Water electrolysis on La(1-x)Sr(x)CoO(3-δ) perovskite electrocatalysts

Decoupling the roles of carbon and metal oxides on the electrocatalytic reduction of oxygen on La1-xSrxCoO3-δ perovskite composite electrodes

Perovskite oxides are active room-temperature bifunctional oxygen electrocatalysts in alkaline media, capable of performing the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) with lower combined overpotentials relative to their precious metal counterparts. However, their semiconducting nature necessitates the use of activated carbons as conductive supports to generate applicably relevant current densities. In efforts to advance the performance and theory of oxide electrocatalysts, the chemical and physical properties of the oxide material often take precedence over contributions from the conductive additive. In this work, we find that carbon plays an important synergistic role in improving the performance of La1-xSrxCoO3-δ (0 ≤ x ≤ 1) electrocatalysts through the activation of O2 and spillover of radical oxygen intermediates, HO2- and O2-, which is further reduced through chemical decomposition of HO2- on the perovskite surface. Through a combination of thin-film rotating disk electrochemical characterization of the hydrogen peroxide intermediate reactions (hydrogen peroxide reduction reaction (HPRR), hydrogen peroxide oxidation reaction (HPOR)) and oxygen reduction reaction (ORR), surface chemical analysis, HR-TEM, and microkinetic modeling on La1-xSrxCoO3-δ (0 ≤ x ≤ 1)/carbon (with nitrogen and non-nitrogen doped carbons) composite electrocatalysts, we deconvolute the mechanistic aspects and contributions to reactivity of the oxide and carbon support.

Anion-Based Pseudocapacitance of the Perovskite Library La1- xSr xBO3-δ (B = Fe, Mn, Co)

We have synthesized a library of perovskite oxides with the composition La_{1- x}Sr _xBO_3-δ ( x = 0-1; B = Fe, Mn, Co) to systematically study anion-based pseudocapacitance. The electrochemical capacitance of these materials was evaluated by cyclic voltammetry and galvanostatic charging/discharging in 1 M KOH. We find that greater oxygen vacancy content (δ) upon systematic incorporation of Sr²⁺ linearly increases the surface-normalized capacity with a slope controlled by the B-site element. La_0.2Sr_0.8MnO_2.7 exhibited the highest specific capacitance of 492 F g^-1 at 5 mV s^-1 relative to the Fe and Co oxides. In addition, the first all-perovskite asymmetric pseudocapacitor has been successfully constructed and characterized in neutral and alkaline aqueous electrolytes. We demonstrate that the asymmetric pseudocapacitor cell voltage can be increased by widening the difference between the B-site transition metal redox potentials in each electrode resulting in a maximum voltage window of 2.0 V in 1 M KOH. Among the three pairs of asymmetric pseudocapacitors constructed from SrCoO_2.7, La_0.2Sr_0.8MnO_2.7, and brownmillerite (BM)-Sr₂Fe₂O₅, the BM-Sr₂Fe₂O₅//SrCoO_2.7 combination performed the best with a high energy density of 31 Wh kg^-1 at 450 W kg^-1 and power density of 10 000 W kg^-1 at 28 Wh kg^-1.

Exceptionally active iridium evolved from a pseudo-cubic perovskite for oxygen evolution in acid

Exploring robust catalysts for water oxidation in acidic electrolyte is challenging due to the limited material choice. Iridium (Ir) is the only active element with a high resistance to the acid corrosion during water electrolysis. However, Ir is rare, and its large-scale application could only be possible if the intrinsic activity of Ir could be greatly enhanced. Here, a pseudo-cubic SrCo_0.9Ir_0.1O_3-δ perovskite, containing corner-shared IrO6 octahedrons, is designed. The Ir in the SrCo_0.9Ir_0.1O_3-δ catalyst shows an extremely high intrinsic activity as reflected from its high turnover frequency, which is more than two orders of magnitude higher than that of IrO₂. During the electrochemical cycling, a surface reconstruction, with Sr and Co leaching, over SrCo_0.9Ir_0.1O_3-δ occurs. Such reconstructed surface region, likely contains a high amount of structural domains with corner-shared and under-coordinated IrO_x octahedrons, is responsible for the observed high activity.

