Robust Crystal Phase Separation with Distinct Charge, Orbital, and Spin Orders in AgMn7O12.

An AA'3B4O12-type A-site-ordered quadruple perovskite oxide AgMn7O12 was prepared by high-pressure and high-temperature methods. At room temperature, the compound crystallizes into a cubic Im3̅ symmetry with a charge distribution of AgMn33+Mn43.5+O12. With the temperature decreasing to TCO,OO ≈ 180 K, the compound undergoes a structural phase transition toward a monoclinic C2/m symmetry, giving rise to a B-site charge- and orbital-ordered AgMn33+Mn23+Mn24+O12 phase. Moreover, this charge-/orbital-ordered main phase coexists with the initial cubic AgMn33+Mn43.5+O12 phase in the wide temperature range we measured. The charge-/orbital-ordered phase shows two antiferromagnetic phase transitions near 125 and 90 K, respectively. Short-range ferromagnetic correlations are found to occur for the initial B-site mixed cubic phase around 35 K. Because of the robust phase separation, considerable magnetoresistance effects are observed below TCO,OO in AgMn7O12.

Spin-Polarization Strategy for Enhanced Acidic Oxygen Evolution Activity.

Spin-polarization is known as a promising way to promote the anodic oxygen evolution reaction (OER), since the intermediates and products endow spin-dependent behaviors, yet it is rarely reported for ferromagnetic catalysts toward acidic OER practically used in industry. Herein, the first spin-polarization-mediated strategy is reported to create a net ferromagnetic moment in antiferromagnetic RuO2 via dilute manganese (Mn2+ ) (S = 5/2) doping for enhancing OER activity in acidic electrolyte. Element-selective X-ray magnetic circular dichroism reveals the ferromagnetic coupling between Mn and Ru ions, fulfilling the Goodenough-Kanamori rule. The ferromagnetism behavior at room temperature can be well interpreted by first principles calculations as the interaction between the Mn2+ impurity and Ru ions. Indeed, Mn-RuO2 nanoflakes exhibit a strongly magnetic field enhanced OER activity, with the lowest overpotential of 143 mV at 10 mA cmgeo -2 and negligible activity decay in 480 h stability (vs 200 mV/195 h without magnetic field) as known for magnetic effects in the literature. The intrinsic turnover frequency is also improved to reach 5.5 s-1 at 1.45 VRHE . This work highlights an important avenue of spin-engineering strategy for designing efficient acidic oxygen evolution catalysts.

Iridium single atoms incorporated in Co3O4 efficiently catalyze the oxygen evolution in acidic conditions.

Designing active and stable electrocatalysts with economic efficiency for acidic oxygen evolution reaction is essential for developing proton exchange membrane water electrolyzers. Herein, we report on a cobalt oxide incorporated with iridium single atoms (Ir-Co3O4), prepared by a mechanochemical approach. Operando X-ray absorption spectroscopy reveals that Ir atoms are partially oxidized to active Ir>4+ during the reaction, meanwhile Ir and Co atoms with their bridged electrophilic O ligands acting as active sites, are jointly responsible for the enhanced performance. Theoretical calculations further disclose the isolated Ir atoms can effectively boost the electronic conductivity and optimize the energy barrier. As a result, Ir-Co3O4 exhibits significantly higher mass activity and turnover frequency than those of benchmark IrO2 in acidic conditions. Moreover, the catalyst preparation can be easily scaled up to gram-level per batch. The present approach highlights the concept of constructing single noble metal atoms incorporated cost-effective metal oxides catalysts for practical applications.

Correlation of the crystal structure and ion storage behavior of MoO3 electrode materials for aluminum-ion energy storage studied using in situ X-ray spectroscopy.

