KOH (Merck) solution. The resulting mixture was exposed to microwave radiation for 5 min at 800 Watt power with a break of 15 s after every 30 s, left overnight, centrifuged, washed with acetone and then dried in a vacuum oven at 100 1C overnight. For comparison, 40 wt% Pd/GNS, 80 wt% Pd–MnCo2O4, 17 wt% MnCo2O4/GNS, 40 wt% Pt/GNS and 40 wt% Pt/NC (nanocarbon) were also prepared under similar experimental conditions. H2PtCl6
6H2O (Aldrich) was used as a precursor for Pt. In each preparation, constituents of each hybrid were taken by mass and their compositions are expressed in wt%. However, for simplicity, hybrid materials: 40 wt% Pd–10 wt% MnCo2O4/GNS, 40 wt% Pd–10 wt% Co3O4/GNS, 40 wt% Pd–10 wt% NiCo2O4/ GNS and 40 wt% Pd/GNS are represented in the text and figures as Pd–10MnC/GNS, Pd–10CoC/GNS, Pd–10NiC/GNS and Pd/GNS, respectively. Hybrid materials containing 5 wt% cobaltites have also been similarly represented. Electrode preparation The catalyst ink was prepared by dispersing 3 mg of the catalyst powders in 600 mL of ethanol–water (2 : 1) mixture through ultrasonication for 30 min. 30 mL of the suspension was placed on pretreated glassy carbon (GC = 0.5 cm2) and dried in air. 10 mL of 1% Nafion solution (Alfa Aesar) was then dropped over the catalyst film on GC. The catalyst loading on GC was 0.3 mg cm 2. Pretreatment of GC, electrical contact with the catalyst over-layer and electrode mounting were carried out as described previously.28 In the case of the study of the ORR, 4.2–12.6 mL of the suspension was placed over the GC disk electrode surface (0.07 cm2) and 1.4 mL of Nafion solution was added on top to fix the electrocatalyst. Material characterization X-ray diffraction (XRD) powder patterns of the catalysts were recorded on an X-ray diffractometer (Thermo Electron) using CuKa as the radiation source (l = 1.541841 Å). The morphology

20334

Phys. Chem. Chem. Phys., 2013, 15, 20333--20344

of the catalytic films has been studied using transmission electron microscopy (TEM: TECNAI G2 FEI). To obtain TEM pictures, the catalyst was dispersed in methanol and a drop of this suspension was placed onto a carbon coated copper grid (Icon Analytical Equipment PVT. LTD., Prod. Code: 01810) and dried. The oxidation states of metals present on the surface of hybrid materials have been analyzed using an AMICUS-X-ray photoelectron spectrometer. All binding energy values were charge corrected to the C 1s signal (284.6 eV). The peak deconvolution and fitting were performed by using the XPSPEAK Version 4.1 software. Electrochemical characterization Electrochemical studies, namely, cyclic, CO stripping and linear sweep voltammetries and choronoamperometry (CA) have been carried out in a three-electrode single-compartment Pyrex glass cell using a potentiostat/galvanostat Model 273A (PARC, USA). Pure Pt-foil and Hg/HgO/1 M KOH were used as auxiliary and reference electrodes, respectively. The Hg/HgO/1 M KOH electrode (Ecalibrated = 0.082 V vs. SHE) was calibrated as described elsewhere29 with respect to a reversible hydrogen electrode (RHE). For this purpose, a CV of pure Pt wire (dia. = 0.5 mm, Aldrich, 99.99% purity) in hydrogen (pure)-saturated 1 M KOH was recorded in a small potential range from 0.960 to 0.90 V vs. Hg/HgO at the scan rate of 1 mV s 1 at 25 1C. However, the potentials mentioned in the text are given against the RHE (E1 = 0.828 V vs. SHE) electrode only. Cyclic voltammetry (CV) CV of each electrocatalyst has been carried out at a scan rate of 50 mV s 1 between 0.110 and 1.210 V vs. RHE in 1 M KOH with and without containing methanol at 25 1C. Before recording the voltammogram, each electrode was cycled for five runs at the potential scan rate of 50 mV s 1 in 1 M KOH. All CV experiments were performed in an Ar deoxygenated solution. CO stripping voltammetry To perform the CO stripping experiment, each hybrid electrode was immersed into the electrolyte, Ar was bubbled for 25 min and a CV (blank) was then recorded at the scan rate of 50 mV s 1 in the potential region from 0.110 to 1.210 V vs. RHE. Then the working electrode was kept at E = 0.110 V vs. RHE and CO was bubbled into the electrolyte for 30 min. After that, Ar was bubbled for 30 min and a CO stripping voltammogram was then recorded under conditions similar to those employed in recording the blank CV. Rotating disk electrode (RDE) Linear sweep voltammetry (LSV) experiments were performed to investigate the ORR activities of hybrid electrocatalysts in O2-saturated 1 M KOH at 25 1C. The potential range and scan rate employed were 0.354–1.084 V vs. RHE and 2 mV s 1, respectively. To obtain O2-saturated and deoxygenated solutions, pure O2 and Ar gas were bubbled for 45 and 30 min, respectively. A flow of O2 was maintained over the electrolyte during the recording of LSV to ensure its continued O2 saturation.

This journal is

c

the Owner Societies 2013

View Article Online

PCCP

Paper

Published on 30 October 2013. Downloaded by University of Michigan Library on 31/10/2014 13:11:04.

Chronoamperometry This study has been performed to examine the stability of catalysts at a constant potential in the cell solution. In the MOR study, the CA experiment was carried out in Ar-saturated 1 M KOH + 1 M methanol at E = 0.610 V vs. RHE for 2 h at 25 1C. However, in the case of the ORR study, the CA experiment was performed in O2-saturated 1 M KOH at a constant cathodic potential (0.654 V vs. RHE) for 6 h at 25 1C. Prior to carrying out each CA experiment, the test electrode was cycled for five runs in an Ar-saturated electrolyte.

