Low-Index Facet Polyhedron-Shaped Binary Cerium Titanium Oxide for High-Voltage Aqueous Zinc-Vanadium Redox Flow Batteries.

Aqueous zinc-vanadium hybrid redox flow battery systems are an efficient strategy to address the problems of low voltage and high cost of conventional all-vanadium redox flow batteries. However, the low electrochemical activity of carbon-based electrodes toward a vanadium redox reaction limits the performance of redox flow batteries. In this study, polyhedral binary cerium titanium oxide (Ce2/3TiO3, CTO) is synthesized using molten salt synthesis. CTO is fabricated by adjusting the temperature and composition. Notably, the prepared CTO obtained at 1000 °C shows the highest catalytic activity for a VO2+/VO2+ redox reaction. Further, CTO is prepared as a composite electrocatalyst and applied to a high-voltage aqueous zinc-vanadium redox flow battery. The cell adopts an alkali zinc electrolyte containing a Zn/[Zn(OH)4]2- redox pair and exhibits a high operating voltage of 2.26 V. Remarkably, a zinc-vanadium redox flow battery using the composite electrocatalyst exhibits a high energy density of 42.68 Wh L-1 at 20 mA cm-2 and an initial voltage efficiency of 90.3%. The excellent cell performance is attributed to structural defects caused by A-site deficiency in the perovskite oxide structure as well as oxygen vacancies resulting from the low valence state of the metal ion, which enhance the catalytic activity of the vanadium ions.

Covalency does not suppress O2 formation in 4d and 5d Li-rich O-redox cathodes.

Layered Li-rich transition metal oxides undergo O-redox, involving the oxidation of the O2- ions charge compensated by extraction of Li+ ions. Recent results have shown that for 3d transition metal oxides the oxidized O2- forms molecular O2 trapped in the bulk particles. Other forms of oxidised O2- such as O22- or (O-O)n- with long bonds have been proposed, based especially on work on 4 and 5d transition metal oxides, where TM-O bonding is more covalent. Here, we show, using high resolution RIXS that molecular O2 is formed in the bulk particles on O2‒ oxidation in the archetypal Li-rich ruthenates and iridate compounds, Li2RuO3, Li2Ru0.5Sn0.5O3 and Li2Ir0.5Sn0.5O3. The results indicate that O-redox occurs across 3, 4, and 5d transition metal oxides, forming O2, i.e. the greater covalency of the 4d and 5d compounds still favours O2. RIXS and XAS data for Li2IrO3 are consistent with a charge compensation mechanism associated primarily with Ir redox up to and beyond the 5+ oxidation state, with no evidence of O-O dimerization.

Advances in Understanding Mechanisms of Perovskites and Pyrochlores as Electrocatalysts using In-Situ X-ray Absorption Spectroscopy.

Metal oxides are some of the most promising candidates as electrocatalysts for electrical-energy-storage (EES) systems. Particularly, perovskite and pyrochlore oxides have been intensively investigated as bifunctional electrocatalysts because of their superior catalytic activities during the oxygen-reduction and -evolution reactions. However, the origin of the outstanding catalytic activities and structural changes of the materials are not clear, in part due to the difficulty in identification during electrocatalysis. In this Minireview, we present a critical overview of recent progress in understanding catalytic mechanisms of perovskite and pyrochlore oxides, highlighting the innovative in-situ X-ray absorption spectroscopy (XAS) analysis for electrochemical tests.

Highly Active Bifunctional Electrocatalysts for Oxygen Evolution and Reduction in Zn-Air Batteries.

To realize the full performance of Zn-air batteries, the co-presence of a highly efficient oxygen reduction reaction (ORR) and an oxygen evolution reaction (OER) in the system is critical. Although copper and nickel are known to be bifunctional catalysts for ORR and OER, sluggish reactions as a result of the exceptionally strong O=O bond on the metal surface make it difficult to achieve high system efficiency. In this study, a metal carbide layer (CuCx and NiCx ) on dendritic copper and nickel is fabricated by a facile electrodeposition process to provide efficient catalytic active sites with moderate binding energy for easy electron transfer in both the OER and the ORR. The dendritic structure provides an enriched catalytic surface and the protective metal carbide layer offers an appropriate O binding energy and durability of Zn-air batteries. Owing to the presence of the stable metal carbide surface on the dendritic metal, the CuCx /Cu and NiCx /Ni catalysts exhibited well-defined limiting current densities of -5.19 and -5.11 mA cm-2 , respectively, and improved ORR and OER activities with lower polarization than the corresponding metal catalysts. Density functional theory revealed a 0.74 eV decrease in the overpotential of NiCx /Ni-catalyzed OER reactions compared with Ni-catalyzed OER reactions. The experimental and theoretical results prove that carbide layers on dendritic metal surfaces can greatly improve the activity of ORR and OER bifunctional electrocatalysts for Zn-air batteries.

