Graphene support for enhanced electrocatalytic activity of Pd for alcohol oxidation

Abstract

Graphene nanosheets (GNS) were synthesized by the NaBH₄ reduction of freshly prepared graphite oxide and utilized as a catalyst support of palladium nanoparticles for the electrooxidation of ethanol and methanol. Home-made GNS were characterized by Raman spectroscopy and XRD. The study has shown that Pd nanoparticles dispersed on GNS were highly active for the electrooxidation of ethanol and methanol in 1 M KOH. It is observed that Pd nanoparticles dispersed on GNS are more active compared to those dispersed on nanocarbon particles (NC) or multiwall carbon nanotubes (MWCNTs) for ethanol/methanol electrooxidation under similar experimental conditions. The enhanced electrochemical activity of Pd/GNS toward alcohol oxidation can be ascribed to the greatly enhanced electrochemical active surface area of Pd nanoparticles on the GNS support.

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Introduction

In recent years, electrooxidation of small organic molecules has attracted considerable attention due to the development of direct liquid fuel cells, which require highly reactive fuels of high energy densities.^1,2 Among different types of fuel cells, direct methanol (DMFCs) and ethanol fuel (DEFCs) cells are excellent power sources due to their high energy density, low pollutant emission, low operating temperature and ease of handling. Pt and Pt-based alloys are the best catalysts for the alcohol oxidation,^3–5 but they are very costly and undergo poisoning by the oxidation intermediates, particularly CO molecule.^6–8 As replacements for Pt catalysts, Pd and Pd based catalysts are emerging as a good choice due to their high abundance and good electrocatalytic activity for alcohol oxidation in alkaline medium.^8–11 In order to enhance the electrocatalytic activity further, Pd catalysts have recently been obtained on different support materials. Shen and coworkers obtained a nanostructured Pd catalyst on Vulcan XC-72,¹² MWCNTs (multiwalled carbon nanotubes),¹² ACFs (active carbon fibers),¹² CMSs (carbon microspheres)¹³ and HCSs (hollow carbon spheres)¹⁴ using PdCl₂ as metal precursor and formic acid/tannic acid as reducing agent. The Pd/MWCNT catalyst indicated better activity than Pd/ACF or Pd/C electrode. The smaller size and higher dispersion of Pd nanoparticles on the Pd/MWCNT surface are considered as the key factors accounting for the higher catalytic activity. Bambagioni et al.⁹ have studied the performance of Pd/MWCNT for the oxidation of methanol, ethanol, and glycerol in 2 M KOH. The results exhibited high activity of the catalyst for the oxidation reactions of all alcohols even at metal loadings as low as 17–20 μg cm⁻². Zhang et al.¹⁵ obtained well dispersed Pd nanoparticles on the surface of vanadium oxide nanotubes (VO_x-NTs) through a simple reductive process and investigated them as electrocatalysts for MOR in alkaline medium. This new material was found to exhibit an excellent electrocatalytic activity and good stability under alkaline condition. Li et al.^15,16 obtained Pd nanoparticles supported on β-MnO₂ nanotubes and VO_x nanotubes and reported that the methanol electrooxidation on both the catalysts in NaOH solution was better than the traditional Pd/C catalyst at comparable metal loadings. Lee et al.¹⁷ obtained Pd nanoparticles on Vulcan XC-72R (Pd/C) by means of a polyol process in poly(vinyl pyrrolidone) and NO³⁻ ion. They observed an excellent stability for the Pd/C catalyst in comparison with commercial Pd.

Recently, Singh et al.^6,18–20 synthesized nanostructured Pd-x wt% C (x = 0.5, 1, 2 and 5) and Pd-y wt% MWCNT (y = 1, 2 and 5) by the borohydride reduction method and investigated for electrocatalysis of methanol^19,20 and ethanol^6,18 oxidation reactions in 1 M KOH at 25 °C. The Pd-1 wt% MWCNT and Pd-0.5 wt% C electrodes exhibited the highest catalytic efficiencies for oxidation of ethanol.

