Fuel cell polarization

Fig. 14.20 Fuel cell polarization curves for CM-FeS04-KB (cyanamide-FeS04-Ketjeblack) ORR catalysts obtained at different heat-treatment temperatures. Operating conditions H2/02,100% RH,...

To obtain a useful fuel cell, polarization must therefore be kept as slight as possible. This can be done by choosing an electrolyte of good conductivity and above all by accelerating the electrode reactions. In low-temperature cells, that operate with aqueous electrolytes the reactions at both cathode and anode can be considerably accelerated by the addition of very active catalysts. These materials are incorporated in the appropriate electrode, so that the electrode not only conducts the current but in addition catalyzes the reaction. The rest of this paper is devoted exclusively to cells of this type. 

FIGURE 8.16 Schematic fuel cell polarization voltage [V] vs. current density [A/cm2] curve. 

The relationship between the cell voltage and the cell current density can be simply written as the following equation if the fuel cell polarization is larger than 60 mV ... 

FIGURE 27.32 Fuel cell polarization curve of a 50 cm MEA made with the new 3M cast membrane (30 pm). (From 3M, www.3m.com.)... 

Several authors have shown the fuel cell performance with varying PTFE content [103, 106, 108, 109]. Figure 7.16 shows the polarizatirais curves of DMFC with different contents of PTFE on the anode and cathode GDL respectively [82]. An excess of PTFE has an adverse effect on the fuel cell polarization as observed in both graphs. The better performance is attained with different PTFE amounts at the anode and the cathode, indicating that an optimum amount has to be chosen according to the particular material and application. This is related to the fact that different amounts of water are present in the anode and the cathode. While an aqueous solution is fed to the anode, water reaches the cathode mainly by transport from the anode through the membrane, because dried O2 or air is typically supply to the cathode. 

The variation of activity for these catalysts, all made under the same experimental conditions, except for the carbon support, was stunning and is presented as fuel cell polarization curves in Figure 3.25. On the one hand, the specific area of all these catalysts was measured and no correlation with the catalytic activity was found. On the other hand, there was a definite correlation between the catalytic activity and the surface nitrogen concentration measured by XPS. This correlation is illustrated in Figure 3.26. It is obvious that the catalytic activity for ORR increases when the surface of the catalyst is richer in nitrogen atoms. This is a logical result, since the precursor of the Fe-N2/C catalytic site (the most... 

Figure 3.25. PEM fuel cell polarization curves at 80°C of several MEAs made with nonnoble metal cathode catalysts. The cathode catalysts were prepared by adsorption of iron acetate (0.2 wt% Fe) on various untreated (NT) (panel a) or heat treated (T) carbon supports (panel b). The (T) carbon supports were obtained by heat beatment of (NT) carbon supports at 900°C in H2 Ar NH3 (1 1 2). The polarization curve for 5 wt% Pt/C at the cathode is given for comparison (according to Figure 3 in ref. [Ill] reproduced with permission of the American Chemical Society).

Figure 7.14 (a) Cross section through a catalytic layer formed by "self-assembly" of the Pickering emulsion and (b) the corresponding fuel cell polarization curve. 

Fig. 8.9 (a) Fuel cell polarization plots recorded with CM-FeS04-KB ORR catalysts obtained at different heat-treatment temperatures (b) fuel cell polarization plots comparison with CM-FeS04-Ketjenblack heat-treated at 1,050 °C and CM-FeAc-KB heat-treated at 1,000 °C. Nafion 117 membrane anode 30 psig H2, 0.25 mgp, cm (catalyzed cloth GDL, E-TEK) cathode 30 psig O2, non-precious catalyst loading 4.0 mg cm 100 % RH anode and cathode humidification 300/500 standard niL per minute anode/cathode flow rates for H2 and O2, respectively MEA surface area 5 cm (reprinted from ref. [48] with permission from Elsevier)... 

Fig. 8.12 ORR performance of CoFe-N-C NPMCs as a function of the Co-to-Fe ratio in the synthesis, (a) RDE measurement in 0.5 M H2SO4 at 25 °C and 900 rpm catalyst loading 0.6 mg cm . (b) Fuel cell polarization plots in a H2-O2 cell anode/cathode back pressure 1.0...

Fig. 8.16 Bottom H2-O2 fuel cell polarization plots recorded with 4 mg cm of a PANI-FeCo-C catalyst in the cathode. Performance of an H2-air fuel cell with a Pt cathode (0.2 mgp, cm ) is shown for comparison. Top Long-term perframance stability test of the PANI-FeCo-C catalyst in a H2-air fuel cell at a constant fuel cell voltage of 0.40 V. Anode and cathode gas pressure 2.8 bar, anode loading 0.25 mgp, cm cell temperature 80 °C (reprinted from ref [57] with permission from AAAS)...

Fig. 22.10 Fuel cell polarization curves before (BOX) and after (EOT) OER testing for 10 ig/cm of E and Ru on 0.15 mg/cm Pt-NSTF. Uncoated Pt-NSTF presented for comparison. Test conditions Galvano-dynamic scans at 80 °C H2/air, constant stoich 2.0/2.5, 68 °C/68 °C, and dew point, 50/50 kPa...

Partial pressures (normalized to atmospheric pressure) depend on anode/cathode gas pressure and composition while standard potential and ohmic impedance are both temperature dependent. Fuel cell polarization losses are generally dependent on partial pressures, temperature, and current density, and are spatially distributed in an actual cell. In this paper the dynamic model is lumped parameter, where outlet properties are equal to average properties (Ding et al, 1997 Lukas et al, 1999 Lukas et al, 2001). 

