Electrochemical methods for catalyst activity evaluation

Abstract

Since a fuel cell is an electrochemical device, electrochemical methods are deemed to play important roles in characterizing and evaluating the cell and its components such as the electrode, the membrane, and the catalyst. The most popular electrochemical characterization methods include potential step, potential sweep, potential cycling, rotating disk electrode, rotating ring-disk electrode, and impedance spectroscopy. Some techniques derived from these methods are also used for fuel cell characterization. An electrochemical reaction involves at least the following steps: transport of the reactants to the surface of the electrode, adsorption of the reactants onto the surface of the electrode, charge transfer through either oxidation or reduction on the surface of the electrode, and transport of the product(s) from the surface of the electrode. The purpose of the electrochemical characterizations is to determine the details of these steps. The characterizations are carried out in various electrochemical cells. There are typically three types of cells: conventional 3-electrode cells, half-cells, and single cells. In those cells, the entity (e.g., catalyst, electrode) to be characterized forms the working electrode, the potential of which or the current passing through which is controlled or monitored. What happens on the working electrode is the sole interest of the investigation. The working electrode and another electrode, called the counter electrode, form a circuit, and the current flowing through this circuit will cause some reaction on the counter electrode as well. However, the investigation has no interest in what happens on the counter electrode, except that the reaction occurring on it should not interfere with the working electrode. In order to minimize the impact of the solution (or electrolyte) resistance on the potential of the working electrode, a reference electrode is often used to form another circuit with the working electrode. Ideally, this electrode is non-polarizable and maintains a stable potential. There is high input impedance in the voltage measurement equipment, which makes the current flowing in this circuit very small. Therefore, the impact of the uncompensated electrolyte resistance on the potential of the working electrode is minimized. In this article, each section starts with a brief introduction to techniques, principles, and instrumentation, followed by examples for the purposes of illustration. The illustration is limited to low-to medium-temperature fuel cells such as the proton exchange membrane (PEM), direct liquid, and phosphoric acid fuel cells that involve protons as the ionic charge transport species. © 2008 Springer-Verlag.