Background

1.1.1 History of fuel cells

The invention of the fuel cell is widely attributed to Sir William Grove, a Welsh judge and amateur scientist. In 1839, he discovered that mixing hydrogen and oxygen in the presence of an electrolyte produced electricity and water. The term “fuel cell” was coined in 1889 by L.

Mond and C. Langer, who attempted to develop a fuel cell that uses industrial coal gas and air.

The technology had no practical value until the 1930’s, when Francis T. Bacon began his groundbreaking research. Bacon applied the platinum catalysts employed by Mond and Langer in a hydrogen-oxygen fuel cell using less corrosive alkaline electrolyte and nickel electrodes. It took Bacon years to overcome technical challenges and it was not until 1959 that he and his co-workers were able to demonstrate a practical five-kilowatt fuel cell stack.

NASA’s influence in the development of fuel cells is not to be understated. In the 1950’s, NASA decided to use fuel cells to supply power during space flight for their manned space missions. NASA funded over 200 research projects on fuel cell technology and consequently, fuel cells have provided on-board electricity and water to the Gemini, Apollo and Space Shuttle missions.

For a long time, the price of fuel cell systems was prohibitive and they were used only in special applications, where good performance was the primary concern. In the last 20 years, ongoing research has produced new materials and solutions, which have led to improving fuel cell economies. That, coupled with concern for the environment and earth’s limited resources,

has led to introduction of commercial applications of fuel cells, mainly in the fields of ground transport and distributed power generation.

1.1.2 About fuel cells

Fuel Cells are electrochemical energy converters. They can be regarded as black-boxes (see Figure 1-1) converting chemical energy contained in a fuel directly into electrical energy while generating heat and water as by-products.

The basic mechanism underlying this conversion is the same as the one for batteries. The primary difference being that the battery contains the reactants (i.e. fuel and oxidant) that generate electricity whereas those reactants need to be supplied externally to the fuel cell. In other words, a battery needs to be thrown away or recharged once those reactants are depleted while the fuel cell can be refueled more easily and quickly by either refilling the tank with

Fig.1-1. Schematic representation of different energy (battery, fuel cell and internal combustion engine) converters in the form of black-boxes.

combustion engines (ICE) which, when provided with fuel and air, generate mechanical power with heat and exhaust gases as byproducts.

Fuel cells are customarily classified according to the electrolyte employed. The five most common fuel cell types are

– Polymer Electrolyte Membrane Fuel Cells (PEMFC), – Alkaline Fuel Cells (AFC),

– Phosphoric Acid Fuel Cells (PAFC), – Molten Carbonate Fuel Cells (MCFC) and – Solid Oxide Fuel Cells (SOFC).

In addition, there are two fuel cell types known as Direct Methanol Fuel Cell (DMFC) and Direct Formic Acid Fuel Cell (DFAFC), which are similar to PEMFC, except they use methanol and formic acid as the fuel, respectively; instead of hydrogen or hydrogen rich gas.

Table 1-1 gives an overview of the main classes of fuel cells with their associated fuels, operating temperatures and electrolyte types.

Table 1-1. Comparison of different fuel cells and their operating characteristics.

Data from [1].

Proton Exchange Membrane Fuel Cells (PEMFC) or Polymer Electrolyte Fuel Cells

(PEFC) are based on a solid polymer electrolyte. Fast startup times, low temperature operation and high power densities make them an easy to use technology especially for portable or transport applications. CO poisons the catalyst and the hydrogen fuel has to be very pure. Because the polymer membrane has to be kept well humidified for good proton conduction, water management is one of the critical aspects of successfully running a PEMFC.

Direct Methanol Fuel Cells (DMFC) are similar in construction to PEM fuel cells. Since

liquid methanol can be used as a fuel, no external fuel processing is required and high energy storage densities can be achieved. Unfortunately, the polymer membrane is not impermeable to liquid methanol and the resulting fuel crossover reduces overall system efficiency.

Direct Formic Acid Fuel Cells (DFAFC) are a subcategory of PEM fuel cells. Similar to methanol, formic acid is a small organic molecule fed directly into the fuel cell, removing the need for complicated catalytic reforming. Storage of formic acid is much easier and safer than that of hydrogen because it does not need to be done at high pressures and (or) low temperatures, as formic acid is a liquid at ambient temperature. Formic acid does not cross over the polymer membrane, so its efficiency can be higher than that of methanol.

Alkaline Fuel Cells (AFC) are based on a liquid, concentrated KOH electrolyte. AFCs can

operate with non-precious metal catalysts (typically nickel) and therefore have a cost advantage over other types of fuel cells. The use of a liquid electrolyte requires an additional electrolyte re-circulation system. Unfortunately, CO2 is a poison for the liquid electrolyte and

needs to be scrubbed from process air. Typically, the use of AFCs has been limited to niche applications such as military and space applications.

Phosphoric Acid Fuel Cells (PAFC) are based on a liquid acid electrolyte. Due to their

higher operating temperature, they are less sensitive to CO impurities in the fuel and water management is less of an issue. Additionally, they exhibit excellent long term stability. Their relatively long start-up times and low power densities limit their application to stationary power or co-generation plants.

Molten Carbonate Fuel Cells (MCFC) are based on a liquid molten carbonate electrolyte

and generally exhibit very high conversion efficiencies. A high operating temperature allows direct use of non noble catalysts along with direct internal processing of fuels such as natural gas. Relatively long start-up times and low power densities again limit their application to stationary power or co-generation plants.

Solid Oxide Fuel Cells (SOFC) are based on a solid oxide electrolyte conducting oxygen O²⁻

ions. As with the MCFC, the high operating temperature translates into non-noble catalysts, direct internal hydrocarbon fuel processing and high quality waste heat that can be utilized in combined-cycle power plants. Additionally, high power densities along with high efficiencies can be attained. Slow start-up times dictate their primary use as stationary power or co-generation plants.