Gas bubbles have controversial effects on Taylor flow electrochemistry

Abstract

Electrochemistry is currently resurging in popularity amongst synthetic chemists due to the unique opportunities it provides to activate organic molecules. Simultaneously, continuous-flow technology has been used to enable scalability and to increase the efficiency of the developed electrochemical processes. Many of these processes involve a gaseous reagent or byproduct generated during the electrochemical process. The presence of a gas phase in flow reactors may lead to the generation of a so-called Taylor flow regime, where gas bubbles and liquid segments alternate. While Taylor flow has almost exclusive positive effects in flow chemistry due to increased mass and heat transfer, we show herein that the ramifications of gas bubbles on flow electrochemistry are essentially negative. Computational fluid dynamics (CFD) was used to gain a detailed understanding of the effects induced by the gas phase on the electrochemical process, taking the reduction of furfural to furfuryl alcohol carried out in an in-house developed electrochemical reactor as benchmark. We show that the gas bubble presents a local situation with infinite electrical resistance leading to a temporary passivation of the electroactive surface, while its presence also intensifies the mixing in the liquid slug reducing mass transfer limitations. Essentially, the larger the bubble, the higher the energy losses become and the less efficient the reactor is used. This results in a higher overall energy consumption for the electrochemical process. Moreover, we investigated the residence time distribution in the liquid slug, and the effect of different operational conditions (bubble size, gas holdup, interelectrode distance, electrolyte velocity and species concentration) on the overpotential and current density, providing guidelines for reactor design and operation. Based on the results described herein, we also discuss potential solutions to increase the efficiency of the electrochemical flow reactor.