Advanced anodes for high-temperature fuel cells

Segregation of Nickel/Iron Bimetallic Particles from Lanthanum Doped Strontium Titanates to Improve Sulfur Stability of Solid Oxide Fuel Cell Anodes

Perovskite derived Ni catalysts offer the remarkable benefit of regeneration after catalyst poisoning or Ni particle growth through the reversible segregation of Ni from the perovskite-type oxide host. Although this property allows for repeated catalyst regeneration, improving Ni catalyst stability towards sulfur poisoning by H2S is highly critical in solid oxide fuel cells. In this work Mn, Mo, Cr and Fe were combined with Ni at the B-site of La0.3Sr0.55TiO3±δ to explore possible benefits of segregation of two transition metals towards sulfur tolerance. Catalytic activity tests towards the water gas shift reaction were carried out to evaluate the effect of the additional metal on the catalytic activity and sulfur stability of the Ni catalyst. The addition of Fe to the Ni perovskite catalyst was found to increase sulfur tolerance. The simultaneous segregation of Fe and Ni from La0.3Sr0.55Ti0.95-xNi0.05FexO3±δ (x ≤ 0.05) was investigated by temperature programmed reduction, X-ray diffraction and X-ray absorption spectroscopy and catalytic tests after multiple redox cycles. It is shown that catalytic properties of the active phase were affected likely by the segregation of Ni/Fe alloy particles and that the reversible segregation of Ni persisted, while it was limited in the case of Fe under the same conditions.

Atomic Scale Origin of Enhanced Ionic Conductivity at Crystal Defects

In oxide materials, the presence of dislocations often strongly affects the properties of thin film and multilayer devices. Although it was reported that ionic conduction can be enhanced by introducing dislocations in ionic conductors, the underlying mechanism of such enhancement remains unclear. Here we analyzed the ionic conduction enhancement in an yttria-stabilized zirconia (YSZ) single edge dislocation from a structural point of view, using atomic resolution scanning transmission electron microscopy (STEM). First, the atomic structure and chemistry of a dislocation in YSZ were characterized by STEM and energy dispersive X-ray spectroscopy (EDS). A relative ionic conduction variation map around the dislocation was then estimated based on the well-established strain-conductivity and chemistry-conductivity relationships in YSZ. We propose that a faster ionic conductivity path can be formed around the dislocation core due to the coupling of the tensile strain field and dopant segregation, which could account for enhanced ionic conductivity along dislocations.

An ultra-flexible modular high vacuum setup for thin film deposition

A modular high vacuum chamber dedicated to thin film deposition is presented. We detail the vacuum and gas infrastructure required to operate two highly flexible chambers simultaneously, with a focus on evaporation techniques (thermal and electron beam) and magnetron sputtering, including baking equipment to remove residual water from the chamber. The use of O-ring-sealed flat flanges allows a tool-free assembly process, in turn enabling rapid changes of the whole setup. This leads to a high flexibility regarding the deposition techniques as the chamber can be adapted to different sources within minutes, permitting the formation of multilayer systems by consecutive depositions onto the same substrate. The central piece of the chamber is a flat flange ground glass tube or cross. The glass recipient permits optical monitoring of the deposition process. Further equipment, such as for the introduction of gases, additional pressure gauges, or evaporators, can be incorporated via specifically designed stainless steel/aluminum interconnectors and blank flanges. In the end, we demonstrate the preparation of an unsupported thin film system consisting of electron-beam-evaporated platinum nanoparticles embedded in magnetron-sputtered zirconia (ZrO₂), deposited onto NaCl single crystals, which subsequently can be removed by dissolution. These films are further analyzed by means of transmission electron microscopy, X-ray photoelectron spectroscopy, and atomic force microscopy.

Failure Modes, Mechanisms, Effects, and Criticality Analysis of Ceramic Anodes of Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) are a highly efficient chemical to electrical energy conversion devices that have potential in a global energy strategy. The wide adoption of SOFCs is currently limited by cost and concerns about cell durability. Improved understanding of their degradation modes and mechanisms combined with reduction–oxidation stable anodes via all-ceramic-anode cell technology are expected to lead to durability improvements, while economies of scale for production will mitigate cost of commercialization. This paper presents an Ishikawa analysis and a failure modes, mechanisms, effects, and criticality analysis (FMMECA) for all-ceramic anode based SOFCs. FMMECA takes into account the life cycle conditions, multiple failure mechanisms, and their potential effects on fuel-cell health and safety.

