Performance characterization of Ti substrate lead dioxide electrode with different solid

Performance characterization of Ti substrate lead dioxide electrode with different solid solution interlayersHaishen Kong •Haiyan Lu •Wenli Zhang •Haibo Lin •Weimin HuangReceived:25March 2012/Accepted:23May 2012/Published online:16June 2012ÓSpringer Science+Business Media,LLC 2012Abstract In this study,Ti substrate was coated by three different mixed oxides (SnO 2–Sb 2O 5,RuO 2–TiO 2,IrO 2–Ta 2O 5),then PbO 2was electrodeposited on them to prepare PbO 2electrode.The microstructure of the solid solution interlayers and PbO 2coatings was characterized by scan-ning electronic microscopy and X-ray diffraction.The results indicated that the above oxides interlayer exited in the form of solid solution and the interlayer covered the Ti substrate largely.Accelerated life test proved that the presence of solid solution interlayer can increase the sta-bility of PbO 2electrodes in electrolysis.Cyclic voltam-mogram indicated that the PbO 2electrode with the solid solution interlayer have more active surface area in sulfuric acid solution.Linear scanning voltammograms showed that the over potentials of the oxygen evolution are decreased with the addition of the solid solution interlayer.Electro-chemical impedance spectroscopy showed that the pres-ence of interlayer can increase the electrochemical activity of oxygen evolution reaction and electrical conductivity.IntroductionTi substrate lead dioxide is a promising anode widely used in practice owing to its high excellent conductivity and chemical inertness [1,2].The PbO 2electrode shows high catalytic activity for contaminants abatement and organic mineralization [3,4].It is also employed as O 2evolution electrode in some practical applications,such as the elec-trowinning of metals [5].Nevertheless,the main problem of the Ti/PbO 2electrode is its poor stability.The known failure mechanisms of Ti/PbO 2electrode include flaking off of the fragile PbO 2coating to the electrolyte and generating TiO 2insulating layer between the substrate and coating [1,6].A lot of efforts have been made to improve the stability of the Ti/PbO 2electrode.The most useful method is introducing a transition layer between Ti substrate and PbO 2coating [7,8].Comn-inellis and Plattner [9]have shown that the presence of Au or Pt thin interlayer can effectively prevent the growth of TiO 2insulating layer between the substrate and the PbO 2coating.These attempts have shown that the successful interlayer should have high conductivity,excellent chemical stability,and corrosion resistance in electrolysis.And the interlayer should combine closely with the PbO 2layer to decrease the interface resistance and improve adhesive strength.The dimensionally stable anodes (DSA)invented by Beer in the late 1960s have become the most important electrodes in electrochemical engineering.These electrodes consist of Ti substrate and oxides coating,such as Ti/SnO 2–Sb 2O 5[10–12],Ti/RuO 2–TiO 2[13,14],and Ti/IrO 2–Ta 2O 5[15,16]electrodes,etc.All the three mixed oxides exist in the form of solid solution,they can cover the Ti substrate completely and combine with the substrate closely.The solid solution electrodes show several advantages of good electrical conductivity,excellent chemical stability andH.Kong ÁH.Lu (&)ÁW.Zhang ÁH.Lin (&)ÁW.Huang College of Chemistry,Jilin University,Changchun 130012,People’s Republic of China e-mail:luhy@ H.Line-mail:lhb910@H.LinState Key Laboratory of Theoretical and Computational Chemistry,Jilin University,Changchun 130012,People’s