cm901928b
5112Chem.Mater.2009,21,5112–5118
DOI:10.1021/cm901928b
Nickel Foam Supported-Co3O4Nanowire Arrays for H2O2
Electroreduction
Guiling Wang,Dianxue Cao,*Cuilei Yin,Yinyi Gao,Jinling Yin,and Lin Cheng College of Material Science and Chemical Engineering,Harbin Engineering University,Harbin150001
P.R.China
Received July1,2009.Revised Manuscript Received September1,2009
Ni foam supported-Co3O4nanowire arrays are prepared by a template-free growth method, followed by a thermal treatment in air,and are characterized by scanning electron microscopy, transmission electron microscopy,X-ray diffraction,infrared spectroscopy,and thermogravimetric and differential thermal analysis.The Co3O4nanowires have a diameter of about250nm,a length up to15μm,and a Brunauer-Emmett-Teller surface area of78.4m2g-1.They grow almost vertically from the surface of Ni foam skeleton,pack densely,and uniformly cover the entire surface of Ni foam skeleton.Electroreduction of H2O2on Co3O4nanowire arrays in alkaline medium is investigated by cyclic voltammetry,chronoamperometry,and electrochemical impedance spectroscopy.The Co3O4 nanowire electrode exhibits superior activity,stability,and mass transport property for H2O2 electroreduction.A current density of90mA cm-2is achieved at-0.4V in0.4mol dm-3H2O2 and3.0mol dm-3NaOH at room temperature.The per gram current density measured at-0.4V on Co3O4nanowires is about1.5times of that on Co3O4nanoparticles.
Introduction
Fuel cells for use in air-free environments(space and underwater)require liquid or constringent oxygen as oxidant.The bulky tank for carrying oxygen significantly reduces the energy density and safety standard of fuel cell systems.H2O2has been investigated as an alternative oxidant to replace oxygen.Several types of fuel cells using H2O2as oxidant have been reported,such as,direct borohydride-hydrogen peroxide fuel cells1,2and metal-hydrogen peroxide semifuel cells.3-5Besides its easy handling and storage,H2O2also has faster reduction kinetics than oxygen.The solid/liquid reaction zone at the H2O2cathode is easier to realize and maintain than the solid/liquid/gas region at the oxygen cathode.There-fore,fuel cells with H2O2oxidant tend to have high performance and are more compact.The problem of H2O2as oxidant is its chemical decomposition leading to the formation of O2,which reduces the utilization efficiency of H2O2,causes a two-phase counter current flow within the electrode,and adversely affects reactant diffusions.
Electrodes of nanowire arrays standing on a current-collecting substrate have been successfully prepared by various methods,such as,template-free growth6,7and template-directed synthesis using anodic aluminum oxide,8-11polymer,12-21and virus22as template,as well as other methods.23,24Owing to the unique structure,this type of electrode usually possesses larger electrochemical
*Corresponding author.E-mail:caodianxue@https://www.wendangku.net/doc/b715041483.html,.
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https://www.wendangku.net/doc/b715041483.html,/cm Published on Web10/07/2009r2009American Chemical Society
Article Chem.Mater.,Vol.21,No.21,20095113
active surface area,higher utilization efficiency of the active materials,and superior mass transport property than conventional electrodes fabricated by mixing and pressing powder of active material with conducting materials(e.g.,carbon black)and polymer binders(e.g., polytetrafluorethylene).Martin and co-workers12-19ex-tensively investigated the preparation of nanowire arrays of V2O5,SnO2,LiMn2O4,and LiFePO4using a porous polymer membrane template method and found that these nanowire arrays as Li-ion battery electrodes show better rate capabilities than conventional electrodes com-posed of the same materials.Recently,the research groups of Belcher22and Wu6,7reported the synthesis of Co3O4nanowire arrays using virus-templated and tem-plate-free methods,respectively.These Co3O4nanowires displayed high capacity and rate capability as the anode material for Li-ion batteries.The reversible capacity reached around700mAh g-1,which is twice that of current graphite anode.Li and co-workers8investigated the performance of Pt and Pt-Ru nanowire array elec-trodes obtained via an anodic aluminum oxide(AAO) template on a Ti/Si substrate as the anode of direct methanol fuel cell and found that these electrodes have a high electrode area and the surface of the metals is nearly one hundred percent used in contrast to the tradi-tional gas diffusion electrode,in which the utilization of the surface of the catalysts is usually less than85%due to the addition of carbon and polymer binder.Our recent study demonstrates that Co3O4nanoparticles exhibited promising activity and stability for catalytic electroreduc-tion of H2O2in alkaline medium.25The reduction occurs mainly via a direct pathway at low H2O2concentration, e.g.,less than0.5mol dm-3.However,the Co3O4nano-particle electrode has to be prepared by a conventional method of mixing Co3O4powder with carbon and poly-mer binder and pressing the mixture onto a conducting substrate.The resulting electrode usually suffers draw-backs of low active material utilization due to presence of inaccessible regions to the electrolyte solution and poorly controlled porous structure which limited the mass trans-port of reactants within the electrode.
