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Advances in materials design in solid-state energy devices have opened up unprecedented opportunities for development in recent years. This work focuses on understanding, controlling and optimising the mechanism of oxygen reduction reactions (ORR) in complex transition metal oxides, in particular La0.6Sr0.4CoO3-δ (LSC) thin films grown at different substrate temperatures by Pulsed Laser Deposition (PLD). We investigated the surface to bulk elemental distribution of the films with low-energy ion scattering spectroscopy and aimed to correlate it with the electrochemical activity and stability of the films.[1] Although the initial ORR activity of the film grown at high substrate temperature was better than the one grown at low substrate temperature, interestingly, it showed 2-times higher degradation rate in the long-term electrochemical tests. The better stability of the film grown at low substrate temperature is attributed to the segregation of Sr into protruding particles. In this way, Co content reached stoichiometry in the remaining surface. This study emphasizes the influence of processing temperature and post thermal treatments on the electrochemical activity and stability of the PLD films. [1] Celikbilek, O., Cavallaro, A., Kerherve, G., Fearn, S., Chaix-Pluchery, O., Aguadero, A., Kilner, J. A., Skinner, S. J. 2020, “Surface Restructuring of Thin-Film Electrodes Based on Thermal History and Its Significance for the Catalytic Activity and Stability at the Gas/Solid and Solid/Solid Interfaces,” ACS Appl. Mater. Interfaces, 12, pp. 34388−34401.

Solid-oxide fuel/electrolyzer cells are limited by a dearth of electrolyte materials with low ohmic loss and an incomplete understanding of the structure-property relationships that would enable the rational design of better materials. Here, using epitaxial thin-film growth, synchrotron radiation, impedance spectroscopy, and density-functional theory, the impact of structural parameters (i.e., unit-cell volume and octahedral rotations) on ionic conductivity is delineated in La0.9 Sr0.1 Ga0.95 Mg0.05 O3- δ . As compared to the zero-strain state, compressive strain reduces the unit-cell volume while maintaining large octahedral rotations, resulting in a strong reduction of ionic conductivity, while tensile strain increases the unit-cell volume while quenching octahedral rotations, resulting in a negligible effect on the ionic conductivity. Calculations reveal that larger unit-cell volumes and octahedral rotations decrease migration barriers and create low-energy migration pathways, respectively. The desired combination of large unit-cell volume and octahedral rotations is normally contraindicated, but through the creation of superlattice structures both expanded unit-cell volume and large octahedral rotations are experimentally realized, which result in an enhancement of the ionic conductivity. All told, the potential to tune ionic conductivity with structure alone by a factor of ≈2.5 at around 600 °C is observed, which sheds new light on the rational design of ion-conducting perovskite electrolytes.

The solid-state batteries considered here operate with ceramic electrolyte materials such as oxides, sulfides or phosphates and offer a new paradigm for batteries with high specific energy and power density. While ceramic electrolytes eliminate potentially flammable liquids and corrosive environments, which has profoundly positive implications for improved safety, many challenges remain. To fully realize the promise of high energy density, new cell and electrode designs are needed. Furthermore, strategies for mitigating dendrite formation as well as accelerating electrode-electrolyte interface kinetics is essential. Suitable electrodes tolerating high chemo-mechanical stresses induced during charging and discharging also must be identified, and an improved understanding of the defect chemistry operative in the bulk and at interfaces is required. Successful design and operation of such temperature-robust ceramic cells have far-reaching implications for new applications like synergetic operations taking advantage of industrial waste heat in large-scale battery storage units, electric vehicles, and grid storage.

Surface-sensitive ion beam techniques, such as Secondary Ion Mass Spectrometry (SIMS) and Low-Energy Ion Scattering (LEIS), are making significant contributions to further our understanding of the materials’ performance and the degradation processes that occur under operating conditions. In this contribution, we explore how recent instrumental developments and analytical approaches have boosted the application of these powerful techniques for the characterization of surfaces and interfaces in energy conversion and storage devices. Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech.

Isotope Exchange/Depth Profiling (IEDP) using Secondary Ion Mass Spectrometry (SIMS) has been used to determine the oxygen tracer diffusion and surface exchange coefficients of (100) oriented 9.5 mol% yttria stabilised zirconia single crystals. Exchange experiments performed with molecular oxygen are compared with the exchange using an oxygen plasma. The surface exchange coefficient for specimens in a plasma is up to 100 times higher compared to measurements with normal molecular oxygen. For the exchange experiments we used an inductively coupled radio frequency (rf) oxygen plasma with a maximum radio frequency power of 250 W. Double probe measurements and optical emission spectrometry are used for the characterisation of the plasma. The measured electron temperatures are within the range of 5 12 eV.

