Electrochemical Energy Conversion in Fuel Cells

Abstract

This chapter is focused on the properties of a direct energy conversion in fuel cells. Here a free energy of a chemical reaction is converted directly into electrical energy. Basic thermodynamics of that reaction is given, and relation of energy and reversible potential, Er, of the cell carrying out chemical reaction. The main properties of energy conversion resulting from its direct feature include the highest conversion efficiency. Possible fuels are discussed, and with H2 as a fuel, the reaction product is H2O. The possibility that fuel cells will be the major source of clean energy, particularly important for automotive application, is analyzed, considering hydrogen, ethanol, and methanol fuels. An overview of the properties of low temperature fuel cells is given. These include: Low-temperature fuel cells are the Proton Exchange Membrane or Polymer Electrolyte Membrane Fuel Cell (PEMFC), Alkaline Fuel Cell (AFC), the Direct Methanol Fuel Cell (DMFC), Direct Ethanol Fuel Cell (DMFC), and the Phosphoric Acid Fuel Cell (PAFC). The high-temperature fuel cells operate at temperatures approx. 600–1000 °C, and two different types have been developed: Molten Carbonate Fuel Cell (MCFC) and the Solid Oxide Fuel Cell (SOFC). Typical polarization curves for anode and cathode and the cell, i.e., anode and cathode potentials and cell voltage as a function of current density, are given. The cell losses under current flow are identified.