Graphene, a sp²-hybridized two-dimensional mono-layer sheet,^8,21–23 has high specific surface area,^23–26 excellent electronic conductivity,^8,23,24 high thermal stability, unique graphitized basal plane structure and low manufacturing cost^8,27,28 and has recently been used as a support material for Pt metal dispersion.^2,24–26 It is reported that the electrocatalytic activity of Pt/graphene nanosheet (GNS) for methanol oxidation was better than Pt/VulcanXC-72^24,25 or Pt/MWCNT.² Very recently, Zhao et al.⁸ carried out cyclic voltammetry of Pd/polypyrrole (PPy)-graphene, Pd/graphene and Pd/Vulcan XC-72 carbon black in 1 M CH₃OH + 0.5 M NaOH and found that the electrocatalytic activity of the Pd/PPy-graphene electrode was higher than that of the Pd/graphene and the Pd/Vulcan electrodes. These results prompted us to prepare the Pd/GNS electrode at varying Pd loadings, namely 15, 20, 25 and 40 wt%, and examine their electrocatalytic activities towards methanol/ethanol oxidation in 1 M KOH (25 °C). Preliminary investigations revealed that the electrocatalytic activity of the catalyst electrode was the highest with 20 wt% Pd loading. Based on values of the alcohol oxidation current density at a constant potential (E = −0.20 V_Hg/HgO), the catalyst electrodes followed the catalytic order: 20 wt% Pd/GNS (methanol: I_p ≈ 413 mA mg⁻¹_Pd and ethanol: 1742 mA mg⁻¹_Pd) >15 wt% Pd/GNS (methanol: I_p ≈ 165 mA mg⁻¹_Pd and ethanol: 763 mA mg⁻¹_Pd) >25 wt% Pd/GNS (methanol: I_p ≈ 162 mA mg⁻¹_Pd and ethanol: 712 mA mg⁻¹_Pd) >40 wt% Pd/GNS (methanol: I_p ≈ 79 mA mg⁻¹_Pd and ethanol: 299 mA mg⁻¹_Pd). Details of results obtained on the electrocatalysis of active 20 wt% Pd nanoparticles dispersed on GNS in the oxidation of methanol and ethanol in 1 M KOH (25 °C) are reported in this paper. For comparison, 20 wt%Pd nanoparticles dispersed on MWCNTs and NC supports were also obtained and studied under similar experimental conditions.

Experimental

Graphene preparation

Graphene nanosheets were obtained through chemical reduction of graphite oxide (GO), which was prepared by a modified Hummers and Offenmans method.^29,30 For the purpose, 0.5 g of graphite (Aldrich), 0.5 g NaNO₃ (Aldrich) and 23 ml H₂SO₄ (Merck) were stirred in an ice bath and to this 3 g of KMnO₄ (Aldrich) was slowly added with constant stirring. This solution was transferred to a 35 ± 5 °C for 1 h and 40 ml re-distilled water was then added and the temperature was kept at 90 ± 5 °C for 30 min. Finally, 100 ml distilled water was added, followed by slow addition of H₂O₂ (30%) (Merck) till the color of the solution changed from dark brown to yellow. The resulting solution was filtered and the residue was washed with hot double distilled water several times and then dried in a vacuum oven at 100 °C over night to obtain GO. 100 mg of GO was dispersed in 100 ml double distilled water and ultrasonicated for 1 h. Subsequently, 200 mg NaBH₄ was added, stirred for 1 h and then heated at 125 °C for 3 h. The black precipitate (GNS), so obtained, was separated by centrifugation, washed with double distilled water and dried in a vacuum oven at 100 °C over night.³¹

Pd catalyst preparation

The catalysts, Pd/GNS, Pd/MWCNT and Pd/NC, with the same Pd loadings (20 wt%) were prepared by a microwave-assisted reduction method.³² In a typical procedure, 16 mg of the support (GNS, activated MWCNT or activated NC) was dispersed in 40 ml ethylene glycol (EG, Merck) solvent by ultrasonication for 1 h and then 3.3 ml of an acidified aqueous solution of 0.01 M PdCl₂ (Merck) (0.05 M HCl) was added and ultrasonicated for 15 min. The pH of this mixture was adjusted to 10 with the aid of 0.8 M KOH. Then, the solution was placed in a microwave oven and heated for 3 min at 800 Watt power. The resultant slurry was centrifuged, washed with acetone and dried in a vacuum oven at 100 °C over night. As mentioned earlier,¹⁸ the activation of NC (Aldrich, 99 + %, particle size ≤30 nm, BET surface area >100 m² g⁻¹)/MWCNT (Aldrich, Pr. No. 659258, dia. = 110–170 nm and length = 5–9 micron) was carried out by refluxing it in concentrated HNO₃ for 5 h.

