Electrochemical behaviors of anode materials and their performance for bauxite desulfurization☆

Yingnan Hu ,Aijing Lü,Mingyong Wang ,Dong Wang ,Junhao Liu ,Zhi Wang ,Xuzhong Gong ,*

1 National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology,Key Laboratory of Green Processand Engineering,Institute of Process Engineering,Chinese Academy of Sciences,Beijing 100190,China

2 University of Chinese Academy of Sciences,Beijing 100049,China

3 State Key Laboratory of Advanced Metallurgy,University of Science and Technology Beijing,Beijing 100083,China

Keywords:Corrosion resistance High sulfur bauxite Electrolysis desulfurization Electrode materials

ABSTRACT Pyrite inside bauxite could be oxidized into soluble S-containing ions by electrolysis,and thus achieving bauxite desulfurization by using fi ltration.How ever,S-containing ions in electrolyte had some corrosion effects on electrode,especially for anode.In this work,six kinds of traditional materials w ere selected as anode,and their corrosion behaviors w ere examined by using electrochemistry characterization.Tafel and CV curves from simulating electrolyte suggested that their corrosion potentials w ere in the follow ing order:Ni＞C＞SS＞Fe＞Cu＞Pb-Ag.Asexpected,the desulfurization ratio and cell voltage from bauxite electrolysiswere in the following order respectively:Cu＞Ni＞Fe＞SS＞C＞Pb-Ag and Ni＞Fe＞SS＞Cu＞C＞Pb-Ag.Finally,Niwasproposed a kind of excellent electrode material for bauxite desulfurization from electrolysis.

1.Introduction

Electrochemical desulfurization w as a practicable w ay to convert pyrite inside fossil fuel into soluble sulfate,more importantly,the organic sulfur can also be removed simultaneously[1].The method has already been introduced into the desulfurization of other high sulfur-containing minerals(e.g.high-sulfur coal).The main advantage of electrochemical desulfurization is mild operating conditions(i.e.low temperatures and pressures)w ith high desulfurization ratio[2].Additionally,the oxidation desulfurization on anode could also improve the current ef fi ciency for hydrogen evolution on cathode,w hich provides a possible method of high ef fi ciency hydrogen production[3,4].

Both organic sulfur and inorganic sulfur in coal w ere oxidized into sulfatesfrom electrolysistogether[5,6].How ever,our previousresearch indicated that inorganic sulfur in coal w aseasily removed by electrolysis[7],w hile the removal ef fi ciency of organic sulfur by coal electrolysis was not high,because of complex organic sulfur structure.It w as w orth noting that S-containing phase in bauxite w as mainly FeS2.Therefore,electrolysis w as highly suitable for bauxite desulfurization[8-10]due to the fact that the sulfur in pyrite phase could be oxidized easily into various anions in alkaline solution,such as S0,SO32-,S2O32-and SO42-[11-15].Fortunately,alkaline leaching w as the fi rst step in bauxite processing,and the leaching mixture was just like bauxitew ater-slurry w hich was directly ready for electrolysis.

For direct oxidation desulfurization or indirect oxidation desulfurization[16],the performance of anode materials directly impacted the current ef fi ciency and desulfurization ef fi ciency.On the one hand,there are some sulfur oxides from water electrolysis on the surface of anode,resulting in the high desulfurization ratio[9].On the other hand,these oxidants also corroded electrode,especially for anode.More importantly,there are some S-containing ions during FeS2electrolysis oxidation[11,12],w hich could corrode electrode obviously[17].What's more,the sulfur ions can in fl uence FeS2oxidation during bauxite electrolysis as w ell.

Previousresearchesrevealed that S2-resulted from FeS2electrolysis could react w ith Cu to form Cu2S[18],S2O32-or SO32-also reacted w ith electrode material to form sul fi des.Therefore,as long as these sulfur ionsexisted in solution,the electrodematerialsw ould be inevitably corroded.Additionally,these sulfur ions had low oxidation over-potential in NaOH solution[19],and thus leading to the production of SO42-.They could be oxidized or reduced on electrode easily,and sulfur ion w ith low valences could easily corrode electrode[20],resulting in several consumptions of anode.Obviously,many materials used as anode could be dissolved directly during the electrolysis process,and then entered into electrolyte[21].Consequently,the selection of electrode materialsw assigni fi cant to improve thedesulfurization ef fi ciency and cut costs.

