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Electrochimica Acta 88 (2013) 565– 570

Contents lists available at SciVerse ScienceDirect

Electrochimica Acta

jou rn al hom epa ge: www.elsev ier .com/ locate /e lec tac ta

raphene ribbon-supported Pd nanoparticles as highly durable, efficientlectrocatalysts for formic acid oxidation

huangyin Wang, Arumugam Manthiram ∗

aterials Science and Engineering Program and Texas Materials Institute, University of Texas at Austin, Austin, TX 78712, USA

r t i c l e i n f o

rticle history:eceived 2 July 2012eceived in revised form 25 October 2012ccepted 26 October 2012vailable online 3 November 2012

a b s t r a c t

Graphene ribbon-supported Pd nanoparticles (Pd/GR) have been prepared by a simple, clean process inaqueous medium, involving only a Pd precursor and graphene ribbon (GR) as both a reducing agent and acatalyst support. A high density of Pd nanoparticles with a small average particle size (∼2.8 ± 0.8 nm)could be successfully deposited on graphene ribbons. The as-obtained Pd/GR electrocatalysts show

eywords:raphene ribbond nanoparticlesormic acid oxidationuel cell

increased electrochemical surface area and significantly enhanced catalytic activity for formic acid oxida-tion reaction (FAOR) compared to the conventional Pd/C electrocatalysts in terms of peak current density(1.4 A mg−1 vs. 0.79 A mg−1) and peak potential (−0.15 V shift). Both the good dispersion and the rela-tively small particle size of the Pd nanoparticles on graphene ribbon and the synergetic effect betweenPd and GR lead to high initial catalytic activity for FAOR. More importantly, a dramatic enhancement indurability is found with Pd/GR for FAOR due to the strong interaction between Pd and GR.

. Introduction

Direct formic acid fuel cells (DFAFCs) have been attracting muchttention as a promising portable power source [1–5]. Pt- and Pd-ased nanomaterials are extensively used as anode electrocatalystso catalyze formic acid oxidation in DFAFCs [1–3]. Compared to Pt,d-based electrocatalysts show much higher catalytic activity forormic acid oxidation reaction (FAOR). However, the poor dura-ility/stability of conventional Pd electrocatalysts has become aajor obstacle for the commercialization of the DFAFC technology

6,7]. Although the reasons for the poor durability of Pd catalystsn DFAFCs are still not fully clear, some possibilities have been pro-osed: aggregation of Pd nanoparticles, fast dissolution of Pd fromhe carbon support under the DFAFCs operating conditions, and COccumulation at relatively high formic acid concentrations [6,7].hese difficulties warrant the development of highly durable andfficient Pd-based electrocatalysts for DFAFCs.

On the other hand, graphene with a 2D structure is beingntensively investigated both from fundamental and technologi-al points of view [1,8–10]. Graphene-based materials have beenxtensively investigated for applications such as biosensors, fuel

ells, supercapacitors, and batteries. When graphene is etchedr patterned along one specific direction, a novel quasi one-imensional (1D) structure, referred to as graphene ribbon (GR),

∗ Corresponding author. Tel.: +1 512 471 1791; fax: +1 512 471 7681.E-mail addresses: rmanth@mail.utexas.edu, manth@austin.utexas.edu

A. Manthiram).

013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2012.10.125

© 2012 Elsevier Ltd. All rights reserved.

is obtained [5–13]. With its remarkable properties, graphene ribb-ons have been suggested to be a potential basic structure forfuture carbon-based nanoelectronics. Even though graphene andgraphene-based nanomaterials have been extensively studied aselectrode materials for various electrochemical energy conversionand storage devices [1,9,10], to the best of our knowledge, there areonly a few reports on the use of graphene ribbon as an electrodematerial for electrochemical energy conversion and storage devices[13]. Therefore, an investigation of the fundamental electrochem-ical properties of graphene ribbons will be of great scientific andtechnological interest.