While water splitting could provide a green means to store energy, there are few materials that can sustain high water oxidation half-reaction rates in acidic electrolytes. Here, authors design a perovskite oxide that generates high performance under-coordinated iridium sites during electrocatalysis.

Electron Correlations Engineer Catalytic Activity of Pyrochlore Iridates for Acidic Water Oxidation

The development of highly efficient oxygen-evolving catalysts compatible with powerful proton-exchange-membrane-based electrolyzers in acid environments is of prime importance for sustainable hydrogen production. In this field, understanding the role of electronic structure of catalysts on catalytic activity is essential but still lacking. Herein, a family of pyrochlore oxides R₂ Ir₂ O₇ (R = rare earth ions) is reported as acidic oxygen-evolving catalysts with superior-specific activities. More importantly, it is found that the intrinsic activity of this material significantly increases with the R ionic radius. Electronic structure studies reveal that the increased R ionic radius weakens electron correlations in these iridate oxides. This weakening induces an insulator-metal transition and an enhancement of IrO bond covalency, both of which promote oxygen evolution kinetics. This work demonstrates the importance of engineering the electron correlations to rationalize the catalytic activity toward water oxidation in strongly correlated transition-metal oxides.

Optimization of iron-doped Ni3S2 nanosheets by disorder engineering for oxygen evolution reaction

Nowadays, disorder engineering of catalytic materials has attracted significant attention because it can increase catalytic active sites and thus enhance their catalytic activity for electrocatalytic reactions. However, it is extremely important to uncover the relationship between disorder engineering and catalytic activity. Particularly, deep exploration of the relationship is very important for fabricating excellent highly active catalysts for oxygen evolution reaction (OER), which is one of the promising technologies in energy transition. In this study, we prepared Fe-doped Ni3S2 materials and simultaneously controlled the disorder degree by regulating the ion concentration to improve the activity for OER. By investigating the as-prepared catalysts with various disorder degrees for OER, we also explored the relationship between the disordered structure and OER catalytic performance. In particular, the optimized electrocatalyst with an appropriate disorder degree showed excellent activity and stability. We hope that this study provides a feasible direction to fabricate and optimize transition metal chalcogenide (TMC) electrocatalysts as efficient and stable electrocatalysts for OER.

The role of oxygen vacancies in water oxidation for perovskite cobalt oxide electrocatalysts: are more better?

We demonstrate that high oxygen vacancies significantly weaken the oxygen-evolving activity for PrBaCo₂O_6−δ due to the formation of vacancy ordering.

Pulsed reactivity on LaCoO3-based perovskites: a comprehensive approach to elucidate the CO oxidation mechanism and the effect of dopants

In this contribution we focus on three lanthanum cobaltate perovskites: undoped, Sr-doped, and Cu-doped to investigate the effect of doping on catalytic activity through pulsed reactivity experiments.

Enhancing oxygen and hydrogen evolution activities of perovskite oxide LaCoO3via effective doping of platinum

In this study, a series of perovskite oxides LaCo_1−xPt_xO_3−δ (x = 0, 0.02, 0.04, 0.06, and 0.08) were prepared by the citric acid–ethylenediaminetetraacetic acid (CA–EDTA) complexing sol–gel method and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). Then, the samples were investigated as OER and HER bifunctional electrocatalysts in alkaline media. Compared with other catalysts, LaCo_0.94Pt_0.06O_3−δ had good stability and presented more activity at a lower overpotential of 454 mV (at 10 mA cm⁻²), a lower Tafel slope value of 86 mV dec⁻¹ and a higher mass activity of 44.4 A g⁻¹ for OER; it displayed a lower overpotential of 294 mV (at −10 mA cm⁻²), a lower Tafel slope value of 148 mV dec⁻¹ and a higher mass activity of −34.5 A g⁻¹ for HER. The improved performance might depend on a larger ECSA, faster charge transfer rate and higher ratio of the highly oxidative oxygen species (O₂²⁻/O⁻). Furthermore, the e_g orbital filling of Co approaching 1.2 in the B site might play a leading role.

