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Oxygen evolution reaction on Ni-substituted Co3O4 nanowire array electrodes

Oxygen evolution reaction on Ni-substituted Co 3O 4nanowire array electrodes

Bangan Lu,Dianxue Cao *,Pan Wang,Guiling Wang,Yinyi Gao

College of Material Science and Chemical Engineering,Harbin Engineering University,Harbin 150001,PR China

a r t i c l e i n f o

Article history:Received 29July 2010Received in revised form 14September 2010

Accepted 18September 2010Available online 20October 2010Keywords:Cobalt oxides Nickel substitution Nanowire arrays

Oxygen evolution reaction

a b s t r a c t

Nanowire arrays of mixed oxides of Co and Ni freely standing on Ni foam are prepared by a template-free growth method.The effects of Ni content on the morphology,structure and catalyst performance for oxygen evolution reaction are investigated by scanning electron microscopy,X-ray diffraction spectroscopy and electrochemical techniques including cyclic voltammetry,chronopotentiometry and electrochemical impedance spectroscopy.A transformation from nanowire arrays to nanoplate arrays is found with the increase of the atomic ratio of Ni to Co in the preparation solution.The Ni x Co 3àx O 4electrode obtained at 1:1of Ni:Co in the preparation solution exhibits nanowire array structure and has better catalytic performance for oxygen evolution reaction than other Ni x Co 3àx O 4and Co 3O 4electrodes.The catalytic activities of the Ni x Co 3àx O 4and Co 3O 4electrodes are correlated with their surface roughness.Superior stability of the Ni x Co 3àx O 4nanowire array electrode is demonstrated by a chronopotentiometric test.The reaction orders with respect to OH àon the Ni x Co 3àx O 4electrode are close to 2and 1at low and high overpotentials,respectively.

1.Introduction

The oxygen evolution reaction (OER)plays an important role in electrochemical science and technology because it is involved in many electrochemical processes,for example,water splitting by electrolysis for production of hydrogen.An effective electrocatalyst can reduce the overpotential and thus enhance the energy ef?ciency.Therefore,considerable research efforts have been made in developing the OER electrocatalysts which are highly active,stable and inexpen-sive.Co 3O 4has long been known to be an active and stable catalyst for the OER in alkaline media [1e 8].

Co 3O 4electrodes for the OER are usually thin ?lms [2,9e 11]or particle agglomerates bound together by polymers [12,13].Recently,nanowire arrays standing on a current collecting substrate have been investigated as electrodes for the OER [14],

hydrogen evolution reaction [15],Li-ion battery [16],super-capacitor [17]and hydrogen peroxide electroreduction [18].Self-standing nanowire array electrodes possess a large elec-trochemical active surface area,high utilization ef?ciency of active materials,and superior mass transport property.The unique structure of this type of electrode is particularly bene-?cial for the OER,which involves evolution of a large volume of gas.During the course of our investigations,Li et al.[14]reported that Co 3O 4and NiCo 2O 4nanowire array electrodes have higher current densities for the OER than their nano-particle ?lm electrodes.Tafel slopes were 60mV for the nano-wire electrodes and 120mV for the nanoparticle ?lm electrodes.

Compared with Co 3O 4nanoparticles and thin ?lms,the self-standing Co 3O 4nanowires have a relatively large diam-eter (hundreds of nanometers)and long length (tens of micrometers).Therefore,increasing the surface roughness

*Corresponding author .Tel./fax:t8645182589036.E-mail address:caodianxue@https://www.doczj.com/doc/c714219034.html, (D.

Cao).

A v a i l a b l e a t w w w.s c i e n c e d i re ct.c o m

j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /h e

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 36(2011)72e 78

and conductivity of Co3O4nanowires is necessary for making high utilization of the nanowires in the OER.It has been reported that the electrocatalytic activity and conductivity can be improved by the addition of Ni to the spinel lattice of Co3O4,i.e.,replacing Co2tin the tetrahedral sites or Co3tin the octahedral sites[11,13,19,20].In this work,the in?uence of doping of Co3O4nanowire arrays with Ni on its electrocatalytic performance for the OER was investigated.The catalytic behavior of the Ni-substituted Co3O4nanowire array electrode was studied by employing various electrochemical techniques.

