Jennifer L. M. Rupp

Solid oxide fuel cells: Systems and materials

A solid oxide fuel cell (SOFC) is a solid-state energy conversion system that converts chemical energy into electrical energy and heat at elevated temperatures. Its bipolar cells are electrochemical devices with an anode,electrolyte, and cathode that can be arranged in a planar or tubular design with separated gas chambers for fuel and oxidant. Single chamber setups have bipolar cells with reaction selective electrodes and no separation between anode and cathode compartments. A nickel/yttria-stabilized-zirconia (YSZ) cermet is the most investigated and currently most widespread anode material for the use with hydrogen as fuel. In recent years, however, doped ceria cermet anodes with nickel or copper and ceria as the ceramic phase have been introduced together with ceria as electrolyte material for the use with hydrocarbon fuels. The state-of-the-art electrolyte material is YSZ of high ionic and nearly no electronic conductivity at temperatures between 800-1000 °C. In order to reduce SOFC system costs, a reduction of operation temperatures to 600-800 °C is desirable and electrolytes with higher ionic conductivities than YSZ are aimed for such as bismuth oxide, lanthanum gallate or mixed conducting ceria and the use of thin electrolytes. Proton conducting perovskites are researched as alternatives to conventional oxygen conducting electrolyte materials. At the cathode, the reduction of molecular oxygen takes place predominantly on the surface. Todays state-of-the-art cathodes are La x Sr 1 - x MnO 3 - δ for SOFC operating at high temperature i.e. 800-1000 °C, or mixed conducting La x Sr 1 - x Co y Fe 1 - y O 3 - δ for intermediate temperature operation, i.e. 600-800 °C. Among the variety of alternative materials, Sm x Sr 1 - x CoO 3 - δ and Ba x Sr 1 - x Co x Fe 1 - x O 3 - δ are perovskites that show very good oxygen reduction properties. This paper reviews the materials that are used in solid oxide fuel cells and their properties as well as novel materials that are potentially applied in the near future. The possible designs of single bipolar cells are also reviewed.

A Microdot Multilayer Oxide Device: Let Us Tune the Strain-Ionic Transport Interaction

In this paper, we present a strategy to use interfacial strain in multilayer heterostructures to tune their resistive response and ionic transport as active component in an oxide-based multilayer microdot device on chip. For this, fabrication of strained multilayer microdot devices with sideways attached electrodes is reported with the material system Gd0.1Ce0.9O(2-δ)/Er2O3. The fast ionic conducting Gd0.1Ce0.9O(2-δ) single layers are altered in lattice strain by the electrically insulating erbia phases of a microdot. The strain activated volume of the Gd0.1Ce0.9O(2-δ) is investigated by changing the number of individual layers from 1 to 60 while keeping the microdot at a constant thickness; i.e., the proportion of strained volume was systematically varied. Electrical measurements showed that the activation energy of the devices could be altered by Δ0.31 eV by changing the compressive strain of a microdot ceria-based phase by more than 1.16%. The electrical conductivity data is analyzed and interpreted with a strain volume model and defect thermodynamics. Additionally, an equivalent circuit model is presented for sideways contacted multilayer microdots. We give a proof-of-concept for microdot contacting to capture real strain-ionic transport effects and reveal that for classic top-electrode contacting the effect is nil, highlighting the need for sideways electric contacting on a nanoscopic scale. The near order ionic transport interaction is supported by Raman spectroscopy measurements. These were conducted and analyzed together with fully relaxed single thin film samples. Strain states are described relative to the strain activated volumes of Gd0.1Ce0.9O(2-δ) in the microdot multilayer. These findings reveal that strain engineering in microfabricated devices allows altering the ionic conduction over a wide range beyond classic doping strategies for single films. The reported fabrication route and concept of strained multilayer microdots is a promising path for applying strained multilayer oxides as active new building blocks relevant for a broad range of microelectrochemical devices, e.g., resistive switching memory prototypes, resistive or electrochemical sensors, or as active catalytic solid state surface components for microfuel cells or all-solid-state batteries.