Results and discussion XRD/TEM XRD powder patterns of Pd/GNS, Pd–5MnC/GNS, Pd–10CoC/ GNS, Pd–10NiC/GNS, and Pd–10MnC/GNS, recorded between 2y = 20 and 2y = 801, are shown in Fig. 1. Similar to Pd/GNS, XRD patterns of the hybrid catalysts also showed three characteristic diffraction peaks at 2y E 401, 46.51 and 68.21 corresponding to planes (111), (200) and (220), respectively for the face centered cubic (fcc) crystalline structure of Pd.5,30 However, the characteristic spinel peaks of cobaltites are not observed in the XRDs of the hybrid catalysts. Further, the nature of the spinel oxide does not seem to influence the position of the diffraction peaks and hence the lattice parameters of Pd. The broad peak observed between 2y = 241 and 2y = 261 is the characteristic peak corresponding to the (002) plane of GNS.25 It is observed that Pd gets transformed into PdO on sintering the catalyst at 350 1C for 3 h in air as is quite apparent from the XRDs of hybrids with 10MnC and 10CoC shown in Fig. 1. In addition to three characteristic peaks for Pd, the thermally-treated hybrids exhibited five diffraction peaks, at 2y = 33.971 (d = 2.6366 Å), 42.01 (d = 2.1494 Å), 54.861 (d = 1.6721 Å), 60.551 (d = 1.5279 Å) and 71.291 (d = 1.3218 Å) corresponding to planes (101), (110),

(112), (103), and (211), respectively, substantiating the formation of tetragonal PdO [JCPDS file 06-0515]. The characteristic diffraction peaks of spinel oxides did not appear even on sintering the hybrid catalysts (Fig. 1e and f). However, the XRDs of pure spinel oxides produced the characteristic diffraction peaks. For instance, the XRD of MnCo2O4 produced the characteristic spinel planes, (220), (311), (400), (511) and (440) at 2y = 30.851 (d = 2.899 Å), 37.061 (d = 2.426 Å), 44.681 (d = 2.028 Å), 59.101 (d = 1.563 Å), and 64.831 (d = 1.438 Å), respectively, which closely match with data shown in JCPDS file 32-0297. The reason for the absence of the characteristic spinel diffraction lines from the XRD powder patterns of hybrid materials is not very clear. Possibly, it could be because of the presence of the oxide component in the hybrids in either nanocrystalline or amorphous states. The crystallite size of Pd was determined by the Scherrer equation30 and employing the most intense Pd(111) diffraction peak. Estimates of the average Pd crystallite size in Pd/GNS, Pd–5MnC/GNS, Pd–10MnC/GNS, Pd–10NiC/GNS and Pd–10CoC/ GNS were 5.8, 6.8, 6.7, 7.5 and 7.3 nm, respectively. Thus, the results indicate that the crystallite size of Pd slightly improves in the presence of transition metal mixed oxides but there is practically no change in the lattice parameter. The crystallite size of Pd increased greatly to 27–28 nm when the hybrid catalysts were sintered at 350 1C for 3 h. TEM images of Pd/GNS, MnC/GNS, Pd–xMnC/GNS (x = 5 and 10), Pd–10CoC/GNS and Pd–10NiC/GNS are shown in Fig. 2A–L. The Pd NPs look to be spherical in shape and exist in a cluster form on the GNS surface (Fig. 2A), whereas the oxide (MnC) particles look like nano dots (o5 nm) and produce a very uniform distribution on the GNS surface (Fig. 2D). It appears that the oxide nanoparticles are embedded in the GNS matrix. The dispersion of NPs seems to be somewhat better in the case of the hybrid containing 10MnC (Fig. 2G). Some dark areas are also observed which may be attributed to over deposition or the mass contrast of NPs on GNS. At high resolution, the lattice fringes are observed which substantiate that the NPs are crystalline in nature. Further, the Pd NPs seem to be highly agglomerated solely when they are deposited onto the NiC/GNS composite (Fig. 2K–L). This can be attributed to similar properties of Pd and Ni, being elements of the same group. Estimates of the sizes of the NPs were B6, B7, B8, B8, and B7 nm in hybrid catalysts containing Pd/GNS, 10MnC, 10NiC, 10CoC and 5MnC, respectively. Thus, the average particle sizes are similar to those of Pd crystallite sizes obtained from XRD. This shows a better dispersion of the Pd NPs in presence of cobalites. X-ray photoelectron spectroscopy (XPS)

Fig. 1 XRD patterns of: (a) Pd–10CoC/GNS, (b) Pd–10NiC/GNS, (c) Pd–10MnC/ GNS, (d) Pd–5MnC/GNS; (e) Pd–10MnC/GNS and (f) Pd–10CoC/GNS sintered at 350 1C for 3 h; inset: Pd/GNS.

This journal is

c

the Owner Societies 2013

XPS of pure cobaltites and hybrid catalysts, Pd–10MnC/GNS, Pd–10NiC/GNS, and Pd–10CoC/GNS were recorded and core level spectra of Pd 3d, Co 2p, Mn 2p and Ni 2p are shown in Fig. 3A–E. The Pd 3d core photoelectron spectra for each sample shown in Fig. 3A exhibit two strong photo peaks, 3d5/2 and 3d3/2, at binding energies (B.E.s) 334.3 & 339.6, 334.2 & 339.5 and 334.3 & 339.6 eV in the case of hybrids Pd–10CoC/GNS, Pd–10MnC/GNS and

Phys. Chem. Chem. Phys., 2013, 15, 20333--20344

20335

View Article Online

PCCP

Published on 30 October 2013. Downloaded by University of Michigan Library on 31/10/2014 13:11:04.

Paper

Fig. 2 TEM images of: (A & B), Pd/GNS; (C & D), MnC/GNS; (E & F), Pd–5MnC/GNS; (G & H), Pd–10MnC/GNS; (I & J), Pd–10CoC/GNS; and (K & L), Pd–10NiC/GNS catalysts.

Pd–10NiC/GNS, respectively. These values of B.E.s are lower than the standard 3d5/2 B.E. of Pd (334.8 eV: KRATOS Analytical). This shows that the Pd 3d5/2 B.E. peak shifts negatively by 0.5–0.6 eV on the Pd–oxide/GNS, showing thereby a transfer of electrons from the oxide to Pd.31 In order to know whether Pd interacts with both GNS and oxide, the Pd 3d spectra for composites, Pd/GNS and Pd/MnC were also recorded (Fig. 3A) and the results, so obtained, indicated that the Pd 3d5/2 peak

20336

Phys. Chem. Chem. Phys., 2013, 15, 20333--20344

(B.E. = 335.6 eV) on GNS shifts by 0.8 eV while that on MnC shifts by 0.6 eV toward the higher B.E. with respect to the standard 3d5/2 B.E. This shows that the Pd interact with both GNS and the oxide. The Co 2p spectra on hybrid catalysts, Pd–10CoC/GNS, Pd–10MnC/GNS, and Pd–10NiC/GNS shown in Fig. 3B seem to have similar features and closely match to those of pure CoC, MnC and NiC (Fig. 3E). Each spectrum consists of two

This journal is

c

the Owner Societies 2013

View Article Online

Published on 30 October 2013. Downloaded by University of Michigan Library on 31/10/2014 13:11:04.