Among the perovskite LaCo_1−xPt_xO_3−δ catalysts, LaCo_0.94Pt_0.06O_3−δ proved best for catalyzing OER/HER, with η = 454/294 mV, which might be attributed to LCP6 having the e_g orbital filling of Co closest to 1.2.

Ferromagnetic ligand holes in cobalt perovskite electrocatalysts as an essential factor for high activity towards oxygen evolution

The definition of the interplay between chemical composition, electro-magnetic configuration and catalytic activity requires a rational study of the orbital physics behind active materials.

Boosting Overall Water Splitting via FeOOH Nanoflake-Decorated PrBa0.5Sr0.5Co2O5+δ Nanorods

The development of an efficient, robust, and low-cost catalyst for water electrolysis is critically important for renewable energy conversion. Herein, we achieve a significant improvement in electrocatalytic activity for both the oxygen-evolution reaction (OER) and the hydrogen-evolution reaction (HER) by constructing a novel hierarchical PrBa_0.5Sr_0.5Co₂O_5+δ (PBSC)@FeOOH catalyst. The optimized PBSC@FeOOH-20 catalyst consisted of layered perovskite PBSC nanorods and 20 nm thick amorphous FeOOH nanoflakes exhibiting an excellent electrocatalytic activity for the OER and the HER in 0.1 M KOH media, delivering a current density of 10 mA cm^-2 at overpotentials of 390 mV for the OER and 280 mV for the HER, respectively. The substantially enhanced performance is probably attributed to the hierarchical nanostructure, the good charge-transfer capability, and the strong electronic interactions of FeOOH and PBSC. Importantly, an alkaline electrolyzer-integrated PBSC@FeOOH-20 catalyst as both the anode and cathode shows a highly active overall water splitting with a low voltage of 1.638 V at 10 mA cm^-2 and high stability during continuous operation. This study provides new insights into exploring efficient bifunctional catalysts for overall water splitting, and it suggests that the rational design of the oxyhydroxide/perovskite heterostructure shows great potential as a promising type of electrocatalysts.

On the Role of Ferromagnetic Interactions in Highly Active Mo-Based Catalysts for Ammonia Synthesis

Reactions involving nitrogen fixation and transfer are of great industrial interest. In this regard, unveiling all the physical principles that determine their activity would be enormously beneficial for the rational design of novel catalysts with improved performance. Within this context, this work explores the activity of bulk molybdenum-based transition metal nitrides in ammonia synthesis. Our results highlight that the most active compositions show increasing ferromagnetism in the metal-nitrogen bonds, which constitute the active sites. We observe that the total spin accumulated in the bonds at the active sites is a physically meaningful descriptor to discriminate optimum catalysts. Higher activities are associated with ferromagnetic phases, and the underlying reason is an enhanced overlapping of the electronic wavefunctions; which also make the reaction steps spin-sensitive. These finding provides strong evidence of the general influence of electrons magnetic moment in catalysis, being part of the specific field of spintro-catalysis.

A Porous Pyrochlore Y2 [Ru1.6 Y0.4 ]O7-δ Electrocatalyst for Enhanced Performance towards the Oxygen Evolution Reaction in Acidic Media

A robust porous structure is often needed for practical applications in electrochemical devices, such as fuel cells, batteries, and electrolyzers. While templating approach is useful for the preparation of porous materials in general, it is not effective for the synthesis of oxide-based electrocatalysts owing to the chemical instability of disordered porous materials thus created. Now the synthesis of phase-pure porous yttrium ruthenate pyrochlore oxide using an unconventional porogen of perchloric acid is presented. The lattice oxygen defects are formed by the mixed-valence state of Ru^4+/5+ through the partial substitution of Ru⁴⁺ with Y³⁺ cations, leading to the formation of mixed B-site Y₂ [Ru_1.6 Y_0.4 ]O_7-δ . This porous Y₂ [Ru_1.6 Y_0.4 ]O_7-δ electrocatalyst exhibits a turnover frequency (TOF) of 560 s^-1 (at 1.5 V versus RHE) for the oxygen evolution reaction, which is two orders of magnitude higher than that of the RuO₂ reference catalyst (5.41 s^-1 ).

