(Digital Presentation) New Trends in the Formation of Composites with Desired Functional Properties

Abstract

The main requirements for anode materials can be divided into two groups: i) overall operating (satisfactory electrical conductivity, availability and reasonable cost, long service life, etc.); ii) specific, due to particular target process (high electrocatalytic activity and selectivity) [1]. The lack of universal anode material requires the creation of new approaches to managing the functional properties of the catalyst. In our opinion, the optimal strategy is to choose a base material that meets the general operational requirements, followed by the creation of an active layer on its basis in accordance with the requirements of particular target processes.The regularities of formation of composites of the TiOx-Pt and TiOx-Pt-Pd systems and their physicochemical properties were studied.Composites were obtained in two stages. First, a thin non-continuous layer of Pt or successive layers of Pt-Pd was applied to the substrate by electrodeposition. Nitrite electrolyte for platinating and phosphate palladation were used. The amount of Pt and Pd on the surface ranged from 0.1 to 2.0 mg cm-2. Then the resulting material was heat treated in an air atmosphere. The surface layers of the composites were formed due to the oxidation of the substrate and the encapsulation of platinum and palladium particles into titanium oxides during this stage. The chemical composition of the bulk and surface of the electrocatalytic coatings obtained was determined by XPS, SEM and EDAX.XPS showed that Pt exists in the metallic state, while Ti is in the form of both metal and intermetallic TixPty, with low Pt content, and in oxide form in Ti(IV) compounds on the surface of non-heat treated electrode As one can see, the coating is not uniform, and the platinum content is significantly reduced from the surface to the substrate (from 92 to 67 at.%), that indicates the local nature of the coating and confirmed by SEM data. As thermal treatment contributes to a more uniform distribution of Pt on the surface, as well as its diffusion in combination with oxygen in the coating bulk, it leads to the formation of a composite of titanium-platinum oxides.The semiconductor properties of such anodes are due to the presence of TiO2, which is an n-type semiconductor. As the temperature increases, the potentials of the flat zones and the carrier concentrations increase, which may be related to the interaction of platinum (electron donor) with TiO2. The use of titanium suboxides as a substrate and increasing the processing temperature also leads to an increase in the potential of flat zones. However, this reduces the number of carriers due to the increase in the degree of stoichiometry of titanium suboxides due to oxygen uptake. The electrodes obtained at 583 K do not correspond to the general dependence. It is likely that the new Ti-O phase formed under such conditions makes an individual contribution to the semiconductor properties of such materials.Palladation of platinum titanium does not lead to a significant change in surface morphology. Platinum, according to EDAX, is fairly evenly distributed on the surface of titanium, which is not observed for palladium. Unlike platinum, the maximum amount of palladium is observed in the protruding parts of the surface, and the minimum - in the hollows, due to the uneven distribution of current on the surface of Ti/Pt during the electrodeposition of palladium. The most noticeable change in morphology occurs at a surface Pd content of 1.0 mg cm-2. The oxygen concentration increases on the surface during heat treatment of Ti/Pt-Pd, there is oxidation of Pd, the surface becomes blue. The distribution of oxygen on the surface coincides with palladium, which indicates the formation of an oxide layer. The XRD method detected the PdO phase on the surface of heat-treated samples with a surface palladium content of 0.2 to 1.0 mg cm-2. At 2θ = 33.8°, the PdO-specific reflex is manifested, the area of which increases with increasing of surface palladium content.Thus, the use of the combined electrochemical-pyrolytic method of obtaining composite anode materials allows to create materials that differ significantly in their structural parameters, composition and physicochemical properties. The main factors influencing the properties of such materials are the nature of the substrate, the amount and nature of platinum and palladium in the surface layer, the temperature and duration of heat treatment in the air. Developed anode materials are characterized by long service life. References Walsh, F.C. Modern developments in electrodes for electrochemical technology and the role of surface finishing / F.C. Walsh // Trans. Inst. Met. Finish. – 2019. – Vol. 97(1). – P. 28-42