2.Experimental

2.1.Preparation and characterization of nanowire array electrodes

Nanowire array electrodes were prepared via a template-free growth method using nickel foam as the supporting substrate (current collector)[14,16,17,21,22].n(?0,2,4,5,6,8)mmol Ni(NO3)2,(10àn)mmol Co(NO3)2and5mmol NH4NO3were dissolved in a solution consisting of35cm3H2O and18cm3 ammonia(25wt.%).The solution was magnetically stirred for 10min in air at room temperature and then heated in an oven at90 C for2h(ready for nanowire growth).Nickel foam substrate(110PPI,320g mà2,Changsha Lyrun Material Co., Ltd.,China)was degreased with acetone,etched with 6.0mol dmà3HCl for15min,rinsed with water,soaked in 0.1mmol dmà3NiCl2for4h,and then rinsed with water extensively.The pre-treated nickel foam was immersed in the reaction solution for12h at90 C to allow the growth of nanowire arrays.After growth,the electrodes were thor-oughly washed with H2O and calcined at250 C for2h in air. The obtained electrodes were denoted as Co3O4,Ni x Co3àx O4-1:4,Ni x Co3àx O4-2:3,Ni x Co3àx O4-1:1,Ni x Co3àx O4-3:2and Ni x Co3àx O4-4:1,with respect to n?0,2,4,5,6,8,respectively.

The morphology was examined by a scanning electron microscope(SEM,JEOL JSM-6480).The structure was analyzed using an X-ray diffractometer(XRD,Rigaku TTR III)with Cu K a radiation(l?0.1514178nm).

2.2.Electrochemical measurements

Cyclic voltammetry(CV),chronopotentiometry and electro-chemical impedance(EIS)experiments were performed in a conventional three-electrode electrochemical cell using a computerized potentiostat(Autolab PGSTAT302,Eco Chemie)controlled by GPES software.Nanowire array elec-trode(1cm2nominal planar area)acted as the working elec-trode.A glassy carbon rod behind a D-porosity glass frit was employed as the counter electrode to eliminate the interfer-ence of evolved hydrogen.A saturated Ag/AgCl,KCl electrode served as the reference.All potentials were referred to the reference electrode.All electrochemical measurements were performed at25 C.The electrolyte was1.0mol dmà3NaOH aqueous solution unless speci?ed.All solutions were made with analytical grade chemical reagents and Millipore Milli-Q water(18M U cm).EIS measurements were performed by applying an AC voltage with5mV amplitude in a frequency range from0.01to100kHz.3.Results and discussion

3.1.Morphology of Co3O4and Ni-substituted Co3O4 electrode

Fig.1shows the SEM images of Co3O4and Ni-substituted Co3O4electrodes.Co3O4nanowires densely grow on the entire surfaces of nickel foam skeletons as we reported previously [17,18].The diameter of the Co3O4nanowires is about 450nm and their length is up to around15m m(Fig.1a).The Ni-substituted Co3O4displayed the same nanowire array structure as Co3O4when the atomic ratio of Ni to Co in the preparation solution is less than1:1.However,when Ni:Co is larger than1:1,a transformation from nanowire arrays to nanoplate arrays was observed with the increase of Ni:Co.For example:Ni x Co3àx O4-1:1is the array of nanowires having diameters of around850nm and lengths up to around20m m (Fig.1b and c);Ni x Co3àx O4-4:1is the array of nanoplates with lengths up to around35m m and thickness of about350nm (Fig.1d),which is similar to Ni(OH)2nanoplates reported in the literature[22].The surface of Ni x Co3àx O4-1:1nanowires is rougher than that of Co3O4nanowires,as demonstrated in Fig.1c and d.

