Triple ionic-electronic conducting oxides for next-generation electrochemical devices

Investigation of Water Impacts on Surface Properties and Performance of Air-Electrode in Reversible Protonic Ceramic Cells

Water, being abundant and readily accessible, gains widespread usage as proton source in many catalysis and energy conversion technologies, including applications like reversible protonic ceramic cells (R-PCCs). Revealing the influence of water on the electrode surface and reaction kinetics is critical for further improving their electrochemical performance. Herein, a hydrophilic air-electrode PrBa0.875Cs0.125Co2O5+δ is developed for R-PCC, which demonstrates a remarkable peak power density of 1058 mW cm-2 in fuel cell mode and a current density of 1354 mA cm-2 under 1.3 V in electrolyzing steam at 650 °C. For the first time on R-PCC, surface protons' behavior in response to external voltages is captured using in situ FTIR characterizations. Further, it is shown that contrary to the bulk proton uptake process that is thought to follow hydrogenation reactions and lead to cation reductions. The air-electrode presents enriched surface protons occurring through oxidizing surface cations, as confirmed by depth-profiling XPS results. H/D isotope exchange experiments and subsequent electrochemical characterization analyses reveal that the presence of protons enhances surface reactions. This study fills the knowledge gap between water-containing atmospheres and electrochemical performance by providing insights into the surface properties of the material. These new findings provide guidance for future electrode design and optimization.

Syngas Production from CO2 and H2O via Solid-Oxide Electrolyzer Cells: Fundamentals, Materials, Degradation, Operating Conditions, and Applications

Highly efficient coelectrolysis of CO2/H2O into syngas (a mixture of CO/H2), and subsequent syngas conversion to fuels and value-added chemicals, is one of the most promising alternatives to reach the corner of zero carbon strategy and renewable electricity storage. This research reviews the current state-of-the-art advancements in the coelectrolysis of CO2/H2O in solid oxide electrolyzer cells (SOECs) to produce the important syngas intermediate. The overviews of the latest research on the operating principles and thermodynamic and kinetic models are included for both oxygen-ion- and proton-conducting SOECs. The advanced materials that have recently been developed for both types of SOECs are summarized. It later elucidates the necessity and possibility of regulating the syngas ratios (H2:CO) via changing the operating conditions, including temperature, inlet gas composition, flow rate, applied voltage or current, and pressure. In addition, the sustainability and widespread application of SOEC technology for the conversion of syngas is highlighted. Finally, the challenges and the future research directions in this field are addressed. This review will appeal to scientists working on renewable-energy-conversion technologies, CO2 utilization, and SOEC applications. The implementation of the technologies introduced in this review offers solutions to climate change and renewable-power-storage problems.

Mn Additive Improves Zr Grain Boundary Diffusion for Sintering of a Y-Doped BaZrO3 Proton Conductor

Yttrium-doped barium zirconate (BZY) has garnered attention as a protonic conductor in intermediate-temperature electrolysis and fuel cells due to its high bulk proton conductivity and excellent chemical stability. However, the performance of BZY can be further enhanced by reducing the concentration and resistance of grain boundaries. In this study, we investigate the impact of manganese (Mn) additives on the sinterability and proton conductivity of Y-doped BaZrO3 (BZY). By employing a combinatorial pulsed laser deposition (PLD) technique, we synthesized BZY thin films with varying Mn concentrations and sintering temperatures. Our results revealed a significant enhancement in sinterability as Mn concentrations increased, leading to larger grain sizes and lower grain boundary concentrations. These improvements can be attributed to the elevated grain boundary diffusion of zirconium (Zr) cations, which enhances material densification. We also observed a reduction in Goldschmidt's tolerance factor with increased Mn substitution, which can improve proton transport. The high proton conduction of BZY with Mn additives in low-temperature and wet hydrogen environments makes it a promising candidate for protonic ceramic electrolysis cells and fuel cells. Our findings not only advance the understanding of Mn additives in BZY materials but also demonstrate a high-throughput combinatorial thin film approach to select additives for other perovskite materials with importance in mass and charge transport applications.

Modulating Lattice Oxygen Activity of Iron-Based Triple-Conducting Nanoheterostructure Air Electrode via Sc-Substitution Strategy for Protonic Ceramic Cells

Iron-based perovskite air electrodes for protonic ceramic cells (PCCs) offer broad application prospects owing to their reasonable thermomechanical compatibility and steam tolerance. However, their insufficient electrocatalytic activity has considerably limited further development. Herein, oxygen-vacancy-rich BaFe0.6 Ce0.2 Sc0.2 O3-δ (BFCS) perovskite is rationally designed by a facile Sc-substitution strategy for BaFe0.6 Ce0.4 O3-δ (BFC) as efficient and stable air electrode for PCCs. The BFCS electrode with an optimized Fe 3d-eg orbital occupancy and more oxygen vacancies exhibits a polarization resistance of ≈ 0.175 Ω cm2 at 600 °C, ≈ 1/3 of the BFC electrode (≈0.64 Ω cm2 ). Simultaneously, BFCS shows favorable proton uptake with a low proton defect formation enthalpy (- 81 kJ mol-1 ). By combining soft X-ray absorption spectroscopy and electrical conductivity relaxation studies, it is revealed that the enhancement of Fe4+ -O2- interactions in BFCS promotes the activation and mobility of lattice oxygen, triggering the activity of BFCS in both oxygen reduction and evolution reactions (ORR/OER). The single cell achieves encouraging output performance in both fuel cell (1.55 W cm-2 ) and electrolysis cell (-2.96 A cm-2 at 1.3 V) modes at 700 °C. These results highlight the importance of activating lattice oxygen in air electrodes of PCCs.

