Electrochimica Acta 190 (2016) 956–963

A facile approach for preparation of highly dispersed platinum-copper/carbon nanocatalyst toward formic acid electro-oxidation

Yiyin Huang, Tianshou Zhao*, Lin Zeng, Peng Tan, Jianbo XuDepartment of Mechanical and Aerospace Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China

A R T I C L E I N F O

Article history:Received 7 September 2015Received in revised form 10 December 2015Accepted 31 December 2015Available online 3 January 2016

Keywords:Fuel cellSupported catalystsFormic acid oxidationPlatinum-copperSeparation

A B S T R A C T

Gaining control over the size and dispersion of binary metal nanoparticles is critical in order tomanipulate their catalytic properties. In this study, we demonstrate a facile and effective solid phaseevolution approach to prepare a highly dispersed PtCu/C catalyst via a surface substitution and etchingseparation process with the Pt-decorated Cu particles on carbon as the precursors. It is demonstrated thatthe dispersion of metal nanoparticles in PtCu/C derived from the present solid phase evolution is betterthan that in PtCu/C (C) prepared from the co-reduction by NaBH4. As a result, the synthesized PtCu/Cshows a larger electrochemically active surface area (ECSA) (48.6 m2g�1), higher mass (0.52 mA mg�1)and area activities (1.07 mA cm�2) than that of the PtCu/C (C) (37.9 m2g�1, 0.34 mA mg�1 and0.89 mA cm�2, respectively). As compared to commercial Pt/C catalyst, PtCu/C exhibits ca. 2.5 timeshigher formic acid oxidation (FAO) activities (0.52 mA mg�1 and 1.07 mA cm�2). The tolerance toward COpoisoning is characterized by CO stripping, the result indicates that both onset (0.43 V) and peak (0.50 V)potentials of PtCu/C for CO oxidation show a negative shift of ca. 70 mV. More significantly, PtCu/C showshigh stability in the acid solution, which can maintain 90.1% retention in ECSA after 1000 CV cycles. Inaddition, the solid separation method offers ease of manipulation, allowing the synthesis of a novel classof highly dispersed binary metal nanoparticles.

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1. Introduction

Platinum catalysts are among the most versatile catalystscapable of mediating a multitude of chemical reactions, andremain difficult to be replaced by other non-corrosive metals in theanodes of low temperature proton exchange membrane fuel cells(PEMFCs) [1–3]. Their popularity is evidenced by a surge of interestin alloying Pt with transition metals for synthesis of low-cost,highly active and stable catalysts [4–9]. Pt alloys have been widelyused in the anodes of PEMFCs, due to their unique structure andnature. The bifunctional effect [10], the ligand effect [11] and thethird-body effect [12–14] are generally applied to promote thesluggish kinetics on electro-catalytic oxidation of small organicmolecules. To commercialize the use of Pt alloy catalysts, it isparticularly important to select a method of synthesis thatmaximizes the dispersion and optimizes the size of Pt-containingnanoparticles to increase the activity and stability. An impregna-tion-reduction method, in which Pt and other metal salt precursorsare impregnated into a carbon support, followed by a reduction

* Corresponding author. Tel.: +852 2358 8647. Fax: +852 2358 1543.E-mail address: metzhao@ust.hk (T. Zhao).

http://dx.doi.org/10.1016/j.electacta.2015.12.2230013-4686/ã 2016 Elsevier Ltd. All rights reserved.

process, has some prevalence due to its simplicity and suitabilityfor large-scale production [15]. However, this method fails to allowfull control over the uniform nanoparticles, particularly againstagglomeration even with the support of strong oxidizing acid-treated carbon [16]. As such, much attention has been recentlyfocused on the use of highly viscous solvents, surfactants,functional polymers and other capping agents [17–20], in aneffort to control the particle size and to acquire high dispersion.Nevertheless, the introduction of these additional compounds inthe preparation procedures increases the complexity of post-processing, which limits the practical applications for fuel celltechnology.