To characterize the correlation of the crystal structure and Al-ion storage behavior, we prepared various crystal structures of MoO3 (α-MoO3, ß-MoO3 and h-MoO3) electrode materials and studied them via in situ X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) techniques. The α-MoO3 electrode material possesses a specific capacitance of 575.4 F g-1 and a gravimetric capacity of 207.8 mA h g-1 at a current density of 1 A g-1. From the in situ XRD results, the crystal structures of α-MoO3 and ß-MoO3 show a significant distortion, whereas that of h-MoO3 is minorly affected during the insertion or extraction of Al3+ ions. Based on the in situ XAS results, the MoO6 octahedral structure and Mo ion valence of α-MoO3 and ß-MoO3 also exhibit a strong variation, whereas those of h-MoO3 are nearly unchanged during the insertion or extraction of Al3+ ions. Notably, in situ XRD and XAS also clearly show a possible phase of AlxMoO3 during the Al3+ insertion and extraction cycles in the α-MoO3 and ß-MoO3 electrode materials, which may play a crucial role in the behavior of the residue of Al3+ ions and poor cycling stability. We provide clear evidence that the Al-ion energy storage performance of various MoO3 electrode materials is strongly associated with the corresponding tunnel space and the stability of their crystal structures. This work also provides new insight into a strong correlation between ion-storage efficiency and the corresponding crystal structure, which is greatly helpful for the development and improvement of new electrode materials for Al-ion energy storage.

1.8 V Aqueous Symmetric Carbon-Based Supercapacitors with Agarose-Bound Activated Carbons in an Acidic Electrolyte.

The specific energy of an aqueous carbon supercapacitor is generally small, resulting mainly from a narrow potential window of aqueous electrolytes. Here, we introduced agarose, an ecologically compatible polymer, as a novel binder to fabricate an activated carbon supercapacitor, enabling a wider potential window attributed to a high overpotential of the hydrogen-evolution reaction (HER) of agarose-bound activated carbons in sulfuric acid. Assembled symmetric aqueous cells can be galvanostatically cycled up to 1.8 V, attaining an enhanced energy density of 13.5 W h/kg (9.5 µW h/cm2) at 450 W/kg (315 µW/cm2). Furthermore, a great cycling behavior was obtained, with a 94.2% retention of capacitance after 10,000 cycles at 2 A/g. This work might guide the design of an alternative material for high-energy aqueous supercapacitors.

Electrochemical properties and mechanism of CoMoO4@NiWO4 core-shell nanoplates for high-performance supercapacitor electrode application studied via in situ X-ray absorption spectroscopy.

Binary transition metal oxide CoMoO4@NiWO4 core-shell nanoplates grown directly on a Ni foam substrate were synthesized via a facile two-step hydrothermal process. The core-shell nanoplates with high electrochemical surface area (2933 cm2) demonstrated excellent electrochemical properties (areal capacity as high as 0.464 mA h cm-2 at a current density of 5 mA cm-2) and great cycle stability (92.5% retention after 3000 cycles with a high current density of 40 mA cm-2). The mechanism of the electrochemical reactions based on the in situ X-ray absorption spectroscopy technique clearly shows that the Co and Ni elements simultaneously participate in the faradaic reactions with the electrolyte. These results indicate that the excellent electrochemical performance of CoMoO4@NiWO4 compared to that of CoMoO4 nanoplates is attributed to a large electrochemical surface area and synergistic effect between NiWO4 and CoMoO4. This combination of two binary transition metal oxides can hence provide an excellent route to develop a high-performance electrode material for supercapacitor applications.

A comparison of nitrogen-doped sonoelectrochemical and chemical graphene nanosheets as hydrogen peroxide sensors.

Nitrogen-doped graphene nanosheet (N-SEGN) with pyrrolic nitrogen and 5-9 vacancy defects has been successfully prepared from a hydrothermal reaction of tetra-2-pyridinylpyrazine and sonoelectrochemistry-exfoliated graphene nanosheet, with point defects. Additionally, based on the same reaction using chemically reduced graphene oxide, nitrogen-doped chemically reduced graphene oxide (N-rGO) with graphitic nitrogen was prepared. The N-SEGN and N-rGO were used as a non-enzymatic H2O2 sensors. The sensitivity of the N-SEGN was 231.3 µA·mM-1·cm-2, much greater than 57.3 µA·mM-1·cm-2 of N-rGO. The N-SEGN showed their potential for being a H2O2 sensor.