All-solid-state lithium batteries are of great interest scientifically as a next-generation of electrochemical energy storage devices, owing to their superior safety features and their potential to enable new chemistries to improve performance. The properties of the solid state electrolyte are integral to the overall cell capability – to date the most promising group of materials are the garnet-structured oxides, based on Li7La3Zr2O12 (LLZO), with high room temperature ionic conductivity and a wide electrochemical stability window. There are several aspects in the development of this relatively new material which are yet to be fully understood – these are the focus of this thesis. In this work, processing cubic doped LLZO as a bulk ceramic was investigated and served as a basis for understanding its stability and electrochemical performance; it was optimised to obtain highly dense microstructures under atmosphere-controlled conditions to prevent reaction with moisture. Chemical inhomogeneities in the pellets, especially at the grain boundaries, as investigated by secondary ion mass spectrometry (SIMS) and low energy ion scattering, were shown to be important in determining the transport properties of the electrolyte - in particular the propensity for dendrite formation during cell cycling. It was shown that aluminium-rich grain boundaries in aluminium-doped LLZO favour the formation of inter-granular lithium dendrites (with a 60 % lower critical current density for cell failure) over gallium-doped LLZO. The use of germanium (Ge4+) as a dopant was studied, and shown to stabilise the cubic LLZO phase through substitution of 0.10 moles of Ge at the lithium sub-lattice (at the tetrahedral 24d sites), giving conductivities of the order 10-4 S cm-1 and redox stability over a 4.5 V range with lithium electrodes. Chemical and electrochemical characterisation of the moisture reactivity of gallium-doped LLZO was also carried out, showing a chemically-altered proton-rich region extending to 1.35 micrometres following 30 minutes immersion in H2O at 100 °C and highly reactive grain boundaries. These chemical changes led to a threefold increase in the resistance of both the electrolyte and the interface with lithium electrodes. Chemical and tracer diffusivity of protons were estimated from the diffusion profiles of H+ and D+ obtained by SIMS depth-profiling. A new methodology for measuring macroscopic lithium tracer diffusion in LLZO was introduced, using SIMS depth-profiling and isotopic labelling, in which a number of experimental parameters were varied to optimise the technique. The preliminary results for lithium diffusivity in doped LLZO obtained from this method were compared with values from other methods (impedance and nuclear magnetic resonance) and used to comment on the mechanism for lithium diffusion in the materials. Open Access

The (La,Sr)(Cr,Fe)O3-δ (LSCrF)-Scandia-Stabilised Zirconia (ScSZ) based dual-phase composite system has been investigated in this work. As the dense layer in an oxygen transport membrane device, transport properties profoundly affect the membrane performance. Isotopic Exchange Depth Profile (IEDP) combined with Secondary Ion Mass Spectroscopy (SIMS) was performed to investigate the oxygen ionic transport in the composite materials and the electrical conductivity was measured by the 4-Probe DC method. In order to achieve improved performance, optimisation strategies were also applied, including varying the Cr substitution level in the LSCrF single phase materials and changing the phase ratios in the composite systems. For the electrical conductivity behaviour, in the composite system a percolation threshold was observed at around 20-30 vol% for all Cr:Fe ratios (LSCrF37, LSCrF55 and LSCrF73). Among these three systems, the LSCrF73-based composites presented the highest electrical conductivity. The diffusion measurements were firstly carried out at the macroscopic scale and the ‘effective’ diffusion kinetics were obtained. In a dry oxygen atmosphere, the predominant phase for the surface exchange process was determined to be the LSCrF phase while the ScSZ-based ionic conductor dominated the bulk diffusion. The percolation limit for the diffusion coefficients was observed at a composition of around 30 vol% ionic conductor for the three measured systems (LSCrF37-10Sc1CeSZ, LSCrF55-10Sc1CeSZ, and LSCrF73-10Sc1CeSZ) and the LSCrF73 based dual-phase composites displayed the best bulk diffusivity. Subsequently, in oxygen diffusion studies at the microscopic scale using Focused Ion Beam (FIB)-SIMS, a synergistic effect between the two phases was observed in the composite materials: a decreased surface exchange coefficient (k) was observed for the MIEC phase LSCrF while an enhanced k was obtained for the pure ionic conductor (10Sc1CeSZ) compared to the corresponding isolated single-phase materials. Further mechanism studies were performed on a specialised sample by applying IEDP, SIMS and Low Energy Ion Scattering (LEIS) techniques. The plausible origin of the synergistic effect is suggested to be a combination of the spillover type mechanism and the self-cleaning behaviour of the LSCrF based upon these observations. Prior to the dual-phase composite materials, the starting single-phase materials, LSCrF and ScSZ were firstly characterised. As the Cr substitution increased, the electrical conductivity increased and the maximum value was achieved in the LSCrF73 phase while the bulk diffusivity of the LSCrF decreased. Fast grain boundary diffusion behaviour was observed in the LSCrF73 single phase material. Additionally, the diffusion behaviour of both the single-phase and dual-phase samples in a pure water vapour environment was lastly studied by using labelled H218O to reflect operating atmospheres. The diffusion coefficient (D*) at 800 ̊C of LSCrF was found to increase dramatically by 3 orders of magnitude compared to an isothermal experiment under dry oxygen atmosphere. The surface exchange coefficient of 10Sc1CeSZ has been improved to 1.5×10-6 cm s-1 at 800 ̊C while in dry O2 conditions almost no 18O has been exchanged into the sample. For the LSCrF-ScCeSZ composite, ScCeSZ single phase dominates both the surface exchange process and the bulk diffusion in pure water vapour atmosphere. Open Access