A comprehensive understanding of the agglomeration mecha-nisms of nanoparticles with an impregnation-reduction methodmust be established to explore novel synthesis techniques ofhighly dispersed alloy nanoparticles. The initial stage of theimpregnation-reduction process involves generating Pt alloynanoparticles from a solution, to be deposited on the surfaces ofsupport, e.g., carbon surfaces. These surfaces and the locationsclose to the particles are prone to subsequent metal deposition,resulting in unwanted particle growth and aggregation asillustrated in Scheme 1A. Rapid reduction of precursors, selectiveuse of solvents and capping agents that separate metal

http://crossmark.crossref.org/dialog/?doi=10.1016/j.electacta.2015.12.223&domain=pdfmailto:metzhao@ust.hkhttp://dx.doi.org/10.1016/j.electacta.2015.12.223http://dx.doi.org/10.1016/j.electacta.2015.12.223http://www.sciencedirect.com/science/journal/00134686www.elsevier.com/locate/electacta

Scheme 1. The illustration of (A) nanoparticle growth and aggregation via liquid phase deposition and (B) solid phase evolution processes.

Y. Huang et al. / Electrochimica Acta 190 (2016) 956–963 957

nanoparticles are effective solutions in mitigating this issue[21–23]. However, these means are insufficient in thoroughlyeliminating the particle growth and aggregation. The root of theproblem lies in the fact that the migration of metal ions or particlesoccurs easily in the liquid phase, increasing the chance of metaldeposition in the same and adjacent positions, which worsens overtime. As such, eliminating this problem requires a more complexmigration process in contrast to the liquid phase migration; aprocess which employs the assembly of particles via a solid phaseis required, as depicted in Scheme 1B. To achieve such a process, Pt-alloy atoms/clusters are choice precursors for forming nano-particles, due to their small size and good stablility in the solidphase. Isolated Pt-containing clusters can be readily acquired fromthe galvanic reaction between small Pt4+ ions and abundant metalparticles [24] with low activity, adding further feasibility to thisapproach. During the assembly of atoms/clusters to form nano-particles, aggregation may occur. It should be noted that someaction exists from the interior of the material, such as supportcoupling action, which can act as a separator for these nano-particles. To date, however, very little exploration on this approachhas been conducted.

Cu particles were selected as a substrate in this study.Polyhedral Cu particles were prepared by a sodium borohydride(NaBH4) reduction process. The substitution reaction betweenabundant Cu particles and fewer Pt4+ ions was conducted to obtainPt atom/cluster-decorated Cu particles. These Pt-decorated Cuparticles were subsequently mixed with carbon. A process of nitricacid etching was then performed to drive the atom/clusterassembly (namely solid phase evolution) at the atomic-scale,removing redundant Cu simultaneously. This synthesis process isstraightforward and easy to control. After etching, the morphology,dispersion and structure of the alloy nanoparticles were analyzedby X-ray power diffraction (XRD), transmission electron micro-scope (TEM), X-ray photoelectron spectroscopy (XPS) and so on. Aformic acid oxidation (FAO) reaction was used to determine thecatalytic properties of the materials. Compared with palladium(Pd), although Pt itself is susceptible to poisoning as oxidizingformic acid via a direct dehydration path [25]; this FAO reactionbecomes an effective reaction to examine the influence of alloyingPt with Cu in catalytic properties [26,27].

