(414a) Microfluidic Electrochemistry: A Versatile Platform to Study Reactions at Electrodes

Microfluidics is the science of manipulation and process of small volumes of fluids in micron size channels. Fundamental electrochemical mechanisms such as electrocatalysis, corrosion, and metallization are based on surface reactions occurring at the electrode-electrolyte interface. The high surface area-to-volume ratio in microchannels increases the importance of surface interactions within them. Both ?top-down? and ?bottom-up? fabrication methods have been developed to manufacture microelectrodes and to assemble them within microfluidic devices. The application of such devices enhances convective transport rates while retaining laminar flow condition required for analytical modeling. Other capabilities such as the ability to use very small quantities of electrolyte, to perform in short time with high sensitivity, to control concentrations of molecules in space and time, and low cost make microfluidic devices a suitable platform to study different electrochemical mechanisms.

The development of surface roughness during electrodeposition of metals has important application for secondary batteries as well as for metallization. During successive battery charging cycles, when metal is re-deposited at the ?anode,? the evolution of dendritic structures leads to shorting and failure, in some cases catastrophic. The phenomenon of dendrite growth has been a major impediment to the use of large-scale secondary zinc alkaline batteries as well as lithium metal batteries. The microfluidic electrochemical platform is uniquely suited to analysis of dendrite formation, which in most cases is closely tied to mass transport conditions. This is because mass transport and diffusion layers are well-defined at the working electrode surface, electrolyte compositions can be rapidly changed in situ, and visual imaging of the electrode surfaces is straightforward.

To construct a microfluidic electrochemical cell, poly(dimethylsiloxane) (PDMS) microchannels are fabricated using soft lithography techniques. PDMS elastomer (Sylgard 184, Dow Corning) is prepared by mixing and degassing a 10:1 ratio of liquid silicone base and a curing agent. The mixture is then poured either over a microchannel mold and cured in an oven, or cured as a sheet, with the channel geometry then fabricated by laser-cutting. The microelectrodes are fabricated separately via one of a number of techniques. Low profile microelectrodes are printed either directly as an ink jet, or thermally following lithography. Electrodes that are substantial in three dimensions are either patterned as a printed circuit board or embedded in epoxy and cross-sectioned. The combination of these techniques enables precise control of electrode geometries on the micron scale, resulting in reliable knowledge of current densities and therefore quantitative electrochemical experiments. Flow of electrolyte is established in the microchannels with a syringe pump and the region of interest is observed using a microscope. We analyze in situ dynamic observations using a high speed camera. Electrolyte flow rate is used to control mass transfer of metal ions to the working electrode surface, while secondary variables such as electrolyte composition, electrode geometry, and current waveform are varied while surface evolution is monitored.

In this work, we study the alkaline zinc electrode to determine conditions maximizing cycle life, as well as to optimize the experimental system. The goal of finding conditions under which a zinc electrode may be cycled repeatedly will require deposition of dendrite-free deposits. We use silver electrodes as a test system for both dendrite formation as well as phase change studies. By flowing a ZnO KOH solution over two electrodes with a fixed potential difference, we notice silver oxidation and corrosion on the positive electrode and zinc deposition on the negative electrode. Studying this system with microfluidic electrochemistry allows us to understand qualitatively and quantitively the current efficiencies at each electrode, the regimes of electrode transformation as a function of cell potential, electrical current and electrolyte velocity, and the overall reversibility of the system.