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The perovskite-type oxide Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) is known as a highly active and stable oxygen evolution reaction (OER) electrocatalyst composited out of non-noble metals. The possibility of using the scalable flame spray synthesis (FSS) technique for the production of BSCF nanoparticles intensified the interest in this material for a future application in an alkaline water electrolyzer. A possible scale-up would require the optimization of the synthesis parameters to maximize the production rate. To further understand the influence of the synthesis parameters of the tunable FSS on the OER activity of BSCF, a systematic study was carried out by producing BSCF with different total metal concentrations (CTM), flow rates of the precursor solution (FRPS) and of the dispersion gas (FRDG). This study reveals that all three parameters have a direct impact on the OER activity of BSCF—measured in a rotating disc electrode (RDE) setup—due to the controllability of the initial Co and Fe oxidation state—indicated by X-ray absorption spectroscopy (XAS) measurements—and with that also of the oxygen vacancy concentration in the as-synthesized BSCF. This controllability enables the optimization of the OER activity of BSCF and emphasizes the importance of having Co in a lower initial oxidation state for reaching a high electrocatalytic performance. [ABSTRACT FROM AUTHOR]

A new endstation to perform operando chemical analysis at solid–liquid interfaces by means of ambient pressure x-ray photoelectron spectroscopy (APXPS) is presented. The endstation is located at the Swiss Light Source and can be attached to the soft x-ray in situ spectroscopy beamline (X07DB) for solid–gas type experiments and to a tender x-ray beamline (PHOENIX I) for solid–liquid interface experiments. The setup consists of three interconnected ultrahigh vacuum chambers: one for sample preparation using surface science techniques, the analysis chamber for APXPS experiments, and an entry-lock chamber for sample transfer across the two pressure regimes. The APXPS chamber is designed to study solid–liquid interfaces stabilized by the dip and pull method. Using a three-electrode setup, the potential difference across the solid-electrolyte interface can be controlled, as is demonstrated here using an Ir(001) electrode dipped and pulled from a 0.1M KOH electrolyte. The new endstation is successfully commissioned and will offer unique opportunities for fundamental studies of phenomena that take place at solid–liquid interfaces and that are relevant for fields such as electrochemistry, photochemistry, or biochemistry, to name a few. [ABSTRACT FROM AUTHOR]

Proton exchange membrane water electrolysis (PEMWE) is a promising technology for electricity-to-fuel conversion which allows for direct production of hydrogen from water. One of the key problems limiting widespread implementation of PEMWE into energy systems is the sluggish kinetics of the anodic process: the oxygen evolution reaction (OER). Additionally, state-of-the-art OER materials contain large amounts of low abundant noble metals (Ru, Ir), and therefore, development of low-cost, highly active and stable OER catalysts remains an important challenge. We developed a synthetic approach to the iridium pyrochlores-complex oxides of iridium with reduced content of the noble metal as compared to IrO2. The materials were synthesized from molten sodium nitrate (Adams fusion method) at moderate temperatures (500-575 °C) and consist of highly crystalline iridium pyrochlore nanoparticles with surface areas of up to 40 m2 g-1, which is a significant improvement compared to the traditional high temperature solid-state synthesis. Electrochemical measurements in acidic media showed that yttrium and bismuth pyrochlore catalysts possess high OER activity approaching the activity of state-of-the-art IrO2 nanoparticles. High intrinsic activities and stability behavior of yttrium iridium catalysts were correlated with the formation of the highly active IrOx surface layer due to leaching of the Y3+ cations into the electrolyte solution, revealed both experimentally and computationally using density functional theory calculations. [ABSTRACT FROM AUTHOR]

