Membrane electrodes characteristics

Yasuda K, Taniguchi A, Akita T, loroi T, Siroma Z. 2006a. Characteristics of a platinum black catalyst layer with regard to platinum dissolution phenomena in a membrane electrode assembly. J Electrochem Soc 153 A1599-A1603. 

Table 16.2 records the characteristics of certain selected crystalline-membrane electrodes. 

Table 16.2 Characteristics of Certain Selected Crystalline Membrane Electrodes...

Response characteristics of the enantioselective, potentiometric membrane electrode for S-captopril [1]... 

Because any potentiometric electrode system ultimately must have a redox couple (or an ion-exchange process in the case of membrane electrodes) for a meaningful response, the most common form of potentiometric electrode systems involves oxidation-reduction processes. Hence, to monitor the activity of ferric ion [iron(III)], an excess of ferrous iron [iron(II)] is added such that the concentration of this species remains constant to give a direct Nemstian response for the activity of iron(III). For such redox couples the most common electrode system has been the platinum electrode. This tradition has come about primarily because of the historic belief that the platinum electrode is totally inert and involves only the pure metal as a surface. However, during the past decade it has become evident that platinum electrodes are not as inert as long believed and that their potentiometric response is frequently dependent on the history of the surface and the extent of its activation. The evidence is convincing that platinum electrodes, and in all probability all metal electrodes, are covered with an oxide film that changes its characteristics with time. Nonetheless, the platinum electrode continues to enjoy wide popularity as an inert indicator of redox reactions and of the activities of the ions involved in such reactions. 

The membrane electrode assembly (MEA), which consists of three components (two gas diffusion electrodes with a proton exchange membrane in between), is the most important component of the PEMFC. The MEA exerts the largest influence on the performance of a fuel cell, and the properties of each of its parts in turn play significant roles in that performance. Although all the components in the MEA are important, the gas diffusion electrode attracts more attention because of its complexity and functions. In AC impedance spectra, the proton exchange membrane usually exhibits resistance characteristics the features of these spectra reflect the properties of the gas diffusion electrode. In order to better understand the behaviour of a gas diffusion electrode, we introduce the thin-film/flooded agglomerate model, which has been successfully applied by many researchers to... 

Selectivity is one of the essential characteristics of the membrane electrodes. Accordingly, it is important that by-products, degradation products, metabolites, and compressing components (excipients) do not interfere, and thus the ISMEs can be successfully used for raw material assays as well as for raw material determinations in pharmaceutical formulations. To improve the selectivity, it is necessary to choose another counter ion for membrane construction, because the selectivity of membrane electrodes can be explained through the stability constants of the ion pair complexes between the counter ion and the ions from the solution. To be selective over one ion from the solution it is necessary for the stability constant of the ion pair complex to be less than the stability constant of the ion pair complex obtained between the main ion and counter ion. 

The validation of ion-selective membrane electrodes is based on the reproducibility of their development, the simplicity and rapidity of their construction, and their response characteristics. Typically, these response characteristics include minimum 50 mV/decade for the slope, 10 6 mol/L for limit of detection, large working range, and low response time, which can be assured only by the best counter ion and matrix (PVC and plasticizer for solid membrane electrodes and solvent for liquid membrane electrodes). 

In ion-selective potentiometry the stability and reproducibility of Eceu depends on the type of ISE used. Carefully optimized liquid membrane electrodes in flow-through arrangements for clinical applications yield standard deviations in Eceu smaller than 0.1 mV. This corresponds to less than z, X 0.4% error in the determination of activities. Ion-selective microelectrodes have poorer characteristics due to technical (e.g., silanization) and electrical (high resistance see below) reasons. 

Membrane electrode assemblies (MEAs) are typically five-layer structures, as shown in Figure 10.1. The membrane is located in the center of the assembly and is sandwiched by two catalyst layers. The membrane thickness can be from 25 to 50 pm and, as mentioned in Chapter 10, made of perfluorosulfonic acid (Figure 11.3). The catalyst-coated membranes are platinum on a carbon matrix that is approximately 0.4 mg of platinum per square centimeter the catalyst layer can be as thick as 25 pm [12], The carbon/graphite gas diffusion layers are around 300 pm. Opportunities exist for chemists to improve the design of the gas diffusion layer (GDF) as well as the membrane materials. The gas diffusion layer s ability to control its hydrophobic and hydrophilic characteristics is controlled by chemically treating the material. Typically, these GDFs are made by paper processing techniques [12],... 