PCCP

Paper

Fig. 3 XPS of: Pd 3d (A) for Pd/GNS, Pd/MnC, Pd–10MnC/GNS, Pd–10CoC/GNS and Pd–10NiC/GNS; Co 2p (B) for Pd–10MnC/GNS, Pd–10CoC/GNS and Pd–10NiC/ GNS; Mn 2p (C) for MnC, Pd/MnC, Pd–10MnC/GNS and Pd–10MnC/GNS (sintered at 350 1C, 3 h); Ni 2p (D) for NiC and Pd–10NiC/GNS; Co 2p (E) for pure cobaltites. Deconvolution spectra of Co 2p (F) and Mn 2p (G) for Pd–10MnC/GNS.

main peaks: 2p3/2 at B.E. B781 and 2p1/2 at B.E. B796 eV with spin–orbit splitting (Dp) of B15.0 eV.32,33 Similarly, the Mn 2p spectra of Pd–10MnC/GNS and Ni 2p spectra of Pd–10NiC/GNS are also similar to those of their respective spectra of pure MnC and NiC (Fig. 3C and D). Thus, these results show that metal oxides maintain their spinel structure in hybrid catalysts. To investigate the oxide surface composition, all Co 2p, Ni 2p, and Mn 2p spectra were deconvoluted. In deconvolution of Co 2p spectra, the Co 2p3/2 peak and two satellite peaks S1 and S2 were considered while both the main peaks, 2p3/2 and 2p1/2 were considered in the case of Ni 2p and Mn 2p spectra. The results, so obtained, are shown in Table 1. Typical deconvoluted Co 2p and Mn 2p spectra in the case of Pd–10MnC/GNS are shown in Fig. 3F and G. The peak located at 779.7–779.9 eV is characteristic of Co3+ species32,33 whereas the component at 780.9–781.4 eV corresponds to Co2+ species.32,33 The broad components at 782.8–785.9 eV and 788.4–789.4 eV contain the contributions of Co2+ and Co3+, respectively.34 Further, deconvolution of the Co 2p1/2 produced two peaks at 794.6–795.8 eV and 796.1–796.4 eV, excepting in the case of MnC. Deconvolution of

This journal is

c

the Owner Societies 2013

Co 2p1/2 peak in the latter case shows only one peak at 795.3 eV. Furthermore, Dp values listed in Table S1 (in ESI†) indicate that in the case of NiC and MnC, Co3+ species are located at tetrahedral sites and that Co2+ are located at octahedral sites in the spinel lattice whereas in the case of CoC, octahedral sites contain both Co2+ and Co3+ species. Deconvolution of the Mn 2p spectra on Pd–10MnC/GNS and Pd/MnC produced two pairs of peaks at 339.5–639.9 and 651.4–652.9 and 642.4–642.9 and 654.1–655.2 eV, showing Mn essentially in +2 and +4 states, respectively,35,36 whereas the Mn 2p spectra of pure MnC produced two pairs of peaks at 639.2 and 652.2 and 641.7 and 655.1 eV, showing the presence of Mn in +2 and +3 states, respectively. Thus, results of deconvoluted 2p spectra of samples, MnC, Pd/MnC and Pd–10MnC/GNS placed in Table 1B indicate that the presence of Mn in a higher state, Mn4+, in the hybrid catalyst can be ascribed to Mn–Pd interaction. Further, Ni 2p spectra of the Pd–10NiC/GNS exhibited 5 peaks at 855.1, 856.2, 862.1, 872.3 and 876.9 eV. The two peaks (855.1 and 856.2 eV) of the main 2p3/2 peak indicate the presence of Ni2+ and Ni3+ whilst that of satellite peaks at

Phys. Chem. Chem. Phys., 2013, 15, 20333--20344

20337

View Article Online

Paper

PCCP

Table 1 (A) Results of deconvolution of the Mn 2p X-ray photoelectron spectra. (B) Results of deconvolution of the Ni 2p X-ray photoelectron spectra. (C) Results of deconvolution of the Pd 3d X-ray photoelectron spectra

Published on 30 October 2013. Downloaded by University of Michigan Library on 31/10/2014 13:11:04.

(A) Spin–orbit doublet – I

Spin–orbit doublet – II

Composite

2p3/2

2p1/2

2p3/2

2p1/2

S1

Dp1

Dp2

Mn+2/Mn+3/Mn+4

MnC

639.2 (27%) 639.5 (21%) 640.0 (20%)

652.2 (36%) 651.4 (19%) 652.9 (24%)

641.7 (24%) 642.4 (42%) 642.9 (40%)

655.1 (6%) 654.1 (18%) 655.2 (16%)

645.1 (7%) — — — —

13.0

13.4

Mn2+/Mn3+ = 1.1

11.9

11.7

Mn2+/Mn4+ = 0.5

12.9

12.3

Mn2+/Mn4+ = 0.5

2p1/2

S1

S2

Dp1

Dp2

Ni+2/Ni+3

872.3 (41%) 872.3 (28%)

862.7 (26%) 862.1 (9%)

878.3 (7%) 876.9 (18%)

17.4

16.3

0.2

17.2

16.1

0.3

Pd–MnC Pd–10MnC/GNS (B)

Spin–orbit doublet Composite

2p3/2

NiC

854.9 (5%) 855.1 (11%)

Pd–10NiC/GNS

856.0 (21%) 856.2 (34%)

(C) Spin–orbit doublet Composite

3d5/2

3d3/2

Dp

Pd/GNS Pd/MnC Pd–10MnC/GNS Pd–10NiC/GNS Pd–10CoC/GNS

335.6 335.4 334.2 334.3 334.3

340.7 340.5 339.5 339.6 339.6

5.1 5.1 5.3 5.3 5.3

Note: the percent relative areas of metal cations are shown in parentheses.

862.1 and 876.9 eV indicate a mixture of Ni2+ and Ni3+. Similar results were also shown by the Ni 2p spectra of pure NiC.34,37,38 The Co2+/Co3+, Ni2+/Ni3+, Mn2+/Mn3+ and Mn2+/Mn4+ ratios listed in Table S1 (ESI†) and Table 1(A and B) are obtained from the respective spectral areas of the spin–orbit doublets of the main Co 2p3/2, Ni 2p3/2 and Mn 2p3/2 lines, respectively, assuming the proportionality constant is the same for different oxidation states of a particular metal. The Co2+/Co3+ ratio in the oxide appears to increase with the introduction of Ni or Mn, the increase being greater with the latter. Similar results were also found by Restovic et al.39 and Gautier et al.33 in the case of Mn introduction in CoC. Thus, XPS results suggest that along with Pd metals, Co2+ and Co3+ ions are also present on the surface of the hybrid catalyst, Pd–10CoC/GNS and that replacement of Co by Ni or Mn in CoC increases Co2+/Co3+ ratio. Thus, results of the XPS study suggest that, in the hybrid catalysts, Pd–10CoC/GNS, Pd–10MnC/GNS and Pd–10NiC/GNS synthesized by us, Co is present on the surface in the form of both Co2+ and Co3+ and that the Co2+/Co3+ ratio increases with addition of Ni or Mn. Further, Ni is present largely in Ni3+ while Mn is present as Mn2+ and Mn4+ ions. Pd 3d XPS spectra of thermally treated catalysts, Pd–10MnC/ GNS and Pd–10CoC/GNS at 350 1C for 3 h also indicated two strong characteristic photo peaks, 3d5/2 at B336.5 and 3d3/2 at B342 eV for the presence of PdO.6,40 Features of Co 2p (2p3/2 at B779.3–780.5 and 2p1/2 at B794–795.8 eV) spectra were also similar to those of unsintered ones. However, the Mn 2p

