Fuel cell Potential

S. Gottesfeld, "Polymer Electrolyte Fuel Cells Potential Transportation and Stationary Applications," No. 10, An EPREGRI Fuel Cell Workshop on Technology Research and Development, Stonehart Associates, Madison, Connecticut, 1993. 

In Table 15 are given some theoretical energy capacities [335] calculated by means of Eq. (129) and the data from Table 14. These data are based on the hypotheses that the fuel cell potential is the reversible potential, the Faradaic efficiency is 100% and the weight taken into the calculation is that of the re-... 

Table 3.1 shows the thermodynamic energy densities and practical energy densities for several battery systems and fuel cells. Fuel cells potentially offer 5-10 times greater energy densities than rechargeable batteries [4]. 

Further progress is expected from new developments and combinations of processes. Thus, it would be possible to make the disposal of the gaseous (and highly pure) waste gas streams (residual propane content of the propylene feed) cost-effective and a source of electric power by connection to novel, compact, membrane fuel cells. Potential synergisms would also occur in the operating temperature of the cells (medium-temperature cells at 120 °C using the residual exothermic heat of reaction from the oxo reaction), the membrane costs by means of combined developments (e.g., for membrane separations of the catalysts [22]), and also in the development of the zero-emission automobile by the automotive industry. The combination of hydroformylation with fuel cells would further reduce the E-factor - thus approaching a zero-emission chemistry. ... 

Figure 19.18 Fuel cell potential as a function of the current density at 70°C, 1-3 atm. Reprinted with permission from Ernst et al. (1999).

Because AG and F are known, the theoretical fuel cell potential of hydrogen/oxygen can also be calculated ... 

At 25 °C, the theoretical hydrogen/oxygen fuel cell potential is 1.23 Volts. This equation gives the reversible open circuit voltage of the hydrogen fuel cell. 

Effect of Operation Conditions on Reversible Fuel Cell Potential... 

A polarization curve, i.e., an 1-V curve of a fuel cell stack under certain operating conditions, is frequently used to describe fuel cell performance and determine performance decay with time. Polarization curve analysis can provide the most important kinetic parameters, such as the Tafel slope, exchange current density, cell resistance, limiting current density, and specific activity of Pt. A single fuel cell potential can be described by ... 

Cationic contaminants tend to build up in the polymer electrolyte. This is because the sulfonate sites have a higher affinity for most other cations than protons and because most other cations do not partake in a suitable reaction to exit the polymer electrolyte phase [2,3]. In the case of ammonia, there is a suitable reaction at the cathode to remove ammonium ions from the system, but this reaction is likely slower than proton reduction. Some other metal ions, such as copper and cobalt, are electrochemically active in the fuel cell potential window and tend to "plate out" of the system. In general, once a cationic contaminant is in the polymer electrolyte phase it tends to stay there until the membrane has an acid treatment. 

The actual fuel cell potential is decreased from its full potential, the Nemst potential, because of irreversible losses. Multiple phenomena contribute to irreversible losses in an actual fuel cell. Eor the hydrogen oxidation reaction, the func-tionahty of fuel cell voltage, E, is typically given by [42-44]... 

To a good approximation, the temperature dependence of the equilibrium fuel cell potential is linear (Kulikovsky, 2010a) ... 

The theoretical hydrogen/oxygen fuel cell potential is 1.23 Volts. As temperature rises from room temperature to that of an operating fuel cell (80 °C), there is a small cell voltage decrease from 1.23 V at 25 °C to 1.18 V at 80 °C. Fuel cell voltage is in general the summation of the thermodynamic potential ENemst/ the activation overpotential ijact (from both anode and cathode overpotentials, i.e., i]act(cathode)- act(anode))/ and the ohmic overpotential ijohmic/ which can be expressed as... 

Abstract The scope of this chapter is to give a brief introduction about fuel cells, types of applications in fuel cell technology, characteristics of fuel cells, potential applications in fuel cell technology, and current research and development and key technology players in fuel cells. 

Using Equation 5.141, the effect of concentration loss on the fuel cell performance is shown in Figure 5.24 for different limit current densities. The plots are shown for n = 1, T = 353 K, and Jl = 1, 2,3 A/cm. From the figure, it is clear that the fuel cell potential is affected by the concentration loss for large current densities. 

In the previous sections, we looked into the losses in the fuel cell potential contributed by the resistance to the reaction kinetics at the cathode and anode (activation losses), resistance to ion or electron transport (ohmic losses), and the mass concentration variation near the electrode (mass transfer losses). In addition to these losses, fuel cells show significant potential losses as a result of a short circuit in the electrolyte and crossover of reactants through the electrolyte. 

The response of the fuel cell is determined by the electrochemical processes and associated kinetics at the electrode and electrode interface. The electrochemical processes depend on the mass and charge transfer between the bulk electrolyte solution and electrode surface. The rates at which these transfers occur are determined by the number of localized phenomena and largely depend on the materials involved. These processes are presented in this chapter and the relations between the fuel cell potential and current density are given in terms of BV and Tafel equations. The key losses in the fuel cell include the activation losses, ohmic losses, mass transport losses, and losses owing to reactant crossover and internal currents that are discussed in this chapter. The fuel cell polarization curve is presented and is discussed for low-temperature and high-temperature fuel cells such as PEMFC and SOFC, respectively. 

Using CRC Handbook tables, calculate the voltage efficiency of a C/O2 fuel cell [C(s) + Ojfg) = C02(g)] at 1000 K when the fuel cell potential is 0.5 V and all activities equal 1. 

FIGURE 9.6 Redox potentials in various methanol-oxygen biological fuel cells. Potentials are at pH 7.5 and given versus SCE. (Reproduced with permission from Ref. [29]. Copyright 1998, Elsevier.)... 

At 25°C, the theoretical hydrogen/oxygen fuel cell potential is 1.23 Volts. 

By introducing Equations (3-23), (3-24), (3-34), and (3-41) into Equation (3-43), a relationship between fuel cell potential and current density, the so-called fuel cell polarization curve, is obtained ... 

In some cases, in applications with highly variable power, the fuel cell is intentionally sized at a power level between average and peak power. A fuel cell provides power up to its nominal power. An increase in demand above this power level further drops the fuel cell potential and in that case the battery steps in and covers the difference in power. In this case, the battery must be sized not only to match the power requirements, but also to match the energy requirements during those periods when the power demand exceeds the fuel cell nominal power. The fuel cell automatically recharges the battery when the load power goes below the fuel cell nominal power. 

MEA should be fabricated from the sample PEMs and then assembled into the single-cell system, which is the same as fuel cell. Potential step experiments were performed to evaluate fuel permeability using an electrochemical interface with a certain flow rate of humidified H2 at the anode and humidified N2 at the cathode. The anode served as both the counter and reference electrodes. The cathode potential was increased gradually, and the steady-state current density corresponds to the H2 crossover current density. Although more complicated than ex situ method, the in situ method is much more accurate and closer to the real situation of fuel cell operation. 

In an ideal cyclic voltammetric (CV) set-up, one would choose the appropriate scan rate and a potential window for scanning. In an aqueous electrolyte, one can scan the 1.5 V potential window to get mechanistic information, but in a real fuel cell, potentials above 0.75 V (vs. SHE) is not advised if the electro-active Pt is supported on carbon. At these elevated potentials, the carbon corrosion current density increases significantly and such a useful technique could impact the cell performance post diagnostics. Scan rates are typically... 

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