Republic of ChinaJ Mater Sci (2012)47:6709–6715DOI 10.1007/s10853-012-6613-xcorrosion resistance in electrolysis.Otherwise,the rough surface of the DSA provides more sites for deposition of PbO2crystallite,the adhesive strength between the inter-layer and the PbO2coating may be improved.In the study,we applied three different mixed oxides (SnO2–Sb2O5,RuO2–TiO2,and IrO2–Ta2O5)as interlayer between the Ti substrate and the PbO2coating.The objec-tives of this study are increasing the stability of Ti/PbO2 electrode and searching for the function of different solid solution interlayers on Ti/PbO2electrode.The electro-chemical behaviors of the PbO2electrode with the solid interlayer were characterized by electrochemical and phys-ical pared with the Ti/PbO2electrode without interlayer,Ti/solid solution interlayer/PbO2anode showed more superior in stability,electric conductivity,and elec-trochemical activity.And the Ti/IrO2–Ta2O5/PbO2electrode has the best stability and electrochemical activity of oxygen evolution.ExperimentalPreparation of interlayer onto Ti substratePrior to coating,the Ti substrate(geometric size:2.09 1.090.1cm3)underwent sandblasting,30min of etching in boiling10%hydrochloric acid and5min of ultrasonic cleaning in ultra pure water(18.25M X cm).Three kinds of thin interlayers(SnO2–Sb2O5,RuO2–TiO2,and IrO2–Ta2O5)were prepared by thermal decomposition tech-nique.The applied mole ratio of SnCl4Á5H2O to SbCl3, RuCl3Áx H2O to Ti(C4H9O)4and H2IrCl6Á6H2O to TaCl5is 9:1,1:5,and7:6in this paper,respectively[12,13,17,18]. The total mass of oxides loading are about1.5,4,and 0.8mg cm-2,respectively.Ti/solid solution interlayer/PbO2electrode preparation PbO2was deposited onto Ti/interlayer(geometric area: 1.091.0cm2)under the current density of20mA cm-2at 65°C for2h.The electrolyte was composed of0.5M Pb(NO3)2,0.04M NaF and0.1M HNO3.The resultant electrodes were called Ti/SnO2–Sb2O5/PbO2,Ti/RuO2–TiO2/PbO2,and Ti/IrO2–Ta2O5/PbO2electrode.PbO2 coating weights are about0.163,0.154,and0.150g, respectively.Ti/PbO2electrode without interlayer made in the same method was used as a reference,the coating weight is about0.140g.Microstructure characterizationThe morphology and particle size of solid solution inter-layer and PbO2coating were studied by JSM-6700F SEM (Japan Electron Co.,Japan).The crystalline phase was identified by XRD using a Ragaku diffractometer(Japan) with CuK a-radiation operating at40kV and200mA. Composition analysis was carried out by XPS(Thermo Scientific ESCALAB250,USA)with monochromatized radiations(Al,K a).Electrochemical measurementElectrochemical measurements were performed by a conventional three electrode glass cell and an electro-chemical workstation(PARSTAT2273,Princeton Applied Research,USA).Ti/PbO2,Ti/SnO2–Sb2O5/PbO2, Ti/RuO2–TiO2/PbO2,and Ti/IrO2–Ta2O5/PbO2electrodes were used as working electrode(geometric area: 1.091.0cm2),Ti/RuO2–TiO2–SnO2net(geometric area: 3.093.0cm2)was used as the counter-electrode because it was stable in the media and saturated calomel electrode (SCE)was used as the reference electrode in a separate compartment with a Luggin capillary.All the potentials were referred to SCE unless otherwise stated in this paper. Linear sweep voltammetry was performed to acquire their oxygen evolution potential in0.5M H2SO4solution.CV curves were recorded between0.8and1.95V in0.5M H2SO4solution.EIS were recorded at constant potential (1.95V vs.SCE)located in the oxygen evolution reaction (OER)domain.It was obtained in the frequency range of 100mHz–100kHz.The amplitude of the ac signal was 10mV.All