For electrode materials,the catalytic activity of metal oxide w as better than that of the noble metal[22,23].Some noble metal and itsoxide anodeshave high catalytic activitiesfor ore or organic compound oxidation[24,25],but they were hardly adopted in bauxite electrolysisdesulfurization because of the high cost.

Due to the strong corrosion resistance to Cl2and HClO,the graphite anode showed a high current ef fi ciency of desulfurization in NaCl solution[7].Nickel and iron are good electrode materials,w hich had high catalytic activity and good corrosion resistant[26-28].Stainless steel had better corrosion resistance for the reason that the Cr(OH)3layer formed and suppressed the stainless steel corrosion continuously during the electrolysis process[29,30].While,lead-silver alloy is a kind of stable binary alloy anode[31,32],silver could reduce the potential,and the catalytic oxidant such as PbO2could be produced from electrolysis.

In this w ork,NaOH solution w as used as electrolyte,and six kindsof conventional materials w ere selected as anode,including iron(Fe),stainless steel(SS),nickel(Ni),copper(Cu),lead-silver alloy(Pb-Ag)and graphite(C).To reveal the reactivity and corrosion-resistant property of electrode materials,their electrochemical behaviors on bauxite electrolysis and desulfurization were studied.

2.Experimental

2.1.Samples

High sulfur bauxite from Guizhou province of China w as used as sample.The bauxite composition w as analyzed by X-ray fl uorescence(XRF,AXIOS,Holland),asshown in Table 1.Bauxitephaseswerecharacterized by X-ray diffraction(XRD,X'Pert PRO MPD,Holland)[33].Aluminum in bauxite mainly exists in the form of diaspore,and the sulfur mainly exists in the form of the pyrite.In addition to titanium dioxide and silica impurities,there are also potassium manganite and other impurities.The total sulfur content in bauxite is 3.95 wt%,which w as analyzed by a C-Sanalyzer(LECOCS-344,USA).

Table 1 Composition analysis of bauxite

2.2.Electrolysisapparatus

The electrolysis apparatus w as described in detail elsew here[33].Iron(99.9%Fe),stainless steel(316L),nickel(99.9%),copper(99.9%),lead-silver alloys(99%Pb,1%Ag)and graphite(99.99%)w ere used as the electrode(anode and cathode)materials.The electrode active area w as alw ays 2×2 cm2.Electrolysis conditions w ere as follow s:0.6 A of the current intensity,90°Cof the electrolysis temperature,400 ml of the electrolyte volume,25 g·L-1of the bauxite w ater slurry concentration,500 r·min-1of the stirring speed,1.0 mol·L-1of NaOHconcentration and 4 h of the electrolysis time.Bauxite phase after electrolysis w as analyzed by XRD,and sulfur content in bauxite after electrolysis w as analyzed by a C-S analyzer(LECOCS-344,USA).

The Tafel,CV,ACimpedance,and polarization curves were all measured by a three-electrode system on the electrochemical w orkstation(CHI604B,Shanghai ChenHua instrument Company),and the w orking electrodes(1×1 cm2effective areas)w ere iron,stainless steel,nickel,copper and graphite electrodes respectively.The counter electrode w as platinum electrode(2×2 cm2effective areas).The reference electrode w as a saturated calomel electrode(SCE).The electrolyte w as 1 mol·L-1NaOH solution,w ith a slurry concentration of 25 g·L-1,stirring speed of 500 r·min-1,scanning rate of 30 m V·s-1and temperatureof 90°C.In the meanw hile,to relieve bauxite particle interference,electrolyte composition for the bauxite electrolysis desulfurization process w as also simulated by using 1 mol·L-1NaOH w ith 0.1 mol·L-1Na2S2O3(FeS2w as oxidized into S2O32-by reactive oxygen from w ater electrolysisat fi rst,and then S2O32-w asoxidized into SO42-).Other conditions were controlled to remain unchanged(NaOH concentration of 1 mol·L-1,pulp concentration of 25 g·L-1,scan rate of 30 m V·s-1),and then theanodic polarization curvesof nickel electrodewerecharacterized at 30 °C,50 °C,70 °Cand 90 °C.

