Fuel temperature

In low temperature fuel ceUs, ie, AEG, PAEC, PEEC, protons or hydroxyl ions are the principal charge carriers in the electrolyte, whereas in the high temperature fuel ceUs, ie, MCEC, SOEC, carbonate and oxide ions ate the charge carriers in the molten carbonate and soHd oxide electrolytes, respectively. Euel ceUs that use zitconia-based soHd oxide electrolytes must operate at about 1000°C because the transport rate of oxygen ions in the soHd oxide is adequate for practical appHcations only at such high temperatures. Another option is to use extremely thin soHd oxide electrolytes to minimize the ohmic losses. 

For high temperature fuel ceUs, there is stiU a strong need to develop lower cost materials for ceU components. In the case of SOFCs, improved fabrication processes and materials that permit acceptable performance in fuel ceUs at lower operating temperatures are also highly desirable. 

One factor contributing to the inefficiency of a fuel ceU is poor performance of the positive electrode. This accounts for overpotentials of 300—400 mV in low temperature fuel ceUs. An electrocatalyst that is capable of oxygen reduction at lower overpotentials would benefit the overall efficiency of the fuel ceU. Despite extensive efforts expended on electrocatalysis studies of oxygen reduction in fuel ceU electrolytes, platinum-based metals are stiU the best electrocatalysts for low temperature fuel ceUs. 

Moehida, I., Kawabuehi, Y., Kawano, S., Matsumura, Y. and Yoshikawa, M., High catalytic activity of pitch-based activated carbon fibres of moderate surface area for oxidation of NO to NOj at room temperature. Fuel, 1997, 76(6), 543 548. 

Fig. 19. Example temperatures, fuel vapor flow rates, and mass fractions during a three-day diurnal test sequence...

The advantages of thermal incineration are that it is simple in concept, has a wide application, and results in almost complete destruction of pollutants with no liquid or solid residue. Thermal incineration provides an opportunity for heat recovery and has low maintenance requirements and low capital cost. Thermal incineration units for small or moderate exhaust streams are generally compact and light. Such units can be installed on a roof when the plant area is limited. = The main disadvantage is the auxiliary fuel cost, which is partly offset with an efficient heat-recovery system. The formation of nitric oxides during the combustion processes must be reduced by control of excess air temperature, fuel supply, and combustion air distribution at the burner inlet, The formation of thermal NO increases dramatically above 980 Table 13.10)... 

Because NO, formation is a function of the temperature, fuel-air mixture, and fluid dynamics in the furnace, the goal of a combustion modification is to mix fuel and air more gradually to reduce the flame temperature (lower thermal NO, production), and to stage combustion, initially using a richer fuel-air mixture, thus reducing oxidation of the nitrogen in the fuel. LNBs sewe the role of staged combustion. 

Cooling strongly depends on fuel cell operating temperature and also depends on the fuel cell s external environment. For low temperature fuel cells, cooling imposes a significant energy debit because pumps need to force coolant out to a heat... 

Concerns about global climate change have motivated new interest in low-carbon or noncarbon fuels. Recent rapid progress and industrial interest in low-temperature fuel cells (which prefer hydrogen as a fuel) for transportation and power applications have also led to a reexamination of hydrogen as a fuel. 

One leading prototype of a high-temperature fuel cell is the solid oxide fuel cell, or SOFC. The basic principle of the SOFC, like the PEM, is to use an electrolyte layer with high ionic conductivity but very small electronic conductivity. Figure B shows a schematic illustration of a SOFC fuel cell using carbon monoxide as fuel. 

This reaction is of great technological interest in the area of solid oxide fuel cells (SOFC) since it is catalyzed by the Ni surface of the Ni-stabilized Zr02 cermet used as the anode material in power-producing SOFC units.60,61 The ability of SOFC units to reform methane "internally", i.e. in the anode compartment, permits the direct use of methane or natural gas as the fuel, without a separate external reformer, and thus constitutes a significant advantage of SOFC in relation to low temperature fuel cells. 

