Active control of the depletion boundary layers in microfluidic electrochemical reactors

Abstract

In this paper, we describe three methods to improve the performance of pressure-driven laminar flow-based microreactors by manipulating reaction-depletion boundary layers to overcome mass transfer limitations at reactive surfaces on the walls, such as electrodes. The transport rate of the reactants to the reactive surfaces is enhanced by (i) removing the depleted zone through multiple periodically-placed outlets; (ii) adding fresh reactants through multiple periodically-placed inlets along the reactive surface; or (iii) producing a spiraling, transverse flow through the integration of herringbone ridges along the channel walls. For approaches (i) and (ii), the network of microfluidic channels needs to be designed such that under the operating conditions used the right amount of boundary layer at each outlet or inlet is removed or replenished, respectively. Here, we report a set of design rules, derived with the help of a fluidic resistance circuit model, to aid in the design of appropriate microfluidic networks. Also, the actual enhancement of the performance of the electrochemical microreactor, i.e. chemical conversion efficiency, using multiple inlets, multiple outlets, or herringbone ridges is reported.

New Concepts in the Chemistry and Engineering of Low‐Temperature Fuel Cells

“…Using such a focusing technique, both fuel utilization and power density may be increased as it enables the use of higher concentrations of fuel and oxidant. Altering structural parameters of the microchannel in which the reactants flow can further improve performance and fuel utilization [108][109][110]125,126]. For example, Ahmed et al developed a "tridentshaped" design that used electrolyte stream in the channel center to focus both the fuel and oxidant streams onto their respective electrodes [108].…”

“…For example, Ahmed et al developed a "tridentshaped" design that used electrolyte stream in the channel center to focus both the fuel and oxidant streams onto their respective electrodes [108]. In another interesting example, Yoon et al investigated different active and passive methods of minimizing the depletion boundary layer, which limits cell performance: (1) using multiple outlets to remove the depleted regions, (2) using multiple inlets to replenish the depleted regions, and (3) using a herringbone structure to generate secondary transverse flow, enabling chaotic mixing within a single laminar stream, to replace the depleted layer with fluid of higher fuel concentration [126]. In all three examples, the performance improvements obtained from the design modifications must be balanced with either the increased system complexity and parasitic pumping losses associated with additional fluidic streams, or the possibility of increased fuel crossover associated with the chaotic advection.…”

Enzyme-Based Microfluidic Biofuel Cell to Generate Micropower

“…The depletion layer can be also manipulated to overcome mass transfer limitations by the development of original microfluidic configurations. This strategy was pointed out by simulation and experiments in the case of a microfluidic fuel cell working from formic acid (Yoon et al, 2006). These configurations featured, along the electrodes, either multiple periodically located outlets to remove consumed species or multiple periodically located inlets to add fresh reactants in the microfluidic channel.…”

Gold coated optical fibers as three-dimensional electrodes for microfluidic enzymatic biofuel cells: Toward geometrically enhanced performance

“…22,23 As for 2D electrodes, it is indeed likely that thicker depletion layers develop along longer GCOFs and restrict their performance. 6,10,24 The most straightforward way to reduce the effect of depletion layers is to increase the flow rate. Nevertheless, we did not try to further increase the reference flow rate as in one of our former work 23 because we wanted to minimize reactants consumption.…”

“…Indeed, microtextured 3D electrodes have the potential to enhance mass transfer of fresh reactants to the reactive electrode surface. 6 Moreover, 3D electrodes can increase the enzyme loading by offering enlarged reactive surface areas. 7 Several types of 3D electrode architectures such as herringbone ridges, 6 graphite rods, 8 or flow-through porous electrodes 9 were evaluated in membraneless laminar fuel cells that used various chemical reactants.…”