Exploring backbone-cation alkyl spacers for multi-cation side chain anion exchange membranes
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Zhu, Liang (ORCID:0000000345510963)↗
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Results for “multi-cation”
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Broadband optical phonon scattering reduces the thermal conductivity of multi-cation oxides
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Riffe, William T. (ORCID:0009000348945724)↗
Multi-cation perovskites prevent carrier reflection from grain surfaces
The composition of perovskite has been optimized combinatorially such that it often contains six components (A x B y C 1-x-y PbX z Y 3-z ) in state-of-art perovskite solar cells. Questions remain regarding the precise role of each component, and the lack of a mechanistic explanation limits the practical exploration of the large and growing chemical space. Here, aided by transient photoluminescence microscopy, we find that, in perovskite single crystals, carrier diffusivity is in fact independent of composition. In polycrystalline thin films, the different compositions play a crucial role in carrier diffusion. In this work, we report that methylammonium (MA)-based films show a high carrier diffusivity of 0.047 cm 2 s -1 , while MA-free mixed caesium-formamidinium (CsFA) films exhibit an order of magnitude lower diffusivity. Elemental composition studies show that CsFA grains display a graded composition. This curtails electron diffusion in these films, as seen in both vertical carrier transport and surface potential studies. Incorporation of MA leads to a uniform grain core-to-edge composition, giving rise to a diffusivity of 0.034 cm 2 s -1 in CsMAFA films. A model that invokes competing crystallization processes allows us to account for this finding, and suggests further strategies to achieve homogeneous crystallization for the benefit of perovskite optoelectronics.
36 MATERIALS SCIENCE↗
A First–Principles–Based Sub–Lattice Formalism for Predicting Off–Stoichiometry in Materials for Solar Thermochemical Applications: The Example of Ceria
Theoretical models that reliably can predict off-stoichiometry in materials via accurate descriptions of underlying thermodynamics are crucial for energy applications. For example, transition-metal and rare-earth oxides that can tolerate a large number of oxygen vacancies, such as CeO 2 and doped CeO 2 , can split water and carbon dioxide via a two-step, oxide-based solar thermochemical (STC) cycle. The search for new STC materials with a performance superior to that of state-of-the-art CeO 2 can benefit from predictions accurately describing the thermodynamics of oxygen vacancies. The sub-lattice formalism, a common tool used to fit experimental data and build temperature-composition phase diagrams, can be useful in this context. Here, sub-lattice models are derived solely from zero-temperature quantum mechanics calculations to estimate fairly accurate temperature- and oxygen-partial-pressure-dependent off-stoichiometries in CeO 2 and Zr-doped CeO 2 . Physical motivations for deriving some of the “excess” sub-lattice model parameters directly from quantum mechanical calculations, instead of fitting to minimize deviations from experimental and/or theoretical data, are identified. As a result, important limitations and approximations of the approach used are specified and extensions to multi-cation oxides are also suggested to help identify novel candidates for water and carbon dioxide splitting and related applications.
08 HYDROGEN↗
Combinatorial Discovery of Lanthanum–Tantalum Oxynitride Solar Light Absorbers with Dilute Nitrogen for Solar Fuel Applications
© 2017 American Chemical Society. Oxynitrides with the photoelectrochemical stability of oxides and desirable band energetics of nitrides comprise a promising class of materials for solar photochemistry. Challenges in synthesizing a wide variety of oxynitride materials has limited exploration of this class of functional materials, which we address using a reactive cosputtering combined with rapid thermal processing method to synthesize multi-cation-multi-anion libraries. We demonstrate the synthesis of a LaxTa1-xOyNzthin film composition spread library and its characterization by both traditional thin film materials characterization and custom combinatorial optical spectroscopy and X-ray absorption near edge spectroscopy (XANES) techniques, ultimately establishing structure-chemistry-property relationships. We observe that over a substantial La-Ta composition range the thin films crystallize in the same perovskite LaTaON2structure with significant variation of anion chemistry. The relative invariance in optical band gap demonstrates a remarkable decoupling of composition and band energetics so that the composition can be optimized while retaining the desirable 2 eV band gap energy. We also demonstrate the intercalation of diatomic nitrogen into the La3TaO7structure, which gives rise to a direct-allowed optical transition at 2.2 eV, less than half the value of the oxide's band gap. These findings motivate further exploration of the visible light response of this material that is predicted to be stable over a wide range of electrochemical potential.
