Study On Ti/MO₂ Anode With Intermediate Layers Of Oxide Solid Solution

The selection and preparation of acid-proof anodic materials are always achallenge in the electrochemical applications. A high performance anodicmaterials should have following characteristics: high electrical conduction,good electrocatalytic properties, long service life, corrosion resistance,availability, as well as low cost etc., but due to the strong oxidizability relatedto oxygen evolution and the corrosiveness of acid to anode, the selection ofanodic material used in sulfuric acid electrolyte is more difficult than thatused in mild condition, and it is also of high practical significance in industry.Valve metal titanium, having high mechanical strength, low density,corrosion resistance, good conductivity and low-cost, is recognized as themost suitable substrate among all the acid-proof materials, non-precioussemiconductor oxides (SnO₂, Sb₂O₄, MnO₂, etc.) are appropriate to be theactivated layer of anodes, because they have not only good electrocatalyticproperties, but also good corrosion resistance. So titanium anodes (Ti/MO₂)are very well-known in the electrochemical industry, their applications allowa long and stable operation in several important industrial fields, such as theelectrochemical synthesis and the degradation of pollutant. However, thedisadvantage is that the evolving oxygen diffuses into the substrate to formtitanium dioxide-an insulator-can cause a reduction of conductivity andweakens the binding of activated layer to the substrate, the former will detach from the titanium support, especially in a strong acid electrolyte. View fromabove, addition of oxides (SnO₂, Sb₂O₄, MnO₂, etc.) as an intermediate layerinterposed between substrate and activated layer, all form of a solid solution,provides a way to improve corrosion resistance, binding force andconductivity. Thereby problems arising from above are prevented.This paper presents an investigation on performance of Ti/MO₂ electrode.First, a novel anode with intermediate layers of multiple non-noble metaloxides was prepared by thermal decomposition, sol-gel and the combinationtechnology, the intermediate layers and activated layer were characterized,mechanism of formation of oxide solid solution was discussed, the role ofmatched solid solution in anode was proposed, the effect of crystal structure,crystal defect and oxygen vacancy on the formation and conduction of solidsolution was researched, the service life and catalytic properties of anodeswere also tested. Then the fractal geometry theory was introduced into thefield of titanium oxide electrodes, the correlation between surface fractaldimension and the electrocatalytic performance was proposed. Finally, theseanodes were applied into phenol wastewater treatment and the electrolyticreduction of sodium metaborate to sodium borohydride. The contents andconclusions are as follows:1. The metal titanium and non-noble metal semiconductor oxides (SnO₂,Sb₂O₄, MnO₂, etc.) were singled out as the substrate and the intermediatelayer respectively, the oxides (MnO₂, SnO₂ and PbO₂) are chosen as the activelayers, Ti/PbO₂, Ti/MnO₂ and Ti/SnO₂ anodes with multiple intermediatelayers were prepared by thermal decomposition, electrodepositing, sol-gel andcombination technology, above electrodes morphology, crystal phase, surfacecomposition and valence state were characterized by means of SEM, XRDand XPS, the anticipated service lives of electrodes were measured using accelerated life test in 1.0mol/L H₂SO₄ solution at a current density of 4A/cm².The electrocatalytic properties of electrodes have also been studied. Theresults are as follow:â‘ The nanometer-sized Ti/SnO₂ electrode was prepared by sol-geltechnique. The optimum doping concentration of antimony is 4% and the bestsintering temperature is 600℃. The service life is prolonged, determined bythe accelerated lifetime test, the over potential for oxygen-evolution is highbased on the kinetics test. The mechanism of the oxygen evolution reaction onthe anode was also proposed.â‘¡The Ti/PbO₂ anode with multiple intermediate layers was prepared bythermal decomposition and electrodepositing technology, the mushroomshaped activated layer (Pb₃O₄andβ-PbO₂), possessed a high surface area, leadto a good electrocatalytic property. When the rare earth elements Ce and Ywere doped into the intermediate layer, the service life, kinetic parameters ofoxygen evolution and electrocatalytic property of Ti/PbO₂ in acidic solutionwere better than lead anode.â‘¢A novel Ti/MnO₂ anode with different multiple intermediate layers andactivated layer (β-MnO₂) was prepared by thermal decomposition. The resultsindicated that the kinetic parameters a, b and overpotential are smaller thanprecious metal, the i₀ is bigger than precious metals, The activation energy ofoxygen evolution reaction on Ti/MnO₂ anode (8KJ/mol) is lower thanPtO₂(60.67KJ/mol) and Pt/MnO₂(18.83KJ/mol), the substitution mechanismof oxygen evolution is SN₁. Among all the anodes with different intermediatelayers, the Ti/SnO₂+RuO₂+MnO₂/MnO₂ appears to be an ideal material foroxygen-evolution in acid solution due to its lower overpotential.