In the current scenario of innovative energy systems, reversible solid oxide cells (r-SOCs) are widely recognized as promising devices, thanks to their high conversion efficiency in both power-to-fuel and fuel-to-power modes. A crucial aspect for their scale-up is the development of multi-functional compounds that can work as either air or fuel electrodes. The most reliable alternative to conventional materials for r-SOCs are perovskite oxides. Sr-substituted lanthanum ferrites (LSF) show large mixed ionic and electronic conductivity (MIEC) and versatile catalytic activity upon B-site doping. Nevertheless, their poor stability in highly reducing environments precludes their widespread application. In this work, 5 mol% Ru-doped lanthanum ferrite La 0.6 Sr 0.4 Fe 0.95 Ru 0.05 O 3-δ (LSFR05) is investigated as a multi-functional electrode for reversible and symmetric solid oxide cells (r-SSOCs). Ru-doping promotes LSF stability, retaining a higher lattice oxygen content. Upon reduction, LSFR05 exhibits exsolution of uniformly dispersed catalytically active Fe–Ru nanoparticles. All-perovskite symmetric cells showed remarkably high power density as H 2 –SOFC at 850 °C (602 mW cm−2), while in CO 2 -SOEC mode a current density output of 1.39 A cm−2 at 1.5 V was obtained. With a standard 70:30 CO 2 :CO gas composition, the cell showed a polarization resistance as low as 165 mΩ cm2. The complete SOFC-SOEC reversibility was successfully assessed for over 200 h. [Display omitted] • 5 mol%-Ru doped LSF perovskite as a versatile electrode for reversible symmetric cells. • Ru-doping improves stability and promotes Fe–Ru nanoparticles exsolution. • As a SOFC, symmetric cell (SSOC) shows 602 mW cm−2 power density in H 2 at 850 °C. • As a SOEC for CO 2 electrolysis, SSOC shows 1.39 A cm−2 at 1.5 V, at 850 °C. • SOFC-SOEC reversibility for over 200 h in 50:50 CO 2 :CO. [ABSTRACT FROM AUTHOR]

Pyrochlore oxides (A 2 B 2 O 7) simultaneously containing iridium and ruthenium in the B-site are promising catalysts for oxygen evolution reaction (OER) in acid media. The catalytic activity of the pyrochlore based catalysts is increased by the coexistence of Ir and Ru in the B-site of the pyrochlore structure. Lanthanide (Yb, Gd, or Nd) stabilized mixed pyrochlores with a fraction of Ru in the B-site of x Ru =0.2, 0.4, 0.6, 0.8 were synthesized by the spray-freeze freeze-dry approach. All prepared mixed pyrochlore catalysts are surpassing the OER activity of the corresponding iridium and ruthenium analogues featuring no cation mixing as well as that of the benchmark IrO 2 catalyst. The synergy of Ir and Ru in the B-site of the pyrochlore structure suppresses the effect of the A-site cation radius on the OER activity. The observed OER activity scales with the Ir-Ru bond distance which represents the local structure of the prepared materials. The most active ytterbium catalyst also shows a significant stability improvement under OER operando conditions over the benchmark IrO 2 Image, graphical abstract [ABSTRACT FROM AUTHOR]

It is shown that producing PrBaCo2O5+δ and Ba0.5Sr0.5Co0.8Fe0.2O2+δ nanoparticle by a scalable synthesis method leads to high mass activities for the oxygen evolution reaction (OER) with outstanding improvements by 10× and 50×, respectively, compared to those prepared via the state‐of‐the‐art synthesis method. Here, detailed comparisons at both laboratory and industrial scales show that Ba0.5Sr0.5Co0.8Fe0.2O2+δ appears to be the most active and stable perovskite catalyst under alkaline conditions, while PrBaCo2O5+δ reveals thermodynamic instability described by the density‐functional theory based Pourbaix diagrams highlighting cation dissolution under OER conditions. Operando X‐ray absorption spectroscopy is used in parallel to monitor electronic and structural changes of the catalysts during OER. The exceptional BSCF functional stability can be correlated to its thermodynamic meta‐stability under OER conditions as highlighted by Pourbaix diagram analysis. BSCF is able to dynamically self‐reconstruct its surface, leading to formation of Co‐based oxy(hydroxide) layers while retaining its structural stability. Differently, PBCO demonstrates a high initial OER activity while it undergoes a degradation process considering its thermodynamic instability under OER conditions as anticipated by its Pourbaix diagram. Overall, this work demonstrates a synergetic approach of using both experimental and theoretical studies to understand the behavior of perovskite catalysts. Flame Spray synthesized nanoparticles of PrBaCo2O5+δand Ba0.5Sr0.5Co0.8Fe0.2O2+δyield superior oxygen evolution reaction activities in alkaline electrolyte than those prepared by the state‐of‐the‐art sol–gel method. The functional stabilities of these materials are also assessed and elucidated in parallel with density‐functional theory computed Pourbaix diagrams highlighting the importance of understanding cation dissolution process. [ABSTRACT FROM AUTHOR]

