Efficiencies PEMFC

From these examples, it can be seen that water content has a strong effect upon proton conductivity. Thus, it is clear that water management is an important factor for efficient PEMFC operation. It will be discussed in Section 3.2.3. 

The pore texture plays a major role in the accessibility of the inner surface onto which the metal is deposited [39,67]. Globally, the presence of mesopores or macropores is beneficial to dispersion [68-71], following increased true surface availability. The presence of micropores increases the dispersion of the metal nanoparticles, but also yields metal entrapment in areas inaccessible to either the ionomer or the reactants in operating conditions [51]. As a result, the mesopore to micropore specific surface area ratio should be high for efficient PEMFC electrocatalysts. The support-solvent interactions are also of prime importance should the... 

Since it can operate at ambient temperatures, the PEMFC can startup quickly, but it does have two significant disadvantages lower efficiency and more stringent purity requirements. The lower efficiency... 

Actually, various efforts have been made to develop the compact and efficient microchannel PrOx reactor for portable PEMFC applications. Goerke et al. [2] reported micro PrOx reactor employing stainless steel microchannel foil and Cu/Ce02 catalyst. They showed more than 99% CO conversion at less than 150 C and residence time of 14ms while CO selectivity was about 20%. Chen et al. [3] also developed microchannel reactor made of... 

For the practical use of this CO removal reactor, the microchannel reactor should be operated carefully to maintain operating temperature ranges because the reaction temperature is critical for the microchannel reactor performance such as CO conversion, selectivity and methanation as disclosed in the above results. It also seems that the present microchannel reactor is promising as a compact and high efficient CO remover for PEMFC systems. 

While the PEM fuel cells appear to be suitable for mobile applications, SOFC technology appears more applicable for stationary applications. The high operating temperature gives it flexibility towards the type of fuel used, which enables, for example, the use of methane. The heat thus generated can be used to produce additional electricity. Consequently, the efficiency of the SOFC is -60 %, compared with 45 % for PEMFC under optimal conditions. 

Following a period of slack, decisive improvements were made after 1990 in the area of PEMFCs. Modem models now achieve specific powers of over 600 to 800 mW/cm while using less than 0.4 mg/cm of platinum catalysts and offering a service fife of several tens of thousands of hours. These advances were basically attained by the combination of two factors (1) using new proton-exchange membranes of the Nafion type, and (2) developing ways toward much more efficient utilization of the platinum catalysts in the electrodes. 

Solid alkaline membrane fuel cells (SAMECs) can be a good alternative to PEMFCs. The activation of the oxidation of alcohols and reduction of oxygen occurring in fuel cells is easier in alkaline media than in acid media [Wang et al., 2003 Yang, 2004]. Therefore, less Pt or even non-noble metals can be used owing to the improved electrode kinetics. Eor example, Ag/C catalytic powder can be used as an efficient cathode material [Demarconnay et al., 2004 Lamy et al., 2006]. It has also... 

Oxidation of Adsorbed CO The electro-oxidation of CO has been extensively studied given its importance as a model electrochemical reaction and its relevance to the development of CO-tolerant anodes for PEMFCs and efficient anodes for DMFCs. In this section, we focus on the oxidation of a COads monolayer and do not cover continuous oxidation of CO dissolved in electrolyte. An invaluable advantage of COads electro-oxidation as a model reaction is that it does not involve diffusion in the electrolyte bulk, and thus is not subject to the problems associated with mass transport corrections and desorption/readsorption processes. 

There are six different types of fuel cells (Table 1.6) (1) alkaline fuel cell (AFC), (2) direct methanol fuel cell (DMFC), (3) molten carbonate fuel cell (MCFC), (4) phosphoric acid fuel cell (PAFC), (5) proton exchange membrane fuel cell (PEMFC), and (6) the solid oxide fuel cell (SOFC). They all differ in applications, operating temperatures, cost, and efficiency. 

A fuel cell system for automobile application is shown in Figure 1.5 [41]. At the rated power, the PEMFC stack operates at 2.5 atm. and 80°C to yield an overall system efficiency of 50% (based on lower heating value of hydrogen). Compressed hydrogen and air are humidified to 90% relative humidity at the stack temperature using process water and heat from the stack coolant. A lower system pressure is at part load and is determined by the operating map of the compressor-expander module. Process water is recovered from spent air in an inertial separator just downstream of the stack in a condenser and a demister at the turbine exhaust. 

The utilization principles are shown in Figure 6, where the typical examples are enumerated. Hydrogen turbine has been studied by Japanese WE-NET project and the achieved energy efficiency was as high as about 60 %, which can be competitive with fuel cell system. One of the typical direct energy conversion systems, which have no movable parts and no noise, is fuel cell. Today topics of clean cars have been focused to the cars with PEMFC as was mentioned previously. 

A well-distributed deposition of Pt/C nanocatalyst and Nafion ionomer on bofh hydrophilic and hydrophobic carbon-based electrodes has been successfully obfained using a Pt/C concentration of 1.0 g/L, an electrical field of 300 V/cm, and a deposition time of 5 minutes [118]. The deposition of Pt/C nanocatalysts and Nafion solution via the electrophoretic process gives rise to higher deposition efficiency and a uniform distribution of catalyst and Nafion ionomer on the PEMFC electrodes. 

Wang, X., Waje, M., and Yan, Y. CNT-based electrodes with high efficiency for PEMFCs. Electrochemical and Solid-State Letters 2005 8 A42-A44. 

In a PEMFC, the power density and efficiency are limited by three major factors (1) the ohmic overpotential mainly due to the membrane resistance, (2) the activation overpotential due to slow oxygen reduchon reaction at the electrode/membrane interface, and (3) the concentration overpotential due to mass-transport limitations of oxygen to the electrode surfaced Studies of the solubility and concentration of oxygen in different perfluorinated membrane materials show that the oxygen solubility is enhanced in the fluorocarbon (hydrophobic)-rich zones and hence increases with the hydrophobicity of the membrane. The diffusion coefficient is directly related to the water content of the membrane and is thereby enhanced in membranes containing high water content the result indicates that the aqueous phase is predominantly involved in the diffusion pathway. ... 

The efficiency of PEMFCs ranges from about 40 to 50%, and operating temperature is about 255 K. The PEMFCs and direct methanol fuel cells (DMFCs) are considered to be promising power sources, especially for transportation applications. The PEMFCs with potentially much higher efficiencies and almost zero emissions offer an attractive alternative to the internal combustion engines for automotive applications. This fuel cell has many important attributes such as high efficiency, clean, quiet, low-temperature operation, capable of quick start-up, no liquid electrolyte and simple cell design (Hu et al., 2004). 

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