Mechanical, thermal, and electrochemical properties of Pr doped ceria from wafer curvature measurements

This work demonstrates, for the first time, that a variety of disparate and technologically-relevent thermal, mechanical, and electrochemical oxygen-exchange material properties can 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 Pr0.1Ce0.9O1.95-x (10PCO) films atop 200 μm thick single crystal yttria stabilized zirconia or magnesium oxide substrates were used to measure the biaxial modulus, Young's modulus, thermal expansion coefficient, thermo-chemical 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. Taken together, this work highlights 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.

Conducting Copper(I/II)-Metallopolymer for the Electrocatalytic Oxygen Reduction Reaction (ORR) with High Kinetic Current Density

The oxygen reduction reaction (ORR) is still the most research-intensive aspect of a fuel cell. The sluggish kinetics of the electrocatalysts toward the ORR requires large amounts of platinum to be used as cathode material, which calls for alternatives to replace or minimize the amount of the noble metals used. This study describes the synthesis and complete characterization of a copper metallopolymer (Cu MP) based on a conducting polymer (CP) and single-site catalytic centers for the electrocatalytic ORR. The copper (II) catalyst, embedded in a redox-active and conducting polymeric environment, was pursued as a potential candidate to replace noble metals in fuel cell applications. Performance studies at a rotating disk electrode (RDE) showed that the metallopolymer exhibited a direct four-electron reduction at potentials between -150 and -350 mV vs. the reversible hydrogen electrode (RHE) and high kinetic current densities of over 22.62 mA/cm². The kinetic current densities obtained at the Cu MP electrode outperformed most of the reported state-of-the art electrocatalysts toward the ORR. Further analysis of the Cu/CP hybrid revealed the copper being largely reduced to the oxidation state +I.

High-Performance La0.5Ba0.5Co1/3Mn1/3Fe1/3O3−δ-BaZr1−zYzO3−δ Cathode Composites via an Exsolution Mechanism for Protonic Ceramic Fuel Cells

A novel exsolution process was used to fabricate complex all-oxide nanocomposite cathodes for Protonic Ceramic Fuel Cells (PCFCs). The nanocomposite cathodes with La0.5Ba0.5Co1/3Mn1/3Fe1/3O3−δ-BaZr1−zYzO3−δ nominal composition were prepared from a single-phase precursor via an oxidation-driven exsolution mechanism. The exsolution process results in a highly nanostructured and intimately interconnected percolating network of the two final phases, one proton conducting (BaZr1−zYzO3−δ) and one mixed oxygen ion and electron conducting (La0.5Ba0.5Co1/3Mn1/3Fe1/3O3−δ), yielding excellent cathode performance. The cathode powder is synthesized as a single-phase cubic precursor by a modified Pechini route followed by annealing at 700 °C in N2. The precursor phase is exsolved into two cubic perovskite phases by further heat treatment in air. The phase composition and chemical composition of the two phases were confirmed by Rietveld refinement. The electrical conductivity of the composites was measured and the electrochemical performance was determined by impedance spectroscopy of symmetrical cells using BaZr0.9Y0.1O2.95 as electrolyte. Our results establish the potential of this exsolution method where a large number of different cations can be used to design composite cathodes. The La0.5Ba0.5Co1/3Mn1/3Fe1/3O3−δ-BaZr0.9Y0.1O2.95 composite cathode shows the best performance of 0.44 Ω∙cm2 at 600 °C in 3% moist synthetic air.

