Synergistic Bulk and Surface Engineering for Expeditious and Durable Reversible Protonic Ceramic Electrochemical Cells Air Electrode.

Reversible protonic ceramic electrochemical cells (R-PCECs) offer the potential for high-efficiency power generation and green hydrogen production at intermediate temperatures. However, the commercial viability of R-PCECs is hampered by the sluggish kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) within conventional air electrodes operating at reduced temperatures. To address this challenge, this work introduces a novel approach based on the simultaneous optimization of bulk-phase metal-oxygen bonds and in-situ formation of a metal oxide nano-catalyst surface modification. This strategy is designed to expedite the ORR/OER electrocatalytic activity of air electrodes exhibiting triple (O2-, H+, e-) conductivity. Specifically, this engineered air electrode nanocomposite-Ba(Co0.4Fe0.4Zr0.1Y0.1)0.95Ni0.05F0.1O2.9-δ demonstrates remarkable ORR/OER catalytic activity and exceptional durability in R-PCECs. This is evidenced by significantly improved peak power density from 626 to 996 mW cm-2 and highly stable reversibility over a 100-h cycling period. This research offers a rational design strategy to achieve high-performance R-PCEC air electrodes with superior operational activity and stability for efficient and sustainable energy conversion and storage.

Synergistic dual-phase air electrode enables high and durable performance of reversible proton ceramic electrochemical cells.

Reversible proton ceramic electrochemical cells are promising solid-state ion devices for efficient power generation and energy storage, but necessitate effective air electrodes to accelerate the commercial application. Here, we construct a triple-conducting hybrid electrode through a stoichiometry tuning strategy, composed of a cubic phase Ba0.5Sr0.5Co0.8Fe0.2O3-δ and a hexagonal phase Ba4Sr4(Co0.8Fe0.2)4O16-δ. Unlike the common method of creating self-assembled hybrids by breaking through material tolerance limits, the strategy of adjusting the stoichiometric ratio of the A-site/B-site not only achieves strong interactions between hybrid phases, but also can efficiently modifies the phase contents. When operate as an air electrode for reversible proton ceramic electrochemical cell, the hybrid electrode with unique dual-phase synergy shows excellent electrochemical performance with a current density of 3.73 A cm-2 @ 1.3 V in electrolysis mode and a peak power density of 1.99 W cm-2 in fuel cell mode at 650 °C.

Identifying a Universal Activity Descriptor and a Unifying Mechanism Concept on Perovskite Oxides for Green Hydrogen Production.

Producing indispensable hydrogen and oxygen for social development via water electrolysis shows more prospects than other technologies. Although electrocatalysts have been explored for centuries, a universal activity descriptor for both hydrogen-evolution reaction (HER) and oxygen-evolution reaction (OER) is not yet developed. Moreover, a unifying concept is not yet established to simultaneously understand HER/OER mechanisms. Here, the relationships between HER/OER activities in three common electrolytes and over ten representative material properties on 12 3d-metal-based model oxides are rationally bridged through statistical methodologies. The orbital charge-transfer energy (Δ) can serve as an ideal universal descriptor, where a neither too large nor too small Δ (≈1 eV) with optimal electron-cloud density around Fermi level affords the best activities, fulfilling Sabatier's principle. Systematic experiments and computations unravel that pristine oxide with Δ ≈ 1 eV possesses metal-like high-valence configurations and active lattice-oxygen sites to help adsorb key protons in HER and induce lattice-oxygen participation in the OER, respectively. After reactions, partially generated metals in the HER and high-valence hydroxides in the OER dominate proton adsorption and couple with pristine lattice-oxygen activation, respectively. These can be successfully rationalized by the unifying orbital charge-transfer theory. This work provides the foundation of rational material design and mechanism understanding for many potential applications.

The bio-inspired heterogeneous single-cluster catalyst Ni100-Fe4S4 for enhanced electrochemical CO2 reduction to CH4.