â‘£The Ti/MO₂ anode with dual SnO₂+Sb₂O₄ intermediate layers is anoptimum material used for oxygen evolution in acidic solution. The initial potentials of Ti/SnO₂+Sb₂O₄/MnO₂, Ti/SnO₂+Sb₂O₄/PbO₂ and Ti/SnO₂+Sb₂O₄ anodefor oxygen evolution in 1.0mol/L H₂SO₄ solution are 1.701V, 1.860V and1.918V respectively. So the Ti/SnO₂ and Ti/PbO₂ electrodes are applicable inthe system of anodic oxidation, the Ti/MnO₂ anode is applicable in thesystem of anodic oxygen evolution.⑤The single Sb₂O₄, the dual SnO₂+Sb₂O₄ and the multipleSnO₂+Sb₂O₄+MO₂(M=Ru,Mn) are appropriate oxide intermediate layers forTi/SnO₂, Ti/PbO₂ and Ti/MnO₂ anodes respectively. Due to the same crystalstructure, the SnO₂+Sb₂O₄ (56% Sb₂O₃ and 44% Sb₂O₅) intermediate layercan form a matched solid solution with lead, manganese coatings and titaniumdioxide, activation energy is low (13.965KJ/mol) during the formation, tinyand closely grains were uniformly distributed on the solid solution surfacewithout cracked structure, thereby effectively preventing oxygenated speciestransport toward the titanium substrate to form titanium dioxide insulatinglayer and extending anode service life. The anticipated service lives ofTi/SnO₂+Sb₂O₄/PbO₂, Ti/SnO₂+Sb₂O₄/MnO₂ and Ti/SnO₂+Sb₂O₄ anodes canreach 15.7y, 10.6y and 4.6y respectively in an industrial current density(1000A/m²). Meanwhile the conductivities of these anodes were alsoimproved because of the generation of oxygen vacancy and crystal defectsduring the formation of solid solution. Compared to a lead electrode, theconsumption of electricity by Ti/SnO₂+Sb₂O₄/PbO₂ electrode is 17% lower inthe same condition.2. Geometric factors are usually recognized to govern catalysis ofelectrode. The fractal geometry theory was firstly introduced to describe theanode surface irregularity quantitatively in this article, and the fractaldimensions of the oxide electrode surface were determined by means of boxdimension law and cyclic-voltammetry based on the correlation between different scan rate and peak current. Meanwhile the relationship betweenfractal dimension and electrocatalysis performance of oxide electrode wasalso investigated. The combination of fractal geometry theory and theelectrode electrocatalysis performance has provided a new method to theresearch of electrocatalysis. The results indicated that oxide electrode withmultiple transitional layers is porous and three-dimensional, the fractaldimension ranges from 2 to 3, the electrocatalysis performance of oxygenevolution on anode is related to surface fractal dimension, e.g., theelectrocatalytic activity of Ti/RuO₂ anode with a higher box dimension isbetter than Ti/MnO₂ and Ti/PbO₂ anodes. However, the fractal dimension ofTi/PbO₂ calculated from cyclic-voltammetry method is greater than others,which coincides well with the irregularities of surface layer. Both twomethods have their own merits, can construct the relation between surfacemorphology and catalytic activity.3.The performance of Ti/SnO₂ prepared by sol-gel method wasinvestigated in the application of phenol contained wastewater treatment.Impacts of doping antimony and calcinations temperature on phenoldegradation were studied; kinetic parameters and activated energy of phenoldegradation were measured. A same investigation applied to theTi/SnO₂+Sb₂O₄+MnO₂/PbO₂ electrode prepared by thermal decompositionand electrodepositing. Photoelectrocatalytic degeneration of phenol on theTi/MnO₂ and stainless steel/MnO₂ electrode were conducted and the removalrates were also compared. The results indicated that Ti/SnO₂+Sb₂O₄,Ti/SnO₂+Sb₂O₄/PbO₂ and Ti/SnO₂+Sb₂O₄/MnO₂ anodes are applicable tophenol contained wastewater treatment, the removal rates are 96.5%, 95.8%,and 91.5% respectively. The current efficiencies for Ti/SnO₂+Sb₂O₄, Ti/SnO₂,Ti/SnO₂+Sb₂O₄+MnO₂/PbO₂ and Ti/PbO₂ in the same condition are 73.5%, 66.0%, 62.0%, and 54.9% respectively. So the Ti/SnO₂+Sb₂O₄ with solidsolution intermediate layer is an ideal anodic materials used for phenolcontained wastewater treatment.4 Ti/SnO₂+Sb₂O₄/PbO₂, Ti/SnO₂+Sb₂O₄/MnO₂, Ti/SnO₂+Sb₂O₄ anodesand Ti/Sn cathode were applied into the realization of electrochemicalreduction sodium metaborate to sodium borohydride, i.e., the NaBO₂ wasconverted directly into borohydride by electrochemical reduction (charge), theborohydride produced by-product sodium metaborate again (discharge) whenhydrogen evolution occurs, thus in situ boron circulation was carried out. Thequantitative relationship between nickel electrode opening potential andborohydride concentration (10^-5-10^-3mol/L) was first established in this article,the relatively standard deviation is 2.22%, and the recovery ratio is 98.43%.Kinetics law of electrochemical reduction sodium metaborate to sodiumborohydride was also preliminary proposed.Summarizing the points above, we can make a conclusion that multipleintermediate layers of solid solution play an important role in the performanceof titanium based oxide anodes. Because the addition of oxides asintermediate layer can form matched solid solution, not only the anodeconductivity is improved, but also the binding force between the substrate andactivated layer is strengthened. So the service life of anode is prolonged inacid solution and their electrocatalytic ability is improved. Among all theintermediate layers mentioned above, the dual SnO₂+Sb₂O₄ is recognized asthe best in anodes.