Materials Engineering and Fuel Cell Development

Extract

The direct conversion of chemical energy in a fuel cell has been one of the most challenging technological problems since a simple hydrogen/oxygen fuel cell was first demonstrated 150 years ago by Groves at the London Institution. Aqueous electrolyte fuel cells have been successfully used in space to power electrical equipment in the Apollo spacecraft and in the space shuttle, but terrestrial applications have been principally limited to a few specialized military and telecommunication applications.

In the original concept of a fuel cell, a primary fuel (e.g., hydrogen, hydrocarbon) is reacted with oxygen in an electrochemical cell incorporating an acid electrolyte, namely:

so as to convert the free energy of oxidation (ΔG°) of the fuel to carbon dioxide and water directly into electrical energy.

Fuel cells are attractive in that theoretical thermodynamic energy/fuel conversion ratios given by the appropriate value of ΔG°/ΔH° (free energy change/enthalpy change) can be very high (e.g., ~95% for H₂/O₂ reaction at 298 K) compared to the limitations imposed on heat engines by the Carnot cycle efficiency, (T₂−T₁)/T₂. However, in practice it has proved difficult to realize the high electrical/fuel conversion efficiencies promised by fuel cell systems. This is caused by problems associated with the electrochemical kinetics and/or the requirement that in the Faradaic reaction both mass and electron transfer must occur in a spatially restricted region where an electrode, an electrolyte, and one of the reacting phases (usually a gas) are in intimate contact. This region is often referred to as the three-phase boundary.