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Oxygen evolution on Co3O4 and Li-doped Co3O4 coated electrodes in an alkaline solution
\ PERGAMON
International Journal of Hydrogen Energy 13 "0888# 588Ð696
Oxygen evolution on Co2O3 and Li!doped Co2O3 coated electrodes in an alkaline solution C[ Boccaa\ \ G[ Cerisola\a E[ Magnone\b A[ Barbuccia a
Institute of Chemistry\ Faculty of En`ineerin`\ University of Genova\ P[ le J[ F[ Kennedy 0\ 05018 Genova\ Italy b INFM and Dept[ of Chemistry and Industrial Chemistry\ University of Genova\ Genova\ Italy
Abstract The oxygen evolution reaction "OER# was investigated at 59>C in 04 wt) NaOH solution on Co2O3\ Li!doped Co2O3 and perovskite!type oxide "LaNiO2# electrodes[ Cobalt and Li!doped cobalt oxides were obtained by di}erent preparation techniques and were applied to chemically!pickled nickel plates[ The obtained phases were structurally characterized by X!ray powder di}raction[ The electrodes were subjected to electrochemical measurements\ before and after elec! trochemical ageing "10 days\ 049 mA cm−1#[ The correlation between the coating preparations and their electrocatalytic activity was investigated[ The e.ciency of the studied electrodes seems to depend on the preparation method[ Two Tafel slopes for the OER were observed in most cases[ This behaviour was interpreted on the basis of a change in the valence state of the surface oxide[ Þ 0888 International Association for Hydrogen Energy[ Published by Elsevier Science Ltd[ All rights reserved[
Nomenclature b Tafel slope ðmV dec−0Ł E Electrode potential "V vs SCE# Eeq Equilibrium potential "V vs SCE# i Current density ðmA cm−1Ł i9 Exchange current density ðA cm−1Ł hO1 Oxygen overpotential ðVŁ
0[ Introduction Electrochemical oxygen evolution is one of the most important processes in several electrochemical devices "water electrolysers\ secondary metal!air batteries\ fuel cells\ chloroalkali cells\ etc[#[ Oxygen evolution reaction "OER# is a very interesting process because of its high overvoltage in alkaline solu! tions ð0\ 1Ł[ In a commercial water electrolyser\ nickel anodes have a very high oxygen evolution overpotential\ equal to about 399 mV ð2Ł[ In order to obtain a major anode e.ciency and a greater stability\ new materials
Corresponding author
have been studied\ especially those based on mixed metal oxides ð3Ð7Ł[ The spinel!type NiCo1O3 ð8Ð01Ł and Co2O3 ð0Ð1Ł elec! trodes were studied by several authors and they showed good e.ciency and long term performance[ Moreover\ they have a lower cost than RuO1!based anodes and a good corrosion stability ð1\ 02\ 03Ł[ In our previous work ð01Ł the OER was investigated on Te~on bonded!NiCo1O3 electrodes prepared by di}erent techniques\ in order to show the in~uence of the prep! aration methodology on the electrocatalytic properties of these oxides[ In this study it was observed that Te~on bonded!electrocatalysts prepared via oxalate and hydroxide coprecipitation techniques showed a remark! able activity towards oxygen evolution[ The usual ther! mal decomposition technique lets us produce NiCo1O3 oxides with very good electrocatalytic properties\ too[ In the present study the OER was investigated at 59>C in 04 wt) NaOH solution on both Co2O3 and Li!doped Co2O3 electrodes[ The latter were studied since it has been demonstrated ð04Ł that Li!doping increased electrical conductivity and electrocatalytic properties of Co2O3 oxides[ Since perovskite!type oxides demonstrated good elec! trocatalytic and stability characteristics for the OER ð6\
9259!2088:88:,19[99 Þ 0888 International Association for Hydrogen Energy[ Published by Elsevier Science Ltd[ All rights reserved PII] S 9 2 5 9 ! 2 0 8 8 " 8 7 # 9 9 0 1 9 ! 6
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1[ Experimental
trodes was carried out by thermal decomposition\ "G#[ The La!Ni nitrate aqueous solution "9[0 M# was brushed over the nickel plates\ dried at 009>C for 29 min and thermally decomposed at 299>C for 0 h and 699>C for 2 h[ The obtained coverage was about 6 mg cm−1 for all the electrodes[
1[0[ Electrodes preparation
1[1[ X!ray diffraction and S[E[M[ characterisation
Co2O3\ Li!doped Co2O3 and LaNiO2 oxides were applied to chemically pickled commercial nickel plates\ because this surface _nish permits to obtain anodic sub! strates with good electrocatalytic properties for the OER and good stability\ as previously reported ð01\ 05Ł\ as well as good adhesion of the Te~on!bonded oxides[ All the reagents were of analytic reagent grade purity\ from Aldrich Chemicals[ Cobalt oxide coated electrodes were obtained by the thermal decomposition technique "A#[ The cobalt nitrate aqueous solution "9[0 M# was brushed over the nickel plates "a small amount of ethanol was added#\ _red at 299>C for 29 min and then at 599>C for 09 h[ Te~on bonded!cobalt oxide coated electrodes were also prepared "B#[ In this case the powder obtained by the thermal decomposition of the cobalt nitrate was mixed with a Te~on water dispersion and brushed on the nickel substrate[ The electrodes were dried at 009>C for 29 min and cured at 299>C for 0 h[ Ten at) Li!doped Co2O3 anodes were prepared from cobalt nitrate and lithium nitrate dissolved in distilled water "9[0 M#[ Coprecipitation of Li!Co mixed oxalate\ "C#\ and of Li!Co mixed hydroxide\ "D#\ was performed[ Hot ammonium oxalate solution and potassium hydrox! ide were used to precipitate the Li!Co mixed oxalate and hydroxide respectively[ The precipitates were vacuum _l! tered\ _red at 299>C "29 min# and then at 599>C "09 h#[ A lithium!cobalt nitrate aqueous solution was used for the evaporation technique\ "E#[ The evaporation was made at around 89>C[ The _nal stage of water evap! oration was carried out at 094>C and the dried precipitate was thermally decomposed "see above#[ The powders obtained by methods "C#\ "D# and "E# were mixed with a Te~on water dispersion[ Several cata! lyst:Te~on combinations were investigated[ The best one was a catalyst:Te~on ratio equal to 2 ] 0 "wt)#\ which permits the achievement of the necessary adhesion to nickel substrates\ with the minimum amount of Te~on[ Te~on!bonded Li!Co coated electrodes were submitted to thermal curing as in the case of electrodes B[ Li!doped Co2O3 coated electrodes were also prepared by thermal decomposition "F#[ The preparation of such electrodes was performed brushing the Li!Co nitrate aqueous solution over the nickel substrate "see electrodes A preparation#[ The preparation of the perovskite oxide coated elec!