Trace Y Doping Regulated Bulk/Interfacial Reactions of P2-Layered Oxides for Ultrahigh-Rate Sodium-Ion Batteries

P2-phase layered cathodes play a pivotal role in sodium-ion batteries due to their efficient Na+ intercalation chemistry. However, limited by crystal disintegration and interfacial instability, bulk and interfacial failure plague their electrochemical performance. To address these challenges, a structural enhancement combined with surface modification is achieved through trace Y doping. Based on a synergistic combination of experimental results and density functional theory (DFT) calculations, the introduction of partial Y ions at the Na site (2d) acts as a stabilizing pillar, mitigating the electrostatic repulsions between adjacent TMO2 slabs and thereby relieving internal structural stress. Furthermore, the presence of Y effectively optimizes the Ni 3d-O 2p hybridization, resulting in enhanced electronic conductivity and a notable rapid charging ability, with a capacity of 77.3 mA h g-1 at 40 C. Concurrently, the introduction of Y also induces the formation of perovskite nano-islands, which serve to minimize side reactions and modulate interfacial diffusion. As a result, the refined P2-Na0.65 Y0.025 [Ni0.33 Mn0.67 ]O2 cathode material exhibits an exceptionally low volume variation (≈1.99%), an impressive capacity retention of 83.3% even at -40 °C after1500 cycles at 1 C.

Facile Deficiency Engineering in a Cobalt-Free Perovskite Air Electrode to Achieve Enhanced Performance for Protonic Ceramic Fuel Cells

As a crucial component responsible for the oxygen reduction reaction (ORR), cobalt-rich perovskite-type cathode materials have been extensively investigated in protonic ceramic fuel cell (PCFC). However, their widespread application at a commercial scale is considerably hindered by the high cost and inadequate stability. In response to these weaknesses, the study presents a novel cobalt-free perovskite oxide, Ba0.95 La0.05 (Fe0.8 Zn0.2 )0.95 O3-δ (BLFZ0.95), with the triple-conducting (H+ |O2- |e- ) property as an active and robust air electrode for PCFC. The B-site deficiency state contributes significantly to the optimization of crystal and electronic structure, as well as the increase in oxygen vacancy concentration, thus in turn favoring the catalytic capacity. As a result, the as-obtained BLFZ0.95 electrode demonstrates exceptional electrochemical performance at 700 °C, representing extremely low area-specific resistance of 0.04 Ω cm2 in humid air (3 vol.% H2 O), extraordinarily high peak power density of 1114 mW cm-2 , and improved resistance against CO2 poisoning. Furthermore, the outstanding long-term durability is achieved without visible deterioration in both symmetrical and single cell modes. This study presents a simple but crucial case for rational design of cobalt-free perovskite cathode materials with appreciable performance via B-site deficiency regulation.

MatGPT: A Vane of Materials Informatics from Past, Present, to Future

Combining materials science, artificial intelligence (AI), physical chemistry, and other disciplines, materials informatics is continuously accelerating the vigorous development of new materials. The emergence of "GPT (Generative Pre-trained Transformer) AI" shows that the scientific research field has entered the era of intelligent civilization with "data" as the basic factor and "algorithm + computing power" as the core productivity. The continuous innovation of AI will impact the cognitive laws and scientific methods, and reconstruct the knowledge and wisdom system. This leads to think more about materials informatics. Here, a comprehensive discussion of AI models and materials infrastructures is provided, and the advances in the discovery and design of new materials are reviewed. With the rise of new research paradigms triggered by "AI for Science", the vane of materials informatics: "MatGPT", is proposed and the technical path planning from the aspects of data, descriptors, generative models, pretraining models, directed design models, collaborative training, experimental robots, as well as the efforts and preparations needed to develop a new generation of materials informatics, is carried out. Finally, the challenges and constraints faced by materials informatics are discussed, in order to achieve a more digital, intelligent, and automated construction of materials informatics with the joint efforts of more interdisciplinary scientists.

A fast ceramic mixed OH-/H+ ionic conductor for low temperature fuel cells

Low temperature ionic conducting materials such as OH- and H+ ionic conductors are important electrolytes for electrochemical devices. Here we show the discovery of mixed OH-/H+ conduction in ceramic materials. SrZr0.8Y0.2O3-δ exhibits a high ionic conductivity of approximately 0.01 S cm-1 at 90 °C in both water and wet air, which has been demonstrated by direct ammonia fuel cells. Neutron diffraction confirms the presence of OD bonds in the lattice of deuterated SrZr0.8Y0.2O3-δ. The OH- ionic conduction of CaZr0.8Y0.2O3-δ in water was demonstrated by electrolysis of both H218O and D2O. The ionic conductivity of CaZr0.8Y0.2O3-δ in 6 M KOH solution is around 0.1 S cm-1 at 90 °C, 100 times higher than that in pure water, indicating increased OH- ionic conductivity with a higher concentration of feed OH- ions. Density functional theory calculations suggest the diffusion of OH- ions relies on oxygen vacancies and temporarily formed hydrogen bonds. This opens a window to discovering new ceramic ionic conducting materials for near ambient temperature fuel cells, electrolysers and other electrochemical devices.