2. Experimental

2.1. Synthesis of catalyst

Active carbon (Vulcan XC-72R) was pretreated with 5 M HNO3at 110 �C for 12 h. Then, the mixture was neutralized with sodiumhydroxide, filtered, washed with double distilled water and driedovernight. The acid treating procedure was used to produce someoxygen-containing groups such as carboxyl, carbonyl and hydroxylon carbon surface. These groups provide anchoring sites for metalparticle deposition [28]. PtCu/C alloy catalyst was prepared via thesurface substitution and etching separation processes. In a typicalsynthesis, 165 mg of CuCl2�2H2O was dissolved in 100 ml ofdeionized water under stirring. An excess NaBH4 solution wasdropped slowly into the solution to reduce Cu2+ ions. Afterwards,this mixture was stirred with 30 mg of the pretreated Vulcan XC-72R carbon under ultrasound, and then rapidly filtered and washedto remove the residual NaBH4. The obtained Cu/C composite wasimmediately transferred to a beaker and dispersed in 100 ml ofdouble distilled water, then 100 ml of solution containing 10 mg ofH2PtCl6�6H2O was added into the suspension under vigorousstirring. The galvanic substitution reaction from Pt4+ ions to Pt byoxidizing Cu was conducted for 1 h. 1 ml of concentrated nitric acidwas then dropped very slowly into the solution under constantstirring. The temperature of the system was elevated to 80 �C andsustained for 2 h. Finally, the mixture was filtered, washed anddried overnight. The PtCu alloy material was synthesized via thesame method without adding carbon. For comparison, a PtCu/C (C)catalyst was also prepared by co-reduction of an equimolarmixture of H2PtCl6 and CuCl2 with NaBH4 in a carbon suspension.The Pt content in this material was set at 10 wt.%.

2.2. Physical and electrochemical characterization

Morphology and element mapping of the catalytic materialswere depicted by JEOL-6300F scan electron microscopy (SEM) andan energy-dispersive X-ray (EDS) microanalyser. The Pt loadingsand Pt/Cu atomic ratios in the materials were determined using anUltima2 inductively coupled plasma OES spectrometer (ICP-OES,Jobin Yvon). For sample preparation, carbon in the materials was

Fig. 1. The photographs of (A) Cu particles prepared by reducing ca. 0.934 mmol Cu2+ with excess sodium borohydride (after filtration), (B) etching Cu particles in (A)with nitric acid at 80 �C for 2 h, (C) replacing Cu particles with chloroplatinic acid(Pt:Cu = 1:10, atomic ratio), and (D) etching PtCu particles in (C) with nitric acid at80 �C for 2 h.

958 Y. Huang et al. / Electrochimica Acta 190 (2016) 956–963

removed at 700 �C; the residue was dissolved in aqua regia and thesolution was diluted with double distilled water and transferred tocentrifuge tubes for analyses. The dispersed state and sizedistribution of the metal particles were characterized using aJEOL JEM-2010 transmission electron microscope (TEM) at theaccelerating voltage of 200 kV. The chemical valences of Pt and Cuin the catalysts were analyzed by X-ray photoelectron spectrosco-py (XPS, VG ESCALAB 250) with an Al Ka X-ray source at 1487 eV.The chamber pressure was kept below 3 � 10�10mbar during thetest and specific correction was conducted by using the C 1sbinding energy of 285 eV. X-ray power diffraction (XRD) wasperformed with a Philip X’Pert Pro MPP diffractometer using a CuKa (l = 1.54 Å) radiation source at the scan rate of 5� min�1.

The electrochemical measurements were performed using aCHI604 electrochemical working station (CH Instrument Inc.). Thereference electrode was an Ag/AgCl in saturated KCl solution andthe counter electrode was platinum net. A glassy carbon(0.1256 cm2) electrode covered by the catalyst was used as theworking electrode. For working electrode preparation, 1.0 mg of Ptbalck, 1.9 mg of PtCu alloy, 9.2 mg of PtCu/C or 2.5 mg ofcommercial Pt/C (Johnson Matthey Corp.) (40 wt.%) was dispersedin a suspension of 200 ml of 5 wt.% Nafion solution (DuPont, USA)and 1800 ml of ethanol by ultrasonic stirring. 4 ml of the slurry wasspread on the polished glassy carbon electrode surface and the