Suram, Santosh K.↗
Calcium-based multi-element chemistry for grid-scale electrochemical energy storage
Calcium is an attractive material for the negative electrode in a rechargeable battery due to its low electronegativity (high cell voltage), double valence, earth abundance and low cost; however, the use of calcium has historically eluded researchers due to its high melting temperature, high reactivity and unfavorably high solubility in molten salts. Here we demonstrate a long-cycle-life calcium-metal-based rechargeable battery for grid-scale energy storage. By deploying a multi-cation binary electrolyte in concert with an alloyed negative electrode, calcium solubility in the electrolyte is suppressed and operating temperature is reduced. These chemical mitigation strategies also engage another element in energy storage reactions resulting in a multi-element battery. These initial results demonstrate how the synergistic effects of deploying multiple chemical mitigation strategies coupled with the relaxation of the requirement of a single itinerant ion can unlock calcium-based chemistries and produce a battery with enhanced performance.
25 ENERGY STORAGE↗
Interlaced crystals having a perfect Bravais lattice and complex chemical order revealed by real-space crystallography
The search for optimal thermoelectric materials aims for structures in which the crystalline order is disrupted to lower the thermal conductivity without degradation of the electron conductivity. Here we report the synthesis and characterization of ternary nanoparticles (two cations and one anion) that exhibit a new form of crystal-line order: an uninterrupted, perfect, global Bravais lattice, in which the two cations exhibit a wide array of distinct ordering patterns within the cation sublattice, form-ing interlaced domains and phases. Partitioning into domains and phases is not unique; the corresponding boundaries have no structural defects or strain and entail no energy cost. We call this form of crystalline order “interlaced crystals” and present the example of hexagonal-CuInS 2 . Interlacing is possible in multi-cation tetrahedral-ly-bonded compound with an average of two electrons per bond. Interlacing has min-imal effect on electronic properties, but should strongly reduce phonon transport, making interlaced crystals attractive for thermoelectric applications.
36 MATERIALS SCIENCE↗
Selective reduction in epitaxial SrFe 0.5 Co 0.5 O 2.5 and its reversibility
Oxygen-vacancy engineering in transition metal oxides enables programmable functionalities by modulating the valence states and local coordination of constituents. Here, we report the selective reduction of cobalt ions in epitaxial SrFe 0.5 Co 0.5 O 2.5 thin films under reducing gas environments, while iron ions remain unchanged. X-ray absorption spectroscopy reveals an absorption edge shift of 1.65 eV in the Co L-edge upon reduction, and multiplet simulations estimate a decrease in the average Co valence from Co 2.91+ to Co 2.00+ . This site- and element-specific reduction leads to the formation of a structurally distinct oxygen-deficient phase stabilized by oxygen vacancies at tetrahedral sites, as confirmed by density functional theory. Optical spectroscopy reveals an increase in the bandgap from 2.47 eV to 3.04 eV, accompanied by enhanced transparency. Furthermore, simultaneous in situ diffraction and transport measurements confirm fully reversible redox-driven transitions among three phases: reduced defective perovskite, brownmillerite, and oxygen-rich perovskite phases. These findings demonstrate that selective redox control in multi-cation oxides enables the realization of chemically and functionally distinct oxygen-deficient phases.
Fuel cells↗
Synthesis and structure of high-purity BaCe 0.25 Mn 0.75 O 3 : an improved material for thermochemical water splitting
Solar thermochemical hydrogen production (STCH) via redox-active metal oxides is an approach for direct solar-driven hydrogen generation typically using a high-temperature redox cycle involving refractory oxides and steam. Typical cycles involve high-temperature reduction of oxides to form oxygen vacancies, followed by lower temperature reaction between oxygen vacancies and steam where the oxide is re-oxidized and the steam is reduced to hydrogen. Only a few materials have demonstrated reversible cycling under the typically harsh STCH conditions (e.g. 1500°C reduction, 900°C re-oxidation) and critical questions remain on the true reversibility of non-stoichiometric multi-cation oxide systems, significantly hampered by the lack of single-phase samples for these material systems. To date, most STCH processes have relied on CeO 2 as a benchmark active material, but more recently, the 12R phase of BaCe 0.25 Mn 0.75 O 3 (BCM) has demonstrated greater hydrogen-generation potential at lower peak temperatures. However, previous reports of 12R-BCM have included large fractions, > 10 wt%, of secondary phases, which complicate analysis of the stability and performance. A comprehensive understanding of the redox mechanism and reversibility of the process in BCM can only be achieved with nearly single-phase samples which, to date, have been difficult to produce. Here two approaches to BCM synthesis are reported: solid state and sol–gel-based routes. It is demonstrated that both routes can be tuned to produce the 12R structure with > 97 wt% yield when annealed ≥1450°C. Herein synchrotron-based diffraction measurements of rhombohedral 12R-BCM enabled characterization of the anisotropy between thermal expansion along the c-axis and within the ab plane. The impact of high-temperature redox cycling on the stability and phase fraction of the 12R-BCM polytype was also investigated. Our results offer two viable routes for synthesis of high-purity 12R-BCM critically needed for evaluating the efficacy of BCM as a STCH material and validate its ability to split water at lower temperatures over extended numbers of redox cycles.