A dual-phase composite material comprised of scandia-stabilized zirconia (ScSZ) and doped lanthanum chromite (La,Sr)(Cr,Fe)O3-δ (LSCrF) was investigated in this work as a potential oxygen transport membrane material in the production of synthesis gas. The main focus of this work was to examine the effects of exposure to three aging environments (air, 4% H2-96% N2, 5% H2-95% CO2) for two durations (300 hours, 1000 hours) on the oxygen transport kinetics and surface chemistry of ScSZ/LSCrF. Determination of the oxygen ion conductivity via Electrochemical Impedance Spectroscopy of single-phase ScSZ showed a decrease in the activation energy above 600˚C corresponding to a change in crystal phase at this temperature. Determination of the oxygen transport kinetics in dry oxygen (18O2) atmosphere on single-phase ScSZ via Isotopic Exchange Depth Profiling combined with Secondary Ion Mass Spectrometry showed the material exhibited an oxygen self-diffusion coefficient on the order of 10-8 cm2 s-1 at 700˚C for 30 minutes but required the application of a catalytic layer to boost oxygen surface exchange to detectable levels. Examinations of the surface chemistry of polished single-phase ScSZ and LSCrF via Low Energy Ion Scattering and X-ray Photoelectron Spectroscopy showed minimal differences to the ideal bulk stoichiometry. Determination of the crystal structure of dual-phase ScSZ/LSCrF via X-ray diffraction showed secondary phase formation in the samples after 300 hours of aging in air and 5%H2-95%CO2, and in all samples after 1000 hours of aging. Oxygen transport kinetics of dual-phase ScSZ/LSCrF under dry oxygen showed minor differences in D* and k* after exposure to the aging environments for 300 hours when compared to the unaged material, but an increase in surface exchange kinetics was observed for the materials aged in reducing conditions for 1000 hours. D* and k* in dual-phase ScSZ/LSCrF were on the order of 10-8 cm2 s-1 and 10-8 cm s-1 respectively before and after aging up to 1000 hours. Oxygen surface exchange kinetics are increased by about one order of magnitude when performed in water vapor (H218O) over dry oxygen. Examinations of the surface chemistry of as-prepared dual-phase ScSZ/LSCrF showed Cr depletion on the outer atomic surface of all samples, in addition to elevated surface fraction of Zr within ~10 nm of the sample surface. Open Access

The LSCF material system is a desirable material for a Solid Oxide Fuel Cell (SOFC) cathode due to its Mixed Ionic-Electronic Conducting (MIEC) properties. It has been utilised commercially for many years, however, the cell lifetime is inhibited by degradation processes passivating the surface of the electrode. A-site cation segregation is believed to be a primary degradation mechanism due to the formation of electronically insulating secondary phase particles reducing the active surface area that facilitates the oxygen reduction reaction. Numerous reports have studied the overall effect that degradation has on SOFC cell performance, however, it is still unclear the extent to which microstructural changes affect a material’s oxygen exchange properties. To date, many studies measuring the oxygen exchange rate utilised pure oxygen atmospheres in order to isolate the effect of oxygen. However, for cost and practicality reasons, the desired gas stream for SOFC is ambient air. Multiple oxygen-containing gaseous components and impurities (such as CO, CO2, H2O, SO2 and NOx) are contained within ambient air and have been shown to alter the oxygen exchange rate or enhance the degradation of MIEC materials. This work focuses upon characterising the effect ambient air has upon the surface microstructure and oxygen exchange rate of the LSCF system. In-situ High-Temperature Environmental Scanning Electron Microscopy (HT-ESEM) was used to analyse the growth rate and growth behaviour of strontium-based particles in pure O2, pure H2O, ambient air and vacuum environments. The HT-ESEM data was directly compared to Electron Backscattered Diffraction crystal orientation data in order to understand what effect the LSCF domain structure had upon the segregation of strontium. It was observed that the surface microstructure has a strong influence on the growth behaviour and growth kinetics of the particles. A common methodology for measuring oxygen self-diffusivity and surface exchange rates is Isotopic Exchange Depth Profiling (IEDP). This traditionally has used pure oxygen as the anneal environment to isolate the exchange properties of the O2 species, however, in the presence of other oxygen containing species the exchange process will be more complicated. In order to analyse the surface exchange rate in ambient air (or any other atmosphere containing a consistent oxygen partial pressure), the novel ‘back-exchange’ technique was developed. Initial development of the technique has demonstrated its validity and confirmed enhancement of the oxygen exchange rate in ambient air over pure oxygen. Time-of-Flight (ToF) SIMS was used to measure isotopically exchanged diffusion profiles. For materials with a high oxygen self-diffusivity, such as LSCF, the ‘line-scan’ method must be employed instead of a standard depth profile. The ToF-SIMS utilises a statistical method in order to correct for detector 'dead time', however, this method relies upon the total ion count to remaining constant across each pixel of the raster area. The line-scan method relies upon analysis of a surface perpendicular to the original exchange surface and as such will not have a constant ion count near the sample edge. Errors associated with the measured diffusion profile are discussed and an optimised sample preparation has been proposed. Open Access

