I.C. Vinke

Bulk and electrochemical properties of BiVO4

The mixed (oxygen ionic-electronic) conductor bismuth vanadate (BiVO4) was studied with respect to its electrochemical properties. The ionic transference numbers, measured by the concentration cell method in the temperature range of 800 to 1000 K, vary from 0.7 to 0.3. The total conductivity of this ceramic material isone order of magnitude lower than found for cubic yttria-stabilized zirconia (YSZ). The activation enthalpy for the electronic conductivity is high (193 kJ/mol) compared to the ionic conductivity (71 kJ/mol). The PO2 dependence of the conductivity data in combination with the Seebeck measurements showed electrons to be the majority charge carriers, indicating that BiVO4 is an n-type mixed conductor.

The oxygen transfer process on solid oxide/noble metal electrodes, studied with impedance spectroscopy, dc polarization and isotope exchange

The electrochemical oxygen transfer process at the yttria stabilized zirconia (YSZ) and Fe-implanted YSZ, and at the erbia stabilized bismuth oxide (BE25) surface is studied with dc polarization and impedance spectroscopy using gold electrodes, and with 18O gas phase exchange. The surface modification by Fe-implantation increases the exchange current density up to a factor of 50, but analysis of the impedance spectra at different polarization levels indicates that the type of electrode reaction is not changed by the implantation. Inductive effects at cathodic polarizations are interpreted with a stepwise transfer of electrons. Isotope exchange experiments show an increase in adsorption/reaction sites at the surface after implantation. The high exchange current density, I0, for BE25 is independent of type of electrode but does depend on electrode morphology. I0 can be equated to the surface oxygen exchange rate, indicating that the entire electrolyte surface is active in the electrode exchange process. Qualitative interpretation of the impedance spectra measured at different levels of polarization results in a model where adsorbed oxygen species diffuse over the oxide surface, while charge transfer occurs across the surface.

Electrochemical properties of stabilized δ-Bi2O3. Oxygen pump properties of Bi2O3-Er2O3 solid solutions

The oxygen pumping characteristics of the solid electrolyte material (Bi0.75Er0.25)2O3with gold and platinum electrocodes have been studied by three-electrode I–V measurements. These experiments show only little influence of the electrode material used on the electrode behavior. Exchange current density values are high and are confirmed by 18O2 exchange data on the bare electrolyte. This leads to the conclusion that (Bi0.75Er0.25)2O3 has good properties for oxygen pumping applications and is an electrochemically active material in oxygen transfer processes.

Three-electrode current-voltage measurements on erbia-stabilized bismuth oxide with sputtered noble metal electrodes

The anodic and cathodic polarization behaviour of sputtered porous gold electrodes on (Bi2O3)0.75(Er2O3)0.25 (abbreviated BE25) was studied as function of temperature and oxygen partial pressure using a three-electrode cell. The anodic polarization is smaller than the cathodic polarization, allowing current densities of 2 × 103 A m−2 at 0.1 V and 1041 K in oxygen for oxygen evolution. Comparison with the polarization behaviour of similar sputtered platinum electrodes on BE25 shows little effect of the electrode material on the exchange current densities. This indicates that the electrolyte surface is active in the oxygen transfer process, while the noble metal electrode serves merely as current collector. Analysis of the electrode impedance shows strong influence of surface diffusion on the electrode reaction(s). For the exchange current density an unusual Po2 dependence is observed.

Surface oxygen exchange properties of bismuth oxide-based solid electrolytes and electrode materials

The surface oxygen exchange coefficient, ks, has been measured for the solid solution (Bi2O3)0.75(Er2O3)0.25 and (Bi2O3)0.6(Tb2O3)0.4 (abbreviated BE25 and BT40), using gas-phase 18O exchange techniques. The activation enth alpy of ks amounts to ΔE=110 kJ/molforBT40 andΔE=130 kJ/molforBE25. The magnitude of ks for the purely ionic conducting BE25 is comparable with values obtained from electrode polarization (I−V) measurements (ΔE=140 kJ/mol.) The comparatively high ks values show a (PO2)n dependence on oxygen pressure with values of n close to 0.5, indicating surface control in the oxygen transport process. Bismuth oxide containing solid solutions show a large activity in the oxygen exchange reaction with the gas phase.