Seed-mediated atomic-scale reconstruction of silver manganate nanoplates for oxygen reduction towards high-energy aluminum-air flow batteries.

Aluminum-air batteries are promising candidates for next-generation high-energy-density storage, but the inherent limitations hinder their practical use. Here, we show that silver nanoparticle-mediated silver manganate nanoplates are a highly active and chemically stable catalyst for oxygen reduction in alkaline media. By means of atomic-resolved transmission electron microscopy, we find that the formation of stripe patterns on the surface of a silver manganate nanoplate originates from the zigzag atomic arrangement of silver and manganese, creating a high concentration of dislocations in the crystal lattice. This structure can provide high electrical conductivity with low electrode resistance and abundant active sites for ion adsorption. The catalyst exhibits outstanding performance in a flow-based aluminum-air battery, demonstrating high gravimetric and volumetric energy densities of ~2552 Wh kgAl-1 and ~6890 Wh lAl-1 at 100 mA cm-2, as well as high stability during a mechanical recharging process.

A Ternary Ni46 Co40 Fe14 Nanoalloy-Based Oxygen Electrocatalyst for Highly Efficient Rechargeable Zinc-Air Batteries.

Replacing noble-metal-based oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) electrocatalysts is the key to developing efficient Zn-air batteries (ZABs). Here, a homogeneous ternary Ni46 Co40 Fe14 nanoalloy with a size distribution of 30-60 nm dispersed in a carbon matrix (denoted as C@NCF-900) as a highly efficient bifunctional electrocatalyst produced via supercritical reaction and subsequent heat treatment at 900 °C is reported. Among all the transition-metal-based electrocatalysts, the C@NCF-900 exhibits the highest ORR performance in terms of half-wave potential (0.93 V) in 0.1 m KOH. Moreover, C@NCF-900 exhibits negligible activity decay after 10 000 voltage cycles with minor reduction (0.006 V). In ZABs, C@NCF-900 outperforms the mixture of Pt/C 20 wt% and IrO2 , cycled over 100 h under 58% depth of discharge condition. Furthermore, density functional theory (DFT) calculations and in situ X-ray absorption spectroscopy strongly support the active sites and site-selective reaction as a plausible ORR/OER mechanism of C@NCF-900.

Unveiling the Catalytic Origin of Nanocrystalline Yttrium Ruthenate Pyrochlore as a Bifunctional Electrocatalyst for Zn-Air Batteries.

Zn-air batteries suffer from the slow kinetics of oxygen reduction reaction (ORR) and/or oxygen evolution reaction (OER). Thus, the bifunctional electrocatalysts are required for the practical application of rechargeable Zn-air batteries. In terms of the catalytic activity and structural stability, pyrochlore oxides (A2[B2-xAx]O7-y) have emerged as promising candidates. However, a limited use of A-site cations (e.g., lead or bismuth cations) of reported pyrochlore catalysts have hampered broad understanding of their catalytic effect and structure. More seriously, the catalytic origin of the pyrochlore structure was not clearly revealed yet. Here, we report the new nanocrystalline yttrium ruthenate (Y2[Ru2-xYx]O7-y) with pyrochlore structure. The prepared pyrochlore oxide demonstrates comparable catalytic activities in both ORR and OER, compared to that of previously reported metal oxide-based catalysts such as perovskite oxides. Notably, we first find that the catalytic activity of the Y2[Ru2-xYx]O7-y is associated with the oxidations and corresponding changes of geometric local structures of yttrium and ruthenium ions during electrocatalysis, which were investigated by in situ X-ray absorption spectroscopy (XAS) in real-time. Zn-air batteries using the prepared pyrochlore oxide achieve highly enhanced charge and discharge performance with a stable potential retention for 200 cycles.

Carbon-Coated Core-Shell Fe-Cu Nanoparticles as Highly Active and Durable Electrocatalysts for a Zn-Air Battery.