The development of unified regenerative fuel cells (URFCs) necessitates an active and stable bifunctional oxygen electrocatalyst. The unique challenge of possessing high activity for both the oxygen reduction (ORR) and oxygen evolution (OER) reactions, while maintaining stability over a wide potential window impedes the design of bifunctional oxygen electrocatalysts. Herein, two design strategies are explored to optimize their performance. The first incorporates active sites for the ORR and OER, Mn and Co, into a single perovskite structure, which is achieved with the perovskites Ba 0.5 Sr 0.5 Co 0.8 Mn 0.2 O 3- δ (BSCM) and La 0.5 Ba 0.25 Sr 0.25 Co 0.5 Mn 0.5 O 3- δ (LBSCM). The second combines an active ORR perovskite catalyst (La 0.4 Sr 0.6 MnO 3- δ (LSM)) with an OER active perovskite catalyst (Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3- δ (BSCF)) in a physical mixed composite (BSCF/LSM). The success of the two strategies is investigated by measuring the catalysts' catalytic performance and response to alternating reducing and oxidizing potentials to mimic the dynamic conditions experienced during the operation of URFCs. Additionally, the continuous, potentiodynamic change in Mn, Co, and Fe oxidation states during the ORR and OER is elucidated with operando X-ray absorption spectroscopy (XAS) measurements, revealing key insights into the nature of the active sites. The results reveal important catalyst physiochemical properties and provide a guide for future research and design principles for bifunctional oxygen electrocatalysts., Competing Interests: The authors declare no conflict of interest., (This journal is © The Royal Society of Chemistry.)

The development of a sustainable and environmentally friendly energy economy encompasses efficient hydrogen production from renewable energy via electrolysis. In this context, great efforts have recently been dedicated to the development of more efficient and cost-effective electrocatalysts. Understanding the mechanism of the oxygen evolution reaction (OER) on transition metal oxide catalysts is of great interest, but the reaction and system complexity render the characterization of active sites and the understanding of reaction mechanisms challenging. Time resolved Quick X-ray Absorption Spectroscopy (XAS) can provide dynamic snapshots of the electronic and local structure of nanocatalysts, revealing the 'real active phase' of the catalyst, which can substantially differ from the as-prepared catalyst powder or the catalyst in form of an electrode under non-operating conditions. In this contribution, several examples will be presented showing how operando XAS can reveal catalyst-support interactions, changes in the reaction mechanism, and dynamic reversible/irreversible changes in the electronic and local structure of OER catalysts., (Copyright 2024 Emiliana Fabbri, Thomas J. Schmidt. License: This work is licensed under a Creative Commons Attribution 4.0 International License.)

Co-based catalysts are promising candidates to replace Ir/Ru-based oxides for oxygen evolution reaction (OER) catalysis in an acidic environment. However, both the reaction mechanism and the active species under acidic conditions remain unclear. In this study, by combining surface-sensitive soft X-ray absorption spectroscopy characterization with electrochemical analysis, we discover that the acidic OER activity of Co-based catalysts are determined by their surface oxidation/spin state. Surfaces composed of only high-spin Co II are found to be not active due to their unfavorable water dissociation to form Co III -OH species. By contrast, the presence of low-spin Co III is essential, as it promotes surface reconstruction of Co oxides and, hence, OER catalysis. The correlation between OER activity and Co oxidation/spin state signifies a breakthrough in defining the structure-activity relationship of Co-based catalysts for acidic OER, though, interestingly, such a relationship does not hold in alkaline and neutral environments. These findings not only help to design efficient acidic OER catalysts, but also deepen the understanding of the reaction mechanism., (© 2024. The Author(s).)

Co-based perovskite oxides are intensively studied as promising catalysts for electrochemical water splitting in an alkaline environment. However, the increasing Co demand by the battery industry is pushing the search for Co-free alternatives. Here we report a systematic study of the Co-free layered perovskite famil y RBaCuFeO 5+ δ (R = 4f lanthanide), where we uncover the existence of clear correlations between electrochemical properties and several physicochemical descriptors. Using a combination of advanced neutron and X-ray synchrotron techniques with ab initio DFT calculations we demonstrate and rationalize the positive impact of a large R ionic radius in their oxygen evolution reaction (OER) activity. We also reveal that, in these materials, Fe 3+ is the transition metal cation the most prone to donate electrons. We also show that similar R 3+ /Ba 2+ ionic radii favor the incorporation and mobility of oxygen in the layered perovskite structure and increase the number of available O diffusion paths, which have an additional, positive impact on both, the electric conductivity and the OER process. An unexpected result is the observation of a clear surface reconstruction exclusively in oxygen-rich samples ( δ > 0), a fact that could be related to their superior OER activity. The encouraging intrinsic OER values obtained for the most active electrocatalyst (LaBaCuFeO 5.49 ), together with the possibility of industrially producing this material in nanocrystalline form should inspire the design of other Co-free oxide catalysts with optimal properties for electrochemical water splitting., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

