Fuel Cells

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Abstract: Fuel cells are electrochemical devices that convert the chemical energy of a fuel into electricity. The proton exchange membrane fuel cell (PEMFC) uses hydrogen as a fuel and oxygen as an oxidant. In addition to the fuel cell stack, a fuel cell system needs a number of other components, including an air compressor, fuel pump or compressor, coolant pump, heat exchangers, a controller and a boost converter. While the fuel cell system efficiency at low load is higher that the diesel engine, high load efficiency can be similar or lower. Fuel cell vehicles are at the initial stage of commercialization, due to still insufficient maturity of the technology and a lack of hydrogen fueling infrastructure.

Introduction

Historically, Sir William Grove is credited with creating the first fuel cell in 1839. However, he could not produce enough power to compete with other sources of power available in his time [395]. In the 1930s, approximately 100 years later, Francis Bacon made significant engineering advances in fuel cell technology. By 1959, after 27 years of research and development, he was able to produce a 5 kW fuel cell system that powered a fork-lift truck. Since this modest beginning, fuel cells have been further developed and used in various applications. They are used to provide power for life-support systems aboard space shuttles, to power homes and businesses, and as propulsion systems for vehicles.

Fuel cells are electrochemical devices that convert the chemical energy of a fuel and oxidizer into electricity through a pair of redox reactions. The basic components include an anode, a cathode, and an electrolyte. The fuel is oxidized at the anode with the aid of a catalyst to yield ions and electrons that pass through the electrolyte and an external circuit respectively to the cathode where, with the aid of another catalyst, they react with oxygen to yield water, possibly other reaction products and heat. While hydrogen is a common fuel used in fuel cells, other fuels can also be used. Unlike batteries, which must be periodically recharged, fuel cells can produce power as long as they are supplied with fuel and oxidant.

Figure 1 shows an example of a fuel cell that uses a solid electrolyte consisting of proton-conducting membrane. This membrane is sandwiched between two electrodes. Hydrogen is fed through channels on one side of the membrane and air flows through channels on the other side. The hydrogen side is designated the anode where a platinum catalyst ionizes the hydrogen molecules into negatively charged electrons and hydrogen ions (protons). The latter migrate through the electrolyte toward the cathode. Oxygen is supplied to the cathode where it combines with the hydrogen ions to form water vapor which eventually exits from the fuel cell. The result is an electrical voltage between the negative terminal of the anode and the positive terminal of the cathode. Stacks of electrodes sandwiching proton exchange membranes, a fuel cell stack, are used to generate the power required for a given application.

In addition to the fuel cell stack, a fuel cell system needs a number of other components to ensure proper operation, Figure 2 and Figure 3 [5631][5632]. These include an air compressor, fuel pump or compressor, coolant pump, heat exchangers, a controller and a boost converter that converts the unregulated electrical output from the fuel cell to a stable and usable voltage that matches the load characteristics. These balance-of-plant (BOP) components have varying power demands that can affect the net output from the fuel cell system.