Cells with solid polymer electrolytes

The necessary porosity for thicker layers was introduced by appropriate current densities [321-323], by co-deposition of composites with carbon black [28, 324] (cf. Fig. 27), by electrodeposition into carbon felt [28], and by fabrication of pellets from chemically synthesized PPy powders with added carbon black [325]. Practical capacities of 90-100 Ah/kg could be achieved in this way even for thicker layers. Self-discharge of PPy was low, as mentioned. However, in lithium cells with solid polymer electrolytes (PEO), high values were reported also [326]. This was attributed to reduction products at the negative electrode to yield a shuttle transport to the positive electrode. The kinetics of the doping/undoping process based on Eq. (59) is normally fast, but complications due to the combined insertion/release of both ions [327-330] or the presence of a large and a small anion [331] may arise. Techniques such as QMB/CV(Quartz Micro Balance/Cyclic Voltammetry) [331] or resistometry [332] have been employed to elucidate the various mechanisms. 

When cells with solid polymer electrolytes (SPE) are used, no electrolytes need to be dissolved in the medium, and solvents, which usually cannot be employed in electrolysis, may be used [74—79]. 

A membrane ionomer, in particular a polyelectrolyte with an inert backbone such as Nation . They require a plasticizer (typically water) to achieve good conductivity levels and are associated primarily, in their protonconducting form, with solid polymer-electrolyte fuel cells. 

Fig. 12.2 Plate-and-frame electrolyser schemes (a) undivided cell (b) membrane-divided cell (c) solid polymer electrolyte (SPE) reactor (d) membrane-divided cell with GDE (e) SPE-GDE reactor. Liquid compartments are denoted in grey...

Major tasks of the hydrogen energy program in the Russian Federation in which about 20 scientific organisations are involved include research on fuel cells and electrolyzers both with solid polymer electrolyte, on H2 / O2 steam generator (experiments with 20- 100 kW and 10-20 MW thermal power), and on catalytic combustion [83]. 

Instrumentation. The interface within a suitably constructed electrochemical cell to be investigated is placed in the sample position of a standard DRIFT accessory for an infrared spectrometer for a typical design, see [328,329]. Examples reported so far deal with solid polymer electrolyte fuel cells where the surface of the anode layer exposed to a mixed gas atmosphere containing both water and methanol is separated from the environment via a Cap2 window [331, 332]. Various oxidized species and penetrating methanol were observed. 

Ion conducting solid polymer electrolytes, such as those used in battery and fuel cell membranes, have been explored for use in supercapacitors [153,159,200,201]. While these electrolytes are environmentally benign and do not leak, conductivities are typically much lower than liquid or gel electrolyte systems, especially at subambient temperatures (important for military and space applications). Nevertheless, capacitance in supercapacitors prepared with solid polymer electrolytes has been reported to be as good as or better than the same devices constructed using liquid electrolytes. Nafion [200], polyethylene oxide [153], and polyvinyl alcohol [153] are the polymers of choice for this application. 

Polymer electrolytes are used in lithium ion rechargeable batteries. Pure polymer electrolyte systems include polyethylene oxide (PEO), polymethylene-polyethylene oxide (MPEG), or polyphosphazenes. Chlorinated PVC blended with a terpoly-mer comprising vinylidene chloride/acrylonitrile/methyl methacrylate can make a good polymer electrolyte. Rechargeable lithium ion cells use solid polymer electrolytes. Plasticized polymer electrolytes are safer than liquid electrolytes because of a reduced amount of volatiles and flammables. The polymer membrane can condnct lithinm ions. The polymer membrane acts as both the separator and electrolyte [7],... 

A possible solution to this problem is to use an electrolyte, such as a solid polymer electrolyte, which is less reactive with lithium metal [3]. Another simple solution is the lithium-ion cell. 

The most promising fuel cell for transportation purposes was initially developed in the 1960s and is called the proton-exchange membrane fuel cell (PEMFC). Compared with the PAFC, it has much greater power density state-of-the-art PEMFC stacks can produce in excess of 1 kWA. It is also potentially less expensive and, because it uses a thin solid polymer electrolyte sheet, it has relatively few sealing and corrosion issues and no problems associated tvith electrolyte dilution by the product water. 

The experimental setup is shown in Figure 9.23. The Pt-black catalyst film also served as the working electrode in a Nafion 117 solid polymer electrolyte cell. The Pt-covered side of the Nafion 117 membrane was exposed to the flowing H2-02 mixture and the other side was in contact with a 0.1 M KOH aqueous solution with an immersed Pt counterelectrode. The Pt catalyst-working electrode potential, Urhe (=Uwr)> was measured with respect to a reversible reference H2 electrode (RHE) via a Luggin capillary in contact with the Pt-free side of the Nafion membrane. 

