Arnold Lundén

Proton conductivity in fuel cells with solid sulphate electrolytes

Abstract Fuel cell and concentration cell experiments have been carried out with various solid sulphates as the solid electrolyte. Platinum, nickel sponge and some perovskites are found to be good alternatives as electrode materials. It is evident that proton conductivity is dominating in the studied cells, while it still is an open question whether there also can be a certain contribution from oxygen ion conductivity. Although the electrodes are blocking for Li + , Na + , etc., there are correlations between the fuel cell currents and the electrical conductivity of the salt. The observed phenomena are not limited to sulphates but occur also for other salts, such as phosphates.

Sulphate-based solid electrolytes: properties and applications

Sulphate-based solid electrolytes can have conductivities as high as 3 (Ω cm)−1, as is the case in pure Li2SO4 at a temperature of 800°C. For binary systems highly conducting phases can be found down to at least 415°C. If additional components are added conductivities exceeding 10−3 (Ω cm)−1 can be obtained at room temperature. The conductivity is due to cation mobility and, in contrast to other solid electrolytes such as beta-alumina and AgI-based double salts, both mono- and divalent cations are mobile. Phase diagrams for the systems containing Li2SO4 combined with Na2SO4, K2SO4, Rb2SO4, Cs2SO4, MgSO4, CaSO4, and ZnSO4 have been constructed. A number of galvanic cells using anodes made from Mg, Ca, and Zn have been tested at elevated temperatures. A Mg|Li1.72Mg0.14SO4|MnO2 cell gave an open-circuit voltage of 2.3 V and a power density of 400 W/kg at 745°C. Ag|electrolyte|I2 cells have been tested at room temperature.

Paddle-wheel versus percolation mechanism for cation transport in some sulphate phases

Abstract Lithium sulphate and a few other compounds have high temperature phases which are both solid electrolytes and plastic crystals (rotor phases). Three types of experiments are here considered in order to test the validity of a “paddle-wheel mechanism” that has been proposed for cation conductivity in these phases. A single-crystal neutron diffraction study has been performed for cubic lithium sulphate. The refinement of the data gives a very complex model for the location of the lithium ions. There is definitely a void at and near the octahedral (1/2, 1/2, 1/2) position. 90% of the lithium ions are located at the tetrahedral 8c-sites (1/4, 1/4, 1/4), although significantly distored in the directions of the four neighbouring sulphate ions. The remaining 10% of the lithium ions are refined as an evenly distributed spherical shell which is sorrouding the sulphate ions. The lithium ions are transported along a slightly curved pathway of continous lithium occupation to a corressponding to a distance of about 3.7 A. Thus, lithium transport occurs in one of the six directions [110], [1 1 0], [101] etc. The electrical conductivity has been studied for solid solutions of lithium tungstate in cubic lithium sulphate. The conductivity is reduced in the one-phase region, while it is increased in a two phase (solid-melt) region. There are pronounced differenes between the rotor phases and other phases concerning how partial cation substitution affects the electrical conductivity of solids solutions. Regarding self and interdiffusion, all studied mono- and divalent cation are very mobile in the rotor phases, which lack the pronounced correlation with ionic radii that is characteristic for diffusion in other of solid classes of solid electrolytes. The quoted studies are to be considered as strong evidence against a percolation model proposed by Secco.

Enhancement of cation mobility in some sulphate phases due to a paddle-wheel mechanism

The four high-temperature phases fcc Li 2 SO 4 , bcc LiNaSO 4 , bcc LiAgSO 4 and non-cubic Li 4 Zn(SO 4 ) 3 are characterized by a rotational motion of the translationally static sulphate ions which enhances the cation mobility. This model is supported by evidence from X-ray, neutron and light scattering, extremely high latent heats for the solid-solid phase transition, large diffusion coefficients for both mono- and divalent cations, similar effects on the electrical conductivity of doping with mono- or divalent cations, rheological properties, molecular dynamics simulations, etc. Some recent conductivity studies have been suggested as counter-evidence to the paddle-wheel mechanism, but their accuracy is seriously affected by the choice of a questionable experimental technique.

On the ionic conductivity and phase transitions in the Li2SO4Li2WO4 system and their relation to ion transport mechanism

Abstract Secco et al. have performed several measurements of ionic conductivity, which they have considered as “convincing evidence” that the “paddle-wheel” mechanism does not contribute significantly to ion conductivity in Li 2 SO 4 -based compositions. However, a comparison of their results in the high-conductivity range with those of other investigators suggests that their data are artifacts. The cause of this is that the resistance of their sulfate-rich samples is about 0.1 ohm at high temperatures. Thus, their results are reliable only for “normal,” i.e., low, conductivities. It is briefly summarized why the “paddle-wheel” mechanism for ion transport is superior to a percolation-type mechanism for a few high-conducting phases.

