Overcoming diffusion limitations in electrochemical microreactors using acoustic streaming

Abstract

We provide a theoretical background on laminar flow electrochemical reactors and rationally map typically encountered operating regimes in literature. At the highest (mass transfer limited) current densities attainable, the Fourier number (Fo) and the Sherwood number (Sh) are identified as important dimensionless groups for design considerations. The critical Fourier number places a severe constriction on the production capacity of a microreactor, as it determines the maximum throughput for a reactor of fixed length and inter-electrode distance. This critical Fourier number indicates that the absence of convective mixing and the reliance on diffusion of the electroactive species is the key problem to address for further intensifying electrochemical microreactors. Computer simulations are then used to illustrate the steady-state as well as transient behavior of generic case studies. These simulations also show that the transient response to a voltage step can be used to experimentally verify that the absence of mixing is a major bottleneck of laminar flow reactors. A versatile tubular electrochemical reactor was developed and applied to an acetonitrile solution containing ferrocene and benzylbromide, which is a simple model system for investigating self-supported synthesis (i.e., electrosynthesis without intentionally added supporting electrolyte). The reactor was then fitted with piezoelectric elements that excite a resonating standing wave in the flow direction, resulting in pairs of counter-rotating vortices in the fluid (acoustic streaming). The sonicated experiments prove that ultrasound assisted mixing reduces the critical Fourier number, enabling self-supported synthesis at elevated flow rates and throughput.