ORIGINAL PAPER
狄克森州立大学
High electrocatalytic activity of non-noble Ni-Co/graphene catalyst for direct ethanol fuel cells
Zaihua Wang &Yongling Du &Fengyuan Zhang &Zhixiang Zheng &Yuzhen Zhang &Chunming Wang
Received:10July 2012/Revid:17August 2012/Accepted:26August 2012/Published online:11September 2012#Springer-Verlag 2012
Abstract The electrocatalytic oxidation of ethanol is studied on the non-noble catalysts Ni-Co/graphene and Ni/graphene supported on glass carbon electrode (GCE)in alkaline medi-um.The synthesized materials are characterized by X-ray diffraction,X-ray photoelectron spectroscopy,transmission electron microscopy,and scanning transmission electron mi-croscopy.The elements of Ni-Co/graphene and Ni/graphene catalysts are characterized using energy-dispersive X-ray spectroscopy.The electrocatalytic properties of Ni-Co/gra-phene and Ni/graphene for ethanol oxidation are investigated by cyclic voltammetry,chronoamperometry,and Tafel plot.Compared with Ni/graphene catalyst,Ni-Co/graphene has the higher electroactivity and better stability for ethanol oxidation.The rate constant (k s )and charge-transfer coefficient (α)are calculated for the electron exchange reaction of the modified GCE.The results indicate that Co addition could promote the oxidation reaction at the Ni/graphene cata
lyst.Our study demonstrates that the low-cost electrocatalyst Ni-Co/graphene has a great potential for real direct ethanol fuel cells ’application.
Keywords Non-noble catalyst .Nickel –cobalt/graphene .Direct ethanol fuel cells .Alkaline medium
Introduction
connectlandOver the past veral years,there has been an increasing interest in the development of alternative power sources,
both for environmental reasons and the low availability of fossil fuels [1,2].As an alternative to fossil fuels,direct alcohol fuel cells (DAFCs)have been regarded as one of the most appropriate choices [3].The versatility of DAFCs has been shown through the u of a variety of fuels,such as methanol [4]and ethanol [5]as well as with other small alcohols and carbohydrates [6].Recently,ethanol has been gaining considerable attention due to its greater availability and higher energy storage capacity (per volume)than meth-anol [7],although the most common alcohol for DAFCs was methanol (direct methanol fuel cells)in the past.The low reaction kinetics of ethanol oxidation and the high fabrica-tion cost limit the advancement of future energy technolo-gies [8,9].So it is necessary to u new type of electrocatalysts to improve the velocities of ethanol oxida-tion and lower the cost.The best performin
g anode materials for the electrochemical oxidation of ethanol is precious metals,particularly pure platinum or platinum alloys.How-ever,the high cost of the precious metals and the low ethanol electrooxidation kinetics caud by noble metal catalyst poisoning inhibit its commercialization.To solve the problems,two main works have been done:(1)using non-noble metals,such as Co,Ni,Pb,and W,to replace platinum and other noble metals to reduce the cost of cata-lysts [10],and (2)using high conductive materials as sup-ports,like carbon nanotubes (CNTs),nanofibers,mesoporous carbon,and graphene to improve efficiency of electrocatalysts [11,12].
Among non-noble metals,nickel is a versatile catalytic material due to its electrochemical stability and resistance to poisoning.Nickel can be easily converted to Ni(OH)2,and the Ni 2+/Ni 3+redox centers show high catalytic activity toward oxidation of ethanol in alkaline media [13].However,the poor electrical conduction of both nickel hydroxide and its oxidized NiOOH form restricted its catalytic performance to some
Z.Wang :Y .Du :F.Zhang :Z.Zheng :Y .Zhang :C.Wang (*)Department of Chemistry,Lanzhou University,Tianshui Road 222,大家的日语mp3下载
Lanzhou 730000,People ’s Republic of China e-mail:wangcm@lzu.edu
J Solid State Electrochem (2013)17:99–107DOI 10.1007/s10008-012-1855-8
extent.Much work has been focud on using different addi-tives to improve the electrocatalytic property of nickel.For example,ethanol electrooxidation by Pd-Ni[14]and Cu-Ni [15]in alkaline solution has been reported.Some studies have demonstrated that the u of different forms of cobalt additives could improve electrocatalytic performance enabling redox chemistry in an alkaline medium[16]and supplying oxygen atoms at less-positive potentials[17].汽车漆面保养
Compared with other carbon materials,graphene is an ideal electrode material for its higher surface area[18],more excellent conductivity[19],more unique graphitized basal plane structure,and lower manufacturing cost.Some gra-phene/inorganic nanoparticles composites have shown ex-cellent properties,which can be applied in field emission displays,nsors,supercapacitors,batteries,catalysis,and so on[20–24].Therefore,graphene should be explored as a support material to improve electrocatalytic activity of cat-alyst particles for methanol and ethanol oxidations[12,25]. For example,Li et al.prepared Pt/graphene nanocomposites in one pot,and good performance of the composites toward direct alcohol fuel cells was obrved[26].
