Visualization of dragged water and generated hydrogen bubbles in a direct toluene electro-hydrogenation electrolyzer

Methylcyclohexane is an appealing liquid organic hydrogen carrier produced from toluene hydrogenation. The direct toluene electro-hydrogenation technique in proton exchange membrane electrolyzers avoids heat losses and reduces the electricity consumption concerning the conventional methods. However, this technology faces a critical issue. The water dragged by electro-osmosis from the anode side blocks the toluene supply to the cathode reaction site. This paper presents the experimental visualization, for the first time, of water droplets and generated hydrogen bubbles inside the cathode porous transport layer of a direct toluene electro-hydrogenation cell. Experiments were conducted under different current densities and using not only pure water but also dilute sulfuric acid as anode reactant. The visualizations revealed that the bubbling grew sharply as the electric current increased between 20 and 100 mA/cm 2 . It was also observed that water droplets got stuck inside the cell. As for hydrogen, small bubbles left the cell rapidly while larger ones stayed inside for longer. Finally, the experimental visualizations confirmed that the water dragging phenomenon was mitigated by using dilute sulfuric acid as anode reactant instead of pure water. Indeed, the hydrogen generation was halved at 200 mA/cm 2 and the methylcyclohexane Faraday efficiency was boosted by 15%–22% points. • Mass transport was studied in a direct toluene electro-hydrogenation electrolyzer. • First visualization of dragged water and generated hydrogen inside an operating cell. • More liquid water droplets and H2 bubbles were observed as the current rises. • Dragged water increases H2 generation and hampers toluene hydrogenation. • Sulfuric acid minimizes the water flux and boosts the Faraday efficiency.