electrochemical experiments were carried out at room temperature(25±2°C).All solutions were pre-pared with ultra pure water and all reagents used in the experiments were analytic grade.Accelerated life testThe accelerated life test was conducted under galvanostatic electrolysis at a current density of1A cm-2in1M H2SO4 solution with potentiostat–galvanostat(WYJ,Hangzhou Pingguo,China).The cell voltage between the working electrode and the counter-electrode(Ti net geometric area: 3.093.0cm2)was recorded.The cell voltage versus service time curve was recorded simultaneously.The ser-vice life of the electrodes was considered to be terminated when the cell voltage reaches up to10V.Results and discussionSurface morphologyFigure1a–c shows the SEM images of Ti/SnO2–Sb2O5, Ti/RuO2–TiO2,and Ti/IrO2–Ta2O5interlayers,respectively.It is observed that the surface of the solid solution prepared using the thermal decomposition method exhibits a ‘‘cracked-mud’’structure which is typical for DSA[10].The average size of the cracks of Ti/SnO2–Sb2O5,Ti/RuO2–TiO2,and Ti/IrO2–Ta2O5interlayers are about0.2,1,and 3l m,respectively.This structure can increase the contact area between the interlayer and the PbO2layer,which reduce the interface resistance and improve the adhesive strength.Figure2a–c and d shows the SEM images of Ti/PbO2, Ti/SnO2–Sb2O5/PbO2,Ti/RuO2–TiO2/PbO2and Ti/IrO2–Ta2O5/PbO2electrode surfaces,respectively.The SEM images show that the PbO2layer is compact,this structure can prevent free O2-from contacting the titanium substrate to generate an insulating TiO2layer[19].The average size of PbO2grain dimension is about3–4l m,and the average size of crack in IrO2–Ta2O5interlayer is also about3–4l m.Due to the existence of the crack,the PbO2could deposit in the crack of IrO2–Ta2O5interlayer to form an embedded structure with the IrO2–Ta2O5interlayer.The embedded structure can make the combination of surface coating and Ti substrate more compact andfirm[20,21].The IrO2–Ta2O5interlayer could increase the adhesive power between Ti substrate and PbO2coating with the embedded structure.The accelerate service life test demonstrated that the PbO2electrode with IrO2–Ta2O5interlayer has the longest service life for the studied four PbO2electrodes.The stability of Ti/IrO2–Ta2O5/PbO2electrode increase largely. Therefore,there could be the superior adhesive power over the other interlayer between Ti substrate and PbO2coating.XRD analysisTheoretically,whether or not a metal oxide mixture can form a solid solution is highly dependent on the ionic radius difference in the metal elements.The oxidation states of Sn,Sb,Ru,Ti,Ir,Ta,and Pb species are Sn(IV), Sb(V),Ru(IV),Ti(IV),Ir(IV),Ta(V),and Pb(IV),respec-tively.The ionic radii of Sn(IV),Sb(V),Ru(IV),Ti(IV), Ir(IV),Ta(V),and Pb(IV)are71,62,62,68,77,73,and 84nm,respectively(Table1).When the ratio of radii difference between two metal ions in oxides is less than 15%,the metal oxide mixture should be highly intermixed and exist in the form of solid solution[22].The different ratios of ionic radii among Sn4?and Sb5?,Ru4?and Ti4?, Ir4?,and Ta5?radii are13,9,and5%,respectively.These may explain why the oxide mixtures existed in the form of continuous solid solution.The differences ratio of ionic radii between Ti4?obtained for oxidation of Ti substrate and correlative ions in the interlayer also lie in the Hume-Rothery limit(15%),so combination of the substrate and interlayer is successful[22].The different ratios of ionic radii of Sn4?and Pb4?as well as Ir4?and Pb4?are15and Fig.1SEM images of the interlayer surface.a