08 HYDROGEN↗
Investigating the Effect of Lamination on FAMACs: Toward a New Phase Space of Perovskite Solar Cell Fabrication
The rapid progress in the field of perovskite solar cells has led to efficiencies approaching that of crystalline silicon in single junction devices, and increasing emphasis is now being placed on scalable fabrication strategies that have the potential to truly impact the photovoltaics industry. The standard fabrication route of sequential, layer-bylayer depositions to form the device stack is limited by the small range of selective materials that can withstand successive solvent exposures and thermal annealings. In order to overcome this limitation, we have developed a method to fabricate the device stacks through mechanical lamination under moderate pressures and temperatures. The procedure involves fabricating two transparent conductive oxide/transport material/perovskite half stacks and laminating them together at the perovskite/perovskite interface. This procedure previously achieved 10% efficiency in initial attempts based on methyl ammonium lead iodide (MAPI) perovskite compositions. Herein, we reported the fabrication conditions to achieve a reproducible lamination with a complex, multi-cation perovskite, [Cs0.05(MA0.17F A0.83)0.95P b(I0.83Br0.17)3] (FAMACs), laminated at 18 degrees C and with a pressure of 350 psi for 30 minutes. The resulting material showed a sharpening of the optical absorption edge and a slight reduction in the band gap from 1.622 eV to 1.616 eV, indicative of improved ordering and reduced defects. Concurrently, time resolved photoluminescence measurements revealed an increased luminescence lifetime from 85 ns to 123 ns.
14 SOLAR ENERGY↗
Enabling Efficient Water Splitting with Advanced Materials Designed for High pH Membrane Interface
This research focuses on durable, high-performance materials and interfaces for advanced water splitting, enabling a clear pathway for achieving <$2/KgH2 (on scale) via anion exchange membrane (AEM)-based electrolysis. We aim to advance this goal via an improved fundamental understanding of both hydrogen and oxygen evolution reactions (HER/OER) leading to platinum group metal (PGM)-free catalyst materials. Here, we use NiFeCo and NiMo as OER and HER catalysts, respectively, in a full alkaline electrolysis cell. A thermally stable, multi-cation AEM is used to operate the cell at elevated temperatures thereby lowering the operating potential by increasing the kinetics of the respective catalytic reactions. The transition metal catalysts, paired with this novel AEM, have been used to achieve an operating potential of 1.79 V at 1 A/cm2. This potential is 300 mV lower than similar electrolysis cells equipped with PGM-based catalysts. We have also demonstrated the efficacy of the less caustic potassium carbonate solution as a replacement for potassium hydroxide as an electrolyte. By using a carbonate-based electrolyte, the hydroxide ion required for the anodic reaction is continuously replenished by re-establishing an equilibrium with water. This consideration, coupled with the reduced alkalinity of the carbonate solution, yields an electrolysis cell capable of sustained operation with minimal increase in operating potential.
