| 1. |
- KINYANJUI, FRANCIS GACHAO, 1981, et al.
(författare)
-
Crystal structure and proton conductivity of BaSn0.6Sc0.4O3-delta: insights from neutron powder diffraction and solid-state NMR spectroscopy
- 2016
-
Ingår i: Journal of Materials Chemistry A. - : Royal Society of Chemistry (RSC). - 2050-7488 .- 2050-7496. ; 4:14, s. 5088-5101
-
Tidskriftsartikel (refereegranskat)abstract
- The solid-state synthesis and structural characterisation of perovskite BaSn(1-x)ScxO(3-delta)(x = 0.0, 0.1, 0.2, 0.3, 0.4) and its corresponding hydrated ceramics are reported. Powder and neutron X-ray diffractions reveal the presence of cubic perovskites (space group Pm (3) over barm) with an increasing cell parameter as a function of scandium concentration along with some indication of phase segregation. Sn-119 and Sc-45 solid-state NMR spectroscopy data highlight the existence of oxygen vacancies in the dry materials, and their filling upon hydrothermal treatment with D2O. It also indicates that the Sn4+ and Sc3+ local distribution at the B-site of the perovskite is inhomogeneous and suggests that the oxygen vacancies are located in the scandium dopant coordination shell at low concentrations (x = 0.3). O-17 NMR spectra on O-17 enriched BaSn1-xScxO3-delta materials show the existence of Sn-O-Sn, Sn-O-Sc and Sc-O-Sc bridging oxygen environments. A further room temperature neutron powder diffraction study on deuterated BaSn0.6Sc0.4O3-delta refines the deuteron position at the 24k crystallographic site (x, y, 0) with x = 0.579(3) and y = 0.217(3) which leads to an O-D bond distance of 0.96(1) angstrom and suggests tilting of the proton towards the next nearest oxygen. Proton conduction was found to dominate in wet argon below 700 degrees C with total conductivity values in the range 1.8 x 10(-4) to 1.1 x 10(-3) S cm(-1) between 300 and 600 degrees C. Electron holes govern the conduction process in dry oxidizing conditions, whilst in wet oxygen they compete with protonic defects leading to a wide mixed conduction region in the 200 to 600 degrees C temperature region, and a suppression of the conductivity at higher temperature.
|
|
| 2. |
- KINYANJUI, FRANCIS GACHAO, 1981
(författare)
-
Functional Oxide Materials exhibiting Ionic Conductivity for Future Energy Conversion Needs
- 2013
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Fuel cells are electrochemical devices that transform the chemical energy in hydrogen and oxygen into electrical energy at high efficiencies and produce only water vapour emissions. Materials with the perovskite or fluorite structure types are commonly employed as the ion conducting ceramic electrolyte membranes in intermediate and high temperature fuel cells. However, due to the desire for lower operating temperatures of ~ 600 °C, there exists several challenges, namely: i) insufficient ionic conductivity (minimum required ~ 10-2 Scm-1), ii) poor densification and iii) poor conductivity across grain boundaries. Another problem for proton conducting ceramic fuel cells is the lack of suitable cathode materials with appropriate mixed protonic-electronic conduction. In-situ cells to characterize these materials with concurrent techniques are also not available. The works herein try to address these challenges by exploring the structure property interplay of several candidate materials. The main techniques used were: Thermogravimetric Analysis, powder X-ray and neutron diffraction and electrochemical impedance spectroscopy. The proton conductivity of In3+-BaZrO3 was improved through co-doping with Yb3+ compared to individually doped In3+-BaZrO3 and Yb3+-BaZrO3 samples. Spark plasma sintering of In3+-BaZrO3 achieved high densities (92 %) samples, and the grain boundary conductivity was boosted in comparison to conventionally sintered samples. The oxygen deficient perovskite system, Ba3In2ZrO8, substituted with Ga3+ and Y3+ and Gd3+ and Y3+ combinations was shown to posses’ mixed ionic-electronic conduction with the Ga3+ containing sample having the greatest electron hole contribution. The crystal structure and conductivity of an alternative system to BaZrO3, Sc3+ substituted BaSnO3, was explored and BaSn0.6Sc0.4O3-δ found to have a proton conductivity as high as the current leading materials, i.e., 1.07×10-3 S cm-1 at 600 °C. Disorder in the anion sub-lattice of the pyrochlore-fluorite, Y2(Ti1–xZrx)2O7 system, studied using advanced analysis of neutron diffraction data, was found to significantly enhance O2- ion conductivity; in comparison disorder in the cation sub-lattice did not greatly influence the conductivity. This work also demonstrates two cells developed for in-situ conductivity and hydration studies coupled to neutron diffraction using In3+-BaZrO3 samples, and new insights into the hydration behaviour with respect to temperature and the thermal parameters of the oxygen anions were gained.
|
|
| 3. |
- KINYANJUI, FRANCIS GACHAO, 1981, et al.