20338

Phys. Chem. Chem. Phys., 2013, 15, 20333--20344

spectra (2p3/2 at B642.6 and 2p1/2 at B653.6 eV) of the sintered catalyst shifted toward the higher energy showing the presence of Mn mainly in the Mn4+ state (Fig. 3C).34,35 Thus, results show that the spinel structure of transition metal oxides does not change even on sintering the hybrid materials. CV Electrochemically active surface area (EASA). Cyclic voltammograms of hybrid electrodes have been recorded at a scan rate of 50 mV s 1 in Ar-saturated 1 M KOH (without methanol) at 25 1C and reproduced in Fig. 4. Features of these voltammograms were similar to those reported for Pd/GNS25 and Pd–Ru–Sn13 in 1 M KOH. Each voltammogram contains two anodic peaks (I & II) and one cathodic peak (III), which correspond to three electrochemical processes occurring on the surface of each hybrid catalyst. Peak I (Ep = 0.40 V vs. RHE) can be attributed to the electrochemical adsorption of OH (i.e., OH - OHads + e).41 Peak II, which appears above E = 0.850 V vs. RHE can be attributed to the formation of Pd oxide layer through the reaction, Pd–OHads + OH - Pd–O + H2O + e.41 The cathodic Peak III (Ep = 0.650 V vs. RHE) can be ascribed to the electrochemical reduction of Pd–O, formed under anodic conditions, into elemental Pd (i.e., Pd–O + H2O + e - Pd + 2OH ).41 It is observed that the intensity of current peak I increases with the introduction of the oxide into the Pd/GNS matrix, the intensity being the greatest with MnC. Also, the intensity of current peak I increases with increasing addition of MnC.

This journal is

c

the Owner Societies 2013

View Article Online

Published on 30 October 2013. Downloaded by University of Michigan Library on 31/10/2014 13:11:04.

PCCP

Paper

1

Fig. 4 Cyclic voltammograms of Pd–10MC/GNS (M = Mn, Co or Ni) and Pd/GNS catalysts at 50 mV s 1 in 1 M KOH (25 1C).

Fig. 5 Cyclic voltammograms of Pd–10MC/GNS and Pd/GNS catalysts at 50 mV s in 1 M KOH + 1 M CH3OH (25 1C).

Table 2 Electrocatalytic properties of the hybrid electrocatalysts towards methanol electrooxidation in 1 M KOH + 1 M methanol at 25 1C, scan rate = 50 mV s 1, catalyst loading = 0.30 (i.e., Pd = 0.12) mg cm 2, geometrical area of electrode = 0.5 cm2

and declines rapidly thereafter. The rapid decrease in the oxidation current can be due to the manifestation of multiple processes such as CO adsorption,41,43 transformation of active Pd sites into inert palladium(II) oxide41,43–45 and diffusion processes. The Ip,b observed between 0.684 and 0.884 V in negative going scan has been attributed to the resumption of methanol oxidation at the Pd surface which is regenerated from the reduction of palladium(II) oxide, formed during the on-going scan.6,12,44 However, some authors considered that Ipb is caused by the oxidation of carbonaceous species that are not completely oxidized under anodic conditions.12,41,43 Results shown in Table 2 indicate that Eop for methanol oxidation shifts negatively with the introduction of cobalt oxide, the magnitude of the shift being greatest with MnC. Thus, the results show that an addition of a Co-based spinel oxide improves the methanol oxidation process. The peak current density ( jp = Ip,f (mA)/geometrical area (cm2)) also increases significantly on cobaltite introduction and is found to be the greatest with MnC. Table 2 shows that the peak potential (Ep) for the methanol electrooxidation varies with the nature of the electrode material. Therefore, in order to make a meaningful comparison of electrocatalytic activities, the methanol oxidation current densities ( j = observed current, I, mA/geometrical area of electrode, cm2), produced at different electrocatalysts, at a constant potential, E = 0.610 V vs. RHE, were noted from CV curves shown in Fig. 5 and are listed in Table 2. The activity data summarized in Table 2 show that among cobaltite-incorporated Pd/GNS hybrid materials, the Pd–10MnC/GNS electrode has the greatest catalytic activity for methanol electrooxidation in 1 M KOH. Also, the specific activity, SA (= I, mA/EASA, cm2) of the base electrode shown in Table 2 improves by 25–33% with the introduction of cobaltites. This shows that the addition of cobaltites improves both the geometrical as well as the electronic properties of the material. It is noteworthy that at E = 0.626 V vs. RHE and at the scan rate of 50 mV s 1 in 1 M KOH + 1 M methanol (25 1C),

Anodic scan At E = 0.610 V vs. RHE Catalyst

EASA/ cm2

Pd–10MnC/GNS Pd–10NiC/GNS Pd–10CoC/GNS Pd/GNS

49 35 32 30

   

0.6 0.2 0.7 0.5

Eop/V vs. Ep/V vs. jp/mA RHE RHE cm 2 0.331 0.400 0.402 0.408

1.078 1.018 1.016 1.005

83 54 49 45

   

j/mA cm 2

SA/mA cm 2

1.9 15  1.0 0.15 0.9 11  0.5 0.16 2.2 10  1.2 0.16 1.6 7  0.9 0.12

The EASA of each catalyst was estimated, as reported elsewhere,42 by taking into account the charge associated with the reduction of a PdO monolayer and using the relation EASA = Q/S,12 where Q is coulombic charge in mC and S is the charge required for the reduction of the PdO monolayer. The value of S is taken as 0.424 mC cm 2.42 Estimates of the EASA for different electrocatalysts are listed in Table 2. The results show that the EASA of Pd/GNS increased with the introduction of cobaltites, the magnitude of increase being the greatest with MnC. Thus, the result corroborates the findings of the TEM observations that an addition of cobaltites improves the dispersion of Pd NPs and hence the EASA. Methanol electrooxidation. Fig. 5 presents CV curves of Pd–10MC/GNS (M = Mn, Co or Ni) hybrids and Pd/GNS recorded at the scan rate of 50 mV s 1 in the potential region, 0.110–1.210 V vs. RHE in Ar-saturated 1 M KOH + 1 M methanol at 25 1C. The CV curves in the presence of 1 M KOH + 1 M methanol (Fig. 5) show the two characteristic methanol oxidation peaks: one observed on the positive (Ip,f) and the other one on negative (Ip,b) going scans.41,43–46 It is observed that on the positive going scan the onset of methanol oxidation (Eop) takes place at a potential E0.360 V vs. RHE and then the oxidation current increases progressively with potential, attains a maximum

This journal is

c

the Owner Societies 2013

Phys. Chem. Chem. Phys., 2013, 15, 20333--20344

20339

View Article Online

Published on 30 October 2013. Downloaded by University of Michigan Library on 31/10/2014 13:11:04.