2.3.Desulfurization ratio calculation

The desulfurization ratio of bauxite electrolysis can be calculated by Eq.(1),

w here,D isthedesulfurization ratio,%;S1isthesulfur content in bauxite before electrolysis,(mass fraction),wt%;and S2is the sulfur content in bauxite after electrolysis,(mass fraction),w t%.Compared with the calculation equation of desulfurization ratio in the reference[8],the desulfurization ratio from Eq.(1)w as low er,since the mass of bauxite after electrolysis decreased slightly(FeS2changed into Fe(OH)3).However,Eq.(1)w as more signi fi cant for the application of bauxite after desulfurization,because the application process paid more attention to sulfur content of bauxite after desulfurization.

2.4.Electrode reaction kinetics calculation

In the electrochemical reaction,the reaction rate is expressed with the current density,and the relationship betw een the reaction rate and activation energy can be shown by the Arrhenius equation:

w here,j is the current density(A·cm-2),Eais the activation energy(J·mol-1),T is the temperature(K),R is the gas constant(8.314 J·mol-1·K-1),and f is the pre-exponential factor.Take logarithm on both sides for Eq.(2):

There isa linear relationship between lg j and 1/T in the above equation,and itsslope is-Ea/2.303R.According to the polarization curvesat different temperatures,taking diagram of lg j-1/T,the slope of the line w as obtained,and then the AAE(Ea)could be calculated through the slope of the line.

According to the bauxite electrolysis desulfurization process[8-10],the electrode reaction kinetics w as essentially the oxygen evolution reaction kinetics w ith bauxite addition.Since bauxite electrolysis desulfurization was an indirect oxidation process[23],the high oxygen evolution potential leads to the increase in reactive oxygen amount,improving the desulfurization ratio.

3.Results and Discussion

3.1.Electrochemistry behaviors of electrode in S-containing solution

In the process of electrolytic desulfurization,thiosulfate is considered to be stable and the most abundant sulfur-containing species present in the solution,so thiosulfate isselected as a simulated solution.To examine the corrosion behaviors of electrode in S-containing solution,1 mol·L-1NaOH and 0.1 mol·L-1Na2S2O3mixed solution w as simulated the S-containing electrolyte during the bauxite electrolysis process.The corrosion potential could be obtained by Tafel curves,w hich w as an ef fi cient method for examining electrode corrosion behavior.

From Fig.1 and Table 2,the order of corrosion potential(Ecorr)is Ni＞C＞SS＞Fe＞Cu＞Pb-Ag.The corrosion potential(Ecorr)of nickel was the highest while that of lead-silver alloy was the lowest,indicating that the corrosion of lead-silver alloy w as much easier than that of nickel under the examined circumstance.For S-containing NaOH solution,copper and lead-silver alloy could beeasily consumed through chemical and electrochemical corrosion[18].Cu intended to react w ith sulfur ion,such as S-1,S2O32-and SO32-,forming Cu2Sand Cu2Owith defected crystalline structure[34].The blue electrolyte w as seen after electrolysis reaction when copper w as used as anode due to the generation of Cu SO4solution.Nickel had a strong corrosion resistance because of hydroxide occurrence on the surface,such as Ni(OH)2[35].Fe(OH)2and Fe(OH)3w ere formed on thesurfaceof iron at thenegative potential[36],therefore iron anode had good corrosion resistance under electrolysis conditions.As expected,Cr(OH)3w as produced on the surface of stainless steel,contributing to the good corrosion resistance[29,30].In the Tafel curves,the interval with strong polarization has a linear relationship,different materials have different self-etching potentials.The anode Tafel slope(?a)and the cathode Tafel slope(?c)indicate the reaction rate,the larger the slope,the faster the corresponding reaction rate.From Table 1,it can be seen that iron and stainless steel have better corrosion resistance ability,copper has a higher anode slope w ith poor corrosion resistance.

Fig.1.Tafel curves of different electrode materials.(1 mol·L-1 NaOH concentration,0.1 mol·L-1 Na2S2O3 concentration,25 °Cof temperature.)

Table 2 Potential parameters of different anode materials

Fig.2.ACimpedance curves of different electrode materials.(a)Nyquist plots;(b)bode impedance;(c)bod e phase.(1 mol·L-1 NaOH concentration,0.1 mol·L-1 Na2S2O3 concentration,25°Cof temperature.)