Nanotechnologies in ceramic electrochemical cells

Although they are emerging technologies for achieving high-efficiency and green and eco-friendly energy conversion, ceramic electrochemical cells (CECs), i.e. solid oxide electrolysis cells (SOECs) and fuel cells (SOFCs), are still fundamentally limited by their inferior catalytic activities at low temperature, poor thermo-mechanical stability, high material cost, etc. The materials used in electrolytes and electrodes, which are the most important components in CECs, are highly associated with the cell performances. Therefore, rational design of electrolytes and electrodes with excellent catalytic activities and high stabilities at relatively low cost is a meaningful and valuable approach for the development of CECs. Nanotechnology is a powerful tool for improving the material performances in CECs owing to the favourable effects induced by the nanocrystallization of electrolytes and electrodes. Herein, a relatively comprehensive review on the nanotechnologies implemented in CECs is conducted. The working principles of CECs and the corresponding challenges were first presented, followed by the comprehensive insights into the working mechanisms of nanocrystalline materials in CECs. Then, systematic summarization and analyses of the commonly used nano-engineering strategies in the fabrication of CEC materials, including physical and chemical methods, were provided. In addition, the frontiers in the research of advanced electrolyte and electrode materials were discussed with a special emphasis on the modified electrochemical properties derived from nanotechnologies. Finally, the bottlenecks and the promising breakthroughs in nanotechnologies were highlighted in the direction of providing useful references for rational design of nanomaterials for CECs.

Oxygen Electrode PrBa0.5Sr0.5Co1.5Fe0.5O5+δ-BaZr0.1Ce0.7Y0.1Yb0.1O3-δ with Different Composite Proportions for Proton-Conducting Solid Oxide Electrolysis Cells

Proton-conducting solid oxide electrolysis cell (H-SOEC), as a hydrogen production device using proton conductor oxides as an electrolyte, has gained attention due to its various advantages of being more suitable for operating conditions at intermediate and low temperatures. However, its commercialization urgently needs to address the issue of insufficient catalytic activity of the oxygen electrode at lower temperatures. In this work, PrBa0.5Sr0.5Co1.5Fe0.5O5+δ-BaZr0.1Ce0.7Y0.1Yb0.1O3-δ (PBSCF-BZCYYb) series composite materials (denoted as PBSCF-BZCYYb46, PBSCF- BZCYYb55, and PBSCF-BZCYYb64 based on the mass ratios of PBSCF and BZCYYb as 4:6, 5:5, and 6:4, respectively) are prepared and applied as oxygen electrodes for H-SOECs. The H-SOECs with the structure of PBSCF-BZCYYb|BZCYYb|NiO-BZCYYb (active layer)|NiO-BZCYYb (support layer) are prepared and recorded as Cell 1, Cell 2, and Cell 3 with PBSCF-BZCYYb46, PBSCF-BZCYYb55, and PBSCF-BZCYYb64 as oxygen electrodes. The H-SOECs exhibit electrolysis current densities of 669.00, 743.80, and 503.30 mA cm-2 under 1.3 V at 650 °C, respectively. The cells also show considerable stability in the constant voltage electrolysis of 179.5, 152.8, and 83.0 h, respectively. Through the comparison of various electrochemical properties, PBSCF-BZCYYb55 is considered the most promising oxygen electrode material in this work.

Design of Mixed Ionic-Electronic Materials for Permselective Membranes and Solid Oxide Fuel Cells Based on Their Oxygen and Hydrogen Mobility

Oxygen and hydrogen mobility are among the important characteristics for the operation of solid oxide fuel cells, permselective membranes and many other electrochemical devices. This, along with other characteristics, enables a high-power density in solid oxide fuel cells due to reducing the electrolyte resistance and enabling the electrode processes to not be limited by the electrode-electrolyte-gas phase triple-phase boundary, as well as providing high oxygen or hydrogen permeation fluxes for membranes due to a high ambipolar conductivity. This work focuses on the oxygen and hydrogen diffusion of mixed ionic (oxide ionic or/and protonic)-electronic conducting materials for these devices, and its role in their performance. The main laws of bulk diffusion and surface exchange are highlighted. Isotope exchange techniques allow us to study these processes in detail. Ionic transport properties of conventional and state-of-the-art materials including perovskites, Ruddlesden-Popper phases, fluorites, pyrochlores, composites, etc., are reviewed.

Synthesis and Oxygen Mobility of Bismuth Cerates and Titanates with Pyrochlore Structure

Synthesis and study of materials based on bismuth cerates and titanates were carried out. Complex oxides Bi_1.6Y_0.4Ti₂O₇ were synthesized by the citrate route; Bi₂Ce₂O₇ and Bi_1.6Y_0.4Ce₂O₇-by the Pechini method. The structural characteristics of materials after conventional sintering at 500-1300 °C were studied. It is demonstrated that the formation of a pure pyrochlore phase, Bi_1.6Y_0.4Ti₂O₇, occurs after high-temperature calcination. Complex oxides Bi₂Ce₂O₇ and Bi_1.6Y_0.4Ce₂O₇ have a pyrochlore structure formed at low temperatures. Yttrium doping of bismuth cerate lowers the formation temperature of the pyrochlore phase. As a result of calcination at high temperatures, the pyrochlore phase transforms into the CeO₂-like fluorite phase enriched by bismuth oxide. The influence of radiation-thermal sintering (RTS) conditions using e-beams was studied as well. In this case, dense ceramics are formed even at sufficiently low temperatures and short processing times. The transport characteristics of the obtained materials were studied. It has been shown that bismuth cerates have high oxygen conductivity. Conclusions are drawn about the oxygen diffusion mechanism for these systems. The materials studied are promising for use as oxygen-conducting layers in composite membranes.