In this study,cobalt oxide nanowires freely standing on nickel foam were prepared via a template-free growth method.Their morphology,structure,and catalytic ac-tivity for H2O2electroreduction in alkaline medium were investigated.Nickel foam served as both the current collector and substrate of nanowires.Its three-dimen-sional network structure with micro open cages and zigzag flow channels render the electrode with excellent mass transport property and large surface area per unit volume.
Experimental Section
Ni foam supported-cobalt oxide nanowire electrodes were prepared via a template-free growth method.6,7,26Co(NO3)2(10mmol)and NH4NO3(5mmol)were dissolved in a solution consisting of35cm3H2O and15cm3ammonia(30wt%).The solution,after being magnetically stirred for10min in air at room temperature,was transferred to a covered Petri dish and heated in an oven at90(1°C for2h(ready for nanowire growth).Ni foam substrate(length?width?thickness=14?10?1.1mm,110PPI,320g m-2;Changsha Lyrun Material Co.,Ltd.China)was degreased with acetone,etched with 6.0mol dm-3HCl for15min,rinsed with water,soaked in 0.1mmol dm-3NiCl2for4h,and then rinsed with water extensively.The left and right edges(2mm in width)of the Ni foam sheet were wrapped with Teflon tape and folded to90°, which serves as the spacer for keeping the nickel foam substrate away from the dish bottom.Nanowire growth was carried by immersing the Ni foam in the reaction solution for12h at 90(1°C.After the covered edges were cut off,the obtained electrodes(10?10?1.1mm)were thoroughly washed with H2O,dried at60°C for2h(referred to as-prepared),and calcined at temperatures between200and400°C for2h in air.In average,8mg nanowires were grown on the1cm2Ni foam,weighed after calcination at300°C.
The morphology was examined by a scanning electron micro-scope(SEM,JEOL JSM-6480)and a transmission electron microscope(TEM,FEI Teccai G2S-Twin,Philips).The struc-ture was analyzed using an X-ray diffractometer(Rigaku TTR III)with Cu K R radiation(λ=0.1514178nm).Chemical bonding information on metal-oxygen,hydroxyl,and inter-calated nitrate anions were investigated with Fourier transform infrared spectroscopy(FTIR,Equinos55,Bruker)using the potassium bromide pellet technique.Each FTIR spectrum was collected after16scans with a resolution of0.5cm-1.Thermo-gravimetric and differential thermal analysis(TG/DTA)was performed with a Pyris-Diamond thermal analyzer(Perkin-Elmer)in a flow of air(40cm-3min-1)at a heating rate of 20°C min-1from room temperature up to800°C in an Al2O3 sample pan.Brunauer-Emmett-Teller(BET)surfaces were measured by nitrogen adsorption isotherms recorded on a Micromeritics ASAP2020volumetric adsorption system.
Cyclic voltammetric(CV),chronoamperometric,and electro-chemical impedance(EIS)experiments were performed in a conventional three-electrode electrochemical cell using a com-puterized potentiostat(Autolab PGSTAT302,Eco Chemie) controlled by GPES software.Ni foam(1cm2nominal planar area)supported nanowire arrays acted as the working electrode.
