Enhanced ionic conductivity and reduced electrical conductivity in Ce0.8Gd0.2O1.9: Effect of calcium Co-doping

Double-perovskite materials of composition Sr2Mg1–xCoxMoO6−δ (SMCMO, x = 0 to 0.7) were evaluated as potential SOFC anode materials. Their lattice structures, electrical and ionic conductivity, thermal expansion coefficient (TEC), and electrochemical performance were investigated as a function of Co content. Co doping was found to increase the TEC of the Sr2MgMoO6−δ material; however, the TEC was within the range of the commonly used La0.8Sr0.2Ga0.8Mg0.2O3-δ (LSGM) electrolyte. SMCMO also showed good chemical compatibility with the LSGM electrolyte at temperatures below 1300 °C. Both the electronic and ionic conductivity increased with increasing Co doping. To investigate the effect of Co doping on the conduction properties of SMCMO, we performed first-principle calculations. From these results, the weak Co–O bond is considered to be responsible for the enhanced ionic conductivity of SMCMO materials. The substitution of Co was also found to increase the sinterability of SMCMO, resulting in a decrease in t...

Unlocking the nature of the co-doping effect on the ionic conductivity of CeO2-based electrolyte

The effects of climate change have become increasingly noticeable in recent years and have drawn public attention. Renewable and sustainable energy sources are replacing fossil fuels and therefore reducing emissions. Solid oxide electrolysis cells (SOECs) hereby offer a great potential for energy storage and conversion as well as the production of chemical compounds and fuels. Although they are a promising option, there are still many challenges remaining.Gadolinium-doped ceria (GDC) is an interesting alternative to state of art nickel/yttria-stabilised zirconia (Ni/YSZ) SOEC cathodes. This is because GDC exhibits excellent electrochemical performance under reducing conditions due to its mixed electronic and ionic conductivity, enabling its use as a single-phase cathode without the need for an additional Ni phase [1]. Ni-free GDC cathodes provide a potential solution for CO2 electrolysis, as the material itself possesses both high coking tolerance and high electrochemical activity for CO formation [2]. A major drawback to (single-phase) GDC cathodes, however, is their pronounced chemical expansion, which is expected to be a driving force for the formation of cracks and thus subsequently may lead to failure of the cell. An in-depth understanding of chemical expansion is therefore crucial for the successful application of GDC cathodes in high-temperature CO2 electrolysis.The cause of the chemical expansion is the formation of Ce3+ (electrons localized on Ce4+) upon reduction of GDC. It thus depends on the concentration of Ce3+, which in turn is a consequence of the effective pO2 – i.e. the applied cathodic voltage. Since Ce3+ is the minority point defect in GDC, its concentration can be determined relatively easily by measuring the chemical capacitance of this electrode material [3].The aim of this study is to gain further insights into the effect of co-dopants on the chemical capacitance of GDC. This is done by electrochemical impedance spectroscopy (EIS) under the application of different bias voltages at a constant temperature. Therefore, the change in defect concentrations with respect to changes in the electrochemical potential of oxygen in the material (i.e. the equivalent pO2) can be experimentally determined, which provides the defect chemical background to gain a deeper understanding of the chemical expansion behaviour of GDC and co-doped alternatives.To realize this, model-type samples were prepared by pulsed laser deposition of GDC thin films with various co-dopant concentrations on YSZ single crystals, which act as the electrolyte in subsequent electrochemical experiments. The electronic current collector was implemented either as microelectrodes, prepared via sputtering, photolithography and ion beam etching, or as a dense Pt layer sputtered onto GDC thin-film electrodes.A porous Pt/YSZ counter electrode was applied after model electrode preparation by brushing and sintering Pt/YSZ on the opposite side of the single crystal. These GDC thin film electrodes were thoroughly investigated by means of impedance spectroscopy under different atmospheres. Upon electrochemically polarizing the GDC model electrodes with varying bias voltages, the chemical potential of oxygen in the material can be manipulated. The Nernst equation was used to calculate the equivalent oxygen partial pressure for each polarization, while the chemical capacitance was extracted from the impedance response. Thereby, we obtained an isothermal variation of volume-specific chemical capacitance with respect to the effective pO2 present in the bulk of GDC.By correlating electrochemical polarization, chemical capacitance, and GDC co-doping, we thus lay the foundation for a basic understanding of the material’s chemical expansion when used as a SOEC cathode. Acknowledgements: This research was funded in whole by the Austrian Science Fund (FWF): [10.55776/COE5] (Cluster of Excellence MECS) and [10.55776/I5478].