Molten Salt Flux Synthesis, Crystal Facet Design, Characterization, Electronic Structure, and Catalytic Properties of Perovskite Cobaltite

We present a simple and cost-effective molten salt synthetic route toward phase-pure perovskite cobaltite microcrystallines and successfully regulate different crystal facets for perovskite LaCoO₃ by the strong interaction between Cl^- anions and Sr²⁺ cations in molten salt system and polar plane. We then take LaCoO₃ (100 and 110), LaCoO₃ (111), and La_0.7Sr_0.3CoO₃ (111) as comparison models, and we characterize their crystal structure, morphology, composition, electronic state, and catalytic properties. X-ray photoelectron spectroscopy (XPS) shows that the prepared samples with high-energy (111) crystal facets contain more surface oxygen species and active Co ions than La enrichment perovskite LaCoO₃ (110 and 100) on the surface. Furthermore, combining with ambient-pressure XAS, valence band spectroscopy, and density functional calculations, we find that exposed high-energy (111) crystal facets and doping Sr ions can enhance the hybridization between Co cations and O anions and their O p-band center is closer to the Fermi level, compared with that of LaCoO₃ (100 and 110). As expected, the samples with high-energy (111) crystal facets show better CO oxidation activity than LaCoO₃ (100 and 110), and La_0.7Sr_0.3CoO₃ (111) exhibits the highest catalytic activity. Our findings provide a new avenue to prepare high-energy facet perovskite catalysts and we also clearly reveal the relationship between surface electronic structure and intrinsic CO oxidation activity of perovskite cobaltite.

Exceptional electrocatalytic oxygen evolution via tunable charge transfer interactions in La0.5Sr1.5Ni1-xFexO4±δ Ruddlesden-Popper oxides

The electrolysis of water is of global importance to store renewable energy and the methodical design of next-generation oxygen evolution catalysts requires a greater understanding of the structural and electronic contributions that give rise to increased activities. Herein, we report a series of Ruddlesden-Popper La_0.5Sr_1.5Ni_1-xFe_xO_4±δ oxides that promote charge transfer via cross-gap hybridization to enhance electrocatalytic water splitting. Using selective substitution of lanthanum with strontium and nickel with iron to tune the extent to which transition metal and oxygen valence bands hybridize, we demonstrate remarkable catalytic activity of 10 mA cm^-2 at a 360 mV overpotential and mass activity of 1930 mA mg^-1_ox at 1.63 V via a mechanism that utilizes lattice oxygen. This work demonstrates that Ruddlesden-Popper materials can be utilized as active catalysts for oxygen evolution through rational design of structural and electronic configurations that are unattainable in many other crystalline metal oxide phases.

Exceptional electrocatalytic oxygen evolution via tunable charge transfer interactions in La0.5Sr1.5Ni1-xFexO4±δ Ruddlesden-Popper oxides

The electrolysis of water is of global importance to store renewable energy and the methodical design of next-generation oxygen evolution catalysts requires a greater understanding of the structural and electronic contributions that give rise to increased activities. Herein, we report a series of Ruddlesden–Popper La_0.5Sr_1.5Ni_1−xFe_xO_4±δ oxides that promote charge transfer via cross-gap hybridization to enhance electrocatalytic water splitting. Using selective substitution of lanthanum with strontium and nickel with iron to tune the extent to which transition metal and oxygen valence bands hybridize, we demonstrate remarkable catalytic activity of 10 mA cm⁻² at a 360 mV overpotential and mass activity of 1930 mA mg⁻¹_ox at 1.63 V via a mechanism that utilizes lattice oxygen. This work demonstrates that Ruddlesden–Popper materials can be utilized as active catalysts for oxygen evolution through rational design of structural and electronic configurations that are unattainable in many other crystalline metal oxide phases.