The structure of the powder oxides\ at room tempera! ture\ was determined by a PHILIPS PW 0609 X!ray powder di}ractometer using CuKa radiation "Ni!_lter^ E 39 kV and I 39 mA#[ For accurate determination of lattice parameters\ Si was used as internal standard[ Bragg peaks with 1u values between 19>Ð79> were used for calculating the lattice parameters by a least squares _tting program[ Surface morphology was observed by a Scanning Elec! tron Microscope equipped with Energy Dispersive X!ray Analyzer\ EDX\ "Link System Stereoscan 199\ Cam! bridge Instruments#[
7Ł\ LaNiO2 electrodes were prepared and studied in our work\ in order to have a comparison of the elec! trochemical properties of Co2O3 and Li!doped Co2O3 electrodes[
1[2[ Electrochemical testin` The investigation of the electrochemical properties of the electrodes was carried out by D[C[ electrochemical methods such as potentiodynamic curves and cyclic vol! tammetry[ The experimental details were reported in pre! vious works ð01\ 05Ł[ All the measurements were made at 59>C[ In order to obtain information about the stability of the various electrocatalytic coatings under operating con! ditions\ the electrodes were submitted to galvanostatic electrochemical ageing for 10 days "anodic current 049 mA cm−1\ T 59>C#[ After this treatment\ the above mentioned electrochemical measurements were run again on the aged samples[
2[ Results and discussion 2[0[ X!ray diffraction and S[E[M[ characterization The X!ray powder di}raction revealed the formation of single!phase products for all the powders[ Weak extra peaks from second impurities were especially detected on the LaNiO2 sample[ A similar impurity problem was reported by Bockris et al[ ð6Ł[ The crystallographic space group for all the samples was Fd2m\ as revealed by the indexed X!ray data[ Typical X!ray powder patterns for the Co2O3 sample in com! parison with XRD pattern simulations\ generated using the F2dm unit cell of cubic Co2O3\ are shown in curves "a# and "b# respectively of Fig[ 0[ The model provided a good agreement between observed and calculated
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Fig[ 0[ X!ray powder di}raction pattern] "a# sample B^ "b# Co2O3 lattice "F2dm#\ simulation of theoretical pattern[
pro_les[ The lattice parameters of the di}erent spinel samples are listed in Table 0[ However\ the morphological characterization revealed that important di}erences in the powder aspect are related to the preparation technique of the oxides[ A comparison of two oxide powders is displayed in Fig[ 1aÐ b[ Lath!like structures were detected in Li!doped Co2O3 prepared by oxalate coprecipitation "Fig[ 1a#^ these oxide morphologies were not revealed when the evaporation technique was followed "Fig[ 1b#[ In Fig[ 2\ a typical Te~on!bonded Li!doped Co2O3 electrode is shown[ The oxide particles are trapped in a porous Te~on matrix\ which allows both the faradaic process at the oxide!solu! tion interface and electron transmission through the coat! ing paths to take place[
2[1[ Electrochemical results Figures 3 and 4 refer to the current!overpotential curves at 59>C in 04 wt) NaOH of all the electrodes\ before and after electrochemical ageing respectively[ Before the ageing the polarization curves show an anodic peak at overpotential values ranging between ca 9[19Ð9[39 V "P0 6 P2#\ which is more evident on Li! doped electrodes[ This peak probably refers to the oxi! dation of the cobalt ions to higher oxidation states\ and it is followed by an area where the OER occurs[ The above mentioned anodic peak tends to disappear after the ageing[ It is worth noticing that on the perovskite it is not detected[ In the oxygen evolution region the various electrodes
Table 0 Lattice parameters of samples obtained by di}erent preparation techniques
Powder Co2O3 Co2O3¦Li Co2O3¦Li Co2O3¦Li Co2O3¦Li
Preparation method
Crystal system
Space group
Lattice constants a b c "_#
V "_2#
Thermal decomposition
Cubic Cubic Cubic Cubic Cubic
Fd2m Fd2m Fd2m Fd2m Fd2m
7[977"0# 7[980"0# 7[989"0# 7[982"0# 7[976"0#
418[97 418[56 418[36 429[95 418[77
Oxalate coprecipitation Hydroxide coprecipitation Evaporation
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Fig[ 1["a\b# Micrography of Li!doped Co2O3 powders prepared by "a# oxalate coprecipitation and "b# evaporation techniques[
exhibit di}erent values of current densities\ which could depend on di}erent active surface areas "Table 1#[ Sam! ples D and G display the higher current densities at hO1 9[44 V "ca 23[85 and 45[25 mA cm−1 respectively#[ After the electrochemical ageing an increase of current density values on electrodes A\ D\ E\ F and G is displayed[ It may be due to the above mentioned treat! ment which produces an increase of the active surface\
especially on samples A\ D and G[ On the contrary\ the electrochemical ageing seems to cause the loss of the electrocatalytic properties on electrodes B and C[ Table 1 also reports the oxygen overpotential values hO1 calculated at 09 mA cm−1 "OER region#\ as well as the equilibrium potentials Eeq of the various electrodes[ The equilibrium potentials are not similar for all the electrodes[ An increase in these was determined by elec!