Paper

PCCP

the catalytic activity of our most active catalyst, Pd–10MnC/GNS (I = 17 mA cm 2 = 142 mA mgPd–1) is also found to be greater than those recently reported for Pt/microsphere/graphite (I E 24 mA mgPd–1),42 Pd–NiO (4 : 1)/C (I = 45.3 mA mgPd–1),46 Pd–Co3O4(2 : 1)/C (I = 80 mA mgPd–1),46 Pd–Mn3O4 (2 : 1)/C (I = 41 mA mgPd–1),46 and PdAu41/C (I = 51 mA mgPd–1)47 under similar experimental conditions. CO stripping voltammetry. The performance of hybrid catalysts in relation to the adsorbed CO removal has also been investigated by carrying out CO stripping voltammetry in 1 M KOH at a scan rate of 50 mV s 1 and the results, so obtained, are reproduced in Fig. 6A. Each curve of Fig. 6A, particularly the curve for graphene–cobaltites–Pd, shows two well separated CO oxidation current peaks (I & II) under anodic conditions. Current peaks I are centered at ca. 0.450 V vs. RHE and current peaks II are centered at ca. 0.90 V vs. RHE. Stripping voltammetry of pure Pd under similar condition (Fig. 6B) and expanded curve for Pd/GNS as shown in inset of Fig. 6A also exhibit two similar CO oxidation current peaks. However, the stripping voltammetry

of MnC/GNS did not indicate any oxidation peak (Fig. 6B). These results indicate that there could be, at least, two different kinds of Pd active sites, even on the pure Pd surface, where CO adsorption can take place. It appears that CO gets adsorbed strongly on one kind of Pd site while it is adsorbed less strongly on the other kind. The observation of Fig. 6A further shows that the CO oxidation peaks get considerably enlarged and their corresponding peak potentials shifted toward the negative side in the presence of a cobaltite in the base material, the effect being the greatest with MnC. Thus, these results show that an addition of a cobaltite increases the anti CO poisoning ability of the catalyst. The anti CO poisoning abilities of catalysts followed the order: Pd–10MnC/GNS > Pd–10NiC/GNS > Pd–10CoC/GNS > Pd/GNS. The CO oxidation peak disappeared completely in the next two to three consecutive cycles. The CO oxidation peak II shows a doublet on Pd–10MnC/GNS. This possibly can arise when some of the Pd sites are free from the interaction of MnC. The higher poisoning tolerance associated with hybrid catalysts can be ascribed to the tendency of Co-based spinel oxides to produce adsorbed OH molecule from the electrochemical adsorption of OH ions (OH - OHad + e) in alkaline solutions at lower potentials.26 The latter species (OH) facilitates the CO oxidation. Thus, the greater anti CO poisoning ability observed with the Pd–10MnC/GNS hybrid can be attributed to the higher tendency of MnC to form adsorbed OH molecules, as is quite evident from anodic current peaks for the electrochemical adsorption of OH on different hybrid surfaces (Fig. 4). CA Chronoamperograms of hybrid electrodes recorded at E = 0.610 V vs. RHE for 2 h in 1 M KOH + 1 M methanol at 25 1C are shown in Fig. 7. The features of curves shown in Fig. 7 indicate that all the catalysts exhibit fairly stable performance during their 2 h test at E = 0.61 V vs. RHE almost from the start of the CA experiment. However, the hybrid electrode containing

Fig. 6 CO stripping voltammetry of (A) Pd/GNS and Pd–10MC/GNS (M = Mn, Co or Ni) and (B) MnC/GNS and Pd catalysts at 50 mV s 1 in 1 M KOH at 25 1C. The inset shows the expanded CO stripping voltammetry curve for Pd/GNS.

20340

Phys. Chem. Chem. Phys., 2013, 15, 20333--20344

Fig. 7 Chronoamperograms of Pd/GNS (a), Pd–10MnC/GNS (b), Pd–10NiC/GNS (c), and Pd–10CoC/GNS (d) at E = 0.610 V vs. RHE and of E-TEK Pt/C catalyst at E = 0.670 V vs. RHE (inset) in 1 M KOH + 1 M methanol at 25 1C.

This journal is

c

the Owner Societies 2013

View Article Online

Published on 30 October 2013. Downloaded by University of Michigan Library on 31/10/2014 13:11:04.

PCCP

Paper

MnC demonstrated a much superior electrocatalytic performance in comparison with other catalyst electrodes, as is quite evident from the observed js at 1 h, which were 8.2, 4.9, 3.8 and 2.8 mA cm 2 on Pd–10MnC/GNS, Pd–10NiC/GNS, Pd–10CoC/ GNS, and Pd/GNS, respectively. Thus, the percentage increase in the activities of hybrid electrodes in relation to the base electrodes are 185, 75, and 36% in case of Pd–10MnC/GNS, Pd–10NiC/GNS and Pd–10CoC/GNS, respectively. Thus, the results show that the activity as well as stability of the hybrid electrodes are much superior to the base electrode. Their stability is also found to be much superior to the recently reported Pt/C47 electrode in 1 M KOH + 1 M MeOH. For instance, during the CA test at 0.610 V vs. RHE in 1 M KOH + 1 M MeOH at 25 1C, the initial j = 8.29 mA cm 2 on Pd–10MnC/GNS (Pd mass = 0.12 mg cm 2) measured after 3 min of the electrolysis became 8.2 mA cm 2 during the electrolysis of 30 min duration (i.e. a loss in activity of B1% only or no loss). On the other hand, in a similar CA test at E = 0.670 V vs. RHE in 1 M KOH + 1 M MeOH, Su et al.47 on Pt/C (Pt loading: 0.1 mg cm 2) (inset of Fig. 7) found a reduction in the initial current density of B8.6 mA cm 2, i.e. from B14.5 to B5.9 mA cm 2 (i.e. a loss in activity of B59%), under similar electrolysis conditions. The considerable loss in activity of the Pt catalyst has been ascribed to the poisoning effect of oxidation intermediates, particularly the CO molecule. RDE study. Prior to investigation of the ORR activities, the loading of hybrid materials on the GC disk electrode (3 mm) was adjusted for obtaining a reproducible diffusion limited current density. Catalyst loadings between 0.60 and 0.90 mg cm 2 produced reproducible LSV curves with a diffusion limited current density close to 5 mA cm 2. Representative LSV curves recorded at various loadings in the case of Pd–5MnC/GNS hybrid materials are reproduced in Fig. 8. To measure the ORR activity by RDE the loading on the GC disk was, therefore, kept constant at 0.6 mg cm 2 in the case of each electrocatalyst.

