Fuel cell, solid polymer membrane types

As with batteries, differences in electrolytes create several types of fuel cells. The automobile s demanding requirements for compactness and fast start-up have led to the Proton Exchange Membrane (PEM) fuel cell being the preferred type. This fuel cell has an electrolyte made of a solid polymer. 

A somewhat different type of rechargeable Li cell is being developed by workers who intend to use a solid polymer membrane as separator [see the use of solid polymer electrolyte (SPE) membranes in some fuel cells (Section 13.6.6)]. The advantage here... 

Solid polymer electrolytes, typically perfluorosulfonic acid (PFSA) membranes, are at the core of Polymer electrolyte membrane fuel cells (PEMFCs). These membranes electrically and mechanically isolate the anode and cathode while, when appropriately humidified, allowing for effective ion migration. Nafion, manufactured by DuPont, is one of the most thoroughly used and studied membranes in the PFSA family. Another family of membranes that holds some promise for use in PEMFCs is the group of sulfonated polyaromatic membranes, typically sulfonated polyetherketones. While research is being performed on other types of membranes, as well as hybrid membranes that might have been better-suited properties, information on these is searce [1-10]. 

There exist a variety of fuel cells. For practical reasons, fuel cells are classified by the type of electrolyte employed. The following names and abbreviations are frequently used in publications alkaline fuel cells (AFC), molten carbonate fuel cells (MCFC), phosphoric acid fuel cells (PAFC), solid oxide fuel cells (SOFC), and proton exchange membrane fuel cells (PEMFC). Among different types of fuel cells under development today, the PEMFC, also called polymer electrolyte membrane fuel cells (PEFC), is considered as a potential future power source due to its unique characteristics [1-3]. The PEMFC consists of an anode where hydrogen oxidation takes place, a cathode where oxygen reduction occurs, and an electrolyte membrane that permits the transfer of protons from anode to cathode. PEMFC operates at low temperature that allows rapid start-up. Furthermore, with the absence of corrosive cell constituents, the use of the exotic materials required in other fuel cell types is not required [4]. 

PEM fuel cells use a solid polymer membrane (a thin plastic film) as the electrolyte. The standard electrolyte material currently used in PEM fuel cells is a fully fluorinated Teflon-based material produced by DuPont for space applications in the 1960s. The DuPont electrolytes have the generic brand name Nafion, and the types used most frequently are 113, 115, and 117 [13, 57-62,68]. The Nafion membranes are fully fluorinated polymers that have very high chemical and thermal stability. This polymer is permeable to protons when it is saturated with water, but it does not conduct electrons. 

An electrolyte is an essential component within fuel cells, used to facilitate the selective migration of ions between the electrodes. Fuel cells are typically classified according to the electrolytes used alkaline fuel cell (AFC), polymer electrolyte (or proton exchange membrane) fuel cell (PEMFC), phosphoric acid fuel cell (PAFC), molten carbonate fuel cell (MCFC), and solid oxide fuel cell (SOFC). Typical efficiencies, operating temperatures and output voltage for the various types of fuel cells are shown in Table 2.14. It should be noted that none of these fuel cells... 

Fuel cells can be classified into phosphoric acid fuel cells, molten carbonate fuel cells, solid oxide full cells, polymer electrolyte membrane fuel cells, and alkaline full cells according to the type of electrolyte used (150). All these fuel cells operate on the same principle, but the t) e of fuel used, operating speed, the catalyst used and the electrolyte used are different. In particular, pol5mier electrolyte membrane fuel cells can be used in small-sized stationary power generation equipment or transportation systems due to their high reaction speed, low operating temperature, high output density, rapid startup, and variation in the requested output. 

The membrane has two functions. First, it acts as the electrolyte that provides ionic conduction between the anode and the cathode but is an electronic insulator. Second, it serves as a separator for the two-reactant gases. Some sources claim that solid polymer membranes (e.g., sulfonated fluorocarbon acid polymer) used in PEMFC are simpler, more reliable, and easier to maintain than other membrane types. Since the only liquid is water, corrosion is minimal. Pressure balances are not critical. However, proper water management is crucial for efficient fuel cell performance [6]. The fuel cell must operate under conditions in which the by-product water does not evaporate faster than it is produced, because the membrane must be hydrated. Dehydration of the membrane reduces proton conductivity. On the other hand, excess of water can lead to flooding of the electrodes. 

