E. Skou

An improved method for the determination of the electrochemical active area of porous composite platinum electrodes

In this project, an improved method for determining the electrochemical active area (ECA) of a platinum electrocatalyst has been developed. By the use of cyclic voltammetry, the ECA of a platinum electrocatalyst is measured by hydrogen adsorption. In the procedure, the charge used to form one monolayer of hydrogen is measured. The determination is influenced by the hydrogen evolution reaction and an accurate procedure is needed. Therefore, the hydrogen adsorption is modelled by three Frumkin-like adsorption isotherms. By performing a simulation of the measured cyclic voltammetry curves in the hydrogen adsorption potential region (hydride region), it has been possible to obtain a very accurate measure for the electrochemical active area. In this way, the current originating from the hydrogen evolution reaction can be sorted out. The procedure is used to characterise two different oxygen reduction electrodes both prepared from the same Pt/C catalyst with Nafion® as binder.

Water and methanol uptake in proton conducting Nafion® membranes

Methanol uptake from methanol–water mixtures has been measured at ambient temperature in Nafion®117 membranes initially saturated with water. After equilibration for 18 h the membranes were dried at 70°C in vacuum with a cold trap cooled with liquid nitrogen and the total amount of absorbent was determined from the weight loss. The methanol–water ratio was determined by 1H NMR. A plot of the mole fraction of methanol in the membrane vs. the mole fraction in the water–methanol mixture was linear in the whole range from pure water to pure methanol. The slope indicated that Nafion® does not preferentially take up either water or methanol. The number of solvent molecules was found to be 23 per sulphonic group for both methanol and water.

ac and dc conductivity studies of pelletised 12-tungstophosphoric acid and 12-tungstosilicic acid hexahydrates

Abstract Ionic conduction in pelletised 12-tungstophosphoric acid and 12-tungstosilicic acid hexahydrates has been examined using ac and dc techniques. The studies definitively assign a protonic conduction mechanism, with evidence for conduction via crystallite surfaces. With blocking electrodes, σ (298 K) values are ≈ 10 −4 S cm −1 with Arrhenius-type temperature dependences. With reversible electrodes, conductivities increase with increasing relative humidity of flow gas. dc conductivities are slightly lower than ac values because of electrode polarisation.

Tin-mordenites, syntheses and ionic conductivity

Abstract Tin-mordenites were prepared by heating H-mordenite mixed with tin (II) chloride dihydrate in an oxygen atmosphere. Tin was ion exchanged into the mordenites with up to 3.6 molecules per unit cell. Excess tin (II) chloride was oxidized to tin (IV) oxide by oxygen. The ac and dc conductivities were measured in atmospheres with different water partial pressures. The conductivity measurements showed that the tin exchanged mordenites were proton conductors, that the conductivity was very dependent of the water content, but that the dehydration was reversible. The conductivity increased with increasing tin content until full ion exchange whereafter it remained virtually constant. Tin (IV) oxide in the preparates gave rise to additional grain boundary conduction. The highest conductivity measured was σ=0.6×10-1(ωcm)-1 at 120°C in a water saturated atmosphere.

SOFC cathode kinetics investigated by the use of cone shaped electrodes: The effect of polarization and mechanical load

Abstract The SOFC cathode reaction is investigated on cone shaped electrodes of La1 − x SrxMnO3 + δ; pointing at yttria stabilized zirconia (YSZ) electrolyte pellets at 1000 °C in air. The methods used was impedance spectroscopy superimposed on potential step relaxation curves. Additionally, the mechanical load on the pointed electrodes was varied from 0 to 100 g. The contact areas of electrodes and electrolyte were examined by optical spectroscopy and SEM before and after measurements. The tip of the electrodes consisted of a flat polycrystalline area acting as a multipoint electrode. The experiments showed that area normalization can be used for this kind of electrodes. The experiments also showed that the pretreatment of the electrodes is extremely important in order to obtain reproducible results. The electrodes showed activation within short timescales at both anodic and cathodic polarizations. On a longer timescale, activation was only observed at cathodic polarizations. The electrodes were reversible in the sense that the original catalytical activity was recovered when the polarization was removed. The current time behavior could be represented by two consecutive exponential relations with time constants in the order of 1000 and 10 000 s. When mechanical loads were applied to the unpolarized electrode, the active area increased and the reaction resistance decreased almost linearly with load up to a load of ca. 100 g. When a similar experiment was performed after a cathodic polarization of 83 mV and attainment of steady state, the initial active area was higher and no effect of the load was observed at small loads. At higher loads the area started to increase indicating that at least part of the activation process is the formation of a reaction zone on the YSZ surface. Examination of the impedance diagrams showed the reaction resistance to contain contributions from a charge transfer process and a mass transport process. Upon increasing the load, the charge transfer resistance followed the electrolyte resistance, whereas the mass transport resistance showed a relative increase at higher loads (larger contact areas) indicating that surface transport is an important element in the reduction of oxygen on this type of materials. Also, this behaviour was reversible in the sense that the electrode returned to its original state after removal of polarization and load, and the experiment could be repeated.

