Carbon-supported Pt–Co alloy nanoparticles for oxygen
reduction reaction
Qinghong Huang
a,b
,Hui Yang
a,*
,Yawen Tang b ,Tianhong Lu b ,Daniel L.Akins
c
a
Nanotechnology Laboratory,Shanghai Institute of Microsystem and Information Technology,Chinese Academy of Sciences,
No.865,Changning Road,Shanghai 200050,PR China
b
Department of Chemistry,Nanjing Normal University,Nanjing 210097,PR China
c
CASI and Department of Chemistry,The City College of The City University of New York,NY 10031,USA
Received 28April 2006;received in revised form 31May 2006;accepted 31May 2006
Available online 7July 2006
Abstract
The oxygen reduction reaction (ORR)on carbon-supported Pt–Co alloy nanoparticles has been investigated.As-prepared Pt–Co alloy nanoparticles exhibit a single-phase disordered structure and have a very small particle size.Such alloy catalysts exhibit an enhance-ment of a factor of ca.1.3–3.2in Pt mass activity and of a factor of ca.1.2–2.2in the specific activity for the ORR when compared to Pt/C catalyst.The enhanced electrocatalytic activity is attributed to the favorable Pt–Pt mean interatomic dist
ance caused by alloying and to the high dispersion of the alloy catalysts.Ó2006Elsevier B.V.All rights reserved.
Keywords:Oxygen reduction reaction;Pt–Co alloy;Electrocatalysis;Nanoparticle
1.Introduction
The commercialization of proton exchange membrane fuel cells (PEMFCs)is still hindered by several factors,including the poor kinetics of the cathodic reaction and the high costs of Pt-based electrocatalysts [1].Even under the open-circuit condition,the overpotential for oxygen cathode in PEMFCs is around 0.2–0.3V due to non-revers-ibility of the oxygen reduction reaction (ORR)and to the ‘‘mixed potential’’effect [2].Thus,there is great room for the improvement in oxygen reduction kinetics.In fact,the development of a more active oxygen reduction catalyst than Pt has been the subject of extensive research for a number of decades.
Aiming to increase the catalytic activity of the ORR and to lower the cost of the catalysts,various Pt based alloy catalysts,such as Pt–M,(where M =Co,Ni,Cr and Fe
etc.),have been investigated extensively [3–16].In general,most of the authors reported an activity enh
ancement of the ORR on the alloy catalysts with factors of 1.5–5in comparison to pure Pt.The improvement in the ORR electrocatalysis on Pt alloys has been explained by several factors,such as geometric factor (the more favorable Pt–Pt interatomic distance)[5],dissolution of non-noble element in Pt alloys (an increase in Pt surface area)[6]or electronic factor (an increase in Pt d-band vacancy)[7].Among the different Pt alloy catalysts used for the ORR,Pt–Co alloy catalysts showed the higher electrocatalytic activity [15,16]and have been extensively studied [17–19].The size,the dispersion and the compositional homoge-neity of the Pt alloy nanoparticles were important factors to obtain a good electrocatalytic activity.However,in most of these studies,the carbon-supported Pt alloy catalysts were generally prepared by the impregnation of the second metal on Pt/C,and then by alloying at temperatures above 700°C.This heat treatment gives rise to an undesired alloy particle growth,which could result in the decreases in Pt mass activity (MA)for the ORR since the MA of highly
1388-2481/$-see front matter Ó2006Elsevier B.V.All rights reserved.doi:10.1016/j.elecom.2006.05.027
*
Corresponding author.Fax:+862132200534.E-mail address:hyang@mail.sim.ac (H.Yang).
www.elsevier/locate/elecom
dispersed Pt catalysts decreases for the particle size >3.5nm.Also,the control of the particle size distribution with these preparation methods is quite limited.In this work,a low temperature approach via the carbonyl com-plex route was used to prepare Pt–Co/C alloy catalysts with a small particle size and their electrocatalytic activity for the ORR was evaluated and correlated to the change in their structure parameter.
2.Experimental
The Pt–Co alloy catalysts were prepared via the car-bonyl complex of Pt and Co route,followed by H2reduc-tion in the temperature range of150–300°C[10].Briefly,Pt and Co carbonyl complexes were synthesized through the reaction of Na2PtCl6Æ6H2O and CoCl2Æ6H2O with CO at about50°C for24h.The sodium acetate/metals molar ratio was adjusted to6–1.After the synthesis of the car-bonyl complexes of Pt and Co,Vulcan XC-72carbon was added to the mixture and stirred at about55°C for more than6h.Subsequently,the solvent was removed and then subjected to heat treatment at150–300°C under nitrogen and under hydrogen for1h,respectively.After the reduction,the sample was washed with water until no chlorine ions were detected and then dried.Therefore,we got the Pt–Co/C alloy catalyst with different Pt/Co atomic ratios and a total metal loading of20wt.%.