3.2.XRD patterns of Co3O4and Ni-substituted Co3O4

To investigate the effect of Ni substitution on the crystalline structure of Co3O4,the nanowires and nanoplates were scratched from nickel foam and analyzed by XRD.Fig.2 shows the XRD patterns of Co3O4and Ni-substituted Co3O4 with different Ni substitution levels.The diffraction peaks of the Co3O4nanowires are well indexed to the cubic spinel Co3O4(JCPDS74-2120),which is consistent with the literature results[23,24].For the Ni-substituted Co3O4,when the atomic ratio of Ni to Co in the preparation solution is less than3:2, the diffraction peaks matched with both spinel Co3O4and NiCo2O4(JCPDS73-1702).In addition,peaks corresponding to b-Ni(OH)2phase(JCPDS73-1520)were also observed and their intensity increased with the increase of the ratio of Ni to Co,indicating the formation of Ni(OH)2in the Ni-substituted Co3O4.The dominant phase in Ni x Co3àx O4-4:1 is Ni(OH)2.Since it is dif?cult to distinguish between the XRD patterns of Co3O4and NiCo2O4,the cubic spinel lattice parameter(a0)of Ni x Co3àx O4-1:4,Ni x Co3àx O4-2:3,Ni x Co3àx O4-1:1and Ni x Co3àx O4-3:2was determined and compared with that of Co3O4.This procedure was carried out to obtain information about the incorporation of Ni into the Co3O4 spinel lattice since it was known that the incorporation of Ni into spinel Co3O4causes an expansion of the elementary spinel lattice[13,23].The lattice parameter(a0)was calcu-lated through the d-spacing of the plane(311)and the values are0.8081,0.8085,0.8089,0.8113,and0.8105nm for Co3O4,Ni x Co3àx O4-1:4,Ni x Co3àx O4-2:3,Ni x Co3àx O4-1:1and Ni x Co3àx O4-3:2,respectively.The a0of Co3O4(0.8081nm)is in good agreement with its theoretical value(0.808nm).Simi-larly,the a0of Ni x Co3àx O4-1:1(0.8113nm)is in good agree-ment with the theoretical a0of NiCo2O4(0.8114nm),which suggests that the main phase of Ni x Co3àx O4-1:1is spinel NiCo2O4[5].

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3.3.Cyclic voltammograms of Co3O4and Ni-substituted Co3O4electrodes

Fig.3shows cyclic voltammetric behavior of Co3O4and Ni-substituted Co3O4electrodes with different substitution levels at a scan rate of1.0mV sà1in1.0mol dmà3NaOH.The current was normalized to the amount of activity materials in1cm2 electrode.The cyclic voltammogram of Co3O4exhibits a shoulder peak at around0.40V(its corresponding cathodic peak at about0.25V)and a main peak at0.46V(its corre-sponding cathodic peak at about0.41V).These two pairs of peaks can be attributed to the transition of the redox couple Co(II)/Co(III)and Co(III)/Co(IV),respectively,according to the literature[2,23,25].The low intensity of the shoulder anodic peak may be explained by assuming that this anodic process corresponds to the oxidation of Co(II)cations in the tetrahe-dral positions of the spinel oxide,which is more dif?cult to be oxidized or reduced than cations in octahedral sites[23,26]. Ni x Co3àx O4-1:4displays a similar CV pro?le with Co3O4,but with higher peak currents.The CV of Ni x Co3àx O4-1:1shows only one broad anodic peak at around0.39V and an over-lapped cathodic peak at around0.2V,similar to that of NiCo2O4[2,9,27e30].These anodic peaks can be

correlated

Fig.2e XRD patterns of Co3O4(a),Ni x Co3L x O4-1:4(b),

Ni x Co3L x O4-2:3(c),Ni x Co3L x O4-1:1(d),Ni x Co3L x O4-3:2(e),

and Ni x Co3L x O4-4:1

(f).

Fig.3e Cyclic voltammograms of Co3O4(a),Ni x Co3L x O4-1:4

(b),Ni x Co3L x O4-1:1(c),and Ni x Co3L x O4-3:2(d)in

1.0mol dm L3NaOH at a scan rate of1mV s L1

Fig.1e SEM images of Co3O4(a),Ni x Co3L x O4-1:1(b,c)and Ni x Co3L x O4-4:1(d).

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with the oxidation of Co(II)to Co(III)to Co(IV).The cathodic peaks correspond to the reduction of Co(IV)to Co(III)to Co(II) and Ni(III)to Ni(II)(note:the redox potential of Ni(II)/Ni(III) is very close to Co(III)/Co(IV))[11].The OER potential at 5mA cmà2mgà1increases in the order of Ni x Co3àx O4-1:1 (0.56V)

density(CV peak area)decreases in the same order.This shows the close correlation of the OER catalytic activity with the transition of oxidation state of the catalysts.

