(Invited) Synthesis of TiO2-Based for Electrocatalysts CO2 Reduction Using in-Liquid Plasma Method and Control of Their Reduction Properties
Authors: Takagi Kai , Hunge Yuvaraj Mohan , Serizawa Izumi , Terashima Chiaki 
Published: December 2024

Introduction
In recent years, there has been growing interest in converting carbon dioxide (CO2), a greenhouse gas, into a valuable resource. The electrolytic reduction method, which uses only water and electricity, offers a particularly clean approach to CO2 conversion. Its high conversion efficiency at ambient temperature and pressure, facilitated by gas diffusion electrodes (GDE), makes it a promising option for widespread adoption. Electrocatalytic CO2 reduction using GDE selectively yields carbon monoxide, ethylene, and formic acid with Ag, Cu, or Sn catalysts, respectively 1). These catalysts are often supported on conductive carbon black (CB), valued for its high electrical conductivity and porosity. However, CB-based catalysts encounter challenges like redox-induced degradation and high CO2 overvoltage.
So, we turned to plasma in liquid as a new method for making electrocatalysts. Plasma in liquid is generated by subjecting a metal in solution to high-frequency, high-voltage conditions. This process enables the sputtering of metal onto powder samples dispersed in solution, with the metal surface instantly heated to high temperatures. This process can improve electrochemical properties like oxygen deficiency 2). Consequently, plasma treatment in liquid presents an opportunity to utilize highly durable and chemically resistant metal oxides as catalyst base materials.
The objective of this study was to synthesize a new catalyst for electrocatalytic CO2 reduction using titanium dioxide as the catalyst base via in-liquid plasma. Titanium dioxide offers advantages of affordability, durability, and chemical resistance. Moreover, oxygen-deficient TiO2 with enhanced conductivity finds application as a base material for batteries.
Experimental
In the experiments, high-frequency high voltage (4 kV, 100 kHz, 0.5 µs-Pulse Width) was applied under the same conditions to various metal electrodes in a 5.0 wt%-NH4 solution (200 mL) containing dispersed TiO2 (P25: 0.5 g) to generate plasma in the liquid. Common plasma electrodes included tungsten (W), silver (Ag), and copper (Cu). Each gas diffusion electrode (GDE) was prepared by washing the powder after in-liquid plasma treatment with pure water, followed by spray coating on a gas diffusion layer with Nafion dispersion as a binder. Electrocatalytic CO2 reduction was evaluated in a conventional three-chamber gas diffusion cell at 500 mA/cm2 for 10 minutes. The CO2 supplied to the gas chamber was collected in a Tedlar bag, and the CO2 reduction products were analyzed by gas chromatography.
Results and Discussion
Figure shows the CO2 reduction products and Faraday efficiency of P25 treated with various liquid plasma electrodes. In pristine P25, small amounts of CO and CH4 were detected due to the redox cycle of Ti4+ and Ti3+. The use of tungsten (W), silver (Ag), and copper (Cu) as in-liquid plasma electrodes increased the production of H2, CO, and CH4, respectively. The difference in conversion can be attributed to the fact that the amount of etched Cu is less than that of Ag. Additionally, black nanoparticles were dispersed in the filtered solution when Ag was used, while the solution turned a clear blue color with Cu, suggesting the formation of a complex with ammonia. Generally, oxidized metal nanoparticles are synthesized when plasma is generated in solution. In this study, since an alkaline solution was used, it can be considered that the oxidized Ag nanoparticle surfaces were reduced, resulting in highly dispersed reduced Ag nanoparticles being deposited. In contrast, the generated Cu nanoparticles impregnated TiO2 by dissolving in ammonia and exhibited catalytic activity. CH4 significantly affects Faraday efficiency relative to the amount produced because CO and H are two-electron reactions, while CH4 is an eight-electron reaction, requiring four times as many electrons. Although C2H4 is produced by Cu nanoparticles at sufficient current density, the present results show increased CH4 production. This may be due to TiO2 promoting the hydrogenation of CO2 before Cu forms the C-C bond. Further investigation is required.
Conclusions
In this study, TiO2 underwent treatment with various in-liquid plasma electrodes for electrocatalytic CO2 reduction. As a result, changing the electrode allowed for successful support of metal nanoparticles on TiO2, resulting in selectivity of CO2 reduction products. During the presentation, we will discuss the comparison with conductive carbon black (CB) and the changes in electrocatalytic CO2 reduction characteristics under different plasma conditions in liquid. Compared to pristine P25, the production of CO increased by 9.5-fold from 40 to 380 μmol, and CH4 increased by 2.0-fold from 20 to 40 μmol.
Reference
1) Sassenburg et al., ACS Appl. Energy Mater., 5, 5, 5983 (2022).
2) Takagi et al., Sci. Total Environ., 902 (13), 166018 (2023).

Figure 1