Catalyst electrode preparation

3 mg of the catalyst was dispersed in 600 μl solution of double distilled water, isopropanol and ethanol (1 : 1 : 2). The mixture was then ultrasonicated to get a homogenous suspension containing 5 × 10⁻³ mg μl⁻¹catalyst. 30 μl of the suspension was then applied on a pretreated glassy carbon (GC, geometrical area = 0.5 cm²) plate through a micro pipette (100 μl capacity) and dried in air. Finally, 10 μl of 1 wt% Nafion solution (Alfa Aesar) was dropped over the dried catalyst layer so as to enhance the adherence of the catalytic film. The GC electrode was pretreated as described elsewhere.¹⁸ The electrodes, thus obtained, were finally irradiated with a microwave (800 Watt) for 1 min. Electrical contact with catalyst film was made as already described elsewhere.³³ The loadings of the catalyst (C + Pd) (=0.3 mg cm⁻²) and Pd metal (=0.06 mg cm⁻²) were determined from their respective amounts present in the suspension (30 μl) assuming 100% utilization of the Pd precursor.

Catalyst characterization

X-Ray diffraction (XRD) patterns of catalysts were recorded on an X-ray diffractometer (Thermo Electron) at a sweep rate of 3° min⁻¹ using Cu-K_α as the radiation source (λ = 1.541841 Å).

Electrochemical studies were carried out in a conventional three-electrode single compartment Pyrex glass cell. The potential of the working electrode was measured against the Hg/HgO/1 M KOH electrode (0.098 V vs.SHE). The counter electrode was a Pt plate of ∼8 cm². The potential values mentioned in the text are referred against the Hg/HgO electrode only. Electrochemical studies, namely, cyclic voltammetry (CV) and chronoamperometry, have been carried out by a computer controlled EG & G PAR potentiostat/galvanostat (Model: 273A).

CV of each electrocatalyst was recorded in 1 M KOH with and without containing 1 M alcohol at 25 °C. Before recording the final voltammogram, each electrode was cycled for five runs at a scan rate of 50 mV s⁻¹ in 1 M KOH. All electrochemical experiments were performed in an Ar deoxygenated electrolyte at 25 °C. The mass of the catalytic films on GC was 0.3 mg cm⁻² and the geometrical area of each catalyst electrode used in the investigation was 0.5 cm². To ascertain the reproducibility, triplicate electrodes of each catalyst were employed in the investigation and the data listed in Table 1 are average ones.

Table 1 Result of cyclicvoltammetry of 20 wt% Pd/GNS, 20 wt% Pd/MWCNT and 20 wt% Pd/NC in 1 M KOH +1 M alcohol at 25 °C

Catalyst	Methanol		Ethanol		EASA/cm^2 mg^−1Pd	I/mA mg^−1Pd at −10 mV		SA/mA cm^−2
	I p/mA mg^−1Pd	E p/mV	I p/mA mg^−1Pd	E p/mV		Methanol	Ethanol	Methanol	Ethanol
20 wt% Pd/GNS	1179 ± 23	81 ± 15	1966 ± 26	89 ± 3	5169 ± 18	915 ± 12	1830 ± 25	0.18	0.36
20 wt% Pd/MWCNT	911 ± 3	70 ± 2	1581 ± 14	48 ± 12	4531 ± 20	800 ± 17	1510 ± 20	0.18	0.34
20 wt% Pd/NC	891 ± 10	22 ± 8	1276 ± 19	14 ± 16	2183 ± 20	850 ± 29	1236 ± 14	0.39	0.56

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Results and discussion

X-Ray diffraction (XRD)