Electrochemical conversion of CO2-to-CH4 is a process of converting the inert greenhouse gas into energy molecules. It offers great promise for the transformation of carbon-neutral economy. However, achieving high CH4 activity and selectivity remains a major challenge because the electrochemical reduction of CO2-to-CH4 is accompanied by various C1 intermediates at the catalytic site, involving multiple proton-coupled electron transfer processes. Herein, different from the traditional designing strategy, we propose a bio-inspired theoretical design approach to construct a heterogeneous single-cluster catalyst Ni100-Fe4S4 at the atomic level, which may show high CO2 electroreduction performance. Combined with the crystallographic data and theoretical calculations, Ni100-Fe4S4 and CO dehydrogenase exhibit highly similar catalytic geometric active centers and CO2 binding modes. By exploring the origin of the catalytic activity of this biomimetic structure, we found that the activation of CO2 on Ni100-Fe4S4 theoretically exceeds that on natural CO dehydrogenase. Density functional theory calculations reveal that the dehydrogenase enzyme-liked Fe-Ni active site serves as an electron enrichment 'electro-bridge' (an electron-rich highly active catalytic site), which can activate CO2 molecules efficiently and stabilize various intermediates in multistep elementary reactions to selectively produce CH4 at a low overpotential (0.13 eV). The calculated CO2 electroreduction pathways are well consistent with the nickel-based catalytic materials reported in experimental studies. Our work showcases and highlights the rational design of high-performance catalytic materials via the biomimetic methodology at the atomic level.

Exceptional Anisotropic Noncovalent Interactions in Ultrathin Nanorods: The Terminal σ-Hole.

Nanomaterial is the Holy Grail of material science, which has been widely applied in the fields of energy, environment, chemistry, and biomedicine. Its catalytic merits were usually ascribed to the advantages of size effect, strain effect, and covalent effect. Noncovalent interactions are critical in the catalysis processes but often overlooked. Herein, different from the traditional understandings, we discover for the first time and give systematic insights into a unique noncovalent terminal σ-hole phenomenon in the 3d-metal-based nanorods, which should be one of the key origins of nanomaterial activity. As a proof-of-concept, pure metal and alloyed core-shell nanoclusters/nanorods composed of the two most important 3d metals (Co and Ni) growing from 0.5 to 2.5 nm are investigated. Unlike nanoclusters, the σ-hole only appears at the terminal sites of nanorods and the magnitude of the terminal σ-hole generally enhances with the growing processes. Further investigations show that this terminal σ-hole is closely related to the important physicochemical properties of nanorods. For example, the work function along the axis of the terminal σ-hole is smaller than other directions, contributing to the facile electronic transport along the axis of the terminal σ-hole. Most importantly, we find that the d-orbital center of the atoms around the terminal σ-hole shifts closer to the Fermi level as compared with other atoms, which can endow the terminal sites in nanorods with the higher chemical adsorption capability. We believe that this work will provide critical guidance for the rational design of nanomaterials in many potential applications.

Hydrogen spillover in complex oxide multifunctional sites improves acidic hydrogen evolution electrocatalysis.

Improving the catalytic efficiency of platinum for the hydrogen evolution reaction is valuable for water splitting technologies. Hydrogen spillover has emerged as a new strategy in designing binary-component Pt/support electrocatalysts. However, such binary catalysts often suffer from a long reaction pathway, undesirable interfacial barrier, and complicated synthetic processes. Here we report a single-phase complex oxide La2Sr2PtO7+δ as a high-performance hydrogen evolution electrocatalyst in acidic media utilizing an atomic-scale hydrogen spillover effect between multifunctional catalytic sites. With insights from comprehensive experiments and theoretical calculations, the overall hydrogen evolution pathway proceeds along three steps: fast proton adsorption on O site, facile hydrogen migration from O site to Pt site via thermoneutral La-Pt bridge site serving as the mediator, and favorable H2 desorption on Pt site. Benefiting from this catalytic process, the resulting La2Sr2PtO7+δ exhibits a low overpotential of 13 mV at 10 mA cm-2, a small Tafel slope of 22 mV dec-1, an enhanced intrinsic activity, and a greater durability than commercial Pt black catalyst.

New Undisputed Evidence and Strategy for Enhanced Lattice-Oxygen Participation of Perovskite Electrocatalyst through Cation Deficiency Manipulation.