The aim of this work is to investigate: enhancement of the oxygen reduction reaction at the PrCoO3-δ/Pr2NiO4+ δ (PCO113/PNO214), inteface; and the mechano-chemical effect in mechanically strained lattice mismatched PCO113 films on Ce0.9Gd0.1O2/8-mol% YSZ(002) fabricated using pulsed laser deposition, PLD, and chemically strained A-site deficient PCO113 (Pr1-xCO3- δ) samples. PCO113 films on Si(004) and Ce0.9Gd0.1O2(200)/8-mol% YSZ(002) single crystal substrates were deposited by PLD. Out-of-plane compressive PCO113 lattice strains, calculated from the PCO113(002) reflection, between -0.39% to -0.99% were measured. Single phase PNO214 films could not be deposited for the PLD conditions tested in this work. Isothermal measurements of PNO214 at 800°C in air showed the gradual formation of Pr6O11 and Pr4Ni3O10, suggesting that this process is kinetically limited. Subsequently, investigation of the mechano-chemical effect was limited to the mechanically and chemically strained PCO113 systems. The chemical expansion on introducing Pr-deficiency, x, in PCO113 was attributed to the formation of Pr vacancies, which effect was large enough to offset the decrease in ionic radii as the oxidation state of Co3+ increased to Co4+. High temperature XRD indicated a thermo-chemical expansion wherein oxygen uptake at high temperatures affected the expansion of the PCO113 lattice as a function of temperature. A comparison of the thermos-chemical expansion between the Pr-deficient PCO113 samples confirmed an increase in oxygen vacancy concentration with Pr-deficiency. 18O diffusion profiles of the Pr-deficient PCO113 samples were fitted using Crank's solution for diffusion in a semi-infinite plane. D* and k* values varied between 1.36-2.26 ×10-16 cm2/s and 4.48-10.1×10-10 cm/s. D* and electrical conductivity was found to increase with Pr-deficiency. Visual analysis of 18O diffusions profiles of the strained PCO113 films suggested that the mechano-chemical effect was present and the introduction of a compressive out-of-plane strain inhibited oxide ion diffusion. Open Access

Modified ionic transport at interfaces in oxide conductors has received significant interest recently, yet previous studies have provided inconsistent and often controversial results. Lattice strain occurring at heterogeneous interfaces or surrounding dislocations is often speculated as the cause for enhanced transport properties, yet direct evidence for this is lacking. The aim of this work was to investigate the possibility for modified oxygen conductivity at interfaces and develop a greater understanding of the underlying mechanisms. In this work, it was shown that the crystallographic orientation of yttria-stabilised zirconia (YSZ) films fabricated by pulsed laser deposition (PLD), could be altered by the roughness and the temperature of the substrate during deposition. Furthermore, the films were seen to feature an expanded out-of-plane lattice parameter, which was found to be dependent upon the deposition temperature, substrate, the orientation of film, and the thermal history after growth. The ionic transport of YSZ films fabricated on a range of substrates was assessed as a function of film thickness using electrochemical impedance spectroscopy (EIS) and a novel method of oxygen isotope exchange tracer diffusion (IETD). No enhancement over the conductivity of bulk YSZ was observed, despite expanded out-of-plane lattice parameters, substrates representing both positive and negative lattice mismatch, and a high density of interfacial dislocations. Instead, all interface regions were found to be more resistive due to a higher density of grain boundaries. Finally, gadolinia-doped ceria (CGO) films were analysed by low energy ion scattering (LEIS), to investigate the effects of dopant segregation to the surface and sub-surface regions. Gd segregation was found to occur for all films and observed to increase for higher annealing temperatures. Investigations into the affect of the lattice parameter on the rate of dopant segregation showed a possible link between segregation and lattice strain. Open Access

The performance of Solid Oxide Fuel Cell (SOFC) electrodes is determined both by their porous microstructure and the intrinsic properties of their component materials. This thesis details the development of two characterisation tools for analysis of mass and charge transport processes in SOFC electrode materials. Firstly, a new approach to isotopic exchange is described, which allows the oxygen self- diffusion (D^∗) and effective surface exchange (k^∗) to be measured in ambient atmospheres. This is significant as many similar studies in the literature are limited to investigations in pure, dry oxygen, or other proxy environments, which are not representative of realistic SOFC operating conditions. A finite difference simulation was created to generate profiles that were used to extract material parameters from this “back-exchange” data. The technique was then validated by comparison of the results from two experiments in pure, dry oxygen (both single step and back-exchange), which demonstrated good agreement with values of D ∗ and k ∗ in the literature, for the common SOFC cathode material La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF6428). A third experiment found the surface exchange coefficient to increase by a factor of 5 when exchanged under ambient conditions compared with pure, dry oxygen. Secondly, following an introduction to the tortuosity factor, X-ray tomography was used to 3D image micro-tubular (MT) samples ( c. 1 mm diameter) at three key length-scales. A zirconia based MT-Solid Oxide Electrolyser Cell (SOEC) was imaged at the whole cell level, both before and after 300 hours operation at 750◦ C . Current collector contact was found to be poor even before operation, but afterwards the paste was seen to agglomerate into metallic silver and no longer span the gap to the current collector wire, which further degraded contact. A ceria based MT-SOFC with a hierarchical microstructure was then imaged at both the micro- and nanoscale. The geometry data was used to determine that the tortuosity factor of this radial system was significantly higher when measured at either length-scale in isolation, than when considered together in a multi length-scale model. Open Access