Perovskite-type materials have been investigated extensively in the attempt to discover new electrolyte materials for solid oxide fuel cells (SOFC) operating at low temperatures. These materials include LaAlO3-based perovskite, which, when adequately doped, presents high ionic conductivity. However, despite the high conductivity of doped LaAlO3, two characteristics limit its application as solid electrolyte: low sinterability and p-type electronic conductivity in oxidizing conditions. The purpose of this work was to investigate the effect of Pr co-doping on the densification and electrical conductivity of Sr-doped LaAlO3. The addition of Pr eliminated the deficiencies mentioned above. Pr in solid solution acts as a perfect sintering aid because promoted densification without forming secondary phase which should be detrimental for electrical conduction. The addition of Pr also increased the bulk electrical conductivity measured in air atmosphere but had no effect at low oxygen partial pressure. However, the addition of Pr had a strong effect on the grain boundary, improving the ionic grain boundary electrical conductivity at air atmosphere which was attributed to the Pr influence on the space charge layer.

Effect of Co doping on the transport critical current density of rapidly heat-treated filamentary (Nd,Sm,Gd)–Ba–Cu–O superconductors

Novel fast oxide ion conductor and application for the electrolyte of solid oxide fuel cell

Effects of rock-salt layer on electronic and oxide ionic mixed conductivity in strontium titanate, SrO(SrTiO 3) n ( n = 1, 2, ∞)

Mixed ion conductor shows oxide ion and electronic conduction simultaneously and expecting for application to oxygen separation membrane from air. Because of significant volume change depending on oxygen partial pressure, oxygen separation membrane using mixed ion conductor has disadvantage of narrow range of oxygen partial pressure resulting in the limited permeation rate. Therefore, increased oxide ion conductivity and mechanical strength have been required for mixed ion conducting membrane for oxygen separator. In our previous study, we found that Fe doped LaGaO3 shows reasonably high oxide ion conductivity and chemical stability. In particular, La0.7Sr0.3Ga0.6Fe0.4O3 (LSGF) shows largest oxide permeation rate from air to He. In this study, for further increase in oxygen permeation rate, effects of dopant to Ga site in LSGF was investigated. Application to CH4 partial oxidation was also studied. All LSGF sample was prepared with conventional solid state reaction method using oxide as source materials. Partial substitution was performed for Ga site in LSGF and dense sample without impurity phase was confirmed by XRD measurement. For oxygen permeation measurement, La0.6Sr0.4CoO3 catalyst was coated on the surface of LSGF disk (Φ17mm diameter, 0.5mm thickness) It was found that the oxygen permeation from air to He was sensitively influenced by dopant and the oxygen permeation rate was increased as the following order, Co>Al>In>Ni>Mn at 1273 K. On the other hand, in case of Ru and Ir which is easily reduced to metallic state decreased the oxygen permeation rate significantly. Therefore, it was found that addition of 10 mol % of Co or Al is effective for increasing oxygen permeation rate of LSGF. Effects of Co in LSGF was further studied from oxygen permeation rate and the highest oxygen permeation rate was achieved at 3 mol% in Ga site of LSGC. On this optimized composition, La0.7Sr0.3Ga0.57Fe0.4Co0.03O3, shows 225 μmol min-1 cm-2 at 1273 K and this value is almost 2.5 times larger than that of LSGF. Effects of small amount of Co on electrical conductivity in LSGF were also studied. Although LSGF shows high hole conduction like log(σ/Scm-1)=0.6, the conductivity was increased by Co doping, in particular, at lower temperature. In contrast, Al doping decreases total conductivity and so, increase in oxygen permeation rate could be assigned to the increased mobility of oxide ion in LSGF. Application of Co doped LSGF to CH4 partial oxidation was further studied. Although optimized amount of Co doped for Ga site in LSGF is 3 mol%, the membrane was broken when 3 mol% doped LSGF was used for oxygen membrane. 1% Co doped LSGF was applied for oxygen permeation membrane and it was found that amount of oxygen permeation rate was 1.5 times increased comparing with LSGF. In addition, no crack was formed in membrane. The amount of oxygen permeation was achieved 364μmol cm2 min-1 which is corresponded to 8.9ml/min cm2 at 1273K. In summary, this study reveals that doping small amount of Co to Ga site of LSGF is highly effective for increasing oxygen permeation rate without decreasing chemical stability.