Fig. 2. The SEM images and the corresponding element mappings of PtCu alloy (A1-A3) inset in B4 shows the metal particle size distribution of the PtCu/C catalyst based on thdispersion state.

electrode was dried at room temperature for 30 min. The total Ptloading for every catalytic material on the electrode was 2 mg.Carbon monoxide (CO) stripping tests were performed in thefollowing processes: a H2SO4 solution was first deaerated withhigh purity N2. After that, CO was admitted into the solution andthe adsorption reaction on the catalyst was kept for 15 min. Theexcess CO was eliminated with high purity N2 before the strippingtests. All solutions were first deaerated by purging nitrogen (N2)and the electrochemical measurements were performed at 20 �C.The electrodes were pretreated by cyclic voltammetry at a scan rateof 50 mV s�1 for 50 cycles in order to obtain a stableelectrochemical response.

3. Results and discussion

The galvanic reaction between Cu particles and Pt4+ ions wasobserved as illustrated in Fig.1. The initial atomic ratio of Pt:Cu wasset to be 1:50, in order to obtain Pt atom/cluster decorated Cunanoparticles. From Fig. 1A to 1B, a transformation of the blacksolid into a blue solution is clearly observed, indicating a completedissolution of Cu particles through nitric acid etching. With theintroduction of chloroplatinic acid into Cu particles, however,some black particles were still reserved after acid etching as shownfrom Fig. 1C to 1D, suggesting that the successful substitution of Cuby Pt4+ ions and demonstrating that the substitution reaction canbe used to prepare PtCu alloy materials. It should be noted that theoxidation of Cu by oxygen in air may negatively affect thesubsequent substitution reaction. But the short time intervalbetween the start of filtration and the end of substitution allows usto ignore this influence. Further evidence on this aspect is obtainedfrom metal loadings of the synthetic catalysts determined by usinginductively coupled plasma OES analyses. The practical Pt loadingsin the PtCu/C, PtCu/C (C) and PtCu materials are 10.8 wt.%, 9.7 wt.%and 52.1 wt.%, respectively. The Pt content in PtCu/C is close to theexpected value (11.2 wt.%) without taking consideration of Cu inthe synthesis, which suggests that most of Pt was reduced anddeposited on the carbon support. The practical Cu contents in thePtCu/C, PtCu/C (C) and PtCu materials are 1.9 wt.%, 3.5 wt.% and40.6 wt.%, respectively. Thus, the atomic ratio of Cu/Pt in PtCu/C,

and PtCu/C (B1-B3); the TEM images of PtCu alloy (A4, A5) and PtCu/C (B4, B5). Thee statistics of 1000 particles; the insets in A5 and B5 are some models of the metal

Fig. 3. The TEM images of Cu particles (A1, A2) and a typical SAED pattern (A3). The TEM images of PtCu (A4), PtCu/C (B1, B2) and PtCu/C (C) (B3, B4).

Y. Huang et al. / Electrochimica Acta 190 (2016) 956–963 959

PtCu/C (C) and PtCu are 0.54:1, 1.10:1 and 2.39:1, respectively. It iswell known that the acid etching processes (namely dealloying)result in the formation of Pt-rich shell. Note that if more Cu atoms(core) are enclosed with the same amount of shell atoms (Pt shell),the bigger PtCu particles are prone to be fabricated. Therefore, thesignificant difference in Cu content of the two materials impliesthat the average size of metal particles in the catalysts is different.

