Nanocatalysts for water electrolysis were synthesized through a triple direct current (DC) thermal plasma system, comprising three DC torches, a reactor, and a power supply. This is a simple process that does not require expensive post-treatment methods, such as cleaning, drying, and heat treatment. In this study, starting materials consisting of micronized powders were used as precursors. Cobalt boride, cobalt phosphide, and metal (Cu and Ni)-carbon nanotube (CNT) nanocomposites were synthesized from Co, B, P, Cu, Ni, and CNT. Precursors were evaporated by the high temperature generated from the DC thermal torch; these vapors were converted to nuclei via supersaturation due to decreasing temperature. After nucleation, they were converted to nanoparticles via condensation. The product size could be controlled by adjusting the plasma quenching rate.
Cobalt boride nanoparticles were synthesized by adjusting the composition and flow rate of the plasma-forming gas. The structure of the synthesized nanoparticles varied according to the gas composition. In the Ar-N2 plasma, the dissociated nitrogen reacted with the boron precursor and produced hexagonal boron nitride (h-BN) nanocages, which encapsulated cobalt boride nanoparticles. In the Ar-H2 plasma, spherical cobalt boride nanoparticles (<20 nm) were synthesized, and their size distribution was controlled by the growth time or quenching rate. This was controlled by adjusting the flow rate of the plasma-forming gas. The electrochemical performance of the synthesized cobalt boride nanoparticles was investigated. The electrochemical characteristics were determined according to the decreasing particle sizes of the products. In the oxygen evolution reaction (OER) measurement, the product achieved an overpotential of 355 mV at a current density of 10 mA/cm2 and Tafel slope of 49 mV/dec. Their performance was more efficient than that of cobalt-based catalysts reported to date. In contrast, in the hydrogen evolution reaction (HER), a high Tafel slope of 92 mV/dec was observed.
Cobalt phosphide nanoparticles were synthesized by adjusting the molar ratio of mixed powder used as the precursor; herein, mixed cobalt and phosphorus powders were used as precursors. A triple DC thermal plasma jet was obtained by using mixed Ar-N2 gas as the plasma-forming gas. When the Co:B molar ratio was 1:1, CoP and Co2P crystal phases were synthesized with similar crystallinity and spherical shapes. Products from Reactor 1 were under tens of nanometers with spherical morphology, whereas those from Reactor 3 were covered by phosphorus. Only the Co2P crystal phase existed without CoP in the Co-rich molar ratio of the mixed powder. In contrast, in the P-rich molar ratio powder, the CoP crystal phase exists without Co2P. In addition, bulk P with cobalt phosphide nanoparticles was synthesized in Reactor 3; bulk P was formed from the unreacted P vapor with Co nuclei in a rapid quenching rate environment, and the electrochemical performances showed an overpotential of 0.323 V at 10 mA/cm2 and a Tafel slope of 71.7 mV/dec for OER activity. In HER measurements, this product achieved an overpotential of -0.317 V at a current density of 10 mA/cm2 and Tafel slope of 66.7 mV/dec.
Nanocomposites such as metal nanoparticles attached to the surface of CNT were synthesized on a triple DC thermal plasma jet system with a counter gas. An experiment was carried out by adjusting the flow rate of the counter gas, which is the role of the carrier gas in the CNT. The flow rate of the counter gas was controlled from 10 to 50 L/min. In addition, the thermal flow characteristics inside the reactor were numerically analyzed using the laboratory developed DCPTUN code and ANSYS-FLUENT software. The electrochemical performance of the synthesized metal-CNT nanocomposites was investigated under optimized conditions. They achieved an overpotential of 0.328 V for the OER and -0.193 V for the HER at a current density of 10 ma/cm2. The HER showed a higher efficient Tafel slope (48.8 mV/dec) than cobalt boride and cobalt phosphide.