A Mixed Protonic-Electronic Conductor Base on the Host-Guest Architecture of 2D Metal-Organic Layers and Inorganic Layers

The key to designing and fabricating highly efficient mixed protonic-electronic conductors materials (MPECs) is to integrate the mixed conductive active sites into a single structure, to break through the shortcomings of traditional physical blending. Herein, based on the host-guest interaction, an MPEC is consisted of 2D metal-organic layers and hydrogen-bonded inorganic layers by the assembly methods of layered intercalation. Noticeably, the 2D intercalated materials (≈1.3 nm) exhibit the proton conductivity and electron conductivity, which are 2.02 × 10^-5 and 3.84 × 10^-4 S cm^-1 at 100 °C and 99% relative humidity, much higher than these of pure 2D metal-organic layers (>>1.0 × 10^-10 and 2.01×10^-8 S cm^-1 ), respectively. Furthermore, combining accurate structural information and theoretical calculations reveals that the inserted hydrogen-bonded inorganic layers provide the proton source and a networks of hydrogen-bonds leading to efficient proton transport, meanwhile reducing the bandgap of hybrid architecture and increasing the band electron delocalization of the metal-organic layer to greatly elevate the electron transport of intrinsic 2D metal-organic frameworks.

Insight of BaCe0.5Fe0.5O3- δ twin perovskite oxide composite for solid oxide electrochemical cells

One-pot synthesized twin perovskite oxide composite of BaCe_0.5Fe_0.5O_{3- δ} (BCF), comprising cubic and orthorhombic perovskite phases, shows triple-conducting properties for promising solid oxide electrochemical cells. Phase composition evolution of BCF under various conditions was systematically investigated, revealing that the cubic perovskite phase could be fully/partially reduced into the orthorhombic phase under certain conditions. The reduction happened between the two phases at the interface, leading to the microstructure change. As a result, the corresponding apparent conducting properties also changed due to the difference between predominant conduction properties for each phase. Based on the revealed phase composition, microstructure, and electrochemical properties changes, a deep understanding of BCF's application in different conditions (oxidizing atmospheres, reducing/oxidizing gradients, cathodic conditions, and anodic conditions) was achieved. Triple-conducting property (H⁺/O^2-/e^-), fast open-circuit voltage response (∼16-∼470 mV) for gradients change, and improved single-cell performance (∼31% lower polarization resistance at 600°C) were comprehensively demonstrated. Besides, the performance was analyzed under anodic conditions, which showed that the microstructure and phase change significantly affected the anodic behavior.

Toward Superb Perovskite Oxide Electrocatalysts: Engineering of Coupled Nanocomposites

A significant issue that bedeviled the commercialization of renewable energy technologies, ranging from low-temperature water electrolyzers to high-temperature solid oxide cells, is the lack of high-performance catalysts. Among various candidates that could tackle such a challenge, perovskite oxides are rising-star materials because of their unique structural and compositional flexibility. However, single-phase perovskite oxides are challenging to satisfy all the requirements of electrocatalysts concurrently for practical applications, such as high catalytic activity, excellent stability, good ionic and electronic conductivities, and superior chemical/thermo-mechanical robustness. Impressively, perovskite oxides with coupled nanocomposites are emerging as a novel form offering multifunctionality due to their intrinsic features, including infinite interfaces with solid interaction, tunable compositions, flexible configurations, and maximum synergistic effects between assorted components. Considering this new configuration has attracted great attention owing to its promising performances in various energy-related applications, this review timely summarizes the leading-edge development of perovskite oxide-based coupled nanocomposites. Their state-of-art synthetic strategies are surveyed and highlighted, their unique structural advantages are highlighted and illustrated through the typical oxygen reduction reaction and oxygen evolution reactions in both high and low-temperature applications. Opinions on the current critical scientific issues and opportunities in this burgeoning research field are all provided.

Machine-Learning-Accelerated Development of Efficient Mixed Protonic-Electronic Conducting Oxides as the Air Electrodes for Protonic Ceramic Cells

Currently, the development of high-performance protonic ceramic cells (PCCs) is limited by the scarcity of efficient mixed protonic-electronic conducting oxides that can act as air electrodes to satisfy the high protonic conductivity of electrolytes. Despite the extensive research efforts in the past decades, the development of mixed protonic-electronic conducting oxides still remains in a trial-and-error process, which is extremely time consuming and high cost. Herein, based on the data acquired from the published literature, the machine-learning (ML) method is introduced to accelerate the discovery of efficient mixed protonic-electronic conducting oxides. Accordingly, the hydrated proton concentration (HPC) of 3200 oxides is predicted to evaluate the proton conduction that is essential for enhancing the electrochemical performances of PCCs. Subsequently, feature importance for HPC is evaluated to establish a guideline for rapid and accurate design and development of high-efficiency mixed protonic-electronic conducting oxides. Thereafter, screened (La_0.7 Ca_0.3 )(Co_0.8 Ni_0.2 )O₃ (LCCN7382) is prepared, and the experimental HPC adequately corresponds with the predicted results. Moreover, the PCC with LCCN7382 exhibits satisfactory electrochemical performances in electrolysis and fuel cell modes. In addition to the development of a promising air electrode for PCC, this study establishes a new avenue for ML-based development of mixed protonic-electronic conducting oxides.

Surface Self-Assembly Protonation Triggering Triple-Conductive Heterostructure with Highly Enhanced Oxygen Reduction for Protonic Ceramic Fuel Cells

Triple-conducting (H⁺ /O^2- /e^- ) cathodes are a vital constituent of practical protonic ceramic fuel cells. However, seeking new candidates has remained a grand challenge on account of the limited material system. Though triple conduction can be achieved by mechanically mixing powders uniformly consisting of oxygen ion-electron and proton conductors, the catalytic activity and durability are still restricted. By leveraging this fact, a highly efficient strategy to construct a triple-conductive region through surface self-assembly protonation based on the robust double-perovskite PrBaCo_1.92 Zr_0.08 O_5+δ , is proposed. In situ exsolution of BaZrO₃ -based nanoparticles growing from the host oxide under oxidizing atmosphere by liberating Ba/Zr cations from A/B-sites readily forms proton transfer channels. The surface reconstructing heterostructures improve the structural stability, reduce the thermal expansion, and accelerate the oxygen reduction catalytic activity of such nanocomposite cathodes. This design route significantly boosts electrochemical performance with maximum peak power densities of 1453 and 992 mW cm^-2 at 700 and 650 °C, respectively, 86% higher than the parent PrBaCo₂ O_5+δ cathode, accompanied by a much improved operational durability of 140 h at 600 °C.