Water electrolysis provides a potential means to large-scale renewable fuel generation, although sluggish oxygen evolution kinetics challenges progress. Here, authors report on Ruddlesden–Popper oxides as active oxygen evolution electrocatalysts that provide impetus for overcoming kinetic barriers.

Activation of Surface Oxygen Sites in a Cobalt-Based Perovskite Model Catalyst for CO Oxidation

Anionic redox chemistry is becoming increasingly important in explaining the intristic catalytic behavior in transition-metal oxides and improving catalytic activity. However, it is a great challenge to activate lattice oxygen in noble-metal-free perovskites for obtaining active peroxide species. Here, we take La_0.4Sr_0.6CoO_3-δ as a model catalyst and develop an anionic redox activity regulation method to activate lattice oxygen by tuning charge transfer between Co⁴⁺ and O^2-. Advanced XAS and XPS demonstrate that our method can effectively decrease electron density of surface oxygen sites (O^2-) to form more reactive oxygen species (O^{2- x}), which reduces the activation energy barriers of molecular O₂ and leads to a very high CO catalytic activity. The revealing of the activation mechanism for surface oxygen sites in perovskites in this work opens up a new avenue to design efficient solid catalysts. Furthermore, we also establish a correlation between anionic redox chemistry and CO catalytic activity.

Enlarged CoO Covalency in Octahedral Sites Leading to Highly Efficient Spinel Oxides for Oxygen Evolution Reaction

Cobalt-containing spinel oxides are promising electrocatalysts for the oxygen evolution reaction (OER) owing to their remarkable activity and durability. However, the activity still needs further improvement and related fundamentals remain untouched. The fact that spinel oxides tend to form cation deficiencies can differentiate their electrocatalysis from other oxide materials, for example, the most studied oxygen-deficient perovskites. Here, a systematic study of spinel ZnFe_x Co_2-x O₄ oxides (x = 0-2.0) toward the OER is presented and a highly active catalyst superior to benchmark IrO₂ is developed. The distinctive OER activity is found to be dominated by the metal-oxygen covalency and an enlarged CoO covalency by 10-30 at% Fe substitution is responsible for the activity enhancement. While the pH-dependent OER activity of ZnFe_0.4 Co_1.6 O₄ (the optimal one) indicates decoupled proton-electron transfers during the OER, the involvement of lattice oxygen is not considered as a favorable route because of the downshifted O p-band center relative to Fermi level governed by the spinel's cation deficient nature.

Chemical Activity of the Peroxide/Oxide Redox Couple: Case Study of Ba5Ru2O11 in Aqueous and Organic Solvents

The finding that triggering the redox activity of oxygen ions within the lattice of transition metal oxides can boost the performances of materials used in energy storage and conversion devices such as Li-ion batteries or oxygen evolution electrocatalysts has recently spurred intensive and innovative research in the field of energy. While experimental and theoretical efforts have been critical in understanding the role of oxygen nonbonding states in the redox activity of oxygen ions, a clear picture of the redox chemistry of the oxygen species formed upon this oxidation process is still missing. This can be, in part, explained by the complexity in stabilizing and studying these species once electrochemically formed. In this work, we alleviate this difficulty by studying the phase Ba₅Ru₂O₁₁, which contains peroxide O₂^2- groups, as oxygen evolution reaction electrocatalyst and Li-ion battery material. Combining physical characterization and electrochemical measurements, we demonstrate that peroxide groups can easily be oxidized at relatively low potential, leading to the formation of gaseous dioxygen and to the instability of the oxide. Furthermore, we demonstrate that, owing to the stabilization at high energy of peroxide, the high-lying energy of the empty σ* antibonding O-O states limits the reversibility of the electrochemical reactions when the O₂^2-/O^2- redox couple is used as redox center for Li-ion battery materials or as OER redox active sites. Overall, this work suggests that the formation of true peroxide O₂^2- states are detrimental for transition metal oxides used as OER catalysts and Li-ion battery materials. Rather, oxygen species with O-O bond order lower than 1 would be preferred for these applications.

