Polymer electrolyte response

By comparing impedance results for polypyrrole in electrolyte-polymer-electrolyte and electrode-polymer-electrolyte systems, Des-louis et alm have shown that the charge-transfer resistance in the latter case can contain contributions from both interfaces. Charge-transfer resistances at the polymer/electrode interface were about five times higher than those at the polymer/solution interface. Thus the assignments made by Albery and Mount,203 and by Ren and Pickup145 are supported, with the caveat that only the primary source of the high-frequency semicircle was identified. Contributions from the polymer/solution interface, and possibly from the bulk, are probably responsible for the deviations from the theoretical expressions/45... 

Polymer electrolyte fuel cells (PEFC) deliver high power density, which offers low weight, cost, and volume. The immobilized electrolyte membrane simplifies sealing in the production process, reduces corrosion, and provides for longer cell and stack life. PEFCs operate at low temperature, allowing for faster startups and immediate response to changes in the demand for power. The PEFC system is seen as the system of choice for vehicular power applications, but is also being developed for smaller scale stationary power. For more detailed technical information, there are excellent overviews of the PEFC (1,2). 

The need for an air sampling pump can be eliminated by use of a diffusion tube having a set length to diameter (L/d) ratio in its geometry for introduction of a gas sample. Proper selection of the geometry and L/d ratio of the diffusion tube results in an electrochemical cell with a response which is independent of external gas flow rate. A schematic of a solid polymer electrolyte diffusion head sensor cell is shown in Figure 13. 

Polymer electrolyte fuel cell (PEFC) is considered as one of the most promising power sources for futurist s hydrogen economy. As shown in Fig. 1, operation of a Nation-based PEFC is dictated by transport processes and electrochemical reactions at cat-alyst/polymer electrolyte interfaces and transport processes in the polymer electrolyte membrane (PEM), in the catalyst layers consisting of precious metal (Pt or Ru) catalysts on porous carbon support and polymer electrolyte clusters, in gas diffusion layers (GDLs), and in flow channels. Specifically, oxidants, fuel, and reaction products flow in channels of millimeter scale and diffuse in GDL with a structure of micrometer scale. Nation, a sulfonic acid tetrafluorethy-lene copolymer and the most commonly used polymer electrolyte, consists of nanoscale hydrophobic domains and proton conducting hydrophilic domains with a scale of 2-5 nm. The diffusivities of the reactants (02, H2, and methanol) and reaction products (water and C02) in Nation and proton conductivity of Nation strongly depend on the nanostructures and their responses to the presence of water. Polymer electrolyte clusters in the catalyst layers also play a critical... 

Masuda, H., Ito, K., Kakimoto, Y., Miyazaki, T., Ashikaga, K. and Sasaki, K., (2006) Numerical simulation of two-phase flow and transient response in polymer electrolyte fuel cell, in Proceedings of FUELCELL2006, The 4th International Conference on Fuel Cell Science Engineering and Technology, Irvine, CA, June 19-21. 

Figure 5.32. Impedance plots for single cells at ambient temperature, a Nation 117. Cell voltage and ohmic drop corrected potential (in parenthesis) ( ) 0.9 V (0.9 V) ( ) 0.8 V (0.81 V) (A) 0.70 V (0.76 V) ( ) 0.6 V (0.74 V) ( ) 0.5 V (0.74 V). b Nafion 112. Cell voltage and ohmic drop corrected potential (in parenthesis) ( ) 0.9 V (0.9 V) ( ) 0.8 V (0.81 V) (A ) 0.70 V (0.73 V) ( ) 0.6 V (0.67 V) ( ) 0.5 V (0.61 V). Plots were corrected for the high-frequency resistances. Left detail of the high-frequency regions [29]. (Reprinted from Journal of Electroanalytical Chemistry, 503, Freire TJP, Gonzalez ER. Effect of membrane characteristics and humidification conditions on the impedance response of polymer electrolyte fuel cells, 57-68, 2001, with permission from Elsevier.)...

Freire TJP, Gonzalez ER (2001) Effect of membrane characteristics and humidification conditions on the impedance response of polymer electrolyte fuel cells. J Electroanal Chem 503 57-68... 