Ti/SnO2–Sb2O5,b Ti/RuO2–TiO2,c Ti/IrO2–Ta2O58.3%,respectively,which are also smaller than the limit.It can be inferred that SnO 2–PbO 2and IrO 2–PbO 2can form continuous solid solution.But TiO 2with PbO 2only form a limited solid solution due to the larger difference of their ion radii (19%)in Ti/RuO 2–TiO 2/PbO 2electrode.Figure 3a shows XRD patterns of different interlayers:Ti/SnO 2–Sb 2O 5,Ti/RuO 2–TiO 2,and Ti/IrO 2–Ta 2O 5.As shown in Fig.3a-1,the broad and symmetric peaks at 2h =26.6°(110),33.9°(101),52.0°(211)are attributed to diffraction peaks of SnO 2with tetragonal cassiterite structure.However,the peak positions are slightly different from the characteristic peaks of pure SnO 2crystal (PDF-21-1250)and no Sb 2O 5-rich peaks are detected.These facts reveal that various components in the Sb 2O 5–SnO 2have been highly intermixed.In other words,the SnO 2–Sb 2O 5mixture exists in the form of a solid solution.In addition,to the well-defined peaks of solid solution,the Ti metal peaks from the substrate are also observed.In Fig.3a-2,a series of broad and symmetric peaks were found at 2h =27.7°(110),36.0°(101),54.5°(211),56.9°(220),and 69.2°(301)corresponding to the rutile structure of TiO 2and no RuO2-rich peaks were detected.The facts also reveal that various components in the RuO 2–TiO 2interlayer have been highly intermixed in the form of a solid solution.In Fig.3a-3,a series of broad and symmetric peaks were found at 2h =27.7°(110),34.5°(101),and 53.3°(301)corresponding to the IrO 2–Ta 2O 5solid solution.The above results demonstrated that SnO 2–Sb 2O 5,RuO 2–TiO 2,and IrO 2–Ta 2O 5interlayers all exist in the form of solid solution.Figure 3b shows XRD patterns of Ti/PbO 2,Ti/SnO 2–Sb 2O 5/PbO 2,Ti/RuO 2–TiO 2/PbO 2,and Ti/IrO 2–Ta 2O 5/PbO 2electrodes.The diffractive peaks at 2h =25.4°,31.9°,49.0°,and 62.3°attribute to b -PbO 2structure (PDF-35-1222),they belong to (110),(011),(211),and (301)crystal face,respectively (Fig.3b-2).The results indicate that PbO 2electrode with and without solid solution inter-layer mostly contain b -PbO 2crystalline phases.The introduction of interlayer does not result in significant b -PbO 2phase changes.However,the corresponding diffractive peaks intensities are different among thefourFig.2SEM images of different PbO 2electrodes.a Ti/PbO 2surface,b Ti/SnO 2–Sb 2O 5/PbO 2surface,c Ti/RuO 2–TiO 2/PbO 2surface,d Ti/IrO 2–Ta 2O 5/PbO 2surfaceTable 1Radii of correlative ions in different solid solution Metal ions Ti 4?Sn 4?Sb 5?Ru 4?Ir 4?Ta 5?Pb 4?r (pm)68716262777384PbO 2electrodes.Moreover,the crystal orientations of the PbO 2with interlayer are richer than those without.The results suggest the better crystallinity of PbO 2at Ti/solid solution substrate.The solid solution interlayers affect the preferred orientation of PbO 2electrode.Accelerated life testFigure 4shows the accelerated life test of the Ti/PbO 2,Ti/SnO 2–Sb 2O 5/PbO 2,Ti/RuO 2–TiO 2/PbO 2,and Ti/IrO 2–Ta 2O 5/PbO 2electrode.The experiment is used to compare their electrochemical stabilities in electrolysis.When the cell voltage reaches up to 10V,the electrode service life is considered to be terminated.The service life of the Ti/PbO 2electrode is very short,which is about 11h.The presence of SnO 2–Sb 2O 5,RuO 2–TiO 2,and IrO 2–Ta 2O 5interlayers greatly prolong the service life of the PbO 2electrode.The service life of Ti/SnO 2–Sb 2O 5/PbO 2,Ti/RuO 2–TiO 2/PbO 2,and the Ti/IrO 2–Ta 2O 5/PbO 2elec-trode are about 58,207,and 672h,respectively.The above results show that the solid solution interlayers have greatlyincreased the service life of the