The ORR activity of hybrid electrodes containing 5 and 10 wt% of oxides has been determined by recording LSV curves in the potential region, 1.084–0.354 V vs. RHE, at the scan rate of 2 mV s 1 and at a fixed rotation of 1600 rpm in O2-saturated 1 M KOH at 25 1C and the curves, so obtained, are reproduced in Fig. 9A and B. For comparison, LSV curves for MnC/GNS and Pd/GNS were also recorded under similar conditions and are shown in Fig. 9A. The results shown in Fig. 9A and B demonstrate that the hybrid composites with 5 and 10 MnC have superior ORR activities and positive Eop in comparison to the other composites of the present study. Furthermore, the hybrid with 5 wt% oxide seems to demonstrate a somewhat better activity performance. It is noteworthy that the Pd–5MnC/GNS hybrid prepared by us exhibited superior ORR activity to Pt/NC and Pt/GNS also, particularly in the mixed kinetics diffusion control region, as is quite apparent from the LSV curves shown in inset of Fig. 8. These results indicate that the Pd–5MnC/GNS hybrid

Fig. 8 Linear sweep voltammograms of Pd–5MnC/GNS at different catalyst loadings (a–d) and of Pd–5MnC/GNS, Pt/NC and Pt/GNS at a fixed catalyst loading (0.60 mg cm 2) (inset) in O2-saturated 1 M KOH; scan rate = 2 mV s 1; rotation = 1600 rpm and T = 25 1C.

Fig. 9 Linear sweep voltammograms of Pd–5MC/GNS (M = Mn, Ni or Co), MnC/ GNS and Pd/GNS (A) and of Pd–10MC/GNS (B) at 1600 rpm in O2-saturated 1 M KOH; scan rate = 2 mV s 1; catalyst loading = 0.60 (i.e., Pd = 0.24) mg cm 2 and T = 25 1C.

This journal is

c

the Owner Societies 2013

Phys. Chem. Chem. Phys., 2013, 15, 20333--20344

20341

View Article Online

Published on 30 October 2013. Downloaded by University of Michigan Library on 31/10/2014 13:11:04.

Paper

PCCP

catalyst could be considered as a promising electrocatalyst material for use as a cathode in alkaline fuel cells. Based on values of the current density observed at a constant potential, E = 0.850 V vs. RHE, the catalysts can be placed in the activity order: Pd–5MnC/GNS ( j = 4.06 mA cm 2) > Pd–5NiC/GNS ( j = 3.01 mA cm 2) > Pd/GNS ( j = 2.86 mA cm 2) > Pd–5CoC/ GNS ( j = 2.72 mA cm 2). However, values of the specific activity (= I, mA/EASA, cm2) of all three hybrid catalysts including the base one (Pd/GNS) were found to be nearly the same, i.e., 0.062  0.003 mA cm 2. Estimates of the EASA were 4.4, 3.5 and 3.2 cm2 for Pd–5MnC/GNS, Pd–5NiC/GNS and Pd–5CoC/GNS whereas they were 5.6, 3.6 and 3.3 cm2 for Pd–10MnC/GNS, Pd–10NiC/GNS and Pd–10CoC/GNS, respectively. The mass of the catalytic film over the disk electrode and the scan rate used in the EASA determination were 0.60 mg cm 2 and 2 mV s 1, respectively. The electrode kinetics study has been performed on active Pd–5MC/GNS (M = Mn, Co and Ni) only. For this purpose, the LSV curves of hybrid electrodes containing Pd–5MC/GNS were recorded at varying rotations in O2-saturated 1 M KOH at 25 1C. Features of the LSV curves obtained for all the hybrid catalysts were similar. A set of seven LSV curves determined on one active hybrid electrode, Pd–5MnC/GNS at rotations: 500, 800, 1200, 1600 and 2000 rpm are given in Fig. 10. To extract the kinetic data, a suitable potential region, wherein all 4–5 LSV curves determined at varying rotations deviate significantly, is generally chosen.48 According to Fig. 10, the mixed kinetic diffusion control region, 0.950–0.80 vs. RHE is taken for evaluation of the kinetic information. At more negative potentials (o0.80 V), the mass transport-limited current becomes significant where a dependence of j upon rotation rate is observed. With increasing the rotation rate, the limiting current increased due to an increase in the oxygen diffusion rate from bulk to the electrode surface. Thus, the overall measured j is mainly the sum of contributions of the

Fig. 10 Linear sweep voltammograms of Pd–5MnC/GNS at varying rotations in O2-saturated 1 M KOH; scan rate = 2 mV s 1; catalyst loading = 0.60 (i.e., Pd = 0.24) mg cm 2 and T = 25 1C.

20342

Phys. Chem. Chem. Phys., 2013, 15, 20333--20344

kinetic current density, jk, and the diffusion limiting current density, jd. Both have been analyzed from the RDE data using the Koutecky–Levich (K–L) eqn (1),49 1/j = 1/jk + 1/jd = 1/jk + B/o1/2 = 1/jk + 1/0.2nFCO2u

1/6

DO22/3o1/2 (1)

where o is the electrode rotation rate in revolutions per minute, B is Levich constant, n is the number of electrons transferred per O2 molecule, F is the Faraday constant (96 485 C mol 1), CO2 is the concentration of O2 in the bulk (8.4  10 4 mol L 1), u is the kinematic viscosity of the solution (1.1  10 2 cm2 s 1) and DO2 is the diffusion coefficient of oxygen (1.65  10 5 cm2 s 1).50 From eqn (1) the K–L plots ( j 1 vs. o 1/2) were constructed and values of the intercept at 1/o1/2 = 0 and the slope of each K–L curve were determined. Values of the intercept and slope, thus determined, were used to estimate values of jk, B, jd, and n (Table 3). A set of seven K–L plots constructed for the electrode, Pd–5MnC/GNS, at varying potentials in the potential region, 0.774–0.879 V vs. RHE, are shown in Fig. 11. As jk represents the intrinsic activity and is a function of potential, its value is given at a constant potential (E = 0.834 V vs. RHE) for all the hybrid materials in Table 3. Values of the slope (B) seem to slightly decrease with the increase of the cathodic potential; the average values of slope obtained on each catalyst are given in Table 3. The value of n is found to be 3.6, which is close to 4 in the case of the ORR study Table 3 Estimate of electrode kinetic parameters for the ORR on Pd–5MC/GNS (M = Mn, Co and Ni) in O2-saturated 1 M KOH at 25 1C; geometrical area of GC disk = 0.07 cm2, catalyst loading = 0.60 (i.e., Pd = 0.24) mg cm 2, scan rate = 2 mV s 1, rotation speed = 500–2000 rpm

M

jk/mA cm 2 at E = 0.834 V vs. RHE

B/mA

Mn Ni Co

12.4 7.95 5.87

7.9  0.9 (0.879–0.774 V) 7.4  0.9 (0.844–0.764 V) 6.8  0.8 (0.844–0.764 V)

Fig. 11

1

cm2 rpm1/2

jd/mA cm 2 at 1600 rpm

n

B4.9 B5.5 B5.9

B3.6 B3.3 B3.1

1/j versus 1/o1/2 plots constructed from Fig. 10 at constant potentials.