High-Entropy Perovskite Oxide: A New Opportunity for Developing Highly Active and Durable Air Electrode for Reversible Protonic Ceramic Electrochemical Cells

Reversible proton ceramic electrochemical cell (R-PCEC) is regarded as the most promising energy conversion device, which can realize efficient mutual conversion of electrical and chemical energy and to solve the problem of large-scale energy storage. However, the development of robust electrodes with high catalytic activity is the main bottleneck for the commercialization of R-PCECs. Here, a novel type of high-entropy perovskite oxide consisting of six equimolar metals in the A-site, Pr_1/6La_1/6Nd_1/6Ba_1/6Sr_1/6Ca_1/6CoO_3-δ (PLNBSCC), is reported as a high-performance bifunctional air electrode for R-PCEC. By harnessing the unique functionalities of multiple elements, high-entropy perovskite oxide can be anticipated to accelerate reaction rates in both fuel cell and electrolysis modes. Especially, an R-PCEC utilizing the PLNBSCC air electrode achieves exceptional electrochemical performances, demonstrating a peak power density of 1.21 W cm^-2 for the fuel cell, while simultaneously obtaining an astonishing current density of - 1.95 A cm^-2 at an electrolysis voltage of 1.3 V and a temperature of 600 °C. The significantly enhanced electrochemical performance and durability of the PLNBSCC air electrode is attributed mainly to the high electrons/ions conductivity, fast hydration reactivity and high configurational entropy. This research explores to a new avenue to develop optimally active and stable air electrodes for R-PCECs.

Proton Transport in the Gadolinium-Doped Layered Perovskite BaLaInO4

Materials capable for use in energy generation have been actively investigated recently. Thermoelectrics, photovoltaics and electronic/ionic conductors are considered as a part of the modern energy system. Layered perovskites have many attractions, as materials with high conductivity. Gadolinium-doped layered perovskite BaLaInO4 was obtained and investigated for the first time. The high values of conductivity were proved. The composition BaLa0.9Gd0.1InO4 demonstrates predominantly protonic transport under wet air and low temperatures (<400 °C). The doping by rare earth metals of layered perovskite is a prospective method for significantly improving conductivity.

Electrokinetic Insights into the Triple Ionic and Electronic Conductivity of a Novel Nanocomposite Functional Material for Protonic Ceramic Fuel Cells

Triple ionic and electronic conductivity (TIEC) in cathode materials for protonic ceramic fuel cells (PCFCs) is a desirable feature that enhances the spatial expansion of active reaction sites for electrochemical oxygen reduction reaction. The realization of optimal TIEC in single-phase materials, however, is challenging. A facile route that facilitates the optimization of TIEC in PCFC cathodes is the strategic development of multiphase cathode materials. In this study, a cubic-rhombohedral TIEC nanocomposite material with the composition Ba(CeCo)_0.4 (FeZr)_0.1 O_{3- δ} (BCCFZ) is designed via self-assembly engineering. The material consists of a mixed ionic and electronic conducting phase, BaCo_{1-( x + y + z )} Ce_x Fe_y Zr_z O_{3- δ} (M-BCCFZ), and a dominant proton-conducting phase, BaCe_{1-( x + y + z )} Co_x Zr_y Fe_z O_{3- δ} (H-BCCZF). The dominant cerium-rich H-BCCFZ phase enhances the material's oxygen vacancy concentration and the proton defects formation and transport with a low enthalpy of protonation of -30 ± 9 kJ mol^-1 . The area-specific resistance of the BCCFZ symmetrical cell is 0.089 Ω cm² at 650 °C in 2.5% H₂ O-air. The peak power density of the anode-supported single cell based on BCCFZ cathode reaches 1054 mW cm^-2 at 650 °C with good operation stability spanning over 500 h at 550 °C. These promote BCCFZ as a befitting cathode material geared toward PCFC commercialization.

Structural and Transport Properties of E-Beam Sintered Lanthanide Tungstates and Tungstates-Molybdates

Lanthanide tungstates and molybdates are promising materials for hydrogen separation membranes due to their high protonic conductivity. A promising approach to fabricating ceramics based on these materials is radiation thermal sintering. The current work aims at studying the effect of radiation thermal sintering on the structural morphological and transport properties of (Nd,Ln)_5.5(W,Mo)O_11.25-δ as promising materials for hydrogen separation membranes. The defect fluorite structure was shown to be preserved during radiation thermal sintering at 1100 °C. The presence of protons in hydrated samples was confirmed by TGA. According to four-electrode studies and the isotope exchange of oxygen with C¹⁸O₂, the samples demonstrate a high proton conductivity and oxygen mobility. Residual porosity (up to 29%) observed for these samples can be dealt with during membrane preparation by adding sintering aids and/or metal alloys nanoparticles. Hence, sintering by e-beams can be applied to the manufacturing of hydrogen separation membranes based on these materials.