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Fig[ 2[ Typical morphology of Te~on!bonded Li!doped Co2O3 electrode "oxides prepared by oxalate coprecipitation#[
Fig[ 3[ Current! overpotential potential curves for oxygen evolution on fresh electrodes in 04 wt) NaOH solution at 59>C "Ž A\ , D\ E E\ r F\ G#[ "Sweep rate 9[1 mV s−0#[
trochemical ageing[ This change may be due to the higher oxide formation on the electrode surface\ as will be described in the following[ The analysis of the experimental data reported in Table
1 demonstrates that under the same current density\ the lowest hO1 values are typical of samples D\ E and G "ranging between 9[23Ð9[33 V#[ It is noteworthy that the hO1 values decrease with Li
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Fig[ 4[ Current! overpotential curves for oxygen evolution on aged electrodes in 04 wt) NaOH solution at 59>C "Ž A\ , D\ E E\ r F\ G#[ "Sweep rate 9[1 mV s−0#[
Table 1 Experimental current density values in the range of oxygen evol! ution "hO1 9[44 V#\ working electrode equilibrium potential and overpotential values "at i 09 mA cm−1# for the OER on spinel! and perovskite!type oxide electrodes\ in 04 wt) NaOH solution at 59>C Sample A B C D E F G
Fresh Aged Fresh Aged Fresh Aged Fresh Aged Fresh Aged Fresh Aged Fresh Aged
i "mA cm−1#
Eeq "V:SCE#
hO1 "V#
4[49 61[03 1[53 !!! 5[59 !!! 23[85 001[03 06[60 07[14 4[04 18[57 45[25 039[26
−9[029 ¦9[033 −9[199 !!! ¦9[914 !!! Ð9[909 ¦9[119 −9[049 ¦9[213 −9[035 ¦9[920 ¦9[064 ¦9[091
9[475 9[249 9[612 !!! 9[321 !!! 9[327 9[143 9[339 9[399 9[463 9[379 9[231 9[297
Sample that lost its electrocatalytic properties after 10 days of ageing at 59>C[
doping[ This may be due to both an increase in electrical conductivity and a higher presence of Co2¦ ions with Li doping[ In fact\ these results cannot only be explained by
an increase of electrical conductivity\ but if all the Li¦ enter in the tetrahedral sites\ the amount of Co2¦ ions in the spinel will increase\ which is in agreement with the oxygen overpotential decrease ð04Ł[ After 10 days of elec! trochemical ageing\ only samples B and C lose their elec! trocatalytic properties for the OER\ while the other elec! trodes maintain them[ Moreover\ the oxygen overpotential decreases especially on samples A and D[ This probably suggests that the surface oxide is endowed with a higher stability and hence with better elec! trocatalytic activity than other electrodes ð06Ł[ The kinetic parameters for the OER of the various electrodes\ i[e[\ Tafel slope\ b\ and exchange current density\ i9\ are summarized in Table 2\ according to the di}erent coating preparation techniques[ The polarization curves for most of the electrodes dis! played two Tafel regions] one at low overpotentials and the other at high overpotentials[ The values of the _rst one ranged between 60Ð88 mV decade−0 and the second one between 098Ð067 mV decade−0[ The presence of two Tafel slopes may be ascribed to the transition to a higher valence state of cobalt ions in the lattice ð04Ł[ Co2O3 has the spinel structure Co1¦ðCo12¦ŁO31−\ hence the possible transitions to higher valence states are] Co1¦ : Co2¦
"0#
Co2¦ : Co3¦
"1#
C[ Bocca et al[ : International Journal of Hydrogen Energy 13 "0888# 588Ð696 Table 2 Tafel parameters of the OER on spinel! and perovskite!type oxide electrodes in 04 wt) NaOH solution at 59>C[
Sample A B C D E F G
Fresh Aged Fresh Aged Fresh Aged Fresh Aged Fresh Aged Fresh Aged Fresh Aged
b "mV dec!0#
i9 "09−4 A cm−1#
Low h
Low h
High h
034 74 078 * 73 * 87 88 60
00[60 9[68 06[43 006 * 013 067 082
* 0[08 * 4[54 1[18 9[07
069
0[74
098 75
4[55 * 7[57 08[50 09[50 0[70
040 67 81
High h
12[55 8[17
033 067
9[76 0[97
01[52 04[03
Sample that lost its electrocatalytic properties after 10 days of ageing at 59>C[
The above experimental results are consistent with the most probable mechanism of oxygen evolution ð1\ 07Ł\ which is] M¦OH− MOH¦e−
"2#
MOH¦OH− MH1O1¦e−
"3#
1 M H1O1 1 M¦1 H1O¦O1
"4#
where M is a tetravalent cobalt cationic site[ The OER on Co2O3 and Li!doped Co2O3 have been studied by several authors[ Depending on the oxide prep! aration method and experimental condition\ di}erent values for the Tafel slope "ranging between ½49н69 mV decade−0 at low overpotentials and ½099н039 mV decade−0 at high overpotentials\ depending on the tem! perature# were obtained ð08Ł[ The electrocatalytic performance of Te~on!bonded Co2O3 and Li!doped Co2O3 electrodes towards the OER were studied by Rasiyah and Tseung ð04Ł in KOH med! ium at 14>C[ Tafel slopes of ½59 mV decade−0 at low overpotentials and ½019 mV decade−0 at high over! potentials were observed[ Singh et al[ ð19Ł\ reported a Tafel slope ranging between ½42н57 mV decade−0 at low overpotentials and between ½099н039 mV decade−0 at high over! potentials[ Such values were obtained for the OER in 0 M KOH solution at 14>C on thin _lms of Co2O3 applied to nickel substrates[ In the case of electrodes that showed only a Tafel slope "fresh samples A and B\ and aged electrodes C and F#\ it
