A fuel cell is an electrochemical energy device that converts the chemical energy in the fuel directly into electrical energy. It is becoming an increasingly attractive alternative to other conversion technologies, from small-scale passive devices like batteries to large-scale thermodynamic cycle engines. Unlike conventional power devices, i.e., steam turbines, gas turbines, and internal combustion engines, which are based on certain thermal cycles, the maximum efficiency of fuel cells is not limited by the Carnot cycle principle. Figure 1 is a schematic of a general fuel cell (Faghri and Guo, 2005). A fuel cell generally functions as follows: electrons are released from the oxidation of fuel at the anode, protons (or ions) pass through a layer of electrolyte, and the electrons are used in reduction of an oxidant at the cathode. The desired output is the largest possible flow of electrons over the highest electric potential. Although other oxidants such as halogens have been used where high efficiency is critical, oxygen is the standard because it is readily available in the atmosphere. Fuel cells typically use hydrogen, carbon monoxide, or hydrocarbon fuels (i.e., methane, methanol). The hydrogen and carbon monoxide fuels may be the products of catalytically-processed hydrocarbons. Hydrogen from processed ammonia is also used as fuel.
Two cases are possible for the electrolyte: it may be a conductor for anions or for cations. In the first case, the oxidant at the cathode combines with electrons, which tend to circumvent the electrolyte, and become anions which travel through the electrolyte to the anode (Fuel Cell Handbook, 2000). At the anode, the anions give up their electrons and combine with hydrogen to form water. The water, depleted fuel, and products are exhausted from the anode surface, and the depleted oxidant and products are exhausted from the cathode surface. In the second case, where the electrolyte conducts cations, the hydrogen-containing fuel is decomposed electrochemically, releasing electrons and leaving hydrogen cations to travel through the electrolyte. Upon reaching the cathode, the cations combine electrochemically with the oxidant and electrons, which tend to circumvent the electrolyte to form water. The water, depleted oxidant, and other gases present are exhausted from the cathode, while the depleted fuel and product gases are exhausted from the anode.
There are several types of fuel cells, and they each belong to one of the two cases just described. Anion-conducting electrolyte fuel cells are (Larminie and Dicks, 2000): alkaline fuel cells – for example, those using potassium hydroxide molten carbonate that operates at about 650 C – and solid oxide fuel cells that operate to 1000 °C. Cation-conducting electrolyte fuel cells include phosphoric acid fuel cells and polymer electrolyte membrane fuel cells. The latter, with power capacities from small batteries to automotive use, are receiving the most commercial and research attention.
Among different types of fuel cells, the proton exchange membrane fuel cell (PEMFC) is one of the best candidates as an alternative energy source in the future because it offers advantages in light weight, durability, high power density and rapid adjustment to power demand. The fuel for PEMFCs can be either hydrogen or methanol. Figure 1.20 shows the basic structure of a PEMFC, which can be subdivided into three parts: the membrane electrode assemblies (MEAs), the gas diffusion layers (GDLs), and bipolar plates (Faghri, 2006; Faghri and Guo, 2005). The key component of the PEMFC is the MEA, which is composed of a proton exchange membrane sandwiched between two fuel cell electrodes: the anode, where hydrogen is oxidized, and the cathode, where oxygen from air is reduced. A gas diffusion layer is formed from a porous material that must have high electric conductivity, high gas permeability, high surface area and good water management characteristics. One side of the bipolar plate is next to the cathode of a cell, while the other side is next to the anode of the neighboring cell. The fuel cell stack consists of a repeated, interleaved structure of MEAs, GDLs and bipolar plates. It is evident that flow channels are an essential component for flow distribution in many PEMFC designs. The flow channels in a PEMFC are typically on the order of a 1 mm hydraulic diameter, which falls into the range of minichannels (i.e., hydraulic diameters from 0.2 to 3 mm). As shown in Fig. 2, one channel wall is porous (gas diffusion layer); mass transfer occurs on this wall along its length. Hydrogen is consumed on the anode side along the main flow dimension in minichannels. Oxygen from air is introduced on the cathode side to form water at catalyst sites at the cathode; this water is transported into the minichannels through the gas diffusion layer, and eventually it is removed from the cell by the gas flow – and gravity – if so oriented.
Several factors affect the efficiency of fuel cells. The operating temperature determines the maximum theoretical voltage at which a fuel cell can operate. Higher temperatures correspond to lower theoretical maximum voltages and lower efficiencies. However, a higher temperature at the electrodes increases electrochemical activity, which in turn increases efficiency. A higher operational temperature also improves the quality (exergy) of the waste heat. In addition, increasing pressure increases both maximum theoretical voltage and electrochemical activity. However, electrical resistance in the electrodes and corresponding connections, ionic resistance, and electrical conductivity in the electrolyte all lower cell efficiency. Efficiency is also affected by mass transport of products to the electrodes, as well as product permeation through the electrolyte. Development and application of fuel cell technology has increased significantly through analysis and improvement of the heat and mass transfer in the fuel cell stack and auxiliary components and by implementation of innovative heat systems that address these issues. A detailed review of these problems related to fuel cell technology, including challenges and opportunities is presented by Faghri and Guo (2005). Current technical limitations can be overcome through a combination of novel technologies, increased efficiencies of components, decreased component size and mass and detailed modeling. There has been extensive development of fuel cell modeling that has been reviewed by Wang (2004), and Faghri and Guo (2005). A successful fuel cell model will capture all of the physics of the problem including reaction kinetics, the electric potential field, the fluid flow as well as thermal field (Rice and Faghri 2006; 2008).
Faghri, A., 2006, “Unresolved Issues in Fuel Cell Modeling,” Heat Transfer Engineering, Vol. 27, pp. 1-3.
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Fuel Cell Handbook, CD, 5th edition, U.S. Department of Energy, Office of Fossil Energy, Federal Energy Technology Center, October 2000.
Larminie, J. and Dicks, A., 2000, Fuel Cell Systems Explained, Wiley, New York.
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Rice, J., and Faghri, A., 2008, “Thermal Startup Characteristics of a Miniature passive Liquid Fuel DMFC System,” ASME Journal of Heat Transfer, Vol. 421, No. 3 pp. 475-486.
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