We present here, for the first time, graphene ribbon-supportedPd nanoparticles (Pd/GR) as advanced catalysts for DFAFCs. Thesynthesis of the electrocatalysts does not involve any extra reduc-ing agent or stabilizing agent. Graphene ribbon itself acts as boththe reducing agent and the catalyst support for the reduction anddeposition of Pd nanoparticles [14]. The driving force for the syn-thesis and deposition of Pd nanoparticles on graphene ribbon isthe redox potential difference between PdCl42− (0.83 V vs. SCE) andgraphene ribbon (0.48 V vs. SCE) [14]. This fast and in situ synthe-sis leads to a uniform deposition of Pd nanoparticles with smallparticle size on graphene ribbon. For a comparison, Pd nanopar-ticles supported on carbon black (Pd/C) were also prepared withthe conventional polyol reduction method [3]. Electrochemicalcharacterizations demonstrate that Pd/GR exhibits significantly

enhanced initial electrocatalytic activity for formic acid oxida-tion in terms of peak potential and peak current density. Moreinterestingly, Pd/GR shows unusually high durability compared toPd/C.

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66 S. Wang, A. Manthiram / Elect

. Experiment

.1. Synthesis of graphene ribbon

The synthesis of graphene ribbon by unzipping multi-walledarbon nanotubes (MWCNTs) was performed according to Tour’sethod [12]. MWCNTs were received from Cheaptubes.com

10–20 �m in length and 30–50 nm in diameter). MWCNTs wererst dispersed in concentrated sulfuric acid for 12 h, followed byhe addition of potassium permanganate (KMnO4). The reaction

ixture was then stirred for 1 h at room temperature and heatedo 70 ◦C for an additional 1 h. Finally, the reaction was stopped byouring the reaction mixture over ice containing a small amountf hydrogen peroxide. The product was collected by centrifuga-ion after thorough washings sequentially with acid, water, andthanol. The as-obtained graphene ribbon was re-dispersed ineionized water at a concentration of 0.5 mg mL−1. The harsh con-ition (strong acid and strong oxidizer) of the synthesis process

ntroduces significant amounts of defect sites on graphene ribbons,hich are beneficial for the deposition of the metal nanoparticles

15].

.2. Synthesis of Pd/GR

To synthesize Pd/GR, 50 mL of graphene ribbon suspension0.5 mg mL−1) was mixed with 6.5 mL of 10 mM Na2PdCl4 aqueousolution under vigorous stirring. The reaction mixture was stirredor 30 min to get the Pd/GR product, followed by washing withater and centrifugation. In the process of Pd/GR synthesis, no

xtra reducing agents (e.g., sodium borohydride, ethylene glycol, orthanol) or stabilizers (e.g., a polymer or a surfactant) were used,nd graphene ribbon itself served as the reducing agent, stabilizer,nd catalyst support for Pd.

.3. Synthesis of Pd/C

Pd/C electrocatalyst was prepared by the polyol reductionethod with ethylene glycol (EG) as the reducing agent. Briefly,

Fig. 1. TEM images of the (A) as-obtained graphene rib

ica Acta 88 (2013) 565– 570

25 mg of carbon black (XC-72) was dispersed in EG by sonicationand stirring, followed by the addition of 6.5 mL of 10 mM Na2PdCl4aqueous solution. The pH of the mixture was adjusted to around10 by adding 2 M NaOH in EG. The mixture was heated to and keptat 160 ◦C for 3 h. The product was collected by centrifugation afterthorough washing with water.

2.4. Physical and electrochemical characterization

The scanning electron microscopy (SEM) and transmission elec-tron microscopy (TEM) images were collected with a Hitach S-5500machine at 30 kV. The XRD patterns were recorded with a Phillips X-ray diffractometer. The electrochemical tests were carried out witha typical three-electrode cell. The electrocatalysts were loaded ontoa glassy carbon electrode as the working electrode and Pt wire wasused as the counter electrode. All the potentials reported here arein reference to saturated calomel electrode (SCE). The formic acidelectro-oxidation was carried out in 0.5 M H2SO4 + 0.5 M HCOOHelectrolyte solution, while the working electrode was rotated at1000 rpm to facilitate the removal of the generated CO2 product.All the electrochemical tests were performed at room temperature.

