A Novel Flow Electrochemical Reactor for Intensifying Diffusion-Limited Electrochemical Processes

Abstract

Electrochemical processing is attracting renewed interest in industry for synthesis of chemicals or for the removal of a range of contaminants because it is cleaner than most alternatives. Redox reactions that are controlled by applying a potential between two electrodes avoid the use of potentially harmful auxiliary chemicals, an increasingly important advantage that offsets the electrical energy that is required. However, electrochemical processing is still limited in many applications by electrochemical engineering issues such as electrode material selection and construction, efficiency losses caused by mass transfer limitations and batch mode of operation. 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 static mixer working electrode is made using additive manufacturing techniques that allows complex designs to be realised using a range of different metals with different surface roughnesses. 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. Two ports at either end of the electrode, manufactured as an integral part of the design provide connections for fluid flow. As with all static mixers, the momentum of the solution induces mixing as it flows past the electrode surface. In the current version of the cell, 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. Separate ports provide fluid flow into this latter compartment. Figure 1 shows a schematic drawing of one configuration of the electrochemical cell with the arrangement of the electrodes, the fluidic setup including the peristaltic pump(s) used to control the solution flow into and out of the cell and the power supply connection to the cell. The efficiency with which the cell works may be evaluated by comparing the limiting current measured at various flow rates with results from a Rotating disk electrode (RDE) in the same solution. These comparisons are useful indicators of performance only, and are not used to draw any conclusions about the hydrodynamic conditions at the static mixer surface. 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 by nearly forty-five times. As expected, the rate of this acceleration is highest when the reaction is diffusion limited, so the impact is seen most clearly in dilute solutions. By comparison, results obtained with an RDE were not as promising confirming the effectiveness of the CSIRO designed flow electrochemical cell. Exhaustive electrolysis experiments were also undertaken to show how effectively the cell can remove contaminants from a fixed volume of a contaminated aqueous solution. A two Litre solution of copper contaminated water (i.e. 100 pm CuSO4.4H2O in 0.01M H2SO4) was processed using the flow cell at a constant flow rate of 100 mL/min for 5 hours. SEM/EDS results confirmed the deposition of copper ions onto the static mixer working electrode. ICP results showed that a 50% reduction in the copper concentration from 100 ppm to 51 ppm was achieved in 5 hours which is encouraging. Figure 1