Fuel Cell Types

The electrolyte of the PEMFC consists of a proton-exchange membrane. The operating temperature is around 80°C. Cold start, below 0°C, is proven. For transport applications, the PEMFC is the fuel cell of choice. Although for stationary applications many alternatives exist, PEM fuel cells are being applied more and more, taking advantage of the impressive cost reductions in the last five years. Especially when fast start-up and load following dynamics are important and the supply of hydrogen is not an issue, PEM fuel cells offer a clear advantage over high temperature fuel cells. For those applications where reformed fuels are preferred, so-called high temperature PEMFC’s are being developed and applied. The rated power density of the PEMFC is nowadays 0.7 W.cm^-2 and higher, depending on operating conditions

The direct methanol fuel cell is a variation of the PEMFC; it uses the same type of electrolyte. Instead of using hydrogen as fuel, methanol as solution in water is directly oxidized to CO₂. The power density of the DMFC is considerably lower than that of the PEMFC. Maximum power densities, 0.25 W.cm^-2 are obtained at a cell voltage as low as 0.4V. Compared to the PEMFC, high noble metal loadings are used, 1.2 mg.cm^-2 or higher.

The direct methanol fuel cell is mostly applied for low power applications, such as portable electronics, where they replace batteries, and as battery charger in the kW range for e.g. leisure and military applications.

Liquid Phosphoric Acid is the electrolyte of the PAFC. The operating temperature is around 200°C. The PAFC can use reformate with CO concentrations up to 1-2%. The power density of the PAFC is in the range of 0.14 W.cm^-2. Although the PAFC used to dominate the demonstration market in the 100 – 200 kW range, it seems to be overtaken by both PEMFC and MCFC systems in this segment. Typical applications are in the industrial and commercial combined heat and power.

The electrolyte of the AFC consists of liquid Potassium Hydroxide. The operating temperature is around 80°C, but can be as high as 200°C. The AFC is currently being used for power generation on space crafts. The use of AFCs is limited because practically only pure hydrogen can be used as fuel. Air needs to be cleaned from CO₂, which limits the application for terrestrial applications considerably. The power density of the AFC is in the range of 0.1 – 0.3 W.cm^-2. A big potential advantage of alkaline fuel cells is that non-platinum containing electrodes can be used for both anode and cathode. In practice however, platinum is often used to enable higher power densities.

Recent developments are towards applying anion-exchange membranes, to get rid of the disadvantage of using a liquid electrolyte.

A molten mixture of lithium, sodium and potassium carbonate is used as the electrolyte in the MCFC. They also require carbon dioxide to be delivered to the cathode. As a consequence, contrary to other fuel cells, they require CO₂ emission control. The operating temperature is between 600 and 700°C. Due to the high operating temperature, internal reforming of hydrocarbon fuels is possible. The power density of the MCFC is in the range of 0.1 – 0.12 W.cm^-2. The power of MCFC systems is in the 50 kWe - 5 MWe range. Typical applications are in the industrial and commercial combined heat and power.

Yttrium stabilized Zirconia is generally used as the solid electrolyte in the SOFC. Depending on the electrolyte and the material composition of the electrodes, the SOFC can be operated between 600°C and 1000°C. Fuels ranging from hydrogen to natural gas and higher hydrocarbons can be used. The SOFC is mainly in development for stationary power generation for systems in the 1 kWe - 5 MWe range, although this range appears to be narrowed to systems of below 100 kW. While it used to be considered an important option for auxiliary power units on board of vehicles, this has not received much attention the last years. The power density of the SOFC is in the range of 0.15 – 0.7 W.cm^-2.