Hydrogen Fuel

General Fuel Cell Characteristics

Fuel cells are electrochemical devices that convert the chemical energy of a fuel directly to usable energy - electricity and heat - without combustion. This is quite different from most electric generating devices (e.g., steam turbines, gas turbines, reciprocating engines), which first convert the chemical energy of a fuel to thermal energy, then to mechanical energy and finally to electricity. Fuel cells are similar to batteries containing electrodes and electrolytic materials to accomplish the electrochemical production of electricity.
Batteries store chemical energy in an electrolyte and convert it to electricity on demand, until the chemical energy has been depleted. Applying an external power source can recharge depleted secondary batteries, but primary batteries must be replaced. Fuel cells do not store chemical energy, but rather, convert the chemical energy of a fuel to electricity. Thus fuel cells do not need recharging and can continuously produce electricity as long as fuel and oxidant are supplied. Figure 1 presents the basic components of a fuel cell, which include a positive electrode (anode), negative electrode (cathode) and an electrolyte. Fuel is supplied to the anode (positive electrode) while oxidant is supplied to the cathode (negative electrode). Fuel is electrochemically oxidized on the anode surface and oxidant is electrochemically reduced on the cathode surface. Ions created by the electrochemical reactions flow between anode and cathode through the electrolyte. Electrons produced at the anode flow through an external load to the cathode completing an electric circuit.

A typical fuel cell requires gaseous fuel and oxidant flows. Hydrogenis the preferred fuel because of its high reactivity, which minimizes the need for expensive catalysts, and because electro-oxidation of hydrogen leads only to water emission. Hydrocarbon fuels can be supplied but typically require conversion to hydrogen or a hydrogen-rich mixture before electrochemical reaction can occur. This fuel processing step can be accomplished prior to entering the fuel cell (for lower temperature fuel cells) or within the fuel cell (for higher temperature fuel cells). Oxygen in air is the preferred oxidant because of its availability in the atmosphere.As indicated in Figure 1, the electrolyte serves as an ion conductor.The direction of ion transport depends upon the fuel cell type, which determines the type of ion that is produced and transported across the electrolyte between the electrodes. The various fuel cell types are described in a subsequent section.A single fuel cell is only capable of producing about 1 volt, so typical fuel cell designs link together many individual cells to form a “stack” that produces a more useful voltage. A fuel cell stack can be configured with many groups of cells in series and parallel connections to further tailor the voltage, current and power produced. The number of individual cells contained within one stack is typically greater than 50 and varies significantly with stack design. Figure 2 presents the basic components that comprise the fuel cell stack. These components include the electrodes and electrolyte of Figure 1 with additional components required for electrical connections and to provide for the flow of fuel and oxidant to each cell in the stack. These key components include current collectors, separators, and gas flow channels, which are often integrated into one design as in the “interconnect” design pictured in Figure 2. This interconnect serves as current collector and gas separator and provides the flow channels for both fuel and oxidant. The interconnect provides the electrical connections between cells and physically separates the oxidant flow of one cell from the fuel flow of the adjacent cell. The channels serve as the distribution pathways for the fuel and oxidant. The preferred fuel for most fuel cell types is hydrogen. Hydrogen is not readily available, but, and the infrastructure for provision of hydrocarbon fuels is well established in our society. Thus, fuel cell systems that have been developed for practical power generation applications to-date have been designed to operate on hydrocarbon fuels. This typically requires the use of a fuel processing system or “reformer” as shown in Figure 3. The fuel processor typically accomplishes the conversion of hydrocarbon fuels to a mixture of hydrogen rich gases and, depending upon the requirements of the fuel cell, subsequent removal of contaminants or other species to provide pure hydrogen to the fuel cell.
In addition to the fuel cell system requirement of a fuel processor for operation on hydrocarbon fuels, Figure 3 presents the need for a power conditioning or inverter system component as well. This is required for the use of current end-use technologies that are designed for consuming alternating current (AC) electricity, and for grid connectivity in distributed power applications. Since the fuel cell produces direct current (DC) electricity, the power conditioning section is a requirement for fuel cell systems that are designed for distributed generation today. In the future, systems and technologies may be amenable to the use of DC electricity, which would allowsignificant cost savings.