Electrolytes in fuel cells

Ionic conductors, used in electrochemical cells and batteries (Chapter 6), have high point defect populations. Slabs of solid ceramic electrolytes in fuel cells, for instance, often operate under conditions in which one side of the electrolyte is held in oxidizing conditions and the other side in reducing conditions. A signihcant change in the point defect population over the ceramic can be anticipated in these conditions, which may cause the electrolyte to bow or fracture. 

The transport number is an important characteristic of a solid. Materials for use as electrolytes in batteries need cation, especially Li+ or Na+, transport numbers to be close to 1, while the electrolytes in fuel cells need O2- or H+ transport numbers close to 1 at the operating temperature of the cell. 

Proton, that is, H+ ion, conductors are of importance as potential electrolytes in fuel cells. There are a number of hydroxides, zeolites, and other hydrated materials that conduct hydrogen ions, but these are not usually stable at moderate temperatures, when water or hydroxyl tends to be lost, and so have only limited applicability. 

Biichi, F. N., Gupta, B., Haas, O. and Scherer, G. G. 1995. Study of radiation-grafted FEP-g-polystyrene membranes as polymer electrolytes in fuel cells. Electrochimica Acta 40 345-353. 

The use of multilayered films of zirconia-based solid electrolytes in fuel cells is more promising than the use of one-layer films. 

The key was the insertion of a zirconia buffer layer between the silicon substrate and the superconducting film on top of it. (Zirconia is a white crystalline compound used as an insulator in enamels and as an electrolyte in fuel cells.) The buffer, deposited with superconducting film onto the silicon by electron-beam evaporation, served as an effective barrier, preventing the elements from intermingling during the annealing process. 

A key factor in the possible application of oxygen ion conducting ceramics is that, for use as solid electrolyte in fuel cells, batteries, oxygen pumps or sensors, their electronic transport number should be as low as possible. Given that the mobilities of electronic defects typically are a factor of 1000 larger than those of ionic defects, a band gap of at least 3 eV is required to minimize electronic contributions arising from the intrinsic generation of electrons and holes. 

The perfluorosulfonic acid (Nafion) membrane found its application in fuel cells long before its introduction to the chlor-alkali industry (26-28). The Nafion membrane is used as the solid polymer electrolyte (separator/electrolyte) in fuel cells. Figure 2 shows the schematic of such an SPE fuel cell. 

The role of the electrolyte in fuel cells is to prevent the molecular forms of the fuel and oxidant from mixing and to provide a means for ionic transport. It must also ensure that electrons pass from the fuel to the oxidizing electrode only via the external current. An ideal fuel cell electrolyte is one which is permeable to only one ionic species. 

Actual electrolytes already contain mobile ions such as molten salts or salt solutions (molten table salt or a solution of table salt, for example), but in some cases also solids (solid electrolytes in fuel cells). The substances referred to are already composed of ions in the solid state. Almost all salts are like this one example is... 

We have confined our goal to reviewing the state-of-the-art in the development of radiation grafted proton-exchange membranes. This review provides an up-to-date summary of the synthesis, properties, and appHcations of radiation grafted membranes as solid polymer electrolytes in fuel cells. 

A difficulty of principle arises when using a solid electrolyte in direct carbon fuel cells. In fact, in fuel cells with a liquid electrolyte (solution or melt) the entire surface area of the carbon material is in contact with the electrolyte (is wetted by the electrolyte). In fuel cells with a solid electrolyte, to the contrary, the contact between the solid carbon material and the solid electrolyte is a mere point contact, and the working surface area is much smaller. 

Aoki M, Uchida H, Watanabe M (2005) Novel evaluation method for degradation rate of polymer electrolytes in fuel cells. Electrochem Commun 7 1434-1438... 

Molten salts are not only useful as solvents in chemical synthesis, electrolysis, soldering, enameling, de-enameling, metal recycling and preparation, coal gasification, and desulfuration, but they are also reactants, catalysts, and ambients for heat storage and heat transfer, as well as electrolytes in fuel cells (molten carbonates). The solvent can participate in the reaction that is carried out in fluxes. BaTiOa is made in molten TiO, as a solvent and Bio(AlClJ, can only be made in a very acidic cryolite (NaAlClJ. 

F.N. Buchi, B. Gupta, O. Haas, G.G. Scherer, Study of radiation-grafted FEP-G-polysty-rene membranes as polymer electrolytes in fuel cells, Electrochim. Acta 40 (3) (1995) 345-353. 

PILs are potentially very nsefiil as ionic-condncting electrolytes in electrochemical systems and as proton-condncting electrolytes in polymer membrane fuel cells (PBMFCs). While the properties and use of BAN as a hquid electrolyte have been investigated over a number of years, it was only recently that the potential application of PILs as electrolytes in fuel cells was identified. The nse of PILs as electrolytes are described below, and their use in PBMFCs is described in the following section. 

A large number of stoichiometric imidazolium, alkylammonium, and other heterocyclic amine based PILs were developed and characterized, with particular emphasis on their ionic conductivity. " Some of these PILs were identified as having potential uses as electrolytes in fuel cells, with preliminary fuel cell tests conducted. ... 

Reiss, I., The possible use of mixed ionic electronic conductors instead of electrolytes in fuel cells. Solid State Ionics, 52, 127-134 (1992). 

The concept of the usage of an ion exchange membrane as electrolyte in fuel cells dates back to the 1940s [1,2]. Firstly, the development of ion... 

The utility of AEMs as potential electrolytes in fuel cells arises not only from the prospects for the use of non-Pt-group metal (non-PGM) catalysts and cheaper fuel ceU components (less corrosive enviromnent) but also from potential for use of alternative fuels. 

It is extremely important that the ionic transference number is high enough to be used as a solid electrolyte in fuel cells. The Ce" " cation has a tendency to be reduced easily, so that electronic conduction caused by the reductimi of Ce" irais becomes a problem in the case of the Ce02-containing electrolyte. BaCeOs-based oxides contain Ce ions, exhibiting certain electronic conductivity by reductimi [157]. Transport parameters including activation enthalpies of hole and proton conductimi have been reported [157, 159]. 

The aromatic polyethers bearing pyridine moieties described in the previous sections have been effectively used as polymer electrolytes in fuel cells. Because of the use of several different... 

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