In situ X-ray CT visualization of hydrogen bubbles inside the porous transport layer of a direct toluene electro-hydrogenation electrolyzer.

Abstract

The organic hydride toluene/methylcyclohexane is an attractive hydrogen carrier. The direct electro-hydrogenation of toluene to methylcyclohexane has a higher energy efficiency compared to the conventional two-step method. Yet, there are factors inhibiting the toluene supply to the cathode reaction site and reducing the conversion rate to methylcyclohexane. Thus, hydrogen may be generated as a side reaction. Understanding how hydrogen bubbles are distributed is crucial regarding the design optimization of these electrochemical devices. This work presents, for the first time in a direct toluene electro-hydrogenation electrolyzer, the X-ray visualization of hydrogen bubbles inside the porous electrode. The concentration of bubbles was higher near the cathode catalyst and tended to increase with the electric current and operating time. Bubble distribution spreads across the active area when using flat field plates. As for rib-and-channel field plates, despite bubbles mostly concentrated beneath ribs, the total hydrogen accumulated inside the porous electrode could be reduced.