Design of Active Sites on Nickel in the Anode of Intermediate-Temperature Solid Oxide Fuel Cells using Trace Amount of Platinum Oxides

In recent years, the lowering of the operation temperature of solid oxide fuel cells (SOFCs) has attracted much attention owing to the trade-off between the best performance and the life span of SOFCs. For this challenge, new active sites on the Ni surfaces in a Nickel-Yttria-Stabilized Zirconia (Ni-YSZ) cermet anode of SOFCs have been created by deposition of trace amounts of platinum oxide (PtO_x ) followed by an activation step of the anode at 1073 K in a hydrogen flow. The internal resistance (IR) free value (185 mA cm^-2 at 0.8 V) observed for the single cell with an anode sputtered with a trace amount of PtO_x (Pt content in anode: from 9 to 91 ppm) at 973 K is conspicuously higher than that of a similar single cell with a nonsputtered cermet anode (85 mA cm^-2 ) at 0.8 V and 1073 K. Transmission electron microscopy microanalysis shows that the defect structure is formed on a partially oxidized Ni surface by active Pt species. Also, surface atomistic simulation on NiO (111) predicts the formation of Frenkel defect clusters with Pt cations, which partially cover the Ni surface. The formation of Frenkel defect clusters on the partially oxidized Ni surface (i.e., creation of new active sites for formation of water molecules) promotes the anode reaction, resulting in improvements in the anode performance of SOFC single cells at 973 K. Design of the aforementioned new active sites on Ni through sputtering of trace amounts of PtO_x provides a great opportunity for "radical innovation" in the design of intermediate-temperature SOFCs.

A review of high temperature co-electrolysis of H2O and CO2 to produce sustainable fuels using solid oxide electrolysis cells (SOECs): advanced materials and technology

High-temperature solid oxide electrolysis cells (SOECs) are advanced electrochemical energy storage and conversion devices with high conversion/energy efficiencies. They offer attractive high-temperature co-electrolysis routes that reduce extra CO₂ emissions, enable large-scale energy storage/conversion and facilitate the integration of renewable energies into the electric grid. Exciting new research has focused on CO₂ electrochemical activation/conversion through a co-electrolysis process based on the assumption that difficult C[double bond, length as m-dash]O double bonds can be activated effectively through this electrochemical method. Based on existing investigations, this paper puts forth a comprehensive overview of recent and past developments in co-electrolysis with SOECs for CO₂ conversion and utilization. Here, we discuss in detail the approaches of CO₂ conversion, the developmental history, the basic principles, the economic feasibility of CO₂/H₂O co-electrolysis, and the diverse range of fuel electrodes as well as oxygen electrode materials. SOEC performance measurements, characterization and simulations are classified and presented in this paper. SOEC cell and stack designs, fabrications and scale-ups are also summarized and described. In particular, insights into CO₂ electrochemical conversions, solid oxide cell material behaviors and degradation mechanisms are highlighted to obtain a better understanding of the high temperature electrolysis process in SOECs. Proposed research directions are also outlined to provide guidelines for future research.

Structural investigations of La0.6Sr0.4FeO3−δ under reducing conditions: kinetic and thermodynamic limitations for phase transformations and iron exsolution phenomena

The crystal structure changes and iron exsolution behavior of a series of oxygen-deficient lanthanum strontium ferrite (La_0.6Sr_0.4FeO_3−δ, LSF) samples under various inert and reducing conditions up to a maximum temperature of 873 K have been investigated to understand the role of oxygen and iron deficiencies in both processes. Iron exsolution occurs in reductive environments at higher temperatures, leading to the formation of Fe rods or particles at the surface. Utilizing multiple ex situ and in situ methods (in situ X-ray diffraction (XRD), in situ thermogravimetric analysis (TGA), and scanning X-ray absorption near-edge spectroscopy (XANES)), the thermodynamic and kinetic limitations are accordingly assessed. Prior to the iron exsolution, the perovskite undergoes a nonlinear shift of the diffraction peaks to smaller 2θ angles, which can be attributed to a rhombohedral-to-cubic (R3̄c to Pm3̄m) structural transition. In reducing atmospheres, the cubic structure is stabilized upon cooling to room temperature, whereas the transition is suppressed under oxidizing conditions. This suggests that an accumulation of oxygen vacancies in the lattice stabilize the cubic phase. The exsolution itself is shown to exhibit a diffusion-limited Avrami-like behavior, where the transport of iron to the Fe-depleted surface-near region is the rate-limiting step.