Transference number measurements on a hybrid polymer electrolyte

Abstract Two methods were used to measure the transference number of a hybrid polymer electrolyte at different salt concentrations. The hybrid polymer electrolyte was based on a radiation induced cross-linked polyether network with propylene carbonate in an interpenetrating liquid phase and LiAsF 6 as electrolyte salt. One method for transference number measurements was based on the Hittorf method, which gave values ranging from 0.15 at 0.1 mole kg −1 to 0.31 at 2.1 mole kg −1 . The other method was the DC-polarization method, which gave values from 0.31 at 0.1 mole kg −1 to 0.72 at 2.1 mole kg −1 . The difference in the transference numbers between the two methods is explained by contribution from mobile ion-pairs.

Proton conduction in H2Ti4O9, 1.2 H2O

Abstract H 2 Ti 4 O 9 , x H 2 O was prepared from K 2 Ti 4 O 9 by hydrolysis with 0.4 M nitric acid as described by Marchand and co-workers. The K 2 Ti 4 O 9 was prepared by a conventional solid state reaction, and by a sol/gel procedure. The ac conductivity was measured conventionally, and also under a load of 0.5 V. The temperature range was from 20–100°C. The measuring cell was purged with gas (N 2 or H 2 ) saturated with water vapor at the measuring temperature. Carbon electrodes were used. The dc conductivity was measured on tablets with platinum black electrodes in an atmosphere of hydrogen saturated with water vapour at 75% RH. The maximum conductivity was 1.8 × 10 −3 (Ω cm) −1 .

α-zirconium hydrogenphosphate, monohydrate. preparation, chemical properties and ac conductivity

Abstract The preparation of zirconium hydrogenphosphate, monohydrate (αZrP) was performed by two methods giving materials with slightly different properties. It is shown that the particle size of αZrP depend upon the method of preparation and the washing procedures, and that the ac conductivity decreases with increasing particle size (5 x 10 -6 to 5 x 10 -5 Ω -1 cm -1 at 20°C). The activation energy has been determined to 0.25 eV/mol (between 20°C and about 100°C) independent of the particle size, showing αZrP to be a surface ion conductor.

Study of preparation and ac/dc conductivity of tin-zeolite-Y materials

Abstract Tin-zeolite-Y materials were prepared by heating zeolite H-Y mixed with tin (II) chloride dihydrate. Excess of tin (II) was oxidized to tin (IV) oxide by oxygen. The ac and dc conductivities were measured in atmospheres of constant relative humidity in the temperature range 25°C to 116°C. The materials are protonic conductors. The highest measured conductivity was 0.9×10 -3 (Ω cm) -1 .

Ac and dc conductivity of crystalline pyrochlore antimonic acid, Sb2O5·nH2O

Abstract Ionic conduction in pelletised pyrochlore-form antimonic acid (Sb 2 O 5 · n H 2 O, prepared on acid digestion of KSb(OH) 6 ) has been examined using ac and dc techniques. ac studies employing blocking electrodes give σ (298 K)=4×10 -4 ω -1 cm -1 and an Arrhenius temperature dependence (283 T /K E a =30kJ mol -1 . ac and dc studies employing reversible electrodes (Pt-black, flowing H 2 (g)) (i) definitively assign the conductivity to protonic conduction, (ii) show conductivity to increase with increasing relative humidity of the flow gas and (iii) indicated conduction via both bulk and grain boundary pathways. dc conductivities are lower (by ca. an order of magnitude) than ac values, this being due to grain boundary conduction.