In this study,we ud a facile,economic,and environ-mentally friendly method to synthesize non-noble electro-catalysts Ni-Co/graphene and Ni/graphene.The behavior of the catalysts for ethanol oxidation was studied in alkaline medium by cyclic voltammetry and chronoamperometry. Our aim is to evaluate the performance of non-noble catalyst Ni-Co/graphene for direct ethanol fuel cells(DEFCs)and study the enhancement of cobalt and graphene for ethanol electrooxidation.
Experimental
Reagents and instruments
Graphite flake(natural,−325mesh)was from Alfa Aesar. All other chemicals were analytical reagent grade which were ud without further purification.
X-ray diffraction(XRD)was carried out using a D/Max 2400Rigaku diffractometer with Cu-Kαradiation(k0 0.15418nm).The diffraction data were recorded for2θangles between10°and80°.X-ray photoelectron spectros-copy(XPS)was performed on a VG ESCA LAB210 electron spectrometer using a Mg-Kαline excitation source with the reference of C1s at285.0eV.The morphologies of the samples and energy-dispersive X-ray spectroscopy (EDX)analys were carried out using transmission electron microscopy(TEM,TecnaiG2F30,FEI,USA)equipped with an EDX microanalyzer.
Electrochemical measurements were performed on a CHI660C workstation(Shanghai Chenhua,China)with a conventional three-electrode system at room temperature.Synthesis of Ni-Co/graphene and Ni/graphene
Nickel and cobalt nanoparticles were deposited on graphene by a simple solution synthesis method using hydrazine hydrate as a reducing agent and ethylene glycol as a solvent. The typical procedure is as follows:131.4mg of NiSO4·6H2O,145.5mg of Co(NO3)·6H2O(atomic ratio of Ni/Co01:1),and100mg of graphene oxide(GO),which was synthesized from graphite powder by a modified Hum-mers method[27,28],and they were all added into60.0ml ethylene glycol in a100-ml flask.The resulting mixture was ultrasonicated for1h and stirred for24h at ambient tem-perature,followed by the addition of ammonia solution into the solution with stirring to reach a final pH of10.5.Then, 1.0ml of hydrazine hydrate was added into the above solution,and the reduction reaction was performed at85°C for12h under constant stirring.The resulting product was filtrated and washed copiously with ethanol and distilled water for veral times and then dried in a vacuum desicca-tor at room temperature.For comparison,Ni/graphene and graphene were also synthetized by the same procedure.
Preparation of Ni-Co/graphene/glass carbon electrode, Ni/graphene/glass carbon electrode,and grap
hene/glass carbon electrode
The glass carbon electrode(GCE)was first polished with 0.3and0.05μm alumina slurry and sonicated in distilled water for5min.After drying,5μl of2.5gl−1Ni-Co/graphene in dimethylformamide–Nafion(49:1volume ratio)was grad-ually deposited on the surface of pretreated GCE and dried in a desiccator.For comparison,Ni/graphene/GCE and graphene/ GCE were also prepared by the same procedure.
Results and discussion
X-ray diffraction and X-ray photoelectron spectroscopy The XRD patterns of Ni-Co/graphene,Ni/graphene,and graphene are shown in Fig.1.There is a typical reflection peak at2θ022.4°in all the three curves which can be attributed to graphene(002).The strong diffraction peaks at2θ044.4°,51.8°,and76.5°(curve b of Fig.1a)can be assigned to the characteristic(111),(200),and(222)plane of face-centered cubic(fcc)structure Ni(JCPDS,04–0850). The typical diffraction peaks at2θ044.3°,51.6°,and76.5°(curve c of Fig.1a)can prove that the Ni-Co alloy was synthesized,which can be en more clearly in Fig.1b.
To further confirm the formation of Ni-Co alloy,the Ni-Co/ graphene sample was analyzed by XPS sp
ectroscopy. Figure2a displays the peaks of C1s(285.0eV),O1s (531.4eV),Ni2p,and Co2p.The two obvious peaks at 778.4and793.9eV(Fig.2b)can be attributed to the electrons’multiplet splitting of Co2p3/2and2p1/2,indicating the zero
valence of Co,whereas for Ni 2p (Fig.2c ),the peaks located at 853.2and 870.6eV are assigned to the binding energy of Ni 2p 3/2and Ni 2p 1/2,respectively,which is also in agreement with Ni (0).Owing to the different environments,the signals of metallic bonding energy in as-obtained Ni-Co alloy are slightly shifted compared with the pure nickel and cobalt bulk metals.Both of them reveal the formation of Ni-Co alloy [29].Morphologies and size distributions
The morphologies of the synthesized catalysts have been characterized by TEM and scanning transmission electron microscopy (STEM).From Fig.3a,c ,it can be clearly en that the metal particles of Ni-Co/graphene are more uniform
and denr than Ni/graphene.The result indicates that Co addition may help the dispersion of the metal particles.The main diameters of metal particles of Ni-Co/graphene and Ni/graphene catalysts are 20and 30nm (Fig.3e,f ),respective-ly.When the cobalt content is less than 50wt%in the Ni-Co alloy,increasing the Co content results in a gradual decrea in the grain size of the alloy.So the phe
,the metal size of Ni-Co/graphene is smaller than Ni/graphene,could be well explained by Srivastava [30].Highly dis-perd metal nanoparticles on supports with larger surface areas have advantages in catalytic activity and nsor nsi-tivity [31].Therefore,Ni-Co/graphene hybrids may have better catalytic properties than Ni/graphene.Figure 3b,d displays STEM images of Ni/graphene and
Ni-Co/graphene.
