Fuel Cell Membranes

Abstract

Fuel cells are electrochemical devices that convert the energy liberated in a chemical reaction directly into electricity. The basic building blocks of a fuel cell are an ionically conducting electrolyte that separates two electrodes, an anode where oxidation reactions occur, and a cathode for reduction reactions. During operation, fuel (e.g., H2 gas or methanol) is fed continuously to the anode, and an oxidant (normally oxygen from air) is supplied to the cathode. Fuel oxidation and oxygen reduction reactions occur spontaneously on the electrodes with the production and consumption of electrons (at the anode and cathode, respectively) and the generation of heat (because the electrical energy conversion process is not 100% efficient). Electrical energy is extracted from the electrons as they flow from the anode to the cathode via an external circuit. A fuel cell, although having components and characteristics similar to those of a battery, differs in one primary way. The battery is an energy storage device, where the maximum energy available is determined by the amount of chemical reactant stored within the battery itself. The fuel cell, on the other hand, is an energy conversion device with separate (external) storage of fuel and oxidant. Thus, a fuel cell has the capability of producing electrical energy for as long as reactants are supplied to the electrodes. There are a variety of different fuel cells that are normally classified according to the electrolyte type (see Table 29.1). The amount of power produced by a fuel cell depends upon several factors, such as fuel cell type, cell size, the temperature at which the cell operates, and the pressure at which the gases are supplied to the cell. A single fuel cell element (one anode and cathode) produces enough electricity for only the smallest applications. Therefore, individual fuel cells are typically combined into a stack configuration. Two types of fuel cells that employ polymeric cation exchange membranes as the electrolyte material are the proton exchange membrane fuel cell (PEMFC) and the direct methanol fuel cell (DMFC). The membrane has a multifunctional role in these devices: (1) It physically separates the anode and cathode to prevent an electrical short circuit, (2) it separates the fuel and oxidant to eliminate a chemical short circuit, and (3) it provides