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Study on Electrode Materials of Ru-la Based Oxide Coating

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Tutor: LiQingFen
School: Harbin Engineering University
Course: Materials Science
Keywords: electrocatalytic activity,lanthanum,ruthenium oxide,stability,selectivity,unifor
CLC: TB34
Type: PhD thesis
Year:  2012
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Abstract:
The Tisupported oxide electrodes £¨DSA¡ðR£©applied for electrolytic seawater and otherfields often have some problems such as low electrocatalytic activity and high cost. Thesurface modification by adding low cost active element to anodes is frontier topics in this field.Rubased oxides and rare earth La were investigated using the material analytic methods,including SEM/EDS¡¢AFM¡¢XRD¡¢XPS and TG-DSC, and the electrochemical testingtechnology, including Tafel curve, open circuit potential, cyclic voltammetry, differentialcapacitance and accelerated life test. The effect of La on electrocatalytic activity of Ru oxidecoatings in NaCl solution was studied systematically, with the aim to determining therelationship of the La electro-catalysis and electronic and geometric factors. The influence ofthe double structure on the surface activity, and those of Ir and Sn on the electro-catalyticactivity, stability, selectivity and corrosion resistance were observed. The optimization designof compositions of RuIrSnLa oxide coatings was investigated by uniform design combinedwith partial least square £¨PLS£© and grey relational degree analysis£¨GRD£©.The electrocatalytic properties of the Ti/Ru£¨1-x£©LaxO2£¨0¡Üx¡Ü0.7£© oxide coatings wereinvestigated by different characterization methods, including the cyclic voltammetry, surfaceroughness, porosity, exchange current density, polarization resistance, apparentelectrocatalytic activity and true electrocatalytic activity. The results show that the rutheniumoxide coatings by adding La species exhibit high voltammetric charges. The surface activitycan be effectively enhanced due to increase of surface roughness and porosity. In particular,voltammetric charges reach a maximum value at the nominal La content of30mol%.The surface morphology and structure of Ti/Ru£¨1-x£©LaxO2£¨0¡Üx¡Ü0.7£© coatings wereinvestigated. The results show that cracked-mud morphology is not the main cause ofenhancing actice surface. Three reasons of the increase of apparent electrocatalytic activity byadditing La to Ru oxide coatings are: grain refinement, the existence of amorphous structure,and the formation of high non-stoichiometric oxides.The structure of electric double layer of Ti/Ru£¨1-x£©LaxO2coatings in3.5%NaCl solutionwas investigated by electrochemical impedance spectroscopy £¨EIS£©. The results show that theequivalent circuit of the coatings is Rs(RctQdl) in the absence of the chlorine evolution reaction,and that is Rs£¨RfQf£©(RctWQdl) in the chlorine evolution reaction. The change trend of thepseudocapacitance of oxide film at the double layer agrees with that of voltammetric charges with the addition of La species. Base on EIS, the electric conductivity of coatingoxide films dramatic decreases at La nominal content large than70mol%. A typical featurewith a Warburg impedance appears at low-frequency region, and impedances increase withincreasing La content.The specific adsorption, the effect of La on the ClER and the kinetic mechanism wereinvestigated. Results show that the coating surface exhibits significantly specific adsorption ofCl ions in NaCl neutral solution, which has effect on the kinetics of the chlorine evolutionprocess, resulting in an increase of Tafel slope and a decrease of reaction order. Comparativeanalysis based on experimental data shows that chlorine evolution reaction of the oxidecoatings accord with recombination controlled mechanisms.The electrode properties such as electrical catalysis, stabality, selectiity and corrosionresistance for RuO2¡¢Ru-La¡¢Ru-Ir-La and Ru-Ir-Sn-La oxide coatings were comparativelystudied. And the effects of La, Ir and Sn on the RuO2based coatings were investigated.Results show that La can effectively enhance electrocatalytic activity, but decrease stabalityand service life. Ir based on Ru-La oxide coating can increase the stabality and service life,but has little influence on catalysic activity and selectivity. The addition of Sn based onRu-Ir-La oxide coatings can increase electrical catalysis, stabality and selectivity, resulting inbest synergistic effect of all components of coating electrodes.The prescriptions of quaternary metallic oxide coatings of the Ti/RuIrSnLa wereobtained by uniform design. The effect of component on preperties was investigated by partialleast square £¨PLS£© and grey relational degree analysis£¨GRD£©. Results show that Ir plays animportant role to increase service life of Ti/RuIrSnLa oxide coatings. La exerts the strongestinfluence on properties including electrocatalytic properties, chlorine-evolution potential andpotential difference between oxygen-evolution and chlorine-evolution. And of all ingredients,Sn has major effect on oxygen-evolution potential. According to PLS regression model, theresults of uniform design were optimized, and the optimum formula is Ir0.66Ru0.02Sn0.29La0.03.
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