A dependence of structural transformation and iron exsolution on chemical environment and reducing conditions is proven for the perovskite La_0.6Sr_0.4FeO_3−δ.

Study on Zinc Oxide-Based Electrolytes in Low-Temperature Solid Oxide Fuel Cells

Semiconducting-ionic conductors have been recently described as excellent electrolyte membranes for low-temperature operation solid oxide fuel cells (LT-SOFCs). In the present work, two new functional materials based on zinc oxide (ZnO)-a legacy material in semiconductors but exceptionally novel to solid state ionics-are developed as membranes in SOFCs for the first time. The proposed ZnO and ZnO-LCP (La/Pr doped CeO₂) electrolytes are respectively sandwiched between two Ni_0.8Co_0.15Al_0.05Li-oxide (NCAL) electrodes to construct fuel cell devices. The assembled ZnO fuel cell demonstrates encouraging power outputs of 158-482 mW cm^-2 and high open circuit voltages (OCVs) of 1-1.06 V at 450-550 °C, while the ZnO-LCP cell delivers significantly enhanced performance with maximum power density of 864 mW cm^-2 and OCV of 1.07 V at 550 °C. The conductive properties of the materials are investigated. As a consequence, the ZnO electrolyte and ZnO-LCP composite exhibit extraordinary ionic conductivities of 0.09 and 0.156 S cm^-1 at 550 °C, respectively, and the proton conductive behavior of ZnO is verified. Furthermore, performance enhancement of the ZnO-LCP cell is studied by electrochemical impedance spectroscopy (EIS), which is found to be as a result of the significantly reduced grain boundary and electrode polarization resistances. These findings indicate that ZnO is a highly promising alternative semiconducting-ionic membrane to replace the electrolyte materials for advanced LT-SOFCs, which in turn provides a new strategic pathway for the future development of electrolytes.

Catalytic Layer Optimization for Hydrogen Permeation Membranes Based on La5.5WO11.25-δ/La0.87Sr0.13CrO3-δ Composites

(LWO/LSC) composite is one of the most promising mixed ionic-electronic conducting materials for hydrogen separation at high temperature. However, these materials present limited catalytic surface activity toward H₂ activation and water splitting, which determines the overall H₂ separation rate. For the implementation of these materials as catalytic membrane reactors, effective catalytic layers have to be developed that are compatible and stable under the reaction conditions. This contribution presents the development of catalytic layers based on sputtered metals (Cu and Pd), electrochemical characterization by impendace spectroscopy, and the study of the H₂ flow obtained by coating them on 60/40-LWO/LSC membranes. Stability of the catalytic layers is also evaluated under H₂ permeation conditions and CH₄-containing atmospheres.

Proton-Conducting La-Doped Ceria-Based Internal Reforming Layer for Direct Methane Solid Oxide Fuel Cells

Performance degradation caused by carbon deposition substantially restricts the development of direct methane solid oxide fuel cells (SOFCs). Here, an internal reforming layer composed of Ni supported on proton conducting La-doped ceria, such as La₂Ce₂O₇ (LDC) and La_1.95Sm_0.05Ce₂O₇ (LSDC) is applied over conventional Ni-Ce_0.8Sm_0.2O_2-x (SDC) anodes for direct methane SOFCs. The proton conducting layer can adsorb water for internal reforming thus significantly improving the performance of the direct methane SOFCs. In situ Raman and FTIR results confirm the water adsorption capacity of LDC and LSDC. They also exhibit excellent phase stability in wet CO₂ at 650 °C for 10 h, which ensures that the additional catalyst layer maintains structure stability during the internal reforming. In wet methane at 650 °C, the peak power density of the conventional cell is only 580 ± 20 mW cm^-2, and increases to 699 ± 20 and 639 ± 20 mW cm^-2 with the addition of Ni-LDC and -LSDC layers, respectively. For the stability test in wet methane at 650 °C and 0.2 A cm^-2, the voltage of the conventional cell starts to drop dramatically in 10 h, while the Ni-LDC and -LSDC catalyst layers operate stably in 26 h under the identical conditions. These catalyst layers even show comparable stability in dry and wet methane in 26 h, but for longer operation, the wet methane is still preferred for maintaining the stability of the cell.