Electrochemical Response of Mixed Conducting Perovskite Enables Low-Cost High-Efficiency Hydrogen Sensing

High-performance noble metal-free gas sensors are crucial for widespread applications in various areas. Non-Nernstian electrochemical sensors have attracted tremendous attention, but are limited by the high cost and low efficiency of Pt electrode. Moreover, responses from different electrodes usually have the same polarity, degrading the sensor performance. Here we report a reverse response on a series of mixed ionic-electronic conductors (MIECs). Exemplary SrFe0.5Ti0.5O3-δ (SFT50) perovskite shows excellent H2 sensing properties, including high sensitivity and selectivity, humidity resistance, and long-term stability. Strikingly, the response is positive, as opposed to the usual one. Such an unusual response is ascribed to the change of the surface electrostatic potential due to the gas chemical reaction, which outcompetes traditional mechanisms, thereby reversing the response polarity. A conceptual noble-metal-free sensor with dual oxide electrodes of opposite polarity is designed by substituting SFT50 for the benchmark Pt, achieving a 1.5-2.0× increase in H2 response, sensitivity, and selectivity and a low limit of detection of 16 ppb. The ideal unity of excellent sensing and unusual polarity for MIECs can be used to optimize the performance of a variety of conventional sensors while reducing the cost. Our findings provide new insights into electrochemical gas sensing and offer a facile approach for developing low-cost high-performance gas sensors.

Hydration Entropy and Enthalpy of a Perovskite Oxide from Oxygen Tracer Diffusion Experiments

Water incorporation into perovskite oxides generates protonic defects in the form of hydroxide ions. In this study, an indirect method to probe the thermodynamics of water incorporation is demonstrated. Acceptor-doped single-crystal samples of SrTiO₃ were subjected to H₂¹⁸O/H₂¹⁶O exchange annealing at temperatures of 723 < T/K < 1023 at a water partial pressure of pH₂O = 0.1 bar; from ¹⁸O diffusion profiles, measured by secondary ion mass spectrometry, oxygen tracer diffusion coefficients DO* were obtained. The decreased values of DO* for wet (relative to dry) conditions yielded Δ_hydH = -(73 ± 15) kJ mol^-1 and Δ_hydS = -(148 ± 18) J mol^-1 K^-1 as the hydration enthalpy and entropy of SrTiO₃. For T < 1023 K and this pH₂O, the experiments also indicate that oxygen exchange from H₂O(g) is faster than that from O₂(g) (with a lower activation enthalpy) and that the surface space-charge potential is decreased under wet conditions.

Ion Intercalation in Lanthanum Strontium Ferrite for Aqueous Electrochemical Energy Storage Devices

Ion intercalation of perovskite oxides in liquid electrolytes is a very promising method for controlling their functional properties while storing charge, which opens up its potential application in different energy and information technologies. Although the role of defect chemistry in oxygen intercalation in a gaseous environment is well established, the mechanism of ion intercalation in liquid electrolytes at room temperature is poorly understood. In this study, the defect chemistry during ion intercalation of La_0.5Sr_0.5FeO_3-δ thin films in alkaline electrolytes is studied. Oxygen and proton intercalation into the La_1-xSr_xFeO_3-δ perovskite structure is observed at moderate electrochemical potentials (0.5 to -0.4 V), giving rise to a change in the oxidation state of Fe (as a charge compensation mechanism). The variation of the concentration of holes as a function of the intercalation potential is characterized by in situ ellipsometry, and the concentration of electron holes is indirectly quantified for different electrochemical potentials. Finally, a dilute defect chemistry model that describes the variation of defect species during ionic intercalation is developed.

Solution-Processed Titanium Dioxide Ion-Gated Transistors and Their Application for pH Sensing

Titanium dioxide (TiO2) is an abundant metal oxide, widely used in food industry, cosmetics, medicine, water treatment and electronic devices. TiO2 is of interest for next-generation indium-free thin-film transistors and ion-gated transistors due to its tunable optoelectronic properties, ambient stability, and solution processability. In this work, we fabricated TiO2 films using a wet chemical approach and demonstrated their transistor behavior with room temperature ionic liquids and aqueous electrolytes. In addition, we demonstrated the pH sensing behavior of the TiO2 IGTs with a sensitivity of ∼48 mV/pH. Furthermore, we demonstrated a low temperature (120°C), solution processed TiO2-based IGTs on flexible polyethylene terephthalate (PET) substrates, which were stable under moderate tensile bending.

Manipulating Electrocatalytic Activity of Perovskite Oxide Through Electrochemical Treatment

Perovskite oxides are widely used in electrochemical cells, profiting from their excellent accommodation of different elements and structure stability. Here, it is reported that when rapidly exceeding the electrochemical stability window of a perovskite oxide through electrochemical treatment, nanoparticles can dynamically exsolve from the perovskite lattice, yielding a nanoparticle decorated material (NDM) with fascinating particle population and distribution. It is reported that as compared to the NDM produced by chemical gas reduction, electrochemical treatment fabricated NDM shows much better electrochemical performance. At 900 °C, a peak power density (PPD) of 896 mW cm^-2 (more than tenfold enhancement) is obtained for a yttrium stabilized zirconia (YSZ) electrolyte-supported symmetrical cell with La_0.43 Ca_0.37 Ti_0.8 Co_0.1 Fe_0.1 O_{3- δ} (LCTCF) electrode after electrochemical treatment for several minutes, while it only reaches to 210 mW cm^-2 after chemical gas treatment for tens of hours using humidified hydrogen as fuel. The study establishes a new fairyland for tuning the performance of-but not limited to-the electrochemical cells.