Table 3 Resistance parameters of different anode materials

From Fig.2(a)and Table3,theorder of electrodereaction resistances(Rct)w as Ni＞C＞Pb-Ag＞SS＞Fe＞Cu,w hereas the order of solution resistance(Rs)for six electrode materials w as Ni＞SS＞Fe＞C＞Cu＞Pb-Ag.Through the fi tted equivalent circuit,the dynamic process and mechanism contained in the electrode system can be analyzed.Additionally,as shown in Fig.2(b),at low frequency,the polarization impedance of six electrode materials w as Ni＞SS＞C＞Cu＞Pb-Ag＞Fe,indicating that Niand SShad good corrosion resistance,w hilethecorrosion resistance of Fe w as very subtle.How ever,it is seen that the phase maximum is increased in the case of Fe in Fig.2(c),w hich denotes a decrease in the corrosion rate[37].From Fig.2(c),Fe and Ni could form the constant phase element(CPE)[29],and there is a peak on bode phase for six electrodes,and thus an electric equivalent circuit w ith CPE and Rpw as proposed,as show n in Fig.2(a).The reaction resistances for nickel and stainlesssteel anode were bigger than others,indicating that they had better corrosion resistance activity.

3.2.Corrosion difference of anode in S-containing solution

The electrochemistry behavior of electrode corrosion is attributed to their surface reaction and structure.How ever,CV curves from the S-containing electrolyte with different electrodes could describe the reaction on the surface of electrodes,w hich could analyze the reaction and production.

Asshow n in Fig.3(a),there isarelatively low and fl at oxidation peak at 0.3-0.45 V on the CV curve for nickel electrode,and the current density increases gradually w ith increasing time.The reaction resulted from the adsorption of oxygen and anodic oxide layer(Ni(OH)2)on the electrode surface[17].The oxidation current increased rapidly to above 0.45 V,being accompanied by a large number of oxygen gas evolution.Our previousstudies[17,38]show ed that these positive peaks at 0.3-0.45 V belonged to sulfur ion oxidation,w hile the negative peaks 0.2-0.35 V should come from the reduced sulfur ions.How ever,sulfur ion concentration w as very low in electrolyte in this work,therefore these peaks should belong to Ni(OH)2formation and reduction.Conversely,thereare no obviousoxidation peakson Fe electrode during potential positive direction process at 0.3-0.45 V,as show n in Fig.3(b).Furthermore,there are several Fe oxidation and reaction peaks at the range of-0.6 V to-1.3 V[39].There are a few oxidation peaks for iron and nickel electrode materials,and their formation hindered the electrode oxidation process continuously.

How ever,there are some big oxidation peaks for copper and Pb-Ag electrode materials,as show n in Fig.3(b).According to references[18,34],some oxides have been formed on the surface of electrode.And the current density of oxidation peak w as big,indicating that they w ere oxidized easily in the solution.The generation of CuOand Cu2O could not hinder the copper oxidation process continuously,therefore the copper w as corroded seriously[29,34].PbOand PbO2were formed on the surface of Pb-Ag electrode,follow ed by reacting w ith NaOH,thus continuously falling off from the electrode surface.It w as noted that PbO2has high over-potential for oxygen evolution in H2SO4solution,therefore it w as an ideal anode material for oxidation degradation of organic compounds in waste w ater[40].

As show n in Fig.3(c),Cr(OH)3(0.2 V-0.3 V)and Fe(OH)3(-0.6 V)peaks were found on the CV curve for stainless steel[29,30],indicating thenature of corrosion resistance for stainlesssteel.There are no formed oxidesasprotection layer on thesurfaceof graphite,thereforethe graphite layer structure would gradually be corroded by the electrolysis.

Fig.4 show sthe XRDpattern of several anodes after and before electrolysis.There are some compounds on the surface of copper and iron anodes after electrolysis,such as Cu2O,Cu O,Cu S,Fe(OH)3and FeS.

Fig.3.CV curves of different electrode materials.(1 mol·L-1 NaOH concentration,0.1 mol·L-1 Na2S2O3 concentration,25 °Cof temp erature)100 m V·s-1 of scan rate,20 times.

These compounds on the surface of electrodes veri fi ed the analysis from CV curves for iron and copper.How ever,the surfaces of nickel and stainless iron anode w ere not changed obviously after the electrolysis,indicating that nickel and stainlessiron w ere corroded dif fi cultly in S-containing electrolyte.

Fig.4.XRDpattern of different anode materials after and before electrolysis.(a)Copper,2 h;(b)nickel,4 h;(c)iron,(d)stainlessiron,8 h.(1 mol·L-1 NaOH concentration,0.1 mol·L-1 Na2S2O3 concentration,area of 2×2 cm2,25°Cof temperature.)