The role of the perovskite lattice oxygen in the oxygen evolution reaction (OER) is systematically studied in the PrBaCo 2 O 5+δ family. The reduced number of physical/chemical variables combined with in-depth characterizations such as neutron dif-fraction, O K-edge X-ray absorption spectroscopy (XAS), electron energy loss spectroscopy (EELS), magnetization and scanning transmission electron microscopy (STEM) studies, helps investigating the complex correlation between OER activity and a single perovskite property, such as the oxygen content. Larger amount of oxygen vacancies appears to facilitate the OER, possibly contributing to the mechanism involving the oxidation of lattice oxygen, i.e., the lattice oxygen evolution reaction (LOER). Furthermore, not only the number of vacancies but also their local arrangement in the perovskite lattice influences the OER activity, with a clear drop for the more stable, ordered stoichiometry., (© 2021 Wiley-VCH GmbH.)

Mixed oxides derived from the perovskite structure by combination of A- and B-site elements and by partial substitution of oxygen provide an immense playground of physico-chemical properties. Here, we give an account of our own research conducted at the Paul Scherrer Institute on perovskite-type oxides and oxynitrides used in electrochemical, photo(electro)chemical and catalytic processes aimed at facing energy relevant issues.

Density functional theory (DFT) has proven to be an invaluable and effective tool for identifying highly active electrocatalysts for the oxygen evolution reaction (OER). Herein, we take a computational approach to first identify a series of rare-earth pyrochlore oxides based on Ir and Ru as potential OER catalysts. The DFT-based phase diagrams, Pourbaix diagrams (E vs pH), projected density of states, and band energy diagrams were used to identify prospective OER catalysts based on rare-earth Ir and Ru pyrochlores. The predicted materials were synthesized using the spray-freeze freeze-drying approach to afford nanoparticulate oxides conforming to the pyrochlore structural type A 2 B 2 O 7 where A = Nd, Gd, or Yb and B = Ir or Ru. In agreement with the computed Pourbaix diagrams, the materials were found to be moderately stable under OER conditions. All prepared materials show higher stability as compared to the benchmark IrO 2 catalyst, and the OER mass activity of Yb 2 Ir 2 O 7 and the ruthenate pyrochlores (Nd 2 Ru 2 O 7 , Gd 2 Ru 2 O 7 , and Yb 2 Ru 2 O 7 ) were also found to exceed those of the benchmark IrO 2 catalyst. We find that the OER activity of each pyrochlore series (i.e., iridate or ruthenate) generally improves as the size of the A-site cation decreases, indicating that maintaining control over the structure can be used to influence the electrocatalytic properties.

Co/Fe oxyhydroxide catalysts have been deposited onto the surface of amorphous carbons or different perovskite oxides. By performing electrochemical characterizations and operando X-ray absorption spectroscopy measurements, novel insights into Co/Fe oxyhydroxide catalysts and their interactions with perovskite oxides have been revealed. The addition of Fe into Co oxyhydroxide catalysts greatly enhances the oxygen evolution reaction (OER) activity by stabilizing the Co cations into a lower oxidation state under operative conditions compared to the case of undoped Co oxyhydroxide. A beneficial Co/Fe electronic interaction for OER can also be achieved by depositing Co oxyhydroxide on Fe-containing oxide supports, such as the LaFeO 3 perovskite. Finally, it was found that, despite the lower Fe content in the Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ (BSCF) perovskite structure, Co oxyhydroxides supported on this perovskite exhibit the highest OER activity. Therefore, our findings suggest that perovskite structures presenting a large content of oxygen vacancies and undergoing surface reconstruction, such as BSCF, offer the best interface for Co oxyhydroxides. Finally, profiting from the beneficial Co/Fe electronic interaction and perovskite interface interaction, the highest OER activity has been achieved by depositing Co/Fe oxyhydroxide on the surface of BSCF perovskite.