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may be supposed that even at low overpotential a high surface coverage by hydroxyl ions is achieved[ LaNiO2 electrodes were studied in order to compare the performance of Co2O3 and Li!doped Co2O3 oxides\ because of the known good electrocatalytic properties of perovskite!type oxides towards the OER ð6\ 7\ 07Ł[ The results obtained in this work con_rmed that also in our case the perovskite!type oxides may be considered very good electrocatalysts for OER[ Cyclic voltammograms were performed before and after electrochemical ageing[ Before ageing\ the highest current densities were found on samples that preserved their electrocatalytic performance after ageing "electrodes A\ D\ E\ F and G#[ On Co2O3 and Li!doped Co2O3 elec! trodes the current density values at overpotentials close to 9[2 V ranged between ca 1Ð03 mA cm−1[ After the ageing the current density values at such potential tend to increase on all the samples[ Also the LaNiO2 electrode showed similar values of current density "½2 mA cm−1#\ which increased after the electrochemical ageing[ As examples of the typical behaviour obtained from samples which maintain their electrocatalytic activity\ the comparison of the CV on samples A\ D and G before and after the ageing are displayed in Fig[ 5[ From the present work and some previous results ð01Ł it was evident that the oxide preparation technique plays a fundamental role in obtaining electrocatalytic mixed oxides endowed with good electrocatalytic performance for the OER[ The comparison of the main electrochemical par! ameters also obtained from other stable electrodes pre! pared by di}erent methods is referred to in Table 3[ The data reported for Co2O3 and Li!doped Co2O3 coated electrodes and other di}erently prepared NiÐCo coated oxide anodes show that the thermal decompo! sition of painted precursor salts generally led to the best electrocatalytic performance[ Te~on!bonded oxides obtained by hydroxide coprecipitation again supplied excellent results[ Other preparation methods were demonstrated to work well depending on the oxide type[ The decreasing of the electrocatalytic activity displayed by Co2O3 and Li!doped Co2O3 after the ageing was not encountered when NiCo1O3 coated electrodes were studied[ However\ these arguments strongly depend on both the electrochemistry of the oxide type and on the way the Te~on!bonded electrodes are made^ the practical par! ameters "oxide morphology\ Te~on content\ etc[# play a relevant role in the _nal result[
3[ Conclusions This study con_rms that the electrocatalytic activity of Te~on!bonded electrodes toward OER as well as the
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Fig[ 5[ Cyclic voltammograms for fresh and aged electrodes A\ D and G in 04 wt) NaOH solution at T 59>C "sweep rate 4 mV s−0#[ "Ð Ðfresh electrodes\ Ð ÐŽÐ Ðaged electrodes#[
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Table 3 Experimental current density values in the range of oxygen evolution "hO1 9[44 V# and oxygen overpotential values "at i 09 mA cm−1# obtained for the OER on mixed oxide coated electrodes from di}erent methods i "mA cm−1# Samples
Preparation method
hO1 "V#
Fresh
Aged
Fresh
Aged
NiCo1O3 Co2O3 Co2O3¦Li LaNiO2
Thermal decomposition
0[17 4[49 4[04 45[25
2[95 61[03 18[57 039[25
9[304 9[475 9[463 9[231
9[204 9[249 9[379 9[297
NiCo1O3
Oxalate coprecipitation
1[29
1[84
9[337
9[031
Co2O3¦Li
Evaporation
06[60
07[14
9[339
9[399
NiCo1O3 Co2O3¦Li
Hydroxide coprecipitation
00[85 23[85
07[99 001[03
9[435 9[327
9[237 9[143
morphological aspect of the oxides are greatly a}ected by the preparation techniques[ Nevertheless XRD analysis of the di}erently prepared powders displayed lattice parameters consistent with Co2O3 formation[ Two Tafel slopes were detected in the anodic region\ this is in agreement with the most probable OER mech! anism[ However\ the electrode performance generally decayed when the electrochemical ageing was performed[ A comparison with some electrochemical data obtained on other Te~on!bonded oxides under similar conditions is reported and discussed[ From these results coprecipitation of the mixed hydroxides seems to guaran! tee the best reliability when Te~on!bonded oxide elec! trodes are prepared[
Acknowledgements Financial support from the MURST "39) Research Funds from {Elettrocatalisi ed Elettrosintesi| Group# is gratefully acknowledged[
References ð0Ł Gennero de Chialvo MR\ Chialvo AC[ Electrochim Acta 0889^24Ð1]326[
ð1Ł Singh RN\ Koenig JF\ Poillerat G\ Chartier P[ J Elec! trochem Soc 0889^026Ð4]0397[ ð2Ł Hall DE[ J Electrochem Soc 0874^021Ð1]30C[ ð3Ł Tseung ACC\ Jasem S[ Electrochim Acta 0866^11]20[ ð4Ł Schrebler Guzman RS\ Vilche JR\ Arvia AJ[ Corr Science 0867^07]654[ ð5Ł Lu PWT\ Srinivasan S[ J App Electrochem 0868^8]158[ ð6Ł Bockris JO|M\ Otagawa T\ J Electrochem Soc 0873^020Ð 1]189[ ð7Ł Singh RN\ Bahadur L\ Singh SP\ Chartier P\ Poillerat G[ J Appl Electrochem 0883^13]038[ ð8Ł Hall DE\ J Electrochem Soc 0870^017] 639[ ð09Ł Willems H\ Kobussen AGC\ De Wit JHW\ Broers GHJ[ J Electroanal Chem 0873^066]116[ ð00Ł Davidson R\ Kissel G\ Srinivasan S[ J Electroanal Chem 0871^021]018[ ð01Ł Bocca C\ Barbucci A\ Delucchi M\ Cerisola G[ Int J Hydro! gen Energy "in press#[ ð02Ł Rasiyah P\ Tseung ACC[ J Electrochem Soc 0872^029Ð 01]1273[ ð03Ł Daghetti A\ Lodi G\ Trasatti S[ Mater Chem Phys 0872^7]0[ ð04Ł Rasiyah P\ Tseung ACC[ J Electrochem Soc 0872^029Ð 1]254[ ð05Ł Bocca C\ Barbucci A\ Cerisola A[ Int J Hydrogen Energy 0887^12Ð3]136[ ð06Ł Schumacher LC\ Holzhueter IB\ Hill JR Dignam MJ[ Elec! trochim Acta 0889^24Ð5]864[ ð07Ł Trasatti S[ Electrodes of conductive metallic oxides\ Else! vier Scienti_c Publishing Co[\ Amsterdam\ 0879[ ð08Ł Trasatti S[ Electrochim Acta 0873^18Ð00^0492[ ð19Ł Singh SP\ Samuel S\ Tiwari SK\ Singh RN[ Int J Hydrogen Energy 0885^10Ð2]060[
International Journal of Hydrogen Energy 13 "0888# 588Ð696
Oxygen evolution on Co2O3 and Li!doped Co2O3 coated electrodes in an alkaline solution C[ Boccaa\ \ G[ Cerisola\a E[ Magnone\b A[ Barbuccia a
Institute of Chemistry\ Faculty of En`ineerin`\ University of Genova\ P[ le J[ F[ Kennedy 0\ 05018 Genova\ Italy b INFM and Dept[ of Chemistry and Industrial Chemistry\ University of Genova\ Genova\ Italy
