Ohmic PEMFC

Antoine O, Bultel Y, Durand R, Ozil P. 1998. Electrocatalysis, diffusion and ohmic drop in PEMFC particle size and spatial discrete distribution effects. Electrochim Acta 43 3681-3691. 

In a PEMFC, the power density and efficiency are limited by three major factors (1) the ohmic overpotential mainly due to the membrane resistance, (2) the activation overpotential due to slow oxygen reduchon reaction at the electrode/membrane interface, and (3) the concentration overpotential due to mass-transport limitations of oxygen to the electrode surfaced Studies of the solubility and concentration of oxygen in different perfluorinated membrane materials show that the oxygen solubility is enhanced in the fluorocarbon (hydrophobic)-rich zones and hence increases with the hydrophobicity of the membrane. The diffusion coefficient is directly related to the water content of the membrane and is thereby enhanced in membranes containing high water content the result indicates that the aqueous phase is predominantly involved in the diffusion pathway. ... 

The internal resistance of a fuel cell includes the electric contact resistance among the fuel cell components, and the proton resistance of the proton-conducting membrane. In PEMFCs, the proton resistance of the polymer electrolyte membrane contributes the most to the total ohmic resistance. 

It has been demonstrated that EIS can serve as a standard analytical diagnostic tool in the evaluation and characterization of fuel cells. Scientists and engineers have now realized that the entire frequency response spectrum can provide useful data on non-Faradaic mechanisms, water management, ohmic losses, and the ionic conductivity of proton exchange membranes. EIS can help to identify contributors to PEMFC performance. It also provides useful information for fuel cell optimization and for down-selection of the most appropriate operating conditions. In addition, EIS can assist in identifying problems or predicting the likelihood of failure within fuel cell components. 

Figure 6.20. Nyquist plots for the electrodes fabricated according to the same preparation procedure [19]. Note NSGA stands for novel silica gel additive, and TNPA stands for traditional Nafion polymer additive. The values in parentheses are the ohmic drop corrected cell potential. (Reproduced from Wang C, Mao ZQ, Xu JM, Xie XF. Preparation of a novel silica gel for electrode additive of PEMFCs. Journal of New Materials for Electrochemical Systems 2003 6(2) 65-9, with permission from JNMES.)...

Electrocatalysts One of the positive features of the supported electrocatalyst is that stable particle sizes in PAFCs and PEMFCs of the order of 2-3 nm can be achieved. These particles are in contact with the electrolyte, and since mass transport of the reactants occurs by spherical diffusion of low concentrations of the fuel-cell reactants (hydrogen and oxygen) through the electrolyte to the ultrafine electrocatalyst particles, the problems connected with diffusional limiting currents are minimized. There has to be good contact between the electrocatalyst particles and the carbon support to minimize ohmic losses and between the supported electrocatalysts and the electrolyte for the proton transport to the electrocatalyst particles and for the subsequent oxygen reduction reaction. This electrolyte network, in contact with the supported electrocatalyst in the active layer of the electrodes, has to be continuous up to the interface of the active layer with the electrolyte layer to minimize ohmic losses. 

If activation losses of the electrodes, ohmic losses and concentration losses of the DBFC were on the same level as the PEMFC with an operation voltage of 0.75 V, the DBFC would obtain 1V of operation voltage under the similar operation conditions, as shown in Fig. 8.20. This high operation voltage will benefit the stack and the system design for fuel ceU users because the number of cells can be reduced by 25 % compared with the PEMFC stacks. 

FIGURE 21.42 (a) U-I curve and (b) ohmic resistance of a PEMFC using Pt-Ti02-PEM operated at 80°C and ambient pressure with no external humidification at the reactant utilization of H2 56% and O2 54%. An OCV was measured at a flow rate of 7 mL min- for both dry H2 and dry O2. The amount of Pt dispersed in the PEM = 0.1 mg cm-, the amount of Ti02 = 0.42 mg cm- (4 wt%). Full symbols measured on increasing current density, and open symbols measured on decreasing current density. (Reproduced from Uchida, H. et al., J. Electrochem. Soc., 150, A57, 2003. With permission of the Electrochemical Society, Inc.)... 

