Extracting the electrical energy from the simple reaction between hydrogen and oxygen to produce water is an extremely attractive proposition, which is exactly what a fuel cell does:

2H2 + O2—> 2H2O + electricity + heat

A basic fuel cell consists of an anode and a cathode separated by an electrolyte as shown in Fig. 9.1 of the following chapter. At the anode, hydrogen is separated into protons and electrons, and because the electrolyte only conducts protons, electrons are forced through an external circuit, providing the current to do work. At the cathode, oxygen reacts with protons and electrons to produce water and heat. It is possible to capture the heat for cogeneration, leading to overall efficiencies as high as 90%. Fuel cells are fundamentally more efficient than combustion systems, and efficiencies of 40-50% are gained in today’s applications. We note that the ubiquitous internal combustion engine used in transport has an efficiency below 20%. The main challenge facing fuel cell use is probably system cost, with esti­mates for automotive applications being * $50/kW, compared with the * $30/kW that is required to make fuel-cells favourable over internal combustion. Internal combustion engines are highly developed and matching their durability, reliability, weight and infrastructure with fuel cells is a considerable hurdle to the commercial uptake of fuel cells, but one which can be overcome by understanding the funda­mental limitations of fuel cells.

Fuel cells are typically distinguished by the type of electrolyte used in charge transport. The major classes of fuel cells include: alkaline fuel cells (AFC), solid oxide fuel cells (SOFC), phosphoric acid fuel cells (PAFC), molten carbonate fuel cells (MCFC) and proton-exchange (or polyelectrolyte) membrane fuel cells (PEMFCs). In Chap. 9 we concentrate on solid oxide fuel cells (SOFC) which use ceramic materials as the electrolyte, enabling their operation at high temperatures using a variety of fuels. Higher temperatures are required for adequate diffusion rates of protons and other charged species, which are measurable via neutron scattering, but impose materials problems and significant start-up delays. Chapter 10 presents neutron scattering studies of the operation of PEM fuel cells,

where the aqueous environment provides rapid diffusion of the charged species at much lower temperatures, but poses other challenges. These two fuel cell chapters make use of a wide range of neutron techniques because they are concerned with structure and dynamics over a variety of length scales, from atomic through to macroscopic. The reader is referred to Chap. 1 for an outline of these techniques for a more thorough description. Chapter 9, in particular, is an excellent demonstration of how combining information from a variety of neutron techniques of analysis leads to a more complete understanding of structure/dynamics-function relations.