3. Results and discussion

Fig. 1A shows the SEM image of graphene ribbons, which showsmuch larger diameter than the unzipped carbon nanotubes inaddition to the disappearance of the tubular morphology. Oncemixed with the Pd precursor, the defect sites on graphene ribbonserve as anchoring sites to catch the Pd2+ ions and then formingthe Pd nanoparticles. The chemical potential difference betweenthe Pd precursor and graphene ribbon leads to the reduction anddeposition of Pd nanoparticles on graphene ribbon. Besides, thefunctional groups generated by the unzipping process may facili-tate the adhesion between Pd nanoparticles and graphene ribbons.

A large amount of defect sites are introduced on the grapheneribbons during the harsh synthesis process, which leads to a highloading of Pd nanoparticles on graphene ribbon. Fig. 1B and C showsthe TEM images of Pd/GR at different magnifications, in which a

bon, (B and C) Pd/GR, and (D) conventional Pd/C.

S. Wang, A. Manthiram / Electrochimica Acta 88 (2013) 565– 570 567

hpiFopctbtsaot

Fig. 2. SEM image of Pd/GR.

igh density of Pd nanoparticles with uniform dispersion and finearticle size (∼2.8 nm) could be seen. Both Fig. 1B and the SEM

mage in Fig. 2 show the cross-linked network structure of Pd/GR.or a comparison of the electrocatalytic activity on Pd/GR with thatn conventional electrocatalysts, Pd/C electrocatalysts were pre-ared by the classical polyol reduction approach (see Section 2), asonfirmed by the TEM image in Fig. 1D. The particle size distribu-ions of Pd nanoparticles on graphene ribbon (Pd/GR) and carbonlack (Pd/C) are plotted in Fig. 3. It can be seen that the average par-icle size of Pd nanoparticles on graphene ribbon (∼2.8 nm) is muchmaller than that on carbon black (5.5 nm). The fast nucleation of Pd

toms under the present synthesis condition and the confinementf Pd atoms/nanoparticles by the defect sites ensure the small par-icle size. The crystalline nature of Pd nanoparticles of Pd/GR and

Fig. 3. Particle size distribution of Pd nanoparticles in Pd/GR and Pd/C.

Fig. 4. XRD patterns of Pd/C and Pd/GR.

Pd/C was examined by the X-ray diffraction (XRD), as seen in Fig. 4.The diffraction peaks at 2� = 40.0◦, 46.5◦, 68.1◦, and 82.5◦ could beindexed, respectively, as the Pd (1 1 1), Pd (2 0 0), Pd (2 2 0), and Pd(3 1 1) reflections, consistent with that of face-centered cubic (fcc)palladium phase [3]. The Pd (2 2 0) peak at 68.1◦ is broader andweaker in Pd/GR than that in Pd/C due to the smaller size of thePd nanoparticles on GR [16,17], which agrees well with the TEMdata (Figs. 1C and 3). Both the TEM and XRD results demonstratethat the graphene ribbon, acting as both the reducing agent andsupport material, leads to a good dispersion and fine particle sizeof Pd, which is beneficial to increase the electrochemical surfacearea and thus the utilization efficiency of the Pd catalysts. It shouldbe emphasized that the Pd nanoparticle deposition would prefer tobe located onto the defect sites introduced by the harsh unzippingprocess, and a strong interaction between the Pd nanoparticles andgraphene ribbons via defect sites would enhance their durabilityfor the formic acid electro-oxidation, as discussed below.

Fig. 5 shows the cyclic voltammograms (CVs) of formic acidelectro-oxidation in a 0.5 M H2SO4 + 0.5 M HCOOH electrolyte solu-tion on Pd/GR and Pd/C electrodes at a scan rate of 50 mV s−1. Theanodic peak current density (forward scan) reflects the amountof formic acid oxidized at the Pd electrocatalysts, and the anodic

peak potential represents the ability of electrocatalysts to cat-alyze formic acid oxidation. Therefore, the anodic peak currentdensity and the peak potential are two important parameters toevaluate the electrocatalytic activity of catalysts. As summarized

Fig. 5. CV curves of formic acid electro-oxidation on Pd/C and Pd/GR electrocatalystsin 0.5 M H2SO4 + 0.5 M HCOOH at a scan rate of 50 mV s−1.