Fig. 1 shows the XRD powder patterns of compounds, GO, Graphene nano sheet (GNS), 20 wt% Pd/GNS, 20 wt% Pd/MWCNT and 20 wt% Pd/NC, respectively. In Fig. 1, the diffraction peak observed at 2θ ≈ 10.65° (d = 8.3 Å) is the characteristic diffraction peak (002) of GO^2,34 and the diffraction peak at 2θ ≈ 42.9° (d = 2.1 Å) corresponds to the (100) plane of the hexagonal structure of carbon.^2,35 The observation of Fig. 1 further shows that after chemical reduction of GO with NaBH₄, the typical diffraction peak (002) of GO shifts towards a higher angle, 2θ ≈ 27.32° (d ≈ 3.26 Å). Furthermore, the broadening of diffraction peak (002) indicates the formation of graphene nano sheets (GNS). The peak (002) shifted to a slightly lower angle by ∼0.8° in the XRD pattern of 20 wt% Pd/GNS. XRD patterns of 20 wt% Pd/GNS, 20 wt% Pd/MWCNT and 20 wt% Pd/NC show the three characteristic peaks of Pd at 2θ ≈ 40° (d ≈ 2.25 Å), 46.5° (d ≈ 1.95 Å) and 68.2° (d ≈ 1.37 Å) corresponding to planes (111), (200) and (220), respectively.^8,17 The most intense diffraction peak (111) is used to estimate the Pd particle size using Scherer formula.⁸ Estimates of the particle size of Pd were 2.5, 8.4 and 12.6 nm, respectively, on GNS, MWCNT and NC supports. This result shows that the size of Pd particles gets significantly reduced on the GNS support. Applying Lorentzian fit for the diffraction peak (002) and using Scherer formula, the estimate of the crystallite size of GNS was found to be 1.2 nm. Values of the d-spacing and the crystallite size were used to estimate the number of the graphene layers.^36,37 The result showed that the GNS sample contains ∼3 layers. The d-spacing was calculated by using the Bragg equation.

Fig. 1 XRD patterns of GO, GNS, 20 wt% Pd/GNS, 20 wt% Pd/MWCNT and 20 wt% Pd/NC.

Raman spectroscopy

Fig. 2 shows Raman spectra of GO, GNS and 20 wt% Pd/GNS, respectively. All the three samples exhibited the characteristic D, G and 2D bands at ∼1350, ∼1580 and ∼2650 cm⁻¹.^22,31 The D band is ascribed to edges, other defects and disordered carbon, whereas G band is produced from the zone center E_2g mode, corresponding to the ordered sp² bonded carbon atom.^22,31 After reduction of GO with NaBH₄, the G band shifted towards a slightly lower wave number (∼4 cm⁻¹), indicating that GNS were produced.²⁵ The ratio of the intensities of the D (I_D) and of G (I_G) band, I_D/I_G, is a measure of the degree of the disorder and average size of the sp² domain^22,31/the in-plane crystallite (L_a) few-layer graphene samples.³⁷ The I_D/I_G ratio is found to be higher in graphene (1.17) compared to that in GO (1.02). This indicates the higher level of the disorder of the graphene layers and increased number of defects in GNS.²² The increase in this ratio also suggests a decrease in the average size of the sp² domain upon the reduction of the exfoliated GO.³⁸ Value of the average size of the in-plane crystallite (L_a) for the graphene sample prepared by us was estimated by employing the relation, L_a = 4.4(I_G/I_D)³⁷ and found to be ∼4 nm. The strong D band in the Raman spectrum of 20 wt% Pd/GNS arises mostly from the edge and steps present in the GNS.²²

Fig. 2 Raman spectra of GO, GNS and 20 wt% Pd/GNS.

The 2D band is the second order of the D band and is a characteristic of graphene and used to determine the number of layers of graphene in sample. Broad and less intense 2D band observed for the graphene (Fig. 2B) clearly indicates that it has few layers.^36,39 The number of layers (N) was calculated using the relationship,^36,40 the full width at half maximum (FWHM) of 2D = (−45(1/N) + 88) [cm⁻¹]. This calculation showed that the graphene contains approximately two layers. However the number of layers obtained from the XRD study is ∼3. Thus, results show that the graphene sample used in the present study contains 2–3 layers.

