Elsevier

Carbon

Volume 51, January 2013, Pages 282-289
Carbon

Graphene-supported Pd–Ru nanoparticles with superior methanol electrooxidation activity

https://doi.org/10.1016/j.carbon.2012.08.055Get rights and content

Abstract

Pd–Ru bimetallic nanoparticles dispersed on graphene nanosheets (GNS) have been obtained by a microwave-assisted polyol reduction method and investigated for methanol electrooxidation in 1 M KOH + 1 M CH3OH at 25 °C. Structural and electrochemical characterizations of electrocatalysts are carried out by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, cyclic voltammetry, CO stripping voltammetry and chronoamperometry. The study shows that introduction of Ru (1–10 wt.%) into 40 wt.%Pd/GNS produces an alloy of Pd and Ru with the face centered cubic crystal structure. The electrocatalytic activity increased with increasing percentage of Ru in the Pd–Ru alloy showing maximum with 5 wt.%Ru. The electrocatalytic activity of the 40 wt.%Pd–5 wt.%Ru/GNS electrode at E = −0.10 V vs. Hg/HgO was ∼2.6 times greater than that of the base (40 wt.%Pd/GNS) electrode. Based on the methanol oxidation current, measured at 1 h during the chronoamperometry tests at E = −0.10 V vs. Hg/HgO, the active 40 wt.%Pd–5 wt.%Ru/GNS electrode exhibited ∼72% and ∼675% higher poisoning tolerance as compared to 40%Pd/GNS and 40%Pd/multiwalled carbon nanotube electrodes, respectively.

Introduction

Among different kinds of fuel cells, polymer electrolyte membrane fuel cell (PEMFC) and direct alcohol fuel cell (DAFC) are most promising. As a fuel, alcohols show superiority over hydrogen due to easier handling and storage, lower operating temperature and higher energy density [1], [2]. Among different fuel candidates methanol has been considered as one of the most appropriate fuel for the DAFC because of its low molecular weight, simplest structure and very high energy density (6.1 kWh kg−1) [3], [4]. Successful commercialization of direct methanol fuel cell (DMFC) could not be made so far. Several difficulties such as sluggish electrode kinetics, high cost of platinum electrocatalysts, methanol crossover through membrane and electrode poisoning are the major difficulties in the process of commercialization of DMFC [2], [3], [5], [6]. The methanol oxidation reaction (MOR), in fact, requires active multiple catalytic sites for the adsorption of methanol and formation of OH species [7]. The latter can promote desorption of the adsorbed methanol residues [7], [8]. In view of this, several bimetallic composites of platinum with Ru or other elements such as Sn, Pb, Mo, etc., were investigated and found to improve the electrocatalytic efficiency of Pt greatly [8], [9], [10], [11], [12], [13], [14], [15]. But, the cost problem still persists. In order to minimize the cost, it is desired to search out a suitable substitute for Pt having lower cost and comparable activity. Among the Pt metals series, Pd stands well as the Pt substitute, because it has similar properties, lower cost and high availability [4], [6], [16]. Palladium exhibits inert behavior towards alcohol oxidation in acidic medium (like H2SO4) while it is highly catalytically active in alkaline solution [4], [6], [16]. To improve its catalytic activity and poisoning tolerance, the research work has been recently carried out with good results on alcohol oxidation by alloying Pd with other metals like Ni, Ag, Ni–Zn, etc. [6], [16], [17], [18] in presence of a high surface area carbon support e.g. multiwalled carbon nanotube (MWCNT) [19], [20], carbon microsphere (CMS) [21], nanowire array (NWA) [22], graphene nanosheet (GNS) [1], [23], [24]. Recently Wang et al. [6] prepared bimetallic Pd–Ag/C catalysts with different Ag loadings by the borohydride reduction method and investigated for methanol electrooxidation. The Pd–Ag(1:1)/C catalyst exhibited the greatest catalytic activity among the series. Yi et al. [25] prepared titanium-supported binary Pd–Ru particles in different atomic ratios by the hydrothermal method and investigated their electrocatalytic activity for ethanol oxidation reaction (EOR) in alkaline medium. Among the electrocatalysts investigated, the Pd87Ru13 catalyst displayed the greatest electrocatalytic activity towards EOR in alkaline medium. He et al. [26] prepared carbon supported Pd4Au and Pd2.5Sn electrocatalysts by a chemical reduction method and observed that the Pd4Au alloy nanoparticles displayed better catalytic activity towards EOR in alkaline medium. Anindita et al. [27], [28] prepared Pd–0.5 wt.%C  x wt.%Ru (x = 1, 2, 5, 10, 20, 30 and 50) composites by the borohydride reduction method and investigated their electrocatalytic activities toward MOR and EOR in 1 M KOH. Among the series, the Pd–0.5 wt.% C-20 wt.%Ru composite exhibited the greatest catalytic activity as well as poisoning tolerance.

Very recently, we have obtained bimetallic 40%Pd–x%Ru (where x = 0, 1, 3, 5, 6 and 10) nanoparticles (NPs) on recently discovered GNS and studied them in electrocatalysis of MOR in 1 M KOH. Results have shown an increase in the catalytic activity with the increase of Ru content in the Pd–Ru NPs/GNS electrodes. The greatest electrocatalytic activity and poisoning tolerance was observed with 40 wt.%Pd–5 wt.%Ru NPs/GNS. Details of results of the investigation are reported in this communication.

Section snippets

Preparation of composites

The catalyst support, GNS, was prepared through the reduction of graphite oxide (GO) by NaBH4 [23], [24], [29]. The oxide (GO) was prepared by the modified Hummers and Offenmans method [23], [24], [30]. Pd–Ru–GNS composites were prepared by a microwave-assisted polyol reduction method [24]. In a typical procedure, 12–10 mg GNS was dispersed in 40 ml ethylene glycol (EG, Merck) solvent by ultrasonication for 1 h and then 20.5–205 μL of 0.096 M RuCl3 (Merck) and 6.6 ml of 0.01 M PdCl2 (Merck) were added

XRD

The XRD powder patterns of 40%Pd/GNS, 40%Pd–5%Ru/GNS and 40%Pd–10%Ru/GNS are shown in Fig. 1. The XRD for the Pd/GNS catalyst shown in Fig. 1 exhibits the three characteristic diffraction peaks at 2θ  39.5°, 46° and 68° corresponding to (1 1 1), (2 0 0) and (2 2 0) planes, respectively. Thus, results indicate that the Pd NPs follow the face centered cubic (fcc) structure. The relatively broad diffraction peak observed at 2θ  25° corresponds to the (0 0 2) plane of the GNS support. Similar diffraction

Conclusions

The study demonstrates that the use of GNS in place of MWCNT as support material for Pd-based active alcohol fuel cell catalysts is advantageous because it makes them catalytically more active and CO poisoning tolerant. In 40%Pd NPs-catalyzed electrooxidation of methanol in 1 M KOH + 1 M CH3OH, the rate of MOR is found to improve ∼1.6 times and CO poisoning tolerance ∼5 times (350%) when the GNS was used in place of MWCNT as the support materials. With introduction of Ru from 1% to 10%, the

Acknowledgements

The authors thank Prof. A.K. Shukla, Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India, for helping in obtaining the XPS data and their interpretations. One of authors (R.A.) acknowledges the financial support received from the Council of Scientific and Industrial Research (CSIR), Government of India, New Delhi as Senior Research Fellow (Grant No. 09/013(0126)/2007-EMR-I) to carry out the studies.

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