3.4.Steady-state polarization curves of Co3O4and

Ni-substituted Co3O4electrodes

Steady-state polarization curves of the OER on electrodes of Co3O4and Ni-substituted Co3O4were measured to evaluate the catalytic activity.Fig.4shows the ohmic drop corrected j e E curves and Tafel plots(insert).The polarization curves were taken in1.0mol dmà3NaOH at a scan rate of0.1mV sà1at room temperature.It can be seen that the catalytic activity increased in the order:Co3O4

The surface roughness of Co3O4and Ni-substituted Co3O4 electrodes was measured by cyclic voltammetry and corre-lated with their catalytic activity.Fig.5shows the represen-tative cyclic voltammograms of Ni x Co3àx O4-1:1electrode at different scan rates in a potential region of0.125e0.175V (D E?50mV).The CV exhibits the typical rectangular feature of an electrical double layer capacitor.In this potential region, charge transfer electrode reactions were considered to be negligible and the current is solely from electrical double layer charging and discharging.The plot of current against poten-tial scan rate has a linear relationship(insert of Fig.5)and its slope is the double layer capacitance(i?C(d E/d t)).The roughness factor,R f,was calculated by dividing the electrode capacitance(C)with the capacitance of smooth surface of NiCo2O4(60m F cmà2)(R f?C/60m F cmà2)[25,32].The correla-tion of R f with the OER current at0.6V(taken from Fig.4)for various electrodes is presented in Fig.6.As can be seen,with the increase of the atomic ratio of Ni to Co in the preparation solution,both the OER current and R f increase initially,reach their maxima at Ni:Co?1:1,and then decrease.The consis-tent changing trend of R f and catalytic activity of electrodes suggests that the electrode surface roughness plays an important role for high activity.The roughness factor of our Ni x Co3àx O4-1:1nanowire array electrode(5483)is larger than that of NiCo2O4?lm and particle electrodes prepared by different methods.For example,NiCo2O4?lm deposited on Ni substrate by a microwave assisted thermal decomposition method gives a roughness factor of3252[9].The NiCo2O4 particles coated on Ni have a roughness factor of3226[25].The large roughness factor of our Ni x Co3àx O4-1:1electrode can be attributed to its unique three dimensional nanowire array structure.

The enhancement of electrocatalytic activity of Co3O4by substituting Co with Ni has long been noticed.Ni substitution is believed to improve Co3O4activity by either enlarging its speci?c surface area and roughness factor(geometric effect) or by increasing its conductivity(electronic effect)or both [9,14].In this study,we found the roughness factor of Ni x Co3àx O4-1:1electrode is about5times that of Co3O4elec-trode.Correspondingly,the catalytic activity of Ni x Co3àx O4

-1:1

Fig.5e Cyclic voltammograms of Ni x Co3L x O4-1:1electrode in the potential range of0.125e0.175V in1.0mol dm L3 NaOH(insert is the plot of current densities at0.15V against scan rates).

Fig.4e Steady-state polarization curves for the OER on

electrodes of Co3O4(a),Ni x Co3L x O4-1:4(b),Ni x Co3L x O4-2:3(c),

Ni x Co3L x O4-1:1(d),Ni x Co3L x O4-3:2(e),and Ni x Co3L x O4-4:1

(f).The insert represents the corresponding Tafel plots.

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electrode is also about 5times that of Co 3O 4electrode.This good correlation suggests that the increase of roughness factor by Ni substitution is the main factor being responsible for the increase in the catalytic activity towards the OER.

3.5.Effect of NaOH concentration on the OER over Ni x Co 3àx O 4-1:1

The in?uence of NaOH concentration on the OER over Ni x Co 3àx O 4-1:1nanowire array electrode was investigated by the steady-state polarization method.Fig.7a shows the polari-zation curves measured in (3àx )mol dm à3NaNO 3tx mol dm à3NaOH solutions with x from 0.5to 3.0.NaNO 3was added to maintain a constant concentration (3.0mol dm à3)of cations and anions in the solution.Obviously,higher NaOH concen-tration produced larger OER current density.The current density at 0.6V increased from 20to 153mA cm à2when the NaOH concentration increased from 0.5to 3.0mol dm à3.Tafel slope is nearly independent of NaOH concentration (around 70mV),which is an indication that the reaction mechanism of the OER remains the same within the whole range of NaOH concentration studied in this work.log j at various potentials was plotted against log[OH à]and a close linear relationship was observed,as shown in Fig.7b.The slope of the plot,which is the reaction order of the OER with respect to NaOH,was found to be close to 2at low potentials (e.g.0.52V)and 1at high potentials (e.g.0.60V).This result is in good agreement with that reported in the literature for the OER on NiCo 2O 4[2,11,33,34].Two types of reaction mechanisms accounting for reaction orders with respect to [OH à]at low and high over-potentials have been discussed in the literature.