Electrochemical Reactor Design and Configurations, Fig. 3 Contributions of component overpotentials to a fuel cell polarization curve (a) simulated polarization curve for water-saturated H2 and air at 80 °C and 1 atm. The curves are (—) cell potential E, (— —) reversible potential Erev, ( ) transport-limited overpotential r)n, (— —) ohmic resistive overpotential r), and (— —)... 

A sufficiently accurate approximation of the fuel cell polarization curve may be obtained by the following equation ... 

Figure IIL7. Effect of operating temperature on fuel cell polarization curve [13],... 

Figure 1.27. A sample fuel cell polarization curve obtained from the diagnostic modeling PEM fuel cell with an active area of 192 cm, designed by NRC-IFCI...

Figure 4.2. DMFC polarization behavior using aqueous methanol and solid polymer electrolyte (SPE Nafion 117) [52]. a) Anode half-cell polarization comparison between SPE and 0.5 M H2SO4, PtRu/C 0.5 mg cm, b) fuel cell polarization curve using 2 M CH3OH in water effect of temperature. PtRu/C coated Nafion 117 membrane. Note the anode and cathode catalyst loads in the case of fuel cell experiments were not specified in [52]. (Reproduced Ifom Journal of Power Sources, 47(3), Surampudi S, Narayanan SR, Vamos E, Frank H, Halpert G, LaConti A, Kosek J, Surya Prakash GK, and Olah GA, Advances in direct oxidation methanol fuel cells, 377-85, 1994, with permission fi-om Elsevier.)...

Figure 4.38. Direct ethanol fuel cell polarization curves at 353 K. Ethanol concentration 2 M, Nation 117, O2 pressure 3 bar [183]. (Reproduced from Journal of Power Sources, 158(1), Rousseau S, Coutanceau C, Lamy C, Leger J-M, Direct ethanol fuel cell (DEFC) electrical performances and reaction products distribution under operating conditions with different platinum-based anodes, 18-24, 2006, with permission from Elsevier.)...

Illustrative fuel cell polarization with losses noted. (From Ye, S. 2007. Electrocatalysts for PEM fuel cells Challenges and opportunities. 14th National Meeting of Chinese Society of Electrochemistry, Yangzhou, China.)... 

Fig. 2.1 Direct methanol fuel cell polarization curves at 145 °C for MEAs containing different inorganic fillers top). Methanol feed 2 M, 2.5 atm (gauge pressure) oxygen...

FIGURE 10.27 Fuel cell polarization curve under different RH. (Reprinted from handouts of P. Pei. With permission.)... 

The single characteristic that encompasses phenomena at all scales and in all fuel cell components is the fuel cell polarization curve. The polarization curve of a membrane-electrode assembly (MEA), a single cell, or a fuel cell stack furnishes the link between microscopic structure and physicochemical properties of distinct cell components on the one hand and macroscopic cell engineering on the other. It thus condenses an exuberant number of parameters, which lies in the 50s to 100s, into a single response function. Analysis of parametric dependencies in the polariz-tion curve could be extremely powerful at the same time, it could as well be highly misleading if applied blindly. ... 

Figure I.6a displays a selection of fuel cell polarization curves extracted from publications that span a period of 125 years. The ordinate depicts the fuel cell voltage Eceii (jo) as given in Equation 1.21. The abscissa represents, on a logarithmic scale (to the base of 10), the fuel cell current density that has been normalized to the surface area-specific mass loading of Pt, mpf.

This chapter mainly deals with the fundamentals of H2/air PEM fuel cells, including fuel cell reaction thermodynamics and kinetics, as well as a brief introduction to the single fuel cell and the fuel cell stack. The electrochemistry and reaction mechanisms of H2/air fuel cell reactions, including the anode HOR and the cathode ORR, are discussed in depth. Several concepts related to PEM fuel cell performance, such as fuel cell polarization curves, OCV, hydrogen crossover, and fuel cell efficiencies, are also introduced. With respect to fuel cell stmctures and components, the material properties and effects on fuel cell performance are also discussed. In addition, several important conditions for fuel cell operation, including temperature, pressure, RH, and gas stoichiometries and flow rates, and their effects on fuel cell operation, are also briefly presented. This chapter provides the requisite baseline knowledge for the remaining chapters. 

FIGURE 2.14 Air and oxygen fuel cell polarization curves for directly catalyzed developmental Dow membrane with a catalyst loading of 0.13 mg Pt cm [39]. 

If these four equations are substituted into Eqns (9.14), (9.9), and (9.12), a semiempitical equation can be obtained, from which the fuel cell polarization curve can be calculated at a desired temperature, backpressure (obtained using gas partial pressure, according to Eqn (9.9) or (9.12)), and RH. Figure 9.2 shows the calculated polarization curves at two different backpressures (2.0 and 3.0 atm absolute backpressure, respectively). Evidently, the backpressure has an effect on performance. Note that during calculation, the values of both and / m were measured from their... 

FIGURE 9.2 Calculated fuel cell polarization curves at backpressures of 2.0atm and 3.0atm. 

The fuel-cell determined mass activities (MA-FC) were also compared with the RDE determined mass activities determined (MA-RDE) (Figure 11.12) in the kinetic region (0.9 V from IR-free fuel-cell I- ceu curves and form RDE curves in 0.1 M HQO4 electrolyte), hi general, the mass activities obtained from RDE and the fuel-cell polarization data showed one trend (i.e., MA decreases with increasing temperature), whereas the specific activities obtained from RDE and the fuel-cell polarization data showed another trend (i.e., SA increases with increasing temperature). 

Fuel cell polarization curves owing to internal losses, activation losses, ohmic losses, and concentration losses. Note that the actual magnitude of each loss is different for each fuel cell design and construction. 

As seen from Equation 5.155, the activation and mass transfer losses are similar in the anode and cathode though often the cathode losses dominate. The fuel cell polarization can be written in approximate form as... 

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. 

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