Perovskite La0.6Sr0.4Cr1−xCoxO3−δ solid solutions for solar-thermochemical fuel production: strategies to lower the operation temperature

Storing abundant solar energy in synthetic fuels is key to ensure a sustainable energy future by replacing fossil fuels and reducing global warming emissions. Practical implementation of the solar-to-fuel technology is predicated on finding new materials with higher efficiency and lower operation temperature than state-of-the-art materials. We use criteria aimed for designing such efficient solar-to-fuel conversion materials in the perovskite system. Based on thermodynamic considerations, the first perovskite solute–solution series, La0.6Sr0.4Cr1−xCoxO3−δ, is investigated to gain fundamental understanding on the role of B-site cationic doping in water and CO2 splitting to produce synthetic fuel. Notably, all of the novel material compositions operate in a strongly lowered temperature regime of 800–1200 °C compared to state-of-the-art binary oxides in the field. We found an optimum in doping for fuel production performance, namely La0.6Sr0.4Cr0.8Co0.2O3−δ, which viably splits both CO2 and H2O. Based on thermogravimetric analysis, we show that the highest performing perovskite splits 25 times more CO2 compared to the current state-of-the-art material, ceria, for two-step thermochemical cycling at 800–1200 °C. No adverse formation of carbonates in a CO2 atmosphere or cation segregation was observed in near and long range structural investigations, which highlight the durability and potential of these solid solutions. These new perovskite compositions enable lowering of the standard solar-to-fuel reactor temperature by 300 °C. The lowered operating temperature has tremendous implications for solar-synthesized fuels in a reactor in terms of lowered heat loss, increased efficiency, and reactor materials.

A shortcut to garnet-type fast Li-ion conductors for all-solid state batteries

Ga-doped Li7La3Zr2O12 garnet structures are among the most promising electrolytes for all-solid state Li-ion-batteries. The synthesis and processing of garnet-type fast Li-ion conductors depend on conventional sol–gel and solid state syntheses and sintering that are usually done at temperatures above 1050 °C to reach the high Li-ion conducting cubic phase. This process results in micron-sized particles and potential Li-loss, which are unfavorable for further processing and electrode–electrolyte assembly. Here, we tackle this problem and report a novel low temperature synthesis-processing route to stabilize the cubic phase of Li7La3Zr2O12, while keeping the nanocrystallites at ∼200–300 nm. Li7La3Zr2O12 phases are obtained at temperatures as low as 600 °C by a modified sol–gel combustion method utilizing mainly nitrate precursors, and the sintering temperature is lowered by ∼200 °C compared to the state-of-art. Through a new model experiment, we also shed light on the conditions influencing the tetragonal to cubic phase transformation via homogeneous Ga-diffusion and incorporation occurring at a surprisingly low temperature of ∼100 °C for a post-annealing step. The sintered pellets of the newly obtained Li6.4Ga0.2La3Zr2O12 deliver high bulk Li-ion conductivities in the range of ∼4.0 × 10−4 S cm−1 at 20 °C, and a wide thermal operation window is accessible through its characteristic activation energy of ∼0.32 eV. We report that there is an optimum in sintering-processing conditions for the cubic c-Li6.4Ga0.2La3Zr2O12 solid state electrolytes and their Li-ionic conductivity and the (Raman) near order characteristics that can be tracked through changes in Li–O vibrational modes. Based on this alternative route, low-temperature synthesized powders can be sintered to relatively dense pellets at around only 950 °C. At higher sintering temperatures (e.g. 1100 °C), Li-losses progress as confirmed by structural studies and a reduction of both ceramic pellet density and ionic conductivity, as well as distortions in the Li-sublattice, are found. Through this work, an alternative low temperature processing route for Ga-doped Li7La3Zr2O12 garnet type electrolytes for all-solid state batteries is suggested. The new synthesis method and the use of c-Li6.4Ga0.2La3Zr2O12 nanoparticles could open pathways in terms of preventing Li-loss during the process and advancing future solid electrolyte–electrode assembly options for all-solid state Li-ion batteries.