X-ray diffraction(XRD)measurements of Pt-based cat-alysts were carried out on an X0Pert Pro MPD X-ray dif-fractometer using Cu-K a1radiation.The XRD spectra were recorded in the range of10°–90°with a counting time of10s per0.1°.The particle size of as-prepared catalyst was evaluated by transmission electron microscopy (TEM)using JEM-1011microscope.The sample was pre-pared by placing one drop of catalysts,which was dispersed in ethanol,on a copper grid covered by carbonfilm and by evaporating the solvent.Elemental analysis of as-prepared catalysts was performed on an inductively coupled plasma (ICP)atomic emission spectrometer(PS-I,Leeman).
Porous electrodes were prepared as described previously [10].Twenty milligrams of catalysts,0.5mL of Nafion solution(5wt.%,Aldrich),and2.5mL of ultrapure water were mixed ultrasonically.A measured volume(3l L)of this ink was transferred via a syringe onto a freshly pol-ished glassy carbon disk(3mm in diameter).After the sol-vents were evaporated,the electrode was used as the working electrode.Each electrode contained about 56l g cmÀ2of the metal.All chemicals used were of analyt-ical grade.All the solutions were prepared with ultrapure water.Electrochemical measurements were performed using a CHI Potentiostat/Galvanostat and a conventional three-electrode cell.The counter electrode was a Pt plate, and the Ag/AgCl(3M KCl)electrode was used as the ref-erence electrode,but all the potentials are quoted with respect to the reversible hydrogen electrode(SHE).The electrolyte us
ed was0.5M HClO4.The catalytic activity for the ORR was measured with the rotating disk electrode technique(Autolab speed control).High-purity nitrogen or oxygen was used for deaeration of the solutions.During the measurements,a gentle nitrogen or oxygenflow was kept above the electrolyte surface.All electrochemical experiments were performed in a thermostated cell at 30°C.
3.Results and discussion
In this paper,we employed Pt and Co carbonyl com-plexes as the catalyst precursors.Experimental results by infrared spectroscopy confirmed that as-synthesized pre-cursor is a mixture of the carbonyl complex of Pt and Co.The practical composition of as-prepared Pt–Co bime-tallic catalysts was performed by an ICP analysis.The obtained practical compositions for all alloy catalysts investigated in this paper are nearly the same as the nomi-nal values.Additional thermogravimetric analysis also indicates that the total metal loading within all the catalysts including E-Tek Pt/C is about20wt.%,which is in fairly good agreement with the nominal loading.
Fig.1shows the XRD patterns of the Pt–Co bimetallic catalysts with a metal loading of20wt.%and different Pt/ Co atomic ratios,all heat-treated under hydrogen at 300°C.For a comparison,the homemade Pt/C catalyst prepared with the similar procedure is also shown in this figure.Thefirst peak l
ocated at about24.8°in all the XRD patterns is associated to the Vulcan XC-72carbon support.The other four peaks are characteristic of face-centered cubic(fcc)crystalline Pt(JCPDS card:04-802), corresponding to the planes(111),(200),(220),and (311)at2h values of ca.39°,47°,67°,and83°,respectively, indicating that all the catalysts are principally single-phase disordered structures(solid solutions).Relative to the same reflections in Pt/C,the diffraction peaks for the Pt–Co cat-alysts are shifted slightly to higher2h values,showing the effect of increasing amounts of Co in the Pt–Co alloy cat-alysts.The higher angle shifts of the Pt peaks,which can be clearly seen in Table1,indicative of formation of an alloy involving the incorporation of Co in the fcc structure
Q.Huang et al./Electrochemistry Communications8(2006)1220–12241221
of Pt.Since XRD is mass sensitive,a small fraction of lar-ger particles within the samples would produce the nar-rower diffraction peaks.Thus,the broad diffraction peaks suggests that the as-prepared Pt–Co alloys exist in small particle sizes with a narrow size distribution and in a disor-dered form.To obtain the precise values of the maximum peak (h max )and the full-width at half-maximum (B 2h ),the Pt (220)crystal face was fitted to a Gaussian line shape.The lattice parameters (a fcc )and the Pt–Pt mean inter-atomic distances calculated from h max and B 2h for the Pt–Co catalysts are provided in Table 1.The lattice parameters for the Pt–Co/C catalysts are smaller than that for Pt/C,indicating a lattice contraction with increasing content of Co,and signaling a progressive increase in the