Fuel cell conventional

Methanol can be considered as a hydrogen carrier in a fuel cell. Conventionally, methanol has been reformed/shift converted to produce hydrogen. A low concentration of carbon monoxide formed during this process leads to a strong poisoning of the anode, and even after cleaning of the... 

A fascinating point, especially to physical chemists, is the potential theoretical efficiency of fuel cells. Conventional combustion machines principally transfer energy from hot parts to cold parts of the machine and, thus, convert some of the energy to mechanical work. The theoretical efficiency is given by the so-called Carnot cycle and depends strongly on the temperature difference, see Fig. 13.3. In fuel cells, the maximum efficiency is given by the relation of the useable free reaction enthalpy G to the enthalpy H (AG = AH - T AS). For hydrogen-fuelled cells the reaction takes place as shown in Eq. (13.1a). With A//R = 241.8 kJ/mol and AGr = 228.5 under standard conditions (0 °C andp = 100 kPa) there is a theoretical efficiency of 95%. If the reaction results in condensed H20, the thermodynamic values are A//R = 285.8 kJ/ mol and AGR = 237.1 and the efficiency can then be calculated as 83%. 

Metal-air batteries are different from conventional batteries because metal-air batteries are connected to the atmosphere, and need this access to operate. Metal-air batteries are also different from fuel cells because metal-air batteries have a self-contained anode within the battery case itself. Metal-air batteries are part conventional" battery and part fuel cell. Sometimes metal-air batteries are called semi-fuel cells. "Conventional" batteries have the active components of both the anode and cathode within the battery case (see Figure 1.1). Fuel cells have both of the "active" components (or fuels) of the anode and cathode supplied from outside the fuel cell case. A metal-air battery, or semi-fuel cell, has the solid anode within the case (like a battery), while the cathode fuel is brought into the cell (like a fuel cell). Oxygen gas... 

The conventional electrochemical reduction of carbon dioxide tends to give formic acid as the major product, which can be obtained with a 90% current efficiency using, for example, indium, tin, or mercury cathodes. Being able to convert CO2 initially to formates or formaldehyde is in itself significant. In our direct oxidation liquid feed fuel cell, varied oxygenates such as formaldehyde, formic acid and methyl formate, dimethoxymethane, trimethoxymethane, trioxane, and dimethyl carbonate are all useful fuels. At the same time, they can also be readily reduced further to methyl alcohol by varied chemical or enzymatic processes. 

In the fuel cell hydrogen is used two to three times as efficiendy as in an internal combustion engine. Hence, when utilized in a fuel cell, hydrogen can cost two to three times that of more conventional fossil fuels and stiU be competitively priced, ie, as of this writing the market price for hydrogen when used in a fuel cell and produced by electrolysis is competitively priced with gasoline. 

The problem is that hydrogen, even at 10,000 psi (or 690 bar), requires five to ten times the volume of today s gasoline tank, depending on the fuel cell vehicle s real world efficiency. Packaging volume is compromised even further because pressurized tanks require thick carbon fiber walls and are, therefore, nonconformable. Moreover, they may cost several thousand dollars more than a conventional gasoline tank. 

In the longer term, more exotic technologies, such as fuel cells powered by hydrogen, may be feasible. These technologies are fai from being economically feasible, but rapid progress is being made. However, as conventional vehicles become cleaner, the relative emissions-reduction benefits from alternative fuels declines. 

Fuel cells have attracted considerable interest because of their potential for efficient conversion of the energy (AG) from a chemical reaction to electrical energy (AE). This efficiency is achieved by directly converting chemical energy to electricity. Conventional systems burn fuel in an engine and convert the resulting mechanical output to electrical power. Potential applications include stationary multi-megawatt power plants, battery replacements for personal electronics, and even fuel-cell-powered unmanned autonomous vehicles (UAVs). 

Note that fuel cells do not provide a new source of energy. They are designed to use conventional, currently available fuels. Their main advantage lies in the efficiency of their operation, which creates fewer byproducts to threaten the environment. 

An exciting emerging technology is the biofuel cell. A biofuel cell is like a conventional fuel cell however, in place... 

Other possible applications of smart elastomers are in the area of polymer engine which can produce maximum power density (4 W/g) and output both in terms of electrical and mechanical power without any noise. These features are superior compared to conventional electrical generator, fuel cell, and conventional IC engine. Many DoD applications (e.g., robotics, MAV) require both mechanical and electrical (hybrid) power, and polymer engine can eliminate entire transducer steps and can also save engine parts, weight, and is more efficient. 

Fuel cells are the subject of vast amounts of research and most experts now predict that by about 2020 they will be widely used for mass transportation. There are four major potential benefits to using fuel cell technology compared to more conventional sources of energy ... 

The overall efficiency of fuel cells is higher than that of conventional heat engines. Running on pure hydrogen, a fuel cell has a theoretical efficiency up to 80%, and in some kinds of fuel cell practical efficiencies of over 70% have been reported. For most practical purposes modem fuel cells generally have efficiencies of over 40%. 

There is a major potential for energy conservation in transportation, by increasing the energy efficiency of automobiles. The recent commercialization of hybrid vehicles, which combine electric and gasoline motors, demonstrates how much more efficient automobile transport can be. Hybrid power systems deliver double the fuel efficiency of conventional engines. Moreover, as fuel cells are perfected, even greater energy efficiencies may be achieved. 

The advantage of a fuel cell over a conventional battery is that the fuel for electrical power can be replenished easily. Just as we pull into a service station to refill the gas tank, owners of automobiles powered by fuel cells will refill their fiiel tanks with hydrogen or butane. 

Interestingly, the PEMFC may also operate directly on methanol. Naturally, the problems associated with high coverage of various intermediates will be present, as mentioned above, as well as additional problems such as loss of methanol over the membrane. Nevertheless, it is possible to operate a methanol fuel cell with a voltage around 0.4 V and a reasonable current, to power small mobile devices such as portable computers and cell phones and make them independent of connection to the conventional power net. For more details on fuel cells we refer the reader to L. Carr-ette, K.A. Friedrich and U. Stimming, Fuel Cells 1(1) (2001) 5-39. 

Figure 8.33. Schematic of the efficiencies of a fuel cell driven car and a conventional car with a combustion engine. Note the advantage of the fuel cell car at low load, prevailing under urban driving conditions.

Such bimetallic alloys display higher tolerance to the presence of methanol, as shown in Fig. 11.12, where Pt-Cr/C is compared with Pt/C. However, an increase in alcohol concentration leads to a decrease in the tolerance of the catalyst [Koffi et al., 2005 Coutanceau et ah, 2006]. Low power densities are currently obtained in DMFCs working at low temperature [Hogarth and Ralph, 2002] because it is difficult to activate the oxidation reaction of the alcohol and the reduction reaction of molecular oxygen at room temperature. To counterbalance the loss of performance of the cell due to low reaction rates, the membrane thickness can be reduced in order to increase its conductance [Shen et al., 2004]. As a result, methanol crossover is strongly increased. This could be detrimental to the fuel cell s electrical performance, as methanol acts as a poison for conventional Pt-based catalysts present in fuel cell cathodes, especially in the case of mini or micro fuel cell applications, where high methanol concentrations are required (5-10 M). 

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