Paganin VA, Oliveira CLF, Ticianelli EA, Springer TE, Gonzalez ER (1998) Modelistic interpretation of the impedance response of a polymer electrolyte fuel cell. Electrochim Acta 43 3761-6... 

Figure 6.1. Impedance spectra at several electrode potentials for H2/02 PEFCs with 30 wt% Pt/C, 0.4 mg Pt/cm2 and 0.7 mg Nafion /cm2 electrodes, Nafion 117 membrane at T = 25°C and atmospheric gas pressure a full range of frequency (0.05 Hz to 10 kHz) b details of the high-frequency region (o) 0.925 V, (A) 0.9 V, (V) 0.875 V, and (+) 0.65 V [4], (Reprinted from Electrochimica Acta, 43(24), Paganin VA, Oliveira CLF, Ticianelli EA, Springer TE, Gonzalez ER. Modelistic interpretation of the impedance response of a polymer electrolyte fuel cell, 3761-6, 1998, with permission from Elsevier and the authors.)...

A major challenge in the development of these two sensors was the selection of the proper electrolyte. For the Back Cell design, the polymer electrolyte required a unique set of properties. The polymer must have relatively high viscosity upon fabrication to prevent it from entering the pores of the substrate and blocking the triple points. The polymer must have sufficient ionic conductivity to eliminate pick-up of electrical noise. In addition, the polymer must have a stable water content at the high relative humidities found in respirator circuits. An obvious requirement is that the gas must dissolve in the polymer electrolyte. For the CO2 sensor, an enzyme, carbonic anhydrase, is used to improve response time. Therefore, the hydrogel must provide an hospitable environment for the enzyme to retain its activity. 

N. Fujiwara, K. Asaka, Y. Nishimura, K. Oguro and E. Torikai, Preparation of gold-solid polymer electrolyte composites as electric stimuli-responsive materials, Chem. Mater., 2000, 12, 1750-1754. 

N. Mayo, R. Harth, U. Mor, D. Marouani, J. Hayon and A. Bettelheim, Electrochemical response to H2, 02, C02 and NH3 of a solid-state based on a cation- and anion-exchange membrane serving as a solid polymer electrolyte, Anal. Chim, Acta, 1995,310, 139-144. 

K. Oguro, Bending response of polymer electrolyte actuator, Proceedings of SPIE Reprint (The International Society for Optical Engineering) Preprint, Electroactive Polymer Actuators and Devices, Vol. 3987, Newport Beach, California, 6-8, March, 2000. 

K. Onishi, S. Sewa, K. Asaka, N. Fujiwara and K. Oguro, Morphology of electrodes and bending response of the polymer electrolyte actuator, Electrochim. Acta, 2000, 46, 737-743. 

AC impedance measurements were carried out on 300 0.m thick polymer electrolyte samples, with n=16 and an exposed area of 0.20 cm, using a Pt/PEGi6Mg(CI04)2/Pt cell configuration results for the other molar ratios will be reported later. Regarding the experimental set-up, major equipment included a Schlumberger 1255 HF Frequency Response Analyser, electrochemically interfaced to a Western Systems 486 PC via a PAR 273A... 

Greszczuk et al. [252] employed the a.c. impedance measurements to study the ionic transport during PAn oxidation. Equivalent circuits of the conducting polymer-electrolyte interfaces are made of resistance R, capacitance C, and various distributed circuit elements. The latter consist of a constant phase element Q, a finite transmission line T, and a Warburg element W. The general expression for the admittance response of the CPE, Tcpr, is [253]... 

It is complex to attempt to offer general specifications for a solid polymer electrolyte which will be useful under all conditions. However, a few generalizations are possible. To achieve a reasonable transient response — say of the order of a minute or less — the electrolyte should be of the order of 25 pm in thickness. Since the response time goes as the square of the thickness it is clear that a two or three-fold increase is the most that is likely to be tolerable. 

Electrolyte Thickness. A response time of the order of 10 to 20 seconds (T99) is desirable. This implies a solid polymer electrolyte thickness less than 25 jum. This is more or less the same for both oxygen and carbon dioxide electrodes. 

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