This journal is

c

the Owner Societies 2013

View Article Online

Published on 30 October 2013. Downloaded by University of Michigan Library on 31/10/2014 13:11:04.

PCCP

Paper

Fig. 12 E vs. log jk plot for the ORR on Pd–5MnC/GNS at 1600 rpm, scan rate = 2 mV s 1 at 25 1C.

on Pd–5MnC/GNS, over the whole range of potentials employed. This shows that the ORR in 1 M KOH at room temperature follows essentially a 4e pathway in the potential region, 0.879–0.774 V vs. RHE. Recently, Seo et al.51 also reported a 4e pathway of ORR on 60 wt% Pd/GNS and 60 wt% Pt/GNS in 0.1 N NaOH; the 4e pathway of ORR on Pt has been reported in many studies.52–54 To construct the Tafel plot, the kinetic current was determined using the relation, jk = j  jd/( jd j), at varying potentials and at a constant electrode rotation, 1600 rpm. jd/( jd j) is the mass-transport correction term.55 The jd value was noted from the LSV curve determined at 1600 rpm for each hybrid catalyst. The Tafel plot (E vs. log jk) on Pd–5MnC/GNS shown in Fig. 12 shows two linear regions, one with a slope of 45 mV at low overpotentials (E = 0.934–0.884 V vs. RHE) and the other with a slope of 70 mV at higher overpotentials (E = 0.854–0.784 V vs. RHE). Two similar Tafel slopes on 30% Pt/C in 1 M NaOH were also observed by Ramaswamy and Mukerjee.56 Liang et al.29 have also recently reported a low Tafel slope of 42 mV on Co3O4/ N–GNS in 0.1 M KOH. However, E vs. log jk plots for other hybrids containing CoC and NiC prepared by us exhibited only a single slope of 68–70 mV over the potential range, B0.984–0.834 V vs. RHE. The Tafel slope of 70 mV for ORR observed on Pd–5MnC/GNS electrode is close to those found on platinum in acid–alkaline solutions.53,55,57,58 The stabilities of the ORR active hybrid electrodes Pd–10MnC/ GNS and Pd–5MnC/GNS were also examined at E = 0.654 V vs. RHE for 6 h in 1 M KOH solution at 25 1C (Fig. 13). For comparison, Pt/NC and Pt/GNS electrodes were also tested for their stabilities under identical experimental conditions (Fig. 13). Pt electrodes were prepared by employing a similar procedure as was used for the preparation of other electrodes in the investigation. The chronoamperograms in Fig. 13 show that in the case of Pd–5MnC/GNS, after application of the constant potential, the ORR current attains its highest value within 4 min and remained practically constant throughout the experiment. Whereas, the ORR current initially observed on Pd–10MnC/GNS (Fig. 13b) and Pt electrodes (Fig. 13c and d) undergoes a gradual

This journal is

c

the Owner Societies 2013

Fig. 13 Chronoamperograms of Pd–5MnC/GNS, Pd–10MnC/GNS, Pt/GNS and Pt/NC at E = 0.654 V in O2-saturated 1 M KOH at 1600 rpm (25 1C).

decay with the passage of time, the magnitude of the decrease in current being less on the Pt/GNS electrode. Recently, Liang et al.20 has also observed a gradual decay in the ORR current density on the Pt/C catalyst with passage of time because of surface oxide and particles dissolution in alkaline solutions.20 Thus, results substantiate that the ORR activity performance of the Pd–5MnC/GNS hybrid electrode is superior to Pt/NC or Pt/GNS electrode. A similar stability has not been found, to our knowledge, for other Pt- or Pd-based catalysts under alkaline conditions.

Conclusions We have developed new Pd–MC/GNS (M = Mn, Co or Ni) hybrid materials which efficiently oxidize methanol and reduce oxygen in alkaline solutions. Also, they have excellent CO poisoning tolerance vis-a-vis stability under both anodic and cathodic conditions. The electrocatalytic activity of the Pd–10MnC/GNS hybrid catalyst towards MOR was the greatest among the hybrid electrodes studied by us. However, in the case of the ORR study, the activity performance of the Pd–5MnC/GNS and Pd–10MnC/ GNS hybrid catalysts were the best. Similar bifunctional catalysts with excellent CO poisoning tolerance as well as activity for ORR and MOR are scarce in the literature. The study reflects that addition of a controlled amount of a suitable transition complex oxide is highly beneficial with regard to improvement in the catalytic activity, CO poisoning tolerance and stability of the Pd/graphene composite. Efforts are continuing to improve the catalytic properties of the active Pd–MnCo2O4/GNS electrode further by doping of suitable metals/nonmetals.

Acknowledgements Authors thank the Council of Scientific and Industrial Research (CSIR), Government of India through a research project (Ref. No. 01(2320/09-EMR-II)).

Phys. Chem. Chem. Phys., 2013, 15, 20333--20344

20343

View Article Online

Paper

PCCP

Published on 30 October 2013. Downloaded by University of Michigan Library on 31/10/2014 13:11:04.

