Intermediate Sr2Co1.5Fe0.5O6-δ Tetragonal Structure between Perovskite and Brownmillerite as a Model Catalyst with Layered Oxygen Deficiency for Enhanced Electrochemical Water Oxidation

Three-dimensional ordered macroporous design of heterogeneous cobalt-iron phosphides as oxygen evolution electrocatalyst

Oxygen evolution reaction (OER) plays a key role in electrochemical conversion, which needs efficient and economical electrocatalyst to boost its kinetics for large-scale application. Herein, a bimetallic CoP/FeP2heterostructure with a three-dimensional ordered macroporous structure (3DOM-CoP/FeP2) was synthesized as an OER catalyst to demonstrate a heterogeneous engineering induction strategy. By adjusting the electron distribution and producing a lot of active sites, the heterogeneous interface enhances catalytic performance. High specific surface area is provided by the 3DOM structure. Additionally, at the solid-gas-electrolyte threephase interface, the electrocatalytic reaction exhibits good mass transfer.In situRaman spectroscopy characterization revealed that FeOOH and CoOOH reconstructed from CoP/FeP2were the true OER active sites. Consequently, the 3DOM-CoP/FeP2demonstrates superior OER activity with a low overpotentials of 300/420 mV at 10/100 mA cm-2and meritorious OER durability. It also reveals promising performance as the overall water splitting anode.

Introducing gradient Er ions and oxygen defects into SrCoO3 for regulating structural, electrical and magnetic transport properties

The SrCoO3-δ system has broad application potential due to its diverse crystal structures, oxidation stoichiometric ratio, and significant electrical and magnetic properties. However, it faces the challenges of a complex crystal structure and oxygen defect control in this material system. Herein, we introduce oxygen defects into SrCoO3-δvia Er doping to regulate the structural, electrical and magnetic transport properties. Sr1-xErxCoO3-δ (x = 0-0.25) undergoes an evolution of structure and oxygen content (measured using the iodometric method) from hexagonal SrCoO2.626 (H + Co3O4) to cubic perovskite Sr0.9Er0.1CoO2.689 (CP) and finally to ordered tetragonal Sr0.8Er0.2CoO2.635 (OT). Among the three phases, Sr0.9Er0.1CoO2.689 (CP) exhibits the lowest resistivity, only 4.06 mΩ cm at room temperature, which is attributed to its high three-dimensional symmetry, overlap of O 2p and Co 3d orbitals at high oxygen ion concentration. Further introduction of Er ions and oxygen defects promotes the transformation from low spin Co4+ (LS, t52ge0g, S = 1/2) to high spin Co3+ (HS, t42ge2g, S = 2), and from the CoO6 octahedron (low magnetic moment transformation) to the CoO4.25 tetrahedron (high magnetic moment). The oxygen-deficient CoO4.25 layer appears, which can enhance the ordering of A sites and oxygen vacancies, and the CP phase transforms into room-temperature ferromagnetic Sr0.8Er0.2CoO2.635 (OT, TC∼330 K). Er ions provide unpaired electrons in the 2f orbital, which results in a strong magnetization of Sr0.8Er0.2CoO2.635 (OT, 4.66 μB/Co) at low temperatures.

High-Valence Oxides for High Performance Oxygen Evolution Electrocatalysis

Valence tuning of transition metal oxides is an effective approach to design high-performance catalysts, particularly for the oxygen evolution reaction (OER) that underpins solar/electric water splitting and metal-air batteries. Recently, high-valence oxides (HVOs) are reported to show superior OER performance, in association with the fundamental dynamics of charge transfer and the evolution of the intermediates. Particularly considered are the adsorbate evolution mechanism (AEM) and the lattice oxygen-mediated mechanism (LOM). High-valence states enhance the OER performance mainly by optimizing the eg -orbital filling, promoting the charge transfer between the metal d band and oxygen p band. Moreover, HVOs usually show an elevated O 2p band, which triggers the lattice oxygen as the redox center and enacts the efficient LOM pathway to break the "scaling" limitation of AEM. In addition, oxygen vacancies, induced by the overall charge-neutrality, also promote the direct oxygen coupling in LOM. However, the synthesis of HVOs suffers from relatively large thermodynamic barrier, which makes their preparation difficult. Hence, the synthesis strategies of the HVOs are discussed to guide further design of the HVO electrocatalysts. Finally, further challenges and perspectives are outlined for potential applications in energy conversion and storage.

Cationic Defect Engineering in Perovskite La2CoMnO6 for Enhanced Electrocatalytic Oxygen Evolution

The urgent need to promote the development of sustainable energy conversion requires exploration of highly efficient oxygen evolution reaction (OER) electrocatalysts. Defect engineering is a promising approach to address the inherent low electrical conductivity of metal oxides and limited reaction sites, for use in clean air applications and as electrochemical energy-storage electrocatalysts. In this article, oxygen defects are introduced into La₂CoMnO_6-δ perovskite oxides through the A-site cation defect strategy. By tuning the content of the A-site cation, oxygen defect concentration and corresponding electrochemical OER performance have been greatly improved. As a result, the defective La_1.8CoMnO_6-δ (L_1.8CMO) catalyst exhibits exceptional OER activity with an overpotential of 350 mV at 10 mA cm^-2, approximately 120 mV lower than that of the pristine perovskite. This enhancement can be attributed to the increase in surface oxygen vacancies, optimized e_g occupation of transition metal at the B-site, and enlarged Brunauer-Emmett-Teller surface area. The reported strategy facilitates the development of novel defect-mediated perovskites in electrocatalysis.