Kinetics of CO2 Reduction over Nonstoichiometric Ceria

The kinetics of CO2 reduction over nonstoichimetric ceria, CeO2−δ, a material of high potential for thermochemical conversion of sunlight to fuel, has been investigated for a wide range of nonstoichiometries (0.02 ≤ δ ≤ 0.25), temperatures (693 ≤ T ≤ 1273 K), and CO2 concentrations (0.005 ≤ pCO2 ≤ 0.4 atm). Samples were reduced thermally at 1773 K to probe low nonstoichiometries (δ < 0.05) and chemically at lower temperatures in a H2 atmosphere to prevent particle sintering and probe the effect of higher nonstoichiometries (δ < 0.25). For extents greater than δ = 0.2, oxidation rates at a given nonstoichiometry are hindered for the duration of the reaction, presumably because of near-order changes, such as lattice compression, as confirmed via Raman Spectroscopy. Importantly, this behavior is reversible and oxidation rates are not affected at lower δ. Following thermal reduction at very low δ, however, oxidation rates are an order of magnitude slower than those of chemically reduced samples, and rates monotonically increase with the initial nonstoichiometry (up to δ = 0.05). This dependence may be attributed to the formation of stable defect complexes formed between oxygen vacancies and polarons. When the same experiments are performed with 10 mol % Gd3+ doped ceria, in which defect complexes are less prevalent than in pure ceria, this dependence is not observed.

Engineering disorder in precipitation-based nano-scaled metal oxide thin films

Distinctive microstructure engineering of amorphous to nanocrystalline functional metal oxide thin films for MEMS devices is of high relevance to allow for new applications, quicker response times, and higher efficiencies. Precipitation-based thin-film techniques are first choice. However, these often involve organic solvents in preparation. Their relevance on the disorder states of amorphous to fully crystalline metal oxides is unclear, especially during crystallization. In this study the impact of organic solvents on the as-deposited amorphous state and crystallization of the metal oxide, CeO(2), is reported for thin-film preparation via the precipitation-based method spray pyrolysis. The choice of organic solvent for film preparation, i.e. glycols of different chain lengths, clearly affects the structural packing and Raman bond length of amorphous states. Organic residues act as space fillers between the metal oxide molecules in amorphous films and affect strongly the thermal evolvement of microstructure, i.e. microstrain, crystallization enthalpy, crystallographic density, grain size during crystallization and grain growth. Once the material is fully crystalline, equal near- and long-range order characteristics result independent of organic solvent choice. However, the fully crystalline films still show decreased crystallographic densities, presence of microstrain, and lower Raman shifts compared to microcrystalline bulk material. The defect state of amorphous and fully crystalline thin-film microstructures can actively be modified via explicit use of organic glycols with different chain lengths for metal oxide films in MEMS.

Uncovering Two Competing Switching Mechanisms for Epitaxial and Ultrathin Strontium Titanate-Based Resistive Switching Bits

Resistive switches based on anionic electronic conducting oxides are promising devices to replace transistor-based memories due to their excellent scalability and low power consumption. In this study, we create a model switching system by manufacturing resistive switches based on ultrathin 5 nm, epitaxial, and grain boundary-free strontium titanate thin films with subnanometer surface roughness. For our model devices, we unveil two competing nonvolatile resistive switching processes being of different polarities: one switching in clockwise and the other in counterclockwise direction. They can be activated selectively with respect to the effective switching voltage and time applied to the device. Combined analysis of both processes with electrical DC-methods and electrochemical impedance spectroscopy reveals that the first resistive switching process is filament-based and exhibits counterclockwise bipolar resistive switching. The R(OFF)/R(ON) resistance ratio of this process is extremely stable and can be tuned in the range 5-25 depending on the switching voltage and time. Excitingly, at high electric field strength a second bipolar resistive switching process was found. This process is clockwise and, therefore, reveals the opposite polarity switching direction when compared to the first one. Both processes do not obstruct each other, consequently, stable 1, 2, or even 3 crossover current-voltage (I-V) characteristics can be addressed for the memory bits. Equivalent circuit model analysis and fitting of impedance characteristics unequivocally show for the created grain boundary free switches that the oxides defects and its carrier distribution close to the electrode interface contribute to the resistive switching mechanism. The addressability of two sets of resistive ON and OFF states in one device through electric field strength and switching time offers exciting new operation schemes for memory devices.

