Fuel Cell Calculations

Sulzer Hexis went into bankruptcy in late 2005, so the following account is history. The Sulzer Hexis fuel cell was exclusively aimed at the domestic market (Batawi 1996 1999 2001 Schuler, 2001 Ballhausen, 2001). After a long development history the project matured as the Sulzer Hexis Premiere, an incomplete fuel cell, calculated using calorific value theory. 

If this combustion reaction could be harnessed as a fuel cell, calculate the theoretical voltage that could be produced at standard conditions. Hint Use AG° values from Appendix 4.)... 

Electrolyte characteristics and preliminary steam-electrolysis fuel-cell calculations and performance... 

Appendix B Useful Equations for Fuel Cell Calculations... 

Figure 2 shows calculated open circuit potentials for this idealized carbon fuel cell, calculated from thermodynamic data. (These are our calculations, after Hemmes and Cassir, 2004). This figure shows that at practical temperatures for carbon fiiel cell operation (T > 650°C), the open circuit potential will be in excess of the standard potential of reaction (1). The concentration of CO2 is suppressed because of the reaction of CO2 with carbon to produce CO according to the Boudouard reaction ... 

Stoichiometry has important practical applications, such as predicting how much product can be formed in a reaction. For example, in the space shuttle fuel cell, oxygen reacts with hydrogen to produce water, which is used for life support (Fig. L.l). Let s look at the calculation space shuttle engineers would have to do to find out how much water is formed when 0.25 mol 02 reacts with hydrogen gas. 

The aluminum-air fuel cell is used as a reserve battery in remote locations. In this cell aluminum reacts with the oxygen in air in basic solution, (a) Write the oxidation and reduction half-reactions for this cell, (b) Calculate the standard cell potential. See Box 12.1. 

Hydrogen, a fuel that releases a large amount of chemical energy when it bums, is used as an energy source in fuel cells. Use standard enthalpies of formation to calculate the change in mass that occurs when 1.00 mol of H2 is burned. 

Significant (and even spectacular) results were contributed by the group of Norskov to the field of electrocatalysis [102-105]. Theoretical calculations led to the design of novel nanoparticulate anode catalysts for proton exchange membrane fuel cells (PEMFC) which are composed of trimetallic systems where which PtRu is alloyed with a third, non-noble metal such as Co, Ni, or W. Remarkably, the activity trends observed experimentally when using Pt-, PtRu-, PtRuNi-, and PtRuCo electrocatalysts corresponded exactly with the theoretical predictions (cf. Figure 5(a) and (b)) [102]. 

In case of fuel cell cathodes, theoretical considerations were directed towards optimizing catalysts for O2 reduction [103]. This has led to the synthesis of Pt3Co/C nanocatalyst systems and preliminary results again indicate perfect agreement between the calculations and the wet electrochemical results obtained with metal nanoparticles of the composition which theory had recommended [106]. 

Paradoxically, all these significant recent contributions to the theory of the ORR, together with most recent experimental efforts to characterize the ORR at a fuel cell cathode catalyst, have not led at aU to a consensus on either the mechanism of the ORR at Pt catalysts in acid electrolytes or even on how to properly determine this mechanism with available experimental tools. To elucidate the present mismatch of central pieces in the ORR puzzle, one can start from the identification of the slow step in the ORR sequence. With the 02-to-HOOads-to-HOads route appearing from recent DFT calculations to be the likely mechanism for the ORR at a Pt metal catalyst surface in acid electrolyte, the first electron and proton transfer to dioxygen, according to the reaction... 

Some PEM fuel cell performance data were obtained using an electrical resistor to provide a variable load. Two digital multimeters and a shunt resistor were used to measure the voltage and current, so we could calculate the power produced. 

Fig. 23. (a) Experimental IR-free overpotentials in MCFC-based separator. Cell performance 0.25% C02 Feed. All curves calculated [32] (b) C02 production scheme using molten carbonate fuel cell stack. 

For the calculation of WTW energy requirements and GHG emissions we have made the simplification that the fuel consumption of a vehicle fuelled with ethanol (e.g., E85) is the same as that of a vehicle fuelled with pure gasoline. Methanol is used in fuel cell vehicles with on-board fuel processors. Table 7.2 shows the properties of different transportation fuels. 

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%. 

There are two other possible scenarios. In one, the installer offers the fuel-cell heating appliance just as he does other heating systems then he has to cope with additional training needs, because of the double qualification and the calculations for operation efficiency. In the other scenario, the installer works for the energy supplier, an alternative that is not profitable, as he loses the profit margin of the heating system. 

This calculation is based on hydrogen fuel-cell cars with a hydrogen consumption of 26 kWh/100 km, gasoline or diesel cars with a consumption of 51/100 km and a refinery efficiency of 90%. In addition, - parity is assumed. 

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