In the search for solid oxide fuel cell (SOFC) materials, which exhibit improved oxide ion conductivity at lower temperatures, attention has recently been focussed towards materials with atypical structural chemistry and diffusion pathways. For example high oxygen diffusivity is reported in fergusonite structured CeNbO4+δ, which can incorporate a range of oxygen excess stoichiometries by oxidation of Ce3+ to Ce4+ and as a result, four commensurate or incommensurately modulated superstructures of the monoclinic parent cell are possible. An alternative strategy to reduce migration energy barriers is to change the charge carrier from oxide ions to protons, and through the introduction of oxide ion vacancies, develop materials that can conduct both species through a O2(g)/H+(g) partial pressure dependence. Alkaline earth doped rare earth niobates RE1-xAxNbO4-δ (RE=La, Nd, Gd, Tb, Er A=Ca, Sr, Ba) have been shown to exhibit proton conductivities that can be maintained to much higher temperatures compared to traditional perovskite materials, with reported stability to both protonic and acidic media. However they are hindered by low dopant solubility, and an intermediate temperature monoclinic to tetragonal phase transition that is problematic for device applications. Owing to the unique structural and redox behaviours of the cerium analogue of the RENbO4 series, this worked aimed to investigate the transport properties of acceptor doped Ca1-xAxNbO4±δ. This work has shown that Ce1-xAxNbO4±δ can cycle between oxygen hyper- and hypostoichiometry, and in the process alternate between mixed electronic interstitial oxide ion conduction, and (mixed electronic) protonic conduction respectively. Under both oxidising and reducing conditions the conductivity increases by over one order of magnitude relative to both CeNbO4+δ and the most protonically conductive of the series La1-xAxNbO4-δ respectively. This is a result of the greater solid solubility of strontium and calcium in CeNbO4+δ, relative to LaNbO4 (1-2%), and is speculated to arise from the charge compensation mechanism of electron hole formation via Ce3+-Ce4+ oxidation, where the change in both cerium valance and coordination may act to stabilise the dopant. The likelihood of electronic charge carriers in the hypostoichiometric phases of Ca1-xAxNbO4±δ, is of interest due to limited number of mixed proton-electron conductors if the properties can be optimised further. Furthermore the transition temperature raises from ~520 °C to ~650 °C on exchange of lanthanum with cerium. Unusually the hyperstoichiometric phases are also suggested to show proton conductivity, indicating Ca1-xAxNbO4±δ may exhibit more unusual defect reactions. Open Access

Mixed Ionic Electronic Conductors (MIECs) such as La0.6Sr0.4Co0.2Fe0.8O3­δ (LSCF) and La2NiO4+δ (LNO) are gaining much attention as candidate cathode materials at reduced working temperatures (600-800°C), because they exhibit better oxygen transport properties, in comparison to earlier cathode materials such as LaxSr1­xMnyO3­δ (LSM). The main objective of the study in this thesis is establishing the relationship between the amount of chromium and performance degradation of LSCF and Sr-free LNO cathodes, and improving understanding of the mechanism. LSCF were screen printed onto Ce0.9Gd0.1O1.95 (CGO10) electrolyte pellets and infiltrated with chromium nitrate solutions to different Cr levels up to 2wt%. Electrochemical impedance spectroscopy at 500 ~ 800C showed that even very low levels of Cr give a significant increase in cathode polarisation resistance, which increases with Cr concentration. The impedance response was analysed using the model of Adler, Lane and Steele (ALS model) to extract oxygen self-diffusion (Do) and surface exchange (ko) parameters for the LSCF. The results show that Cr reduces both Do and ko, the latter being the more affected. However, the activation energies for polarisation resistance, Do and ko are not significantly affected by Cr content. This indicates that the Cr poisoning mechanism involves the de-activation of sites for oxygen exchange on the LSCF surface and that the cathode's residual activity is by means of remaining active sites. The surface reaction and diffusion of oxygen in dense LSCF was studied by oxygen isotope exchange using bulk specimens and depth profile analysis. The specimen surfaces were coated with different thicknesses of Cr2O3 by sputtering prior to exchange. The results showed that the Cr2O3 surface layer had a large inhibiting effect on ko, in agreement with the electrochemical measurements, but a negligible effect on Do because diffusion of Cr into the bulk was very slow. The effect of Cr on Sr-free LNO electrodes deposited symmetrically onto Ce0.9Gd0.1O1.95 (CGO10) electrolytes by screen printing was studied in a similar way. XRD of LNO/Cr2O3 powder mixtures annealed at 1000C for 24h showed that LNO reacts readily with Cr2O3 to form LaCrO3 and LaNiO3. Despite this reactivity, the electrochemical experiments show that the polarisation resistance of LNO cathodes is insensitive to Cr introduced by solution infiltration for Cr concentrations up to 1%. For concentrations above this level the Cr increases the polarisation resistance, but to a much lesser degree in comparison to LSCF. Cr reduces the rate of the oxygen reduction reaction on the LNO surfaces, but does not change the activation energy significantly. The relatively good tolerance of LNO cathodes to Cr ingress is probably due to the reaction products themselves having some useful catalytic activity for oxygen reduction. Open Access