Fig.1a XRD patterns of graphene (a ),Ni/graphene (b ),Ni-Co/graphene (c ).b The 40°–60°region of a .The symbol full-width asterisk stands for the peak corresponding to nickel crystal and black up-pointing triangle stands for the peak as-cribed to
carbonapr
Fig.2a XPS spectra for Ni-Co/graphene nanocomposites,b detailed spectra of Co 2p ,c detailed spectra of Ni 2p
The tissue-like structure of graphene can be en more clearly in the STEM images.The high-resolution transmis-sion electron microscope (HRTEM)image (bottom int of Fig.3c )of a Ni-Co nanoparticle attached on the graphene nanosheets reveals the crystalline character of Ni-Co alloy with a lattice spacing of 0.205nm,which can be indexed to the (111)plane of fcc Ni-Co crystals [32,33].The lected area electron diffraction (SAED)pattern (top int of Fig.3c )obtained reveals the crystalline fcc structure,exhib-iting the fringe pattern with indices (111),(200),and (220)of pure fcc Ni-Co alloy nanoparticles.
Figure 4shows the EDX spectrum of the Ni/graphene and Ni-Co/graphene catalysts.The EDX spectrum in Fig.4a reveals that Ni/graphene catalyst is compod of C,O,and Ni elements.The atomic ratio of Co and Ni in the catalyst is about 1:1which can be calculated from Fig.4b ,indicating that Co and Ni contents in the Ni-Co/graphene catalyst are about the same as tho in the initial mixtures in the solution.The
small oxygen peak showed in Fig.4means that graphene contains a small amount of carboxyl and hydroxyl group.Electrocatalytic oxidation of ethanol at different electrodes To investigate the electrocatalytic activity of Ni/graphene and Ni-Co/graphene for ethanol oxidation,cyclic voltammetry (CV)methods are carried out in 0.1M NaOH containing 0.1M ethanol.In Fig.5,a redox couple of Ni/graphene catalyst appears in the blank NaOH solution at potential values of +430and +280mV in the anodic and the cathodic direction,which is attributed to Ni(OH)2/NiOOH transformation [34,35]:
Ni OH ðÞ2þOH À! NiOOH þH 2O þe
À
ð1Þ
The increa of current density in the forward direction after +600mV and the decrea in the backward direction before +600mV are ascribed to redox reactions ofamerican family
GO.
graphene,b STEM image of Ni/graphene,c TEM image of Ni-Co/graphene,d STEM image of Ni-Co/graphene,e nanoparticle diameter distribution of Ni/gra-phene,and f nanoparticle di-ameter distribution of Ni-Co/graphene.The bottom int of Fig.3c shows the HRTEM im-age of a Ni-Co nanoparticle at-tached on the graphene
nanosheets,and the top in t shows the SAED pattern
After adding 0.1M ethanol to the supporting electrolyte,a new peak located in +754mV could be contributed to ethanol oxidation on Ni/graphene catalyst.In the backward scan,ethanol is reoxidized at a potential value of +725mV as a result of a refreshed surface with Ni(OH)2/NiOOH
redox couple.This phenomenon coincides with Ni(OH)2/NiOOH transformation.NiOOH has the higher electrocata-lytic activity for ethanol oxidation.Hence,we can predict the mechanism to Fleischmann [36]:
NiOOH þethanol !Ni OH ðÞ2þoxidation product
5ingð2Þ
Moreover,a decrea in the current density of the reduc-tion peak in the backward direction suggests that a great percentage of NiOOH species has been consumed in the ethanol oxidation process.
Figure 6a reprents the cyclic voltammograms of Ni/graphene catalyst in 0.1M NaOH solution at various scan rates of 0.01–0.8Vs −1.The peak currents of the voltammo-grams are linearly proportional to sweep rates in the range of 0.01–0.05Vs −1in Fig.6b ,which can be attributed to the electrochemical activity of an immobilized redox couple at the surface.From the slope of the lines and using the following equation [37],I p ¼u A Γ
*
n 2F 2
4RTugc是什么意思
ð3Þ
jaywalkwhere Γ*is the surface coverage of the redox species,υbeing the potential sweep rate,A is the surface area
of
graphene and b Ni-Co/graphene
catalystsjust beat it歌词
Fig.5Cyclic voltammograms of Ni/graphene catalyst in 0.1M NaOH solution in the abnce (a )and in the prence (b )of 0.1M ethanol at 0.05Vs −1at the potential range of 0to +1,000mV