Quaternary Ammonium Ions. In a recent study (17), 1200 EW Nafion has been used to construct a membrane ion selective electrode. The electrode was placed in both the tetrabutylammonium ion and cesium ion forms, and the response characteristics of each form were measured. These electrodes show Nernstian responses, and the tetrabutylammonium ion electrode has no interference from inorganic cations such as Na" ", K" ", and Ca2" ". However, this electrode shows a marked interference with decyltri-methylammonium ion. In addition the cesium ion electrode response is sensitive to the presence of tetrabutylammonium ion and especially dodecyltrimethylammonium ion. Although membrane electrode sensitivities are not in general proportional to thermodynamic selectivity coefficients, the results do indicate that these large, hydrophobic cations are preferred over smaller inorganic cations by the polymer. The authors suggest that the surfactant character of the two asymmetric tetraalkylammonium ions may lead to non-electrostatic interactions with the fluorocarbon regions of the polymer, which would enhance their affinities (17). 

Ion-selective membrane electrodes as amperometric and potentiometric biosensors cannot be successfully used for ion monitoring in water. Their main characteristic is detection of an ion in the sample continuously and without any prior separation. The sampling process for a solid sample is reduced at its dissolution in distilled water. Due to the complexity of the matrix for wastewater or for seawater samples, there are a number of interfering inorganic and organic ions. Using biosensors for water analysis, one can obtain the total quantity of organic substances that are contained in a class it is practically impossible to discriminate the content of every compound from within the same class. 

Ion-selective membrane electrodes have as a main characteristic their selectivity. They are constructed to be utilized to determine an analyte directly in the solution without any prior separation from the matrix. This is achieved assuming a high selectivity of the electrode vs. the possible interfering ions. The selectivity is characterized through the potentiometric selectivity coefficient. The values of the coefficients that can be taken into account for validation are those obtained through the mixed solutions method at a ratio between analyte and interferent of 1 10. The method is... 

The E-pO plot of the cell with the gas membrane oxygen electrode at 600 °C is presented in Fig. 2.4.4, sections 1 and 2 [187], This plot contains an inflection point at pO 2, which is characteristic of such membrane electrodes. The positive e.m.f. values of the cell (2.4.28) correspond to running the following potential-determining reaction ... 

P. Millet, J. Alleau and R. Durand, Characteristics of membrane-electrode assemblies for solid polymer electrolyte water electrolysis, J. Appl. Electrochem., 1993, 23, 322. 

The ignition/extinction results and responses to changes in load provide information about the time scales for the response of the fuel cell. The time constant for transitioning to steady state during startup is 100 s. Five of the key time constants associated with PEM fuel cells are listed in Table 3.1.They include the characteristic reaction time of the PEM fuel cell (ti), the time for gas phase transport across the diffusion layer to the membrane electrode interface (T2), the characteristic time for water to diffuse across the membrane from the cathode to the anode (ts), the characteristic time for water produced to be absorbed by the membrane (T4), and the characteristic time for water vapor to be convected out of the fuel cell (T5). Approximate values for the physical parameters have been used to obtain order of magnitude estimates of these time constants. 

After extended operation of an STR PEM fuel cell with the same membrane electrode assembly (> 2500 h), autonomous oscillations were observed under conditions where the STR PEM fuel cell exhibited 5 steady states [23]. An example of the oscillations is shown in Figure 3.11.These oscillations have periods of 10 -10 s and show characteristics of a capacitively coupled switch. The oscillations transition very rapidly (<10s) between high and low states with an overshoot on the rise and undershoot and recovery on the fall. The period, magnitude and on/off times for these oscillations varied with temperature, and load resistance. Benziger and co-workers have suggested that these unusual dynamics are associated with mechanical relaxations of the polymer membrane driven by changing water content, but the detailed physical processes causing these unusual dynamics are not yet understood. 

Krewer U, Park JY, Lee JH, Cho H, Pak C, You DJ, Lee YH (2009) Low and high temperature storage characteristics of membrane electrode assemblies for direct methanol fuel cells. J Power Sources 187 103-111... 

It is useful to discuss the performance characteristics of an eCMR in terms of a polarization plot to help understand its various modes of operation depicted in Figure 15.4. This analysis follows our earlier approach (Choi et al., 2004 Thampan et al., 2001 Vilekar Datta, 2010), in which we consider the membrane electrode assembly (MEA) for a fuel cell cCMR as consisting of the live layers, an electrical analog of which is shown in Figure 15.5. 

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