What can be learnt from XPS about electrochemical processes will be demonstrated and discussed in the main part of this chapter by means of specific examples. Thereby a survey of new XPS and UPS results on relevant electrode materials will be given. Those electrode materials, which have some potential for a technical application, are understood as practical and will be discussed with respect to the relevant electrochemical process. The choice of electrode materials discussed is of course limited. Emphasis will be put on those materials which are relevant for technical solid polymer electrolyte cells being developed in the author s laboratory. 

For Cl2 or 02 evolution the stability of ruthenium based electrodes is not sufficient on a technical scale. Therefore the possibility of stabilizing the ruthenium oxide without losing too much of its outstanding catalytic performance was investigated by many groups. For the Cl2 process, mixed oxides with valve metals like Ti or Ta were found to exhibit enhanced stability (see Section 3.1), while in the case of the 02 evolution process in solid polymer electrolyte cells for H2 production a mixed Ru/Ir oxide proved to be the best candidate [68, 80]. 

The solid polymer electrolyte cells are viewed as being particularly appropriate for the treatment of high purity water systems, including the provision of ultra pure water for the pharmaceutical industry, cf. Ref. [205], The process is often coupled with UV radiation which serves to decompose unwanted, residual ozone [133],... 

Platinum chemically deposited on a Nafion membrane was used as a platinum SPE (Solid Polymer Electrolyte) electrode. The electrochemical measurements were performed using the half cell shown in Fig. 2-2. The cell body is made from Teflon (PTFE). The cell is divided into two compartments one for backside gas supply one for the electrolyte. SPE electrodes are placed between them with the deposited side facing the gas compartment. A gold foil with a hole was placed behind the SPE electrode... 

Shukla, A.K., Jackson, C.L., Scott, K., Murgia, G. 2002. A solid-polymer electrolyte direct methanol fuel cell with a mixed reactant and air anode. J Power Sources 111 43-51. 

Ion-exchanger resins as solid polymer electrolytes, impregnated with the cations of the chosen anode metal, may prove applicable. Their use in the fuel-cell/electrolyzer single module concept is already under investigation as to complexity and operability (115). Doubtless better SPE s will be discovered. 

Electrochemical gas detection instruments have been developed which use a hydrated solid polymer electrolyte sensor cell to measure the concentration of specific gases, such as CO, in ambient air. These instruments are a spin-off of GE aerospace fuel cell technology. Since no liquid electrolyte is used, time-related problems associated with liquid electrolytes such as corrosion or containment are avoided. This paper describes the technical characteristics of the hydrated SPE cell as well as recent developments made to further improve the performance and extend the scope of applications. These recent advances include development of NO and NO2 sensor cells, and cells in which the air sample is transported by diffusion rather than a pump mechanism. 

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. 

Inaba et al. [29] have introduced a different cell to work with gaseous compounds (Fig. 4). A metal-plated solid polymer electrolyte (SPE) composite electrode faces the gas to be reduced. On the other side, the SPE is in contact with 0.1 M NaOH in which a Pt wire and an Ag/AgCl reference electrode are immersed. This system permits the electroreduction of insoluble reactants in water without employing organic solvents. For example, 2-chloro-l,l,l,2-tetrafluoroethane (HCFC 124) is transformed into 1,1,1,2-tetrafluoroethane (HFC 134a). The cathodic reaction can be written as follows ... 

Figure 4 Schematic diagram of the electrolytic cell with a solid polymer electrolyte composite electrode. SPE = Neosepta AM-1 CE = Pt wire RE = Ag/AgCl WEC = cathode compartment CEC = anode compartment. (From Ref. 29.)...

The description given here is a basic outline of the principles of the solid polymer electrolyte fuel cell used in the first Gemini space flights with nonfluorinated membranes (Fig. 13.23). Because the cell is slated for development as part of the electrochemical engine in cars, stages in its modern development are described in another section. 

Fig- 13.29. Distribution of particle sizes in 10 wt. % Pt-C electrocatalyst. Particle sizes are average diameters. (Reprinted from E. A. Ti-cianelli, M. N. Beery, and S. Srinivasan, Dependence of Performance of Solid Polymer Electrolyte Fuel Cells with Low Platinum Loading on Morphologic Characteristics of the Electrodes, J. Appl. Electrochem. 21 601 copyright 1991, Fig. 9. 

The key achievements at Ballard (Wilkinson 1998)15 are low Pt loading (1 mg cm-2) in 50-kW fuel cell power plants, and that of a power-to-weight ratio of 1 kW/kg. The development of the solid polymer electrolyte membrane was by no means in a final state in the late 1990s. The quality of the membrane controls the highest current density at which the cell is viable. There are open areas, too, with regard to the composition (as apart from the loading and particle size) of the catalyst a PtRu alloy... 

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