Introduction 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. Figure 1

Introduction Energy transport and storage system should be considered with new energy source installation such as solar and wind power. There are some methods for it, and organic hydride methods are highly paid attention. In these methods, toluene-methylcyclohexane (MCH) conversion system is a good candidate as an energy transport and storage system with some reasons such as easy handling, affinity with Japanese current infrastructure, and so on. In the toluene-MCH conversion system, direct-toluene hydrogenation is especially paid attention since the system can be easier and exothermic heat loss reaction is less than other methods, so total efficiency of direct method gets higher than other methods. However, there are some problems on it, and the most serious problem is optimizing water management in cathode [1,2]. In cathode, toluene and MCH are flowing, and water appears in cathode porous transport layer (PTL) by being transported from anode then the water interrupts fuel supplies and the conversion rate from toluene to MCH gets lower. Therefore, water management should be optimized for higher performance of the system. However, there is no research which is focusing on water movement in this system as far as we know. Therefore, in this study, the original cell was investigated which had a visualization window on the cathode surface and allowed us to visualize the water movement during operating the system, and water movement was investigated. Fig. 1 shows the cell configuration and the actual picture from visualization window. As shown in Fig.1(a), the original cell was developed. In the left side, there is a visualization window with sapphire glass and the surface of the cathode PTL can be shot by hi-speed camera and so on. Fig.1(b) shows the example of visualization results. The cell was set with the gravity direction lower side and flow direction upper. The image shows us cathode PTL and toluene-MCH flow with water droplet and hydrogen bubble. As it can be seen in Fig.1(b), there are some droplets with several sizes. The 0.5 – 1.5 mm diameter droplets can be recognized as water droplet by seeing the movement with movie. By using this system, the effects of different anode supplies were discussed on water and hydrogen generation. Reference [1] K. Nagasawa et al., Electrochemistry, 339-344, 86 (2018) [2] K. Nagasawa et al., Electrocatalysis, 164-169, 8 (2017) Figure 1