Direct observation of the dynamic reconstructed active phase of perovskite LaNiO3 for the oxygen-evolution reaction

Ni-based transition metal oxides are promising oxygen-evolution reaction (OER) catalysts due to their abundance and high activity. Identification and manipulation of the chemical properties of the real active phase on the catalyst surface is crucial to improve the reaction kinetics and efficiency of the OER. Herein, we used electrochemical-scanning tunnelling microscopy (EC-STM) to directly observe structural dynamics during the OER on LaNiO3 (LNO) epitaxial thin films. Based on comparison of dynamic topographical changes in different compositions of LNO surface termination, we propose that reconstruction of surface morphology originated from transition of Ni species on LNO surface termination during the OER. Furthermore, we showed that the change in surface topography of LNO was induced by Ni(OH)2/NiOOH redox transformation by quantifying STM images. Our findings demonstrate that in situ characterization for visualization and quantification of thin films is very important for revealing the dynamic nature of the interface of catalysts under electrochemical conditions. This strategy is crucial for in-depth understanding of the intrinsic catalytic mechanism of the OER and rational design of high-efficiency electrocatalysts.

Surface Reconstruction of Co4N Coupled with CeO2 toward Enhanced Alkaline Oxygen Evolution Reaction

Constructing the active interface in a heterojunction electrocatalyst is critical for the electron transfer and intermediate adsorption (O*, OH*, and HOO*) in alkaline oxygen evolution reaction (OER) but still remains challenging. Herein, a CeO₂/Co₄N heterostructure is rationally synthesized through the direct calcination of Ce[Co(CN)₆], followed by thermal nitridation. The in situ electrochemically generated CoOOH on the surface of Co₄N serves as the active site for the OER, and the coupled CeO₂ with oxygen vacancy can optimize the energy barrier of intermediate reactions of the OER, which simultaneously boosts the OER performance. Besides, electrochemical measurement results demonstrate that oxygen vacancies in CeO₂ and optimized absorption free energy originating from the electron transfer between CeO₂ and CoOOH contribute to enhanced OER kinetics. This work provides new insight into regulating the interface heterostructure to rationally design efficient OER electrocatalysts under alkaline conditions.

Co-Fe-P Nanosheet Arrays as a Highly Synergistic and Efficient Electrocatalyst for Oxygen Evolution Reaction

The rational design and synthesis of highly efficient electrocatalysts for oxygen evolution reaction (OER) is of critical importance to the large-scale production of hydrogen by water electrolysis. Here, we develop a bimetallic, synergistic, and highly efficient Co-Fe-P electrocatalyst for OER, by selecting a two-dimensional metal-organic framework (MOF) of Co-ZIF-L as the precursor. The Co-Fe-P electrocatalyst features pronounced synergistic effects induced by notable electron transfer from Co to Fe, and a large electrochemical active surface area achieved by organizing the synergistic Co-Fe-P into hierarchical nanosheet arrays with disordered grain boundaries. Such features facilitate the generation of abundant and efficiently exposed Co³⁺ sites for electrocatalytic OER and thus enable Co-Fe-P to deliver excellent activity (overpotential and Tafel slope as low as 240 mV and 36 mV dec^-1, respectively, at a current density of 10 mA cm^-2 in 1.0 M KOH solution). The Co-Fe-P electrocatalyst also shows great durability by steadily working for up to 24 h. Our work thus provides new insight into the development of highly efficient electrocatalysts based on nanoscale and/or electronic structure engineering.

The Pivotal Role of s-, p-, and f-Block Metals in Water Electrolysis: Status Quo and Perspectives

Transition metals, in particular noble metals, are the most common species in metal-mediated water electrolysis because they serve as highly active catalytic sites. In many cases, the presence of nontransition metals, that is, s-, p-, and f-block metals with high natural abundance in the earth-crust in the catalytic material is indispensable to boost efficiency and durability in water electrolysis. This is why alkali metals, alkaline-earth metals, rare-earth metals, lean metals, and metalloids receive growing interest in this research area. In spite of the pivotal role of these nontransition metals in tuning efficiency of water electrolysis, there is far more room for developments toward a knowledge-based catalyst design. In this review, five classes of nontransition metals species which are successfully utilized in water electrolysis, with special emphasis on electronic structure-catalytic activity relationships and phase stability, are discussed. Moreover, specific fundamental aspects on electrocatalysts for water electrolysis as well as a perspective on this research field are also addressed in this account. It is anticipated that this review can trigger a broader interest in using s-, p-, and f-block metals species toward the discovery of advanced polymetal-containing electrocatalysts for practical water splitting.