Giant Functional Properties in Porous Electroceramics through Additive Manufacturing of Capillary Suspensions

Dedicated hierarchical structuring of functional ceramics can be used to shift the limits of functionality. This work presents the manufacturing of highly open porous, hierarchically structured barium titanate ceramics with 3-3 connectivity via direct ink writing of capillary suspension-type inks. The pore size of the printed struts (∼1 μm) is combined with a printed mesostructure (∼100 μm). The self-organized particle network, driven by strong capillary forces in the ternary solid/fluid/fluid ink, results in a high strut porosity, and the distinct flow properties of the ink allow for printing high strut size to pore size ratios, resulting in total porosities >60%. These unique and highly porous additive manufactured log-pile structures with closed bottom and top layers enable tailored dielectric and electromechanical coupling, resulting in an energy harvesting figure of merit FOM₃₃ more than four times higher than any documented data for barium titanate. This clearly demonstrates that combining additive manufacturing of capillary suspensions in combination with appropriate sintering allows for creation of complex architected 3D structures with unprecedented properties. This opens up opportunities in a broad variety of applications, including electromechanical energy harvesting, electrode materials for batteries or fuel cells, thermoelectrics, or bone tissue engineering with piezoelectrically stimulated cell growth.

Enhanced mixed proton and electron conductor at room temperature from chemically modified single-wall carbon nanotubes

A chemically modified single-wall carbon nanotube showing efficient mixed proton and electron conduction at room temperature is demonstrated.

Progress and prospects of reversible solid oxide fuel cell materials

Reversible solid oxide fuel cell (RSOFC) is an energy device that flexibly interchanges between electrical and chemical energy according to people's life and production needs. The development of cell materials affects the stability and cost of the cell, but also restricts its market-oriented development. After decades of research by scientists, a lot of achievements and progress have been made on RSOFC materials. According to the composition and requirements of each component of RSOFC, this article summarizes the research progress based on materials and discusses the merits and demerits of current cell materials in electrochemical performance. According to the efficiency of different materials in solid oxide fuel cell (SOFC mode) and solid oxide electrolyzer (SOEC mode), the challenges encountered by RSOFC in the operation are evaluated, and the future development of RSOFC materials is boldly prospected.

Synthesis of LaWN3 nitride perovskite with polar symmetry

[Figure: see text].

High-Conductive Protonated Layered Oxides from H2 O Vapor-Annealed Brownmillerites

Protonated 3d transition-metal oxides often display low electronic conduction, which hampers their application in electric, magnetic, thermoelectric, and catalytic fields. Electronic conduction can be enhanced by co-inserting oxygen acceptors simultaneously. However, the currently used redox approaches hinder protons and oxygen ions co-insertion due to the selective switching issues. Here, a thermal hydration strategy for systematically exploring the synthesis of conductive protonated oxides from 3d transition-metal oxides is introduced. This strategy is illustrated by synthesizing a novel layered-oxide SrCoO₃ H from the brownmillerite SrCoO_2.5 . Compared to the insulating SrCoO_2.5 , SrCoO₃ H exhibits an unprecedented high electronic conductivity above room temperature, water uptake at 250 °C, and a thermoelectric power factor of up to 1.2 mW K^-2 m^-1 at 300 K. These findings open up opportunities for creating high-conductive protonated layered oxides by protons and oxygen ions co-doping.

Surface and Bulk Oxygen Kinetics of BaCo0.4Fe0.4Zr0.2-XYXO3-δ Triple Conducting Electrode Materials

Triple ionic-electronic conductors have received much attention as electrode materials. In this work, the bulk characteristics of oxygen diffusion and surface exchange were determined for the triple-conducting BaCo_0.4Fe_0.4Zr_0.2−XY_XO_3−δ suite of samples. Y substitution increased the overall size of the lattice due to dopant ionic radius and the concomitant formation of oxygen vacancies. Oxygen permeation measurements exhibited a three-fold decrease in oxygen permeation flux with increasing Y substitution. The DC total conductivity exhibited a similar decrease with increasing Y substitution. These relatively small changes are coupled with an order of magnitude increase in surface exchange rates from Zr-doped to Y-doped samples as observed by conductivity relaxation experiments. The results indicate that Y-doping inhibits bulk O²⁻ conduction while improving the oxygen reduction surface reaction, suggesting better electrode performance for proton-conducting systems with greater Y substitution.

Defect engineering of oxide perovskites for catalysis and energy storage: synthesis of chemistry and materials science

Oxide perovskites have emerged as an important class of materials with important applications in many technological areas, particularly thermocatalysis, electrocatalysis, photocatalysis, and energy storage. However, their implementation faces numerous challenges that are familiar to the chemist and materials scientist. The present work surveys the state-of-the-art by integrating these two viewpoints, focusing on the critical role that defect engineering plays in the design, fabrication, modification, and application of these materials. An extensive review of experimental and simulation studies of the synthesis and performance of oxide perovskites and devices containing these materials is coupled with exposition of the fundamental and applied aspects of defect equilibria. The aim of this approach is to elucidate how these issues can be integrated in order to shed light on the interpretation of the data and what trajectories are suggested by them. This critical examination has revealed a number of areas in which the review can provide a greater understanding. These include considerations of (1) the nature and formation of solid solutions, (2) site filling and stoichiometry, (3) the rationale for the design of defective oxide perovskites, and (4) the complex mechanisms of charge compensation and charge transfer. The review concludes with some proposed strategies to address the challenges in the future development of oxide perovskites and their applications.