Fig.5 show s the SEM and EDSof iron(a),copper(b)and nickel(c)electrodes after electrolysis at a scale of 500 μm and 10 μm respectively.From Fig.5(a),it can beseen that the surface of theiron electrode washeavily corroded,somedendritic iron oxidesadhered to thesurface of the iron and then w ere identi fi ed as iron hydroxide later.There are also many sul fi des and iron oxides.From Fig.5(b),it can be seen that the surface of the copper electrode had fallen off,resulting in three different degrees of corrosion morphology.Combined w ith XRD analysis,copper sul fi de,copper oxide and other substances w ere generated on the surface of the copper electrode.Therefore,oxygen and sulfur elements w ere evenly distributed on the surface of the electrode in the EDS.From Fig.5(c),there is no obvious corrosion morphology on thesurface of thenickel electrode,but only some tiny corrosiveparticles are found in the fractional electrode,which bene fi tsfrom the formation of hydroxides.

3.3.Effects of anode materials on bauxite electrolysis

Besidescorrosion behavior of electrode material in electrolyte,effect of anode on bauxite electrolysis w as also very signi fi cant to select the applicable anode,including desulfurization ratio and cell voltage for bauxite electrolysis.

Fig.6 show s effects of electrode materials on desulfurization ratio.When copper was used as anode,the highest desulfurization ratio of 64.97%w as obtained.The desulfurization ratio of bauxite electrolysis w ith lead-silver alloy as anode w as the low est,only reaching 33.11%.When nickel and iron were used as anode,the desulfurization ratio reached 54.82%and 53.45%,respectively.

Anode activity considerably depended on the oxygen evolution potential w hich also determined the lifetime and concentration of oxidant(·OH)from w ater electrolysis[40].The high concentration of oxidant w ould normally give rise to the high desulfurization ratio.Electrolysis desulfurization ratios using copper,nickel and iron as anode were higher than those using other anode materials.Due to the dissolution of Cu or Cu(OH)2in thissolution,copper anodew ould beconsumed obviously.

Fig.7 show s that the order of cell voltage w as Ni＞Fe＞SS＞Cu＞C＞Pb-Ag.The higher the cell voltage w as,the higher energy consumption w as.Compared w ith other electrode materials,the cell voltage of bauxite electrolysis w ith lead-silver alloy w as the low est,because there w ere no oxide fi lms found on the surface of electrode.It w as noted that the result above could also be veri fi ed by data in Fig.2 and Table 3.Some oxides on the surface of electrode have suppressed the electrode corrosion,meanwhile resulting in the increase in cell voltage.Due to the low corrosion resistance,lead-silver alloy and copper w ere not ideal materials to be used as anode for bauxite electrolysis.Our researchesimplied that the anodematerial corrosion w aslargely caused by the low valence sulfur ion oxidation[17,38],such as S-1,S2O32-and SO32-,especially at high temperatures.Various sulfur ions circulated between anode and cathode have corroded the anode continuously.

Fig.5.SEM and EDSof iron(a),copper(b)and nickel(c)electrodes after electrolysis.

Fig.6.Effectsof electrodematerialson desulfurization ratio.(0.6 Aof the current intensity,90 °Cof the electrolysis temperature,400 ml of the electrolyte volume,25 g·L-1 of the BWSconcentration,500 r·min-1 of the stirring speed,1.0 mol·L-1 of the concentration of NaOH and 4 h of the electrolysis time.)

Fig.7.Change of cell voltageof different electrodematerials.(0.6 Aof thecurrent intensity,90 °Cof the electrolysis temperature,400 ml of the electrolyte volume,25 g·L-1 of the BWSconcentration,500 r·min-1 of the stirring speed,1.0 mol·L-1 of the concentration of NaOH and 4 h of the electrolysis time.)

3.4.Polarization curves and electrode reaction kinetics

Fig.8 show s the polarization curves of different anode materials.As can be seen in Fig.8(a),at the same current density,the over-potential order of the electrode material was Ni＜SS＜Cu＜C＜Fe.The low overpotential for anode could easily generate the reactive oxygen.How ever,the high over-potential for anode could result in the increase in reactive oxygen.Because the indirect oxidation w asthe dominant reaction[41],the reactive oxygen generation was the critical step for the electrolysis desulfurization[42].From Fig.8(a),the low oxygen evolution potential hasresulted in·OHformation easily on thesurface of nickel anode,leading to the organic pollutant oxidation rather than electrochemistry combustion[43].The electrochemistry combustion took place more easily at high oxygen evolution potential due to the increased amount and residence time of·OH[39].Therefore,iron w as also suitable to be used as anode materials for bauxite electrolysis desulfurization as the high oxygen evolution potential facilitated the formation of Fe(OH)3.