(författare)
-
In-situ conductivity and hydration studies of proton conductors using neutron powder diffraction
- 2012
-
Ingår i: Solid State Ionics. - : Elsevier BV. - 0167-2738. ; 225, s. 312-316
-
Tidskriftsartikel (refereegranskat)abstract
- An impedance spectroscopy cell and humidifier system has been developed to study the ion conducting properties and hydration/dehydration reactions in-situ during neutron powder diffraction experiments. As a model system, acceptor doped barium zirconate, BaZr0.5In0.5O2.75, has been studied during in-situ hydration and dehydration. Rietveld refinement of the collected data shows an increase in lattice parameter, a, upon hydration of BaZr0.5In0.5O2.75. In addition, we show the existence of a two-phase region, containing deuterated and non-deuterated barium zirconate, in the temperature interval ~ 573 K to ~ 873 K. Impedance spectroscopy data collected in-situ on heating and cooling and the corresponding ionic conductivity agree well with the change in deuteron content, as determined from neutron diffraction as a function of temperature. The newly developed in-situ cells make it possible to correlate conducting properties to structural changes under identical conditions and will have applicability to many other proton conducting solids.
|
|
| 4. |
- KINYANJUI, FRANCIS GACHAO, 1981
(författare)
-
Proton Conducting Ceramics; Synthesis, Characterization and Sample Cell Environments
- 2012
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Acceptor doped Perovskite ceramics have been shown to exhibit high levels of protonic conduction in humid atmospheres. However significant grain boundary resistance and long sintering times required to achieve sufficient grain size growth has hindered their widespread application as electrolyzers, fuel cells and gas separation membranes. Furthermore there is insufficient detailed knowledge on the local environment of the H+ and its behavior in relation to the crystal structure under real working conditions.The works of the thesis describe the synthesis, structural characterization and conductivity of BaZr0.5In0. 5O3—δ and BaSn1-xScxO3—δ (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5). Traditional solid state sintering was used to prepare the samples which were then characterized by a variety of methods such as electrochemical impedance spectroscopy, thermogravimetric analysis, X-ray and neutron diffraction. Spark plasma sintering processing technique has been explored. BaZr0.5In0. 5O3—δ processed using the spark plasma sintering method were found to exhibit higher grain boundary conductivity compared to solid state sintered samples that had fewer grain boundaries under wet H2 (5% H2 + 95% Ar). The highest proton conductivity was found in the SPS samples under wet H2 atmosphere.In BaSn1-xScxO3—δ the deuteron was found to occupy the 24k crystallographic position by Rietveld analysis of room temperature neutron powder diffraction data. A detailed understanding on the environment of the deuteron in Scandium doped BaSnO3 has been developed. Conductivity was found to peak at x ≈ 0.3-0.4. Experimental facilities at the ISIS neutron facility at Oxford, UK, for in-situ studies of fuel cell materials were demonstrated and their capability to bring about new insights on the workings of protonic conduction shown. The diffraction experiments as a function of temperature using in-situ impedance and in-situ gas flow cells gave detailed insight into the conductivity and hydration/dehydration behavior using deuterated BaZr0.5In0. 5O3—δ samples with the crystal structure remaining intact over the whole temperature interval despite expanding on deuteration and contacting on de-deuteration. The oxygen anisotropic temperature factor u11 was found to replicate the cell unit expansion-contraction behavior as the deuteron was inserted or removed from the crystal structure.Keywords: proton conduction, perovskites, neutron diffraction, in-situ cell, Rietveld refinements, deuteron position, BaSnO3, BaZrO3
|
|
| 5. |
- KINYANJUI, FRANCIS GACHAO, 1981, et al.
(författare)
-
Proton conduction in oxygen deficient Ba3In1.4Y0.3M0.3ZrO8 (M = Ga3+ or Gd3+) perovskites
- 2014
-
Ingår i: Journal of Alloys and Compounds. - : Elsevier BV. - 0925-8388. ; 605, s. 56-62
-
Tidskriftsartikel (refereegranskat)abstract
- B-site disordered, oxygen deficient Ba3In1.4Y0.3M0.3ZrO8 (M = Gd3+ or Ga3+) perovskites of space group Pm (3) over barm, were prepared by a solid-state reactive sintering method. Thermogravimetric analysis of the as-prepared samples revealed 79.3% and 55.5% protonation of the available oxygen vacancies by OH groups in the Gd-3: and Ga-3: containing samples, respectively. Conductivity was found to be in the range of 0.3-1.1 x 10 (3) S cm (1) (M = Gd3+) and 1.1-4.6 x 10 (4) S cm (1) (M = Ga3+) for the temperature interval 300-600 degrees C in wet Argon. Ba3In1.4Y0.3Ga0.3ZrO8 shows an approximate one order of magnitude increase in conductivity at T > 600 degrees C under dry oxygen indicating a significant p-type contribution whereas Ba3In1.4Y0.3Gd0.3ZrO8 reveals a smaller enhancement. Ba3In1.4Y0.3Ga0.3ZrO8 displays considerable mixed proton-electronic conduction in the interval 400-800 degrees C under wet oxidising conditions suggesting possibility of Ga-containing compositions as a cathode materials in a proton conducting fuel cell.
|
|
| 6. |
- Norberg, Stefan, 1972, et al.
(författare)
-
Pyrochlore to Fluorite Transition: The Y2(Ti1-xZrx)2O7 (0.0 ≤ x ≤ 1.0) System
- 2012
-
Ingår i: Chemistry of Materials. - : American Chemical Society (ACS). - 1520-5002 .- 0897-4756. ; 24:22, s. 4294-4300
-
Tidskriftsartikel (refereegranskat)abstract
- The structural properties of the system Y-2(Ti1-xZrx)(2)O-7 have been investigated using the neutron powder diffraction technique, including a detailed analysis of the "total scattering" using reverse Monte Carlo modeling to probe the short-range ion-ion correlations over sample range 0.0
|
|
