Adding a Spinodal Decomposition Retarder: An Approach to Improving Electrochemical Properties of Ruthenium–Tin Complex Oxides

Abstract

Adding a certain amount of preformed SnO 2 nano-particles in the thermal co-decomposition synthesis of Ru 0.45 Sn 0.55 O 2 significantly improves its electrochemical properties. By optimizing the amount of the added SnO 2 , the total voltammetric charge for Ru 0.45 Sn 0.55 O 2 prepared at 500 • C has been increased by one fold. Improvement in true electrocatalytic activity of the oxide anode for oxygen evolution has been confirmed in sulfuric acid solutions. Electrochemical characterizations and microstructure analyses reveal that the added SnO 2 retards/suppresses the spinodal decomposition of Ru 0.45 Sn 0.55 O 2 , which in turn results in an increased active surface area and volumetric percentage of the active phases of the oxide, and therefore, improves the electrochemical properties. This additive-induced retardation of spinodal decomposition can be interpreted from thermodynamics and kinetics of spinodal decomposition, providing a general approach to controlling the microstructures and properties of various complex oxides. Life time analyses showed that adding a small amount of SnO 2 as the spinodal decomposition retarder does not degrade the predicted service life-time of the electrode.Ruthenium oxide (RuO 2 ) coating is a highly important anode material in electrochemical industry. 1-4 RuO 2 -based oxides have been widely used in various industrial electrochemical processes including water splitting, chlorine evolution and organics oxidation. They are also being investigated for supercapacitor applications. 5,6 One popular strategy that has been used to improve/modify electrochemical performance of a RuO 2 -based oxide is to mix/dope RuO 2 with other metal oxide(s) such as TiO 2 , ZrO 2 , SnO 2 , NbO 2 , SiO 2 , etc. [7][8][9][10][11][12][13][14][15][16][17][18][19] The "mixed oxide" strategy has the great advantages in reduced consumption of Ru (reduced cost) and scalability with industrial applications via conventional coating processes. In particular, Ru-Sn-O ternary oxides exhibit almost the best electrochemical properties among all RuO 2 -based complex oxide formulations, and therefore, have been studied extensively. 1,6,8,12,17,18 One fundamental challenge for optimization of Ru-Sn-O has been the very limited understanding of phase separation and microstructure evolution of Ru-Sn-O. It is known that Ru-Sn-O prepared by a conventional sol-gel process or a thermal co-decomposition process of mixture precursors is, by nature, predominantly composed of simple physical mixtures of the component oxides, RuO 2 and SnO 2 . We attributed this phenomenon to the large driving force for spinodal decomposition of RuSn-O, 19,20 meaning a virtual solid solution phase, if instantaneously existing, would spontaneously decompose into a Ru-rich phase and a Sn-rich phase, with their compositions being very close to pure RuO 2 and SnO 2 , respectively. In thermodynamics, spinodal decomposition, being contrast to a regular nucleation-growth mechanism, is defined as a fast spontaneous phase separation mechanism that is acc...