Abstract The oxygen evolution reaction "OER# was investigated at 59>C in 04 wt) NaOH solution on Co2O3\ Li!doped Co2O3 and perovskite!type oxide "LaNiO2# electrodes[ Cobalt and Li!doped cobalt oxides were obtained by di}erent preparation techniques and were applied to chemically!pickled nickel plates[ The obtained phases were structurally characterized by X!ray powder di}raction[ The electrodes were subjected to electrochemical measurements\ before and after elec! trochemical ageing "10 days\ 049 mA cm−1#[ The correlation between the coating preparations and their electrocatalytic activity was investigated[ The e.ciency of the studied electrodes seems to depend on the preparation method[ Two Tafel slopes for the OER were observed in most cases[ This behaviour was interpreted on the basis of a change in the valence state of the surface oxide[ Þ 0888 International Association for Hydrogen Energy[ Published by Elsevier Science Ltd[ All rights reserved[
Nomenclature b Tafel slope ðmV dec−0Ł E Electrode potential "V vs SCE# Eeq Equilibrium potential "V vs SCE# i Current density ðmA cm−1Ł i9 Exchange current density ðA cm−1Ł hO1 Oxygen overpotential ðVŁ
0[ Introduction Electrochemical oxygen evolution is one of the most important processes in several electrochemical devices "water electrolysers\ secondary metal!air batteries\ fuel cells\ chloroalkali cells\ etc[#[ Oxygen evolution reaction "OER# is a very interesting process because of its high overvoltage in alkaline solu! tions ð0\ 1Ł[ In a commercial water electrolyser\ nickel anodes have a very high oxygen evolution overpotential\ equal to about 399 mV ð2Ł[ In order to obtain a major anode e.ciency and a greater stability\ new materials
Corresponding author
have been studied\ especially those based on mixed metal oxides ð3Ð7Ł[ The spinel!type NiCo1O3 ð8Ð01Ł and Co2O3 ð0Ð1Ł elec! trodes were studied by several authors and they showed good e.ciency and long term performance[ Moreover\ they have a lower cost than RuO1!based anodes and a good corrosion stability ð1\ 02\ 03Ł[ In our previous work ð01Ł the OER was investigated on Te~on bonded!NiCo1O3 electrodes prepared by di}erent techniques\ in order to show the in~uence of the prep! aration methodology on the electrocatalytic properties of these oxides[ In this study it was observed that Te~on bonded!electrocatalysts prepared via oxalate and hydroxide coprecipitation techniques showed a remark! able activity towards oxygen evolution[ The usual ther! mal decomposition technique lets us produce NiCo1O3 oxides with very good electrocatalytic properties\ too[ In the present study the OER was investigated at 59>C in 04 wt) NaOH solution on both Co2O3 and Li!doped Co2O3 electrodes[ The latter were studied since it has been demonstrated ð04Ł that Li!doping increased electrical conductivity and electrocatalytic properties of Co2O3 oxides[ Since perovskite!type oxides demonstrated good elec! trocatalytic and stability characteristics for the OER ð6\
9259!2088:88:,19[99 Þ 0888 International Association for Hydrogen Energy[ Published by Elsevier Science Ltd[ All rights reserved PII] S 9 2 5 9 ! 2 0 8 8 " 8 7 # 9 9 0 1 9 ! 6
699
C[ Bocca et al[ : International Journal of Hydrogen Energy 13 "0888# 588Ð696
1[ Experimental
trodes was carried out by thermal decomposition\ "G#[ The La!Ni nitrate aqueous solution "9[0 M# was brushed over the nickel plates\ dried at 009>C for 29 min and thermally decomposed at 299>C for 0 h and 699>C for 2 h[ The obtained coverage was about 6 mg cm−1 for all the electrodes[
1[0[ Electrodes preparation
1[1[ X!ray diffraction and S[E[M[ characterisation
Co2O3\ Li!doped Co2O3 and LaNiO2 oxides were applied to chemically pickled commercial nickel plates\ because this surface _nish permits to obtain anodic sub! strates with good electrocatalytic properties for the OER and good stability\ as previously reported ð01\ 05Ł\ as well as good adhesion of the Te~on!bonded oxides[ All the reagents were of analytic reagent grade purity\ from Aldrich Chemicals[ Cobalt oxide coated electrodes were obtained by the thermal decomposition technique "A#[ The cobalt nitrate aqueous solution "9[0 M# was brushed over the nickel plates "a small amount of ethanol was added#\ _red at 299>C for 29 min and then at 599>C for 09 h[ Te~on bonded!cobalt oxide coated electrodes were also prepared "B#[ In this case the powder obtained by the thermal decomposition of the cobalt nitrate was mixed with a Te~on water dispersion and brushed on the nickel substrate[ The electrodes were dried at 009>C for 29 min and cured at 299>C for 0 h[ Ten at) Li!doped Co2O3 anodes were prepared from cobalt nitrate and lithium nitrate dissolved in distilled water "9[0 M#[ Coprecipitation of Li!Co mixed oxalate\ "C#\ and of Li!Co mixed hydroxide\ "D#\ was performed[ Hot ammonium oxalate solution and potassium hydrox! ide were used to precipitate the Li!Co mixed oxalate and hydroxide respectively[ The precipitates were vacuum _l! tered\ _red at 299>C "29 min# and then at 599>C "09 h#[ A lithium!cobalt nitrate aqueous solution was used for the evaporation technique\ "E#[ The evaporation was made at around 89>C[ The _nal stage of water evap! oration was carried out at 094>C and the dried precipitate was thermally decomposed "see above#[ The powders obtained by methods "C#\ "D# and "E# were mixed with a Te~on water dispersion[ Several cata! lyst:Te~on combinations were investigated[ The best one was a catalyst:Te~on ratio equal to 2 ] 0 "wt)#\ which permits the achievement of the necessary adhesion to nickel substrates\ with the minimum amount of Te~on[ Te~on!bonded Li!Co coated electrodes were submitted to thermal curing as in the case of electrodes B[ Li!doped Co2O3 coated electrodes were also prepared by thermal decomposition "F#[ The preparation of such electrodes was performed brushing the Li!Co nitrate aqueous solution over the nickel substrate "see electrodes A preparation#[ The preparation of the perovskite oxide coated elec!
