Mixed oxide electrode

Angelinetta C., Trasatti S., Atanososka Lj. D., Atanasoski R.T., Surface properties of Ru02 + Ir02 mixed oxide electrodes, /. Electroanal. Chem., 214(1-2), 535-546,1986. 

Angelinetta, C., Trasatti, S., Atanasoska, L. D., Minevski, Z. S. and Atanasoski, R. T. (1989), Effect of preparation on the surface and electrocatalytic properties of Ru02 + Ir02 mixed oxide electrodes. Mater. Chem. Phys., 22(1-2) 231-247. 

Oxygen evolution on Ru02 and Ru02/Ti02 mixed oxide electrodes is characterized by a relatively low Tafel slope, 40 mV decade 1, at low current densities. For plots of log j vs. E, an increase in slope is observed in these plots at high current densities. The deviation from Tafel linearity in this current density region has been related to, for example, uncorrected iR drops [227], i.e. within the film or between the oxide and Ti substrate. 

Kondintsev, I.M., and Trasatti. S. 1994. Electrocatalysis of Hj evolution on RuOj -I- IrOj mixed oxide electrodes. Electrochimica Acta 39, 1803-1808. 

Xiong, W. and Kale, G.M. (2006) Novel high-selectivity NO2 sensor incorporating mixed-oxide electrode. Sens. Actuators B, 114, 101-8. 

At mixed oxide electrodes, free orbitals of the metals from the platinum group (Pt, Ir, Ru) are able to catalyze direct reactions such as the formation of chlorine. First, chloride is bound to the surface of the electrode oxidation happens in the second step. 

Electrochemical reaction behavior of organic compounds is difficult to assess. Research works sometimes show that intermediates may be formed that are more toxic than the initial system. The formation of chlorate [31], bromate on mixed oxide electrodes [32], peroxodisulfate, and H2O2 [19] is well known as well as the formation of DPDs such as THMs and AOX. 

Xiong W, Kale G (2005) Novel high-selectivity N02 sensor incorporating mixed-oxide electrode. Sens Actuators B Chem 114 101-108... 

De Faria LA, Boodts JFC, Trasatti S. Electrocatalytic properties of Ru + Ti + Ce mixed oxide electrodes for the CI2 evolution reaction. Electrochim Acta... 

Santana MHP, De Faria LA. Oxygen and chlorine evolution on RUO2+Ti02 + Ce02 + Nb205 mixed oxide electrodes. Electrochim Acta 2006 51 3578-85. 

Kitiyanan, A., S. Ngamsinlapasathian, and S. Pavasupree, 2005. The preparation and characterization of nanostructured Ti02-Zr02 mixed oxide electrode for efficient dye-sensitized solar cells. J Solid State Chem 178 1044-48. 

Appreciable interest was stirred by the sucessful use of nonmetallic catalysts such as oxides and organic metal complexes in electrochemical reactions. From 1968 on, work on the development of electrocatalysts on the basis of the mixed oxides of titanium and ruthenium led to the fabrication of active, low-wear electrodes for anodic chlorine evolution which under the designation dimensionally stable anodes (DSA) became a workhorse of the chlorine industry. 

To the contrary, mnlticomponent nonmetallic systems such as mixed oxides often provide the possibility for a smooth or discontinuous variation of electrophysical parameters, and thns for some adjustment of their catalytic properties. In a number of cases, one can do without expensive platinum catalysts, instead using nonmetallic catalysts. Serious research into the properties of nonmetallic catalytic electrodes was initiated in the 1960s in connection with broader efforts to realize various kinds of fuel cells. 

Of considerable interest was the demonstration that metalloporphyrins and the like can be used as nonmetallic catalysts in electrochemical reactions, nourishing hopes that in the future, expensive platinum catalysts could be replaced. Starting in 1968, dimensionally stable electrodes with a catalyst prepared from the mixed oxides of titanium and ruthenium found widespread use in the chlorine industry. 

In the following chapter examples of XPS investigations of practical electrode materials will be presented. Most of these examples originate from research on advanced solid polymer electrolyte cells performed in the author s laboratory concerning the performance of Ru/Ir mixed oxide anode and cathode catalysts for 02 and H2 evolution. In addition the application of XPS investigations in other important fields of electrochemistry like metal underpotential deposition on Pt and oxide formation on noble metals will be discussed. 

For Cl2 or 02 evolution the stability of ruthenium based electrodes is not sufficient on a technical scale. Therefore the possibility of stabilizing the ruthenium oxide without losing too much of its outstanding catalytic performance was investigated by many groups. For the Cl2 process, mixed oxides with valve metals like Ti or Ta were found to exhibit enhanced stability (see Section 3.1), while in the case of the 02 evolution process in solid polymer electrolyte cells for H2 production a mixed Ru/Ir oxide proved to be the best candidate [68, 80]. 

In order to understand the observed shift in oxidation potentials and the stabilization mechanism two possible explanations were forwarded by Kotz and Stucki [83], Either a direct electronic interaction of the two oxide components via formation of a common 4-band, involving possible charge transfer, gives rise to an electrode with new homogeneous properties or an indirect interaction between Ru and Ir sites and the electrolyte phase via surface dipoles creates improved surface properties. These two models will certainly be difficult to distinguish. As is demonstrated in Fig. 25, XPS valence band spectroscopy could give some evidence for the formation of a common 4-band in the mixed oxides prepared by reactive sputtering [83],... 

The results of the above mentioned study on mixed oxides prepared by thermal decomposition [84] are not in contradiction to the results obtained on reactively sputtered electrodes. A premise for common d-band formation is the formation of a solid solution with homogeneous properties which is probably not obtained during thermal decomposition. Indeed the authors find a trend towards the behaviour of the sputtered electrodes when homogeneity is improved by changing the solvent for the starting compounds. 

Stabilization of Ru based oxides by valve metal oxides has not been studied in such detail using photoelectron spectroscopy. The most common compositions, however, with relatively high valve metal content, are not in favor of formation of a solid solution. Studies of the phase formation in Ru/Ti mixed oxides has shown [49] that homogeneous solutions are formed for compositions with Ru < 2% or Ru > 98% (see Section 3.1.1). Therefore electrodes with other compositions are better described as physical mixtures and the electrochemical behaviour is most likely that of a linear superposition of the single components. It has to be considered, however, that the investigations performed by Triggs [49] concern thermodynamic equilibrium conditions. If, by means of the preparation procedure, thermodynamic equilibrium is... 

Depending on the fabrication techniques and deposition parameters, the pH sensitive slope of IrOx electrodes varies from near-Nemstian (about 59 mV/pH) to super-Nemstian (about 70mV/pH or higher). Since the compounds in the oxide layers are possibly mixed in stoichiometry and oxidation states, most reported iridium oxide reactions use x, y in the chemical formulas, such as lr203 xH20 and IrOx(OH)y. Such mixed oxidation states in IrOx compounds may induce more H+ ion transfer per electron, which has been attributed to causing super-Nerstian pH responses [41],... 

Figure 24. Models illustrating the source of chemical capacitance for thin film mixed conducting electrodes, (a) Oxygen reduction/oxidation is limited by absorption/de-sorption at the gas-exposed surface, (b) Oxygen reduction/ oxidation is limited by ambipolar diffusion of 0 through the mixed conducting film. The characteristic time constant for these two physical situations is different (as shown) but involves the same chemical capacitance Cl, as explained in the text.

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