Layered Ruddlesden-Popper (RP) type La_n+1Ni_nO_{3n+1} (n = 1, 2 and 3) oxides have recently been suggested as candidates for solid oxide fuel cell (SOFC) cathodes. However, the transport properties of the higher order (n = 2 and 3) phases have not been well-understood. The aim of this work is to achieve well-defined epitaxial La_3Ni_2O_{7-δ} and La_4Ni_3O_{10-δ} films deposited by the pulsed laser deposition (PLD) technique in order to fundamentally investigate their intrinsic anisotropic properties for SOFC applications. This research involved PLD target synthesis and PLD deposition for these RP materials. The obtained films were evaluated through crystallographic, compositional, surface morphological and microstructural characterisation. Their electrical, oxygen diffusion and surface exchange properties were characterised by 4-point DC van der Pauw and the isotopic exchange depth profile (IEDP) with secondary ion mass spectrometry (SIMS) method. Finally, the surface and interface information were also obtained via SIMS and low energy ion scattering (LEIS) techniques. An effect of target degradation during deposition was found to have significant influence on the microstructural and compositional variation with increasing film thickness. Successful epitaxial growth of the higher order phases (n = 2 and 3) of lanthanum nickelate have been demonstrated by PLD for the first time. The total planar electrical conductivity of these higher order RP epitaxial films exhibits a more promising conductivity value than La_2NiO_{4+δ}. These dense higher order RP epitaxial films enabled the first information about the oxygen diffusion and surface exchange properties along the c-axis to be obtained. Also, an outermost surface structure of LaO-termination, followed by a Ni-enrichment in the sub-surface region for these RP materials was found by LEIS analysis. These studies provide useful information to help the further development of these materials and related RP-phases for the potential SOFC applications in the future. Open Access

La0.8Sr0.2Ga0.8Mg0.2O3-δ (LSGM) is an oxygen ion conducting electrolyte material widely used in solid oxide fuel cells (SOFC). La2NiO4+δ (LNO) is a mixed ionic-electronic conducting layered perovskite with K2NiF4 type structure which conducts oxygen ions via oxygen interstitials. LNO has shown promising results as an SOFC electrode in the literature. In this work the compatibility and performance of LNO electrodes on the LSGM electrolyte material for solid oxide electrolysis cell (SOEC) is investigated. The materials were characterised as SOEC/SOFC cells by symmetrical and three electrode electrochemical measurements using Electrochemical Impedance Spectroscopy (EIS). Conductivity and ASR values were obtained in the temperature range 300 – 800oC with varying atmospheres of pH2O and pO2. The cells were also subjected to varied potential bias, mimicking fuel cell or electrolysis use. Enhancement of LNO performance was observed with the application of potential bias in both anodic and cathodic mode of operation in all atmospheres with the exception of cathodic bias in pO2 = 6.5x10-3 atm. In ambient air at 800oC LNO ASRs were 2.82Ω.cm2, 1.83Ω.cm2 and 1.37Ω.cm2 in OCV, +1000mV bias and -1000mV bias respectively. In low pO2 at 800oC LNO ASRs were 9.17Ω.cm2, 1.74Ω.cm2 and 456.9Ω.cm2 in OCV, +1000mV bias and -1000mV bias respectively. The increase in ASR with negative potential bias in low pO2 is believed to be caused by an increase in mass transport and charge transfer impedance responses. Material stability was confirmed using X-Ray Diffraction (XRD), in-situ high temperature pH2O and pO2 XRD. In-situ XRD displayed single phase materials with no observable reactivity in the conditions tested. Scanning Electron Microscopy images of cells tested by EIS in all atmospheres displayed no microstructure degradation except for those cells tested in a humid atmosphere which display a regular pattern of degradation on the LNO surface attributed to reaction with the Pt mesh current collector. Open Access

The effects of low concentration Co, Cr and Mn oxide, singly and in combination, on the sintering and electrical properties of Ce0.9Gd0.1O1.95 (CGO) have been investigated with possible mechanisms suggested to explain this modified behaviour. The influence of these dopants on the densification kinetics of CGO were primarily investigated using constant heating rate dilatometry. Whilst low concentration Co and Mn-oxide were found to improve the sinterability of CGO, the addition of Cr-oxide was found to inhibit the densification kinetics of the material. The location and concentration of these dopants were investigated as a function of relative density using scanning transmission electron microscopy combined with energy dispersive x-ray mapping. All materials showed a gradual reduction in the grain boundary dopant concentration with sintering time, leading eventually to the formation of a second phase that was subsequently analysed by either electron energy loss spectroscopy or synchrotron x-ray powder diffraction. The improved densification of both the Co-doped and Mn-doped materials was believed to be related to an increased rate of lattice and grain boundary cation diffusion, associated with the segregation of the transition metal dopant to the grain boundary. In both cases the onset of rapid densification was correlated with the reduction of the transition metal cation leading to an increase in cerium interstitials, which are suggested to be the defects responsible for cerium diffusion. The inhibiting effects of Cr-addition were similarly related to changes in the defect chemistry, with the Cr ions creating a blocking effect that hindered the dominant grain boundary pathway for cation diffusion. The effects of these dopants on the electrical conductivity of CGO were examined using a combination of AC impedance spectroscopy and Hebb-Wagner polarisation measurements. Whilst Co-doping was found to enhance the specific grain boundary conductivity of CGO, the addition of either Cr or Mn resulted in an approximate 2 orders of magnitude decrease, even at dopant concentrations as low as 100 ppm. Despite these differences in ionic conductivity, both Co and Cr-doping were found to significantly enhance the electronic contribution to the conductivity along the boundaries, particularly within the p-type regime. The modified electrical behaviour was related to the formation of a continuous, transition metal-enriched grain boundary pathway and a change in the driving force for grain boundary Gd segregation, leading to a depletion of oxygen vacancies within the space charge regions and the consequent reduction of oxygen transport across the boundaries. The effects of this segregation were finally examined with mono-layer sensitivity using low energy ion scattering incorporating a novel method of self-standardisation. These analyses provided strong support for the conductivity mechanisms previously outlined. Open Access