A glassy carbon rod behind a D-porosity glass frit was employed as the counter electrode to minimize the effect of H2O2decom-position.A saturated Ag/AgCl,KCl electrode served as the reference.All potentials were referred to the reference electrode. All electrochemical measurements were performed at room temperature.The electrolyte was3.0mol dm-3aqueous NaOH solution.All solutions were made with analytical grade chemical reagents and Millipore Milli-Q water(18MΩcm).EIS mea-surements were performed by applying an AC voltage with5mV amplitude in a frequency range from0.01to100kHz.
Results and Discussion Characterization of Cobalt Oxide Nanowire Arrays. Figure1shows the SEM images of the electrode calcined at300°C.Clearly,the skeletons of Ni foam were com-pletely covered by nanowires,which grow densely and almost vertically from the substrate.The shiny Ni foam substrate turned to black after nanowire growth.The nanowires have diameters of around250nm,lengths up
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5114Chem.Mater.,Vol.21,No.21,2009Wang et al.
to around 15μm,and fairly good uniformity.The as-prepared electrode demonstrated similar morphology with that calcined,indicating that the nanowire arrays were formed within the growth period in the solution.Figure 2shows the TEM images of a single nanowire as-prepared (Figure 2A)and calcined at 300°C (Figure 2B -D).The as-prepared nanowire looks assembled by nanospheres.After calcination,nanospheres break down to nanoparticles with irregular shape and are packed roughly layer by layer to form the nanowires which have a more flat surface (Figure 2B,C).The high resolution TEM (Figure 2D)shows lattice fringes,which combined with the X-ray diffraction (XRD)results below indicate the high-crystal-line nature of the building blocks of the nanowires.
Figure 3shows the XRD patterns of the nanowires.The as-prepared nanowires have the mixed brucite β-Co(OH)2phase and spinel Co 3O 4phase,which sug-gested that partial Co 2twas oxidized to Co 3tduring the period of the reaction solution preparation and the nanowire growth.This is supported by the color change of reaction solution from the original pink to black.The XRD pattern of the nanowires heated at 300°C in air matches the pattern of pure spinel phase Co 3O 4,showing that Co(OH)2was converted into Co 3O 4by the thermal treatment,probably via the reaction between Co(OH)2and O 2in air.Peak broadening reflects the nanoplatelet nature of Co 3O 4building blocks as shown in Figure 2C.It should be pointed out that the presence of R -type cobalt hydroxide in the as-prepared nanowires could not be excluded even though they were not observed in the XRD spectra.The synthesis of R -type cobalt hydroxide using Co(NO 3)2and ammoniacal solutions under similar
conditions used in this work has been reported.27,28R -Type cobalt hydroxides are layered compounds con-sisting of positively charged layers with anions and solvent molecules residing in the gallery to restore charge neutrality.They were usually obtained in the form of poorly or turbostratically crystallized aggregates,owing to their metastable feature,and may not be observed in the XRD.27-32
The FTIR spectra of the as-prepared and calcined sample are shown in Figure 4.The difference between the two spectra is the presence of two clear peaks at 1384and 3627cm -1in the spectrum of the as-prepared nano-wires.The weak peak around 1384cm -1can be attributed to a ν3vibrational mode with D 3h symmetry of NO 3-,which possibly is adsorbed in the sample or intercalated in the interlayers of R -type cobalt hydroxide (an indication of the presence of R -type cobalt hydroxides).The strong sharp peak around 3627cm -1comes from the O -H stretching vibrations of Co(OH)2,which,combining with the XRD analysis (Figure 3),supports the conclusion that the as-prepared sample contains Co(OH)2.
The
Figure 1.SEM images of nanowires calcined at 300°
C.
Figure 2.TEM images of a single nanowire:(A)as-prepared and (B -D)calcined at 300°
C.