Nanostructured Co-Mn containing perovskites for degradation of pollutants: Insight into the activity and stability

The efficient oxidative removal of persistent organic components in wastewater relies on low-cost heterogeneous catalysts that offer high catalytic activity, stability, and recyclability. Here, we designed a series of nanostructured Co-Mn containing perovskite catalysts, LaCo_1-xMn_xO_3+δ (LCM, x = 0, 0.3, 0.5, 0.7, and 1.0), with over-stoichiometric oxygen (δ > 0) to show superior catalytic activity for the degradation of a variety of persistent aqueous organic pollutants by activating peroxymonosulfate (PMS). The nature of LCM for catalysis was comprehensively investigated. A "volcano-shaped" correlation was observed between the catalytic activity and electron filling (e_g) of Co in LCM. Among these compounds, LaCo_0.5Mn_0.5O_3+δ (LCM55) exhibited an excellent activity with e_g = 1.27. The high interstitial oxygen ion diffusion rate (D_O^2- = 1.58 ± 0.01 × 10^-13 cm² s^-1) of LCM55 also contributes to its catalytic activity. The enhanced stability of LCM55 can be ascribed to its stronger relative acidity (3.22). Moreover, an increased solution pH (pH ≥ 7) generated a faster organic degradation rate and a decrease in metal leaching (0.004 mM) for LCM55 perovskite, justifying it as a potential material for environmental remediation.

Perovskites in catalysis and electrocatalysis

Catalysts for chemical and electrochemical reactions underpin many aspects of modern technology and industry, from energy storage and conversion to toxic emissions abatement to chemical and materials synthesis. This role necessitates the design of highly active, stable, yet earth-abundant heterogeneous catalysts. In this Review, we present the perovskite oxide family as a basis for developing such catalysts for (electro)chemical conversions spanning carbon, nitrogen, and oxygen chemistries. A framework for rationalizing activity trends and guiding perovskite oxide catalyst design is described, followed by illustrations of how a robust understanding of perovskite electronic structure provides fundamental insights into activity, stability, and mechanism in oxygen electrocatalysis. We conclude by outlining how these insights open experimental and computational opportunities to expand the compositional and chemical reaction space for next-generation perovskite catalysts.

Electronic Origin and Kinetic Feasibility of the Lattice Oxygen Participation During the Oxygen Evolution Reaction on Perovskites

Density functional theory is employed to investigate the electronic origin and feasibility of surface lattice oxygen (O_surf) participation during the oxygen evolution reaction (OER) on perovskites. O_surf participation occurs via the nonelectrochemical pathway in which adsorbed atomic oxygen (O*) diffuses from the transition-metal site to the oxygen site, and then O_surf shifts out of the surface plane to react with O* to form O_surf-O* and a surface oxygen vacancy. The different thermodynamic driving forces of O_surf participation on LaMO_3-δ (M = Ni, Co, and Cu) are explained by the changes in the oxidation state of the transition-metal site throughout the reaction. We show that O_surf participation on LaNiO₃ cannot be hindered by O_surf protonation in the OER potential range. By including the coverage effect and utilizing the implicit solvent model, we finally show that lattice oxygen mechanism is more feasible than the conventional mechanism for OER on LaNiO₃.