Oxygen evolution reaction (OER) is a key half-reaction in many electrochemical transformations, and efficient electrocatalysts are critical to improve its kinetics which is typically sluggish due to its multielectron-transfer nature. Perovskite oxides are a popular category of OER catalysts, while their activity remains insufficient under the conventional adsorbate evolution reaction scheme where scaling relations limit activity enhancement. The lattice oxygen-mediated mechanism (LOM) has been recently reported to overcome such scaling relations and boost the OER catalysis over several doped perovskite catalysts. However, direct evidence supporting the LOM participation is still very little because the doping strategy applied would introduce additional active sites that may mask the real reaction mechanism. Herein, a dopant-free, cation deficiency manipulation strategy to tailor the bulk diffusion properties of perovskites without affecting their surface properties is reported, providing a perfect platform for studying the contribution of LOM to OER catalysis. Further optimizing the A-site deficiency achieves a perovskite candidate with excellent intrinsic OER activity, which also demonstrates outstanding performance in rechargeable Zn-air batteries and water electrolyzers. These findings not only corroborate the key role of LOM in OER electrocatalysis, but also provide an effective way for the rational design of better catalyst materials for clean energy technologies.

A New Durable Surface Nanoparticles-Modified Perovskite Cathode for Protonic Ceramic Fuel Cells from Selective Cation Exsolution under Oxidizing Atmosphere.

A high-performance cathode of a protonic ceramic fuel cell (PCFC) should possess excellent oxygen reduction reactivity, high proton/oxygen-ion/electron conductivity, and sufficient operational stability, thus requiring a delicate tuning of both the bulk and surface properties of the electrode material. Although surface modification of perovskites with nanoparticles from reducing-atmosphere exsolution has been demonstrated effective at improving the electrochemical anodic oxidation, such nanoparticles would easily re-incorporate into the perovskite lattice causing a big challenge for their application as a cathode. Here, a durable perovskite-based nanocomposite cathode for PCFCs is reported, which is facilely prepared via the exsolution of nanoparticles in an oxidizing atmosphere. Through composition and cation nonstoichiometry manipulation, a precursor with the nominal composition of Ba0.95 (Co0.4 Fe0.4 Zr0.1 Y0.1 )0.95 Ni0.05 O3-δ (BCFZYN-095) is designed, synthesized, and investigated, which, upon calcination, gives rise to the formation of a perovskite-based nanocomposite comprising a major perovskite phase and a minor NiO phase enriched on the perovskite surface. The major perovskite phase enabled by the proper cation nonstoichiometry manipulation promotes bulk proton conduction while the NiO nanoparticles facilitate the oxygen surface exchange process, leading to a superior cathodic performance with a maximum peak power density of 1040 mW cm-2 at 650 °C and excellent operational stability of 400 h at 550 °C.

One Pot-Synthesized Ag/Ag-Doped CeO2 Nanocomposite with Rich and Stable 3D Interfaces and Ce3+ for Efficient Carbon Dioxide Electroreduction.

Electrochemical CO2 reduction (ECR) technology is promising to produce value-added chemicals and alleviate the climate deterioration. Interface engineering is demonstrated to improve the ECR performance for metal and oxide composite catalysts. However, the approach to form a substantial interface is still limited. In this work, we report a facile one-pot coprecipitation method to synthetize novel silver and silver-doped ceria (Ag/CeO2) nanocomposites. This catalyst provides a rich 3D interface and high Ce3+ concentration (33.6%), both of which are beneficial for ECR to CO. As a result, Ag/CeO2 exhibits a 99% faradaic efficiency and 10.5 A g-1 mass activity to convert CO2 into CO at an overpotential of 0.83 V. The strong interfacial interaction between Ag and CeO2 may enable the presence of surface Ce3+ and guarantee the improved durability during the electrolysis. We also develop numerical simulation to understand the local pH effect on the ECR performance and propose that the superior ECR performance of Ag/CeO2 is mainly due to the accelerated CO formation rate rather than the suppressed hydrogen evolution reaction.

Exceptionally Robust Face-Sharing Motifs Enable Efficient and Durable Water Oxidation.

