The experimentation conducted was directed, on the one hand, to the synthesis of an anodic electrode material that could be coupled with the Na0.44MnO2 (NMO) cathode material developed in previous years at the RSE laboratories, and on the other hand, to the subsequent hybridization of the synthesized anode material by employing different types of carbonaceous materials in order to obtain an anode electrode material that could operate in an aqueous environment and thus obtain a first hybrid electrochemical cell.
The hybridization process of the solvo-thermal synthesized NaTi2(PO4)3 (NTP) anode material was conducted initially using different types of graphitic materials: Active Carbon (AC), Conductive Carbon (CC), Carbon Nanofibers (CNF), and carbon (C) coming from the decomposition of organic material, such as sucrose. Synthesis tests were carried out using two different techniques: solid-state reaction and hydrothermal synthesis. The hydrothermal tests showed the formation of the species of interest with high purity. For the hybridization process, it was verified that the most promising material was obtained using a hydrothermal synthesis process in the presence of carbon nanotubes (CNTs) made at Politecnico di Milano laboratories. The initial minimum amount of CNTs for obtaining a material with good electrochemical properties was at least 0.2 grams, corresponding to a content of about 20% at the end of the synthesis. The test conditions were modified so as to obtain a material with a synthesis yield as high as possible in terms of active material and with a carbonaceous material content such that the obtained electrode material acquired good electronic conductivity. The different materials were characterized with the help of the X-ray diffraction (XRD) technique, which allows the qualitative and semi-quantitative determination of the different crystalline species formed during the synthesis process. Determination of the electronic conductivity of the synthesized materials was performed with the measuring cell developed during the previous year. This measurement makes it possible to verify whether the hybridized material has an electronic conductivity comparable to the solid mixtures made for the preparation of the expanses, namely mixing between active material and polytetrafluorethylene (PTFE) binder for making the electrode. The CNT species can hardly be determined via X-ray diffractometry; for this reason, the content of “C” was correctly determined via the TGA (thermal gravimetric analysis) technique, which relies on a process of thermal decomposition (combustion) of the graphitic component (C). Therefore, the quantitative analyses performed by X-ray diffraction were corrected according to the “C” content determined by said technique.
The hybridized cathode electrode material NMO was coupled with hybridized NTP to make a complete single cell. In this experimental phase, the two electrodes were placed within the same solution without any separators, as the electrochemical tests were to be performed in the presence of a reference electrode that would allow the system to monitor the behavior of the two electrode materials during the charge and discharge processes to which they were subjected. The results led to the determination of the Ragone diagram, thanks to which the entire system can be characterized.