PEM Fuel Cell Operation

The principle of how a PEM fuel cell generates electricity is straightforward. However, the cell power output depends on material properties, cell design and structure, and operation conditions, such as the gas flow, pressure regulation, heat, and water management. High performance of a PEM fuel cell requires maintaining optimal temperature, membrane hydration, and partial pressure of the reactants 

A PEM fuel cell can be operated at ambient pressure or at a higher pressure. A fuel cell usually obtains better performance when the pressure is increased. But note that to increase the operating pressure, extra compression power is needed. From the system point of view, die net gain of pressurized operation is questionable when compression power is taken into account. In addition, die issue of pressurization is related to the issue of water management [1]. 

Very often, the reactant gas is fed from a pressurized tank to die fuel cell at the inlet. The pressure, known as the backpressure, is controlled by a pressure regulator installed at the outlet. This backpressure regulator keeps the desired pressure at the fuel cell outlet, while the inlet pressure is sometimes not even recorded. However, the inlet pressure, which is always higher than the outlet pressure as there is a pressure drop between die inlet and outlet along the flow channels, is sometimes the pressure that matters. For example, there are two types of air supply systems for the cathode inlet [1]. 

Air blowing In an air-blowing system, air is supplied to the cathode by a mechanical device, a compressor or a blower. The compressor or the blower must be able to deliver the air at the required flow rate and desired pressure. The backpressure regulator may still be installed to control the backpressure either at ambient pressure or at a higher pressure. 

The fuel eell reaction is exothermal therefore it generates heat as a by-product. To maintain the desired temperature, heat must be removed from the system. Some heat dissipates from the outer surface of the fuel cell and the rest must be taken away with a eooling system. The cooling medium may be air, water, or a special coolant. The inner design of the fuel cell must allow the coolant to pass through, for example, a coolant plate or coolant chaimel on the back of the anode or cathode plate. Small fuel cells need a heater to reach the operating temperature because so much heat is being taken away from the outer surface [1]. The heat balance within a fuel cell can be written as 

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For both reactions to occur, a three-phase boundary is required where the reactant gas, protons, and electrons react at the catalyst surface. The CLs should be able to facilitate transport of protons, electrons, and gases to the catalytic sites. Under normal PEM fuel cell operating conditions (<80°C), the reactants are gaseous phase H2 and O2 (from air), and the product is water, primarily in the liquid phase. Water removal is a key factor affecting catalyst... 

J. Yeom, G. Z. Mozsgai, B. R. Flachsbart, et al. Microfabrication and characterization of a silicon-based millimeter scale, PEM fuel cell operating with hydrogen, methanol, or formic acid. Sensors Actuators B 107 (2005) 882-891. 

PEM fuel cells operate at relatively low temperatures, around 80°C. Low temperature operation allows them to start quickly (less warm-up time) and results in less wear on system components, resulting in better durability. However, they require that a noble-metal catalyst (typically platinum) be used to separate the hydrogen s electrons and protons, adding to system cost. The platinum catalyst is also extremely sensitive to CO poisoning, making it necessary to employ an additional reactor to reduce CO in the fuel gas if the hydrogen is derived from an alcohol or hydrocarbon fuel. This also adds cost. Developers are currently exploring platinum/ruthenium catalysts that are more resistant to CO. 

A PEM electrolyzer is literally a PEM fuel cell operating in reverse mode. When water is introduced to the PEM electrolyzer cell, hydrogen ions (protons) are drawn into and through the membrane, where they recombine with electrons to form hydrogen molecules. Oxygen gas remains behind in the water. As this water is recirculated, oxygen accumulates in a separation tank and can then be removed from the system. Hydrogen gas is separately channeled from the cell stack and captured. 

In recent decades, research has intensified to develop commercially viable fuel cells as a cleaner, more efficient source of energy, due to the global shortage of fossil fuels. The challenge is to achieve a cell lifetime suitable for transportation and stationary applications. Among the possible fuel cell types, it is generally believed that PEM fuel cells hold the most promise for these uses [10, 11], In order to improve fuel cell performance and lifetime, a suitable technique is needed to examine PEM fuel cell operation. EIS has also proven to be a powerful technique for studying the fundamental components and processes in fuel cells [12], and is now widely applied to the study of PEM fuel cells as well as direct methanol fuel cells (DMFCs), solid oxide fuel cell (SOFCs), and molten carbonate fuel cells (MCFCs). 