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Article Chem.Mater.,Vol.21,No.21,20095115 absorption bands shown at665and576cm-1in the
spectra of both as-prepared and calcined sample originate
from the stretching vibrations of the metal-oxygen bond
in the spinel metal oxide,further demonstrating the
presence of Co3O4in the as-prepared sample.The broad
peak around3440cm-1can be assigned to hydrogen
bonded O-H stretching of OH in brucite Co(OH)and
adsorbed and intercalated H2O as well.The water bend-
ing mode is also found at around1650cm-1.All these
peaks are well documented in the literature.27-29,33The
3627and1384cm-1peaks completely disappeared in the
spectrum of the calcined sample,showing that cobalt
hydroxides were completely converted to Co3O4after
calcination at300°C.
Figure5represents the TG and DTA curve of the as-prepared sample measured under air flow.The weight loss of the as-prepared sample was clearly observed during the heating process,which takes place roughly in three stages noted by stage I,II,and III on the graph.The weight loss in the range of23-165°C(domain I)can be assigned to the evaporation of water physically adsorbed on the surface,trapped in intercrystallite pores,and intercalated in the interlayer space.28,29,34Other weakly absorbed species might also contribute to this loss.The domain II ranging from165to220°C is associated with the loss of water produced by dehydroxylation of the cobalt hydroxides through oxidation with oxygen com-bined with the loss of parts of nitrate anionic species.35,36 The domain III ending at about580°C can be attributed to the loss of residue nitrate anions and hydroxyl groups.28The water content in the as-prepared sample was about1%estimated from the weight loss in domain I. The total weight loss in domains II and III(165-580°C) is12%,which is lower than the theoretical weight loss corresponding to the departure of hydroxyl groups in Co(OH)2together with the conversion into Co3O4 (13.6%).The discrepancy originates from the existence of Co3O4in the as-prepared sample and indicates the very low content of Co3O4in the as-prepared sample.
Catalytic Behavior of Cobalt Oxide Nanowire Array Electrode.Cyclic voltammograms of as-prepared and calcined samples were recorded in NaOH solution at a scan rate of5mV s-1(Figure6).Three(A,B,and C)and two(B and C)anodic peaks were observed on the as-prepared sample and the sample calcined at300°C, respectively.These peaks can be attributed to the conver-sion between different cobalt oxidation states,that is, Peaks A,B,and C correspond to the oxidation of Co-(OH)2to Co3O4,Co3O4to Co(OH)3,and Co(OH)3to CoO2,respectively.37-39Peak A did not occur on the sample calcined at300°C,indicating that Co(OH)2was converted to Co3O4after calcination,which is consistent with the result of XRD(Figure3)and FTIR analysis (Figure4).Peaks B and C for Co3O4occurred at more positive potential than that for Co(OH)2.This peak shift likely results from the structure difference of the two compounds.The cathodic peaks can be assigned to the reduction of Co(OH)3to Co3O4(B0)and Co3O4to Co-(OH)2(A0),respectively.The redox couple of Co(OH)2/ Co3O4(A-A0)is likely responsible for the catalytic activity of H2O2reduction because its redox potential is close to the onset potential for H2O2reductions(around -0.1V;see figure7).25
Figure7shows the influence of calcination temperature on catalytic activity for H2O2electroreduction in
NaOH
Figure5.TD/DTA curves of the as-prepared nanowires scratched down
from Ni foam
substrate.
Figure6.Cyclic voltammograms of the Ni foam supported nanowire
electrodes:dash line,as-prepared;solid line,calcined at300°C.Electro-
lyte:3.0mol dm-3NaOH.Scan rate:5mV s-1.
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5116Chem.Mater.,Vol.21,No.21,2009Wang et al.
solution.The polarization curves demonstrate that H 2O 2electroreduction starts at around -0.1V and the current density increases with the potential going negatively.The sample calcined at 300°C (Co 3O 4)exhibits higher activity than that calcined at 200°C (mixed phase of Co(OH)2and Co 3O 4)and the as-prepared nanowires (Co(OH)2in great majority).Since the specific surface area of the samples calcined below 300°C does not change significantly (Table 1),the activity difference shows that Co 3O 4has much higher catalytic activity for H 2O 2electroreduction than Co(OH)2.A further increase in the calcination temperature,higher than 300°C,results in a decrease in catalytic activity even though the nanowires remain to be Co 3O 4.The loss of activity can be attributed to the decrease of surface area (Table 1),which results from the merge of Co 3O 4nanowires as clearly shown by the SEM images (Figure 8).Therefore,calcination tempera-ture is critical for obtaining Co 3O 4nanowires with high surface area.