Fig. 2 shows typical scan electron microscopy (SEM) andtransmission electron microscope (TEM) images, as well as theSEM-energy-dispersive X-ray (EDS) element mappings of Pt and Cuin PtCu and PtCu/C. It can be seen from Fig. 2A1 and B1 that thePtCu alloy is nonuniform at the microscale, unlike the morphologyof PtCu/C. This observation is supported by nonuniform Pt/Cucomposition profiles, shown as large darkened areas inFig. 2A2 and A3 as compared to Fig. 2B2 and B3. There existsparticle agglomeration inside the nonuniform PtCu alloy, asindicated by the closer observation from TEM images inFig. 2A4 and A5. In contrast, the PtCu/C material exhibits highlydispersed metal nanoparticles with an average diameter of ca.2.7 nm which was estimated based on the statistics of 1000 par-ticles, as shown in Fig. 2B4 and B5. The result suggests that theexistence of carbon is a prerequisite for the evolution of Pt-decorated Cu particles into small, highly dispersed nanoparticles.

The materials were further characterized by a high-resolutionTEM (HRTEM) analyses. In Fig. 3A1 and 3A2, it is found that themajority of the as-prepared Cu particles are well-definedpolyhedra with sizes between 5–50 nm. Chloride ions, producedfrom CuCl2 in the solution, play an important role in theformation of Cu polyhedra, based on their preferential adsorptionon certain crystal faces. Similar results were also reported in theliterature [29]. A selected area electron diffraction (SAED) patternin Fig. 3A3 shows two sets of spots. The inner set with a latticespacing of 2.34 Å is indexed to the [111] reflection of CuO; theouter set with a spacing of 1.17 Å corresponds to the [�313]reflection of CuO. The formation of CuO was attributed to thepreservation of the Cu/C sample in ambient atmosphere beforeTEM analyses. During acid etching, the small Pt-decorated Cuparticles evolve into individual alloy nanoparticles. These alloynanoparticles are able to readily agglomerate without carbonsupport anchoring, as displayed by the (b) region indicated as ared ellipse in Fig. 3A4. The (a) region in Fig. 3A4 indicates alloy

aggregation with a continuous structure, which may be evolvedfrom a big Pt-decorated Cu particle. Interestingly, the phenomenarelated to particle aggregation disappeared after the introductionof carbon, suggesting that the nanoparticles were initiallyanchored on the carbon surface before the occurence ofagglomeration. The observed alloy nanoparticles in the HRTEMimages of Fig. 3B1 and B2 are even smaller than those in the PtCu/C materials prepared via the same galvanic substitution processes[30–32]. In addition, some agglomerations of metal particles areobserved for the PtCu/C (C) material, as shown by the typicalimages in Fig. 3B3 and B4, again suggesting that the solid phaseevolution facilitates the uniform dispersion of metal nano-particles. Morphological evolution was observed by samplingaliquots of the reaction solution at various etching times for TEManalyses and some typical images are shown in Fig. 4. Thecorresponding Pt contents in the intermediate at the etching timeof 0, 5, 15, 30 and 60 min are 2.9 wt.%, 4.8 wt.%, 5.4 wt.%, 10.5 wt.%and 11.0 wt.%, respectively, suggesting the dissolution of Cuduring etching. In Fig. 4 we find that the PtCu precursors have theparticle size range (5–50 nm) similar to that of Cu particles. Afterthe etching reaction, however, big particles with the size close toprecursors were not found, suggesting this unique etchingprocess based on carbon anchoring effect enables the synthesisof other small, highly dispersed alloy nanoparticles, regardless ofthe sizes of the initial metal particles.

The X-ray power diffraction (XRD) analysis of PtCu/C is shownin Fig. 5A. The four characteristic peaks for metal particles areshown to have a positive shift as compared with those of Pt,indicating that the PtCu nanoparticles were crystallized in theface-centered cubic (fcc) phase along with lattice contraction. Thelattice contraction may tune the intrinsic activity of the catalyst ina manner similar to that of surface lattice strain effects [33,34]. Theaverage particle sizes (d) of nanoparticles in the PtCu/C and PtCu/C(C) catalysts were estimated using the Scherrer’s formula based onthe (111) peak in the corresponding XRD pattern [35]:

d ¼ 0:9lbcosu

ð1Þ

where l is the wavelength of X-ray (0.154 nm), u is the diffractionangle at Pt (111) crystal facet (in degree), and b is the full-width athalf-maximum (FWHM) of Pt (111) peak (in radian).

