Mass Transport-Enhanced Electrochemical Processes Using Newly Developed Axial Flow Electrochemical Reactor

Abstract

Electrochemical processing is attracting renewed interest in industry, particularly for the partial or complete synthesis of chemicals and the removal of a range of contaminants and pathogens from water. This renewed interest is driven by the cleanliness of electrochemistry; direct reduction or oxidation of target species at an electrode avoids the use of potentially harmful auxiliary chemicals, which is an increasingly important benefit that offsets the electrical energy that is required.Although electrochemical processing is used widely, there is still scope to improve its efficiency in many industrial applications. In this respect, a particularly common problem is mass transport. At the atomic level, electrons are transferred between an electrode and a reactant by tunneling - a process whose probability diminishes exponentially with distance - so the reactant species must reside within just a few atomic layers of the surface for electron transfer is to take place at a reasonable rate. However, this means the solution must be mixed at all practical scales of length if mass transport is to be efficient, a result that is particularly difficult to achieve. Although mixing is well-studied both practically and theoretically, it is still an imperfect science. In a relatively recent advance, the term chaotic advection was introduced to describe the exponential dividing of fluid filaments during flow that produces particularly efficient mixing. Two significant points about chaotic advection are: it commonly includes both laminar and turbulent flow regimes, andstatic mixers can be efficient devices for generating chaotic advection. An axial flow electrochemical cell is conceptually simple but practical examples are infrequently encountered in both industry and research. Achieving efficient mixing throughout the length of the cell is perhaps the greatest obstacle to the adoption of this otherwise efficient geometry. The CSIRO flow electrochemical cell aims to address these issues by using advanced computational fluid dynamics (CFD) to design, manufacture and optimize an electrode that can maximize mixing within an axial flow cell. The current design of the CSIRO electrochemical flow cell employs an additively manufactured, high surface area static mixer as a central working electrode that fits snugly within a tubular porous polymeric separator which defines a working compartment. In addition, an inert tubular counter electrode surrounds the working compartment at a small distance from the separator, creating a low volume counter compartment and formed the outside casing of the cell (Figure 1).In this work, the CSIRO flow electrochemical cell has been characterized using the reduction of ferricyanide, [Fe(CN)6]3+) at three different reactant concentrations and seven different flow rates. Results show that the cell can accelerate the rate of the reduction reaction compared to other classical electrochemical systems. Thus, the suitability of the flow cell was studied for water treatment (purification/disinfection) applications. Extensive physical, chemical and electrochemical characterization techniques were used in this investigation to evaluate the cells performance. Figure 1