Fuel cells are usually classified by the electrolyte employed in the cell. An exception to this classification is DMFC (direct methanol fuel cell) that is a fuel cell in which methanol is directly fed to the anode. The electrolyte of this cell does not determine the class. The operating temperature for each of the fuel cells can also determine the class. There are, thus, low- and high-temperature fuel cells. Low-temperature fuel cells are alkaline fuel cells (AFCs), polymer electrolyte membrane fuel cells (PEMFCs), DMFC, and phosphoric acid fuel cells (PAFCs). The high-temperature fuel cells operate at temperatures —600-1000 °C and two different types have been developed, molten carbonate fuel cells (MCFC) and solid oxide fuel cells (SOFCs). AU types of fuel cells are presented in the following sections in order of increasing operating temperature. An overview of the fuel cell types is given in Table 1.1 [1,5-7]. 

Membrane-type fuel cells. The electrolyte is a polymeric ion-exchange membrane the working temperatures are 60 to 100°C. Such systems were first used in Gemini spaceships. These fuel cells subsequently saw a rather broad development and are known as (solid) polymer electrolyte or proton-exchange membrane fuel cells (PEMFCs). 

Fuel cells can be broadly classified into two types high temperature fuel cells such as molten carbonate fuel cells (MCFCs) and solid oxide polymer fuel cells (SOFCs), which operate at temperatures above 923 K and low temperature fuel cells such as proton exchange membrane fuel cells (PEMs), alkaline fuel cells (AFCs) and phosphoric acid fuel cells (PAFCs), which operate at temperatures lower than 523 K. Because of their higher operating temperatures, MCFCs and SOFCs have a high tolerance for commonly encountered impurities such as CO and CO2 (CO c)- However, the high temperatures also impose problems in their maintenance and operation and thus, increase the difficulty in their effective utilization in vehicular and small-scale applications. Hence, a major part of the research has been directed towards low temperature fuel cells. The low temperature fuel cells unfortunately, have a very low tolerance for impurities such as CO , PAFCs can tolerate up to 2% CO, PEMs only a few ppm, whereas the AFCs have a stringent (ppm level) CO2 tolerance. 

The PEFC was first developed for the Gemini space vehicle by General Electric, USA. In this fuel cell type, the electrolyte is an ion-exchange membrane, specifically, a fluorinated sulfonic acid polymer or other similar solid polymer. In general, the polymer consists of a polytetrafluoroethylene (Teflon) backbone with a perfluorinated side chain that is terminated with a sulfonic acid group, which is an outstanding proton conductor. Hydration of the membrane yields dissociation and solvation of the proton of the acid group, since the solvated protons are mobile within the polymer. Subsequently, the only liquid necessary for the operation of this fuel cell type is water [7,8],... 

In this book the focus is on PEMFCs therefore, in the following sections we will only discuss several major types of PEMFCs, such as H2/air (02) fuel cells, direct liquid fuel cells, PAFCs, and alkaline fuel cells. PEMFCs, also called solid polymer electrolyte fuel cells, use a polymer electrolyte membrane as the electrolyte. They are low-temperature fuel cells, generally operating below 300°C. 

PEMFC (Proton Exchange Membrane Fuel Cell) and SPFC (Solid Polymer Fuel Cell) are the two competing mnemonics of a low-temperature fuel cell type originated for use in space by General Electric, USA. To reflect present practice, the author will use PEFC (Proton Exchange Fuel Cell). The DMFC (Direct Methanol Fuel Cell) also uses proton exchange membranes, but is referred to by its own mnemonic. Proton exchange between polar water molecules is discussed by Koryta (1991 1993) and in the introduction to this book. 

Polymer electrolyte membrane (PEM) fuel cells use a solid polymer as an electrolyte and porous carbon electrodes containing a platinum catalyst. They need only hydrogen, oxygen front the air, and water to operate and do not require corrosive fluids like some fuel cells. They are typically fueled with pure hydrogen supplied from storage tanks or onboard reformers. Such reformers could use different type of fuels, for instance methanol (Figure 7.9). 

Depending on the type of measurement (absorption or emission), different types of cells and samples have been employed. A slightly modified working fuel cell using an ion exchange polymer membrane as solid electrolyte coated with various catalyst layers has been described [573] that enables investigations of the platinum and ruthenium atoms incorporated in the catalyst layers. In order to avoid convolution of signals of both electrode layers (which are penetrated by the X-ray beam), one catalyst layer (the cathode) was removed (cut out) in the area of the beam path. 

Current research is centred on making compact cells of high efficiency. They are described in terms of the electrolyte that is used. The principle types are alkali fuel cells, described above, with aqueous KOH as electrolyte, MCFCs (molten carbonate fuel cells), with a molten alkali metal or alkaline earth carbonate electrolyte, PAFCs (phosphoric acid fuel cells), PEMs (proton exchange membranes), using a solid polymer electrolyte that conducts ions, and SOFCs, (solid oxide fuel cells), with solid electrolytes that allow oxide ion, 0 , transport The... 

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