Inkjet printing infiltration of Ni-Gd:CeO2 anodes for low temperature solid oxide fuel cells

ABSTRACT

The effect of inkjet printing infiltration of Gd0.1Ce0.9O2-x in NiO-Gd0.1Ce0.9O2-x anodes on the performance of symmetrical and button cells was investigated. The anodes were fabricated by inkjet printing of suspension and sol inks. Symmetrical cells were produced from composite suspension inks on Gd0.1Ce0.9O2-x electrolyte. As-prepared scaffolds were infiltrated with Gd0.1Ce0.9O2 ink. Increasing the number of infiltration steps led to formation of "nano-decoration" on pre-sintered anodes. High resolution SEM analysis was employed for micro-structural characterization revealing formation of fine anode sub-structure with nanoparticle size varying in the range of 50-200 nm. EIS tests were conducted on symmetrical cells in 4% hydrogen/argon gas flow. The measurements showed substantial reduction of the activation polarization as a function of the number of infiltrations. The effect was assigned to the extension of the triple phase boundary. The i-V testing of a reference (NiO-8 mol% Y2O3 stabilized ZrO2/NiO-Gd0.1Ce0.9O2-x /Gd0.1Ce0.9O2-x /Gd0.1Ce0.9O2-x -La0.6Sr0.4Co0.2Fe0.8O3-δ ) cell and an identical cell with infiltrated anode revealed ~2.5 times improvement in the maximum output power at 600 °C which corresponded with the reduction of the polarization resistance of the symmetrical cells at the same temperature (2.8 times). This study demonstrated the potential of inkjet printing technology as an infiltration tool for cost effective commercial SOFC processing.

GRAPHICAL ABSTRACT

Insights into the Surface Reactivity of Cermet and Perovskite Electrodes in Oxidizing, Reducing, and Humid Environments

Understanding the surface chemistry of electrode materials under gas environments is important in order to control their performance during electrochemical and catalytic applications. This work compares the surface reactivity of Ni/YSZ and La_0.75Sr_0.25Cr_0.9Fe_0.1O₃, which are commonly used types of electrodes in solid oxide electrochemical devices. In situ synchrotron-based near-ambient pressure photoemission and absorption spectroscopy experiments, assisted by theoretical spectral simulations and combined with microscopy and electrochemical measurements, are used to monitor the effect of the gas atmosphere on the chemical state, the morphology, and the electrical conductivity of the electrodes. It is shown that the surface of both electrode types readjusts fast to the reactive gas atmosphere and their surface composition is notably modified. In the case of Ni/YSZ, this is followed by evident changes in the oxidation state of nickel, while for La_0.75Sr_0.25Cr_0.9Fe_0.1O₃, a fine adjustment of the Cr valence and strong Sr segregation is observed. An important difference between the two electrodes is their capacity to maintain adsorbed hydroxyl groups on their surface, which is expected to be critical for the electrocatalytic properties of the materials. The insight gained from the surface analysis may serve as a paradigm for understanding the effect of the gas environment on the electrochemical performance and the electrical conductivity of the electrodes.

Physical and chemical properties of Cu(i) compounds with O and/or H

The electronic structure and chemical bonding of Cu(i) compounds with O and/or H are investigated using ab initio calculations based on density functional theory. A hybrid functional PBE0 is employed, which accurately reproduces an experimental band gap of cuprite Cu₂O. Cuprous hydroxide CuOH (cuprice) is found to be an indirect band gap semiconductor. Depending on the bond network configuration of CuOH, its band gap is found to vary between 2.73 eV and 3.03 eV. The presence of hydrogen in CuOH has little effect on the character of Cu-O bonds, as compared to Cu₂O, but lowers the energy levels of the occupied states upon O-H bond formation. The bonding charge density and electron localization function calculations reveal that a closed-shell Cu-Cu interaction takes place in Cu₂O and CuOH between the neighbouring Cu cations belonging to different bond networks. Besides, three structures of cuprous hydride CuH are investigated. We find that the halite structure of CuH can be stabilized at high pressure (above 32 GPa) while wurtzite is the most stable structure of CuH at ambient pressure. The H-H interaction contributes to the dynamical stabilization of the halite structure. The wurtzite and sphalerite structures of CuH are predicted to be semiconducting with small band gaps, while the halite structure is calculated to be metallic.

