Abdulkadir Olatunbosun Biffo Biffo

Solid oxide cells are electrochemical energy storage and conversion devices that are strategic for enabling a sustainable and carbon-free economy. The commercialization of these sustainable energy devices is hampered by their high operating temperatures, which reflect the high temperatures required to activate ionic conduction in one of their critical components, namely, the solid oxide electrolyte. Addressing these drawbacks requires operating the solid oxide cells at an intermediate temperature range of 400 – 600 °C, which makes improving ionic conduction in solid oxide electrolyte materials at this temperature range pertinent. This project aims to elucidate the structural features that underpin conduction properties in novel solid oxide electrolyte materials. Knowledge of the structural properties that control the fast dynamics of ions in solid oxide materials will be pivotal not only for designing better electrolyte materials but also for identifying new materials with tunable properties. In this project, we studied three families of oxide ion conductors: the hexagonal perovskite derivatives, lanthanum molybdenum oxide (LAMOX), and the palmierite structure. Using a combination of experimental and computational techniques, we identified the structural features that facilitate high oxide ion conduction in these materials. In addition, we tuned the performance of these materials via tailored cationic substitution to enhance oxide ion conduction and to design better solid oxide electrolytes. Our results are presented in four chapters. In Chapter 2, we investigated Ba7Nb4MoO20-based hexagonal perovskite derivatives, a promising family of oxide ion-conducting solid-oxide electrolyte materials. We studied a new vanadium-substituted derivative, Ba7Nb3.9-xVxMo1.1O20.05, to assess the effect of vanadium substitution on the structural features conducive to high ionic conductivity. Structural changes upon V incorporation were investigated using X-ray and Neutron diffraction, together with 51V and 93Nb solid-state nuclear magnetic resonance spectroscopy. In the undoped composition at room temperature, two distinct oxygen sites (O1 and O5) are observed along the palmierite-like layer, corresponding to a mixture of four- and six-fold coordination for adjacent M2 cations. At high temperature (527 °C), oxygen reorganization results in major occupation of O1 and fourfold (tetrahedral) coordination of the M2 cations. The same rearrangement is observed upon V substitution, but already at room-temperature. From 51V NMR, V5+ cations are identified as tetrahedrally coordinated, indicating their preferential occupation of the M2 site, which is correlated with an increased tetrahedral coordination of Nb5+ cations as observed by 93Nb NMR. Altogether, these observations indicate that V-substitution alters the oxygen sublattice to mimic the high-temperature structure. Additionally, BVSE calculations indicate a lower energy barrier for O2- migration in the presence of vanadium in the structure. This conclusion corroborates the hypothesis that vanadium’s propensity for a lower coordination number is beneficial for promoting high O2- mobility in this promising class of oxide ion conductors. In Chapter 3, we set out to unravel the structural origins of the abrupt two orders of magnitude increase in ionic conductivity around 580 °C in the superionic La2Mo2O9 oxide ion conductor. The atomic-scale structure, conduction mechanism, and pathways in La2Mo2O9 have remained elusive for the past two decades. We uncover the structural origin and the specific oxygen environments involved in the exchange responsible for the superionic transition in this material, and establish a structural model that provides unprecedented insight into the structural disorder of its oxygen sublattice. This understanding, combined with mapping the oxide ion energy landscape, reveals a concerted merry-go-round-type migration mechanism, in which cooperative oxygen motion is driven by repulsive interactions towards stationary oxygen sites and facilitated by an interstitial site that bridges Mo–O polyhedra into a continuous conduction pathway. Our findings redefine the role of oxygen disorder in oxide superionic conductors and delineate design principles for engineering high oxide ion conductivity beyond vacancy-mediated transport. In Chapter 4, we explored the promising palmierite family of ion conductors, aiming to understand how compositional modification affects their structure and ionic conductivity. We investigated the Sr3V2-xMoxO8+x/2 series (0 ≤ x ≤ 0.4), establishing a solubility limit at x = 0.2, and demonstrating that Mo-substituted compositions exhibit bulk O2- conductivity approximately two orders of magnitude higher than the pristine composition at 600°C. Combined X-ray and neutron diffraction refinement shows little change in the average crystallographic structure, prompting investigation of the local structure and dynamics using ab initio molecular dynamics, bond valence site energy calculations, and 51V and 17O solid-state nuclear magnetic resonance supported by density functional theory-nuclear magnetic resonance calculations. We illustrated that Mo6+ incorporation generates a new oxygen environment consistent with the formation of a charge-compensating interstitial O3 site. While V5+ remains