Fig. 4. Some typical TEM images of PtCu particles sampled at 0, 5, 15, 30, 60 and 120 minutes during nitric acid etching at 80 �C.

960 Y. Huang et al. / Electrochimica Acta 190 (2016) 956–963

The size of crystalline PtCu alloy particles in PtCu/C and PtCu/C(C) was calculated to be ca. 3.0 and 3.1 nm, respectively. The valuein PtCu/C is close to the size observed in the TEM analysis. Thechemical valences of Pt and Cu in the materials were analyzed byXPS. Pt (0)/Pt (II) and Cu (0)/Cu (II) atomic ratios in PtCu/C weredetermined to be 1.03:1 and 11.3:1 from the valance fitting inFig. 5B and C, respectively. Cu (II) species may have a highersolubility in acid solution compared to Pt (II) species, resulting inthe less Cu (II) species reserved after nitric acid etching. In addition,

Fig. 5. The XRD patterns of PtCu/C and PtCu/C (C) at the scan rate of 5� min�1 (A), the higregions of Pt/C (JM) and PtCu/C. The specific correction in XPS data was conducted by

some species are inset into Pt lattice to form Pt (II) compounds asshown in Fig. 5C. These species are able to draw the electrons of thePt d-orbital, which can neutralize the electron-donating effectfrom Cu to Pt [36], resulting in the same binding energy of Pt 4fpeaks in PtCu/C and Pt/C as shown in Fig. 5D.

The FAO reaction was used to evaluate the electro-catalyticproperties of PtCu/C, PtCu/C (C), PtCu alloy, commercial Pt/C and Ptblack catalysts. Both the mass activity with respect to Pt contentand the area activity relative to electrochemically active surface

h-resolution Cu 2p (B) and Pt 4f (C) XPS spectra of PtCu/C. (D) A comparison in Pt 4fusing the C 1s binding energy of 285 eV.

Table 1Summary on electrochemical parameters of the catalysts.

PtCu/C PtCu/C-after 1000 cycles Pt/C (JM) Pt/C (JM)-after 1000 cycles

ECSA from H desorption/adsorption (m2 g�1) 48.6 43.8 44.3 38.1ECSA retention after cycles (%) — 90.1 — 86.0Mass activity at 0.69 V (mA mg�1) 0.52 0.33 0.18 0.07Mass activity retention (%) — 63.5 — 38.9Area activity at 0.69 V (mA cm�2) 1.07 0.75 0.41 0.19Area activity retention (%) — 70.1 — 46.3ECSA from CO oxidation (m2 g�1) 53.2 — 61.0 —Onset potential for CO oxidation (V) 0.43 — 0.49 —Peak potential for CO oxidation (V) 0.50 — 0.58 —

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area (ECSA) can serve as indicators for the properties of catalysts.The ECSA was calculated by the following formula and some resultsare summarized in Table 1:

ECSA ¼ QH½Pt� � 210 ð2Þ

where [Pt] is the Pt loading (2 mg) in the electrode, QH is the chargefor hydrogen adsorption/desorption (mC), and 210 (mC cm�2) is thecharge required to convert a monolayer H on Pt [37]. On the basis ofthe small nanoparticles with high dispersion, PtCu/C has an ECSAof 48.6 m2g�1, similar to that of Pt/C (44.3 m2g�1) as listed inTable 1. PtCu/C (C) has an ECSA of 37.9 m2g�1.