Corner-sharing oxides usually suffer from structural reconstruction during the bottleneck oxygen-evolution reaction (OER) in water electrolysis. Therefore, introducing dynamically stable active sites in an alternative structure is urgent but challenging. Here, 1D 5H-polytype Ba5 Bi0.25 Co3.75 FeO14- δ oxide with face-sharing motifs is identified as a highly active and stable candidate for alkaline OER. Benefiting from the stable face-sharing motifs with three couples of combined bonds, Ba5 Bi0.25 Co3.75 FeO14- δ can maintain its local structures even under high OER potentials as evidenced by fast operando spectroscopy, contributing to a negligible performance degradation over 110 h. Besides, the higher Co valence and smaller orbital bandgap in Ba5 Bi0.25 Co3.75 FeO14- δ endow it with a much better electron transport ability than its corner-sharing counterpart, leading to a distinctly reduced overpotential of 308 mV at 10 mA cm-2 in 0.1 m KOH. Further mechanism studies show that the short distance between lattice-oxygen sites in face-sharing Ba5 Bi0.25 Co3.75 FeO14- δ can accelerate the deprotonation step (*OOH + OH-  = *OO + H2 O + e- ) via a steric inductive effect to promote lattice-oxygen participation. In this work, not only is a new 1D face-sharing oxide with impressive OER performance discovered, but also a rational design of dynamic stable and active sites for sustainable energy systems is inaugurated.

High-Performance Perovskite Composite Electrocatalysts Enabled by Controllable Interface Engineering.

Single-phase perovskite oxides that contain nonprecious metals have long been pursued as candidates for catalyzing the oxygen evolution reaction, but their catalytic activity cannot meet the requirements for practical electrochemical energy conversion technologies. Here a cation deficiency-promoted phase separation strategy to design perovskite-based composites with significantly enhanced water oxidation kinetics compared to single-phase counterparts is reported. These composites, self-assembled from perovskite precursors, comprise strongly interacting perovskite and related phases, whose structure, composition, and concentration can be accurately controlled by tailoring the stoichiometry of the precursors. The composite catalyst with optimized phase composition and concentration outperforms known perovskite oxide systems and state-of-the-art catalysts by 1-3 orders of magnitude. It is further demonstrated that the strong interfacial interaction of the composite catalysts plays a key role in promoting oxygen ionic transport to boost the lattice-oxygen participated water oxidation. These results suggest a simple and viable approach to developing high-performance, perovskite-based composite catalysts for electrochemical energy conversion.

Thermal-expansion offset for high-performance fuel cell cathodes.

One challenge for the commercial development of solid oxide fuel cells as efficient energy-conversion devices is thermo-mechanical instability. Large internal-strain gradients caused by the mismatch in thermal expansion behaviour between different fuel cell components are the main cause of this instability, which can lead to cell degradation, delamination or fracture1-4. Here we demonstrate an approach to realizing full thermo-mechanical compatibility between the cathode and other cell components by introducing a thermal-expansion offset. We use reactive sintering to combine a cobalt-based perovskite with high electrochemical activity and large thermal-expansion coefficient with a negative-thermal-expansion material, thus forming a composite electrode with a thermal-expansion behaviour that is well matched to that of the electrolyte. A new interphase is formed because of the limited reaction between the two materials in the composite during the calcination process, which also creates A-site deficiencies in the perovskite. As a result, the composite shows both high activity and excellent stability. The introduction of reactive negative-thermal-expansion components may provide a general strategy for the development of fully compatible and highly active electrodes for solid oxide fuel cells.

Fast operando spectroscopy tracking in situ generation of rich defects in silver nanocrystals for highly selective electrochemical CO2 reduction.

Electrochemical CO2 reduction (ECR) is highly attractive to curb global warming. The knowledge on the evolution of catalysts and identification of active sites during the reaction is important, but still limited. Here, we report an efficient catalyst (Ag-D) with suitable defect concentration operando formed during ECR within several minutes. Utilizing the powerful fast operando X-ray absorption spectroscopy, the evolving electronic and crystal structures are unraveled under ECR condition. The catalyst exhibits a ~100% faradaic efficiency and negligible performance degradation over a 120-hour test at a moderate overpotential of 0.7 V in an H-cell reactor and a current density of ~180 mA cm-2 at -1.0 V vs. reversible hydrogen electrode in a flow-cell reactor. Density functional theory calculations indicate that the adsorption of intermediate COOH could be enhanced and the free energy of the reaction pathways could be optimized by an appropriate defect concentration, rationalizing the experimental observation.