Notes and references 1 K. Scott and A. K. Shukla, Mod. Aspects Electrochem., 2007, 40, 127–227. 2 E. Antolini and E. R. Gonzalez, J. Power Sources, 2010, 195, 3431–3450. 3 E. H. Yu, U. Krewer and K. Scott, Energies, 2010, 3, 1499–1528. 4 I. Danaee, M. Jafarian, F. Forouzandeh, F. Gobal and M. G. Mahjani, Int. J. Hydrogen Energy, 2008, 33, 4367–4376. 5 C. Bianchini and P. K. Shen, Chem. Rev., 2009, 109, 4183–4206. 6 R. K. Pandey and V. J. Lakshminarayanam, J. Phys. Chem. C, 2009, 113, 21596–21603. 7 R. N. Singh, A. Singh and Anindita, Carbon, 2009, 47, 271–278. 8 R. N. Singh, A. Singh and Anindita, Int. J. Hydrogen Energy, 2009, 34, 2052–2057. 9 Y. Wang, Z. M. Sheng, H. Yang, S. P. Jiang and C. M. Li, Int. J. Hydrogen Energy, 2010, 35, 10087–10093. 10 Q. He, W. Chen, S. Mukerjee, S. Chen and F. Laufek, J. Power Sources, 2009, 187, 298–304. 11 R. Awasthi and R. N. Singh, Int. J. Hydrogen Energy, 2012, 37, 2103–2110. 12 R. Awasthi and R. N. Singh, Carbon, 2013, 51, 282–289. 13 R. Awasthi and R. N. Singh, Catal. Sci. Technol., 2012, 2, 2428–2432. 14 Y. Wang, X. Wang and C. M. Li, Appl. Catal., B, 2010, 99, 229–234. 15 V. Bambagioni, C. Bianchini, A. Marchionni, J. Filippi, F. Vizza, J. Teddy and P. Serp, J. Power Sources, 2009, 190, 241–245. 16 C. Xu, L. Cheng, P. K. Shen and Y. Liu, Electrochem. Commun., 2007, 9, 997–1001. 17 F. Cheng, H. Wang, Z. Sun, M. Ning, Z. Cai and M. Zhang, Electrochem. Commun., 2008, 10, 798–801. 18 E. Antolini, Appl. Catal., B, 2012, 123–124, 52–68. 19 D. A. C. Brownson, D. K. Kampouris and C. E. Banks, J. Power Sources, 2011, 196, 4873–4885. 20 Y. Liang, Y. Li, H. Wang, J. Zhou, J. Wang, T. Regier and H. Dai, J. Am. Chem. Soc., 2012, 134, 3517–3523. 21 M. Hamdani, R. N. Singh and P. Chartier, Int. J. Electrochem. Sci., 2010, 5, 556–577. 22 R. N. Singh, T. Sharma, A. Singh, Anindita and D. Mishra, Int. J. Electrochem. Sci., 2007, 2, 762–777. 23 S. Trasatti, in The Electrochemistry of Novel Materials, ed. J. Lipkowski and P. N. Ross, Wiley VCH, New York, 1994, ch. 5, p. 207. 24 J. P. Singh, N. K. Singh and R. N. Singh, Int. J. Hydrogen Energy, 1999, 24, 433–439. 25 R. N. Singh and R. Awasthi, Catal. Sci. Technol., 2011, 1, 778–783. 26 L. J. Cote, F. Kim and J. Huang, J. Am. Chem. Soc., 2009, 131, 1043–1049. 27 J. P. Singh and R. N. Singh, J. New Mater. Electrochem. Syst., 2000, 3, 137–145. 28 R. N. Singh, T. Sharma, A. Singh, Anindita, D. Mishra and S. K. Tiwari, Electrochim. Acta, 2008, 53, 2322–2330. 29 Y. Liang, Y. Li, H. Wang, J. Zhou, J. Wang, T. Regier and H. Dai, Nat. Mater., 2011, 10, 780–786. 30 Y. Zhao, L. Zhan, J. Tian, S. Nie and Z. Ning, Electrochim. Acta, 2011, 56, 1967–1972. 31 B. D. Adam, C. K. Ostrom and A. Chen, Langmuir, 2010, 26, 7632–7637.

20344

Phys. Chem. Chem. Phys., 2013, 15, 20333--20344

32 T. J. Chuang, C. R. Bridle and D. W. Rice, Surf. Sci., 1976, 59, 413–429. 33 J. L. Gautier, E. Rios, M. Gracia, J. F. Marco and J. R. Gancedo, Thin Solid Films, 1997, 311, 51–57. 34 J. F. Marco, J. R. Gancedo, M. Gracia, J. L. Gautier, E. I. Rios, H. M. Palmer, C. Greaves and F. J. Berry, J. Mater. Chem., 2001, 11, 3087–3093. 35 G. C. Allen, S. J. Harris and J. A. Jutson, Appl. Surf. Sci., 1989, 37, 111–134. 36 A. L. Audi and P. M. A. Sherwood, Surf. Interface Anal., 2002, 33, 274–282. 37 H. Wang, C. M. B. Holt, Z. Li, X. Tan, B. S. Amirkhiz, Z. Xu and B. C. Olsen, Nano Res., 2012, 5, 605–661. 38 E. Rios, H. N. Cong, J. F. Marco, J. R. Gancedo, P. Chartier and J. L. Gautier, Electrochim. Acta, 2000, 45, 4431–4440. 39 A. Restovic, E. Rios, S. Barbato, J. Ortiz and J. L. Gautier, J. Electroanal. Chem., 2002, 522, 141–151. 40 Z. Cui, P. J. Kulesza, C. M. Li, W. Xing and S. P. Jiang, Int. J. Hydrogen Energy, 2011, 36, 8508–8517. 41 S. S. Mahapatra and J. Datta, Int. J. Electrochem., DOI: 10.0461/2011/563495. 42 M. Simoes, S. Baranton and C. Coutanceau, J. Phys. Chem. C, 2009, 113, 13369–13376. 43 H. Li, G. Chang, Y. Zhang, J. Tian, S. Liu, Y. Luo, A. M. Asiri, A. O. Al-Youbi and X. Sun, Catal. Sci. Technol., 2012, 2, 1153–1156. 44 Z. Zhang, L. Xin, K. Sun and W. Li, Int. J. Hydrogen Energy, 2011, 36, 12686–12697. 45 X. Wang, C. Hu, Y. Xiong, H. Liu, G. Du and X. He, J. Power Sources, 2011, 196, 1904–1908. 46 C. Xu, Z. Tian, P. K. Shen and S. P. Jiang, Electrochim. Acta, 2008, 53, 2610–2618. 47 Y. Z. Su, M. Z. Zhang, X. B. Liu, Z. Y. Li, X. C. Zhu, C. W. Xu and S. P. Jiang, Int. J. Electrochem. Sci., 2012, 7, 4158–4170. 48 K. J. J. Mayrhofer, D. Strmcnik, B. B. Blizanac, V. Stamenkovic, M. Arenz and N. M. Markovic, Electrochim. Acta, 2008, 53, 3181–3188. 49 E. Borja-Arco, R. H. Castellanos, J. Uribe-Godinez, A. AltamiranoGutierrez and O. Jimenez-Sandoval, J. Power Sources, 2009, 188, 387–396. 50 C. L. Lee, H. P. Chiou, C. M. Syu, C. R. Liu, C. C. Yang and C. C. Syu, Int. J. Hydrogen Energy, 2011, 36, 12706–12714. 51 M. H. Seo, S. M. Choi, H. J. Kim and W. B. Kim, Electrochem. Commun., 2011, 13, 182–185. 52 R. Adzic, in Electro Catalysis, ed. J. Lipkowski and P. N. Ross, New York, Wiley-VCH, 1998, ch. 5. 53 A. Sarapuu, A. Kasikov, T. Laaksonen, K. Kontturi and K. Tammeveski, Electrochim. Acta, 2008, 53, 5873–5880. 54 B. Li and J. Prakash, Electrochem. Commun., 2009, 11, 1162–1165. 55 C. V. Rao, A. L. M. Reddy, Y. Ishikawa and P. M. Ajayan, Carbon, 2011, 49, 931–936. 56 N. Ramaswamy and S. Mukerjee, J. Phys. Chem. C, 2011, 115, 18015–18026. 57 L. Jiang, A. Hsu, D. Chu and R. J. Chen, J. Electrochem. Soc., 2009, 156, B370–B376. 58 T. Cochell and A. Manthiram, Langmuir, 2012, 28, 1579–1587.

This journal is

c

the Owner Societies 2013