Figure 9shows the dependence of the catalytic perfor-mance of Co 3O 4nanowires on the concentration of H 2O 2.The limiting current increased almost linearly with the increase of H 2O 2concentration below 0.4mol dm -3.O 2evolution was observed when H 2O 2concentration exceeds
0.4mol dm -3.The onset potential for H 2O 2electroreduc-tion on Co 3O 4nanowires is similar to that on Co 3O 4nanoparticles (around -0.1V).25However,the Ni foam supported-Co 3O 4nanowire array electrode demonstrated a much lower potential for currents reaching their limits than a Co 3O 4nanoparticle/carbon/PTFE electrode (e.g.,around -0.6V for nanowire vs -0.35V for nanoparticles measured in 0.4mol dm -3H 2O 2at 5mV s -1).25This suggests that the Co 3O 4nanowire electrode has much better mass transport performance than the conventional pressed powder electrode.
In order to investigate the stability of the electrode,H 2O 2electroreduction at various constant potentials was performed.The chronoamperometric curves are shown in Figure 10.Currents remain nearly constant within a 30min test period,indicating that the Co 3O 4nanowire electrode has a superior stability for H 2O 2electroreduc-tion in an alkaline medium.Table 2lists the mass current density for H 2O 2electroreduction on a Co 3O 4
nanowire
Figure 7.Current -potential polarization curves for H 2O 2electro-reduction on electrodes calcined at various temperatures.Electrolyte:3.0mol dm -3NaOH t0.4mol dm -3H 2O 2.Scan rate:5mV s -1.Figure 8.Current -potential polarization curves for H 2O 2electroreduction
at various H 2O 2concentrations on the electrode calcined at 300°C.Electrolyte:3.0mol dm -3NaOH tH 2O 2.Scan rate:5mV s -1
.
Figure 9.SEM images of nanowires calcined at 350(A)and 400°C (B).
Table 1.BET Surface Area of the Nanowires Calcined at Different
Temperatures
BET surface area/m 2g -1
76.280.378.426.8
13.5Figure 10.Chronoamperometric curves for H 2O 2electroreduction on the electrode calcined at 300°C.Electrolyte:3.0mol dm -3NaOH t0.4mol dm -3H 2O 2
.
Article Chem.Mater.,Vol.21,No.21,20095117 (taken from Figure10)and on a Co3O4nanoparticle
electrode(taken from ref25).The nanowire loading is
8mg cm-2.The nanoparticles have an average diameter
of around17nm,and their loading in the electrode is
around15mg cm-2.25Clearly seen from Table2,Co3O4
nanowires show higher steady mass current density than
nanoparticles.The current-time curves measured at
-0.4V for the nanowire and nanoparticle electrode also
display different features.For the nanoparticle electrode,
the currents fluctuate with the increase of time due to the
perturbation of oxygen from H2O2decomposition.25This
negative effect was obviously depressed for the nanowire
electrode indicated by the smooth current-time curve.
Therefore,the nanowire electrode outperformed the na-
noparticle electrode in terms of both activity and mass
transport property.
Figure11shows the electrochemical impedance spectra
of the Co3O4nanowire electrode(the sample calcined at
300°C)measured at different potentials in NaOH solu-
tion with and without H2O2.In the absence of H2O2,the
spectrum displays a semicircle at a high frequency region
and a straight line at low frequency region.In the presence of H2O2,the spectra at all potentials consist of two slightly depressed semicircles,and the one at the high frequency region closely resembles the one without H2O2 and is nearly independent of the potential.It can be attributed to the response of the transition between Co-(OH)2/Co3O4.40The one at low frequency region ob-viously relates with H2O2because it appears only in the presence of H2O2.The diameter of this semicircle becomes smaller when the potential becomes more negative, which suggests that this semicircle most likely corresponds to the electroreduction of H2O2because the reduction rate depends upon the electrode potential,and the lower the potential,the faster the reaction(Figure10)and the smaller the diameter of the semicircle(Figure11).40The experimen-tal data are simulated by an electrical equivalent circuit composed of two constant phase elements(CPE)/resistive elements in series(inset of Figure11).The fitting results show that a good agreement between the experimental(symbols) and simulated(solid lines)data was obtained.The fitting parameters of the circuit elements are given in Table3.