Efficient Water Splitting Actualized through an Electrochemistry-Induced Hetero-Structured Antiperovskite/(Oxy)Hydroxide Hybrid.

Exploring active, stable, and low-cost bifunctional electrocatalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is crucial for water splitting technology associated with renewable energy storage in the form of hydrogen fuel. Here, a newly designed antiperovskite-based hybrid composed of a conductive InNNi3 core and amorphous InNi(oxy)hydroxide shell is first reported as promising OER/HER bifunctional electrocatalyst. Prepared by a facile electrochemical oxidation strategy, such unique hybrid (denoted as EO-InNNi3 ) exhibits excellent OER and HER activities in alkaline media, benefiting from the inherent high-efficiency HER catalytic nature of InNNi3 antiperovskite and the promoting role of OER-active InNi(oxy)hydroxide thin film, which is confirmed by theoretical simulations and in situ Raman studies. Moreover, an alkaline electrolyzer integrated EO-InNNi3 as both anode and cathode delivers a low voltage of 1.64 V at 10 mA cm-2 , while maintaining excellent durability. This work demonstrates the application of antiperovskite-based materials in the field of overall water splitting and inspires insights into the development of advanced catalysts for various energy applications.

Single-phase perovskite oxide with super-exchange induced atomic-scale synergistic active centers enables ultrafast hydrogen evolution.

The state-of-the-art active HER catalysts in acid media (e.g., Pt) generally lose considerable catalytic performance in alkaline media mainly due to the additional water dissociation step. To address this issue, synergistic hybrid catalysts are always designed by coupling them with metal (hydro)oxides. However, such hybrid systems usually suffer from long reaction path, high cost and complex preparation methods. Here, we discover a single-phase HER catalyst, SrTi0.7Ru0.3O3-δ (STRO) perovskite oxide highlighted with an unusual super-exchange effect, which exhibits excellent HER performance in alkaline media via atomic-scale synergistic active centers. With insights from first-principles calculations, the intrinsically synergistic interplays between multiple active centers in STRO are uncovered to accurately catalyze different elementary steps of alkaline HER; namely, the Ti sites facilitates nearly-barrierless water dissociation, Ru sites function favorably for OH* desorption, and non-metal oxygen sites (i.e., oxygen vacancies/lattice oxygen) promotes optimal H* adsorption and H2 desorption.

Utilizing ion leaching effects for achieving high oxygen-evolving performance on hybrid nanocomposite with self-optimized behaviors.

Ion leaching from pure-phase oxygen-evolving electrocatalysts generally exists, leading to the collapse and loss of catalyst crystalline matrix. Here, different from previous design methodologies of pure-phase perovskites, we introduce soluble BaCl2 and SrCl2 into perovskites through a self-assembly process aimed at simultaneously tuning dual cation/anion leaching effects and optimizing ion match in perovskites to protect the crystalline matrix. As a proof-of-concept, self-assembled hybrid Ba0.35Sr0.65Co0.8Fe0.2O3-δ (BSCF) nanocomposite (with BaCl2 and SrCl2) exhibits the low overpotential of 260 mV at 10 mA cm-2 in 0.1 M KOH. Multiple operando spectroscopic techniques reveal that the pre-leaching of soluble compounds lowers the difference of interfacial ion concentrations and thus endows the host phase in hybrid BSCF with abundant time and space to form stable edge/face-sharing surface structures. These self-optimized crystalline structures show stable lattice oxygen active sites and short reaction pathways between Co-Co/Fe metal active sites to trigger favorable adsorption of OH- species.

Bulk and Surface Properties Regulation of Single/Double Perovskites to Realize Enhanced Oxygen Evolution Reactivity.