Paddle-wheel versus percolation model, revisited

Abstract Secco has proposed a percolation model for the large cation mobility in some inorganic rotor phases. He originally based this model on a couple of measurements where it was overlooked that the recorded impedances actually were some 10 to 100 times larger than the actual resistances of the thin and wide pellet samples (thickness 1–2 mm, cross-section about 1 cm 2 ). This explains why the results reported by Secco et al. deviate from those of all other investigations of the conductivity of these high-temperature rotor phases. Several other properties of these phases are in conflict with Seccos percolation model, but in accordance with the so-called paddle-wheel mechanism. The activation energies of cation and anion diffusion will be taken as an example. In addition, some comments will be made on the fact that Secco makes no distinction between pure stoichiometric compounds, solid solutions and phase mixtures in his argumentation for his “percolation type” of transport mechanism. What Secco calls “new positive mixed alkali and mixed anion effects” have previously been considered by a number of authors as composite electrolytes. Furthermore, the conductivity is strongly enhanced in some isovalent solid solutions due to the presence of a large number of cation vacancies, e.g. in hexagonal Na 2 SO 4 (Li).

Pressure dependence of the transition to the proton conducting phase of CsHSO4, CsHSeO4 and RbHSeO4 studied by differential scanning calorimetry

Abstract The temperature and enthalpy of the transition to the proton conducting α-phase have been determined for CsHSO 4 , CsHSeO 4 and RbHSeO 4 in the pressure range up to 1 GPa by means of differential scanning calorimetry (DSC). At normal pressure the transition occurs at 415, 400 and 447 K, respectively, the transition enthalpy is 5.53, 5.74 and 10.5 kJ/mole, respectively, and the volume increase is of the order of 0.3% for CsHSO 4 , while there is instead a volume decrease of the order of 1.3% for RbHSO 4 . For CsHSO 4 the transition temperature has a flat maximum near 0.6 GPa and 422 K. For CsHSeO 4 the transition temperature is close to 400 K in the pressure range below about 0.56 GPa, while d T /d P is about 55 K/GPa at higher pressures. A new high-pressure pressure phase exists only in a very limited temperature and pressure range. For RbHSeO 4 the phase diagram has either a triple point or a minimum in the vicinity of 430 K and 0.45 GPa. The volume increase is of the order of 2% for the transformation to the α-phase at 0.55 GPa.

The structure of the solid electrolyte LiAgSO4 at 803 K and of LiNaSO4 at 848 K

Abstract The structures of the high-temperature phases of LiAgSO 4 and LiNaSO 4 have been determined from neutron and X-ray powder diffraction data. In both cases the sulphate ions form a bcc lattice where the lithium ions are found to occucy the ( 1 4 , 0, 1 2 ) tetrahedral positions, while the other cations are in the (0, 1 2 , 1 2 ) octahedral positions. The temperature factors are found to be exceptionally high. While the structural models are derived solely from the diffraction lines, additional support of them is obtained from the diffuse scattering. Comparisons are made with fcc Li 2 SO 4 concerning structure, electrical conductivity, cation diffusion and anion reorientation.

Bulk phase transitions of cesium hydrogen sulphate initiated by surface processes, grinding or external pressure

Abstract The conditions have been studied for the existence at room temperature of three phases of CsHSO 4 , here called the β-, γ-phases. Crystallization of an aqueous solution produces the δ-phase, while the metastable β-phase can be obtained after annealing at an elevated temperature. Both these phases can be transferred by a mechanical treatment, grinding, to the γ-phase, which has the lowest energy of them all. Transformation to the δ-phase is instead favoured by applying either hydrostatic or uniaxial pressure. Transitions between all three phases can be controlled by modifying the water vapour pressure above the salt, i.e. a surface process triggers phase transitions in the bulk.

Solid sulphate electrolytes; the first examples of a strange ION transport mechanism

Abstract Neutron and X-ray diffraction studies reveal that fcc LJ2SO4, bcc LiNaSO4, and bcc LiAgSO4 are characterized by a strong rotational disorder of the sulphate ions which strongly enhances the mobility of the cations. Single crystal neutron scattering studies have been performed on fee Li2SO4 and the quasielastic scattering supports the previous conclusions, and more detailed information should be obtainable by this technique. The elastic constants of fee Li2SO4 have been determined by Brillouin scattering. The information obtained so far by Raman scattering concerning the motion of lithium ions in fcc Li2SO4 supports earlier conclusions from conductivity measurements.