The structure of the powder oxides\ at room tempera! ture\ was determined by a PHILIPS PW 0609 X!ray powder di}ractometer using CuKa radiation "Ni!_lter^ E 39 kV and I 39 mA#[ For accurate determination of lattice parameters\ Si was used as internal standard[ Bragg peaks with 1u values between 19>Ð79> were used for calculating the lattice parameters by a least squares _tting program[ Surface morphology was observed by a Scanning Elec! tron Microscope equipped with Energy Dispersive X!ray Analyzer\ EDX\ "Link System Stereoscan 199\ Cam! bridge Instruments#[
7Ł\ LaNiO2 electrodes were prepared and studied in our work\ in order to have a comparison of the elec! trochemical properties of Co2O3 and Li!doped Co2O3 electrodes[
1[2[ Electrochemical testin` The investigation of the electrochemical properties of the electrodes was carried out by D[C[ electrochemical methods such as potentiodynamic curves and cyclic vol! tammetry[ The experimental details were reported in pre! vious works ð01\ 05Ł[ All the measurements were made at 59>C[ In order to obtain information about the stability of the various electrocatalytic coatings under operating con! ditions\ the electrodes were submitted to galvanostatic electrochemical ageing for 10 days "anodic current 049 mA cm−1\ T 59>C#[ After this treatment\ the above mentioned electrochemical measurements were run again on the aged samples[
2[ Results and discussion 2[0[ X!ray diffraction and S[E[M[ characterization The X!ray powder di}raction revealed the formation of single!phase products for all the powders[ Weak extra peaks from second impurities were especially detected on the LaNiO2 sample[ A similar impurity problem was reported by Bockris et al[ ð6Ł[ The crystallographic space group for all the samples was Fd2m\ as revealed by the indexed X!ray data[ Typical X!ray powder patterns for the Co2O3 sample in com! parison with XRD pattern simulations\ generated using the F2dm unit cell of cubic Co2O3\ are shown in curves "a# and "b# respectively of Fig[ 0[ The model provided a good agreement between observed and calculated
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Fig[ 0[ X!ray powder di}raction pattern] "a# sample B^ "b# Co2O3 lattice "F2dm#\ simulation of theoretical pattern[
pro_les[ The lattice parameters of the di}erent spinel samples are listed in Table 0[ However\ the morphological characterization revealed that important di}erences in the powder aspect are related to the preparation technique of the oxides[ A comparison of two oxide powders is displayed in Fig[ 1aÐ b[ Lath!like structures were detected in Li!doped Co2O3 prepared by oxalate coprecipitation "Fig[ 1a#^ these oxide morphologies were not revealed when the evaporation technique was followed "Fig[ 1b#[ In Fig[ 2\ a typical Te~on!bonded Li!doped Co2O3 electrode is shown[ The oxide particles are trapped in a porous Te~on matrix\ which allows both the faradaic process at the oxide!solu! tion interface and electron transmission through the coat! ing paths to take place[
2[1[ Electrochemical results Figures 3 and 4 refer to the current!overpotential curves at 59>C in 04 wt) NaOH of all the electrodes\ before and after electrochemical ageing respectively[ Before the ageing the polarization curves show an anodic peak at overpotential values ranging between ca 9[19Ð9[39 V "P0 6 P2#\ which is more evident on Li! doped electrodes[ This peak probably refers to the oxi! dation of the cobalt ions to higher oxidation states\ and it is followed by an area where the OER occurs[ The above mentioned anodic peak tends to disappear after the ageing[ It is worth noticing that on the perovskite it is not detected[ In the oxygen evolution region the various electrodes
Table 0 Lattice parameters of samples obtained by di}erent preparation techniques
Powder Co2O3 Co2O3¦Li Co2O3¦Li Co2O3¦Li Co2O3¦Li
Preparation method
Crystal system
Space group
Lattice constants a b c "_#
V "_2#
Thermal decomposition
Cubic Cubic Cubic Cubic Cubic
Fd2m Fd2m Fd2m Fd2m Fd2m
7[977"0# 7[980"0# 7[989"0# 7[982"0# 7[976"0#
418[97 418[56 418[36 429[95 418[77
Oxalate coprecipitation Hydroxide coprecipitation Evaporation
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Fig[ 1["a\b# Micrography of Li!doped Co2O3 powders prepared by "a# oxalate coprecipitation and "b# evaporation techniques[
exhibit di}erent values of current densities\ which could depend on di}erent active surface areas "Table 1#[ Sam! ples D and G display the higher current densities at hO1 9[44 V "ca 23[85 and 45[25 mA cm−1 respectively#[ After the electrochemical ageing an increase of current density values on electrodes A\ D\ E\ F and G is displayed[ It may be due to the above mentioned treat! ment which produces an increase of the active surface\
especially on samples A\ D and G[ On the contrary\ the electrochemical ageing seems to cause the loss of the electrocatalytic properties on electrodes B and C[ Table 1 also reports the oxygen overpotential values hO1 calculated at 09 mA cm−1 "OER region#\ as well as the equilibrium potentials Eeq of the various electrodes[ The equilibrium potentials are not similar for all the electrodes[ An increase in these was determined by elec!
