Visualization of Hydrogen Bubbles Using Synchrotron X-Ray CT inside Porous Transport Layer of Direct Toluene Electro-Hydrogenation Electrolyzers

Abstract

Toluene is a highly versatile solvent commonly used in many industries due to its high energy density and effective hydrogen carrier properties. It can store a considerable amount of hydrogen in a relatively small volume, making it a promising hydrogen carrier for various applications. Toluene's high volumetric energy density also makes it one of the best hydrogen carriers available. Additionally, toluene is easy to handle and transport as it is a liquid at room temperature, and it can be stored and transported like any other liquid fuel. Furthermore, toluene is a safer alternative for hydrogen storage compared to other hydrocarbons due to its low toxicity.Direct toluene-electro hydrogenation electrolyzer is a promising method that enables the simultaneous performance of water electrolysis and toluene hydrogenation in a single system. However, during the toluene direct electrolytic hydrogenation process, the formation of hydrogen bubbles from the electrolysis of water hinders the toluene hydrogenation reaction, leading to a reduced conversion rate of methylcyclohexane (MCH) and Faraday efficiency.In this study, we aimed to understand the conditions for hydrogen bubble generation and transport. We quantitatively evaluated the generation amount and distribution of hydrogen bubbles by visualizing them in an operating electrochemical cell using Synchrotron X-ray CT. Figure 1 displays the distribution of hydrogen bubbles along the porous transport layer (PTL) of the electrochemical cell. These bubbles were visualized using the raw images obtained from Spring-8 and subsequently reconstructed. The cell was operating at a current density of 200 mA/cm2 for a period of 30 minutes with pure water supplied to the anode side. Darker areas in the images indicate regions with hydrogen bubbles, which have a lower density and atomic weight than other substances in the PTL. Therefore, they are more transparent to X-rays and appear darker in the image. Higher current values and longer operation times led to increased hydrogen bubble generation, with a significant increase observed at 30 minutes. By visualizing these hydrogen bubbles, we were able to evaluate the generation distribution, amount, and transport of hydrogen, which can help improve the direct toluene-electro hydrogenation electrolyzer system. Acknowledgement This study was based on results obtained from the Development of Fundamental Technology for Advancement of Water Electrolysis Hydrogen Production in Advancement of Hydrogen Technologies and Utilization Project (P14021) commissioned by the New Energy and Industrial Technology Development Organization (NEDO). References Nagasawa K, Sugita Y, Atienza-Márquez A, Kuroda Y, Mitsushima S. Effect of the cathode catalyst loading on mass transfer in toluene direct electrohydrogenation. Journal of Electroanalytical Chemistry 2023:117431. https://doi.org/10.1016/J.JELECHEM.2023.117431.Shigemasa, A. Atienza-Márquez, K. Inoue, S. Jang, F. Reyna-Peña, T. Araki, “Visualization of dragged water and generated hydrogen bubbles in a direct toluene electro-hydrogenation electrolyzer,” J. Power Sources 2023; 554:232304. https://doi.org/10.1016/j.jpowsour.2022.232304.Nagasawa, A. Kato, Y. Nishiki, Y. Matsumura, M. Atobe, S. Mitsushima, “The effect of flow-field structure in toluene hydrogenation electrolyzer for energy carrier synthesis system. Electrochemical Acta 2017; 246:459-65. https://doi.org/10.1016/j.electacta.2017.06.081. Nagasawa, K. Tanimoto, J. Koike, K. Ikegami, S. Mitushima, “Toluene permeation through solid polymer electrolyte during toluene direct electro-hydrogenation for energy carrier synthesis”, Journal of Power Sources, Vol.439, (2019) 22707. https://doi.org/10.1016/j.jpowsour.2019.227070. Figure 1 SPring-8 CT Visualization Reconstructed image after 30 minutes of operation at 200 mA/cm². Figure 1