Synthesis of hydrous ruthenium oxide nanoparticles in sub- and supercritical water and their capacitive properties

Hydrous ruthenium oxide (RuO₂ · nH₂O) nanoparticles with various particle sizes and water contents were synthesized in sub- and supercritical water in a very short reaction time of 15 min. The particle size, surface area, morphology, crystalline structure, and electrochemical properties were analyzed and compared with those of commercial RuO₂ particles. Ultrafine spherical RuO₂ · 0.6H₂O nanoparticles with an average size of 4.2 nm were produced in subcritical water (250°C, 300 bar), while larger and more highly crystalline rod-shaped RuO₂ · (0.3-0.5)H₂O particles were produced in supercritical water (400°C, 300 bar). The use of NaOH under the supercritical water conditions resulted in a decrease in particle size. The hydrous RuO₂ nanoparticles synthesized in subcritical water exhibited a much higher specific capacitance (255 F g^-1) at a scan rate of 10 mV s^-1 than those synthesized in supercritical water (77 F g^-1) and commercial RuO₂ (8 F g^-1).