Lead Dioxide Based Oxide-Surfactant Composites

Abstract

Great attention is paid to various electrochemical methods of coatings synthesizing in connection with the simplicity of their implementation, low cost equipment and the ability to control the composition and properties of the materials obtained by changing the electrolysis regimes and the composition of electrolytes [1, 2]. Composite materials based on lead dioxide, containing surfactants are of considerable interest for research, since while maintaining the basic properties of PbO2, the composition, physico-chemical properties, and electrocatalytic activity of such materials can vary widely. At the same time, there is a lack of information [4, 5] regarding the effect of surfactants on the regularities of electrodeposition of materials based on lead dioxide, especially the physico-chemical properties of the oxides obtained and their electrocatalytic activity. In this connection, herein, we report the regularities of synthesizing, the effect of deposition conditions on the composition, physico-chemical properties of composite PbO2-sodium laureth sulfate (SLES) materials obtained from nitrate electrolytes. The addition of surfactants has a significant impact on the kinetics of lead dioxide electrodeposition, without changing the mechanism of the process. In this way, at low anodic polarizations second electron transfer stage is rate determining, whereas at high anodic polarizations such stage is diffusion transport of the lead ions to the electrode surface. The presence of sodium laureth sulfate in the deposition electrolyte leads to a slight inhibition of the deposition of lead dioxide. The content of the additive in lead dioxide is determined mainly by its adsorption on the oxide. An increase in the adsorption of additive due to an increase in its concentration in the solution and an increase in electrostatic attraction (an increase in the anodic current density and the acid content in the deposition electrolyte) leads to the enrichment of the composite material with organic substance. Anionic surfactants, SLES in particular, are included in the growing lead dioxide. The content of organic substance in the oxide can vary from 3.4 to 12.5 w.%, forming a surfactant-oxide composite coating. The changing in deposition conditions (temperature, pH and anodic current density) allows one to control the content of surfactant in the resulting oxide. Based on the data obtained, a colloidal-electrochemical mechanism for the formation of composites was proposed, according to which the process proceeds in several stages: the formation of colloidal oxide particles in the near-electrode zone; adsorption of anionic polyelectrolyte on colloidal particles; electrophoretic delivery of oxide particles with adsorbed surfactant to the electrode surface; sedimentation and crystallization of the composite at the interface. The surface morphology and chemical composition of obtained materials were investigated by SEM, EDAX, phase composition and texture was determined by X-Ray powder diffraction. The morphology and structure of composite materials differs significantly from lead dioxide. With an increase in the additive content in the composite, there is a transition from large-grained deposits to materials with submicron and nano-sized crystals. The electocatalytic activity of the materials involved was investigated in respect to oxygen evolution reaction and the oxidation of 4-chlorophenol. The calculated value of Tafel slope is 229 on nonmodified PbO2, while on 10.2 wt.% SLES-PbO2 it is 178 mV/dec. Slope decreasing in the case of PbO2-SLES denotes the increasing of lead dioxide infilling degree by oxygen-containing radicals, which are directly involved in the electrochemical stages of oxygen evolution. The most likely, surfactants on the electrode surface play the role of mediators in the process of oxygen evolution and inhibit the surface blocking by sulfate ions on such materials. The processes of electrooxidation of 4-chlorophenol on lead dioxide and PbO2-based composite materials proceed qualitatively in the same way and differ only in the rate. Thus, lead dioxide based surfactant-oxide composite coatings are of great interest because of high electrocatalytic activity in respect to oxygen transfer reactions. References R. Vargas, C. Borras, D. Mendez, J. Mostany, B.R. Scharifker, J. Solid State Electrochem. 20, 875 (2016).M. Krishna, E. J. Fraser, R. G. A. Wills, F. C. Walsh, J. Energy Storage, 15, 69 (2018).X. Li, H. Xu, W. Yan, J. Alloy Compd., 718, 386 (2017).A. B. Velichenko, T. V. Luk'yanenko, N. V. Nikolenko, R. Amadelli, F. I. Danilov, Russ. J. Electrochem., 43, 118 (2007).