Characterization of Ultrasonic Cavitation Plasma to Synthesize Carbon Catalysts for Oxygen Reduction Reaction

Carbon materials exhibit great potential as catalysts for oxygen reduction reaction (ORR). In fuel cells or air batteries, precious-metals are employed as catalysts for ORR. However, carbon materials are expected to replace precious-metals. In this study, carbon nanoparticles were synthesized by plasma in an organic solvent. This plasma was generated by the solution plasma method, which is to generate plasma by applying a bipolar high voltage to electrodes in solvent. By using a homogenizer probe as one of the electrodes, microbubbles and flow were introduced between the electrodes. We call a solution plasma generated with cavitation as a cavitation plasma.

In cavitation plasma in xylene, two types of plasma were observed. One was a blue small plasma, while the other was an orange large plasma. The occurrence of these plasmas was determined by electrode gap length, cavitation intensity, and voltage settings. To characterize the two types of plasmas, optical emission spectroscopy (OES) measurements were conducted through a glass reactor using a spectrometer. The emissions from these plasmas exhibited distinct features, particularly emissions originating from C2 and CH, which are essential for carbon material synthesis from xylene. Additionally, emissions from hydrogen, utilized for the estimation of electron temperature and electron density, were also detected. Notably, there was an intense broad emission only from the orange plasma, indicating a substantial differentiation between the two plasmas.