Other Types of Fuel Cells

The situation changed drastically in the mid-1990s in view of the considerable advances made in the development of membrane hydrogen-oxygen (air) fuel cells, which could be put to good use for other types of fuel cells. At present, most work in methanol fuel cells utilizes the design and technical principles known from the membrane fuel cells. Both fuel-cell types use Pt-Ru catalyst at the anode and pure platinum catalyst at the cathode. The membranes are of the same type. 

Hydrocarbons such as natural gas or methane can be reformed internally in the SOFC, which means that these fuels can be fed to the cells directly. Other types of fuel cells require external reforming. The reforming equipment is size-dependent which reduces the modularity. 

These selection and evaluation criteria were applied systematically to four technological fields, three of which contribute to new energy-efficient solutions. Passive houses, for example, with their major components of insulation solutions, window systems, ventilation and control techniques are close to market diffusion within the next ten years. Fuel cells for mobile uses in vehicles, however, are still a long way from market introduction, for instance, because of unresolved problems regarding the deactivation of the membrane electrode assembly (MEA) and the need for cost reductions by about one order of magnitude. Other types of fuel cells for stationary uses may be closer to market introduction, owing to less severe technical bottlenecks and better economic competitiveness. 

The net result of current flow in a fuel cell is to increase the anode potential and to decrease the cathode potential, thereby reducing the cell voltage. Figure 2-3 illustrates the contribution to polarization of the two half cells for a PAFC. The reference point (zero polarization) is hydrogen. These shapes of the polarization curves are typical of other types of fuel cells. 

Discusses principles for selecting materials for other types of fuel cells and electrochemical devices... 

A solid oxide fuel cell (SOFC) consists of two electrodes anode and cathode, with a ceramic electrolyte between that transfers oxygen ions. A SOFC typically operates at a temperature between 700 and 1000 °C. at which temperature the ceramic electrolyte begins to exhibit sufficient ionic conductivity. This high operating temperature also accelerates electrochemical reactions therefore, a SOFC does not require precious metal catalysts to promote the reactions. More abundant materials such as nickel have sufficient catalytic activity to be used as SOFC electrodes. In addition, the SOFC is more fuel-flexible than other types of fuel cells, and reforming of hydrocarbon fuels can be performed inside the cell. This allows use of conventional hydrocarbon fuels in a SOFC without an external reformer. 

After thirty years of basic research, it was demonstrated that SOFCs potentially have many significant advantages over traditional generators of electricity and other types of fuel cells. These advantages are essentially related to the high operating temperature and to the solid state of the cell components, mainly the electrolyte. They can be summarized as follows ... 

In principle, this synergistic power generation method can be applied to other carbon resources such as petroleum, coal and biomass, and to other types of fuel cell such as SOFC and PEFC. 

The SOFCs have practically the same advantages as the MCFCs for applications in electric utility companies and chemical industries. An additional advantage is that, because the SOFC power plant is a two-phase system (gas and solid) whereas all other types of fuel cells are three-phase systems (gas, liquid, and solid), the complex problems associated with liquid electrolytes are eliminated... 

This cell works optimally at 80 °C using relatively inexpensive materials. When it is switched on in the cold, it produces about one-quarter of the power finally produced after it warms up. This is an advantage compared with other types of fuel cells operating at intermediate (200 °C) or high (650 to 1000 °C) temperatures, which need an auxiliary power source to start them and warm them up. The alkaline environment means that a wide range of electrode catalysts are available, while cells using acid solutions can only use noble metal electrode materials, which is a distinct economic disadvantage for terrestrial applications. 

General. There is no doubt that a comparison (updated in the 1990s) of the potential vs. current density plot for the various fuel cells (see Fig. 13.27) shows that the proton exchange membrane fuel cell with a perfluoropolymer sulfuric acid has superior performance (i.e., higher cell potential and hence efficiency) compared with the other types of fuel cells. Because this cell has been chosen for development by the majority of the automotive manufacturers, special attention is given here to its development. 

Figure 6.2. Scenario results for market introduction of fuel cell vehicles in Japan, giving the stock of fuel cell vehicles as a function of time, along with the decline in cost of passenger fuel cell cars (the stock comprises other types of fuel cell vehicles as well) (based on Tsuchiya et ah, 2004).

There are other types of fuel cells of which proton exchange membranes (PEMFC), molten carbonate (MCFC), and solid oxide (SOFC) are the most promising. These fuel cells are at various stages of technology demonstration and are not commercially available. Each type of fuel cell has its own preferred range... 

PEM-type fuel cells have the advantage of operating at pressures from 101 to 810 kPa. In addition, they work well at lower temperatures (a feature not present in other types of fuel cells). These fuel cells can operate at 55% efficiency versus a maximum of 30% for internal combustion engines [1], In order to achieve the necessary energy to power the motors for propulsion, fuel cells are manufactured in a stack arrangement. In these arrangements, other components are needed (such as humidifiers to keep the cell moist, air compressors, and gas filters) to make up the fuel-cell propulsion unit. Each stack has a flat cathode sheet and a flat anode sheet, which are stacked in order to meet the particular vehicle s power requirements [1], Current vehicle testing... 

Proton exchange membrane (PEM) fuel cells are the best choice for automobiles due to their high-power density, low mass, and low operating temperatures as compared to other types of fuel cells. 

Extend the models to other types of fuel cells. 

As an example of areal fuel cell, therefore, we consider the hydrogen-oxygen fuel cell with an aqueous acid electrolyte and its special features. The special features of other types of fuel cells are described in the following chapters. 

---