PbO 2electrodes,especially IrO 2–Ta 2O 5interlayer.The rough surface of solid solution interlayers can increase the adhesive strength between Ti substrate and PbO 2coating.The presence of solid solution interlayers can inhibit inactivation of Ti substrate and avoid flaking of PbO 2layer.Thus,the stability of Ti/solid solution inter-layer/PbO 2electrodes is improved.Electrochemical activity of oxygen evolutionFigure 5shows the relationship of q*against the reciprocal of square root of scan rate.The charge q*was obtained by integration of the voltammetric curves over the whole potential range from 0.8to 1.95V.q*is considered as proportional to the active surface area of oxide electrodes [23].The results show that the introduction of solid solu-tion interlayer could enhance voltammetric charge quantity q*.The q *values increase in the order of Ti/PbO 2,Ti/SnO 2–Sb 2O 5/PbO 2,Ti/RuO 2–TiO 2/PbO 2,and Ti/IrO 2–Ta 2O 5/PbO 2.A possible explanation for the phenomenon is that there are richer crystalline orientations for the PbO 2electrodes with the solid solution interlayers than that without.Con-sequently,there is more activity surface area in Ti/solid solution interlayer/PbO 2electrode.Figure 6shows the liner scanning voltammograms of Ti/PbO 2,Ti/SnO 2–Sb 2O 5/PbO 2,Ti/RuO 2–TiO 2/PbO 2,and Ti/IrO 2–Ta 2O 5/PbO 2electrodes in 0.5M H 2SO 4solution.The oxygen evolution potential decreased as the following order of Ti/PbO 2,Ti/SnO 2–Sb 2O 5/PbO 2,Ti/RuO 2–TiO 2/PbO 2,and Ti/IrO 2–Ta 2O 5/PbO 2.The presence of solid solu-tion interlayer can increase the activity of oxygen evolution.For evaluating the oxygen evolution activity of different PbO 2electrode,EIS measurements were performed at constant potential (1.95V)located in the OER domain.Figure 7shows the Nyquist diagrams of different electrode in 0.5M H 2SO 4solution.The equivalent circuit which best fits the experimental EIS data is a R s (R ct Q dl )(R f Q f )com-bination (Fig.8).In the previous literature [19,24],itwasFig.3XRD patterns ofdifferent interlayers and PbO 2electrodes.a-1Ti/SnO 2–Sb 2O 5,a-2Ti/RuO 2–TiO 2,a-3Ti/IrO 2–Ta 2O 5interlayers,b-1Ti/PbO 2,b-2Ti/SnO 2–Sb 2O 5/PbO 2,b-3Ti/RuO 2–TiO 2/PbO 2,b-4Ti/IrO 2–Ta 2O 5/PbO 2electrodeFig.4Accelerated life test of different interlayers and PbO 2electrodes in 1M H 2SO 4under 1A cm -2.1Ti/PbO 2,2Ti/SnO 2–Sb 2O 5/PbO 2,3Ti/RuO 2–TiO 2/PbO 2,4Ti/IrO 2–Ta 2O 5/PbO 2electrodeoften used to simulate the impedance data for OER on metal oxide anodes.In this circuit,R s represents the uncompensated solution resistance,first capacitive loop (R f Q f )at high frequency region reflects electron transfer information through the oxide film,the R f and Q f stands for resistance through the oxide layer and the pseudo-capacitance of the oxide layer,respectively.The second capacitive loop (R ct Q dl )at low frequency region reflects the charge transfer information at the interface PbO 2/electro-lyte,R ct and Q dl indicate the charge transportation resis-tance at the interface and the double layer capacitance,respectively.In this experiment,R ct stands for the activity of oxygen evolution.The fitted electrical parameters are listed in Table 2.As shown in the table,the R f of the Ti/PbO 2,Ti/SnO 2–Sb 2O 5/PbO 2,Ti/RuO 2–TiO 2/PbO 2,and Ti/IrO 2–Ta 2O 5/PbO 2elec-trode are 0.0328,0.0172,0.0201,and 0.0187X cm 2,while the R ct are 2.021,1.749,1.512,and 1.15X cm 2,respectively.From above experimental data,it can be concluded that the presence of the solid solution interlayer can reduce the R f and R ct ,which indicates the conductivity and electrochemical activity