Electrode tailoring improves the intermediate temperature performance of solid oxide fuel cells based on a Y and Pr co-doped barium zirconate proton conducting electrolyte

A solid oxide fuel cell was developed with BaZr0.7Pr0.1Y0.2O3-δ, a proton-conducting electrolyte showing high conductivity, and good sinterability and chemical stability. A maximum power density of 163 mW cm−1 at 600 °C was achieved combining BZPY10 with tailored electrodes; a composite anode promoting the electrolyte densification, and a composite cathode made of two mixed conductors.

Perovskite oxides – a review on a versatile material class for solar-to-fuel conversion processes

Thermochemical water and carbon dioxide splitting with concentrated solar energy is a technology for converting renewable solar energy into liquid hydrocarbon fuels as an alternative to fossil fuels, which are dominating in todays energy mix. For the conversion reaction to be efficient, special redox materials are necessary to perform the necessary chemical reactions in a thermochemical cycle. Through this review we carefully examine perovskite oxides to design and optimize next generation solar-to-fuel conversion materials operating on thermochemical cycles. To date efforts have primarily been directed to binary oxides among which most prominently ceria was selected. Despite the promise, ceria has an unfavorable high reduction temperature and is restricted in its opportunities to manipulate through extrinsic doping the oxygen nonstoichiometry and thermodynamic properties for oxygen exchange towards H2O and CO2 splitting. In contrast, recent reports highlight new opportunities to use and alter perovskite oxides in terms of elemental composition over a wider range to affect reduction temperature, oxygen exchange characteristics needed in the catalytic reactions and fuel yield. To further foster perovskites for solar-to-fuel conversion, we review basic concepts such as the lattice structure and defect thermodynamics towards CO2 and water splitting, discuss the role of oxygen vacancies and present strategies for an efficient search for new perovskite compositions. Summarizing, recent efforts on perovskite oxide compositions investigated are based on Fe, Mn, Co, or Cr with reported fuel yields of up to several hundred μmol per g per cycle in the literature. This article reviews the underlying principles, the latest advances and future prospects of perovskite oxides for solar-to-fuel technology.

Liquid-phase deposition of ferroelectrically switchable nanoparticle-based BaTiO3 films of macroscopically controlled thickness

BaTiO3 films are extensively used in many electrical devices, because they offer remarkable dielectric and ferroelectric properties. Here, we demonstrate a powerful, nanoparticle-based deposition route towards BaTiO3 films with systematic thickness control over a wide range up to several microns. The unusual control over the film thickness with the maintenance of crack free nanostructures, phase and ferroelectric properties of the BaTiO3 films allows us to fabricate various future devices of different thicknesses by a single deposition method. For this, films are deposited from stable dispersions of BaTiO3 nanocrystals, synthesized via an efficient microwave-assisted non-aqueous sol–gel approach. Crack-free films of controlled thickness are obtained by a carefully elaborated, alternating process of spin-coating and intermediate drying. According to X-ray diffraction and confocal Raman microscopy, the final, sintered films consist of BaTiO3 nanocrystals of about 20 nm in a hexagonal–tetragonal phase mixture. The nanoparticulate films display outstanding optical characteristics exceeding 90% transparency above 500 nm and a band gap of 3.5 eV. The latter, band gap, is larger than the classic bulk materials band gap of 3.2 eV, indicating a more electrically insulating nature of the films. Piezoresponse force microscopy gives evidence for potent ferroelectric switching. This newly accessible film processing route with wide film thickness tuning allows for desired ferroelectric response with the advantage of a wide film thickness to implicate building blocks for various applications e.g. ferroelectric random access memory devices, microelectromechanical system devices or Bragg reflectors.