Electrochemical Properties of Ti/Mn2O3-Co3O4 Coating Anodes Prepared with Different Thermal Decomposition Temperatures
The anode potential in the metal electrowinning process accounts for more than 50％of the tank voltage,and reducing the overpotential of the anode is an effective way to reduce the industrial operation cost.In order to reduce the energy consumption and pro-long the service life,researchers have developed a series of anodes,among which the titanium-based dimensionally stable anodes(DSAs),which have the characteristics of dimensional stability,low pollution,and excellent electrocatalytic performance,have at-tracted wide attention.Titanium-based noble metal-coating anodes have caused a technological revolution in the chlor-alkali industry,but the high price limits their large-scale application.The transition metal oxides generally have the advantages of low cost,low toxici-ty,and good catalytic performance,which are expected to replace the existing anode materials for solution electrolysis.Ti/MnO2 and Ti/Mn2O3-coating anodes,although showing good oxygen evolution reaction(OER)electrocatalytic performance,still fall short of the noble metal-coating anodes.Co3O4 has the advantage of low cost,low toxicity and good catalytic performance,and it is proposed to im-prove OER electrocatalytic performance of Ti/MnOx coated anode by cobalt doping.At present,the preparation processes for DSAs mainly include thermal decomposition,electrodeposition and sol-gel method,among which the thermal decomposition method is sim-ple and low-cost,which is ideal for large-scale industrialized production.The temperature of thermal decomposition can change the morphology and structure of the anode as well as the composition of the physical phase,thus affecting its electrochemical performance.Cobalt-doped titanium-based manganese oxide(Ti/MnOx-Co3O4)coating anodes were prepared by thermal decomposition method un-der different temperature conditions.The physical phase composition,surface morphology,chemical state,molecular structure,and OER electrocatalytic performance of the coatings were analyzed by X-ray diffraction(XRD),scanning electron microscopy(SEM),X-ray photoelectron spectroscopy(XPS),linear sweep voltammetry(LSV),cyclic voltammetry(CV)and electrochemical impedance spectroscopy(EIS)tests,etc.XRD patterns showed that MnO2 in Ti/MnOx-Co3O4 coatings was gradually transformed to Mn2O3 with the rise of temperature,and MnO2 existed in the form of the typical"bun peak",while the characteristic peak of Mn2O3 increased in inten-sity and became narrower.The former indicated that the crystallinity of MnO2 was small and had more defects,while the latter in-creased in crystallinity,which was unfavorable to the presence of active components on the anode surface.Theoretically,Mn3+had higher oxygen precipitation activity than Mn4+,and with the increasing temperature,the increase in Mn3+improved the electrocatalytic activity of the anode.Combined with SEM,Ti/MnOx-Co3O4 coating anode surface gradually became dense,resulting in the decrease of catalytic active sites decreasing OER electrocatalytic activity.CV curves showed asymmetry and poor reversibility of Ti/MnOx-Co3O4 coating anodes,with oxidation peaks and reduction peaks occurring at 1.45～1.55 and 1.20～1.40 V,respectively,and were Co(Ⅱ)/Co(Ⅲ),Co(Ⅲ)/Co(Ⅳ),Mn(Ⅲ)/Mn(Ⅳ)and Mn(Ⅱ)/Mn(Ⅳ)redox pair conversion resulted from the combined action of oxygen and hydrogen precipitation reactions.Voltammetric charge(q*)first rose and then fell at the thermal decomposition temperature of 350 ℃,voltammetric charge value was the highest of 228.83 C·cm-2,peak current was the highest,OER overpotential was the lowest of 0.334 V,Tafel slope as the lowest of 329.82 mV·dec-1,andRctis the charge transfer resistance whose value was the lowest of 0.13 Ψ.With the temperature rising,the crystallinity of Ti/MnOx-Co3O4 coating anodes decreased,the defects on the anode surface increase making the catalytic active sites increase,so OER catalytic activity of the anode was enhanced.When the temperature exceeded 350 ℃,Ti/MnOx-Co,O4 coating anode was converted from MnO2 to Mn2O3,the crystallinity increased,the catalytic active sites decreased,and the catalytic activity of the anode decreased.In addition,the rise in the content of Mn3+in the anode could improve the catalytic activi-ty of Ti/MnOx-Co3O4 coating anodes,but the coating became denser,which restrained the catalytic activity sites of the anode,thus re-ducing the catalytic activity of the anode.The content of Mn3+and Mn4+in the coating and the change of catalytic activity sites caused by the anode surface morphology factors jointly affected the electrochemical performance of the anode,so that OER catalytic activity of the anode increased and then decreased with increasing temperature,and the activity of the anode prepared at 350 ℃ was the best.Ad-ditionally,by comparing voltammetric charge values of Ti/MnO2,Ti/Mn2O3 and Ti/MnOx-Co,O4 prepared at 350 ℃ coating anodes,the latter was 28.32 and 8.32 times higher than the former,respectively,and thus,cobalt doping significantly improved OER electrocatalyt-ic activity of the coated anodes.XPS results showed that the relative content of Co2+/(Co3++Co2+)in all Ti/MnO,-Co3O4 coating anodes in the layers was about 0.5,which was consistent with the ratio of Co3+to Co2+in Co3O4,and matched with XRD patterns,suggesting that Co existed mainly in the form of Co3O4 in Ti/MnOx-Co3O4 coating anodes.The relative contents of Mn3+/(Mn3++Mn4+)and adsorbed oxy-gen Oa/(Ol +O,+Oa)in Ti/MnOx-Co3O4 coating anodes firstly increased and then decreased,which could be seen that the anode prepared at 350 ℃ had the largest percentage of Mn3+,and this anode has more adsorbed oxygen,which was favorable for the catalytic reaction.
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