conversion of Co into the alloyed state.Nearly linear relationship between the lattice parameter and practical composition (plot not shown)exhibit Vegard’s law behavior for solid solution,suggesting that Co is completely alloyed with Pt within all the as-prepared Pt–Co catalysts.No peak for pure Co and its oxides was found.By using Scherrer’s equation,the average diameter of all as-prepared catalyst is less than 2.5nm as listed in Table 1,which is much smal-ler than that of E-Tek Pt–Co alloy catalysts with the same metal loading,assessing that syntheses procedure used here is a good method to obtain alloy catalysts with small par-ticle size,and thus a good dispersion.In addition,it is ver
y interesting that all the catalysts prepared with the similar procedure have nearly the same structure and similar particle size;thus,under these conditions it is suitable to compare their catalytic activity for the ORR.Fig.2shows a typical TEM image of carbon-supported Pt–Co alloy catalyst with a Pt/Co atomic ratio of 2:1and its corresponding particle size distribution histogram.The as-prepared Pt–Co alloy nanoparticles are found to have a mean diameter of ca.2.1nm with a standard deviation of 1.2nm.Such a mean particle size is much smaller than that of E-Tek Pt–Co/C catalyst.The mean particle size of as-synthesized Pt–Co alloy catalysts is also smaller than those of Pt–Ni and Pt–Cr alloy catalysts prepared with the similar method [10,11].To the best of our knowledge,such a mean particle size for the Pt–Co alloy catalyst is the smallest when compared to the published results.Very small particle size of as-prepared Pt–Co alloy catalysts could be due to the lower alloying temperature.
Fig.3presents the cyclic voltammograms (CVs)of the Pt/C and Pt–Co/C catalysts in 0.5M HClO 4at a scan rate of 50mV/s between 0.07and 1.07V.From the CV of Pt/C shown in Fig.3,the hydrogen adsorption/desorption peaks and preoxidation/reduction peaks of the Pt surface on Pt are clearly seen,indicating the presence of polycrystalline Pt.However,no well-defined hydrogen adsorption/desorp-tion peaks on the Pt–Co alloy catalysts were found,sug-gesting that the high dispersion of the catalysts with the disordered surface structure was obtained.The current density of th
e hydrogen region decreases with the increase in Co amount,indicative of a change in electrochemical active surface area with Co content within the catalysts.The real surface area of the nanodivided Pt was determined from the H adsorption/desorption peaks.The commonly accepted value is 210l C cm À2for platinum [20].The
Table 1
The Structural parameters of Pt/C and Pt–Co/C catalysts Catalysts Maximum 2h of Pt (111)(°)Lattice parameter
a fcc (A ˚)Pt–Pt interatomic
distance d (A ˚)Particle size
(nm (from XRD))Pt Real surface area (m 2g À1)Pt/C
39.78 3.9103 2.7650
2.288.81Pt:Co(5:1)39.97
3.8996 2.7574 2.570.48Pt:Co(3:1)40.13 3.8774 2.7417 2.463.99Pt:Co(2:1)40.26 3.8691 2.7359 2.25
4.82Pt:Co(1:1)
40.49
3.8089
2.6933
2.5
56.61
0.5
1.0
1.5
2.0
2.5
3.0
3.5
0.0
0.10.20.30.4
0.5F r e q u e n c y %
Partical diameter /nm
1222Q.Huang et al./Electrochemistry Communications 8(2006)1220–1224
obtained specific surface areas for the Pt/C and Pt–Co
bimetallic catalysts are listed in Table1.In addition,all the Pt/C and Pt–Co alloy catalysts have a similar double layer behavior,suggesting that all the catalysts have the same resistances to transfer charges.Moreover,the onset of the oxide formation and the reduction peak potential of the oxide slightl
y shifted to more positive potentials with the increasing of Co amount,indicating that the alloying inhibited the chemisorption of OH on the Pt sites at high potentials(above0.8V)by the change in electronic effects. This may be beneficial to the oxygen adsorption at low overpotential,and thus the ORR kinetic enhancement [5,14].It is noticed that no marked changes in the shape and size of the CVs are observed during the whole electro-chemical measurements and that the Pt real surface areas of all the catalysts keep almost constant,suggesting that the catalysts are stable and that no Pt or Co dissolution happens under the experimental conditions.The long-term stability of Pt alloy catalysts for the application in fuel cells is a key issue because of the possible dissolution of non noble metal components in Pt nanoalloys during fuel cell reactions.However,in this paper,the upper potential was set to ca.1.0V so that the particle surface change could be avoided and the catalysts are stable[8].