Challenges in developing direct carbon fuel cells

A direct carbon fuel cell (DCFC) can produce electricity with both superior electrical efficiency and fuel utilisation compared to all other types of fuel cells.

Surface chemistry of pure tetragonal ZrO2 and gas-phase dependence of the tetragonal-to-monoclinic ZrO2 transformation

Phase-pure stable structurally defective tetragonal ZrO₂ with surprising surface and bulk chemical properties.

Local structure and controllable thermal expansion in the solid solution (Mn1−xNix)ZrF6

Controllable thermal expansion in the cubic solid solutions of (Mn_1−xNi_x)ZrF₆through atomic linkage flexibility.

Modeling the impedance spectra of mixed conducting thin films with exposed and embedded current collectors

This article studies the electrochemical impedance spectroscopy response of mixed ionic-electronic conducting (MIEC) films with embedded current collectors (CCs). Even though the MIEC surface is fully exposed, the impact of the CCs can be significant.

Tin doped PrBaFe2O5+δ anode material for solid oxide fuel cells

PrBaFe_1.9Sn_0.1O_5+δ shows excellent redox stability, high electrical conductivity, and ASR of a symmetrical cell as low as of 0.095–0.285 Ω cm² from 850–750 °C in wet hydrogen, better than or comparable to the best of ceramic anodes in the open literature.

Magnetic and Magnetoresistive Properties of 3D Interconnected NiCo Nanowire Networks

Track-etched polymer membranes with crossed nanochannels have been revealed to be most suitable as templates to produce large surface area and mechanically stable 3D interconnected nanowire (NW) networks by electrodeposition. Geometrically controlled NW superstructures made of NiCo ferromagnetic alloys exhibit appealing magnetoresistive properties. The combination of exact alloy compositions with the spatial arrangement of NWs in the 3D network is decisive to obtain specific magnetic and magneto-transport behavior. A proposed simple model based on topological aspects of the 3D NW networks is used to accurately determine the anisotropic magnetoresistance ratios. Despite of their complex topology, the microstructure of Co-rich NiCo NW networks display mixed fcc-hcp phases with the c-axis of the hcp phase oriented perpendicular to their axis. These interconnected NW networks have high potential as reliable and stable magnetic field sensors.

Bimetallic Nickel/Ruthenium Catalysts Synthesized by Atomic Layer Deposition for Low-Temperature Direct Methanol Solid Oxide Fuel Cells

Nickel and ruthenium bimetallic catalysts were heterogeneously synthesized via atomic layer deposition (ALD) for use as the anode of direct methanol solid oxide fuel cells (DMSOFCs) operating in a low-temperature range. The presence of highly dispersed ALD Ru islands over a porous Ni mesh was confirmed, and the Ni/ALD Ru anode microstructure was observed. Fuel cell tests were conducted using Ni-only and Ni/ALD Ru anodes with approximately 350 μm thick gadolinium-doped ceria electrolytes and platinum cathodes. The performance of fuel cells was assessed using pure methanol at operating temperatures of 300-400 °C. Micromorphological changes of the anode after cell operation were investigated, and the content of adsorbed carbon on the anode side of the operated samples was measured. The difference in the maximum power density between samples utilizing Ni/ALD Ru and Pt/ALD Ru, the latter being the best catalyst for direct methanol fuel cells, was observed to be less than 7% at 300 °C and 30% at 350 °C. The improved electrochemical activity of the Ni/ALD Ru anode compared to that of the Ni-only anode, along with the reduction of the number of catalytically active sites due to agglomeration of Ni and carbon formation on the Ni surface as compared to Pt, explains this decent performance.