Fig.8.Anodic polarization curves of different electrode materials.(NaOH concentration 1 mol·L-1,pulp concentration 25 g·L-1,a sw eep speed of 30 mV·s-1,90 °C of temperature.)

Fig.8(b)show s a peak of nickel hydroxide at 0.32 Vfor nickel anode,which wasthe main reason for the nickel to show an excellent corrosion resistancein NaOHsolution containingsulfur.For stainlesssteel electrode,there is a peak at 0.38 V at w hich the chromium hydroxide(Cr(OH)3)might form[29,30],thus suppressing itscorrosion continuously.

Fig.9 shows the polarization curves of anode at different temperatures.Under the same potential condition,thecurrent density increased w ith increasing temperature due to the increase in reaction rate.The diffusion and migration of ions in solution improved the active oxygen generation rate.Additionally,the current density of nickel anode was higher than that of iron anode under the same condition,indicating that nickel anode had a better performance as anode.

As can be seen from Fig.10,the AAEof nickel and iron electrode reaction w as higher than 40 kJ·mol-1w hen the potential w as low er than 0.8 V,indicating the oxygen evolution reaction for bauxite electrolysis w as controlled by electrochemical reaction[44-48].At low potential,there is low electric quantity on the surface of anode,while OH-concentration w as high on the surface of anode,so electrode reaction w as slow,w hich w as a control step for bauxite electrolysis.And AAEof iron electrode reaction w as higher than that of nickel electrode reaction.When the electrode potential w as higher than 0.8 V,the AAE of nickel and iron electrode reaction w as low er than 40 kJ·mol-1,indicatingtheoxygen evolution reaction for bauxiteelectrolysisbelonged to the diffusion control region.At high potential,OH-w as exhausted fast,and diffusion rate of OH-in bulk solution w as slow,so the diffusion process of OH-controlled the w ater electrolysis.Our previous results demonstrated that the bauxite electrolysis desulfurization w as controlled by electrode reaction[8,9].Because bauxite or waste water electrolysis w as an indirect oxidation process[15],the desulfurization process w as controlled by the reactive oxygen generation directly,but rather the sulfur phase oxidation.With increasing electrode potential,the AAEof the electrode reaction reduced gradually.Meanw hile,the electrolysis desulfurization w as dependent on the diffusion of liquid phase and solid particle[8,10].The decrease in the AAEresulted in the increase in the reaction rate.Additionally,our previous research suggested that the oxidation rate of S2-to SO42-w as higher on the nickel anodethan on thestainlesssteel anode[16,32].Asexpected,thebauxite electrolysis was the oxidation process transferring S2-to SO42-.Therefore,the result indicated that nickel anode w asa preferred candidate as anode material for bauxite electrolysis desulfurization.

4.Conclusions

·Based on the Tafel analysis,the order of their corrosion potentials(Ecorr)w as slightly changed to the follow ing:Ni＞C＞SS＞Fe＞Cu＞Pb-Ag.

·The order of electrode reaction resistances(Rct)was Ni＞C＞Pb-Ag＞SS＞Fe＞Cu,w hile the order of solution resistance(Rs)for the six examined electrode materials w as Ni＞SS＞Fe＞C＞Cu＞Pb-Ag.

·For the given experimental conditions,the desulfurization ratio and cell voltage of the electrode materials w ere in the follow ing order respectively:Cu＞Ni＞Fe＞SS＞C＞Pb-Ag and Ni＞Fe＞SS＞Cu＞C＞Pb-Ag.

·The AAE of nickel and iron electrode reaction w as higher than 40 kJ·mol-1when the potential was low er than 0.8 V,w hich means that the oxygen evolution reaction for BWSelectrolysis w as controlled by electrochemical reaction.How ever,w hen the electrode potential w as higher than 0.8 V,the AAEof nickel and iron electrode reaction turned into the diffusion control region(AAE ＜ 40 kJ·mol-1).Compared to iron,nickel anode activity w as high for the oxygen evolution process.