of oxygen evolution for the Ti/solid solution inter-layer/PbO 2electrodes are higher than those for the Ti/PbO 2electrode.The reason of high conductivity could be explained as follows:first,the metal oxide formed by thermal decom-position is non-stoichiometric,there are more free electron and oxygen vacancies in the solid solution interlayer.The free electron and oxygen vacancies can enhance the elec-trical conductivity of the electrode.Second,the solid solution interlayer can prevent from forming an insulating TiO 2layer on Ti surface [25,26].Third,the solid solution interlayer can closely combine with the Ti substrate and the electrodeposited PbO 2coating,this structure can reduce interface resistance and increase the electrical conductivity of the whole Ti substrate PbO 2electrode.An explanation for high electrochemical activity of oxygen evolution is that there is more activity surface area on the Ti/solid solution interlayer/PbO 2electrodes.The mechanism for oxygen evolution on oxide electrode in acid solution might be proposed as follows [27]:Fig.5Relationship of voltammetric charge quantity (q *)versus the reciprocal of square root of scan rate in 0.5M H 2SO 4solution.a Ti/PbO 2,b Ti/SnO 2–Sb 2O 5/PbO 2,c Ti/RuO 2–TiO 2/PbO 2,and d Ti/IrO 2–Ta 2O 5/PbO 2electrodeFig.6Linear scanning voltammograms at different PbO 2electrode in 0.5M H 2SO 4:scan rate at 1mV s -1.1Ti/PbO 2,2Ti/SnO 2–Sb 2O 5/PbO 2,3Ti/RuO 2–TiO 2/PbO 2,4Ti/IrO 2–Ta 2O 5/PbO 2electrodeFig.7Nyqusit diagrams of different PbO 2electrodes in 0.5M H 2SO 4.a Ti/PbO 2electrode,b Ti/SnO 2–Sb 2O 5/PbO 2electrode,c Ti/RuO 2–TiO 2/PbO 2electrode,d Ti/IrO 2–Ta 2O 5/PbO 2electrode.E =1.95V (vs.SCE)Fig.8Equivalent circuit used in the analysis of the experimental EIS dataSþH2O¼SÁÁÁOHþHþþeÀð1ÞSÁÁÁOH¼SÁÁÁOþHþþeÀð2ÞSÁÁÁOHþSÁÁÁOH¼SÁÁÁOþSþH2Oð3ÞSÁÁÁOþSÁÁÁO¼2SþO2ð4ÞS is an active surface area on the oxide surface.SÁÁÁO and SÁÁÁOH denote the adsorption state.For the same PbO2 electrode,the number of S is contributive to the high electrochemical activity of oxygen evolution.When the S is large,there may be high electrochemical activity in the electrode surface.The results of q*is consistent with R ct. Therefore,Ti/solid solution interlayer/PbO2electrodes with low R ct have high oxygen evolution activity.Because the q* value of Ti/IrO2–Ta2O5/PbO2is the highest compared the other three electrodes,it has the highest electrochemical activity of oxygen evolution.Summary and conclusionsThe SnO2–Sb2O5,RuO2–TiO2,and IrO2–Ta2O5mixture oxides coated on Ti substrate exist in the form of a solid solution.The solid solution interlayers can completely cover the titanium substrate and improve the bond with the Ti substrate and PbO2coating.The PbO2electrodes with solid solution interlayers greatly increased the service life of the PbO2electrode.In particular,Ti/IrO2–Ta2O5/PbO2 electrode greatly prolongs the service life of the PbO2 electrode to672h,which is61times longer than that of the PbO2electrode without interlayer.The overpotential of the oxygen evolution decreases with the addition of the solid solution interlayer.The presence of interlayer can increase the electrochemical activity of oxygen evolution and electrical conductivity.The oxygen evolution activity increases in the order of Ti/PbO2,Ti/SnO2–Sb2O5/PbO2, Ti/RuO2–TiO2/PbO2,and Ti/IrO2–Ta2O5/PbO2electrodes.Acknowledgements The authors would like to acknowledge the support provided by the National Natural Science Foundation of China(No.20873051),key project in Jilin Province(No.20080306). 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