Fig.4shows the ORR on the Pt/C and Pt–Co catalysts under the similar experiment conditions.From thefigure, the ORR on all the catalysts is diffusion-controlled when the potential is less than0.7V and is under mixed diffu-sion-kinetic control in the potential region between0.7 and0.85V.In the Tafel region(higher than ca.0.85V) and mixed potential region,the ORR activities of all the catalysts are roughly identical with respect to the geometric surface area.Based on the MA of Pt,however,all the Pt–Co alloy catalysts are slightly more active than the Pt/C catalyst for the ORR,which is qualitatively simil
ar to the E-Tek Pt–Ni and Pt–Co alloy catalysts[8].As compared to the Pt/C catalyst,the maximum MA was found with a Pt/Co atomic ratio of2:1,an enhanced factor of ca.3.2, which is comparable with those of Pt–Cr and Pt–Ni alloy catalysts prepared via the carbonyl route.Furthermore,a comparison of the ORR on the homemade and E-Tek Pt/C catalysts indicates that both catalysts exhibit similar catalytic activity for the ORR.
To compare the specific activity(SA)of the Pt based cat-alysts for the ORR,Tafel plots of the kinetic current den-sities based on the Pt real surface area within the catalysts are shown in Fig.5.The obtained Tafel slopes at low cur-rent density for the Pt/C,Pt–Co(5:1)/C,Pt–Co(3:1)/C,Pt–Co(2:1)/C,Pt–Co(1:1)/C catalysts are64,64,61,62and 72mV decadeÀ1,respectively,which are very close to À2.3RT/F[14].The Tafel slopes at high current density for the Pt/C,Pt–Co(5:1/C),Pt–Co(3:1)/C,Pt–Co(2:1)/C, Pt–Co(1:1)/C catalysts are120,125,122,119and 128mV decadeÀ1,respectively,very close toÀ2.3·2RT/ F[14].Within thefitting error,the Tafel slopes did not show any dependence on the composition and structural parameters of the catalysts.Thus,it can be concluded that the ORR pathway and rate-determining step are the same on all the catalysts investigated here.Furthermore,it is evi-dent that the SA of the Pt–Co alloy catalysts for the ORR
Q.Huang et al./Electrochemistry Communications8(2006)1220–12241223
is higher than that of the Pt/C and that the maximum SA was found with a Pt/Co atomic composition of2:1.Gener-ally,the kinetics enhancement in SA for the ORR on the as-prepared Pt–Co alloy catalysts is a factor of1.2–2.2, which depends on the catalyst composition and on the polarization potential.Such an enhancement factor for the ORR is slightly lower than those on the Pt–Cr and Pt–Ni alloy catalysts prepared with the similar method, probably due to the particle size effect.
It is known that the enhancement in the ORR activity on Pt based alloy catalysts is generally correlated to the change in Pt–Pt interatomic distance(d).The change in SA of the ORR at0.90V and0.85V on Pt based catalysts as a function of Pt–Pt mean interatomic distance is plotted in Fig.6.Volcano-type curves between SA and d are found with the best electrocatalytic activity for the ORR when the Pt–Pt interatomic distance is0.273–0.274nm,which is in good agreement with the data reported in the literature [5,7].As we indicated above,no marked changes in the shape and size of the CVs are observed during the whole electrochemical measurements,suggesting that the activity enhancement should not be ascribed to the increase in Pt roughness.Thus,it is believed that the Pt–Pt mean inter-atomic distance plays an important role in the ORR electrocatalysis.
4.Conclusions
In summary,carbon-supported Pt–Co alloy nanoparti-cle catalysts with a very small particle size and disorder structure can be easily prepared via the carbonyl route. Compared with the Pt/C catalyst with similar metal load-ing,the same disordered fcc structure and similar particle size,the bimetallic catalysts with different Pt/Co atomic ratios showed an enhancement factor of ca.1.2–2.2in the specific activity for the ORR.The maximum activity of Pt–Co based catalysts was found with a Pt/Co atomic ratio of2:1.Such enhanced activity in SA of the Pt based cata-lysts has been ascribed to the change in
the more favorable Pt–Pt mean interatomic distance.A very good dispersion of the Pt–Co alloy catalysts is also an important factor in determining catalyst activity for oxygen reduction. Acknowledgements
We thank the National Natural Science Foundation of China(20003005,20573057)and Knowledge Innovation Engineering of Chinese Academy of Sciences for support of this work.D.L.A.thanks the NSF-IGERT program (DGE-9972892),NSF-MRSEC program(DMR-0213574) and DoD-ARO(DAAD19-01-1-0759)for support of this work.
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