Mixed conductivity in terbia-stabilized bismuth oxide

The mixed conducting solid solution 0.75Bi2O3−0.25Tb4O7 (BT40) was studied by impedance techniques using ionically blocking electrodes. These measurements confirmed the p-type electronic conductivity suggested in literature. In air at temperatures between 600 and 900 K the ionic transference number ranges from 0.28 to 0.70. The total conductivity at 833 K in air is 0.93 Ω−1 m−1. The activation enthalpy for the electronic conductivity varies from 77 kJ/mol in pure O2 to 83 kJ/mol in an ambient with PO2=2×10−4 atm. The electronic conductivity seems to be a function of the sample geometry and increases with decreasing cross section to height ratio. The activation enthalpy for the ionic conductivity being 110 kJ/mol, is independent of the PO2 and does not depend on the sample geometry.

Three-electrode current-voltage measurements on erbia stabilized bismuth oxide with co-pressed gold gauze electrodes

The polarization behaviour of (Bi2O3)0.75(Er2O3)0.25(BE25) with a co-pressed gold gauze electrode was studied as a function of temperature and oxygen partial pressure in a three electrode cell. The anodic polarization is smaller than the cathodic polarization. The cathodic charge transfer coefficient, αc, is about 0.5 while the anodic one, αa, is about 1.5. The exchange current density shows a (PO2) dependence for partial pressures below 1 atm with an activation enthalpy of ˜126 kJ mol−1. These values compare well with results obtained from 18O exchange experiments. Current densities for the co-pressed gold gauze electrodes are about a factor of 5 to 10 larger than found for the previously reported porous sputtered gold electrodes. Analysis of the electrode impedance shows strong influence of surface diffusion on the electrode reaction, which must take place at the surface of the electrolyte.

Surface Oxygen Exchange Kinetics of Solid Oxide Ion Conductors

For oxides with high oxygen ion mobility the exchange of lattice oxygen at the surface with ambient molecular oxygen is modelled with a simple global two step model. The first global step describes the dissociative adsorption reaction chain, while the second step describes the exchange of adsorbed mono-atomic oxygen species with the lattice oxygen. By monitoring the evolution of the ratio’s of 16O2, 16O18O and 18O2 in the gas phase during exchange, the exchange rates for the dissociativeadsorption, r dis , and the lattice exchange, r b , can be obtained. The measured √PO2 dependence of r dis indicates possibly the formation of adsorbed O- species. Using this two step exchange model for the 25 at% Er doped δ-Bi2O3 a good agreement is obtained between the isotope exchange experiments and electrochemical experiments using inert gold electrodes.

Influence of electrode geometry and NLLS fit analysis of I-V measurements in a three-electrode cell

The analysis of electrode polarisation (I-V) measurements of oxygen electrodes on δ-Bi2O3-based solid electrolytes is complicated by an ohmic polarisation correction which is of the order of the electrode resistance. The analysis can be performed with a NLLS fit technique, which includes this correction resistance, Ru, as adjustable parameter. By an appropriate choice of the electrode geometry the factor Ru can be minimized.

OXYGEN TRANSPORT AND TRANSFER PROPERTIES OF ERBIA-STABILIZED BISMUTH OXIDE

The electrode resistances of solid solutions of 75 mol% Bi2O3−25 mol% Er2O3 with sputtered and with copressed gold gauze electrodes were compared. In contrast with literature no enhancement of the electrode process could be observed for the copressed electrodes. The measurements show an oxygen partial pressure dependence of power−0.5 for the electrode resistance. Additionally 18O2 exchange results also point to a low oxygen coverage and dissociative adsorption of oxygen. The electrode surface contributes significantly to the electrode process.