Steffen Møller-Holst

Transport and equilibrium properties of Nafion® membranes with H+ and Na+ ions

Abstract Transference coefficients, ionic conductivities and equilibrium properties are reported for Nafion® 117 membranes containing a mixture of two cations and water. The membranes were equilibrated in 11 kinds of aqueous solutions of HCl and NaCl, with mole fractions of HCl between 0 and 1, all having cCl−=0.03 kmol m−3. Some experiments were repeated for Nafion® 112 and 115 membranes. By electron probe microanalysis (EPMA), it was found that Nafion® 117 membranes have a slightly higher affinity for Na+ than for H+. The water content of the membrane was determined gravimetrically. It decreased from 20.6±0.1 to 18.4±0.2 molecules water per cationic site, as the membrane changed from the pure H-form (HM) to the pure Na-form (NaM). The water transference coefficient, tH2O, obtained by streaming potential measurements, increased in Nafion® 117 from 2.5±0.1 to 9.2±0.1 as the mole fraction of Na+ in the membrane, xNaM, increased from 0 to 1. Most of the increase occurred for xNaM>0.5. The transference number of H+ in the membrane, tH+(m), which was determined by an improved emf-method, showed a rapid decrease for xHM

Exergy effeciency and local heat production in solid oxide fuel cells

Fuel cells are expected to play an important role in future production of electrical energy. The solid oxide fuel cell (SOFC) converts hydrogen and oxygen to water around 1000°C. Hydrogen may be produced outside or inside the cell before conversion. The success of the SOFC relative to other fuel cells or other power generating systems may depend on exploit of the heat production of the cell[l]. The heat production may be used for the reforming reaction which produces hydrogen[2] from natural gas, and for cogeneration of electricity[l]. Takehara and coworkers show that the heat production differs between electrodes in the SOFC unit cell[3]. They also discuss how the temperature distribution in the cell may vary in a tubular cell construction[4]. Rosen[l] concludes that exergy analyses should be used to evaluate efficiencies of power generating systems. We shall give the results of an exergy analysis of the SOFC, using a new method, the electric work methodC5, 63. Our conclusion agrees on a general basis with that of Rosen[l], but more details of the process are given. The characteristics of the method are detailed descriptions of causes and location of local heat and energy changes per unit time. The analysis extends the work of Takehara and coworkers[3, 41 on this point, because the electric work method is more accurate than previously published methods. The analysis reveals the main factors for further improvement of the cell efficiency. The effect of the reformer reaction on the exergy efficiency will be quantified. Electrochemical conversion of methane will be compared with the process where hydrogen is * Author to whom correspondence should be addressed. externally produced from methane and further converted in the cell. A detailed discussion of the energy gain by cogeneration must, however, be postponed. We shall limit ourselves to the calculation of a unit cell. Unit cell descriptions are required for computer modelling of the system[7]. Both reversible and irreversible heat effects are taken into account, but losses due to diffusion will be neglected.