Effect of Co doping on oxygen permeation in Sr 3Ti 2O 7 with Ruddlesden-Popper structure

Effect of Co doping on the electrochemical properties of Sr2Fe1.5Mo0.5O6 electrode for solid oxide fuel cell

The effects of Co doping in a series of La3Ni2−xCoxO7−δ samples before and after high oxygen pressure annealing have been investigated. The structural refinements suggest that the Co ions could be incorporated into the La3Ni2O7−δ matrix with a high solution level of x = 0.3. Despite of substantial increase in electrical conductivity with increasing Co doping, the as-prepared La3Ni2−xCoxO7−δ samples exhibit insulator-like conductivity. Noticeably, the high oxygen pressure annealing treatment results in an increase in the oxygen content and the average valence state of Ni ions of the La3Ni2−xCoxO7−δ samples. As a result, the annealed La3Ni2−xCoxO7−δ samples exhibit an insulator-to-metal transition, and the metallic-like conductivity could be achieved above ∼20 K in Co-doped samples.

Effect of co doping on the electric and dielectric properties of Bi3.8−xEr0.2YbxTi3O12 lead-free ceramics

Half-Heulser thermoelectric materials ZrNi1−y Co y Sn (y = 0, 0.02, 0.04, 0.08, 0.12) were prepared by a time-efficient levitation melting and spark plasma sintering procedure. X-ray diffraction analysis and electron probe microanalysis showed that single-phase half-Heusler compounds without compositional segregation have been obtained. The effects of Co doping on the electrical conductivity, Seebeck coefficient, and thermal conductivity of ZrNiSn-based half-Heusler alloys have been investigated from 300 K to 900 K. The Seebeck coefficient displayed a change from negative to positive values above nominal Co doping content of y = 0.02, indicating a transition in the conduction behavior from n-type to p-type. The maximum dimensionless figure of merit ZT of undoped ZrNiSn sample reached 0.5 at 870 K.

Crystal structure, thermal expansion and electrical conductivity of Pr(Ga 1- xCo x) 0.9Mg 0.1O 3-δ ( x=0, 0.1, 0.2, 0.3)

In the present study, the electrical resistivity (ρ) as a function of the temperature (T) has been measured in polycrystalline ZnO, Co-doped ZnO (ZCO) and H irradiated ZCO (HZCO) samples, in the 300-20 K range. The achieved results show impressive effects of Co doping and H irradiation on the ZnO transport properties. The Co dopant increases the ZnO resistivity at high T (HT), whereas it has an opposite effect at low T (LT). H balances the Co effects by neutralizing the ρ increase at HT and strengthening its decrease at LT. A careful analysis of the ρ data permits to identify two different thermally activated processes as those governing the charge transport in the three materials at HT and LT, respectively. The occurrence of such processes has been fully explained in terms of a previously proposed model based on an acceptor impurity band, induced by the formation of Co-oxygen vacancy complexes, as well as known effects produced by H on the ZnO properties. The same analysis shows that both Co and H reduce the effects of grain boundaries on the transport processes. The high conductivity of HZCO in the whole T-range and its low noise level resulting from electric noise spectroscopy make this material a very interesting one for technological applications.