Perovskite oxides have been at the forefront among catalysts for the oxygen evolution reaction (OER) in alkaline media offering a higher degree of freedom in cation arrangement. Several highly OER active Co-based perovskites have been known to show extraordinary activities and stabilities when the B-site is partially occupied by Fe. At the current stage, the role of Fe in enhancing the OER activity and stability is still unclear. In order to elucidate the roles of Co and Fe in the OER mechanism of cubic perovskites, two prospective perovskite oxides, La 0.2 Sr 0.8 Co 1- x Fe x O 3-δ and Ba 0.5 Sr 0.5 Co 1- x Fe x O 3-δ with x = 0 and 0.2, were prepared by flame spray synthesis as nanoparticles. This study highlights the importance of Fe in order to achieve high OER activity and stability by drawing relations between their physicochemical and electrochemical properties. Ex situ and operando X-ray absorption spectroscopy (XAS) was used to study the local electronic and geometric structure under oxygen evolving conditions. In parallel, density function theory computational studies were conducted to provide theoretical insights into our findings. Our findings show that the incorporation of Fe into Co-based perovskite oxides alters intrinsic properties rendering efficient OER activity and prolonged stability.

Multilayered heterostructures of Ce 0.85 Sm 0.15 O 2-δ and Y 0.16 Zr 0.92 O 2-δ of a high crystallographic quality were fabricated on (001)-oriented MgO single crystal substrates. Keeping the total thickness of the heterostructures constant, the number of ceria-zirconia bilayers was increased while reducing the thickness of each layer. At each interface Ce was found primarily in the reduced, 3+ oxidation state in a layer extending about 2 nm from the interface. Concurrently, the conductivity decreased as the thickness of the layers was reduced, suggesting a progressive confinement of the charge transport along the YSZ layers. The comparative analysis of the in-plane electrical characterization suggests that the contribution to the total electrical conductivity of these interfacial regions is negligible. For the smallest layer thickness of 2 nm the doped ceria layers are electrically insulating and the ionic transport only occurs through the zirconia layers. This is explained in terms of a reduced mobility of the oxygen vacancies in the highly reduced ceria.

The growing need to store increasing amounts of renewable energy has recently triggered substantial R&D efforts towards efficient and stable water electrolysis technologies. The oxygen evolution reaction (OER) occurring at the electrolyser anode is central to the development of a clean, reliable and emission-free hydrogen economy. The development of robust and highly active anode materials for OER is therefore a great challenge and has been the main focus of research. Among potential candidates, perovskites have emerged as promising OER electrocatalysts. In this study, by combining a scalable cutting-edge synthesis method with time-resolved X-ray absorption spectroscopy measurements, we were able to capture the dynamic local electronic and geometric structure during realistic operando conditions for highly active OER perovskite nanocatalysts. Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ as nano-powder displays unique features that allow a dynamic self-reconstruction of the material's surface during OER, that is, the growth of a self-assembled metal oxy(hydroxide) active layer. Therefore, besides showing outstanding performance at both the laboratory and industrial scale, we provide a fundamental understanding of the operando OER mechanism for highly active perovskite catalysts. This understanding significantly differs from design principles based on ex situ characterization techniques.

A novel sintering additive based on LiNO(3) was used to overcome the drawbacks of poor sinterability and low grain boundary conductivity in BaZr(0.8)Y(0.2)O(3-δ) (BZY20) protonic conductors. The Li-additive totally evaporated during the sintering process at 1600 °C for 6 h, which led to highly dense BZY20 pellets (96.5% of the theoretical value). The proton conductivity values of BZY20 with Li sintering-aid were significantly larger than the values reported for BZY sintered with other metal oxides, due to the fast proton transport in the "clean" grain boundaries and grain interior. The total conductivity of BZY20-Li in wet Ar was 4.45 × 10(-3) S cm(-1) at 600 °C. Based on the improved sinterability, anode-supported fuel cells with 25 μm-thick BZY20-Li electrolyte membranes were fabricated by a co-firing technique. The peak power density obtained at 700 °C for a BZY-Ni/BZY20-Li/La(0.6)Sr(0.4)Co(0.2)Fe(0.8)O(3-δ) (LSCF)-BZY cell was 53 mW cm(-2), which is significantly larger than the values reported for fuel cells using electrolytes made of BZY sintered with the addition of ZnO and CuO, confirming the advantage of using Li as a sintering aid., (This journal is © the Owner Societies 2011)