568 S. Wang, A. Manthiram / Electrochimica Acta 88 (2013) 565– 570

Table 1Summary of the electrochemical data, derived from Figs. 5 and 8.

Sample ESA (m2 g−1) Mass activity (A mg−1) Specific activity (mA cm−2) Peak potential (V) Current retention at50th cycle (%)

Pd/C 35.6 0.79 2.2 0.28 19Pd/GR 45.4 1.4 3.1 0.13 114

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Fig. 7. Continuous CV scans of formic acid oxidation in 0.5 M H2SO4 + 0.5 M HCOOH.

ig. 6. The i–t curves of formic acid oxidation on Pd/C and Pd/GR electrocatalysts in.5 M H2SO4 + 0.5 M HCOOH at a constant potential of 0.1 V.

n Table 1, the anodic peak current density of formic acid oxi-ation on the Pd/GR electrocatalyst is 1.4 A mg−1, which is muchigher than 0.79 A mg−1 observed on Pd/C, indicating Pd/GR hasuch higher mass activity for formic acid oxidation. It is believed

hat the enhanced mass activity of Pd/GR relative to Pd/C coulde attributed to the improved dispersion and smaller particle sizef Pd nanoparticles on graphene ribbon. The improved dispersionnd smaller particle size of Pd on graphene ribbons significantlynhance the utilization efficiency of Pd electrocatalysts [18]. Sim-larly, the anodic peak potential of formic acid oxidation on Pd/GRlectrocatalysts is shifted negatively by ∼0.15 V relative to that ofhe conventional Pd/C electrocatalyst, indicating that the formiccid oxidation occurs more readily and easily on Pd/GR, whichight be attributed to the synergetic effect between Pd nanoparti-

les and graphene ribbon with unique electronic properties. Also,he presence of oxygen-containing functional groups on grapheneibbon could improve the hydrophilicity of Pd/GR electrocatalysts,aking them electrochemically more accessible during electro-

atalysis. To further evaluate the activity and stability of the Pd/GRnd Pd/C catalysts, chronoamperometry tests were conducted in.5 M H2SO4 and 0.5 M HCOOH at 0.1 V for 1800 s (see Fig. 6). Ashown in Fig. 6, the steady current density of Pd/GR at 1800 s wasuch higher than that of Pd/C, indicating that graphene ribbon as a

atalyst support can significantly enhance the activity and stabilityf Pd nanoparticles toward formic acid oxidation.

As discussed above, Pd-based electrocatalysts suffer from poorurability for formic acid oxidation in DFAFCs, although they showigher catalytic activity compared to Pt-based catalysts. Pd elec-rocatalysts could be easily aggregated, dissolved from the carbonupport into the electrolyte, or poisoned by the intermediates,esulting in poor durability and a hindrance of the commercial via-ility of DFAFCs. Accordingly, the long-term durability of the Pdlectrocatalysts was evaluated by continuously scanning the CVurves to follow the changes in the anodic peak current densityith cycle number, as shown in Figs. 7 and 8. Fig. 7 compares

he continuously scanned CV curves of Pd/GR and Pd/C electro-atalysts measured in 0.5 M H2SO4 + 0.5 M HCOOH. As seen, thehanges in the anodic current density of formic acid oxidation ond/C are much wider than that on Pd/GR. Fig. 8 shows the changes

Fig. 8. Durability of Pd/C and Pd/GR electrocatalysts for formic acid electro-oxidation in 0.5 M H2SO4 + 0.5 M HCOOH.

in the anodic current density for formic acid oxidation on both

Pd/GR and Pd/C electrodes with cycle number. As seen, the peakcurrent density on the conventional Pd/C electrocatalyst decreasessharply and only 19% of the initial current remains after 50 cycles(Table 1), confirming the poor durability of Pd/C as reported in the