Cyclic voltammograms (CVs) are usually featured by the peaksof hydrogen adsorption/desorption and surface oxidation, whilethe sizes of these peaks are determined by the number of Pt activesites. The severe aggregation in the PtCu alloy reduced the Pt activesites. Therefore, it shows a featureless cyclic voltammogram (CV)as depicted in Fig. 6A. The resulting mass catalytic activity towardFAO is extremely low even compared to that of Pt black in Fig. 6B.However, PtCu alloy exhibits a dramatic enhancement on FAO assupported on the carbon support. As shown in Fig. 6C and Table 1,PtCu/C has the mass and area activities of ca. 0.52 mA mg�1 and1.07 mA cm�2 in terms of the peak current density at 0.69 V,respectively, increasing by a factor of 1.5–2.0 compared to

Fig. 6. Cyclic voltammograms (CVs) of PtCu alloy and Pt black at the sweep rate of 50 mVPtCu/C, PtCu/C (C) and Pt/C (JM) at 50 mV s�1 in N2-saturated 0.25 M HCOOH + 0.5 M H2background correction in N2-saturated 0.5 M H2SO4.

commercial Pt/C. The mass and area activities of PtCu/C are alsoenhanced by a factor of 0.2–0.6 relative to PtCu/C (C) (0.34 mA mg�1

and 0.89 mA cm�2). FAO on Pt undergoes a dual-pathway, whichwas revealed by the two peaks at the positive-going sweep CVcurves: the peak at ca. 0.35 V is related to direct dehydrogenationin which formic acid is oxidized to carbon dioxide. The peak at0.69 V corresponds to dehydration involved in generating carbonmonoxide from the step of dissociative adsorption [38]. The arearatio (A1/A2) between the two peaks with background correctioncan be used to evaluate the reaction proportion of the twopathways [38]. The evaluation was conducted by fitting the curveswith a Lorentz model, as shown in Fig. 6D. It was found that directdehydrogenation stood for a larger proportion on PtCu/C relative toPt/C during FAO because the former showed a higher A1/A2 ratio of0.58 than did the latter (0.21). This result is supported by the Tafelslopes at the low potentials in Fig. 7A. In addition, the onsetpotential determined from the Tafel curve was �0.04 V for PtCu/C,an apparent lower value than that of Pt/C (0.00 V), again showingits superior FAO property.

The chronoamperometric response indicates that PtCu/Cexhibits higher current densities at both low and high potentialscompared to those of Pt/C as shown in Fig. 7B, revealing that PtCu/Chas preferable catalytic durability for FAO. This result may beattributed to its superior catalytic activity and selectivity in

s�1 in N2-saturated (A) 0.5 M H2SO4 and (B) 0.25 M HCOOH + 0.5 M H2SO4. (C) CVs ofSO4. (D) The Lorentz model fitting of positive-going sweep CV curves for FAO with

Fig. 7. (A) Tafel curves at 2 mV s�1 and (B) current-time curves at 0.2 V (left) and 0.6 V (right) of PtCu/C and Pt/C (JM) in N2-saturated 0.25 M HCOOH + 0.5 M H2SO4. CVs ofPtCu/C and Pt/C (JM) at 50 mV s�1 in N2-saturated (C) 0.25 M HCOOH + 0.5 M H2SO4 and (inset in C) 0.5 M H2SO4 after 1000 cycles between 0.6 and 1.2 V at 100 mV s�1 in 0.5 MH2SO4. (D) The attenuation ratio of ECSA, area and mass activities after 1000 cycles. (E) CV curves of PtCu/C, PtCu/C (C) and Pt/C (JM) at 50 mV s�1 in N2-saturated 0.5 M H2SO4.(F) CO stripping curves on the catalysts in H2SO4 solution. Scan rate: 50 mV s�1.

