Currently, the use of renewable energy is being promoted as an approach to decarbonization. However, renewable energy requires stable energy storage and supply because the amount of electricity is affected by time and climate. One of the solutions is use of hydrogen as an energy source. As the demand for hydrogen increases in the future, a large amount of hydrogen will be needed, and the storage and transportation of large amounts of hydrogen will also be necessary. However, the methods for storing and transporting large amounts of hydrogen have not yet been established, and this is one of the challenges in the current hydrogen utilization. One of the methods for storing and transporting large amounts of hydrogen is the organic hydride method. The organic hydride method is a method of storing and transporting hydrogen using hydrogenation and dehydrogenation of organic compounds. Among these, the use of toluene and methylcyclohexane (MCH) has attracted attention. The advantages of using toluene and MCH include the following: because toluene and MCH are liquids at normal temperature and pressure, they can be transported at about 1/500th the volume of direct hydrogen gas transport; toluene and MCH can be reused; hydrogenation and dehydrogenation are the only reactions, so no byproducts are generated; and they can be transported in the same way as petroleum. Hydrogen can be transported in the same way as petroleum. Therefore, it is expected to be used as a storage and transport medium for hydrogen.

A toluene direct electro hydrogenation electrolyzer is available as a means of hydrogenating toluene. In the toluene direct electro hydrogenation electrolyzer, hydrogen and toluene are supplied to the electrolyzer, and electricity is applied to protonate the hydrogen and supply it to toluene to produce MCH. The advantages are that cost and time losses can be reduced because water electrolysis and toluene hydrogenation can be performed simultaneously, there is no heat loss in the reaction, and the theoretical decomposition voltage can be reduced compared to electrolysis of water and hydrogenation of toluene separately. However, during hydrogenation of toluene, a part of the water being electrolyzed is transferred to the toluene reaction surface, reducing the reaction efficiency to MCH. In addition, part of the protonated hydrogen becomes hydrogen bubbles, which inhibit the fuel supply and reduce the reaction area. Furthermore, the phenomena of water movement and hydrogen bubbles in toluene hydrogenation have not yet been studied.

In this study , we aim to understand the generation of hydrogen bubbles by visualizing the phenomena in toluene hydrogenation using a toluene direct electro hydrogenation electrolyzer with an X-ray CT system, and to provide a guideline for future research on the suppression of hydrogen bubble generation and water movement. Fig. 1 shows a schematic diagram of the toluene direct electro- hydrogenation electrolyzer used in this experiment. The electrolyzer consists of a catalyst layer, a gas diffusion layer (GDL), and a separator, in that order. The electrolyzer is operated by supplying toluene to the anode side and hydrogen to the cathode side, and electricity is supplied. Visualization of the interface between the GDL and catalyst layer on the cathode side of the electrolyzer during operation using an X-ray CT system allows the phenomena on the reaction surface to be visualized. Figure 2 Visualization of the inside of the electrolyzer during operation. Fig. 2 (a) shows the electrolyzer filled with MCH and (b) shows the electrolyzer filled with toluene. The toluene electrolyzer generates fewer hydrogen bubbles, which confirms that the reaction is normal, and the correlation between the current density and the generation of hydrogen bubbles is examined.