Ion Transfer in Layer-by-Layer Films

Abstract

The need for miniaturized devices has driven the exponential development of nanotechnology during the past two decades. One special field of paramount practical applications is electrochemical systems at the nano-scale. For successful development in such fields, an in-depth analysis of mass and/or charge transfer mechanism is highly desired. In such a system, mass transfer often takes the form of ion transfer, which can also be viewed as charge transfer. For example, in fuel cell development, the catalyst layer deposited on a polyelectrolyte membrane would demand higher ionic (protonic) conductivity. In the electrochemical sensors, the overall performance depends largely upon sensitivity of the thin films to recognize the analyte and the speed to communicate the resultant signals with the underlying electrodes. These phenomena are closely related to the (ionic) mass transfer within such films. A clear understanding of mass transfer in such an electrochemical system is the prerequisite for any significant progress in devices. Fundamentally, three different types of mass-transfer phenomena exists for ionic species in electrolytes at electrodes: (1) diffusional transport under concentration gradient, (2) migration transport of oppositely charged ions under electric field of the electrode, and (3) convection transport due to physical stirring of the electrolyte. For electrodes modified with electroactive or redox films, the redox behavior is much more complicated. When the applied potential reaches the oxidation potential of the redox-active species in the film, the electron transfer from the electrode surface to the film is coupled with the simultaneous ionic transfer from electrolyte to the film for maintaining electro-neutrality. Thus, we observe two simultaneous mass-transfer processes at the same time and each one needs to be characterized individually. A detailed study of mass-transfer within such films would first require conceptual understanding of the main characterization technique used. We will conduct a literature survey on various models used to characterize such mass transfer in layer-by-layer (LbL) films, and then propose a model for characterizing mass-transfer in LbL films that contain nanoparticles.