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Fig[ 2[ Typical morphology of Te~on!bonded Li!doped Co2O3 electrode "oxides prepared by oxalate coprecipitation#[
Fig[ 3[ Current! overpotential potential curves for oxygen evolution on fresh electrodes in 04 wt) NaOH solution at 59>C "Ž A\ , D\ E E\ r F\ G#[ "Sweep rate 9[1 mV s−0#[
trochemical ageing[ This change may be due to the higher oxide formation on the electrode surface\ as will be described in the following[ The analysis of the experimental data reported in Table
1 demonstrates that under the same current density\ the lowest hO1 values are typical of samples D\ E and G "ranging between 9[23Ð9[33 V#[ It is noteworthy that the hO1 values decrease with Li
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Fig[ 4[ Current! overpotential curves for oxygen evolution on aged electrodes in 04 wt) NaOH solution at 59>C "Ž A\ , D\ E E\ r F\ G#[ "Sweep rate 9[1 mV s−0#[
Table 1 Experimental current density values in the range of oxygen evol! ution "hO1 9[44 V#\ working electrode equilibrium potential and overpotential values "at i 09 mA cm−1# for the OER on spinel! and perovskite!type oxide electrodes\ in 04 wt) NaOH solution at 59>C Sample A B C D E F G
Fresh Aged Fresh Aged Fresh Aged Fresh Aged Fresh Aged Fresh Aged Fresh Aged
i "mA cm−1#
Eeq "V:SCE#
hO1 "V#
4[49 61[03 1[53 !!! 5[59 !!! 23[85 001[03 06[60 07[14 4[04 18[57 45[25 039[26
−9[029 ¦9[033 −9[199 !!! ¦9[914 !!! Ð9[909 ¦9[119 −9[049 ¦9[213 −9[035 ¦9[920 ¦9[064 ¦9[091
9[475 9[249 9[612 !!! 9[321 !!! 9[327 9[143 9[339 9[399 9[463 9[379 9[231 9[297
Sample that lost its electrocatalytic properties after 10 days of ageing at 59>C[
doping[ This may be due to both an increase in electrical conductivity and a higher presence of Co2¦ ions with Li doping[ In fact\ these results cannot only be explained by
an increase of electrical conductivity\ but if all the Li¦ enter in the tetrahedral sites\ the amount of Co2¦ ions in the spinel will increase\ which is in agreement with the oxygen overpotential decrease ð04Ł[ After 10 days of elec! trochemical ageing\ only samples B and C lose their elec! trocatalytic properties for the OER\ while the other elec! trodes maintain them[ Moreover\ the oxygen overpotential decreases especially on samples A and D[ This probably suggests that the surface oxide is endowed with a higher stability and hence with better elec! trocatalytic activity than other electrodes ð06Ł[ The kinetic parameters for the OER of the various electrodes\ i[e[\ Tafel slope\ b\ and exchange current density\ i9\ are summarized in Table 2\ according to the di}erent coating preparation techniques[ The polarization curves for most of the electrodes dis! played two Tafel regions] one at low overpotentials and the other at high overpotentials[ The values of the _rst one ranged between 60Ð88 mV decade−0 and the second one between 098Ð067 mV decade−0[ The presence of two Tafel slopes may be ascribed to the transition to a higher valence state of cobalt ions in the lattice ð04Ł[ Co2O3 has the spinel structure Co1¦ðCo12¦ŁO31−\ hence the possible transitions to higher valence states are] Co1¦ : Co2¦
"0#
Co2¦ : Co3¦
"1#
C[ Bocca et al[ : International Journal of Hydrogen Energy 13 "0888# 588Ð696 Table 2 Tafel parameters of the OER on spinel! and perovskite!type oxide electrodes in 04 wt) NaOH solution at 59>C[
Sample A B C D E F G
Fresh Aged Fresh Aged Fresh Aged Fresh Aged Fresh Aged Fresh Aged Fresh Aged
b "mV dec!0#
i9 "09−4 A cm−1#
Low h
Low h
High h
034 74 078 * 73 * 87 88 60
00[60 9[68 06[43 006 * 013 067 082
* 0[08 * 4[54 1[18 9[07
069
0[74
098 75
4[55 * 7[57 08[50 09[50 0[70
040 67 81
High h
12[55 8[17
033 067
9[76 0[97
01[52 04[03
Sample that lost its electrocatalytic properties after 10 days of ageing at 59>C[
The above experimental results are consistent with the most probable mechanism of oxygen evolution ð1\ 07Ł\ which is] M¦OH− MOH¦e−
"2#
MOH¦OH− MH1O1¦e−
"3#
1 M H1O1 1 M¦1 H1O¦O1
"4#
where M is a tetravalent cobalt cationic site[ The OER on Co2O3 and Li!doped Co2O3 have been studied by several authors[ Depending on the oxide prep! aration method and experimental condition\ di}erent values for the Tafel slope "ranging between ½49н69 mV decade−0 at low overpotentials and ½099н039 mV decade−0 at high overpotentials\ depending on the tem! perature# were obtained ð08Ł[ The electrocatalytic performance of Te~on!bonded Co2O3 and Li!doped Co2O3 electrodes towards the OER were studied by Rasiyah and Tseung ð04Ł in KOH med! ium at 14>C[ Tafel slopes of ½59 mV decade−0 at low overpotentials and ½019 mV decade−0 at high over! potentials were observed[ Singh et al[ ð19Ł\ reported a Tafel slope ranging between ½42н57 mV decade−0 at low overpotentials and between ½099н039 mV decade−0 at high over! potentials[ Such values were obtained for the OER in 0 M KOH solution at 14>C on thin _lms of Co2O3 applied to nickel substrates[ In the case of electrodes that showed only a Tafel slope "fresh samples A and B\ and aged electrodes C and F#\ it