tetrahedrally coordinated throughout the series, Mo6+ exhibits both MoO4 and distorted trigonal bipyramidal MoO5 polyhedra, with the latter arising from O3 occupation. This O3 site is proposed to bridge neighboring MO4 tetrahedra along the ab-plane, shifting the conduction mechanism from a 3D cooperative rotational process to an interstitial-based process and shortening the O2- jump distances. The increased conductivity in the Mo-substituted Sr3V2O8 composition is therefore attributed not solely to an increased carrier concentration, but to the mechanistic transition enabled by molybdenum’s propensity for higher coordination polyhedra and longer M-O bonds than vanadium. These findings suggest a design principle for palmierite electrolytes: the incorporation of transition-metal cations with variable coordination polyhedra introduces oxygen interstitials, activating new conduction pathways and thereby enhancing bulk oxide ion conductivity. Finally, in Chapter 5, we investigated the origin of the enhanced oxide ion conductivity in the V-substituted Ba3NbMoO8.5 hexagonal perovskite derivatives by resolving the local oxygen environments and their dynamic exchange, which underpin enhanced bulk conductivity. Ba3NbMoO8.5 is a promising oxide ion conductor exhibiting a hybrid 9R hexagonal perovskite and palmierite structure. Yet, the structural mechanism by which V substitution enhances the bulk oxide ion conductivity is not fully understood. To address this, the isovalent-substituted series Ba3Nb1-xVxMoO8.5 (0 ≤ x ≤ 0.4) was investigated by combined X-ray and neutron diffraction, multinuclear 51V, 93Nb, and 17O solid-state nuclear magnetic resonance (supported by density functional theory calculations), and ab initio molecular dynamics simulations. The incorporation of V5+ into the Ba3NbMoO8.5 structure is observed to affect the palmierite-like layers, such that V5+ exclusively occupies the tetrahedral M1 site and progressively increases the tetrahedral Nb5+ population at this site with x. Across the series, the MO6 population decreases systematically in favor of lower-coordinated MO5 and MO4 polyhedral units. On the local scale, five distinct oxygen environments were resolved across the series, corresponding to oxygen coordinated to V, identical (M–O–M) and mixed (M–O–M’) transition metal sites, where the latter signifies intermixing of cations across the M1 and M2 sites. A low barrier (Ea = 0.69 eV), fast-exchange dynamic is observed in oxygen environments bridged by identical cations, representing the dominant oxide ion conduction pathway. The V-coordinated oxygen environments are essentially immobile, but the incorporation of V reorganizes the Mo/Nb coordination toward lower-coordination environments that constitute this conduction pathway. This study shows that tailored cationic substitution that drives the hybrid 9R hexagonal perovskite and palmierite structure towards the palmierite component governs the enhancement in oxide ion conductivity, offering a design principle for further improving conductivity in this family of oxide ion conductors. Altogether, this work demonstrates that lower-coordination polyhedra and interstitial oxygen sites are recurring structural features driving high oxide ion conductivity across three distinct families of solid oxide electrolytes. In hexagonal perovskite derivatives, vanadium substitution reorganizes the oxygen sublattice and the surrounding Mo/Nb coordination towards lower coordination polyhedra, thereby lowering the energy barrier for oxide ion migration. In La2Mo2O9, the superionic transition is driven by a concerted migration mechanism that relies on an interstitial site bridging Mo–O polyhedra into a continuous conduction pathway. In the palmierite Sr3V2O8 system, molybdenum incorporation creates interstitial oxygen sites, shifting the conduction mechanism from a rotational to an interstitialcy-based process, thereby shortening jump distances and increasing conductivity by two orders of magnitude. Taken together, these findings establish that the design of oxide ion conductors should prioritize conduction mechanisms that exploit interstitial sites and lower-coordination polyhedra over purely vacancy-mediated transport, and that a thorough understanding of structure-property relationships should precede attempts to improve conductivity through compositional tuning. Beyond these fundamental contributions, these insights have direct implications for the commercialization of solid oxide cells. A central barrier to widespread adoption is the need for high operating temperatures, driven largely by the poor ionic conductivity of current electrolyte materials at intermediate temperatures (400 – 600 °C). By identifying the structural features that enable high oxide ion conductivity and demonstrating that targeted cationic substitution can activate conduction pathways through lower-coordination polyhedra and interstitial sites, this work provides concrete design principles for developing electrolyte materials that perform well in the intermediate-temperature range. Lowering the operating temperature would reduce material degradation, broaden the choice of compatible cell components, and lower manufacturing and operational costs, all of which are critical steps toward making solid oxide cells commercially viable for sustainable energy storage and conversion.