Sol-gel techniques have been widely used to prepare ceramic or glass materials with controlled microstructures. Applications of the sol-gel method in fabrication of high-temperature fuel cells are steadily reported. Modification of electrodes, electrolytes or electrolyte/electrode interface of the fuel cell has been also performed to produce components with improved microstructures. Recently, the sol-gel method has expanded into inorganic-organic hybrid membranes for low-temperature fuel cells. This paper presents an overview concerning current applications of sol-gel techniques in fabrication of fuel cell components. 

Sol-gel techniques have been successfidly applied to form fuel cell components with enhanced microstructures for high-temperature fuel cells. The apphcations were recently extended to synthesis of hybrid electrolyte for PEMFC. Although die results look promising, the sol-gel processing needs further development to deposit micro-structured materials in a selective area such as the triple-phase boundary of a fuel cell. That is, in the case of PEMFC, the sol-gel techniques need to be expanded to form membrane-electrode-assembly with improved microstructures in addition to the synthesis of hybrid membranes to get higher fuel cell performance. 

The electrocatalytic oxidation of methanol has been widely investigated for exploitation in the so-called direct methanol fuel cell (DMFC). The most likely type of DMFC to be commercialized in the near future seems to be the polymer electrolyte membrane DMFC using proton exchange membrane, a special form of low-temperature fuel cell based on PEM technology. In this cell, methanol (a liquid fuel available at low cost, easily handled, stored, and transported) is dissolved in an acid electrolyte and burned directly by air to carbon dioxide. The prominence of the DMFCs with respect to safety, simple device fabrication, and low cost has rendered them promising candidates for applications ranging from portable power sources to secondary cells for prospective electric vehicles. Notwithstanding, DMFCs were... 

When surveying the central milestones in the development of electrocatalysis for low temperature fuel cells operating in acidic environments, the following, listed in chronological order, seem to be the most outstanding ... 

Varcoe JR, Slade RCT. 2005. Prospects for alkaline anion-exchange membranes in low temperature fuel cells. Fuel Cells 5 187-200. 

Hogarth MP, Ralph TR. 2002. Catalysis for low temperature fuel cells. Part III Challenges for the direct methanol fuel cell. Platinum Metals Rev 46 146-164. 

Christoffersen E, Liu P, Ruban A, Skriver UL, Nprskov JK. 2001. Anode materials for low temperature fuel cells—A density functional theory study. J Catal 199 123. 

Russell AE, Rose A. 2004. X-ray absorption spectroscopy of low temperature fuel cell catalysts. Chem Rev 104 4613-4635. 

In addition to their proven capacity to catalyze a highly efficient and rapid reduction of O2 under ambient conditions (e.g., cytochrome c oxidase, the enzyme that catalyzes the reduction of >90% of O2 consumed by a mammal, captures >80% of the free energy of ORR at a turnover frequency of >50 O2 molecules per second per site), metalloporphyrins are attractive candidates for Pt-free cathodes. Probably the major impetus for a search for Pt-free cathodic catalysts for low temperature fuel cells is... 

The prevalence of the heme in O2 metabolism and the discovery in the 1960s that metallophthalocyanines adsorbed on graphite catalyze four-electron reduction of O2 have prompted intense interest in metaUoporphyrins as molecular electrocatalysts for the ORR. The technological motivation behind this work is the desire for a Pt-ffee cathodic catalyst for low temperature fuel cells. To date, three types of metaUoporphyrins have attracted most attention (i) simple porphyrins that are accessible within one or two steps and are typically available commercially (ii) cofacial porphyrins in which two porphyrin macrocycles are confined in an approximately stacked (face-to-face) geometry and (iii) biomimetic catalysts, which are highly elaborate porphyrins designed to reproduce the stereoelectronic properties of the 02-reducing site of cytochrome oxidase. 

The DMFC is a potentially attractive alternative to the high temperature fuel cells currently available which are primarily based on H2/02, since ... 

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