Fuel cells current density-potential

Fig. 4. Potential-current density curves for some electrogenerative and fuel cell reactions (O) benzene reduction on Pt (4S) (A) vinyl chloride reduction on Pt (31) (A) ethylene reduction on Pt (25) ( ) ethylene reduction on Pd (48a) (O) oxygen reduction on Pt (49). (Reprinted by permission of the publisher, The Electrochemical Society, Inc.)...

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).

Development of fuel cells accelerated with the energy crisis of the 1970s. The fuels used most frequently are hydrogen, hydrocarbons, and alcohols. Normally, hydrogen is not a primary fuel but is produced from the others and used primarily in low-temperature cells. Table 17.2 describes the characteristics of the most important types, and Table 17.3 summarizes their current status [27]. Figure 17.7 shows their potential-current density behavior [28]. 

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 ... 

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. 

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. 

Ethylene glycol (EG, C2H6O2) is ubiquitously used in the automotive industry as an engine coolant, and hence a distribution infrastructure already exists. Also, EG has a crossover current density roughly half that of methanol [69]. However, PEFC performance with EG is still relatively low, with a fuel cell specific energy density about 20-40% less than that of the same fuel cell utilizing methanol. Additionally, EG has been shown to rapidly degrade PEFC elecfiolyte material, which obviously limits its potential PEFC applications. 

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 ... 

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... 

Graphs of operating potential versus current density are called polarization curves, which reflect the degree of perfection that any particular fuel cell technology has attained. High cell operating potentials are the result of many years of materials optimization. Actual polarization curves will be shown below for several types of fuel cell. 

Further, as the current density of the fuel cell increases, a point is inevitably reached where the transport of reactants to or products from the surface of the electrode becomes limited by diffusion. A concentration polarization is estabhshed at the elec trode, which diminishes the cell operating potential. The magnitude of this effect depends on many design and operating variables, and its value must be obtained empirically. 

This is considerably higher than that of an H2-O2 fuel cell (i.e., 83%). However, under normal operating conditions, at a current density j, the electrode potentials deviate from their equilibrium values as a result of large overpotentials, r, at both electrodes (Fig. 5) ... 

Figure 5. Typical half-cell potentials vs. current density curves tor an Flj-Oj fuel cell and aDMFC.

Depending on current density, the working potential of steady-state methanol oxidation varies within the range 0.35 to 0.65 V (RHE). Therefore, the working voltage of a methanol-oxygen fuel cell will have values between 0.4 and 0.7 V. 

Figure 1.5 The slope of E ath versus log /orr through the fuel-cell-relevant potential range has an apparently constant value near RT/F (measured current density, here designated i, is corrected for hydrogen crossover current, designated i and the measured cell voltage is ir-corrected to provide the cathode potential E) [Neyerlin et al., 2006].

The performance of fuel cells is affected by operating variables (e.g., temperature, pressure, gas composition, reactant utilizations, current density) and other factors (impurities, cell life) that influence the ideal cell potential and the magnitude of the voltage losses described above. Any number of operating points can be selected for application of a fuel cell in a practical system, as illustrated by Figure 2-4. 

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