Low-temperature fuel cells PEMFC

In the case of low-temperature fuel cells (PEMFCs, AFCs and PAFCs), some simplified expressions can be obtained. 

Solid (e.g., carbon supports), liquid (e.g., water or dimethylsulfoxide electrolytes), and gas phases (e.g., O2) in devices like low-temperature fuel cells (PEMFCs) [1] and metal-air batteries [6]. 

The low-temperature fuel cells (PEMFC and PAFC) require pure hydrogen, as carbon monoxide is a poison to the platinum anode. A PSA unit as used in large-scale hydrogen plants (Figure 2.4) carmot be used at the small scale in question, and the final purifrcation is made either by methanation (as used in ammonia plants, Section 2.5) or by preferential partial oxidation (PROX) of carbon monoxide to carbon dioxide over a... 

The net flow of fuel and electrons through the electrolyte is small, typically the equivalent of only a few mA/cm. However, for the low-temperature fuel cell PEMFC, though the Nemst equation predicted that the open-circuit voltage is 1.2 V, the measured values are at 1V. The voltage loss 0.2 V represents significant efficiency loss. The electrolyte supports a very small amount of electronic conductivity, so that small short-circuiting currents are possible. 

The last three fuel cells (PEMFC, DAFC and SAMFC) are low-temperature fuel cells. In this chapter, the discussion will be focused on these fuel cells, particularly the PEMFC and the DAFC, since they can accommodate biomass fuels, either after fuel processing to obtain reformate hydrogen or directly with bioethanol. 

The PEMFC is nowadays the most advanced low-temperature fuel cell technology [19, 20], because it can be used in several applications (space programs, electric vehicles, stationary power plants, auxiliary power units, portable electronics). The progress made in one application is greatly beneficial to the others. 

Over the last decade, novel carbonaceous and graphitic support materials for low-temperature fuel cell catalysts have been extensively explored. Recently, fibrous nanocarbon materials such as carbon nanotubes (CNTs) and CNFs have been examined as support materials for anodes and cathodes of fuel cells [18-31], Mesoporous carbons have also attracted considerable attention for enhancing the activity of metal catalysts in low-temperature DMFC and PEMFC anodes [32-44], Notwithstanding the many studies, carbon blacks are still the most common supports in industrial practice. 

According to the electrolyte and working temperature, one distinguishes the low-temperature fuel cell technologies (i) alkaline fuel cell, AFC (70 to 80°C), (ii) proton exchange membrane fuel cell, PEMFC (70 to 80°C), (iii) phosphoric acid cell, PAFC (200°C) from the high-temperature technologies, (iv) molten carbonate fuel cell, MCFC (650 to 700°C), and (v) solid oxide fuel cell, SOFC (1000°C). 

Concentration overpotentials can be very high in low temperature fuel cells, where they often cause the occurrence of limiting currents (such as in PEMFCs, which operate at 80°C). However, they are less critical in SOFCs, due to the high operating temperatures (about 1000°C). 

In this chapter, after recalling the working principles and the different kinds of fuel cells, the discussion will be focused on low-temperature fuel cells (AFC, PEMFC, and DAFC), in which several kinds of carbon materials are used (catalyst support, gas-diffusion layer [GDL], bipolar plates [BP], etc.). Then some possible applications in different areas will be presented. Finally the materials used in fuel cells, particularly carbon materials, will be discussed according to the aimed applications. To read more details on the use of carbon in fuel cell technology, see the review paper on The role of carbon in fuel cell technology recently published by Dicks [6],... 

For low-temperature fuel cells (AFC, PEMFC, and DAFC) carbon is mainly used... 

In low-temperature fuel cells (AFC, PEMFC, DAFC, etc.), carbon materials are important since they are involved in the fabrication of BP, GDL, and CL. It appears that no other materials can replace carbon with the same properties (good electronic conductivity, good thermal and chemical stabilities, and low cost). But much work is needed to optimize carbon materials for fuel cell applications and to ensure that they meet the performance targets for conductivity, physical properties, and lifetime within operating stacks. 

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. 

In cases where high purity hydrogen is valued, dense metal membranes are an attractive option over polymeric membranes and porous membranes that exhibit much lower selectivities. Two examples where this is true are low-temperature fuel cells (e.g., proton exchange membrane fuel cells [PEMFCs] and alkaline fuel cells [AFCs]) and hydrogen-generating sites where the product hydrogen is to be compressed and stored for future use. 

The reforming process (as applied to a hydrocarbon or alcohol) yields a product stream that consists predominantly of hydrogen, carbon monoxide, carbon dioxide, water, unconverted feedstock, and trace by-products. This product stream mixture, called reformate, is unsuitable for direct use in low-temperature PEMFC and AFC, and some trace by-products (notably organosulfur compounds) will poison both high-temperature fuel cells and low-temperature fuel cells. A membrane for separating and purifying hydrogen from reformate must also be chemically compatible with the compounds in the reformate stream. 

Polymer electrolyte fuel cells, also sometimes called SPEFC (solid polymer electrolyte fuel cells) or PEMFC (polymer electrolyte membrane fuel cell) use a proton exchange membrane as the electrolyte. PEEC are low-temperature fuel cells, generally operating between 40 and 90 °C and therefore need noble metal electrocatalysts (platinum or platinum alloys on anode and cathode). Characteristics of PEEC are the high power density and fast dynamics. A prominent application area is therefore the power train of automobiles, where quick start-up is required. 

The situation is different for HT-PEMFC and LT-PEMFC. The adsorption of carbon monoxide on electro catalyst is driven by the temperature. The CO adsorption is promoted at lower temperatures. A high amount of absorb carbon monoxide on the electro catalysts decreases the performance of the fuel cell dramatically. So low temperature fuel cell requires a hydrogen-rich gas with a very low content of CO in the range of a few ppm. High temperature PEMFC can tolerate a few percent of CO in the hydrogen rich gas and SOFC can use CO directly due to the high operation temperatures. 

Low-temperature fuel cells proton exchange membrane fuel cells (PEMFCs) and direct methanol fuels (DMFCs)... 

So as it can not be considered to provide a fuel cell functioning at higher temperatures than 80 or even 100°C for the user s safety, the choice in the type of fuel cell to use in portable devices is limited to low temperature fuel cells such as PEMFC (for Proton Exchange Membrane Fuel Cell or sometimes Polymer Electrolyte Membrane Fuel Cell) and DMFC (for Direct Methanol Fuel Cell). 

Fuel Cell Reactions. Low temperature fuel cells such as proton exchange membrane fuel cells (PEMFC) or direct methanol fuel cells (DMFC) employ large amounts of noble metals such as Pt and Ru. There has been extensive research to replace these expensive metals with more available materials. A few studies considered transition metal nitrides as a potential candidate. In an anode reaction of DMFC, Pt/TiN displayed the electroactivity for methanol oxidation (53). Pt/TiN deposited on stainless steel substrate showed the high CO tolerance in voltammogram performed with a scan rate of 20 mV/s and 0.5 M CH3OH - - 0.5 M H2SO4 electrolyte. The bifunctional effect of Pt and TiN for CO oxidation was mentioned as observed between Pt and Ru in commercial PtRu/C catalysts. 

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