Noble Metal-Free Nanocatalysts with Vacancies for Electrochemical Water Splitting

The fast development of nanoscience and nanotechnology has significantly advanced the fabrication of nanocatalysts and the in-depth study of the structural-activity characteristics of materials at the atomic level. Vacancies, as typical atomic defects or imperfections that widely exist in solid materials, are demonstrated to effectively modulate the physicochemical, electronic, and catalytic properties of nanomaterials, which is a key concept and hot research topic in nanochemistry and nanocatalysis. The recent experimental and theoretical progresses achieved in the preparation and application of vacancy-rich nanocatalysts for electrochemical water splitting are explored. Engineering of vacancies has shown to open up a new avenue beyond the traditional morphology, size, and composition modifications for the development of nonprecious electrocatalysts toward efficient energy conversion. First, an introduction followed by discussions of different types of vacancies, the approaches to create vacancies, and the advanced techniques widely used to characterize these vacancies are presented. Importantly, the correlations between the vacancies and activities of the vacancy-rich electrocatalysts via tuning the electronic states, active sites, and kinetic energy barriers are reviewed. Finally, perspectives on the existing challenges along with some opportunities for the further development of vacancy-rich noble metal-free electrocatalysts with high performance are discussed.

Manipulating the Structural and Electronic Properties of Epitaxial SrCoO2.5 Thin Films by Tuning the Epitaxial Strain

Structure determines material's functionality, and strain tunes the structure. Tuning the coherent epitaxial strain by varying the thickness of the films is a precise route to manipulate the functional properties in the low-dimensional oxide materials. Here, to explore the effects of the coherent epitaxial strain on the properties of SrCoO_2.5 thin films, thickness-dependent evolutions of the structural properties and electronic structures were investigated by X-ray diffraction, Raman spectra, optical absorption spectra, scanning transmission electron microscopy (STEM), and first-principles calculations. By increasing the thickness of the SrCoO_2.5 films, the c-axis lattice constant decreases, indicating the relaxation of the coherent epitaxial strain. The energy band gap increases and the Raman spectra undergo a substantial softening with the relaxation of the coherent epitaxial strain. From the STEM results, it can be concluded that the strain causes the variation of the oxygen content in the BM-SCO2.5 films, which results in the variation of band gaps with varying the strain. First-principles calculations show that strain-induced changes in bond lengths and angles of the octahedral CoO₆ and tetrahedral CoO₄ cannot explain the variation band gap. Our findings offer an alternative strategy to manipulate structural and electronic properties by tuning the coherent epitaxial strain in transition-metal oxide thin films.

B-Site Cation-Ordered Double-Perovskite Oxide as an Outstanding Electrode Material for Supercapacitive Energy Storage Based on the Anion Intercalation Mechanism

Perovskite oxides are highly promising electrodes for oxygen-ion-intercalation-type supercapacitors owing to their high oxygen vacancy concentration, oxygen diffusion rate, and tap density. Based on the anion intercalation mechanism, the capacitance is contributed by surface redox reactions and oxygen ion intercalation in the bulk materials. A high concentration of oxygen vacancies is needed because it is the main charge carrier. In this study, we propose a B-site cation-ordered Ba₂Bi_0.1Sc_0.2Co_1.7O_6-δ as an electrode material with an extremely high oxygen vacancy concentration and oxygen diffusion rate. A maximum capacitance of 1050 F g^-1 was achieved, and a high capacitance of 780 F g^-1 was maintained even after 3000 charge-discharge cycles at a current density of 1 A g^-1 with an aqueous alkaline solution (6 M KOH) electrolyte, indicating an excellent cycling stability. In addition, the specific volumetric capacitance of Ba₂Bi_0.1Sc_0.2Co_1.7O_6-δ reaches up to 2549.4 F cm^-3 based on the dense construction and high tap density (3.2 g cm^-3). In addition, an asymmetric supercapacitor was constructed using activated carbon as a negative electrode, and it displayed the highest specific energy density of 70 Wh kg^-1 at the power density of 787 W kg^-1 in this study.

Layer-by-Layer Epitaxial Growth of Defect-Engineered Strontium Cobaltites

Control over structure and composition of (ABO₃) perovskite oxides offers exciting opportunities since these materials possess unique, tunable properties. Perovskite oxides with cobalt B-site cations are particularly promising, as the range of the cation's stable oxidation states leads to many possible structural frameworks. Here, we report growth of strontium cobalt oxide thin films by molecular beam epitaxy, and conditions necessary to stabilize different defect concentration phases. In situ X-ray scattering is used to monitor structural evolution during growth, while in situ X-ray absorption near-edge spectroscopy is used to probe oxidation state and measure changes to oxygen vacancy concentration as a function of film thickness. Experimental results are compared to kinetically limited thermodynamic predictions, in particular, solute trapping, with semiquantitative agreement. Agreement between observations of dependence of cobaltite phase on oxidation activity and deposition rate, and predictions indicates that a combined experimental/theoretical approach is key to understanding phase behavior in the strontium cobalt oxide system.