Perovskite-based oxides have emerged as promising oxygen evolution reaction (OER) electrocatalysts. The performance is closely related to the lattice, electronic, and defect structure of the oxides, which determine surface and bulk properties and consequent catalytic activity and durability. Further, interfacial interactions between phases in a nanocomposite may affect bulk transportation and surface adsorption properties in a similar manner to phase doping except without solubility limits. Herein, we report the development of a single/double perovskite nanohybrid with limited surface self-reconstruction capability as an OER electrocatalyst. Such superior performance arises from a structure that maintains high crystallinity post OER catalysis, in addition to forming an amorphous layer following the self-reconstruction of a single perovskite structure during the OER process. In situ X-ray absorption near edge structure spectroscopy and high-resolution synchrotron-based X-ray diffraction reveal an amorphization process in the hybrid single/double perovskite oxide system that is limited in comparison to single perovskite amorphization, ensuring high catalytic activity.

Direct evidence of boosted oxygen evolution over perovskite by enhanced lattice oxygen participation.

The development of oxygen evolution reaction (OER) electrocatalysts remains a major challenge that requires significant advances in both mechanistic understanding and material design. Recent studies show that oxygen from the perovskite oxide lattice could participate in the OER via a lattice oxygen-mediated mechanism, providing possibilities for the development of alternative electrocatalysts that could overcome the scaling relations-induced limitations found in conventional catalysts utilizing the adsorbate evolution mechanism. Here we distinguish the extent to which the participation of lattice oxygen can contribute to the OER through the rational design of a model system of silicon-incorporated strontium cobaltite perovskite electrocatalysts with similar surface transition metal properties yet different oxygen diffusion rates. The as-derived silicon-incorporated perovskite exhibits a 12.8-fold increase in oxygen diffusivity, which matches well with the 10-fold improvement of intrinsic OER activity, suggesting that the observed activity increase is dominantly a result of the enhanced lattice oxygen participation.

Screening highly active perovskites for hydrogen-evolving reaction via unifying ionic electronegativity descriptor.

Facile and reliable screening of cost-effective, high-performance and scalable electrocatalysts is key for energy conversion technologies such as water splitting. ABO3-δ perovskites, with rich constitutions and structures, have never been designed via activity descriptors for critical hydrogen evolution reaction (HER). Here, we apply coordination rationales to introduce A-site ionic electronegativity (AIE) as an efficient unifying descriptor to predict the HER activities of 13 cobalt-based perovskites. Compared with A-site structural or thermodynamic parameter, AIE endows the HER activity with the best volcano trend. (Gd0.5La0.5)BaCo2O5.5+δ predicted from an AIE value of ~2.33 exceeds the state-of-the-art Pt/C catalyst in electrode activity and stability. X-ray absorption and computational studies reveal that the peak HER activities at a moderate AIE value of ~2.33 can be associated with the optimal electronic states of active B-sites via inductive effect in perovskite structure (~200 nm depth), including Co valence, Co-O bond covalency, band gap and O 2p-band position.

Super-Exchange Interaction Induced Overall Optimization in Ferromagnetic Perovskite Oxides Enables Ultrafast Water Oxidation.

Oxygen evolution reaction (OER) is crucial in many renewable electrochemical technologies including regenerative fuel cells, rechargeable metal-air batteries, and water splitting. It is found that abundant active sites with favorable electronic structure and high electrical conductivity play a dominant role in achieving high electrocatalytic efficiency of perovskites, thus efficient strategies need to be designed to generate multiple beneficial factors for OER. Here, highlighted is an unusual super-exchange effect in ferromagnetic perovskite oxide to optimize active sites and enhance electrical conductivity. A systematic exploration about the composition-dependent OER activity in SrCo1 x Rux O3- δ (denoted as SCRx) (x = 0.0-1.0) perovskite is displayed with special attention on the role of super-exchange interaction between high spin (HS) Co3+ and Ru5+ ions. Induced by the unique Co3+ -O-Ru5+ super-exchange interactions, the SCR0.1 is endowed with abundant OER active species including Co3+ /Co4+ , Ru5+ , and O2 2- /O- , high electrical conductivity, and metal-oxygen covalency. Benefiting from these advantageous factors for OER electrocatalysis, the optimized SCR0.1 catalyst exhibits a remarkable activity with a low overpotential of 360 mV at 10 mA cm-2 , which exceeds the benchmark RuO2 and most well-known perovskite oxides reported so far, while maintaining excellent durability. This work provides a new pathway in developing perovskite catalysts for efficient catalysis.