High PEM Fuel Cell Advantage and Characteristics of its AC Impedance PEM fuel cells operated at high temperatures (> 100°C) have several advantages over those operated at lower temperatures (1) faster electrochemical kinetics, (2) improved and simplified water management, (3) effective thermal management,... 

When connected through an external circuit, the net result of these two half-cell reactions is the production of H2O and electricity from H2 and O2. Heat is also generated in the process. In the absence of a proper catalyst, however, neither of these two half reactions takes place at meaningful rates under PEM fuel cell operating conditions (50 to 80°C, 1 to 5 atm). Despite decades of effort in search of cheaper alternatives, platinum is still the catalyst of choice for both the HOR and ORR. 

Polyarylenes, in particular different types of poly(arylene ether ketone)s, have been the focus of much hydrocarbon membrane research in recent years. - - With good chemical and mechanical stability under PEM fuel cell operating conditions, the wholly aromatic polymers are considered to be the most promising candidates for high-performance PEM fuel cell applications. Many different types of these polymers are readily available and with good process capability. Some of these membranes are commercially available, such as poly(arylene sulfone)s and poly(arylene... 

Stanic, V. and Hoberecht, M., MEA failure mechanisms in PEM fuel cells operated on hydrogen and oxygen, in Extended Abstracts of2004 Fuel Cell Seminar, San Antonio, TX, November 1-5, 2004, p. 85. 

The membrane layer consists of a pol3mrier structure capable of transporting hydrogen ions with high conductivity, inspiring the name "proton exchange membrane". The membrane is thus a solid electrolyte. For the current emphasis on PEM fuel cells operating below 100°C, the Nation (trademark of... 

Consider the replicated impedance measurements presented in Table 3.9 for a 5 cm Polymer Electrolyte Membrane (PEM) fuel cell operating at a current of 1 A. The measurements were collected at a frequency of 1 Hz on the same cell using two different sets of instrumentation, the 850C provided by Scribner Associates and the FC350 provided by Gamry Instruments. 

Figure 23.4 Comparison of the impedance response for a PEM fuel cell operated at 0.2 A/cm to model predictions generated using a reaction sequence involving formation of hydrogen peroxide and a reaction sequence involving formation of PtO. (Taken from Roy et al. and reproduced with permission of The Electrochemical Society.)...

To avoid these problems, engineers have focused on a cell in which the fuel is hydrogen gas, the oxidant is oxygen from the air, and the product is water vapor. One of the more promising hydrogen fuel cells is one in which the half-cell reactions are separated by a thin polymer sheet called a proton-exchange membrane (PEM). The PEM fuel cell operates at approximately 100°C, and the moist membrane itself is the electrolyte. 

Several properties of cellulose phosphate have been evaluated. The material has good thermal stability and low hydrogen crossover, two requirements that are important to meet DOE fuel cell program targets. Further characterization of the material is required, especially the determination of proton conductivity. In addition, testing of the material in an MEA will allow the effect of acid stability and swelling properties of cellulose phosphate be evaluated under typical PEM fuel cell operating conditions. 

Improve overall PEM fuel cell operating efficiency. 

Modify baseline chemical sensing technology to create sensors capable of making the measuring the parameters of Table 3.2 in a PEM fuel cell operating environment. 

PEM fuel cells operate with hydrogen. At the moment, there is no hydrogen storage available which is suitable for miniature applications. For direct methanol fuel cells (DMFCs) a better storage opportunity exists in form of methanol cartridges. 

The greatest utility of the STR PEM fuel cell reactor is to study the dynamics of PEM fuel cell operation. Specific questions that have been explored are as follows ... 

While the model presented applies to a wide range of PEM fuel cell operational regimes there are some caveats to its application. We assume the flux of gas and heat out of the catalyst layer scale with the current density. This would not be the case, for example, with a dry anode feed and a wet cathode feed, which would generate water transport independent of the current level. We assume there is no lateral pressure gradient imposed in the x direction across the GDL such as would arise in interdigitated or serpentine flow fields we consider straight flow fields. 

Franco AA (2013) Toward a bottom-up multiscale modeling framework for the transient analysis of PEM fuel cells operation. In Franco AA (ed) Polymer electrolyte fuel cells science, applications and challenges. CRC PressATaylor Francis Group, Boca Raton... 

This is also called the Solid Polymer Fuel Cell (SPFQ and the Direct Methanol Fuel Cell (DMFC) is included in this classification. These cells use a solid perfluorinated sulfonated polymer ion exchange membrane (e.g. DuPont Nafion) [65] in the form of a thin plastic film, which serves as the electrolyte in the PEM fuel cell operating at 50-100°C. 

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