962 Y. Huang et al. / Electrochimica Acta 190 (2016) 956–963

decreasing the generation of carbon monoxide during FAO. Toevaluate the tolerance of the catalysts toward detrimentalelectrochemical corrosion, the accelerated durability tests (ADT)were conducted between 0.6 and 1.2 V at 100 mV s�1 in 0.5 MH2SO4 for 1000 cycles. The results are displayed in Fig. 7C and Dand Table 1. It should be noted that PtCu/C may suffer Pt dissolutionand partial carbon corrosion over the course of cycles [39], causingan attenuation in catalytic activity. However, Cu etching from thenanoparticles [40,41] occurred along with the cycles, releasingsome new Pt active sites on the nanoparticle surfaces. Based onthis, PtCu/C still retains 90.1% of the initial ECSA, and shows 63.5%of retention in mass activity as well as 70.1% in specific area activityafter 1000 cycles. These values are significantly higher than thoseof Pt/C, revealing superior electrochemical stability toward FAO.

Given that the average particle size of nanoparticles in PtCu/C,PtCu/C (C) and Pt/C is close to each other, the activity differenceresulted from the effect of particle size can be ignored. In addition,the evaluations from hydrogen desorption/adsorption and COoxidation indicate that the PtCu/C and Pt/C catalysts have thesimilar ECSA as listed in Table 1, both higher than that of PtCu/C (C).Thus, the highest activity of PtCu/C toward FAO can be mainlyascribed to the following two factors: one is the suppressionagainst the generation of surface oxygenated species. As depictedin Fig. 7E, the onset potentials of surface oxidation are 0.60,0.61 and 0.57 V for PtCu/C, PtCu/C (C) and Pt/C, respectively. Thelower tendency for PtCu surface to chemisorb oxygenated speciesfacilitates FAO, because these oxygenated species likely block theadjacent vacant active sites that are necessary for the decomposi-tion of formate (an intermediate in FAO) to carbon dioxide via the

direct dehydrogenation pathway [14]. The other is the bettertolerance toward CO poisoning. As shown in Fig. 7F and Table 1,PtCu/C shows the onset and peak potentials toward CO oxidation at0.43 and 0.50 V, respectively, lower than the values at commercialPt/C (0.49 and 0.58 V) and at PtCu/C (C) (0.50 and 0.57 V). On thebasis of the electronic and/or lattice contraction effects [33,34], theintroduction of moderate amounts of Cu can optimize the catalyticselectivity of PtCu/C toward FAO, tune the generation andadsorption of surface oxygenated species, and facilitate oxidativeremoval of CO on catalyst surface. All these aspects contribute to itselectro-catalytic oxidation toward formic acid in a high-efficiencyapproach.

4. Conclusions

In summary, with the use of PtCu precursors with a high Cu/Ptatomic ratio, we have demonstrated a straightforward solid phaseevolution approach to the synthesis of small, highly dispersed PtCualloy nanoparticles. This solid phase evolution approach facilitatedthe high dispersion of metal nanoparticles compared to co-reduction by NaBH4. The introduction of a carbon substrate foranchoring played critical roles in separating Pt-decorated Cuparticles during acid etching. Derived from the better tolerancetoward CO poisoning and weak adsorption for oxygenated species,the PtCu/C catalyst exhibited the enhanced catalytic activity forFAO by a factor of 1.5–2.0 compared to commercial Pt/C. The highdispersion of particles in PtCu/C resulted in a larger ECSA(48.6 m2g�1) compared to PtCu/C (C) (37.9 m2g�1). The catalyticselectivity and stability of PtCu/C toward FAO were also improved

Y. Huang et al. / Electrochimica Acta 190 (2016) 956–963 963

compared to Pt/C. The solid phase evolution approach can also beapplied to the synthesis of other small, highly dispersed binarymetal nanoparticles for various types of reactions.

Acknowledgments

The work described in this paper was fully supported by a grantfrom the Research Grants Council of the Hong Kong SpecialAdministrative Region, China (Project no. 623313).

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A facile approach for preparation of highly dispersed platinum-copper/carbon nanocatalyst toward formic acid electro-oxida...1 Introduction2 Experimental2.1 Synthesis of catalyst2.2 Physical and electrochemical characterization

3 Results and discussion4 ConclusionsAcknowledgmentsReferences