Degradation Kinetics during Oxygen Electrocatalysis on Perovskite-Based Surfaces in Alkaline Media

The oxygen evolution reaction (OER) during alkaline water electrolysis is the bottleneck of water splitting. Perovskite materials have been particularly proposed as good and economically reasonable electrocatalysts for the OER, showing promise and advantages with respect to classic metallic electrodes. However, the degradation of perovskites during catalysis limits their service lifetime. Recently, the material BaCo_0.98Ti_0.02O_3-δ:Co₃O₄ was shown to be electrocatalytically and chemically stable during water electrolysis even under industrially relevant conditions. The lifetime of this perovskite-based system is prolonged by a factor of 10 in comparison to that of Pr_0.2Ba_0.8CoO_3-δ and is comparable to that of industrially applied electrodes. Here we report on the degradation kinetics of several OER catalysts at room temperature, comparatively studied by monitoring the oxygen evolution at microelectrodes. A decrease in the reaction rate within a maximum of 60 s is observed, which is related to chemical and/or structural changes at the oxide surface.

Monitoring surface transformations of metal carbodiimide water oxidation catalysts by operando XAS and Raman spectroscopy

Chemical and structural transformations at the electrode surface of metal carbodiimides MNCN (M = Co, Ni, Mn, Cu), were studied by operando Raman and XAS spectroscopy during electrocatalytic water oxidation

First-Row Transition Metal Based Catalysts for the Oxygen Evolution Reaction under Alkaline Conditions: Basic Principles and Recent Advances

Owing to its abundance, high gravimetric energy density, and environmental friendliness, hydrogen is a promising renewable energy to replace fossil fuels. One of the most prominent routes toward hydrogen acquisition is water splitting, which is currently bottlenecked by the sluggish kinetics of oxygen evolution reaction (OER). Numerous of electrocatalysts have been developed in the past decades to accelerate the OER process. Up to now, the first-row transition metal based compounds are in pole position under alkaline conditions, which have become subjects of extensive studies. Recently, significant advances in providing compelling catalytic performance as well as exploring their catalytic mechanisms have been achieved in this area. In this review, we summarized the fundamentals and recent progresses in first-row transition metal based OER catalysts, with special emphasis on the pathways of promoting catalytic performance by concrete strategies. New insight into material design, particularly the role of experimental approaches in the electrocatalytic performance and reaction mechanisms of OER are expected to be provided.

Regulating p-block metals in perovskite nanodots for efficient electrocatalytic water oxidation

Water oxidation represents the core process of many sustainable energy systems, such as fuel cells, rechargeable metal-air batteries, and water splitting. Material surface defects with high-energy hanging bonds possess superb intrinsic reactivity, whose actual performance is limited by the dimension and conductivity of the electrocatalyst. Herein we propose a surface defect-rich perovskite electrocatalyst through a p-block metal regulation concept to achieve high performance for oxygen evolution. As a typical p-metal, Sn⁴⁺ dissolves from the solid phase from model SnNiFe perovskite nanodots, resulting in abundant surface defects with superior water oxidation performance. An oxygen pool model and a fusion-evolution mechanism are therefore proposed for the in-depth understanding of p-block metal regulation and the oxygen evolution reaction. The energy chemistry unveiled herein provides insights into water oxidation and helps to tackle critical issues in multi-electron oxygen electrocatalysis.

Electrocatalysts that possess high densities of surface defects show great promise for efficient water oxidation. Here the authors demonstrate that regulating the p-block metal content in perovskite nanodots imparts these materials with abundant surface defects and excellent electrocatalytic activity.