Chemical energy fuel cells

A fuel cell is basically an energy conversion device. This means that it takes one type of energy and changes it to another type of energy. Fuel cells use hydrogen and oxygen to cause a chemical reaction. The chemical reaction produces water and, with it, electricity. 

Cogeneration strategies were described in Chapter 10 with regard to the simultaneous production of energy and chemicals in fuel cells. A similar concept can be applied in environmental chemistry because electrochemical degradation of most contaminants can yield useful products and eventually produce energy. 

Normal batteries have the advantage of being relatively small, which makes them easily inserted, removed, and transported from place to place. Such batteries are limited in the amount of current they produce by the amount of the reagents inside the battery. When the oxidizable reagent in the battery is consumed, the battery is dead unless it is a rechargeable battery. One way to overcome this problem is to use fuel cells which, like batteries, have an electrode where oxidation takes place and an electrode where reduction takes place. However, fuel cells do not depend on chemicals stored inside the electrode compartments for their energy. Fuel cells produce energy from reactants that continuously flow into their compartments while the chemical reaction products flow out of them. 

A fuel cell is an electrochemical device that continuously and directly converts the chemical energy of externally supplied fuel and oxidant to electrical energy. Fuel cells are customarily classified according to the electrolyte employed. The five most common technologies are polymer electrolyte membrane fuel cells (PEM fuel cells or PEMFCs), alkaline fuel cells (AFCs), phosphoric acid fuel cells (PAFCs), molten carbonate fuel cells (MCFCs) and solid oxide fuel cells (SOFCs). However, the popularity of PEMFCs, a relatively new type of fuel cell, is rapidly outpacing that of the others. 

As you will learn in thermodynamics, the conversion from chemical energy to heat to mechanical energy and finally to electrical energy is accompanied by unavoidable energy losses. Because fuel cells convert chemical energy directly to electrical energy, fuel cells are potentially more efficient. Recent reports indicate that fuel cells can replace burners and turbines in electricity generation plants. These plants would be much smaller, with capacity on the order of 1 MW. 

The GC spontaneously converts chemical energy into electric energy. In an EC, electric energy of a battery (a dc power supply) is used to produce chemicals. A fuel cell is a GC in which fuel is used as one of the cell chemicals. The cathode (and the anode) can be either positive or negative. In a GC, the cathode is positive, and in an EC, the anode is positive. If an electrochemical cell can work in both modes (galvanic and electrolytic), each of the electrodes can be either the cathode or the anode. However, the electrode polarity remains the same. In a rechargeable battery, the electrodes should be called positive electrode and negative electrode. 

MEMS have the potential to revolutionize many technologies, and the number of commercial applications is increasing rapidly. Many applications, such as pumps, motors, and actuators, can be improved with onboard power supphes, and various technologies are being explored to provide such power. Obvious choices, such as chemical batteries, fuel cells, and fossil fuels, show some promise, but none of them can match radioisotope power for long, unattended operation (Blanchard et al., 2001). This is becanse of the larger energy density available with nnclear sonrces. 

In fuel cells an attempt is made to make the fullest possible use of the free energy of reactions, such as the combustion of fuels, to produce electrical energy. Processes are chosen which occur as nearly reversible as possible in order to obtain the maximum useful proportion of AG. The mode of operation of fuel cells is fundamentally different from that of batteries. While batteries store electrical energy, fuel cells convert energy obtainable from chemical processes directly into electricity. 

Hydrogen use as a fuel in fuel cell appHcations is expected to increase. Fuel cells (qv) are devices which convert the chemical energy of a fuel and oxidant directiy into d-c electrical energy on a continuous basis, potentially approaching 100% efficiency. Large-scale (11 MW) phosphoric acid fuel cells have been commercially available since 1985 (276). Molten carbonate fuel cells (MCFCs) ate expected to be commercially available in the mid-1990s (277). 

Fuel Cell Catalysts. Euel cells (qv) are electrochemical devices that convert the chemical energy of a fuel direcdy into electrical and thermal energy. The fuel cell, an environmentally clean method of power generation (qv), is more efficient than most other energy conversion systems. The main by-product is pure water. 

Electrochemical systems convert chemical and electrical energy through charge-transfer reactions. These reactions occur at the interface between two phases. Consequendy, an electrochemical ceU contains multiple phases, and surface phenomena are important. Electrochemical processes are sometimes divided into two categories electrolytic, where energy is supplied to the system, eg, the electrolysis of water and the production of aluminum and galvanic, where electrical energy is obtained from the system, eg, batteries (qv) and fuel cells (qv). 

Michael Krumpelt, Ph.D., Manager, Fuel Cell Technology, Argonne National Laboratory Member, American Institute of Chemical Engineers, American Chemical Society, Electrochemical Society (Section 27, Energy Resources, Conversion, and Utilization)... 

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