Ohmic resistance dispersion

H. Uchida, Y. Mizuno, and M. Watanabe. Suppression of methanol crossover and distribution of ohmic resistance in Pt-dispersed PEMs under DMFC operation. Journal of the Electrochemical Society 149, A682-A687 2002. 

Electrolytically evolved gas bubbles affect three components of the cell voltage and change the macro- and microscopic current distributions in electrolyzers. Dispersed in the bulk electrolyte, they increase ohmic losses in the cell and, if nonuniformly distributed in the direction parallel to the electrode, they deflect current from regions where they are more concentrated to regions of lower void fraction. Bubbles attached to or located very near the electrodes likewise present ohmic resistance, and also, by making the microscopic current distribution nonuniform, increase the effective current density on the electrode, which adds to the electrode kinetic polarization. Evolution of gas bubbles stirs the electrolyte and thus reduces the supersaturation of product gas at the electrode, thereby lowering the concentration polarization of the electrode. Thus electrolytically evolved gas bubbles affect the electrolyte conductivity, electrode current distribution, and concentration overpotential and the effects depend on the location of the bubbles in the cell. Discussed in this section are the conductivity of bulk dispersions and the electrical effects of bubbles attached to or very near the electrode. Readers interested in the effect of bubbles dispersed in the bulk on the macroscopic current distribution in electrolyzers should see a recent review of Vogt.31... 

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.)... 

In addition to the effect of support on the catalyst dispersion, the interaction with the ionomer (Nafion ) is crucial for the electrochemical performance of the catalyst layer in the fuel cell. Catalyst nanoparticles that are isolated from the ionomer network are electrochemically inactive. Furthermore, the distribution of the ionomer will affect the ohmic resistance and the mass transport of the reactants and/or products in the catalyst layer. Hence, the interface between the catalyst/support/ionomer will influence the overall polarization behavior of the anode. 

Although the catalyst activity is the most important factor in improving cell performance of HT-PEMFC, the catalyst layer structure can also be optimized to increase the concentration of O2 in phosphoric acid near the catalyst sites. To establish a diffusion path for O2 in gas phase, two possible approaches can be taken. One is the dispersion of the hydrophobic binder such as PTFE within the catalyst layer. For the MEAs that contain the high content of phosphoric acid, it would be best to use the PTFE binder in the catalyst layer. The other way is to control the pore size within the catalyst layer so that the pores that are filled with phosphoric acid and pores that provide path for O2 diffusion in the gas phase can be separated according to the pore size. The piimaiy pores between catalyst particles are known to be filled with phosphoric acid, while the larger secondary pores between the agglomerates of catalyst particles provide O2 diffusion path [47]. If the O2 diffusion path within the catalyst layer can be established and maintained without the use of the hydrophobic binder which increases the ohmic resistance in MEAs [45], the cell performance of HT-PEMFC can be improved. 

At high temperatures, the isotherms are characterized by a plateau nearly independent on frequency, followed by a dispersive regime. Moreover, whereas the plateau for the KNIO isotherms coexists with = 0 in a wide range of temperatures, this only occurs for the KN8 system at high temperatmes. For low temperatures the values of / / at the frequencies studied are higher than zero, indicating that in these cases the ohmic resistance should be... 

The ohmic drop across the electrolyte and the separator can also be calculated from Ohm s law usiag a modified expression for the resistance. When gas bubbles evolve at the electrodes they get dispersed ia and impart a heterogeneous character to the electrolyte. The resulting conductivity characteristics of the medium are different from those of a pure electrolyte. Although there is no exact description of this system, some approximate treatments are available, notably the treatment of Rousar (9), according to which the resistance of the gas—electrolyte mixture, R, is related to the resistance of the pure electrolyte, R ... 

It has been observed that solid oxide fuel cell voltage losses are dominated by ohmic polarization and that the most significant contribution to the ohmic polarization is the interfacial resistance between the anode and the electrolyte (23). This interfacial resistance is dependent on nickel distribution in the anode. A process has been developed, PMSS (pyrolysis of metallic soap slurry), where NiO particles are surrounded by thin films or fine precipitates of yttria stabilized zirconia (YSZ) to improve nickel dispersion to strengthen adhesion of the anode to the YSZ electrolyte. This may help relieve the mismatch in thermal expansion between the anode and the electrolyte. 

The main problem in accurate measurement of resistivity is one of contact resistance between the measurement electrodes and the specimen. This is clearly the case for samples with low resistivity, but can also be a problem for more resistive samples if either the contact resistance is high or the contact is non-ohmic. Contact resistance may be reduced by painting electrodes directly on to the surface of the specimen instead of relying on pressure contact with metal plates or foils. Suitable paints are silver dispersions or Aquadag (an... 

An electronic conduction-type humidity sensor was developed by Ishida et al. [56J. The sensor structure is very similar to the one illustrated in Figure 20-41. A cross-linked hydrophilic polymer in which carbon particles are dispersed is used as the humidity-sensitive film. The swelling of the polymer disturbs the ohmic contacts between dispersed carbon particles and thus the electronic resistance of the element increases sharply as the relative humidity approaches 100< o, as shown in Figure 20-41. This element was put into practical use as a dew detector for the VTR cylinder in 1978. 

The calculation of the distance between the anode and the separator is slightly complex as the asbestos diaphragm is deposited on the cathode screen, and the anode blade is located 4-5 mm from the diaphragm with the anolyte in between, and contains dispersed chlorine gas bubbles. The gas-solution mixture is circulated effectively by the convection of the two-phase flow resulting in a relatively low gas void fraction in the electrolysis zone. The superficial resistivity, Pmix. based on Eq. (163), would be about 1.2 times the resistivity of the solution free of gas bubbles, which is 1.2/0.59 = 2.03 cm. The ohmic drop between the anode and the diaphragm can then be calculated from Eq. (165) and the current as ... 

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