Insulator-transition-induced Degradation of Pyrochlore Ruthenates in Electrocatalytic Oxygen Evolution and Stabilization through Doping.

Ru-based pyrochlores (e.g., Y2Ru2O7-d) are promised to replace IrO2 in polymer electrolyte membrane (PEM) electrolyzers. It is significant to reveal the cliff attenuation on the oxygen evolution reaction (OER) performance of these pyrochlores. In this work, we monitor the structure changes and electrochemical behavior of Y2Ru2O7-d over the OER process, and it is found that the reason of decisive OER inactivation is derived from an insulator transition occurred within Y2Ru2O7-d due to its inner ²perfecting² lattice induced by continuous atom rearrangement. Therefore, a stabilization strategy of the Ir-substituted Y2Ru2O7-d is proposed to alleviate this undesirable behavior. The double-exchange interaction between Ru and Ir in [RuO6] and [IrO6] octahedra leads the charge redistribution with simultaneous spin configuration adjustment. The electronic state in newly formed octahedrons centered with Ru 4d3 (with the state of eg'2--a1g-1 eg0) and Ir 5d6 (eg'4a1g-2 eg0) relieves the uneven electron distributions in [RuO6] orbital. The attenuated Jahn-Teller effect alleviates atom rearrangement, represented as the mitigation of insulator transition, surface reconstruction, and metal dissolution. As results, the Ir-substituted Y2Ru2O7-d presents the greatly improved OER stability and PEM durability. This study unveils the OER degradation mechanism and stabilization strategy for material design of Ru-based OER catalysts for electrochemical applications.

Iridium Doped Pyrochlore Ruthenates for Efficient and Durable Electrocatalytic Oxygen Evolution in Acidic Media.

Developing highly active, durable, and cost-effective electrocatalysts for the oxygen evolution reaction (OER) is of prime importance in proton exchange membrane (PEM) water electrolysis techniques. Ru-based catalysts have high activities but always suffer from severe fading and dissolution issues, which cannot satisfy the stability demand of PEM. Herein, a series of iridium-doped yttrium ruthenates pyrochlore catalysts is developed, which exhibit better activity and much higher durability than commercial RuO2 , IrO2 , and most of the reported Ru or Ir-based OER electrocatalysts. Typically, the representative Y2 Ru1.2 Ir0.8 O7 OER catalyst demands a low overpotential of 220 mV to achieve 10 mA cm-2 , which is much lower than that of RuO2 (300 mV) and IrO2 (350 mV). In addition, the catalyst does not show obvious performance decay or structural degradation over a 2000 h stability test. EXAFS and XPS co-prove the reduced valence state of ruthenium and iridium in pyrochlore contributes to the improved activity and stability. Density functional theory reveals that the potential-determining steps barrier of OOH* formation is greatly depressed through the synergy effect of Ir and Ru sites by balancing the d band center and oxygen intermediates binding ability.

Heteroepitaxial Control of Fermi Liquid, Hund Metal, and Mott Insulator Phases in Single-Atomic-Layer Ruthenates.

Interfaces between dissimilar correlated oxides can offer devices with versatile functionalities, and great efforts have been made to manipulate interfacial electronic phases. However, realizing such phases is often hampered by the inability to directly access the electronic structure information; most correlated interfacial phenomena appear within a few atomic layers from the interface. Here, atomic-scale epitaxy and photoemission spectroscopy are utilized to realize the interface control of correlated electronic phases in atomic-scale ruthenate-titanate heterostructures. While bulk SrRuO3 is a ferromagnetic metal, the heterointerfaces exclusively generate three distinct correlated phases in the single-atomic-layer limit. The theoretical analysis reveals that atomic-scale structural proximity effects yield Fermi liquid, Hund metal, and Mott insulator phases in the quantum-confined SrRuO3 . These results highlight the extensive interfacial tunability of electronic phases, hitherto hidden in the atomically thin correlated heterostructure. Moreover, this experimental platform suggests a way to control interfacial electronic phases of various correlated materials.

Magnetic structures and electronic properties of cubic-pyrochlore ruthenates from first principles.

The magnetic ground states ofR2Ru2O7andA2Ru2O7withR= Pr, Gd, Ho, and Er, as well asA= Ca, Cd are predicted devising a combination of the cluster-multipole (CMP) theory and spin-density-functional theory (SDFT). The strong electronic correlation effects are estimated by the constrained-random-phase approximation (cRPA) and taken into account within the dynamical-mean-field theory (DMFT). The target compounds feature d-orbital magnetism on Ru4+and Ru5+ions forRandA, respectively, as well as f-orbital magnetism on theRsite, which leads to an intriguing interplay of magnetic interactions in a strongly correlated system. We find CMP + SDFT is capable of describing the magnetic ground states in these compounds. The cRPA captures a difference in the screening strength betweenR2Ru2O7andA2Ru2O7compounds, which leads to a qualitative and quantitative understanding of the electronic properties within DMFT.

Optical spectroscopy study of Ca3(Ru0.91Mn0.09)2O7 single crystal in high magnetic fields.

The magneto-optical spectrum, with magnetic fields up to 42 T, of double layered ruthenates Ca3(Ru0.91Mn0.09)2O7 (CRMO) single crystal is studied. Both the temperature and magnetic field induced insulator-to-metal transitions (IMTs) are observed via magneto-optical properties of the crystal. The critical magnetic field (H // c) of IMT for CRMO is found to be as large as 35 T at 5 K. The fine structure of optical spectra identified the antiferromagnetic/ferro-orbital-ordering configurations of Ru 4d orbitals at low temperatures. Meanwhile, the configuration of orbital polarization of such double-layer CRMO single crystal is discussed. These results suggest that the orbital degree of freedom plays an important role in the field induced IMT of multi-orbital system.

Strain and vector magnetic field tuning of the anomalous phase in Sr3Ru2O7.

A major area of interest in condensed matter physics is the way electrons in correlated electron materials can self-organize into ordered states, and a particularly intriguing possibility is that they spontaneously choose a preferred direction of conduction. The correlated electron metal Sr3Ru2O7 has an anomalous phase at low temperatures that features strong susceptibility toward anisotropic transport. This susceptibility has been thought to indicate a spontaneous anisotropy, that is, electronic order that spontaneously breaks the point-group symmetry of the lattice, allowing weak external stimuli to select the orientation of the anisotropy. We investigate further by studying the response of Sr3Ru2O7 in the region of phase formation to two fields that lift the native tetragonal symmetry of the lattice: in-plane magnetic field and orthorhombic lattice distortion through uniaxial pressure. The response to uniaxial pressure is surprisingly strong: Compressing the lattice by ~0.1% induces an approximately 100% transport anisotropy. However, neither the in-plane field nor the pressure phase diagrams are qualitatively consistent with spontaneous symmetry reduction. Instead, both are consistent with a multicomponent order parameter that is likely to preserve the point-group symmetry of the lattice, but is highly susceptible to perturbation.