Bipolar Electrochemistry: A Powerful Technique for Chemical Analysis and Property Screening

Abstract

The phenomenon of bipolar electrochemistry was first exploited about three decades ago in the industrial sector to minimize power dissipation and to carry out electrochemical reactions in poorly conducting media. However, following the rigorous mathematical analysis of the potential distribution at the interface of bipolar electrodes by Duval and co-workers in 2001, bipolar electrochemistry has emerged as a powerful technique for electroanalysis and surface patterning. It is particularly the lack of direct electrical connection to the bipolar electrode that makes the technique quite unique. This aspect has facilitated electroanalysis in miniaturized systems such as capillary electrophoresis where microelectrodes are employed and the high electric fields in the separation channel often lead to fluctuations in the potential of the working electrode, making it difficult to control electrochemical reactions. The variation in the potential difference along the interface of a bipolar electrode makes bipolar electrochemistry a very straight forward technique for decorating substrates with chemical gradients that can be used, for example, to screen chemical or physical phenomena or to drive certain transport processes. Even more interesting is the possibility to control the position, width and shape of the gradients by simply changing the electric field in solution. The simplicity of the instrumentation used to create these gradients makes bipolar electrochemistry an inexpensive alternative to lithographic techniques for gradient formation. Chapter 1 gives a brief introduction to traditional electrochemistry. It describes the experimental setup for carrying out electrochemical reactions and how these reactions can be controlled. The fundamental principles of bipolar electrochemistry are also presented in this chapter and its advantages over traditional electrochemistry are stated. It also reviews recent applications of bipolar electrochemistry in areas such as device fabrication, micromotors, analyte enrichment and detection in microfluidic devices, and surface patterning. Finally, a review of spectroelectrochemistry is presented, focusing on Raman spectroscopy. In chapter 2, basic measurements to provide insight into the potential profile in a bipolar electrochemical setup are presented. The bipolar effect is also demonstrated experimentally using copper deposition as an example. The existence of a potential gradient along a bipolar electrode is demonstrated by evaluating the reactivity gradient on Au and Ag electrodes modified by a self-assembled monolayer (SAM) of a thiol-functionalized quinone upon the application of an electric field. In situ Raman microscopy is used to characterize the gradient. The ability to control the position and width of gradients formed on bipolar electrodes is also demonstrated. In chapter 3, the applications of cobalt oxide are briefly reviewed. Preparation of cobalt compounds involving different oxidation states of the metal is also reported. Its catalytic, optical and electrochromic properties are investigated using cyclic voltammetry and Raman microscopy. In chapter 4, a one-dimensional chemical compositional gradient of a cobalt oxide electrocatalyst for water oxidation is displayed on a bipolar electrode. The composition of the material gradient composition is screened using in situ Raman microscopy while its catalytic activity is investigated using Scanning electrochemical microscopy (SECM), leading to the identification of the active phase. In chapter 5, similar studies in chapter 4 are reported for nickel oxide water oxidation catalyst and comparism between both catalysts is made. Chapter 6 gives a summary of this dissertation and a quick overview of future projects.