R is the ohmic resistance of the electrolyte and electrode. R1and R2are the charge transfer resistance of the redox reduction of Co(OH)2/Co3O4and H2O2reduction,respec-tively.Q1and Q2are the corresponding constant phase angle elements.Its impedance is defined as Z CPE=1/(Q(jω)n),in which Q represents the frequency independent parameter,ωis the radial frequency,and n has values-1e n e1.If n=-1, CPE behaves as an inductor;if n=0,the CPE behaves as a pure resistor,and if n=1,CPE behaves as a pure capacitor.41As can be seen from Table2,R1is around 0.57Ωcm2and is independent of the potential.n1is between 0.6and0.8,indicating the deviation of Q1from the ideal behavior of a perfect capacitor,which might result from the roughness and porosity of the nanowire electrode. R2decreases with the potential becoming more negative, showing the H2O2reduction is faster at a more negative potential,which is in good agreement with the results shown in Figure10.When n2=1(-0.2to-0.4V),it implies Q2 behaves like a capacitor.The two semicircle features in the EIS further suggest that the redox couple of Co(OH)2/ Co3O4might be responsible for the catalytic activity of H2O2electroreduction on the Co3O4nanowire electrode.
Conclusions
Growth of spinel Co3O4nanowire arrays on Ni foam was successfully demonstrated.The nanowires have dia-meters of around250nm and lengths up to around15μm. They densely and vertically stand on the frame of a Ni foam substrate to give a Co3O4nanowire loading of0.8 mg cm-2.The thermal treatment temperature is very important for the formation of the active Co3O4phase and for the retainment of high active surface area of the electrode.The resulting electrode shows better perfor-mance for H2O2electroreduction in an alkaline medium in terms of both activity and mass transport property than a Co3O4nanoparticle/carbon/PTFE electrode.The
Table2.Mass Current Density(mA mg-1Co3O4)for H2O2Electro-reduction on a Co3O4Nanowire and a Co3O4Nanoparticle Electrode at
Different Potentials
potential/V-0.2-0.3-0.4 Co3O4nanowire0.85 4.0 6.8 Co3O4nanoparticle 3.77.6
10.8
Figure11.Electrochemical impedance spectra of the nanowire electrode
calcined at300°C,measured at different bias potentials in the solution of
3.0mol dm-3NaOH and3.0mol dm-3NaOHt0.4mol dm-3H2O2.
Scattered symbols:experimental data points.Solid lines:simulated
results.Inset is the equivalent electrical circuit used to fit the experimental
data.
Table3.Values of the Fitting Parameters Evaluated from the Equivalent
Circuit at Different Potentials
E/V R/Ω
cm2
R1/Ω
cm2
Q1/mS
cm-2s n n1
R2/Ω
cm2
Q2/mS
cm-2s n n2
0 1.530.5834.360.8 1.74 1.5740.8 -0.2 1.520.5736.820.6 1.43 1.5991 -0.3 1.510.5632.970.60.880.0171 -0.4 1.540.5632.160.80.620.0181
(40)Palmas,S.;Ferrara,F.;Vacca,A.;Mascia,M.;Polcaro,A.M.
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redox couple of Co(OH)2/Co3O4might play an important role in the electrocatalytic reduction of H2O2on the Co3O4nanowire electrode.
Acknowledgment.We gratefully acknowledge the financial support of this research by National Nature Science Foundation of China(20973048),Heilongjiang Provincial Natural Science Foundation(ZJG2007-06-02),Heilongjiang Postdoc Founda-tion(LBH-Q06091),Key Laboratory of Superlight Materials and Surface Technology of Ministry of Education,and Harbin Engineering University(HEUFT07030,HEUFT07051,and HEUFT08008).