The aim of this work was to develop oxide fuel cell (SOFC) cathodes made from (LaNiO3)nLaO Ruddlesden-Popper (R-P) phases, and to investigate novel in situ characterisation techniques for SOFC cathodes. Cathodes were developed from La2NiO4+δ (L2N1) and La4Ni3O10-δ (L4N3), R-P phases known to have attractive conductivities at SOFC temperatures. These phases were shown to be chemically stable, both with each other and with the common electrolyte material La0.8Sr0.2Ga0.8Mg0.2O3-δ (LSGM). LSGM-supported symmetrical cells were fabricated with electrodes of single phase L2N1 and L4N3, and a range of L2N1+L4N3 composites. The performance of these was tested from 500 – 700 °C with the composites giving the lowest area-specific resistance (ASR); a 50:50 wt.% L2N1:L4N3 composition being optimal. Functionally graded electrodes were developed consisting of a thin compact L2N1 layer deposited onto the LSGM, topped by a thicker porous L2N1+L4N3 composite layer, completed by a thin porous L4N3 current collector. These gave a lower ASR than the ungraded electrodes. Using a 50:50 composite was optimal with ASRs of 15.59, 2.29, and 0.53 Ωcm2 at 500, 600, and 700 °C respectively; amongst the best-in-class for electrodes made from this type of material. X-ray absorption near-edge spectroscopy was chosen as a method to gain in situ information on the redox chemistry of elements within SOFC materials. Initial studies were carried out on powder samples of L2N1 and L4N3; the nickel oxidation state in these was found to reduce on heating to SOFC operating temperatures. Bespoke equipment was developed to enable such studies to be carried out on symmetrical cells under polarisation and with simultaneous AC impedance spectroscopy. The bulk nickel redox chemistry was correlated with the changing concentration of ionic charge carriers in the materials, and was found to be dominated by thermal effects. These techniques were then used to explore in situ chromium poisoning of state-of-the-art perovskite cathodes. The surface chemistry of SOFC materials is key to performance. Low-energy ion scattering was used to find the composition of the outer monolayer for the entire (LaNiO3)nLaO R-P series; lanthanum termination was found for each phase. Open Access

Electrochemical impedance spectroscopy (EIS) is recognized as a powerful tool for characterizing the charge transfer reaction on heterogeneous interfaces, thus has been widely applied in various fields of research and in many applications, such as corrosion, development of various types of batteries and fuel cells. Due to the limitation of linearization inherent to the EIS technique the nonlinear information of an investigated system is automatically abandoned. This work focuses on extending the traditional linear EIS technique into the nonlinear domain, nonlinear EIS (NLEIS), and has demonstrated that the nonlinear information can be utilized for gaining a more comprehensive understanding of a system. In this work, the NLEIS technique is discussed from a basic theoretical point of view, including the fundamental definition of nonlinear “impedance” and experimental issues on which a consensus has yet to be reached within the community. A LabView programmed experimental setup was developed during the project to perform NLEIS measurements and preliminary data analysis, which appeared superior to commercial systems (higher frequency range) and systems developed at other institutes (lower noise level). A qualitative approach for data analysis in NLEIS is discussed and prompted in this work, which is more intuitive and suitable for fast first stage analysis compared to an in-depth quantitative modelling approach. To test the NLEIS methodology developed, a classic redox couple- ferri-ferrocyanide- was examined. Information on nonlinearity and symmetry of the system was directly investigated by measuring harmonic responses up to 5th order and it was demonstrated that in contrast to EIS a full set of kinetic parameters could be directly calculated from a single measurement. In addition, features generic to the methodology were discovered, which are independent of systems under investigation. Finally, the NLEIS technique was used in investigating the oxygen reduction reaction on strontium and iron doped lanthanum cobaltite (LSCF) cathode materials of intermediate-temperature solid oxide fuel cells (IT-SOFCs). The decrease of the level of nonlinearity and the increase of asymmetry indicates a potential mechanism change, with increased operating temperature.

Recently there has been a great level of interest in the effect of interfaces in oxide ionic conductors with a view to eventual application in solid oxide fuel cells (SOFCs). Enhancements in electrical conductivity of one half to eight orders of magnitude have been reported in simplified thin film and multi-layered heterostructure systems. Often this is reported to be enhanced oxygen ion conduction with little supporting evidence. The aim of this work is to investigate these reports and develop an understanding of the underlying mechanisms. Several series of samples were investigated to achieve this goal. The first, designed to emulate an anomalous result from literature with alternating samarium doped ceria (SDC) and undoped ceria layers, featured an increasing total number of layers of equal individual thickness, and thus a constant interfacial density. This system featured no enhancement in conductivity and exhibited a level of tracer diffusion comparable with that in bulk SDC. Electron energy loss spectroscopy (EELS) studies, however, revealed a significant level of Ce III in the undoped layers. The second and third series used similar materials but tested the hypothesis proposed in many works, that conductivity enhancement was related to tensile strain in the conducting material at the heterointerfaces. The second, manipulating the strain at the interface by varying the dopant (Nd, Sm, Y) in films with alternating doped and undoped ceria layers and a range of interfacial densities. This series exhibited minimal change in conductivity with strain or interfacial density. The third series replaced the doped ceria with yttria-stabilised zirconia (YSZ) in order to achieve a higher level of tensile strain. This again featured minimal change in conductivity. Tracer diffusion and secondary ion mass spectrometry (SIMS) studies suggested that the undoped ceria layers featured vacancy-rich regions, close to the interfaces, possibly with compensating Ce III. The final series of multilayers comprised alternating praseodymium nickel copper gallate (Pr1.91 Ni0.71 Cu0.24 Ga0.05 O4) and SDC layers which exhibited a high level of conductivity and evidence of reduced levels of p-type conduction with decreasing SDC layer thickness, suggesting enhanced ionic conductivity. Oxygen tracer studies revealed, however, that the dominant charge carrier was not oxygen. Finally a study of the effect of dislocations in ionic conductors was performed on deformed single crystal YSZ. Impedance measurements revealed a small enhancement in conductivity in the orientation parallel to the dislocation cores however diffusion measurements showed a change that could be negated by the consideration of the inherent errors.