3.6.Electrochemical impedance measurements for Ni x Co 3àx O 4-1:1electrode

Fig.8shows the electrochemical impedance spectra of the Ni x Co 3àx O 4-1:1nanowire electrode measured in 1.0mol dm à1NaOH at different potentials.As can be seen,the Nyquist plot recorded at open circuit potential (OCP)displays a depressed semicircle at high frequency region and a straight line at low frequency region.The spectra measured at 0.55V and 0.65V consist of two slightly depressed semicircles.The one at high

frequency region is nearly the same as the one measured at

OCP and is almost independent of the potential.The one at low frequency region is clearly potential dependent.This two-semicircle feature is similar to that reported in Ref.[35],but is different from that in Refs.[31,36,37],in which only one semicircle was observed.The semicircle at high

frequency

Fig.6e Dependence of roughness factor and current

density (at 0.6V from Fig.4)on the Ni doping level of Co 3O 4

Fig.7e Steady-state polarization curves for the OER on Ni x Co 3L x O 4-1:1electrode at different [OH L ](a)and plots of

log j versus log c OH L (b).Scan rate:0.1mV s

L 1

.Fig.8e Nyquist plots of Ni x Co 3L x O 4-1:1electrode in 1.0mol dm L 3NaOH at different potentials.Scattered

symbols:experimental data points.Solid lines:simulated results.Insert is the equivalent electrical circuit used to ?t the experimental data.

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might relate to the response of the transition of redox couples of Co and Ni species[17,18,35].The one at low frequency region can be attributed to the OER process because its diameter is smaller at higher potential(i.e.,faster reaction).

The experimental data are simulated by an electrical equivalent circuit composed of two constant phase elements (CPE)/resistive elements in series(insert of Fig.8).The?tting parameters of the circuit elements are given in Table2.R is the ohmic resistance of the electrolyte and electrode.R1and R2are the charge transfer resistances of redox reactions of Ni x Co3àx O4-1:1and the OER,respectively.Q1and Q2are the corresponding constant phase angle elements.The imped-ance of CPE is de?ned as Z CPE?1/Q(j u)n,in which,Q repre-sents the frequency independent parameter,u is the radial frequency,n has valuesà1n 1.For n?1,CPE behaves like a perfect capacitor.As can be seen from Table2,R1and Q1are nearly independent of the potential.n1is around0.7,indi-cating the deviation of Q1from the ideal behavior of perfect capacitor,which might result from the roughness of the nanowire electrode.R2signi?cantly decreased with the potential increase from0.55V to0.65V,indicating a faster charge transfer process for the OER at higher potential.EIS results support that the OER takes place on the higher oxida-tion state species of Co and/or Ni[2,11,33,34].

3.7.Stability of Ni x Co3àx O4-1:1nanowire array electrode

The stability of the Ni x Co3àx O4-1:1nanowire array electrode for the OER in1.0mol dmà3NaOH solution was tested at various constant current densities.The potential e time current is shown in Fig.9.Starting from5mA cmà2,the current was increased to70mA cmà2by?ve steps and held for2h at each current density.As seen,at both low and high constant current densities,the potentials remain nearly constant within each 2h test period,and the electrode is stable within the overall 10h test period,indicating that the electrode has a superior stability for the OER in an alkaline medium.

4.Conclusions

Nanowire arrays of Ni-substituted Co3O4freely standing on nickel foam substrate were successfully prepared and eval-uated as the OER electrocatalyst.Ni substitution remarkably enhances the catalytic activity of Co3O4nanowire by increasing its surface roughness.Both the physical shape and the electrocatalytic performance of the mixed oxides depend upon the Ni substitution level.With the increase of atomic ratio of Ni to Co in the preparation solution, Ni x Co3àx O4converts from nanowire arrays to nano-?ake arrays.Ni x Co3àx O4nanowire array electrode obtained when Ni:Co in the preparation solution is1:1exhibits the best catalytic performance among all the mixed oxides of Ni and Co studied,and has good stability for the OER in NaOH solution.The three dimensional network structure of nickel foam combined with the free-standing nanowire arrays of the electrocatalytic active materials provides the electrodes with a large surface area and superior mass transport prop-erty.This,plus the low cost and ease of fabrication on a large scale,renders the Ni foam supported Ni x Co3àx O4nanowire arrays a promising high-performance electrode for the OER in alkaline media.

Acknowledgements

We gratefully acknowledge the?nancial support of this research by the National Nature Science Foundation of China (20973048),the Fundamental Research Funds for the Central Universities,and the Key Laboratory of Superlight Materials and Surface Technology,Ministry of Education,China.

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