Effect of Cathode PTL Wettability on Gas-Water-Oil Transport in a Direct Toluene Electro-Hydrogenation Electrolyzer
Authors: Shiono Ryuhei , Tanaka Hiroya , Reyna-Peña Fatima Isabella , Takimoto Yosei , Araki Takuto , Mitsushima Shigenori 
Published: December 2024

To achieve carbon neutrality, it is envisioned that electricity generated from renewable energy sources will be converted to hydrogen for use. There are several methods for storing and transporting hydrogen, but one of the most promising is the toluene-MCH organic hydride process. These organic compounds are liquid at room temperature and pressure and can utilize petroleum infrastructure. Direct toluene electro-hydrogenation electrolyzer is an attractive method that can simultaneously perform water electrolysis and toluene hydrogenation in a single system. This alternative is gaining attention because of its simple system, low energy loss, and low cost. However, one problem with this method is the presence of water droplets and hydrogen bubbles in the cathode PTL. The water dragged by electro-osmosis from the anode side blocks the toluene supply to the cathode reaction site. As a result, protons unable to react with toluene then convert into hydrogen bubbles on the cathode side. These bubbles also block the supply of toluene, resulting in significant decrease in toluene-MCH conversion efficiency.
A simulation approach is essential for the practical implementation of the system. We have conducted numerical analyses using single-cell scale models, but the parameters for modeling and verification were insufficient. Thus, experimental methods such as visualization of inside the PTL using an X-ray CT have been conducted [1]. In addition to this, a high-speed camera was used in this study to visualize dragged water, which is a difficult task to do with CT alone.
Figure A, shows an overview of the electrolyzer and visualization method employed. The electrolyzer with a sapphire glass window allowed visualization of the surface of the cathode PTL during its operation. Pure water was pumped to the anode side and toluene to the cathode side. Constant-current electrolysis at 100 mA/cm² was operated with a reaction area of 1 cm². Hydrophilic, hydrophobic, and lipophilic coatings were applied to the PTL and compared to the non-coated PTL, in order to investigate the effect of PTL wettability of toluene, hydrogen bubbles, and dragged water transport. Figure B is the non-coated and lipophilic PTL at 30 minutes after beginning electrolysis, with hydrogen bubbles shown in red and dragged water in blue. Under both conditions, hydrogen bubbles were quickly discharged, while the dragged water remained on the PTL surface and channel wall. Comparing the figures, the amount of water observed on the surface of the lipophilic PTL increased while the diameter of the hydrogen bubbles tended to be larger in the non-coated PTL. Calculated Faraday efficiencies for toluene-MCH were 60.7% for the non-coated PTL and 84.4% for the lipophilic PTL, indicating that lipophilicity improved the performance of the electrolyzer. Considering these results, lipophilic coating likely promoted the discharge of dragged water from inside the PTL to outside and enhanced toluene supply to the cathode reaction site.
Overall, the utilization of the high-speed camera enabled the visualization of the three phases of gas-water-oil, suggesting that the PTL wettability influences the transport behavior of these phases. These findings provide valuable insights for developing models to simulate the three-phase transport within the PTL.

References

[1] F. I. Reyna-Peña, et al. Int. J. Hydrogen Energy, 50, D787 (2024).

Figure 1