Polymer electrolytes operational temperature

The electrolyte is a poly(ethylene oxide) (PEO) film with a dissolved hthium salt LiTFSI. In order to obtain a sufficiently good ionic conductivity, we need to operate the battery at a temperature greater than the glass transition temperature of the polymer. The operating temperature required is 60-80°C, which limits the field of application to large batteries, which notably precludes the mobile market. Under no circumstances must the thermal degradation temperature of the PEO, which is around 120 C, be surpassed. 

Polymer Electrolyte Fuel Cell. The electrolyte in a PEFC is an ion-exchange (qv) membrane, a fluorinated sulfonic acid polymer, which is a proton conductor (see Membrane technology). The only Hquid present in this fuel cell is the product water thus corrosion problems are minimal. Water management in the membrane is critical for efficient performance. The fuel cell must operate under conditions where the by-product water does not evaporate faster than it is produced because the membrane must be hydrated to maintain acceptable proton conductivity. Because of the limitation on the operating temperature, usually less than 120°C, H2-rich gas having Htde or no (

Solid Polymer E,kctroljte. The electrolyte in soHd polymer electrolyte (SPE) units is Nafion, a soHd polymer developed by Du Pont, which has sulfonic acid groups attached to the polymer backbone. Electrodes are deposited on each side of the polymer sheet. H" ions produced at the anode move across the polymer to the cathode, and produce hydrogen. The OH ions at the anode produce oxygen. These units have relatively low internal resistances and can operate at higher temperatures than conventional alkaline electrolysis units. SPE units are now offered commercially. 

As can be seen from Eigure 11b, the output voltage of a fuel cell decreases as the electrical load is increased. The theoretical polarization voltage of 1.23 V/cell (at no load) is not actually realized owing to various losses. Typically, soHd polymer electrolyte fuel cells operate at 0.75 V/cell under peak load conditions or at about a 60% efficiency. The efficiency of a fuel cell is a function of such variables as catalyst material, operating temperature, reactant pressure, and current density. At low current densities efficiencies as high as 75% are achievable. 

Polymer electrolyte fuel cells (PEFCs) have attracted great interest as a primary power source for electric vehicles or residential co-generation systems. However, both the anode and cathode of PEFCs usually require platinum or its alloys as the catalyst, which have high activity at low operating temperatures (<100 °C). For large-scale commercialization, it is very important to reduce the amount of Pt used in fuel cells for reasons of cost and limited supply. 

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). 

Both temperature and pressure have a significant influence on cell performance the impact of these parameters will be described later. Present cells operate at 80°C, nominally, 0.285 MPa (30 psig) (5), and a range of 0.10 to 1.0 MPa (10 to 100 psig). Using appropriate current collectors and supporting structure, polymer electrolyte fuel cells and electrolysis cells should be capable of operating at pressures up to 3000 psi and differential pressures up to 500 psi (4). 

Improvements in solid polymer electrolyte materials have extended the operating temperatures of direct methanol PEFCs from 60 C to almost 100 C. Electrocatalyst developments have focused on materials that have higher intrinsic activity. Researchers at the University of Newcastle upon Tyne have reported over 200 mA/cm at 0.3 V at 80 C with platinum/ruthenium electrodes having platinum loading of 3.0 mg/cm. The Jet Propulsion Laboratory in the U.S. has reported over 100 mA/cm at 0.4 V at 60 C with platinum loading of 0.5 mg/cm. Recent work at Johnson Matthey has clearly shown that platinum/ruthenium materials possess substantially higher intrinsic activity than platinum alone (45). 

These three approaches to reject heat and exhaust fuel recovery with power generation apply primarily to the higher temperature, solid oxide (1800 F) and molten carbonate (1200 F), fuel cell systems operating on CH4 fuel. The lower operating temperatures of the phosphoric acid (400 F) and polymer electrolyte (175 F) fuel cells severely limit the effectiveness of thermal cycle based power generation as a practical means of heat recovery. 

In addition to Nafion-based catalyst layers, additional types have been developed, including CLs with different ion exchange capacities (lECs) [57,58] or with other hydrocarbon-type ionomers such as sulfonated poly(ether ether ketone) [58-60], sulfonated polysulfone [61,62], sulfonated polyether ionomers [63], and borosiloxane electrolytes [64], as well as sulfonated polyimide [65]. These nonfluorinated polymer materials have been targeted to reduce cost and/or increase operating temperature. Unfortunately, such CLs still encounter problems with low Pt utilization, flooding, and inferior performance compared wifh convenfional Nafion-based CLs. 

The polymer electrolyte fuel cell (PEFC) or proton exchange membrane fuel cell—also known as the polymer electrolyte membrane fuel cell (PEMFC)—is a lower temperature fuel cell (typically less than 100°C) with a special polymer electrolyte membrane. This lower temperature fuel cell is well suited for transportation, portable, and micro fuel cell applications because of the importance of fast start-up and dynamic operation. The PEMFC has applicability in most market and application areas. 

For higher temperature operation, a polybenzimid-azole-based polymer electrolyte may be preferred. The PEMFC structures have good mechanical integrity under compression and expansion from differential temperature and pressure gradients that occur during operation. This system has minimal materials problems, except for the cost and operation characteristics of the membrane. The PEMFC operates at 1 A/cm at 0.7 V. The electrode reactions in acidic media have been discussed above. 

Redox shuttles based on aromatic species were also tested. Halpert et al. reported the use of tetracyano-ethylene and tetramethylphenylenediamine as shuttle additives to prevent overcharge in TiS2-based lithium cells and stated that the concept of these built-in overcharge prevention mechanisms was feasible. Richardson and Ross investigated a series of substituted aromatic or heterocyclic compounds as redox shuttle additives (Table 11) for polymer electrolytes that operated on a Li2Mn40g cathode at elevated temperatures (85 The redox potentials of these... 

A polymer electrolyte with acceptable conductivity, mechanical properties and electrochemical stability has yet to be developed and commercialized on a large scale. The main issues which are still to be resolved for a completely successful operation of these materials are the reactivity of their interface with the lithium metal electrode and the decay of their conductivity at temperatures below 70 °C. Croce et al. found an effective approach for reaching both of these goals by dispersing low particle size ceramic powders in the polymer electrolyte bulk. They claimed that this new nanocomposite polymer electrolytes had a very stable lithium electrode interface and an enhanced ionic conductivity at low temperature. combined with good mechanical properties. Fan et al. has also developed a new type of composite electrolyte by dispersing fumed silica into low to moderate molecular weight PEO. 

The initial drop in the polarization curve is due to the sluggish kinetics of the ORR at the temperatures normally used for current polymer-electrolyte fuelcell operation (<100 °C). A typical electrochemical... 

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