Cyclic voltammetry (CV)

Fig. 3–5 show CVs of 20 wt% Pd/GNS, 20 wt% Pd/MWCNT and 20 wt% Pd/NC electrodes in the potential range, −0.8–0.7 V, in 1 M KOH and 1 M KOH + 1 M alcohol at 25 °C. All the three CV curves shown in Fig. 3 are similar, regardless of the nature of the support and tally exactly the CV curves of pure Pd as already reported in the literature.¹⁸ CV curves of catalysts determined in 1 M KOH + 1 M alcohol also appear to be similar and that each curve displays two characteristic anodic (oxidation) current peaks, one in the forward scan (i.e. under anodic condition) and the other one in the reverse scan (i.e. under cathodic condition). For electrochemical activity comparison, values of the forward oxidation current peak and of the corresponding peak potential for methanol/ethanol oxidation are given in Table 1.

Fig. 3 Cyclicvoltammograms of 20 wt% Pd/GNS, 20 wt% Pd/MWCNT and 20 wt% Pd/NC in 1 M KOH at 25 °C.

Fig. 4 Cyclicvoltammograms of 20 wt% Pd/GNS, 20 wt% Pd/MWCNT and 20 wt% Pd/NC in 1 M KOH + 1 M CH₃OH at 25 °C.

Fig. 5 Cyclicvoltammograms of 20 wt% Pd/GNS, 20 wt% Pd/MWCNT and 20 wt% Pd/NC in 1 M KOH + 1 M C₂H₅OH at 25 °C.

Based on the alcohol oxidation current peak values, the rate of oxidation of methanol/ethanol is higher on the 20 wt% Pd/GNS electrode than that observed on 20 wt% Pd/MWCNT or 20 wt%/NC under similar experimental conditions. The rate of methanol oxidation observed on the 20 wt% Pd/GNS electrode (I_p ≈ 1179 mA mg⁻¹_Pd, E_p ≈ 0.08 V_Hg/HgO) in 1 M KOH + 1 M CH₃OH also seems to be higher than those recently reported on Pd/PPy-GNS⁸ (I_p ≈ 359 mA mg⁻¹_Pd, E_p ≈ −0.16 V_SCE ≈ −0.02 V_Hg/HgO) in 0.5 M NaOH + 1 M CH₃OH, Pt/GNS²⁴ (I_p ≈ 261 mA mg⁻¹_Pt, E ≈ 0.61 V_SCE ≈ 0.75 V_Hg/HgO) in 0.5 M H₂SO₄ + 0.5 M CH₃OH and 60% Pt/GNS²⁵ (I_p ≈ 300 mA mg⁻¹_Pt, E_p ≈ 0.70 V_Ag/AgCl ≈ 0.80 V_Hg/HgO) in 0.5 M H₂SO₄ + 2 M CH₃OH. Investigation on electrocatalytic activities of Pd or Pt dispersed nanoparticles on GNS towards ethanol oxidation is scarce in the literature.

As the peak potential for oxidation of methanol/ethanol varies with the nature of support, the electrocatalytic activities of 20 wt% Pd electrodes have been compared at a common and constant potential (E = −10 mV), chosen prior to the alcohol oxidation peak current on the forward scan of CV curves (Fig. 4 and 5). At this potential the current produced due to oxidation of alcohol on different electrodes shown in Table 1 clearly demonstrate that, among the electrodes investigated, the 20 wt% Pd/GNS electrode has the greatest catalytic activity for oxidations of both methanol and ethanol in 1 M KOH. At E = −10 mV, the rate of oxidation of ethanol on each electrode is, however, approximately 2 times higher than that of methanol. The observed higher rates of ethanol oxidation on the Pd-based electrocatalysts may be ascribed to the fact that ethanol is selectively oxidized to acetic acid, which is soon transformed into acetate ion in the alkaline environment of the reaction. On the other hand, methanol produces carbonatevia a CO intermediate.^41,42 The CO is known as a highly poisonous intermediate in the alcohol oxidation reaction. In fact, during the alcohol oxidation reaction CO gets strongly adsorbed on the active Pt/Pd sites and thereby reduces the catalytic efficiency of the electrode.^6,7,41,42 Thus, based on the observed current (mA mg⁻¹_Pd) at E = −10 mV, the Pd electrodes of the present study follow the catalytic order: 20 wt% Pd/GNS > 20 wt% Pd/MWCNT > 20 wt% Pd/NC.