Recent Progress in DNA Hybridization Chain Reaction Strategies for Amplified Biosensing

With the continuous development of DNA nanotechnology, various spatial DNA structures and assembly techniques emerge. Hybridization chain reaction (HCR) is a typical example with exciting features and bright prospects in biosensing, which has been intensively investigated in the past decade. In this Spotlight on Applications, we summarize the assembly principles of conventional HCR and some novel forms of linear/nonlinear HCR. With advantages like great assembly kinetics, facile operation, and an enzyme-free and isothermal reaction, these strategies can be integrated with most mainstream reporters (e.g., fluorescence, electrochemistry, and colorimetry) for the ultrasensitive detection of abundant targets. Particularly, we select several representative studies to better illustrate the novel ideas and performances of HCR strategies. Theoretical and practical utilities are confirmed for a range of biosensing applications. In the end, a deep discussion is provided about the challenges and future tasks of this field.

Ammonia-fed reversible protonic ceramic fuel cells with Ru-based catalyst

The intermediate operating temperatures (~400-600 °C) of reversible protonic ceramic fuel cells (RePCFC) permit the potential use of ammonia as a carbon-neutral high energy density fuel and energy storage medium. Here we show fabrication of anode-supported RePCFC with an ultra-dense (~100%) and thin (4 μm) protonic ceramic electrolyte layer. When coupled to a novel Ru-(BaO)₂(CaO)(Al₂O₃) (Ru-B2CA) reversible ammonia catalyst, maximum fuel-cell power generation reaches 877 mW cm^-2 at 650 °C under ammonia fuel. We report relatively stable operation at 600 °C for up to 1250 h under ammonia fuel. In fuel production mode, ammonia rates exceed 1.2 × 10^-8 NH₃ mol cm^-2 s^-1at ambient pressure with H₂ from electrolysis only, and 2.1 × 10^-6 mol NH₃ cm^-2 s^-1 at 12.5 bar with H₂ from both electrolysis and simulated recycling gas.

Instrument for spatially resolved, temperature-dependent electrochemical impedance spectroscopy of thin films under locally controlled atmosphere

We demonstrate an instrument for spatially resolved measurements (mapping) of electrochemical impedance under various temperatures and gas environments. Automated measurements are controlled by a custom LabVIEW program, which manages probe motion, sample motion, temperature ramps, and potentiostat functions. Sample and probe positioning is provided by stepper motors. Dry or hydrated atmospheres (air or nitrogen) are available. The configurable heater reaches temperatures up to 500 °C, although the temperature at the sample surface is moderated by the gas flow rate. The local gas environment is controlled by directing flow toward the sample via a glass enclosure that surrounds the gold wire probe. Software and hardware selection and design are discussed. Reproducibility and accuracy are quantified on a Ba(Zr,Y)O_3-δ proton-conducting electrolyte thin film synthesized by pulsed laser deposition. The mapping feature of the instrument is demonstrated on a compositionally graded array of electrocatalytically active Ba(Co,Fe,Zr,Y)O_3-δ thin film microelectrodes. The resulting data indicate that this method proficiently maps property trends in these materials, thus demonstrating the reliability and usefulness of this method for investigating electrochemically active thin films.

A Fusion Parameter Method for Classifying Freshness of Fish Based on Electrochemical Impedance Spectroscopy

Compared with using a single characteristic parameter of electrochemical impedance spectroscopy (EIS) to classify the freshness of fish samples from different origins, more characteristic parameters could bring higher accuracy as well as complexity, subjectivity, and uncertainty. In order to eliminate the disadvantages of the multiparameter model, a data fusion method based on model similarity (DFMS) was proposed in this study. The similarity relation between the freshness models based on EIS characteristic parameters and physicochemical indicator was analyzed and quantified accordingly, and then, the weighting factors of the fusion model were determined. The classification accuracy rate of fish freshness based on DFMS was 9.2∼15% greater than that of a single EIS characteristic parameter. The novel dimensionless fusion parameter method proposed in this article might provide a simple yet effective indicator for EIS-based food quality evaluation.

Characterization of the high temperature properties of BaCe0.4Zr0.4Pr0.2O3−δ perovskite as a potential material for PC-SOFCs

This work presents a systematic study of the high temperature properties of BaCe_0.4Zr_0.4Pr_0.2O_3−δ perovskite in view of its potential application in proton conducting solid oxide fuel cells.

Mechanistic origin of the time-dependence of the open-circuit voltage of a galvanic cell involving a ternary or higher compound

It has previously been predicted [H.-I. Yoo and M. Martin, Phys. Chem. Chem. Phys., 2010, 12, 14699] and observed [E. Kim, et al., Solid State Ionics, 2013, 235, 22] that the open-circuit voltage U of a galvanic cell, involving a ternary or higher compound with more than one kind of mobile ionic carrier, is path- and time-dependent upon imposition or removal of the mobile components' chemical potential differences, in contradistinction to the cell involving a binary compound. This has been attributed [H.-I. Yoo and M. Martin, Phys. Chem. Chem. Phys., 2010, 12 14699; J.-Y. Yoon, et al., Solid State Ionics, 2012, 213, 22] to the decoupled redistributions of multiple mobile components or multi-fold relaxation. We hereby experimentally demonstrate with SrTi0.982Al0.018O3-Δ, known to have an appreciable water solubility depending on temperature, that introduction of a secondary ionic carrier H+ in addition to the native O2- indeed renders the otherwise time-independent U time-dependent; and that this phenomenon may, thus, be employed to probe the presence of a secondary ionic carrier, e.g., H+ in addition to the primary O2- in BaTi0.982Al0.018O3-Δ whose water solubility is yet to be known. The temporal behavior of U of SrTi0.982Al0.018O3-Δ subjected to the two fixed chemical potential differences, ΔμO and ΔμH, is precisely delineated in terms of two-fold relaxation of H and O, yielding their chemical diffusivity values, and consequently, the ambiguity with the EMF-method to determine the ionic transference numbers of a multinary compound is cleared away.