Promoting oxygen vacancy formation and p-type conductivity in SrTiO3via alkali metal doping: a first principles study

Strontium titanate (SrTiO₃, STO) is a prototypical perovskite oxide, widely exploited in many technological applications, from catalysis to energy conversion devices. In the context of solid-oxide fuel cells, STO has been recently applied as an epitaxial substrate for nano-sized layers of mixed ion-electron conductive catalysts with enhanced electrochemical performances. To extend the applications of such heterogeneous nano-cathodes in real devices, also the STO support should be active for both electron transport and oxide diffusion. To this end, we explored using first-principles calculations the strategy of doping of STO at the Sr site with sodium and potassium. These two ions fit in the perovskite structure and induce holes in the STO valence band, so as to obtain the desired p-type electronic conduction. At the same time, the doping with alkali ions also promotes the formation of oxygen vacancies in STO, a prerequisite for effective oxide diffusion. Analysis of electron density rearrangements upon defect formation allows relating the favorable vacancy formation energies to an improved electronic delocalization over the oxide sub-lattice, as observed in closely related materials (e.g. Sr₂Fe_1.5Mo_0.5O₆). Overall, our results suggest the alkali-doped STO as a new potential substrate material in nanoscale heterogeneous electrodes for solid oxide electrochemical cells.

High-Performance Anode Material Sr2FeMo0.65Ni0.35O6-δ with In Situ Exsolved Nanoparticle Catalyst

A metallic nanoparticle-decorated ceramic anode was prepared by in situ reduction of the perovskite Sr2FeMo0.65Ni0.35O6-δ (SFMNi) in H2 at 850 °C. The reduction converts the pure perovksite phase into mixed phases containing the Ruddlesden-Popper structure Sr3FeMoO7-δ, perovskite Sr(FeMo)O3-δ, and the FeNi3 bimetallic alloy nanoparticle catalyst. The electrochemical performance of the SFMNi ceramic anode is greatly enhanced by the in situ exsolved Fe-Ni alloy nanoparticle catalysts that are homogeneously distributed on the ceramic backbone surface. The maximum power densities of the La0.8Sr0.2Ga0.8Mg0.2O3-δ electrolyte supported a single cell with SFMNi as the anode reached 590, 793, and 960 mW cm(-2) in wet H2 at 750, 800, and 850 °C, respectively. The Sr2FeMo0.65Ni0.35O6-δ anode also shows excellent structural stability and good coking resistance in wet CH4. The prepared SFMNi material is a promising high-performance anode for solid oxide fuel cells.

Direct conversion of methane to aromatics in a catalytic co-ionic membrane reactor

Nonoxidative methane dehydroaromatization (MDA: 6CH4 ↔ C6H6 + 9H2) using shape-selective Mo/zeolite catalysts is a key technology for exploitation of stranded natural gas reserves by direct conversion into transportable liquids. However, this reaction faces two major issues: The one-pass conversion is limited by thermodynamics, and the catalyst deactivates quickly through kinetically favored formation of coke. We show that integration of an electrochemical BaZrO3-based membrane exhibiting both proton and oxide ion conductivity into an MDA reactor gives rise to high aromatic yields and improved catalyst stability. These effects originate from the simultaneous extraction of hydrogen and distributed injection of oxide ions along the reactor length. Further, we demonstrate that the electrochemical co-ionic membrane reactor enables high carbon efficiencies (up to 80%) that improve the technoeconomic process viability.

Solid oxide fuel cell interconnect design optimization considering the thermal stresses

The mechanical failure of solid oxide fuel cell (SOFC) components may cause cracks with consequences such as gas leakage, structure instability and reduction of cell lifetime. A comprehensive 3D model of the thermal stresses of an anode-supported planar SOFC is presented in this work. The main objective of this paper is to get an interconnect optimized design by evaluating the thermal stresses of an anode-supported SOFC for different designs, which would be a new criterion for interconnect design. The model incorporates the momentum, mass, heat, ion and electron transport, as well as steady-state mechanics. Heat from methane steam reforming and water–gas shift reaction were considered in our model. The results examine the relationship between the interconnect structures and thermal stresses in SOFC at certain mechanical properties. A wider interconnect of the anode side lowers the stress obviously. The simulation results also indicate that thermal stress of coflow design is smaller than that of counterflow, corresponding to the temperature distribution. This study shows that it is possible to design interconnects for an optimum thermal stress performance of the cell.