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may be supposed that even at low overpotential a high surface coverage by hydroxyl ions is achieved[ LaNiO2 electrodes were studied in order to compare the performance of Co2O3 and Li!doped Co2O3 oxides\ because of the known good electrocatalytic properties of perovskite!type oxides towards the OER ð6\ 7\ 07Ł[ The results obtained in this work con_rmed that also in our case the perovskite!type oxides may be considered very good electrocatalysts for OER[ Cyclic voltammograms were performed before and after electrochemical ageing[ Before ageing\ the highest current densities were found on samples that preserved their electrocatalytic performance after ageing "electrodes A\ D\ E\ F and G#[ On Co2O3 and Li!doped Co2O3 elec! trodes the current density values at overpotentials close to 9[2 V ranged between ca 1Ð03 mA cm−1[ After the ageing the current density values at such potential tend to increase on all the samples[ Also the LaNiO2 electrode showed similar values of current density "½2 mA cm−1#\ which increased after the electrochemical ageing[ As examples of the typical behaviour obtained from samples which maintain their electrocatalytic activity\ the comparison of the CV on samples A\ D and G before and after the ageing are displayed in Fig[ 5[ From the present work and some previous results ð01Ł it was evident that the oxide preparation technique plays a fundamental role in obtaining electrocatalytic mixed oxides endowed with good electrocatalytic performance for the OER[ The comparison of the main electrochemical par! ameters also obtained from other stable electrodes pre! pared by di}erent methods is referred to in Table 3[ The data reported for Co2O3 and Li!doped Co2O3 coated electrodes and other di}erently prepared NiÐCo coated oxide anodes show that the thermal decompo! sition of painted precursor salts generally led to the best electrocatalytic performance[ Te~on!bonded oxides obtained by hydroxide coprecipitation again supplied excellent results[ Other preparation methods were demonstrated to work well depending on the oxide type[ The decreasing of the electrocatalytic activity displayed by Co2O3 and Li!doped Co2O3 after the ageing was not encountered when NiCo1O3 coated electrodes were studied[ However\ these arguments strongly depend on both the electrochemistry of the oxide type and on the way the Te~on!bonded electrodes are made^ the practical par! ameters "oxide morphology\ Te~on content\ etc[# play a relevant role in the _nal result[
3[ Conclusions This study con_rms that the electrocatalytic activity of Te~on!bonded electrodes toward OER as well as the
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Fig[ 5[ Cyclic voltammograms for fresh and aged electrodes A\ D and G in 04 wt) NaOH solution at T 59>C "sweep rate 4 mV s−0#[ "Ð Ðfresh electrodes\ Ð ÐŽÐ Ðaged electrodes#[
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Table 3 Experimental current density values in the range of oxygen evolution "hO1 9[44 V# and oxygen overpotential values "at i 09 mA cm−1# obtained for the OER on mixed oxide coated electrodes from di}erent methods i "mA cm−1# Samples
Preparation method
hO1 "V#
Fresh
Aged
Fresh
Aged
NiCo1O3 Co2O3 Co2O3¦Li LaNiO2
Thermal decomposition
0[17 4[49 4[04 45[25
2[95 61[03 18[57 039[25
9[304 9[475 9[463 9[231
9[204 9[249 9[379 9[297
NiCo1O3
Oxalate coprecipitation
1[29
1[84
9[337
9[031
Co2O3¦Li
Evaporation
06[60
07[14
9[339
9[399
NiCo1O3 Co2O3¦Li
Hydroxide coprecipitation
00[85 23[85
07[99 001[03
9[435 9[327
9[237 9[143
morphological aspect of the oxides are greatly a}ected by the preparation techniques[ Nevertheless XRD analysis of the di}erently prepared powders displayed lattice parameters consistent with Co2O3 formation[ Two Tafel slopes were detected in the anodic region\ this is in agreement with the most probable OER mech! anism[ However\ the electrode performance generally decayed when the electrochemical ageing was performed[ A comparison with some electrochemical data obtained on other Te~on!bonded oxides under similar conditions is reported and discussed[ From these results coprecipitation of the mixed hydroxides seems to guaran! tee the best reliability when Te~on!bonded oxide elec! trodes are prepared[
Acknowledgements Financial support from the MURST "39) Research Funds from {Elettrocatalisi ed Elettrosintesi| Group# is gratefully acknowledged[
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