S. Wang, A. Manthiram / Electrochimica Acta 88 (2013) 565– 570 569

d Pd/C (B) after 200 cycles of CV scans.

litsbPi(fegcaiatt

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Fig. 9. TEM image of Pd/GR (A) an

iterature [6,7]. In contrast, the peak current density on Pd/GRncreases initially and then levels off with no obvious degrada-ion even for 200 cycles. Fig. 9 gives the TEM images and Fig. 10hows the histograms of Pd/GR and Pd/C after the long-term dura-ility test, and no obvious change is seen relative to the size ofd/GR before the long-term durability test (Fig. 1C). In contrast,ncrease in particle size and agglomerations were observed for Pd/CFig. 9B). The data indicate that the durability of Pd nanoparticlesor formic acid oxidation is dramatically improved with the Pd/GRlectrocatalyst. In addition, in order to electrochemically investi-ate the property changes of the electrocatalysts, background CVurves, which could be used to estimate the electrochemical surfacerea, were collected in 0.5 M H2SO4 before and after the durabil-

ty test, as shown in Fig. 11. For both Pd/GR and Pd/C, the currentround the hydrogen adsorption/desorption area (−0.2–0.1 V) andhe reduction of Pd oxide (∼0.5 V in the cathodic scan) could reflecthe electrochemical surface area (ESA) of the Pd nanoparticles. As

ig. 10. Particle size distribution of Pd nanoparticles in Pd/GR and Pd/C after dura-ility testing.

Fig. 11. CV curves of formic acid electro-oxidation on Pd/C and Pd/GR electrocata-lysts in 0.5 M H2SO4 before and after durability testing at a scan rate of 50 mV s−1.

seen, before the durability test, Pd/GR shows much higher surfacearea than Pd/C due to the relatively fine particle size. After the dura-bility test, the surface area of Pd/C has decreased significantly, whileonly a slight decrease in surface area was observed for Pd/GR. TheESA values of Pd/GR and Pd/C were calculated by integrating thereduction region of Pd oxide, and the values are summarized inTable 1 [19,20]. It can be clearly seen that the Pd/GR shows muchhigher ESA (45.4 vs. 35.6 m2 g−1) due to the improved dispersionand smaller particle size. As another important parameter, specificactivity is usually used to evaluate the intrinsic activity of electro-catalysts. Based on the ESA values and the mass activity, the specificactivity could be calculated as summarized in Table 1. As seen, thePd/GR also shows higher specific activity than Pd/C.

The graphene ribbon produced by the harsh reaction process

inherits a lot of defect sites, and we believe that the defect sitesstrongly anchor the Pd nanoparticles and prevent them fromaggregating or dissolving into the electrolyte during formic acid

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70 S. Wang, A. Manthiram / Elect

lectro-oxidation [14]. To the best of our knowledge, the Pd/GRlectrocatalysts presented here show the most durable perfor-ance [21].

. Conclusions

In conclusion, graphene ribbon-supported Pd nano-lectrocatalysts obtained by a novel synthesis approach areound as highly durable and efficient electrocatalysts for formiccid electro-oxidation in DFAFCs. The synthesis of Pd/GR involvesnly the Pd salt precursor, graphene ribbon as the Pd support,nd water as the reaction solvent, without any extra reducinggent or stabilizing agent. A high density of Pd nanoparticles withne particle size was successfully deposited on graphene ribbon,

acilitated by the efficient reduction of Pd2+ ions by grapheneibbon and the confinement effect of Pd atoms/nanoparticles byhe defect sites on the graphene ribbons. Compared to the conven-ional Pd/C electrocatalyst, Pd/GR shows dramatically enhancedlectrocatalytic activity for formic acid electro-oxidation in termsf peak current density and peak potential. More importantly,d/GR displays a significantly improved durability, which couldotentially overcome the important bottle-neck problem in formiccid electro-oxidation.

cknowledgments

This work was supported by the Office of Naval Research MURIrant No. N00014-07-1-0758 and Welch Foundation Grant F-1254.

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