Kendrick, Ian↗
Durable, Impermeable Brazes for Solid Oxide Fuel Cells (Final Report)
To develop a new braze to replace the current state-of-the-art Ag-CuO reactive air braze (RAB), two approaches were proposed: 1) development of a new, self-passivating, silver-free braze and 2) development of a CuO-free, silver-based braze. For the first approach, an integrated computational-experimental method was used. Thermo-Calc® was utilized to quickly screen through hundreds of alloy systems to identify candidate braze compositions with the appropriate melting ranges. Material compositions, melting ranges, mechanical properties, oxidation resistance, and wetting characteristics of the candidate alloys were then analyzed experimentally. Substrate surface pre-treatments, active element additions, and novel brazing schemes were also investigated. Unfortunately, the current study failed to identify new silver-free braze compositions suitable for SOFC applications. To help measure the passivation characteristics of surface passivating braze oxide coatings, work was conducted to demonstrate, for the first time, that a variety of disparate and technologically-relevant thermal, mechanical, and electrochemical oxygen-exchange thin film material properties could all be obtained from in situ , current-collector-free wafer curvature measurements. Specifically, temperature or oxygen partial pressure induced changes in the curvature of 230 nm thick (100)-oriented Pr 0.1 Ce 0.9 O 1.95_x (10PCO) films atop 200 mm thick single crystal yttria stabilized zirconia or magnesium oxide substrates were used to measure the biaxial modulus, Young’s modulus, thermal expansion coefficient, thermochemical expansion coefficient, oxygen nonstoichiometry, chemical oxygen surface exchange coefficient, oxygen surface exchange resistance, thermal stress, chemical stress, thermal strain, and chemical strain of the model mixed ionic electronic conducting material 10PCO. The (100)-oriented thin film 10PCO thermal expansion coefficient, thermo-chemical expansion coefficient, oxygen nonstoichiometry, and Young’s modulus (which is essentially constant, at ~200 MPa, over the entire 280–700 °C temperature range in air) measured here were similar to those from other bulk and thin film 10PCO studies. In addition, the measured PCO10 oxygen surface coefficients were in agreement with those reported by other in situ , current collector- free techniques. However, the measured PCO10 oxygen surface exchange coefficients were significantly lower than those obtained from literature studies with large amounts of intentional or inadvertent precious metal surface coverage/contamination (suggesting that uncontaminated (100)-oriented 10PCO may not be a desirable SOFC cathode material). Taken together, this highlighted the advantages of using a sample’s mechanical response, instead of the more traditional electrical response, to probe the electrochemical properties of the ion-exchange materials used in solid oxide fuel cell, solid oxide electrolysis cell, gas-sensing, battery, emission control, water splitting, water purification, and other electrochemically-active devices. For the second approach, a novel silver-nickel brazing method was developed. It was demonstrated that transient porous nickel interlayers, instead of reactive element additions, could be used to promote Ag wetting on yttria stabilized zirconia (YSZ) and produce high-quality YSZ stainless steel (SS) braze joints. Mechanical tests on these reactive-element-free, silver-based SOFC braze joints, both before and after 500 hours of 750°C oxidation in air, showed that the braze and braze interface strength were higher than the underlying YSZ|NiO-YSZ substrate. The microstructural and compositional evolution of SS|Ag-Ni|YSZ braze joints exposed to dual atmosphere (air on one side and 4%H 2 -96%N 2 on the other side) for 300 hours of isothermal 750 °C aging and 300 25°C/min 35-830 °C rapid thermal cycles were compared to a Ag-3CuO braze joint. In contrast to conventional Ag-CuO RAB brazes, the Ag-Ni brazes remained pore free and well bonded, suggesting that Ag-Ni brazes may be more suitable for long term SOFC operation than Ag-CuO brazes. Other applications inspired by the silver-nickel wetting process on various ceramic substrates were also explored. In support of efforts to identify new CuO-free Ag-based brazes, density functional theory and ab initio molecular dynamics calculations were performed to 1) understand silver wetting on YSZ, and 2) identify new oxides to promote silver wetting on YSZ. These simulations found that while the formation of dissolved oxygen clusters within molten silver at the YSZ interface promoted wetting, much greater wetting angle reductions came from the formation of metal oxide interlayers at the Ag-YSZ interface. Through the development of a simple descriptor, many simple metal oxides (single cation) were examined. Unfortunately, their ability to promote Ag wetting were less than that of CuO. However, expanding the search to multi-cation oxides led to several promising candidates, such as CuAlO 2 , CuGaO 2 , and Cu 3 TiO 4 ; all of which are also stable in the reducing SOFC conditions. Depending upon their solubility in molten Ag, these newly-identified oxides could either be pre-applied as wetting promoting interlayers or directly incorporated into Ag to form new reactive air brazes.
36 MATERIALS SCIENCE↗
Complex Amorphous Dielectrics
This work focused on synthesizing a wide range of oxides containing two or more metals, and measuring their properties. Many simple metal oxides such as zirconium oxide, have been extensively studied in the past. We developed a technique in which we create a large number of compositions simultaneously and examine their behavior to understand trends and identify high performance materials. Superior performance generally comes in the form of increased responsiveness; in the materials we have studied this may mean more electrical charge for a given voltage in a capacitor, faster switching for a given drive in a transistor, more current for a given voltage in an ionic conductor, or more current for a given illumination in a solar cell. Some of the materials we have identified may find use in decreasing the power needed to operate integrated circuits, other materials could be useful for solar power or other forms of energy conversion.
32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