Bultel Y, Ozil P, Durand R. Modelling the mode of operation of PEMFC electrodes at the particle level influence of ohmic drop within the active layer on electrode performance. / Appl Electrochem 1998 28(3) 269-76. 

The PEMFC (see Table 17.2 for identification) has the greatest potential to reach high power densities. DMFCs suffer from the high activation potential of the cathodic reduction of oxygen and anodic oxidation of methanol. MCFCs operate at 650°C and SOFCs at 1000°C, their electrolytes being, respectively, molten carbonates and solid metal oxides. Their activation overpotentials are small, but ohmic overpotentials at the... 

Another way to consider the impact of membrane conductivity on fuel cell performance is shown in Fig. 17.5. Figure 17.5a shows the conductivity of a few different EW membranes as a functiOTi of temperature with the atmosphere inside the conductivity cell held at a fixed dew point of 80°C [17]. When the conductivity cell is at 80°C, the %RH is 100%. As the temperature of the cell increases, the %RH at a fixed dew point decreases, causing a decrease in the membrane conductivity. This is similar to the situation in some PEMFC applications where the cell temperature may rise while the humidity level of the incoming gases remains constant. The graph in Fig. 17.5b uses the same data. Here the conductivity is used to calculate the resistance of a 25 pm membrane, and using Ohms law, that resistance is used to calculate the voltage loss (ohmic loss) one would see in a fuel cell at a 0.6 A/cm current density [17]. This represents the fuel cell performance loss due to the loss of membrane conductivity (certainly not the only performance loss under these conditions ). 

At 80°C, 100% RH, the performance loss is low, about 10 mV. Further, the performance difference between the different EW membranes is also quite low, less than 4 mV. As the temperature increases, the performance losses also increase and the effect of the different EW ionomers becomes apparent. At 120°C, the 1,0(X) EW membrane has a large ohmic loss of about 180 mV. This represents a > 20% loss in the operating voltage of a typical PEMFC at this current density, or about > 15% of the energy contained in the hydrogen fuel being converted to heat. The lower EW membranes do provide a significant improvement, but even at the lowest EW shown here, 650, the ohmic loss is still 6 times that of the fully humidified cell. 

Figure 1.30. Voltage transient (thin line) and fitted average voltage (bold hue) for the whole stack. Extrapolation is indicated with a dotted hne. Air supply free convection i = 200 mA cm 171. (Reprinted from Journal of Power Sources, 112(1), Mennola Tuomas, Mikkola Mikko, Noponen Matti, Hottinen Tero and Lund Peter, Measurement of ohmic voltage losses in individual cells of a PEMFC stack, 261-72, 2002, with permission from Elsevier.)...

Meimola T, Mikkola M, Noponen M. Measurement of ohmic voltage losses in individual cells of a PEMFC stack. J Power Sources 2002 112 261-72. 

Figure 7.4. Actual fuel cell voltage/current characteristie of (a) PEMFC and (b) DMFC. OCV - open eireuit voltage Ohmic - ohmic overpotential Anodie - anodic overpotential and Cathodie - eathodie overpotential.

Figure 21.2 shows a typical polarization curve (or current-voltage curve) of PEMFCs. This curve results from both the anodic HOR and the cathodic ORR reactions. The actual celt voltage is much lower than the ideal celt voltage and the theoretical cell voltage. When the current is drawn from a fuel cell, the actual cell voltage will drop from its ideal due to several types of irreversible losses, as shown in Figure 21.2. The drop is mainly caused by mixed potential and fuel crossover, activation overpotential, ohmic overpotential, as well as mass transfer (concentration) overpotential.

The macroscopic modeling described for the PEMFCs can also be apphed to the case of an SOFC fed with hydrogen if we have tests which enable us to identify the different parameters. The activation overvoltage term is far smaller in the case of an SOFC. In fact, the open circuit voltage of a cell is nearest to the reversible potential of the cell. Because of the high temperature, all the species are in gaseous form, and their activity is equal to their partial pressure. The Ohmic resistance of the MEA (the electrodes and the membrane) is highly dependent on the temperature. 

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