Understanding the interaction between a catalyst and oxygen has been a key step in designing better electrocatalysts for the oxygen reduction reaction (ORR) as well as applying them in metal-air batteries and fuel cells. Alloying has been studied to finely tune the catalysts' electronic structures to afford proper binding affinities for oxygen. Herein, we synthesized a noble-metal-free and nanosized transition metal CuFe alloy encapsulated with a graphitic carbon shell as a highly efficient and durable electrocatalyst for the ORR in alkaline solution. Theoretical models and experimental results demonstrated that the CuFe alloy has a more moderate binding strength for oxygen molecules as well as the final product, OH(-), thus facilitating the oxygen reduction process. Furthermore, the nitrogen-doped graphitic carbon-coated layer, formed catalytically under the influence of iron, affords enhanced charge transfer during the oxygen reduction process and superior durability. These benefits were successfully confirmed by realizing the catalyst application in a mechanically rechargeable Zn-air battery.

All-solid-state cable-type flexible zinc-air battery.

A cable-type flexible Zn-air battery with a spiral zinc anode, gel polymer electrolyte (GPE), and air cathode coated on a nonprecious metal catalyst is designed in order to extend its application area toward wearable electronic devices.

Fabrication of Ba0.5Sr0.5Co0.8Fe0.2O(3-δ) catalysts with enhanced electrochemical performance by removing an inherent heterogeneous surface film layer.

A heat-treatment approach for Ba0.5Sr0.5Co0.8Fe0.2O(3-δ) (BSCF5582) is introduced as a way of enhancing the electrocatalytic performance of perovskite catalysts. The perovskite made by heat-treatment in oxygen atmosphere loses around 30 nm of spinel layer on the surface relative to the untreated version, and demonstrates enhanced oxygen reduction reaction and oxygen evolution reaction catalytic activities.

Metal-free Ketjenblack incorporated nitrogen-doped carbon sheets derived from gelatin as oxygen reduction catalysts.

Electrocatalysts facilitating oxygen reduction reaction (ORR) are vital components in advanced fuel cells and metal-air batteries. Here we report Ketjenblack incorporated nitrogen-doped carbon sheets derived from gelatin and apply these easily scalable materials as metal-free electrocatalysts for ORR. These carbon nanosheets demonstrate highly comparable catalytic activity for ORR as well as better durability than commercial Vulcan carbon supported Pt catalysts in alkaline media. Physico-chemical characterization and theoretical calculations suggest that proper combination of graphitic and pyridinic nitrogen species with more exposed edge sites effectively facilitates a formation of superoxide, [O2(ad)](-), via one-electron transfer, thus increasing catalytic activities for ORR. Our results demonstrate a novel strategy to expose more nitrogen doped edge sites by irregular stacked small sheets in developing better electrocatalysts for Zn-air batteries. These desirable architectures are embodied by an amphiphlilic gelatin mediated compatible synthetic strategy between hydrophobic carbon and aqueous water.

STA1, an Arabidopsis pre-mRNA processing factor 6 homolog, is a new player involved in miRNA biogenesis.

MicroRNAs (miRNAs) are small regulatory RNAs that have important regulatory roles in numerous developmental and metabolic processes in most eukaryotes. In Arabidopsis, DICER-LIKE1 (DCL1), HYPONASTIC LEAVES 1, SERRATE, HUA ENHANCER1 and HASTY are involved in processing of primary miRNAs (pri-miRNAs) to yield precursor miRNAs (pre-miRNAs) and eventually miRNAs. In addition to these components, mRNA cap-binding proteins, CBP80/ABA HYPERSENSITIVE1 and CBP20, also participate in miRNA biogenesis. Here, we show that STABILIZED1 (STA1), an Arabidopsis pre-mRNA processing factor 6 homolog, is also involved in the biogenesis of miRNAs. Similar to other miRNA biogenesis-defective mutants, sta1-1 accumulated significantly lower levels of mature miRNAs and concurrently higher levels of pri-miRNAs than wild type. The dramatic reductions of mature miRNAs were associated with the accumulation of their target gene transcripts and developmental defects. Furthermore, sta1-1 impaired splicing of intron containing pri-miRNAs and decreased transcript levels of DCL1. These results suggest that STA1 is involved in miRNA biogenesis directly by functioning in pri-miRNA splicing and indirectly by modulating the DCL1 transcript level.