Phase transfer of oxide particles using hydroxamic acid derivatives and application for supercapacitors

Particle extraction through liquid-liquid interface (PELLI) methods are developed for the colloidal processing of ZnO, CeO2 and FeOOH particles. The direct transfer of the particles from an aqueous synthesis medium to a device processing medium allows for reduced nanoparticle agglomeration, because drying and re-dispersion processes are avoided. Different PELLI methods are developed, which are based on the use of chelating hydroxamates molecules, such as octanohydroxamic acid (OCHA) and bufexamac (BFXM) as extractors. OCHA and BFXM show strong adsorption on the ZnO, CeO2 and FeOOH particles due to chelating bonding mechanisms and allow for efficient particle extraction from water to organic solvents. Bottom–up and top-down methods are used, depending on the density of the organic solvents. OCHA shows improved extraction of particles, compared to BFXM due to the difference in the chemical structure. The unique feature of hydroxamates is related to their pH-dependent solubility and their strong adsorption on particles at high pH, despite the electrostatic repulsion of the negatively charged particles and dissociated hydroxamic acids. It is found that OCHA can be used as a capping agent for particle synthesis and an extractor for their extraction to an organic phase. The advantages of PELLI methods are demonstrated by the fabrication and testing of FeOOH-carbon nanotube electrodes for supercapacitors, which show enhanced performance and high capacitance at high active mass loading. The enhanced electrochemical performance is achieved due to reduced particle agglomeration, which allows improved particle mixing with conductive carbon nanotubes. The capacitive properties are investigated at different charge-discharge rates. The impedance spectroscopy data shows dependence of capacitance on electrode potential and provides an insight into the strategies for further improvement of electrode performance.