To examine the role of a substrate in the improvement of electrocatalytic properties of Pd, knowledge of the electrochemically active surface area (EASA) of the electrodes is required. It is known that for Pd-electrodes, in contrast with Pt, the charge of a monolayer of adsorbed hydrogen is difficult to determine due to the ability of bulk Pd to absorb hydrogen. Therefore, the EASA of Pd electrodes has been measured by determining the coulombic charge for the reduction of palladium oxide.^8,20,43,44 As shown in Fig. 3, the oxide reduction peak appears at E = −0.3–0.4 V on the cyclic voltammograms of electrodes in 1 M KOH. The coulombic charge (Q) for the reduction of palladium oxide has been determined by integrating the area under oxide reduction peak through an integrator facility provided in the instrument (potentiostat/galvanostat) and values were 62 mC, 55 mC and 27 mC for 20 wt% Pd/GNS, 20 wt% Pd/MWCNT and 20 wt% Pd/NC, respectively. The EASA of the electrode was then obtained using the relation, EASA = Q/Sl, where Q is the coulombic charge (in mC), S is a proportionality constant used to relate charge with area and l is the catalyst loading in mg. A charge value of 0.405 mC cm⁻² is assumed for the reduction of a PdO monolayer.^8,20,43 Estimates of EASA of electrodes are shown in Table 1. Results show that the EASAs of 20 wt% Pd dispersed on different supports follow the order: 20 wt% Pd/GNS > 20 wt% Pd/MWCNT > 20 wt% Pd/NC. The reported values of the BET surface area of graphene nano sheet, MWCNTs and NC supports are 450,²³ 299⁴⁵ and 100 m² g⁻¹, respectively. Thus, the higher the BET surface area of the support the greater is the EASA of the dispersed Pd catalyst. Estimates of the specific activity (SA = I, mA mg⁻¹_Pd/EASA, cm² mg⁻¹_Pd) listed in Table 1 show that the graphene nano sheet support improves only geometrical properties (i.e.EASA) of a Pd catalyst while its electronic properties remains unaffected. So, the improved electrocatalytic activity of the synthesized catalyst, i.e. 20 wt% Pd/GNS compared to the Pd supported on other carbon nanostructures, is believed to be due to the improved electroactive surface area of Pd nanoparticles on the GNS support. In fact, the structure of the support reduces the particle size and hence increases the EASA and in turn, electrocatalytic activity of Pd.

Chronoamperometry

Fig. 6 represents the chronoamperograms of electrodes in 1 M KOH +1 M alcohol at the potential of −0.2 V. It is clear that during the whole testing time the current density produced on 20 wt% Pd/GNS catalyst is higher compared to that on 20 wt% Pd/MWCNT or 20 wt% Pd/NC in the case of both methanol and ethanol. Thus, the 20 wt% Pd/GNS electrode has considerably improved performance as well as poising tolerance compared to 20 wt% Pd/MWCNT and 20 wt% Pd/NC.

Fig. 6 Chronoamperograms of 20 wt% Pd/GNS, 20 wt% Pd/MWC NT and 20 wt% Pd/NC in (a) 1 M KOH + 1 M CH₃OH and (b) 1 M KOH + 1 M C₂H₅OH at 25 °C.

Conclusions

In summary the study indicates that the graphene nanosheet is a much superior support for obtaining highly dispersed and active Pd nanoparticles for ethanol/methanol electrooxidation in alkaline solutions. It is observed that 20 wt% Pd nanoparticles dispersed on GNS are electrochemically more active than those dispersed on MWCNTs or NC support under similar experimental conditions. All the three Pd catalysts, 20 wt% Pd/GNS, 20 wt% Pd/MWCNT and 20 wt% Pd/NC, have the higher electrocatalytic activity for ethanol oxidation than that for methanol oxidation in 1 M KOH, the activity, however, being the greatest for 20 wt% Pd/GNS catalyst.

Acknowledgements

This work was supported by the Council of Scientific and Industrial Research (Grant No. 09/013(0126)/2007-EMR-I), and the Department of Science and Technology (Grant No. SR/S1/PC-41), Government of India.

Notes and references

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