In order to lower SOFC operating temperature between the range of 500 and 800°C, research has been focussed on finding alternative cathode materials, as it is where the highest area specific resistivity in the ceramic cell is exhibited. There is a constant requirement for novel high performance mixed conducting oxides for a variety of applications including cathodes for solid oxide fuel cells and as oxygen separation membranes. The conventional method of approaching this problem is by synthesising oxide compositions obtained from consideration of crystal structure and chemistry. One consequence of this is that synthesis is limited to a few chosen compositions because of the time consuming nature of the synthesis process. A way round this is the use of combinatorial methods to explore much wider ranges of compositions based upon rational choices of starting compositions. An interdisciplinary project was launched to use an ink jet based robot system, LUSI (London University Search Instrument), in order to synthesise combinatorial arrays of ceramic dot samples (2mm dia 500μm high) suitable for measurement of oxygen transport. More information on the robot and the project can be found on the related project web page www.foxd.org. The aim is to use oxygen 18 isotopic exchange with an array together with high resolution SIMS to identify materials with a high degree of exchange with oxygen gas at a given temperature. The first part of this programme was aimed at producing arrays of materials for which the transport properties are well known and to this end we have started with the perovskite materials with the general formula La1-xSrxCo1-yMnyO3, for which we have enough previous information to verify that the experimental protocols are correct. This study was followed by the study of a less known composition space with the general formula La1-xSrxCo1-y-zMnyFezO3. Quality synthesis of automated ink-jet printed samples has been carried out and composition (Induced Couple Plasma Mass Spectrometry), structure determination (X-ray diffraction and Raman Spectrometry) and oxygen content (Thermogravimetric and Catalysis Temperature-programmed Desorption) have been characterised systematically over the full range of compositions prior to performing the isotopic exchange procedure. Isotopic exchange procedures have been written, and tested on one composition. A new diffusion algorithm has been coded to take into account the shape of the combinatorial samples. Design of a new high-throughput technique has been created and this technique has been tested to identify new SOFC cathode materials. A first systematic study has been carried out over the full range of composition of general formula La1-xSrxCo1-y-zMnyFezO3. An automated production of thick-film perovskites has been achieved to a certain extent: composition, phase and oxygen non stoichiometry have been characterised over the full composition of the La0.8Sr0.2Co1-y-zMnyFezO3 pseudo-ternary solid solution. The isotopic exchange technique has been carried out on the La0.8Sr0.2MnO3 and La0.8Sr0.2CoO3 samples.

Oxygen excess lanthanum nickelate, La2NiO4+δ (LNO), is a candidate cathode material for intermediate temperature solid oxide fuel cells (IT-SOFCs). The aim of this work is to investigate the properties of LNO in the intermediate temperature regime (500 – 700°C). The structure and stability of LNO has been studied by in-situ high resolution synchrotron x-ray diffraction and thermal analysis. A bi-phasic orthorhombic room temperature structure was identified, which undergoes a transition to a tetragonal phase. The phase change occurs over the temperature range 250°C to 450°C and is associated with loss of oxygen on heating. LNO undergoes an oxidation reaction, catalysed by platinum, above 800°C where it begins to form the higher order Ruddlesden-Popper phases, La3Ni2O7-δ and La4Ni3O10-δ. The oxygen ion transport properties of LNO have been studied by determining the oxygen tracer diffusion and surface exchange coefficients (D* and k*, respectively). LNO displays high D* and reasonable k* values and exhibits low activation energies for these processes (0.54eV and 0.63eV, respectively). The low activation energy for diffusion is associated with a high oxygen interstitial concentration between 350°C – 550°C. The compatibility of LNO with IT-SOFC electrolytes was investigated using high resolution x-ray synchrotron diffraction techniques. The stability of composites of LNO with Ce0.9Gd0.1O2-δ was found to be highly dependent on oxygen partial pressure and temperature and no reaction phase was observed in composites exposed to atmospheric oxygen. Studying composites in-situ revealed a series of reaction processes that have not previously been identified from ex-situ diffraction techniques. The performance of LNO as a cathode was studied by AC impedance of symmetrical cells with Ce0.9Gd0.1O2-δ and La0.8Sr0.2Ga0.8Mg0.2O3-δ electrolytes. Significant enhancement of the cathode performance was achieved by the addition of a thin compact layer of LNO at the electrode/electrolyte boundary; an area specific resistance (ASR) of 0.5 Ω.cm2 was measured at 800°C in a symmetrical cell with this layered structure. The decrease in ASR is believed to be a result of improved contact at the electrolyte/cathode boundary enhancing the oxygen ion transfer to the electrolyte, and an increase in the cathode surface area for the oxygen reduction reaction to occur.